The Role of Low Intrinsic Emittance in Modern Photoinjector Brightness

Reducing the intrinsic emittance of photocathodes is one of the most promising routes to improving the brightness of electron sources. However, when emittance growth occurs during beam transport (for example, due to space-charge), it is possible that this emittance growth overwhelms the contribution of the photocathode and, thus, in this case source emittance improvements are not beneficial. Using multi-objective genetic optimization, we investigate the role intrinsic emittance plays in determining the final emittance of several space-charge dominated photoinjectors, including those for high repetition rate free electron lasers and ultrafast electron diffraction. We introduce a new metric to predict the scale of photocathode emittance improvements that remain beneficial and explain how additional tuning is required to take full advantage of new photocathode technologies. Additionally, we determine the scale of emittance growth due to point-to-point Coulomb interactions with a fast tree-based space-charge solver. Our results show that in the realistic high brightness photoinjector applications under study, the reduction of thermal emittance to values as low as 50 pm/um (1 meV MTE) remains a viable option for the improvement of beam brightness.