Abstract Details
Abstracts
Author: Xu Chu
Requested Type: Consider for Invited
Submitted: 2026-02-28 23:18:08
Co-authors: S.C.Cowley, F.I.Parra
Contact Info:
Princeton Plasma Physics Laboratory
100 Stellarator Rd.
Princeton, New Jersy 08540
Mercer
Abstract Text:
Stellarators have exhibited disruptive instabilities, such as core density collapse in LHD, that release much of the stored energy. Such instabilities would be problematic in reactors. We investigate when ballooning mode instabilities can provide such a “hard” limit. We adapt the model developed by Ham et al. [Plasma Phys. Control. Fusion,60 075017(2018)] that calculates the displacement of an isolated flux tube as the saturated state of a ballooning mode. This model is extended to stellarators. Saturated flux tube states that cross 10-20% of the plasma minor radius are shown to exist for stellarators with linearly ballooning-unstable profiles. Comparisons of the calculated displaced flux tubes with flux tube structures observed in a W7X extended MHD simulation [Phys. Rev. Lett. 133,135102(2024)] show good agreement. Stellarator equilibria that are metastable to ballooning modes have been found, opening the possibility that ballooning modes can set a hard stability limit in stellarators, like Edge Localized Modes (ELMs) do in tokamaks.
The nonlinear ballooning theory is further extended to describe flux tubes of finite thickness. The bending of the magnetic field outside the flux tube due to the finite thickness of the tube is balanced by the compression of the fields inside the flux tube. According to this balance, the aspect ratio of the flux tube cross-section scales with the square of its width along the direction of the flux tube motion, unlike the linear balloon scaling, where the aspect ratio scales linearly with that width. A closed set of equations is derived to describe the magnetic field inside and outside the finite-sized flux tube, and it is solved numerically using a Picard iteration method. Comparisons with equilibrium flux tube shapes in a fully nonlinear MHD simulation using Athena++ show good agreement. This model can be potentially used to estimate ELM filament sizes and the induced transport to the divertor target from the ELM filaments.
Characterization: 1.0
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