j j thomson electron experiment

• Structure of Atom

Cathode Ray Experiment

What is cathode ray tube.

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

Table of Contents

Cathode ray tube, recommended videos.

• J.J.Thomson Experiment

Apparatus Setup

Procedure of the experiment.

• Frequently Asked Questions – FAQs

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

The cathode ray tube (CRT), invented in 1897 by the German physicist Karl Ferdinand Braun, is an evacuated glass envelope containing an electron gun a source of electrons and a fluorescent light, usually with internal or external means to accelerate and redirect the electrons. Light is produced when electrons hit a fluorescent tube.

The electron beam is deflected and modulated in a manner that allows an image to appear on the projector. The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

J. J. Thomson Experiment – The Discovery of Electron

The Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electrons and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

J. J. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes. He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and is called a cathode ray tube for the entire construction.

The experiment Cathode Ray Tube (CRT) conducted by J. J. Thomson, is one of the most well-known physical experiments that led to electron discovery . In addition, the experiment could describe characteristic properties, in essence, its affinity to positive charge, and its charge to mass ratio. This paper describes how J is simulated. J. Thomson experimented with Cathode Ray Tube.

The major contribution of this work is the new approach to modelling this experiment, using the equations of physical laws to describe the electrons’ motion with a great deal of accuracy and precision. The user can manipulate and record the movement of the electrons by assigning various values to the experimental parameters.

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

• Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
• High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.
• The electricity starts flowing as the circuit was complete.
• To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
• The positive pole and negative pole were kept on either side of the discharge ray.
• When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
• This was further confirmed by placing the phosphorescent substance at the end of the discharge ray. It glows when hit by a discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were on the positive side. So the constituents of the discharge tube were negatively charged.

After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom . And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, from all this we learn that the electrons are in fact the basic constituent of all the atoms.

Most of the mass of the atom and all of its positive charge are contained in a small nucleus, called a nucleus. The particle which is positively charged is called a proton. The greater part of an atom’s volume is empty space.

The number of electrons that are dispersed outside the nucleus is the same as the number of positively charged protons in the nucleus. This explains the electrical neutrality of an atom as a whole.

Uses of Cathode Ray Tube

• Used as a most popular television (TV) display.
• X-rays are produced when fast-moving cathode rays are stopped suddenly.
• The screen of a cathode ray oscilloscope, and the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall off the screen pictures are visible on the screen.

Frequently Asked Questions – FAQs

What are cathode ray tubes made of.

The cathode, or the emitter of electrons, is made of a caesium alloy. For many electronic vacuum tube systems, Cesium is used as a cathode, as it releases electrons readily when heated or hit by light.

Where can you find a cathode ray tube?

Cathode rays are streams of electrons observed in vacuum tubes (also called an electron beam or an e-beam). If an evacuated glass tube is fitted with two electrodes and a voltage is applied, it is observed that the glass opposite the negative electrode glows from the electrons emitted from the cathode.

How did JJ Thomson find the electron?

In the year 1897 J.J. Thomson invented the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Thomson realized that the accepted model of an atom did not account for the particles charged negatively or positively.

What are the properties of cathode rays?

They are formed in an evacuated tube via the negative electrode, or cathode, and move toward the anode. They journey straight and cast sharp shadows. They’ve got strength, and they can do the job. Electric and magnetic fields block them, and they have a negative charge.

What do you mean by cathode?

A device’s anode is the terminal on which current flows in from outside. A device’s cathode is the terminal from which current flows out. By present, we mean the traditional positive moment. Because electrons are charged negatively, positive current flowing in is the same as outflowing electrons.

Who discovered the cathode rays?

Studies of cathode-ray began in 1854 when the vacuum tube was improved by Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker. In 1858, Plücker discovered cathode rays by sealing two electrodes inside the tube, evacuating the air and forcing it between the electrode’s electric current.

Which gas is used in the cathode ray experiment?

For better results in a cathode tube experiment, an evacuated (low pressure) tube is filled with hydrogen gas that is the lightest gas (maybe the lightest element) on ionization, giving the maximum charge value to the mass ratio (e / m ratio = 1.76 x 10 ^ 11 coulombs per kg).

What is the Colour of the cathode ray?

Cathode-ray tube (CRT), a vacuum tube which produces images when electron beams strike its phosphorescent surface. CRTs can be monochrome (using one electron gun) or coloured (using usually three electron guns to produce red, green, and blue images that render a multicoloured image when combined).

How cathode rays are formed?

Cathode rays come from the cathode because the cathode is charged negatively. So those rays strike and ionize the gas sample inside the container. The electrons that were ejected from gas ionization travel to the anode. These rays are electrons that are actually produced from the gas ionization inside the tube.

What are cathode rays made of?

Thomson showed that cathode rays were composed of a negatively charged particle, previously unknown, which was later named electron. To render an image on a screen, Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electrical or magnetic fields.

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Discovering the electron: JJ Thomson and the Cathode Ray Tube

Concept Introduction: JJ Thomson and the Discovery of the Electron

The discovery of the electron was an important step for physics, chemistry, and all fields of science. JJ Thomson made the discovery using the cathode ray tube. Learn all about the discovery, the importance of the discovery, and JJ Thomson in this tutorial article.

Further Reading on the Electron

Electron Orbital and Electron Shapes Writing Electron Configurations Electron Shells What are valence electrons? Electron Affinity Aufbau Principle

Who was JJ Thomson?

JJ Thomson was an English physicist who is credited with discovery of the electron in 1897. Thompson was born in December 1856 in Manchester, England and was educated at the University of Manchester and then the University of Cambridge, graduating with a degree in mathematics. Thompson made the switch to physics a few years later and began studying the properties of cathode rays. In addition to this work, Thomson also performed the first-ever mass spectrometr y experiments, discovered the first isotope and made important contributions both to the understanding of positively charged particles and electrical conductivity in gases.

Thomson did most of this work while leading the famed Cavendish Laboratory at the University of Cambridge. Although he received the Nobel Prize in physics and not chemistry, Thomson’s contributions to the field of chemistry are numerous. For instance, the discovery of the electron was vital to the development of chemistry today, and it was the first subatomic particle to be discovered. The proton and the neutron would soon follow as the full structure of the atom was discovered.

What is a cathode ray tube and why was it important?

Prior to the discovery of the electron, several scientists suggested that atoms consisted of smaller pieces. Yet until Thomson, no one had determined what these might be. Cathode rays played a critical role in unlocking this mystery. Thomson determined that charged particles much lighter than atoms , particles that we now call electrons made up cathode rays. Cathode rays form when electrons emit from one electrode and travel to another. The transfer occurs due to the application of a voltage in vacuum. Thomson also determined the mass to charge ratio of the electron using a cathode ray tube, another significant discovery.

How did Thomson make these discoveries?

Thomson was able to deflect the cathode ray towards a positively charged plate deduce that the particles in the beam were negatively charged. Then Thomson measured how much various strengths of magnetic fields bent the particles. Using this information Thomson determined the mass to charge ratio of an electron. These were the two critical pieces of information that lead to the discovery of the electron. Thomson was now able to determine that the particles in question were much smaller than atoms, but still highly charged. He finally proved atoms consisted of smaller components, something scientists puzzled over for a long time. Thomson called the particle “corpuscles” , not an electron. George Francis Fitzgerald suggested the name electron.

Why was the discovery of the electron important?

The discovery of the electron was the first step in a long journey towards a better understanding of the atom and chemical bonding. Although Thomson didn’t know it, the electron would turn out to be one of the most important particles in chemistry. We now know the electron forms the basis of all chemical bonds. In turn chemical bonds are essential to the reactions taking place around us every day. Thomson’s work provided the foundation for the work done by many other important scientists such as Einstein, Schrodinger, and Feynman.

Interesting Facts about JJ Thomson

Not only did Thomson receive the Nobel Prize in physics in 1906 , but his son Sir George Paget Thomson won the prize in 1937. A year earlier, in 1936, Thomson wrote an autobiography called “Recollections and Reflections”. He died in 1940, buried near Isaac Newton and Charles Darwin. JJ stands for “Joseph John”. Strangely, another author with the name JJ Thomson wrote a book with the same name in 1975. Thomson had many famous students, including Ernest Rutherford.

Discovery of the Electron: Further Reading

Protons, Neutrons & Electrons Discovering the nucleus with gold foil Millikan oil drop experiment Phase Diagrams

Subatomic science: JJ Thomson's discovery of the electron

Read about how JJ Thomson announced his discovery of the electron at the Royal Institution in this blog by our Head of Heritage and Collections. 

JJ Thomson, while familiar to scientists, is not necessarily a name most people would recognise; however, anyone who has undertaken any science at school will have heard of an electron.

It is Thomson we have to thank for discovering this fundamental breakthrough in science and announcing his discovery to the world during a lecture here at the Royal Institution in 1897.

What is an electron?

The technical definition is:

"An electron is a stable subatomic particle with a negative electrical charge. Unlike protons and neutrons, electrons are not constructed from even smaller components."

As a non-scientist this definition is something I have heard before but must confess is not something that means a great deal to me. It is an explanation in its basic form but doesn’t convey really what an electron is or what the impact of its discovery made

John Dalton's atomic theory

Prior to 1897, scientists had hypothesised about the makeup of the universe at the atomic and subatomic level but had not been able to prove any theories. The atom had been known about for many years.

In 1808, chemist John Dalton developed an argument that led to a realisation: that perhaps all matter, the things or objects that make up the universe are made of tiny, little bits.

These are fundamental and indivisible bits and named after the ancient Greek words ‘a’ meaning not and ‘tomos’ meaning cut therefore ‘atomos’ or uncuttable. Atoms.

JJ Thomson's cathode ray tube experiments

Thomson, a highly respected theoretical physics professor at Cambridge University, undertook a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube – he was attempting to solve a long-standing controversy regarding the nature of cathode rays, which occur when an electric current is driven through a vessel from which most of the air or other gas has been pumped out.

This was something that many scientists were investigating at the time. It was Thomson that made the breakthrough however, concluding through his experimentation that particles making up the rays were 1,000 times lighter than the lightest atom, proving that something smaller than atoms existed.

Thomson likened the composition of atoms to plum pudding, with negatively-charged ‘corpuscles’ dotted throughout a positively charged field.

G Johnstone Stoney coins the term 'electron'

Thomson explained within his lecture all of his experiments and the results, never mentioning the word electron but instead sticking to corpuscles to explain these tiny particles in the same terms as biological cells (corpuscles are a minute body or cell in an organism).

Such would they have remained if not for the term 'electron' coined by G Johnstone Stoney who in 1891 denoted the unit of charge found in experiments that passed electrical current through chemicals.

It was then in 1897 after Thomson’s publication of his research that Irish physicist George Francis Fitzgerald suggested that the term be applied to Thomson's research instead of corpuscles to better describe these newly discovered subatomic particles.

JJ Thomson and the Royal Institution

Thomson had a long-standing relationship with the Royal Institution during his long academic career in Cambridge, lecturing many times on the development of physics through Discourses and educational lectures to all ages.

Thomson was a great friend of Sir William Henry Bragg and Sir William Lawrence Bragg, who jointly won the Nobel Prize in 1915 for the development of x-ray crystallography, and who were both former Director’s of the Royal Institution.

JJ Thomson's Nobel Prize

Thomson received the Nobel Prize for his work in Physics in 1906 and was knighted in 1908. The studies of nuclear organisation that continue even to this day and the further identification of elementary particles have all followed the accomplishments of Thomson and his discovery in 1897.

More about the history of the Ri

Art, culture and society History of science

Letters to gwendoline – wwi bragg family correspondence.

One story of Gallipoli told through letters home in memory of Anzac Day

History of science

The birth of electric motion.

As we celebrate the bicentenary of Faraday's invention of the electric motor in 1821, our Head of Heritage and Collections

Who discovered the greenhouse effect?

John Tyndall set the foundation for our modern understanding of the greenhouse effect, climate change, meteorology, and weather

J. J. Thomson and the Existence of the Electron

J. J. Thomson (1856 – 1940)

On April 30 , 1897 , English physicist Joseph John Thomson  gave the first experimental proof of the electron , which had been already theoretically predicted by Johnstone Stoney . Thomson was awarded the 1906   Nobel Prize in Physics for the discovery of the electron and for his work on the conduction of electricity in gases.

“As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.” – J. J. Thomson, “Cathode rays” Philosophical Magazine, 44, 293 (1897).

Background J. J. Thomson

Joseph John Thomson was born in 1856 in Manchester, England and was taught mainly in private schools at the beginning. In 1876 , he enrolled at Trinity College, Cambridge where he received his  Bachelor’s and Master’s degree . When Thomson became Cavendish Professor of Physics , Ernest Rutherford was among his students and later on, he succeeded Thomson in the post. Thomson was known to be an excellent teacher. Seven of his research assistants and his son were able to win the Nobel Prizes in physics . Thomson himself was awarded the famous prize in 1906 “ in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases. ” Two years later, he was knighted. In 1918 , Thomson became Master of the Trinity College in Cambridge .

The Discovery of the Electron

“The electron: may it never be of any use to anybody!” – A popular toast or slogan at J. J. Thomson’s Cavendish Laboratory in the first years of the 1900s, as quoted [9]

The fact that atoms were built up from a more fundamental unit was already suggested by scientists like William Prout or Norman Lockyer . However, J. J. Thomson was the first known scientist to suggest that the fundamental unit was over 1000 times smaller than an atom. Today, the subatomic particle is known as the electron . To achieve this discovery, Thomson used his explorations on the properties of cathode rays. He published his suggestion on 30 April 1897 following his discovery that Lenard rays could travel much further through air than expected for an atom-sized particle. At first, he estimated the mass of cathode rays through the heat that was generated when the rays hit a thermal junction. Then Thomson compared his result with the magnetic deflection of the rays.

J. J. Thomson’s cathode ray tube with electromagnetic deflection coils

Discovering Evidence for Isotopes

Next to this famous discovery, Thomson and his research assistant  F. W. Aston channeled a stream of neon ions through a magnetic and an electric field . They measured its deflection and observed two patches of light on the photographic plate , which was placed in the path. They concluded, that neon is composed of atoms of two different atomic masses ( isotopes ), which was the first evidence for isotopes of a stable element. Also, Thomson’s separation of neon isotopes by their mass was the first example of mass spectrometry . In 1905 , Thomson discovered the natural radioactivity of potassium and one year later he managed to demonstrate that hydrogen had only a single electron per atom.

In the bottom right corner of this photographic plate are markings for the two isotopes of neon: neon-20 and neon-22.

In 1884 Thomson was elected a Fellow of the Royal Society, which awarded him the Royal Medal in 1894, the Hughes Medal in 1902 and the Copley Medal in 1914. In 1902 he was elected to the American Academy of Arts and Sciences, in 1903 to the National Academy of Sciences. In 1906 he was awarded the Nobel Prize in Physics for his research on the electrical conductivity of gases. Thomson was awarded a Knight Bachelor’s degree in 1908 and was admitted to the Order of Merit in 1912. Since 1907 he was a corresponding member of the Bavarian Academy of Sciences. In 1911 he was elected as an external member of the Göttingen Academy of Sciences.

From 1918 until his death in 1940 he was Head of Trinity College and from 1916 to 1920 President of the Royal Society . Thomson’s ashes were buried at Westminster Abbey (near Sir Isaac Newton).

