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 analysis of a forced-flow cooled superconducting coil: numerical simulation of multiple stability
Abstract The stability of a forced-flow cooled superconducting coil is investigated by use of numerical simulation. A numerical code to integrate the simultaneous partial differential equation system composed of the 1D hydrodynamic equations and the 1D thermal conduction equation has been developed. By using the code, stability margins are evaluated as functions of coolant mass flow rate, operation current and imposed magnetic field. The results of computations show that the stability margin is multivalued with respect to these operation parameters, as expected from the experimental results. It is also shown that the appearance of the first unstable regime is closely related to the existence of the stagnant region located on the upstream side of the heated zone and that the second stable regime appears because the heat transfer is appreciably enhanced by the induced fast backflow due to the thermal expansion of coolant.
Transient cooling of an internally cooled superconducting magnet
Abstract Large forced convection-cooled superconducting magnets can locally become normal conducting due to a temperature rise initiated by an energy disturbance. If this disturbance is too large the conductor will quench. For a small disturbance the coolant, a supercritical helium flow, will be able to remove the dissipated energy and the conductor will return to its superconducting state. In this paper the critical disturbance from which recovery is still possible is calculated with the simulation program STABIL, for the Sultan conductor as developed by the Netherlands Energy Research Foundation ECN.