Improvement of the crossed undulator design for effective circular polarization control in X-ray FELs

TL;DR

This paper proposes an extended cascade crossed undulator scheme for X-ray FELs that achieves over 95% circular polarization, fast helicity switching, and high output power near saturation levels, improving upon previous designs.

Contribution

The paper introduces a cascade crossed undulator configuration that enhances circular polarization control and output power in X-ray FELs compared to traditional single-stage setups.

Findings

Achieves over 95% circular polarization.

Enables fast helicity switching.

Provides output power near saturation levels.

Abstract

The production of X-ray radiation with a high degree of circular polarization constitutes an important goal at XFEL facilities. A simple scheme to obtain circular polarization control with crossed undulators has been proposed so far. In its simplest configuration the crossed undulators consist of pair of short planar undulators in crossed position separated by an electromagnetic phase shifter. An advantage of this configuration is a fast helicity switching. A drawback is that a high degree of circular polarization (over 90%) can only be achieved for lengths of the insertion devices significantly shorter than the gain length, i.e. at output power significantly lower than the saturation power level. The obvious and technically possible extension considered in this paper, is to use a setup with two or more crossed undulators separated by phase shifters. This cascade crossed undulator scheme is distinguished, in performance, by a fast helicity switching, a high degree of circular polarization (over 95%) and a high output power level, comparable with the saturation power level in the baseline undulator at fundamental wavelength. We present feasibility study and exemplifications for the LCLS baseline in the soft X-ray regime.