The world's largest superconducting solenoid magnet, built for the CMS experiment at the Large Hadron Collider (LHC), reached full field on 28 August. In addition, elements of the detector already in place within the magnet have been successfully recording the tracks of cosmic rays as part of the magnet test and cosmic challenge (MTCC).

The CMS magnet is a marvel of modern technology. Weighing in at more than 10,000 tonnes, the magnet is built around a 6 m diameter, 13 m long superconducting coil that generates a field of 4 T and stores 2.5 GJ of energy. When it was designed in the early 1990s, it was beyond the state-of-the-art. What makes it remarkable is not just its high magnetic field, but also the fact that the field is maintained with high uniformity over such a large volume. New techniques have had to be developed, allowing the solenoid coil to be more compact than 1990s technology could have achieved.

Construction of the magnet was approved in 1996, and began in earnest in 1998. By 2002, fabrication of the superconducting wire was complete. Winding the cable for the solenoid coil began in 2000 and took five years. By the end of 2005, the solenoid was ready for testing, and in February 2006, it was cooled to its operating temperature of 4.5 K. Following the insertion of various particle detectors, the MTCC was ready to begin at the end of July (see CERN Courier July/August 2006 p28).

During these tests, which lasted until the end of August, more than 25 million cosmic events were recorded at a trigger rate of around 200 Hz. It was a big task to provide the trigger, optimize the performance of the various detector systems and ensure the data integrity. Highlights included data transfer to some Tier-1 centres of the LHC Computing Grid and fast off-line running at Fermilab. Now that the maximum field has been reached in this first phase of the MTCC, the next step is to map it with a precision of 1 in 10,000 in the space that will later be filled by the electromagnetic barrel calorimeter and tracking detectors.

• The magnet is a common project to which all 155 institutes of the CMS Collaboration have contributed financially. Major innovative and technical contributions have been made by the French Atomic Energy Commission in Saclay (CEA) for the original concept and general engineering; CERN for the project coordination, all ancillaries, and the magnet yoke and assembly; the Swiss Federal Institute of Technology (ETH Zurich) for the development and production of the compound superconductor and organization of major magnet procurement including the barrel yoke; the US Department of Energy's Fermi National Accelerator Laboratory near Chicago for the superconducting wire and field mapping; the Italian National Institute of Nuclear Physics (INFN) in Genoa for the design and execution of the winding operation; the Russian Institute for Theoretical and Experimental Physics (ITEP) in Moscow; and the University of Wisconsin for the endcap yoke.