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July 1964 Radio-Electronics
 [Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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This "Multiple-Beam
Klystron Pushes Back Microwave Frontiers" article in a 1964 issue of
Radio-Electronics magazine explores the klystron, a revolutionary vacuum
tube capable of operating at ultra-high frequencies (UHF to gigacycles) where
conventional tubes fail. Unlike standard designs, klystrons use internal
resonant cavities instead of external coils, enabling efficient velocity
modulation - bunching electrons into pulsating AC for microwave generation. The
piece details GE's breakthrough multiple-beam klystron (6601), which integrates
10 electron beams with shared cavities to deliver 45 kW at 8.4 GHz while
maintaining redundancy (a single beam failure only reduces output by 10%). Key
innovations include tunable resonators, magnetic beam focusing, and water
cooling (30 gallons/minute). The article also highlights GE's more powerful
ZM6602 (100 kW) and foresees even larger designs, cementing the klystron's role
in radar, space communications, and high-power RF systems by pushing the limits
of microwave technology.
Multiple-Beam Klystron Pushes Back Microwave Frontiers
Cover Story: New breed of tube puts out more continuous power
at higher frequencies than anyone thought possible
By Eric Leslie
The Klystron - the tube of the future - handles far higher frequency signals
than the tubes we are used to. A very different kind of tube, it has a special approach
to the job of generating or amplifying signals.
To find out how klystrons work, let's see what happens in the output circuit
of an ordinary tube. For instance, take an audio signal generator, working at, say,
1,000 cycles-the type that would supply a bridge or similar device. The output is
produced across the secondary of the transformer in Fig. 1. But the current in the
plate circuit of the tube, and therefore in the primary of the transformer, is not
alternating current. It is a direct current of varying strength. We have little
surges of electrons flowing 1,000 times a second through the primary winding. As
each of these bunches of electrons flows through the primary, it creates a stronger-than-average
field around the core. This field acts on the secondary and produces a true alternating
current.
Fig. 1 - Pulsating dc in transformer primary becomes ac in
secondary.
Thus there is not essentially such a tremendous difference between alternating
and pulsating direct current. (A mathematician would call the pulsating current
in the primary a direct current with an alternating current superimposed on it.)
The klystron gets the same result in a different way - a way that works at extremely
high frequencies, where ordinary tubes become too inefficient. Klystrons work from
the uhf band to well up in the gigacycles. (A gigacycle is 1,000 mc.) One reason
for its far better high-frequency performance is that instead of external coils
and capacitors (which would be mere bits of wire above 1,000 mc), its tuning elements
are inside the tube. They are resonant cavities.
You have probably read articles that derived the resonant cavity from a coil
and capacitor. The description starts out by showing the capacitor as two plates
with a single half-turn of wire between them. Then more half-turns are placed in
parallel with the first - reducing the inductance each time - until a solid wall
has been built between the two plates. If you haven't run into this, Fig. 2 is the
classical illustration, and you can get more detail in any text-book on klystrons.
(Also, see "Klystron, Tube for Outer Space," Radio-Electronics, February 1961.)
Fig. 2 - Classic derivation of microwave resonant cavity
from lumped-constant L-C circuit. In (a), schematic of ordinary coil-capacitor tank;
b - highest possible frequency for discrete L-C components;
c - "transition"
step: paralleled "turns" to reduce inductance;
d, cylindrical resonant cavity,
logical outcome of step c;
e - klystron cavity.
Fig. 3 - Principle of single-beam, multiple-cavity klystron.
Here, let's just say that a resonant cavity tunes to a much higher frequency
than any coil-capacitor combination could, unless it were fantastically smaller
than the cavity.
This resonant cavity - or a number of them, in most klystrons - is used to bunch
the electrons in the stream flowing through the tube to produce an alternating current
like the one in Fig. 1. This is done by varying the velocity of the electron stream.
For that reason, klystrons are called velocity-modulated tubes.
