The Influence of Parameters of the Stabilizing Layer of 2G HTS Tapes and the Specifics of Heat Removal on the CVC Shape in the Resistive State and on the Critical Voltage Inducing Irreversible Changes in Superconducting Properties

Resumo

The actual form of the resistive section of the AC current–voltage curve (CVC) is determined experimentally. It is found that the CVC is shaped both by the electrical properties of the superconducting layer and by the thermomechanical properties of layers in the tape structure. The specific features of CVC behavior in different current ranges are determined. It is demonstrated that small amounts of heat released in stabilizing and superconducting layers do not affect the CVC shape below the critical current. Above the critical current, combined heat releases in stabilizing and superconducting layers raise the temperature and the resistance of an HTS tape in the resistive state. It is found that the stabilizing layer has a critical thickness. Below this critical level, the current in the resistive state flows primarily along the HTS layer, and its slight overheating initiates the transition of the sample to the normal state due to the thermal instability of heat removal to liquid nitrogen. In tapes with thicker copper layers, the current in the resistive state is distributed between stabilizing and HTS layers, and the transition to the normal state occurs near the critical temperature of the superconductor at zero current. The dependence of critical voltages characterizing different damage levels of HTS tapes on the thickness of the copper layer is determined. It is found that the breakdown occurs at higher voltages if step voltage is applied.