New results and remaining issues in superconducting magnets for ITER and associated R&D in Europe

Both the CS (Central Solenoid) coils and the TF (Toroidal Field) coils in ITER will use Nb3Sn as superconducting material, which proved very sensitive to applied strain and has a limited production, when 517 t will be required for ITER. PF and CC coils will use NbTi strands, although no conductors, carrying 45 kA and operating in pulse mode in a tokamak, have ever been produced. For Nb3Sn conductors, the main milestones of the R and D programme were the manufacture and tests of two model coils, the Central Solenoid Model Coil (CSMC) and the Toroidal Field Model Coil (TFMC). Despite the achievement of their nominal operation in terms of current and magnetic field, these coils showed a reduced margin, compared to what was expected from strand measurements. Following a revision of the design of these conductors in 2003 a complementary R and D programme was launched to qualify the modified design. For NbTi conductors for PF coils a specific development is carried out and an important milestone will be the tests of a 50 m PF conductor, wound in a single layer solenoid and inserted in the CSMC bore. Although manufacturing techniques for TF and CS coils have been qualified by the construction of the model coils, nevertheless, several points require further development. The metallic screen inside the insulation of the PF conductor, aiming at control of the dielectric quality of the insulation through the life of the machine, is the first one. A second item is the insulation system of the TF coils. Whereas the TFMC used a classical multilayer glass-polyimide composite, impregnated by epoxyde resin, a specific development is being carried out, to demonstrate the feasibility of using in the TF coils radiation-resistant resins, such as cyanate-ester. Contrary to the model coils, the CS, TF coils and PF coils will be wound into multiple pancakes, which implies the insertion of helium inlets at the innermost turn. First results of this development will be presented, including as well mechanical qualification as hydraulic qualification. A dedicated development is also carried out to demonstrate the manufacturing feasibility of the radial plates into which the TF conductor will be wounded. Another development includes prototypes of the precompression rings. Endly, fatigue testing of a prototype mock-up is planned to demonstrate feasibility and effectiveness of a PF tail design.