Accurate modelling of intrabeam scattering and its impact on photoinjectors for free-electron lasers

Intrabeam scattering (IBS) is a fundamental effect that can limit the performance of high-brightness electron machines but has so far been neglected in standard modeling of RF photoinjectors. Recent measurements at SwissFEL show that the slice energy spread (SES) in the injector is significantly underestimated in standard beam dynamic simulations. In this paper, we employ a dedicated Monte Carlo simulation model that accurately predicts IBS-induced SES growth in the photoinjector of an X-ray free-electron laser. The simulations are benchmarked against SES measurements at the SwissFEL and are supported by a new analytical model. The results show that IBS-induced SES growth occurs throughout the injector, most prominently in the electron source, and must be included in performance assessments. We further demonstrate that while 5D brightness is largely conserved, the 6D brightness degrades with propagation, highlighting the need to account for IBS in accurate photoinjector design and optimization.