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Nuclear fusion decoded - A technical look into the heart of the reactor
How plasma is tamed - and what really happens in the reactor
07.07.2025
How can we achieve the breakthrough to the clean energy of the future? Facilities such as the international research reactor ITER are working at full speed on the energy source of the future. The principle: extremely hot plasma and strong magnetic fields should make nuclear fusion controllable. However, although the technology is making great progress, the road to an operational fusion power plant has not yet been paved. What final challenges do science and technology still have to overcome before fusion power plants can feed electricity into the grid? Here, Prof. Dr. Hartmut Zohm, fusion researcher at the Max Planck Institute for Plasma Physics, explains where we stand today - and what hurdles still need to be overcome to turn hope into reality.
Why is the plasma state necessary for magnetic fusion?
Prof. Dr. Zohm: The plasma state is necessary for any kind of fusion. We had talked about the fact that we have to reach these high temperatures in the hydrogen gas, in the order of 100 million degrees, so that the atomic nuclei come so close that they fuse together. When you do this, the energy of the electrons and the atomic nuclei is so high that they are no longer bound to each other. This means that we are no longer dealing with neutral atoms, but with the plasma state. This is a consequence of the high temperatures. So all fusion gases that we know of are fusion plasmas.
How can this plasma be controlled and stabilized?
Prof. Dr. Zohm: It is a thermodynamically unstable state. If you only have a high temperature gradient and you try to heat it up in the middle, it will initially disperse and try to distribute the energy in space as evenly as possible. We do this in magnetic fusion with magnetic fields. We have strong magnetic fields that are generated with electromagnets. And these guide the particles in a toroidal geometry, i.e. like a donut. The magnetic field lines run around in it and the particles follow the field lines. This essentially keeps them away from the wall. The whole thing is structured in such a way that you have magnetic field lines that span so-called magnetic surfaces, like annual rings on a tree. These become hotter and hotter towards the inside. This is how you get the temperature gradient. It must be very hot in the center of the system. And at the latest when the plasma comes into contact with the wall, it is no longer 100 million degrees, but only 500 degrees, depending on the material.
How does the confinement of this fusion plasma differ between the tokamak and the stellerator?
Prof. Dr. Zohm: Tokamak and stellerator use a toroidal arrangement where the magnetic field lines close toroidally so that the particles cannot hit the wall along the magnetic field lines, but always run in a circle. The difference is that in the stellerator the entire magnetic field is generated with external electromagnets, i.e. coils, while in the tokamak a strong current also flows in the plasma, which contributes to the magnetic field configuration required for confinement.
How do the superconducting magnets required for the two principles work and what distinguishes them from conventional magnets?
Prof. Dr. Zohm: We are mainly familiar with permanent magnets from the household. These are horseshoe magnets, for example, or the kind you use to pin notes to the blackboard. Here we are talking about electromagnets, i.e. coils. If you send current through a coil, it generates a magnetic field. You may be familiar with this from technical applications where things are lifted. This is the case with a powerful electromagnetic scrap press, for example, when a car is lifted. We also use electromagnets like this in terms of their functional principle. As we have to generate very strong magnetic fields, this also means that we have to allow high currents to flow in the coils. This requires relatively high power. This is where superconductivity helps us. Superconductivity is an interesting physical phenomenon. When a material cools down, the electrical resistance suddenly disappears and once you have brought the material into this state and put the current into it, it flows forever without you having to apply energy to drive it further. We make use of this to keep the external energy consumption of a power plant as low as possible. In experiments that are 20 to 30 years old, copper was still used for the coils. In more recent experiments, such as the ITER that is being built, superconducting coils are already being used.
Does this superconductivity only work at very low temperatures in the material?
Prof. Dr. Zohm: Yes, although there have also been breakthroughs here in recent years. Around 20 years ago, materials were discovered that are so-called high-temperature superconductors. These are always low temperatures by human standards, but already significantly higher than is the case with classic superconductors. This has many advantages. You don't have to cool them down as much. You also have more temperatures at which you can operate the system. You can also generate even higher fields. These so-called high-temperature superconductors are actually also a very important by-product and therefore a major benefit that can be transferred from fusion research to other technical areas.