References and Further Reading:

• [1]  The Discovery of the Electron
• [2]  Thomson’s discovery of the isotopes of Neon
• [3] J. J. Thomson’s  Nobel Lecture
• [4]  Ernest Rutherford Discovers the Nucleus , SciHi Blog
• [5] J. J. Thomson at Wikidata
• [6]  Works by or about J. J. Thomson  at  Internet Archive
• [7]  A short film of Thomson lecturing on electrical engineering and the discovery of the electron  (1934)
• [8]  J.J. Thomson (1897),  Cathode rays ,  Philosophical Magazine , 44, 293
• [9] Proceedings of the Royal Institution of Great Britain, Volume 35 (1951), p. 251.
• [10]  Kathy Joseph,  J.   J. Thomson Cathode Ray Tube Experiment: the Discovery of the Electron ,  Kathy Loves Physics & History @ youtube
• [11]  Rayleigh (1941).  “Joseph John Thomson. 1856-1940” .  Obituary Notices of Fellows of the Royal Society .  3  (10): 586–609.
• [12]  “Joseph John “J. J.” Thomson” .  Science History Institute . June 2016 .
• [13]    Thomson, J. J. (June 1906).   “On the Number of Corpuscles in an Atom” .   Philosophical Magazine .   11   (66): 769–781
• [14]  Thomson, J. J. (1905).  “On the emission of negative corpuscles by the alkali metals” .  Philosophical Magazine . Series 6.  10  (59): 584–590.
• [15] Timeline for J. J. Thomson, via Wikidata

Tabea Tietz

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Supplement to Experiment in Physics

Appendix 7: evidence for a new entity: j.j. thomson and the electron.

In discussing the existence of electrons Ian Hacking has written, “So far as I’m concerned, if you can spray them then they are real” (Hacking 1983, p. 23). He went on to elaborate this view. “We are completely convinced of the reality of electrons when we set out to build—and often enough succeed in building—new kinds of device that use various well-understood causal properties of electrons to interfere in other more hypothetical parts of nature” (p. 265).

Hacking worried that the simple manipulation of the first quotation, the changing of the charge on an oil drop or on a superconducting niobium sphere, which involves only the charge of the electron, was insufficient grounds for belief in electrons. His second illustration, which he believed more convincing because it involved several properties of the electron, was that of Peggy II, a source of polarized electrons built at the Stanford Linear Accelerator Center in the late 1970s. Peggy II provided polarized electrons for an experiment that scattered electrons off deuterium to investigate the weak neutral current. Although I agree with Hacking that manipulability can often provide us with grounds for belief in a theoretical entity, [ 1 ] his illustration comes far too late. Physicists were manipulating the electron in Hacking’s sense in the early twentieth century. [ 2 ] They believed in the existence of electrons well before Peggy II, and I will argue that they had good reasons for that belief.

The position I adopt is one that might reasonably be called “conjectural” realism. It is conjectural because, despite having good reasons for belief in the existence of an entity or in the truth of a scientific law, we might be wrong. At one time scientists had good reason to believe in phlogiston and caloric, substances we now have good reason to believe don’t exist. My position includes both Sellars’ view that “to have good reason for holding a theory is ipso facto to have good reason for holding that the entities postulated by the theory exist” (Sellars 1962, p. 97), and the “entity realism” proposed by Cartwright (1983) and by Hacking (1983). Both Hacking, as noted above, and Cartwright emphasize the manipulability of an entity as a criterion for belief in its existence. Cartwright also stresses causal reasoning as part of her belief in entities. In her discussion of the operation of a cloud chamber she states, “…if there are no electrons in the cloud chamber, I do not know why the tracks are there” (Cartwright, 1983, p.99). In other words, if such entities don’t exist then we have no plausible causal story to tell. Both Hacking and Cartwright grant existence to entities such as electrons, but do not grant “real” status to either laws or theories, which may postulate or apply to such entities.

In contrast to both Cartwright and Hacking, I suggest that we can also have good reasons for belief in the laws and theories governing the behavior of the entities, and that several of their illustrations implicitly involve such laws. [ 3 ] I have argued elsewhere for belief in the reality of scientific laws (Franklin 1996). In this section I shall concentrate on the reality and existence of entities, in particular, the electron. I agree with both Hacking and Cartwright that we can go beyond Sellars and have good reasons for belief in entities even without laws. Hacking and Cartwright emphasize experimenting with entities. I will argue that experimenting on entities and measuring their properties can also provide grounds for belief in their existence.

In this section I will discuss the grounds for belief in the existence of the electron by examining J.J. Thomson’s experiments on cathode rays. His 1897 experiment on cathode rays is generally regarded as the “discovery” of the electron.

The purpose of J.J. Thomson’s experiments was clearly stated in the introduction to his 1897 paper.

The experiments discussed in this paper were undertaken in the hope of gaining some information as to the nature of Cathode Rays. The most diverse opinions are held as to these rays; according to the almost unanimous opinion of German physicists they are due to some process in the aether to which—inasmuch as in a uniform magnetic field their course is circular and not rectilinear—no phenomenon hitherto observed is analogous: another view of these rays is that, so far from being wholly aetherial, they are in fact wholly material, and that they mark the paths of particles of matter charged with negative electricity (Thomson 1897, p. 293).

Thomson’s first order of business was to show that the cathode rays carried negative charge. This had presumably been shown previously by Perrin. Perrin placed two coaxial metal cylinders, insulated from one another, in front of a plane cathode. The cylinders each had a small hole through which the cathode rays could pass onto the inner cylinder. The outer cylinder was grounded. When cathode rays passed into the inner cylinder an electroscope attached to it showed the presence of a negative electrical charge. When the cathode rays were magnetically deflected so that they did not pass through the holes, no charge was detected. “Now the supporters of the aetherial theory do not deny that electrified particles are shot off from the cathode; they deny, however, that these charged particles have any more to do with the cathode rays than a rifle-ball has with the flash when a rifle is fired” (Thomson 1897, p. 294).

Thomson repeated the experiment, but in a form that was not open to that objection. The apparatus is shown in Figure 14. The two coaxial cylinders with holes are shown. The outer cylinder was grounded and the inner one attached to an electrometer to detect any charge. The cathode rays from A pass into the bulb, but would not enter the holes in the cylinders unless deflected by a magnetic field.

Figure 14. Thomson’s apparatus for demonstrating that cathode rays have negative charge. The slits in the cylinders are shown. From Thomson (1897).

When the cathode rays (whose path was traced by the phosphorescence on the glass) did not fall on the slit, the electrical charge sent to the electrometer when the induction coil producing the rays was set in action was small and irregular; when, however, the rays were bent by a magnet so as to fall on the slit there was a large charge of negative electricity sent to the electrometer…. If the rays were so much bent by the magnet that they overshot the slits in the cylinder, the charge passing into the cylinder fell again to a very small fraction of its value when the aim was true. Thus this experiment shows that however we twist and deflect the cathode rays by magnetic forces, the negative electrification follows the same path as the rays, and that this negative electrification is indissolubly connected with the cathode rays (Thomson 1897, p. 294–295, emphasis added).

This experiment also demonstrated that cathode rays were deflected by a magnetic field in exactly the way one would expect if they were negatively charged material particles. [ 4 ]

Figure 15. Thomson’s apparatus for demonstrating that cathode rays are deflected by an electric field. It was also used to measure \(\bfrac{m}{e}\). From Thomson (1897).

There was, however, a problem for the view that cathode rays were negatively charged particles. Several experiments, in particular those of Hertz, had failed to observe the deflection of cathode rays by an electrostatic field. Thomson proceeded to answer this objection. His apparatus is shown in Figure 15. Cathode rays from C pass through a slit in the anode A, and through another slit at B. They then passed between plates D and E and produced a narrow well-defined phosphorescent patch at the end of the tube, which also had a scale attached to measure any deflection. When Hertz had performed the experiment he had found no deflection when a potential difference was applied across D and E. He concluded that the electrostatic properties of the cathode ray are either nil or very feeble. Thomson admitted that when he first performed the experiment he also saw no effect. “on repeating this experiment [that of Hertz] I at first got the same result [no deflection], but subsequent experiments showed that the absence of deflexion is due to the conductivity conferred on the rarefied gas by the cathode rays”. [ 5 ] On measuring this conductivity it was found that it diminished very rapidly as the exhaustion increased; it seemed that on trying Hertz’s experiment at very high exhaustion there might be a chance of detecting the deflexion of the cathode rays by an electrostatic force (Thomson 1897, p. 296). Thomson did perform the experiment at lower pressure [higher exhaustion] and observed the deflection. [ 6 ]

Thomson concluded:

As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a negatively electrified body moving along the path of these rays, I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter. (Thomson 1897, p. 302) [ 7 ]

Having established that cathode rays were negatively charged material particles, Thomson went on to discuss what the particles were. “What are these particles? are they atoms, or molecules, or matter in a still finer state of subdivision” (p. 302). To investigate this question Thomson made measurements on the charge to mass ratio of cathode rays. Thomson’s method used both the electrostatic and magnetic deflection of the cathode rays. [ 8 ] The apparatus is shown in Figure 15. It also included a magnetic field that could be created perpendicular to both the electric field and the trajectory of the cathode rays.

Let us consider a beam of particles of mass \(m\) charge \(e\), and velocity \(v\). Suppose the beam passes through an electric field F in the region between plates D and E, which has a length \(L\). The time for a particle to pass through this region \(t = \bfrac{L}{v}\). The electric force on the particle is \(Fe\) and its acceleration \(a = \bfrac{Fe}{m}\). The deflection d at the end of the region is given by

Now consider a situation in which the beam of cathode rays simultaneously pass through both \(F\) and a magnetic field \(B\) in the same region. Thomson adjusted \(B\) so that the beam was undeflected. thus the magnetic force was equal to the electrostatic force.

This determined the velocity of the beam. Thus,

Each of the quantities in the above expression was measured so the \(\bfrac{e}{m}\) or \(\bfrac{m}{e}\) could be determined.

Using this method Thomson found a value of \(\bfrac{m}{e}\) of \((1.29\pm 0.17) \times 10^{-7}\). This value was independent of both the gas in the tube and of the metal used in the cathode, suggesting that the particles were constituents of the atoms of all substances. It was also far smaller, by a factor of 1000, than the smallest value previously obtained, \(10^{-4}\), that of the hydrogen ion in electrolysis.

Thomson remarked that this might be due to the smallness of \(m\) or to the largeness of \(e\). He argued that \(m\) was small citing Lenard’s work on the range of cathode rays in air. The range, which is related to the mean free path for collisions, and which depends on the size of the object, was 0.5 cm. The mean free path for molecules in air was approximately \(10^{-5}\) cm. If the cathode ray traveled so much farther than a molecule before colliding with an air molecule, Thomson argued that it must be much smaller than a molecule. [ 9 ]

Thomson had shown that cathode rays behave as one would expect negatively charged material particles to behave. They deposited negative charge on an electrometer, and were deflected by both electric and magnetic fields in the appropriate direction for a negative charge. In addition the value for the mass to charge ratio was far smaller than the smallest value previously obtained, that of the hydrogen ion. If the charge were the same as that on the hydrogen ion, the mass would be far less. In addition, the cathode rays traveled farther in air than did molecules, also implying that they were smaller than an atom or molecule. Thomson concluded that these negatively charged particles were constituents of atoms. In other words, Thomson’s experiments had given us good reasons to believe in the existence of electrons.

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Cathode Ray Experiment

The electric experiment by j.j. thomson.

J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly contributed to our understanding of the modern world.

This article is a part of the guide:

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• 1 Physics Experiments
• 2 Ben Franklin Kite
• 3 Brownian Movement
• 4 Cathode Ray Experiment

Like most scientists of that era, he inspired generations of later physicists, from Einstein to Hawking .

His better-known research proved the existence of negatively charged particles, later called electrons, and earned him a deserved Nobel Prize for physics. This research led to further experiments by Bohr and Rutherford, leading to an understanding of the structure of the atom.

What is a Cathode Ray Tube?

Even without consciously realizing it, most of us are already aware of what a cathode ray tube is.

Look at any glowing neon sign or any ‘old-fashioned’ television set, and you are looking at the modern descendants of the cathode ray tube.

Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. This cathode ray also became known as an ‘electron gun’.

Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it.

With more experimentation, researchers found that the ‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be particles.

Physicists knew that the ray carried a negative charge but were not sure whether the charge could be separated from the ray. They debated whether the rays were waves or particles, as they seemed to exhibit some of the properties of both. In response, J. J. Thomson constructed some elegant experiments to find a definitive and comprehensive answer about the nature of cathode rays.

Thomson’s First Cathode Ray Experiment

Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.

His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.

He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.

Thomson's Cathode Ray Second Experiment

Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.

Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.

For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.

As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.

Thomson's Third Experiment

The third experiment was a brilliant piece of scientific deduction and shows how a series of experiments can gradually uncover truths.

Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis .

He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.

Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.

Later Developments

Thomson came up with the initial idea for the structure of the atom, postulating that it consisted of these negatively charged particles swimming in a sea of positive charge. His pupil, Rutherford, developed the idea and came up with the theory that the atom consisted of a positively charged nucleus surrounded by orbiting tiny negative particles, which he called electrons.

Quantum physics has shown things to be a little more complex than this but all quantum physicists owe their legacy to Thomson. Although atoms were known about, as apparently indivisible elementary particles, he was the first to postulate that they had a complicated internal structure.

Thomson's greatest gift to physics was not his experiments, but the next generation of great scientists who studied under him, including Rutherford, Oppenheimer and Aston. These great minds were inspired by him, marking him out as one of the grandfathers of modern physics.

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Martyn Shuttleworth (Sep 22, 2008). Cathode Ray Experiment. Retrieved Sep 09, 2024 from Explorable.com: https://explorable.com/cathode-ray-experiment

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J.J. Thomson

by: Ann Johnson

• 1.1 Biography
• 2 Electron Discovery
• 3 Cathode Ray Experiments
• 4 Isotopes and Mass Spectrometry
• 5.1 Further reading
• 5.2 External links
• 6 References

The Main Idea

J. J. Thomson was a Nobel Prize winning English physicist who used cathode rays to discover electrons. He also developed the mass spectrometer.

J. J. Thomson was born on December 18th, 1856 in England. His father wished he would become an engineer, however he could not find an apprenticeship. He attended Trinity College at Cambridge, and eventually headed the Cavendish Laboratory. Thomson married one of his students, Rose Paget, in 1892. They had two children, Joan and George Thomson. George eventually became a physicist and earned a Nobel Prize of his own. J. J. Thomson published over 200 papers and 13 books. He died on August 30th, 1940 in Cambridge and is buried in Westminster Abbey.

Electron Discovery

J. J. Thomson discovered the electron in 1897 while performing experiments on electric discharge in a high-vacuum cathode ray tube. He interpreted the deflection of the rays by electrically charged plates and magnets as "evidence of bodies much smaller than atoms." He later suggested that the atom is best represented as a sphere of positive matter, through which electrons are positioned by electrostatic forces.

Cathode Ray Experiments

A cathode ray tube is a glass tube with wiring inserted on both ends, and as much air as possible pumped out of it. Cathode rays were discovered to travel in straight lines, just like waves do. Physicists knew that the ray had an electric charge, and they were trying to figure out if that electric charge could be separated from the ray.

Thomson had the hypothesis that the ray and charge were inseparable, and designed experiments using a magnetic field to prove this was true. He first built a cathode ray tube with a metal cylinder at the end. The cylinder had slits in it that were attached to electrometers, that could measure electric charges. When he applied a magnetic field across the tube, no activity was recorded by the electrometers. This meant the charge had been bent away by the magnet. This proved his theory that the charge and the ray were inseparable.

