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†‡ Langmuir probe plasma parameter data are presented over a two-dimensional array of spatial locations in the discharge chamber of a 40-cm diameter ring-cusp NEXT ion thruster. The discharge plasma structure is determined by the magnetic circuit, which reduces the diffusion of electrons across magnetic field lines. Number density profiles exhibit a highly collimated plume structure confined by the axial magnetic field near the discharge cathode assembly (DCA). A maximum value of 2x10 13 cm -3 is measured along centerline at the keeper exit plane. A number density of ~5x10 11 cm -3 is observed outside of the discharge cathode plume. Electron temperatures in the cathode plume range from 2 – 4 eV increasing off-axis in the radial direction to 4 – 8 eV in the bulk discharge plasma. This increase is explained by electron acceleration across the free-standing potential gradient, termed a double layer, forming the transition between the discharge cathode plume and bulk discharge plasma. Between the magnetic cusps, near the anode, the electron temperature increases significantly. Only the high-energy electrons can cross the magnetic field lines to occupy these regions resulting in an increased electron temperature. Shorting of the discharge keeper to discharge cathode common, a condition of interest from ground-based NSTAR testing, does not have an effect on the near plasma structure outside of the DCA keeper sheath.

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