THE QUALITY CONTROL OF THE LHC CONTINUOUS CRYOSTAT INTERCONNECTIONS

The interconnections between the Large Hadron Collider (LHC) magnets have required some 40 000 TIG welded joints and 65 000 electrical splices. At the level of single joints and splices, non-destructive techniques find limited application: quality control is based on the qualification of the process and of operators, on the recording of production parameters and on production samples. Visual inspection and process audits were the main techniques used. At the level of an extended chain of joints and splices – from a 53.5 m half-cell to a complete 2.7 km arc sector – quality control is based on vacuum leak tests, electrical tests and RF microwave reflectometry that progressively validated the work performed. Subsequent pressure tests, cryogenic circuits flushing with high pressure helium and cool-downs revealed a few unseen or new defects. This paper presents an overview of the quality control techniques used, seeking lessons applicable to similar large, complex projects. INTRODUCTION The Large Hadron Collider (LHC), its installation and the interconnection work between magnets have been extensively described at various stages of the project [1]. The project management aspects of the interconnection of the arc continuous cryostat are described elsewhere [2]: this paper specifically presents the quality control aspects. Three main joining technologies are involved in interconnection work: induction soldering for ~ 10 000 13 kA splices, ultrasonic welding for ~ 53 000 600 A splices and TIG welding for ~ 37 000 joints. Other activities involve essentially assembly. QA STRATEGY AND ORGANISATION Interconnections (IC) in the continuous cryostat have been designed to be performed using mechanical/automatic processes. Indeed the main contractor IEG employed “operators” for TIG welding, not “welders”. Moreover, the geometry of these interconnections implies that non-destructive testing methods find only limited application. Consequently the Quality Assurance (QA) strategy is based on four concepts: Prior qualification of the processes, the equipment and the operators on samples; On-line monitoring and recording of critical parameters of all junctions; Strict application of the qualified procedures and full traceability; Production samples that can be tested off-line using destructive methods. The human resources directly involved in the quality control (QC) of interconnection work in the period January 2005 to June 2008 represent ~ 120 man-years, or ~ 38% of the total workload. At peak, in the period January to June 2007, ~ 72 persons were involved simultaneously in QC. The relative contribution of the different teams is shown in Fig. 1: IEG: operators were responsible for the first level of control, followed by their team leaders and dedicated inspectors. ICIT: Project Associates under a CERN-HNINP collaboration agreement, who were trained to perform visual inspection work. AT-MCS: the organisation of the quality control was ensured by a team of CERN staff from AT-MCS, together with a few technicians from the Institut de Soudure. VAC: responsible for the vacuum performance and leak testing, CERN staff from AT-VAC together with the contractor ALL43. ELQA: responsible for the electrical testing, CERN staff from AT-MEL together with Project Associates from the same CERN-HNINP collaboration.