Isotopes and Mass Spectrometry

After discovering the electron, Thomson started studying positive rays. Positive rays behaved very differently from cathode rays, and he found that each ray followed its own parabolic path based on its detection on the photographic plate. He reasoned that no two particles would follow the same path unless they possessed the same mass-to-charge ratio. He correctly suggested that the positively charged particles were formed by the loss of an electron (isotopes). This created the field of mass spectrometry, which is still used very heavily today.

Properties of matter, including mass and charge, are related to Thomson's work with electrons and the mass spectrometer.

Further reading

Thomson, J. J. (June 1906). "On the Number of Corpuscles in an Atom". Philosophical Magazine 11: 769–781. doi:10.1080/14786440609463496. Archived from the original on 19 December 2007. Retrieved 4 October 2008. Leadership and creativity : a history of the Cavendish Laboratory, 1871 - 1919

External links

http://www.cambridgenetwork.co.uk/news/cambridge-physicist-is-streets-ahead/

http://thomson.iqm.unicamp.br/thomson.phphttp://www.chemheritage.org/discover/online-resources/chemistry-in-history/themes/atomic-and-nuclear-structure/thomson.aspx http://www.biography.com/people/jj-thomson-40039 http://study.com/academy/lesson/jj-thomsons-cathode-ray-tube-crt-definition-experiment-diagram.htmlhttps://explorable.com/cathode-ray-experiment

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• J.J.. Thomson - Nobel Lecture: Electronic Waves

George Paget Thomson

Nobel lecture.

Nobel Lecture, June 7, 1938

Electronic Waves

Ever since last November, I have been wanting to express in person my gratitude to the generosity of Alfred Nobel, to whom I owe it that I am privileged to be here today, especially since illness prevented me from doing so at the proper time. The idealism which permeated his character led him to make his magnificent foundation for the benefit of a class of men with whose aims and viewpoint his own scientific instincts and ability had made him naturally sympathetic, but he was certainly at least as much concerned with helping science as a whole, as individual scientists. That his foundation has been as successful in the first as in the second, is due to the manner in which his wishes have been carried out. The Swedish people, under the leadership of the Royal Family, and through the medium of the Royal Academy of Sciences, have made the Nobel Prizes one of the chief causes of the growth of the prestige of science in the eyes of the world, which is a feature of our time. As a recipient of Nobel’s generosity I owe sincerest thanks to them as well as to him.

The goddess of learning is fabled to have sprung full-grown from the brain of Zeus, but it is seldom that a scientific conception is born in its final form, or owns a single parent. More often it is the product of a series of minds, each in turn modifying the ideas of those that came before, and providing material for those that come after. The electron is no exception.

Although Faraday does not seem to have realized it, his work on electrolysis, by showing the unitary character of the charges on atoms in solution, was the first step. Clerk Maxwell in 1873 used the phrase a “molecule of electricity” and von Helmholtz in 1881 speaking of Faraday’s work said “If we accept the hypothesis that elementary substances are composed of atoms, we cannot well avoid concluding that electricity also is divided into elementary portions which behave like atoms of electricity.” The hypothetical atom received a name in the same year when Johnstone Stoney of Dublin christened it “electron”, but so far the only property implied was an electron charge.

The last year of the nineteenth century saw the electron take a leading place amongst the conceptions of physics. It acquired not only mass but universality, it was not only electricity but an essential part of all matter. If among the many names associated with this advance I mention that of J.J. Thomson I hope you will forgive a natural pride. It is to the great work of Bohr that we owe the demonstration of the connection between electrons and Planck’s quantum which gave the electron a dynamics of its own. A few years later, Goudsmit and Uhlenbeck, following on an earlier suggestion by A.H. Compton showed that it was necessary to suppose that the electron had spin. Yet even with the properties of charge, mass, spin and a special mechanics to help it, the electron was unable to carry the burden of explaining the large and detailed mass of experimental data which had accumulated. L. de Broglie , working originally on a theory of radiation, produced as a kind of by-product the conception that any particle and in particular an electron, was associated with a system of waves. It is with these waves, formulated more precisely by Schrödinger, and modified by Dirac to cover the idea of spin, that the rest of my lecture will deal.

The first published experiments to confirm de Broglie’s theory were those of Davisson and Germer, but perhaps you will allow me to describe instead those to which my pupils and I were led by de Broglie’s epoch-making conception.

A narrow beam of cathode rays was transmitted through a thin film of matter. In the earliest experiment of the late Mr. Reid this film was of celluloid, in my own experiment of metal. In both, the thickness was of the order of 10 -6 cm. The scattered beam was received on a photographic plate normal to the beam, and when developed showed a pattern of rings, recalling optical halos and the Debye -Scherrer rings well known in the corresponding experiment with X-rays. An interference phenomenon is at once suggested. This would occur if each atom of the film scattered in phase a wavelet from an advancing wave associated with the electrons forming the cathode rays. Since the atoms in each small crystal of the metal are regularly spaced, the phases of the wavelets scattered in any fixed direction will have a definite relationship to one another. In some directions they will agree in phase and build up a strong scattered wave, in others they will destroy one another by interference. The strong waves are analogous to the beams of light diffracted by an optical grating. At the time, the arrangement of the atoms in celluloid was not known with certainty and only general conclusions could be drawn, but for the metals it had been determined previously by the use of X-rays. According to de Broglie’s theory the wavelength associated with an electron is h/mv which for the electrons used (cathode rays of 20 to 60,000 volts energy) comes out from 8 X 10 -9 to 5 X 10 -9 cm. I do not wish to trouble you with detailed figures and it will be enough to say that the patterns on the photographic plates agreed quantitatively, in all cases, with the distribution of strong scattered waves calculated by the method I have indicated. The agreement is good to the accuracy of the experiments which was about 1%. There is no adjustable constant, and the patterns reproduce not merely the general features of the X-ray patterns but details due to special arrangements of the crystals in the films which were known to occur from previous investigation by X-rays. Later work has amply confirmed this conclusion, and many thousands of photographs have been taken in my own and other laboratories without any disagreement with the theory being found. The accuracy has increased with the improvement of the apparatus, perhaps the most accurate work being that of v. Friesen of Uppsala who has used the method in a precision determination of e in which he reaches an accuracy of I in 1,000.

Before discussing the theoretical implications of these results there are two modifications of the experiments which should be mentioned. In the one, the electrons after passing through the film are subject to a uniform magnetic field which deflects them. It is found that the electrons whose impact on the plate forms the ring pattern are deflected equally with those which have passed through holes in the film. Thus the pattern is due to electrons which have preserved unchanged the property of being deflected by a magnet. This distinguishes the effect from anything produced by X-rays and shows that it is a true property of electrons. The other point is a practical one, to avoid the need for preparing the very thin films which are needed to transmit the electrons, an apparatus has been devised to work by reflection, the electrons striking the diffracting surface at a small glancing angle. It appears that in many cases the patterns so obtained are really due to electrons transmitted through small projections on the surface. In other cases, for example when the cleavage surface of a crystal is used, true reflection occurs from the Bragg planes.

The theory of de Broglie in the form given to it by Schrödinger is now known as wave mechanics and is the basis of atomic physics. It has been applied to a great variety of phenomena with success, but owing largely to mathematical difficulties there are not many cases in which an accurate comparison is possible between theory and experiment. The diffraction of fast electrons by crystals is by far the severest numerical test which has been made and it is therefore important to see just what conclusions the excellent agreement between theory and these experiments permits us to draw.

The calculations so far are identical with those in the corresponding case of the diffraction of X-rays. The only assumption made in determining the directions of the diffracted beams is that we have to deal with a train of wave of considerable depth and with a plane wave-front extending over a considerable number of atoms. The minimum extension of the wave system sideways and frontways can be found from the sharpness of the lines. Taking v. Friesen’s figures, it is at least 225 waves from back to front over a front of more than 200 Å each way.

But the real trouble comes when we consider the physical meaning of the waves. In fact, as we have seen, the electrons blacken the photographic plate at those places where the waves would be strong. Following Bohr, Born, and Schrödinger, we can express this by saying that the intensity of the waves at any place measures the probability of an electron manifesting itself there. This view is strengthened by measurements of the relative intensities of the rings, which agree well with calculations by Mott based on Schrödinger’s equation. Such a view, however successful as a formal statement is at variance with all ordinary ideas. Why should a particle appear only in certain places associated with a set of waves? Why should waves produce effects only through the medium of particles? For it must be emphasized that in these experiments each electron only sensitizes the photographic plate in one minute region, but in that region it has the same powers of penetration and photographic action as if it had never been diffracted. We cannot suppose that the energy is distributed throughout the waves as in a sound or water wave, the wave is only effective in the one place where the electron appears. The rest of it is a kind of phantom. Once the particle has appeared the wave disappears like a dream when the sleeper wakes. Yet the motion of the electron, unlike that of a Newtonian particle, is influenced by what happens over the whole front of the wave, as is shown by the effect of the size of the crystals on the sharpness of the patterns. The difference in point of view is fundamental, and we have to face a break with ordinary mechanical ideas. Particles have not a unique track, the energy in these waves is not continuously distributed, probability not determinism governs nature.

But while emphasizing this fundamental change in outlook, which I believe to represent an advance in physical conceptions, I should like to point out several ways in which the new phenomena fit the old framework better than is often realized. Take the case of the influence of the size of the crystals on the sharpness of the diffracted beams, which we have just mentioned. On the wave theory it is simply an example of the fact that a diffraction grating with only a few lines has a poor resolving power. Double the number of the lines and the sharpness of the diffracted beams is doubled also. However if there are already many lines, the angular change is small. But imagine a particle acted on by the material which forms the slits of the grating, and suppose the forces such as to deflect it into one of the diffracted beams. The forces due to the material round the slits near the one through which it passes will be the most important, an increase in the number of slits will affect the motion but the angular deflection due to adding successive slits will diminish as the numbers increase. The law is of a similar character, though no simple law of force would reproduce the wave effect quantitatively.

Similarly for the length of the wave train. If this were limited by a shutter moving so quickly as to let only a short wave train pass through, the wave theory would require that the velocity of the particle would be uncertain over a range increasing with the shortness of the wave train, and corresponding to the range of wavelengths shown by a Fourier analysis of the train. But the motion of the shutter might well be expected to alter the velocity of a particle passing through, just before it closed.

Again, on the new view it is purely a matter of chance in which of the diffracted beams of different orders an electron appears. If the phenomenon were expressed as the classical motion of a particle, this would have to depend on the initial motion of the particle, and there is no possibility of determining this initial motion without disturbing it hopelessly. There seems no reason why those who prefer it should not regard the diffraction of electrons as the motion of particles governed by laws which simulate the character of waves, but besides the rather artificial character of the law of motion, one has to ascribe importance to the detailed initial conditions of the motion which, as far as our present knowledge goes, are necessarily incapable of being determined. I am predisposed by nature in favour of the most mechanical explanations possible, but I feel that this view is rather clumsy and that it might be best, as it is certainly safer, to keep strictly to the facts and regard the wave equation as merely a way of predicting the result of experiments. Nevertheless, the view I have sketched is often a help in thinking of these problems. We are curiously near the position which Newton took over his theory of optics, long despised but now seen to be far nearer the truth than that of his rivals and successors.

“Those that are averse from assenting to any new Discoveries, but such as they can explain by an Hypothesis, may for the present suppose, that as Stones by falling upon water put the Water into an undulating Motion, and all Bodies by percussion excite vibrations in the Air: so the Rays of Light, by impinging on any refracting or reflecting Surface, excite vibrations in the refracting or reflecting Medium or Substance, much after the manner that vibrations are propagated in the Air for causing Sound, and move faster than the Rays so as to overtake them; and that when any Ray is in that part of the vibration which conspires with its Motion, it easily breaks through a refracting Surface, but when it is in the contrary part of the vibration which impedes its Motion, it is easily reflected; and, by consequence, that every Ray is successively disposed to be easily reflected, or easily transmitted, by every vibration which overtakes it. But whether this Hypothesis be true or false I do not here consider.”

Although the experiments in diffraction confirm so beautifully the de Broglie-Schrödinger wave theory, the position is less satisfactory as regards the extended theory due to Dirac. On this theory the electron possesses magnetic properties and the wave requires four quantities instead of one for its specification. This satisfies those needs of spectroscopy which led to the invention of the spinning electron. It suggests however that electronic waves could be polarized and that the polarized waves might interact with matter in an anisotropic manner. In fact detailed calculations by Mott indicate that if Dirac electrons of 140 kV energy are scattered twice through 90° by the nuclei of gold atoms the intensity of the scattered beam will differ by 16% according to whether the two scatterings are in the same or in opposite directions. Experiments by Dymond and by myself have established independently that no effect of this order of magnitude exists, when the scattering is done by gold foils. While there is a slight possibility that the circumnuclear electrons, or the organization of the atoms into crystals might effect the result, it seems very unlikely. Some of the theorists have arrived at results conflicting with Mott, but I understand that their work has been found to contain errors. At present there seems no explanation of this discrepancy which throws doubt on the validity of the Dirac equations in spite of their success in predicting the positive electron.

I should be sorry to leave you with the impression that electron diffraction was of interest only to those concerned with the fundamentals of physics. It has important practical applications to the study of surface effects. You know how X-ray diffraction has made it possible to determine the arrangement of the atoms in a great variety of solids and even liquids. X-rays are very penetrating, and any structure peculiar to the surface of a body will be likely to be overlooked, for its effect is swamped in that of the much greater mass of underlying material. Electrons only affect layers of a few atoms, or at most tens of atoms, in thickness, and so are eminently suited for the purpose. The position of the beams diffracted from a surface enables us, at least in many cases, to determine the arrangement of the atoms in the surface. Among the many cases which have already been studied I have only time to refer to one, the state of the surface of polished metals. Many years ago Sir George Beilby suggested that this resembled a supercooled liquid which had flowed under the stress of polishing. A series of experiments by electron diffraction carried out at the Imperial College in London has confirmed this conclusion. The most recent work due to Dr. Cochrane has shown that though this amorphous layer is stable at ordinary temperature as long as it remains fixed to the mass of the metal, it is unstable when removed, and recrystalizes after a few hours. Work by Professor Finch on these lines has led to valuable conclusions as to the wear on the surfaces of cylinders and pistons in petrol engines.

It is in keeping with the universal character of physical science that this single small branch of it should touch on the one hand on the fundamentals of scientific philosophy and on the other, questions of everyday life.

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Key Questions

Thomson's experiments with cathode ray tubes helped him to discover the electron.

This ushered in a model of atomic structure referred to as the plum pudding model. I like to think of it like a sphere shaped chocolate chip cookie since plum pudding is not super popular in the US.

The cookie dough (they didn't know what it was yet) is positively charged and the chocolate chips (electrons) are negatively charged and scattered randomly throughout the cookie (atom). The positive and negative charges cancel producing a neutral atom.

JJ Thompson’s Discovery of Electron: Cathode Ray Tube Experiment Explained

JJ Thomson discovered the electron in 1897 and there are tons of videos about it.  However, most videos miss what JJ Thomson himself said was the motivating factor: a debate about how cathode rays move.  Want to know not only how but why electrons were discovered?

Table of Contents

The start of jj thomson, how thomson discovered electrons: trials and errors, thomson’s conclusion.

A short history of Thomson: Joseph John Thomson, JJ on papers, to friends, and even to his own son [1] , was born in Lancashire, England to a middle class bookseller.  When he was 14 years old, Thomson planned to get an apprenticeship to a locomotive engineer but it had a long waiting list, so, he applied to and was accepted at that very young age to Owen’s college. 

