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.

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

“…Plasma parameters such as the plasma potential, electron density and temperature, and electron energy distributions were measured by single and emissive probes under various discharge conditions, such as various rf power levels, source pressures, magnetic field tunings, and probe positions [3]. Single probe (SP) was used for the plasma density, temperature, and EEDF determination, while the emissive probe (EP) was used for measuring the plasma potential and compared with that by SP [4].…”

Selected Topics in Plasma Physics

“…For example, plasma in some stars (white dwarfs) has a density of 10 25 -10 26 cm �3 , but in the interstellar space, plasma has a density of 1-10 cm �3 . The ratio is 10 26 . The ratio of other parameter values is a bit less.…”

“…The MW is launched in right (R) hand mode (R mode is extraordinary type) using the four step ridge waveguide. This makes theẼ-field intensity to be maximum in the center of the cavity that is propagating in parallel to the externally applied magnetic field [25,26]. As soon as the MW is launched, it continues to interact with the gas particles on their propagation timescales (ns).…”

Evolution of Microwave Electric Field on Power Coupling to Plasma during Ignition Phase

“…The MW is launched in right (R) hand mode (R mode is extraordinary type) using the four step ridge waveguide. This makes theẼ-field intensity to be maximum in the center of the cavity that is propagating in parallel to the externally applied magnetic field [25,26]. As soon as the MW is launched, it continues to interact with the gas particles on their propagation timescales (ns).…”

“…But as time passes (see Figure 6b-d), the power deposition location gets shifted in the off-ECR or upper hybrid resonance (UHR) regime. The UHR zone is a region where the two conditions n e < n crit and B < B ECR are satisfied [26][27][28]. The term n crit represents the critical density for the MW frequency, 2.45 GHz that is 7:4 Â 10 16 m À3 .…”