Stability tests were performed of small test coils using two kinds of superconducting wire wound respectively on a FRP or a SUS bobbin. One wire is 0.50 mm-diameter NbTi composite wire with the copper ratio of 1.3, and with no insulation film. The other wire is a 0.80 mm-diameter NbTi composite wire with the copper ratio of 6.5, and with the PVF insulation. The stability limit was determined as the maximum direct current that could be applied to the test coil without spreading of a normal zone after giving a pulse current to a small heater located at a center part of the test coil winding. The stability limits were obtained for magnetic fields from 1.1 T to 7.6 T and bulk liquid temperatures from 1.6 K to 4.2 K at atmospheric pressure. The critical current at the stability limit under a constant magnetic field increased slightly with the decrease of liquid He temperature from 4.2 K down to near the λ-temperature. The stability limit increased dramatically by shifting to He II cooling from He I cooling. The degradation of heat transfer in the Kapitza conductance regime was observed on the wire with insulation film, and was not seen on the wire with no insulation film.
展开▼
