Wall conditioning using 2.45 GHz ECR-DC on superconducting tokamak TRIAM-1M

Wall conditioning using electron cyclotron resonance-discharge cleaning (ECR-DC) was carried out for 35 h in a hydrogen plasma on superconducting tokamak, TRIAM-1M after ventilation of vacuum vessel for 2 months. The baking-out on extension ports was carried out before ECR-DC. As the wall of plasma chamber was cooled by water during ECR-DC about 25°C, the pure effect of ECR-DC could be investigated. The ECR plasma of the electron density ∼2×1010 cm−3 and electron temperature ∼6 eV was maintained by the microwave power up to 1 kW provided by a magnetron of frequency in 2.45 GHz. The decay time of the partial pressure of mass number 18 (H2O) and 28 (C2H4 and/or CO) were monitored during ECR-DC. The time evolution shows that oxygen and carbon was well removed from the surface of the wall by ECR-DC. The plasma resistivity of the first ohmic heated (OH) plasma just after the ECR-DC was the same as that of the final OH plasma after exposing the wall to tokamak plasmas for more than 7000 s. Intensity of the O II line, which was investigated in lower hybrid current drive (LHCD) plasma, did not change during an experimental campaign. These indicate that the wall conditioned by only ECR-DC is good in both OH and LHCD plasmas. A quantitative model of the efficiency for ECR-DC is presented. The model can explain the time evolution of partial pressure. From this model the duration required to clean up using ECR-DC was derived. This result may help in the design of the pump in large machines.