  Ultrafast electron diffraction (UED) is a technique in which short-pulse
electron beams can probe the femtosecond-scale evolution of atomic structure in
matter driven far from equilibrium. As an accelerator physics challenge, UED
imposes stringent constraints on the brightness of the probe electron beam. The
low normalized emittance employed in UED, often at the 10 nm scale and below,
is particularly sensitive to both applied field aberrations and space charge
effects. The role of aberrations is increasingly important in small probe
systems that often feature multiple orders of magnitude variations in beam size
during transport. In this work, we report the correction of normal quadrupole,
skew quadrupole, and sextupole aberrations via dedicated corrector elements in
an ultrafast electron micro-diffraction beamline. To do this, we generate
precise 4-dimensional phase space maps of rf-compressed electron beams, and
find excellent agreement with aberration-free space charge simulations.
Finally, we discuss the role a probe-forming aperture can play in improving the
brightness of bunches with appreciable space charge effects.
