Construction and RF conditioning of EUV-FEL accelerator for SACLA

In order to increase an opportunity of free-electron laser (FEL) user experiments and to expand generation region of the laser wavelength, we constructed an accelerator for the SACLA beamline-1, which is use for an extreme ultraviolet-FEL (EUV-FEL). Many of the accelerator components, such as an electron gun, an injector part and an RF power sources, were relocated from those of the SCSS test accelerator, which was the prototype machine for SACLA and whose operation was terminated in 2013. A C-band accelerating structure was a newly designed disk-loaded type structure in order to obtain a high shunt impedance and to increase the electron beam energy within the limited space. The permanent magnet segments of an undulator were replaced to SACLA type with 18 mm periodic-length from the previous 15 mm periodic-length. We expect the electron beam energy is 420 MeV, the wavelength and the pulse energy of the EUV-FEL is at 42 nm and 100 μJ, respectively. Installation of the accelerator components was performed during summer shutdown periods in 2013 and 2014. An RF conditioning was started in autumn 2014. Targets of a trip rate of an accelerating structure after the rf conditioning aims at 1 times a hour below. After the RF conditioning of 350 hours, the acceleration gradient of the C-band accelerator was 46 MV/m by a conditioning period of 350 hours. The trip rate decreased to be less than 0.27 times an hour with an acceleration gradient of 42 MV/m. (author)