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A detailed study of the spatial variation of plasma density, temperature, and potential in hollow cathodes using miniature fast scanning probes has been undertaken in order to better understand the cathode operation and to provide benchmark data for the modeling of the cathode performance and life described in a companion paper. Profiles are obtained throughout the discharge and in the very high-density orifice region by pneumatically driven Langmuir probes, which are inserted directly into the hollow cathode orifice from either the upstream insert region inside the hollow cathode or from the downstream anode-plasma region. A fast transverse-scanning probe is also used to provide radial profiles of the cathode plume as a function of position from the cathode exit. The probes are extremely small to avoid perturbing the plasma; the ceramic tube insulator is 0.05cm in diameter with a probe tip area of 0.002cm2. A series of current-voltage characteristics are obtained by applying a rapid sawtooth voltage wave form to the probe as it is scanned through the plasma at speeds of up to 2m∕s to produce the profiles with a spatial resolution of about 0.05cm. At discharge currents of 10–25A from the 1.5-cm-diameter hollow cathode, the plasma density inside the cathode is found to exceed 1014cm−3, with the peak density occurring upstream of the orifice. The plasma potentials on axis inside the cathode are found to be in the 10–20V range with electron temperatures of 2–5eV, depending on the discharge current and gas flow rate. A potential discontinuity or double layer of less than 10V is observed in the orifice region, and under certain conditions appears in the bright “plasma ball” in front of the cathode. This structure tends to change location and magnitude with discharge current, gas flow, and orifice size. A potential maximum proposed in the literature to exist in or near the cathode orifice is not observed. Instead, the plasma potential increases from the orifice exit both radially and axially over several centimeters to values of 5–10V above the anode voltage. The potential and temperature profiles inside the cathode are insensitive to anode configuration changes that alter the discharge voltage at a given flow. Application of an axial magnetic-field characteristic of the cathode region found in ring-cusp ion thrusters increases the plasma density in the cathode plume, but does not significantly change the potential or temperature. Measurements of the plasma profiles and the internal cathode parameters for a hollow cathode operating at discharge currents of up to 25A in xenon are shown and discussed.

The successful performance of the NSTAR ion thruster in Deep Space 1 mission, coupled with the recently completed 30,032 hour life test of the flight spare thruster, has accelerated the implementation of electric propulsion in NASA missions. However, while the NSTAR thruster was still operational when the life test stopped, post-test analysis showed that the face-plate of the keeper electrode had completely eroded away and the cathode assembly was nearing end of life. Proposed nuclear electric propulsion missions require life of thrusters to exceed 10 years, and the life of the cathode assembly and emitter are of obvious concern. In an effort to diagnose the cathode keeper erosion and establish the cathode insert emitter life, several different probe diagnostics have been used internal and external to the hollow cathode to measure the local plasma parameters. Axially scanning miniature high speed pneumatic Langmuir probes have been used to investigate the plasma parameters inside the cathode orifice and in the keeper region. A radially-scanning probe miniature pneumatic Langmuir probes have been used to generate plasma density and potential profiles directly in front of the keeper. A retarding potential analyzer has detected ions in the discharge with energies significantly in excess of the discharge voltage and the source of the ions has been found to be turbulent plasma oscillations in the cathode plume localized near the keeper exit. Measurements of the plasma parameters internal to the cathode and external to the keeper will be presented for discharge currents up to 13 A.

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