Klystron: Tube for Outer Space
February 1961 Radio Electronics Article
you have been in the RF and microwaves business for any length of time,
you are probably familiar with a company named Varian. In the days before
you did your parts shopping online, Varian catalogs populated the desks
and bookshelves of many RF engineers who worked in the radar field,
including mine. Did you know that it is named after the brothers Russell
and Sigurd Varian, who started the business in 1948 to market their
high power klystron tubes? Following a number of reorganizations, it
was purchased by Agilent technologies in 2010. This story from Radio
Electronics magazine does a real nice job explaining the workings
of a klystron without getting too deep into the gory theoretical detail.
Thanks to Terry W. for providing this article.
See all available
vintage Radio-Electronics articles.
Klystron - tube for outer space
By Tom Jaski
When we get to talking to those intelligent beings "out there" on other
planets or even in other solar systems, very likely klystrons will be
the transmitter tubes that will make our communications possible. Large-power
klystrons have been used as amplifiers in the equipment that bounced
radar signals off the moon, Venus and satellites in orbit. Klystrons
have been used to "interrogate" satellites, and to trigger into action
the electronic and mechanical equipment in them.
only for outer space, but for its usefulness wherever microwaves
must be generated, the importance of this tube grows with the industry's
use of higher and higher frequencies
but even more practical, are other applications for klystrons. Large-power
klystrons are used widely in Europe for UHF television transmitters.
Here UHF television has not become common enough to need many power
klystrons. Klystrons are also the heart of the new "scatter" communications
systems in which the line-of-sight rule about microwave transmission
is violated simply by using very high-power transmitters, large antennas
and ultra-sensitive receivers.
Another major use of klystrons
is in experiments with food sterilization. These use high-speed electrons
issuing from linear electron accelerators, and these in turn are powered
by large klystron tubes.
In linear accelerators, the klystrons
provide a mighty push to the electrons passing through successive drift
tubes, eventually speeding them up to almost the speed of light.
What then are these klystrons, what do they look like and how do
Klystrons were invented just before the start
of World War II by the Varian brothers, then graduate students at Stanford
University. I remember their little shack behind the Ryan laboratory
in the hills behind the university, and the excited talk of a resident
in the area who had seen the barbed-wire fence around this little shack
develop a mysterious red-hot glowing section of wire, True or not, the
klystron has played an enormously important role in the development
of radar and microwave communications, and is now on the verge of taking
over industrial jobs from other tubes.
To start the explanation of klystrons, let us first
look into another item, resonant cavities. Understanding cavities is
essential to understanding klystrons. All RF oscillating circuits contain
resonant elements (Fig. 1-a). As frequency increases, we must decrease
the inductance and capacitance of the resonant circuits. We decrease
the inductance by decreasing the turns until we have nothing left but
a straight wire or even a flat strip of metal. The capacitance is reduced
by lowering the number of plates in our capacitor and finally by further
separating the plates (Fig, 1-b). Eventually we get to paralleling inductances
(Fig. 1-c) since paralleling two inductors halves their inductance,
and the entire process winds up as in Fig. 1-d or 1-e. The final product
is a box or cavity, the top and bottom representing the capacitor plates
and the sides the paralleled inductors.
Cavities follow certain hard and fast rules, which can be determined
easily from common-sense observation. For example, regarding the top
and bottom plates of the cavity as plates of a capacitor, we see that
they are virtually short-circuited at the edges. This means that at
the edges of the plates we cannot have a charge, and therefore no field.
From this follows our first rule about cavities: the electric field
parallel to a wall must be zero at that wall. Now to maintain any charge
which has a field in the center of the plate and none at the edges,
the voltage distribution must look something like a sine-wave half-cycle
from wall to wall. In fact, this is the simplest way we can maintain
a field in a cavity, the simplest "mode" in which we can operate it.
It follows that the width of the cavity should be just about a half-wavelength
of the microwave energy, or any multiple of that. And the same goes
for the length, if the cavity is rectangular.
Fig. 1 - Evolution of a klystron cavity: a - lumped tuned circuit;
b - same, highest possible frequency ; - turns paralleled to decrease
inductance; d, e - rectangular and cylindrical resonant cavities;
f - klystron cavity. The last three are all derived from c.
Fig. 2 - Typical klystron, cutaway view.
Fig. 3 - A 10-kw multi-cavity klystron.
Fig. 4 - Cross-section, reflex klystron.
Fig. 5 - Three old-time klystrons, the 417A, 707B 2K25. The 2K25
is still used to generate 3-centimeter waves.
