Quench-limited SRF cavities: Failure at the heat-affected zone

With the recent progress in surface cleaning, the performance of superconducting RF cavities is mostly limited by a quench. It is important to understand the nature of the quench origin. In a common SRF cavity design the RF magnetic field is concentrated near the equatorial weld of the cavity. This weld has long been the major suspect in forming a surface defect, either as an impurity or in an increased surface roughness, that eventually gives rise to a quench. We used surface mounted thermometers to obtain a temperature map of the cavity in the quench region. A high temperature, temporal, and spatial resolution of the thermometry system allows us to pinpoint the quench origin with an accuracy of a few millimeters. We found that the hot-spot precursor forms in the weld heat-affected area rather than in the melted zone. The high resolution optical inspection found surface defects in exactly the same locations as the temperature mapping system. We will describe the measurement techniques and discuss possible scenarios of formation of these defects.