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The first module of CMS superconducting magnet is leaving towards Cern: a huge solenoid, which will hold the world record of stored energy

January 20, 2004

The first module of the five constituting the CMS superconducting magnet is sailing on January 21st from Genova port to Cern. CMS (Compact Muon Solenoid) is one of the experiments that will take place at the accelerator Lhc (Large Hadron Collider), under construction at Cern in Geneva. The device will arrive after a 10-days travel. One of the most ambitious goals of CMS is to provide information about the elusive Higgs boson: the elementary particle which is associated with the mechanism giving rise to the masses of all particles. The theoretical models predicted its existence, but it has not been directly observed yet.

Cms will analyse the products of the collisions of the proton beams steered in Lhc. It will reconstruct their tracks and measure their energy. The superconducting magnet, containing the heart of Cms experiment generates a very high magnetic field, necessary to recognize the particles produced by the collisions. Indeed the tracks of the charged particles crossing a magnetic field are deflected in different ways according to their mass and charge. Therefore, observing particles tracks we can trace back to their identity. To obtain observable deflections we need a magnetic field as high as the energy of the outgoing particles. Since in Lhc particles beams with a very high energy are produced and made to collide (these particles in a very small scale reproduce the conditions of our universe in the very first instants of its birth) it is necessary to have a very high magnetic field.

The superconducting magnet is the result of an international cooperation among different research centres. The participants are in fact the Infn (Italian National Institute for Nuclear Physics) the Cern, the Cea (Commissariat pour l'Energie Atomic), the Eth-Z (Polytechnic of Zurich), with the cooperation of Ansaldo Superconductors of Genova. The latter was entrusted with the construction of the five modules constituting the magnet and with the preparation of an area of about 1.500 square meters for their construction.

Besides producing a very high magnetic field, the magnet must have extraordinary sizes, so that it can contain the whole of the detectors necessary to carry out precise measurements. Therefore Cms has an inside diameter of 6.3 meters, a length of 12.5 meters and generates a magnetic field of 4 T (about 80.000 times stronger than the Earth's). Once completed, the Cms superconducting magnet will boast a notable record: with its 2.6 Gigajoule of energy it will hold the world record of energy ever stored in a magnet. Another feature of this apparatus is that it must operate at a very low temperature, so that the cables, where the electric current flows, can be in a particular status named superconductor. Thanks to this characteristic, the high current necessary to generate the required magnetic field can flow trough a few wires of about a millimetre of diameter. If a superconducting material was not used, cables of so huge sizes would be needed, to preclude the construction of the entire structure.

The notable sizes of the superconducting solenoid representing the "living part" of the magnet, required a modular construction in order to allow the transportation from the fabrication site to the Cern laboratories. "A long work of engineering optimization was necessary leading us to subdivide the solenoid into five modules of length 2.5 meters and of weight 45 tonnes. The modules will be constructed and transported one by one to Cern, where the assembling will take place" says Pasquale Fabbricatore, Infn researcher in Genova. In particular Infn is responsible for the activities of design and construction of the so-called cold mass, i.e. the winding and the mechanical structures that will be cooled at - 268 centigrade degrees (4.2 kelvin degrees).

The construction of the superconducting magnet of Cms ha required the development of innovative technologies. "Since a very high current flows in the superconductor magnet, and since the produced magnetic field is so high and the device is so large, inside the solenoid high electromagnetic forces are generated causing mechanical deformation that could make it not working. In order to prevent this problem the standard solution was putting a reinforcing mechanical structure containing the solenoid. In our case this would not have been sufficient. In order to avoid also the smallest deformation, making the cables loosing superconducting properties the reinforcement has been inserted directly inside the cables: an innovative solution requiring remarkable technical skills. However, at this point it has become very difficult to wind the cable in the right way, so it has been necessary to develop a sophisticated completely automate winding system allowing us to perform the work with high geometrical precision", concludes Pasquale Fabbricatore.

Istituto Nazionale di Fisica Nucleare (INFN)