Radar System Engineering

SEC. 10.6] MAQNETRON CHARACTERISTICS 353
produce a frequency modulation of 5 amp X 0.4 Me/see per amp =
2 Me/see. Since the bandwidth of the receiver for a l-psec pulse would
be about 2 Me/see, frequency modulation of this magnitude will result
in a serious loss of received energy. When longer pulses and correspondingly
narrower bandpass receivers are used, this problem becomes much
more critical and may place very severe requirements on the flatness of
the current pulse.
Instabilities.—It has so far been found impossible to construct magnetrons
that do not occasionally present to the pulser either a very low
impedance as a result of a gas discharge (sparking) within the tube, or a
very high impedance due to a failure of the magnetron to oscillate in the
proper manner (mode-changing). Either of these events may occur only
once or twice per million pulses, but when such an event does take place,
voltage and current surges are frequently produced in the pulser which
may cause failure of some component. The pulser designer must therefore
over-design components and provide special protective circuits to
guard against events that may happen only once in a million pulses. All
magnetrons change mode or spark occasionally, but the frequency of
sparking or mode-changing can be reduced by operating at moderate peak
anode currents and short pulse durations. Magnetron performance and
life are materially increased if the pulser design is such that, in the event
of sparking or mode-shifting, an excessive discharge does not take place
through the magnetron.
These two types of instability, mode-changing and sparking, are difficult
to distinguish in practice, since mode, changing usually produces
sparking and vice versa. In spite of this, it is advantageous to consider
them separately since the cure for each is quite specific and distinct.
Sparking is an internal discharge in the magnetron which arises as a
consequence of the generation of bursts of gas withh the tube. The
gas may be liberated from the anode or from the cathode; in either event
the frequency of the phenomenon is multiplied by an increase in anode
voltage, anode current, or pulse length. Operation of a magnetron under
.conditions which exceed specifications for any of these quantities results
in a very rapid increase in sparking rate. Sparking limits the maximum
pulse length at which magnetrons can be operated.
Of all the modes of oscillation possible in magnetrons, only the ~-mode
is ordinanl y used but all magnetrons wil 1 sometimes oscillate in an
unwanted mode, or alternate erratically between two modes. This
tendency is responsible for some of the most troublesome problems in
magnetron and pulser design. Considerable progress toward an under.
standing of the phenomenon has now been made, 1 and when mode.
1F. F. Rieke and R. Fletcher,“Mode Selectionin Magnetrons,” RL Report No.
S09, Sept. 28, 1945. MicrowaveMa@etrom, Vol. 6, Chap. 8, Radiation Laboratory
Series.