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- Third-Generation Nuclear Weapons
-
- During the early 1950's American weapon laboratories were
- exceptionally productive. They not only achieved dramatic improvements in
- the performance of fission bombs, which represent the first generation of
- nuclear weapons, but also succeeded in establishing a second generation of
- nuclear weapons by harnessing the explosive power of fusion in the form of
- the hydrogen bomb and its various derivatives. By the end of the 1950's the
- warheads in the U.S. nuclear armament bore little resemblance to the bombs
- that had ushered in the nuclear age over Hiroshima and Nagasaki.
-
- Today a third generation of nuclear weapons is technologically
- feasible. By altering the shape of the nuclear explosive and manipulating
- other design features, weapons could be built that generate and direct
- beams of radiation or streams of metallic pellets or droplets at such
- targets as missile-launch facilities on the ground, missiles in the air and
- satellites in space. These weapons would be as removed from current nuclear
- weapons in terms of military effectiveness as a rifle is technologically
- distant from gunpowder.
-
- The surge of technical creativity that produced the first two
- generations of nuclear weapons can be explained largely by the fact that
- the national laboratories had massive funding, a mandate to pursue new
- weapon possibilities and unqualified Government support. Yet speaking as
- one who worked at that time on the design of nuclear weapons, perhaps the
- most stimulating factor of all was simply the intense exhilaration that
- every scientist or engineer experiences when he or she has the freedom to
- explore completely new technical concepts and then to bring them into
- reality.
-
- The Strategic Defense Initiative, under which a vigorous military
- research and development program is currently being carried out, could well
- generate conditions at the U.S. weapon laboratories similar to those in the
- 1950's. The daunting technical challenge implied in President Reagan's call
- to search for a way to defend the nation against ballistic missiles is
- likely to spur modern-day weaponeers to consider radically new types of
- nuclear weapons--quite apart from concurrent advances in delivery and
- command-and-control systems.
-
- It would be logical for a weapon designer to build on the legacy of
- the first- and second-generation nuclear weapons, all of which transform
- mass into an abundance of energy that is then uniformly dissipated in a
- roughly spherical pattern. Such a new generation of nuclear weapons might
- selectively enhance or suppress certain types of energy from the vast
- energy source provided by a nuclear explosion. Moreover, the lethal effects
- of a selected energy carrier (such as electromagnetic radiation, subatomic
- particles or expelled material) might be increased by distorting its normal
- pattern of emission into a highly asymmetrical one--in essence
- concentrating the energy in a certain direction.
-
- Indeed, nuclear weapons that deliver 1,000 or more times the energy
- per unit area on a target than does a conventional nuclear weapon are
- entirely plausible. Special components or materials attached to the
- exterior of a nuclear device could convert the energy released by its
- detonation into a different form; configuring the nuclear explosive and its
- casing in certain ways could channel most of the energy in certain
- directions. Alternatively, the energy released from a nuclear explosion
- could be converted and directed by exploiting the effect such an explosion
- has on natural surroundings. Regardless of their original intent, if such
- weapons are built, they will undoubtedly be modified for application in a
- wide variety of strategic and tactical missions--offensive as well as
- defensive --in all kinds of environments.
-
- Like previous generations of nuclear weapons, members of the new
- generation would derive their enormous explosive energy from fission (the
- splitting of a nucleus by a neutron into two nuclei of comparable size) or
- a combination of fission and fusion (the joining of two light nuclei to
- form a heavier nucleus). Fission explosions are easier to produce and
- essentially amount to bringing together, in the space of about a
- microsecond (a millionth of a second), enough fissile material (such as
- uranium 235 or plutonium 239) in a sufficiently small volume so that a huge
- number of fission-inducing neutrons can be quickly generated in the
- material. The high-speed assembly of the fissile material is generally
- achieved by precisely detonating chemical-explosive charges in such a way
- as to propel subunits of the material together to form a single compressed
- mass.
-
- Initiating a fusion explosion is a much more complex affair, because
- extremely high temperatures (on the order of hundreds of millions of
- degrees Kelvin) are required. In fact, the only practical mechanism by
- which to generate such temperatures in a transportable device is a fission
- explosive. A pure-fusion explosive--without a fission trigger--reportedly
- still eludes weapon designers.
