The giant magnet of the world’s largest reactor

ITER, the world’s largest fusion experiment, is moving closer to operational status after all the special magnets used to build the reactor core were delivered to southern France. Interesting Engineering This marks the end of a two-decade reactor design process, with component manufacturing spanning three continents.

As the world searches for better ways to produce carbon-free energy, fusion offers a viable solution that can be switched on and off on demand. Recent advances in the field have demonstrated that energy can be harvested from fusion reactions. Currently, more than 30 countries are collaborating on the construction of the International Experimental Thermonuclear Reactor (ITER) in France.

ITER’s design also uses a tokamak reactor, in which hydrogen is pumped into a doughnut-shaped vacuum chamber and heated to create plasma, mimicking the conditions at the core of the Sun. At an incredibly high temperature of 150 million degrees Celsius, fusion reactions begin. However, the plasma must be contained within the reactor walls by giant superconducting magnets.

ITER’s tokamak design uses niobium-tin and niobium-titanium as fuel for the magnets. The coils are electrically activated, then cooled to -269 degrees Celsius to turn them into superconducting magnets. ITER will deploy the magnets in three different ways to create an invisible magnetic cage that will confine the plasma. The doughnut shape on the outside comes from 18 D-shaped magnets. A set of six magnets surround the tokamak horizontally, helping to control the shape of the plasma. Meanwhile, a central solenoid will use pulses to generate current in the plasma. ITER’s plasma current peaks at 15 million amps, a record for a tokamak anywhere in the world. In terms of magnetic fields, the design’s total magnetic energy is 41 gigajoules, 250,000 times stronger than Earth’s magnetic field.

Each of the D-shaped magnets is 17 meters high, nearly 9 meters wide, and weighs 360 tons. Ten of the magnets were manufactured in Europe by Fusion for Energy, while the remaining eight magnets and a spare were made by the Quantum Science and Technology Institute (QST) in Japan. The manufacturing process begins with a niobium-tin filament that is wound with copper wires into a rope-like structure and placed in a steel housing designed with a central channel through which helium can pass. This structure is called a conductor. Engineers needed more than 87,000 kilometers of niobium-tin filament to produce the conductors for the 19 D-shaped magnets.

To create the D-shaped magnet, nearly 750 meters of the conductor were bent into a double helix and heated to 650 degrees Celsius. It was then placed in a D-shaped plate made of stainless steel. The conductor was encased and insulated with glass and Kapton tape, and laser-welded to the cladding to form a double-layer structure. This double-layer structure was insulated, air pockets removed, and sprayed with a synthetic resin to increase strength. Seven such double-layer structures were used to create the core of the D-shaped magnet. Finally, engineers placed the assembly in a 200-ton stainless steel cage that is strong enough to withstand the forces of plasma movement and fusion power generation.

Once assembled, the ITER fusion reactor will produce 500 MW at full power. When connected to the grid, it will produce 200 MW of continuous electricity, enough to power 200,000 homes.