Direct Measurements of Plasma Properties nearby a Hollow Cathode Using a High Speed Electrostatic Probe

A method is described for mapping the plasma parameters nearby a hollow cathode using a high speed positioning system and a segment-shielded electrostatic probe. Contour maps of plasma density, plasma potential, and electron temperature are presented for wide ranges of discharge current (2 20 A) and xenon flow rate (1.5 – 4.5 sccm). The contour maps are constructed from the analysis of 320 I-V traces, containing 1,300 data points each, taken over an axial and a radial region 2.5-mm to 32.5-mm and 0-mm to 20-mm, respectively, relative to the orifice of the hollow cathode. A comparison of plasma properties is presented for different cathode configurations including (a) a keeper-free cathode, (b) three enclosed-keeper configurations, (c) three anode configurations, and (d) an enclosedkeeper configuration with an applied magnetic field. The final configuration is meant to simulate the NSTAR thruster discharge cathode. Plasma densities range from 1x10 to 1x10 cm, with maximums occurring on centerline and close to the cathode orifice. Plasma potential ranges from 9 to 25 V, with contours that form valley depressions falling away from the cathode in cases without a magnetic field and axial trench depressions with a magnetic field. Electron temperatures were observed to loosely follow plasma potential structures and ranged from 1.5 to 3 eV for most operating conditions.