Contribution of leading edge shape to a damaging of castellated tungsten targets exposed to repetitive QSPA plasma loads

Abstract

The castellated tungsten monoblock is considered one of the preferred reference designs of components faced to plasma in the divertor of a fusion reactor. The behavior of such components during the development of transient events (disruptions or/and of Edge Localized Modes (ELMs)) is still among the most critical issues for future thermonuclear devices. Experimental studies of castellated tungsten surface erosion have been performed within the powerful quasi-stationary plasma accelerator QSPA Kh-50 in conditions relevant to unmitigated ITER Type I ELMs. The surface energy load from the impacting hydrogen plasma streams was 0.9 MJ m−2 during 0.25 ms to achieve pronounced melting of the target surface. The studies of solid and liquid particle generation were performed at sequentially oblique plasma exposure of the different chosen edges of the castellated structure. It was determined that most of the W droplets are ejected from the sharp leading edge of the target due to the development of instabilities in the plasma-liquid metal interface. At the same time, the formation of solid dust particles is mainly attributed to the cracking during the surface cooling after the plasma pulse. Particle velocities typically achieve tens m s−1. Both the re-solidified droplets as well as dust particles have been collected on the special plate near the exposed surface. The maximal size of collected particles reached tens of micrometers. Such particles could either fly away with the next plasma pulses or mix with the material of the collecting plate in the course of repetitive plasma pulses. Thin (submicron) layers of re-deposited tungsten also formed on the surface of the collecting plate.