On the role of rf-scattering in the electron losses from minimum-B ECR plasmas

The measurement of the axially lost electron energy distribution escaping from a minimum-B electron cyclotron resonance ion source (ECRIS) in the range of 4-800 keV is reported. The experiments have revealed the existence of a clear hump at 150-300 keV energy, containing up to 15\% of the lost electrons and carrying up to 30\% of the measured energy losses. The mean energy of the hump is independent of the microwave power, frequency and neutral gas pressure but increases with the magnetic field strength, most importantly with the value of the minimum-B field. Experiments in pulsed operation mode have indicated the presence of the hump only when microwave power is applied, confirming that the origin of the hump is rf-induced velocity space diffusion. Particle-in-cell simulation results and a quasi-linear diffusion model are presented to limit the possible mechanisms populating the loss cone at electron energies corresponding to the experimental observations.