Identification and reduction of sources of error fields in layer-wound short solenoids

Abstract

We report on a study of the error fields of a set of layer-wound short solenoids of the type used in high-current electron linear-induction accelerators. These error fields are mainly caused by the azimuthal variation of the winding pitch angle. The conventional winding pattern design tends to concentrate the error fields in a dipole component perpendicular to the solenoid axis. Compounding the problem are smaller contributions due to the asymmetric coil feeds and conductor crossovers from one layer to the next. We have constructed and tested a prototype quadrufilar solenoidal magnet which utilizes symmetry to cancel the effects of current leads and crossovers and which, in the absence of winding errors, should have an intrinsically straight magnetic axis. This design tends to concentrate the error field in the much less harmful octapole component. Measurements of the dipole error field yield a value of the order of one milliradian for the quadrufilar magnet as compared with about six milliradians for the conventional ETA-II magnets and twelve milliradians for the ATA magnets. Several approaches for improvement are suggested. The improved cooling of the quadrufilar design allows dc operation at fields almost three times higher than the conventional magnets.