INFLUENCE OF INTER-STRAND COUPLING CURRENT ON FIELD QUALITY OF SUPERCONDUCTING ACCELERATOR MAGNETS

The coils of superconducting accelerator magnets are wound from two la yer Rutherford type cables, in which large inter-strand coupling curre nts can arise when the magnets are operated at a high current ramp rat e. The coupling currents produce energy losses which heat the magnet c ables and may degrade magnet quench performance. Furthermore, they gen erate field distortions which may result in particle beam loss. A numb er of dipole magnet prototypes developed for the Superconducting Super Collider (SSC) appeared to exhibit strong ramp rate sensitivities in quench currents, energy loss, and field distortions. A model of inter- strand coupling current was developed to simulate this behavior. The m odel successfully reproduces the observed field distortions by conside ring low resistances at the crossovers between the strands of the two cable layers which vary from coil turn to coil turn, The results of th ese analyses are consistent with azimuthal localizations of high ramp rate quenches as well as with in situ measurements of crossover resist ance. The same model can be extended to any kind of accelerator magnet s using Rutherford type cables and is applied to the case of an are qu adrupole magnet developed for the Large Hadron Collider (LHC). The ana lysis results are consistent with a behavior expected from the type of cable used for this prototype.