Fitting ALICE

The support structures for the detectors inside the ALICE solenoid magnet (the L3 magnet) were finished in December 2003. After commissioning and testing, over the next year, the structures will be lowered into the cavern and installed in the magnet by spring 2005.

At first sight you might mistake them for scaffolding. But a closer look reveals unusual features: Two are made of austenitic (non-magnetic) stainless steel with a cross section that looks like an "H". Another is made of 8 centimetre aluminium square tubes. "Them" are the support structures for the detectors and services inside the ALICE solenoid magnet (the L3 magnet) which were finished in December 2003.
«The physicists don't want to have a lot of material close to their detectors; it has to be as few as possible,» says Diego Perini, who is responsible for the common support structures of ALICE. «We therefore had the very difficult task to design something relatively light that is still able to hold a lot of weight.»

The support structures will carry almost 100 tons of detectors and services inside the ALICE solenoid. The central structure, the so-called Space Frame, is 8 metres in diameter and is 7 metres long. Made of austenitic stainless steel and weighing less than 13 tons, it will hold 77 tons of detectors. The baby Space Frame at the front of the detectors is also made of stainless steel, but is only 2.5 metres long. Intended to hold 10 tons of services, it also has to support parts of the detectors while they are being installed. Weighing only 1 ton, the aluminium back frame behind the detectors is much lighter than the other two frames, since this will only have to hold the backside 7 tons of services for the detectors. The three frames will be positioned on twelve metre long austenitic stainless steel rails spanning the ALICE solenoid from one side to the other.

However, weight was not the only problem faced in designing and building the support structures. As it will be installed inside the ALICE solenoid, the material has to be non-magnetic. Special care must be taken with the welding of the structure, because the welding process disturbs the structure of the material and therefore might introduce small magnetic areas. To make things even more difficult, the welds must fully penetrate the entire profile thickness, to maximise resistance to high stresses introduced by the heavy weight of the detectors. Despite the large deformations due to this technique it is possible to obtain high precision for the welded structures. By optimising the welding procedures, accuracy of a few millimetres for the whole 8 metre wheel can be achieved. "There are more than 600 main welds for the central structure," says Diego Perini. "To obtain the high precision needed, we welded one piece at a time, measured it against reference points on the ground, and corrected it in the next weld."
The metal was extruded and some of the structures pre-fabricated in six companies all over Europe. Welding and final assembly at CERN started about four months ago and was finished by Christmas 2003. Throughout the next year, the structures will be tested using dummies of the detectors, before the real thing is lowered into the cavern and installed in the ALICE solenoid in spring 2005.