Abstract-The world-wide procurement of Nb 3 Sn and NbTi for the ITER superconducting magnet systems will involve eight to ten strand suppliers from six Domestic Agencies (DAs) on three continents. To ensure accurate and consistent measurement of the physical and superconducting properties of the composite strand, a strand test facility benchmarking effort was initiated in August 2008. The objectives of this effort are to assess and improve the superconducting strand test and sample preparation technologies at each DA and supplier, in preparation for the more than ten thousand samples that will be tested during ITER procurement.The present benchmarking includes tests for critical current ( c ),-index, hysteresis loss ( hys ), residual resistivity ratio ( ), Future benchmarking efforts will include an annual cross-check of supplier and DA facilities, and also a round of internal tin Nb 3 Sn samples to assess each contributor's sample-preparation techniques. A separate round of NbTi benchmarking is also envisioned.
Heat treatments and phase formation of Nb 3 Sn internal tin superconductors are more complicated than bronze route conductors due to the need to convert low melting/low decomposition temperature Sn-rich phases to higher temperature Cu-rich Cu-Sn phases. Conventionally, the Cu-Sn phase development in internal tin wires and hence heat treatment optimization and microstructure control are typically interpreted as a matter of outward Sn diffusion from the Sn core towards the Nb filaments, and Cu diffusion in the opposite direction is simply assumed. In this paper, we present a perspective of Cu diffusion, based on our investigation of phase development. We shall show that the conventional Sn diffusion perspective cannot explain some of our observations, in particular the subelement core phase development. We shall also show that the distribution of Kirkendall pores is opposite to that of the coarse Nb 3 Sn grains, thus establishing a direct relationship between copper diffusion and coarse Nb 3 Sn grain formation and distribution. We shall compare wires of different local Cu:Nb area ratio (LAR) and show how Cu diffusion appears to control the Cu-Sn phase formation across the subelement and the final Nb 3 Sn microstructure (and hence influences the critical current density). Drawing from what we learnt from our observation, we managed to modify a standard heat treatment and obtained up to over 20% improvement in critical current density in some of the wire designs we investigated.
Two-dimensional angular correlation of positron-annihilation radiation in a lithium single crystal has been measured. A continuous determination of the Fermi-surface anisotropy in the (110) crystallographic plane is obtained for the first time in lithium. The maximum anisotropy of the Fermi surface is found to be 2.8 g 0.6o/o.
Abstract-The longitudinal and transverse cross-sectional microstructures of several internal tin strands have been systematically investigated. The critical current densities of these strands were then correlated with their design parameters. It is observed that the occurrence of certain coarse grain structures is related to the location of the filaments with respect to the subelements as well as to the strand. Experimental evidence suggests that the existence of these coarse grains is related to Sn distribution during the early stages of the heat treatment. It is also noticed that some coarse grains have high aspect ratio features, confirming the need to study the longitudinal fracture surface. We report in this paper the observation of some unusual grain sizes and morphologies. It appears that, of the high-Sn content conductors investigated in this paper, a strand's global composition has a weaker influence on the critical current density than local and structural factors, such as the local Cu:Nb area ratio (LAR) and filament layout.
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