Non-Ambipolarity of Microturbulent Transport

TL;DR

This paper investigates how chaotic magnetic fields in microturbulence lead to non-ambipolar electron and ion transport, affecting plasma confinement and the need for electric fields.

Contribution

It introduces a model linking chaotic magnetic fields to electron transport and non-ambipolar ion diffusion, providing calculations for maximum non-ambipolarity and plasma beta.

Findings

Chaotic magnetic fields induce electron transport similar to diffusion.

Electron transport can counterbalance non-ambipolar ion diffusion.

Derived maximum non-ambipolarity and plasma beta thresholds.

Abstract

When restricted to magnetic flux tubes, the gyrokinetic theory of microturbulence gives the same radial transport for ions and electrons. But, exact magnetic surfaces do not exist in the presence of what is called electrostatic microturbulence. At a finite plasma pressure, a turbulent electric potential is accompanied by a turbulent magnetic field $\tilde{B}$, which makes the magnetic field lines chaotic. Quasi-neutrality along the chaotic magnetic field lines requires a potential that obeys $en \vec{B}\cdot \vec{\nabla} \Phi = \vec{B}\cdot \vec{\nabla} p_e$, where $p_e$ is the electron pressure. This potential produces radial transport similar to that of diffusion coefficient $D_{ef}= (\Delta/a_T)T_e/eB$. $\Delta$ is the radial distance over which the potential $\Phi$ is correlated by the electron motion along the chaotic magnetic field, and $|dT_e/dr| = T_e/a_T$. The chaos-produced electron transport gives an effective viscosity on the electron flow, which can counterbalance a non-ambipolar part of the ion radial particle diffusion that is $f_{na}$ times gyro-Bohm diffusion. This non-ambipolarity would otherwise require a radial electric field that confines ions and hence impurities. The maximum $f_{na}$ that can be counterbalanced and the required plasma beta to avoid shielding the magnetic perturbations $\tilde{B}$ are calculated.