Jiho Park

Ramping Operation of the Conduction-Cooled High-Temperature Superconducting Magnet for an Active Magnetic Regenerator System

An active magnetic regenerator (AMR) system requires fast ramp rate of magnetic field variation. The magnetic field change enables a magnetic refrigerant to create magnetocaloric effect for magnetic refrigeration. In this paper, a conduction-cooled high-temperature superconducting (HTS) magnet has been thermally analyzed and tested for an AMR system. A GdBCO conductor insulated with polyimide tape was wound by dry winding and standard double-pancake coil method. Stycast 2850FT was applied to the edge of the tape conductor so that the cooling plates, which were located at the top and bottom of the coil, should effectively cool the magnet. The whole thermal bridging between the magnet and a two-stage GM cryocooler is analyzed by the numerical simulation and a Joule heating experiment of the HTS magnet at normal state. The inductance of the HTS magnet was measured as 350 mH. The HTS magnet is intended to be operated with maximum 1 T/s at 20 K, cooled by the cryocooler. A solenoid switch and an external dump resistor were employed in order to discharge the HTS magnet. A continuous ramping operation test was conducted with a 60-A peak current and a maximum 20-A/s ramp rate by 3-kW dc power supply. This paper describes the experimental results of the ramping operation of the HTS magnet for an AMR system, and the technical issues on the results are discussed.

Free-piston reciprocating cryogenic expander utilizing phase controller

In a free-piston expander which eliminates mechanical linkages, a prescribed behaviour of the free-piston movement is the key to an expander performance. In this paper, we have proposed an idea of reducing complexity of the free-piston expander. It is to replace both multiple solenoid valves and reservoirs that are indispensable in a previous machine with a combination of a single orifice-reservoir assembly. It functions as a phase controller like that of a pulse tube refrigerator so that it generates time-delay of pressure variation between the warm-end and the reservoir resulting in the intended expansion of the cold-end volume down to the pre-set reservoir pressure. The modeling of this unique free-piston reciprocating expander utilizing phase controller is developed to understand and predict the performance of the new-type expander. Additionally, the operating parameters are analysed at the specified conditions to enable one to develop a more efficient free-piston type cryogenic expander.

Experimental Investigation on Conduction-Cooled Fast-Ramping Layer-Wound (RE)BCO Superconducting Magnet for Magnetic Refrigeration

Magnetic refrigeration requires a strong magnetic field and its alternation for the magnetization and demagnetization processes. A conduction-cooled (RE)BCO coil was designed and fabricated to exert the maximum central magnetic field of 3.5 T with a field homogeneity of 90% on the magnetic refrigerant bed for an adiabatic demagnetization refrigerator (ADR) operating between 22 and 20 K. The polyimide tape insulated (RE)BCO tape conductor was wound on the stainless steel bobbin with standard layer-wound method whose outer diameter and thickness were 25.4 and 0.3 mm, respectively. The fast-ramping (RE)BCO coil was conductively cooled by a GM cryocooler to approximately 20 K. The (RE)BCO coil winding has a novel thermal drain structure to withstand the high thermal loads during alternating current (AC) operation which consists of high purity copper strip arrays installed between the inter-layers of the coil winding. Alternating magnetic field was stably generated with the peak of 3.0 T at the current of 130 A (10.5 A s-1). In this paper, the technical issues regarding to the fast-ramping conduction-cooled (RE) BCO coil itself and the integrated operation with the magnetic refrigeration system are discussed.

Investigation on Cryogenic Refrigerator and Cooling Schemes for Long Distance HTS Cable

High-temperature superconducting (HTS) cable is one of the most prospective superconducting technologies for its low electric power transmission loss and high power density. One difficulty of practicing this technology is maintaining the HTS cable at a cryogenic temperature approximately at 70 K. The types of cryogenic refrigerators are briefly reviewed to apply in long-distance HTS cables. Recuperative-type refrigerators, such as Brayton and Joule-Thomson (JT) refrigerators, are more appropriate to satisfy the required high cooling power. The theoretical reliability of the newly proposed dual mixed refrigerant (DMR) JT refrigerator is analyzed. The cycle configuration of the DMR JT refrigerator is more complex than that of a Brayton refrigerator; however, the degradation of reliability was not significant. Furthermore, a remote operation apart from the compressor is possible in the JT refrigerator without moving components inside the cold box. This is the most desirable characteristic for long-distance HTS cable cooling. Finally, the advantage of using an oxygen and nitrogen mixture as the circulating coolant is discussed. The number of cooling stations can be reduced in a long-distance HTS cable by lowering the coolant feeding temperature.

