# Numerical study of tearing mode seeding in tokamak X-point plasma

## TL;DR

This study uses numerical simulations to analyze how magnetic perturbations seed tearing modes in tokamak plasmas, revealing the dynamics, thresholds, and hysteresis effects relevant for controlling plasma stability.

## Contribution

The paper provides the first detailed numerical analysis of tearing mode seeding phases and thresholds using the JOREK code, validated against experimental data from ASDEX Upgrade.

## Key findings

- Three seeding phases observed in simulations match experiments.
- Thresholds for mode penetration depend on plasma rotation and RMP current.
- Hysteresis effects and parity transitions during mode seeding are identified.

## Abstract

A detailed understanding of island seeding is crucial to avoid (N)TMs and their negative consequences like confinement degradation and disruptions. In the present work, we investigate the growth of 2/1 islands in response to magnetic perturbations. Although we use externally applied perturbations produced by resonant magnetic perturbation (RMP) coils for this study, results are directly transferable to island seeding by other MHD instabilities creating a resonant magnetic field component at the rational surface. Experimental results for 2/1 island penetration from ASDEX Upgrade are presented extending previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge with low collisionality and active RMP coils. Our numerical studies are performed with the 3D, two fluid, non-linear MHD code JOREK. All three phases of mode seeding observed in the experiment are also seen in the simulations: first a weak response phase characterized by large perpendicular electron flow velocities followed by a fast growth of the magnetic island size accompanied by a reduction of the perpendicular electron velocity, and finally the saturation to a fully formed island state with perpendicular electron velocity close to zero. Thresholds for mode penetration are observed in the plasma rotation as well as in the RMP coil current. A hysteresis of the island size and electron perpendicular velocity is observed between the ramping up and down of the RMP amplitude consistent with an analytically predicted bifurcation. The transition from dominant kink/bending to tearing parity during the penetration is investigated.