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Development of superconducting mechanical persistent current switch

The authors developed and tested a superconducting mechanical persistent current switch for SMES applications. For the purpose of improving electric current carrying capacity of one contact up to the kA level, multiple rods of NbTi with a copper matrix were selected. Pre-experiments surveyed current carrying capacity and electrical resistance of valuable contact samples, that were different in diameter of NbTi, shape of contacts and surface treatment. Vacuum valves were manufactured on trial with most suitable contacts for this purpose, and tested in liquid helium. A quench current up to 1900-2000 A DC at single valves, and 4570 A DC at four parallel valves, resulted. The electrical contact resistance before quench was lower than the measuring resolution (<1/spl times/10/sup -10/ ohm). Vacuum valve open/close speed was improved using coned disk springs and electromagnetic repulsion coils for the driving mechanism. With repeating open/close cycles, quench currents were observed to decrease. Therefore the closing mechanism should be improved.