The heating of the electronic distribution of a copper photocathode due to anintense drive laser pulse is calculated under the two-temperature model usingfluences and pulse lengths typical in RF photoinjector operation. Using thefinite temperature-extended relations for the photocathode intrinsic emittanceand quantum efficiency, the time-dependent emittance growth due to the samephotoemission laser pulse is calculated. This laser heating is seen to limitthe intrinsic emittance achievable for photoinjectors using short laser pulsesand low quantum efficiency metal photocathodes. A pump-probe photocathodeexperiment in a standard 1.6 cell S-band gun is proposed, in which simulationsshow the time dependent thermal emittance modulation within the bunch fromlaser heating can persist for meters downstream and, in principle, be measuredusing a slice emittance diagnostic.
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