Effects of wall conditioning on plasma performance in the Advanced Toroidal Facility stellarator

Various wall conditioning techniques have been used on the Advanced Toroidal Facility (ATF). Continued improvement in plasma parameters has been obtained. In ATF, a stellarator, wall conditioning is more important for obtaining favorable plasma parameters than it is in plasma devices with current. Discharge cleaning, baking, and gettering with both chromium and titanium have been used as conditioning techniques. Gettering with titanium has increased the pumping speed of nitrogen and oxygen by more than a factor of 10, significantly reducing impurity effects, and has allowed better control over wall recycling and thereby plasma density. This has increased the operating space for many of the plasma parameters. The effects of these conditioning techniques on plasma parameters are discussed.