Accurate measurement of uncorrelated energy spread in electron beam

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.

Slice energy spread measurement in the low energy photoinjector

“…As mentioned already, recent slice energy spread measurements at the photoinjectors of SwissFEL and European XFEL have shown slice energy spread much higher than expected from simulations [16,17]. This is either due to high slice energy spread already from the low energy section, or due to slice energy spread growth during the high energy acceleration and transportation.…”

“…The first is demonstrated at the SwissFEL by scanning the beam energy γ [16]. The second is demonstrated at the European XFEL by scanning the dispersion function D [17]. Both methods require a constant slice emittance and constant beta function at the measurement screen, which is not easy to achieve for a space charge dominated low energy photoinjector like PITZ.…”

“…Taking the parameters from the European XFEL injector as an example [17], beam energy is 130 MeV, dipole screen resolution is 28 µm with an energy dispersion of 1.2 m, nominal emittance is 0.4 µm for 250 pC [17,26]. The screen-induced energy spread resolution is 3 keV, but it can be reduce to 0.5 keV if the beam energy is lowered to 20 MeV.…”

“…Once the slice energy spread from TDS is precisely removed, slice energy spread σ γ measurements are limited by screen resolution and transverse emittance effect. Energy scan or dispersion scan methods demonstrated for high energy injector at SwissFEL and European XFEL [16,17] can't be easily applied to low energy beamlines, where constant slice emittance and beta function are difficult to maintain under the space charge effect. Instead, we propose to measure the screen resolution and emittance-induced beam size directly.…”

“…Recently, two dispersion based methods were proposed and demonstrated for accurate injector slice energy spread measurements with rms uncertainty down to the 0.1 to 0.3 keV level [16,17]. Both measured much higher slice energy spread than expected for 200 to 250 pC bunch charge, which reduced the laser heater role in improving XFEL lasing [18].…”