Ikuo Itoh

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

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.

Engineering research and development of magnetically levitated high-temperature superconducting coil system for mini-RT project

A magnetically levitated superconducting coil system is being developed using high temperature superconductors for examining a new magnetic confinement of high-beta plasmas. A miniature double-pancake coil was fabricated with a Bi-2223 Ag-sheathed tape for the purpose of developing a floating control using laser displacement gauges. The coil was inductively excited with liquid nitrogen cooling and successfully levitated in the air. A persistent current switch is also being developed with a Bi-2223 Ag-0.3wt%Mn-sheathed tape, and a prototype model was successfully tested.

Experiments of the HTS floating coil system in the mini-RT project

A magnetically levitated superconducting coil device, Mini-RT, has been constructed using a high-temperature superconductor for the purpose of examining a new magnetic confinement scheme of high-beta plasmas. The floating coil is wound with Bi-2223 Ag-sheathed tapes, and it is operated in the temperature range 20-40 K. The excitation tests of the coil were carried out and the rated current was successfully achieved by overcoming many difficulties. The first magnetic levitation was realized for one hour and the plasma production was initiated.

Evaluation of superconducting current feeder system for the Large Helical Device (LHD)

A superconducting (SC) current feeder system for LHD has been operated since March of 1998. It consists of 9 SC bus-lines, 9 pairs of current leads and 3 current lead cryostats, SC bus-lines have a total length of 497 m, rated current of 32 kA and withstand voltage of 5 kV under 77 K helium gas condition. After 2 years operation of this system it was demonstrated that the SC current feeder system is reliable and useful. On the basis of these operational experiences, the authors have evaluated the performance, reliability and operation cost of the SC current feeder system in comparison with nonSC type current feeder system. They have concluded that the SC current feeder system with high current capacity is more suitable than other current feeder systems such as using a water-cooled busbar, especially for large-scale SC fusion experimental devices.

Stable long-term operation of superconducting current-feeder system for the LHD

A superconducting (SC) current-feeder system is used as the current transmission lines for the experimental fusion device, LRD. It consists of nine flexible SC bus lines with total length of 497 m, and nine pairs of gas-cooled current leads. To avoid the propagation of the ice on the leads, the temperature of the terminals had been kept in the range between 5 and 20 degrees C by the heaters. The measured voltage drops of all leads were less than 20 mV. The liquid helium levels of the leads and the sub-cooler tank will equalize by the siphon method. The total time of the coil excitations exceeds 3000 hours. We have demonstrated successfully that the SC current-feeder system was stable and easy to handle, and is useful for the SC experimental fusion device.