WHAT THEY ARE

Superconducting wires, also known as strands, are multi-filamentary composites in which hundreds of superconducting filaments are embedded in a copper matrix. The multi-filament arrangement is necessary to thermally stabilize the wire and to reduce the losses. The figure below shows three examples of arrangement of the superconducting filaments. In the following figure, the SEM cross section of a superconducting wire with the copper partially removed by chemical etching is reported: the superconducting filaments are clearly visible.

At liquid helium temperature and for application in magnetic fields up to about 6T, the most used superconducting material is NbTi. NbTi is a ductile material and the wires can be produced by standard metallurgy. In case of higher applied magnetic fields, on the other hand, NbTi is no longer superconducting and therefore cannot be used. In this case, the more expensive Nb3Sn is the preferred superconducting material.

The TF coils are designed to produce the toroidal field to confine the plasma. Although the toroidal field is 6T, the maximum field acting on the winding producing the field itself is about 12T, therefore the use of Nb3Sn-based superconducting strands is necessary. To fabricate the 18 TF coils to be mounted on DTT, and a spare coil, 11500 km of strands will be needed, equivalent to 55 tons. The Nb3Sn superconducting strands will also be used to produce the Central Solenoid. On the other hand, the PF coils operate at a lower magnetic field and will be produced with NbTi strands.