Impurity origin during ICRF heating in JIPP T-IIU Tokamak

Replacing stainless steel limiters by graphite limiters, it was found that radiations from iron and titanium ions were significantly reduced. Total radiation and loop voltage also decreased. This indicates the limiters are the major impurity sources both in Ohmic and RF heating phases. Although titanium radiations increased with RF power injected by an antenna with a titanium Faraday shield, the maximum intensity was much smaller than those in experiments where the titanium-flashed stainless steel limiters were used. Thus it was found that the Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for OII radiations suggests that the energetic charge-exchange neutrals play a role in releasing oxygen from the wall and that those energetic particles are relatively abundant in the toroidal sections near the antenna. The radiation H(alpha) + D(alpha) decreases during RF pulse around the limiter, which may be due to the change in hydrogen/deuterium recycling at the limiter. This reduction is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed.