Full scale simulation of a field-emitter arrays based electron source for free-electron lasers

We describe the computer modeling of relativistic electron guns (0.5MeV) using pulsed field emitter arrays. The special challenge lies in the fact, that current distributions vary at a submicron scale, whereas structural dimensions are in the millimeter range. The general approach uses two steps. The first one is the computation of individual field-emitter tips including gate and focusing layers. Real world influences as, e.g., the effect of adsorbates on the emitted currents are taken into account by parameterizing the phase space of the tips. Together with a stochastic distribution of emitter properties, this leads to an equivalent current distribution on the cathode itself, which is used in the second step for the calculation of the electron dynamics in the gun itself. We present results for a source using a field-emitter array of 17 700 tips. For the current field-emitter geometry, we see a large high base line effect resulting from nonlinear focusing forces inside the emitter itself. Of special interest are the effects of spatial correlations in the stochastic distribution on the emittance, showing pronounced performance degradations in the case of large correlation lengths.