Low-emittance thermionic-gun-based injector for a compact free-electron laser

A low-emittance thermionic-gun-based injector was developed for the x-ray free-electron laser (XFEL) facility known as the SPring-8 angstrom compact free-electron laser (SACLA). The thermionic-gun-based system has the advantages of maintainability, reliability, and stability over a photocathode radio-frequency (rf) gun because of its robust thermionic cathode. The basic performance of the injector prototype was confirmed at the SPring-8 compact self-amplified spontaneous emission source (SCSS) test accelerator, where stable FEL generation in an extreme ultraviolet wavelength range was demonstrated. The essential XFEL innovation is the achievement of a constant beam peak current of 3--4 kA, which is 10 times higher than that generated by the SCSS test accelerator, while maintaining a normalized-slice emittance below 1 mm mrad. Thus, the following five modifications were applied to the SACLA injector: (i) a nonlinear energy chirp correction; (ii) the optimization of the rf acceleration frequency; (iii) rf system stabilization; (iv) nondestructive beam monitoring; and (v) a geomagnetic field correction. The SACLA injector successfully achieved the target beam performance, which shows that a thermionic-gun-based injector is applicable to an XFEL accelerator system. This paper gives an overview of the SACLA injector and describes the physical and technical details, together with the electron beam performance obtained in the beam commissioning.