Compact HTS cryogen-free magnet for magneto-optics research setup

Small working distances between the objective lenses and the inspected samples in magneto-optical (MO) setups limit the volume available for external magnetic field generation coils. Air or water-cooled copper coils are commonly used for field generation, however these coils usually make it possible to attain limited fields of the order of 10-50 mT, in continuous operating mode. HTS coils offer a unique and cost effective solution in such cases of limited space. A compact, 0.4 T, cryogen-free, HTS magnet has been designed, built and tested in a MO setup. The magnet is made of 3, 100 turns single pancakes, mounted on the second stage of a two-stage cryocooler. The HTS coil operates at about T=45 K and generates 0.4 T in a warm bore of 30 mm diameter. Minimization of metal components in the coil makes it possible to attain field ramp rates of 7.4 T/sec with a coil charging voltage of 20 V. Initial cool-down time of the magnet takes less than 5 hours. The magnet design and performances are described in detail and serve as an example for solutions that HTS technology may offer in limited space applications.