The cold and hot electron populations, temperatures and their transports in the edge plasma of the ORNL CAPRICE ECR ion source

Abstract

The 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 distribution characteristics at different rf power levels, at various pressures, at various axial magnetic field strengths and different distances from the resonant zone. It is found that the edge plasma can be approximated to be bi-Maxwellian, whose characteristics become more pronounced at the more distant positions from the ECR zone, at lower source pressures and at increased magnetic field strengths. These trends are consistently explained in terms of long-range electron-electron and electron-ion Coulomb scattering collisions that occur during the transport of hot electrons from the ECR plasma to the probe and their frequency dependence on electron temperature and density.