High-gain x-ray free-electron laser driven by a laser-plasma accelerator using a vertical gradient optical undulator

Abstract

Recent advances in the studies of laser-plasma accelerators (LPAs) and optical undulators enable the possibility of designing a compact x-ray free-electron laser (FEL). However, the LPA beams usually have a percent-level energy spread that will degrade the FEL performance. To compensate for the energy spread effect, an optical undulator with a transverse gradient has been proposed, in which a dispersed LPA beam is injected into a horizontally polarized Gaussian laser with a horizontal offset relative to the laser center. Such a scheme, however, leads to an inherent orbit deflection and calls for accurate orbit correction. In this paper, we propose an alternative method that does not induce any orbit deflection and requires less total laser power. In this method, a dispersed LPA beam is sent into an optical undulator with a vertical gradient generated by the combination of a Hermite-Gaussian TEM01 mode laser and a TEM00 mode laser. Simulations suggest that this method is feasible to generate megawatt-level soft x-ray FEL by using an LPA beam with the energy of a few tens of MeV.