Differences of ICR cleanings in He, D2 and O2 for deposit removal and hydrogen release in HT-7

The differences in deposit removal and hydrogen release between ICR cleanings with various working gases, He, D2, O2, and gas mix of He/O2, were investigated on hot walls with a temperature of 410–470 K after long deuterium plasma operation in the HT-7 superconducting tokamak. Different mechanism of hydrogen release and deposit removal, such as chemical erosion, physical sputtering, ion induced desorption and isotopes exchange, could explain the difference of ICR cleanings in various gases. The wall conditions, such as deposits and hydrogen retention, would also influence on the efficiency of cleanings for deposit removal and hydrogen release. For each working gas, ICR cleanings with high power and/or high pressure promoted H release. Both the pure O-ICR and D2-ICR cleanings had a higher removal rate for hydrogen than that in He-ICR cleanings by a factor of 4–6. The O-ICR cleaning had a much higher deposits removal rate than the He-ICR and D2-ICR cleanings by a factor of a few tens. ICR cleaning techniques in a superconducting tokamak, such as EAST and ITER, should be reasonably arranged, which depend on deposits and hydrogen retention on walls, and requirement of plasma operation and safety.