The effect of magnetic islands on ITG turbulence driven transport

TL;DR

This paper investigates how magnetic islands influence ITG turbulence-driven transport, revealing that turbulence can cross separatrices and affect critical island sizes, with implications for plasma confinement.

Contribution

It demonstrates the role of turbulence in crossing magnetic separatrices and its impact on island growth thresholds, providing new insights into turbulence-island interactions.

Findings

Turbulence enhances perpendicular transport across magnetic islands.

Turbulence can re-establish temperature gradients within islands.

Critical island size is larger than collisional predictions but smaller than anomalous estimates.

Abstract

In this work, we address the question of the influence of magnetic islands on the perpendicular transport due to steady-state ITG turbulence on the energy transport time scale. We demonstrate that turbulence can cross the separatrix and enhance the perpendicular transport across magnetic islands. As the perpendicular transport in the interior of the island sets the critical island size needed for growth of neoclassical tearing modes, this increased transport leads to a critical island size larger than that predicted from considering collisional conductivities, but smaller than that using anomalous effective conductivities. We find that on Bohm time scales, the turbulence is able to re-establish the temperature gradient across the island for islands widths $w \lesssim \lambda_{turb}$, the turbulence correlation length. The reduction in the island flattening is estimated by comparison with simulations retaining only the perpendicular temperature and no turbulence. At intermediate island widths, comparable to $\lambda_{turb}$, turbulence is able to maintain finite temperature gradients across the island.