Characterization of cathode keeper wear by surface layer activation

The erosion rates of the discharge cathode keeper in a 30 cm NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) configuration ion thruster were measured using a technique known as surface layer activation (SLA). This diagnostic involved producing a radioactive tracer in the keeper surface by high-energy proton bombardment. The decrease in activity of the tracer material was monitored with a gamma spectroscopy system after the surface was subjected to wear processes. Correlation with a depth calibration curve yielded the eroded depth. The primary objectives were to validate the technique by reproducing erosion data from previous wear studies and to determine the effect of different engine operating parameters on erosion rate. The erosion profile at the TH 15 (2.3-kW) setting observed during the 8200-h life demonstration test was reproduced with a measured maximum erosion rate of 0.085 mum/h. Testing at the TH 8 (1.4-kW) setting demonstrated that variations in keeper voltage had a significant effect on the erosion, with a positive bias with respect to cathode potential decreasing the wear rate significantly. Measurements were achieved after operating times of 40-90 h, with a typical uncertainty of 0.003 mum/h.