The Innovation Life podcasts on fusion energy

Nuclear fusion - the solution to the energy problem?

The sun as the engine of our energy needs

How does nuclear fusion work?

Prof. Dr. Zohm: Nuclear fusion is the fusion of atomic nuclei. Our matter is made up of atoms and an atom in turn is made up of nuclei with an electron shell around them. And these tiny atomic nuclei have to come so close together that they fuse. When they do, and this is particularly the case with light atomic nuclei, they fuse to form a new round object, for example hydrogen to helium. And this releases energy.

What exactly does the sun have to do with this?

Prof. Dr. Zohm: Nuclear fusion is the energy source of stars, including our sun. All the energy that we receive from the sun in the entire radiation spectrum ultimately has its source in the interior, in nuclear fusion. This is where the energy is generated. The sun burns hydrogen to helium via a reaction, i.e. not directly. In total, several hydrogen nuclei are put together in such a way that helium comes out at the end and energy is released.

And that is a continuous and relatively stable process, isn't it?

Prof. Dr. Zohm: It is very stable, as you can see. The sun has been alive for four and a half billion years and the calculations say that it will continue to do so for another four and a half billion years. The sun burns the normal hydrogen, i.e. the H that we know from the formula H2O from water, into helium. The efficiency with which this works is relatively low. This means that the sun melts its hydrogen supply very slowly. And that's why it's so stable and we don't actually see any change over human periods of time.

Can this nuclear fusion be replicated in the same way on Earth, or what would it take for this process to take place here on Earth?

First of all, hydrogen is not a scarce commodity. When you turn on the tap, two hydrogen atoms come out with every molecule. But the fact is that hydrogen that can be used scientifically or technically has to be processed first, so to speak. But that's not really a big problem, because nuclear reactions are very efficient. Such a reaction releases around ten million times more energy than chemical combustion. This means that the mass conversion of hydrogen is very low. You can calculate the following: a fusion power plant would burn 54 kilograms of hydrogen for every gigawatt it generates for a year. That is a vanishingly small amount.

What about the availability of deuterium and tritium?

Prof. Dr. Zohm: Well, we also have plenty of deuterium. It is much rarer than normal hydrogen, but there is any amount of it. Seawater, for example, contains a certain amount of it. The fuel supply of deuterium would last for many millions of years to satisfy the world's hunger for energy. That is not a problem. The situation is fundamentally different with tritium. Tritium is radioactive, but with a very short half-life of twelve years. This means that after these twelve years there is nothing left of it. We have almost no natural tritium here on Earth. We would use the neutrons that are released in the fusion reaction to breed it directly on site in the power plant and then burn it again immediately. You need lithium for this and there is enough of it. As already mentioned, the quantities of material that are converted are very small. This lithium could also be recycled from batteries. This can then be used to generate tritium from it. So in principle, the raw materials are deuterium and lithium. And these are available on a large scale.

What is needed to carry out nuclear fusion on Earth in a controllable way? What temperatures and what materials or starting materials are needed?

Prof. Dr. Zohm: I said at the beginning that very small atomic nuclei fuse together. They have to come so close that they practically touch each other. However, the fact that both nuclei are positively charged speaks against this. This means that they actually repel each other. That's why you first have to apply energy so that atomic nuclei can come close enough to each other to fuse. This is also good for us, otherwise we would all merge, so to speak, and dissolve into energy. This is also known from the example of two magnets that you try in vain to bring together with the two positive sides.

The energy is very high, so to speak, per individual particle. We enclose a hydrogen gas and heat it to these high temperatures. This becomes a so-called plasma. And the temperatures are more than a hundred million degrees Celsius in our experiments, in the sun it's about fifteen million degrees in the center. This is because we use a more efficient nuclear reaction of other hydrogen isotopes. There are different types of hydrogen nuclei. We have to use the ones that are not simple H, but so-called deuterium and so-called tritium. These are relatives of hydrogen, chemically the same, but they have different masses. You need these as starting materials in order to lock them together and bring them to these high temperatures. That's where you can see the real problem: heating the whole thing and treating it in containers that can withstand this heat of up to one hundred million degrees in the center.

"When it comes to nuclear fusion, many branches of industry can benefit: e.g. the magnet industry, mechanical engineering and high-tech companies. My message: Network to form an industrial ecosystem!"
-Prof. Dr. Hartmut Zohm, Scientific Member, Max Planck Institute for Plasma Physics and Honorary Professor, LMU Munich

Are there alternatives to deuterium and tritium as fuels?

Prof. Dr. Zohm: There are alternatives. There are other nuclear reactions involving light nuclei that can be used to release energy. However, the threshold for these reactions to start is much higher than for the deuterium-tritium reaction. We have a hard enough time getting deuterium and tritium to fuse. I don't see any direct way to get there in the next few decades using other reactions.

