The structure of the JET vacuum vessel is quite complex, with a large number of components and materials. Pumping it to a very good vacuum is not straightforward, namely because the gas molecules tend to adsorb on the surfaces of the solid state materials of the vessel. A very basic and efficient method to release the gas molecules from their hiding places is material baking. At JET, the whole structure of the vacuum vessel can be baked at up to 320 °C, and the baking system keeps the JET vessel hot continuously (even during plasma experiments), usually at about 200 °C.
The JET vessel baking is driven by two systems: hot gas and electrical. To allow for the hot gas baking, the JET vacuum vessel was built in two layers so that the baking gas can circulate in their interspace. Helium, which is used as the baking gas, runs in a closed loop – from the JET vacuum vessel to a massive blower that forces 22 m3 of the gas every second through heat exchangers (total 780 kW of heating power) and back into the interspace of the double-layer vacuum vessel. An addtional electrical baking system sustains the baking process on vessel components which project from the doughnut-shape vessel, (eg the diagnostic windows). It consists of hundreds of electrical heaters mounted directly onto the outside surface of the vessel components
Vessel baking is a key tool in the ‘first wall conditioning’.However its effect can be boosted if, in parallel, the inner surfaces are bombarded by charged particles. While by keeping materials hot the gas particles adsorbed to surfaces get ‘kicked out’ under bombardement. In most tokamaks, including JET, the walls are conditioned by baking combined with the effect of particle bombardment using “cold” gas glow discharge as well as “hot” plasma discharges.
JET uses either deuterium or helium for ‘cold’ glow discharge cleaning, and very rarely hydrogen. After long shutdowns, more than 24 hour continuous glow discharge cleaning in deuterium followed by a similarly long glow discharge cleaning in helium is not exceptional. During experimental campaigns, an overnight glow discharge cleaning may be requested to improve the first wall condition.
High temperature plasma discharges themselves act as a rather efficient tool to further clean the first wall from adsorbed atoms and molecules, as the plasma particle energies are much higher than in the glow discharge. For fusion experiments this is an adverse effect, as it increases the amount of impurities in the plasma. However, after a shutdown it is a common practice to run a few standard, scientifically uninteresting plasma discharges prior to the actual research.