December 16, 2009

More Powerful Superconducting Magnets will Make More Powerful Particle Colliders

The completed long quadrupole shell magnet (LQS01) in the Building 77A assembly area of Berkeley Lab's Engineering Division.

The Large Hadron Collider (LHC) at CERN has just started producing collisions, but scientists and engineers have already made significant progress in preparing for future upgrades beyond the collider’s nominal design performance, including a 10-fold increase in collision rates by the end of the next decade and, eventually, higher-energy beams.

Increased luminosity will mean more collision events in the LHC’s interaction regions; the major experiments will thus be able to collect more data in less time. But it will also mean that the “inner triplet” magnets, which focus the beams to tiny spots at the interaction regions and are within 20 meters of the collision points, will be subjected to even more radiation and heat than they are presently designed to withstand.

The superconducting inner triplet magnets now in place at the LHC operate at the limits of well-established niobium-titanium (NbTi) magnet technology. One of the LARP goals is to develop upgraded magnets using a different superconducting material, niobium tin (Nb3Sn). Niobium tin is superconducting at a higher temperature than niobium titanium and therefore has a greater tolerance for heat; it can also be superconducting at a magnetic field more than twice as strong.

Unlike niobium titanium, however, niobium tin is brittle and sensitive to pressure; to become a superconductor when cold it must be reacted at very high temperatures, 650 to 700 degrees Celsius. Advanced magnet design and fabrication methods are needed to meet these challenges

The LARP effort initially centered on a series of short quadrupole models at Fermilab and Berkeley Lab and, in parallel, a four-meter-long magnet based on racetrack coils, built at Brookhaven and Berkeley Lab. The next step involved the combined resources of all three laboratories: the fabrication of a long, large-aperture quadrupole magnet.

In 2005 DOE, CERN, and LARP agreed to set a goal of reaching, before the end of 2009, a gradient, or rate of increase in field strength, of 200 tesla per meter (200 T/m) in a four-meter-long superconducting quadrupole magnet with a 90-millimeter bore for housing the beam pipe.

This goal was met on December 4 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 aluminum 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 – also was excellent.

Although the successful test of the long model was a major milestone, it is only one of several steps needed to fully qualify the new technology for use in the LHC. One goal is to further increase the field gradient in the long quadrupole, both to explore the limits of the technology and to reproduce the performance levels demonstrated in short models. A second goal is to address other critical accelerator requirements, such as field quality and alignment, through a new series of models with an even larger aperture (120 millimeters).

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