-
- Fusion reactions not only release substantially more energy per unit
- weight than fission reactions but also produce more high-energy neutrons.
- The additional neutrons can in fact "boost' the yield of a fission weapon
- if they are allowed to interact with uranium or plutonium in the weapon's
- core. Hence placing small quantities of thermonuclear fuel such as tritium
- or deuterium (both are isotopes of hydrogen) in a fission weapon increases
- the overall yield-to-weight ratio of the weapon, since the added weight
- needed for boosting is insignificant.
-
- Unlike boosted weapons, in which the energy released by fusion does
- not significantly contribute to the overall weapon yield, so-called
- thermonuclear weapons derive a substantial part of their explosive energy
- from fusion reactions. The relative amounts of energy attributable to
- fusion and fission depend on the design of the weapon. If a considerable
- amount of lithium deuteride (which, when it is irradiated with neutrons,
- produces tritium) is compressed and heated by the energy released from a
- small fission-explosive trigger, the fraction of the total yield due to
- fusion in relation to the fraction due to fission can become very large.
- Such weapons are sometimes called "clean' thermonuclear weapons, because
- they release relatively few radioactive fission products.
-
- At the other extreme are weapons in which the thermonuclear fuel is
- enclosed in a substantial quantity of ordinary uranium (uranium 238). The
- high-energy neutrons produced by fusion in the thermonuclear fuel can
- induce fission in the surrounding uranium, multiplying the total fission
- yield considerably.
-
- The yield-to-weight ratios of pure fission warheads have ranged from a
- low of about .0005 kiloton per kilogram to a high of about .1 kiloton per
- kilogram. (One kiloton is equivalent to the detonation of about 1,000 tons
- of TNT.) The overall yield-to-weight ratio of strategic thermonuclear
- warheads has been as high as about six kilotons per kilogram. Although the
- maximum theoretical ratios are 17 and 50 kilotons per kilogram respectively
- for fission and fusion reactions, the maximum yield-to-weight ratio for
- U.S. weapons has probably come close to the practical limit owing to
- various unavoidable inefficiencies in nuclearweapon design (primarily
- arising from the fact that it is impossible to keep the weapon from
- disintegrating before complete fission or fusion of the nuclear explosive
- has taken place). Yet even the lowest yield-to-weight ratio of a pure
- fission weapon is orders of magnitude higher than the ratio of chemical
- explosives.
-
- Indeed, the discharge of energy from a detonated nuclear weapon is so
- massive and violent that it immediately vaporizes and ionizes the weapon
- itself, converting it into plasma: an extremely hot gas of positively
- charged ions and negatively charged electrons. In addition substantial
- quantities of gamma rays and neutrons are emitted as by-products of the
- fission and fusion reactions. The kinetic energy of the weapon-debris
- plasma as well as the nuclear emanations constitute what could be called
- the primary effects of a nuclear explosion; they arise in any nuclear
- burst, regardless of the environment in which it takes place.
-
- Plasma at the temperatures prevailing just after a nuclear explosion
- radiates X rays. Indeed, about 70 percent of the energy emitted in the
- first few microseconds after an explosion consists of this radiation. The
- exact fraction of the total explosive energy released in the form of
- primary X rays tends to increase with the yield-to-weight ratio, since the
- ratio determines the overall temperature of the weapon-debris plasma. The
- greater the amount of energy dissipated in the form of X rays, the less the
- kinetic energy of the expanding weapon-debris plasma. A typical plasma
- velocity for a thermonuclear weapon with a high yield-to-weight ratio would
- be about 1,000 kilometers per second, representing some 10 percent of the
- total explosive energy.
