This research addresses the issue of damage to the wall of a fusion reactor due to plasma surface inter- action in the exhaust region, the divertor. The occurrence and damage mechanisms of unipolar arcing on fuzzy tungsten were investigated in the linear divertor-plasma simulator PISCES-A at UCSD. A pa- rameter scan was performed to obtain the ignition conditions for arcing in PISCES-A in terms of the fuzz-forming exposure time, power load, bias voltage and surface temperature. It was found that already after 10 minutes, arcing could occur, and that an important parameter in the ignition of arcing was the bias voltage. A threshold of -100 V was found for the surface temperature in range 973-1223 K. Based on these findings the conditions expected in the divertor of the ITER reactor are in the parameter window for arcing. In order to investigate the damage mechanisms during arcing, two arc diagnostics - shadowg- raphy and spectroscopy - were devised and tested on laser-induced plasmas. Shadowgraphy was used to explore the arc phenomenology, but could not be used at the (low) densities in PISCES-A. Spectroscopy was used in PISCES-A to measure the electron temperature and density of the arcing plasma, which were found to be in the range 0.5-0.8 eV and 7 · 1019 ? 2 · 1020 m?3, respectively, and to increase as function of the bias voltage. The arcs had a duration of 2-6 ms, and sustained a current of 20-70 A. The average mass loss rate per arc based on mass measurements was 35 ± 30 mgs?1, and the average damaged surface area per second was 0.9 ± 0.3 cm2s?1. From the arc current measurement, a mass loss rate of 10-30 mgs?1 was obtained. The effective arc track depth was estimated to be 40-150 nm, corresponding to one tenth of the fuzz layer, and the effective erosion rate was 10-40 µms?1. It was estimated that the effective erosion rate on the ITER divertor would be 10 ± 5 nms?1 for a single arc. In the presence of multiple emission centers or a high floating potential this could lead to a total arc-induced erosion of the same order as the steady state erosion rate, which is estimated at 1-10 µms?1 in the literature. The conclusion of this scoping study therefore is that arcing in the ITER divertor potentially is an important cause for damage. Therefore it needs to be investigated more comprehensively, and mitigation measures must be developed.
Arcing on fuzzy tungsten in PISCES-A and possible implications for ITER
Aussems, D. U. B. (Author). 31 Dec 2013
Student thesis: Master