Inst. Phys. Conf. Ser. No 167, pp.653-656 (2000)
Proceedings of EUCAS'99
Magneto-optical study of multifilament Bi-2223 tapes in currentcarrying states: Behaviour of individual filaments
T H Johansen1, Y M Galperin1,2, M Bazilievich1,
A V Bobyl1,2, D V Shantsev1,2 and M E Gaevski2
1
2
Department of Physics, University of Oslo, P. O. Box 1048 Blindern, 0316 Oslo, Norway
Ioffe Physico-Technical Institute, Polytechnicheskaya 26, St.Petersburg 194021, Russia
ABSTRACT: The magneto-optical imaging technique is used to measure magnetic flux
distributions on the surface of a multifilamentary Ag-sheathed Bi-2223 tape at 20 K. The results are
analysed quantitatively on the filamentary scale using a critical-state model approach. The flux
distributions on a global scale are found to be similar to the Bean model predictions for a monocore
tape. In addition, a fine structure associated with the arrangement of individual filaments is observed.
We show that the detailed flux profiles in a number of different magnetic and current-carrying states
can be well described by the thin-strip Bean model applied to the individual filaments.
1.
INTRODUCTION
In order to improve the performance of the high-Tc superconducting multifilamentary tapes it
is essential to control the flux and current distributions in the tape. To assist this development, there
are available several powerful spatially resolved methods for flux visualization, such as magnetooptical (MO) imaging and micro Hall-probe measurements. Up to now, most of these studies have
been carried out in applied fields only, and very few (Oota 1997, Herrmann 1998) have been devoted
to multifilamentary tapes carrying a transport current. Moreover, very little is today known about the
behaviour of the individual filaments in current-carrying states.
In the present work we apply MO imaging to investigate quantitatively the flux distributions
in tapes on a filament-scale in various states; (i) in applied magnetic field, (ii) with transport current
and (iii) in subsequent pulses of field and current. The experimental results are analyzed using a
critical-state model approach, which proves to be sucessful in describing the behaviour both on the
global and on the filament scale.
2.
EXPERIMENT
The sample under study is a 55 filament Bi-2223 tape, prepared by the powder-in-tube
method with subsequent drawing and rolling (Bodin, 1997). The tape width including the Ag sheath
is 3.7 mm, and the critical current at 77 K equals 45 A.
Three types of experiments were carried out, see Fig. 1. In the first, the MO images were
recorded in a perpendicular magnetic field, and also in the remanent after a maximum field of
105 mT was removed. In the second experiment, observations were made with the tape carrying a
transport current of 20 A. Finally, we first applied field and subsequently passed a transport current
while MO images were recorded at time intervals shown in the lower part of Fig. 1. The current was
applied with short pulses of 200 ms duration synchronised with the camera recording. Typically, the
camera exposure time was 33 ms. All the measurements were made at 20 K.
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Inst. Phys. Conf. Ser. No 167, pp.653-656 (2000)
Ba (mT)
105
a
b
time
0
c
20
I (A)
Proceedings of EUCAS'99
time
B >0
0
B =0
d
Ba (mT)
105
e
B <0
time
I (A)
0
20
time
0
Fig. 1. Time profiles of the applied magnetic field
and transport current. The exact intervals of the
MO image recording are indicated by the gray
columns.
Fig. 2. MO images of the tape in the various states
labelled a,b,c,d,e in Fig. 1. The two lower MO images
are obtained by subtraction. Bottom: Optical image of
the tape cross section, where the filaments appear black.
The MO imaging of the magnetic flux is
obtained using a Faraday-active Bi:YIG
indicator film with in-plane magnetization.
Details of the MO imaging technique itself are
given in the review (Koblischka and Wijngaarden, 1995).
3.
RESULTS
MO images of the tape in different states are shown in Fig. 2. The images were taken with
uncrossed polarisers, giving a monotonous relation between image brightness and the local flux
density. Two main features can be seen on all the images. First, there are numerous stripes along the
tape corresponding to the arrangement of the filaments. Second, one sees large-scale features
expected for a monocore superconductors. In particular, these are in (a); maxima of flux density at
the tape edges in an applied field of 5 mT, in (b); a broad maximum of flux density showing trapped
flux in the tape centre, and in (c); field maxima of opposite sign at the tape edges in the current
carrying state. For data analysis we calculate flux density profiles from the MO images by averaging
grey levels over a 100 microns band across the tape.
Present theories based on the critical-state model describe B-profiles either for a single
super-conducting strip (Brandt 1993, Zeldov 1994) or for an infinite array of strips (Mawatari 1996,
Müller 1997). Unfortunately, none of them can be directly applied to a real multifilamentary tape
with a finite number of filaments. To analyze our experimental data quantitatively we use the theory
for a single strip to model the behavior of the individual filaments, and also of the tape as a whole.
