  Magnetic fields generated from a laser-foil interaction are measured with
high fidelity using a proton radiography scheme with in situ x-ray fiducials.
In contrast to prior findings under similar experimental conditions, this
technique reveals the self-generated, Biermann-battery fields extend beyond the
edge of the expanding plasma plume to a radius of over 3.5 mm by t=+1.4 ns, a
result not captured in state-of-the-art magneto-hydrodynamics simulations. An
analysis of two mono-energetic proton populations confirms that proton
deflection is dominated by magnetic fields far from the interaction (>2 mm) and
electric fields are insignificant. Comparisons to prior work suggest a new
physics mechanism for the magnetic field generation and transport in
laser-solid interactions.
