Niobium tin magnet performance scales up
Giorgio Ambrosio, leader of the LARP lonq-quadrupole R&D program, wrote this week's column.
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Giorgio Ambrosio
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While the performance of the Large Hadron Collider at CERN is improving almost daily, members of the US LHC Accelerator Research Program collaboration already are developing the next generation of superconducting magnets that may allow the LHC to exceed its initial design goals.
The key to this improvement is niobium tin (Nb3Sn), a material that is superconducting at higher temperatures and magnetic fields than niobium titanium (NbTi), which is presently used in LHC superconducting magnets.
But using the better performing material comes at a price: niobium tin is a brittle material with superconducting properties that can degrade under strain. It requires different coil-fabrication technologies and magnet-assembly procedures from the traditional niobium titanium. In particular, its brittleness and strain sensitivity make the fabrication of long coils-significantly longer than 1 meter—even more challenging.
The LARP collaboration, which comprises magnet experts at Berkeley, Brookhaven, Fermilab and SLAC national laboratories, achieved an important milestone in December 2009 when the first four-meter-long quadrupole magnet with niobium-tin coils (LQS01) reached a magnetic field gradient of 200 Tesla per meter in a test here at Fermilab.
But the collaboration knew that it could achieve more. In earlier tests with one-meter-long magnets, the best magnets had achieved 222 T/m. So, the collaboration set out to show that it is possible to scale up the length of these superconducting magnets without a loss in performance. To make our point, we took apart the LQS01 magnet and reassembled its four coils, applying a higher and more uniform pre-load on the structure. When we tested the magnet earlier this month at a temperature of 4.5 K, we reached our goal of 222 T/m.
This exceptional result by the LARP collaboration is the fruit of an extraordinary effort to which Fermilab's Technical Division makes many contributions, including coil design and fabrication; coil-splice assembly; conductor tests; magnet test preparation and magnet tests; and project leadership.
Future LARP plans include demonstrating the reproducibility of the four-meter test results with new coils made with the same superconducting material; improving magnet performance by using a conductor with thinner filaments; and developing a coil insulation suited for the production of even longer coils.
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