Optical fibres bringing the LHC into focus
New components are being added to CERN's optical fibre network, which will transport the torrents of data produced by the LHC. 1500 kilometres of cables will be installed in the tunnels and at ground level.
It will take a lot of huffing and puffing to install the LHC's optical fibre network, as the optical fibre cables are literally "blown" into their protective mini-tubes. This operation is obviously not performed by human "blowers", but by high-pressure air-blowing devices. As with so many other LHC items, where the frontiers of technology are being forced back, the specialist supplier in this case has had to pull out all the stops. The firm entrusted with the supply and installation of the LHC optical fibre network, Draka Comteq-NKF, has even broken a world record since being at CERN - at 2:00 hrs on June 12th, they managed to blow 3,100 metres of mini-cable into position between Points 7 and 8 in the space of one hour, using a pressure of 14 bars in the tube.
A network of over 1500 kilometres of optical fibre cables needs to be installed for the LHC machine alone, not including the experiments. These fibres, each capable of transmitting data at a rate of up to 1000 gigabits per second will play a decisive role in real-time data transmission for many purposes: measurements of the beam and the machine status, transfer of the massive quantities of data produced by the experiments, synchronisation of the accelerators, etc. One of the challenges is "to set up a network like the one you might find in a town, with many different branches," explains Luit De Jonge, Head of the ST/EL Group's Optical Fibres section. Furthermore, redundancy must be built into the system to ensure that the connections are maintained even in the event of breakdowns.
As soon as LEP dismantling commenced, work began to complete the existing surface links, namely those which did not transit via the LHC tunnels. This involved connecting Points 5 and 7 to the existing network, which links all the other Points to the accelerator control room at Prévessin (PCR). A special fibre manufactured by the Japanese firm Sumitomo is used for synchronisation loops towards Point 4. This fibre, used for the LEP200 project, has already proved its good performance. A special coating, with a negative temperature coefficient stops the cable from dilating under the effect of daily and seasonal temperature variations. This means that the fibre length hardly varies at all and that the transmission time remains the same - to a few billionths of a second. This precision is crucial, given that the accelerators must be perfectly synchronised. The same fibre was used for the LEP200 project.
For the optical fibre network running through the LHC tunnels a new "mini-tube" technique has been used. Instead of installing one large cable containing hundreds of optical fibres, several mini-tubes are inserted into a protective tube. Each mini-tube can receive one mini-cable containing up to 24 fibres. This system has the advantage of being highly flexible and economical. Whenever it is necessary to connect a new user along the line, a single mini-tube can be branched off. Similarly, in the event of a breakdown anywhere along the line, only the faulty mini-tube needs to be replaced. Last but not least, if requirements were to increase, one can simply add more mini-cables.
After two years of studies, feasibility tests, a market survey and call for tenders, all undertaken in collaboration with the SPL Division, the contract for the supply and installation of these mini-tubes was awarded to Draka Comteq-NKF. This Dutch firm has now begun installing a 27-km long tube around the LHC ring, in conjunction with the Swiss installation firm Mauerhofer & Zuber. This tube, which holds 10 mini-tubes, each containing a maximum of 24 fibres, will be used for communications and control. In addition, 5 cables containing 7 mini-tubes are installed in each octant around the ring to connect the instrumentation on the surface to all beam observation stations. These fibres will transmit information for the beam position monitoring and beam loss monitoring. "The mini-tubes will be very useful for these connections as we will be connecting a beam observation station every 50 metres", explains Luit De Jonge.
It was while a connection of this type was being installed between Points 7 and 8 that the firm beat the world record for length of cable blown into a tube. Once this section has been completed, the sector between Points 2 and 3 will be fitted out in autumn, and so on. The mini-tubes have proved so useful that they will also be used for the surface connections between the Prévessin control room and Points 2, 32, 33 and 4.
In total, 2500 kilometres of mini-tubes will be installed. Not all the tubes will be used immediately, but can be used for housing new cables to reinforce the network in the future.
