Curling Effect in Superconducting Wire

The experimental results of curling effect investigation (sharp increasing of the normal phase propagation speed under influence of the magnetic flux avalanche) in superconducting Nb-Ti and Nb-Zr wires are presented. The external magnetic field was imposed parallel to an axis of a sample of a superconducting wire by thickness from 0.16 mm up to 0.30 mm without a normal stabilizing covering, the transport current was put, and after initiation of a normal phase on a short part of a sample the normal phase propagation and the magnetic flux avalanche along a sample were registered simultaneously. It was found, that the thickness of the magnetic flux avalanche Lmfa and the thickness of the normal phase front Lnpf (length of the part of a sample engaged by S-N transition) achieves 3 - 4 mm, that is more than 10 diameters of the sample. The heating of a superconducting wire by the magnetic flux avalanche is accompanied by increase of the speed of normal phase propagation Vnpp. In a narrow range of change of a transport current the sharp increasing of the speed of normal phase propagation Vnpp up to the speed of a magnetic flux avalanche Vmfa occurs. The further increasing of a transport current is accompanied by reduction of a distance (spacing) between the normal phase and the magnetic flux avalanche. In turn Vmfa increase with increasing of critical current dencity Jc and thickness of a superconducting wire. In particular the speeds Vmfa and Vnpp achieve the value 9 - 13 km/s for the wire Nb-25%Zr by thickness of 0.24 mm with the critical current Jc = 2 - 4 MA/cm2 in magnetic fields 0.5 - 1 T