Thompson later recalled that, “the authorities at Owens College thought my admission was such a scandal – I expect they feared that students would soon be coming in perambulators  – that they passed regulations raising the minimum age for admission, so that such a catastrophe should not happen again.

[2] ”  While in school, his father died, and his family didn’t have enough money for the apprenticeship.  Instead, he relied on scholarships at universities – ironically leading him to much greater fame in academia. In 1884, at the tender age of 28, Thomson applied to be the head of the Cavendish Research Institute. 

He mostly applied as a lark and was as surprised as anyone to actually get the position!  “I felt like a fisherman who…had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land. [3] ”  Suddenly, he had incredible resources, stability and ability to research whatever he wished. 

He ended up having an unerring ability to pinpoint interesting phenomena for himself and for others. In fact, a full eight of his research assistants and his son eventually earned Nobel Prizes, but, of course, like Thomson’s own Nobel Prize, that was in the future.

Why did J. J. Thomson discover the electron in 1897?  Well, according to Thomson: “the discovery of the electron began with an attempt to explain the discrepancy between the behavior of cathode rays under magnetic and electric forces [4] .”  What did he mean by that? 

Well, a cathode ray, or a ray in a vacuum tube that emanates from the negative electrode, can be easily moved with a magnet.  This gave a charismatic English chemist named William Crookes the crazy idea that the cathode ray was made of charged particles in 1879! 

However, 5 years later, a young German scientist named Heinrich Hertz found that he could not get the beam to move with parallel plates, or with an electric field.  Hertz decided that Crookes was wrong, if the cathode ray was made of charged particles then it should be attracted to a positive plate and repulsed from a negative plate. 

Ergo, it couldn’t be particles, and Hertz decided it was probably some new kind of electromagnetic wave, like a new kind of ultraviolet light.  Further, in 1892, Hertz accidentally discovered that cathode rays could tunnel through thin pieces of metal, which seemed like further proof that Crookes was so very wrong.

Then, in December of 1895, a French physicist named Jean Perrin used a magnet to direct a cathode ray into and out of an electroscope (called a Faraday cylinder) and measured its charge.  Perrin wrote, “the Faraday cylinder became negatively charged when the cathode rays entered it, and only when they entered it; the cathode rays are thus charged with negative electricity .

[5] ”  This is why JJ Thomson was so confused, he felt that Perrin had, “conclusive evidence that the rays carried a charge of negative electricity” except that, “Hertz found that when they were exposed to an electric force they were not deflected at all.”  What was going on?

In 1896, Thomson wondered if there might have been something wrong with Hertz’s experiment with the two plates.  Thomson knew that the cathode ray tubes that they had only work if there is a little air in the tube and the amount of air needed depended on the shape of the terminals.

Thomson wondered if the air affected the results.  Through trial and error, Thomson found he could get a “stronger” beam by shooting it through a positive anode with a hole in it.  With this system he could evacuate the tube to a much higher degree and, if the vacuum was good enough, the cathode ray was moved by electrically charged plates, “just as negatively electrified particles would be.

[6] ” (If you are wondering why the air affected it, the air became ionized in the high electric field and became conductive.  The conductive air then acted like a Faraday cage shielding the beam from the electric field.)

As stated before, Heinrich Hertz also found that cathode rays could travel through thin solids.  How could a particle do that?  Thomson thought that maybe particles could go through a solid if they were moving really, really fast.  But how to determine how fast a ray was moving? 

Thomson made an electromagnetic gauntlet.  First, Thomson put a magnet near the ray to deflect the ray one-way and plates with electric charge to deflect the ray the other way.  He then added or reduced the charge on the plates so that the forces were balanced and the ray went in a straight line. 

He knew that the force from the magnet depended on the charge of the particle, its speed and the magnetic field (given the letter B).  He also knew that the electric force from the plates only depended on the charge of the particle and the Electric field.  Since these forces were balanced, Thomson could determine the speed of the particles from the ratio of the two fields. 

Thomson found speeds as big as 60,000 miles per second or almost one third of the speed of light.  Thomson recalled, “In all cases when the cathode rays are produced their velocity is much greater than the velocity of any other moving body with which we are acquainted. [7] ”  

Thomson then did something even more ingenious; he removed the magnetic field.  Now, he had a beam of particles moving at a known speed with a single force on them.  They would fall, as Thomson said, “like a bullet projected horizontally with a velocity v and falling under gravity [8] ”.  

Note that these “bullets” are falling because of the force between their charge and the charges on the electric plates as gravity is too small on such light objects to be influential.  By measuring the distance the bullets went he could determine the time they were in the tube and by the distance they “fell” Thomson could determine their acceleration. 

Using F=ma Thomson determine the ratio of the charge on the particle to the mass (or e/m).  He found some very interesting results.  First, no matter what variables he changed in the experiment, the value of e/m was constant.  “We may… use any kind of substance we please for the electrodes and fill the tube with gas of any kind and yet the value of e/m will remain the same.

[9] ”  This was a revolutionary result.  Thomson concluded that everything contained these tiny little things that he called corpuscles (and we call electrons).  He also deduced that the “corpuscles” in one item are exactly the same as the “corpuscles” in another.  So, for example, an oxygen molecule contains the same kind of electrons as a piece of gold!  Atoms are the building blocks of matter but inside the atoms (called subatomic) are these tiny electrons that are the same for everything .

The other result he found was that the value of e/m was gigantic, 1,700 times bigger than the value for a charged Hydrogen atom, the object with the largest value of e/m before this experiment.   So, either the “corpuscle” had a ridiculously large charge or it was, well, ridiculously small.   

A student of Thomson’s named C. T. R. Wilson had experimented with slowly falling water droplets that found that the charge on the corpuscles were, to the accuracy of the experiment, the same as the charge on a charged Hydrogen atom!   Thomson concluded that his corpuscles were just very, very, tiny, about 1,700 times smaller then the Hydrogen atom [1] .  These experiments lead Thomson to come to some interesting conclusions:

• Electrons are in everything and are well over a thousand times smaller then even the smallest atom. 
• Benjamin Franklin thought positive objects had too much “electrical fire” and negative had too little.  Really, positive objects have too few electrons and negative have too many.  Oops.
• Although since Franklin, people thought current flowed from the positive side to the negative, really, the electrons are flowing the other way.  When a person talks about “current” that flows from positive to negative they are talking about something that is not real!   True “electric current” flows from negative to positive and is the real way the electrons move. [although by the time that people believed J.J. Thomson, it was too late to change our electronics, so people just decided to stick with “current” going the wrong way!]
• Since electrons are tiny and in everything but most things have a neutral charge, and because solid objects are solid, the electrons must be swimming in a sea or soup of positive charges.  Like raisons in a raison cookie.

The first three are still considered correct over one hundred years later.  The forth theory, the “plum pudding model” named after a truly English “desert” with raisins in sweet bread that the English torture people with during Christmas, was proposed by Thomson in 1904. 

In 1908, a former student of Thomson’snamed Ernest Rutherford was experimenting with radiation, and inadvertently demolished the “plum pudding model” in the process.  However, before I can get into Rutherford’s gold foil experiment, I first want to talk about what was going on in France concurrent to Thomson’s experiments. 

This is a story of how a new mother working mostly in a converted shed discovered and named the radium that Rutherford was experimenting with.  That woman’s name was Marie Sklodowska Curie, and that story is next time on the Lightning Tamers.

[1] the current number is 1,836 but Thomson got pretty close

[1] p 14 “Flash of the Cathode Rays: A History of JJ Thomson’s Electron” Dahl

[2] Thompson, J.J. Recollections and Reflections p. 2 Referred to in Davis & Falconer JJ. Thompson and the Discovery of the Electron 2002 p. 3

[3] Thomson, Joseph John Recollections and Reflections p. 98 quoted in Davis, E.A & Falconer, Isabel JJ Thomson and the Discovery of the Electron 2002 p. 35

[4]   Thomson, JJ Recollections and Reflections p. 332-3

[5] “New Experiments on the Kathode Rays” Jean Perrin, December 30, 1985 translation appeared in Nature, Volume 53, p 298-9, January 30, 1896

[6] Nobel Prize speech?

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• Published: 08 September 2024

Stable Deuterium-Tritium plasmas with improved confinement in the presence of energetic-ion instabilities

• Jeronimo Garcia   ORCID: orcid.org/0000-0003-0900-5564 1 ,
• Yevgen Kazakov 2 ,
• Rui Coelho 3 ,
• Mykola Dreval   ORCID: orcid.org/0000-0003-0482-0981 4 ,
• Elena de la Luna 5 ,
• Emilia R. Solano   ORCID: orcid.org/0000-0002-4815-3407 5 ,
• Žiga Štancar   ORCID: orcid.org/0000-0002-9608-280X 6 ,
• Jacobo Varela   ORCID: orcid.org/0000-0002-6114-0539 7 , 8 ,
• Matteo Baruzzo 9 ,
• Emily Belli   ORCID: orcid.org/0000-0001-7947-2841 10 ,
• Phillip J. Bonofiglo 11 ,
• Jeff Candy 10 ,
• Costanza F. Maggi 6 ,
• Joelle Mailloux 6 ,
• Samuele Mazzi   ORCID: orcid.org/0000-0001-6491-8759 1 ,
• Jef Ongena 2 ,
• Juan R. Ruiz 12 ,
• Michal Poradzinski   ORCID: orcid.org/0000-0002-1858-4046 6 ,
• Sergei Sharapov 6 ,
• David Zarzoso   ORCID: orcid.org/0000-0002-7220-8092 13 &

JET contributors

Nature Communications volume  15 , Article number:  7846 ( 2024 ) Cite this article

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• Experimental nuclear physics
• Magnetically confined plasmas

Providing stable and clean energy sources is a necessity for the increasing demands of humanity. Energy produced by Deuterium (D) and Tritium (T) fusion reactions, in particular in tokamaks, is a promising path towards that goal. However, there is little experience with plasmas formed by D-T mixtures, since most of the experiments are currently performed in pure D. After more than 20 years, the Joint European Torus (JET) has carried out new D-T experiments with the aim of exploring some of the unique characteristics expected in future fusion reactors, such as the presence of highly energetic ions in low plasma rotation conditions. A new stable, high confinement and impurity-free D-T regime, with reduction of energy losses with respect to D, has been found. Multiscale physics mechanisms critically determine the thermal confinement. These crucial achievements importantly contribute to the establishment of fusion energy generation as an alternative to fossil fuels.

Introduction

Modern societies are eager to increase their energy resources, which are currently largely provided by fossil fuels. In this context, plasmas, i.e. the fourth state of matter, which are characterized by the presence of free-charged particles, can provide a carbon-free energy source through fusion reactions of light atom nuclei. For that purpose, plasmas must be well confined and reach high pressure in order to overcome the electrostatic repulsion of particles. Magnetic containment is one of the most promising routes toward this goal. The tokamak concept, in which plasmas are confined both by a high toroidal electric current, I p , and a toroidal magnetic field, B T , is particularly promising. Tokamaks have made great progress from their initial proposal 1 , 2 . Plasmas with good thermal confinement have been achieved in high-performance tokamak discharges when turbulence-driven energy losses (one of the main physics mechanisms by which plasmas lose their thermal confinement) are strongly reduced. This is the case for the H-modes 3 or internal transport barrier (ITB) regimes 4 , in which steep pressure gradients are formed, leading to high plasma pressure. Recently, the EAST 5 and KSTAR 6 tokamaks have reported significant advances. Long-sustained high-performance plasmas were obtained in conditions of low turbulent heat transport with simultaneous avoidance of dangerous magnetohydrodynamic (MHD) bursts, called edge localized modes (ELMs) 7 , which are characteristic of H-modes. Such bursts can lead to rapid expulsion of edge plasma and hence to high levels of heat and particle flux to the tokamak wall.

On the one hand, these advances have clarified the route towards a potential fusion energy commercial reactor. On the other hand, the results obtained are not enough to provide a clear insight into how future energy-producing reactor plasmas are expected to behave. The reason is that in significant contrast to plasmas produced nowadays, formed almost exclusively by pure Deuterium (D), energy-producing plasmas will use Deuterium and Tritium (T) to produce 14.1 MeV neutrons and 3.5 MeV 4 He nuclei (alpha particles). The presence of T has been identified as a potential source of significant changes with respect to pure D plasmas, particularly in terms of turbulence or MHD characteristics 8 , 9 . Furthermore, the presence of a small and yet very energetic population of alpha particles, which will provide the main self-heating mechanism through collisions to the thermal plasma, leads to conditions that are mostly unexplored and have been identified as potentially detrimental. The highly energetic fusion-born alpha particles mainly transfer energy to electrons (rather than to thermal ions) by collisions, thus providing strong electron heating. Conditions in which the electron temperature, T e , is significantly higher than the ion temperature, T i , have been identified as leading to unfavorable destabilization of turbulence and clamping of the ion temperatures at very low values, as shown in plasmas with pure electron heating by means of electromagnetic waves 5 , 10 . Moreover, alpha particles do not provide significant torque and therefore lead to low toroidal rotation, reducing rotation-driven suppression of turbulence 11 , 12 . Finally, they can resonantly destabilize Alfvén waves that can produce stochastic transport of alpha particles and hence reduction of fusion energy generation 13 , 14 , 15 , 16 .

These novel characteristics are in contrast to present-day experiments which are typically heated by neutral beam injection (NBI) at energies ∼ 100 keV. NBI dominantly heats ions, not electrons, and provides significant torque. Unlike fusion-generated alpha particles, they produce a sizeable fast-ion density fraction with T i / T e  > 1. In such conditions, it is well known that high confinement can be obtained 6 .

The Joint European Torus (JET) tokamak 17 , which is the only tokamak in the world capable of operating with T, has undergone a new experimental campaign in D-T, DTE2 18 , with the aim of providing solid evidence on the characteristics of D-T plasmas. A world record of fusion energy was produced in JET at the end of 2021 19 by developing H-mode plasmas with ELMs. However, such an impressive result was obtained under conditions of high NBI power. In this paper, we describe DTE2 plasmas that, rather than maximizing the production of fusion energy, aim to capture other important characteristics expected in future fusion reactors, such as the simultaneous development of conditions with dominant total heating fraction transferred to the electrons, low input torque, and the triggering of Alfvén wave instabilities. Since the simultaneous development of plasmas with dominant alpha particle heating and low NBI input power is not possible at JET, the expected conditions in future fusion tokamak reactors were reproduced by using external heating with electromagnetic waves resonating at the ion cyclotron frequency (ICRF). In these conditions, low external torque is applied while generating ions in the MeV energy range and significant energetic ions-related perturbations. These characteristics were not studied in the DTE2 fusion energy record, nor in the first D-T campaign in JET (DTE1) 9 or TFTR 8 in the 90s, since in both cases the maximization of the fusion power by using high NBI power was mostly explored and in such conditions plasmas with T i / T e  > 1 and high ion heating and rotation are obtained. Therefore, the study presented in this paper represents the first time that this path has been pursued in D-T plasmas.

We show that very good properties are achieved in terms of energy confinement and stability. The type of confinement expected in baseline reactor plasmas is obtained because of the better energy confinement in D-T with respect to the same conditions in D. In particular, for the electrons, the energy losses by transport are low, which allows temperatures of about 110 million K. For the ions, the core heat transport is significantly reduced in D-T compared to D when instabilities generated by the energetic ions are observed. This type of plasma provides an integrated solution for future tokamak reactors since, despite the development of an H-mode, deleterious ELMs are avoided. These results suggest improved energy confinement in future D-T plasma conditions.

