Direct measurement of magnetic fields generated in Nb3Sn films for SRF cavities during the superconducting transition

Nb3Sn-coated superconducting radio frequency (SRF) cavities are being developed for operation at 4 K to significantly improve the energy efficiency of SRF-based accelerators. However, trapped magnetic flux generated during cooldown by thermoelectric currents in the bimetallic system can enhance their losses considerably. We present direct measurements of magnetic flux density generated during cooldown in Nb 3 Sn-coated samples prepared by DC magnetron sputtering and by the bronze-route technique. During cooldown, magnetic fields several times larger than the Earth's magnetic field are generated and subsequently trapped as the samples become superconducting. The generated field increases with increasing temperature gradient across the sample, and the effect varies by more than two orders of magnitude depending on the sample preparation. We also present a simulation of thermoelectric currents that provides insight into the underlying mechanism and suggests strategies to reduce the effect. These results are relevant for the production and operation of Nb 3 Sn cavities.