NbTi superconducting materials are generally multi-core composites (NbTi/Cu, NbTi/Cu-CuNi, with barrier layers inside). The NbTi multi-core twisted composite superconducting wire can effectively prevent the degradation of superconducting magnets and is conducive to the stability of superconducting magnets. Therefore, according to the dynamic stability theory of superconductors, the stability design of NbTi superconducting materials must be carried out.
The design mainly involves the selection of parameters such as the copper/superconducting ratio of niobium-titanium superconducting material, the diameter of the core wire, the outer diameter of the conductor, the number of core wires, the twisting pitch, the barrier layer, and the repositioning length of the cable material.
Copper / Superconducting Material Ratio The thermal conductivity of copper is approximately four orders of magnitude higher than that of niobium-titanium superconducting materials. When copper is used as the coating for superconducting wires, it not only prevents local superconductivity loss in the superconducting core but also serves as a low-resistance bypass during superconductivity loss. Considering the stability and reliability of niobium-titanium multi-core composite superconducting wire materials, choosing an appropriate ratio of copper cross-sectional area to superconducting material cross-sectional area is crucial. The copper/superconducting material ratio for general wire materials is typically 1.3 to 2.0. However, those with a larger copper/superconducting material ratio are much larger than those of wire materials.
Core wire diameter: The refinement of the core wire diameter in niobium-titanium superconducting wire is necessary to ensure thermal insulation stability. To stabilize the niobium-titanium superconducting material, the core wire diameter (d) of niobium-titanium should comply with the formula where S is the specific heat capacity of the niobium-titanium superconductor (1.01×10-3 J/cm2•K); T0 = -Jc / eJc / eT ≈ 5K; Jc is the critical current density under the working magnetic field. After calculation, the core diameter (d) of niobium-titanium used in DC superconducting magnets should be less than 45 μm. However, when used in pulsed superconducting magnets, the core diameter of the niobium-titanium superconducting material is much smaller (a few micrometers or even smaller).
Conductor outer diameter and number of strands By increasing the number of strands and increasing the cross-sectional area of the conductor to enhance the current-carrying capacity of the entire conductor, a self-field effect will be generated, resulting in severe performance degradation. For niobium-titanium superconducting materials with a large current-carrying capacity requirement, the superconducting wire is generally made into a fully staggered multi-strand cable (flat, circular, etc., secondary conductors).
Twisting pitch: Under the influence of varying external fields, an induced coupling occurs between the core wires of superconducting wire materials, thereby reducing their current-carrying capacity. To overcome this degradation, niobium-titanium superconducting materials should be twisted along their axial direction. After any point on the superconducting material rotates 360 degrees around the central axis, the relative displacement is the twisting pitch (LD), which has the following two points: (1) When designing NbTi/Cu-CuNi multi-core composite superconducting wire materials, in addition to having a finer core diameter, the assembly sleeve of the composite single-core rods is: the center is a NbTi rod, the outer barrier layer is niobium, the outer Cu-Ni alloy tube, and the outermost is a copper tube. Through multiple assembly sleeves, tens of thousands of multi and fine-core niobium-titanium superconducting materials can be designed and manufactured. (2) To obtain niobium-titanium superconducting materials with high current-carrying capacity, the selected appropriate heat treatment conditions (temperature, time), the number of cold processing - heat treatment cycles, the cold processing rate between the two times, and the cold processing rate after the last aging heat treatment must all be reasonably designed and determined. This of course requires experimentation as the basis, combined with the possibility of production process conditions, for a comprehensive consideration.
