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E-Bombs Part 2 of 4
High
Power Microwave Sources - The Vircator
Whilst
FCGs are potent technology base for the generation of large electrical power
pulses, the output of the FCG is by its basic physics constrained to the
frequency band below 1 MHz. Many target sets will be difficult to attack even
with very high power levels at such frequencies, moreover focusing the energy
output from such a device will be problematic. A HPM device overcomes both of
the problems, as its output power may be tightly focused and it has a much
better ability to couple energy into many target types.
A
wide range of HPM devices exist. Relativistic Klystrons, Magnetrons, Slow Wave
Devices, Reflex triodes, Spark Gap Devices and Vircators are all examples of the
available technology base [GRANATSTEIN87, HOEBERLING92]. From the perspective of
a bomb or warhead designer, the device of choice will be at this time the
Vircator, or in the nearer term a Spark Gap source. The Vircator is of interest
because it is a one shot device capable of producing a very powerful single
pulse of radiation, yet it is mechanically simple, small and robust, and can
operate over a relatively broad band of microwave frequencies.
The physics of the Vircator tube are substantially more complex than those of the preceding devices. The fundamental idea behind the Vircator is that of accelerating a high current electron beam against a mesh (or foil) anode. Many electrons will pass through the anode, forming a bubble of space charge behind the anode. Under the proper conditions, this space charge region will oscillate at microwave frequencies. If the space charge region is placed into a resonant cavity which is appropriately tuned, very high peak powers may be achieved. Conventional microwave engineering techniques may then be used to extract microwave power from the resonant cavity. Because the frequency of oscillation is dependent upon the electron beam parameters, Vircators may be tuned or chirped in frequency, where the microwave cavity will support appropriate modes. Power levels achieved in Vircator experiments range from 170 kiloWatts to 40 GigaWatts over frequencies spanning the decimetric and centimetric bands.
The
two most commonly described configurations for the Vircator are the Axial
Vircator (AV) (Fig.3), and the Transverse Vircator (TV). The Axial Vircator is
the simplest by design, and has generally produced the best power output in
experiments. It is typically built into a cylindrical waveguide structure. Power
is most often extracted by transitioning the waveguide into a conical horn
structure, which functions as an antenna. AVs typically oscillate in Transverse
Magnetic (TM) modes. The Transverse Vircator injects cathode current from the
side of the cavity and will typically oscillate in a Transverse Electric (TE)
mode.
Technical
issues in Vircator design are output pulse duration, which is typically of the
order of a microsecond and is limited by anode melting, stability of oscillation
frequency, often compromised by cavity mode hopping, conversion efficiency and
total power output. Coupling power efficiently from the Vircator cavity in modes
suitable for a chosen antenna type may also be an issue, given the high power
levels involved and thus the potential for electrical breakdown in insulators.
The
Lethality of Electromagnetic Warheads
The
issue of electromagnetic weapon lethality is complex. Unlike the technology base
for weapon construction, which has been widely published in the open literature,
lethality related issues have been published much less frequently.
While
the calculation of electromagnetic field strengths achievable at a given radius
for a given device design is a straightforward task, determining a kill
probability for a given class of target under such conditions is not.
This
is for good reasons. The first is that target types are very diverse in their
electromagnetic hardness, or ability to resist damage. Equipment which has been
intentionally shielded and hardened against electromagnetic attack will
withstand orders of magnitude greater field strengths than standard commercially
rated equipment. Moreover, various manufacturer’s implementations of like
types of equipment may vary significantly in hardness due the idiosyncrasies of
specific electrical designs, cabling schemes and chassis/shielding designs used.
The
second major problem area in determining lethality is that of coupling
efficiency, which is a measure of how much power is transferred from the field
produced by the weapon into the target. Only power coupled into the target can
cause useful damage.
Coupling
Modes
In
assessing how power is coupled into targets, two principal coupling modes are
recognised in the literature:
·
Front
Door Coupling occurs typically when power from a electromagnetic weapon is
coupled into an antenna associated with radar or communications equipment. The
antenna subsystem is designed to couple power in and out of the equipment, and
thus provides an efficient path for the power flow from the electromagnetic
weapon to enter the equipment and cause damage.
·
Back
Door Coupling occurs when the electromagnetic field from a weapon produces large
transient currents (termed spikes, when produced by a low frequency weapon ) or
electrical standing waves (when produced by a HPM weapon) on fixed electrical
wiring and cables interconnecting equipment, or providing connections to mains
power or the telephone network. Equipment connected to exposed cables or wiring
will experience either high voltage transient spikes or standing waves which can
damage power supplies and communications interfaces if these are not hardened.
Moreover, should the transient penetrate into the equipment, damage can be done
to other devices inside.
