Can electrons collide with each other

The sun in the tank

How fusion research captures the starfire

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The sun's energy production comes from the fusion of atomic nuclei.
The sun's energy production comes from the fusion of atomic nuclei.
© SOHO Collaboration, ESA & NASA
© SOHO Collaboration, ESA & NASA

There is no life without the sun - our ancestors already knew that. For the ancient Greeks, their god Helios swung himself on his sun chariot in the morning to provide light and warmth. But what really makes the sun fire seem to burn forever? The brightest thinkers puzzled over this for a long time in vain. In 1852 Hermann von Helmholtz came to the horrific conclusion that the sun must have burned out after 3,021 years. The famous physicist assumed the oxyhydrogen reaction as an energy source, in which hydrogen is chemically burned with oxygen to form water. It was not until 1938 that the German-American physicist and later Nobel Prize winner Hans Bethe solved the riddle: It is not chemical combustion processes that are the source of solar embers, but the fusion of atomic nuclei - mainly hydrogen nuclei to helium nuclei. These Nuclear fusion releases around four million times more energy per hydrogen atom involved than the oxyhydrogen reaction. Thanks to this enormous efficiency, the sun will fortunately last another 4.5 billion years with its fuel supply.

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Fig. A1: The "proton-proton reaction 1" in the sun.
Fig. A1: The "proton-proton reaction 1" in the sun.
© R. Wengenmayr / CC BY-NC-SA 4.0
© R. Wengenmayr / CC BY-NC-SA 4.0

Several fusion reactions of the light hydrogen take place inside. A reaction that is called “proton-proton reaction 1” dominates (Fig. A1): four hydrogen atomic nuclei, i.e. protons, merge via intermediate steps to form a helium nucleus made up of two protons and two neutrons. The neutrons are created from protons. In the process, positrons, the antimatter opponents of electrons, carry away the excess positive electrical charge. This fusion reaction, however, requires enormous temperatures. No problem for the sun: there are around 15 million Kelvin in its center. The nuclei of the light atoms separate completely from their electrons. They form a hot gas from electrically charged particles plasma. In addition, there is enormous pressure inside the sun due to the enormous gravitation: the equivalent of 200 billion earth's atmospheres compress the plasma in such a way that one cubic centimeter of it on earth would weigh almost as much as 20 iron cubes of the same size.

It is only under such extreme conditions that the protons overcome their resistance to the fusion wedding. Normally, because of their similar electrical charge, they repel each other strongly. But in the hot interior of the sun, the protons whiz around so quickly that they can still collide - in the micro-world, heat is nothing more than kinetic energy. They approach up to 10–15 Meter (i.e. a femtometer or a trillionth of a millimeter), and this is where the “transition point” begins Nuclear power to dominate. This strongest force in physics only has a short range, but exceeds the electrical force within this range. The nuclear force can therefore also combine the stubborn protons into atomic nuclei; without them there would be neither atoms nor us. The density of the pressed solar plasma also ensures a sufficient number of collisions and thus keeps the solar fusion furnace warm.

In Greek mythology, a certain Prometheus stole the fire from Helios ’sun chariot to give to the people. The modern descendants of Prometheus include researchers like the late Lyman Spitzer. In a lecture on May 11, 1951, the American astronomer from Princeton University outlined how solar fire could be brought to earth. He had the decisive idea of ​​how to enclose the millions of degrees hot plasma on earth in such a way that controlled nuclear fusion becomes possible in it. Because contact with a material vessel wall would be fatal: the plasma would suddenly cool down and the sensitive fusion reaction would immediately freeze to death. Spitzer suggested floating the plasma in a magnetic cage. Since plasma consists of electrically charged particles, this is possible because magnetic fields exert force on electrical charges. With this, Spitzer outlined the basic principle of future fusion reactors. However, magnetic forces have the disadvantage that they are quite weak. They can only contain an extremely thin plasma, about 250,000 times thinner than air at sea level. That is why the hot plasma will never build up more pressure than air in a bicycle tire, even in large reactors. The sun cannot be copied that easily.

HEATED HYDROGEN CORES

This also applies to the fusion reaction. In an artificial reactor, the solar proton-proton reaction would take place far too slowly. Fortunately, however, nature allows alternative fusion reactions, and one of them is particularly well suited for technical use. This enabled plasma physicists to start controlled nuclear fusion as early as the 1990s, at the European research facility JET, Joint European torus, in English Abingdon and on Tokamak Fusion Test Reactor (TFTR) from Princeton University in America. This alternative fusion reaction takes two types of heavy hydrogen as fuel components: that is Hydrogen isotope deuteriumwhose nucleus contains a neutron in addition to the proton, and the even heavier one Tritium with a nucleus of one proton and two neutrons. One deuterium and one tritium nucleus fuse to form a helium nucleus (Fig. A2). However, this only works above 100 million Kelvin, 300 million Kelvin are ideal. Only then are the heavy hydrogen nuclei sufficiently in motion to fuse efficiently. Temperatures ten to twenty times higher than the sun seem like a crazy target. But they have long since become routine in today's plasma experiments. The research facility ASDEX upgrade at the Max Planck Institute for Plasma Physics in Garching has already reached over 250 million Kelvin.

