Vacuum vessel.

The plasma will be confined inside a steel torus called the vacuum vessel. Larger plasma volumes generally improve energy confinement, which is one reason why the ITER vacuum vessel is being built on an unprecedented scale.

Dimensions

This doughnut-shaped vessel will measure 19.4 metres across and 11.4 metres high and will weigh approximately 5,200 tonnes. Its cross-section is D-shaped to suit the required plasma geometry. With an interior volume of about 1,400 m³, it will be the largest tokamak vacuum vessel ever built. The plasma will occupy approximately 840 m³, about six times the plasma volume of today’s largest operating tokamak.

In-Vessel Components

During deuterium–tritium operation, ITER will produce around 10²⁰ high-energy fusion neutrons per second. These neutrons will activate in-vessel components and parts of the vacuum vessel, so maintenance inside the vessel will have to be carried out remotely. The main in-vessel components are the blanket and the divertor. The blanket consists of modules with plasma-facing first-wall panels and shield blocks that protect the vacuum vessel and magnets from heat and high-energy neutrons. The divertor removes heat, helium ash, residual fuel and impurities from the plasma edge. During later stages of operation, selected blanket locations will be used for Test Blanket Modules to study tritium-breeding concepts.

Shielding

Cryopump.

The vacuum vessel has double walls containing in-wall shielding blocks that weigh from about 50 to 500 kg each. These blocks provide neutron shielding for components outside the vessel, particularly the superconducting magnets. Ferromagnetic shielding elements also help reduce toroidal field ripple, improving fast-particle confinement and reducing the risk of localised heat loads on plasma-facing components.

Cooling water circulates through the double-wall structure of the vacuum vessel to remove heat generated during operation. Outside the vessel, a thermal shield made from stainless-steel panels reduces radiative heat transfer to the superconducting magnets. Its low-emissivity surfaces are highly polished or silver-coated; nearly 800 kg of silver is used for the coating. The shield is actively cooled by helium gas flowing through tubes welded to the panels.

The thermal shield minimises heat transfer from the warm vacuum vessel to the superconducting magnets operating at about 4 K. A second thermal-shield layer is installed between the magnets and the cryostat, further reducing the heat load on the cryogenic system.

Ports

The vacuum vessel is surrounded by the magnet system and enclosed within the cryostat. Forty-four large openings, or ports, provide access for remote handling, diagnostics, heating, fuelling and vacuum systems. Equatorial ports will accommodate systems such as neutral beam injection, while lower ports provide access for divertor cassette replacement and vacuum pumping.

Vacuum

A high vacuum is required inside the vessel before plasma operation to minimise unwanted gases and impurities that would increase radiation losses and contaminate the plasma. During operation, the exhaust system removes helium ash, unburned deuterium and tritium, impurities and other neutral gases. Tritium recovered from the exhaust is processed in the fuel cycle and can be reused as fuel.

The ITER vacuum system combines mechanical pumps with powerful cryopumps to reduce the pressure in the vacuum vessel and cryostat to about one millionth of normal atmospheric pressure. The main cryogenic pumping systems include six torus exhaust pumps, four cryopumps for the neutral beam injectors and two cryopumps for the cryostat. Evacuating the large ITER vacuum volumes will take approximately 24–48 hours.

The cryopumps use cryopanels cooled with supercritical helium and coated with activated charcoal as a sorbent material for gases such as helium and hydrogen isotopes. Research showed that activated charcoal made from finely ground coconut shells has particularly suitable density and porosity for this purpose.

One sector of the ITER vacuum vessel.