Influence of supercurrents on the stability of superconducting magnets

Supercurrents were recently identified as a source of reduced magnet stabil ity which can explain the measured ramp rate limitation in large supercondu cting magnets. They also explain an unexpected periodic field modulation al ong the axes of superconducting accelerator dipoles. Supercurrents are extr a coupling currents between the strands of a cable which are induced by a v ariable field sweep rate (B)over dot(x) along the length of the cable. They flow over the whole cable length and have time constants many orders of ma gnitudes larger than 'normal' interstrand coupling currents. Supercurrents may lead to a highly inhomogeneous current distribution and to additional c oupling losses ('supercurrent losses'), even in magnet sections with (B)ove r dot = 0. Both effects can drastically reduce the magnet stability during fast ramping up. The complete solution of the space and time dependence is given for a two-strand model cable. The theory of supercurrents can explain typical results of ramp rate limitation in large magnets. (C) 1998 Elsevie r Science B.V. All rights reserved.