Constructing 45 Tesla large bore magnets
Research activities performed at high magnetic fields require large bore magnets with far stronger magnetic fields, but such large magnets pose various construction problems with wire production, coil winding and magnet protection. The Dutch scientists involved in this project observed these problems and were eventually able to construct two 45 Tesla magnetic coils with inner bores of 100 millimetres. The first coil failed during testing due to insulation problems linked to insufficient bounding of the traditional Kapton wrap insulation around the wire. Therefore the second coil was wound from wire insulated with Kapton tape that could be better sealed and was additionally protected by a glass fibre wrap. This insulation had also the advantage of being better adapted to the high forces occurring during winding of this stiff, large section wire. Several new techniques had to be developed to wind such a coil: the production of large quantities of large-section, high-strength wire, a technique of electrically and mechanically joining several lengths of this wire inside the coil and finally the protection of the coil. The protection of the coil against incidents inside the bore was realized by having the inside of the coil protected by a thick-walled stainless steel tube and two concentric thin-walled G10 cylinders. The 2mm space in between these G10 cylinders is filled with grease that can be removed by melting in case the system has to be dismounted after a failure that could be accompanied by a remarkable expansion of the protecting steel cylinder. Further, the scientists are now intending to build a third large bore magnet, which will be a copy of the second one. The results obtained so far will be helpful in view of the construction of large coils for the new 50 MJ laboratory in Dresden and the renewed 20 MW installation in Nijmegen.