The first of the two new beam transfer lines to the LHC was successfully commissioned in autumn 2004. At the first attempt a low-intensity proton beam passed down the line to a few metres before the LHC tunnel.
When the Large Hadron Collider (LHC) begins operation, two new beam transfer lines, with a combined length of 5.6 km, will bring 450 GeV proton beams or ions from the Super Proton Synchrotron (SPS) to the new machine. Line TI 2 leads from the extraction in long straight section LSS6 in the SPS to the injection point into the clockwise ring of the LHC near interaction point 2. The other line, TI 8, leads from the extraction in LSS4 to the injection point of the anti-clockwise ring near interaction point 8. The first 100 m of this transfer line, called TT40, are shared with the primary proton line to the CNGS facility and were commissioned together with the new extraction system in LSS4 in 2003. In October 2004 the complete TI 8 line became operational, with protons travelling the 2.5 km to the LHC tunnel.
Studies on how to transport beam from the SPS to the LHC began in the early 1990s. Various configurations were investigated, one of them even implying a polarity reversal of the SPS. The use of cryogenic magnets was also considered. Eventually a system using room-temperature magnets was chosen because it was more economical overall, since the transfer lines will operate only during the short periods of LHC filling.
Between them the two transfer lines required the excavation of more than 5 km of new tunnels and enlargements. Excavation for TI 8 began in autumn 1998 with a civil-engineering shaft near the SPS, some 50 m deep and 8 m in diameter. The first enlarged part of the tunnel, TT40, and some adjacent underground works were excavated using machines known as “road headers”. However, for drilling the 2.3 km towards the LHC a tunnel-boring machine was used. This had to work its way down to the tunnel that housed the still operational Large Electron Positron (LEP) collider, through a height difference of some 70 m, although this is not usually the preferred way of working. Excavation finished in June 2000 and was followed by lining with concrete, leaving a finished tunnel 3 m in diameter.
By contrast, TI 2 was entirely excavated by road headers. Although the inclination of the LHC tunnel means that the SPS extraction and LHC injection sections are nearly at the same height above sea-level, this tunnel needed a Z-shape vertical profile because of geological constraints (an underground river bed!). Additional magnet groups were required for the vertical bending. The construction of TI 2 and TI 8 involved the excavation of 60,000 m3 of material and the use of 21,000 m3 of concrete.
Geodetic referencing work on TI 8 started in autumn 2002, followed by the installation of general services, such as electricity and water cooling, and pulling the power and controls cables. Installation of the magnet system began in December 2003 and finished in May 2004. The relatively restricted space of the transfer tunnels required the development of a new system to transport and install the magnets. This is based on a modular system of “buggies” in the form of very compact tractors with a payload of 9 t each, which are fitted with air cushions and in-wheel motors. The wheels can turn on the spot under the load and allow the magnets to be displaced laterally towards their installation position. An automatic guidance system enables the travelling convoy to reach a typical driving speed of around 3.5 km/h. Using this system together with various girders and adapters, more than 400 magnets have been placed in TT40/TI 8, from 300 kg correctors to 13.5 t bending magnets recovered from earlier installations, as well as the 22 t beam dumps. In addition to work on TI 8 and TI 2, the system will be used to install magnets in the main LHC tunnel as well as for the CNGS project, thanks to its versatility.
All 348 main dipole magnets, 179 main quadrupoles and 93 corrector magnets for TI 2 and TI 8, as well as the bulk of the vacuum system, have been built by the Budker Institute for Nuclear Physics (BINP) in Novosibirsk, as part of the contribution of the Russian Federation to the LHC project. These have been transported to CERN by lorry over the 6000 km between the two laboratories. In addition, 73 dipoles and quadrupoles have been reused from the decommissioned PS-to-SPS electron transfer line and the SPS-to-LEP transfer lines. Because of the small emittance of the beam, the apertures of the lines could be relatively small – sometimes no bigger than a postage stamp.
The next stage was to install the beam instrumentation devices, set up the vacuum system and make the necessary electrical and water connections. TI 8 then entered 11 weeks of hardware commissioning to check all the systems individually, such as the magnet powering and polarities, the magnet temperature interlock system, and the read-out of the beam instrumentation devices. Special measures were taken to ensure a sufficient air flow from the ventilation system, and a final verification of the alignment of the beam-line elements took place. The last two weeks before the first beam test in October were used to operate all the systems together from the control room, and a series of “dry runs” allowed the many new components of the control system to be deployed and tested in advance.
For the actual beam tests, the beam dump at the end of the line was supplemented temporarily by additional iron and concrete shielding blocks. This was to minimize the radiological impact on the LHC tunnel and the cavern for the LHCb experiment, where installation is still in full swing. The entire LHC point 8 and several hundred metres in the adjacent LHC arcs were closed. Also the beam tests, spread over two weekends, were scheduled to minimize the impact on the ongoing installation work.
A single-bunch beam with 5 x 109 protons was prepared for the first beam tests. The line was set to 449.2 GeV, the SPS energy measured during the 2003 lead-ion run, and the LSS4 extraction system was set up and re-steered. As soon as the beam dumps at the beginning of the line were retracted, the first bunch of particles travelled through to the end of the installed part of the line, without the need for any “threading” (all corrector elements were set to zero current). In the following hours the necessary calibrations of the beam instrumentation were made and many measurements were carried out, such as energy acceptance, aperture scans, dispersion and optical matching, in part also using higher single-bunch intensities of 3-4 x 1010 protons. On the second test weekend, at the beginning of November, some commissioning was also done with multiple bunches per extraction, accumulating a total intensity at the end of the line of 8.6 x 1013 protons over the two weekends.
Although the data are still being analysed, the basic theoretical model of the lines seems to be well confirmed. The trajectory stability was found to be very good and the layout of the beam diagnostics, which performed well, was shown to be appropriate. The new control system, with its extensive array of applications, performed excellently, greatly facilitating the smooth progress of the tests.
The last part of the TI 8 line, in the LHC tunnel itself, and the injection system will be set up soon. The upstream part of the other transfer line, TI 2, is being installed. Since the main LHC magnets will be brought down through a shaft in TI 2 nearly halfway to the LHC, the downstream part of this transfer tunnel must remain empty of line elements to facilitate the transport of the LHC elements into the ring. It will be completed and commissioned once the installation of the main LHC magnets is over.
The commissioning of TI 8 was quickly and successfully achieved thanks to the dedication of the many people who have worked over the years on the two transfer lines. Following on from the commissioning of the LSS4 extraction and TT40 a year ago, this has served as a large-scale test-bed for components and concepts that will be used in the LHC. It also provided an early understanding of the behaviour of the transfer line, which should help to focus attention during the LHC sector test, planned for 2006, on the injection system and the main ring.
