Characterization of individual spontaneous arcs on fuzzy tungsten surfaces irradiated in a radiofrequency helium plasma

Tungsten fuzz layers formed on the plasma-facing material surfaces of thermonuclear facilities due to helium ion irradiation are known to promote the initiation of electrical discharges on the first wall. Previous studies have indicated increased arcing probability under helium ion irradiation in steady plasmas with increasing ion energy to several hundred eV. Here, we experimentally consider the growth of nanostructures on tungsten surfaces in an RF He plasma and the spontaneous initiation of arcs when applying the pulsed bias voltage to the sample, up to several keV. The arc current and voltage waveforms were obtained, and the scanning electron images of the arc traces were analyzed. The behavior of the arc at the smooth/fuzzy surface boundary directly demonstrated the beneficial arcing conditions of the nanostructured layer. The proposed approach of pulsed biasing enables controlling the amount and duration of arcs for better analysis of their dimensions and dynamics. From the experimental results, the total arc trace length and arc velocity were calculated and compared with literature data, giving a good agreement both between experiments and theoretical data and between experiments in different arc initiation conditions.