D-T plasmas development and comparison to D

Several plasma configurations and D-T concentrations were explored in JET to cover a wide range of possible configurations in future tokamak reactors. To minimize the external torque and hence the toroidal rotation, the plasmas presented in this study were mostly heated with ICRF, which ensures that a low external torque is applied. ICRF heating can accelerate ions to MeV energies, and hence it was used as a proxy for alpha particle-heated dominated plasmas. In these plasmas, 1% of H was used as a minority wave resonator. Additional lower heating power levels of NBI were used with the aim of reaching high temperatures. The discharge #99896, with major radius, R 0  = 3 m, and minor radius, a  = 0.91 m, shown in Fig.  1 A, represents the type of plasma performed. The chosen D-T concentration was ∼ 50% D –50% T as it is expected to deliver the maximum fusion power in future tokamak fusion reactors. A summary of the main global characteristics of this discharge is shown in Supplementary Table  1 in the  Supplementary Information .

A Time evolution of discharge #99896, with toroidal current I p = 1.9 MA, magnetic field B T = 2.75 T, and q 95 = 4.5, heated mainly with ICRF power, P I C R F  = 4.5 MW. The NBI power, P N B I ∼ 3.5 MW, was also injected with deuterium beams, before 9 s, and tritium beams, after 9 s. The radiated power, P r a d represents 60% of the total input power. The power produced by D-T fusion reactions obtained reaches a maximum of P f u s ∼ 0.5 MW. B Time evolution of edge fluctuations as obtained from the B e I I line emission from the inner divertor. C Time evolution of β N , defined as β N  =  β a B T / I p [%] with β the ratio between magnetic and thermal pressure and a the plasma minor radius, H 98 ( y , 2) and \({f}_{Gr}={\bar{n}}_{e}/{n}_{Gr}\) the Greenwald fraction with \({\bar{n}}_{e}\) the average density and n G r the Greenwald density defined as n G r  =  I p / π a 2 . D Accumulated NBI and ICRF radial power deposition for the discharge #99896. E Time evolution of magnetic perturbations detected by the Mirnov coils.

During the phase with ICRF and NBI heating, the thermal confinement time of this plasma, τ , calculated excluding the contribution of energetic ions, is the reference in the ITER baseline conditions as H 98 ( y , 2) =  τ / τ I P B 98 ≥ 1 (Fig.  1 C) with τ I P B 98 the energy confinement time predicted by the IPB98(y,2) scaling 20 . This result is obtained under low rotation conditions, since the Mach number at ρ = 0.5, M  =  v t o r / c s ∼ 0.15, with v t o r the plasma rotation, \({c}_{s}=\sqrt{{T}_{e}/{m}_{i}}\) the sound speed and m i the Deuterium ion mass, is lower than that expected in ITER D-T plasmas 21 .

As shown in Fig.  1 B these results are obtained in the presence of small edge fluctuations rather than ELMs with β N  = 1.2 and f G r ∼ 0.45 (Fig. 1 C). Analysis using the TRANSP suite of modeling codes 22 indicates that 56% of the total auxiliary heating is transferred to electrons as shown in Fig.  1 D. The high electron heating is confirmed by the observation that T e  >  T i , notably in the region ρ  < 0.4 with T e / T i ∼ 1.4 on the magnetic axis. On the contrary, during the NBI-only phase, H 98 ( y , 2) = 0.7, indicating a lower confinement compared to the phase with injected ICRF heating. In terms of power produced by D-T fusion reactions, P f u s ∼ 0.5 MW was obtained. Finally, the radiated power reaches 60% of the total power injected and is located at the plasma separatrix close to the X-point, which indicates that the accumulation of core impurities, usually a concern in high confinement plasmas in a metallic wall environment such as in JET 23 , is avoided.

The good thermal confinement of discharge #99896 is obtained in the presence of generally-believed deleterious electromagnetic perturbations over a wide range of frequencies when ICRF heating is added, as shown in Fig.  1E . We can identify perturbations generated by the interplay between energetic ions and Alfvén waves, such as toroidal Alfvén eigenmodes (TAE) 24 , 25 and reversed-shear Alfvén eigenmodes (RSAE) 26 . Fishbone instabilities 27 , 28 related to the interaction between energetic ions and MHD are also present. All of this activity corroborates the presence of highly energetic ions in the plasma. MHD activity and in particular neoclassical tearing modes (NTM) 29 , 30 are also found.

An equivalent discharge, #100871, in terms of input power, radiated power, and density, with pure D was also carried out to compare to the characteristics of D-T discharges. The match was successful, including a similar pattern of magnetic instabilities. The comparison between the temperature and electron density profiles between D-T and D is shown in Fig.  2 A, B. Although the electron density is nearly identical in D-T and D, both T e and T i are higher in D-T for the entire plasma radius. In particular, in the plasma core, an increase in T i slope is obtained. Since the heating deposition profiles are very similar in D-T and D in the plasma inner core, as verified with TRANSP, such a change in T i could have its origin in differences in thermal transport. Indeed, a significant change is found, as demonstrated by calculating the power balance heat diffusivity for ions, χ i , and electrons, χ e , (see Fig.  2 C). In D-T, χ i shows a drop in the plasma core starting from ρ  = 0.4 and reaching χ i , D − T / χ i , D ∼ 0.5 at ρ  = 0.2. The electron heat diffusivity, χ e , remains very low for D-T and D, which leads to very peaked T e , similar to ITB plasmas observed in other high electron heated plasmas such as the super I-mode developed in EAST 5 . Importantly, χ i , D − T / χ e , D − T ∼ 2 while it is doubled for pure D indicating deteriorated ion thermal confinement.

A Comparison between T i and T e for the D-T discharge #99896 and the D counterpart #100871. T i is measured by the charge-exchange technique on impurity ions. T e is obtained by means of LIDAR and high-resolution Thomsom scattering (HRTS). An average over 8.5 s-8.7 s is performed. ρ is defined as the square root of the normalized toroidal magnetic flux. Shaded error bars represent the standard deviation of the time-averaged signals and the systematic diagnostic uncertainties. B Comparison between electron density, n e , for the D-T discharge #99896 and the D counterpart #100871. n e is measured with HRTS. Shaded error bars represent standard deviation of the time-averaged signals and the systematic diagnostic uncertainties. C Comparison between χ i and χ e obtained by power balance analysis for the D-T discharge #99896 and the D counterpart #100871. Shaded error bars represent standard deviation.

D-T density-ratio scans were performed to evaluate the impact of T on plasma characteristics, as shown in Supplementary Figs.  1 and 2 . As an example, the discharge #99817 was performed in conditions similar to #99896 but with 85% T fraction and B T = 3.7 T. A summary of the main global characteristics of this discharge is shown in Supplementary Information Table  1 . The total heating to electrons under these conditions was 70% as obtained from TRANSP, with P f u s ∼ 1 MW and T e reaching 110 million K and T i 60 million K with only 8 MW of input power. Similar to discharge #99896, a broad range of energetic-ion instabilities was obtained. Compared to the case with ∼ 50% D –50% T , χ i , D − T / χ e , D − T is reduced to ∼ 1, which means that the presence of T in the plasma is a key player in reducing ion thermal energy losses due to heat transport.

Core instabilities analyses

Beyond large-scale MHD and Alfvén-driven fluctuations, small-scale microturbulence driven by the Ion Temperature Gradient (ITG) instability 31 is one of the major threats to plasma thermal confinement. In JET plasmas, ITG modes, destabilized at ion-gyroradius scales, ∼ 1 cm, are usually responsible for enhancing core radial energy transport even in strongly electron-heated plasmas 32 . Instead, energy transport driven by instabilities at electron-gyroradius scales, ∼ 0.1 mm, i.e. by electron temperature gradient modes (ETG), is found not to contribute significantly to overall outward transport in the plasma core 33 . In this section, an analysis of the fluctuations found in plasma #99896 is given, and their role in producing good thermal confinement is clarified.

A Fourier analysis of the measured magnetic fluctuations is shown in the Supplementary Information Fig.  3 . A full range of activity is found at both low and high frequencies. In the frequency range 1 kHz  <  f  < 40 kHz, modes with toroidal mode number n  = 0 − 7 are detected, while for 120 kHz  <  f  < 200 kHz the modes detected cover n  = − 5 to n  = 6. However, nonlinear mode-mode interactions are at the origin of some of these fluctuations. This is demonstrated by performing a mode-mode bi-coherence analysis 34 . A nonlinear interplay between high-frequency TAE and low-frequency NTM is detected, as shown in Fig.  3 A. In particular, toroidal mode numbers n  = 1 and n  = 2 among other interactions are a result of such nonlinear interplays (this is shown in detail in the  Supplementary Information ). A necessary condition for such an interaction to occur is that the radial locations of TAE and NTM are close to each other. This requirement is further verified by the analysis of the location of the different perturbations in real space using several techniques, as clarified in the methods section. RSAE and fishbones are located inside q  = 1, at ρ ∼ 0.25, with q the safety factor defined as q  ≡  d Ψ t / d Ψ p with Ψ t the toroidal magnetic flux and Ψ p the poloidal magnetic flux. RSAE are destabilized very close to the magnetic axis. TAE and NTM are located just outside q  = 1, at 0.25 <  ρ  < 0.45.

A Bicoherence analysis of the perturbations found in discharge #99896. The analysis is performed at t  = 7.7 s. B Logarithmic power of the density fluctuations as obtained from reflectometry at major radius R ∼ 3.36 m, ρ ∼ 0.35, and t  = 8.4 s, for the D-T discharge #99896 with only NBI heating or with full NBI and ICRF heating and comparison to the pure D discharge #100871. C Electrostatic potential, Φ , fluctuations obtained for the D-T discharge #99896 by the global code FAR3D when considering two species of energetic ions, H and D, accelerated by the ICRF power. The yellow circle represents the q  = 1 surface. Inside q  = 1, a n  = 1 perturbation is obtained which is identified as a fishbone instability. Outside q  = 1, TAEs are obtained.

Further evidence of radial and temporal overlap of MHD and TAE beyond q  = 1, located at R ∼ 3.25 m, is obtained from density fluctuations using a reflectometry diagnostic 35 at R ∼ 3.36 m. It is shown in Fig. 3 B that high-frequency fluctuations for both D-T and D are detected in the TAE and NTM frequency ranges. Furthermore, except for the frequencies corresponding to TAE and NTM, density fluctuations are lower for D-T than for D, notably in the typical range of ITG fluctuations f  < 150 kHz. This supports the improvement in confinement for the thermal ions shown in Section 2 although further analyses are required in order to fully characterize turbulence reduction by means of radial correlation from reflectometry. Density fluctuations do not increase significantly when 4.5 MW of ICRF power is added on top of NBI power, except at the TAE and NTM frequencies. This is important because, in general, turbulence increases with increasing input power leading to the so-called thermal confinement degradation with input power 12 .

The origin of the destabilization of magnetic fluctuations by energetic ions has been analyzed with the global gyrofluid code FAR3D 36 . Two energetic ion species, H and D, were considered because they are both ICRF-accelerated by means of the first and second harmonic absorption. Their characteristics are obtained from TRANSP. The frequency and location of the fishbone and TAE perturbations obtained from linear simulations with FAR3D agree with experimental data as shown in the Supplementary Figs.  4 and 5 in the Supplementary Information, which clarifies that the perturbations are destabilized by ICRF-accelerated ions. Nonlinear simulations including both energetic ion species are shown in Fig.  3 C. The radial extension of the electrostatic potential perturbation, 2.4 m  <  R  < 3.5 m, agrees well with the experimental location and clearly shows that the energetic-ion-induced perturbations extend up to mid-radius, coinciding with the radial extension of the decrease in thermal energy transport losses in D-T compared to D.

In addition to the nonlinear interplay between TAE and NTM, other complex and multi-scale interactions are of paramount interest and critically determine the performance. It is found with FAR3D that alpha particles do not destabilize any perturbation due to their low density, however, nonlinear interplay with fishbones, located inside q = 1 at ρ ∼ 0.25, can induce radial transport and losses of alpha particles, partially depleting the plasma axis of such particles, as shown in Fig. 4 A. This is experimentally corroborated, as shown in Fig.  4 B, by means of the fast ion loss detector (FILD) 37 . In the initial phase of the discharge, when the activity of the fishbone is especially strong, alpha particle losses are detected, as can be seen at t ∼ 8 s. In the later phase of the discharge, fishbone activity is reduced in intensity and no further losses are detected with origin in the fishbone perturbation, but rather in the NTM at f ∼ 15 kHz. Unlike fishbones, no alpha losses with origin in TAE are detected in both the FAR3D simulations and the experiment. Importantly, it is found in FAR3D that losses with origin on ICRF accelerated protons are much lower than those from alpha particles in the presence of fishbones. This is an important result, as it shows that alpha particles are very sensitive to magnetic perturbations even if they are not at the origin of such instabilities.

A Alpha particle density profile as calculated with TRANSP assuming no alpha particle transport ( n α , e q ) and comparison to the profile obtained from the FAR3D code after the full development of the fishbone instability ( n α , f i n a l ). B Alpha particle loss frequency spectrum obtained by using the fast ion loss detector (FILD) with channels that are receptive to 3.5 MeV alpha particles.

Therefore, it becomes clear that although magnetic perturbations do not prevent the reach of high confinement, they can lead to loss of fusion power. These results show the critical interaction between magnetic perturbations and alpha particles, and we conclude that it is essential to control such interactions to produce high fusion power in future tokamak reactors.

Nevertheless, magnetic perturbations induced by energetic ions can also lead to beneficial effects that may have a positive impact on reducing energy losses by heat transport. This is the case of the interplay with the so-called zonal flows, i.e. thermal plasma flows with f ∼ 0 and poloidal and toroidal perturbation mode numbers, n ,  m  = 0, 0. Zonal flows were theoretically predicted 38 and also obtained in dedicated numerical simulations in the presence of energetic ions 36 , 39 , 40 . They are known to reduce transport driven by ITG turbulence 41 , in particular in the presence of energetic MeV ions as shown in D- 3 He plasmas 32 , 42 .

The generation of zonal flows is studied for discharge #99896 by analyzing the energy transfer between fishbone and TAE perturbations and the thermal plasma. The 2D pattern of n = 0 structures for zonal poloidal flows, as obtained from FAR3D nonlinear simulations, are shown in Fig.  5A, B, D, E considering two different energies for the energetic ions. Clearly, fishbones and TAE drive zonal flows, with higher intensity with increased energetic ion energy, and hence stronger instability drive. To study the influence of T on the zonal flow generation, an additional simulation with FAR3D is done artificially replacing T by D and keeping the total amount of thermal ions constant. As shown in Fig.  5C, F , although the generation of zonal flow by fishbone and TAE is also present in pure D, the intensity of zonal flow is lower than in D-T, which could explain the lower turbulent transport found in D-T compared to D, as hinted previously in numerical studies performed in D-T plasmas when turbulent transport is close to threshold 43 , 44 .

2D pattern of n  = 0, m  = 0 structures of zonal poloidal flows, V t h (0, 0), for the TAE and fishbone instabilities. V t h (0, 0) is defined as V t h (0, 0) =  E r (0, 0) ×  B T with E r (0, 0) the n ,  m  = 0, 0 component of the perturbed radial electric field. The dependence of the zonal flow intensity on the perturbation strength is studied by scanning the energetic ion equivalent temperature ( T f ) using two values, T f  = 1 MeV and T f  = 500 keV in the FAR3D code. Zonal flow generation increases with increasing perturbation intensity for both TAEs ( A , B ) and fishbones ( D , E ). The radial extension of zonal flow activity coincides with the extension of the two perturbations. The zonal flow activity in D-T is compared to the one in pure D ( C  and F ) by artificially replacing T by D in FAR3D. The intensity of the zonal flow is lower in D than in D-T.

