A facility for testing bulk superconducting hollow MgB$_2$ cylinders for the production and shielding of magnetic fields for polarized targets and nuclear fusion fuels

The challenging magnetic problem of producing internal fields in compact spaces can be solved by high temperature superconducting bulk materials, such as MgB$_2$, promising tools for trapping magnetic fields around polarized substances, while shielding out external fields, as required for fundamental physics studies in scattering experiments. They are also of great interests, as they allow to easily generate holding fields for accumulation and transport of polarized fuel in nuclear fusion tests. A facility has been commissioned, which allows to control the bulk superconductor temperature down to 8 K with to a cold head, driven by a helium compressor, thus satisfying eco-sustainability requirements. The facility is able to test various superconducting hollow MgB$_2$ cylinders, each sintered starting from boron powders having different grain sizes, and it allows to measure the holding and shielding capabilities, together with the corresponding long--term stability. The facility is equipped to map the trapped fields, inside the cylinder along the symmetry axis and radially, as a function of both the temperature and the applied magnetic field. The measurements have been performed in transverse magnetic fields up to 1.2 T, limit due to the available magnet in our lab. After its preparation for transverse field generation, the sample can be moved into longitudinal magnetic fields, shielding the latter, while still keeping the former fields. In the context of an electron scattering experiment, such a solution minimizes beam deflection and the energy loss of reaction products, while also eliminating the heat load to the target cryostat from current leads that would be used with conventional electromagnets. In the context of polarized fuel for fusion its use is straightforward, because the system can trap the magnetic field required during fuel production, and then it can provide the holding field for its transfer in fusion test facilities.