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.
Highlights
The small emittance and the low energy spread of the electron beam required at X-Ray Free Electron Lasers (XFELs) can cause the microbunching instability [1,2] and destroy the lasing
If we take into account that the estimated instrumental errors in the setup of the transverse deflecting structure (TDS) voltage and dispersion are smaller than 2% we can state that the uncorrelated energy spread in the core of the beam is equal to ð5.9 Æ 0.1Þ keV
We have described two methods for measurement of the slice energy spread of an electron bunch
Summary
The small emittance and the low energy spread of the electron beam required at X-Ray Free Electron Lasers (XFELs) can cause the microbunching instability [1,2] and destroy the lasing. In order to measure the slice energy spread a standard approach with a transverse deflector and the dispersive section is used It shows only a low resolution (of several keV) due to impact of OTR screen resolution, the betatron beam size and the deflector strength on the measurement. The deflector is placed after the acceleration section, and the beam energy in it changes during the experiment This case was mentioned in [4] too and it was suggested to use a polynomial equation of the third order with additional sets of measurements in order to exclude the impact of the deflector. In the measurements we have failed to obtain reliable data which would allow an accurate reconstruction We developed another method and have used the data from the energy scan experiments only to show the consistence with the results obtained by the second method. IV, the results of the measurements at the European XFEL injector and their analysis are presented
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