DISCLOSURE / FILE
DIA Defense Intelligence Reference Document DIA-08-0912-005, dated 28 January 2010, is a 37-page AAWSA-program technical survey of pulsed high-power microwave (HPM) source technologies, covering insulation, switching, cathodes, pulse sources, HPM source architectures, and antennas.
DIA AATIP Defense Intelligence Reference Documents (38 DIRDs), FOIA Release, DIRD_03-DIRD_Pulsed_High-Power_Microwave_Source_Technology.pdf
DIA Defense Intelligence Reference Document DIA-08-0912-005, dated 28 January 2010, is a 37-page AAWSA-program technical survey of pulsed high-power microwave (HPM) source technologies, covering insulation, switching, cathodes, pulse sources, HPM source architectures, and antennas.
Authored under the Defense Intelligence Agency's Advanced Aerospace Weapon System Applications (AAWSA) Program in FY 2009 and issued 28 January 2010, this DIRD surveys pulsed high-power microwave (HPM) source technology for directed-energy weapons applications, with peak powers described as exceeding 10 gigawatts. The summary frames HPM as the post-HEMP successor whose effects became fully known after the 1962 Starfish Prime high-altitude nuclear test disrupted electronics 800 miles away in Hawaii. It catalogs critical sub-technologies including dielectric insulation, high-voltage switching, cathode materials, Marx and transformer-based pulse generators, and source families such as BWOs, TWTs, RKAs, virtual cathode oscillators, magnetrons, gyrotrons, and named systems Phoenix, GEM II, Jolt, SNIPER, and EMBL. The paper cites a 28 May 2001 incident in which a U.S. Comanche helicopter testing HPM weapons in New York reportedly generated a pulse that disrupted GPS landing systems for commercial aircraft in Albany.
HEMP effects became fully known in 1962, when a high-altitude nuclear test (codenamed "Starfish Prime") over the Pacific Ocean disrupted radio stations and electronic equipment 800 miles away in Hawaii.p.6
HPM sources developed for this purpose provide peak powers in excess of 10 gigawatts. The goal of such a weapon is to disable communications and computer systems prior to any troop movements and render the enemy unable to stage a response.p.7
As an example of the possible effects of HPM weapons, on 28 May 2001, a U.S. Commanche helicopter, flying in New York state while performing tests involving HPM weapons, was reported to have generated a low-level energy pulse that disrupted the Global Positioning System devices used to land commercial aircraft in Albany.p.7
Failures in semiconductors owing to thermal effects occur when junction temperatures are raised above 600° Kelvin.p.7
In addition, it is very difficult to detect these sort of pulsed sources, since the pulses are very short (typically 1-500 nanoseconds), and even in burst mode, the bursts are usually less than 10 seconds.p.8
The UWB sources would be the most difficult to detect, since they have nearly zero energy at any one frequency and so would not be detected at all by instruments such as spectrum analyzers.p.8
This product is one in a series of advanced technology reports produced in FY 2009 under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace Weapon System Applications (AAWSA) Program.p.2
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UNCLASSIFIED//FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Acquisition Threat Support 28 January 2010 ICOD: 1 December 2009 DIA-08-0912-005 Pulsed High-Power Microwave Source Technology UNCLASSIFIED//FOR OFFICIAL USE ONLY
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UNCLASSIFIED//FOR OFFICIAL USE ONLY
Pulsed High-Power Microwave Source Technology
Prepared by:
(b)(3):10 USC 424
Defense Intelligence Agency
Author:
(b)(6)
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized.
This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or questions pertaining to
this document should be addressed to (b)(3):10 USC 424;(b)(6) AAWSA Program
Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington,
DC 20340-5100.
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UNCLASSIFIED/ /FOR OFFICIAL USE ONLY
Conclusion
This paper provided an overview of the current state of HPM sources and the
technologies driving their development. Advanced cathode materials, computer codes
for more predictive ability in electron beam generation and propagation, high-speed
switching at high power, and low-loss insulation techniques are just a few of the areas
where advancement clearly is needed for progress to occur. Advancements in
photoconductive solid-state (PCSS) and other solid-state switching and sharpening
devices appear to finally be reaching a level of development such that they may soon
be of use in high-voltage systems. If PCSS switching devices capable of higher
operating voltage and extended lifetime are made available, variations of the phased
array will become the HPM source design of choice, given the inherent advantages.
More compact antennas are greatly desired; however, the physics of the radiation
process requires structure sizes on the order of one-half wavelength of the lowest
radiated frequency. With demanding levels of directivity and gain also requirements,
compact UWB antenna designs will continue to be difficult, if not impossible, to realize.
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UNCLASSIFIED/ /FOR OFFICIAL USE ONLYConcatenated page-by-page transcript. Born-digital pages came through pdf.js; scanned pages were transcribed by Claude vision OCR. Pages marked unreadable failed multiple OCR retries (heavy redaction, microfilm artifacts, or blank separators) and are kept in place for audit.
UNCLASSIFIED//FOR OFFICIAL USE ONLY Defense Intelligence Reference Document Acquisition Threat Support 28 January 2010 ICOD: 1 December 2009 DIA-08-0912-005 Pulsed High-Power Microwave Source Technology UNCLASSIFIED//FOR OFFICIAL USE ONLY
UNCLASSIFIED//FOR OFFICIAL USE ONLY
Pulsed High-Power Microwave Source Technology
Prepared by:
(b)(3):10 USC 424
Defense Intelligence Agency
Author:
(b)(6)
Administrative Note
COPYRIGHT WARNING: Further dissemination of the photographs in this publication is not authorized.
This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, (b)(3):10 USC 424 Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or questions pertaining to
this document should be addressed to (b)(3):10 USC 424;(b)(6) AAWSA Program
Manager, Defense Intelligence Agency, ATTN: (b)(3):10 USC 424 Bldg 6000, Washington,
DC 20340-5100.
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Contents
Summary.................................................................................................................vi
Critical Technologies ........................................................................................ 1
Insulation ................................................................................................ 1
Uniform Homogeneous..............................................................................2
Solid.........................................................................................................2
Plastics..................................................................................................2
Epoxies .................................................................................................3
Urethanes and Silicones ........................................................................4
Liquids ....................................................................................................4
Gaseous ...................................................................................................4
Laminated ................................................................................................6
Plastic-Paper-Oil .......................................................................................6
Plastic-Paper-Epoxy ..................................................................................6
Dielectric Tapering ....................................................................................7
Cathode Materials .....................................................................................7
Velvet.......................................................................................................9
Carbon .....................................................................................................9
Ceramics ................................................................................................10
Cesium Iodide Coated .............................................................................10
High-Voltage Switching............................................................................11
Gaseous Switching ..................................................................................12
High-Speed Liquid Switching....................................................................14
Solid-State Switching ...............................................................................14
High-Voltage Pulse Sources .............................................................................15
Marx Generators ......................................................................................15
Transformer Based Generators .................................................................16
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Explosively Driven Generators ..................................................................16
Pulsed High-Power Microwave Sources..............................................................17
Pulsed Electron Beam Sources ..................................................................17
BWOs, TWTs, and RKAs...........................................................................17
Split-Cavity Oscillators..............................................................................18
Virtual Cathode Oscillators........................................................................18
Magnetrons .............................................................................................18
Gyrotrons ................................................................................................19
Impulse HPM Sources ..............................................................................20
SNIPER....................................................................................................20
EMBL .....................................................................................................20
H-Series HPM Sources.............................................................................20
The Phoenix HPM Source..........................................................................22
The GEM II HPM Source ..........................................................................24
The Jolt HPM source ................................................................................24
Mesoband Sources ...................................................................................24
HPM Antennas..................................................................................................25
Narrowband Antennas..............................................................................25
Wideband and Ultrawideband Antennas.....................................................27
Conclusion.........................................................................................................29
Figures
Figure 1. Paschen Curve for Air.....................................................................13
Figure 2. Example of Marx Generator Circuit .................................................16
Figure 3. Orion HPM Testing Facility.............................................................19
Figure 4. Active Denial System With FLAPS Antenna .....................................20
Figure 5. H2 With Large TEM Horn and PGC Output......................................21
Figure 6. Cross-Section Drawing of H5 With Point Geometry Converter, Brewster
Angle Window, and Extended-Ground-Plane Antenna ..........................21
Figure 7. H5 Output Section With the Point Geometry Converter Feeding an
Extended-Ground-Plane Antenna Through a Brewster Angle Window...22
Figure 8. Phoenix Radiated Pulse at 8.5 Meters .............................................23
Figure 9. Phoenix Radiated Spectral Content .................................................23
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Figure 10. Jolt Hyperband HPM Source..........................................................24
Figure 11. Jolt Radiated Electric Field Waveform at 85 Meters........................24
Figure 12. FLAPS Antenna With a Cross-Shorted Dipole Array ........................26
Figure 13. Mode Converter Vlasov Antenna and Vlasov Antenna Attached to a
Coaxial MILO ...................................................................................27
Tables
Table 1. Dielectric Properties of Some HPM Plastics...........................................3
Table 2. Relative Spark Breakdown Strength of Gases .......................................5
Table 3. Cathode Study Findings .....................................................................11
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Pulsed High-Power Microwave Source Technology
Summary
This paper provides an overview of the major types of high-power microwave
(HPM) sources and the critical technologies required to build them. Pulsed
HPM technology has been of scientific and military interest for several decades.
Originally, the interest focused on the area of high-altitude electromagnetic
pulse (HEMP) concerns.
