What's next for the post-merger force

Controlled solar fire on earth

Controlled nuclear fusion has been considered feasible since the 1960s. A long, complex development led to plans for a first reactor that produces more energy than it needs: the International Thermonuclear Experimental Reactor ITER.

Scheme of the fusion reaction

The long-term energy supply for mankind is a major challenge. The resources of fossil fuels are limited: crude oil and natural gas will only meet the demand for a finite long time. In addition, because of the increasing CO2-The content of the atmosphere must be reduced. With renewable energies and nuclear energy, humanity currently has two options at its disposal, energy services practically CO2- to be able to provide them free of charge. Whether this will be sufficient in the long term for an ever increasing energy demand of the world population cannot be conclusively answered today. For this reason, in the global division of labor with nuclear fusion, another CO2-free energy sources can be tapped. This means that mankind could, in principle, use the same process for its energy supply that the stars use to generate their energy. An attempt is being made to fulfill an age-old dream, namely "to bring the sun to earth".

What is the physics behind it?

Like all stars, the sun derives the energy it radiates from fusion, the merging of light atomic nuclei of different isotopes of hydrogen to form helium. A small part of the mass m of the atomic nuclei changes according to the Einstein relationship E = mc2 into kinetic energy of the fusion products (see Fig. 1). In this way the sun generates the immense radiant power of 1026 Watt. The aim of global research and development efforts is to operate nuclear fusion in a power plant in a controlled manner so that it can also be used directly to generate energy on earth. Although the realization of an "earthly sun fire" has turned out to be more difficult than anyone could have known in the 50s and 60s, with the planning for the first test reactor ITER, one is much closer to the realization of a fusion power plant than a few times ago Years. Controlled nuclear fusion is a long-term goal. In view of the great energy potential that could be tapped, it becomes understandable why the highly industrialized countries are making such great and varied efforts in this field of physics.

Fusion of hydrogen

In order to fuse two atomic nuclei charged with the same name, their repulsion due to the Coulomb force must first be overcome. This can only succeed if the cores fly towards each other at high speed. To do this, the gas mixture that contains the nuclei has to be brought to temperatures of around 100 to 200 million degrees. Fig. 1 shows the most favorable fusion reaction that occurs between heavy hydrogen (D, deuterium) and superheavy hydrogen (T, tritium). The atoms of a gas are ionized at temperatures of around 10,000 degrees. Then hydrogen ions and electrons move separately from each other. This condition is known as plasma. Since a plasma essentially contains charged particles, it can be confined with magnetic fields. This is a very large and demanding technical problem. But it has to be solved, because no material wall would even come close to withstanding the high temperatures that are necessary for fusion.

Tokamak and Stellarator

Magnetic confinement of hot plasmas

While many different concepts for magnetic confinement were being investigated in the 1960s and 1970s, two configurations have emerged: the tokamak and the stellarator (see Fig. 2). In both cases, coils generate an annular magnetic field. Because of its curvature, however, this field is too inhomogeneous to be able to enclose the plasma. In order to make the magnetic field homogeneous, one has to "wind" its field lines helically around the ring with the help of another field. In the tokamak, this is done by a central transformer coil, the magnetic field of which induces a ring current in the plasma as the “secondary coil” of this transformer. Its magnetic field twists the previously circular field lines. At the same time, the current heats the plasma very effectively. With the stellarator, on the other hand, the twisting of the field lines is generated solely by external coils. As a result, the magnetic field can be completely specified from the outside and optimized with regard to the inclusion of particles. In addition, the stellarator can in principle work continuously. With the tokamak, both are only possible with great effort. The tokamak, however, has the ability to heat the plasma very effectively, giving it a major development lead, so that it is currently the most advanced type of construction. But work on the stellarator is also making rapid progress. The optimized Wendelstein 7-X stellarator is currently being built in Mecklenburg-Western Pomerania in the Greifswald branch of the Max Planck Institute for Plasma Physics (see Fig. 3).

