Proton radiography of Nernst effect in magnetized underdense plasma*

Magnetic flux advection in high energy density plasmas with large temperature gradients is often dominated by the Nernst effect. In this effect, magnetic fields are advected down temperature gradients in the plasma. In particular, the preheat phase of Magnetized Liner Inertial Fusion (MagLIF) is susceptible to outward radial Nernst advection due to uneven laser heating. The goal of the preheat phase of MagLIF is to raise the temperature of the plasma before imploding the surrounding liner. The experimental benchmarking of extended-magnetohydrodynamic (extended-MHD) models, which include the Nernst effect, is crucial to optimize the preheat phase. 

We have developed an experimental platform at OMEGA laser facility for direct measurements of Nernst velocity in the regime when Nernst advection dominates. In the experiment, the gas jet is heated by a laser propagating parallel to an externally-applied magnetic field to generate magnetized underdense plasma. The magnetic field evolution in time is measured by proton radiography with 3 and 15 MeV protons produced by DHe3 backlighter. We present results at two different times to show the dynamics of the magnetic field providing a measurement of Nernst velocity. Field profiles are inverted using the PROBLEM algorithm [1] to obtain the path integrated magnetic fields in the plasma. This research aims to validate extended-MHD models and understand the Nernst effect in the preheat phase of MagLIF experiments.