Passive persistent current correctors for accelerator magnets

Persistent currents in the superconducting material of conductors in superconducting particle accelerator magnets produce large nonuniformities in the magnetic field at low (injection) fields. The purpose of this paper is to discuss a simple, passive approach to reduce the multipole content of accelerator magnets at low magnetic fields. Calculations were performed to determine methods of reducing the b/sub 2/, b/sub 4/, and b/sub 6/ multipoles. Thin nickel strips are added at precise locations along the outer surface of the beam tube in the magnet. The length of the strips is about 1 meter. Magnetization of the nickel counters the multipoles caused by the persistent currents up to about 2 T. A sample beam tube was prepared with nickel strips and inserted into a prototype collider dipole magnet (CDM) cold mass. The magnet was tested warm at currents up to 30 amperes. Measurements of radial flux density were performed at a series of points along the inner surface of the beam tube to assess the effect of the nickel strips. The test results were compared with predicted values from a finite element model of the magnet. The corrector strips are found to correct the field nonuniformities, in qualitative agreement with calculated results.<>