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Title: Why Nb3Sn cavities quench far below their predicted field
Plain: Nb3Sn should hold about twice the field of niobium, but real cavities fail at roughly half of niobium's best.
Precise: 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.
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.
Kind: well-posed
Answer type: mechanism
References: (empty)
Literature status: open
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.
Posed since: (empty)