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

Abstract: 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. Most high energy density (HED) laser experiments on magnetic reconnection and unmagnetized collisionless shocks rely on either Biermann or Weibel mechanisms to self-consistently 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. Models based on Particle-in-Cell (PIC) simulations have been proposed to explain magnetization, including plasma interpenetration-driven Weibel, temperature gradient-driven Weibel, and adiabatic expansion-driven Weibel. Here, we conducted 2D collisional PIC simulations with a laser ray-tracing module to simulate plasma ablation, expansion, and subsequent 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 $10^{13}$-$10^{14}$ W/cm$^2$, which is relevant to HED and ICF experiments where collisions must be considered. We find that above a critical intensity, the plasma rapidly self-magnetizes via an expansion-driven Weibel process, generating plasma beta of 100 ($\beta = 2k_B n_eT_e/B^2$) with the Hall parameter $\omega_{\rm ce}\tau_{e}>1$ within the first few hundreds of picoseconds. Implications of plasma magnetization for heat transport are also discussed.