 Hello everybody and welcome to the second video of the Fusion Research lecture. You remember that last time I gave a historical overview about the early time of fusion research and one of the important messages from last time was that pinches to the configurations which were researched at the beginning of fusion research are not suitable for fusion reactors. One solution to overcome the problems with pinches were proposed by those three men shown in the picture here. On the left hand side we have Lyman-Spitzer, one of the fathers of the Hubble Space Telescope program. Then we have Igor Tam and Andrei Sakharov from the Soviet Union. And what they suggested was add a strong axial magnetic field to get a better confinement. So the solution proposed by them was add an additional strong axial magnetic field. The solution by Lyman-Spitzer from the US was using coils only. That is called the Stellarator. There the magnetic field is completely produced by coils. And the other solution suggested and pursued by the Soviet colleagues was the Tokamak. And that is also produced by induction. And the additional strong axial magnetic field results in a twisted magnetic field lines. Now why do we need twisted magnetic field lines? That is an important question. Why do we need twisted field lines? That is a good question. Let's look at the case if we would not have twisted field lines. What would happen to them? Let's try to draw a cross-section. Something like this. This is a cross-section where we have the magnetic field pointing into this direction. We have a magnetic field gradient pointing into this direction. And in the fluid picture, you remember that from Plasma Physics 1 lecture, in the fluid picture we have the diamagnetic drift. The diamagnetic drift which describes the drift motion being caused by a pressure gradient and a magnetic field. And since we have a pressure gradient here, so in that cross-section the Plasma pressure is the highest in the center. And we have a magnetic field direction. And this is divided by the charged particles density and the square of the magnetic field. And from the diamagnetic drift motion, the diamagnetic current follows. Which can be written as minus, again it's the pressure gradient and the cross product of the magnetic field. Divided by the squared of the absolute value of the magnetic field strength. Now, if we draw the magnetic, sorry, the diamagnetic current into the cross-section. And since it depends on the square of the magnetic field, sorry, on the inverse of the square magnetic field, it is larger on the output side. So this is the direction of the current, remember there's a minus in front of the fraction. And so maybe something like this, right? I've tried to draw a thick arrow here, so this is supposed to be the J-diah, the diamagnetic current. And on the other side, it's a bit smaller indicated here by a thinner arrow. So this is also J-diah, so-called diamagnetic current. And this current, which is stronger on the output side than on the inverse side, would now lead to charge separation. Having positive charge on the upper side and negative charges on the bottom side. Meaning, if we come back to the original question, why do we need twisted field lines? If we do not have the twisted field lines, we would get charge separation. Charge separation, which generates an electric field. So I've only drawn here the charges. But of course, you know that this makes an electric field like this. This is an electric field, E. And the electric field and the magnetic field, that results in E cross B drift, as you know. E cross B drift. And you know that this is a charge-independent drift, meaning it pushes the whole plasma to the output side. If you make a cross product of the two quantities, you see that the plasma is pushed to the wall. So pushes plasma to wall. And thus we would have no confinement. So not suitable for a fusion reactor, no confinement. Now obviously, twisted field lines are a solution to that. Twisted field lines are a solution to that. And why is that the case? We come back to our cross-section here and extend that plot a little bit by trying to draw another cross-section here. And then trying to make a plot where these are connected. These are two cross-sections, right? Something like this. And if we... So this is the same cross-section magnetic field now pointing outwards, but we would have the same charge separation here with plus and minus. And now adding twisted field lines. So let me try to draw a field line on the torus here. If it's twisted, it might look for example like this. So this is a magnetic field line here, giving the direction of the magnetic field line at each point. And this field line allows now for the charges to freely move along them and thus to compensate for the charge accumulation. So the twisted field lines are therefore necessary to compensate for the charge accumulation. Since they allow currents to flow along the field lines. Allow currents along the field lines. And these currents have a name and they are called furschlüter currents. And since these are quite important, give them a yellow box. And by the way, I have