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An introduction to the technical and operational aspects of the electromagnetic bomb.

The rapid growth of the computing and communications infrastructure over the last decade has produced a significant dependency in modern industrialized economies. and this dependency produces a major vulnerability to attack by electromagnetic weapons. The maturing High-Power Microwave and Flux Compression Generator technology base makes the design of practical, deployable electromagnetic munitions technically feasible.

Modern industrialized nations are now heavily dependent upon their fundamental computing and communications infrastructures. Virtually all computing and communications technologies composing the technological foundation of this infrastructure share a common attribute, in that they are built with modern high-density semiconductor components. This fundamental dependency upon the modern semiconductor device produces a global and pervasive vulnerability to attack by weapons which are specifically designed to damage or destroy semiconductor components. Importantly, such weapons are now both technically feasible and relatively economical to build, in comparison with established weapons of mass destruction such as the nuclear bomb.[1] A wide range of existing targeting and delivery techniques may be employed in using such weapons. These devices are electromagnetic weapons, and the foremost of these is the electromagnetic bomb, or E-bomb.[2]

E'S THE BOMB

In principle, an electromagnetic weapon is any device which can produce an electromagnetic field of such intensity that targeted items of electronic equipment experience either a soft or hard kill.

A soft kill is produced when the effects of the weapon cause the operation of the target equipment or system to be temporarily disrupted. A good example is a computer system, which is caused to reset or transition into an unrecoverable, or hung, state. The result is a temporary loss of function, which can seriously compromise the operation of any system which is critically dependent upon the computer system in question. A hard kill is produced when the effects of the weapon cause permanent electrical damage to the target equipment or system, necessitating either the repair or the replacement of the equipment or system in question. An example is a computer system which experiences damage to its power supply, peripheral interfaces and memory. The equipment may or may not be repairable, subject to the severity of the damage, and this can in rum render inoperable for extended periods of time any system which is critically dependent upon this computer system.

The non-nuclear electromagnetic bomb or warhead is the foremost of the emerging generation of electromagnetic weapons. Applied en masse, it has the potential to significantly alter the balance of military power in any situation where one or both players have a strong dependency upon semiconductor-based military and supporting technologies. Given the ubiquitous nature of modern semiconductor devices, particularly in modern military technology, the electromagnetic bomb promises its user the means of rapidly crippling an opponent's military, economic and arguably political systems. Faced with the electromagnetic bomb, the semiconductor device becomes a common single point of failure for most modern systems, unless extensive hardening measures are applied.

The potential utility of electromagnetic weapons as warfighting tools first became apparent during the period of atmospheric nuclear weapons testing, when it was found that a nuclear weapon detonated in the upper atmosphere produced an intense electromagnetic field transient over a geographically significant area. This phenomenon, termed the electromagnetic pulse (EMP) effect, resulted from the ionization effects produced by the radiation from the nuclear device.[3] A nuclear EMP is a short, 0.5[micro]sec-duration (nominally a 10 ns rise time and 500 ns fall time) pulse which by virtue of its extremely fast rise time produces a spectrum rich in harmonics and capable of coupling quite effectively into unshielded wiring and cabling infrastructure. The result of exploding an EMP weapon will be a high-voltage electrical "spike" propagating along any exposed conductive cables. The high-voltage spike can, if sufficiently intense, produce breakdown effects in semiconductors and, if the intensity is high enough, thermal damage effects in conductive materials [ILLUSTRATION FOR FIGURE 1 OMITTED].

THE TECHNOLOGY BASE

To construct a non-nuclear electromagnetic weapon, it is necessary to build a device which can generate a very large amount of electromagnetic energy very quickly and deliver this energy onto a target or set of targets. A diverse range of technologies may be applied to this purpose, many of which are quite mature.

The key technologies which may be applied to electromagnetic munitions design in the near term are explosively pumped flux compression generators (FCGs) and high-power microwave (HPM) devices, the most important of which is the virtual cathode oscillator, or vircator. Much unclassified literature exists which details experimental work in these areas. The results of this work clearly demonstrate that the construction of deployable electromagnetic warheads is now very feasible.

