Feasibility study of the shorter focusing lattice for soft X-ray undulator line at PAL-XFEL

At Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL), there is an ongoing consideration of employing elliptically polarizing undulators throughout the entire undulator line to enable polarization control in the soft X-ray undulator line. To utilize the elliptically polarizing undulators, the shorter focusing lattice for soft X-ray undulator line at PAL-XFEL is proposed. The FEL performance of the shorter focusing lattice is estimated by using Ming Xie’s fitting formula and is compared with the current focusing lattice. Due to the increased need for intersections in the shorter focusing lattice, there is a challenge where the effective length of the undulator becomes shorter compared to the current focusing lattice within the same length of the undulator line. Therefore, the use of quadrupoles employed in the hard X-ray undulator line at PAL-XFEL has also been taken into consideration to overcome the reduced effective length of the undulator by increasing the betatron phase advance. Based on the time-dependent FEL simulation results, it can be confirmed that the proposed shorter focusing lattice is sufficiently feasible without compromising FEL performance.