Advances in superconducting strands for accelerator magnet application

Abstract

Considerable advances have recently been obtained in the critical current densities J_c of Nb₃Sn based superconductors - the prime candidates for the next generation of superconducting accelerator magnets. The non-Cu critical current densities now approach 3000 A/mm² at 12 T and 4.2 K in engineering quality strand. The design of new strands minimizes the amount of Cu in the package from which the Nb₃Sn is formed and increases the Sn level beyond that required to simply achieve A15 stoichiometry. The result is an A15 layer that is significantly more uniform than earlier generations of wire, both chemically and microstructurally, and wires that significantly surpasses previous Nb₃Sn strands in layer critical current density and in the specific grain boundary pinning force. Remarkably, these developments have been achieved in internal Sn based strands manufactured using both the modified jelly-roll technique with Nb-Ti alloy and the rod-in-tube approach with Nb-Ta alloy. The rod-in-tube approach is particularly exciting because it offers greater manufacturing flexibility. Advances have also been made in strand designs that offer the potential to reduce the large effective filament diameters, which are an issue with these new high-J_c strands. We review the latest developments in Nb₃Sn superconductors and compare their performance and potential with other round-wire high-field superconductors.

Full text

Page 1

ADVANCES IN SUPERCONDUCTING STRANDS FOR ACCELERATOR
MAGNET APPLICATION
Peter J. Lee and David C. Larbalestier, Applied Superconductivity Center, University of Wisconsin-
Madison, Madison, WI 53706-1609, USA
Abstract
Considerable advances have recently been obtained in
the critical current densities Jc of Nb3Sn based
superconductors - the prime candidates for the next
generation of superconducting accelerator magnets. The
non-Cu critical current
3000 A/mm² at 12 T and 4.2 K in engineering quality
strand. The design of new strands minimizes the amount
of Cu in the package from which the Nb3Sn is formed and
increases the Sn level beyond that required to simply
achieve A15 stoichiometry. The result is an A15 layer that
is significantly more uniform than earlier generations of
wire, both chemically and microstructurally, and wires
that significantly surpasses previous Nb3Sn strands in
layer critical current density and in the specific grain
boundary pinning force. Remarkably, these developments
have been achieved in internal Sn based strands
manufactured using both the modified jelly-roll technique
with Nb-Ti alloy and the rod-in-tube approach with Nb-Ta
alloy. The rod-in-tube approach is particularly exciting
because it offers greater manufacturing flexibility.
Advances have also been made in strand designs that offer
the potential to reduce the large effective filament
diameters, which are an issue with these new high-Jc
strands. We review the latest developments in Nb3Sn
superconductors and compare their performance and
potential with other
superconductors.*
densities now approach
round-wire high-field
INTRODUCTION
The LHC marks the end of a series of colliders that have
capitalized on increasing current densities available in
Nb-Ti alloy based superconducting strand. Nb-Ti has
proven to be a remarkably durable superconductor,
dominating superconducting magnet design from the
FNAL Tevatron (on-line in 1983) to the LHC (expected
completion 2006). Multifilamentary superconducting
strands based on Nb-Ti alloys are strong, ductile, and
relatively inexpensive but are limited in operation to
fields below ~11 T (2 K). In Figure 1 we compare the
critical current density variation with applied magnetic
field for superconductors of interest to accelerator magnet
designers. With the exception of the Bi2223 tapes and the
MgB2-SiC [1] data, the critical current densities shown

