Impurity generation and suppression during IBW heating in HT-7

Impurity production during high power ion Bernstein wave (IBW) heating has been a persistent problem when the input RF power is above 100 kW, resulting in a substantial increase in the radiated power. The core electron temperature drops quickly by a factor up to 3 within 20 ms, which is partially caused by fast particle loss induced by high toroidal magnetic ripple. By using the 24 ferritic boards, which makes the reduction of ripple amplitude from 4% to 1.6%, high quality water-cooled graphite limiter, the new IBW antenna and RF boronization, the impurity generation, especially high-Z impurity was almost disappeared. Zeff increased a little with 300 kW, changed from 1.1 to 1.24. The radiated power was reduced to only one-third of the RF power, essentially solving the impurity problem during IBWH. Both electron and ion temperatures were increased during IBW heating. Very good heating efficiency has been obtained.