Electron Emission Model of Ba Dispenser Scandate Cathodes Based on the Beamlet Effect

Abstract

Currently Ba scandate cathodes exhibit the highest electron emission capability of all thermionic cathodes and are very promising for future applications. They are essentially based on Ba dispenser matrix cathodes modified with differently distributed additions of scandia. The high emission capability is due to a very low work function of 1.15 eV for different types of Ba scandate cathodes but is also accompanied by a low Richardson constant in the range from 2 to 8 A cm⁻² K⁻², depending on the type, and is also an indication of incomplete surface coverage. High resolution characterization after activation and some operation time have shown that Sc is concentrated in small patches on the surface, which also contain Ba and O. Based on the beamlet effect first introduced by Hasker, it is shown by model calculations that the high emission and the corresponding Richardson constant of a given type can be quantitatively explained from the sum of beamlets or micro-beams originating from these fine patches, which are rather uniformly distributed over the impregnated cathode surface.