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arXiv · 2505.10153

Resonant theory of kinetic ballooning modes in general toroidal geometry

P. Mulholland·A. Zocco·M. C. L. Morren·K. Aleynikova·M. J. Pueschel·J. H. E. Proll·P. W. Terry
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Abstract

The linear theory of the kinetic-ballooning-mode (KBM) instability is extended to capture a weakly-driven regime in general toroidal geometry where the destabilization is caused by the magnetic-drift resonance of the ions. Such resonantly-destabilized KBMs are characterized by broad eigenfunctions along the magnetic field line and near-marginal positive growth rates, even well below the beta threshold of their non-resonant counterparts. This unconventional (or sub-threshold) KBM, when destabilized, has been shown to catalyze an enhancement of turbulent transport in the Wendelstein 7-X (W7-X) stellarator [1, 2]. Simplifying the energy dependence of key resonant quantities allows for an analytical treatment of this KBM using the physics-based ordering from the more general equations of Tang, Connor, and Hastie [3]. Results are then compared with high-fidelity gyrokinetic simulations for the (st)KBM in W7-X and the conventional KBM in a circular tokamak at both high and low magnetic shear, where good agreement is obtained in all cases. This reduced KBM model provides deeper insight into (sub-threshold) KBMs and their relationship with geometry, and shows promise for aiding in transport model development and geometry-based turbulence optimization efforts going forward.

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P. Mulholland, A. Zocco, M. C. L. Morren, K. Aleynikova, M. J. Pueschel, J. H. E. Proll, P. W. Terry. 2025-05-15. Resonant theory of kinetic ballooning modes in general toroidal geometry. https://doi.org/10.1017/s002237782510086x

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