* This work was supported in part by the U.S. Department of Energy
under Grants DE-FG02-91ER40643 (High Energy Physics) and DE-
FG02-86ER52131 (Office of Fusion Energy Sciences).
The authors may be contacted at the Applied Superconductivity Center
at the University of Wisconsin-Madison, Madison WI 53706, USA
(phone: 608-263-1760; fax: 608-263-1087; e-mail: peterlee@wisc.edu).
D. C. Larbalestier is also with the Department of Materials Science and
Engineering and Department of Physics.
are available in multifilamentary round-wire form suitable
for magnet fabrication. Development of superconductors
for accelerator magnets with fields greater than 11 T has
focused on Nb3Sn. Being brittle, the A15 structure of
Nb3Sn must be made from ductile components that can be
drawn to wire, meaning that, unlike Nb-Ti, the materials
package needed to make the superconductor contains
more than just the A15 filament. Raising the Jc of Nb3Sn
is then both a question of maximizing the quantity of A15
within this package and of optimizing the A15 properties.
The latest generation of Nb3Sn strands can support non-
stabilizer critical current densities in excess of 1000
A/mm² at fields up to 17 T at 4.2 K. Bi-2212, also a brittle
material, can support ~1000 A/mm² out beyond 28 T.
However, whereas Nb3Sn conductors can be made with
small Cu-stabilizer cross-sections, at present the
engineering critical current density, Je, of Bi-2212
10
100
1,000
10,000
0510152025 30
Applied Field, T
Critical Current Density, A/mm²
2212 round wire
2223
tape B|_
1.8 K
Nb-Ti
Nb3Sn Internal Sn
1.8 K
Nb-Ti-Ta
Nb3Sn
1.8K
NbTi
2223
tape B||
Nb3Sn
ITER
Nb3Al 2-stage JR
Nb3Al: ITER
Nb3Sn
PIT
MgB2-SiC
Figure 1: A comparison of critical current density with
applied magnetic field for superconductors of actual or
potential interest for accelerator magnets.
0-7803-7739-9 ©2003 IEEE 151
Proceedings of the 2003 Particle Accelerator Conference

Page 2

conductors is still much reduced by the large Ag area in
the strand. The latest generation of strand from OI-ST
achieves Je of 600 A/mm² at 12 T, 4.2 K with a 28 % Bi-
2212 area [2]. Nb3Al offers a high strength alternative but
is much more expensive to manufacture than Nb3Sn,
which has not, so far, shown strength limitation in well
designed high field accelerator magnets. MgB2 has future
long term potential for low cost and exhibits critical
current densities that, although presently lagging behind
Nb3Sn, continue to show progress [e.g. 1]. A remarkable
development in the past year has been the enhancement of
the upper critical field by manipulation of the resistivity,
with the upper critical field, Hc2, vs. temperature surface
exceeding Nb3Sn. [3]. In previous reviews we have
examined the potential
Superconductors [4] and MgB2 [5] to high energy physics;
in this paper we will focus on developments in Nb3Sn.
for High Temperature
PROGRESS IN NB3SN
Of the available superconductors, Nb3Sn is the closest
to targets set for the next generation of accelerator
magnets [6]. The critical current density, Jc, (non-Cu,
12 T, 4.2 K) is very close to the 3000 A/mm² target,
although the residual resistance ratio, RRR, is very low
(2-13) in recent billets, due to diffusion barrier through-
reaction in most cases. The effective filament size is still
2-3 times the 40 µm target, except for strands fabricated
by the powder-in-tube (PIT) process. Piece length is
reasonable good, considering the (250-1500 m) small
billet sizes and the developmental nature of the strand for
which two billets are rarely identical in design. New, more
scaleable designs are showing promising results, for
instance the Rod Restack Process at OI-ST that has
achieved 2900 A/mm² (12 T, 4.2 K). There has been little
progress in reducing heat treatment times for internal Sn
to less that 150 hrs but PIT can be optimized at the desired
50 hr reaction. PIT wire costs are still limited by the small
production scale and technical difficulties in scale-up.
Nb3Sn has a big advantage over competing
superconductors with respect to large-scale strand, cable
and magnet production experience, such as record-setting
Soft Sn distorts to
outer circle
Figure 2: Cu-split sub-element high Jc strand produced by
IGC-AS (now Outokumpu Advanced Superconductors).
Figure 3: Hot Extruded Rod billet cross-section Image
courtesy of Jeff Parrell, OI-ST.
Figure 4: Single-stack, Internal Tin conductor “MEIT”
manufactured by Supergenics LLC/Outokumpu Advanced
Superconductors under a DOE-SBIR program. The single
central Sn core reduces Sn distortion. It incorporates Nb-
Ta fins to reduce the effective filament diameter. Image
courtesy of Bruce Zeitlin of Supergenics, LLC.
152
Proceedings of the 2003 Particle Accelerator Conference

