Oxygen removal with different cleaning techniques under various wall conditions in HT-7

The effectiveness of several types of oxygen removal, including baking, ion cyclotron resonant discharge cleaning, glow discharge cleaning and disruptive plasma cleaning were surveyed on hot walls of 402–470 K in the HT-7 machine. Oxygen removal from ‘clean’ walls and from walls with oxygen contamination has been examined. The wall conditions, such as material, structure of plasma facing components and oxygen retention, would influence the oxygen removal efficiency of ICR cleanings. On ‘clean’ metal walls before 2000, high power He-ICR produced high oxygen removal. However, on ‘clean’ walls with carbon limiters after 2004, both He-ICR and D2-ICR cleanings would induce oxygen retention but not removal. On heavy contaminated walls, such as after oxidation experiment, the removal rate of oxygen during He-ICR cleanings depended on oxygen retained on the walls, ICR power and pressure. Filling helium promoted oxygen removal during baking. He-GDC cleaning possibly has oxygen removal efficiency at far position, but comparison between it and He-ICR cleaning also depended on oxygen retention status. Disruptive plasma discharges with cryo-pumping are essentially for removal of the remained oxygen from the walls after wall cleanings and for re-building normal plasma. To apply those oxygen removal techniques in a superconducting tokamak device, such as EAST and ITER, should depend on oxygen retention and their distribution on walls.