-
- Gamma rays that are emitted within a second or so of the explosion
- (so-called prompt gamma rays) account for about 3.5 percent of the total
- energy released by fission and for as much as 20 percent of the energy
- released from some cycles of thermonuclear reactions. In current types of
- nuclear explosives all but a few percent of these gamma rays are absorbed
- within the weapon. The kinetic energy of excess neutrons accounts for about
- another 1.8 percent of the energy released by fission and, depending on the
- type of thermonuclear fuel, between 40 and 80 percent of the energy
- released by fusion. High-energy neutrons, however, tend to be slowed down
- by inelastic scattering or collision with light elements in the materials
- of implosion systems. The average energy of the neutrons that actually
- escape capture in the weapon materials and are released into the
- environment is therefore typically much lower. This effect is particularly
- pronounced in thermonuclear weapons, since the fuel consists of light
- elements. Indeed, in such weapons the energy of the neutrons is
- deliberately deposited within the thermonuclear fuel, since neutrons play a
- vital role in maintaining the elevated temperatures needed to achieve high
- reaction rates.
-
- Most nuclear-weapon development for the past 40 years has not had the
- aim of significantly enhancing or suppressing particular forms of energy
- other than by adjusting the relative amounts of fission and fusion taking
- place in the warhead. One exception is the so-called neutron bomb [see
- "Enhanced-Radiation Weapons,' by Fred M. Kaplan; SCIENTIFIC AMERICAN, May,
- 1978]. A nuetron bomb is a low-yield thermonuclear explosive specifically
- designed for an increased output of high-energy neutrons per kiloton of
- total yield. It is intended to be a nuclear antipersonnel weapon that
- produces minimal concomitant blast damage and radioactive fallout.
-
- Yet just as a nuclear weapon can be designed to enhance its output of
- primary neutrons at the expense of blast and radioactive fallout, virtually
- any other primary energy released by a nuclear explosive could similarly be
- enhanced by placing appropriate materials in suitable geometries close to
- the explosive. Significant control over the amount and energy of
- X-radiation, for example, could be achieved by changing the average
- molecular weight of the materials in the weapon, the weapon's exterior
- surface area and the way the energy generated in its core is distributed
- over the expanding front of weapon debris after detonation.
-
- Changes in the design of thermonuclear weapons could also
- substantially increase the energy accounted for by prompt gamma rays. One
- possibility is to encase the weapon with an isotope that, when it is
- bombarded with neutrons, emits gamma rays. In this way excess fission or
- fusion neutrons escaping from the weapon's core could induce the emission
- of gamma rays, nearly half of which would leave the expanding explosion
- debris. (The other half would radiate inward and be absorbed by the debris
- material.)
-
- The quantities of radioactive fission products (the main component of
- fallout) among the weapon debris could similarly be controlled over very
- wide ranges, particularly for thermonuclear weapons with yields greater
- than a few hundred kilotons. Furthermore, by blanketing the weapon with
- isotopes that, when they are irradiated with neutrons, produce radioactive
- nuclei having selected half-lives and decay modes, the lethality of the
- radioactive fallout could be increased.
-
- The effects of a nuclear explosion could also be made directional in
- the same way high-explosive devices such as conventional shaped charges can
- produce armor-penetrating jets of molten metal or directional shrapnel. By
- considering how explosive charges of nonspherical shape release their
- energy some insight can be gained on how this could be done [see
- illustration on next page].
-
- Detonating a disk of high explosive all at once, for example, causes
- the explosion products to be flung out in a characteristic double-cone
- pattern. The reason is that the velocity of the explosion products in a
- direction perpendicular to the disk's two surfaces will be higher than
- their radial velocity. The apex angle of the cones will
- direction perpby
- the ratio of the thickness of the disk to its diameter. The average total
- kinetic-energy flux (energy per unit area per unit time) of the explosion
- products crossing a plane perpendicular to the axis of the double cone
- could therefore be considerably greater than it would be if the same mass
- of high explosive expels its products spherically. If the average velocity
- of the explosion products in the direction of the cone's axis is 40 times
- their average radial velocity (corresponding to a cone angle of about three
- degrees), the enhancement factor would be about 3,000.
-
- Another example is the detonation of a long, thin cylinder of high
- explosive. In this case the highest explosion-product velocities would be
- perpendicular to the axis of the cylinder. Hence the explosion products
- would tend to preserve a cylindrical pattern; the energy-flux enhancement
- factor in this example tends to be smaller than the factor in the preceding
- one.
-
- A final example is a charge of high explosive that is tamped, or
- restricted, by dense material in all directions except forward. In such a
- case the explosion products would be projected primarily forward. The
- additional weight entailed by the inert mass around the explosive is more
- than balanced by the concentration of the energy through the opening in the
- tamper. That is why a rifle bullet can produce much greater damage to a
- target than the detonation of a mass of high explosive having the same
- weight as the rifle.
