TM 9-4931-294-15/2
Figure 2-3. Base Structure of a BWO Tube.
a velocity determined by the cathode to collector
(3) An examination of figure 24 reveals that if the
potential. The process of oscillation is triggered initially
axial velocity of the electric wave and the velocity of the
by noise energy at the terminated end of the helix. This
electron beam are approximately equal, although
noise energy produces electric waves which travel
opposite in direction, an electron in the beam will
backward along the helix towards the output end. Some
experience the same polarity of the electric field
of these electric waves have an axial velocity which is
whenever it passes a space between the helix structure.
approximately that of the electron beam, and interaction
At a given instant electrons opposite that portion of the
occurs between the electron beam and the field wave
component. This interaction process is called "bunching"
accelerated while those electrons opposite the part of the
and can be demonstrated with the aid of figure 2-4 which
field marked minus () are decelerated. Since electrons
shows an instantaneous view of a section of the electron
experience the same electric field polarity each time they
beam moving in close proximity to the electric field wave
move past a space between the helix structure, this
on the helix. The beam is initially unmodulated (all
effect is cumulative and causes the electrons to form
electrons are traveling at a constant velocity from left to
bunches as they progress towards the collector. This
right). The field intensity of the electric wave is greatest
process becomes regenerative in that the electron
opposite the spaces between the helix structure and
bunches induce an electric field on the helix, fortifying the
smallest opposite the structure itself. Therefore, an
original electric field and creating still more bunches.
electron traveling in a straight line in close proximity to
the helix is subject to the effect of the electric wave when
(4) Actually, the electron beam velocity at the
opposite the spaces but is shielded from the wave when
electron gun is slightly greater than the axial velocity of
opposite the helix. From the viewpoint of an electron
the electric wave, since bunching is accomplished
traveling in the beam, the electric field, which it is
primarily by decelerating electrons. As a result, energy
subjected to, is pulse modulated by the shielding effect
from the electron beam is transferred to the electric
of
the
delay
line.
wave. The loss of kinetic energy by the decelerating
electrons contributes to the total energy extracted
2-5