Stability analysis of NbTi-Ta- based high field conductor cooled by pool boiling below 4 K

Stability analysis has been performed for cabled NbTi-Ta-based superconductors intended for the high field (12 T) toroidal field coils for a large scale tokamak device such as ETF. Ternary NbTi-Ta was selected as the superconductor because of its superior critical current density at high field as compared to the binary alloy NbTi. The operating temperature was chosen to be 2.5 K or below to optimize the performance of the superconductor. A cabled conductor was selected to minimize the pulsed field losses. The conductor is cooled by pool boiling in a subcooled (∼ 2.5 K, 0.25 atm) bath, or in a superfluid helium (He-II) bath (∼ 1.8 K, 0.02 atm). The analysis was based on numerically simulating the evolution of a normal zone in the conductor. Appropriate superconductor properties and heat transfer characteristics were utilized in the simulation. In the case of subcooled bath, the low bath temperature reduces both the peak nucleate boiling flux (PNBF) and the minimum film boiling flux (MFBF). In the case of He II bath, the heat transfer characteristic is determined by the cooling channel size, bath pressure and the Kapitza resistance. Results indicated that in both cases of cooling the NbTi-Ta-based conductor can be designed to satisfy the commonly followed stability performance criterion for such large coils. In particular, He II cooling was found to offer significant enhancement in the stability performance of the conductor. The implications of the results are discussed.