Re: How strong must the magnets in a magnetron tube be?

Greetings Nathan:

References:
1. Reference Data for Radio Engineers, 5th Edition, 1968
International Telephone and Telegraph Corporation (ITT)
Chapter16-25 Electron Tubes

2. The Magnetron Tube Structure and Operation
The Complete Microwave Oven Service Handbook
http://www.gallawa.c om/microtech/magnetron.html

3. Cougar Labs, High Rate Magnetron Sputtering
http://www.cougarlabs.com/ma gsput1.html

4. Fredrick E. Terman, Electronic and Radio Engineering
McGraw-Hill, 1955

Background Information

Currently there are two major applications for the magnetron: first, as a
high power microwave oscillator or amplifier as those used in radar and
microwave ovens (See Reference 2); second, as a source for magnetron
sputtering to deposit thin films, particularly for use in semiconductor
processing (See Reference 3.). Small magnetrons are also used as vacuum
pumps on many large microwave tubes. Your question mentions tuned cavities
so I'll discuss the first application.

There are several configurations for magnetron microwave power tubes;
however, I'll discuss the original coaxial form similar to that described
and animated in Reference 2.The device contains two electrodes; a
cylindrical heated cathode surrounded by a cylindrical metallic anode with
a cylindrical space between them. This space is called the electron drift
space. The anode structure has several microwave resonant circuits called
cavities located within it with each cavity having a small opening into
the drift space. This opening is called a coupling hole which allows a
portion of the microwave magnetic field within the cavity to extend into
the drift space. One of the cavities has a wire coupling loop located
within it to couple microwave power out of the magnetron and transmit the
microwave power to the load. Depending on the application, the load can be
anything from a radar transmitting antenna to food for cooking in a
microwave oven. The drift space and the cavities are evacuated to a vacuum.
A magnetic field is generated by an electromagnet or a permanent magnet
with the magnet's poles placed across the ends of the cylindrical magnetron
structure so that the magnetic field lines are aligned with the axis of the
cylindrical electrodes.

The important parameters for the operation of a magnetron are:

(a) the voltage potential and current flow between the anode and the cathode.
(b) the strength of the magnetic field.
(c) the microwave output power
(d)the efficiency of converting direct current (DC) power to microwave power.
(e)the load impedance and variations in the expected load impedance during
operation and it's effect on efficiency, power output and frequency stability.

For a given magnetron configuration parameters (a), (b), (c) and (d) are
plotted on a Magnetron Performance Chart. Parameter (e) is plotted on a chart
called a Rieke Diagram that shows the variation of microwave power output,
anode voltage efficiency and frequency changes caused by changes in the
load's electrical parameters. Each type of magnetron has a unique Magnetron
Performance Chart and a unique Rieke Diagram. A microwave system design
engineer must match the performance parameters that are required for a given
application with these charts and diagrams in order to choose the correct
magnetron for the system. Needless to say selecting the right magnetron for
a specific application is a complex process and beyond the scope of this
answer.

Answers to Questions

To answer your question I will discuss a typical magnetron configuration
that is used for many radar applications and then I will vary the parameters
and note the effect. The magnetron is excited with a negative high voltage
pulse about one microsecond in duration applied between the cathode and the
grounded anode of the device.

Variation of Parameters (See references 1 and 4)

If the magnetron has no microwave cavities in the anode and
no magnetic field is applied, the electrons will flow in a straight lines
radially from the cathode to the anode during the voltage pulse. As we
increase the pulse voltage the electrons will cross the drift space more
quickly and the cathode current will increase. Then for a given fixed
voltage, we add the magnetic field and the electron trajectories will curve
before reaching the anode; however the cathode to anode current will be the
same. For a given voltage the magnetic field required to cause the electrons
to circle and just miss the anode and return back to the cathode is called
the cutoff field. At this point no current flows in the anode- cathode
circuit. If we increase the magnetic field even more, the electrons will
travel in even smaller circles near the cathode and no current flows.

When the current is just cutoff by the magnetic field and we then reduce the
cathode to anode voltage, the electrons will travel more slowly and in wider
circles in the magnetic field and will again intercept the cathode and
cathode to anode current will again flow. This means that for each level of
voltage there is a different value of magnetic field for current cutoff!

Next we add the microwave cavities to the anode and microwave magnetic fields
extend through the cavity coupling holes into the drift space. Now electrons
that are circling near the anode are slowed by the microwave magnetic field
giving up energy to the microwave field. The reduced velocity of the
electrons then causes them to intercept the anode. By adjusting both the
pulse voltage and the magnetic field we can optimize the energy transferred
to the microwave fields in the cavities by the slowed electrons which in
turn increases the microwave power output of the magnetron and increases
or perhaps decreases the conversion efficiency. This complex interaction
can be seen in the Magnetron Chart in Reference 1; however, it is much to complex
to describe in this answer.

Typical Performance Parameters

The Magnetron Performance Chart in Reference 1 has peak pulse cathode to
anode currents ranging from 8 to 32 amperes plotted on the horizontal axis
of the chart. Peak pulse cathode to anode voltages ranging from 8 thousand
volts (8 kilovolts) to 24 kilovolts are plotted on the vertical axis of the
chart. Magnetic field values ranging from 1100 Gausses on the lower voltage
end of the chart to 2100 Gausses on the higher voltage end of the chart are
plotted as straight lines with a slight upward slope as current increases.
Downward sloping curves as current increases of constant microwave power
are also plotted on the chart for output powers ranging from 80 thousand
watts (80 kilowatts) to 240 kilowatts. Finally U shaped curves for constant
conversion efficiencies ranging from 30 % to 50 % are also plotted on the
chart. The Reike Diagram for this particular magnetron is not presented in
Reference 1.

A typical operating point for this particular magnetron is also plotted on
the chart in Reference 1 with a peak pulse voltage value of 16 kilovolts
corresponding to a peak pulse current of 20 amperes. This operating point
occurs at a magnetic field of 1600 Gausses providing a microwave output power
of 140 kilowatts at a conversion efficiency of 41%.

Best regards, Your Mad Scientist
Adrian Popa