Thermal Stability Properties of ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7}$ Coated Conductor Tape Under the Cryocooling Condition

From the viewpoint of a cryogen-free high-field superconducting magnet, the thermal stability properties of YBa 2 Cu 3 O 7 (Y123) coated conductor tapes before thermal runaway are examined under the conduction-cooling condition by a GM-cryocooler. Under such cryocooling condition, Ag-sheathed Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) wires with J c ~ 10 4 A/cm 2 reveal stable behaviors of 3 times larger current properties before thermal runaway than critical currents, although conventional Nb 3 Sn wires with J c ~ 10 4 A/cm 2 quench almost at their critical currents under the cryocooling condition. In order to determine the maximum current-carrying capacity of the Y123 tape in high fields, we use a zero-dimensional heat balance model. On the basis of the measured basic properties for the Y123 tape, the calculations were done at B =20 T, bath temperature T 0 =4.2 K, J c ~ 10 7 A/cm 2 , and cryocooling heat transfer coefficient h =10 -3 W/cm 2 K. We found that the Y123 tape with J c ~ 10 7 A/cm 2 does not have the over-critical current but rather the sub-critical current before thermal runaway.