THERMAL-INSTABILITY, MAGNETIC-FIELD SHIELDING AND TRAPPING IN SINGLE-GRAIN YBA2CU3O7-DELTA BULK MATERIALS

We report results on the temperature dependence of the trap field BT and the thermal instability (flux jump) field H-fj in single-grain YBa2Cu3O7_delta disks. Above 45 K, B-T is limited by the critical current density J(c) and B-T=B-TO(1- T/T-c)(q) with q approximate to 1.67. At 4.2 K, the first flux jump field H-fj of similar to 3 T is about an order of magnitude larger than theoretical prediction based on the adiabatic thermal instability model. We also find that the occurrence of large flux avalanches probably causes severe microcracking damages to the superconducting disks.