The Explosive Flux Compression Generator

The explosively pumped FCG is a mature technology, first demonstrated in the late 1950s by C.M. Fowler at Los Alamos National Laboratory[4] and later by Soviet researchers.[5] Much effort has been expended both by the United States and the Soviet Union/Commonwealth of Independent States in the intervening period, as a result of which a wide range of generator configurations has been developed.

The flux generator is a device which can produce peak electrical energies of megajoules within tens to hundreds of microseconds. This is accomplished in a compact and lightweight package. With delivered power levels of terawatts to tens of terawatts, a large flux generator can produce electrical currents which are three orders of magnitude greater than those produced by a typical lightning stroke.[6]

The FCG is built upon the idea of using a fast explosive to rapidly transfer a large amount of mechanical energy into a magnetic field, thereby significantly increasing the strength of the field. A wide range of generator configurations are possible - published literature to date details cylindrical or coaxial generators, conical generators, cylindrical implosion generators and plate, strip, spiral or helical and spherical generators.[7] This discussion will focus upon the helical FCG [ILLUSTRATION FOR FIGURE 2 OMITTED], as this configuration is most readily applied to bomb and warhead designs.

A flux generator must initially be primed with a magnetic flux - termed a seed or priming field - before the explosive charge can be initiated. The priming field is most commonly produced by discharging an electrical current through the generator; this current is usually termed a start or priming current. The priming current can be produced, in principle, by any device which is capable of producing a current pulse which is often hundreds of thousands to millions of amps of current. The device most commonly used for this purpose is a high-voltage capacitor bank, although small magneto-hydrodynamic generators, homopolar generators and smaller FCGs could also be used for this purpose.

In operation, the explosive is initiated when the priming current has peaked. The explosive burn will distort the armature, shorting the armature and the stator and bypassing the start current source. As the burn progresses, the armature will form a conical shape, with a typical angle of 12-19 [degrees] of arc, which propagates along the length of the generator. The propagating short compresses the magnetic field and reduces the winding inductance, which causes the winding current to ramp up until the generator disintegrates.

Ramp times of several tens to hundreds of microseconds have been demonstrated, with peak currents of tens of mega-amperes and peak energies of tens of megajoules. The generator is in effect a large current amplifier, and current multiplication ratios of hundreds and higher have been described. Flux generators have been successfully cascaded in experiments conducted by the Los Alamos National Laboratory and the Air Force Wright Laboratory, with a smaller generator priming a larger generator.[8]

For munitions applications, the cylindrical form factor of the helical generator is most suitable, as it allows the axial stacking of components. The principal technical issues in adapting existing designs lie in matching the generator's output current to the intended load, such as a microwave tube. This may be accomplished by using passive pulse-shaping networks, high-current transformers and explosive switching devices.[9] The appropriate use of such components will produce a current pulse with suitable waveform shape and timing to satisfy the requirements of the load device. Where a generator is to be directly used as a low-frequency munition, a simple low-inductance coaxial load may be attached to the end of the generator.

It is worth noting that while some FCG designs can be fabricated for a cost on the order of thousands of dollars, the FCG is often a very difficult device to design well and can require a substantial effort in analysis, modeling and prototype testing.

High-Power Microwave Devices - the Virtual Cathode Oscillator

The lethality of the flux generator is constrained by the limited coupling efficiency of a low-frequency pulse, which is typically spectrally constrained to below 1 MHz. Substantially better coupling efficiencies may be achieved by the use of HPM devices, which are more lethal.

A wide range of HPM devices have been described in the published literature. The relativistic klystron, the magnetron, the slow wave device, the reflex triode and the spark gap generator may all be used to produce useful levels of HPM emission.[10]

The vircator is the most suitable of the current generation of HPM devices for use in munition applications, as it is a simple, cheap, robust, one-shot broadband device capable of producing tens of gigawatts of microwave power. The operating principles of the vircator are significantly more complex than those of the flux generator. The vircator is based upon the principle of accelerating a powerful electron beam to relativistic velocities, which causes electrons to punch through a foil or mesh anode. The electrons which have passed the anode form a bubble of space charge, termed a virtual cathode, behind the anode. Under the proper conditions, the virtual cathode is unstable and, if placed in a microwave cavity, will oscillate in the microwave band. Large peak-power levels may be extracted from the oscillating virtual cathode, using established microwave engineering techniques. The anode will typically vaporize or melt after about 1[micro]sec of operation.

