An Inverse Study of Transient Response of a No-Insulation Coil with AC Currents Based on Simplified Equivalent Circuit Models

With the ability to bypass current, no-insultation (NI) superconducting coils can tolerate small over-critical currents, local defects and hotspots, which improves its stability and simplifies quench protection system. To describe the overall behaviors of the NI coils, inductance, variable resistance of superconductors, and characteristic resistance are the three necessary parameters. In general, the variable resistance of superconductors is neglected when the supplied current is below critical current. Besides, the inductance and characteristic resistance are regarded as constants during the transient process, for instance, charge and sudden discharge process. However, many tests have demonstrated that the time constant which is only related to characteristic resistance and inductance changes with different ramping rates or frequencies of AC currents, or even amplitude of current can affect its values. In order to better analyze the transient response of the NI coils, and to figure out how and why the abovementioned three parameters change with frequency, a NI coil was tested under DC biased AC currents with frequency range of 0.5 Hz-200 Hz. An inverse calculation of these parameters based on the simplified equivalent circuit model from the measured center magnetic field, terminal voltage and supplied current is carried out. Two widely used simulation models, a more complicated circuit model meshing with radial and azimuthal elements and a finite element model with rotated anisotropic resistivity, are employed to validate the inverse calculation and to investigate the current density distribution or current distribution in turn level. It is found that the variable resistance of superconductors under AC conditions are non-neglectable, and it increases with frequency. Moreover, the inductance and characteristic resistance are not constant with AC currents and change with frequency.