Abstract
A persistent current switch (PCS) in the GWh class SMES system for load leveling needs to have high OFF resistance as well as zero ON resistance for high efficient SMES operations. So far, the authors have successfully developed NbTi mechanical PCS which has zero ON resistance, i. e. zero contact resistance, and can flow the current up to 200A without quench. The PCS should break current of a few tens amperes even with the counter pulsed technique to reduce the breaking current. The current breaking generates arc discharge which alters the contact condition. Since the superconducting connection strongly depends on conditions of contact surfaces, it is necessary to clarify the influence of the arc discharge on the PCS performance. In this study, arc discharge characteristics of NbTi mechanical PCS and influence of arc on superconducting connection at contact are experimentally investigated using a circuit including a superconducting coil of 1 H and a damping resistance in liquid helium. Arc discharge characteristics are almost similar although arc maintain voltage of 12 V is smaller than that observed with copper contact. Arc discharge does not degrade but rather improves the PCS performance as long as the breaking current is smaller than 50 A. The improvement is attributed to enhanced bridge formation on the contact surfaces and removal effect of the surface oxide film by arc discharge.
