Kazuo Funaki

Development of a 1 T cryocooler-cooled pulse coil with a Bi2223 superconducting parallel conductor for SMES

The authors designed and fabricated a 1 T cryocooler-cooled pulse coil operating at 40 K. A 4-strand interlayer-transposed parallel conductor composed of Bi-2223 rectangular cross-sectional multifilamentary wires was adopted to realize a uniform current distribution and to reduce the AC loss density down to the level of that of a single strand. The pulse coil is a 16-layer solenoidal one with an inner diameter of 52 mm, an outer diameter of 111 mm and a height of 120 mm. The heat drains of AlN plates, which are insulators, are arranged between layers for the cooling of the heat due to the AC loss by heat conduction. They could continuously operate the coil in a triangular waveform mode with an amplitude of 1 T and a frequency of 1 Hz. The AC loss was 10.6 W and the other thermal load was 13 W.

Electromagnetic properties in parallel conductors composed of Bi2223 multifilamentary wires for power transformer windings

The authors study, theoretically, the AC losses in superconducting parallel conductors exposed to a transverse alternating magnetic field in relation with transposition among the strands. They obtained an analytical expression of the additional coupling loss for the deviation of transposition from an optimum condition. The AC losses for the alternating external field were measured in parallel conductors composed of Bi2223 and NbTi multifilamentary wires. The additional losses can be well explained by the theoretical expression in a wide range of the amplitude of external field. They also estimated, experimentally, the current distribution among the strands in solenoidal coil of the parallel conductor and discussed the effect of transposition on the current distribution.

Current distribution in superconducting parallel conductors wound into pancake coils

We made a preliminary investigation of the applicability of high-T/sub c/ superconducting parallel conductors to pancake coils. For the sake of a uniform current distribution and low AC loss, the constituent strands need to be transposed so as to be inductively equivalent with each other. We adopted an interdisk transposition where the strands are not transposed inside a single-pancake coil but only at the joint between the pancake coils. The fabrication process is simple. We have only to fabricate the same double-pancake coils and connect the strands individually with transposition outside the winding. We searched theoretically for the optimum transposition in the case of 3-strand and verified the theoretical result by using small test coils wound with NbTi 3-strand parallel conductors for convenience.

Effects of artificial pins on the flux pinning force and other superconducting properties in NbTi superconductors

A marked enhancement of global pinning strength in Nb-50 wt.%Ti alloy was obtained by introducing ribbon-shaped artificial pins. Simultaneously, however, depression of B/sub c2/ was observed. This behavior is analyzed theoretically, and a satisfactory explanation of the experimental results is obtained.<<ETX>>

Development of a 1 kWh-class module-type SMES-design study

The authors are planning to build a 1 kWh/1 MW (maximum stored energy/maximum power capability) module-type SMES (named ESK; experimental SMES of Kyushu Electric Power) as a first step towards the realization of practical SMESs for power line control. The main points of the design are those of: module-type coils for the development of SMES capacity scale-up; the choice of low loss stranded cables for reducing pulse operating loss; the choices of modified D shape coils and the reduction of stresses in the conductor-which become more serious in scaling-up and high-T/sub c/ superconductor (HTSC) current leads for covering weak points due to thermal loss in a module-type SMES which need many current leads. Some other points are also studied such as the design of the cooling system in which a single coil quench does not induce that of others, and harmonics suppression in the SMES power converter system.

Experimental results of the model coil for cooling design of a 1 T cryocooler-cooled pulse coil for SMES

The authors have been developing high-Tc superconducting coils for SMES applications. Their primary goal is to make a HTS coil which is cooled to 40 K by a single-stage cryocooler and continuously operated at 1 Hz with a field amplitude of 1 T. The coil has heat drains of AlN plates to remove heat because of AC losses. They made a cooling model coil system to study the effective arrangement of the heat drains. The system consisted of a model coil using Cu conductors, current leads and a cryocooled system. The test coil was divided into three sections in different arrangement of heat drains. The model coil was daubed with a high thermal conductivity epoxy resin to improve thermal contact resistance between the conductors and AlN plates. They tested the coil by Joule heating which was equal to AC losses. They measured the temperature distribution in the coil and the temperature difference between Cu conductors and AlN plates. The temperature difference was measured between 0.2 K and 0.7 K. The results will be applied to the 1 T HTS coil design.

Quench development and ultimate normal zone propagation "velocity" in superconductors under fast current change

Normal zone evolution or quench development is a major feature for resistive type fault current limiters design. We studied quench development under fast current rise in several samples of multifilament superconducting wires with a highly resistive matrix. At very fast current rise rates simultaneous quench of the entire sample takes place. It may be described by a characteristic time of a quench. We found that at the highest current rise rate this time is the same for samples with different lengths and cooling conditions made from the same wire. Apparent normal zone velocity determined by this time is an ultimate velocity for a given length of a superconducting wire of certain type. We present experimental data about quench development under fast current rise and provide the theoretical estimations of quench parameters.

Possible solution of the "single strand stability" problem-special cable design

The widely used multistrand superconducting cables demonstrate quench current degradation in AC mode. Often it happens due to premature quench of one strand. It was suggested that changing of the cables design may improve the single strand stability, or stability of a cable in relation to the quench of a single strand. We are checking this idea by testing several AC multistrand superconducting cables made from insulated superconducting strands with different cabling technique. We determined their quench current at different frequencies and current redistribution process in case of the quench of one single strand. The experimental results are presented and influence of the cable design on the single strand stability is discussed.