We applied hot pressing to in situ powder-in-tube-processed (PIT-processed) MgB 2 tapes. We enhanced the superconducting properties by adding nanometer-sized SiC powder and ethyltoluene (C 9 H 12 ) to the mixed powder. Hot pressing was performed at 100 MPa and 630 • C in an Ar gas atmosphere for 2-10 h. Hot pressing reduced the cross-sectional area of the MgB 2 tapes from ∼0.55 mm 2 for conventionally heat-treated tapes to ∼0.44 mm 2 . This increased the MgB 2 core density from 50% to 70%. Undoped MgB 2 tapes hot pressed for 5 h exhibited a transport J c of 90 A mm −2 at 4.2 K and 10 T, which is about three times greater than that for a tape heat treated without hot pressing. The value of J c for codoped tape hot pressed for 5 h was 450 A mm −2 at 10 T, which is larger than that for codoped tape heat treated without hot pressing (280 A mm −2 ). These results clearly demonstrate that hot pressing is effective in increasing the MgB 2 core density, and hence in enhancing the J c values of PIT-processed MgB 2 tapes.
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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