vacuum vessel is the sealed toroidal chamber at the heart of a 📝Tokamak that holds the ultra-high vacuum in which the plasma lives.
The vacuum comes first because purity does: a 📝fusion plasma of a few grams must not collide with stray air molecules, so the vessel is pumped to pressures around a billionth of an atmosphere before any 📝plasma is made. The vessel is also the machine's structural spine and its primary containment barrier — typically a double-walled steel torus whose interior is lined by the 📝first wall and 📝blanket, with the 📝divertor at its floor, and whose ports admit heating systems, diagnostics, fueling, and maintenance equipment. ITER's vessel illustrates the scale: 1,400 cubic meters of interior volume and roughly 5,200 tonnes of steel.
The vacuum vessel is commonly confused with the cryostat, the second vacuum chamber that encloses it. The two hold different vacuums for different reasons: the vessel's vacuum is the plasma's habitat, while the cryostat's is thermal insulation, keeping room-temperature air away from the superconducting magnets that operate at cryogenic temperature between the two walls of vacuum.