HEMP is produced when a nuclear weapon is detonated high above the earth's
surface, creating gamma radiation that interacts with the atmosphere to
create an intense electromagnetic (EM) energy field that is harmless to people
as it radiates outward but can overload computer circuitry with effects similar
to, but causing damage more swiftly than, a lightning strike. HEMP effects
became fully known in 1962, when a high-altitude nuclear test (codenamed
"Starfish Prime") over the Pacific Ocean disrupted radio stations and
electronic equipment 800 miles away in Hawaii. The HEMP effect can span
thousands of miles, depending on the altitude, design, and power of the
nuclear burst. It is speculated that a single device detonated at high altitude
over the central United States could affect the entire country, since the HEMP
would be picked up by conductors, such as wires and power cables, acting as
antennas to conduct the electrical energy into various electronic systems. This
EM radiation was found to have field levels on the order of several hundreds of
kilovolts per meter with onset or rise times of a few nanoseconds and a
duration of nearly a microsecond. Fields of these magnitudes were found to
have severe detrimental effects on numerous electrical systems and items,
such as the power grid, automobiles, communications equipment, and aircraft.
Much testing was done in the 1960s and 1970s to determine vulnerabilities
and to find mitigation solutions. The HEMP is essentially a wideband
microwave pulse, and several EMP simulators were devised and built for use in
testing its effect on various electronic systems. In addition, several programs
were established to investigate the possibility of generating and radiating
other spectrums of EM radiation that may have some of these same effects
without requiring a nuclear detonation. This was the impetus for the
advancement of pulsed HPM technologies for weapons.
HPM radiation is composed of shorter waveforms at higher frequencies than is
HEMP, which makes it highly effective against electronic equipment and more
difficult to harden against. Whereas HEMP weapons are large in scale and
require a nuclear capability along with technology to launch high-altitude
missiles, HPM weapons are smaller in scale, involve a much lower level of
technology, and are within the capability of almost any state. HPMs can
damage computers and electronics similar to the way HEMP can, although the
effects are limited to a much shorter range. Technical accessibility, lower cost,
and the vulnerability of U.S. electronic equipment could make small-scale HPM
weapons attractive to terrorist groups. HPM devices are now categorized as
directed-energy weapons.
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One major use of HPM by the military is for electronic attack, or what is
referred to by the media as an "ebomb." HPM sources developed for this
purpose provide peak powers in excess of 10 gigawatts. The goal of such a
weapon is to disable communications and computer systems prior to any troop
movements and render the enemy unable to stage a response. The technology
used to drive such sources has its roots in pulsed power, and for narrowband
sources requires the use of tools that have been developed in the plasma
physics community. Reliance on microprocessors that have an increasing
density of circuits packaged onto each chip makes such systems ever-more
vulnerable to HPM effects. As an example of the possible effects of HPM
weapons, on 28 May 2001, a U.S. Commanche helicopter, flying in New York
state while performing tests involving HPM weapons, was reported to have
generated a low-level energy pulse that disrupted the Global Positioning
System devices used to land commercial aircraft in Albany.
The use of this type of weapon can be based on several scenarios, depending
on the asset it will be used against. Sometimes, it may only be necessary to
upset a data bus transfer to produce a success; other times, success may not
be so simple. Some of the kill mechanisms obtained from microwave weapons
include semiconductor overheating or burnout, arc generation, computer
upsets, voltage induction into sensitive circuits, display upset, and overvoltage
in discrete components. The asset to be neutralized will often determine the
specific type of microwave source to be used; for example, assets with slots
designed for communications purposes may be most vulnerable to narrowband
HPM of a specific frequency, while assets with several computers linked by a
communications bus may be more vulnerable to ultrawideband (UWB) pulses
of a specific pulse repetition rate (PRR). Often, the variety of EM radiation that
would be most effective is not obvious, and therefore several must be
evaluated.
If EM radiation is able to penetrate a target, the issue then becomes the
susceptibility of the many semiconductor devices, which make up the various
circuits of the target. Failures in semiconductors owing to thermal effects
occur when junction temperatures are raised above 600° Kelvin. Since thermal
energy diffuses through the semiconductor, failure mechanisms depend on the
microwave pulse duration. If the pulse duration is short compared with
thermal diffusion times, then the temperature increases in proportion to the
deposited energy. Pulse durations (t) shorter than about 100 nanoseconds fall
into this regime, and the threshold power for damage varies as 1/t.
Experimental testing has shown that for pulse durations between 100
nanoseconds and 10 microseconds, the power required for damage scales as
1/t1/2. And for pulses longer than 10 microseconds, a steady state in which the
thermal diffusion rate equals the rate of energy deposition and temperature is
proportional to power, resulting in a constant power requirement for damage.
In this case, the power requirement scales as t. The consequence of these
scaling factors is that short pulses require very high power but little energy,
while very long pulses require large amounts of energy but little power. This
analysis results in a vast range of HPM sources capable of damaging
semiconductor devices. The above applies to single-shot pulse durations, but if
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a PRR is applied such that there is insufficient time for thermal diffusion
between pulses (about 1 millisecond), then there will be an overall constant
rise in temperature. For this reason, the PRR capability is of extreme
importance for any HPM source.
As a matter of course, assets to be tested include those of friend and foe alike,
the goal being to find vulnerabilities in both and correcting those found in our
own assets. Several techniques are employed to mitigate vulnerabilities found
in assets, including filtering of conductor lines, using metallic enclosures,
eliminating any unnecessary openings in the outer enclosure, and using ferrite
or other magnetic materials. Any electronics inside a completely sealed
metallic container (Faraday cage) would have no vulnerability to HPM of any
variety; however, such a scenario is also of little or no use, since there could
be no communication to or from the enclosure. Assets therefore must include
some openings for communications, instruments, air flow, and sensors,
sometimes as a matter of fulfilling their function.
From the HPM source perspective, care must be taken to prevent fratricide and
harm to friendly assets. To prevent fratricide, all connections to the source
must be filtered to prevent fast transients from returning to the control unit.
Some signals can be transmitted using fiber-optic cable; however, there is
usually a piece of equipment at the source end that must be filtered. Some
connections, such as the high-voltage power supplies, can make good use of
high inductance filtering to eliminate fast transients, while others, such as
trigger lines, must make use of other filtering means, such as transformer
coupling, lightning arrestors, transorbs, and fast-acting, high-voltage diodes.
Preventing harm to friendly assets is difficult and is a major reason why HPM
has rarely been employed in actual battlefield settings. To ensure there is no
harm to friendly assets, all assets would have to be tested for vulnerabilities,
something that is not done at present. The antenna is a major factor in this
matter. Unfocused antennas radiate a pattern that spreads as it progresses
outward and, thus, the area subjected to the EM fields increases with distance.
It is then harder to separate one's own assets from the radiated fields.
In addition, it is very difficult to detect these sort of pulsed sources, since the
pulses are very short (typically 1-500 nanoseconds), and even in burst mode,
the bursts are usually less than 10 seconds. The short burst mode operation is
necessary because of the high peak powers and subsequent heating of key
components such as switches. The UWB sources would be the most difficult to
detect, since they have nearly zero energy at any one frequency and so would
not be detected at all by instruments such as spectrum analyzers.
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Critical Technologies
Several technology areas are critical to the design and fabrication of a working HPM
source. First, because the pulsed-power section of the source must operate at high
voltages, it must contain insulating materials capable of withstanding the required
voltage. The insulating scheme chosen is critical to the success of the project;
therefore, several insulation techniques will be discussed, along with the merits and
drawbacks of each. Another pulsed-power technology that is critical to the success of
any source is high-voltage switching; various types of switches will be discussed, along
with applications. Finally, cathode materials are a technology area that has been
thoroughly researched and is critical to narrowband HPM generation; several cathode
materials will be discussed, along with needs for future cathodes.
INSULATION
Electrical insulating materials or dielectrics are essential not only for pulsed power and
HPM generation but for the proper functioning of all electrical and electronic equipment.
In fact, usually the size and operating limitations of a piece of equipment are
determined by the choice of insulating material. In the past, all manner of varnishes,
tars, petroleum asphalts, natural resins, gums, saps, and minerals were used for
electrical insulation. Now there is an almost endless list of possible insulating materials.
The question now is, which material is the most appropriate for the task at hand? In
pulsed power, and even more so in HPM applications, the choice is critical. All
properties of a material must be weighed against one another to make the proper
choice. Such properties as voltage breakdown, dielectric loss, dielectric constant, cure
temperatures, hardness, tensile strength, flow modulus, and the variation of all these
with frequency, voltage, and temperature must be considered before an appropriate
insulating material can be selected. Many of these properties have never been
published for most materials, and even when they have been published, they are
typically known only at one or two frequencies. Designing insulation for challenging
applications is at best a compromise between evils. Most of the material studies are
carried out for the power industry, making them valid only at 50 or 60 hertz.
Measurements at these low frequencies usually provide little or no clue about the
values at much higher frequencies. Therefore, it is often up to diligent engineers to
obtain materials data on their own.
One recent research area of interest is in developing what are termed artificial
dielectrics in an effort to decrease insulation weight. This material is made by
suspending hollow glass microspheres in a lightweight dielectric medium. Depending on
the concentration of these spheres, the dielectric constant can be lowered and tailored
for the application, and the loss tangent of the media can also be reduced. Coating the
spheres with a conductor such as aluminum can also raise the dielectric constant. Thus
far, as might be expected, the dielectric strength of such materials is much lower than
that of many thermoplastics, but they have been useful for applications such as
radomes.
