Heat Propagation Velocities in Coated Conductors for Fault Current Limiter Applications

The development of superconducting fault current limiters (SFCL) are mainly based on coated conductors (CC) in spite of the low electric fields the CC can sustain due to their poor thermal behavior and the very low propagation velocities of the normal zone. Nowadays the SFCL demonstrators are made of coil but another way of building them is to use plates with a meandered line. Its main advantage is the increase of the propagation velocities of the normal zone thanks to the longitudinal and lateral diffusion of the heat. Indeed in a meander the heat generated by a local dissipative zone diffuses also laterally and switches the adjacent lines, increasing the apparent propagation velocity of the normal zone. There is also a possibility to increase the lateral propagation velocity of the heat by adding onto the Hastelloy substrate a thermal link (an Ag film for instance) between the lines without an electrical short circuit. These test structures were developed on CC allowing the simultaneous measurement of the propagation of a heat front in the substrate and in the Hastelloy/Ag bilayer. They are made of a heater line and several parallel superconducting strips where the propagation of the T_c temperature front is recorded. We will discuss the benefit of this approach as compared to the standard coil geometry.