Strong gradient neoclassical transport in the plateau regime

TL;DR

This paper extends neoclassical transport theory into the plateau regime to account for strong gradients in tokamak regions like pedestals, revealing significant effects on transport predictions.

Contribution

It introduces a new framework that incorporates strong gradient effects into plateau regime neoclassical theory using a large aspect ratio expansion.

Findings

Strong gradients cause in-out and up-down poloidal asymmetries.

Strong gradient effects can both enhance and reduce transport compared to standard theory.

The framework applies to regions with gradient lengths comparable to ion poloidal gyroradius.

Abstract

Strong gradient regions in tokamaks such as the pedestal or internal transport barriers are regions of reduced turbulence where neoclassical transport can play a dominant role. In pedestals, gradient lengths comparable to the ion poloidal gyroradius have been measured. Standard neoclassical theory can miss important strong gradient effects in these regions because it assumes that the gradient length scales of density, temperature and potential are larger than the ion poloidal gyroradius. We extend plateau regime neoclassical theory into regions of gradients of the order of the ion poloidal gyroradius to capture strong gradient effects on transport processes in the pedestal and internal transport barriers. The fundamental idea behind our new framework is to keep a scale separation between the orbit widths and the gradient length scales by performing a large aspect ratio expansion. In the plateau regime, strong gradients cause poloidal variation that is in-out as well as up-down asymmetric. We study two different test cases assuming either radial force balance or the absence of turbulence and show that strong gradient effects can enhance or reduce standard neoclassical theory predictions in the plateau regime in strong gradient regions.