# Field-line localized destabilization of ballooning modes in 3D tokamaks

## TL;DR

This paper reports the observation and analysis of field-line localized ballooning modes at the edge of high confinement plasmas in 3D tokamaks, showing how 3D magnetic distortions influence stability.

## Contribution

It demonstrates the localization of ballooning modes to specific field-lines and links this to 3D magnetic shear distortions affecting stability predictions.

## Key findings

- Ballooning modes are localized to specific field-lines during rotation.
- 3D magnetic shear distortions lower stability compared to axisymmetric cases.
- Localization matches predictions from 3D ideal MHD stability calculations.

## Abstract

Field-line localized ballooning modes have been observed at the edge of high confinement mode plasmas in ASDEX Upgrade with rotating 3D perturbations induced by an externally applied n = 2 error field and during a moderate level of edge localized mode-mitigation. The observed ballooning modes are localized to the field-lines which experience one of the two zero-crossings of the radial flux surface displacement during one rotation period. The localization of the ballooning modes agrees very well with the localization of the largest growth rates from infinite-n ideal ballooning stability calculations using a realistic 3D ideal magnetohydrodynamic equilibrium. This analysis predicts a lower stability with respect to the axisymmetric case. The primary mechanism for the local lower stability is the 3D distortion of the local magnetic shear.