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. Wendelstein 7-X, for example, has 300 such apertures, which are brazed to the vessel vacuum-tight or connected by metal flanges.