Page 3

dipole magnets at LBNL, the huge (150 ton, 13 T) ITER
Central Solenoid Model Coil (ITER-CSMC)[7], It is also
available from multiple vendors worldwide. Although the
brittleness of Nb3Sn dictates that it be used in the wind-
and-react configuration for small-radius magnets, it can
also be used in the react-and-wind approach for larger
magnets [8].
Issues with Sn elements in Conductors
The use of elemental or weakly alloyed Sn has been
crucial to the production of high critical current densities
in internal Sn and even PIT strands. Higher Sn content
pushes the Sn:Nb ratio in the A15, never always Nb3Sn
composition, closer to the stoichiometric ratio of highest
Hc2 and Tc [9]. Sn however is soft and not only becomes
distorted itself during final wire drawing but also distorts
the surrounding filament pack (see Figure 1), limiting the
amount of post-billet-assembly processing that can be
performed. Furthermore, in order to achieve useful piece
length, the filament pack must be metallurgically bonded,
which normally requires warm processing incompatible
with the low melting point Sn (232 °C). One way around
this problem is to assemble and warm-extrude the sub-
element with Cu in the place of the Sn and then drill a
hole in the Cu for the Sn after the other components are
bonded. However, the smallest gun-drill diameter in a
typical full-length 0.9 m billet is 4.7 mm. Taking account
of the Sn distortion after assembly, this limits the number
of sub-elements that can be stacked into a final billet
without severe piece length problems.
One method to circumvent this problem being explored
by OI-ST, uses salt in the place of the Sn, allowing hot
extrusion of both the sub-element and final composite
assemblies [10]. This process, termed HER for Hot
Extruded Rod, is illustrated by the billet cross-section
shown in Figure 3. After the first extrusion, the salt cores
can be washed away and replaced with Sn at a late stage
in the processing. With hot extrusion permitted for both
sub-element and final composite assemblies, this process
has excellent scalability.
The lack of Sn distortion in the center of the composite
in Figure 2 suggests another alternative. The Mono
Element Internal Tin conductor “MEIT” [11], uses a
single central Sn core with a concentric extrusion-bonded
Nb/Cu filament stack (gun-drilling of the solid Cu core is
used to introduce the Sn after extrusion). Because there is
now only one sub-element, the strand must be drawn to
fine wire (< 0.2 mm diameter). Although such a fine wire
may require a 2-level cable, the high symmetry means
minimal Sn distortion (see Figure 4), high potential for
good piece length, and low cost.
Issues with Sub-Element Size
To produce high overall Jc, the non-Cu sub-element
must have a minimum of Cu filler and a high Sn
concentration. This can enhance the A15 fraction to ~0.5
and dramatically enhances the layer critical current
density. Figure 6 contrasts the high-Cu, low hysteresis
loss ITER-CSMC strands with recent, low-Cu, “high-Jc“
designs. Although some ITER-CSMC strands had high
Sn:Nb ratios, the additional Cu required to isolate the
individual filaments for low hysteresis loss (see Figure
5a) resulted in inferior Nb3Sn-layer quality, even though
the overall Nb3Sn composition difference between the 2
designs was only ~1 at.% Sn [12]. Unfortunately the low-
Cu requirement for highest Jc results in complete physical
bonding of the individual Nb3Sn filaments into a
continuous ring of Nb3Sn (see Figure 5b). Not only is the
effective filament diameter, deff, increased by physical
joining of the filaments but the core of the sub-element is
Figure 5: Comparison of Sub-element cross-sections in a)
a high-Cu low hysteresis loss ITER-CSMC MJR strand
fabricated by TWC, and b) a low-Cu, high Jc MJR strand
fabricated by OI-ST.
0
2000
4000
6000
8000
10000
12000
14000
789 1011121314 1516
Applied Field, T
Layer Critical Current Density, A/mm²
High Jc Internal Sn IGC
EP2-1-3-2 700°C HT
High Jc Internal Sn:
ORe110(695/96)
SMI-PIT Nb-Ta Tube: 64
hrs@675 °C-small grains
ITER: High Jc Internal Sn
TWC1912
504 Filament SMI-PIT,
small grains only
ITER: Mitsubishi Internal
Sn
ITER: LMI Internal Sn
ITER: Furukawa Bronze
Process
ITER: VAC 7.5% Ta
Bronze Process
OI-ST 6445 RRP 0.9 mm
(Parrell et al. ASC2002)
"High Jc"
ITER CSMC
Figure 6: Comparison of the layer critical current
densities for ITER-CSMC (low hysteresis loss) and recent
“high Jc” Nb3Sn composites. Note how different the
intrinsic performance of the Nb3Sn is.
153
Proceedings of the 2003 Particle Accelerator Conference

