Non-Ambipolarity of Microturbulent Transport

Abstract: Even what is called electrostatic microturbulence produces a plasma-beta-dependent 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 $\vec{B}\cdot \vec{\nabla} Φ= \vec{B}\cdot \vec{\nabla} p_e$, where $p_e$ is the electron pressure. This potential produces radial transport similar to that of diffusion coefficient $D_{ef}= (Δ/a_T)T_e/eB$. $Δ$ is the radial distance over which the potential $Φ$ 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 counter balance a non-ambipolar part of the ion radial particle diffusion $f_{na}$. 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.