Abstract
An electron linac using a $C$-band rf frequency, 5.712 GHz, has enabled us to obtain an acceleration gradient of more than $35\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ 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 $38\text{ }\text{ }\mathrm{MV}/\mathrm{m}$. The typical trip rate for each accelerator unit at $35\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ 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.
Highlights
The x-ray free-electron laser (XFEL) is a fourthgeneration light source, which provides ultrabrilliant, highly coherent, femtosecond x-ray pulses to many frontier experiments
The XFEL radiation is generated by extremely high-dense, high-energy electron beams that pass through a long undulator after being accelerated in a linac
A quadrupole magnet, a steering magnet, electron beam monitors [13] and a gate valve were installed between every four accelerator units, which are grouped as a “section.” In the SPring-8 Angstrom compact free-electron laser (SACLA) linac, 12 accelerator units are used from BC2 to BC3, and 52 units are used after BC3
Summary
The x-ray free-electron laser (XFEL) is a fourthgeneration light source, which provides ultrabrilliant, highly coherent, femtosecond x-ray pulses to many frontier experiments. In the world’s first XFEL facility, the Linac Coherent Light Source (LCLS) [1] at SLAC, one-third of the existing S-band linac was reused to accelerate an electron beam up to 14 GeV, with an acceleration gradient of about 17 MV=m. If the pulsed power from an rf source is fixed, the shorter pulse length allows us to obtain higher peak power, Pin, by using an rf pulse compression technique, such as SLAC’s energy doubler, SLED [3] For these reasons, the acceleration gradient is roughly proportional to its operating rf frequency. When our XFEL project was proposed in 2001 [9], a 50-MW class C-band (5.712 GHz) klystron was commercially available For these reasons, a C-band frequency linac was considered the most suitable for a compact XFEL facility to obtain a high acceleration gradient reliably and to have manageable alignment tolerances. The operational performance of the C-band accelerator is discussed
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