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07.07.2025
Science is working intensively on making the sun's energy source - nuclear fusion - usable on Earth. It represents a huge opportunity: if this process can be controlled, a significant proportion of our energy needs could be met with minimal use of materials - efficiently, cleanly and almost inexhaustibly. It sounds like an elegant solution to a highly complex problem. But it's not that simple: extremely high temperatures are required to enable nuclear fusion - conditions that have so far only been possible with immense energy input and major technical challenges. Prof. Dr. Hartmut Zohm is a scientist at the Max Planck Institute and an honorary professor at the Ludwig Maximilian University in Munich. He is researching the possibilities of nuclear fusion and explains the opportunities behind it and what is driving science.
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?
Prof. Dr. Zohm: First of all, hydrogen is not a scarce resource. When you turn on the tap, two hydrogen atoms come out with every molecule. However, hydrogen that can be used scientifically or technically must first be processed, 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 straight away. 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. We know this 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.
"Many branches of industry can benefit from nuclear fusion: 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 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 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 up 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.
Whatadvice 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.
Understanding nuclear fusion: Is this the energy source of the future? (21.05.2025)
In this episode, host Christoph Raithel and Prof. Dr. Hartmut Zohm, Scientific Member of the Max Planck Institute for Plasma Physics and Honorary Professor at the Ludwig-Maximilians-Universität München, talk about an energy source that is as old as the universe itself: nuclear fusion. Tune in to find out what fusion is and why it is becoming increasingly important.
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