Physics Requirements of the PAL-XFEL Undulator System

Pohang Accelerator Laboratory(PAL) is planning a 0.3 nm SASE (self amplification of spontaneous emission) XFEL (X-ray free electron laser) based on a 3.7 GeV linear accelerator. It is targeting utilization of 0.1 nm X-ray radiation from 3rd haxmonic SASE FEL radiation. With a 3.7 GeV electron beam energy, achieving 0.3 nm fundamental FEL lazing is very challenging and requires a very low emittance electron beam with minimum energy spread. It also requires a small period undulator with a very small gap. In this context, application of a SPring8 type in vacuum undulator is seriously considered. This reflects the experiences from the SPring8 SCSS project. In this report, the physics design efforts of the undulator is described. The periodic parts are optimized to achieve the highest field with other requirements. The end structures were designed to be asymmetric along the beam direction to ensure a systematic zero 1st field integral. The thicknesses of the last magnets were adjusted to minimize the transition distance to the fully developed periodic field. The final design features a 4 mm minimum pole gap, a 15 rum period and a peak effective field of 1.09 Tesla.11sciescopuskc