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

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 ω (351 nm) beam delivering 5 kJ over 10 ns and a uniform background field B0 = 0–50 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 J×B force and radially confines the flow. We show that the collimation strengthens with increasing applied magnetic field, as stronger fields reduce the plasma β and correspondingly enhance the confining J×B force. We discuss scaling to solar coronal jets and argue that low-β, magnetic-pressure-dominated ambient conditions promote similar diamagnetic cavity formation and J×B collimation in coronal outflows.