Because the frequency of the oscillation is critically dependent upon the parameters of the electron beam, vircators have a propensity to mode hop and drift in frequency with variations in beam current. If the beam current is suitably manipulated, the vircator can be chirped over a relatively wide band of frequencies. Published experiments suggest peak power levels ranging from 170 kW up to 40 GW within the centimetric and decimetric (D through K) bands.[11]

A number of vircator configurations exist, the most common of which are the axial and the transverse vircators. The axial vircator is the simplest of the two and has produced the best power levels in experiments [ILLUSTRATION FOR FIGURE 3 OMITTED]. It is built into a cylindrical waveguide structure and very commonly uses a transition to a conical horn antenna as a means of extracting power from the cavity. Whereas the axial vircator typically oscillates in a transverse magnetic mode, the transverse vircator oscillates in a transverse electric mode. Current is typically injected into the side of a transverse viractor cavity.

TECHNICAL ISSUES IN THE DESIGN OF AN ELECTROMAGNETIC WARHEAD

The design of an electromagnetic munition, either for bomb or missile applications, is not a trivial task. A number of complex issues must be addressed, including the necessary electrical characteristics to achieve required lethality, packaging, weight, reliability, robustness and integration with the delivery vehicle.

Electromagnetic warheads may in principle be built as low-frequency devices, using a flux generator alone, or as microwave devices, using a flux generator to power a microwave vircator. Combined-effects warheads, which use an oversized flux generator together with a vircator to produce low-frequency and microwave damage effects, are also a possibility.

Such warheads may be packaged as bombs to be delivered by aircraft as free-fall munitions or used in glide bombs as unpowered stand-off munitions. Provided that sufficient performance can be packaged tightly enough, such warheads could also be fitted to stand-off missiles, cruise missiles, anti-ship missiles, surface-to-air missiles and air-to-air missiles.

Semiconductor Susceptibility

The primary electrical damage mechanism we are interested in is electrical breakdown resulting from the effects of exposure to high voltages. Electrical breakdown mechanisms will be quite specific to the type of device exposed and importantly require very little energy to initiate.

In bipolar semiconductor devices, a high voltage across a reverse biased PN junction will rip carriers from the lattice, eventually producing an avalanche effect. If the power supply in the equipment can deliver sufficient energy, thermal damage will subsequently result and the device will be destroyed. Silicon radio frequency (RF) bipolar junction transistors (BJTs), which are widely employed in communications, radar and EW equipment, typically have safe voltage ratings between 15 V and 65 V.[12]

In metal oxide semiconductor (MOS) and other field effect transistor (FET) devices, the primary damage mechanism is an electrical breakdown of the device gate dielectric. The result of exposing a FET device to excessive gate voltages will be a leakage current which may be sufficient to render the device inoperable or degrade its performance significantly. As with BJTs, further secondary damage effects may be produced by the equipment power supply. Breakdown voltages against equipment earth and supply rails are typically less than 10 or 15 V for generic Si CMOS, NMOS, GaAs FETs and high-density DRAMs. Microprocessors running with 3.3- or 5-V rails will tolerate only several volts beyond the rail voltages.[13]

Many devices employ internal structures to absorb electrostatic discharges at the device pins; however, such structures may or may not survive repeated or sustained exposure to high RF voltages. Many communications interfaces employ protection transformers in order to meet regulatory requirements; such transformers have typical ratings between 2 and 3 kV.[14]

If the defense provided by shielding or protection devices is breached, RF voltages as low as tens of volts may damage or destroy semiconductor components, while lesser voltages may produce temporary disruption of operation. Devices which are damaged may continue to operate but fail intermittently, resulting in a substantial expense in equipment debugging time and a significant disruption of operations.

