Obtaining high degree of circular polarization at X-ray FELs via a reverse undulator taper

TL;DR

This paper introduces a reverse taper technique in the main undulator of X-ray FELs to suppress linearly polarized background radiation, enabling high-power circularly polarized X-ray pulses suitable for various FEL configurations.

Contribution

The authors propose a novel reverse taper method that maintains electron bunching while significantly reducing linear polarization, allowing efficient generation of circularly polarized X-ray FEL radiation.

Findings

Suppression of linearly polarized radiation by orders of magnitude.

Production of >100 GW circularly polarized X-ray pulses.

Method applicability across different FEL types and parameters.

Abstract

Baseline design of a typical X-ray FEL undulator assumes a planar configuration which results in a linear polarization of the FEL radiation. However, many experiments at X-ray FEL user facilities would profit from using a circularly polarized radiation. As a cheap upgrade one can consider an installation of a short helical (or cross-planar) afterburner, but then one should have an efficient method to suppress powerful linearly polarized background from the main undulator. In this paper we propose a new method for such a suppression: an application of the reverse taper in the main undulator. We discover that in a certain range of the taper strength, the density modulation (bunching) at saturation is practically the same as in the case of non-tapered undulator while the power of linearly polarized radiation is suppressed by orders of magnitude. Then strongly modulated electron beam radiates at full power in the afterburner. Considering SASE3 undulator of the European XFEL as a practical example, we demonstrate that soft X-ray radiation pulses with peak power in excess of 100 GW and an ultimately high degree of circular polarization can be produced. The proposed method is rather universal, i.e. it can be used at SASE FELs and seeded (self-seeded) FELs, with any wavelength of interest, in a wide range of electron beam parameters, and with any repetition rate. It can be used at different X-ray FEL facilities, in particular at LCLS after installation of the helical afterburner in the near future.