The impact of plasma shaping on the stability of Neoclassical Tearing Modes in DIII-D

Mitigating neoclassical tearing modes (NTMs) is critical for future tokamaks, as they are a leading cause of disruptions in present-day fusion experiments. While classical tearing drive is primarily influenced by the plasma current profile, whose gradients are affected by plasma shape, the quantitative role of shaping in NTM onset remains unresolved. Dedicated experiments on DIII-D, scanning plasma squareness, show that shaping alone is insufficient to suppress NTM onset in the ITER Baseline Scenario. To assess the generality of this result, we analyzed shaping parameters (squareness, triangularity, and elongation) across multiple targeted sub-databases: ITER Baseline Scenario plasmas (~300 discharges), negative triangularity plasmas (~500 discharges), and a broad H-mode dataset (~13,000 discharges) resolved by edge safety factor including Super H-mode, hybrid, and steady-state advanced scenarios. In all cases, NTM stability showed only weak dependence on plasma shape within the typical range of these parameters, suggesting that shaping alone is not an effective actuator for avoiding NTM onset in most operational regimes.