EXPERIMENTAL-STUDY OF STABILITY AGAINST MAGNETIC PULSE FIELDS OF THE KFK POLOIDAL FIELD-COIL CABLE

The stability of a superconducting cable, developed for operation in a 15 kA poloidal field coil of a tokamak experiment, was investigated in rapidly changing magnetic fields. The test arrangement consists of a hairpin-shaped closed loop of the cable, into which the transport current is induced inductively. A magnetic field pulse is supplied to the straight section of the sample by a capacitor discharge into a dipole magnet. The cooling conditions of the superconductor can be varied between liquid and supercritical helium in a closed volume. The results are discussed in terms of a stability model, which was successful in describing the stability of the basic strand. This model compares the energy input into the conductor during the field pulse with the transient heat removal capability of the coolant. The stability behaviour showed a distinct difference between the low and the high current range of the cable: in the lower current range the stability limit is well described by the stability model; in the high current range (well above the design current), however, the stability limit is much lower than expected from the strand experiment and from the model. This is understood qualitatively by an inhomogeneous distribution of the cable a.c. losses and by inhomogeneous current distribution within the cable. The reduced stability in the upper current range might be a serious problem for large superconducting coils exposed to rapidly changing fields.