Comparison of the calculated and measured stability of a NbTi cable-in-conduit conductor
Calculated curves of cable-in-conduit conductor (CICC) energy stability margins vs. current are compared to experimental curves obtained at Oak Ridge National Laboratory for NbTi single triplex conductors. The conductors ranged in length from 1.8 m to 4.8 m, and had no imposed helium flow. The initiating heat pulse was applied for 16.7 ms over the entire conductor length. The calculated stability curves display the large decrease in energy margin from the low current and high energy margin "well-cooled" stability region, to the high current and low energy margin "ill-cooled" region that was determined experimentally. The calculated "limiting current" of 250 A (boundary between the ill-cooled and well-cooled regions) also agrees with experiment. The multi-valued stability margins measured for lengths of 3.1 and 3.8 m could not be obtained by the computer model. Excluding the multiple stabilities, the calculated margins are generally 30% lower than the experimental values. The decrease in energy margin in the ill-cooled region was found to be due to the low critical to operating temperature difference (2.2 K at 250 A). This prevents an initiating pulse in the ill-cooled region from being able to generate significant Joule heating without quenching, limiting the energy margin.<<ETX>>
Parametric study of the stability margins of cable-in-conduit superconductors: Experiment
In a previous experiment on the stability of cable-in-conduit superconductors, we sometimes observed multivalued stability margins, which we attributed to strong heating-induced transient flows. We proposed a schematic theory from which we derived a scaling relation for the limiting current below which the stability margin is always singlevalued. Measurements at different magnetic fields are used to test the scaling with critical temperature and resistivity. We also examine the scaling with heated length and heat pulse duration. The results of these experiments are given and compared with theory.
Thermal Runaway Phenomena of Composite Superconductors
A theoretical and experimental investigation on thermal runaway phenomena of composite superconductors has been performed. It has been theoretically found that there are two substrate limited currents, a “thermal runaway current” Ih and a “completely stable current” Is, in addition to the take-off current and the recovery current. For a current above Ih the temperature of a composite conductor rises rapidly, and for a current below Is the conductor always recovers to superconducting state no matter how large a disturbance has occurred. The thermal runaway phenomena can be explained by considering the temperature dependences the resistivity of the substrate and heat transfer rate to liquid helium.The cornposite comductor tested consists of a copper strip 0.86mm thick by 2.16mm wide on which a copper-clad Ti-Nb-Ta superconducting wire of 0.37mm diameter (core dia.: 0.25mm) is soldered. The test results for the thermal runaway currents agreed well with the predicted. For example, the conductor in an external magnetic field of 43kOe began to runaway thermally at Ih=360A when the temperature reached about 30K. The teermal runaway current may be used as an important standard for designing a metastable superconducting coil.