Partial Meissner effect measurement by a superconducting magnetic flux lens

Abstract: Magnetic flux trapping in the Meissner transition of superconducting radio frequency cavities can substantially increase dissipation which impacts cryogenic costs and necessitates expensive magnetic shielding. Recent findings point at the material preparation and the cool down dynamics at the vortex state as the main ways to counteract flux trapping. Most of the related measurements are performed on the cavity, with risk of results being impacted by the cool down specifics of each facility. We demonstrate an alternative experiment in which flux expulsion is quantified from sheet material, with samples conduction cooled in a stand-alone instrument designed to collimate expelled flux at controlled cool down conditions. A series of tests reliably and reproducibly relate the amount of expelled flux to the cooling rate, the velocity of the superconducting front and the spatial temperature gradient. The results are in line with tests performed on bare cavities and show that expulsion of trapped flux improves with the spatial temperature gradient, independent on the cooling rate. Our concept offers means to control the dynamics of the Meissner effect and can be used for material qualification prior to cavity fabrication.