Silver coated thermal shield of tokamak ITER cryostat. (Credit © ITER Organization, www.iter.org)

The outer coating of the vacuum vessel of a tokamak or stellarator. It has strong reflective properties (e.g. silver plated) and is actively cooled by flowing water. It is designed to minimize heat transfer from the vacuum vessel, where thermonuclear fusion takes place, to the superconducting magnetic coils cooled to 4 K. To further minimise heat transfer, the vacuum vessel covered by the heat shield and the coils are placed in a vacuum, in a giant thermos called a cryostat.