  The measurement of the axially lost electron energy distribution escaping
from a minimum-B electron cyclotron resonance ion source in the range of 4-800
keV is reported. The experiments have revealed the existence of a 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 momentum space diffusion. Possible mechanism
of the hump formation is considered basing on the quasi-linear model of
plasma-wave interaction.
