Off-axis parameters of PAL-XFEL planar undulator

Abstract

The electron beam path in a planar undulator is usually chosen at the undulator magnetic center in both the vertical and horizontal directions. In the case of the vertical-gap undulators of x-ray free-electron lasers, best efforts are paid to align the vertical magnetic center to the reference electron beam path. The twenty undulator segments are aligned vertically within 50 μm to obtain the target 𝐾-values of the undulator segments in the PAL-XFEL hard X-ray beamline. Besides 𝐾value, undulator phase error and beam trajectory deflection are also concerned when the undulator is misaligned to the beam path. In some cases, an off-set in the vertical direction is applied intentionally when a transverse field gradient is required. In this report, we present the systematic measurements of the phase error and trajectory deflection at vertical off-axes of a PAL-XFEL hard X-ray undulator.