Influence of a longitudinal variation of <i>dB</i>/<i>dt</i> on the magnetic field distribution of superconducting accellerator magnets

Abstract

Unexpected periodic variations of the magnetic field were recently found along the axis of superconducting accellerator magnets, which reduces the field quality of the magnets. The field modulation shows a complicated time dependence containing very long time constants. In this letter it is shown that an inhomogeneous distribution of dB/dt along the superconducting cable leads to superposed coupling currents flowing over long lengths, which can explain the observed effects. The solution of the diffusion equation allows one to calculate the space and time dependence of the currents and hence of the amplitude of the field modulation.