Measurement of magnetic cavitation driven by heat flow in a plasma*

Under extreme conditions, heat flow and magnetic fields in plasmas are strongly coupled. However, the Nernst effect, where heat flow drives advection of the magnetic fields, is seldomly described and is often neglected from magneto-hydrodynamic models. Using laser-driven proton radiography, we demonstrate that this heat-flow-driven advection in fact dominates changes to the magnetic field in underdense plasmas on the nanosecond timescale. Through laser-heating of a gas within an applied magnetic field, we measure expulsion of the magnetic field from the hottest regions of the plasma. We reconstruct the magnetic field map and find almost complete cavitation of an applied magnetic field, with the field advected by heat flow at velocities around (6±2)x10⁵ m/s, in advance of the hydrodynamic expansion. Furthermore, we show how changes in the magnetic field can be used as a proxy for heat flow and use the measured advection to explore non-local effects in the heat transport.