Current instability mechanisms in liquid-coolant-cooled high-temperature superconductors

The stable current distribution in Bi₂Sr₂CaCu₂O₈, Bi₂Sr₂Ca₂Cu₃O₈, and YBa₂Cu₃O₇ high-temperature superconductors depending on conditions of cooling thereof by liquid cryocoolants—helium, hydrogen, and nitrogen, respectively—is studied. It is shown that the current instability mechanism may change in going from one coolant to another. Consequently, stable states may be disturbed, first, when the conditions of cooling the superconductor surface change from nucleate boiling to film boiling. Such a thermal mechanism of stable current state disturbance is observed largely when a superconductor is cooled by liquid helium. Second, even for the nucleate boiling of a liquid coolant, current instability may result from the stable formation of the voltage-current characteristic of the superconductor. This type of injected current stability disturbance is most likely when a superconductor is cooled by liquid nitrogen. Criteria for determining a current instability mechanism in relation to the properties of the superconductor and coolant are given.