Radar System Engineering

SEC. 10.10] LINE-TYPE PULSERS 381
The hydrogen-filled thyratron (Fig. 10.41) can satisfactorily pulse at
much higher frequencies than is possible with mercury or other heavy-gas
fillings, because of the high ion mobility inherent in the light-gas filling.
Of course, the average-power rating of the tube must not be exceeded
when going tohigh pulse rates. In going tohighpulse rates, the pulse
width must bedecreased faster than l/f, since the average anode drop at
durations of a few tenths of a microsecond is higher than that during a
long pulse. The hydrogen in the discharge seems to require about 10–7
sec to become fully ionized, so that a high tube drop is required to deliver
the necessary initial current. It is therefore desirable to reduce the pulse
power output as well as pulse width if tube life is to be maintained at
increased repetition rates. Pulse rates up to 40,000 cps have been
obtained at reduced power output.
Hydrogen thyratrons in production are the 3C45, 4C35, and 5C22,
rated at 25, 250, and 1000 kw respective y; they are shown in Fig. 10”42.
The development of such tubes for still higher power levels is under way.
A nonlinear inductance can also be used as a switch. An inductance
with a special alloy core may be placed between points A’A (Fig. 10.36).
This device has the property of Fixedsparkpoints
possessing a high inductance when
the current through it is small, and A’ ~
b
a very low inductance when the current
is large. Since this type of
N
switch has generally been used with
a modulated energy source, there is
initially no voltage across A’A and
hence the impedance is high from A’
to A. As the voltage is built up the
current gradually increases until
suddenly, and in a regenerative
way, the impedance of A ‘A drops,
thereby allowing the line capacity to
FIG.lC.43.—Rotarysparkgap.
discharge through the” switch,” lowering its impedafice still more. A d-c
bias winding is sometimes used to control the point at which the impedance
suddenly drops from its high value. Virtues claimed for the nonlinear
inductance switch are long life, ruggedness, and simplicity. The switch
can be operated at pulse repetition frequencies up to 4000 pps and at
power levels in excess of several hundred kilowatts.
The rotary spark gap, because of its great simplicity and high power
handling ability, has been widely used as a switching device. Figure
10.43 shows schematically how a rotating insulating disk pierced by
tungsten pins serves the function of a switch. As the disk revolves and a
rotating pin approaches a fixed pin, the electric field strength becomes