AAWSAP DIRD, Pulsed High-Power Microwave Source Technology, January 2010

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UNCLASSIFIED/} P'Olt OP'P'l@IAL l:ISE &P•LY
Table 3 shows findings of cathode material studies at SNL and at the AFRL.
Table 3. Cathode Study Findings
Material
Emission Threshold
(kV)
Lifetime
(# of Shots)
Outgassing
(Neutrals/Electron)
Csl-Carbon
Micro.fibers
< 3 kV/cm
> 72,000
4 ­ 6.5 (substrate)
"Sandia Red" Velvet
8 kV/cm
- 8,000
10
"MILO Green" Velvet
10 kV/cm
- 4,000
10-14
Velveteen Low
Low
<600
> 12
F-Velvet
Low
< 100
> 12
Ceramic Cloth
> 120 kV/cm
--­
--­
Ceramic Felt
> 100 kV/cm
--­
--­
Carbon Pyramids
> 80 kV/cm
--­
--­
Carbon Nanotubes
20-50 kV/cm
Arc rate of ­
2%
-4
Bare Carbon
Microfiber
(packing density)
15-40 kV/cm
> 36,000
4.3 ­ 6.5
(substrate)
Csl-Carbon Fiber
Tufts
< 3 kV/cm
> 200,000
-4
Metal I Ceramic
95 kV/cm (diode
collapse
unless> 150 kV/cm)
--­
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HIGH-VOLTAGE SWITCHING
High-voltage switching is among the most challenging of technologies for HPM sources.
Although high-voltage switches have been used for several decades, and thousands of
experiments have been performed on the mechanisms involved in liquid and gaseous
breakdown, there are still many aspects of the phenomenon that defy explanation. This
is especially true as the time required to reach the fully conducting state becomes
extremely short. The usual explanation for this process involves Townsend avalanching,
whereby electron streamers begin at the cathode in an average electric field of only 20­
25 kV/cm and, by virtue of an enhanced electric field at their tip, progress in an orderly
fashion to the anode. At this time, a heating phase begins, and an increasing amount of
current is passed through the streamer until the switch finally reaches the fully
conducting state. The problem with this explanation is that it is most likely incorrect
and relies on exaggerated ion densities to explain how switches can reach full
conduction in less than a billionth of a second. Alternative explanations involving
runaway electron generation provide a better match to observations. Very fast
switching is critically important to the concept of UWB HPM. The basic concept is to
generate a square pulse with the fastest rise time possible. A Fourier transform of this
waveform results in a frequency spectrum containing frequencies determined by the
width and rise time of the square pulse. The period of the lowest frequency is twice the
pulse width, and the rise time is about one-quarter the period of the highest frequency.
UNCLASSIFIED/ ,<FQA OFFICIO I. Uili OPII.V
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