Alternating currents in superconducting wires
Birchak, J. Robert
Houston, William V.
Master of Arts
The transition currents and the power losses were observed for superconducting wires that were carrying 60 cps. currents. The wires were made of niobium, niobium alloyed with 10%, 25% and 33% zirconium, and PbBi 18%. The wire diameters ranged from 10 mils to 62.5 mils. Near 4.2°K all the wires containing niobium remained superconducting until the peak value of the 60 cps. magnetic field at some point on the wire surface had reached or exceeded the bulk critical field of niobium. For the lead alloy the peak value was higher than expected for pure lead. The d.c. transition currents were also measured and were usually greater than the peak a.c. values. In addition, the experiment showed that alternating current power losses occurred while the peak field was far below the transition field. For any given wire, the losses were observed to increase as In. The magnitude of these power losses also varied with the material and the diameter of the wire. The results agreed reasonably well with data from other experiments. To study the results a model was used which had a superconducting cylinder surrounded by a shell of normal material. Maxwell's and London's equations were solved for this situation. The solutions did not predict the obsered relation between the current and the power loss, but they suggested that some form of flux trapping or hysteresis behavior-should have been included in the model.