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

Cathode Pulsers: Hard-Tube Modulators (10) 197
in such a low-impedance primary loop was series inductance. The inductance of
the as-built primary loop-which included the capacitor bank, the eight switch
tubes, the electronic crowbar connection, and the pulse-transformer primary connections
(see Fig. 10-35)—was 1.7 LH. When the primary-loop current of 4000 A
is brought to zero in 0.1 p, or 10-7 seconds, the rate-of-change of current is 4x101O
A/s, and the impulse, or L di/dt, voltage across 1.7 WH would be 68 kV, which
more than doubled the instantaneous anode voltage. This factor, of course, came
in addition to the voltage spike produced by the pulse-transformer back-swing
clipper circuit, which shunted the primary winding. Substantial dv/dt snubbing
was required before this component of stress was brought to more reasonable
levels.
Before giving up on fall time, however, engineers made another attempt to
further reduce the primary-loop inductance. A low-impedance, parallel-plate
transmission line was used. It had a very large width-to-thickness ratio to make
the interconnections, as illustrated in Fig. 10-36. The intent was good but the
implementation was doomed to failure because of the electric-field enhancement
NDIPbvehighvoltage
160 kVdcmw”mum
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VKS-6250 klya!ron
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~ 130-kVpulse
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Figure 10-37. Simplified schematic diagram of 18-A4Wpeak-power, high-precision, hard-tube modulator
using L.-5O97BSTS.