Analysis of wall conditioning by using plasma impact desorption in the tandem mirror GAMMA 10

A method for evaluating wall condition by using plasma impact desorption (PID) technique has been developed and successfully applied to the tandem mirror GAMMA 10 as a monitor for wall conditioning. A magnetically shielded quadrupole mass spectrometer was installed in the vacuum chamber of the GAMMA 10 central cell. The behaviour of the partial pressure of various gas molecules desorbed by ICRF-heated plasma discharges were analyzed. The predominant increase of the partial pressure due to PID ( ΔP PID ) was hydrogen ( M = 2) and a small amount of impurity as CO ( M = 28), CH 3 ( M = 15), H 2O ( M = 18) and CO 2 ( M = 44) was observed in the wall-conditioning discharges. The reduction of hydrogen pressure as well as ΔP PID of the above impurities was observed with the progress of wall conditioning. This behavior has a good correlation with the increase of partial pressures due to electron-impact desorption measured at the same period. The relation between ΔP PID and the charge-exchange flux was investigated.