Insulation generally falls into one of three categories: (1) homogeneous insulation,
where the entire insulating volume is filled with the same media, be it solid, liquid, or
gas; (2) laminated insulation, where the insulating volume is filled with some manner of
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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
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section on laminated insulation). Many plastics come in a wide variety of shapes, forms,
and grades, including bulk volumes, a variety of sheet thicknesses, and various rod
diameters. The subject of using plastics as insulation fills volumes in reference books
and has yet to be exhausted. Table 1 shows selected dielectric properties, collected
over several years, on some of the most common plastics for high-voltage use.
Table 1. Dielectric Properties of Some HPM Plastics
Breakdown Voltage
Material Trade Name (kV/mil)
Acetal Delrin 4.0
Polypropylene 6.0
Polyetherimide Ultem 7.0
Polysulfone Ultrason S 7.5
Polyethersulfone Ultrason E 5.8
Polycarbonate Lexan 6.3
Polyphenylene Ether Noryl 0.6
Polyphenylene Sulfide Ryton 0.4
Polyethylene 5.0
Polyvinylchloride 1.8
Epoxies
One of the greatest advantages of casting epoxies is that a high dielectric strength can
be attained with low maintenance, a long shelf life, and ease of transportation
compared with liquid or laminated insulation schemes. Some of the best epoxies ever
used for high-voltage insulation have only recently become available. These
advancements are due mainly to efforts by the automotive industry to miniaturize the
ignition coil to the point where a separate coil could be incorporated into the spark plug
cap at each cylinder. Technologies have been devised for casting several varieties of
epoxy to allow larger volume castings. The goals are to minimize voids and bubbles,
deal with any exothermal effects, and reduce shrinkage. In addition, a good candidate
material for high-voltage casting must have a high dielectric strength at the frequencies
required, a long pot life, good adhesion, and an unlimited cure depth at a low
temperature. With many epoxies, shrinkage and the glass transition point are functions
of the cure temperature. New, state-of-the-art epoxies have several desirable
characteristics never before available in a single product that make them ideal for high-
voltage applications. Two such characteristics are a low viscosity at room temperature
and a long pot life. This means the epoxy can be mixed (resin and hardener) and the
unit to be insulated can be filled under vacuum to eliminate voids and bubbles. Some of
these epoxies have the viscosity of milk at about 100 degrees Fahrenheit and a pot life
of several hours. A third desirable characteristic is a very low, almost imperceptible
exotherm. This allows insulation of items sensitive to heat, such as thin plastics, paper,
and electronic components or integrated circuits. A fourth desirable characteristic is low
shrinkage, even in large castings. This allows insulation of regions where dimensional
stability is important, such as at distances from high-voltage sections and resonant
structures. A fifth desirable characteristic is good adhesion, both to itself and to
components to be insulated. This is important because any separation from a
component creates a void region where the dielectric strength will be compromised.
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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.
GASOUS
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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thus, implies heightened safety concerns. Even low-volume vessels can contain
hundreds of joules of energy in the compressed gas, and a housing failure can hurl
fragments at deadly velocities. Highly compressed gases are used only in cases where
some prized benefit is worth the increased cost and design trials to be exacted. One
example of this is extremely fast switching where the electrode spacing is proportional
to the added inductance during conduction—the smaller the electrode spacing requires
higher gas pressure for insulation. Almost every common gas has been used as
insulation, and many have attributes making them desirable for certain applications.
Sulfur hexafluoride, nitrogen, air, argon, helium, oxygen, and hydrogen are commonly
used. Of these, only sulfur hexafluoride is an electronegative gas, meaning it has the
ability to remove electrons from the volume through the formation of negative ions and
thereby increase the dielectric strength. Electrical discharges in sulfur hexafluoride
result in foul-smelling sulfur compounds that also deposit on the switch housing and
electrodes and require frequent cleaning. These discharge compounds also tend to be
highly corrosive, especially in the presence of water. Other gases with electronegative
species, typically other halogens such as chlorine, also make good insulators. These
gases are usually much denser than air, and breakdown voltage is roughly proportional
to density, thus higher voltages can be supported even at low pressures. The
halogenated hydrocarbon refrigerants, such as CCl4, CCl2F2, CCl3F, and C2Cl2F4, are also
popular for insulation. The breakdown of air has been thoroughly researched, and in
fact the breakdown voltage of a calibrated gap can be used to determine the magnitude
of high voltages. Table 2 shows the breakdown voltages of several insulating gases
relative to that of air.
Table 2. Relative Spark Breakdown Strength of Gases
Gas N2 Air NH3 CO2 H2S O2 Cl2 H2 SO2 C2Cl2F4 CCl2F2
V/Vair 1.15 1 1 0.95 0.9 0.85 0.85 0.65 0.30 3.2 2.9
Gaseous insulation as a switch medium limits the pulse repetition rate (PRR) to 500-
600 pulses per second because of the creation of numerous metastable states and
elevated energy levels by the previous pulses. This is true for all gases listed here
except hydrogen. Hydrogen can be used at a much higher PRR; however, its dielectric
strength is only 65 percent that of air and, thus, almost twice as much pressure is
required for the same operating voltage. When the pressure is doubled, the energy
content increases by a factor of four, leading to elevated safety concerns. Using
hydrogen for switch insulation poses no explosive danger provided the oxygen content
in the gas is kept below about 5 percent. Other handling problems associated with
hydrogen include hydrogen embrittlement—it will leak through even tiny holes,
including the pores in metal tanks, eventually causing the metal to become brittle and
fail. In addition, hydrogen is flammable when mixed with oxygen. A hydrogen flame is
colorless but very hot, which can be dangerous if leaks develop in pressurized switches
or gas lines. One class of hydrogen switches, hydrogen thyratrons, makes use of the
low-pressure characteristics of gases to eliminate the safety concerns associated with
high pressures (these are discussed in a later section dedicated exclusively to gas
switches).
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LAMINATED
Laminated insulation has been used in some very demanding applications in which size
is of primary importance and very small repeating structures are required, including
high-energy-density capacitors, high-voltage transformers, and high-voltage
transmission lines or pulse-forming lines. Laminated insulation schemes make use of
the nonlinearity of electrical breakdown strength with thickness mentioned earlier for
plastics. Such schemes offer the possibility of significant improvements in state-of-the-
art insulation, including a reduction in the size and an increase in the energy density of
high-voltage pulsed systems. As new and improved materials become available, the
possibilities for such improvements will become more substantial.
PLASTIC-PAPER-OIL
With appropriate attention to process details, very high dielectric strength is routinely
achieved using this lamination scheme. This is in fact the insulation method used in
most high-voltage and high-energy-density capacitors, with the addition of foil layers
on either side of the plastic (usually biaxially oriented polypropylene) to form the
capacitor. Use of corona-processed oil dramatically improves the utility of this insulating
scheme. The plastic is frosted on at least one side and, together with the very thin (1
mil or less) paper layer, allows the oil to penetrate throughout the volume during the
impregnation process. Without the oil, the tightly wound plastic layers can become
sealed around small volumes of air that will not be filled with oil, and breakdowns will
occur. Once the paper is impregnated with the oil, tests have shown that it attains
essentially the same dielectric strength as the oil. Often, vacuum and pressure are
alternately applied to ensure full penetration of the oil into the full volume. It is vitally
important that no bubbles or voids be left in the insulation volume. For this reason,
once the insulating volume is ready for impregnation, it should be left under vacuum at
slightly elevated temperature for at least 24 hours. This not only ensures air pockets
are removed but also allows the removal of surface moisture from the plastic and
paper, which will also contribute to voltage breakdown. The paper not only aids
impregnation but also serves as a path for residual charge to dissipate between voltage
applications. The plastic has a very high surface resistivity, and some residual charge
can become trapped on the surface after each discharge, resulting in charged regions of
different magnitudes and even polarities, which can eventually lead to dielectric failure.
Using this insulation scheme with biaxially oriented polypropylene as the plastic and
Shell Diala AX as the impregnating oil, average dielectric strength of more than 2.1
kV/mil and operating voltages higher than 1.3 MV have been attained in large volumes.
PLASTIC-PAPER-EPOXY
Since the oil is the weakest dielectric medium in the preceding insulation scheme, it is
reasonable to assume that replacing it with a stronger dielectric medium can improve
the overall dielectric strength. Another advantage of this scheme is that in the end we
would have a solid insulated volume with the advantages mentioned earlier. Thus far,
only smaller volumes (1-2 gallons) have been successfully insulated with this scheme.
The problem is that the increase in viscosity over the oil, although small, makes it more
difficult to ensure that full impregnation is achieved. Meanwhile, the programs for which
this scheme is desired insist on nearly 100-percent certainty of success. The most
successful process to date involves using quarter-inch sections of 1-mil paper followed
by quarter-inch open sections for each layer. This is a tedious task in large volumes but
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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 50 percent. The technique works best when the volume dielectric
fluid has a low relative dielectric constant, such as mineral oil has (εr =2.2), since
coatings are readily available for εr = ~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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These mechanisms include:
• Thermionic emission (apply heat – 1,000 °Celsius)
• Secondary emission (electron bombardment; > 100 eV)
• Field emission (apply a very strong electric field; 107 V/cm)
• Explosive emission (form a plasma on the surface; εon= 0)
The emission mechanisms of major importance for HPM at present are thermionic
emission and explosive electron emission; however, field emission shows some hope
with the advancements in nanostructures. Explosive emission, creating a dense plasma
at the cathode surface, is of primary importance at this point. A review of pure metals
reveals a direct correlation between the work function (εm) and melting temperatures.