The magnetic field
always associated with an electric field, and always at right angles
to it, will then be parallel to the top and bottom of the cavity. Thus
it would cut the end plates. But since it is a changing magnetic field,
it will induce a current in any conductor within the field, and the
end plates have currents induced in them which set up counter-magnetic
fields equal to and thus cancelling the first fields.
have the second rule about cavities: the magnetic field must be zero
at any wall which it cuts at right angles. Thus the magnetic field is
confined to the box as well. But with the magnetic field we do not have
the same dimensional problem, for we can swap density for space. Therefore,
the top-to-bottom dimension of the cavity is not as critical, but does
determine the capacity of the cavity to maintain a certain field amplitude.
For just as a capacitor dielectric would break down if it were too thin
for the voltage on the plates, so a cavity can break down, dielectrically
speaking, when the voltage gets too high between top and bottom plates.
Because we design the cavity carefully as far as dimensions are concerned,
we can then set up standing waves in it, and the cavity can easily be
excited with small charges on the top and bottom plates.
we make the cavity an integral part of a vacuum tube, and make part
of the top and bottom into a grid area (punch holes in it or slot it),
this does not drastically change the properties of the cavity. It can
still be excited easily by charge differences between top and bottom
plate. The klystron incorporates one or more of these cavities with
grids in top and bottom. Fig. 2 is a cutaway representation of a typical
The Bunching Action
At the bottom of the tube we have an electron gun that produces
a narrow beam of electrons. This beam leaves the gun under the influence
of the accelerating grid, which you can see just below the first cavity.
Then the electrons travel on through the two cavities, and the space
between them - called the drift space - to the collector, which can
collect the electrons because of a positive charge on it. As the electrons
travel through the first cavity grids, they constitute a current through
these grids, from one grid to the next - after all, a current is nothing
more than a flow of electrons. But, since this is a steady flow of electrons,
the best that we could expect would be a steady potential difference
on the grids.
If we manage to excite the cavity between the
grids in some way creating an alternating potential between these grids,
we will affect the electrons between them. An electron traveling toward
a grid that is positive will be attracted and speed up and one traveling
toward a negative grid will slow down. If the bottom grid of the lower
cavity is momentarily negative, and the top grid positive, the electrons
approaching the bottom grid from the cathode will be retarded, while
those between the grids approaching the top grid of the first cavity
will be accelerated.
In the next half-cycle of applied RF, the
lower grid will be positive and the top one negative. Thus electrons
which then approach the lower grid will be accelerated, and the electrons
which are then between the two grids will be retarded. In this way,
the grids and cavity with applied RF will form bunches of electrons,
some of which move faster than when they left the cathode and some of
which move a bit slower.
When the RF applied to the cavity goes
through zero, the electrons then passing through the grids will not.
be affected, and will just travel on at the same velocity. The lower
cavity and grid assembly, forming the bunches, is appropriately called
the buncher. (The Varians named this a rhumbatron.) In the space between
the cavities, the drift space, the electrons that are moving at the
original "from-the-cathode" velocity will join some of those which were
slowed down. They in turn will be joined by some of those that speeded
up. Thus the bunches of electrons in the drift space become denser,
and the space between bunches has fewer and fewer electrons.
Were we to let the bunches drift too long, the repulsion between
electrons would again scatter them. But we don't give them time to do
that. The denser bunches, now with more electrons, pass through the
second set of grids. Through these grids then pass alternately dense
bunches of electrons and spaces .with none or just a few. This is, in
effect, a pulsed dc. Pulsed dc can look very much like ac if we shift
the base line (different zero level).
The bunches then constitute
a periodically changing current capable of inducing an RF voltage in
the second cavity. Note that the acceleration and deceleration of electrons
between the buncher grids lasted nearly a half-cycle. The bunches which
reach the "catcher" grid are also about a half-cycle long. They will
induce in the catcher cavity an RF of the same frequency as was applied
to the buncher.
Getting Power from a Klystron
To induce a field in the second cavity, the electrons
must give up energy. It is easy to see how this happens after the field
has built up. Electrons approaching a negative grid are retarded and
impart energy to the grid. Electrons leaving a positive grid are also
retarded, giving off energy. Thus if we time the bunches (by regulating
the initial velocity of the electrons) to be between the catcher grids
only when the first catcher grid is positive and the second catcher
grid is negative, while we make sure that we have virtually no electrons
between the grids when this situation is reversed, then we draw the
maximum energy from our bunches of electrons. This is the way a klystron
is operated. The collector and accelerator voltages must be precisely
adjusted to get this kind of timing.
If we feed back a portion
of the catcher energy to the buncher, the tube will oscillate. If our
timing is correct, the phase of the RF will of course be exactly right
for the feedback situation, for the bunching occurs when the second
buncher grid is negative, and we get the most energy when the second
catcher grid is also negative. Amplification is obtained, because the
bunches going through the catcher contain many more electrons, thanks
to the time spent in the drift space, than the bunches coming out of
The energy is coupled into the buncher and out
of the catcher cavities with a small loop, which will contain some of
the magnetic lines of force of the fields and will thus have a current
induced in them.
