Accurate measurement of uncorrelated energy spread in electron beam

TL;DR

This paper introduces two precise methods for measuring the uncorrelated energy spread in electron beams at the European XFEL, achieving sub-keV accuracy and revealing lower energy spread than previous measurements at SwissFEL.

Contribution

It presents two novel measurement techniques for slice energy spread with high accuracy, simplifying procedures and improving precision in electron beam diagnostics.

Findings

Measurement error less than 0.3 keV with energy scaling method.

Measurement error below 0.1 keV with optics adjustment method.

Measured energy spread nearly three times lower than at SwissFEL.

Abstract

We present measurements of slice energy spread at the injector section of the European X-Ray Free Electron Laser for an electron bunch with charge of 250 pC. Two methods considered in the paper are based on measurements at the dispersive section after a transverse deflecting structure (TDS). The first approach uses measurements at different beam energies. We show that with a proper scaling of the TDS voltage with the beam energy the rms error of the measurement is less than 0.3 keV for the energy spread of 6 keV. In the second approach we demonstrate that keeping the beam energy constant but adjusting only the optics we are able to simplify the measurement complexity and to reduce the rms error below 0.1 keV. The accuracy of the measurement is confirmed by numerical modelling including beam transport effects and collective beam dynamics of the electron beam. The slice energy spread measured at the European XFEL for the beam charge of 250 pC is nearly 3 times lower as the one reported recently at SwissFEL for the same cathode material and the beam charge of 200 pC.