FEL Polarization Control Studies on Dalian Coherent Light Source

TL;DR

This study compares two approaches for controlling polarization in free-electron lasers at Dalian Coherent Light Source, demonstrating that elliptical permanent undulators offer more stable polarization and can produce high-energy circularly polarized pulses.

Contribution

The paper provides a comparative analysis of crossed planar and elliptical permanent undulators for FEL polarization control, including a detailed simulation model considering real-world imperfections.

Findings

Elliptical permanent undulators are less sensitive to errors and imperfections.

Proper configuration can generate circularly polarized FEL pulses exceeding 100 μJ.

Residual beam energy chirp affects polarization quality in crossed planar undulators.

Abstract

The polarization switch of a free-electron laser (FEL) is of great importance to the user scientific community. In this paper, we investigate the generation of controllable polarization FEL from two well-known approaches for Dalian coherent light source, i.e., crossed planar undulator and elliptical permanent undulator. In order to perform a fair comparative study, a one-dimensional time-dependent FEL code has been developed, in which the imperfection effects of an elliptical permanent undulator are taken into account. Comprehensive simulation results indicate that the residual beam energy chirp and the intrinsic FEL gain may contribute to the degradation of the polarization performance for the crossed planar undulator. And the elliptical permanent undulator is not very sensitive to the undulator errors and beam imperfections. Meanwhile, with proper configurations of the main planar undulators and additional elliptical permanent undulator section, circular polarized FEL with pulse energy exceeds 100 $\mu$J could be achieved at Dalian coherent light source.