Nb3Sn material development in Russia

From February 2007 to May 2008, 18 short length conductor sections have been tested in SULTAN for design verification and manufacturer qualification of the ITER Toroidal Field (TF) conductor. The test program is focussed on the current sharing temperature, T-cs, at the nominal operating conditions, 68 kA current and 11.15T effective field, which can be fully reproduced in the SULTAN test facility. A broad range of results was observed, with over 2 K difference among the T-cs of the conductors. In average, the results are poorer compared to the potential performance estimated from the strand scaling law. The key parameters to mitigate the degradation are not yet clearly identified. The experimental challenges to test conductors with performance degradation are highlighted, including enhanced instrumentation sets, the application of gas flow calorimetry to sense the current sharing power and the post-processing of voltage data to cancel the transverse potential across the cable. The updated schedule of the tests in SULTAN is presented with the short-term action plan for conductor test. (C) 2008 Elsevier B.V. All rights reserved.

Qualification tests for ITER TF conductors in SULTAN

The results on research of the recent design Nb3Sn bronze strand with increased critical current capacity are presented. The strand has been designed and fabricated for use in full scale ITER TF conductor sample for testing in the SULTAN facility. The verification and qualification of strands have been done. The critical current measurements as a function of temperature in the range from 4.2 K to 15 K in magnetic fields up to 12 T have been performed. The results of testing of current sharing temperature Tcs of strand are discussed. The cable in conduit conductor was manufactured in accordance with ITER recommended scheme option 2. The cable design, thickness of wraps of subcable, void fraction and other characteristics of conductor for TF coil have been investigated to produce sample for testing in the SULTAN facility.

Development of the ${\rm Nb}_{3}{\rm Sn}$ Bronze Strand of TF Conductor Sample for Testing in SULTAN Facility

The structure and morphology of superconducting Nb3Sn layers in multifilamentary internal tin (IT) wires have been studied by transmission electron microscopy and scanning electron microscopy. The composites under study differed in the initial Sn concentration, the presence of strengthening Cu-Nb inserts, and the number of Nb filaments and their diameter in the final wire. In all the analyzed wires, columnar Nb3Sn grains were always present. The Nb3Sn grains are coarser and their size distribution is wider in the filaments of IT strands compared with the bronze-processed wires, the Jc in the former being as high as 2276 A/mm2. Much lower Jc values (850 A/mm2) were observed in a wire with a smaller diameter and a higher number of Nb filaments, which is not only due to the lower Sn concentration in the matrix but also to larger sizes and size scattering of equiaxed Nb3Sn grains, as well as to wider zones of columnar grains.

Effect of Diffusion Annealing and Design of Internal Tin Wires on the Structure and Morphology of Superconducting Nb 3 Sn Layers

The microstructure of Nb3Sn layers in multifilamentary composites manufactured by bronze technology and the internal-tin (IT) method differing in the strand design has been studied. The IT strands of International Thermonuclear Experimental Reactor (ITER) type and high-Jc type are characterized by both the higher Sn concentration and the higher fraction of equiaxed grains than bronze-route composites. In all the samples under study, the fraction of equiaxed grains correlates with the average concentration of Sn in Nb3Sn layers. The Ti doping of the bronze matrix in the bronze-processed wires results in the reduction in average grain sizes of Nb3Sn grains and in a higher fraction of equiaxed grains with a higher Sn concentration, which leads to a higher non-Cu Jc (calculated as a ratio of critical current Ic to a cross-sectional area of a strand without stabilizing Cu), 997 A/mm2 at 12 T, 4.2 K, compared to a composite of the same design with artificially Ti-doped Nb filaments. The ITER type IT strand demonstrates a somewhat higher fraction of equiaxed Nb3Sn grains with much larger average grain sizes (120 nm). The grain coarsening in this IT strand results in a decrease in Jc compared to other ITER type strands studied. The highest values of Jc are ensured in the high-Jc type of IT wires with 7 extended tin sources by the highest fraction (92%) of equiaxed Nb3Sn grains despite their coarser sizes (92 nm).

Structure of superconducting layers in bronze-processed and internal-tin Nb3Sn-based wires of various designs

The results are presented on characterization of internal-tin Nb3Sn strands designed and fabricated for use in full scale ITER TF conductor sample. The microstructure of strands has been investigated and interpreted in correlation with measured dependences of Jc (B) and Jc(B, T). The process of cabling in accordance with ITER recommended scheme have been successfully performed. The alternative new design of TF conductor with lower voids fraction was proposed and experimentally verified. The analysis of ITER related performance of both designed and fabricated conductors has been done on the base of SULTAN samples testing.

Development of a New RF Produced Internal Tin ITER TF Conductor Sample for Testing in SULTAN Facility

A hybrid composite material fabricated from Al-Mg system using high-pressure torsion (HPT) with 10, 20 and 30 anvil rotations was studied in the present work. Fabrication of three-layer Al-Mg-Al and Mg-Al-Mg composites was performed at room temperature on Bridgman anvils in constrained mode under a pressure of 5 GPa and N=10, 20 and 30 revolutions, at a rotation speed of ω=2 rpm. In case of Mg-Al-Mg system bulk composite structure was not formed during HPT. The obtained monolithic samples had a layered structure. In case of Al-Mg-Al disc set, monolithic composite samples were formed after N=10 and 20 anvil rotations. Increase of rotations number to N=30 results in the loss of bulk structure due to an emergence of cracks in areas between the layers. The most intensive fragmentation and mechanical alloying of Mg in Al were observed after HPT with N=20 revolutions. Three zones with different microstructure types were observed after HPT: the central zone where mixing of the components did not occur, the mid-radius zone with vortice-type foldings, and peripheral zone with a homogeneous distribution of magnesium in the aluminium matrix. The microhardness measured along the diameter of the obtained composite materials changed nonmonotonically. Obtained results contribute to fundamental understanding of processes taking place during mechanical alloying induced by HPT. 

Selection of high pressure torsion processing modes for fabrication of Al-Mg metal matrix composites

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Development of a New RF Produced Internal Tin ITER TF Conductor Sample for Testing in SULTAN Facility

Selection of high pressure torsion processing modes for fabrication of Al-Mg metal matrix composites