On-field monitoring of the magnetic axis misalignment in multi-coils solenoids

A method for monitoring the coils alignment in multi-coil magnets, directly during their operation in particle accelerators, is proposed. Few measures of the magnetic field by Hall transducers and a local field model allows one to monitor the magnetic axis position of each coil. Thus, the particle beam can be kept strongly focused over its operation time by means of feedback. Furthermore, the uncertainty in measuring the magnetic axis was studied with the aim of finding the optimal placing of the transducers, where the minimum uncertainty is achieved. The method was validated on a challenging case study on the European project "ELI Nuclear Physics" (ELI-NP). In particular, simulations results, based on both analytical and finite-element models of the ELI-NP multi-coil solenoid, point to an uncertainty below 100 mu m, for misalignment of the magnetic axes in rotation up to pi / 1000 and in translation of 1/1000 times the length of a single coil. This uncertainty is limited by the Hall transducers technology, but has potential for large improvement margins, because the measurement model error is below 1 mu m.