Neoclassical transport and profile prediction in transport barriers

TL;DR

This paper extends neoclassical transport theory to accurately predict plasma profiles in transport barriers of tokamaks, accounting for steep gradients and their effects on plasma behavior.

Contribution

It introduces a modified neoclassical theory applicable to transport barriers with short gradient scales, revealing new nonlinear couplings and multiple solution states.

Findings

Particle and momentum transport are interconnected.

Density and potential profiles are poloidally varying in barriers.

Multiple co-existing plasma states may explain H-L transitions.

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. However, standard neoclassical transport theory assumes that the gradient length scales of density, temperature, and potential are of the order of the system size. In the pedestal, gradient length scales are much shorter and are measured to be of the order of the ion poloidal gyroradius. We present an extension of neoclassical theory that is applicable in transport barriers of large aspect ratio tokamaks. We show that particle and momentum transport are connected in such a way that a source of parallel momentum can drive a significant neoclassical ion particle flux. In strong gradient regions, density, electric potential, mean parallel flow, and ion temperature are shown to no longer be flux functions. Instead, they have a small but important poloidally varying piece that modifies the transport equations to lowest order. This introduces a nonlinearity in the transport problem through the coupling with quasineutrality that yields multiple co-existing solutions when solving for the plasma profiles. The different solutions could be connected to low and high transport states and jumps between solutions could be an indication of H-L back-transitions.