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

Abstract

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.

Gallium Nitride Film Growth Using a Plasma Based Migration Enhanced Afterglow Chemical Vapor Deposition System

“…A value of 9 Â 10 11 cm À3 was found for a plasma generated with an RF power of 600 W at 750 mTorr, which seems a reasonable value given the range of values that have been reported for hollow cathode sources, [21][22][23] and is similar to values expected for inductively coupled plasma sources. 24) It should be noted that a parrallel plate capacitively coupled source would only give a value of about 1 Â 10 10 cm À3 , 24) and that the RF electrode for our source would actually have an electron density somewhat higher than the value measured here for the grounded hollow cathode array.…”

Enhanced performance of cylindrical Hall thrusters

“…This effect can be partially explained by a reduced cathode voltage drop required to sustain the thruster discharge at low currents. 16 For the 2.6 cm CHT operated at the discharge voltage of 250 V and the anode gas flow rate of 4 sccm, the maximum reduction of a half plume angle was almost 20' (from 74' to about 55'), when the discharge current was increased from 0.57A to 0.66A.…”

The use of hollow cathodes in deposition processes: A critical review

“…If excessive electrical power is applied to a hollow cathode system, the energy deposited in the cathode causes it to reach high temperatures [115,116,119,174,175]. Obviously, the plasma power versus cathode temperature increase depends very much on the degree of cooling, the electrode geometry, gas type, and pressure.…”