Time resolved deposition measurements in NSTX

Abstract

Time-resolved measurements of deposition in current tokamaks are crucial to gain a predictive understanding of deposition with a view to mitigating tritium retention and deposition on diagnostic mirrors expected in next-step devices. Two quartz crystal microbalances have been installed on NSTX at a location 0.77m outside the last closed flux surface. This configuration mimics a typical diagnostic window or mirror. The deposits were analyzed ex-situ and found to be dominantly carbon, oxygen and deuterium. A rear facing quartz crystal recorded deposition of lower sticking probability molecules at 10% of the rate of the front facing one. Time resolved measurements over a 4-week period with 497 discharges, recorded 29.2μg/cm2 of deposition, however surprisingly, 15.9μg/cm2 of material loss occurred at 7 discharges. The net deposited mass of 13.3μg/cm2 matched the mass of 13.5μg/cm2 measured independently by ion beam analysis. Monte Carlo modeling suggests that transient processes are likely to dominate the deposition.