[MUSIC] In this second module of our introductory course in particle physics, we are discussing the physics of the atomic nucleus. And in this eleventh video, we are fortunate to be invited to visit the experimental Tokamak of the Federal Institute of Technology in Lausanne, or more precisely of their Swiss Plasma Center. This is a research tool for nuclear fusion, which is the source, I remind you, of energy produced in the sun, and other stars. If we ever manage to use fusion energy in commercial power plants, mankind will have a virtually inexhaustible source of energy, safe and environmentally friendly. Fusion power plants impact the environment in fact very little and produce no greenhouse gases. One can also exclude any serious accident with large release of radioactivity because the fusion process does not by itself produce any radioactive waste from its fuel; only the structural components inside the reactor become somewhat radioactive. Nuclear fusion has also the major advantage of using fuels that are available on Earth, in virtually inexhaustible quantities. It could thus become one pillar of our energy supply that will provide the necessary conditions for sustainable development. I remind you that the fusion reaction between deuterium and tritium has already been introduced in video 2.10. It produces a strongly bound helium nucleus and a neutron, releasing a large amount of energy, approximately 17.5 MeV per reaction So today we will visit a medium sized reactor, which is behind me, called the TCV, Tokamak with Variable Configuration, of the Swiss Plasma Center. In such an instrument the issues of containment and stability of the plasma are studied, which are necessary for the realisation of a fusion reactor. To realise an efficient fusion reaction, and to study burning plasma which produces energy, will require larger machines like ITER, which we will discuss at the end of this video. [NOISE] So we are now in the lobby of the Swiss Plasma Center, and I present to you my two colleagues from EPFL, who will be our guides during the visit. Here on my left is Dr. Yves Martin, who is the Administrative Director, and also the person responsible for communication of the Swiss Plasma Center; and on my right is Dr. Antoine Pochelon, who is senior lecturer, admittedly in retirement, but who has lost none of his enthusiasm for plasma physics and nuclear fusion. Our subject today is nuclear fusion, and how to put it to use in the transformation of nuclear energy into usable energy. So what does this have to do with plasmas, Yves? >> In fact, the fusion reaction we seek to produce in a Tokamak can be obtained only at very high temperatures, because one must overcome the Coulomb repulsion. And to overcome this Coulomb repulsion, it is necessary to heat to temperatures at which matter takes the form of a plasma. A plasma is a gas in which the atoms are ionised, in which the electrons are dissociated from the nucleus of their atom And from that point onwards, we have a mixture of charged particles, but globally neutral. >> Okay. >> And the benefit of this state of matter is that we can guide the particles using a magnetic field. >> Ok. So therefore there are several steps. First one must create a plasma, confine it, magnetically I imagine, and heat it to relatively high temperatures, indeed, incredible temperatures. So can you describe a bit these steps? >> The beginning of plasma formation? >> Yes. >> Well, it actually begins with a gas in which an electric field is applied. And this electrical field short-circuits the plasma it causes in fact a discharge in a rarefied gas. And then, the electric field draws a current, this current heats the plasma, heats the gas which is becoming a plasma when it reaches a temperature, typically of 10’000 degrees Kelvin, corresponding to a kinetic energy of typically one eV. >> That is to say, to the binding energy between a typical electron and a typical nucleus. >> It's the order of magnitude, effectively to separate the electron from its nucleus. We are producing this discharge in a magnetic field, which guides the particles, and forms a magnetic bottle. And we have behind us an example of such a bottle, called a Tokamak. >> Yes, is it a model of one-to-one scale? >> It's a 1:1 model of the machine that you'll see in a moment, and different colours allow us to see the many different elements. First one starts, on the inside, by the vacuum chamber, wherein the plasma will be installed. The main magnetic field of the magnetic bottle is generated by the blue coils, toroidal coils, which create a field around the center in a toroidal sense. >> A field that revolves around the machine's center. >> That's right, here we have a transverse section of the machine. And then, to generate the electric field, which we spoke about, we have these coils, which form the primary circuit of a power transformer, which induces an electric field, and thus the plasma current. In addition, it is a special feature of this machine, that we have as many as 16 additional coils, in yellow here… >> The yellow ones, yes. >> …powered by independent generators. …powered by independent generators. This allows controlling the plasma shape and position. It also helps to extend the plasma, since ultimately currents of the same sign repel each other, opposite currents attract each other. >> To simplify somewhat, the blue coils create a field that is confining, and then the yellow coils are used to manipulate the shape of the containment. So, Yves, can explain how, then, the plasma is heated beyond the initial temperature? So first of all, as in any Tokamak, ohmic heating is used; simply, as Antoine has said, there is current flowing in the plasma. >> Okay. So it is the plasma resistance that will heat the plasma. >> Absolutely. But the temperature, which can be achieved with this heating, is