Why Nb3Sn cavities quench far below their predicted field
In plain words
Nb3Sn should hold about twice the field of niobium, but real cavities fail at roughly half of niobium's best.
Precise statement
Vapor-diffusion Nb3Sn films ($\sim 2e-4\,\mathrm{cm}$ thick) on Nb cavities quench at $H_{\mathrm{pk}} \sim 800\text{-}1200\,\mathrm{G}$, compared with a predicted $H_{\mathrm{sh}}$ near $4000\,\mathrm{G}$. 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.
What would settle it
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 in the literature
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.