Coherence properties of the radiation from FLASH

TL;DR

This paper analyzes the spatial and temporal coherence of FLASH free electron laser radiation, highlighting how coherence varies near saturation and discussing stability issues and potential improvements.

Contribution

It provides a detailed analysis of coherence properties of FLASH radiation, including the effects of higher harmonics and mode suppression on stability and coherence.

Findings

Coherence peaks near FEL saturation

Post-saturation coherence degrades significantly

Pointing stability is limited by mode suppression issues

Abstract

FLASH is the first free electron laser user facility operating in the vacuum ultraviolet and soft x-ray wavelength range. Many user experiments require knowledge of the spatial and temporal coherence properties of the radiation. In this paper we present an analysis of the coherence properties of the radiation for the fundamental and for the higher odd frequency harmonics. We show that temporal and spatial coherence reach maximum close to the FEL saturation but may degrade significantly in the post-saturation regime. We also find that the pointing stability of short FEL pulses is limited due to the fact that non-azimuthal FEL eigenmodes are not sufficiently suppressed. We discuss possible ways for improving the degree of transverse coherence and the pointing stability.