Experimental verification of `supercurrents' in superconducting cables exposed to AC-fields

Abstract

Supercurrents were recently identified as a source of reduced magnet stability which can explain the measured ramp rate limitation in large superconducting magnets. They also explain an unexpected periodic field modulation along the axes of superconducting accelerator dipoles. Supercurrents are extra coupling currents between the strands of a cable which are induced by a variable field sweep rate Ḃ( x) along the length of the cable. They flow over the whole cable length and have time constants many orders of magnitudes larger than `normal' interstrand coupling currents. In an experiment using a two-strand model cable, supercurrents were induced by an external field variation, or by ramping of a transport current. Their time dependence was measured using a Hall probe. The experimental results demonstrate the appearance of coupling currents much larger than the normal ac-coupling currents flowing around half twist pitch loops. Their time constant was τ=65 s for a cable length of L=4.7 m. A stepwise reduction of the cable length revealed the expected τ ∝ L 2 dependence. A model calculation with a theoretical model proposed previously for a two-strand cable shows a very good agreement with the experimental results.