Simulation of Short Pulse Photoemission in a Microdiode 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 the 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 emitter spot size is an important factor when estimating the critical charge density for short-pulse electron emission and that simple capacitive models may considerably underestimate the total charge emitted.