On the Tokamak Soft Beta Limit Near β_N ≈ π³/15

Tokamak operation is commonly discussed in terms of the disruption-scale Troyon coefficient βN ∼2.8. Published experimental studies also describe a lower normalized-beta region in which neoclassical tearing modes (NTMs) appear frequently, typically βN ∼2.0–2.1 in large devices (DIII-D, JET). This note records the proximity of that reported onset regime to the dimensionless number π3/15 ≈2.067. Using two literature sources—an ITER-relevant stability-region statement and a large-sample onset-probability study—we summarize the reported ranges and their offsets from π3/15. The result is presented as a quantitative observational reference point for models of NTM onset in terms of βN.