Advanced Examination Techniques Applied to the Assessment of Vacuum Pressure Impregnation (VPI) of ITER Correction Coils

Abstract

The ITER Magnet System includes a set of 18 superconducting correction coils (CC) which are used to compensate the error field modes arising from geometrical deviations caused by manufacturing and assembly tolerances. The turn and ground insulation are electrically insulated with a multi-layer fiberglass polyimide interleaved composite, impregnated with epoxy resin using vacuum pressure impregnation (VPI). Adequate high voltage insulation (5 kV), mechanical strength and rigidity of the winding pack should be achieved after impregnation and curing of the insulation system. VPI is an effective process to avoid defects such dry spots and incomplete wet out. This insulation technology has also been developed since several years for application to large superconducting coils and more recently to ITER CC. It allows the coils to be impregnated without impacting on their functional characteristics. One of the critical challenges associated with the construction of the CC is the qualification of the VPI insulation. Sections issued from representative VPI test samples with real scale side correction Coil (SCC) cross-section have been delivered and characterized at CERN. High resolution micro-optical inspections have been carried out on large areas through digital microscopy. The aim was to identify lack of impregnation, areas of pure resin and void entrapments. The areas near the filling fibre glass rope received special attention. High precision dimensional and geometrical assessments have been performed with the help of image analysis. Compression and pull-out tests have been also carried out. Finally, high-resolution 3D-computed tomography has been applied for a full volumetric inspection of the sections, enabling the reconstruction in three dimensions of the VPI samples and allowing to fully detect, confirm, and image the volume defects already identified by micro-optical observations.