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
See all available vintage Radio
- 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
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
Less romantic, 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
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 they operate?
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.
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.
Here we 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.
If 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 two-cavity klystron.
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 buncher.
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 oscillator.
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
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
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
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 out.
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.