Seog-Whan Kim

Joint resistances between two parallel high Tc superconducting tapes

Eight different types of joining between two parallel high Tc superconducting (HTS) tapes were prepared and current-voltage (I-V) characteristic curves were investigated at 77 K, liquid nitrogen temperature. Contact resistances of joint parts with 25 mm in length were estimated from I-V curves. Lowest contact resistance among eight samples having different types of joining was 0.497 /spl mu//spl Omega/. The best joining method is to do crossly joining with HTS tapes on two parallel HTS tapes after filling solder paste into their gap. In this case, lowest joint resistance was 0.15 /spl mu//spl Omega/ at 77 K and 0.05 /spl mu//spl Omega/ at 28 K. Joule heat of a joint sample generated at 30 K and at 100 A was calculated 0.492 mW. Also, in I-V curve of samples crossly joined with HTS tapes, it was found that joint resistance does not depend on variation of magnetic fields (0/spl sim/7000 G) at 77 K. From consideration of Joule heat generation, such joining method is available for fabricating the field winding of superconducting rotating machine.

Influence of tape's critical currents and current distributions on AC loss measurement in a multi-tape conductor

AC loss is an important issue to design high temperature superconductor power cables, which consist of a number of Bi-2223 tapes wound on a former. In the cables, the tapes have intrinsically different critical currents, and they are electrically connected to each other and current leads. This makes loss measurements considerably complex, especially for short samples. In this work, we have prepared a multi-tape conductor composed of Bi-2223 tapes. The ac losses of the conductor have been experimentally investigated. The loss tests indicate that the effect of critical currents of each tape on AC loss measurement in the multi-tape conductor is negligible only if currents in the tapes flow uniformly. Moreover, the measured loss of the conductor is in good agreement with the sum of the transport loss in each tape. However, in the case of nonuniform current distribution, the measured AC loss considerably depends on the current distribution parameter /spl gamma/, and the positioning of a voltage lead. Thus it should be very careful to measure the AC losses in the short power cable samples.

Stabilities of the Rutherford cables with Cu matrix and CuMn barrier

Superconducting coils used for high energy accelerator magnets are usually fixed with epoxy resin in the fabrication processes called curing. The assembled magnets in the processes have shown different critical current degradation figures due to excitation ramp rate even if they were made in the same manner. The ramp rate dependences of degradation seem to be affected by AC losses between strands of the Rutherford cables (inter-strand AC losses), which are easily affected by the curing conditions. Although they can be reduced by increasing cross-over resistances between strands, the stability problems caused by current sharing between strands may arise. It is important for such kinds of superconducting accelerator magnets to clarify the mechanisms of the AC losses and stabilities of the Rutherford cables. Experimental studies on the AC losses and the current sharing problems for several types of cable samples have been performed.<<ETX>>

The effect of nano-powder additions on the superconducting properties of MgB/sub 2/

To investigate the effect of Ag or SiC nano-powder additions on the superconducting properties of MgB/sub 2/, a series of superconducting (Ag)/sub (x)wt.-%/(MgB/sub 2/)/sub (100-x)wt.-%/(Ag/sub x/-MgB/sub 2/) and (SiC)/sub (x)wt.-%/(MgB/sub 2/)/sub (100-x)wt.-%/((SiC)/sub x/-MgB/sub 2/)(0/spl les/x/spl les/20), containing Ag and SiC nano-powders, respectively, of different diameters (30 nm and 130 nm), were prepared by a simple solid-state reaction route, cold-pressed into a pellet form and investigated. To maintain the same environment of MgB/sub 2//stainless-steel tapes/wires, Ag/sub x/-MgB/sub 2/ and (SiC)/sub x/-MgB/sub 2/ pellets made out of the mixed powders were put inside stainless steel tubes and then sintered at 900/spl deg/C for two hours in Ar atmosphere. Characterization performed included both X-ray diffraction and magnetization. No impurity phase was identified for as-rolled samples. However, both the Ag/sub x/-MgB/sub 2/ and (SiC)/sub x/-MgB/sub 2/ composite pellets, when sintered, contain various impurity phases. The isothermal magnetizations M(H) of a series of samples were measured at temperatures between 5 and 50 K in fields up to 5 T, using a PPMS-9 (Quantum Design). The optimal amounts of Ag and SiC nano-powder in Ag/sub x/-MgB/sub 2/ and (SiC)/sub x/-MgB/sub 2/ to obtain the largest flux pinning effect are /spl sim/8 and /spl sim/4 wt.-%, respectively. The two-step structures in ZFC M(T) curves of (SiC)/sub x/-MgB/sub 2/ were more developed than Ag/sub x/-MgB/sub 2/. The best flux pinning centers can be created by adding a suitable size and amount of SiC nano-powder, not too large to increase the decoupling between the MgB/sub 2/ grains.

