Kaufman-type xenon ion thruster coupling plasma: Langmuir probe measurements

Abstract

This paper details an investigation into the physics of the external plasma in a Kaufman-type electrostatic ion thruster operating with xenon gas. The external plasma, which is located outside the hollow cathode between the keeper and the anode electrodes, was investigated with a single Langmuir probe. Detailed scans give a peak temperature of eV at the orifice of the keeper decreasing to eV at the anode. The temperature is essentially constant over the external plasma volume. The radial electron density distribution peaks on the axis at decreasing radially to at the boundaries of the plasma; these values have not been corrected for any perturbation of the plasma caused by the presence of the probe. The velocity distribution of the electrons was studied using the second derivatives of the probe characteristics assuming a collisionless sheath. It was shown that the electrons have a Maxwellian distribution. This allowed essentially independent calculations of the electron temperature and plasma potential distribution to be made. These measurements were taken in the stable `spot mode' discharge. Observations in the less stable `plume mode' were also made. However, electrical noise due to plasma instabilities masked the probe signal rendering the analysis inaccurate.