Field emission and unwanted beam propagation simulations in the SRF gun at SEALab

The ideal beam coming from an RF photoemission electron gun is composed only of electrons that are produced by the incidence of the drive laser in the photocathode. The timing of the drive laser with respect to the RF fields in the gun is carefully chosen to tailor the beam properties. There are, however, sources of unwanted electrons that degrade the performance of RF photoemission guns. Field emission in superconducting radio-frequency (SRF) guns contributes to unwanted electron generation, known as dark current. This work presents simulations based on the Fowler–Nordheim (FN) model~\cite{FN} to study field emission in the SEALab SRF gun cavity. By analyzing 2D field maps and using ASTRA simulations~\cite{Astra}, emission hotspots are identified, and particle trajectories are evaluated. While most field-emitted electrons are lost within the cavity, a small but significant portion escapes, contributing up to 25\% of the emitted power. The analysis offers key insights into mitigating performance-limiting effects in SRF guns.