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in order to determine the effective filament size of strands.
Measurements at low fields were performed to study
magnetic instabilities (flux jumps) in strands with high
critical current density and large effective filament size.
Effects of strand deformation and heat treatment as well as
test temperature and sample cooling conditions were
studied. A superconducting current transformer with a
maximum DC current of 28 kA was used for cable testing
at self-fields under various experimental conditions. The
cable tests confirmed that magnetic instabilities cause a
significant reduction of current carrying capability at low
fields. These results are in good agreement with magnet
test data as well as with experimental studies of similar
strands and cables. The strand and cable parameters and
heat treatment procedures, equipment and measurement
methods used, and the results of the measurements are
herein described. The effect of magnetic instabilities is
analyzed and compared to the predictions of an instability
model.
THA01PO06
Composition analysis of Nb3Sn bronze wires by TEM
and Auger spectroscopy
D. Uglietti, R. Flukiger, University of Geneva, Group of
Applied Superconductivity; M. Cantoni, P. Buffat, CIME,
Swiss Fed. Institute of Technology; A. Reginelli, H. J.
Mathieu, Surface Analysis Group, Swiss Fed. Institute of
Technology.
Multifilamentary Nb3Sn bronze route wires exhibit a
gradient of the Sn concentration in the A15 layer, which
affects the transport properties. Little is known about the
microstructure and composition of the superconducting
layer, the last investigation on this subject having been
undertaken in the 80's: a better characterisation of the
superconducting layer at nanometric scale is needed in
order to improve the critical current density in these wires.
In this work the composition gradient across the
superconducting layer has been measured on different
alloyed Nb3Sn bronze wires at the nanometric scale by
means of TEM/EDX. The Sn, Ti and Ta composition
gradients across the filaments and across the wire have
been correlated to the grain morphology and to the
transport properties. At the bronze/A15 interface the grains
are equiaxed and the Sn content is close to 25 %, but
decreases to a value of 18% at the A15/Nb interface,
where the grain shape is columnar. Only the equiaxed
regions of the A15 filaments have a sufficiently high critical
field to carry supercurrents at high magnetic fields, while
the Bc2 values of the columnar regions is lower and does
not contribute to the current transport. At fields > 12 T,
about 75% of the A15 area is superconducting, while less
than half is superconducting at higher fields (>21T). It
follows that the critical current of bronze wires can be
drastically raised by enhancing the high Sn content section
of the filaments.
THA01PO07
Axial tensile stress-strain characterisation of a 36
strands cable
Y. Ilyin, A. Nijhuis, W.A.J. Wessel, N. van den Eijnden,
H.H.J. ten Kate, University of Twente.
During the various manufacturing steps and operation of
the Cable-In-Conduit Conductors (CICC) destined for the
high field coils in the International Thermonuclear
Experimental Reactor (ITER), the Nb3Sn wires of such
cables are subjected to various (thermo-) mechanical
deformations, originating from cabling and compaction, coil
winding, heat treatment, cooling down to liquid helium
temperature and finally electromagnetic forces. As the
strain state of the Nb3Sn strands determines their critical
properties, it is essential for optimal cable and magnet
design to gain knowledge on the deformation of strands
inside CICC’s and its impact on the transport properties.
The stress-strain and deformation of these cables is being
analysed by computation. However, experimental
verification is required at least on sub-cable level. For
validation of existing mechanical models, a new set-up has
been built, which allows the stress-strain testing of ITER
relevant sub-size cables. The cable consists of 36 nonreacted
Nb3Sn strands and the total effective length is four
times the twist pitch of the last cabling stage (480 mm). A
cable sample is subjected to tensile axial stress, while the
axial effective elongation (strain) is being recorded using
extensometers. The radial deformation is monitored at
room temperature by a digital camcorder. The test results
at room and liquid helium temperatures are reported
together with the single strand mechanical characteristics.
THA01PO08
Axial tensile stress-strain characterization of ITER type
Nb3Sn strands in TARSIS
N. van den Eijnden, A. Nijhuis, Y. Ilyin, W.A.J. Wessel,
H.H.J. ten Kate, University of Twente.
Since the strain state of the Nb3Sn filaments in strands
determines the transport properties, it is crucial for
optimizing the cable and magnet design to investigate in
detail the effective deformation state of the strand in
CICC’s and to determine the impact on the transport
properties and superconducting transition. The stressstrain
and deformation of ITER Nb3Sn cables is being
modeled and analysed by computation and basic
experimental verification is required on strand level. In
order to validate the mechanical models, a new set-up is
developed, which allows efficient tensile stress-strain
testing of wires. This paper reports on the first axial tensile
stress-strain measurements that were performed on
several types of ITER Nb3Sn strands at liquid helium,
nitrogen and room temperature in the TARSIS (Test
ARangement for Strain Influence on Strands) setup. A
double extensometer connected to the sample enables to
determine the strain level whereas a load cell is used to
determine the stress level. The details of the take-off origin
of the measured stress-strain curves are discussed and the
data are evaluated with respect to expressions for the
stress-strain curves.
THA01PO09
Extended characterization of European advanced
Nb3Sn strands for ITER
L. Muzzi, S. Chiarelli, A. della Corte, A. Di Zenobio, M.
Moroni, A. Rufoloni, A. Vannozzi, ENEA; E. Salpietro, A.
Vostner, EFDA CSU Garching.
Within the framework of ITER-related projects, new tasks
have been recently launched by EFDA CSU Garching
(European Fusion Development Agreement Close Support
Unit Garching), for the definition and production on
industrial scale of advanced Nb3Sn strands, to be used in
the manufacturing of the ITER high field CS and TF
magnets. We performed an extended characterization of
the advanced Nb3Sn strands coming from different
European companies, in terms of strand layout (diameter,
thickness of Cr coating, Cu:non-Cu ratio, twist pitch length
and orientation), critical transport current and n-value,
RRR, and hysteresis losses. The results of the
measurement campaign show that the upgraded strands
meet the latest ITER requirements, with an overall critical
transport current of at least 200A (at 12T, 4.2K), equivalent
to a non-Cu Jc of about 800A/mm2, a Cu:non-Cu ratio of
123 MT-19 2005, Genova