Experimentally Determined Neutral Density and Plasma Parameters in a 30 CM Ion Engine

Abstract

range of 1 to 7.9 eV and an electron density range of 4e10 to 1e13 cm-3, throughout the discharge chamber, consistent with the results in the literature [1-2]. Plasma potential estimates, computed from the first derivative of the probe characteristic, indicate potential from 0.5V to 11V above the discharge voltage along the thruster centerline. These values are believed to be excessively high due to poor saturation of the probe IV characteristic in the low-density regions of the plasma. Relative neutral density profiles are also obtained with a fiber optic probe sampling photon flux from the 823.1 nm excited to ground state transition. Plasma parameter measurements and neutral density profiles will be presented as a function of probe location and engine discharge conditions. A discussion of the measured electron energy distribution function will also be presented, with regards to variation from pure maxwellian. It has been found that there is a distinct primary population along the thruster centerline, which causes estimates of electron temperature, electron density, and plasma potential, to err on the high side, due this energetic population. Computation of the energy distribution function of the plasma clearly indicates the presence of primaries, whose presence become less obvious with radial distance from the main discharge plume.