Defense Intelligence Reference Document Pulsed High-Power Microwave Source Technology

UNCLASSIFIED/ /FOR OEFICICP l!Slii Si'l.bl/
layering of different materials; and (3) insulation by other means, such as magnetic
insulation-these are usually used only where special conditions apply.
UNIFORM HOMOGENEOUS
Uniform homogeneous insulation implies that the insulating material is consistent
throughout the volume. However, in some cases, such as that of epoxies, there is a
uniform loading of some other material, usually to increase some desired characteristic
of the final product. Examples are loading with silica to increase dielectric strength and
loading with glass fibers to increase mechanical strength. The loaded material is
typically of such small dimensions that it has only a very small effect on other material
parameters from the truly homogeneous case. The use of uniform homogeneous
insulation also results in a more easily modeled design.
SOLID
Solid insulation is often the easiest and typically the most desirable form of insulation
since it does not require maintaining or replacing a liquid level or containing or
monitoring pressure. This fact is often critical to a source project if maintenance or long
shelf lives are important factors.
Plastics
The true title for this section should be "Thermoplastic Polymers (Plastics)," as they
comprise one of the largest groups of insulating materials used in pulsed power and
HPM generation. The term "plastics" includes acetals, acrylics, amides, imides
polyarylate, polybutylene, polycarbonate, polypropylene, styrene, and sulfone
polymers. Plastics were first used as insulation in the 1930s, and it is hard to conceive
of constructing a high-voltage pulse source without them. Plastic materials have been
tailored to suit a wide variety of applications. In the early 1980s, plastics manufacturers
soliciting Sandia National Labs stated that they could engineer plastics to meet any set
of material properties desired. It later became apparent that this was not the case and
that, as usually occurs in nature, when one parameter was made more desirable, others
were made less desirable. In spite of this fact, some well-engineered plastics are now
available for some very demanding applications, such as switch housings and
transmission lines. Nevertheless, virtually no new plastics are being introduced today.
For the past 20 years, engineers have worked with essentially the same plastic
materials, although some improvements have been made in the quality of resins and
extruding and casting methods. In spite of this, there is still much more variation in
specifications (especially mechanical specifications, such as tensile strength) for plastics
from batch to batch than there is for metals. For this reason, the most demanding
plastics applications where the limits of some specification will be approached require
purchasing and independently testing a specific batch to assure confidence. One
interesting and well-documented phenomenon associated with plastics is the
nonlinearity of electrical breakdown strength with thickness. In very thin layers, some
plastics display extremely high breakdown strength. For instance, polypropylene in half-
mil (1 mil= 1/1000 inch) layers yields 7,000-volts-per-mil breakdown strength, while
in one-eighth-inch thickness, this figure drops off to 900 volts per mil. One theory to
explain this is that the proximity of imperfections in the material across the thickness
reduces the dielectric strength in thicker samples. This fact can be used to advantage
by layering thin sheets of insulation together to form thicker insulating regions (see
2
UNCLASSIFIED/) I Oit err1e1111t ""I! 8HLY