Discharge Cathode Electron Energy Distribution Functions in a 40-cm NEXT-type Ion Engine*

Electron energy distribution functions (EEDF’s) are presented over a two-dimensional array of locations in the discharge chamber of a 40-cm diameter ring-cusp ion thruster. The EEDF’s are obtained using two different techniques illustrating comparable results. A distinct difference in the EEDF’s measured through the double layer potential gradient, which transitions between the low-potential discharge cathode plume and the appropriated discharge voltage bulk discharge plasma, is observed for thruster operation with and without beam extraction. This result indicates the coupled nature of the beam and discharge plasmas and highlights the need for beam extraction to reproduce flight-like conditions. For operation with beam extraction, the single-hump near discharge cathode assembly (DCA) distributions transition to double-hump or plateau-like distributions in the double layer and eventually become single-hump distributions with a high-energy tail in the bulk discharge. Radial acceleration across the double layer is responsible for the formation of the second hump in the distribution explaining the rise in electron temperature outside the discharge cathode plume that is observed. Axial acceleration near the DCA is also discernable, but to a lesser extent. These observations agree with measured 40-cm plasma potentials inside the discharge chamber. The discharge-only EEDF’s demonstrate a second hump that is greatly reduced and in many cases not observed. Shorting of the discharge keeper to discharge cathode common did not have an effect on the near DCA plasma structure outside of the keeper sheath.