When cathodes are made from metals with low work functions, there are problems with
metal deposition onto other components. Most cathodes of use in HPM tubes depend on
a surface flashover at a dielectric-metal interface. The surface flashover generates
plasma, typically at tens–of-kilovolts-per-centimeter electric fields. The threshold and
nature of the plasma depend greatly on the cathode materials. Therefore, the choice of
cathode material is of critical importance in the design and operation of any HPM tube.
No discussion of HPM diodes could be complete without mentioning space charge
limited current flow. This stems from the fact that at some magnitude of current
density, the density of electrons in the anode-cathode gap begins to shield the cathode
from further emission owing to their cumulative effect on the electric field at the
cathode surface. The current density at which this happens is given by the Child-
Langmuir law:
Jsc(kA/cm²) = 2.33 x 10⁻⁶ (V(MV)³/² /d(cm)²)
and is dependent on the diode voltage and the anode-cathode spacing. So, if we could
have the ideal cathode material, what would its characteristics be? The response has
not changed much in more than 60 years, as can be seen in the following extraction
from a textbook on the subject.
Primary Characteristics of an Ideal Cathode (J. R. Pierce, 1946):
• Emits electrons freely, without any form of persuasion such as heating or
bombardment (electrons would leak off from it into vacuum as easily as they pass
from one metal to another).
• Emits copiously, supplying an unlimited current density.
• Lasts forever, its electron emission continuing unimpaired as long as it is needed.
• Emits electrons uniformly, traveling at practically zero velocity.
Efforts are still under way to increase the output power, pulsed emission duration,
repetition rate, and emission uniformity by investigating new and existing cathode
materials in an effort to draw closer to the ideal cathode. Some of the materials
currently being investigated are ceramic cloth and felt, carbon structures including
nanotubes and microfibers, and carbon structures coated with cesium iodide.
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VELVET
Velvet has been an explosive electron emission standard for more than 20 years. This
material works by means of the dielectric-metal surface flashover mechanism
mentioned earlier.
Five steps are involved in the explosive emission process for dielectric fibers:
• Surface flashover generates a cold, dense plasma/gas column.
• The applied electric field extracts a space-charge-limited current flow.
• The flow of current resistively heats the gas column.
• The gas columns expand at a rate determined by the gas temperature.
• The gas continues to expand into the anode-cathode gap.
Velvet has several desirable properties that have endeared it to the pulsed power
community and kept it a useful material for all this time. First, it emits at relatively low
field strengths (~10 kV/cm), allowing a wider range of use than do many other
materials. Second, it has a fast turn-on time. Third, the insulating nature of the velvet
fibers provides a sort of built-in ballast during operation. Velvet also has a wide range
of vacuum compatibility (pressures from 10⁻³ to 10⁻⁸ Torr), easing the expense of
vacuum hardware. Finally, velvet is inexpensive and readily available. All these factors
combined have made velvet cathodes common for the past two decades.
However, velvet cathodes also have drawbacks. First, velvet outgases heavily,
especially during and after explosive emission. Significant amounts of material are
released from the velvet during this process. The increased pressure inside the HPM
device then leads to gap closure (conductive bridging of the anode-cathode region) and
early termination of the RF output from the device. The closure rate can be estimated
from:
Velocity of closure (m/s) = 100 (d*/d)²/³ Vd¹/²
where: d is the diode gap, d* is the velvet tuft density, and Vd is the diode voltage.
Second, velvet has a very limited lifetime, partly owing to the material lost during each
shot. Some material lasts for only about 100 shots in single-shot mode. Third, because
of the increase in pressure after each shot, the repetition rate is very limited. Finally,
lack of control over the manufacturing process results in a wide variation in
performance. Results are not reproducible, even between one roll and the next from the
same manufacturer.
CARBON
Carbon cathodes have been used in diodes for more than three decades and have some
appealing characteristics. The outgassing characteristics of carbon cathodes are much
better than those of velvet, although the threshold voltage for emission is generally
much higher. The primary material given off during outgassing from carbon cathodes
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appears to be hydrogen ions, which, because of their low mass, contribute to the
problem of gap closure. The gap closure velocity using carbon cathodes quoted by most
reports researched is 2-2.5 cm/µsec, and diode gaps from the same reports were 1-4
centimeters. Carbon cathodes also have much longer lifetime than velvet. However, one
of the greatest advantages of carbon is the ever-expanding ability to form both macro-
and nanostructures using it as a base or substrate. Structures formed using carbon
include pyramids, fibers, microfibers, nanotubes, and tufts. Many possible carbon
structures still have yet to be formed and tested. Thus far, structures with the most
surface area appear to perform best.
CERAMICS
Ceramics, much like carbon, can be formed into at least microstructures and have some
features that have attracted interest in them for a couple of decades. These include
virtually unlimited lifetimes and extremely low outgassing. A problem with ceramics,
however, is that very high threshold fields are required for diode operation. Coatings to
improve the performance of ceramic cathode structures may exist, and research is
continuing in this area.
CESIUM IODIDE COATED
One of the most recent and impressive materials to be used in cathodes for HPM tubes
is cesium iodide. Cesium is a pure metal that has a work function of only 1.9 eV and a
melting temperature of 28 °Celsius; thus, it is liquid at only slightly above room
temperature. Cesium ions are quite heavy, and that is why this coating was used
initially. It was believed that the gap closure rate would be slowed since the heavy
cesium ions would progress much more slowly across the anode-cathode gap than
would other ion species, given the same electric field. This has proved to be the case,
and closure velocities that are about one-fourth those for velvet or bare carbon (0.4
cm/µsec) have been attained. As a result, the HPM emission times have been extended.
Typically, the cesium salt is dissolved in water as a saturated solution and then the
carbon cathodes are dipped several times. Subsequently, the cathodes must be baked
under vacuum for several hours to remove the water from the surface and leave the
hardened cesium salt. Once completed, the cathodes have a very long lifetime unless
contaminated by back splatter of material from the anode. At present, HPM programs
investigating the performance of cathodes having some form of cesium coating over
carbon nanostructures show the most potential for progress in the state of the art. The
goal of these programs is hundreds of kiloamps for tens of microseconds, resulting in
gigawatt narrow-band HPM sources running at repetition rates of possibly 100 hertz and
thus capable of 100-megajoule energy output per burst. Also of great interest at
present are cathodes termed "hybrids," which utilize multiple emission mechanisms in
beam generation. The cathodes developed by these programs are to be used with the
magnetically insulated line oscillator (Sandia National Laboratories [SNL]), the
relativistic klystron oscillator (Kyle Hendricks, Air Force Research Laboratory [AFRL]),
the reltron (Bruce Miller, SNL), and the super reltron, among others.
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Table 3 shows findings of cathode material studies at SNL and at the AFRL.
Table 3. Cathode Study Findings
Material Emission Threshold Lifetime Outgassing
(kV) (# of Shots) (Neutrals/Electron)
CsI-Carbon < 3 kV/cm > 72,000 4 – 6.5 (substrate)
Microfibers
"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 ~ ~ 4
2%
Bare Carbon 15-40 kV/cm > 36,000 4.3 – 6.5
Microfiber (substrate)
(packing density)
CsI-Carbon Fiber < 3 kV/cm > 200,000 ~ 4
Tufts
Metal / Ceramic 95 kV/cm (diode --- 8
collapse
unless > 150 kV/cm)
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.
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An oft-forgotten aspect of the spectral content is that the spectrum also contains only
the odd harmonics of the lowest frequency by the definition of a Fourier transform.
Thus, the faster the rise time and the wider the pulse, the broader the spectral content.
Therefore, switching speed is the most important parameter of a UWB HPM source. The
types of switches used for HPM are essentially the same as the types of insulation
discussed earlier: gaseous, liquid, and solid state at lower voltages. The single most
important attribute of gases and liquids in switching is their self-healing ability, which
implies some measure of PRR capability.
GASOUS SWITCHING
Gas switches are commonly used for HPM sources as both a prime power and a high-
speed or peaking switch. As mentioned in the discussion of insulation earlier, when
used in a high-speed switch, gas pressure can pose substantial safety concerns. In fact,
all of the discussion regarding gaseous insulation also applies to gas switching, since a
gas switch is simply a gas-insulated region that we wish to fail in a timely fashion. The
higher the voltage impressed across a gas switch, the greater the pressure required to
prevent the switch from conducting until the peak voltage is reached. This is why when
a gas switch is used as a final-stage peaking switch, very high pressures are often
required. A fast-rising pulse is crucial to source design since the rise time determines
the upper frequency content. This is why UWB HPM sources usually contain a peaking
switch at the output to decrease the rise time and increase the spectral content. If the
peaking switch is charged past the DC breakdown level faster than streamers can form
conduction channels, then the final breakdown occurs in an overvolted (compared with
the DC breakdown voltage) switching state. The higher electric field strength between
the switch electrodes results in shortened breakdown times since breakdown develops
in an elevated electric field. All switches exhibit some capacitance to an applied pulse
because of their electrode spacing, resulting in a displacement current as this switch
capacitance charges. This is seen on the other side of the switch as a pre-pulse. The
magnitude of the pre-pulse depends on the rate of change of the charging voltage as
well as the electrode cross-sectional area and spacing. Sometimes efforts to reduce this
pre-pulse are required if it causes problems at the load or undesired spectral content
from the antenna. The pre-pulse phase of breakdown occurs at the speed of light in the
media since it is essentially a field phenomenon. Because of the added inductance and
design of the switch components, pre-pulse has a distinct charging profile. The next
phase of breakdown is a resistive phase as the weakly conducting streamer channel
heats to the final arc or inductive phase and the switch is fully conductive. Since the
final phase is inductive, very low switch inductance and very short gaps are required for
fast rise times. Both the resistive and inductive phase periods contribute to the rise
time as:
τ = (τr²+τL²)1/2
where: τr = (88ns x ρ1/2) / (Z1/3 x E4/3)
and: τL = (Lc + Lh) / Z
with ρ being the gas density as a multiple of that for sea level air, Z is the circuit
impedance in ohms, and E is the electric field between the electrodes in kV/cm. Also, τr
and τL are known as the resistive and inductive rise times, respectively. The resistive
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rise time is the time required to heat the gas channel to full conductivity, and the
inductive rise time is the delay caused by the addition of the switch into the circuit.
