n=1 ideal MHD stability of tokamak plasmas with low aspect ratio and negative triangularity

Abstract Spherical tokamaks (STs) exhibit excellent ideal magnetohydrodynamic (MHD) stability owing to their low-aspect-ratio configuration, while negative triangularity (NT) configurations have attracted extensive attention in recent years for their superior confinement performance, despite relatively inferior MHD stability. This paper investigates the n=1 ideal MHD stability of NT configurations at low aspect ratios, to evaluate the potential compatibility of these two advantageous regimes. Using the MARS-F code, we perform beta-limit scans over various triangularity and aspect ratio values, with fixed safety factor $q$ profiles and normalized pressure gradient $p'$ profiles. The results show that the stability of positive triangularity (PT) configurations is enhanced at low aspect ratios, consistent with classical expectations; however, counterintuitively, the stability of NT configurations deteriorates with decreasing aspect ratio. We present a physical interpretation of this phenomenon, demonstrating that two intuitive concepts ( the ``most natural shape" and ``good/bad curvature") and magnetic well depth can fully explain this geometric effect and the observed stability trend. In addition, the stabilizing effect of the safety factor preserves at low aspect ratios for both PT and NT configurations, partially mitigating the aforementioned stability deterioration of NT plasmas.