2-25
output lead is usually a probe or loop extending into one of the tuned cavities and coupled into a
waveguide or coaxial line. The plate structure, shown in figure 2-18, is a solid block of copper. The
cylindrical holes around its circumference are resonant cavities. A narrow slot runs from each cavity into
the central portion of the tube dividing the inner structure into as many segments as there are cavities.
Alternate segments are strapped together to put the cavities in parallel with regard to the output. The
cavities control the output frequency. The straps are circular, metal bands that are placed across the top of
the block at the entrance slots to the cavities. Since the cathode must operate at high power, it must be
fairly large and must also be able to withstand high operating temperatures. It must also have good
emission characteristics, particularly under return bombardment by the electrons. This is because most of
the output power is provided by the large number of electrons that are emitted when high-velocity
electrons return to strike the cathode. The cathode is indirectly heated and is constructed of a high-
emission material. The open space between the plate and the cathode is called the INTERACTION
SPACE. In this space the electric and magnetic fields interact to exert force upon the electrons.
Figure 2-18.—Cutaway view of a magnetron.
The magnetic field is usually provided by a strong, permanent magnet mounted around the
magnetron so that the magnetic field is parallel with the axis of the cathode. The cathode is mounted in
the center of the interaction space.
BASIC MAGNETRON OPERATION.—Magnetron theory of operation is based on the motion of
electrons under the influence of combined electric and magnetic fields. The following information
presents the laws governing this motion.
The direction of an electric field is from the positive electrode to the negative electrode. The law
governing the motion of an electron in an electric field (E field) states:
The force exerted by an electric field on an electron is proportional to the strength of the field.
Electrons tend to move from a point of negative potential toward a positive potential.
