  We report a technique of proton deflectometry which uses a grid and an in
situ reference x-ray grid image for precise measurements of magnetic fields in
high-energy density plasmas. A D$^3$He fusion implosion provides a bright
point-source of both protons and x-rays, which is split into beamlets by a mesh
grid. The protons undergo deflections as they propagate through the plasma
region of interest, whereas the x-rays travel along straight lines. The x-ray
image therefore provides a zero-deflection reference image. The line-integrated
magnetic fields are inferred from the shifts of beamlets between the deflected
(proton) and reference (x-ray) images. We developed a system for analysis of
this data, including automatic algorithms to find beamlet locations and
calculate their deflections from the reference image. The technique is verified
in an experiment performed at OMEGA to measure a non-uniform magnetic field in
vacuum, then applied to observe the interaction of an expanding plasma plume
with the magnetic field.
