Simulations of self-magnetization in expanding high-energy-density plasmas*

Plasma magnetization is one of the fundamental challenges in both laboratory and astrophysical plasmas. Most high energy density (HED) laser experiments on magnetic reconnection, magnetized and unmagnetized collisionless shocks rely on either Biermann or Weibel mechanism to generate the magnetic fields of interest. Multiple HED experiments have observed the formation of ion-scale magnetic filaments of megagauss strength, though their origin remains debated. Theories based on Particle-in-Cell (PIC) simulations have been proposed to explain magnetization, including plasma interpenetration-driven Weibel [1], temperature gradient-driven Weibel [2], and adiabatic expansion-driven Weibel [3]. Here we consider laser intensity of 10¹³-10¹⁵ W/cm² relevant to HEDP and ICF experiments, where collisions must be considered. We develop a first principles model with collisional 2D PIC simulations including interaction with a laser ray tracing and laser heating module [4] to simulate plasma ablation, expansion, and subsequent magnetization in planar geometry, effectively suppressing the Biermann battery. Results show that the plasma rapidly self-magnetizes, generating plasma beta of 100 (β=2μ₀n_eT_e/B²)  with the Hall parameter ω_ceτ_ei>1 within 1 ns, and the plasma dynamics is largely consistent with the expansion-driven Weibel hypothesis. Implications of plasma magnetization for heat flux suppression are also discussed.