Mechanism of the high-field Q-slope and its cure by baking · revision history

Revision 2

Accepted · core change · record

Changes against revision 1:

Title
Mechanism of the high-field Q-slope and its cure by baking
Plain
Above a certain field, niobium cavities start wasting far more energy; a two-day bake at 120 C removes the effect, and why it works is still debated.
Precise · changed · core
Revision 1: Electropolished bulk Nb cavities at $1.3\,\mathrm{GHz}$ and $T = 2\,\mathrm{K}$ show a steep drop of quality factor $Q_0$ above peak surface magnetic field $H_{\mathrm{pk}} \sim 900-1000\,\mathrm{G}$, removed by in-situ baking at $120\,\mathrm{C}$ for $\sim 48\,\mathrm{h}$. Identify the mechanism (niobium nanohydride precipitation within ~1e-5 cm of the surface, oxygen or vacancy-hydrogen complexes, or other) with a model predicting the onset field and its dependence on bake temperature and duration.
Revision 2: Electropolished bulk Nb cavities at $1.3\,\mathrm{GHz}$ and $T = 2\,\mathrm{K}$ show a steep drop of quality factor $Q_0$ above peak surface magnetic field $H_{\mathrm{pk}} \sim 900-1000\,\mathrm{G}$, removed by in-situ baking at $120\,\mathrm{C}$ for $\sim 48\,\mathrm{h}$. Identify the mechanism (niobium nanohydride precipitation within $\sim 10^{-5}$ cm of the surface, oxygen or vacancy-hydrogen complexes, or other) with a model predicting the onset field and its dependence on bake temperature and duration.
Settled by
Depth-resolved measurement of hydrogen, oxygen and vacancy profiles before and after baking, correlated with $Q_0(H_{\mathrm{pk}})$ of the same cavities and matched by a quantitative model.
Kind
phenomenon
Answer type
mechanism
References
(empty)
Literature status
contested
Status note
Hydride-precipitation and impurity (oxygen, vacancy-hydrogen) explanations each have supporting studies; no consensus was found in records checked through 2025.
Posed since
(empty)

Decisions