Normal-zone propagations inside a layer and between layers in a superconducting coil

We have investigated the propagation of normal zones in an alternate layer structure of conductor and non-conductor parts. In order to evaluate average propagation velocity in the alternate layer structure, a heat balance equation was set up for a coarse-grained spatial unit composed of a pair of conductor and non-conductor layers. An analytical expression of the propagation velocity was obtained for the coarse-grained layer structure with uniform thermal properties. We measured the propagation velocity from turn to turn in a single-layer superconducting solenoid coil with a teflon spacer between turns. The velocity decreased in proportion to the 0.5-1 power of the thickness of the spacer. The experimental results were quantitatively explained by the present theory. We also discussed an anisotropy of the normal-zone propagation in both impregnated and unimpregnated superconducting coils on the basis of the present theoretical results.