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This paper describes design concept, fabrication, and experimental results of a Persistent-Current Switch (PCS) for High Temperature Superconducting (HTS) magnets. Recently large-size HTS magnets have been fabricated with Ag-sheathed Bi2223 wires. Even in the case of HTS magnets, persistent-current mode has the advantage of maintaining a steady magnetic field for long periods. A PCS for an HTS magnet is a key component to realize persistent-current mode. A PCS for an HTS magnet must work at the same operating temperature as an HTS coil, and electrical resistivity for a PCS conductor should be sufficiently large to reduce the energy dissipation during charge and to shorten the switching time. To satisfy these requirements, a YBCO thin film is adopted for a PCS conductor. The PCS is used in a vacuum and conductively cooled by a cryocooler at around 20 K when the switch is "on". To change the PCS from "on" to "off", the temperature of the PCS is increased to around 100 K by electrical heaters. A new design concept of a PCS for an HTS magnet is introduced. And the test results of rated current operation, switching operation and persistent-current operation are shown.

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Synopsis:This paper describes a persistent current HTS magnet that has been developed for Maglev applications. The HTS coil for the magnet consists of 12 single-pancake coils wound with four parallel Ag-sheathed Bi2223 wires. The HTS coil is connected with a persistent current switch (PCS) made of YBCO films and cooled below 20 K by a two-stage GM pulse-tube cryocooler. Detachable current leads are used to reduce heat leakage to the 1st stage of the cryocooler. The initial target for the current decay rate of the magnet was 10%/day. In order to achieve the target, wires with n-value of more than 14 in a range of 10 -9 -10 -6 V/cm at 77 K, 0 T were selected and coil winding processes were improved. As the result, a current decay rate of 0.44%/ day was obtained under persistent current operation of the HTS magnet. To observe the mechanical capability of the magnet, mechanical vibration tests up to ±15 G and electromagnetic vibration tests were carried out.

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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 persistentcurrent 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 HTS 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.