  Achieving maximum electron beam brightness in photoinjectors requires
detailed control of the 3D bunch shape and precise tuning of the beam focusing.
Even in state-of-the-art designs, slice emittance growth due to nonlinear space
charge forces and partial nonlaminarity often remains non-negligible. In this
work we introduce a new means to linearize the transverse slice phase space: a
sacrificial portion of the bunch's own charge distribution, formed into a
wavebroken shock front by highly nonlinear space charge forces within the gun,
whose downstream purpose is to dynamically linearize the desired bunch core. We
show that linearization of an appropriately prepared bunch can be achieved via
strongly nonlaminar focusing of the sacrificial shock front, while the inner
core focuses laminarly. This leads to a natural spatial separation of the two
distributions: a dense core surrounded by a diffuse halo of sacrificial charge
that can be collimated. Multi-objective genetic algorithm optimizations of the
ultra-compact x-ray free electron laser (UCXFEL) injector employ this concept,
and we interpret it with an analytic model that agrees well with the
simulations. In simulation we demonstrate a final bunch charge of 100 pC, peak
current $\sim 30$ A, and a sacrificial charge of 150 pC (250 pC total emitted
from cathode) with normalized emittance growth of $<20$ nm-rad due to space
charge. This implies a maximum achievable brightness approximately an order of
magnitude greater than existing FEL injector designs.
