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

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$ω$ (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 $β$ and correspondingly enhance the confining $\text{J}\times \text{B}$ force.

Figures (8)

• Figure 1: (a) Initial simulation setup. A polystyrene (CH) target, comprising a 100 $\mu$m foil and a 230 $\mu$m washer with a 400 $\mu$m central hole, is driven by a laser beam (purple). A uniform poloidal magnetic field (green arrows) is applied. (b) Electron number density (log scale) for outflows at different applied magnetic field strengths. By 30 ns, outflows with $\text{B}_0$ = 0 and 20 T have expanded beyond the computational domain radially.
• Figure 2: Time evolution of the plasma jet aspect ratio (10–30 ns) for applied magnetic fields ($\text{B}_0$) from 0 to 50 T. A lower aspect ratio indicates stronger collimation. The data for $\text{B}_0$ = 0, 10, and 20 T terminate at 23, 24, and 30 ns, respectively, when the jet's radial expansion exceeds the computational domain, making the width unmeasurable.
• Figure 3: For the $\text{B}_0$ = 50 T case at 30 ns: (a) Logarithm of the electron number density, annotated to show the nozzle jet, high-density shell, diamagnetic cavity, and magnetic field lines. (b) Logarithm of the magnetic field strength, with a schematic overlay indicating the high magnetic pressure region ($\text{B} > \text{B}_0$), the confining $\text{J}\times \text{B}$ force, and the diamagnetic current. (c) Logarithm of the plasma $\beta$, with the red contour line marking $\beta$ = 1.
• Figure 4: Profiles of plasma beta ($\beta$) and magnetic field (B) across the radial direction at the half-height of the outflow for the $\text{B}_0$ = 50 T case at 30 ns. Results are compared for the regular simulation and cases excluding the Nernst effect or resistive diffusion.
• Figure 5: Plasma pressure profile across the radial direction at the mid-height of the outflow for the $\text{B}_0$ = 50 T caseat simulation times of 10, 20 and 30 ns. The $\mathbf{J}_{\text{dia}}$ indicate the diamagnetic current responsible for expelling the magnetic field. The opposing diamagnetic current (Opp $\mathbf{J}_{\text{dia}}$) arises from the pressure gradient directed toward the cavity shell.