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Vacuum thermal treatments (baking) are known to improve the superconductingproperties of the RF surface layer of niobium cavities, and are employed as alast processing step to increase their efficiency determined by intrinsicquality factor Q0. A new method to perform the baking has been demonstrated. Itconsists in annealing of an evacuated cavity with the local heaters installedon its outer surface in a cryostat which ensures an exterior vacuum andprotects the outer cavity surface from oxidation. Such a set-up has a number ofadvantages as it does not require to cool the cavity flanges during baking, andallows to perform the "cold" RF characterization of the cavity in situ,immediately after the thermal treatment without disassembly of heatingelements. Moreover, the air exposure that causes partial degradation of Q0 bysurface reoxidation is avoided. The heat treatment of a single-cell 1.3 GHzniobium cavity at 230 {\deg}C for 24 h demonstrated the doubling of Q0 atEacc=10 MV/m (from 1.20e10 to 2.4e10) and retained the maximal acceleratingfield of 35 MV/m without quenching. The selection of treatment parameters isbased on our previous XPS studies. This treatment ensures incompletedissolution of the native oxide by oxygen diffusion, thereby preventinginteraction of niobium surface with external contaminants. We propose to bakethe cavities directly in a cryomodule, which would allow to use the treatmentto improve their performance. The potential impact of material parameters onthe components of surface resistance has been briefly examined.

Improved RF Performance of Niobium Cavities via In-situ Vacuum Heat Treatment Technique