Measurements of Electron Density in the Charge Exchange Plasma of an Ion Thruster

Abstract

The electron density in the charge exchange plasma emitted by a xenon ion thruster is determined using a langmuir probe. It is shown that the electron density in this plasma is sufŽ ciently low to allow the use of orbital-motion-limited theory. In applying this theory, the in ection point in the experimental current – voltage characteristic is taken as the point where the probe voltage equals the plasma potential. The electron density then follows from letting the theoretical result for the characteristic coincide with the experimental one at a signiŽ cantly larger voltage. Satisfactory agreement is obtained between theoretical and experimental curves. The results obtained for the electron density are compared with corresponding results for the ion density, determined from the ion saturation current. The ion density measurements are valid in the main ion beam, but not in the charge exchange plasma. The electron density measurements are valid in the charge exchange plasma and in the downstream part of the ion beam, but not in its upstream part. In the regions where both results are valid, they agree within experimental accuracy. The composite result covers charged particle densities over the range 10– 10 cm.