Kang Sik Ryu

A "permanent" high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

Abstract A new design for a portable “permanent” superconducting magnet system is explored. The design involves a persistent-mode high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. The system is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium. An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The apparatus includes a room-temperature bore and can function while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20–40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permissible temperature variation exceeding a few degrees kelvin. Models are developed to predict the experimental systems warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. The results indicate that present HTS conductor critical current and index are not yet sufficient to provide field strengths and field decay time constants that are required for typical persistent-mode applications.

Design analysis of a solid nitrogen cooled “permanent” high-temperature superconducting magnet system

Abstract Potential performance advantages of a solid nitrogen cooled “permanent” high-temperature superconducting (SN2/HTS) magnet system over a liquid helium cooled low-temperature superconducting (LHe/LTS) system are explored. The SN2/HTS system design includes a second solid heat capacitor that cools a radiation shield. Recooling of the heat capacitors is performed with a demountable cryocooler. The SN2/HTS system offers both enhanced stability and improved portability over a LHe/LTS system. Design codes are constructed to compare the SN2/HTS system design with a LHe/LTS design for a general permanent superconducting magnet system employing a room temperature bore. The codes predict the system volume and mass that should be expected for a given set of design requirements, i.e. field strength and bore size, and a given set of conductor properties. The results indicate that present HTS conductor critical current and index are not yet sufficient for producing SN2/HTS systems of a size that is comparable with that expected for a LHe/LTS system; however, the conductor properties of Bi2223/Ag have been consistently improving, and new HTS conductors are expected to be developed in the near future. The codes are used to determine the minimum Bi2223/Ag conductor performance required for a SN2/HTS system to be competitive with a LHe/LTS system.

Stand-alone solid nitrogen cooled "permanent" high-temperature superconducting magnet system

This paper describes a stand-alone solid nitrogen cooled permanent high-temperature superconducting magnet system and presents its performance data. The systems cold body includes the magnet, shunted with a superconducting switch and comprised of six double pancakes, each wound with BSCCO-2223/Ag composite tape, and a volume of solid nitrogen and operates within a cyclic temperature range between 20 K and 40 K. At an operating current of 50 A the coil generates a central field of 0.5 T in a 12-mm diameter room temperature bore; the systems 2 liters of solid nitrogen is intended to give a cyclic operating period, from 20 K to 40 K, of about 70 hours. Included also in the paper is a preliminary design code that includes magnetic and thermal requirements of the system.

Fabrication of Bi-2223 HTS magnet with a superconducting switch

A test pancake magnet, equipped with persistent superconducting switch, was fabricated with Bi-2223/Ag-Mg tape. The magnet was operated in persistent mode at 20 and 77 K. For large operating currents that forces the magnet to operate in the flux flow range, the field decays is exponentially with time. A simple circuit model is used to compute field decay time constants. It was also found that Pb-Sn solder joints are unsuitable for persistent mode operation.

Numerical analysis of stability margin and quench behavior of cable-in-conduit NbTi conductors for KSTAR

A numerical model has been proposed to analyze the stability margin and quench characteristics of the cable-in-conduit NbTi conductors for the KSTAR-PF (Korea Superconducting Tokamak Advanced Research) magnets. The dependence of the thermal, hydraulic and electrical properties on the external thermal disturbance was investigated. The algorithm of the program is based on the finite volume method which adopts space discretization and time integration by multi-step Runge-Kutta method to obtain stable numerical solutions. It was confirmed that the disturbance duration can influence the conductor stability and limiting current.