-
- Of course, nuclear reactions release many more forms of energy at much
- higher intensities than chemical high explosives, including gamma rays, X
- rays, neutrons and a wide variety of radioactive nuclei. It is clear that
- even nuclear explosives of very low yield offer many more opportunities
- than chemical explosives to produce such directional effects.
-
- Most of a nuclear explosion's lethal effects are actually secondary
- effects resulting from the interaction of the kinetic energy of the
- weapondebris plasma and the initial radiation (namely X-radiation) with the
- medium in which the detonation takes place. Hence many nuclear-explosion
- phenomena of military interest are determined by properties of the medium
- such as its pressure, density and composition. It is the variations in
- these properties that account for the widely divergent responses associated
- with nuclear bursts in space, in the atmosphere, on the surface of the
- earth and below the earth's surface. By choosing the appropriate primary
- effects to be enhanced or suppressed, depending on the prevailing
- environmental conditions, the secondary effects of the weapon can be more
- efficiently transmitted to targets.
-
- Because space is essentially empty, there is no medium with which to
- interact, and the primary products of a nuclear explosion (X rays,
- weapondebris plasma and nuclear radiation) continue to travel in the same
- directions in which they were released until they hit something or are
- deflected by the earth's magnetic or gravitational field (depending on
- whether they have respectively electric charge or mass). That is why
- initial asymmetries in the distribution of mass in an explosive set off in
- space tend to be preserved out to great distances in the pattern of the
- energy radiated.
-
- If a nuclear explosive is detonated above the atmosphere but within
- the earth's magnetic field, the plasma expanding in directions more or less
- perpendicular to the magnetic field lines will distort the field. When this
- happens, a large fraction of the kinetic energy in the weapon debris is
- converted into electromagnetic energy, resulting in the emission of a
- sudden burst of radiation with a broad range of wavelengths --from a few
- meters to hundreds of kilometers or more. Such an electromagnetic pulse
- (EMP) can represent a substantial fraction of the total energy of the
- explosion and can propagate with little attenuation through the atmosphere
- to the earth's surface.
-
- Nuclear explosions in space or in the high-altitude regions of the
- atmosphere can produce another type of EMP. In this case gamma or
- high-energy X rays striking the upper part of the atmosphere cause
- electrons to be ejected from air molecules. Such a sudden cascade of
- electrons is equivalent to a huge surge of electric current. Since the
- current would not be spherically symmetrical (it would flow predominantly
- in the direction of higher air density, namely downward) and would vary
- with time, it would generate transient magnetic fields that in turn would
- produce electromagnetic radiation in the form of an EMP.
-
- As a result of the approximately exponential increase in the density
- of the atmosphere with decreasing altitude, much of the energy radiated
- downward by a nuclear explosion above the atmosphere is deposited in the
- atmosphere's upper reaches. Deposition of this energy can sometimes produce
- severe secondary effects that then propagate to the surface of the earth. X
- rays and weapon debris at sufficiently high fluences (total energy per unit
- area) can, for example, heat the atmosphere to such high temperatures that
- it radiates visible light and infrared radiation. Gamma rays, neutrons and
- X rays released by the weapon, as well as the decay products of
- radionuclides, can directly or indirectly generate electric currencts in
- the layer of the atmosphere where they deposit their energy. These currents
- can then generate other EMP's whose wavelengths and instantaneous power
- levels extend over a very wide range. Heating of the atmosphere can also
- initiate complex chemical reactions that affect its transmission and
- reflection of radio waves.
-
- In the lower atmosphere, underground or underwater the primary
- X-radiation leaving an exploding nuclear weapon is absorbed by the atoms
- and molecules of the surrounding medium within a few meters of the point of
- detonation. Consequently the medium is quickly heated, forming a fireball,
- which in turn reemits electromagnetic radiation of lower frequencies. Most
- of this radiation is in the visible and infrared regions of the spectrum
- and can travel considerable distances through the air.
