Improving the energy-extraction efficiency of laser-plasma accelerator driven free-electron laser using transverse-gradient undulator with focusing optics and longitudinal tapering

TL;DR

This paper demonstrates that by optimizing dispersion, focusing, and tapering in a laser-plasma accelerator driven FEL with a transverse-gradient undulator, it is possible to enhance energy extraction efficiency and achieve high radiation power.

Contribution

It introduces a numerical study of focusing optics and longitudinal tapering in TGU-based FELs driven by LPAs, showing how to optimize parameters for improved performance.

Findings

Optimized initial dispersion and focusing parameters enable high power with focusing optics.

Longitudinal tapering and transverse-gradient variation further increase radiation power.

Good transverse coherence can be maintained despite non-constant dispersion functions.

Abstract

It is reported that [Z. Huang et al., Phys. Rev. Lett. 109, 204801 (2012)], high-gain free-electron laser (FEL) can be generated by transverse-dispersed electron beams from laser-plasma accelerators (LPAs) using transverse-gradient undulator (TGU) assuming an ideal constant dispersion function without focusing optics. The constant dispersion function keeps electrons beyond the resonant energy bandwidth still being on resonant with the FEL radiation. Instead, in this paper, the case with focusing optics in an LPA-driven FEL using TGU is numerically studied, in which the dispersion function should be monotonously decreasing along the undulator. Even though the FEL resonance is not always satisfied for off-energy electrons in this case, through subtly optimizing the initial dispersion and focusing parameters, it is feasible to achieve a similar radiation power to the case assuming an ideal constant dispersion function without focusing optics, and meanwhile, to attain a good transverse coherence. Moreover, higher radiation power could be expected if longitudinally tapering, transverse-gradient variation in different sections are systematically conducted, which is demonstrated with intense global multiparameters optimizations based on GENESIS simulation.