Performance of the Fast-Ramping High Temperature Superconducting Magnet System for an Active Magnetic Regenerator

Fast magnetic field alternation is indispensable for continuous magnetic refrigeration. An active magnetic regenerative refrigerator (AMRR) utilizes magnetocaloric effect of magnetic materials which occurs during magnetization and demagnetization processes. A conduction cooled high temperature superconducting (HTS) magnet can be one of the prospective candidates as an alternating magnetic field generator. This paper describes the development effort of the cryogen-free HTS magnet for the AMRR. The magnet consists of twelve double pancake GdBCO coils which are insulated with polyimide tape. A two-stage GM cryocooler was employed to cool down the magnet. The critical current of the magnet was measured at the operating temperature before alternating current (AC) operation. In order to remove heat produced by AC loss with small temperature difference, thermal links between the cryocooler and the magnet were carefully designed. This paper presents the test results, AC loss analysis of the HTS magnet in the AMRR system. Maximum central magnetic field of 3 T (150 A) was achieved with the maximum ramping rate of 1 T/s (50 A/s). The AC loss was measured as 11.2 W at the operating conditions and the generated heate was effectively removed by the cryocooler. The AC loss was predicted by the numerical simulation and the simulation results were compared with the experimental results.

Experimental Investigation of Superconducting Linear Compressor (SCLC) for Pulse Tube Refrigerator (PTR)

This research paper focuses on the development of the Stirling-type pulse tube refrigerator (PTR) system. The system includes an innovative cold compression concept by introducing a superconducting linear compressor (SCLC). As one of the key goals of this research, the working gas (helium) is to be compressed directly at cryogenic temperature. The mechanical work (pressure-volume (PV) work) can be directly transmitted to the PTR system without experiencing precooling process. The unique feature of this paper is to utilize a high-temperature superconductor (HTS) coil instead of using metallic conductor coil for a linear motor. By implementing a HTS linear motor to the cold compressor, the ohmic loss can be inherently eliminated, so that the higher motor efficiency can be expected. In this research paper, the experimental validation as a proof of concept is carried out. The HTS linear motor operates at 50xa0Hz without creating overheat problem. The SCLC stably generates the PV work at the temperature of liquid nitrogen and it is subsequently transmitted to the compression volume of the PTR. The whole PTR system, therefore, can readily operate between 80 and 20xa0K with the cooling capacity of 0.26xa0W.

AC Operation of Gd–Ba–Cu–O High

Magnetic refrigeration requires a strong magnetic field and its alternation for the magnetization and demagnetization processes. In this paper, a conduction-cooled HTS magnet system was fabricated and tested for a magnetic refrigerator. It consists of a HTS magnet, a pair of metallic and HTS current leads, and a cryocooler. The Gd-Ba-Cu-O coated conductor with polyimide tape insulation was used for the HTS magnet and wound by standard layer-wound method. Stainless steel bore was assembled with OFHC copper supporting plates of the HTS magnet. The OFHC copper strips were inserted between interlayers of the conductors to enhance axial thermal conduction. Apiezon N grease with hexane solution was applied on every layer of the HTS magnet to minimize thermal contact resistance between the conductor and the OFHC copper strips. The HTS magnet was cooled to 17 K. The charging and discharging of the HTS magnet were achieved by the unipolar power supply and an external dump resistor. Stable magnetic field of 3.25 T was achieved with 150 A. Alternating magnetic field was stably generated with the peak of 2.16 T (0.11 T/s). In this paper, the fabrication method of AC magnet and the operational issues of the conduction-cooled HTS magnet are discussed.

{\rm T}_{\rm C}

An active magnetic regenerative refrigerator (AMRR) is expected to be useful for hydrogen liquefaction due to its inherent high thermodynamic efficiency. Because the temperature of the cold end of the refrigerator has to be approximately liquid temperature, a large temperature span of the active magnetic regenerator (AMR) is indispensable when the heat sink temperature is liquid nitrogen temperature or higher. Since magnetic refrigerants are only effective in the vicinity of their own transition temperatures, which limit the temperature span of the AMR, an innovative structure is needed to increase the temperature span. The AMR must be a layered structure and the thermophysical matching of magnetic field and flow convection effects is very important. In order to design an AMR for liquefaction of hydrogen, the implementation of multi-layered AMR with different magnetic refrigerants is explored with multi-staging. In this paper, the performance of the multi-layered AMR using four rare-earth compounds (GdNi2...