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  • Open Access

Modeling transport in weakly collisional plasmas using thermodynamic forcing

Prakriti Pal Choudhury* and Archie F. A. Bott

  • *Contact author: prakriti.palchoudhury@physics.ox.ac.uk
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Phys. Rev. E 113, 065212 – Published 24 June, 2026

DOI: https://doi.org/10.1103/xtn8-r48v

Abstract

How momentum, energy, and magnetic fields are transported in the presence of macroscopic gradients is a fundamental question in plasma physics. Answering this question is especially challenging for weakly collisional, magnetized plasmas, where macroscopic gradients influence the plasma's microphysical structure. In this paper, we introduce thermodynamic forcing, a new method for systematically modeling how macroscopic gradients in magnetized or unmagnetized plasmas shape the distribution functions of constituent particles. In this method, we propose to apply an anomalous force to those particles inducing the anisotropy that would naturally emerge due to macroscopic gradients in weakly collisional plasmas in which thermal pressure is much larger than magnetic pressure. We implement thermodynamic forcing in particle-in-cell (TF-PIC) simulations using a modified Vay particle pusher and validate it against analytic solutions of the equations of motion. We then carry out a series of simulations of electron-proton plasmas with periodic boundary conditions using TF-PIC. First, we confirm that the properties of two electron-scale kinetic instabilities—one driven by a temperature gradient and the other by bulk-velocity gradient—are consistent with previous results. Then, we demonstrate that in the presence of both macroscopic gradients, heat-flux saturation is mediated by the bulk-velocity-gradient-driven electron firehose instability rather than the temperature-gradient-driven whistler instability. This suggests that saturation mechanisms may differ from our current understanding in the presence of multiple free energy sources. This work enables, for the first time, systematic and self-consistent transport modeling in weakly collisional plasmas, with broad applications in astrophysics, laser-plasma physics, and inertial confinement fusion.

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Phys. Rev. E 113, 065212– Published 24 June, 2026
  • Received 24 April 2025
  • Revised 14 February 2026
  • Accepted 2 June 2026
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