3.6M north10.pdf - Dean-O's Toy Box

Cathode Pulsers: Hard-Tube Modulators (10) 195
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Figure 10-34. Simplified schematic diagram of “200-MW modulator using WL-8461 triodes,
10.7 Some practical examples of hard-tube modulators
Now let’s look at some applications of a couple of these switches.
10,7.1 The 200-MW triode WL-8461) hard-tube modulator
A hard-tube pulse modulator was actually built using eight parallel-connected
WL-8461 triode switch tubes coupled to a super-power klystron load through a
320-kV output-pulse transformer. Its simplified schematic diagram is shown in
Fig. 10-34. Each of the triodes was to be operated with peak plate current of 500
A (for a total primary-loop pulse current of 4000A) and a supply voltage of 65
kVdc, which is adequate to support a 50-kV primary voltage output pulse with
as much as 15-kV switch-tube anode-voltage drop.
Although several identifiable pulses at the 200-MW peak-power level were
actually obtained, the modulator was never capable of continuous, repetitive
operation at this level. The operation of the electronic crowbar in response to
individual switch-tube arcs was far too frequent an occurrence, even at output
levels below 1OO-MW peak. This was an experimental modulator, however, and
much was learned from it. One empirical lesson, which has not added much to
an analytically satisfying data base, was that the performance demonstrated by a
single tube operating alone could not be repeated when the tube was operated in
parallel with others trying to do the same thing. It might be expected, for instance,
that if an individual tube had an average period between internal arc
events called T, an ensemble of n such tubes in parallel might have an average
spark-down interval of T/n. The observed interval was much shorter than that.
Internal switch-tube arcs are presumably random, non-synchronous events.
Extensive diagnostic checks were used, including broad-band delay lines to de-
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