Undulator linear taper control at the European X-Ray Free-Electron Laser facility

Undulator tapering controls the resonance properties of the free-electron laser (FEL) amplification process. Wakefield energy losses in an undulator's vacuum chamber are one of the factors that determine the undulator's linear taper. While another contribution to energy losses, namely the losses due to spontaneous radiation, can be calculated analytically, estimating wakefield energy losses requires detailed knowledge of the chamber geometry and the electron beam current profile. We introduce a method for the automatic estimation of wakefield energy losses, which leverages noninvasive THz diagnostics, a current profile reconstruction algorithm enhanced with machine learning, and a recently developed analytical wakefield function for the European XFEL's undulator beamline. The correctness of this method was validated by directly measuring wakefield-induced electron beam energy losses in the undulator section. This, in turn, enables the prediction of the optimal linear taper in the undulator.