Elimination of Inter-discharge Helium Glow Discharge Cleaning with Lithium Evaporation in NSTX

Authors: R. Maingi, R. Kaita, F. Scotti, V.A. Soukhanovskii, and the NSTX team

Abstract: Operation in the National Spherical Torus Experiment (NSTX) typically used either periodic boronization and inter-shot helium glow discharge cleaning (HeGDC), or intershot lithium evaporation without boronization, and initially with inter-shot HeGDC. To assess the viability of operation without HeGDC, dedicated experiments were conducted in which Li evaporation was used while systematically shrinking the HeGDC between shots from the standard 10 minutes to zero (10 -> 6.5 -> 4 -> 0). Good shot reproducibility without HeGDC was achieved with lithium evaporations of 100 mg or higher; evaporations of 200-300 mg typically resulted in very low ELM frequency or ELM-free operation, reduced wall fueling, and improved energy confinement. The use of HeGDC before lithium evaporation modestly reduced Dα in the outer scrape-off layer, but not at the strike point. Pedestal electron and ion temperature also improved modestly, suggesting that HeGDC prior to lithium evaporation is a useful tool for experiments that seek to maximize plasma performance.

Download PPPL-5279 (pdf 2.4 MB 20 pp)
_________________________________________________________________________________________________