Performance of stabilized superconducting cables under controlled heat transfer conditions

Seven different sample types of compound conductors were tested under almost identical conditions for the heat transfer from the cable surfaces to the liquid helium. Three different types of stability were observed: smooth transition from the critical current Icto a stable flux flow characteristic; an unsteady transition from Icto a stable flux flow characteristic; and an unsteady transition from Icto the normal state. Uniform temperature distribution in the compound conductors was tentatively assumed. Furthermore, the heat flux per unit area varied as the 2.5 power of the temperature difference τ between the conductor and helium. This law proved to be valid except for low τ values for which another heat transfer law could be derived from our measurement. For the majority of experiments, the three stability cases could be satisfactorily predicted. Concerning the final transitions to the normal state of the superconductors ("takeoff points") we found that the transition from nucleate to film boiling is not always decisive.