Experimental investigation of training and degradation in superconducting magnet systems(SMS)

Training and degradation phenomena inherent in SMS, especially in those with complex configuration, are a serious obstacle to the creation of partially stabilized SMS with predictable parameters. The mechanical origin of these phenomena is well recognized, although their more detailed mechanism has not been definitely found. Some of us succeeded in reproducing training and degradation in short samples of superconducting wires strained and charged with transport current in an external magnetic field. No dependence on current charging rate was found-evidence in favor of impulse nature of heat disturbances. Subsequently it was observed that the number of training quenches increases when the heat transfer from the sample decreases ( in these experiments the deterioration of heat transfer was made by placing samples in vacuum instead of liquid helium). All these facts as well as some others argue in favor of serration yielding as the most probable cause of training and degradation. We have carried out a number of experiments studying these phenomena in model circular and oval windings placed in an external superconducting field. The tested windings were separated from the external coil by a separate cryostat. The size of the windings was up to 190 mm, the external field up to 7 T. Potted as well as free-wound samples were tested. They were made of multifilament niobium-titanium wires of different diameter with copper-to-superconductor ratio about 1:1 and 2:1. It was found that the oval windings had much more severe training and degradation than the circular. Some dependences of these phenomena on wire diameter and features of the windings are also reported.