Measurements of accumulated metallic impurities during LiTER operation in NSTX

Abstract

Over 100 g of lithium was evaporated onto the NSTX plasma facing components using two stainless steel ovens (LiTER). Plasma emission was recorded using a 16-channel tangential bolometer and a VUV spectrometer. Confinement improved after lithium deposition and ELM’s were either reduced or eliminated for hundreds of milliseconds. The radiated power profiles were generally, but not universally, highly peaked. Both peakness and a modeled on-axis high-Z impurity concentration were used as metrics for the degree of central metallic impurity accumulation. Several changes in plasma performance were seen, including reduction in plasma density and improvements in energy confinement, however high-Z impurity accumulation correlated most directly with ELM-suppression. In the more severe cases of accumulation, the volume-integrated radiated power exceeded 50% of the total input power and the estimated concentration of metals on-axis (modeled using iron as the representative impurity) reached 0.2% of the electron density.