A
low frequency weapon will couple well into a typical wiring infrastructure, as
most telephone lines, networking cables and power lines follow streets, building
risers and corridors. In most instances any particular cable run will comprise
multiple linear segments joined at approximately right angles. Whatever the
relative orientation of the weapons field, more than one linear segment of the
cable run is likely to be oriented such that a good coupling efficiency can be
achieved.
It
is worth noting at this point the safe operating envelopes of some typical types
of semiconductor devices. Manufacturer’s guaranteed breakdown voltage ratings
for Silicon high frequency bipolar transistors, widely used in communications
equipment, typically vary between 15 V and 65 V. Gallium Arsenide Field Effect
Transistors are usually rated at about 10V. High density Dynamic Random Access
Memories (DRAM), an essential part of any computer, are usually rated to 7 V
against earth. Generic CMOS logic is rated between 7 V and 15 V, and
microprocessors running off 3.3 V or 5 V power supplies are usually rated very
closely to that voltage. Whilst many modern devices are equipped with additional
protection circuits at each pin, to sink electrostatic discharges, sustained or
repeated application of a high voltage will often defeat these.
Communications
interfaces and power supplies must typically meet electrical safety requirements
imposed by regulators. Such interfaces are usually protected by isolation
transformers with ratings from hundreds of Volts to about 2 to 3 kV.
It
is clearly evident that once the defence provided by a transformer, cable pulse
arrestor or shielding is breached, voltages even as low as 50 V can inflict
substantial damage upon computer and communications equipment. The author has
seen a number of equipment items (computers, consumer electronics) exposed to
low frequency high voltage spikes (near lightning strikes, electrical power
transients), and in every instance the damage was extensive, often requiring
replacement of most semiconductors in the equipment.
HPM
weapons operating in the centimetric and millimetric bands however offer an
additional coupling mechanism to Back Door Coupling. This is the ability to
directly couple into equipment through ventilation holes, gaps between panels
and poorly shielded interfaces. Under these conditions, any aperture into the
equipment behaves much like a slot in a microwave cavity, allowing microwave
radiation to directly excite or enter the cavity. The microwave radiation will
form a spatial standing wave pattern within the equipment. Components situated
within the anti-nodes within the standing wave pattern will be exposed to
potentially high electromagnetic fields.
Because microwave weapons can couple more readily than low frequency weapons, and can in many instances bypass protection devices designed to stop low frequency coupling, microwave weapons have the potential to be significantly more lethal than low frequency weapons.
What
research has been done in this area illustrates the difficulty in producing
workable models for predicting equipment vulnerability. It does however provide
a solid basis for shielding strategies and hardening of equipment.
The
diversity of likely target types and the unknown geometrical layout and
electrical characteristics of the wiring and cabling infrastructure surrounding
a target makes the exact prediction of lethality impossible.
A
general approach for dealing with wiring and cabling related back door coupling
is to determine a known lethal voltage level, and then use this to find the
required field strength to generate this voltage. Once the field strength is
known, the lethal radius for a given weapon configuration can be calculated.
A trivial example is that of a 10 GW 5 GHz HPM device illuminating a footprint of 400 to 500 meters diameter, from a distance of several hundred meters. This will result in field strengths of several kilovolts per meter within the device footprint, in turn capable of producing voltages of hundreds of volts to kilovolts on exposed wires or cables. This suggests lethal radii of the order of hundreds of meters, subject to weapon performance and target set electrical hardness.
Maximizing
Electromagnetic Bomb Lethality
To
maximize the lethality of an electromagnetic bomb it is necessary to maximize
the power coupled into the target set.
The
first step in maximizing bomb lethality is to maximize the peak power and
duration of the radiation of the weapon. For a given bomb size, this is
accomplished by using the most powerful flux compression generator (and Vircator
in a HPM bomb) which will fit the weapon size, and by maximizing the efficiency
of internal power transfers in the weapon. Energy which is not emitted is energy
wasted at the expense of lethality.
The
second step is to maximize the coupling efficiency into the target set. A good
strategy for dealing with a complex and diverse target set is to exploit every
coupling opportunity available within the bandwidth of the weapon.
A
low frequency bomb built around an FCG will require a large antenna to provide
good coupling of power from the weapon into the surrounding environment. Whilst
weapons built this way are inherently wide band, as most of the power produced
lies in the frequency band below 1 MHz compact antennas are not an option. One
possible scheme is for a bomb approaching its programmed firing altitude to
deploy five linear antenna elements. These are produced by firing off cable
spools which unwind several hundred meters of cable. Four radial antenna
elements form a “virtual” earth plane around the bomb, while an axial
antenna element is used to radiate the power from the FCG. The choice of element
lengths would need to be carefully matched to the frequency characteristics of
the weapon, to produce the desired field strength. A high power coupling pulse
transformer is used to match the low impedance FCG output to the much higher
impedance of the antenna, and ensure that the current pulse does not vaporize
the cable prematurely.
