Model of the ITER cryostat with its four main sections and numerous access openings.

During ITER operation, the 150-million-kelvin plasma core will be only a few metres away from toroidal field coils operating at about 4 K. Their extremely low temperature is necessary for superconductivity and the generation of the strong magnetic fields that confine the hot plasma.

World’s Largest Thermos

To minimise heat transfer to the superconducting magnets, the ITER machine is enclosed within a huge vacuum chamber called the cryostat. The vacuum suppresses heat transfer by convection, while thermal shields between the warm structures and the magnets reduce radiative heat transfer. These stainless-steel shields are actively cooled by helium gas to about 80–100 K. It will be the world’s largest stainless-steel high-vacuum pressure chamber.

Size

This enormous stainless-steel cylinder is nearly 30 metres wide and about the same height. Its internal diameter of approximately 28 metres is determined largely by the size of the two largest poloidal field coils that it surrounds. The cryostat has a total volume of about 16,000 m³, with an effective pumped volume of approximately 8,500 m³ once the machine components are installed. Its vacuum system is designed to maintain a pressure of about 1 × 10⁻⁴ Pa.

Robustness

The entire cryostat weighs approximately 3,850 tonnes. It contains 23 large penetrations for maintenance and more than 200 additional openings, some up to about four metres across, for cooling systems, magnet feeders, heating systems, diagnostics and remote-handling equipment. All penetrations must remain vacuum-tight to preserve the cryostat environment.

During operation, the cryostat, vacuum vessel and surrounding structures will undergo thermal expansion, contraction and mechanical movement caused by large electromagnetic loads. Flexible bellows are therefore installed between the vacuum vessel and cryostat and between the cryostat and the Tokamak Building to accommodate these relative movements while maintaining vacuum integrity. A total of 93 bellows units are planned, including 83 rectangular and 8 circular units.

Manufacturing of the ITER cryostat base section.

Silver-coated thermal shield.

Segments

The cryostat was assembled on site from 54 large stainless-steel segments manufactured in India. These were welded together to form four main sections: the 1,250-tonne base, the lower cylinder, the upper cylinder and the approximately 665-tonne top lid. The base and lower cylinder have already been installed in the Tokamak Pit, while the upper cylinder and top lid are stored on site for installation during later stages of machine assembly.

Support Function

In addition to providing thermal insulation and a vacuum environment, the cryostat also forms part of the machine’s structural support system. It helps support the Tokamak and transfer mechanical loads to the surrounding structure. The concrete-and-steel pedestal beneath the machine was designed for a total load of about 23,000 tonnes, including approximately 3,850 tonnes for the cryostat itself and 18,900 tonnes for the rest of the ITER machine.

The lower cylinder of the ITER cryostat entering the Cleaning Facility.