The ITER Tokamak What is a TOKAMAK? 10X THE CORE OF THE SUN 150 MILLION DEGREES PLASMA TEMPERATURE

How does it work?The heart of a tokamak is its doughnut-shaped vacuum chamber.

Inside, under the influence of extreme heat and pressure, gaseous hydrogen fuel becomes a plasma—a hot, electrically charged gas. In a star as in a fusion device, plasmas provide the environment in which light elements can fuse and yield energy.

The charged particles of the plasma can be shaped and controlled by the massive magnetic coils placed around the vessel; physicists use this important property to confine the hot plasma away from the vessel walls. The term “tokamak” comes to us from a Russian acronym that stands for “toroidal chamber with magnetic coils” (тороидальная камера с магнитными катушками).

To start the process, air and impurities are first evacuated from the vacuum chamber. Next, the magnet systems that will help to confine and control the plasma are charged up and the gaseous fuel is introduced. As a powerful electrical current is run through the vessel, the gas breaks down electrically, becomes ionized (electrons are stripped from the nuclei) and forms a plasma.

As the plasma particles become energized and collide they also begin to heat up. Auxiliary heating methods help to bring the plasma to fusion temperatures (between 150 and 300 million °C). Particles “energized” to such a degree can overcome their natural electromagnetic repulsion on collision to fuse, releasing huge amounts of energy.

First developed by Soviet research in the late 1960s, the tokamak has been adopted around the world as the most promising configuration of magnetic fusion device. ITER will be the world’s largest tokamak—twice the size of the largest machine currently in operation, with ten times the plasma chamber volume.

Blanket

The 440 blanket modules that completely cover the inner walls of the vacuum vessel protect the steel structure and the superconducting toroidal field magnets from the heat and high-energy neutrons produced by the fusion reactions. As the neutrons are slowed in the blanket, their kinetic energy is transformed into heat energy and collected by the water coolant. In a fusion power plant, this energy will be used for electrical power production.

Each blanket module measures 1 x 1.5 metres and weighs up to 4.6 tonnes. Over 180 design variants exist (related to the position of the modules in the vacuum vessel), but all have a detachable first wall that directly faces the plasma and removes the plasma heat load, and a main shield block that is designed for neutron shielding. The blanket modules also provide passageways for diagnostic viewing systems and plasma heating systems.

The ITER blanket, which covers a surface of 600 m², is one of the most critical and technically challenging components in ITER: together with the divertor it directly faces the hot plasma. Due to its unique physical properties (low plasma contamination, low fuel retention), beryllium has been chosen as the element to cover the first wall. The rest of the blanket modules will be made of high-strength copper and stainless steel.

ITER will be the first fusion device to operate with an actively cooled blanket. The cooling water—injected at 4 MPa and 70 °C—is designed to remove up to 736 MW of thermal power.

During later stages of ITER operation, some of the blanket modules will be replaced with specialized modules to test materials for tritium breeding concepts. A future fusion power plant producing large amounts of power will be required to breed all of its own tritium. ITER will test this essential concept of tritium self-sustainment.

Magnets

The ITER magnet system will be the largest and most integrated superconducting magnet system ever built.

Ten thousand tonnes of magnets, with a combined stored magnetic energy of 51 Gigajoules (GJ), will produce the magnetic fields that will initiate, confine, shape and control the ITER plasma. Manufactured from niobium-tin (Nb3Sn) or niobium-titanium (Nb-Ti), the magnets become superconducting when cooled with supercritical helium in the range of 4 Kelvin (-269 °C).

Superconducting magnets are able to carry higher current and produce stronger magnetic field than conventional counterparts. They also consume less power and are cheaper to operate … making superconducting magnet technology the only option for ITER’s huge magnet systems.

ITER uses high-performance, internally cooled superconductors called “cable-in-conduit conductors,” in which bundled superconducting strands—mixed with copper—are cabled together and contained in a structural steel jacket.

For the most technically challenging raw material—the niobium-tin (Nb3Sn) superconducting strands used in ITER’s toroidal field and central solenoid magnet systems—500 metric tons of strand (more than 100,000 km) were produced by nine suppliers in a procurement effort that lasted from 2008 to 2015. This large-scale industrial effort demanded a ramp-up of global production capacity from 15 metric tons/year to 100 metric tons/year, as well as the introduction of three new strand suppliers.