There are two contributions to the inductive rise time, with Lc being the spark channel
inductance and Lh the housing inductance. A shorter switch gap reduces the inductive
time by lowering the channel inductance but also increases the electric field in the gap,
reducing the resistive time and resulting in a faster rise time. Even though the switch
electrodes are usually designed for minimal cross-sectional area at a given current, the
very short electrode separation required can still result in high interelectrode switch
capacitance. As mentioned earlier, it is also preferable to charge the switch very quickly
to achieve an overvolted switching condition, and, therefore, very fast switches always
have some level of pre-pulse. Because the PRR is also of great importance, hydrogen
has been chosen most often for high-speed gas switching in UWB HPM sources.
Switches of this type have achieved rise times of just over 100 picoseconds (ps) and
PRRs of 1,500 pulses per second.
Another type of gas switch meriting mention for its utility and indispensability in the
HPM pulsed-power driver circuits is the hydrogen thyratron. The thyratron is a partial
vacuum switch. Figure 1 shows what is known as the Paschen curve for air; however,
all gases exhibit the same curve characteristics. At some product of pressure and
electrode spacing, a minimum value of breakdown voltage is reached. While high-
pressure gas switches operate in the region on the right side of the Paschen minimum,
the hydrogen thyratron operates on the left side, beyond the Paschen minimum. The
physics of voltage breakdown in this region results in smaller electrode spacing holding
off higher voltages and reduced pressure at the same spacing enabling greater voltage
holdoff. The single-stage thyratron operates at only tens of kilovolts, while high-
pressure gas switches may operate at several hundreds of kilovolts. When coupled with
a good pulse transformer, a properly chosen thyratron forms the heart of an excellent
driver for HPM sources. The thyratron also has the capability to initiate breakdown
using modest trigger levels (~1 kilovolt) and with nanosecond timing, allowing the use
of multiple switches to share current.
Breakdown Voltage vs. Pressure x Gap
(Air)
100000
10000
Breakdown Voltage - Volts
1000
100
1.00E-02 1.00E-01 1.00E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04
Pressure x Gap - Torr Inches
Figure 1. Paschen Curve for Air
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HIGH-SPEED LIQUID SWITCHING
Liquid switching has also been used in UWB HPM sources with great success. The same
phases of breakdown exist for liquid switches as do for gas switches. The electrode
spacing is typically smaller for liquid switches, and electrodes can be made smaller for
the same level of energy transfer owing to greater thermal diffusion to the liquid as
compared with a gas. Liquid switching does not have the extreme safety concerns
associated with gas switching; however, for repetitively pulsed operation, flow of the
liquid insulating media is required. Filtering, evacuation, and processing may also be
required. Liquid switches have achieved rise times of less than 100 ps and PRRs of
1,500 pulses per second.
SOLID-STATE SWITCHING
Solid-state switches have seen some improvement in voltage holdoff capability but
generally still do not have the capability of operating at tens of kilovolts required of
HPM sources. The current technology in lateral gallium arsenide (GaAs) switches is
greatly improved compared with the old bulk avalanche semiconductor switch
technology of the last decade. The power handling capabilities of this technology are
impressive; however, it still suffers from short lifetimes because of heat dissipation
problems. Source designs using GaAs switches typically involve an array of horns with
one switch per horn. The array can then be phased in time to allow steering of the
beam. GaAs switches operate at about 10 kV and, therefore, in the large arrays
required, several switches fail during any burst mode operation. The most promising
new developments in semiconductor switches today are based on physics pioneered by
I. V. Grekhov and colleagues at the Ioffe Physical-Technical Institute in St. Petersburg.
The AFRL is currently collaborating with Dr. Grekhov and the University of New Mexico
in studies of delayed breakdown devices, silicon avalanche shapers, and drift step
recovery diodes in efforts to improve the performance of these devices. It is also
investigating the use of silicon carbide as an alternative to silicon and GaAs. State-of-
the art pulse generators using these devices currently are capable of 6-8 kV output with
100-ps rise times and 20-ps switching jitter. Conventional solid-state devices such as
junction gate field-effect transistors (JFETs) have not seen substantial improvement
and operate at about a kilovolt with rise times of a few nanoseconds, making them
useful for trigger supplies but not in HPM sources.
Photoconductive solid-state (PCSS) switching is still of great interest because of the
inherent advantages it could provide. PCSS switches have very low jitter, have fast rise
times, and are compact. This technology, sufficiently developed, could allow design of
HPM sources with fewer compression stages, allow greater frequency agility and pulse
width adjustment, and be used in arrays by phasing many lower power sources
together. The technology's main limitations at present are power handling and a limited
lifetime. PCSS switches have three modes of operation. In the linear mode, one
electron-hole pair is generated by each photon absorbed, and so the conductivity is
linearly proportional to the incident photon flux. Linear mode PCSS switches are made
from silicon, doped GaAs, and indium phosphide. The electrical pulse output follows the
amplitude of the optical trigger pulse. Switching in this mode requires about 1 mJ/cm²
of optical energy and thus requires a larger laser trigger than do other operating
modes. PCSS switches also operate in a lock-on mode in which once the optical trigger
causes conduction, carriers remain as long as current still flows, even if the optical
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pulse ceases. This mode requires much less optical power. Finally, PCSS switches may
also be designed to operate in the avalanche mode. Switches operating in this mode
are designed for higher voltages, and above some critical electric field, the switch
remains closed even without optical energy. Carrier multiplication occurs because of the
electric field, and the switching process sustains conduction. Optical energy required for
avalanche mode PCSS switches is about 1-10μJ/cm². Lock-on mode GaAs is the most
commonly used PCSS switch for high-voltage applications. PCSS research at the
University of Texas at Dallas (UTD) has resulted in promising techniques for improving
the longevity of switches. High current densities in PCSS switches normally result in
damage at the metal-semiconductor interface. Research at UTD using amorphic
diamond coatings at the metal-semiconductor interface has resulted in significant
lifetime improvement for the switches. The process uses a conformal coating with the
hardness of natural diamond and extremely high electron emissivity originally called
amorphous ceramic diamond and later shortened to simply amorphic diamond. Used in
stacked Blumlein pulser configurations, these switches have demonstrated 150-ps
switching speed at 100 kV and 10⁵-shot lifetimes.
High-Voltage Pulse Sources
HPM sources have three basic components:
• Electrical or explosive prime power.
• RF generator.
• Antenna.
The prime power is supplied by means of pulsed-electrical-circuit Marx generators and
transformer-based generators or by single-event explosively driven means.
MARX GENERATORS
Marx generators have been used for several decades now and have seen many
improvements in their reliability and repetition rate capabilities. In a Marx generator,
some number of capacitors, referred to as the number of stages, are charged in
parallel. After the charge cycle is complete, gas switches located between each stage
are triggered to conduction, and the capacitor configuration is changed to a series
connection. The result is that the charge on each capacitor is multiplied by the number
of stages, and the effective capacity is the stage capacitance divided by the number of
stages. Because all inductances are also in series, the equivalent inductance is the sum
of all circuit, switch, and capacitor inductances. One problem with Marx generator
circuits is that approximately half the energy used is lost as heat in the charging
resistance for each stage. Using inductive charging can lessen this problem, but care
must be taken, since several LC loops can be formed, all of which resonate at different
frequencies. The result can be extremely high voltages in circuit locations where these
are not expected. This can be especially troublesome in high-repetition-rate circuits.
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Figure 2 shows an example of a Marx generator circuit.
[circuit diagram]
Figure 2. Example of Marx Generator Circuit
TRANSFORMER BASED GENERATORS
Transformer-based pulse generators have also been used for many years as prime
power for HPM sources. Transformers with ferrite cores have been used successfully in
sources with multigigawatt output powers at kilohertz repetition rates. Ferrite
development is an area where substantial gains could be made in HPM sources.
Typically, programs are under time or budget constraints and do not give adequate
attention to this research. As a result, little or no progress in new ferrite materials for
pulsed operation has been made. Ferrites with increased frequency ranges and
increased saturation flux density are needed. Air core transformers are used at higher
flux densities and are often of the resonant variety because of their decreased coupling
levels. Resonant transformers develop peak voltages after multiple cycles owing to
coupling effects. Dual-resonant air core pulse transformers are prevalent and require
coupling coefficients of 0.8, producing peak secondary voltage and maximum energy
transfer after an initial reverse voltage swing. In dual-resonant designs, two
frequencies or resonant modes are generated, and the output is the superposition of
the two modes. Transformer systems generally require the primary circuit to be
matched or tuned to the secondary, or vice versa.