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Fig. A2: In the fusion reactor, a deuterium nucleus fuses with a tritum nucleus to form a helium nucleus (protons: red, neutrons: blue). This releases a neutron with an energy of 14.1 billion electron volts.
Fig. A2: In the fusion reactor, a deuterium nucleus fuses with a tritum nucleus to form a helium nucleus (protons: red, neutrons: blue). This releases a neutron with an energy of 14.1 billion electron volts.
© R. Wengenmayr / CC BY-NC-SA 4.0
© R. Wengenmayr / CC BY-NC-SA 4.0

When deuterium is fused with tritium, the resulting Helium core around twenty percent of the energy released. It uses it to reheat the plasma, which is threatened with cooling. The remaining eighty percent of the fusion energy is carried away by the neutron. As an electrically neutral particle, it escapes the magnet cage and hits the wall of the reactor vessel. In a future power plant, the neutrons will transfer the majority of the fusion heat to a coolant, for example water or helium. This then conveys the heat energy to a turbine system with electric generators, just like in conventional power plants (Fig. B). The energy of the neutron corresponds to 14.1 million electron volts or the equivalent of 2.3 x 10–12 Joules. This seemingly tiny value is gigantic compared to chemical combustion: one gram of fuel can produce around 90 megawatt hours of thermal energy in a fusion reactor. For this you have to burn eight tons of oil or eleven tons of coal. But not only the tiny amounts of fuel would be an advantage of nuclear fusion: Above all, it does not release any climate-damaging carbon dioxide. And their "ashes" are only harmless helium.

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© MPI for Plasma Physics / CC BY-NC-ND 4.0
© MPI for Plasma Physics / CC BY-NC-ND 4.0

But the neutron has another task: it should breed the second fuel component tritium in the wall of the reactor vessel. Tritium is radioactive with a half-life of 12.3 years. That is why the future fusion reactor should produce it in a closed cycle and use it again immediately. The "raw material" for the tritium is lithium. This third element in the periodic table and the lightest of all metals is incorporated into the reactor wall. If a neutron hits the nucleus of the lithium-6 isotope there, it decays into a helium nucleus and the desired tritium nucleus.

Race for the best concept

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Fig. C1: The electrically charged particles of the plasma move along the magnetic field lines (black arrow) on spiral paths. The radius of the spiral depends on the mass of the particles: the heavier protons circumscribe larger spirals than the electrons.

Fig. C1: The electrically charged particles of the plasma move along the magnetic field lines (black arrow) on spiral paths. The radius of the spiral depends on the mass of the particles: the heavier protons circumscribe larger spirals than the electrons.

© R. Wengenmayr / CC BY-NC-SA 4.0
© R. Wengenmayr / CC BY-NC-SA 4.0

The big challenge is an efficient magnetic confinement of the plasma, which consists of the two types of hydrogen, deuterium and tritium. When building the magnetic field cage for the plasma, the fusion researchers take advantage of the fact that the charged plasma particles - the protons and electrons - are forced by electromagnetic forces on spiral paths around the magnetic field lines (Fig. C1). In this way, the particles can be kept away from the walls of the plasma vessel by a suitably shaped magnetic field, as if guided on rails. For a "tight" cage, the field lines within the ring-shaped plasma vessel must span closed, nested surfaces - like the nested annual ring surfaces of a tree trunk (Fig. C2). The plasma pressure is constant on these surfaces, while it decreases from surface to surface - from the hot center to the outside.

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Fig. C2: The magnetic surfaces are neatly nested in one another - like the annual ring surfaces of a tree trunk. So ... [more]
Fig. C2: The magnetic surfaces are neatly nested in one another - like the annual ring surfaces of a tree trunk. This avoids field components pointing outwards that would lead the plasma particles onto the walls. The high ignition temperatures would then be unattainable. [Less]
Fig. C2: The magnetic surfaces are neatly nested in one another - like the annual ring surfaces of a tree trunk. This avoids field components pointing outwards that would lead the plasma particles onto the walls. The high ignition temperatures would then be unattainable.
© MPI for Plasma Physics / CC BY-NC-ND 4.0
© MPI for Plasma Physics / CC BY-NC-ND 4.0

However, these nested “magnetic tubes” would now lose the plasma particles at their ends - together with the valuable thermal energy. Therefore they are closed in a ring. However, this makes the magnetic field stronger on the inside of the ring than on the outside, because the field lines there crowd closer together. As a result, the plasma would be thrown out of the ring. To prevent this from happening, the physicists twist the magnetic field again within itself. The field lines spiral around the “annual rings”: This is how they repeatedly lead the plasma particles from the weaker magnetic field on the outside of the ring back into the denser magnetic field inside - the plasma remains trapped. However, this requires a complicated arrangement of the magnetic field coils. The Stellarators, the "star machines" (lat. stella for Stern), on which the fusion researchers worked in the 1950s and 1960s, initially failed. Only today can supercomputers calculate the geometry of the coils so precisely that the stellarator is back in the running for the best concept for a fusion reactor (Fig. D1). At the end of 2015, the Wendelstein 7-X stellarator went into operation at the Greifswald branch of the Max Planck Institute for Plasma Physics. It should show that stellarators can reliably enclose the hot plasma.

