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

Cathode Pulsers: Hard-Tube Modulators (10) 189
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Crossatron tubes have been built withas much as 100 kV hold-off and closing
currents as great as 12,000 A. When optimized as either a closing-only or opening-only
switch, the 8454H is rated at 50-kV hold-off, 2500-A closing, and 1OOO-A
opening. The Crossatron is one of the few full-control (or at least self-commutation)
switches that is still in the process of being developed.
10.5 Load lines
Although we have discussed and illustrated the voltage/current characteristics
of a number of triode and tetrode modulator switch tubes, we have yet to
relate them to the complementary aspects of the non-linear load impedances that
they will be expected to drive. Figure 10-30 shows how two types of modulators
would interact with three types of loads, described as load lines. In this representation,
load voltage/current relationships are plotted starting with the 0/0
points of the voltage/current axes. The modulator, or source, voltage/current
relationships are plotted backwards, starting at the open-circuit-voltage, zerocurrent
points.
Of the three load characteristics, only one represents an actual device. (It is
also the least linear one.) It is a backward-wave, crossed-field amplifier, the QK-
622 Arnplitron. As voltage is increased across the QK-622, current builds up
slowly, reaching only 4 A at 40 kV, which is near the knee of its characteristic.
Between 40 kV and its operating voltage of 50 kV, current dramatically increases,
from 4 A to 32A. At its operating point, its equivalent linear-resistance impedance,
its second characteristic, would be 1563 ohms, which is the slope of a
straight line from the operating point back to the origin. However, its dynamic,
or incremental, impedance is the slope of the characteristic, or AV/N, as it passes
through the operating point, which is 200 ohms, not 1563 ohms. The third load
characteristic illustrates is a hypothetical linear-beam device whose operating
point is the same as the QK-622 Arnplitron, and it follows the 3/2-power voltagecurrent
relationship. Its incremental impedance at the operating point would be
AV/A.I,or 1024 ohms.
Also shown are the source characteristics for two types of pulse modulators:
line-type and hard-tube. The line-type modulator characteristic starts at 100 kV
and zero current at the right side of the graph and ends at zero voltage and 64A
at the left side, passing through its matched-load operating point of 50 kV and 32
A. The open-circuit voltage is twice the matched-load voltage and the shortcircuit
current is twice the matched-load current, which is a sure-fire criterion for
recognizing a matched source. (It is not, however, matched to the incremental
impedance of either of our microwave-tube loads.) The hard-tube modulator
characteristic starts at 60 kV and zero current at the right side. It is discontinuous,
however, having a knee. In fact, it is much like the Amplitron characteristic only
rotated by 90°. This means that the hard-tube modulator is the Arnplitron’s dual.
As the voltage drop across the hard-tube switch increases beyond the knee, which
occurs at 50 kV (10-kV tube drop, working from right to left), the current tends to
be essentially constant, which is the exact opposite of what happened to the
Arnplitron. Therefore, the hard-tube switch has two distinct incremental impedances:
one relatively low “below the knee” and another relatively high “above
the knee.” The most dramatic difference between the two was exhibited by the