[Paper Review] [Plasma 2020 Decadal] The Material Properties of Weakly Collisional, High-Beta Plasmas

Matthew W. Kunz, Jonathan Squire+15 more|arXiv (Cornell University)|Mar 11, 2019

Microstructure and mechanical propertiesMaterials Science3 citations

TL;DR

This white paper identifies the critical need for a predictive fluid-level theory of weakly collisional, high-beta plasmas—where thermal pressure dominates magnetic pressure and kinetic instabilities govern transport—by integrating kinetic physics into large-scale dynamics. It proposes advancing theory, computation, and experiments to resolve multi-scale interactions, with key progress in modeling microinstabilities, enabling high-fidelity simulations, and leveraging in-situ space data and lab experiments.

ABSTRACT

This white paper, submitted for the Plasma 2020 Decadal Survey, concerns the physics of weakly collisional, high-beta plasmas -- plasmas in which the thermal pressure dominates over the magnetic pressure and in which the inter-particle collision time is comparable to the characteristic timescales of bulk motions. This state of matter, although widespread in the Universe, remains poorly understood: we lack a predictive theory for how it responds to perturbations, how it transports momentum and energy, and how it generates and amplifies magnetic fields. Such topics are foundational to the scientific study of plasmas, and are of intrinsic interest to those who regard plasma physics as a fundamental physics discipline. But these topics are also of extrinsic interest: addressing them directly informs upon our understanding of a wide variety of space and astrophysical systems, including accretion flows around supermassive black holes, the intracluster medium (ICM) between galaxies in clusters, and regions of the near-Earth solar wind. Specific recommendations to advance this field of study are discussed.

Motivation & Objective

• Address the lack of a predictive fluid theory for weakly collisional, high-beta plasmas, where kinetic instabilities dominate transport and thermodynamics.
• Overcome the challenge of coupling large-scale fluid motions with microscale kinetic instabilities that arise due to pressure anisotropy and heat fluxes.
• Develop simplified fluid models that retain essential kinetic physics—such as effective collisionality from microinstabilities—while remaining tractable in complex astrophysical geometries.
• Bridge gaps between theoretical predictions, computational simulations, and observational data from spacecraft and telescopes to validate and refine models.
• Prioritize mid-range supercomputing access and experimental programs to study high-beta plasma behavior in controlled settings.

Proposed method

• Formulate fluid-like equations that incorporate the effects of kinetic gyroscale instabilities (e.g., firehose, mirror, and heat flux-driven instabilities) through effective transport coefficients.
• Use 6D kinetic simulations (PIC and continuum methods) to model ab initio plasma dynamics at fluid scales while resolving ion and electron gyroscales.
• Apply hybrid-kinetic and fully kinetic approaches to study instabilities in the presence of strong field-aligned heat fluxes and anisotropic pressures.
• Integrate theoretical insights into computational frameworks to improve stability, accuracy, and efficiency—especially for high-beta, low-collisionality regimes.
• Leverage in-situ data from MMS and long-duration missions to validate kinetic models and distribution function evolution.
• Design targeted laboratory experiments using helicon sources, mirror configurations, or inertial confinement to access high-beta, weakly collisional conditions.

Experimental results

Research questions

• RQ1 How do kinetic instabilities such as the firehose and mirror modes regulate momentum and energy transport in high-beta, weakly collisional plasmas?
• RQ2 What effective fluid behavior emerges from the nonlinear saturation of microinstabilities, and how can it be captured in simplified fluid models?
• RQ3 How do large-scale fluid motions and magnetic reconnection interact with microscale kinetic instabilities in high-beta environments?
• RQ4 What role do heat fluxes and electron-scale physics play in driving instabilities and modifying plasma transport at high beta?
• RQ5 How can computational methods be optimized to simulate high-beta, weakly collisional plasmas efficiently and accurately at scale?

Key findings

• Microinstabilities such as the firehose and mirror modes are excited when pressure anisotropy exceeds ∼B²/4π, even at small amplitudes, due to the near-conservation of the magnetic moment in weakly collisional plasmas.
• At high β, these instabilities grow and saturate on timescales much faster than large-scale fluid motions, leading to effective collisionality and modified transport properties.
• Kinetic instabilities significantly alter particle distribution functions and regulate heat fluxes, even in the limit of very weak magnetic fields (β ≫ 1).
• Current computational methods, including 6D PIC and continuum simulations, can now model fluid-scale systems with resolved gyroscales, enabling ab initio studies of multi-scale dynamics.
• In-situ spacecraft data—especially from MMS—reveal microinstability-driven non-thermal features in distribution functions, providing critical validation for kinetic models.
• Mid-range supercomputing allocations (10–50 M CPU-hours) are essential for routine high-fidelity kinetic simulations but remain underfunded and underavailable.