Accurate Modelling of Intrabeam Scattering and its Impact on Photoinjectors for Free-Electron Lasers

TL;DR

This work addresses the underestimation of intrabeam scattering (IBS) effects in RF photoinjector modelling and its impact on XFEL performance. It introduces a first-principles Monte Carlo IBS model implemented in REPTIL, complemented by a slice-based analytical formulation based on Piwinski's theory, and validates both against SwissFEL SES measurements. The results show IBS induces significant SES growth throughout the injector—especially at the electron source—and drives notable degradation of 6D brightness during propagation, even as 5D brightness remains largely intact. The findings underscore the necessity of incorporating IBS into photoinjector design and optimization, and lay the groundwork for more accurate modelling of injector upgrades and operating regimes for high-brightness XFELs, including comparisons between Gaussian and uniform longitudinal distributions and prospective traveling-wave gun configurations.

Abstract

Intrabeam scattering (IBS) is a fundamental effect that can limit the performance of high-brightness electron machines, yet it has so far been neglected in standard modelling of RF photoinjectors. Recent measurements at SwissFEL reveal that the slice energy spread (SES) in the injector is significantly underestimated by conventional tracking codes. In this work, we present a dedicated Monte Carlo simulation model that accurately predicts the IBS-induced SES growth in the photoinjector of an X-ray free-electron laser. The simulations are benchmarked against SES measurements at the SwissFEL as well as theoretically supported by a new analytical model. The results demonstrate that IBS-induced SES growth occurs throughout the injector, most prominently in the electron source, and must be taken into account when assessing photoinjector performance. We further show that while the 5D brightness is largely conserved, the 6D brightness undergoes notable degradation with propagation, underscoring the need to include IBS in the accurate design and optimization of photoinjectors.

Figures (5)

• Figure 1: Layout of the SwissFEL injector Prat2022Prat2020_2 from the SES measurements which is used in numerical simulations. For the measurements of SES at the SwissFEL, described within this work, the R$_{56}$ of the laser heater and that of the bunch compressor are set to zero.
• Figure 2: Detail of the longitudinal phase space of the bunch at z = 13 m with (blue) and without (red) IBS included in the space-charge (SPCH) simulations and each performed using the REPTIL code.
• Figure 3: Slice energy spread over the SwissFEL injector, modelled in REPTIL including the IBS module (solid line) for the five different bunch charges used in the measurements Prat2022. The SES induced by IBS is also calculated using Eq. (\ref{['Eqn:Piwinski2']}) and shown in the plot for comparison (dashed line). The SwissFEL baseline (200 pC) in ASTRA's conventional space-charge code (without IBS) is included for comparison.
• Figure 4: Comparison between the measurements at the SwissFEL, the numerical simulations with IBS performed with REPTIL and the analytical calculation using Eq. (\ref{['Eqn:Piwinski2']}). The values are given 111 m downstream of the cathode.
• Figure 5: Evolution of the projected emittance, peak current, central slice emittance, 5D brightness, SES and 6D brightness (top to bottom) calculated for the SwissFEL RF photogun with Gaussian longitudinal distribution (left), SwissFEL RF photogun with uniform longitudinal distribution (middle) and a proposed higher brightness travelling-wave photogun with uniform distribution (right). The project emittance, peak current and slice emittance are simulated with REPTIL and bench-marked against ASTRA. The SES was calculated with a conventional space-charge only simulation (SPCH), using the IBS module of REPTIL (SPCH $+$ IBS) and using the analytical model in Eq. (\ref{['Eqn:Piwinski2']}) (SPCH only with analytical IBS).