Discharge Chamber Plasma Structure of a 30-cm NSTAR-type Ion Engine

Single Langmuir probe measurements are presented for a two-dimensional array of locations in the near Discharge Cathode Assembly (DCA) region of a 30-cm diameter ring-cusp ion thruster over a range of thruster operating conditions encompassing the high-power half of the NASA throttling table. All data were analyzed assuming an electron population consisting of Maxwellian electrons only yielding repeatable results that agree with previous double probe measurements. Unsuccessful attempts were made to analyze the on-axis Langmuir probe data assuming both Maxwellian and primary electrons, yielding inaccurate results that were hyper sensitive to the analysis input parameters. Discharge plasma data taken with beam extraction exhibit a broadening of the higher electron temperature plume boundary compared to similar discharge conditions without beam extraction. The opposite effect is evident with the electron/ion number density as the data without beam extraction appears to be more collimated than the corresponding data with beam extraction. Shorting the discharge cathode keeper to common had no noticeable effect on the discharge plasma.