ALICE at full power
The ALICE dipole magnet recently passed its commissioning tests, which was also an important step forward for the collaboration.
The ALICE collaboration achieved an important milestone when they powered up the newly constructed dipole magnet for the muon spectrometer. On 9 November this very large magnet, now fully assembled in the ALICE experiment cavern, reached its full current of 6000 amps and produced a field of 670 millitesla.
A week of tests to assess the magnet's parameters showed that it fully met all the design values. The tests also included commissioning the new power supply and the magnet control system, and measuring the "stray field," the magnetic field surrounding the magnet.
When the experiment starts, particles collisions at the centre of ALICE's solenoid magnet will produce muons, among other particles, which will then fly through the dipole magnet, 8 metres away. The magnet's two semi-circular coils and the vertical poles of the rectangular yoke create the magnetic field to bend the muons' paths.
At nominal current, the stored energy of the magnet reaches 18 mega joules and the dissipated electrical energy 3.6 megawatts. Demineralized water is pumped through the circuits of the coil at a rate of 115 cubic metres per hour and heats up by 30° C to remove this energy.
During commissioning the mechanical stresses and deformations in the magnet were also measured. The tests proved that the structural stresses in the magnet yoke are well within the safe region. The movement of the coils due to electromagnetic and temperature deformation corresponds to the expected values.
The magnet is the fruit of a truly international collaboration. A team of engineers and technicians of the JINR laboratory in Dubna, Russia, assumed an important part of the design and development work. At CERN many magnetic field and mechanical calculations were performed and the majority of the components were designed. The main components arrived from a number of European countries, either through in-kind contributions or industrial contracts. Russia, for example, supplied more than 800 tonnes of steel for the magnet yoke and the two 30-tonne coils came from France, made from aluminium conductors produced by British industry. The heavy stainless steel coil supports were manufactured in Spain from steel purchased in Sweden. All these big components were then carefully fitted together by a highly motivated team at CERN, where the overall project coordination and management is under the responsibility of Detlef Swoboda, leader of the ALICE dipole magnet project.
The dipole magnet still has to be transferred to its final position in the experiment, next to the big solenoid magnet, which was built for the LEP detector L3 and will be reused by ALICE. To move the dipole magnet, the collaboration has to disassemble it, then move the pieces and rebuild on the other side of the cavern. The collaboration started this rather delicate task before Christmas 2004 and plans to finish by Easter 2005. One of the big challenges for this operation will be the passage of the coils above the ALICE solenoid, which requires millimetre precision in the operation of the overhead crane.
The reconstruction in the final location requires a perfect alignment of the yoke modules with respect to the LHC beam axis and the distance to the interaction point of the particle beams. Alignment dowels between parts of the magnet ensure therefore their relative positions and the CERN survey group verifies all critical dimensions.
Once the magnet is in its final position, between the solenoid and the 300-tonne iron muon filter wall, the collaboration can start mapping the dipole's magnetic field with a sophisticated measuring apparatus. They will use this apparatus to map the field throughout the volume of the complete ALICE magnet system, including inside the solenoid magnet.
