Hollow Cathode Discharge Initiation and Fast Starting Cathode

Conditioning of the insert inside a hollow cathode is believed to be important to ensure reliable discharge initiation and subsequent cathode operation. The desorption of contaminates is reported from hollow cathodes as a function of heater power to quantify cathode conditioning characteristics and identify factors that might prevent the fast initiation of a discharge. In addition to conditioning, a study of hollow cathode emission characteristics under no gas flow (i.e., vacuum) conditions was performed to judge the health of the insert and cathode surfaces. Results of these two studies are described herein along with measurements of the temperature of the cathode as a function of time and heater power. The data are used to validate a transient surface diffusion model that describes the migration and surface coverage fraction of low-work function material from the interior of the hollow cathode to the orifice barrel and to the exterior surface of the orifice plate of a conventional, orificed hollow cathode. A sufficiency condition is presented for our cathode geometry of vacuum emission current level and the ability to start a hollow cathode discharge once gas flow is initiated. Based on the results of the modeling effort and experimental validation campaign, a prototype fast-starting cathode with a directly heated insert design was fabricated. This configuration does not use a traditional hollow cathode tube with an orifice plate. Instead, the insert is simply supported by the coils of a swaged tantalum heater, which allows heat to be transferred directly from the heater to the insert. Therefore the insert heats up and starts to release low work function material faster than in the traditional configuration. More importantly the surface of the insert starts emitting electrons immediately once low work function material is released, while in a traditional hollow cathode the low work function material must first diffuse from the interior of the cathode to the surface of the orifice plate before electron emission becomes possible. Measurements of vacuum emission current on the fast starting cathode are presented that confirm the advantages described above.