  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.
