The world's largest magnet can lift an aircraft carrier

The magnet of the world's strongest fusion reactor can lift a 102 million kg aircraft carrier to nearly two meters high.

The world's largest and strongest magnet is called a central solenoid (CS). When fully assembled, this magnet is 18m high, 4.2m wide, and weighs about 1,000 tons. CS is the core part of the world's largest fusion reactor, the International Thermonuclear Test Reactor (ITER) in 2023.

The first module of the central solenoid coil. (Photo: ITER)

Building such a large magnet is massive work. Engineers and technicians at General Atomics took more than half a decade to design, manufacture, and test various CS components at the production facility. Transporting each part of the magnet was also a challenge, requiring separate roads and cranes to tow. According to John Smith, director of engineering and projects at General Atomics, CS generates a magnetic force strong enough to lift an aircraft carrier weighing about 102 million kilograms to a height of 1.8 meters.

In August 2021, the first module of the magnet was shipped to France. CS includes a total of 6 modules, each module is 2.1 meters high, 4.2 meters wide and weighs 113 kg. Once completed, engineers will stack the module on top of each other in the center of the reactor. Superconducting electromagnets are the "heart" of the ITER tokamak furnace, a device that produces fusion energy under controlled conditions. CS will conduct a current of 15 million amps into the tokamak body, helping to shape and stabilize the hot plasma gas like the core of the Sun inside.

Although the ITER reactor is a prototype of future large tokamak reactors and does not produce electricity, the project will demonstrate fusion as a viable zero-carbon energy solution. To date, no such fusion reactor has come close to producing more energy than it consumes. This is a big problem because the reactor uses a huge amount of electricity. But with CS, ITER can pave the way for sustainable energy.

Transporting one of the magnet's modules alone required dexterity at every turn. The engineers used a very large crane. The body of the crane fits on a 9-wheeler truck. The transportation team mobilized 40 - 60 pickup trucks with attached components to the ITER kiln construction area. They put the magnet module on the truck using a hydraulic system powered by a gasoline engine. After that, the module is loaded onto ships using multiple machines and to the Caribbean Sea. There, engineers loaded it onto a vessel designed to carry easy-to-load containers.

The process of making large magnets like CS is essentially the same as manufacturing any conventional coil of electromagnet. That is winding the electrical conductor around the core coil. Since CS is so large and made of superconducting tin and niobium materials, the problem is a bit more complicated.

First, General Atomics imported wires from a manufacturing plant in Japan. The material is then carefully rolled up and heat treated at 650 degrees Celsius for 5 weeks. When niobium was exposed to the chemical compound Nb3Sn, the scientists converted the conductor into a superconductor. The temperature gradually increased, maintained and then decreased again. Finally, the company loosened the coil springs so that a special machine could wind a total of 5.6 km of wire. The engineering team used nearly 290 meters of adhesive tape to cover all the superconducting wires. Meanwhile, the air inside the magnet module is replaced by 3,785 liters of artificial plastic, treated at 127 degrees Celsius until it hardens.

General Atomics will continue to manufacture five more modules, rotating between different fabrication facilities for steps such as coil heat treatment and adhesive tape coating. According to Smith, the time to produce a module is two years.