Development of HTS Current Leads Prepared by the TFA-MOD Processed YBCO Tapes

HTS current leads have been prepared by the TFA-MOD processed YBCO tapes. A current lead unit is composed of five YBCO tapes soldered to Cu caps at both ends and a GFRP board. The YBCO tape is 5 mm in width, 190 mm in length and about 120 mu m in overall thickness. The YBCO superconducting layer, 1.5 mu m in thickness, is formed on oxide buffer layers of GZO and CcO(2) deposited on Hastelloy substrate tapes. Transport current is supplied through Ag layer deposited on YBCO layer. The YBCO tapes with critical current of about 170 A at 77 K in self-fields have been used for two 500 A-class current lead units. The transport current of 800 A was stably carried in liquid nitrogen without any voltage appearance on each tape. The voltage between both Cu caps linearly increased with increasing current, and it was about 350 mu V at 500 A. The low voltage at Cu joints results from low contact resistance between YBCO tapes and Cu caps, and causes small Joule heating at the Cu joints. Furthermore, the transport current of 1400 A was successfully applied in the current lead combined with two units in parallel. The heat leakage of the current lead unit composed of five YBCO tapes and a GFRP board between 77 K and 4.2 K is calculated to be 47 mW/0.5 kA, which is much smaller than that of conventional Cu current lead (1.2 W/kA). The small heat leakage results from high current performance and low thermal conductivity in the present HTS current lead.