Undulator operation for PAL-XFEL large bandwidth modes

Abstract

Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) is operational for photon users since 2017. The hard X-ray beamline provides FEL with wavelengths from 0.1 and 0.7 nm, and the soft X-ray line provides 1 to 3 nm. The nominal operation of PAL-XFEL is based on the self-amplified spontaneous emission (SASE) process, where the energy spread of an electron beam is 10−3 to 10−4 to reach saturation. The bandwidth of XFEL radiation is naturally a similar order as of the energy spread of the electron beam. For certain types of XFEL user experiments, a large bandwidth of a few percent is required for the soft X-ray beamline. A large XFEL bandwidth may be obtained by increasing the energy spread of an electron beam. In such a case, the SASE process becomes however inefficient. Methods to achieve a large bandwidth X-ray free electron laser were proposed recently. In those methods, an electron beam is deflected transversely and sent to undulators with a vertical offset. The undulators have transverse field gradients. In this paper, we study the undulator operation parameters to achieve a large bandwidth by using the hardware setup of the PAL-XFEL soft X-ray beamline.