Review on Boundary-Induced Coupling Currents

Boundary-Induced Coupling Currents (BICCs) are generated in multistrand superconducting cables during a field sweep if a) the field sweep and/or b) the electrical contacts between the strands of the cable vary along the cable. Typical parts in a coil which cause large BICCs are the connections between two cables in or outside a coil and the coil ends of racetrack magnets. In the first part of the paper several approaches for describing and calculating BICCs are reviewed. Attention is paid on the steady-state as well as the time dependent solutions. In the second part of the paper the consequences of BICCs on the behaviour of magnets are discussed. These are additional field variations along the magnet length, additional coupling losses and a non-uniform distribution of coupling losses and current among the strands, resulting in a reduced stability. Several effects are illustrated by means of measurements on model dipole magnets. It is shown how the additive effect of all the BICCs in a coil is rather unpredictable so that similar coils can have rather different BICC related behaviour.