CMS ready for winding up
End of October, the last lengths of conductor for the CMS superconducting solenoid have been produced. This is another large sub-project of the CMS Magnet being successfully finished, after completion of the Yoke last year (see Bulletin 43/2002).
More than 1600 kilometres of superconducting strand to make 50 km of conductor! That is the length needed to wind the CMS solenoid - and also more than the distance between CERN and Annecy, where the conductor's lengths were fabricated. The full batch comprises 21 lengths of 2500 metres each for the 5 modules of the cold mass which have 4 layers each, plus one to make a short prototype - a 18 million CHF project. "It is a major achievement for the CMS Magnet project and CMS in general," says Domenico Campi, in charge of the CMS magnet group.
The CMS detector needs a high magnetic field (4 Tesla) inside the 13 metres long coil. Only superconductors can achieve this goal, if one wants to avoid a 3 metres thick solenoid! But the high magnetic field also produces high pressure: After powering the magnet, the radial pressure in the CMS Coil reaches 64 atmospheres (equivalent to 64 kg on each cm2)! To avoid building an enormous "pressure vessel" to contain these forces, which in addition would go dangerously through the insulation, an innovative solution has been proposed by CEA: reinforcing the conductor locally where the magnetic pressure is generated.
The complex structure of such a reinforced conductor includes a rectangular cable (the so-called Rutherford type shape) made from 32 strands of niobium titanium superconductor. This is embedded in 99.998 % pure aluminium, forming the so-called insert, which is then reinforced by two sections of aluminium alloy. Steve Horvath from ETHZ and François Wittgen-stein of CERN were at the origin of the first attempts with the CERN Central Workshop colleagues (Thierry Tardy in particular) to obtain such a challenging structure.
The two reinforcement bars are fixed to the insert with the electron beam welding technique. This has the advantage that the energy is concentrated in a narrow melted region which greatly reduces the risk of potential degradation of the superconducting cable due to overheating. Additionally, the metallurgical quality of the weld is excellent. To industrially produce the 50 km of conductor, large and expensive tooling has been employed; in particular, a 80 metres long plant to weld and mill the conductor to its final shape, which has been constructed at TECHMETA in Annecy.
The advantage of such a complicated structure of the conductor over having separate layers of insert and reinforcement is that one has only four layers. This results in high precision both in cylindricity and in the number of turns per module. An oversize of only 0.2 millimetres on the conductor would cause the loss of one full turn, which would reduce the field of the magnet.
Many persons have participated in various phases of the project, but the CMS magnet group would like to mention at least the team who has constantly supervised the production : Bertrand Blau from ETHZ, Benoît Curé, Jean-Paul Grillet, Noël Mezin, Pierluigi Riboni, Sandra Sequeira from CERN, with the help of the EMPA laboratories and the Marti-Supratec AG. This supervision was needed to prevent any problems which could have resulted in the loss of one length of conductor. This would not only have cost about 1 million CHF, but would also have generated incalculable delays!
"The good quality of the conductor has been the starting base for the good quality of the winding that we are experiencing today," says Domenico Campi also in the name of the CMS technical coordinator Alain Hervé.
