Friction Coefficient Between the Ti6Al4V Loading Pole and the 316LN Steel Shims of the HL-LHC 11 T Magnets

As part of the Large Hadron Collider High Luminosity upgrade (HL-LHC) project, 11 Tesla Nb$_3$Sn dipole magnets will replace some of the existing 8.33 T NbTi LHC main dipole magnets. The 11 T dipole coils are mechanically loaded through a removable Ti6Al4V pole wedge. The force exerted on this wedge is transmitted onto the coils via tangential sliding of the wedge against the coil loading shims, which are made from stainless steel 316LN. Thus, the friction coefficient between pole and shims influences the azimuthal coil stress under a certain load, which is applied by the collaring press onto the wedge. Model friction tests revealed a strong stick-slip effect in liquid helium environment. Application of an MoS$_2$-spray to the Ti6Al4V surface resulted in smooth sliding with a friction coefficient lower than 0.1. A remaining static friction peak may be avoided by an optimized deposition method as e.g., PVD.