The first results of O-ICR experiments to remove re-deposited layers and hydrogen in the HT-7 superconducting tokamak

Oxidation experiments associated with ion cyclotron resonance discharges (O-ICR) have been performed in HT-7 in the presence of a permanent magnetic field of 1.5–2.0 T. The influence of ICRH power and filling pressure on hydrogen and carbon removal rates was analysed. Total numbers of 5.70 × 1022 H-atoms, 1.6 × 1022 D-atoms and 2.35 × 1022 C-atoms had been removed within eight O-ICR cleanings. An ∼20.5 nm co-deposited film on average was removed from the limiters and liners with an area of 12 m2. About 1.73 × 1022 O-atom retention in an O-ICR experiment corresponds to a coverage of 1.44 × 1016 O  cm−2. The highest removal rates of H, D and C-atoms of up to 2.64 × 1022 atoms h−1, 7.76 × 1021 atoms h−1 and 1.49 × 1022 atoms h−1, respectively, were obtained in a 40 kW, 9 × 10−2 Pa O-ICR cleaning, corresponding to a removal rate of co-deposits of about 317 nm/day (7.2 g/day for carbon). In a 50 min He-ICR cleaning after the O-ICR experiment about 5.39 × 1021 oxygen retention was removed. Also the influence of the oxidation experiment on the subsequent plasma operation was studied. Normal plasma discharges could be recovered after a few hours of disruptive plasma discharges.