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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.","problem_ref":null,"references":"","settled_by":"High-resolution slice energy spread measurements versus bunch charge and laser-heater setting compared with a start-to-end model including intrabeam scattering.","status_note":"Measurements at FEL injectors in the early 2020s attributed much of the excess to intrabeam scattering; quantitative agreement across facilities is not established.","title":"What sets the uncorrelated energy spread of high-brightness electron 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