derived the necessity for twisted field lines here in the fluid picture. But you can do the same in the particle picture. Then you would need the gradient drift and the curvature drift. But the result would be the same. Okay, now we see that we need twisted field lines. And one solution suggested 1951 by Spitzer was the so-called figure 8 accelerator. Named figure 8 for obvious reasons as you see in the picture below. Figure 8 accelerator in Princeton. That was suggested by Lyman-Spitzer. And you can see one thing in the picture which I have included here. That the experiments at that time could be brought to the conferences actually. So they fit on the table, which is really nice thing. An interesting or funny story is how Lyman-Spitzer got interested in fusion research. Was actually by some science face news. Because in 1951 an Austrian physicist working in Argentina claimed a breakthrough in fusion research. And Lyman-Spitzer once said in an interview that his father-in-law called him in his skiing vacations. Pointing him to that news press article about the fusion breakthrough, which was false. And this got Lyman-Spitzer interested in fusion research. And legend says that the shape of his stellarator, which here looks like an 8 and is a bit twisted. Was inspired by the form of a pretzel, which he had when he went skiing. So here the twisted field lines are achieved by twisting the whole torus. Another way to realize the twisted magnetic field lines is instead of twisting the torus. Using twisted helical coils as depicted on this slide. Here are two realizations of that. On the left hand side is the classical stellarator. On the right hand side the torsotron or heliotron. So let's first briefly talk about the classical stellarator. Where you can see that you have helical coils winding around the torus here, right? Some helical coils winding around the torus. And in these coils the current flows in opposite directions. So the classical stellarator we have oppositely directed currents in the helical coils. That requires the use of additional toroid field coils. We will discuss that in the magnetic configuration section. Here in the drawing these are indicated by the blue color. So we need toroid field coils in addition. In the torsotron or heliotron on the other hand, we also have helical coils winding around the torus as you can see here. But now in these helical coils the current flows in the same direction. So it's equally directed currents in helical coils. And as you can see already maybe in the drawing there are additional type of coils in green. These are vertical field coils which we need here. We need vertical field coils. And again we will discuss that in more detail in the magnetic configuration section. Okay, now these are some nice drawings but let's look at a few stellarator realizations. Here we have the model C stellarator. Model C stellarator, a very early device in being an operation in Princeton. Princeton from 1961 until 1969. If you look at the photography you can see here somebody sitting or standing. So to give you an idea of the size the stellarator is located here. And you can also see it is somewhat elongated. So this is approximately the plasma shape I would say. So it's a racetrack configuration with a length of the racetrack of approximately 1.2 meters. And the plasma had radius of something on the order of 6 centimeters. Not only Princeton stellarators were built but also in Garching in Germany near Munich. So here are some first experiments at the IPP Garching at the Institute for Plasma Physics Garching. You can see a few stellarators here on the bottom. This one, this is also a racetrack configuration. This is Wendelstein 1A. Then here we have another one, this is a linear device. Wendelstein 1B. And by the way if you wonder about the name Wendelstein, Wendelstein is a mountain near Garching. And this was inspired by the Americans who named this stellarator program Matterhorn. So here we have another stellarator. Now this is a circular one. This is Wendelstein 4. And again if you look at the photography you see two scientists sending there to give you an idea about the size of the experiment that they were rather small at that days. Another one, a more recent one. This is a picture from last year, a stellarator called TJK. This is a TJK stellarator. And this is located at the University of Stuttgart. So I have included it here because this is where I work. And this is actually to be more precise a torsaton. And it is a low-temperature stellarator which can be operated at the University because you can see here, you can see that it is a low-temperature stellarator because it emits light in the visible range. Fusion-relevant parameters usually mean that you have temperatures too high for light in the visible range, but rather in the X-ray range. Okay, a very important concept in the stellarator research was the development of modular coils, modular coils. In the year you see two drawings. If you first look at the left drawing here you see the helical coils winding around the torus producing the magnetic field that you need. And on the right drawing you see modular