Linear and Nonlinear Dynamics of Self-Consistent Collisionless Tearing Modes in Toroidal Gyrokinetic Simulations

TL;DR

This paper uses gyrokinetic simulations to explore the nonlinear behavior and saturation mechanisms of collisionless tearing modes in tokamak plasmas, revealing the roles of flows, turbulence, and plasma parameters.

Contribution

It provides new insights into the nonlinear evolution, saturation, and stabilization of tearing modes, including the effects of flows, turbulence, and plasma beta in gyrokinetic simulations.

Findings

Large islands are reduced by current redistribution and turbulence.

Kelvin-Helmholtz instability enhances island decay.

Higher plasma beta suppresses tearing modes and excites kinetic ballooning modes.

Abstract

We investigate tearing modes (TM) driven by current density gradient in collisionless tokamak plasmas by using the electromagnetic gyrokinetic simulation code ORB5. We elucidate the TM width by simulations for flat profiles, as the absence of background diamagnetic flows implies a small rotation-speed, while finite-gradients are included to investigate the TM rotation. For flat profiles, the initial saturation width of nonlinearly driven magnetic islands is related to the TM linear growth rate; however, large islands in the initial saturation phase are prone to current density redistribution that reduces the island width in the following evolution. Island-induced $\boldsymbol{E}\times\boldsymbol{B}$ and diamagnetic sheared flows develop at the separatrix, able to destabilize the Kelvin-Helmholtz instability (KHI). The KHI turbulence enhances a strong quadrupole vortex flow that reinforces the island decay, resulting in a strong reduction of the island width in an eventual steady state. This process is enhanced by trapped electrons. For finite gradients profile, the TM usually rotates in the electron diamagnetic direction, but can change direction when the ion temperature gradient dominates the other gradients. The reduced growth of the TM by diamagnetic effects results in a moderate island size, which remains almost unchanged after the initial saturation. At steady state, strong zonal flows are nonlinearly excited and dominate the island rotation, as expected from previous theoretical and numerical studies. When the plasma beta is increased, the TM mode is suppressed and a mode with the same helicity but with twisting parity, coupled with the neighboring poloidal harmonics, is destabilized, similar to the kinetic ballooning mode.