-
- The radiative energy also combines with the kinetic energy of the
- outwardly expanding plasma to produce a pressure impulse of tremendous
- force on the surrounding medium. Such an impulse forms a shock, or blast,
- wave that propagates through the medium. The denser the medium, the greater
- the amount of energy transformed into the shock wave. Hence for explosions
- in water or earth a larger percentage of the explosion's energy is
- converted into a shock wave than is the case for explosions in air.
-
- Surface, subsurface or very-low-altitude explosions can also fling
- huge quantities of dust, crater debris, manmade structures or water into
- the air that can directly or indirectly cause considerable destruction.
- Moreover, much of this material is likely to be rendered radioactive,
- thereby severely contaminating extensive areas through fallout.
-
- Forms of energy that are not normally released as primary or secondary
- effects can also be generated from the vast energy supply provided by a
- nuclear burst. Furthermore, such energy can be channeled into small
- emission angles. The key question about such weapons (which cannot be
- answered in detail here because the subject is classified) is how to
- convert a substantial fraction of the energy of a nuclear explosion into a
- particular energy that can be emitted with high directional enhancement.
- Suffice it to say that electromagnetic energy with wavelengths typical of
- gamma rays, X rays, visible light and microwaves can be focused by the
- equivalent of lasers: devices that cause the atoms or molecules of a
- material to radiate in phase. Longer-wavelength radiation can be emitted
- directionally if such weapons are equipped with the equivalent of antennas.
- The problem in either case is how to channel the torrential flow of energy
- from a nuclear explosion into an energy-conversion and -direction device in
- the few microseconds before the entire weapon assembly disintegrates.
- Another option, which may simplify the problem somewhat, is to set off
- nuclear devices in a reusable containment structure from which the
- explosive energy could then be tapped. Such structures, designed to
- withstand explosions with yields of up to perhaps one kiloton, have in fact
- been under study for several decades. The Lawrence Livermore National
- Laboratory has recently considered a proposal to construct such a chamber
- in which a variety of nuclear effects could be studied.
-
- For ground-based weapons intended to attack targets in space the
- weight of the needed equipment is not critical; for space-based weapons it
- is, however. It is therefore to be expected that the technical approaches
- for developing ground-based directed-energy nuclear weapons will be
- different from those required for similar weapons in space. Some advantages
- that ground-based weapons have over weapons placed in space include
- avoidance of treaties banning nuclear weapons in space, accessibility to
- large and heavy conversion equipment (with associated higher directivity
- and greater efficiency of conversion of the explosion energy into the form
- radiated), much lower cost and possible reusability of the equipment.
-
- Conversion of the explosion energy into more tractable
- electrical-energy pulses can be accomplished by magnetohydrodynamic
- generators: devices that convert a plasma's kinetic energy directly into
- electricity. (Such devices have been proposed for converting fusion energy
- in a power reactor into electricity.) The pulses of electrical energy could
- then drive devices for conversion of the electricity into electromagnetic
- radiation (with or without an attendant self-destruction of the device)
- that could be tightly focused toward targets in space. In most cases the
- low efficiency of such energy conversion can be more than compensated for
- by a high degree of focusing in the direction of a target.
-
- An extremer possibility is the use of a relatively small nuclear
- explosion deep underground to accelerate very large projectiles through the
- equivalent of a cannon barrel. These so-called hypervelocity projectiles
- would reach velocities close to earth-escape velocity (about 10 kilometers
- per second). Appropriately shaped, compact projectiles can thus penetrate
- the atmosphere in a way that is somewhat analogous to penetration of the
- atmosphere by large meteorites. Such proposals were studied as long ago as
- the late 1950's as a method for placing massive loads of materials in space
- at relatively low cost.
-
- The kinetic energy of, say, 10 tons of material moving at 10
- kilometers per second is the equivalent of about 100 tons of TNT. This
- suggests that reasonably efficient use of a nuclear explosion with a yield
- in the vicinity of one kiloton could provide more than enough propulsive
- energy. If the "cannon barrel' were a few hundred meters long, the average
- acceleration of the projectile would be on the order of 10,000 times the
- acceleration of the earth's gravity, which is not beyond the strain-bearing
- capacity of a compact, high-density projectile. Subsequent fragmentation of
- such a projectile into solid chunks or liquid droplets could make it a
- highly effective weapon for destroying satellites or ballisticmissile
- warheads in space.