EXPLOSIVELY DRIVEN GENERATORS
Explosively driven generators, also called flux compression generators (FCGs), work by
setting up a strong magnetic field between two conductors, usually by discharging a
capacitor bank charged to high voltage through an inductive coil. A conducting hollow
cylinder filled with high explosives is placed in the center of the coil, filling the region
between the two conductors with magnetic flux. The explosives are then used to
compress the initial magnetic flux by driving the conducting cylinder surface, which
contains the flux, outward into the current carrying coil. Work done by the conductors
moving against the magnetic field results in a huge increase in the EM energy. The
additional energy comes from chemical energy stored in the explosives. Thus, FCGs
essentially convert a portion of the chemical explosive energy into EM energy. The
explosively driven conductor is called an armature, and the nondriven inductive coil of
the generator is called the stator. Miniaturizing the generators and fine-tuning the
magnetohydrodynamic aspects takes years and is still an area of intense research.
Material properties under the enormous forces involved are also required for success.
Many hours of research and computer code writing and testing go into the selection of
every single material used. One problem with this form of HPM source is that of
coupling the energy to the load. Attempts to energize the load by direct generation
often result in the development of excessive internal generator voltages and
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breakdown. One solution to these difficulties has been transformer-coupling the load by
having the FCG drive the primary of a transformer. The load for HPM production is
typically some type of loop antenna with very small inductance and driven directly from
the generator. Much research has been done on the effectiveness of this type of
antenna, and one method of improving the operation is to fuse the loop along its length
such that at peak current, the fuses open and radiate large voltage dI/dt spikes.
Pulsed High-Power Microwave Sources
Pulsed HPM sources can be divided into two types:
• Pulsed electron beam sources – these are typically narrow-band HPM sources such
as relativistic klystron amplifiers, backward wave oscillators, traveling wave tubes,
split-cavity oscillators, reltron and super reltron, virtual cathode oscillators,
magnetrons, gyrotrons, and the magnetically insulated line oscillator.
• Impulse HPM sources – these are typically wideband or ultrawideband sources such
as SNIPER, EMBL, Phoenix, Jolt, Thor, GEM II, and the H series.
PULSED ELECTRON BEAM SOURCES
In pulsed electron beam sources, the RF source includes an electron beam generator, a
beam transport, and a wave structure. These sources work by converting the kinetic
energy of an electron beam into EM energy.
BWOs, TWTs, AND RKAs
HPM tubes, such as backward wave oscillators (BWOs), traveling wave tubes (TWTs),
and relativistic klystron amplifiers (RKAs), are also very similar in concept to their
conventional counterparts. The main differences lie in the techniques of beam
formation; the use of pulsed, large magnitude axial magnetic fields for beam transport;
and the application of relativistic voltages and very high beam currents. BWO
efficiencies as high as 35 percent have been obtained at moderate power levels, but as
power levels are increased, the output levels tend to saturate, and the radiated spectra
tends to broaden. These effects are due to beam breakup and turbulent transport. To
maintain high beam quality, large magnetic fields are required for high-power
operation. An approximately 25- to 50-kG applied axial field implies that mechanically
strong solenoidal magnets are required with pulsed capacitor banks to drive them.
These requirements in turn dictate a much larger and heavier HPM source. TWTs can be
made using many of the same techniques as BWOs. The difference is that the beam-
wave interaction is with a forward wave. Thus, TWTs have many of the same issues and
limitations as BWO sources. RKAs use cavities for beam bunching and power extraction
rather than continuous slow wave structures as in BWOs and TWTs. Some RKA designs
use extended structures for power extraction to reduce the power densities and to
increase efficiency. They use high beam currents where space charge forces become
dominant in the bunching process. Tubes have been developed at the 10-GW power
level. They use 0.5 to 1 MeV beams guided by axial magnetic fields of about 10 kG.
Efficiencies are on the order of 40-50 percent. Some of the issues with these tubes are
beam transport, beam loading of the cavities, base pressure of the vacuum system, x-
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ray production, and beam breakup. The combination of x-rays and the high base
pressures has led to breakdowns in the cavities and unstable beam propagation.
SPLIT-CAVITY OSCILLATORS
Split-cavity oscillators (SCOs) utilize transit-time bunching of the beam to generate
microwave energy. They can generate microwave pulses of about 100 MW for durations
on the order of 100 nanoseconds (ns). The SCO can be very compact since no magnetic
field is required and low beam quality requiring only basic pulse power sources is
adequate. Complete systems, including a power supply and mode-converting antenna,
have been built on roll-around laboratory carts.
VIRTUAL CATHODE OSCILLATORS
Virtual cathode oscillators (vircators) operate because of a phenomenon of intense
beam physics. They have no conventional counterpart. They have operating frequencies
tunable from 300 MHz to 40 GHz, no required magnetic field, simple construction, and
low efficiency. Power output varies from 200 MW to 15 GW, and efficiencies range from
1 to 10 percent. Operation is typically in a TM mode in a cylindrical geometry with
mode conversion necessary for efficient radiation. The frequency of oscillation of a free-
running vircator is related to the relativistic beam plasma frequency and typically chirps
upward during the pulse. Plasma closure effects limit the pulse length. Containing the
oscillating beam in a resonant cavity can stabilize the frequency. If the resonant cavity
is driven by an external source, the vircator can be made to lock to the external signal.
Although vircators' simplicity is a major advantage, their very low efficiency poses real
problems for weaponization. They have been used as sources for testing, where
efficiency and size are not an issue.
MAGNETRONS
HPM magnetrons are basically relativistic, cold-cathode versions of their conventional
counterparts. They are characterized by relatively high efficiency, low power densities
internal to the tube, robust operation, and compact size. Their modulators and power
supplies are simple and inexpensive compared with BWOs, TWTs, and RKAs. Relativistic
magnetrons have achieved efficiencies of 10-30 percent in the bands from 0.5 to 10
GHz at power levels of about 5 GW. Pulse widths are on the order of 100 ns, limited by
plasma closure of the anode-cathode gap. Magnetrons may be phase locked for higher
output power.
The magnetically insulated line oscillator (MILO) is essentially a magnetron that uses
the magnetic field of the beam current to provide magnetic insulation between the
cathode and anode. This eliminates the need for pulsed magnets and their power
supplies while also reducing the size and weight of the system. No mode conversion is
required with the MILO.
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The Orion system, first fielded in 1995,
is a self-contained, transportable HPM
test facility housed in five standard
shipping containers. It is computer
controlled via fiberoptic links. The
system is based on four continuously
tunable magnetrons with a tunable
frequency range of 1 to 3.3 GHz. The
thyratron-switched modulator pulse
charges an 11-section pulse-forming
network through a step-up transformer
and a triggered gas output switch. This
provides a 100- to 500-ns pulse at 200
to 500 kV and up to 100 PRR that drives
the magnetrons. The magnetrons are Figure 3. Orion HPM Testing Facility
tuned by stepper-motors and use
explosive emission cathodes. The vacuum of 10⁻⁶ to 10⁻⁷ is provided by cryopumps. The
magnetic field of about 10 kG is provided by cryomagnets. The system includes an
entire shipping container housing a combiner/attenuator network to provide
continuously variable power over five orders of magnitude. The antenna is formed by
two offset, shaped parabolic reflectors, each fed by two pyramidal horns. The antenna
produces a 7 x 15 meter elliptical beam spot at a distance of 100 meters. Figure 3
shows the Orion test facility with its antenna.
GYROTRONS
Gyrotrons tap the energy associated with electrons gyrating about strong magnetic field
lines. The main purpose for gyrotron development thus far has been magnetic
confinement fusion research, in which megawatt-power, long-pulse gyrotron sources
operating at more than 100 GHz provide resonant heating, current drive, and instability
suppression. These devices use an electron gun to launch an electron beam into a
region of slowly increasing magnetic field, where it is compressed. Compression raises
the current density and produces a perpendicular component to the beam velocity.
After compression, electron-guiding structures are placed at the peak electric field
position for the TE₀₁ mode. The beam and guiding center structure then enter a
resonant cavity. Inside the cavity, the electron motion decomposes into three
components: a drift along the magnetic field lines, a slow rotation of the beam about
the magnetic axis owing to the E⃗ × B⃗ drift involving the beam self-electric field, and the
Larmor rotation of individual electrons about the guiding centers. Resonant cavity fields
oscillating faster than the rotational cyclotron frequency of electrons cause the
electrons to bunch on one side of their common guiding centers. This bunching causes
net electron energy to be given up to the cavity fields, which is then extracted.
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Gyrotrons are important in narrowband
HPM production because the only
directed-energy weapon system known
to be fielded today is based on them.
The Active Denial System (ADS) uses a
gyrotron operating at 95 GHz that is
capable of continuous operation. Power
output is 100 kW, and the range is more
than 750 meters. The system uses an
innovative flat parabolic surface (FLAPS)
antenna to radiate a focused beam in a
manner similar to a parabolic dish. The
ADS is effective as a less-than-lethal
weapon for crowd dispersal. The
radiation passes through the
atmosphere with very low loss and is
absorbed in the outer layer of skin,
causing a burning sensation sufficiently Figure 4. Active Denial System With FLAPS Antenna
intense to trigger an involuntary reflex
response in the target. Energy is deposited to a depth of about 0.4 mm. Figure 4 shows
a deployment of the ADS with the FLAPS antenna.
