Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

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Adhesion to itself allows casting in several stages without fear of voids or mechanically
weakened areas. A final desirable characteristic-one that is of obvious importance-is
a very high dielectric strength. With attention to detail and diligence in the casting
procedures, dielectric strengths of more then 4 kV/mil on 0.125-inch thickness have
been achieved. All these advantages have allowed operation of high-voltage pulse
systems at increased power levels and at half the volume of those previously insulated
with mineral oil.
Urethanes and Silicones
These materials are used for casting solid high-voltage equipment, as well as for
coating components to reduce the effects of shrinkage or shock. Typically these
materials are very hard to use with vacuum casting techniques and, thus, have a much
lower dielectric strength than do the best epoxies, especially in larger volumes. Another
drawback is that many urethanes and silicones require either moisture or volatile
ingredients in the curing process, both of which cause problems with high-voltage
systems. Nonetheless, a wide variety of these materials are used in the fabrication of
high-voltage pulse systems for applications that require their characteristics.
LIQUIDS
Liquid insulation has been the primary type of insulation for high-voltage systems since
the beginning of the field. Over the years, mineral oils, vegetable oils, hydrocarbons,
and even tars and saps have been used as insulation. Dielectric liquids have long
served as electrical insulation in power transformers, capacitors, cables, and switching
equipment. Several once commonly used fluids are no longer available because of their
toxicity and environmental impact. As a result, liquids for insulation that do not have
these problems have now been developed for certain applications, including mineral
oils, silicon oils, fluoropolymers, and high-molecular-weight paraffin oils. Most of the
dielectric fluids made are tailored to the power industry, which accounts for about 99
percent of the demand for these liquids. As a result, many such liquids contain additives
that, while necessary for the power industry, are detrimental to high-voltage
applications. These include low-vapor-pressure additives for controlling viscosity and
antioxidants for improved aging. In addition, most insulating liquids also contain
moisture and dissolved gases, which are only weakly bound to the liquid molecules and
are easily freed when high electric field stresses are present. Scientists have for years
worked to extend the usefulness of transformer oils, fuorinert, and castor oil. They have
also developed corona-processing equipment for improving the high-voltage
characteristics of insulating oils. This has allowed state-of-the art insulation design
using insulating oils and oil-impregnated systems. The corona processing involves
flowing the liquid insulation media through a high-field-stress region while under
vacuum to remove dissolved gases and low-vapor-pressure constituents from the oil.
The liquid is then filtered to remove particles larger than 5 microns. This process
improves the corona initiation voltage limit for the liquid and greatly extends the life of
components insulated with the media. It has also allowed a significant reduction in the
size and, thus, energy density of pulsed transformer systems.
GASEOUS
Insulating gases are used in many high-voltage applications where weight is a primary
issue. Typically the use of gases as an insulating media requires pressurization and,
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