Abstract:
A study was made of phase transitions to normal state in high-temperature superconducting wires with current due to gaseous nitrogen occurrence around some part of the wire. The simultaneous presence of liquid and gaseous refrigerant along the wire length means that heat removal non-uniformity from its surface takes place that results in normal zone formation in the wire with current in the site with reduced heat removal. As a result, a jump-like change in the current and voltage occurs that may be used for signal generation by a superconducting sensor in alarm level indicator and/or gaseous inclusion indicator in liquid nitrogen. The sensor’s ability to work is based upon the wire normal state stability maintenance after its operation and upon the reliability of the transient signal registration from the low resistance normal region. These was realized by means of AC current measurements using a transformer-based power supply. The results obtained for the first time are presented of physical modelling of the transients in the superconducting sensors depending on a stabilizing layer presence in the wire, the current supply method (cooled/non-cooled leads), the wire structure (single/bifilar). The importance of the obtained results for applications is confirmed by the recommendations developed on their basis concerning the superconducting sensor application in the alarm indicators for the two types of nitrogen cryostats: bulk type (for winding refrigeration) or long type (cryostat shell for superconducting cable).
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