The title of "largest superconducting toroid magnet in the world" has traditionally been bestowed on the magnets of nuclear fusion reactors. However, that all changed in September when engineers from France's CEA-Saclay and Italy's INFN-LASA put the finishing touches to the ATLAS experiment's barrel module zero (B-0) toroid coil at CEA laboratories near Paris.

The ATLAS collaboration, which is preparing to do physics at CERN's Large Hadron Collider, is building a particle detector like none before. Instead of constructing a compact detector based around a solenoid magnet, ATLAS has opted to use a large air-cored toroidal system enclosing a small central solenoid. Not surprisingly, the collaboration's B-0 toroid coil is a prototype like no other. At 9 m in length it is already by far the largest toroid coil ever built, but it will be dwarfed by the eight 25 m coils forming the toroid of the final magnet system. Its purpose has been to test each stage of the manufacturing process, with the results from each step being fed directly into the manufacture of the larger modules - already well under way. The rationale is that to go from the 5 m dimensions of existing toroids to 25 m in a single step would be too much of a leap into the unknown. CERN therefore entered into partnership with CEA-Saclay and INFN-LASA to produce a coil of intermediate size. Supported by the ATLAS collaboration, CEA and INFN have worked together to finance and build the device, which was delivered to CERN in October for testing to begin early next year.

Although an important milestone for ATLAS in their own right, the tests represent just a small part of the B-0 coil's importance to the ATLAS magnet project. Fabrication of its superconductor was complete by 1998, and with lessons learned from that, production of the superconductor for the full-scale toroids began the same year. Similarly, coil winding for the B-0 coil immediately fed into the winding of the full-scale coils, B-0 cryostat manufacture fed into the full-scale cryostats, and the recently learned lessons of integrating the 9 m coils into their cryostat will soon be helping the companies making the full-scale toroid coils with their integration.

All along, progress on the full-scale coils has proceeded in lock-step with that of their smaller relative, to the extent that when CEA and INFN were preparing to integrate their coil and cryostat in July, the Italian firm Ansaldo was putting the finishing touches to the first of its full-scale coil windings. ATLAS's first 25 m toroid coil is due to arrive at CERN by the end of 2001 for testing at a recently-completed test facility. By that time the B-0 will have fulfilled its final role as commissioning coil of the new test set-up.