Graphite heater optimized for a low-emittance CeB6 cathode

We developed a thermionic cathode assembly using a single-crystal CeB₆ emitter for the x-ray free electron laser project at SPring-8. The CeB₆ cathode has excellent emission properties, i.e., smooth surface, high emission density, uniform emission density, and high resistance to contamination. In order to heat the cathode up to the operational temperature as high as 1500degC, we developed a new-type of graphite heater. Using this heater, a 500 keV pulsed electron beam with more than 1 A peak current can be extracted from the small surface area (3 mm diameter). In this paper, we report on the design and properties of the graphite heater for the low-emittance CeB₆ cathode.