Measurements of ion energy distributions by Doppler shift spectroscopy in an inertial-electrostatic confinement device

Doppler shift spectroscopy was carried out on the discharge in a spherically symmetric inertial-electrostatic confinement system. This enabled the ion energy distributions, types, and densities of ionic species to be determined. A weakly ionized hydrogen radio-frequency discharge was used as the ion source for two spherical and concentric electrostatic grids. The inner and outer grids were the cathode and anode, respectively. It was found that the ion energy distribution consisted of a non-Maxwellian directional component, as well as a spatially isotropic Maxwellian distribution. The directional component consisted of three broadened energy peaks belonging to H3+ (20%), H2+ (60%), and H+ (20%). These ions had energies approximately 20% of the cathode potential. The temperature (in electronvolts) of the Maxwellian distribution was approximately 15% of the cathode potential.