Six poloidal field coils positioned horizontally around the ITER vacuum vessel and D-shaped toroidal field coils will help shape the plasma and keep it in suspension away from the walls. The top poloidal field coil (PF1) will be supplied by Russia; the five lower ring coils are under the procurement responsibility of Europe. Four of these will be produced on site. (PF6 was produced by China under contract with Europe.)
One of only two manufacturing facilities located on the ITER site, the Poloidal Field Coils Winding Facility
was constructed by Europe to house the winding and assembly activities for four of ITER's giant ring-shaped magnets.
Ranging in diameter from 17 to 24 metres and weighing 200 to 400 tonnes, coils PF2, PF3, PF4, and PF5 are simply too big to be manufactured off site and shipped to ITER.
European contractors have equipped the building with the high-tech machinery that is required to carry out the successive stages of the fabrication process, from winding the building blocks of each coil, to stacking and creating the final assembly, to cold testing.
Qualification activities were successfully carried out to commission the winding and impregnation tooling stations. The first pre-dummy pancake—made with several turns of copper conductor instead of the real superconducting materials—was followed by the winding and impregnation of a full two-layer PF5 dummy double pancake.
Now, series production is underway for the double pancakes—flat, spiralled coils that are stacked in up to eight layers to form the coil winding packs. Thirty production double pancakes will be necessary for the four poloidal field coils that will be manufactured by European contractors on site. Fabrication will require at least 18 months per coil.
The contractors have completed all eight double pancakes for the first coil, PF5, and all six for the second coil, PF2. The winding tooling has now been expanded to accommodate the next two coils to be manufactured (PF3 and PF4), which have a diameter of 24 metres.
The winding tooling that served for the winding of double pancakes for coils PF5 and PF2 (similar in size) has been reconfigured to match the dimensions of the two larger coils—PF3 and PF4. Photo: F4E
The raw material for the four coils—niobium-titanium (NbTi) cable-in-conduit superconductor
—is produced in China and delivered to the Poloidal Field Coils Winding Facility on 20-tonne spools.
In the winding
stage of fabrication, the conductor is uncoiled from the spools and straightened before passing through cleaning and sandblasting machines that prepare its surface to receive several layers of glass-fibre insulating tape. Rotating tape heads enter into action as the conductor, fed from two spools simultaneously, joins the tooling table to be "turned" into the precise dimensions required for each coil. The resulting component is a spiralled, two-layer coil called a double pancake that weighs between 20 tonnes (PF2) and approximately 34 tonnes (PF3).
Next, the double pancake is transported by overhead crane to the vacuum pressure impregnation
(VPI) station and lowered into a custom-fit mould. Epoxy resin, injected over the course of several hours, fills all gaps and hardens the glass tape under the effect of heat and pressure. A 36-hour "curing" phase at 140 °C completes the process, which gives the double pancake rigidity and ensures electrical insulation.
The rigid double pancake winding is transferred to the winding pack assembly
area of the building where it is stacked with other double pancakes and joined together. A second vacuum impregnation is performed to harden the entire assembly and components such as clamps, protection covers and helium pipes are added.
The final step is cold testing
, during which the completed poloidal field coils will be cooled to low temperature (approximately 80 K) in order to simulate the thermal stresses and the work conditions that will be experienced during operation. In this way, the performance of the coils will be tested before their installation in the ITER machine.
Poloidal Field Coil Fabrication Facts:
- Coils manufactured on site: PF2, PF3, PF4, PF5
- Diameter of coils: from 17 metres (PF2, PF5) to 24 metres (PF3, PF4)
- Amount of NbTi cable-in-conduit superconductor per coil: from 6 km (PF2) to 14 km (PF3)
- Number of double pancakes per coil: six (PF2) or eight (PF3, PF4, PF5)
- Number of double pancakes produced on site: 30
- Weight of final assemblies: 200 to 400 tonnes
- Start of tooling qualification: 2015
- Start of production winding activities: 2017
- First completed coil: 2020
- Procurement responsibility: Europe
- Contractors: ASG Superconductors, Italy (engineering integration); Sea Alp, Italy (winding tooling), Dalkia-Veolia, France (site and infrastructure); Elytt Energy, Spain and Alsyom/SEIV, France (handling and impregnation); CNIM, France (manufacturing and cold testing)
- Ring coils | A fisheye view
- Magnets | Last steps for poloidal field coil #5
- Poloidal field coils | A decisive year ahead
- Of mega tools and dentist drills
- "Dummy" winding takes shape
- The first 60 metres
- Qualification activities to start on ITER's "ring" magnets