Radar System Engineering

366 THE J1.1G.\-L.’T1N3.V A,VD THE P l:LSER [SE(-. 108
The rate of rise and the overshoot on the leading edge of the ~wltage
wave are important in determining the ability of the magnetron to
operate at high power levels. A multicavity oscillator has several possible
modes of oscillation, in only one of which it operates efficiently and
smoothly. Great effort has been devoted to understanding the conditions
favorable to stable oscillations in a given mode. We now know that
stability depends on both pulser and magnetron. Usually the pulser
can be more easily modified than the magnetron if mode-shifting or modejumping
occurs. In any event, many interacting adjustments must be
made before full power output can be assured. Sometimes a change in
the r-f loading of the magnetron, in jts heater current, or in the shape of
the magnetic field will succeed ~vhere altering the pulse shape has failed.
Almost always, too short a rise time, t,, of the voltage pulse leads to mode
instability, any rate of rise over 100 kv/~sec being considered fast.’
Magnetron sparking can also be caused by high rates of voit,age rise.
A “spike” frequently appears on the leading edge of the voltage pulse.
Since a magnetron operates at a magnetic field far above the nominal
cutoff value, it is impossible for electrons to reach the anode without the
aid of the r-f field. Therefore, until the field is established, little power
is drawn from the pulser and a momentary overvolting of the magnetron
results. This phenomenon is particularly prominent in the case of
line-type pulsers, where the voltage across the magnetron can rise to
twice normal if the magnetron fails to draw power. This high-voltage
“spike” can give rise to all manner of sparking and mode troubles. Its
cure lies not alone in alteration of the pulser design, but also in attention to
magnet ron design. 2
The internal impedance of the pulser plays an important role in affecting
mode stability. There are significant clifferences between the operation
of hard-tube and of line-type pulsers. 3 The hard-tube pulser has an
internal impedance equal to that of the output tube, which varies from
90 to 150 ohms until saturation is reached. The line-type pulser, being
essentially a constant-voltage device in series with an impedance equal
to the load resistance, has a resistance of 400 to 1200 ohms. Consequently,
the conditions of stable operation for a given magnetron w-ill
vary with load and pulser type. Stable operation occurs for those
values of V and 1 which simultaneously satisfy the load and generator
characteristics. This would not concern us except that the various
modes have different V-1 characteristics, thus giving cause for instability.
1The 4J52 magnetron is an exception to the statement made here, being more
stable with a rapid rate of rise.
z More detailed discussionswill be found in Pulse Generators, Vol. 5 of this series,
and in Microwave Magnetrons, Vol. 6.
3RL Report No. 809, Sept. 28, 1945.