After the PAL-XFEL was opened to the public in 2017, beamtime for user service has increased every year to provide more opportunities for user experiments. In 2020, 2,819 hours were provided for user beamtime out of the planned 2,910 hours and the beam availability was 96.9%. The provided beamtime of 2,819 hours was a significant increase from 2,409 hours in 2019, as shown in Table 1. To further increase beamtime, the PAL-XFEL has plans for 24-hour operation and simultaneous operation of hard and soft X-ray beamlines in the near future.
| Year | Planned Beamtime | Provided Beamtime | Availability |
| 2018 | 2,012 h | 1,921 h | 95.5% |
| 2019 | 2,503 h | 2,409 h | 96.2% |
| 2020 | 2,910 h | 2,819 h | 96.9% |
FEL saturation of 0.062 nm (20 keV) was achieved for the first time in PAL-XFEL. The measured FEL energy using the e-loss scan was 408 uJ, the FEL radiation spectrum was 25.3 eV rms (0.127% of the center photon energy), and the FEL pulse duration (FWHM) was 11 fs, which corresponds to 1×1011 photons/pulse. The e-beam energy was 10.4 GeV and the undulator K was 1.4. The undulator gap scan was conducted for 20 undulators to check the FEL saturation as shown in Figure 1. Here, quadratic undulator tapering is applied for the last 6 undulators and the calculated gain length was 3.43 m.
Two-color FEL generation with a single electron bunch has been successfully demonstrated for the hard X-ray undulator line, broadening the research capabilities at the PAL-XFEL. Test experiments have been conducted at two photon energies, 9.7 keV and 12.7 keV. A pump pulse is generated with 8 upstream undulators of the self-seeding section and a probe pulse is generated with 12 downstream undulators of the self-seeding section. The photon energies of the pulses can be independently controlled by changing the undulator parameter K and the time delay between two pulses can be controlled from 0 to 120 femtoseconds by using the magnetic chicane installed at the self-seeding section.
Ultra-bright hard x-ray pulses using the self-seeded FEL were applied to the demonstration of serial femtosecond crystallography (SFX) experiments in 2020. We have consistently improved the spectral purity and peak of the self-seeded FEL using a laser heater and optimized crystal conditions over a hard x-ray range from 3.5 keV to 14.6 keV. The peak brightness for self-seeded hard x-ray pulses was enhanced to almost ten times greater than that of the SASE FEL over hard x-ray ranges. For example, the peak brightness of an x-ray at 9.7 keV is 3.2×1035 photons/(s·mm2·mrad2·0.1%BW), which is the highest peak brightness ever achieved for free-electron laser pulses. Thanks to the ultra-bright x-ray pulse with narrow bandwidth and superior spectral purity, SFX experiment results using the seeded FEL showed better data quality with high resolutions compared with that using the SASE FEL. This work has been published in Nature Photonics (https://doi.org/10.1038/s41566-021-00777-z).
