ABSTRACT
The interaction between an electromagnetic wave and a moving
plasma has been investigated for a long time since the first publi-
cations by Landecker [1] and Lampert [2], and it remains of interest
up to now [3-12]. It is well known that the electromagnetic wave
simultaneously exhibits a change in its amplitude and frequency
and duration compression under such an interaction [13-15]. It has
been shown in previous studies that the electromagnetic wave’s
amplitude and frequency after its reflection from a moving medium
boundary can be enhanced sufficiently. In Refs. 3-8, the method
for multiplication of a wave frequency when a wave reflects from
a plasma moving in the opposite direction has been considered in
detail and the possibility for experimentally realising such a method
has been shown in Refs. 4 and 5. As the reflection is most effective
when the wave phase velocity is near the plasma velocity, a slow-
wave structure has been used in this work. Frequencymultiplication
of 11-20% at an initial frequency of 24.75 MHz has been achieved
with strong wave slowdown (200-300 times) and the plasma
velocity near 6 × 104 m/s. However, the gain factor and frequency multiplication factor in this method are confined by difficulties in
the production of a slow-wave structure, the technology for which
becomes more complicated with increasing frequency and wave
slowdown.Moreover, this approach does allow one to independently
adjust the gain factor and frequency multiplication factor as they
change concurrently by varying the ratio of wave phase velocity to
plasma velocity. These disadvantages can be avoided if one takes
into account the influence of a waveguide on the interaction of an
electromagnetic wave with a medium. It appears that it does not
need any artificial slowdown of awave as anothermechanism comes
into play: the natural slowing of group velocity in a waveguide. In
this case adjustment of the gain factor and frequency multiplication
factor are practically implemented separately. The efficiency of such
changes is characterised by the ratio of the boundary velocity to the
wave group velocity; this is of vital importance in waveguides where
a double dispersion mechanism occurs [10, 11]. The importance of
this mechanism for determining the energy characteristics of re-
flected electromagnetic waves will be shown in this section. Usually
the power characteristics are considered; however, if the plasma
arises and starts to move at a certain time moment then the wave
interaction with the plasma is accompanied by the appearance
of a transient electromagnetic field. The spectral structure of the
secondary electromagnetic waves is of interest even in the case of
a steady uniform movement of the medium boundary [16]. If the
movement is not steady the evolution of the electromagnetic wave
has to be considered as in the previous chapter [17].
