Myung Su Kim

A cryogen-free portable 3 T high-temperature superconducting magnet for an electromagnetic property measurement system has been developed to serve as a user facility at the Korea Basic Science Institute. The metallic insulation method was adopted to reduce the charging delay without sacrificing the self-protecting feature. A genetic-algorithm-aided optimized design was carried out to minimize the superconducting tape consumption while satisfying several design constraints. After the design, the compact high-temperature superconducting magnet composed of eight double-pancake coil modules was wound with high-temperature superconducting tape and stainless steel tape, and integrated with a two-stage cryo-cooler. The 3 T magnet was successfully cooled to approximately 20 K with a cryo-cooler and reached the target field of 3 T without any problems. Long-term measurements and a range of other tests were also implemented to verity the performance of the magnet. Test results demonstrated the feasibility of a cryogen-free portable high-temperature superconducting magnet system for electromagnetic property measurement experiments.

The development of the Electromagnetic Property Measurement System is underway at the Korea Basic Science Institute. The Electromagnetic Property Measurement System is used for electrical, thermal, and magnetic specimen property measurements in variable magnetic fields and temperatures with a low-temperature superconducting magnet. To activate low-temperature superconducting magnet that operates with liquid helium, we propose the use of vapor-cooled current leads. The low-temperature superconducting magnet is connected to the power supply at room temperature through a pair of current leads. Accordingly, heat inflows through the current leads and represents one of the major contributory factors of the entire heat inflow. Therefore, design optimization and evaluation of the current leads is necessary to minimize heat and liquid helium consumption. Vapor-cooled, and hybrid current leads were designed and fabricated, and testing in liquid helium cryostat was conducted. The low-temperature superconducting magnet was charged with electrical current up to the operating level, and the liquid helium boil-off rate was measured with respect to the supplied electrical current level. The performances of the two current leads were accessed based on the thermal load and boil-off rate of the liquid helium. The experimental and analyzed liquid helium boil-off rates associated with the current leads were in close agreement.

A 5 T low-temperature superconducting magnet is fabricated and tested for an EMPS (electro-magnetic property measurement system) whose sample space is 40 mm in diameter. Because the sample space is larger than that of conventional property measurement systems, it will be possible to measure the electro-magnetic properties of several samples simultaneously, thus reducing the measurement time. With a genetic algorithm incorporated with linear-programming, the optimal design of the 5 T LTS magnet was accomplished. A multi-coil 5 T EMPS magnet was fabricated according to the optimal design and tested in liquid helium. During current charging tests up to 43.5 A, the magnet voltage, current, and magnetic field were measured to monitor the magnet performance. Due to the manufacturing errors of the magnet, the field uniformity is usually worse than the design result. The concept of a newly proposed field uniformity improvement method was also introduced to compensate for field errors. The 5 T EMPS magnet system will be integrated with an EMPS console for electro-magnetic properties analyses.