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Identify the limiting mechanism (thin-grain regions, tin-depleted patches with low $H_c$, vortex entry at grain boundaries, thermal runaway in the low-conductivity film) and predict the quench field from measured film microstructure.","problem_ref":null,"references":"","settled_by":"Temperature mapping and post-test microscopy of quench sites in several cavities showing a single microstructural cause whose predicted threshold matches the quench field.","status_note":"Single-cell Nb3Sn cavities reached accelerating gradients of approximately $0.24\\,\\mathrm{MeV}/\\mathrm{cm}$ by the early 2020s; the gap to the predicted superheating field remained.","title":"Why Nb3Sn cavities quench far below their predicted 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