Purified SASE undulator configuration to enhance the performance of the soft x-ray beamline at the European XFEL

TL;DR

This paper proposes a straightforward implementation of the purified SASE (pSASE) undulator configuration at the European XFEL, demonstrating an order of magnitude increase in spectral density for soft x-ray applications between 1.3 keV and 3 keV.

Contribution

It introduces a simple method to enhance spectral density in the soft x-ray range using existing undulator segments without additional hardware.

Findings

Spectral density increased by an order of magnitude between 1.3 keV and 3 keV.

Implementation is compatible with the current SASE3 baseline undulator.

The method complements existing self-seeding techniques.

Abstract

The purified SASE (pSASE) undulator configuration recently proposed at SLAC promises an increase in the output spectral density of XFELs. In this article we study a straightforward implementation of this configuration for the soft x-ray beamline at the European XFEL. A few undulator cells, resonant at a subharmonic of the FEL radiation, are used in the middle of the exponential regime to amplify the radiation, while simultaneously reducing the FEL bandwidth. Based on start-to-end simulations, we show that with the proposed configuration the spectral density in the photon energy range between 1.3 keV and 3 keV can be enhanced of an order of magnitude compared to the baseline mode of operation. This option can be implemented into the tunable-gap SASE3 baseline undulator without additional hardware, and it is complementary to the self-seeding option with grating monochromator proposed for the same undulator line, which can cover the photon energy range between about 0.26 keV and 1 keV.