In magnetic confinement fusion devices, the innermost lining of the vessel wall (socalled first wall) is subject to heat and particle fluxes that are exhausted by the plasma either continuously or in bursts. The effect on these plasma facing components is usually tolerable in present facilities but in future fusion power reactors the power load will be much higher and the duration of the plasma discharges much longer. The potential scale of the damage to the first wall challenges fusion research and technology. This is particularly the case for the development of the divertor, which is the area of the vessel that is in direct contact with the plasma edge. Some plasma facing components will have to withstand peak temperatures of more than 1000 degrees, despite being actively cooled.

The particle fluxes and heat fluxes onto solid surfaces lead to erosion and release surface material into the plasma where it acts as an impurity. Some of the released impurities can migrate to very remote locations inside the machine before they stick to a plasma facing component so forming a layer of an amorphous material. The studies of the processes responsible for erosion, migration and deposition of materials in fusion facilities constitute a significant fraction of the present fusion research program. The migrating particles can also make their way to the confined plasma volume, diluting the fusion fuel and cooling the plasma through increased radiation losses. Impurities can reduce the fusion gain to unacceptably low levels. Therefore, the choice of the first wall materials and control of the power fluxes set important boundary conditions for the performance of the future reactor.

JET is currently investigating a first wall made of tungsten and beryllium, the wall materials chosen for ITER.