Statistical properties of a free-electron laser revealed by the Hanbury Brown and Twiss interferometry

TL;DR

This study uses Hanbury Brown and Twiss interferometry to analyze the statistical properties of the SASE free-electron laser FLASH at multiple wavelengths, revealing high spatial coherence, short pulse durations, and complex beam behaviors.

Contribution

It introduces an advanced theoretical model accounting for multiple beams and jitter, providing new insights into FEL radiation properties and diagnostics.

Findings

High spatial coherence above 50% across wavelengths

Pulse durations below 60 femtoseconds

Presence of multiple beams and significant positional jitter

Abstract

We present a comprehensive experimental analysis of statistical properties of the self-amplified spontaneous emission (SASE) free-electron laser (FEL) FLASH at DESY in Hamburg by means of Hanbury Brown and Twiss (HBT) interferometry. The experiments were performed at the FEL wavelengths of 5.5 nm, 13.4 nm, and 20.8 nm. We determined the 2-nd order intensity correlation function for all wavelengths and different operation conditions of FLASH. In all experiments a high degree of spatial coherence (above 50%) was obtained. Our analysis performed in spatial and spectral domains provided us with the independent measurements of an average pulse duration of the FEL that were below 60 fs. To explain complicated behaviour of the 2-nd order intensity correlation function we developed advanced theoretical model that includes the presence of multiple beams and external positional jitter of the FEL pulses. By this analysis we determined that in most experiments several beams were present in radiating field and in one of the experiments external positional jitter was about 25% of the beam size. We envision that methods developed in our study will be used widely for analysis and diagnostics of the FEL radiation.