Recently, the development of Nb3Al conductors has progressed rapidly with the aim apply them to fusion magnets. In general, the critical current density of Nb3Al is insensitive to strain (or stress) in comparison with that of Nb3Sn. This property is useful in the construction of high-field and large magnets; particularly, fusion magnets. The Nb3Al strands are copper-stabilized multifilamentary composites produced using a jelly-roll process. In this paper, the status of Nb3Al conductor development and the characteristics of the Nb3Al conductor are reported.
This paper presents a numerical analysis of quench propagation in an immersion-cooled, pancake-type superconducting coil. The influences of thermal and electromagnetic conditions on quench propagation are clarified for various values of the thickness of electric-insulation films and transport current. The numerical results presented are in good agreement with the optical observation of quench propagation, therefore proving the validity of this analysis.
This paper presents an experimental study of numerical analysis results for an immersion-cooled, pancake-type superconducting test coil. A thermal disturbance was applied to the test coil by the pulse-heating of film heaters wound in it. The experimental results were compared with calculated results regarding stability limit and quench propagation under various transport currents, durations and positions of thermal disturbance. The experimental values agreed well with the calculated results, thus confirming the reliability of the numerical analysis method.
To make a thermal stability analysis of superconducting magnets, fundamental data on rapid transient boiling heat-transfer giving consideration to the influence of electrically insulated film are needed. In order to simulate an insulated superconductor in practical use, a 20-μm-thick stainless steel strip was applied as the test heater surface in this study. Both surfaces of the strip were coated with a PVF and carbon-paste films. Steady-state and rapid transient boiling heat-transfer were studied utilizing stepwise heat input from the test piece, which was placed horizontally in saturated liquid helium-4. In steady-state boiling, the thermal resistance of the coatings suppressed burn-out and quick transition. Consequently, the heat-transfer data correlated well with a single boiling curve having smooth and continuous transition from nucleate boiling to film boiling. The boiling heat-transfer characteristics are separated into three regions by the steady-state critical flux (CHF) of upward and downward-facing heaters. The nucleate boiling heat-transfer was almost the same as that of the vertically-placed heater reported previously by the authors, but the transition and film boiling heat-transfer were lower. When a stepwise heat flux higher than the steady-state CHF for the downward-facing heater was applied, the wall heat flux sharply dipped once just after the transition from transient nucleate boiling to film boiling. The heat conduction and transient nucleate boiling heat-transfer were almost the same as that of the vertically-placed heater. In contrast, heat-transfer after the transition to film boiling was lower.