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MSC Seminars
Ways for enhancing Jc above 16T in Nb3Sn wires in view of FCC
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Europe/Zurich
30/7-010 (CERN)
Description
Nb3Sn wires are analyzed as possible candidates in view of the FCC accelerator, the question being whether Jc 4.2K above 16 T can be sufficiently enhanced above the highest known values. The potential of Nb3Sn is high: it will be shown that Jc could in principle be enhanced by a factor 4 if both, grain boundary and point pinning could be optimized. Strong Jc enhancements have partially been reached in laboratory wires, but the practical difficulties in transferring these effects on the nanometric scale to industrial wires of several km length are very high, and have so far not been solved. The main problems on the way towards this goal are discussed.
The present discussion starts with the effect of metallic Ta or NbTi nano-inclusions on Jc. These experiments were performed 30 years ago in my laboratory at KIT in Karlsruhe and showed on bronze route wires a marked enhancement of Jc by a factor ~ 1.7 at 4.2K/12T, but were given up after the discovery of HTS materials. The number of deformation steps is higher than for conventional wire processing routes, involving enhanced cold hardening: this causes problems at the very last steps of the deformation cycle, where the distances approach coherence length, the artificial pinning effect being highest. A partial reaction of the inclusion material is observed, but can be compensated and does not constitute a real problem.
The most recent progress was reached in 2014 at the Ohio State University, after a successful transformation of the Zr in the Nb-1%Zr alloy in direct contact with SnO2 to ZrO2 (internal oxidation): the presence of these nano-inclusions during the Nb3Sn reaction process was found to limit the A15 grain size to values of the order of 30 nm: the maximum of the pinning force Fp vs. b is shifted to higher values, reflecting simultaneous point pinning. The value of 9’600 A/mm2 at 4.2K/12T in the A15 layer is twice as high as the current highest value in Internal Sn wires. These results were obtained on monofilamentary wires: in mutifilamentary wires, the homogeneous reduction of the intermediate SnO2 layers to values around 1 m on km long wires will constitute a major problem.
Possible fabrication routes for both methods are sketched.
