Spectacular physics results are expected to follow in short order once CERN's Large Hadron Collider (LHC) turns on next year, but accelerator physicists are already getting ready to upgrade the machine for higher luminosities. In the US's LHC Accelerator Research Program (LARP), Berkeley Lab, Brookhaven, and Fermilab are collaborating on the development of a new generation of powerful superconducting magnets for the LHC's two main interaction regions, those that house the ATLAS and CMS experiments.

What Paolo Ferracin of the Accelerator and Fusion Research Division's Superconducting Magnet Group calls "a major milestone in this development" came with the test at Brookhaven during the week of July 22 of a model magnet that uses niobium-tin windings in a racetrack configuration, dubbed LRS01 (LRS for Long Racetrack Shell). The coils and their supports are called "cold masses" because the coils must be kept at very low temperatures for superconductivity.

The supporting shell structure for the 3.6-meter (almost 12-foot) test magnet's two racetrack coils, which were fabricated at Brookhaven, was designed and built by Berkeley Lab's Superconducting Magnet Group. LRS01 is the first niobium-tin accelerator-type magnet model significantly more than one meter in length, approaching the length of the real magnets needed for the LHC upgrade. Ferracin describes the support structure as "an extension of Berkeley Lab's Subscale Common-Coil Magnet design, based on an aluminum shell pretensioned over iron yokes using pressurized bladders and locking keys." 

Superconducting magnets must be "trained" to reach their optimum field strengths by repeated deliberate quenchings (loss of superconductivity). The training of LRS01 started above 80 percent of the magnet's estimated maximum current density, and after only five quenchings the current had reached 91 percent of its target. This corresponds to a peak magnetic field of 11 tesla, surpassing the 10-tesla limitation of accelerator-type coils based on niobium-titanium technology. This and other results are evidence, says Ferracin, that "the LRS01 magnet is providing key information for the fabrication of long niobium-tin coils and the optimization of shell-based support structures." 

-- Paul Preuss