The JET vacuum system is responsible for pumping out gas from the large volume of the JET torus – more than 200 m3 – similar to the volume of an average apartment. A fusion reactor requires ultra-high vacuum, as the densities of the hydrogen plasma are very low, about one million times lower than the density of air. Even a much smaller amount of non-hydrogen elements remaining in the vessel (e.g. nitrogen or oxygen from the air) would considerably damage discharge performance.
JET can achieve a very good level of vacuum, up to a millionth of a millionth of the density of air (in technical terms, the final pressure of impurities can achieve up to 10-9 mbar, that is 10-7 Pa). The procedure required to achieve and maintain that good vacuum is actually quite complicated, and several techniques must be employed to get after the vessel has been opened during a shutdown.
The first step in vacuum conditioning is to use pumps called ‘roughing pumps’ to achieve a ‘rough vacuum’ of around 0.1 millibar. It takes about 15 hours to go from atmospheric pressure to this level.
Then four large turbomolecular pumps take over. They reduce the pressure to 0.00000001 millibar (1 x 10-8) within a few hours.
The vessel is further pumped by the cryopumps in the divertor region and JET would not be routinely operated with the cryopanels warm. With the pumped divertor panels at helium temperature a well conditioned torus will typically be pumped to ~1 x 10-8 mbar. Several smaller turbomolecular pumps are installed to maintain vacuum in some of the JET diagnostic systems.
At several specific regions of the JET facility, a very high pumping speed is required: in the Neutral Beam Injector box, where it is necessary to prevent the gas flow from the beam neutraliser into the plasma, in the divertor region at the bottom of the vessel, where the plasma exhaust is directed by magnetic field lines, but also in the Lower Hybrid Current Drive system and in the deuterium pellet source. Very fast pumping in these regions is achieved by cryopumps – large surfaces that are at extremely low temperatures. On these surfaces nearly all gases immediately freeze and collect as frost.
During operation, the JET cryopumps are cooled down to -269 °C (5 K) by liquid helium that is supplied from the JET cryoplant. In order to maintain the required amount of liquid helium for the facility, the JET cryoplant has a helium liquefier, an extreme member of the broad family of high capacity refrigerators. During JET operations, the JET’s helium liquefier unit – with two main compressors and several ancillaries – needs around 1 MW power continuously in order to produce about 8,000 litres (i.e. one tonne) of liquid helium per day.
Helium, however, cannot be condensed. One option is to apply argon cooling in the Neutral Beam injector box. Another method, which also improves hydrogen pumping, is to coat the cooling panels with activated charcoal granules. Activated charcoal is a highly porous carbon with millions of tiny pores between the atoms, creating surface areas of several hundreds of square meters per every gram of charcoal, so that it has a unique adsorption capacity.
JET is unique in the world as a fusion research experiment able to work with tritium, and, as a consequence, it has to be operated with all precautions required for active isotope handling. All the gases that are pumped from the vessel must go through a dedicated pipeline to the Active Gas Handling System. In this system, chromatography and cryodistillation processes allow for safe separation and storage of the different isotopes from the pumped gases – namely of tritium (active), deuterium and helium (stable). This procedure is required at all times, even when JET is not operating with tritium, as traces of tritium continuously desorb from the vessel structure into the main pumped volume.
The JET vacuum pumps, including this new cryopanel system, pump all gases from the torus and other systems (e.g. Neutral Beam Injectors) into the Active Gas Handling System, where the different hydrogen species (H, D and T) are sorted out using isotope separation techniques, and deuterium and tritium are stored for future JET fuelling.A new purification system called PERMCAT is also being installed in the Active Gas Handling System to remove impurity gases such as helium, carbon dioxide, water, or methane from the collected gases.