Parametric Investigation of Orifice Aspect-ratio on Low Current Hollow Cathode Power Consumption

As part of an effort to develop efficient hollow cathodes for the International Space Station and low power electric propulsion systems, the effects of orifice length-to-diameter aspect-ratio on the performance of 0.5 to 2 A hollow cathodes were examined to compare with previous experiments and a recently developed model. Cathodes were constructed with nominally identical orifice diameters and lengths that varied from 1 to 6 diameters. The performance of the cathodes was evaluated at the beginning of life and after at least 50 hours of operation. The data generally followed the trends predicted by the model with a few exceptions which appear to be related to the thermal environment of the cathode. Power consumption scaled with the orifice aspect-ratio, while the minimum spot mode flow rate was inversely related to the length-to-diameter ratio. At currents below 1.0 A, the cathode operation became more complex than that which is assumed in the orifice model; low operating temperature caused instabilities in several of the devices. * Graduate Student Researcher, Student Member AIAA † Associate Professor, Senior Member AIAA ‡ Research Engineer, Member AIAA Copyright © 1998 by Matthew T. Domonkos. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. Nomenclature AR = aspect-ratio of the orifice c = speed of light, 3 x 10 m/s h = Planck’s Constant, 6.63 x 10 J-s k = Boltzmann’s Constant, 1.38 x 10 J/K n = index of refraction T = temperature, K ε = emissivity λ = wavelength, m Introduction A parametric investigation has been conducted at the NASA Lewis Research Center in order to evaluate the effect of orifice aspect-ratio on the performance of low current hollow cathodes. The aspect-ratio is defined as the orifice length divided by its diameter. The NSTAR derivative ion thruster for the Deep Space 4 mission, the plasma contactor for the International Space Station, and the NASA Lewis 8 cm ion thruster program all have requirements for low flow rate, low power cathodes. Experimental data exist both to support and to refute the hypothesis that cathode performance is inversely proportional to the orifice aspect-ratio. A recent hollow cathode model developed by Mandell and Katz describes the physics leading to the conclusion that cathode performance scales inversely with orifice aspect-ratio. Ions entering a finite length orifice will undergo collisions as they traverse the channel. Ions lost to collisions with the walls must be balanced by ions created in the orifice in order to satisfy the condition of a quasineutral plasma at the exit of the orifice. As orifice aspect-ratio increases, the number of ionizations occurring increases, and the conductivity of the plasma in the orifice decreases. The energy transport within the orifice also varies inversely with the aspect-ratio, and modeling the orifice processes becomes increasingly important to making an accurate prediction about the performance of the cathode. The physical arguments for the importance of orifice aspect-ratio to cathode performance are sufficiently compelling to warrant a new evaluation of the experimentally observed effects, particularly when trying to minimize the power consumption during low current operation. This report summarizes the experimental findings concerning the impact of