The ITER Toroidal Field Model Coil (TFMC) Development Programme

The TF coils for ITER will use the concept of a circular thin walled Nb3Sn cable in conduit superconductor completely enclosed in an insulated groove in steel plates to form the coil pancakes. These are then stacked together to form the winding pack and supported by a AISI 316LN stainless steel case. The concept is being demonstrated by the fabrication of a racetrack shaped model coil (TFMC) which is designed to operate mecanically, electrically and hydraulically in conditions representative for the ITER T F coils. For this purpose the TFMC will be assembled in the TOSKA facility at FZK Karlsruhe, together with the EURATOM LCT coil which provides an external field. The objectives of the TFMC are as follows: • to develop and verify the full scale TF coil manufacturing techniques; • to establish realistic manufacturing tolerances; • to bench-mark methods for the ITER TF coil acceptance; • to gain information on the coil's behaviour, operating margins and in-service monitoring techniques. The TFMC is actually in an advanced state of manufacture. Thus a large part of the first three objectives is reached. The TFMC will be delivered to FZK in summer 1999 to be prepared there for testing in TOSKA. Connected wit the ITER TFMC two other progammes are running in parallel, namely: • a joint development programme which foresees the production and test of three full size joint samples; • the fabrication of full size sections of the ITER TF coil case and radial plate. 1. DESIGN DESCRIPTION The design of the ITER Toroidal Field Model Coil (TFMC) [1,2] follows the design of the full size ITER TF coils [3, 4]. The coil parameters are reported in Table I and the coil layout is shown in Fig. 1. The conductor used corresponds to the one developed for ITER. The conductor is reacted after winding to form the Nb3Sn compound in a mould at 650°C for about 200 hours, then insulated with a glass/Kapton tape interleaved and transferred to the radial plates. The shape of the radial plate is a racetrack having different radii of curvature. The grooves of the radial plate are closed with a cover laser welded to the plate. After glass-Kapton insulation is applied the double pancake (DP) modules are vacuum impregnated. The 5 DPOs forming the winding pack are assembled together, then the ground insulation is applied and the winding pack is vacuum impregnated. To be able to connect the pancakes the conductor ends are provided with terminations according a technique developed at CEA Cadarache [5]. The terminations of the two pancakes forming a DP module are joined at the inner circumference by soldering and clamping them together. The inter-DP joints are located at the outer circumference. They are made by electron beam welding copper pins between the two terminations. The winding pack is inserted in the case, the space in between filled with graded silica grains and finally epoxy resin impregnated. TABLE I. Coil Parameters ITER TF TFMC Conductor diameter [mm] 45.7 40.7 Conductor insulation thickness [mm] 2.5 2.5 Ground insulation thickness [mm] 8 8 Number of double pancake modules 7 5 Total number of turns 192 98 Winding min/max radius [mm] 2705/9600 600/1161 Case thickness [mm] 75-240 70 80 Overall dimensions [m] 18.7x12x1.4 3.8x2.7x0.77 Weight per coil [t] 695 40 An inter-coil structure (ICS) allows the assembly of the TFMC and the LCT with an angle t o have in addition to the attracting forces between the coils also a pressure on the front part of the coil (Fig.2). The TFMC is supported on the ICS by four wedges to simulate, during testing, mechanical stresses similar to the one experienced by the ITER TF coils during operation. The TFMC case and the ICS are provided with cooling channels. A large number of sensors will be mounted as co-wound voltage taps, strain gauges, temperature and displacement sensors etc., t o be able to operate the coil safely and to monitor the behaviour of the assembly during testing. Coil Case Winding Pack