Vacuum Vessel

The ITER experiments will take place inside the vacuum vessel, a hermetically-sealed steel container that houses the fusion reaction and acts as a first safety containment barrier. In its doughnut-shaped chamber, or torus, the plasma particles spiral around continuously without touching the walls.

The vacuum vessel provides a high-vacuum environment for the plasma, improves radiation shielding and plasma stability, acts as the primary confinement barrier for radioactivity, and provides support for in-vessel components such as the blanket and the divertor. Cooling water circulating through the vessel’s double steel walls will remove the heat generated during operation.

Forty-four openings, or ports, in the vacuum vessel provide access for remote handling operations, diagnostics, heating, and vacuum systems. (Neutral beam injection will take place at equatorial level, for example, while on the lower level, five ports will be used for divertor cassette replacement and four for vacuum pumping.)

The blanket modules lining the inner surfaces of the vessel will provide shielding from the high-energy neutrons produced by the fusion reactions. (Some blanket modules will also be used at later stages to test materials for tritium breeding concepts.) Along with the magnet systems, the ITER vacuum vessel is entirely enclosed in a large vacuum chamber called the cryostat.

In a tokamak device, the larger the vacuum chamber volume, the easier it is to confine the plasma and achieve the type of high energy regime that will produce significant fusion power.

The ITER vacuum vessel, with an interior volume of 1,600 m³, will provide an absolutely unique experimental arena for fusion physicists; the volume of the plasma contained in the centre of the vessel (840 m³) is fully ten times larger than that of the largest operating tokamak in the world today. The ITER vacuum vessel will measure 19.4 metres across (outer diameter), 11.4 metres high, and weigh approximately 5,200 tonnes. (With the installation of the blanket and the divertor, the vacuum vessel will weigh 8,500 tonnes.)

Divertor

Situated at the bottom of the vacuum vessel, the divertor extracts heat and ash produced by the fusion reaction, minimizes plasma contamination, and protects the surrounding walls from thermal and neutronic loads.

Each of the divertor’s 54 “cassette assemblies” has a supporting structure in stainless steel and three plasma-facing components: the inner and outer vertical targets and the dome. The cassette assemblies also host a number of diagnostic components for plasma control and physics evaluation and optimization.

The inner and outer vertical targets are positioned at the intersection of magnetic field lines where particle bombardment will be particularly intense in ITER. As the high-energy plasma particles strike the vertical targets, their kinetic energy is transformed into heat and the heat is removed by active water cooling.

The heat flux sustained by the ITER divertor vertical targets is estimated at 10 MWm² (steady state) and 20 MWm² (slow transients). Tungsten, with the highest melting point of all the elements, has been chosen as the armour material following an international R&D effort, encouraging experimental results, and successful prototype testing.

The fifty four 10-tonne cassette assemblies of the ITER divertor will be installed—and also replaced at least once during the machine’s lifetime—by sophisticated remote handling

The ITER cryostat—the largest stainless steel high-vacuum pressure chamber ever built (16,000 m3)—provides the high vacuum, super-cool environment for the ITER vacuum vessel and the superconducting magnets.

Nearly 30 metres wide and as many in height, the internal diameter of the cryostat (28 metres) has been determined by the size of the largest components its surrounds: the two largest poloidal field coils. Manufactured from stainless steel, the cryostat weighs 3,850 tonnes. Its base section—1,250 tonnes—will be the single largest load of ITER Tokamak assembly.

The cryostat has 23 penetrations to allow access for maintenance as well as over 200 penetrations—some as large as four metres in size—that provide access for cooling systems, magnet feeders, auxiliary heating, diagnostics, and the removal of blanket sections and parts of the divertor.

Large bellows situated between the cryostat and the vacuum vessel will allow for thermal contraction and expansion in the structures during operation. The structure will have to withstand a vacuum pressure of 1 x 10 -4 Pa; the pump volume is designed for 8,500 m3.

The four main cryostat sections will be assembled in an on-site workshop before their transport to the tokamak assembly site.