The winding line for the superconducting TF coils is ready

The winding line, to be used for winding the superconducting Toroidal Field (TF) coils which produce the magnetic field that confines the plasma in the ITER tokamak, is now ready. It is the first time a winding line with such impressive dimensions – 40 metres long, 20 metres wide, 5 metres high – has been built and it gives the opportunity to carry out winding trials which have never been undertaken before: it is the first time a full-size trial winding turn of a large dummy conductor has been carried out.

Located in F4E supplier ASG premises in Italy (ASG is part of a consortium which also includes Iberdrola and Elytt), the winding line will have the task of winding the superconducting coil cables into a D-shaped double spiral called a double pancake. The 7-tonne heavy superconductor coil will be delivered to ASG on spool in a single 760 metre length, so the first task of the winding line will be to unspool and straighten the cable, after which the cable will be cleaned and sandblasted. A continuous length of around 760 metres of superconductor cable will be used and shaped into the 12 metres long and 9 metres wide double pancake, which will then be heat treated at over 650 °C in a specially constructed inert atmosphere oven. After electrical insulation, the double pancake will finally be transferred into the grooves of the stainless steel radial plates, thus forming the double pancake module. As it is necessary that the double pancake fits precisely into the radial plate groove, it is vital to control that the trajectory of the conductor in the double pancakes is very accurate. This is why the winding line is required to achieve a precision on the bending of the conductor of a few tens of parts per million: a very demanding target considering its large dimensions (although the successful result during winding of the trial full size turn has demonstrated the capability of the winding line to achieve the required precisions). After insertion into the radial plates, each double pancake module will be impregnated with epoxy resin, stacked in groups of seven and jointed electrically to form the so-called winding packs. These winding packs will be inserted into stainless steel cases which will be welded in order to form the crucial TF coils.

For the moment, the winding line will however continue to be tested. In total, 70 superconductor lengths are needed to produce the ten TF coils to be procured by F4E (Japan will contribute an additional nine TF coils). They will be produced by five different suppliers, so each type of superconductor will have a slightly different mechanical behavior and therefore, individual tests with prototypes of each superconductor type will have to be carried out in the winding line during the next few months before starting the real production. The final qualification, to take place in the Autumn, will consist of winding a real superconducting cable into a full size double pancake prototype.

Another large machine, a large inert atmosphere oven, which measures 48x20x5 metres and which will be used to carry out the heat treatment of the double pancakes, is also in its final installation phase in ASG premises. The oven will be able to heat-treat up to three double pancakes at a time. After the successful completion of the leak test, carried out in order to verify the capability of the furnace to keep the concentration of impurities during the heat treatment below the required threshold of tens of parts per million, the oven is now in the final phase of the installation. The assembly of external components (electrical connections, sensors, piping, fans and vacuum pumps) is currently being completed and the final testing should start at the end of July.

With the winding line and the oven ready to be used, the main and most complex machinery for the production of the superconductors has been completed.