A novel cryo-cooler-based test-stand for the study of trapped flux dynamics in superconducting materials

When cooling down an ideal superconductor in an ambient magnetic field, the whole field is expelled according to the Meissner effect. In real superconductors, however, the field is partially trapped in the material due to pinning centers such as grain boundaries or impurities. When applying electromagnetic radio frequency fields, this trapped flux causes power dissipation. This is particularly an issue in superconducting cavities for particle accelerators as it increases the demands on the cooling infrastructure. Therefore, understanding the dynamics of trapped flux and the underlying mechanism is of high interest. The influence of the material and cooldown conditions on trapped flux could provide valuable information for material treatments and efficient operation of superconducting cavities. For this purpose, a new test-stand CRAFT (CRyo-cooler based Analysis of Flux Trapping) has been developed. The use of cryo-coolers allows the automation of trapped flux measurements to perform hundreds of superconducting transitions in one measurement run. CRAFT is designed to analyze trapped flux in rectangular samples with different material and cooldown parameters in order to investigate their influence on trapped flux. In this paper, CRAFT is discussed and the measurement procedure of trapped flux is explained. Examples of measurements to characterize CRAFT and illustrate its efficacy are discussed. These data are compared with previous measurements utilizing a bath cryostat [Kramer et al., Rev. Sci. Instrum. 95, 093902 (2024)]. It will be seen that CRAFT not only reproduces the data but also expands the accessible parameter space and improves the quality of the data. First, measurements and the potential for the automation of CRAFT are demonstrated.