A training mechanism in superconducting accelerator magnets

We describe and discuss a mechanism that can contribute to "training" in superconducting accelerator magnets. This is believed to be most relevant when magnet construction and assembly procedures constrain axially compressed cable with friction. Axial compression of a cable causes partial decabling and high axial compliance. Displaced strands are friction-immobilized during subsequent collaring. Lorentz forces attempt to expand the coils and reduce pole friction, thereby allowing displaced strands to slip toward Lorentz equilibrium and heat the conductor. A simple, 2-D model quantifies the intuitive model, and is used to estimate the stress/strain conditions near the inner-layer ends of a Fermi Lab LHC IR quadrupole. Mitigation measures include friction reduction, compliance reduction and pole- turn stretching during collaring. We expect the high temperature reaction in "wind and react". Nb/sub 3/Sn magnets to reduce a coil's axial cable compression, while compacting the ends. This reduced axial compliance may substantially decrease training as a result of this mechanism. (5 refs).