Perfecting Superconducting Niobium for Teravolt and Megawatt Linear Accelerators

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Date/Time:Monday, 15 Nov 2010 from 4:10 pm to 5:10 pm
Location:Physics, Room 5
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Lance Cooley (Superconducting RF Materials Group Leader Technical Division, Fermilab)

While the operation of the Large Hadron Collider at CERN and the final run of the Tevatron at Fermilab hold the attention of particle physicists around the world, engineering plans are being developed for new linear colliders that could one day zoom in on exciting physics of mass, gravity, and the universe. The International Linear Collider (ILC) would make use of some 20,000 superconducting radio-frequency (SRF) cavity resonators, each made from highly pure niobium, to accelerate electrons and positrons along a 40-km collision course to ultimate energies approaching 1 TeV. Superconductivity provides a quality factor that exceeds 1010, five orders of magnitude higher than for copper resonators, so very efficient use of electricity can be realized. This advantage opens up additional possibilities for continuously accelerating protons in beams exceeding 1 MW power, which could be used to produce intense beams of neutrinos (as envisioned by Fermilab's Project X) or drive nuclear reactors while burning spent nuclear fuel. ILC's engineering challenges are daunting: each cavity costs as much as a Ferrari to produce, and almost half of the cavities that roll off of the production line at present require repairs of pits and other defects. The lack of consistency in the present technology stems from complicated fabrication procedures, recipes using hazardous concentrated acids, and empirical understanding about what the target surface should be. New funding and new tools for characterization have started to clarify how different processing steps affect superconducting properties, and I will describe some of the materials science that is guiding advanced processing techniques. Fundamental research conducted by Fermilab and its collaborators has, furthermore, started to build an understanding of how superconducting properties are affected by nanometer-scale features, such as hydrogen-vacancy complexes, roughness, or magnetic defects in the surface oxide, and this understanding could lead to a description of what the perfect SRF cavity surface truly is and how it might be achieved on an engineering scale akin to what is presently done with silicon.