A micro channel-cut crystal X-ray monochromator for a self-seeded hard X-ray free-electron laser

TL;DR

This paper introduces a novel micro channel-cut Si(111) crystal monochromator for hard X-ray free-electron lasers, enabling efficient self-seeding with high spectral purity and improved conversion efficiency.

Contribution

It presents the development and implementation of a small optical delay monochromator that enhances seed pulse quality and efficiency in XFELs.

Findings

Peak reflectivity of 67% achieved, close to the theoretical 81%.

Monochromatic seed pulse obtained with high spectral purity.

Conversion efficiency of ~2×10⁻², ten times higher than previous methods.

Abstract

A channel-cut Si(111) crystal with a channel width of 90 $\mu$m was developed for achieving reflection self-seeding in hard X-ray free-electron lasers (XFELs). With the crystal, a monochromatic seed pulse is produced from a broadband XFEL pulse generated in the first-half undulators with an optical delay of 119 fs at 10 keV. The small optical delay allows a temporal overlap between the seed optical pulse and the electron bunch by using a small magnetic chicane for the electron beam at the middle of the undulator section. A peak reflectivity reached 67%, which is a reasonable value as compared with the theoretical one of 81%. By using this monochromator, a monochromatic seed pulse without broadband background in spectrum was obtained at SACLA with a conversion efficiency from a broadband XFEL pulse of $\sim 2 \times 10^{-2}$, which is $\sim 10$ times higher than that of transmission self-seeding using a thin diamond (400) monochromator.