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Abstract
The continuous-wave free-electron laser (CW-FEL), based on superconducting radiofrequency (SRF) technology with an electron bunch repetition rate of up to MHz levels, is one of the most advanced light sources, providing exceptionally high average and peak-brightness FEL pulses. Among the new CW-FEL facilities worldwide, the recently proposed Dalian Advanced Light Source (DALS) occupies a unique position as an extreme ultraviolet (EUV) facility primarily designed for chemical physics research. Since the beam emittance requirement for DALS is not as stringent as that for X-ray CW-FEL facilities, a direct current (DC) gun is considered as the primary electron source, with a very high frequency (VHF) gun also planned. To demonstrate key technologies and characterize the electron beam performance, a superconducting CW injector testbed, named the Electron Source Test Facility (ESTF), has been designed and is currently under construction. The testbed is uniquely designed to accommodate both guns with minimal switching effort, where the rest of the beamline layout remains unchanged except for the swapped guns. The two-gun switching scheme for the testbed is shown to be a feasible and cost-effective approach. Furthermore, the injector performance with both guns has been evaluated through a start-to-end simulation based on the DALS configuration, including the production of electron beam in the ESTF injector, the following beam acceleration and compression in a superconducting linear accelerator, and finally the beam lasing performance in the undulator section. The evaluation confirms that the DC gun is a promising electron source for CW-FEL facilities, especially for EUV applications, even though all currently constructed CWfacilities have employed the VHF gun. This paper provides a comprehensive description of the injector design and the corresponding performance evaluation.
Highlights
Free-electron laser (FEL) facilities that can provide high peak brightness FEL pulses have enabled groundbreaking research in both fundamental and applied science
To verify the injector performance, the beams produced by the Electron Source Test Facility (ESTF) injector were tracked in the Dalian Advanced Light Source (DALS) superconducting radiofrequency (SRF) linear accelerator (LINAC) section and subsequently evaluated for FEL lasing performance in the undulator
These results suggest that the ESTF injector can provide promising beams for DALS, and the direct current (DC) gun can fulfill FEL lasing requirements in the extreme ultraviolet (EUV) region with performance comparable with that of the very high frequency (VHF) gun
Summary
Free-electron laser (FEL) facilities that can provide high peak brightness FEL pulses have enabled groundbreaking research in both fundamental and applied science. Dalian Coherent Light Source (DCLS) is the first high-gain FEL user facility in China, achieving first lasing in 2016 (Wang, 2017) It provides FEL pulses in the wavelength range 50– 150 nm with a maximum repetition rate of 50 Hz, constrained by the use of NCRF technology (Sun et al, 2024; Wang et al, 2024). Compared with very high frequency (VHF) guns chosen by LCLS-II and SHINE, DC guns with lower cathode surface field and voltage offer the following advantages that could benefit facility operation and future upgrade to even higher average brightness For both VHF and DC guns the photocathode drive laser needs to be synchronized to the RF of the accelerator, DC guns themselves do not require synchronization systems and low-level RF systems for their direct-current voltage operation, thereby simplifying operation and reducing costs. This paper is organized as follows: Section 2 focuses on the beam dynamics optimization of the injector, Sections 3 and 4 present the LINAC design and FEL lasing simulation to verify the injector performance, and Section 5 concludes the study
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