Properties and the influences on plasma performance for the film produced by radio frequency boronization

A new boronization technique focusing on the needs of the future large supe rconducting device has been developed in HT-7 tokamak. The first try on a t okamak gave very promising results. A fine homogeneous and hard a-B/C:H fil m was produced by a pulse ion cyclotron resonance frequency plasma. The fil m shows high adhesion, high thickness and longer lifetime. The ratio of B/C is about 3 up to a depth of 280 nm. X-ray photoelectron spectroscopy analy sis shows that the B-B, B-C, C-C, C-O, and B-O bonds were formed during the boronization. The oxygen content in the film increases from 15% to 25% aft er 250 serious discharges, which demonstrated the strong oxygen gettering b y the film. Good uniformity of the film in both toroidal and poloidal direc tions has been obtained by using long antenna on the high field side. The r ecycling of hydrogen was easily controlled by using helium rf discharge aft er boronization, and very strong wall pumping was observed. Plasma performa nce was significantly improved after boronization. A higher density limit a nd wider operation space were obtained. The strong hard x ray accompanied b y high power lower hybrid current drive was suppressed dramatically. This g ives direct evidence that the thin boron film serves as a protecting layer against the energetic particles, which is very important for future long-pu lse-length discharge. This new technique has been proved to be very effecti ve for conditioning future large magnetic fusion devices. (C) 2000 American Vacuum Society. [S0734-2101(00)01206-9].