not sufficient yet. So we must add other heating systems, and what we normally use, notably here in our Tokamak, is heating by microwaves or radiofrequency heating. It will act on resonances to be found in the plasma. >> But, this will heat the electrons, right? And not nuclei, primarily… >> Indeed, the heating system we have here primarily heats the electrons; but afterwards, by collision with the ions, the electron energy is also transferred to the ions, and therefore, the temperature of the ions also increases. >> So, to learn how one generates the radiofrequencies, which heat the plasma, we will go up, to see gyrotrons producing this radiofrequency. [NOISE] >> I'll go through what a gyrotron is: you have one installed down there, we'll get back to that in a moment. The gyrotron is essentially a vacuum tube that generates 0.5 to 1 mega-watt of power. At the bottom of the image, you find the electron gun producing an electron beam, which passes through the tube. The electrons arrive in the collector, here, at the bottom right. Then, the electron beam passes through a cavity within the magnetic field generated by a superconductor. Then using a wave converter, the microwave power is directed onto a mirror and laterally removed from the tube. Here we see such a gyrotron: Invisible below, the electron gun, in the middle the cylinder housing the superconducting magnet with the cavity. On top, under its yellow lead cover, you have the electron collector. In front, the microwave enters the optical adapter box with various mirrors that elliptically polarise the wave, in the right way to heat the plasma locally in the Tokamak. We use, as you see here, a whole battery of gyrotrons. We have six at a frequency of 80 GHz, with a total power of 3 MW. And back here we have gyrotrons of 120 GHz, which are very long pulse gyrotrons we manufactured in collaboration with French colleagues. [MUSIC] >> So here is the TCV Tokamak with first the magnetic coils that will create the main field, the toroidal field, and a little bit on the back coils that will help shape the plasma, the eight coils. So we see only the outer coils, all the others are hidden inside the machine. And right there behind, one sees the vacuum chamber into which we will inject our plasma. To heat the plasma, we use microwaves to take the plasma to very, very high temperatures. The temperatures required for fusion are of the order of 100 to 150 million degrees, and in this Tokamak, we obtained such temperatures. Note that in the sun, temperatures are much lower, around 1.5 keV, and at this temperature, in fact, only the particles found in the tail of the Maxwell distribution will merge. >> The inside of the TCV vacuum chamber is covered with carbon tiles, in fact, like a space shuttle. And the tile surface is covered with boron carbide by plasma deposition, almost as hard as a diamond layer, which reduces on one hand, the plasma abrasion of the carbon tiles. Moreover, as B4C is formed by elements with low charge Z, it makes that the plasma sees only low Z. This is very important because a source of plasma energy loss is radiation. And this radiation increases sharply with the electric charge, typically as Z to the 4th power. [MUSIC] The role of the TCV we have behind us is to study the properties of the plasma. That is to say for example the density, temperature, or even the rotation of the plasma, because the plasma rotates in the vacuum chamber. With that, we will be able to study the confinement, the transport of heat and the stability. And this, we will study as a function of different characteristics, such as density, temperature and rotation. One thing we will particularly examine with the TCV, this machine where you can vary the shape of the plasma, is the dependence on the form. One can also create a divertor. A divertor is basically a magnetic structure outside the plasma that removes impurities, which are, in the case of fusion, helium, which is the product of the reaction. So we will eliminate it, and also remove impurities at high Z, which as we have seen, make the plasma radiate. Therefore, around the machine, we have an amount of diagnostics, measurements such as this neutral injector measuring the ion temperature and plasma rotation. But we also have tools, i.e. actuators to vary the plasma parameters. We have already spoken about the microwave heating which primarily heats the electrons. We are also installing now 1 megawatt heating by neutral injection to heat directly the ions. So this makes a total of 4.5 megawatts for gyrotrons at frequencies between 80 and 120 GHz, and then to begin with 1 megawatt of neutral injection. [MUSIC] >> So all this means that the TCV is a research tool rather than a prototype reactor, right? But I guess it is part of a more comprehensive program that should lead to a fusion reactor, a true one, right? >> Absolutely, the Tokamak here is an experimental Tokamak, which is used in a vast fusion research program. It is clear that to meet the many challenges of fusion, i.e. to arrive at a commercial fusion reactor, it is necessary to coordinate our efforts at the European level, at the international level, and Switzerland is part since 1978, through a bilateral agreement between Switzerland and Euratom, in all European programs. >> What is the role of the TCV in such a program? Antoine, what are the specifics of the TCV that can contribute to this program? >> The most important specificity of TCV is that it allows varying the shape of the plasma. And this is a program that we are heavily exploiting. For example, most large Tokamaks today have a D-shaped plasma, that is to say the plasma belly looks towards the outside. But in TCV, one can also make plasmas in the shape of an inverted D, that is to say the belly looks inward. And it is, in fact, the only machine in the world to make this