Localization of Electrical Insulation Failures in Superconducting Collared Coils

The localization of possible electrical faults in superconducting accelerator magnets may, in most cases, be a complex, expensive and time consuming process. In particular, inter-turn short circuits and failures of the ground insulation are well detectable when the magnet is collared, often disappear after disassembly for repair due to the release of the internal pre-stress in the coils.

The fault localization method presented here is based on the measurement and analysis of the magnetic field generated inside the magnet aperture by a high voltage pulse. The presence of the faults modifies the distribution of the current in the coils and produces a distorsion of the magnetic field. The methods aims at locating both the azimuthal position of the fault-affected area.

 
Measurement Procedure

On one side of the inter-turn short circuit the current distribution in the coil is asymmetric. On the other side it becomes symmetric again. During the discharge tests performed on the faulty magnet there is also a high current induced in the passive winding deprived of the imposed transport current due to the inter-turn short circuit. This induced current will generate a magnetic field opposing the flux change in the area spawn by the fault-affected loop.The effect is a main cause of an essential degradation of the magnetic field in the aperture of the magnet. The current induced in the loop has also an asymmetric distribution on one side of the short-circuit, and a symmetric one on the other side.

Return

 In terms of magnetic field analysis this corresponds to the vanishing of the even orders of normal field harmonics and of the odd orders of skew field harmonics in the area were the current distribution becomes symmetric.

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