Low-Current Hollow Cathode Evaluation

An experimental investigation of the operating characteristics of 3.2 mm diameter orificed hollow cathodes was conducted to examine low-current and low flow rate operation. Cathode power was minimized with low orifice aspect ratio and the use of an enclosed keeper. Cathode flow rate requirements were proportional to orifice diameter and the inverse of the orifice length. Cathode temperature profiles were obtained using an imaging radiometer, and conduction was found to be the dominant heat transfer mechanism from the cathode tube. Orifice plate temperatures were found to be weakly dependent upon the flow rate and strongly dependent upon the current. Internal cathode pressures were measured to range from 39.2 to more than 66.5 kPa for spot mode emission. A model developed to describe the mass flow achieved good agreement with the experimental cathode pressures for heavy particle temperatures between 2900 and 6100 oC. Plasma properties measured within the insert region exclude the electron partial pressure from being responsible for the elevated cathode pressures. As such, it was concluded that the average heavy particle temperature in the orifice would have to be on the order of several thousand degrees C to account for the high internal pressure. Plasma parameters were also measured in the cathode-tokeeper gap and downstream of the keeper. The structure of the data enabled analysis of the current conduction processes in spot and plume modes. * Graduate Student Research Assistant, Student Member AIAA † Associate Professor, Associate Fellow AIAA ‡ Graduate Student Research Assistant, Student Member AIAA § Research Scientist, Member AIAA Copyright © 1999 by Matthew T. Domonkos. Published by the American Institute of Aeronautics and Astronautics with permission. Nomenclature A = normalization coefficient ARx = cathode with an orifice aspect ratio of x B r = magnetic field vector, G b = coefficient in EEDF exponent Do = orifice diameter, m E = electron energy, eV EEDF = electron energy distribution function f(E) = normalized EEDF Ipr = probe current, A J r = current density vector, A/m KL = coefficient of loss Lo = orifice length, m M = molar mass of Xe, 131 g/mol m& = mass flow rate, kg/s p = pressure, Pa pc = critical sonic pressure, Pa pin = insert region pressure, Pa po = orifice region pressure, Pa R = orifice radius, m R ~ = Ideal gas constant, 8.3145 J/mol-K u = mean flow velocity, m/s Vo = velocity at the orifice exit, m/s Vp = plasma potential, V Vpr = probe voltage, V x = axial distance, m, or exponent on electron energy in EEDF γ = ratio of specific heats, 5/3 μ = dynamic viscosity, N-s/m