IMPULSE HPM SOURCES
Impulse HPM sources are typically ultrawideband. Microwave generation is
accomplished by charging the antenna, a transmission line, or a tuned circuit directly
with an extremely fast rise-time electrical pulse. This is usually done using a well-
designed switch operating in an extremely overvolted condition.
SNIPER
SNIPER (Sub-Nanosecond ImPulse Radiator) is an SNL HPM source operating at 290 kV
that is capable of greater-than-1-kHz PRR. The peak power in the 3.5-ns-wide pulse is
1.25 GW, and the rise time is approximately 150 ps, resulting in spectral content from
100 MHz to 1.5 GHz. The radiated field strength, normalized to a distance of 1 meter, is
120 kV/m using a transverse electromagnetic (TEM) horn.
EMBL
EMBL (EnantioMorphic BLumlein) is an SNL HPM source operating at 750 kV that is
capable of 700-Hz PRR. The peak power in the 3.5-ns-wide pulse is 11 GW, and the rise
time is about 200 ps. EMBL radiates a 285-ps impulse with a TEM horn antenna. Its
spectral content extends from 0.2 to 1.2 GHz, with the peak at 800 MHz. The radiated
field strength normalized to a distance of 1 meter is 350 kV/m.
H-SERIES HPM SOURCES
The H-series HPM sources—H2, H3, and H5—were built at the AFRL and used for
various tests of assets. They are all currently inactive, except for a modified version of
H3, which is being used for research at the University of New Mexico. The H-series HPM
sources are all coaxial line pulsers, meaning they contain a pulsed power section that
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uses a pulse transformer to charge a short section of coaxial transmission line to high
voltage. The transmission line is charged quickly enough to overvolt a high-pressure
(2,000-psi) hydrogen switch, providing 120-ps rise-time pulses at about 350 kV and
2.5-ns pulse width to another 40Ω coaxial transmission line, which uses a point
geometry converter (PGC) to feed an antenna.
Figure 5 shows H2 with a PGC output
driving a large TEM horn. The peak
power is 2 GW, and these sources were
capable of a power supply-limited PRR
of 1.8 kHz. Using a flat-plate TEM horn
antenna, the radiated field at 10 meters
was 25 kV/m. At the AFRL, they
compare HPM sources using what is
termed a figure of merit (FOM), defined
as the field value at some distance
multiplied by the distance. Thus, for H3
the FOM is 250 kV. While developing the
H series of HPM sources, scientists
devised a means of efficiently Figure 5. H2 With Large TEM Horn and PGC Output
converting from a coaxial geometry to a parallel-plate geometry. Radiation from a coax
forms a doughnut pattern with no field on bore site, and thus the PGC was developed to
feed a more interesting antenna.
Figure 6 shows a cross-section drawing of H5 with a PGC feeding a Brewster angle
window and an extended-ground-plane antenna. H5 was developed using a much
smaller volume high-pressure hydrogen switch than its predecessors used; however,
with the pressures involved, personnel were isolated from the source whenever any
pressure was in the switch.
[cross-section diagram]
Figure 6. Cross-Section Drawing of H5 With Point Geometry Converter, Brewster Angle Window, and
Extended-Ground-Plane Antenna
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Figure 7 is a closeup photograph of the same actual components as built and tested at
the High Energy Research and Test Facility antenna range. This source was used
extensively for testing in the 1990s in various remote tests in which the entire system,
including the antenna, a screen room, and far-field diagnostics, was fitted into a trailer
specially modified for the purpose.
[photograph]
Figure 7. H5 Output Section With the Point Geometry Converter Feeding an Extended-
Ground-Plane Antenna Through a Brewster Angle Window
THE PHOENIX HPM SOURCE
The Phoenix was a UWB source developed by the AFRL specifically for asset testing at
the High-Energy Microwave Laboratory (HEML). The HEML has a large anechoic
chamber and is capable of testing small fighter aircraft. The Phoenix had a ferrite core
transformer-based system using two flowed oil switches to generate the fast rise-time
voltage pulse. It was bulkier than the H-series sources and thus was less portable. An
oil-processing platform with 1-micron filtering and a 5-horsepower DC motor driving the
positive displacement oil pump was used. The pumping system was capable of a 7-
gallon-per-minute flow rate through the two switches. The oil switches were designed
into 50Ω parallel-plate transmission lines. The peaking switch electrode spacing was
only 0.015 to 0.020 inches and was highly overvolted. The operating voltage was about
500 kV with a 90-ps rise time and a 1.25-ns pulse width. The peak power was 5 GW,
and the radiated field at 9 meters was 45 kV/m, giving it an FOM of more than 400 kV.
Phoenix had the fastest rise time of any HPM source to date. Figure 8 shows the
waveform of the radiated field at a distance of 8.5 meters. Figure 9 shows the resulting
spectral content. Note that the source has good spectral content beyond 2.5 GHz owing
to the extremely fast rise time.
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4 5
4 0
— Phx Fld 7/94
3 5
- - Phx Fld 7/96
Electric Field on Axis (KV/m)
3 0
2 5
2 0
(KV/m)
1 5
1 0
5
0
-5
0 1 2 3 4 5 6
Time in nanoseconds
Figure 8. Phoenix Radiated Pulse at 8.5 Meters
-6 0
-7 0
- - - Spectrum 7/94
Energy Spectral Density (dBJ/m^2*Hz)
-8 0
— Spectrum 7/96
-9 0
-1 0 0
3J/m²(m²·Hz)
-1 2 0
-1 3 0
-1 4 0
-1 5 0
-1 6 0
0 1 2 3 4 5 6
FREQUENCY (GHz)
Figure 9. Phoenix Radiated Spectral Content
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THE GEM II HPM SOURCE
The GEM II source was based on PCSS switch technology using GaAs bulk avalanche
semiconductor switches (BASS). GEM II was built by Power Spectra, Inc., in the mid-
1990s and was evaluated by the AFRL. The source was built from individual modules,
each containing a rectangular horn output with a BASS switch at the feed point. The
modules were assembled in a 12 x 12 array to form a planar radiator measuring 1.6 x
1.6 x 0.85 meters. By timing the switching sequence of each BASS module, the beam
was steerable up to 30° off center. The HPM output reached 22 kV/m at a distance of
75 meters and was capable of repetition-rate operation at 3 kHz. The rise time of each
module was about 200 ps, and the pulse width was about 2 ns. The major problem with
GEM II was switch lifetime; typically, a few switches failed during each burst.
THE JOLT HPM SOURCE
The Jolt, shown in Figure 10, was designed especially to feed the half-IRA antenna. The
AFRL refers to this source as hyperband, meaning the frequency band ratio is greater
than 10. On the pulsed-power end, Jolt uses a dual-resonant transformer operating at
1.1 MV. The transformer charges an intermediate capacitance discharged by an oil-
insulated peaking switch at the focal point of the half IRA to two flat-plate transmission
lines terminated at the edge of the dish. The voltage rate of rise is 5 x 10¹⁵ V/sec, and
the repetition rate is a maximum of 200 Hz. The radiated electric field waveform
recorded at 85 meters is shown in Figure 11. The half-power point frequencies are an
upper frequency of 2 GHz and a lower frequency of 30 MHz.
[photograph] 8 10⁴
6 10⁴
4 10⁴
Electric field (V/M)
2 10⁴
0
-2 10⁴
4 10⁴
-1 10⁻⁹ 0 1 10⁻⁹ 2 10⁻⁹ 3 10⁻⁹ 4 10⁻⁹ 5 10⁻⁹ 6 10⁻⁹
Time (sec)
Figure 10. Jolt Hyperband HPM Source Figure 11. Jolt Radiated Electric Field Waveform at
85 Meters
MESOBAND SOURCES
The AFRL has defined mesoband or moderate band sources as those with a bandwidth
ratio between one and three. These sources usually generate HPM energy between 50
and 900 MHz. They are designed using two techniques. The first is to feed a damped
sine wave onto a wideband antenna. The second is to switch a transient pulse onto a
resonant transmission line connected to a wideband antenna. One such system at the
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AFRL, called the Matrix, is simply a quarter-wave, high-voltage coax line that is
switched onto a half-IRA antenna by a high-pressure hydrogen switch. This produces a
damped sinusoid with a frequency variable from 180 to 600 MHz by means of different
coax line lengths.
DIEHL Munitiossysteme in Germany manufactures several mesoband sources for sale to
the public. One is a suitcase-sized system using a compact 600-kV Marx generator to
provide an impulse to a loop antenna. It is made tunable by changing the size of the
loop antenna and can radiate a damped sinusoid at 70 kV/m at 2 meters' distance.
BAE Systems in the United Kingdom also sells mesoband sources made using nonlinear
transmission lines and solid-state modulators. These are repetition-rate operated to
more than 1 kHz.
HPM Antennas
Many types of antennas are used for radiating HPM signals, and the choice of which to
use depends on many factors, including power level, bandwidth, size constraints,
efficiency and range requirements, and fratricide concerns. This section discusses the
most common antennas used for high-power applications: horns, parallel-plate
antennas, and impulse radiating varieties.
NARROWBAND ANTENNAS
HPM narrowband antennas have typically been extrapolations of conventional types
modified in some way to prevent air breakdown and to support higher electric fields.
