Beam debunching due to ISR-induced energy diffusion

TL;DR

This paper rigorously analyzes how synchrotron radiation-induced energy diffusion in magnetic beamlines reduces electron bunching, impacting the effectiveness of XFEL seeding schemes.

Contribution

It provides a general calculation of bunching factor reduction due to incoherent synchrotron radiation in arbitrary beamlines, applicable to XFEL seeding.

Findings

Quantifies bunching reduction due to ISR in beamlines

Applies results to common XFEL seeding schemes

Provides a framework for assessing beam coherence loss

Abstract

One of the options for increasing longitudinal coherency of X-ray free electron lasers (XFELs) is their seeding with microbunched electron beam. Several schemes leading to significant amplitude of the beam bunching at X-ray wavelengths were recently proposed. All these schemes rely on beam optics having several magnetic dipoles. While the beam passes through a dipole, its energy spread increases due to quantum effects of synchrotron radiation. As a result, the bunching factor at small wavelengths reduces since electrons having different energies follow different trajectories in the bend. We rigorously calculate reduction in the bunching factor due to incoherent synchrotron radiation while the beam travels in arbitrary beamline. We apply general results to estimate reduction of harmonic current in common schemes proposed for XFEL seeding.