We present experimental and analytical results of quenching for three-coil systems wound with multifilamentary Nb-Ti composites, demonstrating that AC loss, due mostly to interfilament coupling in the composite during quenching, promotes normal-zone propagation (NZP). The experiment used two three-coil systems, each comprised of inner, middle, and outer coils connected in series and shunted with resistors. Both systems shared the same outer and middle coils. Two versions of the inner coil were used, both versions wound with composites that were otherwise identical except for twist pitch lengths. One composite's twist pitch length was chosen sufficiently short to make the interfilament coupling loss to be negligible during quenching, while the other composite's twist pitch length was chosen sufficiently long to make the loss significant. Our model for quenching analysis includes the coupling loss and has been used to simulate the experiment. Agreement between experiment and simulation is excellent. The computed temperature distributions during quenching show a clear effect of heating by coupling loss in the inner coil wound with a composite having a long twist pitch length and virtually no effect of heating in the inner coil wound with a composite having a short twist pitch length. The difference in quenching behaviors between the two systems, different only in twist pitch length in the composites for the inner coils, is so compelling that we conclude that interfilament coupling loss is the key to this difference. We may further conclude that coupling loss may be used to benefit protection of multicoil systems. The twist pitch length that controls this coupling loss is thus a key design parameter; it is best determined through simulation.
- Coupling loss in multifilamentary Nb-Ti composite
- Normal-zone propagation
- Quenching in and protection for superconducting multicoil systems