PPPL-5381
Elimination of Inter-discharge Helium Glow Discharge Cleaning with
Lithium Evaporation in NSTX
Authors: R. Maingi, R. Kaita, 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 inter-shot
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.
Submitted to: Nuclear Materials and Energy
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