Vacuum Vessel Baking
Having cooled down the magnets in the cryostat, now the vacuum vessel is being heated up to 200°C. The purpose of this ´baking´ process is to drive out moisture and other impurities from the surfaces inside the vacuum vessel sufficiently to provide the ultra high vacuum needed for high quality plasma experiments. The presence of too great a quantity of impurities can prevent the reliable formation of a plasma. It can also cause too much energy to be lost from the plasma or cause it to disrupt, ending out of control. In particular the graphite armour covering about 50% of the straight section of the inner wall and the open divertor running around the top of the toroidal vacuum chamber can absorb water which needs to be baked out.
The stainless steel vacuum vessel has a double wall and is heated by passing hot nitrogen gas between the inner and outer shells. The gas is heated electrically in the torus hall basement and is circulated in a closed loop. The baking of the vacuum vessel causes a substantial increase in the heat radiated to the thermal shield around the cryogenic magnets and a corresponding increase in liquid nitrogen consumption by the cryoplant keeping it cold.