On 6 and 7 December a string of powerful superconducting magnets for CERN1's next particle accelerator the Large Hadron Collider (LHC) ran successfully at 8.36 Tesla for 24 hours. 8.36 Tesla is the magnetic field required to accelerate protons to the required energy for LHC and this result demonstrates that the key technical choices made for the construction of the LHC magnets were correct. The test magnets have shown that they can operate reliably under the same working conditions as the future accelerator.
The LHC ring will contain over 1000 superconducting bending magnets, each 14 metres long, and these magnets are among the most technologically challenging components of the machine. For LHC protons to reach their collision energy of 14 TeV (1 Tera electron volt, TeV = 1 million million electron volts) the high technology superconducting magnets have to sustain a magnetic field of 8.36 Tesla, the highest ever used in an accelerator. To achieve this, the cable windings inside the magnets must be cooled to a temperature of 1.8K (-271.2 °C), colder than outer space (2.7K). The validity of the magnet design and the ability of industry to construct reliable magnets is crucial to the LHC project.
In April 1994, one LHC prototype magnet reached the design magnetic field and the next step was to model the conditions of the future accelerator by testing several magnets together. The LHC Test String, one quadrupole and two dipoles, was first cooled to 1.8K with no difficulty, showing the correct functioning of the cryogenic system. On 2 December, the electric current in the magnets was gradually increased to 12,350 A, which corresponds to 8.36 Tesla, the design magnetic field for LHC, and maintained at this level for 30 minutes. On 6 December, in a second experiment, the magnetic field in the string was again raised to 8.36 Tesla and was successfully kept there for 24 hours. At the end of the run, the current was raised until the magnet string attained a field slightly above 8.9 Tesla. This experiment clearly demonstrates that the Test String can operate under the conditions foreseen for the LHC accelerator with a good safety margin.
There are many tests and verifications to be carried out for the full validation of key LHC technical choices, but the success of the Test String is another important milestone in the evolution of the LHC accelerator.