Longitudinal periodicity in superconducting dipole magnets

Recent magnetic measurements sensitive to the persistent currents in superconducting HERA dipole magnets uncovered a longitudinal periodic pattern with a period equal to the pitch of the strands in the cable. Not all the observed facets of this phenomenon have been explained yet. Although the existence of this pattern does not affect the performance of the accelerator, it is of interest to understand its exact source. In this note it is suggested how this pattern could result from differences in the strands making up the cable. More specifically, from differences in their (Jc*D), the average of the product Jc*D of the filaments of a strand, where Jc is the critical current density and D is the diameter of the filament. These strand-to-strand differences in a cable are to be expected from the selection process that probably occurred in the cable manufacture: strands of high and low critical current were mixed in order to maximize cable production yield.