Antenna arrays are in development, albeit mainly for phase-locked multioscillator
systems. The obstacle to using arrays in single-oscillator systems is that little has been
done to develop the required antenna subelements (phase shifters and phase splitters)
capable of high-power operation. Eventually, however, antenna arrays will be dominant
in narrowband HPM because higher powers will require larger area antennas to prevent
breakdown and arrays are the only antennas that are compatible with electronic control
for tracking targets. To achieve good effectiveness and gain, the array size should be
larger than the wavelength to be radiated. The beam width is approximately 2λ/L, and
the gain is L²/λ². Also of importance is the separation between elements in the array.
Larger distances between elements cause grating lobes offcenter from the main beam.
If the distance between elements can be reduced to less than the wavelength, grating
lobes will be minimal. In practice, however, breakdown constraints make this hard to
accomplish.
In practice, only a very small number of antenna types have been used in narrowband
HPM with any success. By far the most common type is the horn in its many forms,
including transverse electromagnetic (TEM), pyramidal, and conical. One advantage of
horn antennas for narrowband use is that the radiation pattern and the gain can be
calculated precisely.
For TEM and pyramidal horns, the gain is given by:
G = 2πab/λ²
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where: a and b are dimensions of the sides and λ is the wavelength.
For conical horns, the gain is given by:
G = 5D²/λ²
where: D is the diameter and λ is the wavelength.
Mode conversion is critically important with horn antennas in order to generate field
patterns of use. The fundamental modes, TE₁₀ in rectangular waveguide and TE₁₁ in
circular waveguide, are preferred since they radiate a field pattern with peak field
strength on axis. These are the most common modes used in electronic vulnerability
testing.
Parabolic dish antennas are the most
common type found in conventional
microwave applications, but their use in
narrowband HPM has been limited
because of breakdown problems in
center feed geometries. Variations of
the parabolic dish have, however, been
very successful. The Active Denial
System (ADS) uses a flat parabolic
surface (FLAPS) antenna. FLAPS uses an
array of crossed dipole scatterers placed
about 1/8 wavelength above a ground
plane to create a geometrically flat
surface that behaves like a parabolic
dish. Each dipole controls its
corresponding polarization. Incident
energy causes a standing wave to be Figure 12. FLAPS Antenna With a Cross-Shorted
established between the dipole and the Dipole Array
ground plane. The interaction of the
dipole reactance and the standing wave causes the incident RF energy to be reradiated
with a phase shift determined by the dipole length, thickness, and distance from the
ground plane, as well as by the angle of incident RF, the dielectric constant of the
media between the dipole and ground plane, and the proximity to adjacent dipoles.
Figure 12 shows the essential elements of the FLAPS antenna with a cross-shorted
dipole array. These antennas are easier to store and have less wind resistance than
conventional parabolic dishes.
The Vlasov antenna is also well suited to narrowband HPM radiation and has some
distinct advantages. It can be fed with a TM₀₁ mode and produce a directed beam with
a nearly Gaussian profile; mates easily to the cylindrically symmetric HPM sources, such
as the MILO and vircators; is easily constructed; and has a large feed aperture to
support high electric fields. Typically sources that generate their power in the TM₀ₙ
modes require a mode converter for radiating anything other than a donut beam, but
the Vlasov antenna does not; the antenna is actually adapted from a mode converter.
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Figure 13 shows (a) Vlasov antenna
origins and (b) a Vlasov antenna
attached to a cylindrical MILO. Vlasov
antennas have one major drawback: the
propagation angle is a function of the
operating frequency. Thus, if the
frequency chirps during the RF pulse,
then the beam direction will sweep. The
propagation angle is given by:
θ = 90° − cos ((1-(fc/f)²)¹/²)
WIDEBAND AND
ULTRAWIDEBAND ANTENNAS
Wideband antennas generally present a
much greater design challenge than do
narrowband antennas. As pulse duration
and rise times are shortened, antenna
design becomes more difficult. A good Figure 13. (a) Mode Converter Vlasov Antenna and
wideband antenna must have low (b) Vlasov Antenna Attached to a Coaxial MILO
dispersion across the entire bandwidth
and high gain with minimal sidelobes. These are difficult to achieve because the
wavelengths are large, requiring large antenna dimensions for high gain. Often, mission
constraints dictate a much smaller antenna, thus the gain will not be constant with
frequency, resulting in a distorted radiated pulse shape. The main consideration for
transmitting UWB signals is minimizing frequency dispersion. For conventional
antennas, the gain is a function of frequency. One approach to solving this problem has
been to correct a conventional antenna (TEM horn) for dispersion. A second approach
has been to use the dispersive characteristics of a conventional antenna, with the
appropriate tailored drive signal, to radiate the desired UWB signal. A third approach
has been to develop a new type of antenna. These three approaches cover the limited
gamut of UWB HPM antennas.
The basic approach to attaining low dispersion in a conventional antenna is to ensure a
slowly varying antenna impedance change along the length, beginning at the source
output impedance and ending somewhere close to the impedance of free space (377Ω).
In practice, it is found that the final impedance does not have to be very close to that of
free space; instead, 220Ω to 280Ω provides the highest efficiency for most TEM horns.
Best results are obtained for any length TEM antenna if the impedance is increased at a
constant percentage rate (that is, is exponentially tapered). The resulting design may
then have electrical breakdown problems at the connection point with the source, since
the antenna impedance changes initially are quite small and, thus, plate spacing also
remains small. Typically, a specially shaped, solid insulating material is required to
obtain a gradual impedance change when transitioning from the source media into air.
This is where the highest electric field strength is found and also where the temptation
to aid impedance tapering by incorporating abrupt transitions in conductor dimensions
is greatest. Any reflections of the pulse from farther down the antenna will also
enhance fields at the feed point. All these factors combine to make the design of the
antenna feed section possibly the most important factor in HPM sources and, in many
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instances, the determinant of the radiated signal characteristics. Sidelobes generated
using TEM antennas tend to fall off sharply and are accompanied by a stretching of the
original pulse duration. The distance from the antenna to where the far field region
begins is defined such that the travel time of the differential distance is less than the
rise time of the pulse. Parameters such as gain and beam width for UWB antennas are
difficult to define because these concepts are from the narrowband world, where the
definitions are for a single frequency. For UWB antenna comparisons, the community
uses the previously described figure of merit concept to aid with this problem. The FOM
is the peak electric field measured at some distance multiplied by that distance, thus
the unit for FOM is volts. Using this concept, gain for UWB antennas is defined as:
UWB gain = FOM/Vp
where: Vp is the peak voltage of the driving pulse.
Typical gains are 3-4 but with extreme designs can reach 6 and above. These are the
antennas of choice for impulse radars since they can be small and lightweight but
radiate UWB pulses quite well. However, sidelobe pulse stretching makes the aiming
accuracy of the transmitting and receiving antennas crucial.
Another technique of interest in WB/UWB antennas uses a log-periodic antenna
designed with dispersion characteristics such that, when driven with the proper input
signal, it produces the fast rise time pulse required. The drive signal in this technique
must have a strong increase in frequency from start to end. No high-power designs
using this technique appear to have been accomplished.
A new type of antenna, the impulse radiating antenna (IRA), incorporates a parabolic
dish as a main component of the design (in fact, it is debatable whether this is actually
a new design because it incorporates a parabolic dish). The distinguishing feature of an
IRA is its use of a final fast switch located at the focal point of the dish to provide a
spherical waveform to the dish. The design also must include a high-voltage
transmission line to feed the peaking switch, which in all probability will not be
dispersionless. However, the frequency content of the feed signal most likely will be
less than that of the wave front from the peaking switch. The design also must include
some number of conical transmission lines from the peaking switch back to the dish,
providing something close to an impedance match for the feed pulser. The crucial
criterion here is that the IRA be driven by a spherical TEM wave front, in which case the
phase center of the wave is then fixed and the IRA is dispersionless. There is also a
much smaller pre-pulse that is radiated from the front side of the switch and is not
reflected from the dish. This signal will be radiated two focal lengths ahead of the main
pulse. For a 4-meter dish, the pre-pulse arrives about 10 ns before the main pulse.
Fast-acting semiconductor protection devices could in principle be effective in negating
the effects of the main pulse. The peaking switch at the focal point must be contained
in some insulating media and, thus, there is a reflection associated with the transition
to air. The wave front generally is not spherical as it enters the air beyond the peaking
switch owing to the physical dimensions of the switch and high-voltage insulation
requirements. Successful IRA designs use the switch insulating media and container to
form a lens designed to give a spherical wave at the air interface. The IRA, like the TEM
horn, transmits a differentiated signal from the applied pulse. This is why the rise time
of the driving pulse is so important to antenna response.
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Conclusion
This paper provided an overview of the current state of HPM sources and the
technologies driving their development. Advanced cathode materials, computer codes
for more predictive ability in electron beam generation and propagation, high-speed
switching at high power, and low-loss insulation techniques are just a few of the areas
where advancement clearly is needed for progress to occur. Advancements in
photoconductive solid-state (PCSS) and other solid-state switching and sharpening
devices appear to finally be reaching a level of development such that they may soon
be of use in high-voltage systems. If PCSS switching devices capable of higher
operating voltage and extended lifetime are made available, variations of the phased
array will become the HPM source design of choice, given the inherent advantages.
More compact antennas are greatly desired; however, the physics of the radiation
process requires structure sizes on the order of one-half wavelength of the lowest
radiated frequency. With demanding levels of directivity and gain also requirements,
compact UWB antenna designs will continue to be difficult, if not impossible, to realize.
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