Electrons in Bunches
Fig. 3 is a simplified sketch of a single-beam klystron. Electrons are emitted
from a cathode at the bottom, and picked up by the collector at the top, which is
maintained at a high voltage. We want to convert this high-voltage power, now being
used to draw the electrons through the tube, into an ac power source that can supply
ultra-high-frequency power to an external circuit.
Let's suppose that the cathode is hot and that the high voltage has just been
turned on. Electrons start from the cathode toward the collector. As they approach
the first section or tunnel in the bottom cavity, they repel electrons in the metal
around them as they approach it. These electrons circulate around the walls of the
first cavity to the top, producing a negative charge at that end. (Because electrons
are repelled from it, the bottom has a slightly positive charge.) The first electrons
traveling on through the resonant cavity now approach the second tunnel, repelling
the electrons from it back toward the bottom section of the first cavity. This makes
it negative, while the top section tends to become positive.
As the bottom section becomes negative, it starts to retard electrons approaching
it from the cathode, while giving those that have just passed it a boost.
If the voltage (or rather the speed of the electron stream) and the dimensions
of the cavity are correctly matched, the cavity will resonate at its natural frequency
and build up high alternating charges at the top and bottom ends of the resonant
cavities. An rf drive signal, injected into the first cavity, will help the process.
This process tends to bunch the electrons into groups by pushing those ahead
of each negatively charged area while retarding those coming toward it or, on opposite
alternations, holding back those above while attracting those below. As the electrons
travel through the tunnels between cavities, the bunches are tightened as they catch
up with slowed-down electrons ahead and are joined by speeded-up ones from the rear.
The electron stream goes through several other cavities, where the effect is
made greater as it passes through each one. By the time the last cavity is reached,
we have a true pulsating electric current, the same as in our transformer primary.
The resonant cavities have slowed down the first electrons in each bunch, and speeded
up the last ones to where we have distinct "clumps" of electrons, traveling toward
the collector.
As the electrons approach the last cavity, they are organized into very tight
clusters or bunches - one such bunch for each rf cycle. These bunches of charge
repel the like charges in the metal gap tips of the last cavity. This action establishes
a very large voltage or electric field across these tips.
To move across the gap against this large electric field, the electrons must
overcome the opposing electric force. In so doing they give up a large portion of
their kinetic energy. This is transformed into microwave energy, which is piped
through a section of waveguide located in the vacuum envelope, then through a vacuum-sealing
window of special alumina ceramic material to the external waveguide.
Very large klystrons have been built. But voltage has to be increased to increase
output, and more power was needed than can be obtained from a practical single klystron.
The natural step would be to couple two klystrons together. This has been done.
In fact, as many as 8 have been coupled. But external coupling circuitry is complex.
Slight misadjustments cut efficiency tremendously and, if a single klystron fails,
the whole group goes out of operation.
Fig. 4 - New G-E multiple-beam klystron, in part.
To avoid this, General Electric engineers developed the idea of building a new
type of tube - one that would in effect be 10 klystrons with a common group of resonant
cavities. Fig. 4 shows a portion of that tube. The broken section in the center
is occupied by a similar set of cavities and 6 more electron beams. This makes it
possible to get 10 time the power of a single klystron, while keeping many of the
advantages of single klystron operation. Since the cavities are common to the 10
beams, they must remain in phase. If a single cathode breaks down, it simply means
that one-tenth less power is produced. (By raising the voltage slightly, it may
even be possible to compensate for that.) The General Electric 6601, shown on our
cover, produces about 45 kilowatts output at about 8.4 gc, with a beam voltage of
12.4 kv and a cathode current of 11 amperes - a little over 1 ampere per gun.
In use, the tube is surrounded by a magnet coil, to focus the electron beams.
Also important: The tube can be tuned over a range of 80 me by varying the capacitance
of the resonator. Cooling is needed to carry away the tremendous heat generated,
so a water jacket circulates 30 gallons per minute around the tube.
Since the photograph on our cover was taken, a larger tube, the ZM6602, with
a 17-kilowatt operating voltage and a cathode current of 17.5 amperes, has been
introduced. It produces 100 kilowatts CW. Even larger tubes may be made in the future.
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