Compact laser-plasma-accelerator-driven free-electron laser using a transverse gradient undulator

Abstract

Laser-plasma accelerators can produce a few GeV electron beams over a distance of a few cm. Such beams typically have relatively low emittance and high peak current but a rather large energy spread and jitter. The large energy spread hinders the potential applications for coherent free-electron laser (FEL) radiation generation. In this paper, we discuss a method to compensate the effects of beam energy spread by introducing a transverse variation of the undulator magnetic field. Such a transverse gradient undulator (TGU) together with a properly dispersed beam can greatly reduce the effects of electron energy spread and jitter on FEL performance. We review the TGU concept and theory and discuss technical implementations. Using particle-in-cell simulations of a GeV laser-plasma accelerator and the FEL simulation code GENESIS that is modified to accommodate TGU, we show a soft x-ray FEL operating in the “water window” wavelengths can reach saturation with a undulator length of about 12 m.