A Tale of Two Polarization Paradoxes I: the Diamagnetic Polarization Paradox

TL;DR

This paper resolves two polarization paradoxes in magnetized plasma physics, clarifies the diamagnetic polarization factor discrepancy, and provides accurate expressions for polarization density considering gyrokinetic effects and anisotropic distributions.

Contribution

It demonstrates the consistency between different theoretical approaches to diamagnetic polarization and derives first-order accurate polarization expressions within gyrokinetic theory.

Findings

Resolved the diamagnetic polarization paradox with a factor of 1/2 discrepancy.

Derived polarization density expressions accurate to first order in amplitude.

Showed that energy and momentum invariants do not produce net polarization effects.

Abstract

An accurate calculation of the total polarization charge density in a plasma is essential for a self-consistent determination of the electric field. Yet, for a magnetized plasma, there are two different polarization paradoxes that are finally resolved in this work. The "diamagnetic polarization paradox'' refers to the fact that there is a paradoxical factor of 1/2 difference between the pressure-driven "diamagnetic polarization'' density calculated using the real space drift theory versus the action-angle space guiding center and gyrokinetic theory that has not been explained before. In this work, it is shown that the results of both approaches can be made consistent with one another. Half of the diamagnetic polarization is due to the transformation from the guiding center density to the real space density. The other half is due to the fact that, within the drift kinetic ordering assumptions, the guiding center density should be expressed as the gyroaverage of the density in the limit of vanishing Larmor radius. Expressions for the diamagnetic polarization density are given that are accurate to first order in amplitude and all orders in gyroradius within gyrokinetic theory for a constant magnetic field. Applications to anisotropic Maxwell-Boltzmann particle distribution functions are presented. Because the total energy and toroidal momentum are local invariants, they do not generate net polarization effects: the electric and thermodynamic polarizations must precisely cancel. In contrast, anisotropic dependance on the magnetic moment generates a net polarization proportional to the temperature anisotropy.