Small amounts of incoming air would already immediately extinguish a burning plasma. The vessel has to be vacuum-tight and capable of being pumped down to a pressure of less than 10-8 millibar, i.e. ultrahigh vacuum.
To withstand the high loads due to pressure and magnetic forces that can be caused by locally induced currents, it is primarily high-grade steel that serves as vessel material. For measuring, heating and control facilities the vessel requires numerous apertures and ports.
The ASDEX Upgrade tokamak, for example, has 115 such apertures, which are brazed to the vessel vacuum-tight or connected by metal flanges.