SPACE-TIME IMAGE OF THE MAGNETIC-FLUX PENETRATING INTO TYPE-II SUPERCONDUCTORS IN AN APPLIED OSCILLATING MAGNETIC-FIELD

We study the nonlinear diffusion of magnetic flux#in soft superconductors in situations where linearization of the diffusion equations for the magnetic induction is impossible. Evolution of the magnetic flux distributions, which begins once a low-frequency applied magnetic field (H(a)>H(c1)) is switched on, is considered. For H(a)(t>0)=H-1sin(omegat+phi), it is shown that the flux, which penetrated into the superconductor after the field was turned on, then gradually flows out of it. In the case of a semi-infinite sample, the time asymptote of the distribution front is shown to be x(f) is-proportional-to t1/4, the full flux averaged over periods of the applied field being m is-proportional-to t-1/4. The value and the sign of the penetrating flux are determined by the phase phi of the applied field at the switching moment. We show that, in general, an applied periodic magnetic field may induce a constant magnetic induction B(infinity) not-equal 0 deep inside the bulk, even if the constant component of the external field is zero. The applied field H-0 + H-1sin omegat produces B(infinity) >> H-0 if H(c2) > H-1 >> H-0 > H(c1).