Strand coating for the superconducting cables of the LHC main magnets

Abstract

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 Sn 95wt. Ag 5wt. 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 con-tact resistance is determined by the strand coating and cabling procedures, oxidation heat treatment, and the magnet coil cu-ring 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. Abstract  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 Sn 95wt. Ag 5wt. 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.

Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN

“…almost 2000 km) was thoroughly inspected at CERN. o Cryogenic measurements on wire of: RRR [19], I c [20], Magnetization [21], R c [18]. Measurements were at 1.9 K, except for I c that, in order to allow a continuous cross check with measurements in Industry, was measured also at 4.2 K. I c measurements were carried out both on virgin strands (to approve the production of a billet) and on strands extracted from cables, to assess the degradation due to cabling, which in practice was very small but was always a potential risk.…”

“…This was essential to determine the correct temperature-time curve of the thermal treatment to reach the desired value of the R c , see above, [17,18]. o Check of all characteristics of cable at room temperature, including dimension and visual inspection on a whole piece length.…”

Superconducting magnets for accelerators and detectors

“…Of course insulation of the strands is not possible because we need current sharing among strands for stability. The cure is a controlled oxidation of the cable to provide a reliable interstrand resistance in the 15-40 µΩ range [12].…”

The effect of CuSn intermetallics on the interstrand contact resistance in superconducting cables for the large hadron collider

“…For this purpose a Sn-Ag coating is applied onto the copper matrix of the strands, which is subsequently submitted to a 200 °C heattreatment in air, lasting typically a few hours. The Sn-Ag coating is applied by a continuous hot-dip process [3,4,5].…”