Hiroyuki Nakao

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 (1)-project overview

This paper describes a project overview for a persistent current HTS magnet, which has been in development for Maglev trains since 1999. The HTS magnet operates with a very small current decay rate of 0.44%/day and can be cooled by a cryocooler below 20 K. The HTS coil consists of 12 single-pancake coils, which were wound with 4 parallel Ag-sheathed Bi2223 tapes. In order to minimize the magnetic field decay rate during persistent current operation, we have made efforts not to decrease the high Tc superconductor characteristics during the winding of the single-pancake coils. The HTS coil is connected with a persistent current switch made of a YBCO thin film, and cooled by a G-M (Gifford-MacMahon) type two-stage pulse tube cryocooler. Detachable current leads were used to reduce heat leakage to the 1st stage of the cryocooler.

Persistent current HTS magnet cooled by cryocooler (4) - persistent current switch characteristics

We developed a persistent current high temperature superconducting (HTS) magnet for Maglev train. The HTS magnet mainly consists of an HTS coil, a persistent current switch (PCS), a GM type two-stage pulse tube cryocooler. A PCS is one of the most important components to maintain persistent current operation. A YBCO thin film was adopted for a PCS conductor because it has a high resistivity over a critical temperature and a high critical current density at lower temperatures. Persistent current mode operation tests were successfully carried out with the PCS. The current decay rate at the rated current operation of 532 A was 0.44%/day which was investigated by measuring the magnetic field at the center of the coil.

HTS Magnet for Maglev Applications (1)— Coil Characteristics

We developed an HTS coil for maglev applications. 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 fabrication and test results of each persistent current HTS coil. The HTS coil consists of 12 single-pancake coils wound with four parallel Ag-sheathed Bi2223 wires and a persistent current switch (PCS) made of YBCO thin films. The coil is conductively cooled by a cryocooler to approximately 20 K. Persistent current operating tests for four HTS coils at 750 kA were carried out and current decay rates of 0.37-0.68%/day were obtained. Mechanical vibration tests up to plusmn15 (plusmn150 m/s2) were carried out to investigate the mechanical properties of the HTS coils. Temperature increasing tests up to 25 K, which is 5 K higher than the rated operating temperature and higher magnetomotive force operating tests up to 800 kA were carried out to investigate the thermal stability of the coils and check the mechanical strength of the coils

Persistent current HTS magnet cooled by cryocooler (3)-HTS magnet characteristics

We fabricated a persistent current HTS magnet wound with conductors composed of four Ag-sheathed Bi2223 wires and an insulated stainless-steel tape. The HTS magnet is composed of 12 racetrack formed single-pancake coils and impregnated with epoxy resin. The magnet is the same size as a magnet for a Maglev train. The stored energy of the magnet is 0.34 MJ and the central magnetic field is 1.3 T at the rated current operation of 532 A. Cooling the magnet to less than 20 K, I-V characteristics in persistent current operations and AC losses in charging and discharging the magnet were investigated.

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.

Development of a low heat leak current-lead system

This paper describes a low heat leak current-lead system for a high temperature superconducting magnet which is refrigerated by conduction cooling and operated in a persistent-current mode. Under the condition that the current lead is cooled by conduction without gas cooling, we investigated the current-lead structure with which the heat load to the cryocooler is sufficiently low. The current lead consists of two parts. One of them spans the temperature interval between room temperature and around 70 K. The other spans the temperature interval between around 70 K and the lower end temperature. The former is made of copper alloy and the latter is made of a high temperature superconductor. To decrease the heat leak to a thermal anchor in the 70 K region, a detachable joint is installed in the copper alloy part. The driving mechanism for the joint is set in the room temperature region. An ultrasonic rotation motor, which works in the vacuum and the magnetic field, is adopted for the driving mechanism. We designed and constructed a detachable current lead of 600 A class, and obtained a test result that the heat leak to the thermal anchor is sufficiently low.

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.

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

Study on Thermal Stability of Conduction-Cooled HTS Coil

One of the features of an HTS coil is its high thermal stability against thermal disturbances and the minimum quench energy of an HTS coil is several orders higher than that of an LTS coil. However, thermal runaways are observed in several conduction-cooled HTS coils. As thermal runaway causes degradation of the HTS coil, it is important to understand how it occurs. We evaluated the thermal stability of a conduction-cooled HTS coil. The HTS coil was composed of four single-pancake coils wound with Ag-sheathed Bi2223 tapes and the inductance of the HTS coil was approximately 3.8 H. The thermal runaway currents of the HTS coil at 15-30 K were measured. The experimental results were found to be in good agreement with the calculated results at 15-30 K.