Demonstration of engineering current density exceeding 1 kA mm$^{−2}$ in ultra-thin no-insulation, soldered coil windings using NbTi/Cu wires with CuNi cladding

The no-insulation, or more precisely, controlled-resistance coil winding method, nowadays being exclusively used for high-temperature superconducting solenoids, has proven its effectiveness for improving quench protection. When considering low-temperature superconductor magnet technology, which is mostly focused on stability and training issues, controlled-resistance insulation windings are directly addressing these aspects as well. Fully soldered coil windings of non-insulated turns can also show superior mechanical properties and feature simplified manufacturing when compared to epoxy impregnated coil windings and are of high practical interest for quasi-stationary magnets provided the related charging time constant can be controlled and kept low enough. For demonstrating the principle feasibility two demonstrator coils were developed using NbTi/Cu wire with CuNi cladding of 1 mm diameter. The wire performance is reported including critical current and n-values at 4.2 K and background magnetic fields from 0 to 9 T, as well as effective transverse resistivity at room temperature and 77 K. Two solenoids with fully soldered windings comprising one layer on a 50 mm bore and three layers on a 100 mm bore, respectively, were manufactured and tested in liquid helium. Their performance is directly compared to data obtained on short wire samples. The drastically enhanced stability of the coils against thermal disturbances allows to avoid any training and enables to operate the coils up, or even slightly beyond, the short-sample critical current, resulting in generated magnetic fields of 2.2 and 3.8 T and time constants of 5 and 55 s, respectively. When initiating a quench deliberately by excessive heating or spontaneously at their limiting currents, the coils entirely switch to the normal state almost instantly, thus requiring no quench protection system. Design, manufacturing and test experiences with the two super stable coils are reported and their use and design constraints for certain applications discussed.