Measurement of Magnetic Cavitation Driven by Heat Flow in a Plasma*

In hot plasmas heat flow and magnetic fields are strongly coupled. Our simulations of high temperature laser-plasma experiments show that strong heat flows cause significant changes in the magnetic field but it has long proven difficult to measure these changes experimentally. A particular challenge in magnetized high-energy-density plasma experiments is Nernst-driven magnetic cavitation, in which heat flow causes expulsion of the magnetic field from the hottest regions of a plasma much faster than the bulk plasma flow. This reduces the effectiveness of magnetized fusion techniques, where strong magnetic fields are used to confine the heat inside the plasma and increase yield.

We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. By reconstructing the magnetic field map using proton radiographs, we demonstrate that the field is advected by heat flow before the plasma expansion. The measured Nernst advection velocity of (600±200) km/s is faster than the ion sound speed, with the magnetic field dynamics dominated by the motion of hot electrons. Despite the steep temperature gradient, we found that the heat flow is localised at relatively low magnetic field strengths. This causes extended magnetohydrodynamic simulations to agree surprisingly well in this regime with both the experimental results and more computationally expensive kinetic simulations.