The magnetically confined plasma in a fusion device is surrounded by the „First Wall“, which is in contact with the plasma: On one hand charged particles, which are confined in the magnetic cage, can lose their charge in collisions with other particles and escape. On the large surface area of the First Wall these energetic atoms can cause erosion processes, which must be kept small. On the other hand, due to the magnetic field geometry, the plasma is in intense contact with the wall at the surface of the divertor. The high electron and ion fluxes can lead to strong heat loads and, depending on the loading conditions, also to significant erosion.
In experimental devices worldwide mostly graphite and carbon-fibre-reinforced carbon were employed as plasma-facing materials. For light chemical elements with low atomic number such as carbon (Z=6) concentrations of up to several percent can be tolerated inside the plasma, without deteriorating the plasma properties significantly. The same is valid for boron or beryllium. As these light materials are easily eroded, future fusion devices require wall materials with a high atomic number as tungsten (Z=74), which is hardly eroded under appropriate conditions. On the other hand, tungsten may only be present in the plasma in very minute concentrations of about 10 ppm (parts per million). However, investigations with ASDEX Upgrade have shown that operation with a completely tungsten-covered “First Wall” is possible.
As mentioned above, the divertor is exposed to exceptional loads: By employing a specific magnetic field geometry the energetic particle flux from the plasma is neutralised there and pumped away. For this reason parts of the divertor surface are exposed to extreme power fluxes of the order of 10 MW/m² at particle fluxes of 1024m-2. Tests of component designs and materials, which are optimised for these conditions, are an important part of the project. They are performed in specialised laboratory devices as well as directly in the fusion experiment ASDEX Upgrade.
Additionally, in a future fusion device materials will be exposed to high neutron fluxes. The neutrons created in a burning plasma deposit a part of their energy in the First Wall. This creates heat inside the material and its atoms are displaced. Furthermore the material is activated and the created transmutation products such as hydrogen or helium atoms influence the material properties. Fortunately, by a subtle choice of the elemental composition of first wall materials the transmutation can be strongly reduced. Materials have been successfully developed which show only small property changes under neutron irradiation.