The influence of plasma-surface interactions on tungsten performance in ITER

Tungsten (W) monoblocks in the ITER divertor targets will be exposed to particle fluences far above the highest values reached in today’s tokamak experiments. Such conditions can lead to significant surface modifications, with a key example being the formation of nano-bubbles and fuzz under He irradiation. To assess whether fuzz may form in ITER, a growth/annealing equilibrium model, including the effect of edge-localized modes (ELM) and the reduced thermal conductivity of fuzz, has recently been proposed which can explain the experimentally observed surface temperature window for fuzz formation.

Applying this model to ITER reveals that for an ELM energy density of 0.1MJ.m^-2 (lower than required to avoid monoblock edge melting and surface roughening), the maximum fuzz thickness is limited to 1-2 mm by the ELM-induced fast annealing. The mechanical and thermal properties of the W material can be modified during operations. In particular, W recrystallization, which leads to a decrease in material strength, is a key effect due  the high surface temperatures in ITER. An approach is proposed to develop an operational budget for the W material, i.e. the time the divertor material can be operated at a given temperature before a significant fraction of the material is recrystallized.