  We study, both theoretically and by experiment, migration of the amplitude
zeros within a fixed cross section of the edge-diffracted optical-vortex beam,
when the screen edge performs permanent translation in the transverse plane
from the beam periphery towards the axis. Generally, the amplitude zeros
(optical-vortex cores) describe spiral-like trajectories. When the screen edge
advances uniformly, the motion of the amplitude zeros is not smooth and
sometimes shows anomalously high rates, which make an impression of
instantaneous "jumps" from one position to another. We analyze the nature,
conditions and mechanism of these jumps and show that they are associated with
the "birth - annihilation" topological reactions involving the optical vortex
dipoles.
