Stability tests were performed for two small test coils wound with NbTi/Cu composite superconducting wires with different surface conditions respectively. One is a 0.5 mm-diameter bare wire with the copper ratio of 1.3. Another is the same wire with chemically oxidized copper surface. The stable limit current under constant magnetic field greatly increased by shifting to He II cooling from He I cooling for both wires, but stationary normal zones were observed in wide current area lower than the stable limit currents for the oxidized wire. The characteristic of the stability of a superconducting wire is more deeply dependent on the conductor surface condition cooled by He II than that in case of He I cooling.
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Ohya et al. (2005) studied this question.
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