Nonlinear Saturation of Ballooning Modes in Stellarators
Abstract
Ballooning mode saturation is investigated in realistic stellarator configurations using the flux tube approach of Ham et. al. [1] [2]. The method is adapted to account for the lack of exact force balance in stellarator equilibrium solvers that assume existence of nested flux surfaces. A variational approach for calculating flux tube energy is developed to overcome this force error problem in stellarator numerical equilibria. Saturated (equilibrium) flux tube states that cross 10-20% of the plasma minor radius are shown to exist for linearly ballooning unstable profiles. It is shown that several features of the displaced flux tube structure in a full nonlinear MHD simulation of Wendelstein 7X are reproduced by our model. Saturated states are found in a compact stellarator equilibrium close but below the marginal ballooning linear instability, i.e. the unperturbed equilibrium is metastable. This suggests that Edge-Localized-Mode-like explosive MHD behavior may be possible in stellarators.
Cite
@article{arxiv.2602.17964,
title = {Nonlinear Saturation of Ballooning Modes in Stellarators},
author = {X. Chu and S. C. Cowley and N. Ferraro and Y. Zhou and F. I. Parra},
journal= {arXiv preprint arXiv:2602.17964},
year = {2026}
}