Effect of electron diamagnetic drifts on cylindrical double-tearing modes

TL;DR

This paper investigates how electron diamagnetic drifts influence the behavior of double-tearing modes in cylindrical tokamak configurations, revealing that pressure profile details critically affect mode saturation and stability.

Contribution

It introduces the use of electron diamagnetic drifts to control double-tearing modes, demonstrating their stabilizing effect through linear and nonlinear simulations.

Findings

Diamagnetic drifts can saturate DTM growth.

Pressure profile asymmetries influence DTM evolution.

Strong outer pressure gradients stabilize the mode.

Abstract

Double-tearing modes (DTMs) have been proposed as a driver of `off-axis sawtooth' crashes in reverse magnetic shear tokamak configurations. Recently differential rotation provided by equilibrium sheared flows has been shown capable of decoupling the two DTM resonant layers, slowing the growth the instability. In this work we instead supply this differential rotation using an electron diamagnetic drift, which emerges in the presence of an equilibrium pressure gradient and finite Larmor radius physics. Diamagnetic drifts have the additional benefit of stabilizing reconnection local to the two tearing layers. Conducting linear and nonlinear simulations with the extended MHD code MRC-3d, we consider an m=2, n=1 cylindrical double-tearing mode. We show that asymmetries between the resonant layers and the emergence of an ideal MHD instability cause the DTM evolution to be highly dependent on the location of the pressure gradient. By locating a strong drift near the outer, dominant resonant surface are we able to saturate the mode and preserve the annular current ring, suggesting that the appearance of DTM activity in advanced tokamaks depends strongly on the details of the plasma pressure profile.