Note: Improved heater design for high-temperature hollow cathodes

We present an improved heater design for thermionic cathodes using a rhenium filament encased in a boron nitride ceramic sleeve. This heater is relatively simple to fabricate, yet has been successfully used to reliably and repeatably light a lanthanum hexaboride (LaB6) hollow cathode based on a previously published design without noticeable filament degradation over hundreds of hours of operation. The high decomposition temperature of boron nitride (2800 C for inert environments) and melting point for rhenium (3180 C) make this heater especially attractive for use with LaB6, which may require operating temperatures upwards of 1700 C. While boron nitride decomposes in air above 1000 C, the heater was used only at vacuum with an inert gas discharge, and no degradation was observed. Limitations of current state of the art cathode heaters are also discussed and compared with the rhenium-boron nitride combination.