Bi-Maxwellian electron energy distributions in the edge of the CAPRICE ECR ion sources

Edge plasma of the ORNL CAPRICE ECR ion source is studied by directly measuring, with electrical probes, its local plasma parameters such as plasma density, temperature and electron energy distributions at different rf power levels and different locations from the resonant zone. Edge plasma can be approximated to be bi-Maxwellian, whose characteristics become more pronounced farther from the ECR zone. These trends are consistently explained by the collective long-range Spitzer collision mechanism in terms of steep temperature gradient between the triangular plasma region and the gap region during the bounce motions between the magnetic poles, so that the low energy electrons are more collisional and are pitch-angle scattered to the loss cones, while the high energy electrons are less collisional and can reach farther edge position.