Plasma impurity content, gas fueling, and exhaust on DIII-D over extended periods between boronizations

Abstract

During the 2006 and 2007 DIII-D experimental campaigns the rate of boronization events was reduced significantly from past campaigns with no detrimental effects on discharges, including high-performance hybrid and advanced tokamak discharges. Boronizations were completed early in both campaigns due to preceding entry vents. Over the 3-month duration of each campaign, a database of edge and core impurity emission and fueling/exhaust rates from many hybrid discharges was developed that demonstrated little secular change over 7000 plasma-seconds of operations. After 6000 s, a set of seven sequential hybrid discharges was executed with no between-shot helium glow-discharge cleaning. While small effects on fueling and exhaust are observed, density remains controlled and fusion performance is held constant. These results, obtained with the all graphite wall on DIII-D, are promising for the next generation of superconducting, long-pulse tokamaks, where studies of stationary, high performance will be of great interest.