Infrared (IR) spectroscopy has several applications. Hefei Infrared Free-Electron Laser Facility (FELiChEM) can supply bright mid/far-infrared radiation to users and provide energy chemistry research with a powerful infrastructure. A beamline must connect the free-electron laser to the experimental stations. The beamline not only efficiently transmits infrared radiation from the laser to the experimental stations but also performs focus and diagnosis during the transmittance. This paper describes the design and performance of a beamline for a Hefei Infrared Free-Electron Laser Facility, including the general requirements, design scheme and layout, optical design, beam evolution, beam transmission, laser beam splitter, online synchronized measurement of macro pulse structure, and laser wavelength.
The beamline consisted of vacuum/prop, optical/focus, and diagnosis subsystems.
As shown in Fig. 1, the vacuum/prop subsystem contained 25 pieces of Φ200 mm stainless steel pipes, 12 sylphon bellows, 15 mirror boxes, and the corresponding support frames, pumps, and gauge valves.
The optical/focus subsystem contained two diamond windows, 13 pieces of Φ150 mm 90° parabolic/planar off-axis mirrors, two beam splitters, and five exit windows (CsI/PTX). The far- and mid-infrared lasers passed through the diamond windows. The 0.5 mm thick diamond plate was placed at the Brewster angle to avoid refractive loss because the refractive index of the diamond was extremely high and the laser was fully polarized. They were then reflected by mirrors M1 and M3 to exit the electron beam. Subsequently, the far-infrared laser was reflected to the right and merged into one beam with the mid-infrared laser reflected by M2 at M4. The beam was further reflected upward by mirror M5, to the right by mirror M6, and penetrated the shielding wall into the experimental hall. In the experimental hall, the beam was reflected upward by mirror M7 and directed forward by mirror M8. The beam splitter reflected approximately 5% for diagnosis. Mirrors M9?M13 distributed the laser to the corresponding experimental stations. All mirrors were first mounted on multidimensional fine adjustable racks, and the racks were then fixed on the flange of the mirror boxes. The focal lengths of the mirrors were optimized using a limited screen function model so that every experimental station could obtain the smallest beam spot, except for experimental station M12, which preferred a parallel beam. The beam transmittance was also optimized. A compromise between focal spot size and transmit efficiency was considered. There were approximately 60% and 50% losses for far- and mid-infrared lasers, respectively. These losses were mainly caused by the absorption of the windows and the beam splitter.
The diagnosis subsystem consisted of two beam splitters (one for far-infrared and one for mid-infrared), four mirrors, one pyroelectric detector, and one spectroscope equipped with three gratings and an arrayed pyroelectric detector (Fig. 10). We developed two synchronized data collecting/transfer circuits for the detector and an arrayed detector to meet the specific macropulse structure. The detector monitored the laser intensity to resolve the macropulses (Fig. 11). Because the bandwidth of the detector was 250 MHz, the detector could 'see' the micropulses, but could not fully resolve them as the measured width was larger than the actual width, which was several picosecond. The arrayed detector recorded the spectrum of the laser pulse using a pulse (macropulse). The diagnostic data were transferred to the EPICS, between the intervals of the macro pulses, and provided to the controlling system and the user to calibrate their data.
The study was conducted in 2015. The vacuum/prop, optical/focus, and diagnosis subsystems were installed in 2017, 2018, and 2019, respectively. The first project commissioning was conducted in 2019. After several adjustments, the designed performances were achieved, and the beamline has been stable and in operation.
After eight years, we constructed a beamline compatible for far- and mid-infrared free-electron lasers. All the designed objectives were achieved. Part of the fine adjustment and calibration may be performed further in future machine studies.
