Early evolution of electron cyclotron driven current during suppression of tearing modes in a circular tokamak

TL;DR

This study investigates the early evolution of electron cyclotron driven current in a circular tokamak, revealing how it spreads and stabilizes within magnetic islands, affecting tearing mode suppression.

Contribution

The paper introduces a detailed fluid model and high-resolution simulations to analyze the initial spreading and steady-state behavior of EC-driven current in magnetic islands.

Findings

EC current initially spreads independently of magnetic flux

Steady-state EC current becomes approximately a flux function with sufficient resolution

Early EC current evolution impacts magnetic island size and stability

Abstract

When electron cyclotron (EC) driven current is first applied to the inside of a magnetic island, the current spreads throughout the island and after a short period achieves a steady level. Using a two equation fluid model for the EC current that allows us to examine this early evolution in detail, we analyze high-resolution simulations of a 2/1 classical tearing mode in a low-beta large aspect-ratio circular tokamak. These simulations use a nonlinear 3D reduced-MHD fluid model and the JOREK code. During the initial period where the EC driven current grows and spreads throughout the magnetic island, it is not a function of the magnetic flux. However, once it has reached a steady-state, it should be a flux function. We demonstrate numerically that if sufficiently resolved toroidally, the steady-state EC driven current becomes approximately a flux function. We discuss the physics of this early period of EC evolution and its impact on the size of the magnetic island.