Clearly, the objective of a warhead designer should be to ensure that a maximum of electrical energy is coupled into the target equipment, to maximize damage effects.

Coupling Mechanisms

The literature recognizes two primary coupling mechanisms through which internal components may be attacked.

Front Door coupling will take place when energy couples in through an antenna, like those used by radar, communications and EW equipment. Antennas are designed to gather energy and thus may efficiently concentrate received energy in receiver circuits. Energy from a weapon can then destroy RF semiconductor devices. Back Door coupling is a more complex mechanism and occurs when energy is coupled into wiring and cables, through which it propagates inside equipment and damages components which may be accessible via conductive, inductive or capacitive paths.

A low-frequency or combined-effects munition using a flux generator will produce a single high-voltage spike and ringing on fixed electrical wiring and cabling. This transient will propagate along the cable and destroy any sensitive components which it may access.[15] Because the fixed power, communications and networking wiring infrastructure typically follows streets, corridors and risers with cable runs of hundreds to thousands of meters, good coupling efficiencies may be achieved. This is because any cable run will comprise multiple linear segments which are typically at close to right angles; therefore, whatever the relative orientation of the weapon field, one or more segments will provide very good coupling efficiency. Networking cables (e.g., 10/100 Base-T Ethernet) use fast low-loss dielectrics and are thus very efficient at propagating such transients with minimal loss.

Flux generator-based munitions also have the potential to destroy data repositories which use magnetic storage media such as tapes. The near field produced in the close proximity of a large flux generator would be easily greater in magnitude than the magnetic coercivity of most modern magnetic materials. Archives using older magnetic media would be significantly more vulnerable to such attack, as a result of the lower coercivity of the medium.

A microwave weapon couples in two back-door modes. The first of these is by producing standing waves on exposed wiring, through which RF energy can directly damage interface devices as well as enter equipment cavities and excite internal resonances. The second mode of coupling is directly through ventilation holes, grills, gaps and poorly secured panels. Any aperture of a suitable size will behave like a slot radiator. Once a resonance is excited within the equipment cavity, a potentially very high field strength may be achieved at an antinode in the standing wave pattern. Internal wiring, printed circuit board tracks and inductive or capacitive paths may then couple energy.

Determining Munition Lethality

As is readily apparent, the exact prediction of a weapon's kill probability is for all practical purposes impossible. However, if we can empirically determine order of magnitude voltages for given damage levels on given types or classes of equipment, we can produce a baseline for estimating the lethality of the munition. Once we know the voltage, we can then determine required field strengths for typical wiring geometries and lengths and in turn determine the required weapon power and distance.

As an example, given the knowledge that a microwave standing wave of kilovolts to tens of kilovolts of amplitude on wiring or cabling associated with a given piece of equipment will produce a hard kill, it is not difficult to determine that a field of kilovolts/m or tens of kilovolts/m at several gigahertz of frequency will produce such voltages. If we then assume a desired lethal footprint for the weapon of 400 to 500 m diameter, or about 0.2 [km.sup.2], we will need a 10-GW microwave warhead operating at about 5 GHz.[16] The choice of frequency in this instance represents a compromise, in that shorter wavelengths generally offer better coupling performance, better power-transfer performance and better antenna performance for a given antenna size. However, shorter wavelengths impose greater demands upon the microwave tube and below 3-cm wavelength ([greater than]10 GHz) begin to suffer from atmospheric quantum absorption effects. A 10-GW warhead operating at about 5 GHz is easily within the reach of current technology.[17]

OPERATIONAL CONSIDERATIONS AND BOMB DAMAGE ASSESSMENT

Bomb damage assessment (BDA) following strikes with electromagnetic warheads is the single most problematic aspect of using such weapons operationally and is likely to become the single greatest impediment to the wider operational use of such munitions. Unlike the case with conventional explosive weapons, determining whether a soft or hard kill has been achieved with an electromagnetic war-head will be quite difficult. This is for a number of good reasons.

