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Note that R_load may actually be Z_load -- the authors used for this figure the 90 ohm impedance of their output transmission line.
It's in H. Schilling et al, "High voltage generator with fast rise time for EMP simulation," in Tenth IEEE Int Pulsed Power Conf Dig Tech Papers, Vol 2, Jul 1995, pp. 1359-1364.
What's interesting is in re-designing their Marx for 1nS-risetime operation, the authors actually put a 60µH inductor in series with the Marx's output "to reduce late time current". This inductor charges a 1nF transfer capacitor, whose output it switched to an 82pF peaking capacitor via a self-closing spark gap. They built their transfer cap with 8 125pF caps arranged in a cylinder around the spark gap. They write: "The peaking capacitor and output inductance are critical to obtaining the 1ns risetime. The peaker is 15.2cm in diameter, with an area equal to 180cm^2. Double transit time between the output switch and the edges of the peaker varies from 0.5 to 1ns because the output switch is not at the center of the peaker. Distance from the high voltage plate to the ground plane is 2.54cm. Voltage holdoff in the dielectric and on its edges is achieved by the rapid charge time and use of sulfur hexaflouride gas at 15 bar [pressurizing the enclosure about the peaker]."
From their diagram, it looks like they built their own peaker with a stack of metal plates interleaved by dielectric.
The "output switch" is just a spark gap directly from (near) the center of the peaker to the angled end of the transmission line, adjustable by a rod passing through the wall of the enclosure.
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For a theoretical treatment of peaking caps, see: Sundararajan, R et al, "Influence of peaking capacitors in reducing rise times of high-voltage nanosecond pulses," IEEE Trans on Industry App, Vol 41 No 3, pp. 690-697, May-June 2005, DOI 10.1109/TIA.2005.847292
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How to calculate peaking capacitor for Marx?