Collimation of diamagnetic laser-driven plasma outflows by an ambient magnetic-pressure gradient

TL;DR

This paper uses magnetohydrodynamic simulations to demonstrate how an ambient magnetic-pressure gradient can effectively collimate laser-driven plasma outflows, mimicking coronal plasma jets.

Contribution

It introduces detailed MHD modeling including non-ideal effects to show magnetic collimation mechanisms in laser-driven plasma flows.

Findings

Stronger magnetic fields enhance plasma collimation.

Diamagnetic cavities form within plasma plumes.

Magnetic pressure gradients exert inward forces that confine the flow.

Abstract

We present magnetohydrodynamic simulations of laser driven plasma outflows propagating along an externally applied poloidal magnetic field, designed to mimic coronal open-field plasma jets. Using the FLASH code with non-ideal terms (resistivity, Biermann battery, and Nernst advection) included, we model a CH target driven by a 3$\omega$ (351 nm) beam delivering 5 kJ over 10 ns and a uniform background field $\text{B}_0$ = 0 to 50 T. Under these conditions, the expanding plume develops a central low-density diamagnetic cavity bounded by a high-magnetic-pressure shell. Magnetic flux is advected from the plume center to its edge, and azimuthal diamagnetic currents form that decrease fields inside the cavity and amplify fields outside, producing a radial magnetic-pressure gradient that exerts an inward $\text{J}\times \text{B}$ force and radially confines the flow. We show that the collimation strengthens with increasing applied magnetic field, as stronger fields reduce the plasma $\beta$ and correspondingly enhance the confining $\text{J}\times \text{B}$ force.