To overcome beam spreading, an axial magnetic
field is used. The action of the magnetic field is to
exert a force on the electrons that keeps them focused
into a narrow beam. The magnetic field may be de-
veloped by a permanent magnet or by one or more
electromagnets. A permanent magnet is used in tubes
that are physically small or of medium power rating.
Unfortunately, the size and the weight of a permanent
magnet are excessive for long or high-power tubes,
making it necessary to use electromagnets. In some
large tubes, several separate electromagnets are used.
The current in each coil is individually adjustable to
optimize the magnetic field shape. The magnetic field
is normally terminated a short distance beyond the
output cavity so that the beam can spread before it hits
the collector. This tends to spread the electron beam
interception over a large surface on the collector,
minimizing collector cooling problems that otherwise
would result from the beam remaining concentrated at
the time of interception.
Even with an axial magnetic field, some electrons
stray from the main electron beam. These electrons
are intercepted by the anode or the klystron drift
tubes. In high-power tubes, it is particularly important
to minimize the number of stray electrons because of
the heat generated when they strike the drift tubes.
And in a high-power klystron, this heating maybe a
very severe problem because drift tubes are quite dif-
ficult to cool. Temperatures may become high enough
to melt the drift tubes, thus destroying them.
Klystron amplifiers normally have actual metal
grid structures across the gaps in the resonant cavities.
Many low-power klystrons have wire mesh grids.
However, most high-power klystrons do not have
actual grids across the gaps. Such grids would inter-
cept sizable quantities of electrons.
It is very difficult to cool grid structures, and a
large amount of beam interception would melt the
grids, thus destroying the tube. Fortunately, by proper
design, the klystron may be made to operate effi-
ciently without actual grid wires across the cavity
gaps. The absence of these grids does not change the
operating principles, but it does have a secondary
effect on klystron performance. If the electron beam
has a small diameter compared to the diameter of the
drift tube, the beam does not couple energy to the
cavities very well. Therefore, the performance of a
klystron amplifier, which does not have gridded gaps,
may sometimes be improved by permitting the elec-
tron beam to be as wide as possible, while keeping the
body current down to the maximum specified for the
tube. The width of the beam may be somewhat con-
trolled by the magnetic field strength.
Body current usually increases with RF input
level, because it is the RF input that causes the
bunches to form. The dense electron concentration in
the bunch causes mutual repulsion of electrons, and
the diameter of the bunch may become larger than the
diameter of the beam with no bunches present.
Consequently, some of the electrons in the bunch may
be lost to the drift tubes, and the body current may
ADDITIONAL EQUIPMENT FOR KLY-
STRON AMPLIFIERS. Additional equipment is
required for a complete amplifier system. Various
power supplies are necessary to deliver required volt-
ages and current. In high-power systems, various RF
circuit components are required to control and meas-
ure the RF input to the klystron tube and to measure
the RF output from the tube. A large collection of
meters and protective devices is needed to monitor
performance and protect operating personnel and
equipment in the event of a malfunction or operator
In most klystron tubes, the anode and the RF
section are connected inside the vacuum envelope.
These connected parts are called the tube body and are
generally operated at ground potential. It is conven-
ient to operate the tube body at ground potential
because the input and output connections (either
waveguide or coaxial) are then also at ground poten-
tial. This makes it easier to connect the klystron into
the rest of the system. In addition, the cavity tuners
are at ground potential, eliminating any danger to per-
sonnel tuning the tube.
The beam supply provides the voltage required to
accelerate the electrons and form the beam. It must
also deliver the required beam current. The crowbar