coils. And these modular coils produce the same magnetic field as on the left-hand side. Now this has the advantage that in case of any damage the coils can be repaired. So it's repairable in case of damage, repairable in case of damage. And in addition, that is maybe even more important, you have more flexibility in your magnetic field configuration and designing your magnetic field configuration, more flexibility. And we will see that this is an important part in stellarator research when we talk about the stellarator in more detail. So more flexibility in magnetic configuration. And the first realization of a modular stellarator was the W7AS stellarator in 1990. W7AS, you see it in the drawing on the left-hand side. And this was the first stellarator with modular coils. And this is what the A stands for in the name for advanced. And that refers to using modular coils. And it was located at IPP. Okay, as a next important experiment I'd like to list here LHD, the large helical device which was put into operation in 1989 in Japan. The picture on the left-hand side, you can see first that it is a heliotron because the coil binds around the torus only in one direction, current flowing in one direction. You see Japanese scientists standing around and engineers here to give you an idea about the size of the experiment. So it is a very large stellarator. It has a large radius of 3.5 meters, an average plasma radius of 0.6 meters. And if you look on the photography on the right-hand side, you see the experiment hidden underneath in a lot of heating systems there. It has superconducting coils. And this allowed for very long-piles operations for conducting coils. And so this was basically an experiment where you could operate plasma-infusion-relevant parameters in steady state. So that was and is still a very important experiment. The next one in the list for the stellarators is W7X, which is an optimized stellarator. And what optimized means, we'll tell you that later. For now optimized here means that losses are reduced. So in that way optimized. To give you an idea about the size, look at the drawing on the left-hand side. In the center there is a human being standing. So it is really a huge device which has of course also superconducting coils to allow for basically steady state operation. It has a large radius of 5.5 meters and the average plasma radius is 0.53 meters and it also uses modular coils. W7X was put into operation in 2015. So 2015 the first plasma was achieved in W7X. And if you look on the photographies on the left-hand side, here you can see the chancellor, Angela Merkel, pushing the button to start the first plasma. Here you can see the head of the Max Bank Institute, Professor Sibel Günder. That was a big press event and the German fusion program got a lot of positive press there. The other photographies, the first plasma is shown top left, the magnetic field lines top right, the plasma. And as I said, that was a very successful experiment so far. Okay, that's it for a brief historical overview about these accelerators. Let's now briefly talk about the tokamak. Tokamak, oops, tokamak. To start with let's first briefly outline the concept of the tokamak because you might know that already based on induction. So we use the transformer principle and what we now try to do is first draw a transformer where we have some kind of primary winding. So some wire winding here around this iron core, something like that. So this is supposed to be the primary winding of the transformer primary winding. This is the original concept where we use an iron core. Now this is the primary winding here and the plasma now is the secondary winding. So we have here some kind of toroidal shaped plasma which might look like this. And around that there are the toroidal field coils, which we had seen earlier. So a lot of coils here. And if we now ramp up a voltage or a current in the primary winding that induces a current in the secondary winding, which is the plasma, it produces a current thus it produces a magnetic field. Together with a magnetic field from the toroidal field coils we get the twisted magnetic field lines necessary for confinement. Now that is the concept with the iron core. In modern devices often we realize the tokamak without an iron core, and let's quickly try to draw that. So again we have some kind of plasma looking like this. And then now we have not an iron core but an air core and we are using a central solenoid as primary winding. So in the center we have some kind of central solenoid. This is supposed to be a long solenoid as you can, I'm sure, easily imagine, right? So this is now the primary winding. Then we of course also have the coils producing the toroidal field here around it. So if we ramp up a current in the primary winding it induces a current in the plasma together with the current from the toroidal field coils we get our twisted magnetic field lines. Now a slightly improved version of the drawing can be seen here. Again in the center the central solenoid divided into several sections as you can see. To being able to ramp up the current in different parts to get a profile