Emitting targets such as radars or communications equipment may continue to transmit even if their receivers, signal processors and data processing subsystems have been electrically damaged or destroyed. Strategic targets such as telephone exchanges, satellite communications, key microwave repeater nodes, government offices, finance industry sites, broadcasting facilities and large production facilities generate continuous electromagnetic emissions and if successfully attacked will cease to do so.

Determining the success of an attack upon a nonradiating and/or hidden target will be even more difficult. It may be necessary to observe enemy actions over a period of time to determine whether the site is still operational. Hidden targets which do not overtly radiate transmissions may, however, be detected and identified via the use of unintended emissions (UE),[18] more commonly known as Van Eck[19] or TEMPEST radiation. UE is result of switching transients in equipment - such as computers, peripherals, switch-mode power supplies, display monitors, local area networks, electrical motors, variable cycle power controllers and internal combustion engine ignition systems - leaking out through ineffective shielding. It can also result from superhet receiver local oscillators leaking out through antennas. Importantly, from an electronic reconnaissance perspective, these emissions are quite unique to their source and therefore can be used to identify it. This is particularly true for emissions from computer equipment and local area networks, as these will exhibit regular repetitive patterns.

While UE typically occurs at power levels many orders of magnitude lower than intentional emissions, regular patterns could allow the use of correlation techniques to significantly increase receiver sensitivity. Further work is needed in this area to determine the feasibility of building an unintentional emitter locating system (UELS) which could be fitted to reconnaissance or strike aircraft or unmanned aerial vehicles. The availability of UELS equipment would resolve much ambiguity in such situations, as it would provide an indication of whether the equipment in the site is still operating. If the UELS is tracking emissions from the target when the weapon is initiated, and the emissions cease and do not reappear after a short period of time, then a hard kill may be assumed with reasonable confidence.

If an electromagnetic munition is used to attack an air base or naval asset, success may become readily apparent from a rapid drop in sortie rates or activity. A critical site or asset, the use of which is required continuously, will not remain inactive unless it is unable to operate. Thus BDA may be accomplished in such instances by observing activity levels after the attack.

Unless specific intelligence is available, the level of hardening which an opponent may have installed at any given site, or the hardness of the equipment in use, may be unknown. Older Soviet Bloc equipment, built with thermionic devices, may resist very high field strengths and may require attack with explosive munitions instead. The wide use of commercial-off-the-shelf equipment in military and civilian systems alike will nevertheless introduce a large degree of susceptibility across a wide range of target sets. Therefore, a combined attack with both electromagnetic and explosive munitions may be a worthwhile strategy to adopt. In this fashion, an electromagnetic munition can be used to suppress the whole site and a conventional munition used to destroy a specific aim point, such as the site radar mast or communications building.

CONCLUSIONS

The design and deployment of electromagnetic warheads for bomb and missile applications is technically feasible in the next decade. Such munitions can be profitably applied to both strategic and tactical targets and may be delivered by a wide range of existing aircraft and missiles. Areas which will require significant research and development efforts in the near term are the packaging of electromagnetic warheads, the integration of warhead components, compact high-energy priming source technology and tools for reconnaissance and BDA using UE.

Provided that satisfactory solutions can be found for these problems, electromagnetic munitions for bomb and missile applications promise to be important and robust weapons in both strategic and tactical operations, offering significantly reduced collateral damage and lower human casualties than established weapons.

REFERENCES

1. Electromagnetic bombs, because of their substantially larger effective footprints in comparison with chemical explosive bombs, are often regarded as weapons of electrical mass destruction (WEMD).

2. Terminology in this area is unfortunately quite unclear. This paper will use the terms electromagnetic bomb (E-bomb), electromagnetic warhead and electromagnetic munition interchangeably, with specific references to low-frequency weapons and high-power microwave (HPM) weapons. Other terms in use are RF munition, EMP munition, EMP bomb and T-bomb.

3. S. Glasstone, editor, The Effects of Nuclear Weapons, US AEC, April 1962 (revised edition February 1964).

4. C.M. Fowler, W.B. Garn and R.S. Caird, "Production of Very High Magnetic Fields by Implosion," Journal of Applied Physics, vol. 31, no. 3, 588-594, March 1960.

