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					Hydrogen Bomb / Fusion Weapons
The process of combining nuclei (the protons and neutrons inside an atomic nucleus) together with a release of
kinetic energy is called fusion. This process powers the Sun, it contributes to the world stockpile of weapons of mass
destruction and may one day generate safe, clean electrical power.

This powerful but complex weapon uses the fusion of heavy isotopes of hydrogen, deuterium, and tritium to release
large numbers of neutrons when the fusile (sometimes termed "fusionable") material is compressed by the energy
released by a fission device called a primary. Fusion (or ‘‘thermonuclear’ weapons derive a significant amount of their
total energy from fusion reactions. The intense temperatures and pressures generated by a fission explosion
overcome the strong electrical repulsion that would otherwise keep the positively charged nuclei of the fusion fuel
from reacting.

The first thermonuclear devices used liquid fuel, such as deuterium, which required significant developments in
cryogenics to keep the fuel below its boiling point of –250°C. Later devices used lithium deuteride fuel, in solid form,
which breeds tritium when exposed to neutrons.

It is inconvenient to carry deuterium and tritium as gases in a thermonuclear weapon, and certainly impractical to
carry them as liquefied gases, which requires high pressures and cryogenic temperatures. Instead, one can make a
“dry” device in which 6Li is combined with deuterium to form the compound 6Li D (lithium-6 deuteride). Neutrons from
a fission “primary” device bombard the 6 Li in the compound, liberating tritium, which quickly fuses with the nearby
deuterium.




The particles, being electrically charged and at high temperatures, contribute directly to forming the nuclear fireball.
The neutrons can bombard additional 6Li nuclei or cause the remaining uranium and plutonium in the weapon to
undergo fission. This two-stage thermonuclear weapon has explosive yields far greater than can be achieved with
one point safe designs of pure fission weapons, and thermonuclear fusion stages can be ignited in sequence to
deliver any desired yield. Such bombs, in theory, can be designed with arbitrarily large yields: the Soviet Union once
tested a device with a yield of about 59 megatons.

In a relatively crude sense, 6 Li can be thought of as consisting of an alpha particle ( 4He) and a deuteron ( 2H) bound
together. When bombarded by neutrons, 6 Li disintegrates into a triton ( 3 H) and an alpha:

6 Li + Neutron = 3 H + 3 He + Energy.
This is the key to its importance in nuclear weapons physics. The nuclear fusion reaction which ignites most readily is

2                 H                     +                    3                   H                   =
4 He + n + 17.6 MeV,
or, phrased in other terms, deuterium plus tritium produces 4He plus a neutron plus 17.6 MeV of free energy:

D + T = 4 He + n + 17.6 MeV.
Lithium-7 also contributes to the production of tritium in a thermonuclear secondary, albeit at a lower rate than 6Li. The
fusion reactions derived from tritium produced from 7 Li contributed many unexpected neutrons (and hence far more
energy release than planned) to the final stage of the infamous 1953 Castle/BRAVO atmospheric test, nearly
doubling its expected yield.
History
The ultimate success of the United States thermonuclear program rested on five factors. First, was the discovery of a
method to overcome the fundamental problem that thermonuclear systems lose as much energy as they create.
Second, Los Alamos had to significantly increase the size of its scientific staff. The hydrogen bomb problem required
complex interactions among physicists, chemists, and metallurgists. Third, to start a thermonuclear fire, smaller and
more efficient fission bombs were needed. Fourth, computational ability had to be greatly enhanced. Fifth, the political
decision had to be made to marshal the resources necessary to accomplish the task.

The idea for a hydrogen bomb came from the thermonuclear study of stars conducted in the 1930s by Hans Bethe.
Unlike fission weapons, which derive their energy from splitting atoms of the heavy elements uranium and plutonium,
hydrogen bombs derive their power from fusing atoms of the light element hydrogen. Since fusion can only be
achieved with stellar temperatures, hydrogen bombs were not possible until such a heat source (fission bombs)
became available.

