- Ford, Kenneth W.
Building the H Bomb.
Singapore: World Scientific, 2015.
ISBN 978-981-4618-79-3.
-
In the fall of 1948, the author entered the graduate program in physics
at Princeton University, hoping to obtain a Ph.D. and pursue a
career in academia. In his first year, he took a course in
classical mechanics taught by
John
Archibald Wheeler and realised that, despite the dry material
of the course, he was in the presence of an extraordinary
teacher and thinker, and decided he wanted Wheeler as his thesis
advisor. In April of 1950, after Wheeler returned from an extended
visit to Europe, the author approached him to become his advisor,
not knowing in which direction his research would proceed. Wheeler
immediately accepted him as a student, and then said that he (Wheeler)
would be absent for a year or more at Los Alamos to work on the
hydrogen bomb, and that he'd be pleased if Ford could join him on
the project. Ford accepted, in large part because he believed that
working on such a challenge would be “fun”, and that it
would provide a chance for daily interaction with Wheeler and other
senior physicists which would not exist in a regular Ph.D. program.
Well before the Manhattan project built the first fission weapon,
there had been interest in fusion as an alternative source of
nuclear energy. While fission releases energy by splitting heavy
atoms such as uranium and plutonium into lighter atoms, fusion
merges lighter atoms such as hydrogen and its isotopes deuterium
and tritium into heavier nuclei like helium. While nuclear fusion
can be accomplished in a desktop apparatus, doing so requires vastly
more energy input than is released, making it impractical as an
energy source or weapon. Still, compared to enriched uranium or
plutonium, the fuel for a fusion weapon is abundant and inexpensive
and, unlike a fission weapon whose yield is limited by the critical
mass beyond which it would predetonate, in principle a fusion weapon
could have an unlimited yield: the more fuel, the bigger the bang.
Once the Manhattan Project weaponeers became confident they could
build a fission weapon, physicists, most prominent among them
Edward Teller,
realised that the extreme temperatures created by a nuclear
detonation could be sufficient to ignite a fusion reaction in
light nuclei like deuterium and that reaction, once
started, might propagate by its own energy release just like
the chemical fire in a burning log. It seemed plausible—the
temperature of an exploding fission bomb exceeded that of the
centre of the Sun, where nuclear fusion was known to occur. The
big question was whether the fusion burn, once started, would
continue until most of the fuel was consumed or fizzle out as its
energy was radiated outward and the fuel dispersed by the explosion.
Answering this question required detailed computations of a rapidly
evolving system in three dimensions with a time slice measured in
nanoseconds. During the Manhattan Project, a “computer”
was a woman operating a mechanical calculator, and even with
large rooms filled with hundreds of “computers” the
problem was intractably difficult. Unable to directly model the
system, physicists resorted to analytical models which produced
ambiguous results. Edward Teller remained optimistic that the
design, which came to be called the “Classical Super”,
would work, but many others, including
J. Robert Oppenheimer,
Enrico Fermi, and
Stanislaw Ulam,
based upon the calculations that could be done at the time, concluded
it would probably fail. Oppenheimer's opposition to the Super or
hydrogen bomb project has been presented as a moral opposition to
development of such a weapon, but the author's contemporary recollection
is that it was based upon Oppenheimer's belief that the classical
super was unlikely to work, and that effort devoted to it would
be at the expense of improved fission weapons which could be
deployed in the near term.
All of this changed on March 9th, 1951. Edward Teller and Stanislaw Ulam
published a report which presented a new approach to a fusion bomb.
Unlike the classical super, which required the fusion fuel to burn
on its own after being ignited, the new design, now called the
Teller-Ulam
design, compressed a capsule of fusion fuel by the radiation pressure of
a fission detonation (usually, we don't think of radiation as having
pressure, but in the extreme conditions of a nuclear explosion it
far exceeds pressures we encounter with matter), and then ignited it
with a “spark plug” of fission fuel at the centre of
the capsule. Unlike the classical super, the fusion fuel would burn at
thermodynamic equilibrium and, in doing so, liberate abundant
neutrons with such a high energy they would induce fission in
Uranium-238 (which cannot be fissioned by the less energetic neutrons of
a fission explosion), further increasing the yield.
Oppenheimer, who had been opposed to work upon fusion, pronounced the
Teller-Ulam design “technically sweet” and immediately
endorsed its development. The author's interpretation is that once
a design was in hand which appeared likely to work, there was no
reason to believe that the Soviets who had, by that time, exploded
their own fission bomb, would not also discover it and proceed to
develop such a weapon, and hence it was important that the U.S.
give priority to the fusion bomb to get there first. (Unlike the
Soviet fission bomb, which was a copy of the U.S. implosion design
based upon material obtained by espionage, there is no evidence the
Soviet fusion bomb, first tested in 1955, was based upon espionage, but
rather was an independent invention of the radiation implosion concept by
Andrei Sakharov
and
Yakov Zel'dovich.)
With the Teller-Ulam design in hand, the author, working with Wheeler's
group, first in Los Alamos and later at Princeton, was charged with
working out the details: how precisely would the material in the bomb
behave, nanosecond by nanosecond. By this time, calculations could
be done by early computing machinery: first the IBM
Card-Programmed Calculator
and later the
SEAC, which
was, at the time, one of the most advanced electronic computers in
the world. As with computer nerds until the present day, the author spent
many nights babysitting the machine as it crunched the numbers.
On November 1st, 1952, the
Ivy Mike device was
detonated in the Pacific, with a yield of 10.4 megatons of TNT. John
Wheeler witnessed the test from a ship at a safe distance
from the island which was obliterated by the explosion. The test
completely confirmed the author's computations of the behaviour
of the thermonuclear burn and paved the way for deliverable
thermonuclear weapons. (Ivy Mike was a physics experiment, not
a weapon, but once it was known the principle was sound, it was basically
a matter of engineering to design bombs which could be air-dropped.)
With the success, the author concluded his work on the weapons project
and returned to his dissertation, receiving his Ph.D. in 1953.
This is about half a personal memoir and half a description of the
physics of thermonuclear weapons and the process by which the first
weapon was designed. The technical sections are entirely accessible
to readers with only a basic knowledge of physics (I was about to say
“high school physics”, but I don't know how much physics,
if any, contemporary high school graduates know.) There is no
secret information disclosed here. All of the technical information
is available in much greater detail from sources (which the author
cites) such as Carey Sublette's
Nuclear Weapon Archive,
which is derived entirely from unclassified sources. Curiously, the
U.S. Department of Energy (which has, since its inception, produced
not a single erg of energy) demanded that the author
heavily
redact material in the manuscript, all derived from unclassified
sources and dating from work done more than half a century ago. The only
reason I can imagine for this is that a weapon scientist who was
there, by citing information which has been in the public domain for
two decades, implicitly confirms that it's correct. But it's not like
the Soviets/Russians, British, French, Chinese, Israelis, and
Indians haven't figured it out by themselves or that others
suitably motivated can't. The author told them to stuff it, and here
we have his unexpurgated memoir of the origin of the weapon which
shaped the history of the world in which we live.
May 2015 