A possible mechanism for enhanced persistent current sextupole decay in SSC dipoles

It has recently been discovered that the HEAR superconducting dipoles have a periodic pattern in their fields as measured along the axis of the dipole. This effect has more recently been observed in the prototype SSC 5 cm dipole. The wavelength of the periods observed in both dipoles is equal to the transposition pitch length of the Rutherford cable. A plausible explanation for the periodic pattern is the existence of a transport current trapped within the cable. That is, a current which runs through one strand from the solder joint at the one end of the cable to the solder joint at the other end, and which returns back by way of another strand. The average current in the strands, which is the current from the power supply, is unaffected by the presence of currents trapped within the cable. The size of the observed field oscillation suggests that the trapped currents might be as large as 100 amps. There is evidence that the trapped currents are time dependent. This is reasonable since the I-V characteristics of the various resistive elements in the cable are all different so the equilibrium distribution of current in the strands should depend on the excitation level of the magnet. It is the purpose of this note to point out that the time dependence of trapped currents is a powerful mechanism for causing the time decay of ''persistent current'' multipoles