CeB₆ electron gun for low-emittance injector

A high-voltage pulsed electron gun has been developed for the low-emittance injector system of the x-ray free electron laser (FEL) project at SPring-8. A single-crystal CeB₆ cathode was chosen as a thermionic emitter because of its excellent emission properties, i.e., smooth surface, high emission density, uniform emission density, and high resistance to contamination. A gun voltage of -500  kV was determined as a compromise between the need for suppressing emittance growth and reducing the risks of high-voltage arcing. We have succeeded in producing a 500 keV beam with 1 A peak current and 3   μs width. We have also measured the beam emittance by means of the double-slit method and obtained an extremely low emittance of 1.1π  mm mrad (normalized, rms). These results are very promising for the x-ray FEL. In this paper, we describe the design and basic performance of the CeB₆ electron gun, and report the emittance measurement experiments.