Abstract
A variety of techniques are used in Tandem Mirror Experiment-Upgrade (TMX-U) to control impurities and reflux: repeated plasma pulses, glow discharge cleaning (GDC), and gettering. We performed a series of experiments under three different plasma-wall conditions: no wall conditioning after a machine maintenance cycle, glow-discharge-cleaned wall, and a gettered wall. We used several plasma diagnostics to determine the effect of these procedures on TMX-U plasma parameters. Spectroscopic measurements show that GDC reduces impurities and increases the electron temperature, enabling full-duration beam-sustained plasma operation without a large number of repeated plasma pulses. Gettering further reduces the impurities and the neutral pressure; this improved condition persists for several shots after gettering is stopped. Measurements from residual gas analyzers and an end-loss ion spectrometer show that hydrogen is present in the plasma during the initial deuterium operation after pumpdown; the hydrogen level decreases after plasma operation with gettering, indicating reduced wall recycling.
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