Abstract
A persistent current switch (PCS) in SMES requires not only small ON resistance but also large OFF resistance in order to perform efficiently during SMES operations. Because a mechanical switch has the advantage of infinite OFF resistance, it can operate without generating Joule losses during coil excitation and energy transfer. Furthermore, it will cause an energy storage loss as long as the ON resistance, which mainly depends on contact resistance, is not decreased to zero. In this study, case studies on energy losses due to the ON resistance of a mechanical switch and OFF resistance of a superconducting switch are carried out in order to show the necessity of a mechanical switch for a PCS in a large-scale SMES. Then, the contact resistance characteristics of a mechanical switch using bulk NbTi contact pieces are investigated in liquid helium as a function of switch driving force, switch pressing force and ON-OFF operation number. Additionally, the effects of surface roughness and shape on contact resistance are examined. It was found that a superconducting connection at contact is realized even if the switch is repeatedly opened and closed many times. The tested mechanical PCS supports a superconducting current flow of more than 200A. Finally, the formation mechanism of the superconducting connection at contact is modeled and analyzed based on theoretical considerations.
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