Thermal Stability Properties of Coated Conductor Tape Under the Cryocooling Condition

From the viewpoint of a cryogen-free high-field superconducting magnet, the thermal stability properties of YBa₂Cu₃O₇ (Y123) coated conductor tapes before thermal runaway are examined under the conduction-cooling condition by a GM-cryocooler. Under such cryocooling condition, Ag-sheathed Bi₂Sr₂CaCu₂O₈ (Bi2212) wires with J_c ~ 10⁴ A/cm² reveal stable behaviors of 3 times larger current properties before thermal runaway than critical currents, although conventional Nb₃Sn wires with J_c ~ 10⁴ A/cm² 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 T₀=4.2 K, J_c ~ 10⁷ A/cm², and cryocooling heat transfer coefficient h=10^-3 W/cm²K. We found that the Y123 tape with J_c ~ 10⁷ A/cm² does not have the over-critical current but rather the sub-critical current before thermal runaway.