Magnetic field sensors for detection of trapped flux in superconducting radio frequency cavities

Superconducting radio frequency (SRF) cavities are fundamental building blocks of modern particle accelerators. They operate at liquid helium temperatures (2-4 K) to achieve very high quality factors (1010-1011). Trapping of magnetic flux within the superconductor is a significant contribution to the residual RF losses, which limit the achievable quality factor. Suitable diagnostic tools are in high demand to understand the mechanisms of flux trapping in technical superconductors, and the fundamental components of such diagnostic tools are magnetic field sensors. We have studied the performance of commercially available Hall probes, anisotropic magnetoresistive sensors, and flux-gate magnetometers with respect to their sensitivity and capability to detect localized, low magnetic flux amplitudes, of the order of a few tens of magnetic flux quantum at liquid helium temperatures. Although Hall probes have the lowest magnetic field sensitivity (∼96 nV/μT at 2 K), their physical dimensions are such that they have the ability to detect the lowest number of trapped vortices among the three types of sensors. Hall probes and anisotropic magnetoresistive sensors have been selected to be used in a setup to map regions of trapped flux on the surface of a single-cell SRF cavity.