Abstract
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.
Highlights
One of the most outstanding scientific issues related to superconducting rf cavities made of high-purity bulk niobium is the occurrence of strong rf losses starting at a peak surface magnetic field (Bp) of about 90–100 mT
In this contribution we present experimental results on the effects of applying a small local thermal gradient to the hot spots, identified on the cavity surface by temperature mapping, while the cavity is immersed in superfluid helium
The experimental results on a BCP-treated large-grain niobium cavity reported in Sec
Summary
One of the most outstanding scientific issues related to superconducting rf (gigahertz range) cavities made of high-purity (residual resistivity ratio >200) bulk niobium is the occurrence of strong rf losses starting at a peak surface magnetic field (Bp) of about 90–100 mT (the socalled Q drop). A possible way to reveal hot spots of magnetic vortices already trapped in the material is to depin them by applying a thermal gradient across and along the cavity wall [4]. In this contribution we present experimental results on the effects of applying a small local thermal gradient to the hot spots, identified on the cavity surface by temperature mapping, while the cavity is immersed in superfluid helium.
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