Near Discharge Cathode Assembly Plasma Potential Measurements in a 30-cm NSTAR- type Ion Engine amidst Beam Extraction

Floating emissive probe plasma potential data are presented over a two-dimensional array of locations in the near Discharge Cathode Assembly (DCA) region of a 30-cm diameter ring-cusp ion thruster. Discharge plasma data are presented with beam extraction at throttling conditions comparable to the NASA TH Levels 8, 12, and 15. The operating conditions of the Extended Life Test (ELT) of the Deep Space One (DS1) flight spare ion engine, where anomalous discharge keeper erosion occurred, were TH 8 and TH 15; consequently, they are of specific interest in investigating discharge keeper erosion phenomena. The data do not validate the presence of a potential hill plasma structure downstream of the DCA, which has been proposed as a possible erosion mechanism. The data are comparable in magnitude to data taken by other researchers in ring-cusp electron-bombardment ion thrusters. The plasma potential structures are insensitive to thruster throttling level with a minimum as low as 14 V measured at the DCA exit plane increasing gradually in the axial direction. Shorting of the discharge keeper to discharge cathode common, at roughly NASA TH 8, did not have a significant effect on the near DCA plasma structure. A sharp increase in plasma potential to the bulk discharge value of 26 – 28 volts, radially past the discharge keeper edge, was observed. Plasma potential measurements indicate a low-potential plume structure emanating from the discharge cathode that may be attributed to a free-standing plasma double layer.