The design and performance of a soft X-ray free-electron laser (FEL) beamline of the SPring-8 Compact free-electron LAser (SACLA) are described. The SPring-8 Compact SASE Source test accelerator, a prototype machine of SACLA, was relocated to the SACLA undulator hall for dedicated use for the soft X-ray FEL beamline. Since the accelerator is operated independently of the SACLA main linac that drives the two hard X-ray beamlines, it is possible to produce both soft and hard X-ray FEL simultaneously. The FEL pulse energy reached 110 µJ at a wavelength of 12.4 nm (i.e. photon energy of 100 eV) with an electron beam energy of 780 MeV.

A high-voltage pulsed electron gun has been developed for the low-emittance injector system of the x-ray free electron laser (FEL) project at SPring-8. A single-crystal ${\mathrm{CeB}}_6$ cathode was chosen as a thermionic emitter because of its excellent emission properties, i.e., smooth surface, high emission density, uniform emission density, and high resistance to contamination. A gun voltage of $-500\mathrm{kV}$ was determined as a compromise between the need for suppressing emittance growth and reducing the risks of high-voltage arcing. We have succeeded in producing a 500 keV beam with 1 A peak current and $3\mu \mathrm{s}$ width. We have also measured the beam emittance by means of the double-slit method and obtained an extremely low emittance of $1.1\pi \mathrm{mm}\mathrm{mrad}$ (normalized, rms). These results are very promising for the x-ray FEL. In this paper, we describe the design and basic performance of the ${\mathrm{CeB}}_6$ electron gun, and report the emittance measurement experiments.

SPring-8-II is a major upgrade project of SPring-8 that was inaugurated in October 1997 as a third-generation synchrotron radiation light source. This upgrade project aims to achieve three goals simultaneously: achievement of excellent light source performance, refurbishment of aged systems, and significant reduction in power consumption for the entire facility. A small emittance of 50 pm rad will be achieved by (1) replacing the existing double-bend lattice structure with a five-bend achromat one, (2) lowering the stored beam energy from 8 to 6 GeV, (3) increasing the horizontal damping partition number from 1 to 1.3, and (4) enhancing horizontal radiation damping by installing damping wigglers in long straight sections. The use of short-period in-vacuum undulators allows ultrabrilliant X-rays to be provided while keeping a high-energy spectral range even at the reduced electron-beam energy of 6 GeV. To reduce power consumption, the dedicated, aged injector system has been shut down and the high-performance linear accelerator of SACLA, a compact X-ray free-electron laser (XFEL) facility, is used as the injector of the ring in a time-shared manner. This allows the simultaneous operation of XFEL experiments at SACLA and full/top-up injection of the electron beam into the ring. This paper overviews the concept of the SPring-8-II project, the system design of the light source and the details of the accelerator component design.

We propose a novel method to achieve a highly brilliant electron bunch with a peak current of several kA in an x-ray free-electron laser (XFEL) by efficiently correcting nonlinearity of the bunch-compression system. In this method, a second-order positive energy chirp is imprinted to the electron bunch by a correction cavity installed at the beginning of the system, namely the nonlinearity is initially overcorrected. The imprinted chirp is enhanced in a compression process at a bunch compressor due to electron bunch rotation in longitudinal phase space, and then continuously corrects the second-order nonlinearity induced in the following sections. Since the frequency and field amplitude of the correction cavity can be significantly lowered to the available technology level, this method enables the use of a high-frequency accelerator, i.e., a high-gradient accelerator, which is indispensable to realize a compact XFEL facility.

The time evolution of beam properties in an electron bunch with the duration of a nanosecond was measured with a time resolution of several tens of picoseconds. A combination of horizontal and vertical slits cuts the beamlet from the original beam, with the current waveform of the beamlet measured using a fast wall-current monitor. The reconstruction of the waveform data obtained by scanning these two slits over the entire beam area provided the time evolution of the spatial profile. A similar measurement using two horizontal (vertical) slits separated by a certain distance also provides the time evolution of the phase–space profile. Using this method, the initial beam extracted from the CeB6 thermionic electron gun of the x-ray free-electron laser (XFEL) SACLA was evaluated. Although the slice emittance in the bunch was measured to be constant, the centroid of the spatial profile moved in the transverse direction by a few hundred micrometers in the 0.6 ns flat-top region. This movement arises from the temporal variation in the rectangular high-voltage pulse of the beam chopper and can cause an increase in the projected emittance. These measurements are important for evaluating the conditions of the initial beam emitted from the cathode and processed downstream of the gun. Hence, the proposed diagnostic system will play an important role in developing an extremely low-emittance electron beam or an artificial electron beam with a multi-bunch or micro-bunch structure that enhances the brightness of the XFEL light.