Hollow Cathode Conditioning and Discharge Initiation Studies

Summary form only given. Orificed hollow cathodes have been developed and used as the plasma electron source for a wide variety of applications including (1) ion and Hall thrusters and (2) plasma contactors for electromagnetic tethers and spacecraft charging control. Many theoretical and experimental efforts have been performed to understand and improve the emission current level, lifetime, and reliability of these devices. The results obtained to date allow optimization of the cathode design and the models provide accurate estimates of cathode performance. Relatively little information is available, however, concerning the process of hollow cathode discharge initiation. In order to develop fast-starting cathodes, which are necessary, for example, in short-duration sounding rocket experiments, a better understanding of the cathode activation process is required. It is also important to study and optimize the process of cathode conditioning, which must precede cathode activation and which can take several hours to perform. In the present work, an experimental study of conditioning and starting processes is presented of barium-dispenser-based cathodes. An important part of this problem is the temperature field (transient and steady-state) throughout the cathode and supporting structure, and, consequently, a transient, three-dimensional finite-element thermal model has been developed to provide this information. Measurements of steady-state temperatures and transient temperature profiles are presented for different heater power levels, and these measurements are compared to the thermal model predictions. Vacuum emission current characteristics are key measures of cathode health and the dependence of the electron emission characteristics on heater power, cathode temperature, and conditioning sequence are presented. These tests are performed in tandem with cathode discharge ignition tests, and the obtained results are used to propose an approach to quickly condition and - start hollow cathode plasma discharges.