Generalized fluid theory including non-Maxwellian kinetic effects

TL;DR

This paper develops a generalized fluid theory that incorporates non-Maxwellian kinetic effects, enabling more accurate and computationally efficient modeling of plasma turbulence and transport phenomena.

Contribution

It introduces a minimal set of fluid equations with analytic non-Maxwellian distribution functions, bridging the gap between kinetic and fluid models in plasma physics.

Findings

Derived fluid closures from Landau Fokker-Planck operator for arbitrary collisionality

Implemented a minimal INMDF fluid model to simulate particle and heat diffusion

Identified the origin of diffusion as a competition between INMDF growth and thermalization

Abstract

The results obtained by the plasma physics community for the validation and the prediction of turbulence and transport in magnetized plasma come mainly from the use of very CPU-consuming particle-in-cell or (gyro)kinetic codes which naturally include non-Maxwellian kinetic effects. To date, fluid codes are not considered to be relevant for the description of these kinetic effects. Here, after revisiting the limitations of the current fluid theory developed in the 19th century, we generalize the fluid theory including kinetic effects such as non-Maxwellian super-thermal tails with as few fluid equations as possible. The collisionless and collisional fluid closures from the nonlinear Landau Fokker-Planck collision operator are shown for an arbitrary collisionality. Indeed, the first fluid models associated with two examples of collisionless fluid closures are obtained by assuming an analytic non-Maxwellian distribution function (e.g., the INMDF [O. Izacard, Phys. Plasmas 23, 082504 (2016)]). One of the main differences with the literature is our analytic representation of the distribution function in the velocity phase space with as few hidden variables as possible thanks to the use of non-orthogonal basis sets. These new non-Maxwellian fluid equations could initiate the next generation of fluid codes including kinetic effects and can be expanded to other scientific disciplines such as astrophysics, condensed matter, or hydrodynamics. As a validation test, we perform a numerical simulation based on a minimal reduced INMDF fluid model. The result of this test is the discovery of the origin of particle and heat diffusion. The diffusion is due to the competition between a growing INMDF on short time scales due to spatial gradients and the thermalization on longer time scales. The results shown here could draw the breaking of some unsolved understandings of the turbulence.