Bohm-like Neoclassical Transport in Highly Collisional Toroidal Plasmas with High Density Gradients

TL;DR

This paper introduces a new neoclassical transport regime in highly collisional toroidal plasmas with high density gradients, characterized by Bohm-like scaling and validated through simulations, improving impurity transport modeling.

Contribution

It identifies a novel transport regime where collisions suppress return flow, leading to Bohm-like scaling, and confirms this with full-orbit particle simulations.

Findings

Collision suppresses return flow in high-density plasmas.

Transport flux is dominated by vacuum toroidal field drift.

The regime is validated by particle simulations.

Abstract

Conventional neoclassical theory in the Pfirsch-Schl\"{u}ter regime fails to accurately model collision-induced transport in toroidal plasmas with high density gradients. In this scenario, we find that collision suppresses the return flow, leading to the dominance of the transport flux by the vacuum toroidal field drift with a reduced Bohm-like scaling. The new regime is also confirmed by full-orbit particle simulations, and can be employed to improve the accurate modeling of impurity transport in toroidal magnetized plasmas.