A focusing magnet based on niobium-tin superconductor, built by members of the US LHC Accelerator Research Program (LARP), has reached the design gradient of 200 T/m. The US group is working on strategies to upgrade the inner triplet quadrupole magnets that perform the final focusing of the particle beams close to the interaction points.
In an upgraded, higher-luminosity LHC the inner triplets will be subjected to still more radiation and heat than the current magnets are designed to withstand. One of the goals of LARP is to develop upgraded magnets using niobium tin (Nb3Sn), which is superconducting at a higher temperature than the niobium titanium (NbTi) currently used. Nb3Sn therefore has a greater tolerance for heat and can remain superconducting at a magnetic field more than twice as strong. However, it is brittle and sensitive to pressure and to become a superconductor when cold, it must first be reacted at temperatures of 650–700 °C.
The LARP effort initially centred on a series of short quadrupole models at Fermilab and Berkeley and, in parallel, a 4-m long magnet based on racetrack coils, built at Brookhaven and Berkeley. The next step involved the combined resources of all three laboratories on the fabrication of a long, large-aperture quadrupole magnet. In 2005 the US Department of Energy (DOE), CERN and LARP set a goal of reaching, before the end of 2009, a gradient of 200 T/m in a 4-m long superconducting quadrupole magnet with a 90 mm bore for housing the beam pipe.
This goal was met on 4 December 2009 by LARP’s first “long quadrupole shell” model magnet. The magnet’s superconducting coils performed well, as did its mechanical structure, based on a thick aluminium cylinder (shell) that supports the superconducting coils against the large forces generated by high magnetic fields and electrical currents. The magnet’s ability to withstand quenches – sudden transitions to normal conductivity with resulting heating – was also excellent.
• LARP is a collaboration of Brookhaven National Laboratory, Fermilab, Lawrence Berkeley National Laboratory and the SLAC National Accelerator Laboratory, founded by the DOE in 2003 to address the challenge of planned upgrades to the LHC’s luminosity.
