Microscopic and macroscopic damage in metals exposed to LHD divertor plasmas

In order to study erosion/damage processes of the tungsten divertor target, material probe experiments were carried out by exposing tungsten specimens to short pulse helium or hydrogen divertor plasma in LHD (10–20 eV, about 1 × 1022 ions/m2 s). A narrow ‘footprint’, a trace of local-melting, of divertor plasma was formed on the material probe head along the divertor-leg for both hydrogen and helium discharges. In the specimens located rather far from the footprint, where less loading of particles is expected, remarkable blistering was confirmed for both helium and hydrogen irradiation cases. The erosion of the surface was estimated to be 6.3 nm/1 s (one discharge), and 4.1 nm/24 s (19 discharges) for helium and hydrogen, respectively. One should note that erosion due to blistering occurs even for the very low energy hydrogen plasma exposure. It is considered that the abrupt change of specimen temperature due to very high flux particle load may enhance the blistering.