13-08-2012, 03:29 PM
Magnetron Theory of Operation
[attachment=31315]
Theory of Operation
A magnetron is a high power microwave oscillator in which the
potential energy of an electron cloud near the cathode is converted
into r.f. energy in a series of cavity resonators similar to the one
shown in Figure 1. As depicted by the low frequency analog, the
rear wall of the structure may be considered the inductive portion,
and the vane tip region the capacitor portion of the equivalent
resonant circuit. The resonant frequency of a microwave cavity is
thereby determined by the physical dimension of the resonator
together with the reactive effect of any perturbations to the
inductive or capacitive portion of the equivalent circuit. This is an
important point and will be recalled later.
In order to sustain oscillations in a resonant circuit, it is necessary
to continuously input energy in the correct phase. Referring to Figure 2, if the instantaneous r.f. field, due to steady state
oscillations in the resonator, is in the direction shown, and, an electron with velocity was to travel through the r.f. field such that
the r.f. field retarded the electron velocity by an amount, the decrease in electron energy will be exactly offset by an increase in the
r.f. field strength.
In a magnetron, the source of electrons is a heated cathode located on the
axis of an anode structure containing a number of microwave resonators.
See Figure 3.
Electrons leave the cathode and are accelerated toward the anode, due to
the dc field established by the voltage source E. The presence of a strong
magnetic field B in the region between cathode and anode produces a
force on each electron which is mutually perpendicular to the dc field
and the electron velocity vectors, thereby causing the electrons to spiral
away from the cathode in paths of varying curvature, depending upon
the initial electron velocity a the time it leaves the cathode.
Typical Magnetron Parameters
The following is a discussion and explanation of typical magnetron specification parameters.
Thermal Drift
At the time high voltage is first applied to a magnetron,
the thermal equilibrium of the device is suddenly
altered. The anode vanes being to heat at the tips due to
electron bombardment and the entire anode/cathode
structure undergoes a transient change in thermal
profile. During the time required for each part of the
magnetron to stabilize at its normal operating
temperature, the output frequency of the magnetron will
"drift." The curve of output frequency vs. time during
the period following initial turn on is called the
"Thermal Drift" curve. Generally speaking, the
maximum drift occurs during the first few minutes after
turn on, and slowly approaches equilibrium over a period ranging from 10 to 30 minutes depending upon the structure mass, power
output, type of cooling and basic magnetron design. Thermal drift curves across a variety of magnetron types operating at the same
frequency and output power may differ radically from each other. Each type is usually designed for a particular purpose and subtle
differences in the internal magnetron configuration can produce radical differences in the thermal drift curve.
Temperature Coefficient
After the thermal drift period has expired and a stable operating frequency has been achieved, changes to ambient conditions which
cause a corresponding change in the magnetron temperature will produce a change in the output frequency. In this content ambient
changes include cooling air temperature or pressure in air cooled magnetrons; mounting plate temperature in heat sink cooled
magnetrons; and flow rate or temperature in liquid cooled magnetrons.
The change in magnetron output frequency for each degree change in body temperature, as measured at a specified point on the
outside surface of the magnetron body, is defined as the Temperature Coefficient for the magnetron and is usually expressed in
MHz/oC. For most magnetrons the temperature coefficient is a negative (frequency decreases as temperature increases) and is
essentially constant over the operating range of the magnetron.
When estimating magnetron frequency change due to temperature coefficient, keep in mind that the temperature coefficient relates
magnetron frequency to body temperature and there is not necessarily a 1:1 relation between body temperature and, for example,
ambient air temperature. In addition, for airborne systems, the cooling effect of lower air temperature at altitude may offset by a
corresponding reduction in air density.
Pushing Figure
The pushing figure of a magnetron is defined as the change in magnetron frequency due to a change in the peak cathode current.
Referring back to the earlier theory discussion, we noted that the resonant frequency of a vane resonator is determined by its
mechanical dimensions plus the reactive effect of any perturbation. The presence of electrons in the vicinity of the vane tips affects
the equivalent capacitance of the resonator by an amount proportional to the density of the electrons and,
since electron density is similarly related to peak pulse current, changes in pulse current level will produce changes in output
frequency. The pushing figure expressed in MHz/Amp is represented by the slope of a frequency vs. peak current curve plotted for
a particular magnetron type.
