Collision frequency dependence of polarization current in neoclassical tearing modes

TL;DR

This study investigates how the neoclassical polarization current varies with collision frequency in tokamak plasmas, revealing a maximum contribution in the intermediate collision regime, which impacts the understanding of tearing mode evolution.

Contribution

It provides the first detailed analysis of the collision frequency dependence of the neoclassical polarization current in the transition regime, including its frequency dependence and impact on island evolution.

Findings

Maximum polarization current occurs at intermediate collision frequencies.

Transition regime analysis bridges collisionless and collisional limits.

Results inform control strategies for neoclassical tearing modes in tokamaks.

Abstract

The neoclassical polarization current, generated when a magnetic island propagates through a tokamak plasma, is believed to influence the initial stage of the neoclassical tearing mode evolution. Understanding the strength of its contribution in the relevant plasma collision frequency regimes for future tokamaks such as ITER is crucial for the successful control and/or avoidance of the neoclassical tearing mode. A nonlinear drift kinetic theory is employed to determine the full collision frequency dependence of the neoclassical polarization current in the small island limit, comparable to the trapped ion orbit width. Focusing on the region away from the island separatrix (where a layer with a complex mix of physics processes exists), we evaluate for the first time the variation of the neoclassical ion polarization current in the transition regime between the analytically tractable collisionless and collisional limits. In addition, the island propagation frequency-dependence of the neoclassical polarization current and its contribution to the island evolution is revealed. For a range of propagation frequencies, we find that the neoclassical polarization current is a maximum in the intermediate collision frequency regime analyzed here - a new and unexpected result.