A container inside which a pressure significantly lower than atmospheric pressure, or vacuum, is artificially maintained. Experiments that require the absence of an atmosphere, such as the creation of hot plasma for thermonuclear fusion, can take place in such a vessel. The shape and structure of the vacuum vessel can vary considerably. From small “table-top” experiments, where the vacuum vessel is simply a closed tube connected to a vacuum pump, to complex, multi-layered vessels with active cooling, such as those being built for the world’s largest tokamak, ITER. The walls of the vacuum vessel are chosen to withstand the conditions that will prevail inside the vessel, which in the case of fusion include temperatures of up to 100 million Kelvin and strong neutron fluxes. The vacuum inside is maintained by one or more vacuum pumps. Entry for e.g. diagnostics, refuelling, or remote handling are provided through ports that guarantee the vacuum tightness of the vessel.