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Title: Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region

Abstract

Despite their long history and wide range of applicability that includes electric propulsion, detailed understanding of the driving physics inside orificed hollow cathodes remains elusive. The theoretical complexity associated with the multicomponent fluid inside the cathode, and the difficulty of accessing empirically this region, have limited our ability to design cathodes that perform better and last longer. A two-dimensional axisymmetric theoretical model of the multispecies fluid inside an orificed hollow cathode is presented. The level of detail attained by the model is allowed by its extended system of governing equations not solved for in the past within the hollow cathode. Such detail is motivated in part by the need to quantify the effect(s) of the plasma on the emitter life, and by the need to build the foundation for future modeling that will assess erosion of the keeper plate. Results from numerical simulations of a 1.2-cm-diam cathode operating at a discharge current of 25 A and a gas flow rate of 5 SCCM show that approximately 10 A of electron current, and 3.45 A of ion current return back to the emitter surface. The total emitted electron current is 33.8 A and the peak emitter temperature is found to bemore » 1440 K. Comparisons with the measurements suggest that anomalous heating of the plasma is possible near the orifice region. The model predicts heavy species temperatures as high as 2034 K and peak voltage drops near the emitting surface not exceeding 8 V.« less

Authors:
; ; ;  [1]
  1. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109 (United States)
Publication Date:
OSTI Identifier:
20714140
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Applied Physics; Journal Volume: 98; Journal Issue: 11; Other Information: DOI: 10.1063/1.2135409; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; AXIAL SYMMETRY; ELECTRON EMISSION; ELECTRON TEMPERATURE; ELECTRONS; EQUATIONS; EROSION; GAS FLOW; GLOW DISCHARGES; HOLLOW CATHODES; ION TEMPERATURE; PLASMA; PLASMA HEATING; PLASMA SIMULATION; TEMPERATURE DEPENDENCE; TEMPERATURE RANGE 1000-4000 K; TWO-DIMENSIONAL CALCULATIONS; VOLTAGE DROP; WALL EFFECTS

Citation Formats

Mikellides, Ioannis G., Katz, Ira, Goebel, Dan M., and Polk, James E. Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region. United States: N. p., 2005. Web. doi:10.1063/1.2135409.
Mikellides, Ioannis G., Katz, Ira, Goebel, Dan M., & Polk, James E. Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region. United States. doi:10.1063/1.2135409.
Mikellides, Ioannis G., Katz, Ira, Goebel, Dan M., and Polk, James E. Thu . "Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region". United States. doi:10.1063/1.2135409.
@article{osti_20714140,
title = {Hollow cathode theory and experiment. II. A two-dimensional theoretical model of the emitter region},
author = {Mikellides, Ioannis G. and Katz, Ira and Goebel, Dan M. and Polk, James E.},
abstractNote = {Despite their long history and wide range of applicability that includes electric propulsion, detailed understanding of the driving physics inside orificed hollow cathodes remains elusive. The theoretical complexity associated with the multicomponent fluid inside the cathode, and the difficulty of accessing empirically this region, have limited our ability to design cathodes that perform better and last longer. A two-dimensional axisymmetric theoretical model of the multispecies fluid inside an orificed hollow cathode is presented. The level of detail attained by the model is allowed by its extended system of governing equations not solved for in the past within the hollow cathode. Such detail is motivated in part by the need to quantify the effect(s) of the plasma on the emitter life, and by the need to build the foundation for future modeling that will assess erosion of the keeper plate. Results from numerical simulations of a 1.2-cm-diam cathode operating at a discharge current of 25 A and a gas flow rate of 5 SCCM show that approximately 10 A of electron current, and 3.45 A of ion current return back to the emitter surface. The total emitted electron current is 33.8 A and the peak emitter temperature is found to be 1440 K. Comparisons with the measurements suggest that anomalous heating of the plasma is possible near the orifice region. The model predicts heavy species temperatures as high as 2034 K and peak voltage drops near the emitting surface not exceeding 8 V.},
doi = {10.1063/1.2135409},
journal = {Journal of Applied Physics},
number = 11,
volume = 98,
place = {United States},
year = {Thu Dec 01 00:00:00 EST 2005},
month = {Thu Dec 01 00:00:00 EST 2005}
}
  • In this paper, we describe results of self-consistent two-dimensional (x-z) particle-in-cell simulations, with a Monte Carlo collision model, of an orificed micro-hollow cathode operating in a planar diode geometry. The model includes thermionic electron emission with Schottky effect, secondary electron emission due to cathode bombardment by the plasma ions, several different collision processes, and a non-uniform xenon background gas density in the cathode-anode gap. Simulated results showing behavior of the plasma density, potential distribution, and energy flux towards the hollow cathode and orifice walls, are discussed. In addition, results of simulations showing the effect of different Xe gas pressures, orificemore » size, and cathode voltage, on operation of the micro-hollow cathode are presented.« less
  • The characteristics of two low-frequency electrostatic flute instabilities of a low-pressure hollow cathode arc discharge are reported. Mode I has azimuthal mode number m = 1, and occurs when the radial electric field is negative (directed inward) while mode II has m = -- 1 and occurs when the field is positive. The radial electric field is controlled by varying the potential of a secondary anode cylinder located close to the outer discharge radius. A linear perturbation analysis, based on the two-fluid equations, is given for a low- BETA , collisionless, cylindrical plasma column, immersed in a uniform axial magneticmore » field, having a Gaussian density profile and an arbitrary radial electric field profile. Reasonable correlation between theory and experiment is demonstrated for both modes, by comparing the calculated and measured frequencies, mode numbers, and also the eigenfunctions of the density and potential fluctuations. The instabilities are basically centrifugal flute modes, driven by E x B drift in the presence of a density gradient and modified by the velocity shear due to nonuniform E x B rotation. (auth)« less
  • 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 alsomore » 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.« less
  • Voltage-current characteristics and the Cu-II 780.8 nm laser performances are described for a novel segmented hollow cathode and for three- and four-slot hollow-anode cathode (HAC) tubes. Each of these operate at a higher voltage and with higher slope resistance than a conventional hollow cathode and produce improved laser performance. The best laser performance is obtained with the segmented tube. The application of a longitudinal magnetic field raises the discharge voltage and enhances the laser performance for the segmented tube and raises the voltage for the four-slot HAC tube. The magnetic field lowers the voltage and reduces the laser performance withmore » the three-slot HAC tube. The voltage effects are attributed to the deflection of the fast electrons by the magnetic field and represent experimental evidence for the oscillation of electrons in a hollow-cathode discharge.« less
  • Low pressure glow discharges in plane-plane geometries have been studied extensively over the years and most of their features are known from experiments and numerical simulation. If a plane cathode is replaced by a cathode with some hollow structure, then, for a certain range of conditions, the negative glows of opposite (adjacent) cathode walls overlap and the discharge behaviour dramatically changes. The voltage is lower at a constant current and the current is even several orders of magnitude higher for a given voltage than for the plane cathode. At the same time, the intensity of the light emission from themore » discharge considerably increases. This effect is called the hollow cathode effect. There are several physical phenomena which could be responsible for the big efficiency of the hollow cathode discharges. The recent investigations based on the Monte Carlo simulation of the electron kinetics have shown that the trapping of energetic electrons in the hollow cathode cavity can explain the order of magnitude of the hollow cathode effect. The configuration of the discharge tube presented in fig. 1 is used here to study the behaviour of glow discharges in a hollow cathode means of numerical simulation.« less