Ions with energies significantly in excess of the applied discharge voltage have been reported for many years in hollow cathode discharges. Models of dc potential hills downstream of the cathode and instabilities in postulated double layers in the cathode orifice have been proposed to explain this, but have not been substantiated. Measurements of the dc and rf plasma density and potential profiles near the exit of hollow cathodes by miniature fast-scanning probes suggests that turbulent ion acoustic fluctuations and ionization instabilities in the cathode plume significantly increase the energy of the ions that flow from this region. Increases in the discharge current and/or decreases in the cathode gas flow enhance the amplitude of the fluctuations and increase the number and energy of the energetic ions, which increases the erosion rate of the cathode electrodes. The transition from the quiescent “spot mode” to the noisy “plume mode” characteristic of these discharges is found to be a gradual transition of increasing fluctuation amplitudes.
Measurements, simplified analyses, and two-dimensional numerical simulations with a fluid plasma model show that classical resistivity cannot account for the elevated electron temperatures and steep plasma potential gradients measured in a 25–27.5A electric propulsion hollow cathode. The cathode consisted of a 1.5cm hollow tube with an ∼0.28cm diameter orifice and was operated with 5.5SCCM (SCCM denotes cubic centimeter per minute at STP) of xenon flow using two different anode geometries: a segmented cone and a circular flat plate. The numerical simulations show that classical resistivity yields as much as four times colder electron temperatures compared to the measured values in the orifice and near-plume regions of the cathode. Classical transport and Ohm’s law also predict exceedingly high electron-ion relative drift speeds compared to the electron thermal speed (>4). It is found that the addition of anomalous resistivity based on existing growth rate formulas for electron-ion streaming instabilities improves qualitatively the comparison between the numerical results and the time-averaged measurements. Simplified analyses that have been based largely on the axial measurements support the conclusion that additional resistivity is required in Ohm’s law to explain the measurements. The combined results from the two-dimensional simulations and the analyses bound the range of enhanced resistivity to be 3–100 times the classical value.
The cross-field electron mobility in Hall thrusters is known to be enhanced by wall collisionality and turbulent plasma fluctuations. Although progress has been made in understanding the plasma-wall interaction and instabilities responsible for the anomalous transport, a predictive model based on the underlying physics of these processes has yet to emerge. Hybrid-PIC simulations of the Hall thruster have typically depended on semi-empirical models of the mobility to provide sufficient electron current to match experimental results. These models are capable of qualitatively predicting the plasma response over a wide range of operating conditions, but have limited quantitative capabilities unless they are calibrated with experimental data. The efficacy of several electron mobility models in reproducing the plasma response of a 6 kW laboratory Hall thruster are assessed. With respect to a two-region mobility model that is frequently reported in the literature, a three-region model for the mobility is shown to significantly improve the agreement with experimentally measured profiles of the plasma potential and electron temperature.
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