Measurement of Ion Energy Distributions Produced within an NSTAR Discharge Chamber

Measurements are presented of ion energy distributions produced within a prototype NSTAR discharge chamber as a function of its operational parameters. The goal of this effort is to document the ion energy distribution characteristics and determine features that could cause rapid sputter erosion of discharge chamber components. Ion energy distributions were measured with a Comstock model AC-901 electrostatic analyzer (ESA) that was located outside of the discharge chamber and sighted through a slit that was cut in the side of the anode and along a diameter across the pseudo-screen grid. Important results are that, at some locations within the discharge chamber at high discharge current operating conditions, high energy ion groups (with drift energies of order 100 eV that are separate and distinct from groups at the discharge energy) can be produced that would cause rapid sputter erosion to most discharge chamber thruster components comprised of molybdenum, stainless steel, tantalum, and tungsten. Under nominal operating conditions, no significant high-energy groups (separate from the discharge ion signal) are detected. However, high energy tails on the discharge ion signal are detected that display effective temperatures of order 10 eV. An additional result is that the discharge ion signal appears to be Maxwellian and ion temperatures of 3 to 4 eV have been measured. In some situations, the discharge ion signal is much stronger than high energy ion groups present to the extent that the discharge ion group tail would cause more sputter erosion than the high energy group (at the location of the ESA).