Nanoscale defect structure within the magnetic penetration depth of ∼100 nm is key to the performance limitations of niobium superconducting radio frequency (SRF) cavities. Using a unique combination of advanced thermometry during cavity RF measurements, and TEM structural and compositional characterization of the samples extracted from cavity walls, we discover the existence of nanoscale hydrides in electropolished cavities limited by the high field Q slope, and show the decreased hydride

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... tion in the electropolished cavity after 120 • C baking. Furthermore, we demonstrate that adding 800 • C hydrogen degassing followed by light buffered chemical polishing restores the hydride formation to the pre-120 • C bake level. We also show absence of niobium oxides along the grain boundaries and the modifications of the surface oxide upon 120 • C bake.