Observations of changes in residual gas and surface composition with discharge cleaning in PLT

Hydrogen discharge cleaning of the PLT vacuum vessel has been studied by mass spectroscopy of desorbed gases and surface analysis of exposed samples. Several modes of vessel conditioning have been studied to date: (1) a high-power discharge cleaning (PDC) mode, with a peak power density to the vessel wall P/sub s/approx.0.6 W/cm2 and a peak electron temperature T/sub e/approx.100 eV; (2) low-power (Taylor-type) discharge cleaning (TDC) with P/sub s/approx.0.05 W/cm2 and T/sub e/< or =5 eV. The predominant residual gases produced during PDC are CH4 (1--5 x 10-6 Torr) and CO (1--10 x 10-7 Torr), whereas TDC produced primarily H2O (1--2 x 10-6 Torr) and CH4 (1-10 x 10-7 Torr). The transient behavior of the residual gases is not simply related to pumping time constants, and provides evidence for the role of surface chemical reactions and diffusion on the formation of these gases. In situ surface analysis of hydrocarbon-covered stainless steel has shown significant decreases in carbon coverage occurring after 103--104 pulses of either cleaning mode. Observed changes in oxygen coverage are more difficult to interpret because of the presence of the passivation oxide layer on the stainless-steel substrates