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>>
Stability of cable‐in‐conduit superconductors
The stability margin of a single triplex of NbTi/Cu strands soldered around a heater wire, sheathed in a stainless‐steel tube, and cooled by pressurized helium is found to be multivalued for certain combinations of transport current, ambient helium pressure, and externally imposed helium flow. A qualitative model based on the idea that good recovery in stagnant or slowly flowing helium is due to heating‐induced flow accounts well for the complex multivalued behavior of the stability margin.
Transient helium heat transfer phase I—Static coolant
Abstract Transient heat-transfer data have been obtained for flat heating surfaces in static liquid and supercritical helium. Measurements start 2(10) −5 s after step power inputs, and cover a heat flux range of 0.05–20 W/cm 2 , pressures from 0.09–0.3 MPa, and four different heater orientations. Initial heat-transfer coefficients, being limited primarily by the Kapitza resistance, are 10–100 times greater than steady state, and the time to reach steady state varies from 10 −5 to 1s. For heat flux below the steady state peak nucleate boiling limit the temperature follows calculations based on pure conduction to the steady state nucleate boiling level. Above that limit the transient conduction period leads to an apparent metastable nucleation period followed by a transition to film boiling.