A study of molybdenum influxes and transport in Alcator C-Mod

Abstract

A characterization is presented of Mo sources ΓMo, core Mo content NMo and their dependences on the Alcator C-Mod operational regimes. The primary impurity source locations are the divertor, the inner wall and the ICRF antenna limiters. Boronization is used to coat the first wall surfaces with a thin layer of B, which greatly reduces the Mo sources for a period of time, eroding away with an e folding period of 15-50 discharges. The penetration of Mo into the core plasma under different conditions is analysed using the concept of penetration factor, PF = NMo/ΓMo (s). In general, the inner wall Mo source is large (∼1018s-1), but is found to be relatively uncorrelated with the core Mo content in diverted plasmas. The outer divertor source is of a similar order to that of the inner wall and has a penetration factor in the range 10-5 - 2 × 10-3 s depending on density and confinement mode. The antenna limiter Mo sources are typically smaller, but with higher penetration factors of 10-3 - 2 × 10-2 s. The behaviour of the antenna limiter sources is consistent with physical sputtering due to the influence of RF sheath rectification. The measurements of the plasma potential on field lines connected to the antenna are very high when the antennas are energized, often reaching hundreds of volts.