LHCb collaboration celebrates magnet's success

The LHCb magnet reached full current and achieved the designed magnetic field on 9 November, a major milestone for the collaboration.


Leading members of the LHCb magnet project, in the Point 8 cavern, in front of the detector's magnet and surrounding iron yoke. The field mapping machine is mounted inside. From left to right: Sylvain Ravat, Marcello Losasso, Jean Renaud (in charge of the assembly), Pierre-Ange Giudici, and Antoine Kehrli.


Champagne bottles were opened in the underground control room of Point 8 during the night of 9 November to celebrate reaching the full magnetic field of the LHCb magnet successfully. In a relaxed atmosphere following the pressure accumulated during the many months of preparation, the LHCb magnet team and all the others "on-board" the commissioning activities eventually assembled together to toast the common success which represents a crucial milestone for the LHCb experiment. Indeed the commissioning had just started to confirm that the almost 60-tonne coils, along with the 1500-tonne iron yoke, perform in the expected way, and that the power, cooling, and control systems work properly.

This April, the magnet was rolled close to its final place in the cavern (see the 26 April 2004 Bulletin). Then, along with members of TS/SU, the magnet team used optical surveys to align the mechanical axis of this huge structure within a few millimetres of its final position relative to the planned interaction point of the LHC beams. Next, over several weeks, they connected and checked the magnet's power supply and water cooling system. The commissioning was the culmination of the joint effort of many people, started more than a year ago, and made the LHCb magnet the first new magnet for an LHC experiment installed in its final position and commissioned, demonstrating that it functions according to specifications.

The magnet has a distinctive funnel shape, designed to capture as much as possible of the cone of particles that will pour from the interaction point where the LHC's two proton beams will collide. It is a conventional rather than superconducting magnet, consuming 4.2 megawatts of electrical power, which means it creates a lot of heat and thus needs a powerful cooling system, with a huge flux of 150 cubic metres per hour of demineralised water flowing through its hollow aluminium conductors and cooling pipes.

In addition to testing the magnet itself, the team began preliminary field measurements around the magnet. First they measured the stray field near the cryogenic plant of the LHC machine, which sits only few metres away from the magnet system. In another test, they perched a thick-walled ARMCO iron box, the size of a refrigerator, on tall concrete blocks high above the cavern floor. The box will shield photon detectors of LHCb's RICH2 system, and the team wanted to measure the magnetic field inside it. Both tests confirmed the design analyses, showing that the stray field is as calculated and should not harm the operation of the detectors to be installed in the shielding box.

Now the LHCb collaboration is preparing to map the magnet's field. A motor-driven mechanical arm will move a bank of special, patented probes through the space around the magnet's poles, measuring the magnetic field at millions of points. The Hall probe cards were developed at CERN in collaboration with the National Institute for Nuclear Physics and High Energy Physics (NIKHEF) of the Netherlands. They are more precise than others available on the market and will also be used to map the magnetic fields in the other LHC experiments.

The collaboration will first make a map of the bare magnet. Then they will install another iron shielding box, this one upstream of the magnet, for the RICH1 detector. This box is expected to affect the magnetic field somewhat around the magnet's poles, which is why the LHCb physicists want to map the field precisely in this configuration as well. Then they will chart a final field map when all surrounding structures are in place.

The field map will allow determining accurately the magnetic axis of the magnet, subsequently used to align it with the beam axis. Also, a good field map is essential for measuring accurately the momentum of the charged particles traversing the detector. The map will also be fed into simulations helping physicists to predict the events they will see when LHCb begins taking data.


On the night of 9 November, magnet project leader Marcello Losasso with members of the LHCb team, celebrating with champagne the successful start of the commissioning, a crucial milestone for the experiment. From left to right: Pierre-Ange Giudici, Carlos De Almeida Martins, Sylvain Ravat, Nicolas Bourgeois, Olivier Jamet, Marcello Losasso, Rolf Lindner, and Wilfried Flegel (former project leader).