Effects of energy spread on Brightness and Coherence of undulator sources

TL;DR

This paper investigates how energy spread affects the brightness and coherence of undulator radiation, emphasizing the importance of accurate models for ultralow emittance sources in advanced synchrotron and FEL facilities.

Contribution

It provides a detailed analysis of energy spread effects on undulator brightness and coherence, refining existing models beyond Gaussian beam approximations.

Findings

Energy spread significantly impacts undulator brightness.

Coherence properties are closely linked to energy spread effects.

Results are relevant for designing diffraction-limited light sources.

Abstract

The (spectral) brightness for partially transverse coherent sources as Synchrotron Radiation (SR) and Free-Electron Laser (FEL) sources can be defined as the maximum of the Wigner distribution. Then, the brightness includes information on both coherence and wavefront characteristics of the radiation field. For undulator sources, it is customary to approximate the single-electron electric field at resonance with a Gaussian beam, leading to great simplifications. Attempts to account for the modified spatial and angular profile of the undulator radiation in the presence of detuning due to energy spread currently build on the simplified brightness expression derived under the assumption of Gaussian beams. The influence of energy spread on undulator radiation properties is becoming important in view of diffraction-limited rings with ultralow emittance coming on-line. Here we discuss the effects of energy spread on the brightness of undulator radiation at resonance, as well as relevant relations with coherence properties.