Tests on JET TF coil insulation

The epoxy glass insulation from a JET toroidal field (TF) coil was extensively tested as part of a study to permit operation of JET at a field 20% higher than the design value. The measurements were performed on specimens taken from a coil that had been in use for several years. The JET TF coils are D-shaped and made of two pancakes of 12 copper turns, with 2 mm inter-turn insulation and 6 mm inter-pancake insulation, all wrapped by 7 mm of ground insulation (the total coil cross-section area being about 0.1 m²). Most of the tests were carried out on specimens taken from the straight leg of the used coil. Tests on inter-turn insulation included shear strength and shear modulus, fracture tests, micrographic analysis, void and glass-resin content and second order glass transition temperature. Tests on ground insulation included tensile strength and elastic moduli, shear strength and shear modulus, inter-laminar shear strength and elastic modulus, mode 1 and mode 2 fracture tests. The results were not as consistent as might be achieved using laboratory specimens, but we consider that they are more representative of the quality achievable in actual manufacture.