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A number of investigations, led over many years1-6, all indicate that defects, viz. focalconic domains, screw and edge dislocations, play an important role in the plasticbehavior of smectics under shear. Although some of the mechanisms in play have beendescribed2,6, the situation is far from being satisfactory, because of a lack of in-situobservations. In this work, we bring some new elements to the question of theinteractions between dislocations at a microscopic level.We have developed an experimental setup designed to observe the motion ofmicroscopic defects under polarized light microscope in a homeotropic slab of a SmAmaterial submitted to a strain7. The deformation of the cell and its temperature arecontrolled and the motion is followed by video. The following results have been reachedand discussed in a theoretical framework.(a) The existence of several regimes for the motion of dislocations and their transitions(b) The existence of a yield stress due to the depinning of the mobile edge dislocationsdislocations from the fixed screw dislocations; the associated forces, energies, andshapes of the moving lines are calculated;(c) The role of the heterogeneous/ homogeneous nucleation of dislocations understrainWe conclude that the interactions between defects actually rule the plastic deformationof the confined smectic and we discuss the origin of observed interactions.References :1 Bartolino and G. Durand, Phys. Rev. Lett., 1977, 39, 1346.2 R.G. Horn and M. Kléman,. Ann. de Phys. 1978, 3, 229.3 L. Bourdon, M. Kléman, L. Lejcek and D. Taupin, J. de Phys. 1981, 42, 261.4 P. Oswald and M. Kléman, J. de Phys. Lettres 1984, 45, L319.5 R. A. Herke, N. A. Clark, and M. Handshy, Science 1995, 267, 651. R. A. Herke, N. A. Clark, and M. Handshy, Phys.Rev. E 1997, 56, 3028.6 C. Meyer, S. Asnacios, and M. Kleman, Eur. Phys. J. E, 2001, 6, 245.7 I.Lelidis, M.Kleman, and J.L.Martin, Mol. Cryst. Liq. Cryst., 2000 , 351,187.