ABSTRACT
Any change in the state of a medium, for example, a change of
its material properties or a movement of its boundaries, affects
the characteristics of an electromagnetic field existing in this
medium. This influence is very strong, even in the simplest non-
dispersive electromagnetic structures. As there are two temporal
processes in this case, medium change and field change, the points
of their origin acquire principal importance, and the corresponding
mathematical problems become initial boundary value ones. It is
evident that a dispersive structure adds new special features to the
change of the electromagnetic field state and can greatly influence
transient electromagnetic processes. In practice, waveguides and
resonators, where the electromagnetic field interacts with matter
in bound areas of space constrained by waveguide or resonator
walls, are very important dispersive structures with the presence
of the walls bringing a dispersive character to electromagnetic
wave propagation in the region considered. The field interaction
with a non-stationary medium acquires new features under these
conditions. In addition, because of the difference between the phase
and the group velocities of the waves conditioned by the dispersion,
the importance of taking into account some initial time of the
interaction process arises. This importance increases in the case
where a medium or its borders moves, when the relationship
between all three velocities, the phase and the group velocities of
the waves and the motion velocity, begin to play a significant role.
Investigations of transients in waveguides have a long history, but
they concern the degradation of pulses in stationary waveguides
and, principally, metallic waveguides.