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Title: The effect of heat treatment on the stability of Nb 3Sn RRP-150/169 strands

Abstract

Here, the magnetic stability of superconductor strands and cables is a key issue in the successful building and operation of high-field accelerator magnets. In this paper, we report the study of a state-of-the-art 0.7 mm Nb 3Sn restacked-rod-process strand manufactured by Oxford Instrument Superconductor Technology. This conductor will be used in Rutherford cable for a 15-T Nb 3Sn dipole demonstrator being built at Fermi National Accelerator Laboratory. Particularly, this study focuses on the impact of varying heat treatment conditions on the stability of the strand. Both the stability against internal flux jumps and external thermal perturbations are studied.

Authors:
 [1];  [1];  [1];  [1]
  1. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1343951
Report Number(s):
FERMILAB-CONF-17-032-TD
Journal ID: ISSN 1051-8223; 1513430
Grant/Contract Number:
AC02-07CH11359
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
IEEE Transactions on Applied Superconductivity
Additional Journal Information:
Journal Volume: 27; Journal Issue: 4; Journal ID: ISSN 1051-8223
Publisher:
Institute of Electrical and Electronics Engineers (IEEE)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY

Citation Formats

Li, Pei, Turrioni, Daniele, Barzi, Emanuela, and Zlobin, Alexander. The effect of heat treatment on the stability of Nb3Sn RRP-150/169 strands. United States: N. p., 2017. Web. doi:10.1109/TASC.2017.2662938.
Li, Pei, Turrioni, Daniele, Barzi, Emanuela, & Zlobin, Alexander. The effect of heat treatment on the stability of Nb3Sn RRP-150/169 strands. United States. doi:10.1109/TASC.2017.2662938.
Li, Pei, Turrioni, Daniele, Barzi, Emanuela, and Zlobin, Alexander. Fri . "The effect of heat treatment on the stability of Nb3Sn RRP-150/169 strands". United States. doi:10.1109/TASC.2017.2662938. https://www.osti.gov/servlets/purl/1343951.
@article{osti_1343951,
title = {The effect of heat treatment on the stability of Nb3Sn RRP-150/169 strands},
author = {Li, Pei and Turrioni, Daniele and Barzi, Emanuela and Zlobin, Alexander},
abstractNote = {Here, the magnetic stability of superconductor strands and cables is a key issue in the successful building and operation of high-field accelerator magnets. In this paper, we report the study of a state-of-the-art 0.7 mm Nb3Sn restacked-rod-process strand manufactured by Oxford Instrument Superconductor Technology. This conductor will be used in Rutherford cable for a 15-T Nb3Sn dipole demonstrator being built at Fermi National Accelerator Laboratory. Particularly, this study focuses on the impact of varying heat treatment conditions on the stability of the strand. Both the stability against internal flux jumps and external thermal perturbations are studied.},
doi = {10.1109/TASC.2017.2662938},
journal = {IEEE Transactions on Applied Superconductivity},
number = 4,
volume = 27,
place = {United States},
year = {Fri Feb 17 00:00:00 EST 2017},
month = {Fri Feb 17 00:00:00 EST 2017}
}

Journal Article:
Free Publicly Available Full Text
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  • From 2009 the mass production of the Nb{sub 3}Sn strands for ITER with the yield of several tens of tons per year operates at JSC Chepetsky Mechanical Plant (Glazov, Russia). In order to enhance the stability of output characteristics of the produced Nb{sub 3}Sn strands, to increase the Nb filaments dimensional homogeneity the manufacture regimes improvement of the used semiproducts such as Nb rods intended for the superconducting filaments formation in the finished strands has been carried out. In the work the investigations of the Nb rheological behavior, the influence of heat treatment in the wide temperature range from 700more » to 1300 °C on the predeformed Nb rods structure and mechanical properties have been performed. Different production routes of the Nb rods, including such operations like forging, extrusion and drawing combined with the recrystallization annealings, were used. Composite Nb{sub 3}Sn strands have been produced and their electrophysical properties have been tested. For the first time influence of the niobium rods manufacture regimes on the current carrying capacity of the industrial Nb{sub 3}Sn strands has been investigated.« less
  • Dipole magnets for the proposed Future Circular Collider (FCC) demand specifications significantly beyond the limits of all existing Nb 3Sn wires, in particular a critical current density (J c) of more than 1500 A mm -2 at 16 T and 4.2 K with an effective filament diameter (D eff) of less than 20 μm. The restacked-rod-process (RRP ®) is the technology closest to meeting these demands, with a J c (16 T) of up to 1400 A mm -2, residual resistivity ratio > 100, for a sub-element size D s of 58 μm (which in RRP ® wires is essentiallymore » the same as D eff). An important present limitation of RRP ® is that reducing the sub-element size degrades J c to as low as 900 A mm -2 at 16 T for D s = 35 μm. To gain an understanding of the sources of this J c degradation, we have made a detailed study of the phase evolution during the Cu–Sn 'mixing' stages of the wire heat treatment that occur prior to Nb 3Sn formation. Using extensive microstructural quantification, we have identified in this paper the critical role that the Sn–Nb–Cu ternary phase (Nausite) can play. The Nausite forms as a well-defined ring between the Sn source and the Cu/Nb filament pack, and acts as an osmotic membrane in the 300 °C–400 °C range—greatly inhibiting Sn diffusion into the Cu/Nb filament pack while supporting a strong Cu counter-diffusion from the filament pack into the Sn core. This converts the Sn core into a mixture of the low melting point (408 °C) η phase (Cu 6Sn 5) and the more desirable ε phase (Cu 3Sn), which decomposes at 676 °C. After the mixing stages, when heated above 408 °C towards the Nb 3Sn reaction, any residual η liquefies to form additional irregular Nausite on the inside of the membrane. All Nausite decomposes into NbSn 2 on further heating, and ultimately transforms into coarse-grain (and often disconnected) Nb 3Sn which has little contribution to current transport. Understanding this critical Nausite reaction pathway has allowed us to simplify the mixing heat treatment to only one stage at 350 °C for 400 h which minimizes Nausite formation while encouraging the formation of the higher melting point ε phase through better Cu–Sn mixing. Finally, at a D s of 41 μm, the Nausite control heat treatment increases the J c at 16 T by 36%, reaching 1300 A mm -2 (i.e. 2980 A mm -2 at 12 T), and moving RRP ® closer to the FCC targets.« less
  • Reaction heat treatments spanning 605 to 750 C and 24 to {approx}400 hours were applied to several sets of RRP strands. Magnetization and transport measurements were used to track the changes of superconducting properties and the amounts of Nb{sub 3}Sn formed. The experiments showed that temperature increases of 15 C produced equivalent properties in half the time within the temperature range 620-680 C. This result was the same whether Ta or Ti was used to alloy the Nb{sub 3}Sn. The bulk pinning force Fp for Ta-alloyed wires displayed a significant drop for temperatures outside the range above, due to tinmore » gradients at low temperatures and grain growth at high temperatures. The F{sub p} drop at high reaction temperatures prevents wire technology from taking advantage of significantly higher Kramer-plot intercept H{sub K} for high-temperature reactions. On the other hand, Ti alloying provides a quick and potent means to increase H{sub K} at reaction temperatures for which F{sub p} remains high.« less
  • Keystoned Rutherford cables made of 28 strands and with a stainless steel core were developed and manufactured using 1 mm Nb3Sn composite wires produced by Oxford Superconducting Technology with 127 and 169 restacks using the Restacked-Rod-Process ®. Furthermore, the performance and properties of these cables were studied to evaluate possible candidates for 15 T accelerator magnets.