Design and electrical characteristics analysis of 100 HP HTS synchronous motor in 21st Century Frontier Project, Korea

A 100 hp class superconducting motor in the 21st Century Frontier R&D Program of Korea has been designed. A theoretical model of high temperature superconducting (HTS) motor was presented by two-dimensional electromagnetic field analysis. The motor is composed of HTS field winding, cold damper shield, air-gap armature winding and laminated machine shield. The HTS field winding consists of racetrack type double pancake coils wound with Bi-2223 HTS tapes operated at about 30 K. The operating current of the HTS tape conductor could be determined by the magnetic field distribution calculated in the HTS field winding and the I/sub c/ - B characteristics of a practical HTS conductor, taking account of its anisotropy.

Transport current loss and I/sub c/ degradation of HTS tapes under mechanical load

Performance of an HTS (high temperature superconductor) tape is evaluated by measuring the critical current I/sub c/, AC loss and mechanical properties of the short samples. However, in coils or magnets for applications, the HTS tapes are under mechanical load caused by winding tension, bending and sometimes twisting. Also thermal contraction by the cooling changes the tension. The mechanical load reduces the critical current of the superconducting wire, and the I/sub c/ degradation affects the AC loss of the wire. We measured the I/sub c/ degradations and AC losses (transport current loss) of commercial Bi-2223 tapes processed by a PIT (powder in tube) method under tension. The I/sub c/ and AC loss measurements are done in the same time with the same voltage taps on the tape conductors. The measurement results and discussions on the relationship between I/sub c/ degradation and AC loss are presented.

The Development of 100 kA Current Leads for a Superconducting Transmission Line Magnet

A pair of current leads to power a transmission line magnet cooled at liquid helium temperature has been designed and developed at Fermilab. The leads were designed to carry 100 kA dc current. Each lead consists of a warm end, a heat exchanger section and a cold end. The warm end is a half moon shaped plate brazed to cylinder. The heat exchanger section is made of 202 copper rods arranged in a staggered pattern. Each rod is 6.35 mm in diameter and 1650 mm in length. The rods were soft-soldered into 12.7 mm deep holes at both warm and cold ends. The helium gas flow, guided by anodized aluminum baffles along the lead length, allows for a relatively high heat transfer coefficient between the current carrying rods and cooling helium gas. The current leads were successfully tested with a ramping current of up to 104 kA. The current lead design, assembly work and the test results are presented

Maximization of flux-linkage in HTS motors using shape design sensitivity analysis with critical current constraint

This paper proposes a design method for optimizing the rotor winding shape of HTS motor. The design objective is to maximize the flux-linkage of stator winding with a given amount of superconductor volume. The increase of flux-linkage results in output-power increase, compact size and quench-reliable characteristic. The optimization algorithm is a design sensitivity analysis where the HTS critical current condition is taken into account. First, the shape of rotor winding is optimized to give a boundary shape of smooth curves. Second, with the shape the rectangular sizes of windings are approximately obtained for easy manufacture. Finally, the rectangular sizes are also optimized for fine-size tuning. The proposed design method is applied to the 100-hp HTS motor.

Design of HTS tape characteristics measurement system

High temperature superconductor (HTS) tapes are now commercially available for practical applications to apply various purposes. However HTS tapes show different electrical and mechanical characteristics, according to the manufacturers who are trying to apply various fabrication processes and treatments. From the viewpoint of an application, it is very important to investigate the properties of HTS tapes under mechanical stress, because the tapes will be wound with twisting and tension in applications such as magnets and cables. Considering of operating situations of applications, HTS tapes might be influenced not only mechanical deformation but also electromagnetic effects. Thus, we developed an equipment to measure the characteristics of HTS tapes, considering of these mechanical and electromagnetic conditions. This system can be used to measure the critical current and the AC loss in magnetic field with tension and twisting. A description of the design and some typical results are presented with discussions.

Design and test of a 1-MJ SMES system

For several decades researches and developments on superconducting magnetic energy storage (SMES) system have been done for efficient electric power management. Korea Electrotechnology Research Institute (KERI) have developed a 1-MJ 300-kVA SMES System for improving power quality in sensitive electric loads. It consists of an IGBT (Insulated Gate Bipolar Transistor) based power conversion module, NbTi mixed matrix conductor superconducting magnet and a cryostat with HTS current leads. The authors developed the code for design of a SMES magnet, which could find the parameters of the SMES magnet having minimum amount of superconductors for the same stored energy, and designed the 1-MJ SMES magnet by using it. And they have designed and fabricated cryostat with kA class HTS current leads for a 1-MJ SMES System. This paper describes the design, fabrication and experimental results for a 1-MJ SMES System.