Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) is the first X-ray free electron laser user facility in China. SXFEL are also the only one which can cover the whole “water window” regime among all the three soft X-ray free electron laser user facilities in the world. SXFEL has been the essential research platform for the Shanghai Zhangjiang Comprehensive National Science Center. The total length of SXFEL is 532 meters, and there are 2 undulator lines, 2 coherent X-ray beam lines, and 5 experimental endstations at present.
This is the overall layout of SXFEL, including the injector section, the main accelerator section, the undulator section, the beam lines and the experimental endstations.
The injector section
The injector section
Including a high brightness photocathode electron gun, a laser heater, and two sets S-band accelerating structures. The electron gun generates a high-quality electron beam with about 10 ps bunch length and about 0.5 nC beam charge. The laser heater can help suppresses the microbunch instability of the electron beam, and the two sets S-band accelerating structures will increase the beam energy to 125 MeV.
The injector section
The main accelerator
Including three main accelerating sections and a magnetic bunch compressor section. The three main accelerating sections increase the beam energy up to 1.5 GeV, an X-band linearizer combined with two chicanes compress the bunch length to about 1 ps, increasing the peak current to about 700 A. The normalized beam emittance at the end of LINAC is about 1.5 mm·mrad.
The main accelerator
The undulator section
Including two undulator lines. One is the straightforward self-amplified spontaneous emission (SASE) line, termed as the SBP line, including 10 segments in-vacuum undulator (IVU16) with a period length of 16 mm and each segment 4 meters long. The SBP line can deliver FEL pulses with the shortest wavelength being down to 2 nm. The other is the seeding line after the beam distribution section, termed as the SUD line, consists of two modulators, two dispersion sections, six 3-meters-long 30 mm-period-length permanent magnet planar undulators (U30) and ten 3-meters-long 23.5 mm-period-length permanent magnet planar undulators (U235). The SUD line can produce fully coherent soft x-ray free electron laser pulses with the shortest wavelength of 3 nm.
The undulator section，left：the SUD line，right：the SBP line
