Large Magnet Structures for Fusion Technology: Lessons from ITER

Abstract

Over a period of 40 years, the ITER project has provided many examples in large magnet structures, from pre-concept to design to manufacture to delivery and assembly, from which lessons can be learned for the future. This presentation is divided into three parts. The first, 'Making and Assembling Large (steel) Cryogenic Structures for Magnets' is based on ITER Coil Structure experience, particularly the Central Solenoid (CS) and Toroidal Field (TF) structures and particularly on large 316LN forgings weighing about 50t individually, requiring then high accuracy machining and deep (up to 0.3m) welds with controlled distortion. New materials development is often proposed. This is easy on a laboratory scale, but often not transferable to an industrial large scale. Examples are provided. The issues to be considered (and solved) are large scale quality (and repair of defects), joining, tolerances and assembly. The second 'Designing Large Steel Support Structures for Magnets' is concerned with structural design codes & criteria, and modern analysis, especially of imperfect, but manufacturable structures. Points that are considered are typical critical issues with cyclically loaded thick components, analysis of partial penetration welds and other welds not permitted under normal American Society of Mechanical Engineers (ASME) welding procedures (including fillet welds and/or those volumetrically not inspectable by Non-Destructive Examinations (NDE)) and allowing for tolerance compensation (i.e. designing with overmetal or designing for shimming). The third 'Avoiding or Reducing Large Steel Support Structures for Magnets' shows how it is possible to choose other routes to include structural materials in magnets, for example (as in ITER TF coils) by using plates to contain the conductor, or by using a thick conductor jacket to at least share the loads with massive steel cases.