Page 4

also shielded, making the entire sub-element cross-section
behave as one and proportionately increasing the loss.
This small size of R&D billets and the problem of Sn
distortion limits the restack numbers of sub-elements so
far to 18-37. By using simple scaling arguments, we can
see that this limits the sub-element diameters to >100 µm
(Figure 7), well above that desired for low hysteresis loss
for both HEP and fusion magnet use.
Supergenics LLC has developed a sub-element splitting
technology using Nb-Ta dividers (patent pending) that
should “un-shield” the sub-element core by subdividing
the sub-element filament pack. In Figure 7 we model the
effect of “un-shielding” the core with one Nb-Ta divider,
as well as the added benefit of un-shielding the core and
splitting the A15 volume with two dividers. Figure 7
shows that there must be at least 60 sub-elements to
approach the desired 40 µm effective filament size target.
Although 504-filament PIT conductors which exceed the
40 µm target have been made, this goal seems much more
feasible for internal Sn than experience with internal Sn is
much more widespread than with PIT.
The first attempts by Supergenics LLC/Outokumpu
Advanced Superconductors under a DOE-SBIR program
at using such Nb-Ta dividers has been successful in
producing composites of 18 and 36 sub-elements,
although significant reaction of the Nb-Ta alloy fin with
Sn has reduced its effectiveness. In Figure 8, we show a
false-color, atomic-number-sensitive, electron-backscatter
image near a Nb-Ta fin after reaction. The inside
filaments (right, yellow) are evidently fully reacted while
the outer filaments (left) furthest away from the original
Sn core have unreacted Nb cores (blue). Near the original
Sn core, the fin is full reacted by Sn, apparently robbing
the outer filaments of needed Sn. The result is a
significantly reduced Jc of the composite. The “fin”,
however, has been successful in stopping the reaction with
the outside Nb diffusion barrier (blue, left), so that a
continuous shielding external Nb3Sn layer is not
produced.
Closer examination of the Nb-Ta reaction region in
Figure 9 reveals a complex reaction producing changes in
composition and thickness of the grain boundaries in the
6
18
36
60
90
126
168
216
270
330
396
504
0
100
200
300
400
500
600
050100150 200
Filament "Diameters", µm
Number of Sub-elements
Physical Sub-element
diameter (Circular), µm
Minimum DPhys, µm
(With 1 Fins core no
longer shielded but no
split - all non-SC in core)
Minimum DPhys, µm
(With 2 Fins core no
longer shielded - all non-
SC in core)
Figure 7: Calculated physical filament diameters based on
simple circular geometries for a typical cable strand wire
diameter (0.8 mm) and for a Cu:Non-Cu ratio of 1. The
typical R&D high-Jc internal-Sn billets have 18-37 sub-
elements in their restack, limiting them to sub-element
diameters >100 µm. The Nb-Ta fins of Figure 4 can avoid
shielding of the residual sub-element Sn-cores (open
circles) by dividing the sub-element into two components,
which do not shield their core (X). This plot was
suggested by one made by R. Scanlan [13].
Figure 8: A false color atomic number sensitive electron
backscatter image of a sub-element cross-section near a
Nb-Ta fin after reaction. Composite fabricated by
Outokumpu Advanced Superconductors/Supergenics and
heat treated by E. Barzi at FNAL.
Figure 9: Atomic-number sensitive, electron-backscatter
image of the Nb-Ta divider in Figure 8 in the fully reacted
region. The grain boundaries in the reacted divider and
the adjacent Nb3Sn are clearly of variable composition.
154
Proceedings of the 2003 Particle Accelerator Conference

