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 °C. 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-to- keeper gap and downstream of the keeper. The structure of the data enabled analysis of the current conduction processes in spot and plume modes.