Design and Tests of Prototype Hybrid Superconducting Fault Current Limiter With Fast Switch

This paper deals with design and operating tests of prototype hybrid superconducting fault current limiter (SFCL) which is series connected non-inductive solenoid resistive coil and asymmetric non-inductively wound coils. The role of resistive SFCL is for reducing the fault current in the first swing. Moreover, resistive SFCL can reduce a load of dielectric strength in hybrid SFCL. Two kinds of high temperature superconductor (HTS) wires were used to fabricate asymmetric HTS coil. These HTS wires had different quench characteristics such as different critical current and n-value. Unbalanced current distribution between two HTS wires in the asymmetric HTS coil during the fault should occur. Thus, the asymmetric HTS coil generates impedance and repulsive force from magnetic flux. The finite element method was performed to calculate maximum repulsive force. From experimental tests and FEM simulation results, proper combination of HTS wires and configuration of asymmetric HTS coil with fast switch were determined to increase maximum repulsive force. Also, the prototype hybrid SFCL with fast switch was fabricated and performed.

Measurement of joint properties of Bi(Pb)-Sr-Ca-Cu-O (2223) tapes by field decay technique

We joined 19-multifilamentary Bi-2223 superconductor tapes and fabricated double-pancake coils by using resistive- and superconducting-joint methods. The critical current ratio (CCR) of the jointed tape and the decay characteristics, joint resistance, and n-value of the pancake coils were evaluated. The joint resistance of the coils was characterized by the field decay technique. It was observed that the CCR was higher in the joined tape made by the resistive-joint method, compared to that by the superconducting-joint method. On the other hand, joint resistance was measured to be 4 orders of magnitude smaller in the superconducting-joint coil; approximately 40% of critical current was retained in the persistent current mode and the joint resistance was 0.18 n/spl Omega/. Better and longer retention of the magnetic field in the superconducting-joint coil is believed to be due to the direct connection between the superconducting cores.

Characteristic Comparison for the Various Winding Methods of HTS Magnets

Recently various high temperature superconducting (HTS) conductors have been commercialized in the power applications. While a BSCCO becomes tape or wire, others coated conductors (CC) are tapes. An HTS magnet utilized with these conductors can be fabricated as a form of pancake winding or layer winding. In the case of HTS magnet using a thin and rectangular tape, the pancake winding method is satisfied in the practical cases because of its suitable features. However, one of the disadvantages of this winding method is increasing brands and splices. From this point of view, we propose a layer winding method using an HTS tape and fabricated HTS layer coils. To verify the feasibility of the proposed winding method, critical current and over-current tests are investigated in liquid nitrogen and the results are compared with those of pancake coils. In addition, operating characteristics on the condition of various winding tensions are analyzed.

Development and test of a 30 kVA superconducting generator

A 3-phase, 30 kVA, 4 pole, 1800 rpm superconducting generator is built and tested. The field winding of the rotor is a racetrack type with NbTi superconducting wire. The rotor is composed of two dampers and a liquid helium container in which the field poles reside. The space between the outermost damper and the container is vacuum insulated. To continuously transfer the liquid helium from the stationary dewar into the rotor and return the helium gas conversely, a special coupling is installed. The stationary coupling is connected to the rotor through a ferrofluid seal. The open circuit characteristic (OCC) and short circuit characteristic (SCC) are obtained. Also, the test results under the light load (up to 3.6 kW) are given. From the design stage, 2-D FE analysis coupled with the external circuit has been performed. The external circuit includes the end winding resistance and reactance as well as two dampers. When compared with the test data, the FE analysis results show a very good agreement.

Fabrication of large area superconducting thin film by pulsed laser scanning

In order to deposit high quality high temperature superconducting (HTS) YBCO thin films with a large area, the pulsed laser scanning (PLS) method was tried and the thickness distribution of the film was investigated. Large area YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films on /spl phi/ 36 mm diameter LaAlO/sub 3/ and /spl phi/ 42 mm diameter Y-ZrO/sub 2/ (YSZ) single crystal substrates were obtained by rotating lenses with radii of 9 and 12.9 mm. The critical temperatures T/sub c/ of the films at the center and edge were all 90 K. The critical current density J/sub c/ at 77 K and zero magnetic field is over 10/sup 6/ A/cm/sup 2/. X-ray diffraction showed that the thin films are completely c-axis oriented. The V-I characteristics of the thin films were also investigated.