Abstract
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.
Highlights
The intense coherent x-ray beams generated by an x-ray free-electron laser (XFEL) are an extremely powerful tool for conducting groundbreaking experiments on the fundamental processes of chemistry, material science, and biology
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)
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
Summary
The intense coherent x-ray beams generated by an x-ray free-electron laser (XFEL) are an extremely powerful tool for conducting groundbreaking experiments on the fundamental processes of chemistry, material science, and biology. The appropriate energy chirp at the injector exit is formed by a C-band rf cavity, so that a peak current of 3 kA is obtained after the magnetic bunch compressors, without overbunching. When a low-energy electron beam is deflected by the geomagnetic field, chromatic dispersion is caused, which increases the projected emittance To avoid this deterioration, air coils were used to compensate for the geomagnetic field in SACLA. We found that the short-term laser-pointing stability was strongly correlated with the electron beam orbit inside the injector The source of this instability and a method of overcoming this problem are described
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