type of plasmas. >> And what is the point of having this form of plasma? >> We were lucky to have as a bonus a rather surprising result, it was not expected of all at first, which is to have the energy confinement increased by a factor of two with the inverted D. This is first interesting in itself. Then, when you have a doubled confinement, you can put half the heat into the plasma, so you pay less to heat the plasma to the same temperature. So that's a first advantage. The second advantage is that it allows having plasmas of equivalent temperature, but with completely different shapes. And who says different confinement, also says different turbulence. So we can compare now with complex codes, which simulate the plasma turbulence and its confinement, and we can compare in this way the experiment with numerical models and this is really what the experiment is good for. >> So it is the turbulent plasma instabilities that limit confinement, confinement time in a Tokamak? >> Actually, at the plasma edge we observe instabilities that strangely resemble the solar prominences, solar flares. We see in this video, which comes from Tokamak MAST in England, these flashes of light, which resemble solar flares. And we develop operational modes precisely to reduce the impact that these solar flares would have on the Tokamak material. [MUSIC] This Tokamak with its shape control system, the electron heating system, and soon its ion heating system, as well as with its set of diagnostic tools, measuring devices, it is an exceedingly productive machine. We obtain large quantities of absolutely fantastic results, which can be created with this machine that is absolutely unique worldwide. Moreover, the fact that we are part of an academic environment in the context of EPFL, gives the possibility to educate a lot of doctoral students in plasmas physics tasks. [MUSIC] >> So the next step towards commercial use of fusion is called ITER. A true reactor this time? >> Yes, absolutely, it is the first reactor, it is likely the first time that we will produce much more energy than what the machine requires to function. They talk about a factor of ten of energy amplification. Here is a model, of a much smaller scale than the model we saw before. So the scale here is 1:50, you see a little man here behind. Otherwise it has exactly the same components that we have in our Tokamak. >> So it is an international program, I imagine, it is not only Switzerland who builds it? >> Absolutely. It is a global collaboration with Japan, South Korea, China, India, Russia, the United States and Europe, including Switzerland. >> So what is the role of Switzerland in promoting this project? >> We contribute first of all scientific aspects: the operation is prepared for this machine, the scientific program, to know which experiments we will perform with it. But we are also preparing physical elements, notably the microwave mirror systems, which allow injecting the microwave, for which we are experts here at the Swiss Plasma Center, with all the knowledge we have accumulated over the years. We also participate in magnetic measurements, and also in the superconductor tests; all superconductors to be used for all the coils of the machine will pass through Switzerland, through our laboratory to be tested. >> So, in contrast to the TCV, it will not be normal magnets, but superconducting magnets that will serve here. >> Absolutely. >> So, up to a large size increase, all this follows somewhat the same principles, that of the Tokamak, right? But are that there are no other means to store a plasma that can undergo fusion? >> In magnetic confinement, there are essentially two major strategies. We talked about the Tokamak, and I will quickly repeat what a Tokamak is. In a Tokamak, the confining field is made at same time outside the plasma by the toroidal coils, and within the plasma, by the field created by the current circulating in the plasma. There is another way of making magnetic bottles, the Stellarator. In that concept, the field, which confines the plasma, is generated completely on the outside, by coils with rather complex shapes. That is why there are not so many Stellarators worldwide right now, they are difficult to build. The stellarator is promising also because there is no disruption like in a Tokamak, that is to say brutal interruptions of the plasma current, and you can get to very high densities. It is therefore extremely promising from the plasma point of view, but it is difficult to build. >> But there is a Stellarator, which was built in Germany. >> Yes, in Greifswald on the shores of the Baltic sea, the Stellarator Wendelstein 7-X was built in recent years and, right now, they are currently testing the shape of the magnetic field produced by the very complex coils outside the machine. There you have a very elegant picture, which shows that the magnetic field, which they had wanted to achieve, is indeed realised. >> And according to you, these two strategic directions will they result in a reactor that works and produces energy soon? >> We explore the Tokamak, which is relatively simple to build, and the Stellarator, which is an alternative. In fact, fusion is a program that spans many generations. It's long, it's a program that includes both fundamental research and also an extremely applied research: we must develop the materials necessary for fusion, we need very large superconductors at high fields, then we must, to name just one example, regenerate tritium from lithium irradiation. Thus, a vast program. >> Thank you very much, Yves and Antoine, for this visit and for this fascinating journey through the world of fusion and plasma physics. I draw your attention to a MOOC offered by EPFL in parallel to this course, entitled "Plasma Physics and Applications", you will find information on the web site shown here. [MUSIC]