By the end of the 1940s, American scientists began to acknowledge the feasibility of a thermonuclear weapon.
Though the technical challenges were daunting, few doubted they could be overcome. However, an even more
fundamental question arose: even if hydrogen bombs could be built, should they be? A debate ensued, which
included world-renowned scientists, politicians, civil servants, and eventually the president himself.

Pressure to build it seemed to mount with the discovery that Manhattan Project scientist Klaus Fuchs had passed
nuclear secrets-including concepts for a hydrogen bomb-to the Soviets. Fuchs left Los Alamos on June 15, 1946. By
January 1949 suspicion of Fuchs’s involvement in espionage had grown. Fuchs soon confessed to his part in the
theft of atomic secrets.

On March 1, 1950, Fuchs was found guilty of communicating information to the Soviets concerning atomic research.
But the theoretical work of 1950 had shown that every important point of the 1946 thermonuclear program had been
wrong. If the Russians started a thermonuclear program on the basis of the information received from Fuchs, Bethe
argued that it must have led to the same failure. Teller later claimed that radiation-implosion -- the key concept behind
the successful hydrogen bomb -- had also been discussed at the Los Alamos meeting. Bethe disagreed, and the
question remained unresolved.

Indeed, the Russian account of matters gives Fuchs credit for radiation implosion. "In the spring of 1946, another
concept, whose paramount importance became evident afterwards, was suggested during work on the `classical
Super.' Klaus Fuchs, with the participation of John von Neumann, proposed a new triggering device. It included an
additional secondary unit with liquid D± T mixture that would be heated, compressed, and, as a result, ignited by
radiation from the primary nuclear bomb. ... Fuchs's configuration was the first physical scheme using radiation
implosion and a precursor of Teller ± Ulam's configuration proposed later. Fuchs's proposal, remarkable for its wealth
of novel ideas, was well ahead of its time and could not be developed, given the current state of the mathematical
modeling of complex physical processes. On May 28, 1946, Fuchs and von Neumann filed a joint patent application
for the invention of the new design of the triggering system for the `classical Super' using radiation implosion." None
of this is attested by American accounts of these matters.

The "Mike" test of Operation Ivy, 1 November 1952, was the first explosion of a true two-stage thermonuclear device.

Some were convinced that there was another spy still at large in the US weapons program, and that the most likely
candidate was Oppenheimer. But the American atmospheric tests of 1954 provided the scientific information
necessary for the Soviets to deduce and confirm key features about its design, leading them to develop their own
bomb in a short time.

Information about the new powerful explosion conducted by the USA team on March 1, 1954, renewed the drive of
Soviet researchers to invent an efficient design of a high-yield thermonuclear bomb. It became clear to the Soviets
that there was an efficient design technique, which had been invented by the American team. The only configuration
left was a two-stage gadget. A new mechanism for compression of the secondary thermonuclear core by radiation
from the primary nuclear charge had been discovered finally. This happened in March and April 1954.

Design Details
The main unknowns to the public are the design of the casing, and the shape and size of the secondary, relative to
the primary. Whether the hot plastic does the pushing or transmits its heat to a designated ablator which does the
pushing a matter of continuing discussion.
It would seem to be difficult to shape the secondary like a cylinder, and get a compression wave traveling just before
fast neutrons from the sparkplug cause fission - although not impossible. Another problem with the cylindrical shape
is that compressing from the sides is like squeezing a tube of toothpaste. If the compression is not fast enough, the
material will squirt out the ends.

The early secondary were cylindrical, because the original goal was to make the largest possible multi-megaton
explosion with a device whose diameter was more tightly constrained than its length, in order to be dropped from a
bomber.

But when the goal became to fit a warhead in the nosecone of the Polaris missile, length and diameter were of
comparable dimensions. The Polaris warhead, the W47, which was tested in 1958 and deployed in the 1960s,
contained the first spherical secondary, an arrangement that was soon to become the standard design. The
advantage of a spherical secondary is higher compression.

				
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posted:5/13/2012
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