The technical systems for CERN’s forthcoming Large Hadron Collider (LHC) reached an important milestone earlier this year with the successful commissioning of String 2 – a chain of prototype LHC magnets complete with all of the necessary powering, control and protection systems. String 2 is the final testbed for validating LHC systems before the new accelerator is installed in its tunnel ready for its start-up in 2006.
When the LHC starts up in five years’ time, it will be the world’s largest superconducting installation of any kind. Nearly all of its main magnets, some 2000 in total, will be bathed in superfluid helium at 1.9 K. Such a low temperature is required to keep the magnets superconducting, but maintaining it presents many challenges. The cold mass of each magnet is installed inside a vacuum vessel and rests on high-tech composite feet that are actively cooled from room temperature to 1.9 K over 25 cm. A similar active cooling scheme is used for the cables that monitor the magnets.
String 2 is the first LHC systems testbed to be built to the LHC’s final design and it is the last chance for LHC engineers to validate their design choices before the installation of the new accelerator underground. In its current configuration, String 2 consists of three prototype dipole magnets, two quadrupoles, and a full-scale prototype distribution line for the cryogenic fluids that cool the magnets. Three more dipoles are scheduled to be added, which will turn String 2 into a full cell of the LHC accelerator. Completing the String 2 set-up are 15 electrical powering circuits with final-design power converters, and a digital current regulation system capable of measuring magnet currents to a few parts per million.
String 2 was cooled down to 1.9 K in mid-September for systems validation tests. The LHC’s superconducting magnets are sensitive devices. If any part of their cable winding heats up, it provokes what is known as a quench – the magnet ceases to superconduct and the energy stored inside it has to be dissipated. Testing the systems that detect quenches and protect the magnets was the first part of the validation programme and was carried out before the magnets were ramped up to nominal current of 11 850 A on 27 September. A full programme of system validation tests in which the entire String is being put through its paces is now under way. All systems are being tested in normal running conditions, during the ramping up and down of the magnet currents, and during provoked quenches.
