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In this paper we present the discovery of a new surface treatment applied tosuperconducting radio frequency (SRF) niobium cavities, leading tounprecedented accelerating fields of 49 MV/m in TESLA-shaped cavities, incontinuous wave (CW); the corresponding peak magnetic fields are the highestever measured in CW, about 210 mT. For TESLA-shape cavities the maximum quenchfield ever achieved was ~45 MV/m - reached very rarely- with most typicalvalues being below 40 MV/m. These values are reached for niobium surfacestreated with electropolishing followed by the so called mild bake, a 120Cvacuum bake (for 48 hours for fine grain and 24 hours for large grainsurfaces). We discover that the addition during the mild bake of a step at 75Cfor few hours, before the 120C, increases systematically the quench fields upto unprecedented values of 49 MV/m. The significance of the result lays notonly in the relative improvement, but in the proof that niobium surfaces cansustain and exceed CW radio frequency magnetic fields much larger than Hc1,pointing to an extrinsic nature of the current field limitations, and thereforeto the potential to reach accelerating fields well beyond the current state ofthe art.

Accelerating fields up to 49 MV/m in TESLA-shape superconducting RF niobium cavities via 75C vacuum bake