Coherence Properties of Individual Femtosecond Pulses of an X-ray Free-Electron Laser

TL;DR

This paper measures the spatial and temporal coherence of femtosecond x-ray pulses from the LCLS free-electron laser, revealing a coherence length of 17 micrometers and a temporal coherence time of 0.6 femtoseconds.

Contribution

First direct measurements of the coherence properties of single femtosecond x-ray pulses from an FEL, providing key insights into their coherence length, temporal coherence, and mode structure.

Findings

Coherence length of 17 micrometers in vertical direction

Temporal coherence time of 0.6 femtoseconds

78% of power in the dominant transverse mode

Abstract

Measurements of the spatial and temporal coherence of single, femtosecond x-ray pulses generated by the first hard x-ray free-electron laser (FEL), the Linac Coherent Light Source (LCLS), are presented. Single shot measurements were performed at 780 eV x-ray photon energy using apertures containing double pinholes in "diffract and destroy" mode. We determined a coherence length of 17 micrometers in the vertical direction, which is approximately the size of the focused LCLS beam in the same direction. The analysis of the diffraction patterns produced by the pinholes with the largest separation yields an estimate of the temporal coherence time of 0.6 fs. We find that the total degree of transverse coherence is 56% and that the x-ray pulses are adequately described by two transverse coherent modes in each direction. This leads us to the conclusion that 78% of the total power is contained in the dominant mode.