Time constants of flat superconducting composites determined from AC loss and relaxation measurements

The basic procedures for determining the time constant of superconductors a re the AC losses (low frequency slope of the power loss density per cycle, position of the maximum loss per cycle vs frequency) and relaxation measure ments of the induced coupling currents. We show that there are fundamental reasons for some differences between them for flat structures (like Rutherf ord type cables, filamentary BSCCO tapes, cable-in-conduit conductors). As the main losses in superconducting cables occur by the induced currents bet ween the opposite sides of the cable (transverse losses), there are many ca ble designs with central insulating layer, mainly for flat cables. Then, th e coupling losses between adjacent strands (parallel losses) can prevail. W e calculate the time constants for both transverse and parallel components of the coupling currents in flat structures with N strands based on differe nt approaches (AC losses, field shielding, current relaxation). For transve rse coupling currents, the time constants from losses differ in the absolut e value. The relaxation time is larger than the loss time constant for N < 18 and smaller for higher N values. For currents induced between adjacent s trands, the ratio of the time constants from the relaxation and AC loss mea surements depends on geometrical parameters of the cable. The relaxation ti me should be generally larger than the time constant from the AC loss measu rements. (C) 2001 Elsevier Science B.V. All rights reserved.