OPERATION OF DIII-D WITH ALL-GRAPHITE WALLS

Recently, additional graphite coverage has been installed in the DIII- D tokamak, increasing the graphite tile coverage from 45% to approxima tely 90% (limited only by openings for ports). Due to a problem with c opper impurity bursts during tokamak discharges, copper foam sheets be tween the graphite tiles and the Inconel wall have been replaced with Grafoil gaskets (a compressible graphite material), which further incr eases the surface area of the graphite inside the DIII-D torus. Althou gh additional graphite has the potential for improving DIII-D performa nce, if not properly conditioned it can also degrade tokamak discharge s by contributing to increased low Z impurity influx (C and O) and hig her particle fueling (D). To address these concerns, improvements were implemented both in tile preparation and wall conditioning techniques . All previously installed tiles were grit blasted with boron-carbide grit to remove codeposited metal impurities and all tiles were outgass ed to 1000-degrees-C prior to installation. The only conditioning tech niques used were baking and helium conditioning (glow and Taylor disch arge cleaning) to avoid producing loosely bound carbon. Previously, H- 2 or D2 Taylor discharge cleaning was used extensively during the vent recovery phase. The glow discharge system used for wall conditioning was modified to include a large area electrode (0.32 m2). Recovery fro m the recent 7 month machine opening was extremely rapid with improved modes of energy confinement (H-mode) transitions observed on the four teenth plasma discharge. In addition, very high confinement (VH-mode) discharges were obtained without boronization, which is further indica tion of the rapid vent recovery. Lower recycling/lower fueling efficie ncy was also observed during operation with all-graphite walls. We wil l also discuss the use of neon and argon glow, and qualification of th e new graphite, including outgassing tests of the Grafoil material.