Photo report
Ring magnet fabrication in full swing
1 Oct 2018
At least as wide as a five-lane highway, ITER's largest ring-shaped magnets are too big to be transported in their finished state. The European Domestic Agency has purpose built a facility at ITER to house their fabrication only a few hundred metres from machine assembly pit. This report will take you inside the building, where a team of 70 is advancing the fabrication of the first production unit—poloidal field coil #5 (PF5).
Beginning with conductor winding and ending—at the other side of the 257-metre-long facility—with final assembly and cold testing, the fabrication of ITER's ring shaped magnets is a precise step-by-step process.
Six ring-shaped poloidal field coils will be positioned horizontally outside of the toroidal field magnet system to shape the plasma and contribute to its stability by "pinching" it away from the walls.
Of the six coils—which range in diameter from 17 metres to 24 metres, and in weight from 200 to 400 tonnes—the European Domestic Agency, Fusion for Energy, is responsible for five. (A sixth is under the procurement responsibility of Russia.) In the facility on site, work on PF5 is 42 percent completed.
See the gallery below and a full report on the European Domestic Agency website.
Two spools of niobium-titanium conductor are simultaneously fed onto the winding table to create a double pancake (a double layer of spiralled conductor.) The winding table in the picture is dimensioned for PF5 and PF2. Later, for the two larger coils PF3 and PF4 (24 metres in diameter), it will be expanded.
During the winding process the conductor is wrapped with several layers of glass-fibre tape, which insulates the conductor turns one from another.
After completing the winding, the next step is to shape the terminations—the areas that connect one conductor length to another. A portion of conductor is opened, the turn insulation is removed and moulds shape the end into a "termination joggle."
Six of the eight double pancakes needed for PF5 have been wound. This staging table is used to prepare the double pancakes for the next stage—impregnation.
For the second phase of operations, the double pancake windings are transferred by overhead bridge crane to the impregnation area in the centre of the winding facility. The cranes lower the double pancakes into moulds for vacuum pressure impregnation (VPI) with epoxy resin. The resin—acting inside of a sealed mould and under the effect of heat—hardens the glass tape to bond each double pancake into a rigid assembly.
A precise mould
The mould is custom built around the double pancake to be impregnated. Current passed through the conductor heats the double pancake to approximately 80 degrees Celsius, all air is removed, and resin is injected slowly to fill all nooks and crannies.
A team of 70 people is working daily to meet a tight production schedule. Here, the technicians are monitoring the impregnation of one double pancake.
The rigid double pancakes will be stacked and joined electrically to form the winding pack (in the case of PF5, eight double pancakes will be stacked). The final assembly will undergo a second impregnation process in this larger mould. Notice in the foreground, a segment of mockup winding that was created from multiple layers of dummy impregnated conductor lengths to test all processes.
This cryogenic chamber will permit a final coil assembly to be tested at 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.
Technicians are active at all of the workstations of the Poloidal Field Coils Winding Facility, either performing work on the first production unit PF5, or qualifying later stages. The European Domestic Agency reports that, all activities combined, the first coil is 42 percent complete.