Amplification of laser imprint in the presence of strongly imposed target-normal magnetic fields*

An experiment was performed on OMEGA EP investigating the predicted amplification of laser imprint by target-normal magnetic fields. A 30-µm-thick CH target was directly driven with a single beam without smoothing by spectral dispersion, allowing spatial modulations to be transferred to the target surface. Magnetic fields up to 45 T were imposed, and Rayleigh-Taylor amplification of the imprint seed was captured using x-ray radiography from a driven Gd backlighter. Analysis of these radiographs showed a 60% increase in the surface perturbation amplitudes in the presence of magnetic fields, consistent for all unsaturated frequencies and at all times probed. Similar 1-D hydrodynamics were observed, indicating that this increase was due to greater levels of laser imprint coming from the suppression of electron motion across the magnetic field lines and subsequent reduction in thermal smoothing. This shows that target-normal magnetic fields remain in the conduction zone of a driven target, consistent with theoretical predictions, though some discrepancy exists in the magnitude of predicted versus observed imprint effects. This provides important insights into the role of Nernst advection in magnetohydrodynamics.