Page 5

Ta(Nb)3Sn and the Nb3Sn. This suggests that an
alternative barrier, perhaps Ta, might be more effective in
implementing this promising concept for reduced deff.
Issues with Ti
Alloying the Nb with Ti (or Ta) is required to raise Hc2
and maximize high field Jc but increases the cost of the
Nb alloy and can reduce piece length. Thus incorporating
Ti into the Sn core has been investigated too.. With ever
decreasing Cu content in the filament pack, however, this
route has become less effective because Ti reduces the
mobility of Sn through the filament pack [15]. Energy
dispersive x-ray analysis has indicated that the Ti forms a
Nb-Sn-Ti ternary phase at the core/filament-pack
interface. The segregation of the Ti can be clearly seen in
the EDS spectral image in Figure 10b.
An extreme case of non-uniform Ti distribution can be
seen in Figure 11, where the Ti only partially penetrates
the Nb filament – leading in this case to 2 distinct A15
layers. New methods of Ti alloying are now being
investigated under the DOE-SBIR program.
ACCELERATOR CONDUCTOR ISSUES
Remarkable progress has been made towards a new
generation of conductors that meet the goals of the high
energy physics community. As the preceding sections
indicate, however, there is still plenty to do. Among the
key issues that still need to be resolved are:
1 How low can the effective filament size for “High
Jc” Nb3Sn strand be reduced?
Can the cost of PIT strand be reduced so that it
competes with internal Sn?
Can the expected cost reduction in internal Sn
conductors be achieved without sacrificing
properties?
How close are we to the intrinsic performance
limits for Nb3Sn strand Jc?
2
3
4
REFERENCES
[1] S.X. Dou, et al., App. Phys. Lett. 81(18), pp. 3419-
3421 (2002).
[2] Private communication from S. Hong and K. Marken.
[3] A. Gurevich, et al. “Anomalous enhancement of the
upper critical field in the two-gap superconductor
MgB2 by selective tuning of impurity scattering”
(submitted to Nature December 2002).
[4] D. C. Larbalestier and P. J. Lee, IEEE 1999 Particle
Accelerator Conference, vol. 1, pp. 177-181, 1999
[5] L.D. Cooley, C.B. Eom, E.E. Hellstrom, and D.C.
Larbalestier, 2001 Particle Accelerator Conference,
Vol.1, pp: 203 –207, 2001.
[6] R. M. Scanlan, IEEE Transactions on Applied
Superconductivity, 11(1), pp. 2150 –2155 (2001).
[7] N. Martovetsky et al., IEEE Trans. Applied
Superconductivity, 12(1), pp. 600-5 (2002).
[8] R. McClusky, K.E. Robins and W.B. Sampson, IEEE
trans. on Magnetics, 27, pp. 1991-1995 (1991).
[9] H. Devantay, et al., J. Mat. Sci., 16, 2145 (1981).
[10] US Patent Number 5534219
[11] B. A.. Zeitlin, et al. Adv. in Cryo. Eng., 48, pp. 978-
985, 2002.
[12] P. J. Lee et al., paper 2MK05 presented at ASC 2002,
to be published in
Superconductivity.
[13] R. Scanlan, “Jc and Deffective: Where are we now, and
what do we need to do?” presentation at the Low
Temperature Superconductor Workshop, Napa, 2002.
[14] J. Uhlrich, University of Wisconsin REU project in
collaboration with M Suenaga, LBNL, 2002.
[15] M. Suenaga, “Sn Diffusion Effects in High Ic Internal
Tin Processed Wires,” presentation at the Low
Temperature Superconductor Workshop, Napa, 2002.
IEEE Trans. Applied
Figure 10: Detail of Core/Filament Pack region of OAS
0.4 mm dia. monocore strand after 150h/530°C heat
treatment in the investigation of Suenaga [15] and Uhlrich
[14]. In a) an atomic-number sensitive electron
backscatter image and b) a spectral image (energy
dispersive x-ray) show that Ti (white) segregates to the
inner side of the Nb (blue) filament pack (Cu in red, Sn in
green).
Figure 11: High resolution field emission scanning
electron microscope image of an outer filament in a 1st
generation OI-ST HER strand where the Ti has only
partially penetrated the Nb filament. Fracture of the
filaments reveals two sizes of A15 grains, The Nb(Ti)3Sn
grains are much smaller than the pure Nb3Sn grains.
155
Proceedings of the 2003 Particle Accelerator Conference