-
- Another possibility is to design nuclear weapons so that the act of
- detonation itself directly accelerates material on the weapon that
- immediately fragments into small pellets or droplets moving at velocities
- substantially greater than 10 kilometers per second. Such weapons could
- readily focus the hypervelocity fragments into a conical volume, but they
- would have to have a mechanism to control the acceleration process in order
- to avoid vaporizing the fragments. In addition they would probably be
- limited to attacking targets in space or in the upper atmosphere, since at
- low altitudes the ranges of such fragments are much less than the distances
- at which the detonation's air blast causes severe damage.
-
- The damage an object is likely to suffer when it is exposed to the
- gamut of energy types emanating from a nuclear explosion can be roughly
- calculated by estimating the type of energy likely to reach the object, the
-
-
- way in which damage could be done and in many cases the rate of deposition
- of the energy. This aspect of the effects of nuclear explosions is
- extremely complex and often not well understood.
-
- Ranges of total energy fluence that can cause temporary malfunction or
- permanent damage in military or civilian targets vary over nine orders of
- magnitude [see illustration below]. The effects of the longer-wavelength
- radiation (such as that produced by an EMP) at the low end of the
- energy-fluence scale are the subtlest and the most difficult to assess and
- are therefore the most uncertain.
-
- A fluence of .1 joule per square meter is one million times greater
- than an easily detectable one-second radio signal emitted by a 10-kilowatt
- spherically symmetrical radio transmitter 100 kilometers away. Yet
- commercial and military communications and radar transmissions producing
- smaller fluences have been known to cause accidental firings of
- high-explosive detonators and malfunctions in computers and other
- electronic and electrical equipment. These effects would be similar to
- those produced by the EMP from nuclear explosions. Indeed, the effects of
- electromagnetic radiation on military ordnance have prompted efforts to
- protect against it. Some measures include enclosure in conducting shields
- and avoidance of components that can be sensitive to even small pulses of
- current induced by electromagnetic radiation that has leaked in. Yet these
- measures have not always been entirely successful.
-
- Some components of electronic systems, such as transistors, can be
- very sensitive to small currents and other effects resulting from gamma-ray
- and neutron bombardment. These effects can be minimized by shielding or by
- avoidance of highly sensitive components. Yet the general lack of
- protective measures in nonmilitary space systems makes them particularly
- vulnerable to such nuclear radiation.
-
- Gamma rays, neutrons, high-energy X rays or radionuclides impinging on
- targets in space can also cause the target to become charged to a potential
- that is on the order of the maximum energy of ejected charged particles. It
- is possible that the electric field strength near the surface could reach
- values on the order of one million volts per meter, sufficient to induce
- malfunctions or permanent damage in some types of internal electrical
- systems that are not well shielded.
-
- Unlike neutrons or gamma rays, hypervelocity fragments would pit the
- surface of a target. Exceedingly rapid ejection of the material during the
- pit formation drives a strong shock wave into the target. Because of their
- high velocities, which are up to about 100 times faster than a high-speed
- rifle bullet, hypervelocity fragments weighing much less than one gram can
- do considerable damage when they are aimed at targets in space.
-
- Visible light or infrared radiation released as a secondary effect
- from the heating of the atmosphere primarily causes damage by igniting
- combustible materials on the surface of targets. Even if the target surface
- is not combustible, nonuniform heating of the surface can nonetheless cause
- damage from the resulting thermal stresses.
-
- Incident high-energy X-radiation or weapon-debris plasma damages a
- target in space principally by the rapid blowoff of vaporized material from
- the target's surface. If X rays are the agent, the resulting shock can be
- transmitted through the outer layers of the object, causing the inside
- surfaces to shatter, presuming the time necessary to deposit the incident
- energy is short compared with the time required for the shock to reach the
- inner surface. Such a process in called spalling. For incident
- weapon-debris plasma, however, spalling does not generally occur. The
- reason is that it takes too long for the weapon-debris plasma to deposit
- its kinetic energy. In any case, the overall momentum transferred inward
- from the surface blowoff can result in incapacitating damage even if there
- is no interior spalling.
