What sets the uncorrelated energy spread of high-brightness electron beams
In plain words
Lasing needs electrons of nearly identical energy, and measured energy spreads in x-ray FEL electron beams have come out larger than models predicted.
Precise statement
In photoinjectors with $1e8\ \text{to}\ 1e9$ electrons per bunch, determine the contributions of intrabeam scattering, shot-noise-seeded microbunching instability and cathode emission to the slice energy spread $\sigma_E$ (few keV) at injector exit and after bunch compression, and predict $\sigma_E$ within $20\ \mathrm{percent}$ of measured values at several facilities.
What would settle it
High-resolution slice energy spread measurements versus bunch charge and laser-heater setting compared with a start-to-end model including intrabeam scattering.
Status in the literature
Measurements at FEL injectors in the early 2020s attributed much of the excess to intrabeam scattering; quantitative agreement across facilities is not established.