inducing a current in the plasma here and then the blue coils here producing the toroidal field together giving us a twist that we need in the field lines. So the tokamak is based obviously on induction. That has a few consequences. One first consequence is that it is an inherently pulsed device, right? It's a pulsed operation because we can only induce a current in the plasma if we ramp up or down a voltage. Now in addition, which is a very big advantage is that driving a current in the plasma provides also a heating mechanism. That is omic heating. Omic heating however has unfortunately the problem that it is, or the drawback that it is not sufficient to achieve fusion parameters alone. So alone it is unfortunately not efficient, sorry not sufficient for fusion parameters to achieve fusion parameters. Now why is that the case? Because when we increase the temperature on the plasma the resistivity drops and thus the heating efficiency from omic heating drops as well. Okay, a bit of an historical overview and the day to start with is certainly 1949 where a Soviet soldier who had basically no, I think he even had no high school diploma if I'm not wrong, Oleg Lavrientiev and I'm pretty sure I pronounced that wrong, I'm sorry, he sent a letter to Stalin proposing the original tokamak concept. Stalin forwarded that to his chief scientist and they found the idea very appealing, slightly adopted it and realized the first tokamak experiments from that. The Kurchatov Institute was founded in Moscow at that name, it was not named like that but nowadays it's known as Kurchatov Institute where a lot of major breakthroughs in tokamak research were achieved. Among others in 1968 where they achieved the remarkable temperature of 2 kilo-electron volt at the T3 tokamak and when they presented their results at conferences the other scientists from the other countries basically didn't believe them, they did not thought the Soviet scientists did something wrong, it was just much better than their values. So the Soviet scientists invited the scientists from the world to their experiment and actually a team of British scientists came over bringing their own diagnostic installing them at the T3 tokamak and they confirmed these values. So this was a much better performance than the Stellarators at that time. So a much better performance much better performance than Stellarators and this is the reason why many labs around the world then decided to switch from Stellarators to tokamaks. In 1973 at the IAEA conference in Novosibirsk the Intor concept was first discussed and proposed and that was the first concept for an international reactor basically if you wanted to predecessor of ETA. However before we come to working fusion reactors the next important reactor to mention or experiment to mention sorry is certainly JET, the joint European Taurus which was put into operation in 1983 and JET really is a massive device. First of all you see it has an iron core then here you see a human being standing there and this gives you an idea of the size of it. The major radius is 2.96 meters the average plasma radius is 1.25 meters and JET is located in Southern England in Cullham and it is also important because it allowed for DT operation. In this photography you see this is showing the inside of JET and again to give you an idea of the size of scientists standing there on the left hand side working on it. That gives you another good idea about the size of JET. JET allowed for real fusion experiments so it was designed for DT operation, so daterium trisium operation and in that sense it is a real fusion experiment and JET holds the record for the highest amount of fusion power released in an experiment which is on the order of 16 megawatt. The next step is ETA the International Thermal Experimental Reactor. ETA is even larger with a radius of 6.2 meters and a small plasma radius of about 2 meters and ETA is designed for the first time to release more energy via fusion processes than was initially needed to heat up the plasma. You can get an idea about the size of ETA looking at the picture on the drawing sorry on the left hand side where you see here a scientist standing next to a human being and ETA is located in southern France. It is a joint international project China, European Union, India, Japan, Korea, Russia and the US are part of it representing half of the world population designing, constructing and building ETA together. Here is a recent photography of the construction site so taken in February. ETA is scheduled to have its first plasma in 2025, first plasma in 2025 and 2035 the full power DT operation will take place so that will be the date when as I said for the first time more power will be released by fusion than was initially needed to heat up the plasma. That will be a remarkable date in fusion research and as you can see it's not too far away and you are right in the place to contribute and help getting making ETA a successful achievement. That's it for the historical overview about Stellarator and Tokamak and in the next video we will talk about key parameters relevant to describe fusion. See you in the next video.