Surface effects on graphite samples exposed to beryllium-seeded plasmas under transient power load on PISCES-B

A transitory positive voltage biasing (called hereafter ‘pulsing’) is applied to a graphite target during deuterium plasma exposure in the PISCES-B linear device to simulate the heat pulse associated with ELMs. The effect of the pulsing on the suppression of chemical erosion and on deuterium retention is investigated, both in pure deuterium and in beryllium-seeded plasmas. In the presence of power transients, spectroscopic observation of CD molecular band at 430 nm and mass loss measurements indicate a shorter time for beryllium-induced suppression of chemical erosion. No effect on erosion due to pulsing is observed for pure graphite surfaces. Deuterium retention is found to be higher when pulsing is applied, by 60% during beryllium seeding and by 30% for pure deuterium. Infrared radiation emitted from the target surface indicates a modification of the surface morphology during the plasma exposure of the graphite samples and a smaller heated emitting area during the pulsing compared to the total exposed area.