5. A.D Sakharov, et al., "Magnetic Cumulation," Doklady Akademii Nauk, 1966, pp. 165, 65-68. Reprinted in Sov. Phys. Usp., 34 (5), May 1991, American Institute of Physics. Also, A.D. Sakharov, "Magnetoimplosive Generators," Usp. Fiz. Nauk 88, 1966, pp. 725-734. Reprinted in Sov. Phys.Usp. 34 (5), May 1991, American Institute of Physics.

6. The EMP - A Triangular Impulse, 2.29, A Handbook Series on Electromagnetic Interference and Compatibility, Don White Consultants, Maryland, 1978. Also, CM. Fowler and R.S. Caird, "The Mark IX Generator," Digest of Technical Papers, Seventh IEEE Pulsed Power Conference, 475, IEEE, New York, 1989. A useful comparison here is that a typical lightning stroke produces a 30,000-amp current, which is typically 1,000 times smaller than the current produced by a large flux generator.

7. R.S. Caird, et al., "Tests of an Explosive Driven Coaxial Generator," Digest of Technical Papers, Fifth IEEE Pulsed Power Conference, p. 220, IEEE, New York, 1985.

R.E. Reinovsky, P.S. Levi and J.M. Welby, "An Economical, 2 Stage Flux Compression Generator System," Digest of Technical Papers, Fifth IEEE Pulsed Power Conference, p. 216, IEEE, New York, 1985.

Also, Fowler and Caird, "The Mark IX Generator."

8. Fact Sheet, High Energy Microwave Laboratory, USAF AFMC, Phillips Laboratory, Kirtland AFB, NM, 1994, and Reinovsky, Levi and Welby, "An Economical, 2 Stage Flux Compression Generator System."

9. J.H. Goforth, et al., "Experiments with Explosively Formed Fuse Opening Switches in Higher Efficiency Circuits," Digest of Technical Papers, Seventh IEEE Pulsed Power Conference, p. 479, IEEE, New York, 1989.

10. V.L. Granatstein and I. Alexeff, High Power Microwave Sources, Artech House, Boston/London, 1987.

R.F. Heoberling and M.V. Fazio, "Advances in Virtual Cathode Microwave Sources," IEEE Transactions on Electromagnetic Compatibility, vol. 34, no. 3, 252, August 1992.

11. L.E. Thode, "Virtual-Cathode Microwave Device Research: Experiment and Simulation," chapter 14 in High Power Microwave Sources, 1987.

12. Motorola RF device data, Motorola Semiconductor Products Inc., Arizona, 1983.

13. CMOS Databook, National Semiconductor Corp., Santa Clara, 1978; Micron DRAM Data Book, Micron Technology Inc., Idaho, 1992; and Motorola RF device data.

14. NPI93 - NPI Local Area Network Products, SMD Transformers. Nano Pulse Industries, Brea. 1993.

15. C.D. Taylor and C.W Harrison, "On the Coupling of Microwave Radiation to Wire Structures," IEEE Transactions on Electromagnetic Compatibility, vol. 34. no. 3, 183, August 1992.

Also, The EMP - A Triangular Impulse, Don White Consultants.

16. J.D. Kraus, Antennas, second edition, McGraw-Hill, 1988; Taylor, "On the Coupling of Microwave Radiation to Wire Structures."

17. Thode, "Virtual-Cathode Microwave Device Research."

18. D. Herskovitz, "The Other SIGINT/ELINT," Journal of Electronic Defense, April 1996.

19. W. van Eck, "Electromagnetic Radiation from Video Display Units: An Eavesdropping Risk," Computers and Security, 1985, p. 269.

Carlo Kopp is is currently working on a Ph.D. in computer science at Monash University in Melbourne, Australia. The author has been actively publishing as a defense analyst in Australia's leading aviation trade journal, Australian Aviation, since 1980 and has specialized in the area of application of modern military technology to operations and strategy. He may be reached at Carlo. Kopp @ aus.net.
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Publication:Journal of Electronic Defense
Date:Jan 1, 1997
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