Origin of the medium-field Q-slope in niobium-on-copper film cavities
In plain words
Cavities made by coating copper with a thin niobium film are cheaper and better cooled than solid niobium, but their energy loss rises steeply with field, for reasons still argued.
Precise statement
Sputtered Nb films $\sim 1e-4 \text{ to } 5e-4\,\mathrm{cm}$ thick on Cu cavities, as used at LEP, LHC and HIE-ISOLDE, show surface resistance $R_{\mathrm{s}}$ rising roughly linearly with RF field from $H_{\mathrm{pk}}$ of a few hundred G, unlike bulk Nb. Identify the mechanism (grain-boundary weak links and local hot spots, trapped flux in a granular film, poor film-substrate thermal contact, or other) and predict $R_{\mathrm{s}}(H)$ from measured film microstructure for different deposition methods, including high-power impulse magnetron sputtering.
What would settle it
$R_s(H)$ of cavities with controlled grain size and grain-boundary density, matched by a model fixed before the tests.
Status in the literature
A 2022 model (Ramiere, Antoine, Amrit, Phys. Rev. Accel. Beams 25, 022001, https://doi.org/10.1103/PhysRevAccelBeams.25.022001) attributed the slope to hot spots in granular films; competing explanations persist.