After the de-spooler, the conductor passes to the winding station shown above. Photo: US ITER
―Lynne Degitz
At the heart of the ITER international fusion reactor-scale experiment now under construction in France is the world’s most powerful pulsed superconducting electromagnet—the ITER central solenoid. The US ITER Project Office at Oak Ridge National Laboratory is beginning fabrication of the magnet modules with vendor General Atomics at the GA facility in Poway, California. GA will produce six superconducting modules plus a spare for the central solenoid. ITER is an international partnership to build and operate a scientific facility that will sustain burning plasma and demonstrate the feasibility of fusion energy for grid-scale power.
In August, GA began winding a mock-up central solenoid coil from non-superconducting material; the mock-up is being used to confirm the readiness of the tooling stations required to fabricate a superconducting module. The conductor arrives at GA as a large spool and has to be separated by a de-spooler and fed to a winding station. The winding station forms the conductor into a spiral-wound coil composed of “pancake” layers. The mock-up coil is now at the stage in the process where the first six-layer set of wound coil, or hexapancake, is formed; the mock-up coil will comprise two hexapancakes and one four-layer quadpancake. The winding station was designed and built by Tauring S.p.A. in Torino, Italy. After winding is completed and evaluated in late December, the mock-up coil will move to other work stations for final qualification of the manufacturing process.
“The approach is to qualify the fabrication process by station,” said David Everitt, the central solenoid system manager at US ITER. “This begins with factory acceptance testing of workstation components, followed by assembly and commissioning, and finally production of the mock-up module.”
Eleven tooling stations will ultimately be installed at GA for fabrication of the central solenoid. While some stations support basic manufacturing tasks such as welding or stacking, others enable more complex processes such as heat treatment, wrapped turn insulation, injection of resin around the coils and cold testing.
The reaction heat treatment furnace station will make the electromagnet material superconducting by heating the niobium-tin and copper conductor to temperatures of 650 degrees Celsius. The furnace, which is 40 feet tall when opened and has a diameter of 18 feet, can hold one module at a time. The modules—weighing about 110 metric tons each—are moved in and out of the furnace with an air-bearing transport tool. Seco Warwick in Meadville, Pennsylvania, received the subcontract from GA for the furnace and fabricated the station at its facility in Świebodzin, Poland.
Following heat treatment, the conductor passes to the turn insulation station, which wraps a combination of fiberglass and Kapton® tape around the conductor bars. The insulation on the conductor ensures that electrical shorts do not occur between turns and layers. The automated wrapping heads were designed and built by Ridgway Machines in Leicester, United Kingdom, for GA. The previously heat treated module needs to be un-sprung like a Slinky without overstraining the conductor, which is now strain-sensitive due to heat treatment. Once the module is un-sprung, the turn insulation machine heads can wrap the insulation around the conductor and then reassemble it exactly as it was before.
Following turn insulation and then ground insulation, the central solenoid modules will move to the vacuum pressure impregnation station. This station injects epoxy resin to saturate the dry turn and ground insulation materials of the module, which is critical for structural integrity of the module as well as contributing to the insulation capability. The station is now under construction at GA and will be ready for testing with the mock-up module in early summer 2015. Before the first superconducting module is vacuum pressure impregnated with epoxy resin, fabrication and testing of the mock-up module will be completed.
The final step for the module fabrication is cold testing at 48.5 kilo amps and 4.7 Kelvin, which is comparable to the operating conditions inside the ITER reactor. Now under design, the cold test station will be ready for the completed mock-up module in February 2016. Eventually, the station will be the final proving ground for the production modules before they are prepared for shipment to the ITER site in France. The mock-up will complete its tour of the work stations by early 2016, and all of the modules are scheduled for completion by February 2019.
Further progress is also underway on the structural supports for the central solenoid. A contract has been awarded to Petersen, Inc., in Ogden, Utah, for fabrication of the lower key blocks that provide the primary support for the massive 1,000 metric ton solenoid. Procurement of the tie-plates, which provide a restraining cage around the solenoid, is now in process. Design of the assembly tooling for putting the central solenoid together at the ITER facility is also well underway; a final design review of the earliest need items was completed in September.
US participation in ITER is sponsored by the DOE Office of Science (Fusion Energy Sciences) and managed by Oak Ridge National Laboratory in Tennessee, with contributions by partner labs Princeton Plasma Physics Laboratory and Savannah River National Laboratory. For more information, see usiter.org.
Oak Ridge National Laboratory is supported by the Office of Science of the US Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, see science.energy.gov.
Media Contact: Lynne Degitz