Tokamak ITER cryostat. (Credit © ITER Organization, www.iter.org)

A container, similar in design to a thermos, in which a very low temperature is maintained. In the case of a tokamak or stellarator, it is a vacuum vessel surrounding a chamber and magnetic coils. The vacuum minimizes heat transfer from the chamber to the coils, which need to be at 4 K temperature to maintain superconductivity.