Discharge Chamber Plasma Potential Mapping of a 40-cm NEXT-type Ion Engine*

Floating emissive probe plasma potential data are presented over a two-dimensional array of locations in the discharge chamber of a 40-cm diameter ring-cusp ion thruster. The data rule out the presence of a potential-hill plasma structure downstream of the discharge cathode assembly (DCA), which has been proposed as a mechanism of high-energy ion formation leading to DCA erosion. The discharge potential structure is dominated by the magnetic circuit, which reduces the diffusion of electrons across magnetic field lines. Shorting of the discharge keeper to discharge cathode common does not have a detectable effect on the near-DCA plasma structure outside of the keeper sheath. The plasma potential structures are insensitive to thruster throttling level with a minimum as low as 14 V at the DCA exit plane increasing gradually in the axial direction, but rising abruptly in the radial direction. A sharp increase in plasma potential to the bulk discharge value (a few volts above the discharge voltage), is observed in the radial direction past the discharge keeper edge. The existence of a free-standing plasma potential structure, termed a double layer, is found to form the transition between the low-potential plume emanating from the discharge cathode and the high-potential bulk discharge plasma. Angular-dependent sputtering yield calculations are used to estimate the NEXT DCA wear rate for the operating condition of the 2000-hr wear test. The calculated erosion rate (49 μm/khr) is slightly lower than the observed keeper erosion rate in the NSTAR 2000-hr and 8200-hr wear tests. This is due to a decreased double-to-single current ratio measured in the beam of the NEXT thruster at the wear test operating condition compared to the measured ratio in the beam of the NSTAR thruster at the full-power throttle point.