Ultrafast laser pulse heating of metallic photocathodes and its contribution to intrinsic emittance

TL;DR

This paper models how ultrafast laser pulses heat metallic photocathodes, affecting their intrinsic emittance, and proposes an experiment to measure this effect, highlighting limitations for photoinjector performance.

Contribution

It introduces a detailed calculation of laser-induced heating effects on metallic photocathodes and suggests a novel experimental approach to measure thermal emittance modulation.

Findings

Laser heating limits intrinsic emittance in metal photocathodes.

Time-dependent emittance growth can be significant during short laser pulses.

Proposed experiment can measure thermal emittance effects downstream.

Abstract

The heating of the electronic distribution of a copper photocathode due to an intense drive laser pulse is calculated under the two-temperature model using fluences and pulse lengths typical in RF photoinjector operation. Using the finite temperature-extended relations for the photocathode intrinsic emittance and quantum efficiency, the time-dependent emittance growth due to the same photoemission laser pulse is calculated. This laser heating is seen to limit the intrinsic emittance achievable for photoinjectors using short laser pulses and low quantum efficiency metal photocathodes. A pump-probe photocathode experiment in a standard 1.6 cell S-band gun is proposed, in which simulations show the time dependent thermal emittance modulation within the bunch from laser heating can persist for meters downstream and, in principle, be measured using a slice emittance diagnostic.