First measurement of energy diffusion in an electron beam due to quantum fluctuations in the undulator radiation

TL;DR

This paper reports the first experimental measurement of energy diffusion caused by quantum fluctuations in undulator radiation within an electron beam, confirming theoretical predictions at the European XFEL.

Contribution

It introduces a novel diagnostic method to measure quantum diffusion effects in an electron beam at an X-ray free-electron laser.

Findings

Quantum diffusion effects are measurable and match theoretical models.

The new diagnostic technique enables detailed phase space analysis.

First experimental validation of quantum diffusion in an operational FEL.

Abstract

We present measurements of slice energy spread growth due to quantum fluctuations of the undulator radiation at the European X-Ray Free-Electron Laser. The method uses a recently installed diagnostic wakefield structure, which enables measurements of the longitudinal phase space after the hard X-ray undulator. The effect of quantum diffusion in the undulator is measured for the first time, and the results are in good agreement with theoretical predictions.