Strong surface magnetic field generation in relativistic short pulse laser-plasma interaction with an applied seed magnetic field

TL;DR

This paper demonstrates that relativistic laser interactions with a thin target containing a seed magnetic field can generate much stronger surface magnetic fields, significantly affecting plasma dynamics and ion acceleration.

Contribution

It introduces a new mechanism for strong surface magnetic field generation triggered by a seed field during relativistic laser-plasma interactions, supported by theoretical scaling and simulations.

Findings

Surface magnetic fields can be amplified by an order of magnitude.

The generated fields can reverse sign relative to the seed.

Altered plasma expansion and ion acceleration dynamics.

Abstract

While plasma often behaves diamagnetically, we demonstrate that the laser irradiation of a thin opaque target with an embedded target-transverse seed magnetic field $B_\mathrm{seed}$ can trigger the generation of an order-of-magnitude stronger magnetic field with opposite sign at the target surface. Strong surface field generation occurs when the laser pulse is relativistically intense and results from the currents associated with the cyclotron rotation of laser-heated electrons transiting through the target and the compensating current of cold electrons. We derive a predictive scaling for this surface field generation, $B_\mathrm{gen} \sim - 2 \pi B_\mathrm{seed} \Delta x/\lambda_0$, where $\Delta x$ is the target thickness and $\lambda_0$ is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to confirm its applicability over a wide range of conditions. We additionally demonstrate that both the seed and surface-generated magnetic fields can have a strong impact on application-relevant plasma dynamics, for example substantially altering the overall expansion and ion acceleration from a $\mu$m-thick laser-irradiated target with a kilotesla-level seed magnetic field.