Twenty years of experience went into the design of the prototype torus cryo vacuum pump for ITER. The design will now be given to a manufacturer to build a 1:1 size prototype, which will be tested at KIT’s test facility for ITER pump models, TIMO-2.

The stakes are high for the ITER torus cryo vacuum pumps: Six pumps will create pressures down to one billionth that of air inside the 1,400 cubic metres of the vacuum vessel. “Just realising the immense pumping speed of 75 cubic metres per second within a pump diameter limited to about two metres is a challenge” says Christian Day.

He is head of KIT’s vacuum technology group, which is in charge of designing all the ITER cryo pumps. The heart of a cryo pump is an extremely cold surface, the cryo panel, which traps the atoms and molecules contained in the gas. At ITER, the cryo pumps will be also used to clean the vessel of helium produced by the fusion reaction. Helium is a headache for cryo pumps, as it hardly sticks to surfaces even as cold as four or five kelvin. KIT’s vacuum experts spent years looking for the most efficient carbon structure to trap helium and finally settled for coconut charcoal from a certain patch of land in Indonesia. Now KIT possesses an entire year’s harvest – enough to supply ITER and several future fusion plants.

ITER holds yet another challenge for the pump engineers: The deuterium and tritium absorbed in the cryo panels must be released regularly and fed back into the fuel cycle. Every ten minutes, the cryo panel of one of the eight pumps is heated to 100 kelvin to set free the trapped gas, and then cooled down again to four kelvin. “The ITER design allows us only 150 seconds for either the heating or the cooling process. That is very hard to realise, as 210 kilograms of steel in the panel have to be brought to temperature“ explains Day. On top of that the mechanical stress which the heating and cooling cycles exert on the pumps has to be taken into account in the design.

As if these were not enough challenges for the pump designers, ITER also wants to use the pre-production prototype as a spare pump during operation if needed. For the KIT team that meant having to come up with a pump design that not only meets the pumping requirements, but also complies with all other ITER regulations – which go as far as demanding resistance against unlikely earthquakes at Cadarache. “We had to run lots of extra simulations and specify the materials to be used in much detail. We also had to integrate all changes made to the ITER requirements during our on-going design process.” says Christian Day.

The pump design has now been approved by F4E and ITER will be passed to a manufacturer, which F4E will identify through a call-for-tender procedure. The finished pump will be fully tested under ITER-like conditions in KIT’s TIMO-2 test facility. In the meantime the KIT vacuum technology group will continue designing the cryo pumps for ITER’s Neutral Beam Heating system. The group has also started to explore pump designs for a future fusion power plant within the EFDA Power Plant Physics and Technology activities.