Physics of Magnetron

1. The Magnetron

2. THE MAGNETRON
• The microwave radiation of
microwave ovens and some radar
applications is produced by a
device called a magnetron.

3. Magnetron Principles
A magnetron is a device that converts high
voltage DC electrical power into microwave
power.

5. Construction
• The magnetron is constructed from a circular
arrangement of microwave cavities that form the
anode. The cathode is arranged so that it is concentric
to the anode vane tips. When the air is extracted, and
the cathode is hot, electrons are emitted, from its
surface, into the space between the cathode and the
anode vane tips. If a DC voltage is applied between the
cathode and the anode and a magnetic field is applied
at right angles to the electric field, then the electrons
will follow a curved path as shown above. Microwave
power is generated as the electrons interact with the
anode resonator structure.

6. Magnetic Circuit Technology
• The applied magnetic field sets the operating
voltage of the magnetron.
• Very high power magnetrons use water-
cooled solenoids to provide a uniform
magnetic field in the interaction space
between the cathode and the anode. In some
applications, such as LINACs, the solenoid
current is varied to enable stable magnetron
operation over a wider power range.

7. Physics Behind Magnetron

8. Physics Behind Magnetron
(Continued)

9. • The magnetron is called a "crossed-field" device
in the industry because both magnetic and
electric fields are employed in its operation, and
they are produced in perpendicular directions so
that they cross. The applied magnetic field is
constant and applied along the axis of the circular
device illustrated. The power to the device is
applied to the center cathode which is heated to
supply energetic electrons which would, in the
absence of the magnetic field, tend to move
radially outward to the ring anode which
surrounds it.
Physics Behind Magnetron (Continued)

10. Applications
Radar
• In radar devices the waveguide is connected to an antenna. The magnetron is
operated with very short pulses of applied voltage, resulting in a short pulse of
high power microwave energy being radiated. As in all radar systems, the radiation
reflected off a target is analyzed to produce a radar map on a screen.
• Several characteristics of the magnetron's power output conspire to make radar
use of the device somewhat problematic. The first of these factors is the
magnetron's inherent instability in its transmitter frequency. This instability is
noted not only as a frequency shift from one pulse to the next, but also a
frequency shift within an individual transmitter pulse. The second factor is that the
energy of the transmitted pulse is spread over a wide frequency spectrum, which
makes necessary its receiver to have a corresponding wide selectivity. This wide
selectivity permits ambient electrical noise to be accepted into the receiver, thus
obscuring somewhat the received radar echoes, thereby reducing overall radar
performance. The third factor, depending on application, is the radiation hazard
caused by the use of high power electromagnetic radiation. In some applications,
for example a marine radar mounted on a recreational vessel, a radar with a
magnetron output of 2 to 4 kilowatts is often found mounted very near an area
occupied by crew or passengers. In practical use, these factors have been
overcome, or merely accepted, and there are today thousands of magnetron
aviation and marine radar units in service. Recent advances in aviation weather
avoidance radar and in marine radar have successfully implemented
semiconductor transmitters that eliminate the magnetron entirely.

11. Heating
• In microwave ovens the waveguide leads to a
radio frequency-transparent port into the
cooking chamber.

12. Lighting
• In microwave-excited lighting systems, such as
a sulfur lamp, a magnetron provides the
microwave field that is passed through a
waveguide to the lighting cavity containing the
light-emitting substance (e.g., sulfur, metal
halides, etc.).

13. LINAC’s
• Magnetrons are also used in LINAC’s to accelerate charged
particles. In accelerator, high voltage flat topped DC pulses
of a few microseconds duration are generated by the
modulator section. These pulses are delivered to the
magnetron and simultaneously to the electron gun. Pulsed
microwaves produced in magnetron are injected into the
accelerator tube or structure via a waveguide system. At
the proper instant electrons produced by the electron gun
are also pulse-injected into the accelerator structure. These
electrons (of about 50 keV initial energy) interact with
electromagnetic field of the microwaves. The electrons gain
energy from the sinusoidal electric field by an acceleration
process analogous to that of a surf rider.

14. Drawback of Magnetron
• The sizes of the cavities determine the resonant
frequency, and thereby the frequency of emitted
microwaves. However, the frequency is not
precisely controllable. The operating frequency
varies with changes in load impedance, with
changes in the supply current, and with the
temperature of the tube. This is not a problem in
uses such as heating, or in some forms of radar
where the receiver can be synchronized with an
imprecise magnetron frequency. Where precise
frequencies are needed, other devices such as
the klystron are used.