Investigation of Cylindrical Anode Diameter Influence on Hollow Cathode Operating Characteristics

This research delves into the effects of cylindrical anode dimensions on the steady-state discharge of open-ended heaterless hollow cathodes. The study assessed anode diameters of 38 mm, 74 mm, and 102 mm. Challenges arose when utilizing the 102 mm diameter anode in diode mode, with a suspected divergent electric field stemming from the anode's size and closeness hindering consistent discharge between the cathode and anode. The cathode was set to operate in triode mode with the keeper adjusted to a discharge current of 1.2 A to navigate this impediment, ensuring the axial electric field's adequacy. In contrast, tests with 74 mm and 38 mm anodes were conducted in diode mode, with the keepers kept unpowered. Analysis of the I-V curves underscored a typical negative resistance in the cathode discharge, a phenomenon attributable to the diminishing emitter sheath voltage with elevating electron energy flux and emitter surface temperatures. A pronounced inverse correlation between the steady-state discharge current and gas flow rate was also discerned, connected to rising emitter temperatures and enhanced plasma drift resistance in the keeper orifice. Notably, anode diameter emerged as a determinant of current leakage, with the 74 mm anode presenting a reduced discharge voltage compared to the 38 mm due to augmented current leakage. Conversely, the 102 mm anode demanded a higher voltage, a requirement linked to its attenuated axial electric field, compelling a rise in voltage for a sufficient current draw to the anode power supply. These discoveries furnish understanding for refining open-ended heaterless hollow cathode designs and pave the way for subsequent inquiries into cathode plumes.