The role of T and energetic ions on the good plasma confinement of discharge #99896 is further analyzed by performing simulations with CGYRO 45 . CGYRO solves the gyrokinetic-Maxwell equations 46 to obtain the electrostatic and electromagnetic fluctuations and corresponding turbulent energy transport. Local simulations are performed at ρ = 0.31, as it is beyond the inverse radius of the sawtooth and is a location in which TAE is detected. The simulations are carried out by including and excluding energetic ions as separate species in addition to the electrons, D and T species. Due to the low energetic ion density compared to the electron density, ∼ 3%, the growth rates obtained in linear simulations are nearly unaffected in the ITG scales by the presence of energetic ions as shown in Fig. 6A , B. However, low k y modes with TAE frequency are destabilized. These results show that the type of plasma found in JET is different to other plasmas dominated by energetic ions effects, such as the FIRE mode 6 , which is characterized by strong turbulence reduction with energetic ion dilution and linear effects 47 .

A Growth rate, γ , and B frequency, ω , spectrum obtained from linear simulations with the CGYRO code. k y is the binormal wavenumber normalized to the proton sound gyroradius ρ s . C Energy flux obtained from gyrokinetic simulations performed with the CGYRO code for discharge #99896 at ρ  = 0.31. The simulations are performed including and excluding the energetic ion component. Values of the ion thermal energy flux deduced from power balance in TRANSP (black horizontal dashed line) are only obtained when the energetic ion component is included in the simulations as a separate species. The total thermal ion energy flux obtained in D-T including energetic ions is compared to the one obtained assuming that all the thermal ions are D while keeping the rest of the parameters fixed. The energy flux in pure D is significantly higher than in D-T (a zoom of those bars is displayed in the inset on the top right). Simulations with \(a/{L}_{{T}_{FI}}=0\) and T F I / T e  = 5.25 are performed to linearly stabilize the low k y energetic ion mode while keeping the energetic ions in the simulations. Fluxes obtained with stabilized mode and energetic ions cannot reproduce the experimental fluxes.

Regarding non-linear effects, as shown in Fig.  6 C, the energy flux, Q , obtained when including energetic ions is close to turbulence threshold and it is comparable to the values calculated from power balance analysis from TRANSP. Importantly, such a strong reduction in thermal energy flux compared to the case without energetic ions is accompanied by a high increase in zonal flow shearing activity, ω E × B , which is more than ten times higher when energetic ions are included, thus confirming the results obtained with FAR3D.

Two extra simulations with CGYRO were performed to reveal the physics mechanism by which turbulent transport is strongly reduced in the presence of energetic ions. In both cases, the aim is to keep the energetic ions in the simulations but to stabilize the energetic ion mode at low k y by assuming either \(a/{L}_{{T}_{FI}}=0\) or by reducing the energy of the energetic ions four times with respect to the standard case, down to T F I / T e  = 5.25 as shown in Fig.  6A, B . As shown in Fig.  6 C, the presence of energetic ions without an energetic ion mode has some stabilizing effect on the thermal fluxes, but the matching of experimental fluxes is only obtained when the energetic ion mode is destabilized.

Furthermore, there is a clear asymmetry between the transport obtained in D and in T, with the T transport systematically lower than that for D, χ i , T ∼ 0.8 χ i , D . Such a difference has an important consequence on the total flux in D-T compared to that of pure D. This is numerically analyzed by performing an alternative simulation in which the T ions are artificially considered as D, thus performing a pure D simulation. The turbulent energy flux ratio Q D − T / Q D  = 0.67 is similar to the power balance obtained for the discharges #100871 in D and #99896 in D-T at the same radial location, Q 99896 / Q 100871  = 0.71. This result confirms expectations from purely numerical simulations performed for D-T plasmas 43 , 48 , 49 . From the numerical point of view, global effects from profile shearing were investigated in CGYRO and found to negligibly affect the thermal fluxes at the radial location studied.

Regarding turbulence in the ETG scales, multiscale simulation capturing both energetic ion modes and ETG is very computationally challenging due to extreme spatial resolution requirements and thus beyond the scope of this work. However, for the case without energetic ions, preliminary multiscale simulations capturing both the ion and electron gyroradius scales indicate that the nonlinear ETG transport is suppressed by the ion-scale fluctuations.

In summary, the analyses of core plasma fluctuations indicate an optimum route towards the generation of fusion power in D-T tokamak plasmas whereby energetic ion instabilities remain in conditions of negligible or weak alpha particle transport, while they can induce thermal energy transport reduction by means of zonal flows, notably in the presence of T. Importantly, this is obtained under conditions of some energetic ions characteristics relevant to ITER burning plasmas, e.g. the energetic ion density is similar to the one expected in ITER, ∼ 1% 48 , avoiding energetic ion dilution as usually happens in strong NBI heated plasmas.

Pedestal formation in D-T

As depicted in Fig.  7 A and B, the D-T discharge #99896 shows the formation of an H-mode with a steep edge temperature gradient, i.e. a pedestal, at ρ p o l ∼ 0.95. The temperature of the D counterpart is lower at the same location, while the density is nearly identical. The fact that the pedestal pressure is higher with increasing isotope mass has been observed in H-mode plasmas with ELMs (ELMy H-mode). However, in ELMy H-mode regimes with typical type-I ELMs 50 , 51 , 52 , an increase in edge density rather than temperature is obtained with increasing isotope mass 53 .

A Comparison between edge n e for the D-T discharge #99896 and D discharge #100871. ρ p o l is defined as the normalized poloidal flux. B Comparison between edge T e for the D-T discharge #99896 and D discharge #100871. The vertical dashed line represents the location of the plasma separatrix. The profiles are obtained from HRTS averaged in the time window 8.5 s-8.9 s for #99896 and 8.4 s-8.8 s for #100871. The evaluation of the error bars in panels A and B is done by deriving the expected signal levels at a given temperature and by calculating the standard deviation based on the photoelectron statistics, the plasma background light variation, and the detector noise.

With the aim of further investigating the origin of the pedestal found in discharge #99896, it is compared to two D-T discharges, one with a clear transition to H-mode with only NBI power and the development of typical type-I ELMy H-mode and another, with NBI and ICRF heating that remains in L-mode, i.e. without temperature pedestal. As shown in Fig.  8 A, the edge density of the discharges #99896 and the one in L-mode is nearly identical, demonstrating that the density of discharge #99896 remains in L-mode, yet the pedestal formation is evident as the edge temperature nearly reaches that of the ELMy plasma (Fig.  8 B). Importantly, discharge #99896 has no ELMs as shown in Fig.  8 C in which the divertor oscillations from Be II divertor emission are compared to those obtained in L-mode and with ELMs. Clearly, the fluctuations are closer to those obtained in L-mode.

Comparison between discharge #99896 and discharge #99502, with P N B I  = 12.5 MW, in H-mode with type-I ELMs, and #99776, in L-mode, with P N B I  = 5.4 MW and P I C R F  = 3.3 MW both obtained at I p =2.5 MA, B T =3.7 T and q 95 =4.5. A Edge n e . B Edge T e . The profiles are obtained from HRTS averaged in the time window 8.5 s-8.9 s for #99896, 7.3 s-7.6 s for #99502, and 8.6 s-8.9 s for #99776. The evaluation of the error bars in panels A and B is done by deriving the expected signal levels at a given temperature and by calculating the standard deviation based on the photoelectron statistics, the plasma background light variation, and the detector noise. C Comparison of the Be II line emission from the inner divertor for the same discharges. The vertical dashed line represents the location of the plasma separatrix.

The spontaneous generation of plasmas with no ELMs and a pedestal for the temperature is systematic in these kinds of plasmas performed in D-T at different I p and B T . It is apparent from the available data that, in those plasma conditions, the input power is close to the L to H-mode transition power threshold, but below the power required to fully develop ELMs. This is supported by the fact that during the phase where the temperature pedestal is sustained an n  = 0 coherent mode can be observed in the magnetic sensors with a frequency of 5 kHz. This mode, known in JET as M-mode 54 , is typically detected in JET immediately after the L to H-mode transition. Long phases with an M-mode present are typically observed in the L-H transition at low density in JET, where pedestal dynamics similar to that described here have been identified 55 . This aspect was further studied in the particular case of discharge #99896, for which it was found that an additional 3 MW of input power lead to the formation of an ELMy H-mode regime. Regarding the magnetic configuration in which these results were obtained, it was used the standard B  ×  ∇ B in JET with the ion ∇ B drift towards the dominant X-point. This configuration is known to be ‘favourable’ in terms of power requirements for the access to H-mode.

These plasmas show similarities with the I-mode regime 56 , 57 , 58 , found in D, which is characterized by the formation of a pedestal for the temperature, while the density remains in L-mode. Similar to the I-mode, the no-ELM regime described here has good impurity transport properties, with no impurity accumulation. However, no signs of the so-called weakly coherent mode (WCM), typically found in I-mode plasmas, have been detected.

A scenario towards D-T burning plasmas

Compared to the more typical H modes that develop ELMs and are heated with NBI power, the D-T discharge #99896 provides an attractive alternative with similar thermal energy, but obtained at lower input power, I p and B T . Such a feature could lead to a more economical and simpler tokamak design. This is shown in Fig.  9 A–C by comparing the discharge #99896 and D-T H-mode discharge #99501, at higher P N B I , I p , and B T and heated with pure NBI power. Clearly, the density is higher at the edge for discharge #99501, since a pedestal is formed for both the density and the temperature, and yet the total thermal energy content of the two discharges, W t h , is similar, W t h ∼ 2.4 MJ. The reason is that the lower edge density is compensated by the higher core temperatures and lower core energy transport losses as shown in Fig. 9 D by comparing χ i , which is nearly ten times lower in the plasma core for discharge #99896.

Comparison between discharge #99896, obtained at P N B I = 3.5 MW, P I C R F = 4.5 MW, I p = 1.9 MA, B T = 2.75 T, and the D-T discharge #99501, obtained at P N B I  = 9.5 MW, I p  = 2.5 MA, B T  = 3.7 T. A n e . B T e . C T i . Shaded error bars represent the standard deviation of the time-averaged signals and the systematic diagnostic uncertainties in panels A – C . D χ i / χ G B . χ G B is the GyroBohm diffusivity defined as \({\chi }_{GB}={{T}_{e}}^{3/2}{{m}_{p}}^{1/2}/({e}^{2}{{B}_{T}}^{2}a)\) with m p the proton mass, e the electron charge, and a the plasma minor radius. Shaded error bars represent standard deviation.

The path towards commercial fusion reactors, although better understood in recent decades, still poses physics and technological uncertainties. The size, magnetic configuration, type of confinement, or power exhaust techniques expected in future tokamak devices are not fully established. Therefore, it is of fundamental importance to further clarify a safe and clear path toward the generation of efficient energy by means of fusion reactions. This is especially important for D-T plasmas. The presence of T, the generation of a high neutron rate at 14.1 MeV energy, or the presence of a significant population of alpha particles, are all characteristics of future fusion reactors in burning D-T plasmas that are not present in ubiquitous pure-D discharges. Studying the impact of such differences is critical in order to properly characterize how D-T fusion reactors might behave. In particular, T can have a strong impact on confinement, impurity generation, and stability, whereas alpha particles can lead to significant destabilization of magnetic perturbations and provide electron heating.

The JET tokamak has recently conducted a new D-T campaign after the first ones were developed at TFTR 8 and JET 9 in the 90’s. In view of clarifying key physics elements that will characterize future D-T plasmas, several scientific directions have been explored. High fusion power generation has been obtained by using NBI heating, which is the main heating mechanism at JET. Other specific experiments have focused on several important topics that were not previously studied in D-T. A particular emphasis has been put on the exploration of some of the unique features expected in future D-T plasmas, i.e. simultaneous dominant electron heating, low rotation, and fully destabilized energetic ions instabilities. Since dominant electron heating by alpha particles is not possible in the JET tokamak at low rotation, the electron heating and high energetic particle generation, able to destabilize magnetic perturbations in stationary conditions, has been externally provided by using the maximum ICRF power available in DTE2.

A major result has been obtained suggesting that some reactor-relevant plasma conditions may be very beneficial. The development of large-scale energetic particle perturbations in the presence of highly energetic ions significantly reduces the conductive-convective energy losses driven by microturbulence in the plasma core leading to a good core energy confinement. The strong impact of zonal flows has been found to play a key role in the reduction of turbulent energy transport in the conditions explored. Importantly, at such low levels of energy losses, there is a clear asymmetry between T and D, since the presence of T significantly enhances the zonal flow activity and further reduces the energy losses by transport, resulting in better global confinement in D-T compared to D.

Such results are obtained in a novel regime developed at JET, similar to the I-mode, consisting of the onset of a pedestal in the temperature while the density remains in L-mode and no damaging ELMs are detected. This new high confinement plasma regime is obtained close to the L-H transition threshold, and therefore requiring low input power. The presence of T is crucial also in the plasma edge, as at equivalent engineering parameters, a pedestal is developed in D-T but not in pure D.

Exploring a broad range of plasma conditions in present-day tokamaks is essential to evaluate how D-T plasmas might behave in the future. This is because the physics mechanisms expected to play a role in ITER and the future fusion reactor cannot be fully reproduced in an integrated way in existing tokamaks, and therefore specific studies must be performed. From this perspective, our studies complement the results obtained in DTE1 as they focus in different plasma regimes. DTE1 provided clear evidence of alpha heating, while differences in energy transport in D-T compared to D were weak and mostly with origin on the pedestal in H-mode plasmas heated with NBI. In DTE2, such results have been reproduced in H-mode plasmas when NBI is used as the main heating system and T i / T e  > 1 52 , 59 . However, our results significantly expand the knowledge about D-T by exploring some other conditions expected in future D-T plasmas that cannot be obtained in high NBI heated conditions. We show that close to the turbulence threshold in the presence of energetic ions instabilities, as expected in ITER 48 , zonal flows can play a significant role and provide a route to lower core energy transport in D-T than in D. Importantly, the change from C-wall in DTE1, to Be and W wall and divertor in DTE2 has expanded the operational regime in which plasmas can be developed, significantly broadening the possibilities to perform experiments under conditions, notably at the plasma edge, not developed in DTE1 44 .

Our findings pave the way for a more economical and simpler design of tokamaks, confirming that nuclear fusion by means of magnetically confined D-T plasmas is a promising source of clean energy. However, further studies, such as compatibility with power exhaust capabilities and exploration at higher density and power, are required to fully qualify these plasmas as a solid route toward tokamak reactors. Furthermore, it is necessary to perform more detailed modeling activities to analyze multiscale effects involving energetic ions and plasma perturbations at different spatial and temporal scales, including the non-linear interaction of alpha particles with magnetic perturbations. The most important aspect is to develop plasmas in which, unlike in the JET results shown in this paper, alpha particle heating is dominant. To this end, experimental and modeling efforts in D-T including a significant population of alpha particles, as expected in ITER 60 or SPARC 61 , are essential.

