Long-term changes in the surface conditions of PLT

Long-term changes in the surface conditions of the PLT vacuum vessel wall have been monitored by the periodic analysis of a variety of sample substrates (stainless steel, alumina, silicon), exposed to PLT discharges for periods of up to several months and subsequently removed for analysis by Auger electron spectroscopy (AES), photoelectron spectroscopy (ESCA), ion backscattering, nuclear reaction analysis, secondary ion mass spectrometry (SIMS), and scanning electron microscopy. Samples exposed for extended time periods (2-6 months) showed deposited films containing limiter (W) and liner constituent metals (Fe, Cr, and Ni) and C and O. The film thicknesses ranged between 100 and 200 Å with 2-15 at.% W and 5-40% Fe as determined by sputter-AES and ion backscattering measurements. Increased deposition of metallic impurities (W, Fe) was noted following the first extensive application of low power discharge cleaning. We discuss the possible mechanisms responsible for the deposition of metals onto the sample surfaces. Deuterium retention was observed in all the exposed samples with the deuterium depth profiles restricted primarily to the deposited films on the stainless steel substrates and extending deeper for Si. The retained deuterium measured in the exposed samples shows a saturation in the retention of (1-11) × 10¹⁵ D atoms/cm² for an estimated variation in the deuterium fluence of 10¹⁶-10¹⁸ D atoms/cm².