Objective Under the financial support of Natural Science Foundation of China, a new infrared free-electron laser (FEL) facility, named FELiChEM, is being developed at University of Science and Technology of China in Hefei. It will be a user facility dedicated for energy chemistry research, and will deliver the infrared laser in the spectral range of 2.5--200 μm to five research stations. National Synchrotron Radiation Laboratory is responsible for the development of the FEL light source and optical beam line.

Methods FELiChEM consists of mid-infrared (MIR) and far-infrared (FIR) free-electron laser oscillators driven by one 60 MeV linac. The linac mainly contains: a 100 kV electron gun, a 476 MHz subharmonic standing wave pre-buncher, a 2856 MHz fundamental frequency traveling-wave buncher and two 2856 MHz fundamental frequency traveling-wave accelerating tubes separated by a magnetic compressor (chicane). Now we have achieved the electron beam with the maximum electron energy of 65 MeV, root mean square value of energy spread of less than 240 keV, normalized transverse emittance of 20--50 mm·mrad and bunch charge of 1.0--1.5 nC. The time structure of the electron beam can be easily tuned with the macrobunch width of less than 10 μs, macrobunch repetition rate of 1--10 Hz and optional microbunch repetition rate within 238,119,59.5,29.75 MHz. The two oscillators cover the wavelength ranges of 2.5--50 μm and 40--200 μm, respectively. In each oscillator, two spherical mirrors with copper base and gold coatings are used to form a symmetrical optical cavity. Two planar permanent-magnet undulators are placed in the center of the two optical cavities. The cavity length for both two oscillators is 5.04 m. The infrared radiation is outcoupled from the downstream cavity, which contains multiple mirrors with different outcoupling hole sizes and mechanical conditioners for switching mirrors. Laser beam generated from the two oscillators transports through the same beam line in low vacuum environment (~10 ^-2Pa) to the experimental hall. The total length of the beam line is about 34 m. In the experimental hall, five research stations in one line are: photon dissociation endstation, photon excitation endstation, and photon detection endstation (containing 3 sub-endstations: sum frequency generation spectroscopy, nanoscale infrared spectroscopy, and in-situ reflection-absorption infrared spectroscopy).The layout of FELiChEM is shown in Fig. 1.

Results and Discussions The design of FELiChEM started from 2015. In July 2018, we began to commission the linac and made the high-quality electron beam travel through the mid-infrared oscillator in one month. Then, after finishing the installment of the optical beamline in May 2019, we successfully detected the spontaneous radiation at the research station in the morning of June 9, 2019 and we achieved the first FEL lasing at 15 μm with the electron beam energy of 35 MeV in the afternoon of the same day. After several months we shut down the machine to install the online laser alignment system and the FEL diagnostics station. We restarted the commissioning of the MIR oscillator from May 2020, and until November 2020, the MIR oscillator had achieved the task object. It generates the laser beam covering the spectral range of 2--50 mm with four electron beam energies of 60 MeV, 48 MeV, 30 MeV and 22 MeV, as shown in Fig. 4. The continuous wavelength tunability is about 300%. Based on the electron beam with the macro-pulse width of 10 μs and micro-pulse repetition rate of 119 MHz, the maximum macro-pulse energy reaches 182 mJ at the window exit of photon dissociation endstation, while the maximum micro-pulse energy is about 170 μJ. In the wavelength range of 6--10 μm, the macro-pulse energy exceeds 100 mJ. An obvious power gap appears around the wavelength of 20.5 μm and the corresponding wavelength of this power gap will be moved to above 50 μm by changing the size of the waveguide in the future. At the wavelengths longer than 40 μm, the laser intensity drops rapidly mainly because the transmittance of the cesium iodide (CsI) window decreases rapidly with the increase of laser wavelength. The spectra measured by the grating spectrometer demonstrated that the full width at half maximum of FEL bandwidth was about 1% (Fig. 5). Currently, each research station is commissioning their instruments online with the mid-infrared FEL, and we will start the commissioning of the far-infrared oscillator soon in 2021.

Conclusions The mid-infrared oscillator of FELiChEM has achieved its task object and can provide 2--50 μm laser for research stations. The commissioning of the far-infrared oscillator will start soon. We expect to finish the commissioning of the FEL source in 2021, and then the facility will be fully open to the users, and at the meantime we will make continuous efforts to improve the stability and reliability of this FEL machine.