Measurement of the absolute energy and energy spread of the ESRF electron beam using undulator radiation

Abstract

Two simple methods of characterizing the average energy and energy spread of the electron beam have been developed at the ESRF. Both are based on analysis of the x-ray spectrum from an undulator. The first allows the absolute energy of the electrons to be determined. It is based on the dependence between the harmonics wavelengths and the electron beam energy. The x-ray beam is monochromatized at 21 keV by a silicon crystal in backscattering geometry. By adjusting the magnetic gap, one makes the third harmonic of the radiation from an undulator coincide with the energy selected by the crystal. The main errors come from the uncertainties in the undulator’s magnetic field and period. By operating the undulator at low field (K=0.36), an absolute accuracy of 10−3 is reached for the electron energy. The energy spread measurement is performed by analyzing the broadening of the harmonics’ profile. It is deduced from the measured ratio between the height of the peak of the seventh harmonic at 29 keV and the height of a secondary maximum at lower energy. The measured low current energy spread is 1.1×10−3±20%. It increases with the single bunch current due to turbulent bunch widening.