Expansion-driven self-magnetization of high-energy-density plasmas

Understanding plasma self-magnetization is one of the fundamental challenges in both laboratory and astrophysical plasmas. Self-magnetization can modify the plasma transport properties, altering the dynamical evolution of plasmas. Multiple high energy density (HED) experiments have observed the formation of ion-scale magnetic filaments of megagauss strength, though their origin remains debated. Here, we conduct 2D collisional particle-in-cell (PIC) simulations with a laser ray-tracing module for a fully-self-consistent simulation of the plasma ablation, expansion, and magnetization. The simulations use a planar geometry, effectively suppressing the Biermann magnetic fields, to focus on anisotropy-driven instabilities. The laser intensity is varied between 10¹³-10¹⁴ W/cm², which is relevant to HED and inertial fusion experiments where collisions must be considered. We find that above a critical intensity, the plasma rapidly self-magnetizes via an expansion-driven Weibel process, producing plasma beta of 100 and Hall parameter ω_ceτ_e>1 within the first few hundreds of picoseconds. The magnetic field is sufficiently strong to modify plasma heat transport and alter the electron temperature profile compared to an unmagnetized simulation. The effect of the transverse laser spot profile on Weibel magnetization is discussed for the parameters of typical OMEGA and NIF beams.