Generation of Single-spike X-ray Pulse from Free-electron Lasers based on Velocity Bunching

Abstract

Intense attosecond X-ray pulses can open new frontier of research in many areas of science. So far several schemes have been proposed to make an attosecond hard X-ray free-electron laser (XFEL) in self-amplified spontaneous emission (SASE) mode of operation. In this thesis we suggest a different scenario to produce an attosecond, single-spike FEL that is relatively simple, economic and compact compared to other methods. Our approach is based on adopting the velocity bunching technique in conjunction with a very low charge beam of the electron bunch, instead of heavily relying on magnetic bunch compressors that are employed in all existing and planned hard XFELs around the world. To show the validity of our proposal, we have designed two example XFELs, a soft XFEL and a hard XFEL. For the hard XFEL the nominal wavelength is 0.1 nm. We show that we can achieve a multi-GW single-spike radiation with a 5-GeV and 5 pC electron beam. In this scheme, the velocity bunching followed by one magnetic bunch compressor is shown to provide a desired single-spike FEL pulse. For the soft XFEL, the nominal wavelength is 1 nm and it is found that a multi-GW single-spike radiation with 5 pC, 3-GeV electron beam can be generated without employing any magnetic bunch compressor. Implementation of the undulator tapering is advantageous to maintain the single-spike radiation after the FEL saturation. Studies on the effect of various component’s errors on the FEL performance are carried out and conclusion is such that our scheme is achievable under the present technology. In parallel, an optimization study of the self-seeding scheme in the Pohang Accelerator X-ray Free Laser (PAL-XFEL) is also performed. Optimizing the number of undulator modules at SASE stage for self-seeding scheme in PAL-XFEL, it is found that a single-spike 0.15 nm radiation can be produced with average power of 32 GW (1.1 mJ in 33 fs) and 0.4-eV BW which is close to the Fourier transform-limited value. The method suggested in the present thesis can be implemented and tested in the existing and planned XFEL facilities.