KAUFMAN-TYPE XENON ION-THRUSTER COUPLING PLASMA - LANGMUIR PROBE MEASUREMENTS

This paper details an investigation into the physics of the external p lasma in a Kaufman-type electrostatic ion thruster operating with xeno n gas. The external plasma, which is located outside the hollow cathod e between the keeper and the anode electrodes, was investigated with a single Langmuir probe. Detailed scans give a peak temperature of 0.4 +/- 0.1 eV at the orifice of the keeper decreasing to 0.3 +/- 0.1 eV a t the anode. The temperature is essentially constant over the external plasma volume. The radial electron density distribution peaks on the axis at 5.0 +/- 0.1 x 10(11) cm(-3) decreasing radially to 1.0 +/- 0.1 x 10(11) cm(-3) at the boundaries of the plasma; these values have no t been corrected for any perturbation of the plasma caused by the pres ence of the probe. The velocity distribution of the electrons was stud ied using the second derivatives of the probe characteristics assuming a collisionless sheath. It was shown that the electrons have a Maxwel lian distribution. This allowed essentially independent calculations o f the electron temperature and plasma potential distribution to be mad e. 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 r endering the analysis inaccurate.