High-power laser pulses hitting solid targets generate their own magnetic fields — not externally applied but self-organized from the plasma dynamics. These fields inhibit heat flow and reshape the plasma evolution, making them critical for inertial fusion and laboratory astrophysics. But measuring their three-dimensional structure in a transient plasma is extraordinarily difficult.
This paper uses multi-view proton tomography at two timings to reconstruct the full 3D magnetic field evolution. Protons from a separate source pass through the plasma; their deflections encode the integrated magnetic field along the line of sight. Multiple views allow tomographic reconstruction, and two timing snapshots reveal the temporal evolution.
The fields transition from target-surface localization at early times to extended coronal structures later. The tomographic inversion directly measures magnetic flux evolution — not inferred from models but reconstructed from data. Comparison with extended-MHD simulations shows good agreement in total magnetic flux (the generation model works) but only moderate agreement in field structure (the transport model needs work). The generation physics is largely correct; the redistribution physics is not. The diagnostic is more accurate than the simulation, which means the measurement is now ahead of the theory it was designed to test.