Studies of plasma breakdown and electron heating on a 14?GHz ECR ion source through measurement of plasma bremsstrahlung

Temporal evolution of plasma bremsstrahlung emitted by a 14 GHz electron cyclotron resonance ion source (ECRIS) operated in pulsed mode is presented in the energy range 1.5-400 keV with 100 mu s resolution. Such a high temporal resolution together with this energy range has never been measured before with an ECRIS. Data are presented as a function of microwave power, neutral gas pressure, magnetic field configuration and seed electron density. The saturation time of the bremsstrahlung count rate is almost independent of the photon energy up to 100 keV and exhibits similar characteristics with the neutral gas balance. The average photon energy during the plasma breakdown is significantly higher than that during the steady state and depends strongly on the density of seed electrons. The results are consistent with a theoretical model describing the evolution of the electron energy distribution function during the preglow transient.