A study on low emittance injector and undulator for PAL-XFEL

For the success of PAL-XFEL, two critical systems, namely a low emittance injector and a variable gap out-vacuum undulator, are under development. In order to realize the target emittance of the PAL-XFEL injector we carried out an optimization study of various parameters, such as the laser beam transverse profile, the laser pulse length, the laser phase, and the gun energy. The transverse emittance measured at the Injector Test Facility (ITF) is ${\it\varepsilon}_{x}=0.48\pm 0.01~\text{mm}~\text{mrad}$ . An undulator prototype based on the EU-XFEL design and modified for PAL-XFEL was built and tested. A local- $K$ pole tuning procedure was developed and tested. A significant reduction (90%) of the local- $K$ fluctuation was observed. The requirement of undulator field reproducibility better than $2\times 10^{-4}$ and the undulator gap setting accuracy below $1~{\rm\mu}\text{m}$ were achieved for the prototype. The optical phase jitter after the pole height tuning at the tuning gap was calculated to be $2.6^{\circ }$ rms, which satisfies the requirement of $5.0^{\circ }$ .