Strand coating for the superconducting cables of the LHC main magnets

The electrical resistance of contacts between strands in the Rutherford type superconducting cables has a major effect on the eddy current loss in cables, and on the dynamic magnetic field error in the LHC main magnets. In order to guarantee the value and constancy of the contact resistance, various metallic coatings were studied from the electrical and mechanical points of view in the past, We report on the molten bath Sn95wt.Ag5wt. coating, oxidized thermally in air after the cabling is completed, that we adopted for the cables of the LHC main magnets. The value of the contact resistance is determined by the strand coating and cabling procedures, oxidation heat treatment, and the magnet coil curing and handling, Chemical analysis helps to understand the evolution of the contacts. We also mention results on two electrolytic coatings resulting in higher contact resistance.