Motivation

• Coupling between filaments affects thetape performance

• MO imaging is ideal for space-resolvedstudies of multifilamentary tapes

Indication of coupling in Bi-2223 tapes:

Complete shielding of a small applied field

-2 -1 0 1 20

5

10

tape

Ba = 5 mT

B (

mT

)

x (mm)

(55 filaments)

Resolving individual filaments

Optical image

Magneto-optical image (remanent state after applied field 105 mT at T=20 K)

Bi-2223 tape: 55 filaments , tape width = 3.7 mm

Flux trapped in individual filaments can be seen

Uncrossedpolarizers

B= 0

B< 0

B> 0

Experiment: flux density profiles

Profiles are obtained from MO images taken in theremanent state after Ba=105 mT

Temperature increase (i) a decrease of the trapped flux(ii) smoothening of the flux profiles.

-0.5 0.0 0.5 1.0

-10

-5

0

5

10

19K 41K 57K 62K 69K 76K

80K 86K 93K 102K 108K

Tape

x (mm)

B (

mT

)

fittingregion

Coupling between filaments

is equivalent to

through random superconducting interconnections

Jcinter Jc

intra

Magneto-optical study of the current distribution in multifilament Bi-2223 tapes with transport current and in applied magnetic fieldA.V. Bobyl, D.V. Shantsev, M.E. Gaevski, T.H. Johansen, Y.M. Galperin and M. Baziljevich e-mail: t.h.johansen@fys.uio.no

University of Oslo, Norway, and Ioffe Institute, Russia homepage: http://www.fys.uio.no/faststoff/ltl/index.htm

• Full penetration: J = Jc

• Field on the tape surface(from Biot-Savart law, 4 filaments):

B(x) = Jcintra F(x-xi) + Jc

inter F(x-x4)

• Fitting experiment with two free parameters:

Jcintra , Jc

inter

• Repeating the experiment and fitting forvarious temperatures gives

Jcintra(T) , Jc

inter (T)

Model

i=1,2,3

Results

Low T

Above 77 K

0 20 40 60 80 1000

10

20

30

40

50

inter-filament

intra-filament

J c (k

A/m

)

T (K)

Fitting

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0

5

10

15

T = 19 K

B (

mT

)

x (mm)

0

2

4

6

8

T = 57 K

-0.2

0.0

0.2T = 108 K

real filaments “coupling”

ExperimentFits

Conclusions

Flux distributions on the surface of a multi-filamentarytape were measured by MO imaging. The results wereanalyzed quantitatively on the filamentary scale using acritical-state model approach. We find that:

• MO imaging shows directly the existence of super-conducting inter-connections between filaments.

• We have extracted the T-dependences of the intra-filament and inter-filament critical currents.

• The relative strength of inter-filament coupling growssteadily as T is raised from 20 K to Tc. At the highest T,the inter-filament current becomes the dominant, whileat low T the global flux profiles are similar to thepredictions for a monocore tape.

• flux creep leads to a noticeable flux redistributionduring a 200 ms pulse of sub-critical current.

We thank NST, Denmark, for the Bi-2223 samples. The finan-cial support from the Norwegian Research Council and NATO

is gratefully acknowledged. One of us (YMG) has receivedsupport from Centre for Advanced Studies, Norway.