Wall conditioning of the JIPP T-II TORUS by AC discharge cleaning

Optimization of discharge cleaning was studied by using 60 Hz AC and cw ECR discharges. Reduction rate of oxygen was investigated for variations of the wall temperature. It has been confirmed that, below 3000C, the reduction rate increases as the wall temperature is raised. A simple model is proposed for the cleaning process and H2O pressure is calculated as a function of plasma density on the basis of this model. By comparing the calculated results to an experimental observation, it has been found that it is essential to raise the electron density of the discharge-cleaning plasma high enough that the dissociation-loss rate of H2O is as large as the sticking-loss rate in order to optimize the cleaning. (author)