Scheme for generation of highly monochromatic X-rays from a baseline XFEL undulator

TL;DR

This paper proposes a novel method using a crystal monochromator within a magnetic chicane to generate highly monochromatic X-rays in XFELs, achieving bandwidths much narrower than traditional self-seeding schemes without disrupting baseline operations.

Contribution

It introduces a pulse doubler technique with a compact monochromator setup that significantly reduces X-ray bandwidth in XFELs, avoiding extensive modifications to the baseline undulator.

Findings

Bandwidth reduced to 10^(-5) with the new method

Compatible with existing XFEL infrastructure

Feasibility demonstrated with European XFEL parameters

Abstract

One goal of XFEL facilities is the production of narrow bandwidth X-ray radiation. The self-seeding scheme was proposed to obtain a bandwidth narrower than that achievable with conventional X-ray SASE FELs. A self-seeded FEL is composed of two undulators separated by a monochromator and an electron beam bypass that must compensate for the path delay of X-rays in the monochromator. This leads to a long bypass, with a length in the order of 40-60 m, which requires modifications of the baseline undulator configuration. As an attempt to get around this obstacle, together with a study of the self-seeding scheme for the European XFEL, here we propose a novel technique based on a pulse doubler concept. Using a crystal monochromator installed within a short magnetic chicane in the baseline undulator, it is possible to decrease the bandwidth of the radiation well beyond the XFEL design down to 10^(-5). The magnetic chicane can be installed without any perturbation of the XFEL focusing structure, and does not interfere with the baseline mode of operation. We present a feasibility study and we make exemplifications with the parameters of the SASE2 line of the European XFEL.