Control of impurity and hydrogen-deuterium dilution by wall conditioning in JT-60U

The effects of wall conditioning techniques used in JT-60U are described. Helium glow discharge cleaning is effective in reducing oxygen and particle recycling. Both carbon and oxygen impurities are reduced by a factor of 2 by reducing the wall temperature from 300 to 150°C. The dilution ratio H/(H+D) decreased by D 2 -TDC, ohmic discharges and He-GDC. At the initial phase of wall conditioning, the dilution ratio increased by He-TDC, and a sequence of tokamak discharges with neutral beam injection, suggesting that hydrogen particles were sputtered from inside the graphite wall. After sufficient wall conditioning had reduced the dilution ratio below 0.25, the dilution rationdid not deteriorate by neutral beam injection or by a sequence of shots.