To get the high temperatures, energy has to be expended first. In the end, however, the process should generate energy. What does the energy balance look like and what role does the so-called Q-value play here?

Prof. Dr. Zohm: You have to be a bit careful here. The Q value, as defined by physicists, only tells us how much energy is introduced into the container or the plasma in order to heat the gas. This is in relation to the heat that is released during fusion. Q is the ratio of heat released by fusion to the heat I have to deposit in the plasma to keep it at these high temperatures. If Q is equal to one, this is known as a break-even point. Then exactly as much heat comes out as I put in to heat it. But that is not yet a functioning power plant. For a functioning power plant, the Q-factor must be much higher, in the order of 30-40, because I get heat out and I normally use electricity to heat the plasma. Ultimately, I have to calculate these factors, the efficiencies in the conversion of process heat into electricity, and then Q equal to one or greater is not really enough. Because then I have a turbine and generators with an efficiency and corresponding losses. So I have to look at the overall system in order to be able to generate energy accordingly.

Is it already possible to keep this overall system positive? How far has science progressed with the Q value?

Prof. Dr. Zohm: We are close to this Q equal to or greater than one. It is a great success for basic research because we can see that more heat comes out than we deposit in the plasma in the gas. We have to build these facilities more efficiently or larger so that the thermal insulation is so good that we don't have to heat as much and still generate energy.

The nuclear reaction of nuclear fusion produces neutrons. These leave this reaction vessel very quickly. However, so-called alpha particles, fast helium nuclei, are also produced and these are used to keep the plasma alive. In other words, to apply the energy that is actually invested in heating from the inside. You practically ignite the whole thing from the outside. Then the energy released during fusion causes it to enter a state known as ignited plasma. When the plasma is ignited, you can turn down the external heating power and then it starts to become really efficient. With inertial fusion, this value is already above one, i.e. two to three. With magnetic fusion, where we enclose a plasma in a magnetic field cage, we are in the order of one and a little below.

It's very exciting. These scientific successes have led to a great deal of attention and also this awareness that fusion energies could provide a building block of our energy supply in a finite time.

Which industries can benefit most from nuclear fusion?

Prof. Dr. Zohm: Fusion with magnetic confinement requires very strong electromagnets and they also have to be superconducting. In other words, they have to be in a state in which the electric current flowing through them no longer feels any resistance. Otherwise you have to apply a lot of energy to generate the magnetic fields. These are the largest superconducting magnets on earth. The magnet industry is certainly one of the branches of industry that can benefit greatly from this. This also applies to other applications, especially in energy technology.

Mechanical engineering also benefits, where large machines are weighted down or vacuum vessels and the like are built. Of course, there is a lot of high-tech involved. There are medium-sized companies that specialize in building certain components that meet the highest specifications. For example, for the heating systems for the plasma. They also benefit greatly from this.

What advice would you give to companies in these sectors in order to be ready for nuclear fusion?

Prof. Dr. Zohm: Networking and specialization are very important. There are start-ups in particular that are currently planning fusion reactors. This is such a complex matter and it requires so many technologies that one industry alone will not be able to do it. And that's why you actually have to build an ecosystem of industries that can do special things, and the German government has recognized this and launched a suitable programme. In this way, you can create a supply chain so that you have the individual components for the fusion reactor. And I would advise all these companies to network and see where they can best make their contribution. That way, we can build a fusion reactor together, because it can't be done by one company or industry alone.

Imagine the sun was a person you could spend the afternoon with. What would you want to ask the sun?

Prof. Dr. Zohm: I would ask it why its corona is so hot. In the periphery of the sun there is the so-called corona. During a solar eclipse, you see the sun in a kind of nebula, surrounded by hot material. This is super exciting plasma physics and I would be interested in it as a scientist. The question is still not entirely clear: how is it that the sun is 5600 Kelvin hot on the surface and further out the temperature rises by several factors? We don't actually know exactly how this heat comes about.

Discover also part 2 and part 3 of our interview with Prof. Dr. Hartmut Zohm, scientific member of the Max Planck Institute for Plasma Physics and honorary professor at the Ludwig-Maximilians-Universität Munich. The interview was conducted by Christoph Raithel, Management Consultant, Bayern Innovativ GmbH, Nuremberg.

Nuclear fusion decoded - A technical look into the heart of the reactor

How plasma is tamed - and what really happens in the reactor

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. If you do this, then the energy of the electrons and the atomic nuclei is so high that they are no longer bound together. 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 let it run 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 sorts 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. Would that 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.