LHC tubes near the end of their journey
Production of the heat exchanger tubes, which will cool down the LHC magnets, and of the cold bore tubes, in which the proton beams will circulate, is due to be completed around the end of 2004. These essential components of the LHC magnets are receiving their finishing touches at CERN.
The work requires facilities of the highest standard, not to mention the right length. The location chosen to apply the finishing touches to all the tubes for the 15-metre long LHC dipole magnets is CERN's large Building 927. There, the heat exchanger tubes are being assembled by electron-beam welding while the cold bore tubes are receiving their insulating coating. These tubes are essential to the operation of the accelerator's magnets. The heat exchanger tubes will play a key role in cooling down the LHC dipole magnets to a temperature of -271°C, and the cold bore tubes will provide the vacuum chamber in which the LHC's proton beams will circulate.
Heat exchanger tubes
About 10 heat exchanger tubes are being completed every day in Building 927. The first operation is to machine the ends of the copper tubes supplied by Outokumpu (Finland), after which the tubes are transferred to Building 118 for ultra-vacuum cleaning in special tanks custom-designed to their length. The copper tubes that will ultimately become the heat exchangers for the LHC magnets are submerged in 17-metre long rinsing tanks in batches of ten.The team of welders, braziers and mechanics involved in the fabrication of the heat exchanger tubes.
At the same time, in Building 112, 10-mm long copper rings are vacuum brazed onto stainless steel transition pieces, which are then electron-beam welded onto the ends of the copper pipes. These stainless steel transition pieces facilitate the connection of the tubes inside the magnets.
'Electron-beam welding preserves the mechanical properties of the copper so the weld is fit to withstand the high pressure exerted on it inside the LHC magnet should a quench, i.e. a transition from the superconducting to the conducting state, occur,' says Gilles Favre, who is responsible for the production of the heat exchanger tubes. After various checks - dimensions, airtightness and X-ray - the tubes are shipped out to the premises of the dipole assembly contractors.
The LHC dipole and quadrupole magnets will be cooled by 1700 of these copper tubes containing superfluid helium. More than 1200 have already been completed.
Cold bore tubes
In another part of Building 927, the cold bore tubes, supplied by the Italian firm DMV, are coated with various layers of insulating material, wrapped in polyester by a machine and then baked in an oven at 190°C.Insulation of the cold bore tubes in which the LHC beams will circulate takes place in Building 927. In the background, Bruno Meunier checks the wrapping machine while, in the foreground, Olivier Vasseur removes the polyester wrapping that covers the tube's insulating layers.
'After baking to bond the insulation to the tube, the polyester mould is removed by hand,' explains Hans Kummer, who is responsible for this insulation process within the Magnets and Superconductors Group (AT-MAS).
Davide Tommasini, Head of the Magnet winding, assembly workshop and electrical insulating materials section (ME) of the AT-MAS Group, says: 'The insulating materials have been developed according to CERN's specifications. Each of the three insulating layers fulfils a different function. The first layer is in polyimide to insulate the cold bore tubes electrically from the superconducting coils around them. The second layer consists of glass-fibre and epoxy resin, providing mechanical protection for the aforementioned dielectric insulation. The third, outermost layer is polyimide coated with a special glue to obtain an ultra-smooth surface that will facilitate the tube's positioning inside the coil.'
A total of 3500 of these stainless steel tubes will be required to make the LHC's twin beams circulate through the magnets. More than 2800 tubes have already been insulated, which represents 36 km out of the 50 required for the LHC.
About 35 tubes are shipped off to the dipole assembly contractors each week.
The insulation of the cold bore tubes started at CERN in the autumn of 2001. 'This is one of the prime examples of the re-insourcing of highly specialised work,' says Lucio Rossi, Head of the Magnets and Superconductors Group (AT-MAS). Hans Kummer adds: 'It enabled us to deliver tubes of the requisite quality, on time and with savings of CHF400 000'.
At the end of 2002 it was also decided to insource the manufacture of the heat exchanger tubes to CERN. 'The technologies were already in use at CERN. The idea was to use the same facilities, such as the ultra-vacuum cleaning, for different long components of the magnet, and to make the most of synergies with existing services, such as the logistics,' explains Francesco Bertinelli, Head of the AT-MAS Group's Component Centre. 'This led to savings on the heat exchanger tubes of CHF500 000 to 700 000.' The production of these tubes will be completed by the end of the year.
Dissecting an LHC dipole...
The cold mass of a 15-metre main dipole magnet has some fifteen different components. All the main components are manufactured under CERN's direct responsibility. Four of them transit through CERN before being shipped to the dipole assembly contractors, namely the cable, which constitutes the magnet's superconducting core (see Bulletin 14/2004), the beam screens, the heat exchanger tubes and the cold bore beam tubes. The two latter components transit via Building 927 where they undergo part of the production process. The 58-mm diameter heat exchanger tubes will remove heat from the magnets using superfluid helium. The 53-mm diameter cold bore tubes will be placed under vacuum to allow the twin beams to circulate around the LHC.