General Approach for Deriving Reference Distribution Functions for Systems out of Equilibrium by Statistical Thermodynamics

TL;DR

This paper presents a general thermodynamic approach to derive reference distribution functions for non-equilibrium systems, exemplified by tokamak plasmas, incorporating multiple forces and entropy principles.

Contribution

It introduces a comprehensive method for deriving distribution functions applicable to systems with multiple thermodynamic forces, extending current models used in plasma physics.

Findings

Derived a more general distribution function than current models.

Linked distribution parameters to external power sources.

Applied the approach to tokamak plasma models.

Abstract

A general approach for deriving the expression of reference (density of) distribution functions, F^0, by statistical thermodynamics and the definition of local equilibrium conditions is illustrated. This procedure may be adopted for a system subject to an arbitrary number of thermodynamic forces. For concreteness, we analyze the case of a system submitted to three independent thermodynamic forces and the local equilibrium corresponds to the configuration of minimum entropy production condition and the maximum entropy principle. In this limit case, we show that the derived expression of distribution function is more general than that one, which is currently used for fitting the numerical steady-state solution obtained by simulating the Ion Cyclotron Radiation Heating (ICRH) FAST-plasmas and for describing various scenarios of tokamak plasmas. Through kinetic theory, we fixed the free parameters linking them with the external power supplies. The singularity at low energy in the proposed distribution function is related to the intermittency in the turbulent plasma. As a matter of fact, this work is not restricted to, but applied to, tokamak plasmas. Tokamak-plasmas are taken as an example of close thermodynamic systems. An application to a simple model of fully ionized tokamak-plasmas submitted to an external Ohmic source is discussed.