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Fig. D1 (left): Stellarator; Fig. D2 (right): Tokamak
Fig. D1 (left): Stellarator; Fig. D2 (right): Tokamak
© MPI for Plasma Physics / CC BY-NC-ND 4.0
© MPI for Plasma Physics / CC BY-NC-ND 4.0

Being ahead of the game currently has a competing principle: the Tokamak (Fig. D2). The name comes from Russian "Toriodalnaya camera s magnetnymi katuschkami" and means in German "annular chamber with magnetic coils". While stellarators build the magnetic field cage exclusively with the help of external coils, tokamaks create part of this field using an electric current flowing in the plasma. This "twists" the magnetic field so that it holds the plasma together like a tube. It also heats up the plasma. The tokamak has a simpler structure than a stellarator. That is why he helped fusion research achieve high temperatures in the plasma and also contained it well. As a transformer, however, it only induces current in the plasma as long as the current strength in its primary coil changes. In contrast to the stellarator, it has to work with pulses. This is not very practical for a power plant operation, even if a pulse can be stretched over hours. That is why the plasma physicists are researching an alternative mode of operation: Additional electromagnetic high-frequency fields should compensate for the ups and downs of the pulses so that a direct current flows in the plasma.

Little radioactivity

The decisive factor is a perfect magnetic enclosure that insulates the hot plasma as well as possible and does not allow it to cool down. The Max Planck scientists in Garching have developed some important ideas. They are now being used in the construction of the large international research reactor ITER (Latin for "the way"), which is being built in Cadarache, southern France. In 2025, ITER is to generate the first plasma, later "ignite" it and for the first time generate more fusion energy than its plasma heating consumes - ten times as much. This could be followed by DEMO: This prototype of a power plant should already generate electricity from the fusion heat. From the middle of this century the first would be commercial Fusion power plants possible. Humanity would then have opened up an almost inexhaustible source of energy. It could meet the world's rapidly growing demand for electrical energy without releasing dangerous greenhouse gases. The fuel supply would be gigantic, because as little as 0.08 grams of deuterium and 0.2 grams of lithium would be enough to generate a family's annual electricity requirement today. The deuterium is in heavy water (D.2O), which occurs naturally in all oceans. Lithium is part of minerals that exist almost everywhere in the earth's crust. The energy supply would no longer be a cause for geopolitical conflicts.

But every form of energy generation has its price: Nuclear power plants contain very strong radioactive fuel elements, the use of fossil fuels is dangerous, and large hydropower plants or wind parks change landscapes. In nuclear fusion, the inside of the reactor vessel is radioactive. However, the amounts of fuel are comparatively tiny, and the sensitive fusion reaction cannot “run away”. Unlike the nuclear fission chain reaction, it is self-locking: If the magnetic field collapses, the plasma touches the wall, cools down suddenly and the fusion reaction stops. The wall survives this almost without damage due to the low plasma density. The worst possible accident would be an escape of tritium from the reactor. The amount would be very small, but the rapidly disintegrating tritium can cause cancer. The planners of a future power plant take this possibility of an accident very seriously, even if its consequences are not remotely comparable to a nuclear power meltdown. Years of neutron bombardment will, however, "activate" part of the reactor vessel radioactively. This is especially true for certain steel alloys in which trace elements are converted into radioactive isotopes. Parts of the reactor wall would have to be stored for a few hundred years before this radioactivity has subsided. Research aims to defuse this problem by developing new materials. And she still has a few years to do that.

The text is published under CC BY-NC-SA 4.0.

Illustration notes:
Fig. A1 and A2: Fusion reactions R. Wengenmayr / CC BY-NC-SA 4.0
Fig. B: Fusion power plant MPI for Plasma Physics / CC BY-NC-ND 4.0
Fig. C: cage for plasma; Part C1: R.Wengenmayr / CC BY-NC-SA 4.0 Part C2: MPI for Plasma Physics / CC BY-NC-ND 4.0
Fig. D: Stellarator and Tokamak: MPI for Plasma Physics / CC BY-NC-ND 4.0

TECHMAX issue 9, updated 7/2020; Author: Roland Wengenmayr; Editor: Tanja Fendt