  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.