Experimental design

The JET tokamak has investigated some of the most important fusion reactor conditions by conducting a new D-T campaign with Be/W wall. To reproduce the simultaneous high electron heating, low torque, and the destabilization of energetic ions-related instabilities expected in future tokamak reactors, JET has mostly used ICRF power rather than NBI. Several experimental conditions were explored to find an optimum plasma state in terms of confinement, energetic ion production, and D-T fusion power yield. One of the limitations of such exploratory work was the ICRF power, and hence the amount of electron heating available, which was limited to ∼ 4.5 MW, whereas the NBI power was used up to 10 MW in pure NBI plasmas. The ICRF frequency used was 55 MHz at B T = 3.7 T and 42 MHz at B T = 2.75 T.

Different scans were performed for I p and B T , e.g. I p was explored in the range 1.9 MA  <  I p  < 2.5 MA at two B T = 2.75 T, 3.7 T. The D-T concentration ratio was scanned from 40% T to 85% T by using different valves injecting neutral D and T gases.

Key diagnostics for D-T operation

The ion temperature profiles in this paper were obtained from charge exchange recombination spectroscopy (CXRS) 62 measurements of impurity ions and electron temperature profiles from combined analysis of LIDAR Thomson scattering 63 and high-resolution Thomson scattering (HRTS) 64 diagnostics. The density profiles were taken from HRTS measurements, with the density normalized to match the line-average density measured by a far-infrared interferometer.

Mirnov coils are used as a standard MHD diagnostic on almost all tokamak devices. The coils are installed within the vacuum vessel close to the plasma boundary and provide a measurement of the time derivative of the magnetic field. Magnetic spectrograms (Fourier decomposition of the Mirnov coil signal) can then be used to identify relevant oscillation frequencies associated with MHD activity. In JET a number of coil arrays with high-frequency response are available, allowing activity in the Alfvén range to be observed.

The time-resolved neutron yield in JET is measured using three fission chambers, containing 235 U and 238 U, located outside the vacuum vessel.

The Alfvén eigenmode active diagnostic (AEAD) 65 is characterized by six toroidally spaced antennas, each with independent power and phasing, whose aim is to actively excite marginally stable TAEs.

Alpha particle losses are detected by the fast ion loss detector (FILD) consisting of a Faraday cup array 37 . The Faraday cup array is composed of multiple cups that span a wide poloidal angle below the outboard midplane at a single toroidal location with a minor radial extent.

ELMs are characterized by the BeII emission signal from the inner divetor region.

The plasma isotopic composition is measured at the divertor comparing the relative amplitude of Balmer D α and T α spectral lines. The D and T ratio in the plasma core is assumed to be equal to the edge, as is usually the case in JET in the presence of multi-ion plasmas when turbulence is driven by ITG 66 .

The JET X-mode reflectometry diagnostic 35 is composed of four distinct radial correlation reflectometers. All these reflectometers probe the mid-plane JET plasma. Plasma fluctuations can be obtained from the phase fluctuations of the reflectometer signal.

Magnetic perturbation spatial location and q profile verification

The q profile for discharge #99896 has been obtained by means of a loop between the EFIT code and the TRANSP code. The EFIT code calculates the magnetic equilibrium with the input data for the energetic ions content from TRANSP simulations. After a few iterations, a converged q profile is obtained. The validation of the q profile obtained from TRANSP and used for modeling with FAR3D was carried out against a series of diagnostics and MHD markers. As markers with the strongest signature in the diagnostics, the destabilised NTMs were identified and their toroidal mode number calculated using a toroidal array of Mirnov coils. The radial location of the associated rational surface q = 4/3 was inferred using two methods: the first uses as proxy the location of the phase inversion of the perturbed electron temperature derived from Electron Cyclotron Emission (ECE) at the NTM frequency, and the second matched the NTM frequency as derived from the Mirnov coils to the Doppler-shifted plasma rotation i.e. n  ×  V p h i , where V p h i is the toroidal rotation of the main plasma ions as derived from CXRS diagnostic. The radial location of the q = 1/1 surface was inferred from the inversion radius of the ECE temperature profile during sawtooth crashes as well as from the fishbones signatures in the perturbed plasma temperature from ECE, evidencing a typical kink-like pattern inside the q = 1 surface. Lastly, the RSAEs and TAEs were located using Soft X-ray cameras, interferometry and reflectometry.

As shown in Supplementary Fig.  6 , in the Supplementary Information, the agreement between the q profile from TRANSP and the MHD markers is good for discharge #99896. The q profile obtained for the D discharge #100871 is very similar compared to the #99896.

Experimental profiles fitting

The profile fitting algorithm makes use of a Gaussian process regression (GPR), which is not limited by a selection of specific fit functions and provides a statistically rigorous estimation of the confidence bounds of the fit. For more details, see the book on the topic written by Carl Edward Rasmussen and Christopher K. I. Williams http://gaussianprocess.org/gpml/chapters/ .

TRANSP simulations

The pulses shown in this article were analyzed by interpretive simulations performed with the TRANSP modelling suite 67 coupled with external heating modules NUBEAM (NBI) 68 and TORIC (ICRF) 69 , and prepared with the OMFIT integrated modelling platform 70 . Interpretive analysis was based on the use of fitted profiles, including electron density and temperatures. The fitting of T e , n e and T i were performed on data obtained from HRTS and CXRS. The fitting of experimental profiles consists on applying a global third-order polynomial fit in the range ρ ≲ 0.8 (with the additional constraint ∂ T i (0)/∂ r  = 0).

FAR3D description and simulations parameters

The gyrofluid FAR3D code solves the linear and nonlinear reduced resistive MHD equations describing the thermal plasma evolution coupled with the first two moments of the gyro-kinetic equation, the equations of the energetic particle density and parallel velocity moments 71 , 72 , introducing the wave-particle resonance effects required for Landau damping/growth. The correct model calibration requires performing gyrokinetic simulations to calculate the Landau closure coefficients in the gyrofluid simulations, matching the analytic TAE growth rates of the two-pole approximation of the plasma dispersion function with a Lorentzian energy distribution function for the energetic particles. The lowest-order Lorentzian is matched with a Maxwellian distribution by choosing an equivalent average energy. Further details of the model equations can be found in references 73 , 74 .

A set of linear simulations is performed to reproduce the Alfven eigenmode (AE) activity observed in the discharge, identifying the resonance induced by populations of energetic particles (EP) as passing D and trapped H. The analysis is based on a parametric study with respect to the EP beta (EP density in the plasma) and energy, calculating AEs consistent with the frequency range, plasma radial location, modes number and AE family observed in the experiment. Nonlinear simulations including passing D and trapped H populations are performed to analyze the saturation phase of the AE instabilities, particularly the energetic particle transport induced, the generation of zonal structure, and the nonlinear interaction between different EP populations. The simulations are performed using the EP model profiles obtained from TRANSP, the measured thermal plasma profiles, and the equilibrium calculated with VMEC code 75 .

CGYRO description and simulations parameters

The CGYRO code 45 solves the electromagnetic gyrokinetic-Maxwell equations 46 . Local simulations were carried out at ρ = 0.31. Shaped, up-down symmetric flux-surface geometry was used and multi-species collisions were included using the Sugama collision operator 76 . Transverse and compressional electromagnetic fluctuations were retained. Rotation effects were assumed to be small in the core and not included. Kinetic electrons, D and T as separate species, were included in the simulations. Regarding the energetic-ion species, a lumped H-D species with effective mass averaged between H and D was assumed and modeled by fitting the energetic particle distribution to an equivalent high-temperature Maxwellian equilibrium distribution.

The simulations used a radial box length of L x = 673 ρ s and a binormal box of length L y = 628 ρ s . N r = 768 radial modes and N α = 64 complex toroidal modes were retained. Other resolution parameters were: N θ = 24 (field line resolution), N ξ = 24 (pitch-angle resolution), N u = 8 (energy resolution) with maximum energy \({u}_{max}^{2}\) = 8. The definitions of the CGYRO numerical resolution parameters can be found in 45 . The energy flux is provided in GB unit defined as \({Q}_{GB}={n}_{e}{T}_{e}{c}_{s}{\rho }_{*}^{2}\) , with \({c}_{s}=\sqrt{{T}_{e}/{m}_{p}}\) , ρ *  =  ρ s / a is the ratio of the proton sound gyroradius, ρ s  =  c s / Ω c , to the system size, with Ω c  =  e B 0 / m p the ion gyrofrequency. Convergence tests have been performed indicating that the CGYRO results are well resolved.

The zonal flow shearing is defined in CGYRO as:

where k x is the radial wavenumber, \(\hat{\phi }({k}_{y},\, {k}_{x})\) is the fluctuating electrostatic potential and 〈〉 denotes the temporal average.

The equilibrium profile and geometry parameters are given in Supplementary Table  2 in the Suplementary Information.

Data availability

The JET experimental data is stored in the PPF (Processed Pulse File) system which is a centralised data storage and retrieval system for data derived from raw measurements within the JET Torus, and from other sources such as simulation programs. These data are fully available for the EUROfusion consortium members and can be accessed by non-members under request to EUROfusion. Numerical data supporting the outcome of this study are available from the corresponding author upon request.

Code availability

The research codes cited in the paper require a prior detailed knowledge of the implemented physics models and are under continuous development. The corresponding author can be contacted for any further information.

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Acknowledgements

J. Garcia would like to thank Gerardo Giruzzi for fruitful discussions. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement nos. 101052200 — EUROfusion). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. This work was supported in part by Grants FIS2017-85252-R and PID2021-127727OB-I00 funded by the Spanish Ministry of Science, Innovation and Universities MICIU/AEI/10.13039/501100011033, by ERDF “A way of making Europe” and by ERDF/EU. An award of computer time was provided by the INCITE program and ALCC program. This research used resources from the Oak Ridge Leadership Computing Facility, which is an Office of Science User Facility supported under Contract DE-AC05-00OR22725. Computing resources were also provided by the National Energy Research Scientific Computing Center, which is an Office of Science User Facility supported under Contract DEAC02-05CH11231. This work was partially supported by the project US DOE under grant DE-FG02-04ER54742. D.Z. received financial support from the AIM4EP Project (ANR-21-CE30-0018), funded by the French National Research Agency (ANR).

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CEA, IRFM, Saint-Paul-lez-Durance, France

Jeronimo Garcia & Samuele Mazzi

Laboratory for Plasma Physics, LPP-ERM/KMS, EUROfusion Consortium member, TEC Partner, Brussels, Belgium

Yevgen Kazakov & Jef Ongena

Instituto de Plasmas e Fusao Nuclear, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal

National Science Center Kharkiv Institute of Physics and Technology, Kharkiv, Ukraine

Mykola Dreval

Laboratorio Nacional de Fusión, CIEMAT, Madrid, Spain

Elena de la Luna & Emilia R. Solano

United Kingdom Atomic Energy Authority, Culham Campus, Abingdon, UK

Žiga Štancar, Costanza F. Maggi, Joelle Mailloux, Michal Poradzinski & Sergei Sharapov

Universidad Carlos III de Madrid, Leganes, Madrid, Spain

Jacobo Varela

Institute for Fusion Studies, Department of Physics, University of Texas at Austin, Austin, TX, USA

Dip.to Fusione e Tecnologie per la Sicurezza Nucleare, ENEA C. R. Frascati, via E. Fermi 45, Frascati (Roma), Italy

Matteo Baruzzo

General Atomics, PO Box 85608, San Diego, CA, USA

Emily Belli & Jeff Candy

Princeton Plasma Physics Laboratory, Princeton, NJ, USA

Phillip J. Bonofiglo

Rudolf Peierls Centre for Theoretical Physics, University of Oxford, Oxford, UK

Juan R. Ruiz

Aix Marseille Univ, CNRS, Centrale Med, M2P2, Marseille, France

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The reported experiments were devised and jointly led by Y.K., J.O., S.S., J.G., and M.B., with the key coordination of E. de la L., C.F.M and J.M. The TRANSP simulations were performed by Ž.Š. and M.P. Gyrokinetic simulations and subsequent analyses were performed by E.B, J.C. and S.M. FAR3D simulations were performed by J.V. with the assistance of D.Z. Reflectometer analyses were performed by M.D. and J.R.R. MHD analyses were performed by R.C. and M.D. Pedestal analyses were performed by E.de la L. and E.S. Alpha particle losses were investigated by P.J.B. The manuscript was written by J.G. and E. de la L. with feedback by all the authors.

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Garcia, J., Kazakov, Y., Coelho, R. et al. Stable Deuterium-Tritium plasmas with improved confinement in the presence of energetic-ion instabilities. Nat Commun 15 , 7846 (2024). https://doi.org/10.1038/s41467-024-52182-z

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Travel Tips to Kabardino-Balkaria: More than Mt. Elbrus!

If you’ve traveled to the North Caucasus before, there is a good chance you’ve already been to Kabardino-Balkaria, and you didn’t even know it!  Kabardino-Balkaria lies in the center of the North Caucasus region, is home to Mt. Elbrus, but more than that is a treasure chest of travel possibilities.  Here is our guide to traveling through the republic of Kabardino-Balkaria, or 9 travel tips to this beautiful land:

1. How do I get there?

Kabardino-Balkaria hosts a large number of both foreign and Russian travelers every year, and has an improving infrastructure able to handle the incoming masses.  Let’s start with the obvious.  You might be a mountain climber or skier coming to enjoy the slopes of Mt. Elbrus.  That means you’re likely arriving on an airplane to Russia.  Here are your travel options:

A. Plane – We advise you fly into the Mineralni Vodi (MRV) airport in the Stavropol Region, which is about 45 minutes from the border of Kabardino-Balkaria.  MRV is the largest airport in the North Caucasus, and has daily direct flights to and from all 3 airpots in Moscow (SVO, DME, and VKO), direct flights from St. Petersburg, and several international flight routes as well, including from Istanbul, Dubai, Greece, Tel Aviv, and Bishkek.  The MRV airport has a growing infrastructure and is the most obvious choice to fly into if going to Elbrus.  From MRV, it’s a 2 hr. drive to Nalchik, and a 3.5 hr. drive to Mt. Elbrus.

That being said, the capital of Kabardino-Balkaria, Nalchik (NAL), also has a small regional airport with a daily flight to/from Moscow as well as weekly flights to Istanbul. As is to be expected in most smaller, regional airports around Russia, the service standard at a small airport like this will be minimal.  As a result, we recommend you flying in and out of MRV if able.  It’s a 2 hr. drive to Elbrus from Nalchik.  You can also fly into other regional airports which are 2 hrs. from Nalchik, such as OGZ in North Ossetia (Vladikavkaz) or IGT in Ingushetia (Magas).

B.  Car/Public Transport – If you have a car, are using a taxi, or are hitch-hiking your way to Kabardino-Balkaria, the region is accessible by a variety of roads and vehicles.  A major Russian federal highway E50 runs through Pyatigorsk into Kabardino-Balkaria, and can take you towards Mt. Elbrus, Nalchik, and deeper into the North Caucasus.  There are daily mini-buses, or “marshrutkas”, that travel to Nalchik from Pyatigorsk, Vladikavkaz, Grozny, and Magas, if you’re coming from a neighboring republic.  From the main Nalchik bus station, there is a marshrutka that goes to Terskol (i.e. Mt. Elbrus) daily around 12:30 pm; for that matter,  marshrutkas run daily into every valley of this beautiful republic.  For the seasoned international traveler, you can drive from the country of Georgia up the famed “Georgian Military Highway” through the heart of the Caucasus Mountains, cross the border into Russia at the “Verkhni Lars” border stop, and be in Nalchik in about 2.5 hours as well. 

