Development of a Persistent Current Switch for HTS Magnets

Abstract

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.

Persistent Current Switch for HTS Superconducting Magnets: Design, Control Strategy, and Test Results

“…The PCS usually achieve above features through the transitions of the superconductor between the superconducting state (zero-resistance state) and the normal state. Thermally-controlled switches trigger the transition though heating the superconductor above its critical temperature [2][3][4][5][6]. This type of PCS can achieve a high resistance, but it usually takes several seconds or even longer to transfer between the two states.…”

An Overview of Flux Pumps for HTS Coils

“…These merits make CC coils ideal for high field magnet uses, like in MRI/NMR insert coil [2] [3]. In these applications, it is desirable to operate CC coil magnets in persistent current mode [4], in which case thick current lead and associated heat losses can be avoided. However, flux leakage is inevitable in closed CC coils.…”

Persistent Current HTS Magnet Cooled by Cryocooler (3)—HTS Magnet Characteristics

“…So far, we have improved the performances of racetrack formed single-pancake coils, especially the n-value in a low electrical field region from to . We also developed a persistent current switch (PCS) made of a YBCO thin film [4] with a higher switching speed and detachable current leads for low heat leak to a cryostat [5]. Integrating these devices, a persistent current HTS magnet of the same size as a magnet for a Maglev train was fabricated and evaluated.…”