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 ∼90%. Due to a problem with copper impurity bursts during tokamak discharges, copper foam sheets between the graphite tiles and the Inconel wall have been replaced with GRAFOIL gaskets, which further increases the surface area of the graphite inside the DIII-D torus. Although additional graphite has the potential for improving DIII-D performance, if not properly conditioned it can also degrade tokamak discharges by contributing to increased low Z impurity influx and higher particle fueling. 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 co-deposited metal impurities and all tiles were outgassed to 1000 degrees C prior to installation. The glow discharge system used for wall conditioning was modified to include a large area electrode. The only conditioning techniques used were baking and helium conditioning to avoid producing loosely bound carbon. Previously, H2 or D2 Taylor discharge cleaning was used extensively during the vent recovery phase. Recovery from the recent 7-month machine opening was extremely rapid with improved modes of energy confinement (H-mode) transitions observed on the 14th plasma discharge. In addition, very high confinement (VH-mode) discharges were obtained without boronization, which is further indication of the rapid vent recovery. Lower recycling/lower fueling efficiency was also observed during operation with all-graphite walls. We will also discuss the use of Ne and Ar glow, and qualification of the new graphite including outgassing tests of the GRAFOIL material