Super hot plasma

In order to heat the plasma to the high temperatures required for nuclear fusion, high-frequency radio waves and microwaves are radiated into the plasma or it is bombarded with particle beams. In this way, temperatures of several hundred million degrees are routinely reached today. However, it does not only depend on the temperature whether a plasma “burns” and generates more energy through fusion than is used to heat the plasma. The most meaningful parameter for evaluating fusion plasmas is the fusion product of plasma density, plasma temperature and the energy containment time, which indicates how well the plasma is insulated against heat loss. The fusion product has been improved by a factor of 25,000 over the past 40 years. In doing so, you had to overcome some unexpected difficulties. In particular, the energy containment time is significantly shorter than expected on the basis of theoretical considerations. Only in recent years has it been shown that this is related to turbulence in the plasma, the eddies of which lead to increased energy losses. The unexpectedly poor containment of energy meant that fusion experiments are now much larger and more complex than originally assumed. By increasing the isolation distance, however, the energy confinement of the plasma can be improved.

Today the fusion product reaches a fifth of the value at which a fusion plasma produces more energy than has to be used for its creation and maintenance. So far, pure deuterium plasma has been used in almost all experiments in order to save the technical effort associated with the use of radioactive tritium. Since one now comes into the state of an energy-generating plasma, the questions directly related to deuterium-tritium plasmas must also be answered. Such experiments with deuterium-tritium plasmas have taken place in two large tokamaks in recent years: first in the European tokamak experiment JET (Joint European Torus) in Culham near Oxford, then also in the TFTR (Tokamak Fusion Test Reactor) in Princeton, New Jersey.

JET - the first step

Computer graphics for the preparation of the Wendelstein 7-X fusion device

JET, as the world's largest fusion experiment, has made a major contribution to the study of hot plasmas. In particular, it generated fusion energy on a large scale for the first time with deuterium-tritium plasmas. Among other things, JET holds the world record with a fusion power of 17.6 MW. In these experiments an energy gain of Q = 0.65 was achieved; H. the power gained through fusion was 65 percent of the power required to maintain and heat the plasma. In order to show the physical and technical feasibility of a fusion power plant, however, one has to generate a plasma that produces significantly more energy than has to be used for heating. B. Q> 10.

ITER - one step further

The fact that fusion research requires large and therefore expensive equipment is offset by close international cooperation. Within Europe, research in the 13 national fusion laboratories is coordinated by the European Atomic Energy Community, Euratom. They also jointly operate the above-mentioned JET project in Culham. The next step on the way to a fusion power plant should even take place in global cooperation: Since 1988, a joint fusion experiment has been planned in cooperation between the European Community, the USA, Japan and Russia, the "International Thermonuclear Experimental Reactor" ITER (see Fig. 4 ). ITER is designed to generate and contain a plasma that produces significantly more power through fusion than is necessary to maintain it. In addition, ITER should also drive the technological developments that are required to implement a fusion power plant.

Design for ITER, the International Thermonuclear Experimental Reactor

While the USA has now withdrawn - at least temporarily - from the ITER project, planning is now entering the final phase. An interim report was submitted in December 1999 and the draft should be finalized in June 2001, so that a building decision can then be discussed. The plant will generate a fusion power of 500 MW and achieve an energy gain of Q = 10. At the same time, ITER should create all the necessary technical prerequisites for the next step, the construction of a demonstration power plant.

With the operation of ITER it is hoped to be able to demonstrate the feasibility of a fusion power plant. After a long period of research with its setbacks, that would be a decisive breakthrough. However, there are still some technical challenges to be overcome. In particular, highly resilient materials for the inner wall of the plasma chamber and neutron-resistant structural materials must be developed. In addition, experience must be gained in the construction of large superconducting magnets and in hatching the fuel tritium from lithium.

And the environment?

Nuclear forces are also used in a merger. The safety and environmental properties of a fusion power plant are, however, much more favorable than those of a conventional nuclear power plant, so that one can expect a much greater willingness of the population to consent to the use of this form of energy. However, the two problem areas characteristic of the merger should not be concealed:

Tritium is a radioactive, highly volatile gas that does not occur in nature because of its short half-life of 12.3 years. It is incubated with the neutrons produced in the reaction from lithium, extracted within the facility and "burned" again. The tritium must be prevented from escaping from the system. The techniques required for this have already been successfully tested. The neutrons produced during the fusion reaction not only carry most of the energy gained, but they also activate the structural materials that surround the plasma as a wall. For this reason, new materials are currently being developed for fusion power plants that can only be (radio) activated to a limited extent. The aim is to ensure that over 90 percent of the materials in a fusion power plant can be released or recycled after a waiting period of no more than 100 years.