Materials for the plasma-facing components of fusion reactors

During reactor operation the plasma-facing materials have to fulfil very complex and sometimes contradicting requirements. At present, tungsten shows the highest promise as plasma-facing material. Experiments in the ASDEX Upgrade tokamak indicate that plasma operation is feasible with walls and divertor surfaces mostly covered with tungsten. Thick tungsten coatings have been deposited by plasma spraying on EUROFER first wall mock-ups and show good adhesion and stability. The performance of tungsten surfaces under intense transient thermal loads is another critical issue, since the formation of a melt layer may favour the generation of highly activated dust particles. Work on `nanocrystalline' tungsten shall improve the mechanical properties under neutron irradiation which is especially important for designs, where tungsten has also to fulfil structural functions. Alternative divertor heat sink materials with very high thermal conductivity like SiC-fibre reinforced copper composites are presently being developed and should allow operation at reactor relevant coolant temperatures.