Review on Boundary-Induced Coupling Currents

Abstract

* LHC DivisionPresented at CEC/ICMC '97, Portland 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.2

AC loss in a stack of flat superconducting cables

“…Oscillations of eddy currents under stationary conditions are already well described [3], [5], [6]. Let us show that (2) provides a solution in a much simpler way.…”

“…To find it, let us use the fact that in linear systems the correct current distribution should provide a minimum energy dissipation [3], [6]. As a result, we have (5) The coefficient of the eddy current oscillations is zero when , where is an integer, and reaches local maximums at .…”

“…It is known that in nonuniform conditions the time constants of the flat cable can be very large [6]- [8], [11]. This is not the case when both the cable and applied magnetic field are uniform along the cable length.…”

“…OR THE last decade lumped circuit models [1]- [6] have been developed to describe eddy currents in a flat superconducting -strand cable subjected to background magnetic field variations. These models divide each strand of the cable into 1 finite elements.…”

Persistent and coupling current effects in the LHC superconducting dipoles

“…Transport current imbalances among the strands (e.g., caused by nonuniform joint resistance), and coupling currents with long time constants (e.g., caused by variations in along the cable, and often referred to as BICC's [10]) cause variations in the local field, resulting in an average change of the magnetization proportional to the change of the current distribution [11]. In turn, current distribution should be affected by the inter-strand resistance .…”