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Saturday, May 16, 2015

Reading List: Building the H Bomb

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.

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