CeB6 electron gun for the soft X-ray FEL project at SPring-8

A pulsed high-voltage electron gun with a thermionic cathode is under development for the injector system of the soft X-ray FEL project at SPring-8 (SCSS project). A CeB6 single crystal of 3 mm diameter was chosen as a thermionic cathode because of its excellent emission properties, i.e., high resistance against contamination, uniform emission density and smooth surface. The CeB6 cathode can produce a 3 A beam with 2 μs FWHM. A gun voltage of −500 kV was chosen as a compromise between the need for controlling emittance growth and minimizing the risks of high-voltage arcing. We have constructed a 500 kV electron gun test stand and have begun performance tests. This paper describes the basic design and the current status of the hardware R&D on the CeB6 gun.