-
- To help make these estimates more accessible, one can consider the
- range within which a particular energy carrier can produce destructive
- effects [see illustration on this page]. Potentially huge damage ranges
- (or, equivalently, large fluences at a given distance) can be readily
- achieved by emitting energy within a narrow angle. Microwaves that have
- wavelengths between three centimeters and one meter are particularly suited
- for such directional enhancement because the atmosphere is essentially
- transparent over this range, making it possible to use the radiation for
- ground-to-space, space-to-ground and space-to-space applications. Also, the
- ranges of the micro-wave-energy fluence needed to cause damage to many
- types of military and civilian targets are the lowest of all forms of
- electromagnetic radiation.
-
- The military potential of directed microwave beams is therefore
- awesome. Suppose, for example, it should become possible to convert 5
- percent of the energy released by a one-kiloton explosion into
- three-centimeter radiation that is emitted by a 50-meter-diameter antenna
- or an equivalent microwave laser. The explosion of such a device in a
- 30,000-kilometer geosynchronous orbit would deposit about 800 joules per
- square meter over an area of 250 square kilometers on the earth's surface
- (larger than the area of Washington, D.C.). This estimated energy fluence
- is greater than the level known to cause severe damage to many types of
- electrical equipment-- computers, antennas, relays and power lines. Of
- course, at much shorter distances the energy fluence would be much larger,
- about five million joules per square meter at a distance of 400 kilometers.
-
- The development and deployment of such a microwave weapon would
- greatly complicate both offensive and defensive military tactics and
- strategy. It could, for example, cause temporary malfunctions or permanent
- damage in the complex electronic and electrical equipment that is typically
- found in military systems for surveillance, tracking, communications,
- navigation and other command-and-control functions. Because the atmosphere
- is virtually transparent to microwaves, either the beam-generating device
- or the intended target could be based in space, in the atmosphere or on the
- earth's surface. In any event, the deployment of such weapons is likely to
- undermine confidence in the wartime reliability of strategic and tactical
- forces, including those forces that constitute the ultimate deterrent to
- nuclear war.
-
- How likely is it that these third-generation nuclear weapons will
- actually be developed and deployed? The answer depends largely on the
- character and extent of support provided by both the U.S.S.R. and the U.S.
- to their respective national weapon laboratories. Since developments in the
- military realm of one country invariably elicit emulative responses from
- the other, the likelihood strongly depends on what is perceived to be the
- pace of the adversary's research and development in this area.
-
- One key indicator of the extent of a country's effort is the frequency
- of nuclear testing. If the U.S. continues and the U.S.S.R. resumes
- underground nuclear testing even at levels substantially lower than the
- 150-kiloton limit stipulated in the Threshold Test Ban Treaty, it will
- probably be just a matter of time before these new types of offensive and
- defensive nuclear weapons are developed.
-
- Photo: PATTERN of energy emission distinguishes current nuclear
- warheads from those likely to be developed in the near future. Current
- warheads (top) release their explosive energy in many forms, each of which
- is radiated uniformly outward. Hence the region in which military equipment
- would be destroyed or incapacitated for each of the major energy types
- (color key above) can be roughly represented as spheres. In contrast,
- warheads of future nuclear weapons could be equipped with devices that
- suppress, convert and direct energy, enabling a significant fraction of the
- explosive energy to be transformed into microwaves that are then
- concentrated on targets (bottom).
-
- Photo: ARRAY OF EFFECTS listed in the key at the left could be
- militarily exploited by the next generation of nuclear weapons, which would
- suppress certain effects, heighten others and perhaps channel them in
- certain directions as well. In space (top row) nuclear weapons could
- radiate incoherent X rays in all directions (a) or coherent X rays in a
- particular direction (b). Microwaves can readily penetrate the atmosphere
- and could therefore reach the surface of the earth from space, particularly
- if they were concentrated (c). Gamma rays also travel a certain distance
- through the air and could be directed to targets in the upper atmosphere
- (d). The ionized weapon debris produced by a nuclear explosion above the
- atmosphere but within the earth's magnetic field could produce a powerful
- pulse of long-wavelength electromagnetic radiation as it distorts the field
- (e). A similar effect can be achieved in the atmosphere (middle row): X
- rays can knock electrons loose from air molecules to create a sudden
- current surge through the air, which results in the emission of the
- radio-wave pulse (f). The more familiar neutron-emission (g), air-blast (h)
- and incendiary (i) effects of nuclear weapons could also be enhanced.
