High-gradient C -band linac for a compact x-ray free-electron laser facility

An electron linac using a C-band rf frequency, 5.712 GHz, has enabled us to obtain an acceleration gradient of more than 35 MV/m reliably. A C-band accelerator system has been developed and constructed for the compact x-ray FEL facility, SACLA, in order to fit within the available site length at SPring-8, and to reduce construction costs. An accelerator unit consists of two 1.8 m-long accelerator structures, a cavity-type rf pulse compressor and a 50 MW pulsed klystron. In order to achieve a compact rf source and to obtain extremely stable rf fields in the accelerator structures, an oil-filled, high-voltage pulse modulator combined with an extremely stable, inverter-type, high voltage charger was developed. SACLA uses 64 sets of these accelerator units in order to achieve a final beam energy of 8.5 GeV. After rf conditioning for 1 700 hours, the maximum acceleration gradient achieved was 38 MV/m. The typical trip rate for each accelerator unit at 35 MV/m and 30 pps is about once per day. Dark current from the accelerator structures is less than 5 pC, which causes a negligible effect on the beam monitors. The phase and amplitude stability of the rf fields were measured to be 0.03 degree and 0.01% rms, respectively, which is sufficient for the XFEL operation of SACLA. Since the first beam commissioning in 2011, the C-band accelerator has demonstrated fairly stable performance under continuous operation for 20 000 hours.