Why $\mathrm{Nb_3Sn}$ cavities quench far below their predicted field
In plain words
$\mathrm{Nb_3Sn}$ should hold about twice the field of niobium, but real cavities fail at roughly half of niobium's best.
Precise statement
Vapor-diffusion $\mathrm{Nb_3Sn}$ films ($\sim 2 \times 10^{-4}\,\mathrm{cm}$ 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
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Single-cell $\mathrm{Nb_3Sn}$ cavities reached accelerating gradients of approximately $0.24\,\mathrm{MeV}/\mathrm{cm}$ by the early 2020s; the gap to the predicted superheating field remained.
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See also
- Prerequisite for Can a $\mathrm{Nb_3Sn}$ cavity exceed the gradient of the best niobium cavities?
- Related Can multilayer coatings raise the RF field above niobium's limit?
- Related Can a $\mathrm{Nb_3Sn}$ cavity exceed the gradient of the best niobium cavities?
- Related Origin of the field-dependent losses in $\mathrm{Nb_3Sn}$ cavities
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