Explosive Lenses

For an implosion device it is necessary to reshape the explosive detonation shockwave in
order that the outer surface of the main high explosive charge becomes detonated
simultaneously, and that a symmetrical implosion becomes generated. This ensures the
shockwave strikes all parts of the surface of the fissile core at the same time, and as a result
there is uniform compression. This is achieved by means of an "explosive lens" system.


An explosive lens is a special type of shaped-charge. The
idea is to use refraction to shape the detonation
shockwave, just as it is used in optics to shape a light
wave.

Optical lenses use combinations of materials in which
light travels with different speeds. This difference in
speed is known as the refractive index, which bends the
light wave when it crosses the boundary between
materials.


In a nuclear weapon, a set of explosive lenses is applied to change several approximately
spherical diverging detonation waves into a singular spherical converging wave. Then the
converging wave is used to collapse the various shells (tamper, reflector etc) and finally it
compresses the core of fissile material to a supercritical state.

Secondary explosives are used in lens systems. This special design of high explosive lens
system creates a smooth, symmetrical implosion shock wave to compress the fissile
material. This compression occurs very rapidly.

Usually explosive lenses are machined from polymer bonded explosives. Materials are
needed which are able to transmit the detonation shockwaves with different speeds. The
original lens designs employed a hollow cone of explosive with a high detonation velocity
(fast explosive), together with an inner cone of explosive with a low velocity (slow explosive).
Some later lens systems used an inert "wave-shaper" made of high density foam or plastic.

The detonator initiates the high velocity explosive at the apex of the cone. A high velocity
detonation wave then travels along the surface of the cone and initiates the inner low
velocity explosive as it goes by. The detonation wave from the low velocity explosive is
slower. This causes the formation of a concave (or planar) detonation wave. In these
systems, multiple initiation points are often used.

The use of the low and high velocity explosives leads to a spherical converging detonation
wave to compress the core. The original "Gadget" device which was used in the American
Trinity test and "Fat Man" device which was dropped on Nagasaki used Baratol as the slow
explosive and Composition B as the fast explosive, but it is possible to use other
combinations.

To create a spherical implosion, a number of in-facing lenses are arranged on the surface of
a sphere in order that the individual shockwaves generated by the lenses coalesce.

There is great advantage from using a large number of lenses. With more lenses this means
that each lens has a small base area and needs to produce a wave with a smaller curvature.
These reduce the thickness of the lens. In addition, it is possible to achieve a higher degree
of implosion symmetry with more lenses.

The lens detonation points need to be spaced as regularly as possible. A 32-lens implosion
system composed of a series of hexagonal and pentagonal lenses, and which is similar to a
soccer ball, was used for the Gadget device and other early American nuclear weapons.

Reports suggest that designs which have 40, 60, 72, and 92 lenses have also been employed.
More lenses leads to a thinner and lighter explosive lens system and greater implosion
uniformity. But more lenses needs more fabrication work and also needs a more powerful
and complex initiation system.

A simple implosion system can be large and heavy. The 32 point systems which were used in
early American nuclear weapons had an external diameter of 1.4 m and weighed over 2000 kg.
It is possible that current systems are less than 30 cm, and weigh only 20 kg, but do not
follow the same design approach as earlier weapons.

It is preferable to use the combination of the fastest and slowest explosives which are
available. This gives the greatest possible refractive index and wave-shaping effect of the
detonation wave, and permits a wider lens angle. Octogen and baratol are the fastest and
slowest explosives which are generally known. Octogen has a detonation velocity of 9110 m/s
at a pressed density of 1.89 g/cc. Baratol (76% barium nitrate/24% TNT, wax binder) has
a velocity of 4870 m/s at cast density 2.55 g/cc.

Later systems have used Octogen as a fast explosive, often as a polymer bonded formulation
which has a high loading of Octogen. It is possible that Plumbatol, a denser and slightly
slower explosive, has been employed in some later lens system designs. It is known that
Boracitol has been used. This was probably in thermonuclear weapon triggers and perhaps
in other types of weapons also.

There are also other types of lens systems, such as air lenses.