- Targets in space could be engaged by microwaves beamed upward (j). The
- energy of subsurface bursts (bottom row) could interact strongly with the
- surrounding medium to produce enhanced ground (k) or water (l) shock waves.
- The amount and distribution of radioactive fallout from nuclear weapons
- could be controlled, depending on the materials chosen to encase the weapon
- as well as on whether the weapon is detonated underground (m) or underwater
- (n). Finally, the blast of a subterranean explosion could conceivably
- propel projectiles through a "cannon barrel' and into space (o).
-
- Photo: FOUR TYPES OF NUCLEAR EXPLOSIVES are depicted schematically;
- all but one rely on fission (the splitting of a nucleus by a neutron into
- two lighter nuclei). A weapon relying solely on fission for its explosive
- energy (a) consists of a core of fissile material (uranium 235 or plutonium
- 239) surrounded by chemical-explosive charges and inert structures that
- focus the charges' blast energy inward, causing the core to implode and
- thereby initiate a runaway fission reaction. The yield of fission
- explosives can be "boosted' (b) by placing deuterium and tritium (isotopes
- of hydrogen) in them. The temperatures produced on detonation of a fission
- explosive cause the hydrogen isotopes to undergo fusion (the joining of
- nuclei), releasing substantial quantities of neutrons, which induce more
- fission reactions. In boosted weapons the fusion reaction does not
- contribute significantly to the total yield of the weapon. Fusion reactions
- can account for most of a nuclear weapon's yield, however, if a substantial
- amount of such a thermonuclear fuel as lithium deuteride is exposed to the
- energy released by fission (c). An outer shell of normal uranium (uranium
- 238) serves to hold the warhead together just a fraction of a microsecond
- longer before it blows apart, enabling the nuclear reactions to produce
- more energy. Also, when it is irradiated with neutrons produced by fusion,
- the U-238 itself undergoes fission. A pure-fusion weapon (d), which
- dispenses with a fission trigger by applying laser, electron or ion beams
- to implode thermonuclear fuel, reportedly eludes weapon designers.
-
- Photo: ATMOSPHERIC PENETRATION of the energy emitted by a nuclear
- burst in space depends on the energy type. Radiation in the microwave,
- infrared and visible ranges of the electromagnetic spectrum could reach the
- ground with relatively little attenuation.
-
- Photo: SHAPED CHEMICAL CHARGES can eject their explosion products
- (primarily blast and weapon debris) in markedly nonspherical patterns. A
- flat disk of chemical explosive, for example, emits its products in a
- characteristic double cone. Setting off a long, thin cylinder of explosive
- produces a cylindrical pattern of emission. Finally, by tamping, or
- restricting, the effects of the explosion with inert, dense material in all
- but one direction, the explosive products can be concentrated in that
- direction. Nuclear explosives could presumably apply such directional
- effects to control the pattern in which their explosive products are
- emitted.
-
- Photo: DESTRUCTIVE EFFECTS of different types of energy are listed in
- this chart as well as the fluence (total energy per unit area) necessary to
- achieve such effects on military equipment. Since relatively small fluences
- of microwave or longer-wavelength radiation are sufficient to cause damage,
- such kinds of radiation may be the energy types emphasized in
- third-generation nuclear weapons.
-
- Photo: MAXIMUM DISTANCE from the detonation of a nuclear weapon at
- which damage can be done to military targets in space depends on the type
- of energy causing the damage and how much of the total explosive energy it
- represents. Two cases are considered: a one-kiloton weapon (black) and a
- one-megaton weapon (color). (A kiloton is the energy equivalent of the
- detonation of 1,000 tons of TNT; a megaton is 1,000 kilotons.) The bars
- indicate the range of damage-radius estimates for plausible
- third-generation weapons, whose energies have been enhanced but not
- directed. The percentage of the total explosive energy funneled into each
- particular energy type is indicated next to each pair of bars. Much greater
- damage radii could be achieved if the weapons focus their energy.
-