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Magnetic flux expulsion properties of the superconducting material such asbulk niobium, widely used for the radio-frequency cavity fabrication,substantially affect the performance characteristics of the cavities. Thequality factor of the SRF resonators can be significantly compromised due tothe presence of the trapped flux vortices causing additional RF energy lossesin the material. Large number of experiments have been carried out by differentresearch groups to establish the correlation of the flux trapping in niobiumcavities with the presence of impurities in niobium as well as various surfacetreatment methods. Majority of these experiments utilize commercially availablecryogenic fluxgate magnetic sensors to measure the field before and after theniobium transition to the superconducting state to quantify the amount of fluxtrapped. One disadvantage of the typically used fluxgates is the size of thesensing volume. As an example, the Barting-ton F and G type cryogenic fluxgateshave a sensing core length of about 30mm, which is comparable to the curvatureradius of the cavity walls and hence the magnetic field lines curvature radiusafter the expulsion. Thus, the measured field value needs to be corrected toaccount for the sensor effective averaging over the sensing volume. In case ofthe sharper geometries, for instance if the flux expulsion to be measured onthe edge of the rectangular niobium flat sheet with a thickness of ~5mm, theuse of the fluxgate would be impractical.

AMR Magnetic Sensors in Flux Expulsion Studies