One of the objectives of the Compass experiment, located on the Super proton synchroton (SPS) at Cern is to study the origin of nucleon spin. This experiment used a target polarised in the 2.5 tesla magnetic field that is produced at the centre of the superconductor magnet Compass. The design and realization of this magnet were fully resumed after a first fabrication did not reach the requested characteristics. Dapnia, through the Nuclear Physics department, the systems engineering department and SACM has played a very important role in taking the project in hand again. Finally, a magnet with the requested parameters has been operational for physics on the CERN experimental areas since the middle of 2006.
After a first unsuccessful tentative for the construction of their superconducting magnet, the Compass collaboration appointed an expert committee to redesign a new magnet able to reach their requests and to follow-up its industrial construction.
SACM was very much involved in this new phase:
. participation to the expert committee,
. follow up of the reconstruction of the magnet by industry,
. realization of the reception tests at Saclay, then at CERN, including in particular two magnetic measurement campaigns to get the requested field homogeneity.
The magnet reached its nominal field without any quench, despite a short which appeared in one of the correction coils. This short makes the operation of the magnet more complex, as this special situation must be taken into account when ramping the main coil.
In collaboration with SIS, SACM performed two magnetic measurement campaigns on the magnet. A first one was done at Saclay at the end of 2005 to characterize the field and to make a first homogenization of the field. Then, beginning of 2006, a second campaign was done at CERN to take into account the magnetic environment, in particular the proximity of the SM1 conventional dipole. By getting independently the magnetic characterization of the main solenoid and of each correction coil, it has been possible to determine a set of correction currents to improve the field homogeneity of the solenoid alone. A homogeneity better than 10-4 was obtained in the useful zone (cylinder of 1.3 m in length and 3 cm in diameter), and this value complied with the physicists' requirements.