Measurement of the inductance of resistive magnets: two case studies

This note concerns the main definitions of self-inductance (i.e. differential, secant and energy-based) that are relevant for the electromagnetic modelling of accelerator magnets. In particular, we consider the case of iron-dominated resistive magnets. The different definitions, which coincide in the linear limit at low field, tend to diverge dramatically as the iron becomes saturated. We describe the measurement technique and the results for two case studies, i.e. the PS MTE octupole and a main SPS dipole of MBB type. We show that the differential inductance may drop dramatically at high current, up to about a factor two, even for magnets where the field non linearity is within a few percent only.