The multinational fusion energy project marks the completion of its most complex magnetic system

After two decades of design, manufacture, fabrication and assembly on three continents, the historic, multinational ITER fusion energy project today celebrates the completion and delivery of the massive toroidal field coils from Japan and Europe.

Masahito Moriyama, Minister of Education, Culture, Sports, Science and Technology of Japan and Gilberto Pichetto Fratin, Minister of Environment and Energy Security of Italy, will attend the ceremony with officials from other ITER members.

Nineteen giant toroidal field coils have been delivered to southern France. They will be key components in ITER, the mega-experimental fusion project that will use magnetic confinement to mimic the process that powers the sun and stars and gives Earth light and warmth.

Fusion research is aimed at developing a safe, abundant and environmentally responsible energy source.

ITER is a collaboration of more than 30 partner countries: the European Union, China, India, Japan, Korea, Russia and the United States. Most of ITER’s funding is in the form of contributed components. This agreement prompts companies such as Mitsubishi Heavy Industries, ASG Superconductors, Toshiba Energy Systems, SIMIC, CNIM and many others to expand their expertise in the latest technologies required for the merger.

The D-shaped toroidal field coils will be placed around the ITER vacuum vessel, a donut-shaped chamber called a tokamak. Inside the vessel, light atomic nuclei will fuse together to form heavier ones, releasing enormous energy from the fusion reaction.

The fuel for this fusion reaction is two forms of hydrogen, deuterium and tritium (DT). This fuel will be injected as gas into the tokamak. By passing an electric current through the gas, it becomes an ionized plasma – the fourth state of matter, a cloud of nuclei and electrons.

The plasma will be heated to 150 million degrees, 10 times hotter than the core of the sun. At this temperature, the speed of light atomic nuclei is high enough for them to collide and fuse. To shape, confine and control this extremely hot plasma, the ITER tokamak must generate an invisible magnetic cage precisely matched to the shape of the metal vacuum vessel.

ITER uses niobium-tin and niobium-titanium as the material for its giant coils. When energized with electricity, the coils become electromagnets. When cooled with liquid helium to -269°C (4 Kelvin), they become superconducting.

To create the precise magnetic fields required, ITER uses three different sets of magnets. Eighteen D-shaped toroidal field magnets confine the plasma within the vessel. Poloidal field magnets, a stacked array of six rings that surround the tokamak horizontally, control the position and shape of the plasma.

At the center of the tokamak, the central cylindrical solenoid uses a pulse of energy to generate a powerful current in the plasma. At 15 million amperes, ITER’s plasma current will be far more powerful than anything possible in current or previous tokamaks.

Ten coils were produced in Europe under the auspices of ITER’s European Country Agency, Fusion for Energy (F4E). Eight coils plus one backup were made in Japan, managed by ITER Japan, part of the National Institute for Quantum Science and Technology (QST).

Each completed spiral is huge: 17 meters long and 9 meters wide, and weighs about 360 metric tons.

The toroidal field coils will work together, in effect, as a single magnet: the most powerful magnet ever created.

They will generate a total magnetic energy of 41 gigajoules. ITER’s magnetic field will be about 250,000 times stronger than Earth’s.

Making toroidal field coils

The fabrication process began with the production of niobium-tin filament. More than 87,000 kilometers of fine wire were needed to create 19 coils of the toroidal field. This thread is produced in China, Europe, Japan, Korea, Russia and the United States.

Hundreds of niobium-tin filaments were wound with copper strands in a rope-like bundle and inserted into a steel jacket, with a central channel to accommodate the forced flow of liquid helium.

The result – a “cable-to-conductor”, or simply “conductor” – forms the core element of the coils. This conductive material was sent to Japan and Europe to begin the fabrication process.

The actual fabrication was even more challenging. To begin with, about 750 meters of conductor was bent into a two-helix trajectory and heat-treated at 650°C. It was then precisely placed on a D-shaped “radial plate”, a stainless steel structure with grooves on both sides into which the conductor is inserted.

The conductor was wrapped and insulated using glass and Kapton tape. The cover plates were fitted and laser welded. This created a “double pancake”, a large but delicate sub-component made of two layers of conductors. The entire double pancake was re-wrapped with duct tape and injected with resin to add structural strength, using a vacuum to remove any air pockets.

In the next stage, seven double pancakes were assembled to make a “winding package”, forming the core of the final magnet. Each double pancake was joined to the other for electrical continuity. The overall winding package was insulated, heat treated and again injected with resin.

Finally, the winding package was encased in a massive, fit-for-purpose stainless steel case weighing around 200 metric tons, strong enough to withstand the immense forces that will be generated during ITER operation.

More than 40 companies were involved in the creation of toroidal field coils (TF). Some of the leading European companies include:

• ASG Superconductors produced 70 dual TF pancakes and 10 winding packages.
• CNIM produced 35 TF radial plates.
• SIMIC produced 35 TF radial plates and completed 10 TF coils,
• Iberdrola coordinated the production of 10 TF coil packs.
• Elytt Energy produced the tooling for the double 70 TF pancakes.
• BNG completed the cold test, at 80 Kelvin, of 10 TF winding packages.

Japan was responsible for the fabrication of all 19 cases of the TF coil, in a collaboration between Mitsubishi Heavy Industries, Toshiba Energy Systems and Hyundai Heavy Industries. In addition, major companies involved in Japan’s TF coil fabrication include:

• Mitsubishi Electric Corporation produced 5 TF winding packages.
• Arisawa Manufacturing produced all the insulating tape.
• Mitsubishi Heavy Industries completed 5 TF coils.
• Toshiba Energy Systems completed 4 TF coils.

“The completion and deployment of the 19 ITER toroidal field coils is a monumental achievement,” said Pietro Barabaschi, ITER Director General. “We congratulate the member governments, internal ITER agencies, the companies involved and the many individuals who dedicated countless hours to this extraordinary effort.”

How does fusion work?

• A small amount of deuterium and tritium (hydrogen) gas is injected into a large donut-shaped vacuum chamber called a tokamak.
• The hydrogen is heated until it becomes an ionized plasma, which looks like a cloud.
• Giant superconducting magnets, built into the tokamak, confine and shape the ionized plasma, keeping it away from the metal walls.
• When the hydrogen plasma reaches 150 million degrees Celsius – ten times hotter than the sun’s core – fusion occurs.
• In the fusion reaction, a small amount of mass is converted into a large amount of energy (E=mc²).
• The ultra-high-energy neutrons produced by the fusion escape the magnetic field and strike the walls of the metal tokamak chamber, transmitting their energy to the walls as heat.
• Some neutrons react with the lithium in the metal walls, creating more tritium fuel for fusion.
• Water circulating in the tokamak walls picks up heat and turns into steam. In a commercial reactor, this steam will drive turbines to produce electricity.
• Hundreds of tokamaks have been built, but ITER is the first designed to achieve a “burnt” or largely self-heating plasma.

How much power will the ITER Tokamak provide?

The plant at ITER will produce about 500 megawatts of thermal energy. If operated continuously and connected to the grid, this would translate into about 200 megawatts of electricity, enough for about 200,000 homes.

A commercial fusion plant would be designed with a slightly larger plasma chamber, for 10-15 times more electricity. A 2,000 megawatt fusion power plant, for example, would supply electricity to 2 million homes.

Fusion power plants are carbon-free; they do not emit CO₂.