A heavy load for heavy ions
On 25 September, the two large coils for the dipole magnet of ALICE, the LHC experiment dedicated to heavy ions, arrived at Point 2 on two heavy load trucks after a 1200 km journey from their assembly in Vannes, France.
An exceptional view presented itself to the residents of the Pays de Gex at the end of last month. The two large coils for the ALICE dipole magnet were transported from the manufacturing plant of Sigmaphi in Vannes, France, to their final destination at Point 2, where they arrived on 25 September. The two gigantic electromagnet coils which are 5 metres long, 6 metres wide, more than 3 metres high, and weigh 20 tonnes each will be installed in the dipole magnet of the forward muon arm spectrometer of the ALICE detector.
To load the huge coils some obstacles had to be overcome. To begin with, the overhead crane of 20 tonnes capacity could not be used to lift the coils including their support structure - therefore the coils had to be jacked up on rollers and pulled outside the hall, where a mobile crane lifted them on to the trucks. Moreover, the big door of the assembly hall was still too small and part of the wall had to be cut open to roll the coils out.
Once on the road, however, the 1200 km long journey did not encounter any problems, and after three and a half days the coils arrived safely at Point 2 escorted by four French police motor cycles. "The trucks could go no faster than 60 km/h and many detours were necessary because of the height of bridges etc." says Detlef Swoboda, who is responsible for the ALICE magnets, "but everything else went very smoothly during transport."
The magnet coils will be inserted in the iron yoke which was manufactured in Russia and is on its way to CERN. The first assembly of the magnet will start in October in the ALICE experiment cavern, where it will be fully tested to verify the characteristics of the magnet and to tune the assembly procedure. This cannot be done in the final location because there the space is too restricted, and the dipole magnet has to be powered up at a distance from the other ALICE magnet -the solenoid reused from the L3 experiment of the LEP- to minimise any interference between both devices. After testing, the dipole magnet will be completely disassembled and lifted over the L3 solenoid component by component to be installed in its final position in the ALICE cavern during summer 2004.
The dipole magnet will be used to identify high momentum muon pairs. The tracks will be analysed by 5 tracking stations located in front, inside, and behind the magnet which will be located at more than 7 metres away from the interaction point. The impressive size and gap width between the poles (3-4 metres) is necessary to obtain the desired acceptance angle of 9 degree.
The coils were wound from pure aluminium conductor, with a cross section of 50.5 by 50.5 millimetres and a central cooling hole of 26 millimetres diameter, through which 110 m3 demineralised water will be pumped per hour, to ensure that the coils do not heat up to more than 30°C. Each coil consists of 12 "pancakes" of 14 windings each, which means that the total length of the aluminium conductor is more than 7 km. "The coils can be considered as prototypes - they are produced only once," says Detlef Swoboda, "so no failure was allowed."
The ALICE dipole magnet is one of the biggest dipoles operating at room temperature. The electrical power dissipation is close to 4 Megawatts. To reach the nominal field of 0.7 T, it will be powered by a DC power converter providing 6000 Ampere, which is already installed at Point 2.
