Cryogenic structural materials for superconducting magnets Page: 3 of 19
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will use 1G-T toroidal magnets and 7-T poloidal magnets. The present MIT design uses
Nb<yTa toroidal coils and NbTi poloidal coils.
Tandenr-mirror fusion confines the linear-cylindrical plasma at its ends using an
arrangement of high-field magnets. The major tandem-mirror fusion facility in United
States is the Mirror Fusion Test Facility (MFTF-B) located at Lawrence Livermore National
Laboratory (LLNL). In MFTF-B, the plasma is contained in a central tube of solenoid
magnets. The solenoid set is completed at each end by a combination of magnets, with a
peak fields of 2 to 3 T in the solenoid, 12 to 13 T in the small axicells, 5 to 6 T in the
transition coils, and 7 to 8 T in the yin-yang coil set. All magnets are
superconducting. Two of the axicells have NbTi outer solenoids and Nb^Sn inserts, with
NbTi conductors in all other magnets. As of this writing (July 1985), all magnets are
2. Materials Needs
Increasing device size and magnetic field increases loads on magnet structures.
Present systems use existing alloys, future designs will require alloys that offer
superior combinations of yield strength (cr^) and fracture toughness (K^) at 4 K.
Precise materials requirements depend on which of two structural support schemes is
used and on the detailed magnet designs. In the most common support scheme,
superconductors are supported by external magnet cases as in MFTF and by severaL coils in
LCP. In an alternative support design, the superconductors are placed inside
high-strength tubes that provide internal support; consequently, the case is a
low-strength aluminum alloy or stainless steel. The superconductor in this support design
is a cable that is “force-cooled” by flowing He. This type of superconductor is used in
the Westinghouse LCP Nb^Sn coil and is also the current choice for the Alcator DCT and
Conventional austenitic stainless steels are used in external magnet cases [lJ. These
alloyB offer combinations of 0^ and K^c at 4 K (Fig. l) that meet MFTF requirements
and can be welded with available techniques and filler metals , Future MFE magnets
will require superior materials. Figure 1 shows the projected design requirements
developed by Japan Atomic Energy Research Institute (JAERI) for a future tokamak machine
13). These requirements call for combinations that are beyond the
capabilities of existing stainless steels. Additional studies suggest that the JAERI
design goals are more readily available.
Because of conductor manufacturing methods, force-cooled Nb^Sn conductors present
unusual materials requirements [4). For these conductors, a thin sheet io wrapped around
conductor-wires and is longitudinally Beam-welded to make tubes before the wire is
heat-treated to form Nb^Sn. The sheet must have good a' and KjC at 4 R after
welding and heat exposure in the range 600 to 800°C for times to 250 hours. An Fe-base
alloy, JBK-75 (a modified alloy A286), was used for the tubet in the Westinghouse LCP coil
and was aged at 700°C for 30 hours to form Nb^Sn. However, JBK-75 forms undesirable
phases when ag.;d at higher temperatures or for longer time periods. The alloy's
coefficient of thermal contraction is too large, and when cooling to 4 K, the tube strains
the Nb^Sn. New materials are needed for Nb^Sn force-cooled conductor tubes .
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Dalder, E.N.C. & Morris, J.W. Jr. Cryogenic structural materials for superconducting magnets, article, February 22, 1985; [Livermore,] California. (digital.library.unt.edu/ark:/67531/metadc1093083/m1/3/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.