Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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has resulted in an average dielectric strength of more than 3 kV/mil for the volumes
mentioned. Adapting this scheme to the manufacture of high-voltage capacitors could
result in significant improvement over the current state of the art of 1 joule per cubic
centimeter.
DIELECTRIC TAPERING
This insulation scheme is little known but has been used with much success in many
high-voltage systems, especially where compact high voltage is required. The basic
scheme is to first design the system while minimizing the peak electric field stress. This
involves hours of small but well-chosen changes to a design in order to shape the field
lines and achieve the least range from minimum to maximum field stress. We then find
the surfaces, which have the highest electric field stresses and therefore the highest
probability of breakdown. In evaluating these parameters, it must be remembered that
dielectric media are much less likely to initiate breakdown than are conducting surfaces
under the same electric field stress. The conductor surfaces under highest field stress
are then layered with high-voltage coatings (usually acrylics, polyurethanes, silicones,
or engineered coatings) with dielectric constants chosen to reduce the electric field
strength at the conductor surface. This technique works because the conductor is the
source of electrons, without which breakdown will not occur. Since the electric field is
excluded from regions of relatively higher dielectric constant, if the insulating volume is
filled with mineral oil (relative dielectric constant of 2.2), then a conducting surface
coated with 10 mils of polyurethane (relative dielectric constant of 3.6) will have a
lower electric field stress than it would without the coating, and the increase in field
stress in the mineral oil will be minimal.
Dialectic tapering can be applied using several layers of coatings with progressively
lower relative dielectric constant from the conducting surface and dramatically reduces
the conducting surface electric field stress. Using finite element electric field solving
codes and several hours of iteration, this technique can often reduce peak electric field
stress for a system by SO percent. The technique works best when the volume dielectric
fluid has a low relative dielectric constant, such as mineral oil has (Er =2.2), since
coatings are readily available for Er= ~3 to 5. In practice, care must be taken in
choosing and applying the coatings to ensure that no voids or bubbles are introduced at
the conductor surface. Careful inspection and repair of any flaws is relatively simple
with this technique. Another, more recent use of this concept is what is termed
continually varying dielectrics in ultrawideband (UWB) guiding structures, such as
transmission lines with greatly reduced dispersion at bends.
CATHODE MATERIALS
This area of research is vitally important to any HPM source requiring electron beam
generation. All high-power microwave tubes, including virtual cathode oscillators and
cavity resonators, rely on a bunched flow of free electrons to set up oscillating electric
fields and thereby generate a radiofrequency (RF) output. The electron flow is usually
initiated by applying a high-voltage pulse to a vacuum diode. For high-power operation,
the cathode must be capable of emitting a very high electron current density using one
of several emission mechanisms.
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