Dust remobilization under normal and ELM-like conditions: controlled experiments in Pilot-PSI

Dust remobilization plays a key role in diverse plasma-wall interaction issues. The most important aspect of remobilization is connected with the identification of in-vessel dust accumulation sites and consequently the development of dust removal techniques. In ITER, PFCs will be castellated, and shaped, and gaps have been assumed to be a preferred dust accumulation site. Despite its importance, remobilization of dust grains in normal operating conditions has only been recently addressed. A novel experimental technique for its study in fusion plasmas has been developed and its main theoretical aspects have been analyzed [1]. The technique is based on controlled adhesion of dust grains on tungsten surfaces combined with detailed mapping of the dust deposition profile prior to and post plasma exposure. Experiments with planar tungsten surfaces have been carried out in Pilot-PSI [1]. Here we report on the recent experiments carried out in Pilot-PSI with the aim to study the effect of ELM-like pulses on remobilization under normal and glancing B field, as well as remobilization from castellated PFCs under steady-state plasma conditions. The observations by two fast cameras with a resolution down to ten micrometres per pixel allowed us for the first time to resolve the details of dust motion in the sheath. Discussion on the difference of the dust remobilization activity and release velocity under normal and ELM-like conditions is provided along with the first results of remobilization of W dust from ITER like castellated structures. We also present evidence of a particular phenomenon taking place under interaction of ELM-like plasma with W dust particles residing on the W surface - multiple molten grains tend to coagulate into a single nearly spherical grain.