[attachment=31315]
Theory of Operation
A magnetron is a high power microwave oscillator in which the
potential energy of an electron cloud near the cathode is converted
into r.f. energy in a series of cavity resonators similar to the one
shown in Figure 1. As depicted by the low frequency analog, the
rear wall of the structure may be considered the inductive portion,
and the vane tip region the capacitor portion of the equivalent
resonant circuit. The resonant frequency of a microwave cavity is
thereby determined by the physical dimension of the resonator
together with the reactive effect of any perturbations to the
inductive or capacitive portion of the equivalent circuit. This is an
important point and will be recalled later.
In order to sustain oscillations in a resonant circuit, it is necessary
to continuously input energy in the correct phase. Referring to Figure 2, if the instantaneous r.f. field, due to steady state
oscillations in the resonator, is in the direction shown, and, an electron with velocity was to travel through the r.f. field such that
the r.f. field retarded the electron velocity by an amount, the decrease in electron energy will be exactly offset by an increase in the
r.f. field strength.
In a magnetron, the source of electrons is a heated cathode located on the
axis of an anode structure containing a number of microwave resonators.
See Figure 3.
Electrons leave the cathode and are accelerated toward the anode, due to
the dc field established by the voltage source E. The presence of a strong
magnetic field B in the region between cathode and anode produces a
force on each electron which is mutually perpendicular to the dc field
and the electron velocity vectors, thereby causing the electrons to spiral
away from the cathode in paths of varying curvature, depending upon
the initial electron velocity a the time it leaves the cathode.
Typical Magnetron Parameters
The following is a discussion and explanation of typical magnetron specification parameters.
Thermal Drift
At the time high voltage is first applied to a magnetron,
the thermal equilibrium of the device is suddenly
altered. The anode vanes being to heat at the tips due to
electron bombardment and the entire anode/cathode
structure undergoes a transient change in thermal
profile. During the time required for each part of the
magnetron to stabilize at its normal operating
temperature, the output frequency of the magnetron will
"drift." The curve of output frequency vs. time during
the period following initial turn on is called the
"Thermal Drift" curve. Generally speaking, the
maximum drift occurs during the first few minutes after
turn on, and slowly approaches equilibrium over a period ranging from 10 to 30 minutes depending upon the structure mass, power
output, type of cooling and basic magnetron design. Thermal drift curves across a variety of magnetron types operating at the same
frequency and output power may differ radically from each other. Each type is usually designed for a particular purpose and subtle
differences in the internal magnetron configuration can produce radical differences in the thermal drift curve.
Temperature Coefficient
After the thermal drift period has expired and a stable operating frequency has been achieved, changes to ambient conditions which
cause a corresponding change in the magnetron temperature will produce a change in the output frequency. In this content ambient
changes include cooling air temperature or pressure in air cooled magnetrons; mounting plate temperature in heat sink cooled
magnetrons; and flow rate or temperature in liquid cooled magnetrons.
The change in magnetron output frequency for each degree change in body temperature, as measured at a specified point on the
outside surface of the magnetron body, is defined as the Temperature Coefficient for the magnetron and is usually expressed in
MHz/oC. For most magnetrons the temperature coefficient is a negative (frequency decreases as temperature increases) and is
essentially constant over the operating range of the magnetron.
When estimating magnetron frequency change due to temperature coefficient, keep in mind that the temperature coefficient relates
magnetron frequency to body temperature and there is not necessarily a 1:1 relation between body temperature and, for example,
ambient air temperature. In addition, for airborne systems, the cooling effect of lower air temperature at altitude may offset by a
corresponding reduction in air density.
Pushing Figure
The pushing figure of a magnetron is defined as the change in magnetron frequency due to a change in the peak cathode current.
Referring back to the earlier theory discussion, we noted that the resonant frequency of a vane resonator is determined by its
mechanical dimensions plus the reactive effect of any perturbation. The presence of electrons in the vicinity of the vane tips affects
the equivalent capacitance of the resonator by an amount proportional to the density of the electrons and,
since electron density is similarly related to peak pulse current, changes in pulse current level will produce changes in output
frequency. The pushing figure expressed in MHz/Amp is represented by the slope of a frequency vs. peak current curve plotted for
a particular magnetron type.