Nonlinear seed island generation by three-dimensional electromagnetic, gyrokinetic turbulence

TL;DR

This study demonstrates how three-dimensional electromagnetic gyrokinetic turbulence influences seed island formation and evolution in neoclassical tearing modes, revealing nonlinear interactions that seed, shape, and modify island growth.

Contribution

First to analyze seed island generation and evolution in a 3D electromagnetic gyrokinetic turbulence framework, highlighting nonlinear seeding and turbulence effects on island dynamics.

Findings

Turbulence does not prevent initial island formation.

Islands are seeded by nonlinear interactions, maintaining coherence.

Turbulence significantly alters island size and structure.

Abstract

Turbulence is shown to be critical to the onset and evolution of the neoclassical tearing mode, affecting both its growth and rotation. The interaction is here studied for the first time in the three dimensional, toroidal gyrokinetic framework. Turbulent fluctuations do not destroy the growing island early in its development, which maintains a coherent form as it grows, in fact the island is seeded and its rotation frequency determined, by nonlinear interaction. This process provides an initial structure that is of the order of an ion gyro-radius wide, allowing the island to rapidly reach a large size. A large degree of stochastisation around the seperatrix, and a complete breakdown of the X-point is seen, which significantly reduces the effective island width. A turbulent modification of the electrostatic field in and around the island greatly affects the size of the resonant layer width, and the island is seen to grow at the linear rate even though the island is significantly wider than the singular layer width.