While the ideal ballooning mode is typically known to be destabilized by the ballooning force term, α, and thus the poloidal plasma beta (βP) through the relation of α∝βP, a recent work J. Y. Kim and H. S. Han, Nucl. Fusion 65, 116013 (2025) shows that a non-negligible destabilization can also arise from the toroidal plasma beta (βT) at a fixed α or βP condition. Considering the potential relevance of this destabilization to the spherical tokamak (ST) with high βT, a modeling study is presented on the dependence of its threshold on plasma shape and aspect ratio, also trying to clarify further its physics origin. It is shown that the threshold βT, which is typically minimized in the intermediate α range, has a large enhancement with increasing plasma shaping or decreasing aspect ratio. While the threshold βT is thus expected to be relatively high in ST devices where the plasma shape is very strong, it can still impose a serious limit on plasma pressure if BT is very low, and an operation approach is suggested to maximize plasma pressure under such conditions. Meanwhile, the βT destabilization is found to arise primarily from the pressure gradient term in the magnetic curvature drift velocity, which is usually negligible but can become significant when βT or its gradient is high.
Kim et al. (Wed,) studied this question.