After a decade of design and fabrication, General Atomics (GA) is preparing to ship the first module of the central solenoid—the largest of ITER’s magnets—to the site in southern France where 35 partner countries are collaborating to build the world’s largest tokamak and the first fusion device to produce net energy.
Earlier this year, GA completed final testing of the first central solenoid module. On June 16, the company announced that the module was being loaded onto a special heavy transport truck for shipment to Houston, where it will be placed on an ocean-going vessel for shipment to southern France. The first module will head to sea in late July and arrive in France in late August; ground transit to the ITER site will take place in early September.
The “beating heart” of ITER: Fully assembled, the six-module central solenoid will be 18 meters (59 feet) tall and 4.25 meters (14 feet) wide and will weigh 1,000 tons. As the world’s strongest magnet, the central solenoid will have a force strong enough to lift an aircraft carrier six feet into the air. At its core, it will reach a magnetic field strength of 13 teslas, about 280,000 times stronger than the earth’s magnetic field. The support structures for the central solenoid will have to withstand forces equal to twice the thrust of a space shuttle lift-off, according to GA.
“This project ranks among the largest, most complex and demanding magnet programs ever undertaken,” says John Smith, GA’s director of engineering and projects. “I speak for the entire team when I say this is the most important and significant project of our careers. We have all felt the responsibility of working on a job that has the potential to change the world. This is a significant achievement for the GA team and US ITER.”
Complex array: Creating the magnetic fields in a tokamak requires three different arrays of magnets. As described by GA, external coils around the ring of the tokamak produce the toroidal magnetic field, confining the plasma inside the vessel. The poloidal coils, a stacked set of rings that orbit the tokamak parallel to its circumference, control the position and shape of the plasma. In the center of the tokamak, the central solenoid uses a pulse of energy to generate a powerful toroidal current in the plasma that flows around the torus. The movement of ions with this current in turn creates a second poloidal magnetic field that improves the confinement of the plasma and generates heat for fusion.
Special facilities: The central solenoid modules are being manufactured at GA’s Magnet Technologies Center in Poway, Calif., under the direction of the US ITER project, which is managed by Oak Ridge National Laboratory. Five additional central solenoid modules, plus a spare, are at various stages of fabrication. Module 2 will be shipped in August.
The Magnet Technologies Center was developed specifically for manufacturing the central solenoid. Detailed information and numerous photos of the manufacturing process are provided in a PDF booklet produced by GA.
Each 4.25-meter-diameter, 110-metric ton module requires more than two years of fabrication from more than 5 kilometers (3 miles) of niobium-tin superconducting cable. The cable is wound into flat layers that must be carefully spliced together before the module is heat treated in a large furnace. Inside the furnace, the module spends approximately 10-and-a-half days at 570°C and an additional four days at 650°C. The entire process takes about five weeks.
Following heat treatment, the cable is insulated to ensure that electrical shorts do not occur between turns and layers. After insulation, the module is enclosed in a mold, and 1,000 gallons of epoxy resin are injected under vacuum to saturate the insulation materials and prevent bubbles or voids. When hardened at 650°C, the epoxy fuses the entire module into a single structural unit.