Pulse-by-pulse measurements of dynamic retention and deposition in NSTX

Three quartz crystal microbalances have been deployed in NSTX to measure deposition/erosion in plasma shadowed areas at the upper and lower divertor and outboard midplane. These show a complex pattern of material gain and loss. At the time of a plasma discharge a transient increase in mass of order ∼0.1 μg cm−2 is observed that decays in the interpulse period to a level either higher, lower or similar to that prior to the discharge. The first discharge of the day always shows a long term step-up in mass. Some correlations of mass gain with plasma duration, stored energy, and change in the plasma shape are observed. Following a days plasma operations, a slow decay in mass is observed. The results are interpreted in terms of dynamic retention and erosion/deposition.