Transverse pressure effect on superconducting Nb$_3$Sn Rutherford and $Re$BCO Roebel cables for accelerator magnets

This thesis seeks to expand the understanding of the transverse pressure effects on state-of-the-art Nb$_3$Sn Rutherford and $Re$BCO Roebel cables for application in accelerator magnets. Various Nb$_3$Sn and $Re$BCO cable samples are exposed to transverse pressure and their critical current is measured at 4.2 K in a perpendicular applied magnetic field of 10 to 10.5 T. For impregnated $Re$BCO Roebel cables, the inter-strand resistance at 4.2 and 77 K, as well as the AC magnetization loss at 4.2 K for different applied magnetic field direction were measured as well. In view of accelerator upgrades such as the High-Luminosity Large Hadron Collider (HL-LHC) or plans for next-generation machines such as the Future Circular Collider, the magnetic field generated by the dipole magnets needs to be enhanced from the present 8.33 T to 11 and to 16 T, respectively. NbTi can no longer deliver these magnetic fields. Instead, the latest Nb$_3$Sn wires technology is used to assemble Restacked-Rod-Process (RRP) and Powder-In-Tube (PIT) based Rutherford cables. When the bending magnetic fields beyond 16 T, $Re$BCO Roebel cables, representing one of the promising routes towards future accelerators, are considered. The Lorentz force generated in such magnets leads to sizeable transverse pressure on the cables of the order of 120 to 200 MPa for $Re$BCO Rutherford cables and of 100 to 150 MPa for $Re$BCO Roebel cables. Different epoxy resins were used to enhance the transverse pressure tolerance of cable samples. The reversible and irreversible critical current performance of the cable samples were investigated at 4.2 K in a background magnetic field 10 and 10.5 T, respectively. Also the dynamic magnetic field quality is a key parameter which is affected by the cable’s inter-strand resistance. Therefore, direct transport measurements of inter-strand resistance in impregnated $Re$BCO Roebel cables were carried out at 4.2 and 77 K. The measured inter-strand resistances are used to estimate AC coupling losses in different magnetic field orientations. For validation, the AC loss of Roebel cables is also measured at 4.2 K. Three analytical models are used to calculate the hysteresis loss and the results are compared with the measured data.