Helium glow wall conditioning of the DIII-D tokamak with large area graphite coverage

Helium glow wall conditioning (HeGWC) is the primary technique for conditioning the DIII-D walls which are composed of Inconel and graphite. Presently, 40% (31 m2) of the DIII-D plasma facing surfaces consist of graphite tiles covering the inside, top, and bottom DIII-D walls which are the high heat flux surfaces for divertor and inside wall limiter discharges. HeGWC with Twall ≤ 50 degree C is effective both in removing low Z impurities and desorbing hydrogenic particles from this large graphite ''reservoir.'' The effective desorption of hydrogen species from the graphite has led to lower recycling and improved particle control during tokamak discharges. We will first discuss the DIII-D glow discharge apparatus, the characterization of the glow discharge, the dependence of particle removal rates on the glow parameters, particularly electrode voltage and the duration of the glow session, and the subsequent discharge particle balance. After installation of graphite tiles covering 40% of the first wall (prior coverage was 9%, primarily in the lower divertor region), previously successful discharge cleaning and wall conditioning techniques in hydrogen failed to give reproducible high-quality discharges. With HeGWC, H-mode discharges were again reproducibly obtained. The main advantages of HeGWC during this phase of initial conditioning were: disruption recovery during tokamak operations by removal of impurities such as CO, and reduced fueling from the walls, allowing reproducible burnthrough and plasma density control. 13 refs., 6 figs