  In an effort to increase spatial and temporal resolution of ultrafast
electron diffraction and microscopy, ultra-high brightness photocathodes are
actively sought to improve electron beam quality. Beam dynamics codes often
approximate the Coulomb interaction with mean-field space charge, which is a
good approximation in traditional beams. However, point-to-point Coulomb
effects, such as disorder induced heating and the Boersch effect, can not be
neglected in cold, dense beams produced by such photocathodes. In this paper,
we introduce two new numerical methods to calculate the important effects of
the photocathode image charge when using a point-to-point interaction model.
Equipped with an accurate model of the image charge, we calculate the effects
of point-to-point interactions on two high brightness photoemission beamlines
for ultrafast diffraction. The first beamline uses a 200 keV gun, whereas the
second uses a 5 MeV gun, each operating in the single-shot diffraction regime
with $10^5$ electrons/pulse. Assuming a zero photoemission temperature, it is
shown that including stochastic Coulomb effects increases the final emittance
of these beamlines by over a factor of 2 and decreases the peak transverse
phase space density by over a factor of 3 as compared to mean-field
simulations. We then introduce a method to compute the energy released by
disorder induced heating using the pair correlation function. This disorder
induced heating energy was found to scale very near the theoretical result for
stationary ultracold plasmas, and it accounts for over half of the emittance
growth above mean-field simulations.
