After a decade of design and fabrication, US-based General Atomics said on 15 June that it is ready to ship the first module of the Central Solenoid to the International Thermonuclear Experimental Reactor (ITER) under construction in Saint-Paul-lès-Durance in southern France. Despite the challenges of Covid-19, ITER is almost 75% built and massive first-of-a-kind components have been arriving in France from three continents over the past 15 months. Construction of ITER is funded mainly by the European Union (45.6%) with the remainder shared equally by China, India, Japan, Korea, Russia and the USA (9.1% each). However, in practice, the members deliver little monetary contribution to the project, instead providing ‘in-kind’ contributions of components, systems or buildings.

ITER construction involves more than 1 million components, manufactured around the world. Many of these components are very large, and the Central Solenoid modules are among the heaviest. The Central Solenoid, the largest of ITER’s magnets, will be made up of six modules. It is one of the largest of the US contributions to the project. Once fully assembled, it will be 18 metres tall and 4.25 metres wide, and will weigh 1,000 tons. It will induce a powerful current in the ITER plasma, helping to shape and control the fusion reaction during long pulses.

The magnetic force of the Central Solenoid is strong enough to lift an aircraft carrier 2 metres into the air. At its core, it will reach a magnetic field strength of 13 Tesla, 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. Earlier this year, General Atomics (GA) completed final testing of the first Central Solenoid module. It will now be loaded onto a special heavy transport truck for shipment to southern France.

The shipping process for the massive magnets requires specialised heavy transport vehicles. The entire process for safely loading and securing the module on the truck, including preparations for lifting, will take about a week.

After loading, the module will be shipped to Houston, Texas, where it will be placed onto a ship for transport to the ITER site. 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. Five additional Central Solenoid modules, plus one spare, are at various stages of fabrication. Module 2 will be shipped in August.

The Central Solenoid modules are being manufactured at GA’s Magnet Technologies Centre in Poway, California, near San Diego, under the direction of the US ITER project, managed by Oak

Ridge National Laboratory (ORNL). The Magnet Technologies Centre was developed specifically for manufacturing the Central Solenoid – the largest and most powerful pulsed superconducting electromagnet ever constructed – in partnership with US ITER.

Creating the magnetic fields in a tokamak requires three different arrays of magnets. 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. Together, ITER’s magnets create an invisible cage for the plasma that conforms precisely to the metal walls of the tokamak.

In the centre 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, as well as generating heat for fusion. At 15 million amperes, ITER’s plasma current will be far more powerful than anything possible in current tokamaks.

The superconductor material used in ITER’s magnets was produced in nine factories in six countries. The 43 kilometres of niobium-tin superconductor for the Central Solenoid was manufactured in Japan.

Fabrication of the first module began in 2015. It was preceded by almost four years of collaboration with experts at US ITER to design the process and tools for fabricating the modules. Each 4.25-metre diameter, 110-tonne module requires more than two years of precision fabrication from more than 5 kilometres of steel-jacketed niobium-tin superconducting cable. The cable is precisely wound into flat, layered “pancakes” that must be carefully spliced together.

To create the superconducting material inside the module winding, the module must be carefully heat treated in a large furnace, which functions similarly to that of a convection oven. The benefit of the convection oven is the ability to shorten the overall process while maintaining uniform “cooking” of the module. Inside the furnace, the module spends approximately ten-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. During turn insulation, the module needs to be un-sprung without overstraining the conductor, which is now strain-sensitive due to heat treatment.

To perform the wrapping, the turns of the module are stretched like a slinky, allowing the taping heads to wrap the fiberglass/Kapton insulation around the conductor. Once the individual turns are wrapped, the external module surfaces are then wrapped with ground insulation, which consists of 25 layers of fiberglass and Kapton sheets. The ground insulation must also tightly fit around complex coil features, such as the helium inlets.

After insulation, the module is enclosed in a mould, and 3,800 litres 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. The finished module is subjected to a series of demanding tests, placing it in the extreme conditions it will experience during ITER operation, including near-complete vacuum and cryogenic temperatures required for the magnet to become superconducting (4.5 Kelvin, which equates to about -270°C). Lessons learned on the first Central Solenoid module have been applied to the fabrication of the subsequent six coils

“This project ranks among the largest, most complex and demanding magnet programmes ever undertaken,” said 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.”

“The ITER project is the most complex scientific collaboration in history,” says Dr Bernard Bigot, Director-General of the ITER Organisation. “Very challenging First-of-a-kind components are being manufactured on three continents over a nearly 10-year period by leading companies such as General Atomics. Each component represents a top-notch engineering team. Without this global participation, ITER would not have been possible; but as a combined effort, each team leverages its investment by what it learns from the others.” He added: “The United States is a vital Member of the ITER project, which they initiated decades ago. General Atomics, with its world-class expertise in both complex manufacturing and precise control of magnetic fields, is a prime example of the remarkable expertise brought to the table by US scientists and engineers.”

Both the engineering insights and the scientific data generated by ITER will be critical for the US fusion programme. Kathy McCarthy,

Director of the US ITER Project Office at ORNL said: “The experience we’re gaining from ITER in integrated, reactor-scale engineering is invaluable for realising a viable, practical path to fusion energy.” US ITER is funded by the Department of Energy’s Office of Science Fusion Energy Sciences programme. UT-Battelle manages ORNL for DOE's Office of Science.

Date: Wednesday, 16 June 2021
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