Asymmetry of thermal propagation velocity in a long force-cooled superconducting test line

We have observed the phenomenon of thermal destruction of superconductivity by current in a force-cooled test model power transmission line of 20 m length at currents up to 9 kA and temperatures between 8 K and 14 K. This line, fabricated by soldering doubly-stabilized Nb3Sn tapes to each side of a bar of 0fhc copper (0.3cm × 1.58 cm), was enclosed in a German silver cryogen flow tube. The cryogen (He gas) was controlled at a flow rate of 0.5 to 0.7 gm/sec. Destruction was initiated by application of a short duration heating pulse to a small section of line. The velocity of N-S interface zone propagation was measured over 1.0 m test sections located downstream and upstream from the initial section. The upstream moving zone is always in a nearly-constant-temperature cryogenic environment. However, downstream portions contact an environment preheated by Joule heating in upstream normal N zones and thus transfer less heat and exhibit a much higher velocity. This qualitative explanation of the observed dramatic velocity asymmetry is to be supplemented by a more precise description of the physics of the moving N-S interface which must include a proper model for the heat conductance into the cryogen. Various models for heat conductance can be tested in velocity calculations.