GLOW DISCHARGE CONDITIONING OF THE PDX VACUUM VESSEL.

A glow discharge technique is described for the conditioning of the large (38 m**3) Poloidal Divertor Experiment (PDX) vacuum vessel. The discharge parameters and working gas (H//2) were chosen to maximize C and O removal and minimize metal sputtering. The glow discharge was produced by biasing one or two internal anodes at 400 V to sustain a discharge current of 2-4 A per anode. Purified H//2 at a pressure of 3 multiplied by 10** minus **2 Torr was flowed through PDX at approximately 10 Torrl/s. The effectiveness of the glow discharge conditioning was monitored by measuring impurity gas (CH//4, C//2H//4, and CO) exhaust rates by mass spectrometry and C and O surface removal rates by in situ AES and XPS. Low Z(eff) high-power discharges were obtained in PDX after 120 h of glow discharge treatment, during which more than 100 equivalent monolayers of C and O were removed from the vessel.