Other alternatives are possible. One is to simply guide the bomb very close to the target, and rely upon the near field produced by the FCG winding, which is in effect a loop antenna of very small diameter relative to the wavelength. Whilst coupling efficiency is inherently poor, the use of a guided bomb would allow the warhead to be positioned accurately within meters of a target. An area worth further investigation in this context is the use of low frequency bombs to damage or destroy magnetic tape libraries, as the near fields in the vicinity of a flux generator are of the order of magnitude of the coercivity of most modern magnetic materials.
Microwave bombs have a broader range of coupling modes and given the small wavelength in comparison with bomb dimensions, can be readily focused against targets with a compact antenna assembly. Assuming that the antenna provides the required weapon footprint, there are at least two mechanisms which can be employed to further maximize lethality.
The
first is sweeping the frequency or chirping the Vircator. This can improve
coupling efficiency in comparison with a single frequency weapon, by enabling
the radiation to couple into apertures and resonances over a range of
frequencies. In this fashion, a larger number of coupling opportunities are
exploited.
The
second mechanism which can be exploited to improve coupling is the polarisation
of the weapon’s emission. If we assume that the orientations of possible
coupling apertures and resonances in the target set are random in relation to
the weapon’s antenna orientation, a linearly polarised emission will only
exploit half of the opportunities available. A circularly polarised emission
will exploit all coupling opportunities.
The
practical constraint is that it may be difficult to produce an efficient high
power circularly polarised antenna design which is compact and performs over a
wide band. Some work therefore needs to be done on tapered helix or conical
spiral type antennas capable of handling high power levels, and a suitable
interface to a Vircator with multiple extraction ports must devised. A possible
implementation is depicted in Fig.5. In this arrangement, power is coupled from
the tube by stubs which directly feed a multi-filar conical helix antenna. An
implementation of this scheme would need to address the specific requirements of
bandwidth, beamwidth, efficiency of coupling from the tube, while delivering
circularly polarised radiation.
Another
aspect of electromagnetic bomb lethality is its detonation altitude, and by
varying the detonation altitude, a tradeoff may be achieved between the size of
the lethal footprint and the intensity of the electromagnetic field in that
footprint. This provides the option of sacrificing weapon coverage to achieve
kills against targets of greater electromagnetic hardness, for a given bomb size
(Fig.7, 8). This is not unlike the use of airburst explosive devices.
In
summary, lethality is maximised by maximising power output and the efficiency of
energy transfer from the weapon to the target set. Microwave weapons offer the
ability to focus nearly all of their energy output into the lethal footprint,
and offer the ability to exploit a wider range of coupling modes. Therefore,
microwave bombs are the preferred choice.
Targeting
Electromagnetic Bombs
The task of identifying targets for attack with electromagnetic bombs can be complex. Certain categories of target will be very easy to identify and engage. Buildings housing government offices and thus computer equipment, production facilities, military bases and known radar sites and communications nodes are all targets which can be readily identified through conventional photographic, satellite, imaging radar, electronic reconnaissance and humint operations. These targets are typically geographically fixed and thus may be attacked providing that the aircraft can penetrate to weapon release range. With the accuracy inherent in GPS/inertially guided weapons, the electromagnetic bomb can be programmed to detonate at the optimal position to inflict a maximum of electrical damage.
Mobile
and camouflaged targets which radiate overtly can also be readily engaged.
Mobile and relocatable air defence equipment, mobile communications nodes and
naval vessels are all good examples of this category of target. While radiating,
their positions can be precisely tracked with suitable Electronic Support
Measures (ESM) and Emitter Locating Systems (ELS) carried either by the launch
platform or a remote surveillance platform. In the latter instance target
coordinates can be continuously datalinked to the launch platform. As most such
targets move relatively slowly, they are unlikely to escape the footprint of the
electromagnetic bomb during the weapon’s flight time.
Mobile
or hidden targets which do not overtly radiate may present a problem,
particularly should conventional means of targeting be employed. A technical
solution to this problem does however exist, for many types of target. This
solution is the detection and tracking of Unintentional Emission (UE). UE has
attracted most attention in the context of TEMPEST surveillance, where transient
emanations leaking out from equipment due poor shielding can be detected and in
many instances demodulated to recover useful intelligence. Termed Van Eck
radiation, such emissions can only be suppressed by rigorous shielding and
emission control techniques, such as are employed in TEMPEST rated equipment.
Whilst the demodulation of UE can be a technically difficult task to perform well, in the context of targeting electromagnetic bombs this problem does not arise. To target such an emitter for attack requires only the ability to identify the type of emission and thus target type, and to isolate its position with sufficient accuracy to deliver the bomb. Because the emissions from computer monitors, peripherals, processor equipment, switch mode power supplies, electrical motors, internal combustion engine ignition systems, variable duty cycle electrical power controllers (thyristor or triac based), super heterodyne receiver local oscillators and computer networking cables are all distinct in their frequencies and modulations, a suitable Emitter Locating System can be designed to detect, identify and track such sources of emission.
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