Underlying reason for differing KBM characteristics within quasi-axisymmetric stellarators

Determine the physical mechanisms responsible for the qualitatively different kinetic ballooning mode characteristics observed among quasi-axisymmetric stellarator configurations, specifically explaining why kinetic ballooning mode destabilization occurs at significantly lower local plasma beta in some quasi-axisymmetric equilibria than the macroscopic magnetohydrodynamic beta limit, whereas prior quasi-axisymmetric studies (e.g., the National Compact Stellarator eXperiment) found the kinetic ballooning mode threshold to coincide with the magnetohydrodynamic limit.

Background

The paper contrasts linear and nonlinear gyrokinetic findings for a reactor-scale quasi-axisymmetric stellarator with prior results from quasi-axisymmetric (NCSX) and quasi-isodynamic (W7-X) devices. In the studied configuration, kinetic ballooning modes (KBMs) destabilize at low local plasma beta, well below the macroscopic MHD beta limit established during equilibrium optimization, diverging from earlier quasi-axisymmetric conclusions that KBM thresholds matched MHD limits.

This discrepancy suggests intrinsic variation in KBM behavior even within the same stellarator class (quasi-axisymmetric), raising a fundamental question about the physical origin of these differences and how magnetic geometry or profile choices govern KBM thresholds. Understanding this mechanism is crucial for reliable prediction and optimization of confinement regimes and beta limits in future quasi-axisymmetric stellarators.

References

This suggests the existence of qualitatively different KBM characteristics even in the same class of stellarator configuration, the underlying reason for which is not understood.

— On the accessibility of stable reactor operating regimes in quasi-symmetric stellarators  (2512.22355 - Wright et al., 26 Dec 2025) in Section 3 (Results and discussion)