Quality monitoring of the electron beam welding of the CMS conductor using ultrasonics

CMS (Compact Muon Solenoid) is a general-purpose detector designed to run at the highest luminosity at the CERN Large Hadron Collider (LHC). Its distinctive features include a 4 T superconducting solenoid with 6 m diameter by 12.5 m long free bore, enclosed inside a 10,000-ton return yoke. Its magnet coil is wound from a pure aluminum stabilized superconductor. In order to withstand the considerable magnetic forces the conductor is reinforced by attaching high-strength aluminum alloy profiles using an electron beam (EB) welding process. 20 unit lengths of conductor, 2.5 km each, are needed. Only a complete bonding ensures the required mechanical strength of the conductor. The quality control of the welding is a key element in the success of the fabrication. Hence, we have introduced into the production line, just after the EB welding unit, a specifically developed ultrasonic imaging system. A high-speed phased-array instrument copes with the challenge of a continuous monitoring of the welding quality over the whole width and the complete length of the conductor. The performance of the ultrasonic inspection system and the assessment of ultrasonic indications with respect to different types of welding flaws are shown. By now over 45 km of the CMS conductor have been successfully welded and inspected.