DYNAMICS OF FIELD EXPULSION IN HTSC FILMS

We present a macroscopic theoretical model which describes the time de pendence of the magnetisation in epitaxial High-Tc superconducting fil ms, when a previously constant external magnetic field parallel to the surface of the film is suddenly decreased. The model assumes that the magnetic field is expelled from the superconductor by the flux-flow m echanism, where the vortices are strongly pinned according to Bean's c ritical state model. We also establish. the conditions required to rea ch the stationary situation.