Simulation of short pulse photoemission in a micro-diode with implications for optimal beam brightness

Molecular dynamics simulations, with full Coulomb interaction are used to model short-pulse photoemission from a finite area in a microdiode. We demonstrate three emission regimes, source-limited emission, space-charge limited emission for short-pulses, and space-charge limited emission for the steady state. We show that beam brightness is at a maximum during transition from the source-limited emission regime to the space-charge limited emission regime for short pulses. From our simulations it is apparent that the important factor is the emitter spot size when estimating the critical charge density for short-pulse electron emission.