Initial wall conditioning in Doublet III

The first year of operation of Doublet III, a large noncircular tokamak (R = 1.43 m, a = 0.45 m, B/sub T/ < or =2.6 T), was highlighted by the achievement of a plasma current in excess of 2 MA. This is partially attributed to the low Z/sub eff/ discharges made possible by wall conditioning using low temperature, low power discharge cleaning, accompanied by preconditioning of the wall and by baking. Approximately 500 h of hydrogen discharge cleaning and 25 h of oxygen discharge cleaning combined with tokamak plasma operations were required to obtain these low Z/sub eff/ tokamak discharges. During the initial cleanup of the wall, copious quantities (>100 monolayers) of carbon were removed, ultimately with greatest efficiency by oxygen discharge cleaning. During the conditioning process, monitoring the impurity removal rate with the residual gas analyzer (RGA) provided the most understandable measure of the rate of the cleanup, while an in situ Auger analysis surface diagnostic supplied and operated by ORNL was most useful in identifying the presence of heavy hydrocarbons (oil), carbides, and oxygen on the wall and monitoring the transport of limiter materials