Free-electron lasers provide powerful scientific research tools for humans to understand and explore the world. Compared with traditional lasers, free-electron lasers have characteristics such as continuously adjustable wavelength over a significant range, high peak power, narrow bandwidth, and full coherence. They have also been widely used in frontier scientific research fields, such as advanced materials, condensed matter physics, atomic and molecular physics, chemistry, biology, and medicine.
In recent years, free-electron laser light sources have rapidly developed. CTFEL is the only high-power terahertz free-electron laser facility driven by a superconducting accelerator in China. However, its capabilities are limited. Currently, the Chinese Academy of Engineering Physics is developing an infrared terahertz free-electron laser facility based on CTFEL. The upgraded facility is a multifunctional user facility for frontier research in the fields of material science, spectroscopy, biology, and medicine.
The upgraded facility adopts resonant optical cavity free-electron laser technology. A straight section of the CTFEL with a 320 kV HV-DC photocathode electron gun and a 2×4-cell superconducting accelerator are used as the injector system. The driving laser and photocathode systems are upgraded for compatibility with both GaAs and Cs2Te. The main accelerator system consists of two 2×9-cell superconducting accelerator modules that can increase the electron beam energy to a maximum of 50 MeV. In addition, two new undulators (U48 and U35) are added to expand the free-electron laser frequency coverage from 0.1‒125 THz, and improve the maximum marco-pulse power to over 100 W.
The upgraded facility has three different working modes FEL, FLASH, and ERL (Fig.5). In the FEL mode, four electron beam transmission paths pass through four undulators to generate FEL radiation in different frequency bands. Notably, the electron beams can choose only one path for transmission. In the FLASH mode, after passing through the main accelerator and the first 180° deflection section, the electron beams directly enter the R101 biomedical experimental station for the X-ray FLASH irradiation experiments. In ERL mode, the facility operates in a quasi-continuous wave mode and can be used as a small energy recovery linear accelerator experimental platform for key physical and technical research on ERL. The average beam current is 1‒3 mA. The injector energy is 6‒8 MeV, and the recirculation energy, approximately 20 MeV.
Two experimental user stations, a biomedical experimental station and a material spectrum experimental station, were developed. The biomedical experimental station was located on the first floor of the laboratory. According to its function, it is divided into two line stations, R101 and R102, which predominantly include the cell biology experiment, neurobiology experiment, and X-ray FLASH radiotherapy experiment platforms. The material spectrum experiment station is located on the second floor of the laboratory, and it is divided into three line stations R201, R203, and R204, including the transient excitation loading-ultrafast spectral detection system, multi-physical field time-resolved pump detection system, off-site lithium battery-infrared THz free-electron laser experimental platform, and terahertz parameter measurement calibration experimental platform. In addition, R202 is a FEL beam quality diagnosis platform for the online real-time diagnosis of FEL parameters.
This study primarily introduces the general design of the infrared terahertz free-electron laser facility of the Chinese Academy of Engineering Physics. The upgraded facility adds two 2×9-cell superconducting accelerator modules and two undulators based on CTFEL, to increase the maximum electron energy to 50 MeV, expand the spectrum coverage to 0.1 THz‒125 THz, and realize the maximum macro-pulse average power to greater than 100 W. Through a track-shaped beamline design, an experimental energy-recovery linear accelerator research platform will be built. In addition, two user experimental stations for material spectroscopy and biomedicine are under construction.