Anyone traveling on their own should download the “Yandex” taxi app, which is Russia’s version of Uber, and has a very user-friendly app with affordable prices.  In smaller villages/towns where Yandex’s service doesn’t reach, just ask a local and they’ll direct you to a friend or relative who can taxi you where you need to go!

C.  Train – Kabardino-Balkaria is also very accessible by the famous cross-country Russian train system if that’s your preferred method of travel.  Almost all trains to the North Caucasus pass through Mineralni Vodi in the Stavropol region to the north, so make sure wherever you are coming from, Mineralni Vodi is one of the stops.  Despite Nalchik having a train station, the city is about 45 minutes from the main railway route that runs diagonal through the North Caucasus, and as a result it’s a bit convoluted to get a train directly to Nalchik.  That being said, the town Prokhladni is a regular stop on trains going to/coming from Baku, Makhachkala, Grozny, Nazran, and Vladikavkaz, so you can always hop off there and find your way by public transport or taxi.

2.  What are the best places to stay?

This list could get exhaustive, fast. 🙂  Let’s first look at an overview of the republic’s geography, followed by hotel recommendations:

A. Nalchik – This is the capital city of Kabardino-Balkaria, with a population of around 250,000.  Nalchik is growing and new, modern hotels are being built regularly.  Here are some of our recommendations:

-Modern and comfortable:  Azimut , Butik Otel

-Budget with less frills:  Hotel Rossia , Korona

You could comfortably spend a week in Nalchik, while doing day trips into Kabardino-Balkaria’s beautiful mountain valleys.

B.  Baksan Valley – This is the most traveled road in Kabardino-Balkaria, the road to Mt. Elbrus.  If you have questions about its safety because of travel warnings, please see our detailed blog here of the drive to erase any doubts or fears.  Needless to say, because of the draw of Mt. Elbrus, there are a huge variety of lodging options at the end of this valley, from 4-star to mid-range to budget to hostel.  Here are just a few we’ll recommend from our experience:

-Modern and comfortable 4-star-ish:  Azau Star , Kristall 139

-Budget with less frills 3-star-ish:   Laguna , Povorot

If you’re a mountain climber with your sites set on the summit of Elbrus, you’ll have to spend at least 3-4 nights at Elbrus’s famous base camp at 13,000 feet.  The “barrel huts” are not easy to book directly with, and we highly recommend you do your climb (and hence, have your bookings handled) through a trusted climbing company.  Here are two shelters at base camp we recommend:

-Modern and comfortable:  Leaprus

-Budget with less frills:  Heart of Elbrus Lodge

If you’re interested in climbing Mt. Elbrus and staying in these barrel huts, click  here  to see our climbing itineraries, pricing, and group dates.

C.  Chegem Valley – Chegem Valley is the adjacent valley to Elbrus’s Baksan Valley, and is famous for its beautiful waterfalls as well as being Russia’s top paragliding location.  The “ Paradrome ” has modest accommodations for those wanting to get to know this beautiful valley for a longer period of time.

D.  Upper Balkaria, or Cherek Valley – This is another beautiful mountain gorge not too far from Nalchik.  There is an authentic lodging complex in Upper Balkaria called Tau-El, with amazing local food for meals as well.

E.  Border Zone lodging – Several of Kabardino-Balkaria’s mountain gorges run into the border zone with neighboring country Georgia, i.e. an area that foreigners cannot enter without a special permit from the local government (often taking 2 months to receive).  There is a famous mountaineering lodge in Bezengi Valley, where several generations of Russian mountain climbers have honed their craft in the Caucasus Mountains.  Perpendicular to Baksan Valley (about 25 minutes from the base of Mt. Elbrus) is Adyr-Suu Valley, where there is a lodge for back-country skiers to stay, while trying their hands (and feet!) on the untouched snow of that valley.  Both these valleys require border permits for foreigners, but are possible to access for the more adventurous!

3.  Top cities to visit?

Most locals would agree that Nalchik is the main city of significance to visit in Kabardino-Balkaria, but let’s be honest, even more would say, “Just go to the mountains!”  Tirnauz is the capital of the Elbrus district, and is an interesting town to spend some time in, with its unique location in the mountains and place in Soviet history as a once-booming mining town.  The main thing to consider in visiting Nalchik and other cities in the lowlands, is the chance to experience Kabardian culture and food.  Whereas the deeper you go into the valleys, the more you’ll encounter Balkar culture and food.

4.  Best local foods to try?

There are 3 types of food that come to mind, when spending time in Kabardino-Balkaria:

A. Khychiny – This is one of the staple national dishes of the Balkar people, and what you’ll inevitably be served if guests of local Balkars.  It’s a thin buttery flat bread, sometimes cooked with fillings of cottage cheese, fresh greens, or potatoes.  It is often slathered in butter, but wow is that some tasty greasy goodness! 🙂

B.  Shashlik – Shashlik is a MUST for any visit anywhere in the North Caucasus!  Most people would agree that it’s the national food of the entire region.  Shashlik is meat shish kabobs; while pork and turkey can be found in some parts of the Caucasus, lamb or chicken are the preferred shashlik meats of choice in Kabardino-Balkaria. 

C.  Soup – No matter where you are in Russia, you’re sure to find a local soup that people love.  Kabardino-Balkaria is no different.  Especially in the winter months in the mountain valleys, there’s nothing better than to come inside from the cold weather and warm your body up to a bowl of hearty Caucasus soup.  Whether Georgian kharcho or local Balkar lakhman, make sure to try your hand at one of these soups with a side of fresh baked bread/lavash!

5.  Top Hole-In-The-Wall restaurants:

Of course, for a republic of this size, we’re bound to leave at least a few great local joints off our list, but here are a few to get you started. ***Note:  Restaurants in the North Caucasus are much better known for their food than their service, so prepare for tasty food, but manage your expectations about service:

-Elbrus – Kogutai Restaurant at Mt. Cheget – While this isn’t a hole-in-the-wall restaurant per se, it’s one of many to choose from in the Cheget tourist village, and we have found them to provide consistently good food and service.  Kogutai has a nice interior, and maybe most important, an English-language menu with good pictures. 🙂  There also is a nice outdoor patio with fantastic views of the surrounding mountains.

-Nalchik #1 – Tameris Restaurant – This is a cafe with a relaxed atmosphere in the capital Nalchik.  Local tour company Elbrus Elevation has taken foreign groups there on multiple occasions and always had good experiences.  Address is ul. Kuliyeva 3. 

-Nalchik #2 – Cafe-Bar Oasis – You have to know where this restaurant is to find it, but once inside, you won’t regret it!  There is a unique cafeteria-style ordering process, that includes several dishes being cooked on the spot once ordered.  You can sample local Kabardian dishes here.  The seating area is very modern and a pleasant atmosphere to have a meal in.  Address is ul. Kuliyeva 2. 

-Upper Balkaria – Tau-El Restaurant – This is the restaurant part of the Tau-El Tourist Complex in Upper Balkaria.  Whether spending the night or just passing through, make sure to stop here for a meal!

6.  Must-See Sites

This republic is so chock full of “must-see” destinations, it’s impossible to narrow the list down.  Here are just a few suggestions to get you started: (***Mt. Elbrus is a no-brainer and we’re assuming that’s on your list)

A. El-Tyubu and Paradrome – This is an amazing area towards the end of Chegem Valley.  Many tourists visit the famous Chegem Waterfalls and don’t drive any further down this gorge, which really is a shame.  El-Tyubu is a picturesque Balkar village with several historical sites to see, including some ancient mausoleums.  The real gem of the area, though, is the Paradrome , which is Russia’s premier paragliding destination.  The combination of the scenic surrounding mountains and constant winds produces almost daily conditions to sail through the beautiful Caucasus sky.  Highly recommend!

B.  Upper Balkaria – Also known as Cherek Valley, the entire drive to the actual village of Upper Balkaria is one big destination.  First, you can spend time at the 3 consecutive “ Blue Lakes ”, one of which is one of Russia’s deepest lakes with an underground spring.  Then, the drive itself becomes an adventure, as you pass by steep rock walls with a huge drop-off on the other side.  If you’re able to walk this part of the road, that is a bonus!  Once you’ve made your way through the valley walls, the region opens up into a beautiful panoramic view.  Many years ago, there were multiple villages in this region, but they’ve since been condensed into one main village.  You can see some of the ancient Balkar towers that their ancestors used to live in as well.

C.  Djili-Suu – Although hard to pronounce and not easy to get to, Djili-Suu is one of those places in the North Caucasus that people rave about that you “have to” visit.  It’s actually on the North side of Mt. Elbrus, and more accessible from the Mineral Waters region (2 hrs. from Kislovodsk).  The base camp for Elbrus climbers summiting the mountain from the North side is at Djili-Suu.  This area is famous in Russia for its numerous natural healing springs, as well as unique climate conditions that make for beneficial, long holidays for seeking a respite from their daily grind.  There are wide swaths of land available for camping, with probably the most unrivaled views of Mt. Elbrus in the North Caucasus.  Make sure to check this out!

7.  Off-the-beaten path destinations

A. King’s Waterfalls (Tsarskie), or Gedmisht – Probably the valley in Kabardino-Balkaria with the least amount of hype is the Malka Valley, which is the northernmost valley and mainly runs through the Kabardian lowlands.  At the point where the villages end, though (Khabas), the asphalt turns into dirt and the hills start to rise, culminating with the incredible King’s Waterfalls, or as one friend put it, Avatar Waterfalls.  These stunning waterfalls are best visited in the early summer, when everything is lush green and the water flow is strong, with many streams of water flowing down the earth’s surface.  The different colors are incredible and it’s hard to look away.  Once you’ve enjoyed the waterfalls, enjoy a meal of shashlik at one of the nearby lunch huts.  Having an off-road vehicle is ideal to visit these falls, but worth the time and effort!

B.  One-seater chair lift at Elbrus – As the infrastructure at Mt. Elbrus has modernized, some of the more “authentic” experiences have gone to the way-side.  This is one experience still available, though!  From the 2nd (11,000 ft.) to 3rd level (12,500 ft.) of Mt. Elbrus (whether skiing, going to base camp, or just touring), there is a single-seater chair lift for 100 rubles each way (less than $2).  This is an amazing experience if you have the time.  It’s 8-10 minutes each way, and a surreal experience of the majestic Caucasus mountain range surrounding you, skiers silently passing you by underneath, and in general enjoying the silent expanse of nature all around.  The chair lifts are from the Soviet times and so it feels like something from a different era.  For mountain climbers, the newer group cable car gives better access to most of base camp, but several huts are pretty close to this chair lift, so it still may be a good option for you.

C.  Abandoned Mines above Tirnauz – Tirnauz is about 1 hr. from Mt. Elbrus, and a town everyone drives through to and from the mountain.  Although today it looks old and half-abandoned, it was a booming mining town in the 20th century.  About a 45-minute drive above the city with an off-road vehicle, you can see the remains of the mining operations.  Learning about this history combined with the breath-taking views of the Baksan Valley and even into Georgia, you’ll wonder why more people aren’t visiting this place.  This is a great spot to see eagles soaring in the sky, as well as admire the Soviet city plan of Tirnauz from above.

8.  What do I need border zone passes to visit?

In Russia, any area within 5-10 km of a neighboring country, without a clearly delineated border (i.e. in the mountains) is considered a special border zone, and patrolled by Russian border guards.  This area IS accessible to all Russian citizens with their passports, but is NOT legally accessible to foreign citizens UNLESS you have a special permit from the FSB (Federal Security Bureau).  These permits are accessible, either through a tour operator or local friend, but require you to submit your application 45-60 days in advance.

Areas in Kabardino-Balkaria that are worth a visit if you have a border zone pass:

A.  Bezengi Wall – This is at the end of the Bezengi Valley, and holds a place of lore among Russian mountain climbers.  Many mountain guides go through training in this valley.  Five of the Caucasus Mountain’ range’s highest seven peaks are a part of the Bezengi Wall, so you can imagine the draw it has for climbers. There are great areas for trekking and camping in this area. 

B.  Adyr-Suu Gorge – This remote valley runs perpendicular to Baksan Valley and is about 25 minutes from the base of Mt. Elbrus.  It’s marked at the entrance by a relic of the past, a car lift from Soviet days that auto-cranks your car (and you) about 50 meters up the mountain.  After 45-60 minutes of driving on gravel road, the gorge opens up into a flat valley with a beautiful view of the surrounding mountains.  The Adyr-Suu Alpine Lodge is at the end of this valley and where back-country skiers base out of during the acclimatization phase of their Mt. Elbrus ski tours.  This is truly a place where you can experience untouched powder!

C.  Mt. Cheget (Elbrus) – Cheget is a neighboring mountain to Mt. Elbrus and where many climbers will acclimatize, both at its base and while doing some hikes.  It also is famous in Russia for its free-ride terrain for more experienced skiers.  Standard access to the chair lifts and mountain are available to all (i.e. mountain climbers don’t need to worry about accidentally crossing into the zone), but anyone wanting to summit the peak of Cheget OR visit the beautiful Cheget Lake needs a border permit. 

Foreigners violating the border zone areas is considered a serious offense in Russia; make sure to do your due diligence if wanting to visit one of these areas!  We highly recommend using a local tour operator and always traveling with a local person if visiting one of these areas.

9.  Any cultural “do’s” or “don’t’s” to be aware of

Kabardino-Balkaria is a fascinating republic with a combination of traditional and modern society.  The more you interact with local people, the more you’ll see a mixture of Muslim faith, post-Soviet mentality, and ancient local traditions all wrapped together.   

Kabardians mainly live in the lowlands (Nalchik, Baksan, and lowland villages), while Balkars primarily live in the mountain valleys (Elbrus, Chegem, Upper Balkaria, etc.).  There is a large population of Russians in the region as well.  Foreigners visit every area of the region regularly, and so local people are used to and will welcome your presence.

Come with an open mind to learn about these peoples, their traditions, and their land.  You won’t regret your trip to Kabardino-Balkaria!

***Want to learn more?  Here are several self-published resources from the podcast “ CaucasTalk ” related to Kabardino-Balkaria:

– Travel Tips to Kabardino-Balkaria (audio version of this blog)

– History of Mt. Elbrus (Part 1)

– History of Mt. Elbrus (Part 2)

– Interview with Local Elbrus guide

– Climbing Elbrus: Interview with American guide

– Who are the Kabardians? (Part 1)

– Who are the Kabardians? (Part 2)

– Skiing in the North Caucasus (Elbrus and more)

READY TO EXPERIENCE KABARDINO-BALKARIA FOR YOURSELF?

Where to find us.

• +1 704-810-4296
• [email protected]
• 1578 Pine Creek Rd., Gastonia, NC 28056

Travel Information

• We no longer offer travel services to Russia. See Caucasus Quest Tours for new destinations
• Is it Safe to Travel to the Caucasus in 2024?
• Climbing Kazbek & Kilimanjaro: Comparing two 5,000+ meter peaks
• How to Train to climb Mt. Kazbek in Georgia

Our Elbrus Climbing Tours

• Climb Elbrus South Route
• Climb Elbrus North Route
• Climb Elbrus & The Capitals
• Climb Elbrus & The Caucasus

Russia Cultural Tours

• Capitals of Russia
• Lake Baikal on Ice
• Delightful Dagestan
• Heart of the Caucasus

ALL Travel Services to Russia and Mt. Elbrus have been indefinitely suspended as of Feb. 2022.

Explore our new tour branch Caucasus Quest to climb Mt. Kazbek (5,054 meters) in Georgia or for immersive cultural touring experiences in Georgia, Armenia, and Azerbaijan.