We can of course use the energy in one of the
catcher cavities to excite additional cavities and grids, and this we
do many times to increase the energy produced by large klystrons. Fig.
3 shows such a large multicavity klystron made by Eimac, capable of
producing 10,000 watts output in the 720-985 mc range.
The Reflex Principle
But there are also klystrons
with but one cavity, The principle is illustrated in Fig. 4. These we
call reflex klystrons because the collector at the end of the tube is
given a negative voltage, thus repelling the electrons. This electrode
is usually called a repeller, What happens here is that the electrons,
after being bunched in the grids, travel on into the drift space above
the cavity for a time, then are repelled back toward the grids. If we
repel them with exactly the right velocity to make them arrive at the
grids when the voltages on these grids are of the correct phase to obtain
energy from the electron bunches, the original field is augmented, and
we have oscillation. So the reflex klystron is used primarily as an
Reflex klystrons come in many shapes. Fig. 5 shows
three of World War II vintage, the 417A made by Westinghouse for the
S-band (10 cm), the 707B with an external cavity, also for the same
frequency range, and the 2K25 used most often as the' local oscillator
in 3-cm (10,000-mc) radar receivers.
All three are tunable to
a certain extent (Fig. 6), The 417A is tuned by changing the cavity
a tuning lever and screws, the 707B by modifying
the electric fields in the cavity with slugs projecting into it, and
the 25K5 by changing the cavity dimensions with the tuning "bow". The
tuning bow is flexed by the screw. This alters the position of the more
or less flexible top portion of the metal enclosure, and the top cavity
grid with it.
A more modern version of the reflex klystron,
using ceramic insulation, is shown in the head photo. Such ceramic klystrons
are now produced and regularly oscillate at 25 kmc, while some laboratory
models have been used to generate frequencies as high as 100 kmc. The
latter are not in production, but are strictly experimental tubes.
Fig. 6 - Three klystron tuning methods.
Klystrons can be modulated
in various ways. One is to vary somewhat the reflector voltage or, in
the power klystron, the collector voltage. This has the effect of changing
the velocity of the electrons, and thus the frequency of oscillation
in the klystron is affected. This kind of modulation is limited within
very narrow ranges. Klystrons specially built with a modulating anode
near the electron gun can be amplitude-modulated by the simple mechanism
of making the electron beam vary in density. Since the amplification
of the tube depends on increasing the density of the electron bunches
in the drift space, the effect of the bunching will be more pronounced
when a lot of electrons are available than when only a few are traveling
through the cavity grids. These anode-modulated klystrons are so constructed
that the total voltage between the cathode and the tube structure (including
the cavities) remains the same. Thus the velocity of the electrons is
constant, but the voltage between the modulating anode and the cathode
can vary and the quantity of electrons with it.
particularly in television transmitters, it is actually unnecessary
to modulate the klystron. Here it acts as a power amplifier, and the
modulation can be introduced at an earlier stage. Thus the klystron
amplifies the already modulated signal.
The klystron can be
pulse-modulated by the anode in the types which have this separately
insulated anode, and by turning the collector voltage on and off in
the types that do not.
Except when we want to modulate the klystron,
the voltages supplied to the elements must be very stable. Usually they
are supplied from well regulated power supplies. The reasons are fairly
obvious. If the dc voltages on the cavities and collector or reflector
varies, the velocity of the electrons also varies. And, since the speed
with which the electrons travel through the buncher determines the frequency
of the generated rf, this too would vary.
In the reflex klystron
the situation is even more critical. The path the electrons travel must
be exactly the right length to allow the electrons on their return voyage
to reinforce the original bunching action. If the path should be altered,
by a varying voltage, the electrons would arrive at the wrong time and
might partly cancel the bunching. The oscillation would then soon die
As a matter of fact, this device is used to allow the reflex
klystron to operate in different "modes". The path of the electrons,
for oscillation, must always be a multiple of a quarter-wave-length.
But whether the tube has a path of 3 3/4. or 4 1/4 wavelengths for the
electrons, the action is the same. However, with the longer path, caused
by a lower (less negative) reflector voltage, the density of the beam
is somewhat affected, and the klystron produces less power. By selecting
one or the other modes the klystron can be made to put out at different
levels of power. The 25K5 for example can operate in about five modes,
all producing the same frequency, but with different power levels.
As UHF television becomes more popular, the klystron will be
used increasingly for high-power amplification in the transmitters.
Further increases in UHF scatter communication and in microwave applications
as we progress in the space age is also to be expected. The klystron,
which has proven its mettle in bouncing signals off our neighboring
planets, will most certainly be the power amplifier for space telephony,
once man takes the big jump and starts traveling between planets in
the solar system and to distant stars. It is a special vacuum tube to
be reckoned with for the next few centuries of man's technological development.