cryostat is the vacuum-insulated vessel that surrounds a machine's superconducting magnets and keeps them at cryogenic temperature — in effect, a thermos bottle built around the entire device.
The name joins the Greek kryos, "icy cold," with -stat, "keeping stationary": a device that holds cold steady. The near-neighbor confusion in a fusion machine is with the 📝vacuum vessel, and the two nest: the vacuum vessel is the inner toroidal chamber holding the ultra-high vacuum in which the 📝plasma lives, while the cryostat is the outer envelope enclosing vessel and magnets alike. Its vacuum serves a different master — not plasma purity but thermal insulation, eliminating the convection that would otherwise pour room-temperature heat into magnets sitting hundreds of degrees colder. Layers of reflective shielding and actively cooled thermal screens intercept the radiant heat the vacuum cannot stop.
The cryostat is where 📝cryogenics meets structure: every pipe, current lead, and support that crosses from warm to cold is a designed-in heat leak to be minimized, and the boundary must survive both vacuum loads and the forces of a magnet system energized or quenching. At 📝ITER, the cryostat is the largest stainless-steel high-vacuum pressure chamber ever built, enclosing roughly 16,000 cubic meters around the machine.