The effect of plasma instabilities on the background impurities in charge breeder ECRIS
Résumé
Experimental observations of plasma instabilities in the 14.5 GHz PHOENIX charge breeder ECRIS are summarized. It has been found that the injection of 133Cs+ or 85Rb+ into oxygen discharge of the CB-ECRIS can trigger electron cyclotron instabilities, which results to sputtering of the surfaces exposed to the plasma, followed by up to an order of magnitude increase of impurity currents in the extracted n+ charge state distribution. The transition from stable to unstable plasma regime is caused by gradual accumulation and ionization of Cs/Rb altering the discharge parameters in 10 - 100 ms time scale, not by a prompt interaction between the incident ion beam and the ECRIS plasma. This time scale is similar to the reported breeding times of the high charge state Cs and Rb ions. Since the commonly applied method of measuring the breeding time, i.e. pulsing the 1+ injection, clearly affects the buffer gas discharge, it is argued that the actual breeding times in continuous operation can differ from those obtained by studying the injection transient.