How long can plasma be kept stable in a fusion power plant and what does that depend on?
Prof. Dr. Zohm: The time scales in such a plasma experiment today are more on the order of hundreds of milliseconds or one second. It sounds a bit paradoxical. In other words, if you have maintained it for 10 seconds, even with copper coils, then that is actually already infinitely long on the plasma time scale. This is because the entire energy content of the plasma has already been exchanged several times during this time, so to speak. It then becomes a technical problem: how well do the technical systems work, how reliable are they, so that the whole thing can then be continued? In fact, the Wendelstein 7-x also started with 500 millisecond and then second pulses, and after it was technically equipped to be able to do this, it was able to carry out an 8-minute discharge. These were limited by the fact that the technical systems had to run reliably. This is a task that industry can solve very well, it is no longer a problem that the Max Planck Institute has to solve in basic research. Our French colleagues with the so-called West Tokamak, which is located near ITER, have achieved a discharge time of up to 20 minutes. Once you have this superconducting coil system, it is quite realistic to run it for that long.
"The problem is that the fusion reaction produces neutrons as well as helium. These are absorbed and damage the material. So how can the materials be made fit enough to survive this environment in the fusion reactor?"
Prof. Dr. Hartmut Zohm
Scientific Member, Max Planck Institute for Plasma Physics and Honorary Professor, LMU Munich
Can we expect almost exponential growth once the technical problems have been solved?
Prof. Dr. Zohm: I wouldn't call it exponential directly. But increases are to be expected. It goes from a basic research experiment that you look at on the plasma time scale to a technical engineering problem that you want to operate for as long as possible because these individual systems work very well. Think of the first flight experiments - people cheered when someone was in the air for 5 seconds and flew down a slope. That proved the principle and then the engineers got to work and today we build planes that fly stationary over the Atlantic with 300 people on board.
But we also talked about extreme heat. What materials are needed for these conditions and what requirements do they have to meet?
Prof. Dr. Zohm: We have indeed learned in recent years that tungsten works best. Tungsten is the metal with the highest melting point, namely 3,400 degrees Celsius. These divertor plates are coated with a layer of tungsten so that they can withstand very high heat loads. We still have to ensure that the plasma is cooled down on the way there, using all kinds of tricks that we have learned in recent years. But the fact is that during these long discharges of, say, eight minutes that I was talking about, all the components in the vessel are in thermal equilibrium.
So is it correct that the material does not set any limits to the continuous operation of such a system?
Prof. Dr. Zohm: You can't say that. We talked about heat loads and temperatures and these are not the problem. The problem is that the fusion reaction produces not only helium but also neutrons, which are absorbed in the material around the plasma. These damage the material and ensure that the material becomes brittle or starts to swell after a long period of time, for example, because helium forms inside. And that is actually the big question, how to make these materials fit to survive this environment in the fusion reactor.
With this, we have actually defined the framework for how nuclear fusion can proceed. There are some areas where there is still work to be done, but nothing that is unsolvable. How do you see that?
Prof. Dr. Zohm: Exactly, that's how it is. We don't see the famous "showstopper". We have really looked into this in great detail. On the other hand, it is a very complex system, and developing it and getting it ready for series production is still a huge task.
Imagine if you had an indestructible space suit and could see into an ongoing fusion reaction. If you could do that, would it be exciting for you?
Prof. Dr. Zohm: It depends on what I'm allowed to take with me. The plasma, especially the core of the plasma, is so hot that it emits practically no light in the visible range. So you just look through it. That means you would probably be in the reaction and see nothing. That's why I would take a pair of glasses with me that translate X-rays into visible light. There is no such thing. But we are completely free in this scenario. And then I could see very well what's going on in there to understand all these dynamics. I would do that immediately!
Also discover Part 1 and Part 2 of our interview with Prof. Dr. Hartmut Zohm, Scientific Member of the Max Planck Institute for Plasma Physics and Honorary Professor at the Ludwig Maximilian University of Munich. The interview was conducted by Christoph Raithel, Management Consultant, Bayern Innovativ GmbH, Nuremberg.
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