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Inst. Phys. Conf. Ser. No 167, pp.653-656 (2000)
3.1
Proceedings of EUCAS'99
12
Global scale
10
B (mT)
8
Shown in Fig. 3 are flux profiles
Ba = 5 mT
6
for the tape in an applied field and with a
transport current I. Interestingly, the
4
a
overall shapes can both be fitted well by
2
the Bean model predictions for one single
0
strip having the full width of the tape
core. The critical current Ic and the
4
distance between the superconductor and
MO indicator h, were taken as fitting
parameters. For the applied field case best
2
fit was achieved for Ic = 60 A, whereas in
the current carrying state we find Ic =
c
32 A, with h = 0.10 mm in both cases.
0
Moreover, we find that Ic strongly
depends on the actual value of Ba and the
magnitude of the transport current. This
-2
variation obviously results from the nonuniformity across the tape.
I = 20.5 A
An important conclusion can be
-4
drawn from the shape of the B-profile in a
small applied field, Fig. 3a. Almost
-2
-1
0
1
2
complete shielding of the field in the tape
x (mm)
center, and pronounced maxima near the
edges show that the shielding currents
Fig. 3. Flux density profiles obtained from MO images
flow in large-scale loops. This means that
taken in an applied field, and in a current-carrying state.
superconducting inter-connections are
The dashed lines show fits of the Bean model for a single
present between the filaments, as
strip. The vertical dotted lines indicate the correlation in
discussed in more detail elsewhere
peak positions.
(Bobyl, 1999). Note that the presence of
coupling cannot be established from Bprofiles for a tape with transport current,
Fig. 3c, since the current can redistribute between filaments even if they have no interconnections.
3.2
Filament scale
There is a clear correlation between the filament positions on the optical image and the locations of
stripes on the various MO images. A similar correlation is also evident in the flux density profiles
shown in Fig. 3. To analyse this in more detail we focus on the behaviour over a short interval x
across the tape. Five profiles obtained from the MO images in Fig. 2 are plotted in Fig. 4 (left). As
seen from the optical image, only two vertical stacks of filaments fall within this range of x, their
position being indicated by grey bars. The filament structure is clearly reflected in all of the profiles.
These experimental profiles are compared to a simple model where we consider only two
filaments behaving in accord with the Bean model for a single strip. Furthermore, it is assumed that
the filaments are identical and do not interact with each other. Thus, they always experience the
same magnetic field and carry the same current. Shown in Fig. 4 (right) are 5 calculated flux density
profiles corresponding to the experimental ones. We find a striking coincidence in the positions of
maxima and minima. The minor deviations are attributed to the influence of the neighbouring
filaments in the tape.
In summary, we conclude that on the global scale it is actually possible to describe the flux
density and current distributions in a multifilamentary tape by the Bean model for a single thin strip
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Inst. Phys. Conf. Ser. No 167, pp.653-656 (2000)
Proceedings of EUCAS'99
0.20
2
a
x 0.3
a
x 0.3
x 0.1
0
0.15
x 0.1
b
c
-4
d-b
-6
e-b
B (arb. un.)
B (mT)
-2
b
c
0.10
d-b
0.05
e-b
-8
0.00
0.4
0.6
0.8
1.0
-3
1.2
-2
-1
0
1
2
3
x (arb. un.)
x (mm)
Fig. 4. Left: Flux density profiles across the tape obtained from the MO images shown in Fig. 2. Gray bars at the
bottom show the positions of two vertical stacks of filaments (see optical image). To avoid overlaps, the profiles
are shifted vertically, and the profiles (a) and (b) are divided by 3 and 10, respectively. Right: Corresponding
profiles calculated using the Bean model for two identical non-interacting filaments.
of width equal to the superconducting core of the tape. On the filament scale, we have shown that the
detailed structures of flux profiles in a number of different magnetic and current-carrying states can
be well described by the thin-strip Bean model applied to the individual filaments.
We thank P. Vase (NST, Denmark) for sample preparation. The work is supported by from
the Research Council of Norway (RCN), and from NATO science fellowship via RCN.
REFERENCES
Bobyl A V et al. 1999 Supercond. Sci. Technol., in press
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Herrmann J et al 1998 Physica C 305, 114
Koblischka M R and Wijngaarden R J 1995 Supercond. Sci. Technol. 8, 199
Mawatari Y 1996 Phys. Rev. B 54, 13215
Müller K H 1997 Physica C 289, 123
Oota A, Kawano K and Fukunaga T 1997 Physica C 291, 188
Zeldov E, Clem J R, McElfresh M, and Darwin M 1994 Phys. Rev. B 49, 9802.
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