Kenji Tasaki

Abnormal quench process with very fast elongation of normal zone in multi-strand superconducting cables

Abstract An abnormal quench process with apparent high propagation velocity of the normal zone, ·fast quench’, was studied in relation to electromagnetic and thermal couplings among strands for multi-strand superconducting cables. The quench process was numerically evaluated by simultaneous equations of heat balance and equivalent electrical circuit of the multi-strand cables. Experimental results are well explained by the numerical simulation concerning threshold current of the fast quench for a two-strand cable. The quench process of a six-strand cable was also calculated numerically and the relation between the fast quench and current commutation among strands is discussed.

The Project Overview of the HTS Magnet for Superconducting Maglev

This paper describes the outline of a development project for the HTS magnet for the superconducting Maglev, which commenced in 1999. A very small current decay rate of 0.44%/day was achieved in 2003, using a prototype HTS coil, and a second HTS magnet, consisting of four persistent current HTS coils, was produced in 2005 for vehicle running tests. The second HTS magnet was operated in a persistent current mode at a rated magneto-motive force of 750 kA, and a top speed of 553 km/h was attained on the Yamanashi Maglev Test Line on December 2,2005.

Persistent current HTS magnet cooled by cryocooler (2) - magnet configuration and persistent current operation test

An high temperature superconducting (HTS) magnet, consisting of an HTS coil, a persistent current switch, a GM type two-stage pulse tube cryocooler, and YBCO current leads was developed. Detachable current leads were adopted to reduce heat leakage during persistent current operation. The HTS coil was cooled to approximately 10 K and persistent current mode operation tests were carried out at various currents up to 532 A, which is the rated current. Current decay at each persistent current mode operation was investigated by measuring the magnetic field at the center of the coil. The current decay rate at the 532 A operation was found to be approximately 0.44%/day.

The Running Tests of the Superconducting Maglev Using the HTS Magnet

An HTS magnet for a Superconducting Maglev, consisting of four persistent current HTS coils, was developed. The HTS coils are installed in a cryostat, and cooled to approximately 15 K by conduction cooling, using two sets of two-stage GM type pulse tube cryocoolers. The HTS magnet is operated in a persistent current mode at a rated magneto-motive force of 750 kA. The running tests were executed on the Yamanashi Maglev Test Line, with a top speed of 553 km/h achieved on December 2, 2005. The test result demonstrated that the HTS coils generated no excessive vibration or heat load.

Quench analysis in a 72 kVA superconducting four-winding power transformer

Abstract Quench processes in a 72 kVA superconducting four-winding power transformer were studied experimentally by sudden short-circuiting of the secondary load. An instantaneous decrease in the transport current was observed in both of the primary and secondary main windings during the first half cycle immediately after the short circuit. The equivalent propagation velocity of normal regions along the wire during the period of instantaneous decrease in current was about two orders of magnitude larger than existing theoretical predictions. Two-dimensional propagation of the normal region was also observed after the very fast propagation. In the primary main winding, the equivalent velocity of the very fast propagation was linearly increased with the voltage applied to the normal region of the winding. This type of very fast progagation was explained by a model in which a broad part of the winding is changed into the normal state via a flux-flow state with very low resistivity, due to the excess current. In the secondary main winding, similar fast propagation was also generated by local heating of the winding above a threshold level in the region of the transport current. The experimental results show that the instantaneous transition to the normal state is attributed to a quench process peculiar to superconducting cables, due to the redistribution of the transport current among strands. It is proposed that the abnormal transitions to the normal state can be useful for self-protection of a.c. superconducting windings from burning.

HTS Magnet for Maglev Applications (2)–Magnet Structure and Performance

An HTS magnet for maglev application has been developed. The magnet consists of four persistent-current HTS coils, and is operated at a rated temperature of 20 K and a rated magnetomotive force of 750 kA for each coil. This paper describes the structure and performance test results of the HTS magnet. The four HTS coils are installed in a cryostat and conductively cooled by two sets of two-stage GM type pulse tube cryocoolers below 20 K. Detachable current leads, which are composed of connectors and ultrasonic motors, are adopted to reduce the heat leakage to the 1st stage cold head of the cryocoolers. The HTS coils are simultaneously charged up to the rated magnetomotive force. Persistent current operating tests were carried out and the current decay rates of 0.4-0.7%/day were attained. Also, in order to evaluate the mechanical capability of the magnet, vibration tests were conducted and permissible vibration responses were obtained