{"schema":"pubphys.bundle/1","record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"94e932f2f82638aeba04595ff2965db3022596e8cceb6b23648d0e94f661e0ea","created":"2026-10-03T07:17:56Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"77fd4e97666003960380c7891a91ef318b8a05472e099a42ec8bee6c99e5a212","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"beams.beam-brightness.photoinjector-brightness-limit","kind":"well-posed","literature_status":"open","n":"1","parents":[],"plain":"Theory gives a maximum brightness set by the field at the cathode and the electrons' sideways energy; real guns fall short by an amount that is not fully explained.","posed_since":"2009","precise":"For short-pulse photoinjectors the maximal 4D normalized brightness of a pancake-shaped bunch is bounded by the cathode field $E_c$ and MTE (bound derived in 2009). Determine whether measured 4D or 5D brightness of DC, normal-conducting RF and superconducting RF guns can come within a factor 2 of this bound at bunch populations of $1\\mathrm{e}8 \\text{ to } 1\\mathrm{e}9\\,\\mathrm{electrons}$, and which effects (image charge, nonuniform emission, imperfect emittance compensation, cathode roughness) cause the observed gap.","problem_ref":null,"references":"","settled_by":"Measured slice brightness of one gun at several $E_{c}$ and MTE values compared with the bound and with a start-to-end model that accounts for the gap.","status_note":"","title":"Can photoinjectors reach the space-charge-limited brightness 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