Ramp-rate limitation in large superconducting magnets due to 'supercurrents'

Unexpected ramp-rate limitations were found in large superconducting accelerators magnets as well as in magnets for fusion experiments. We show in this article that the variation of the field sweep rate dB/dt along the length of a superconducting cable induces extra coupling currents which flow over long lengths. Their time constants are many orders of magnitude larger than the time constant of 'ordinary' coupling currents. These 'supercurrents' can lead to a highly inhomogeneous current distribution in the cable. They furthermore create additional coupling losses ('supercurrent losses'), even in magnet sections where dB/dt = 0. Both of these effects may drastically reduce the stability of magnets during fast ramping up. It is shown that ramp-rate limitations measured in accelerator dipoles can clearly be attributed to the proposed effects. Experimental data that remained unexplained can thus be understood. This article gives the complete solution of time and space dependence of supercurrents and supercurrent losses in the approach of a two-wire model cable, which is obtained by solution of the diffusion equation.