The superconducting material must be clad, or embedded, in ordinary good conductors to provide a low-resistance bypass when certain line segments suddenly turn to normal. The wrapping is also important in preventing the magnetic field lines from jumping forward. In other words, in order to reduce losses and improve stability, the appropriate basic materials must be used. The choice of matrix material depends on conflicting requirements. Wilson and his colleagues' theory suggests that a highly conductive substrate allows for the use of thinner wires, but its multi-strand cables must be twisted in a tighter spiral. The main reason why oxygen free copper with high conductivity is used as the superconducting material of NbTi alloy is that when the superconducting state appears instantaneous disturbance due to local temperature rise, the transport current can be moved to the matrix, so that the existing Joule heat can be kept at a sufficiently small degree. However, it has been noted that copper, which has good stabilizing properties, has never been able to adequately limit the induced current.
Coated stabilizer material in which hundreds of thousands of NbTi filament are embedded in a stabilizer of highly conductive oxygen-free copper or high-purity aluminum to prevent thermal loss or provide current bypass. Remaining resistance ratio (RRR) is often used to describe the quality of stabilized materials. RRR refers to the ratio of the resistivity at room temperature 293K divided by the resistivity at low temperature (4.2K). Generally, the RRR ratio of stabilized materials is required to be above 30 for superconducting magnets.
Bag blocking material. Nb is a widely used barrier layer material to avoid the formation of TiCu4 and other intermetallic compounds between NbTi alloy and Cu matrix, and to prevent the cable from breaking the core and significantly reducing the critical current density (JC).
The composite design of multi-core complex is carried out before hot extrusion. The design is based on the parameter requirements of finished superconducting wire, such as wire diameter, core diameter, copper ratio, etc., to calculate the size of the casing barrier layer, NBI rod and the number of extrusion. In the multi-core composite line, NbTi core filaments are closely arranged in a hexagonal layered arrangement from the core to the outside, and the roots of each layer form an arithmetic series relationship in turn.
