  
    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.

