Transient Magnetic Field Analysis for the Localization of Electrical Faults in Superconducting Collared Coils

The follow-up of the construction of superconducting magnets for accelerators requires setting up of powerful diagnostic tools to detect weak electrical points in the superconducting coils at various stages of their fabrication. In particular some electrical short circuits well detectable after collaring of the magnet often disappear after the coil is uncollared for repair; therefore it is preferable to localize this kind of electrical faults before disassembling the magnet. An R&D work on detection and localization of interturn short circuits is being carried out at CERN in view of the series production of the LHC magnets. The diagnostic methods under study include pulse propagation, time domain reflectometry and transient magnetic field analysis. In this paper special emphasis is put on the analysis of the magnetic field distortions created by the short circuits during a pulsed discharge. A model of LHC dipole allowing the simulation of different fault conditions in the coils has been implemented in ROXIE (static case) and in OPERA-2D (transient case). The model has been verified experimentally on a dedicated short dipole magnet equipped with micro-switches to trigger short circuits in different areas of the coils.