A non-destructive correlated energy spread monitor using multi-stripline electrodes for X-ray free electron lasers

During X-ray free electron laser (XFEL) operation, electron beam parameters such as the correlated energy spread, which affects the self-amplified spontaneous emission (SASE) spectrum, should remain optimized for the best performance. However, the correlated energy spread often varies from the optimized condition owing to the drift of RF stations, even when a feedback system with low-level radio frequency is operating. Non-destructive correlated energy spread monitoring could offer a means to stabilize such variations and improve the performance of X-ray generation by maintaining the spectral quality. Herein, we investigated the feasibility of a non-destructive correlated energy spread monitor based on multi-stripline electrodes for use with the 200 pC electron beam at the Pohang Accelerator Laboratory XFEL (PAL-XFEL). Beam tracking and electromagnetic simulations indicated that the correlated energy spread up to approximately 0.1% could be measured without intercepting the beam at the bunch compressors of the PAL-XFEL. Through the 3D FEL simulation, we confirmed that keeping the SASE energy spectrum bandwidth to 0.05–0.15%, with a photon energy of 9.7 keV at the undulator, requires the correlated energy spread of the electron beam to be within 0.12–0.35% at the third bunch compressor. From the simulations, we conclude that the non-destructive correlated energy spread monitor based on multi-stripline electrodes is applicable to XFEL facilities and could serve as an effective optimization tool.