Hollow cathode theory and experiment. I. Plasma characterization using fast miniature scanning probes

Journal Article · Thu Dec 01 00:00:00 EST 2005 · Journal of Applied Physics

DOI:https://doi.org/10.1063/1.2135417· OSTI ID:20714139

Goebel, Dan M; Jameson, Kristina K; Watkins, Ron M; Katz, Ira; Mikellides, Ioannis G ^[1]

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1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (United States)

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 0.05 cm in diameter with a probe tip area of 0.002 cm{sup 2}. 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 2 m/s to produce the profiles with a spatial resolution of about 0.05 cm. At discharge currents of 10-25 A from the 1.5-cm-diameter hollow cathode, the plasma density inside the cathode is found to exceed 10{sup 14} cm{sup -3}, with the peak density occurring upstream of the orifice. The plasma potentials on axis inside the cathode are found to be in the 10-20 V range with electron temperatures of 2-5 eV, depending on the discharge current and gas flow rate. A potential discontinuity or double layer of less than 10 V 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 5-10 V 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 25 A in xenon are shown and discussed.

• OSTI ID: 20714139
• Journal Information: Journal of Applied Physics, Vol. 98, Issue 11; Other Information: DOI: 10.1063/1.2135417; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-8979
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
ANODES
ELECTRON TEMPERATURE
EV RANGE 01-10
GAS FLOW
GLOW DISCHARGES
HOLLOW CATHODES
ION THRUSTERS
LANGMUIR PROBE
MAGNETIC FIELDS
PLASMA
PLASMA DENSITY
PLASMA INSTABILITY
PLASMA POTENTIAL
PLASMA RADIAL PROFILES
PLASMA SHEATH
SAWTOOTH OSCILLATIONS
SPATIAL RESOLUTION
TEMPERATURE MEASUREMENT
WAVE FORMS
XENON