Abstract
This paper presents a method that uses a gap scan of the phase shifter to optimize the intensity of a free-electron laser (FEL) by matching its phase with that of the electron beam. Phase shifters are essential instruments, especially for a long undulator line, which is segmented by drift sections. The phase-matched (in-phase) and the 180\ifmmode^\circ\else\textdegree\fi{} offset (out-of-phase) conditions are investigated using linear theory, FEL simulations, and experiments to understand how the phase shifter affects FEL amplification. We show that the FEL intensity is dominantly reduced by phase mismatch in the saturation region, where the microbunched electron beam is sufficiently developed, and that the difference of FEL intensity between the in-phase and out-of-phase conditions is an effect of evolution of the bunching factor. At the Pohang Accelerator Laboratory x-ray Free-Electron Laser (PAL-XFEL), the gap scan of the phase shifter at 9.7 keV increased FEL intensity by 4 times compared to the calculated gap of the phase shifter. This intensity increase was obtained dominantly in the saturation region.
Highlights
Most x-ray free-electron laser (XFEL) facilities [1,2,3,4,5] require a long undulator line to obtain sufficient freeelectron laser (FEL) power
Most x-ray free-electron laser (XFEL) facilities [1,2,3,4,5] require a long undulator line to obtain sufficient FEL power. Such a line is normally divided into drift sections, each of which includes a quadrupole to focus electron beams, a corrector to control the electron beam path, a beam position monitor, a beam loss monitor, and a phase shifter to match the phase between the electron beam and the x-ray beam [6]
While the electrons are being bunched as the electron beam propagated further down the undulator modules, the phase bucket can lose electrons when the phase shifter is the out-of-phase condition; this observation demonstrates that FEL optimization using phase shifters is very important in the saturation region with microbunching of the electron beam
Summary
Most x-ray free-electron laser (XFEL) facilities [1,2,3,4,5] require a long undulator line to obtain sufficient FEL power. About the optimal condition of the phase shifter, the Spring-8 Angstrom Compact free-electron Laser (SACLA) reported the changes of the radiation intensity according to the gap of the phase shifter [10], and the European XFEL investigated the spectrum changes by phase mismatch [12]. The key idea is to set the dominant phase of the electron beam to zero, which is the so-called “synchronous phase,” before the beam enters the undulator segment This adjustment ensures that the electrons can stay in the FEL’s amplification region and that the lasing process of FEL can be satisfied. The gap scan of the phase shifter can be conducted to optimize an FEL-lasing condition in various undulator taper configurations with variable-gap undulators [16] Such a process contributes to a reliable performance which has been realized at the Pohang Accelerator Laboratory x-ray Free-Electron Laser (PAL-XFEL) [17].
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