Evidence of high-field radio-frequency hot spots due to trapped vortices in niobium cavities

Superconducting radio-frequency (rf) cavities made of high-purity niobium exhibit strong anomalous rf losses starting at peak surface magnetic fields of about 90-100 mT in the gigahertz range. This phenomenon is referred to as "Q drop.'' Temperature maps of the cavity surface have revealed the presence of "hot spots'' in the high magnetic field region of the cavities. Several models have been proposed over the years to explain this phenomenon but there is still no experimental evidence on the mechanisms behind such hot spots. In this work we show that at least some of the hot spots are due to trapped vortices responsible for the anomalous losses. Here we report experiments in which a local thermal gradient was applied to the hot spot regions of a cavity in order to displace the vortices. Temperature maps measured before and after applying the thermal gradient unambiguously show that the hot spots do move and change their intensities, allowing us to determine changes in the hot spot positions and strengths and their effect on the cavity performance. Results on a large-grain niobium cavity clearly show a different distribution and in some cases a weakening of the intensity of the "hot spots,'' suggesting new ways of improving the cavity performance without additional material treatments.