Plasma-Wall Interaction - A Key Issue in Progress Towards Fusion Power Plants
Generating energy from fusion requires a plasma with a temperature of 100 million degrees. Strong magnetic fields are used to protect the wall of a fusion device
and although this reduces the interaction of the plasma with the walls, they are still exposed to considerable loads that are inherently unavoidable. For this reason, fusion research has focused on plasma-wall interaction right from the outset – and today, particularly at Jülich, it is a central issue.
Fusion research has come a long way, having tested a series of very different concepts for the production and confinement of high-temperature plasmas. Of these, the tokamak concept of magnetic confinement has proven most successful thus far. Over the past decades, the operation of modern tokamaks of different sizes and shapes has created extensive data resources that today provide a solid foundation for the construction of the next generation of fusion experiments. Consequently, the large-scale fusion experiment ITER is currently under construction in Cadarache in the south of France as part of an international cooperation. At ITER, which will generate 500 MW of power, for the first time ten times as much energy will be produced from fusion as is introduced to the plasma via external heating. However, the duration of the plasma pulses produced will initially only be around eight minutes.
Although ITER is a particularly important step towards the goal of realizing the first fusion power plant, it must nevertheless be accompanied by a range of further complementary research institutions. Smaller, flexible test facilities play an important role in this regard. The most significant alternative to the tokamak concept is the method of magnetic confinement employed in a stellarator. The world’s largest, most advanced stellarator experiment, Wendelstein 7-X, is currently under construction in Greifswald based on this principle. In contrast to ITER, Wendelstein 7-X can be operated continuously. However, the development of the stellarator concept in general is at least one generation behind the tokamak. The principles of plasma-wall interaction are the same for both confinement concepts.
A fusion power plant presents several challenges that have not yet been addressed in ITER. While the plasmas in ITER are produced in pulses, a power plant must operate continuously for months at a time. Realizing continuous operation of this magnitude presents the greatest remaining challenge in fusion research, and controlling plasma-wall interaction will to a large extent determine the availability and thus, ultimately, the efficiency and economic success of a future fusion power plant.
Energy produced in this way is the result of the “fusion” of the hydrogen isotopes deuterium (D) and tritium (T):
D + T → He4 + n
Each reaction releases 17.6 MeV or 2.8 10-12 J of energy, which is distributed between a helium nucleus (alpha particle, 3.5 MeV) and a free neutron (14.1 MeV) in the form of kinetic energy. Deuterium, which occurs in natural hydrogen at a ratio of 1:6000, can be extracted easily from water. Tritium, on the other hand, is unstable and has a half-life of 12 years, and must be produced in a nuclear reaction. The neutron released can be used for this purpose, and through the reactions
Li6 + n → T + He4 and Li7 + n → T + He4 + n
tritium can be bred from lithium in the breeding blanket surrounding the reactor’s vacuum vessel. Thus, the raw materials deuterium and lithium have emerged as mankind’s new source of primary energy in controlled nuclear fusion.
The high energy of the fusion products is transferred to heat exchangers inside the walls and, by means of steam, used to produce electricity conventionally.
The neutrons carry 80 % of the energy released and deposit it deep within the walls, and no further interaction with the plasma occurs. 20 % of the energy is transferred by the helium nuclei to the plasma thus heating it. This part of the energy must be removed from the reactor chamber for use, via the interaction of the plasma with the wall.
Another important aspect is the removal of the helium particles via the interaction with the wall. In ITER, for example, fusion creates approximately 2x1020 helium atoms every second. This is equivalent to one milligram of helium per second, which must be continuously pumped off in a state of equilibrium.
This energy and particle exhaust is linked to wall loads, and different concepts have been developed for controlling these. One of the most important is the divertor principle, a key component of ITER. The confining magnetic field is deflected at the plasma edge into the divertor, thus directing the plasma particles to specific wall components (target plates) that are particularly suitable for extreme loads. In addition, neutral gas pressure is formed in the divertor from deuterium, tritium, helium, and other impurities, which enables these particles to be pumped off efficiently.
The thermal loads on certain parts of the wall can be very high. The heat flux is concentrated along the magnetic field lines in a narrow area with a radial extent of only a few centimetres to a relatively small surface area on the target plates. The total area available for this purpose in ITER measures only around 5 m2, which can result in peak loads of up to 10 MW/m2. In contrast, the approximately 700 m2 wall in the main chamber is only exposed to loads of around 0.1 MW/m2, primarily through electromagnetic radiation (light).
Particularly critical for wall components, however, are additional transient thermal loads resulting from plasma instabilities such as plasma disruptions or edge localized modes (ELMs). Plasma disruptions can occur close to a tokamak’s operating limits (e.g. maximum plasma density or plasma pressure) and can rapidly – within a few milliseconds – disperse the total stored plasma energy to the walls. The local thermal peak loads occurring during such instabilities can cause the wall components to melt or crack, thus limiting their lifetime.
Erosion processes at plasma-facing wall components play a major role in the availability of a fusion device. The predominant erosion processes are physical sputtering and chemical reactions, which are linked to the particle flux from the plasma and its energy. Some erosion processes are a direct consequence of critical heat fluxes, such as the melting or sublimation of wall material. Physical sputtering of wall material is caused by the energy and momentum transfer of high-energy plasma particles – with effective yields of a few percent for carbon and much smaller values for heavy metals such as tungsten. For some wall materials such as carbon, chemical erosion processes determined by the surface temperature of the material are also decisive.
Erosion processes release particles, which enter the plasma and are then ionized or dissociated. Transport processes inside the plasma ultimately bring these particles back to the wall, ideally to their location of origin. For hydrogen, which is neutralized at the target plate and transported back to the plasma as an atom or molecule, this process is known as “recycling”. A critical issue here is the potential retention of tritium in surface layers at the walls, as for safety reasons an upper limit has been set for the amount of tritium retained.
Controlling plasma-wall interaction has two facets: firstly, by optimizing plasma properties in front of the wall , and secondly, by selecting and developing suitable wall materials. This means that plasma physics and materials research are the key aspects of this research field. Materials research focuses on materials properties, in particular material combinations, and on understanding damage processes and their effect on material properties. In comparison to the demands of today’s fusion experiments, the loads in a fusion power plant will be considerably higher. In particular, loads resulting from intensive neutron irradiation present new challenges that we must address in today’s research programmes. In light of these problems, it becomes extremely important to develop alternative materials.
Prof. Dr. Christian Linsmeier
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