We report results on the temperature dependence of the trap field B T and the thermal instability (flux jump) field Hfj in single-grain YBa2Cu3O7-δ disks. Above 45 K, BT is limited by the critical current density Jc and BT=BT0(1-T/Tc)q with q≊1.67. At 4.2 K, the first flux jump field Hfj of ∼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.
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