Abstract : Experimental observation of plasma instabilities in 14.5 GHz PHOENIX charge breeder
ECRIS is reported. It is demonstrated with 133Cs and 85Rb that the injection of the 1+
ion beam into oxygen (16O) discharge of the CB-ECRIS can trigger electron cyclotron instabilities,
which restricts the parameter space available for the optimization of the charge
breeding efficiency. It is concluded that the transition from stable to unstable plasma regime
is caused by gradual accumulation and ionization of Cs/Rb and simultaneous change of the
discharge parameters in 10 - 100 ms time scale, not by a prompt interaction between the
incident ion beam and the ECRIS plasma. The delayed appearance of instabilities is presumably
due to the variation of the electron energy distribution in a time scale similar to the
reported breeding times of the high charge states of the injected ions. Since the commonly
applied method of measuring the breeding time, i.e. pulsing the 1+ injection, clearly affects
the charge state distribution of the buffer gas discharge and its electron energy distribution,
it is argued that the actual breeding times in continuous operation can differ significantly
from those obtained by studying the injection transient. Even more importantly, the instabilities
lead to loss of ion confinement, 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+ ion beam. The data demonstrating the instability-induced sputtering is supported
by time-resolved measurements of the biased disc current and ion beam energy spread
conducted with the conventional JYFL 14 GHz ECRIS.
