History of the Teller–Ulam design

The design is named for scientists Edward Teller and Stanisław Ulam, who originally devised the concept in January 1951 for the United States nuclear weapons program, though their individual roles have been subsequently debated. The US Greenhouse George test in May 1951, the world's first artificial thermonuclear fusion, validated the radiation implosion principle. The US first tested the "true" Teller-Ulam design with the very high-yield Ivy Mike test in 1952.

The design was independently devised and then tested by teams of nuclear weapons scientists working for at least four more governments: the Soviet Union in 1955 (RDS-37), the United Kingdom in 1957 (Operation Grapple), China in 1966 (629), and France in 1968 (Canopus). There is not enough public information to determine whether India, Israel, or North Korea possess multi-stage weapons. Pakistan is not considered to have developed them. The Teller-Ulam design is the basis for all nuclear weapons tests above one megaton yield.

History

Teller's "Super"

The idea of using the energy from a fission device to begin a fusion reaction was first proposed by the Italian physicist Enrico Fermi to his colleague Edward Teller in late 1941 during what would soon become the Manhattan Project, the World War II effort by the United States and United Kingdom to develop the first nuclear weapons. Teller soon was a participant at Robert Oppenheimer's 1942 conference on the development of a fission bomb held at the University of California, Berkeley, where he guided discussion towards the idea of creating his "Super" bomb, which would hypothetically be many times more powerful than the yet-undeveloped fission weapon. Teller assumed creating the fission bomb would be nothing more than an engineering problem, and that the "Super" provided a much more interesting theoretical challenge.

For the remainder of the war the effort was focused on first developing fission weapons. Nevertheless, Teller continued to pursue the "Super", to the point of neglecting work assigned to him for the fission weapon at the secret Los Alamos lab where he worked. (Much of the work Teller declined to do was given instead to Klaus Fuchs, who was later discovered to be a spy for the Soviet Union.) Teller was given some resources with which to study the "Super", and contacted his friend Maria Göppert-Mayer to help with laborious calculations relating to opacity. The "Super", however, proved elusive, and the calculations were difficult to perform. The properties of fission and fusion reactions could be studied by cyclotrons. The propagation of a fission detonation could be studied with the help of reactor and critical assembly experiments. But thermonuclear fusion and the extreme case of plasma ignition, were only considered achievable by a nuclear test (ultimately Greenhouse George), for which the justification was lacking. In the meantime, research proceeded via slow manual calculations and the early computer ENIAC.

Even though they had witnessed the Trinity test, after the atomic bombings of Japan scientists at Los Alamos were surprised by how devastating the effects of the weapon had been. Many of the scientists rebelled against the notion of creating a weapon thousands of times more powerful than the first atomic bombs. For the scientists the question was in part technical—the weapon design was still quite uncertain and unworkable—and in part moral: such a weapon, they argued, could only be used against large civilian populations, and could thus only be used as a weapon of genocide. Many scientists, such as Teller's colleague Hans Bethe (who had discovered stellar nucleosynthesis, the nuclear fusion that takes place in stars), urged that the United States should not develop such weapons and set an example towards the Soviet Union. Promoters of the weapon, including Teller and Berkeley physicists Ernest Lawrence and Luis Alvarez, argued that such a development was inevitable, and to deny such protection to the people of the United States—especially when the Soviet Union was likely to create such a weapon itself—was itself an immoral and unwise act. Still others, such as Oppenheimer, simply thought that the existing stockpile of fissile material was better spent in attempting to develop a large arsenal of tactical atomic weapons rather than potentially squandered on the development of a few massive "Supers".

In any case, work slowed greatly at Los Alamos, as some 5,500 of the 7,100 scientists and related staff who had been there at the conclusion of the war left to go back to their previous positions at universities and laboratories. A conference was held at Los Alamos in 1946 to examine the feasibility of building a Super; it concluded that it was feasible, but there were a number of dissenters to that conclusion.

When the Soviet Union exploded their own atomic bomb (dubbed "Joe 1" by the US) in August 1949, it caught Western analysts off guard, and over the next several months there was an intense debate within the US government, military, and scientific communities on whether to proceed with the far-more-powerful Super. On January 31, 1950, US President Harry S. Truman ordered a program to develop a hydrogen bomb.

Many scientists returned to Los Alamos to work on the "Super" program, but the initial attempts still seemed highly unworkable. In the "classical Super," it was thought that the heat alone from the fission bomb would be used to ignite the fusion material, but that proved to be impossible. For a while, many scientists thought (and hoped) that the weapon itself would be impossible to construct.

Ulam's and Teller's contributions

The exact history of the Teller–Ulam breakthrough is not completely known, partly because of numerous conflicting personal accounts and also by the continued classification of documents that would reveal which was closer to the truth. Previous models of the "Super" had apparently placed the fusion fuel either surrounding the fission "trigger" (in a spherical formation) or at the heart of it (similar to a "boosted" weapon) in the hopes that the closer the fuel was to the fission explosion, the higher the chance it would ignite the fusion fuel by the sheer force of the heat generated.

In 1951, after many years of fruitless labor on the "Super", a breakthrough idea from the Polish émigré mathematician Stanislaw Ulam was seized upon by Teller and developed into the first workable design for a megaton-range hydrogen bomb. This concept, now called "staged implosion" was first proposed in a classified scientific paper, On Heterocatalytic Detonations I. Hydrodynamic Lenses and Radiation Mirrors by Teller and Ulam on March 9, 1951. The exact amount of contribution provided respectively from Ulam and Teller to what became known as the "Teller–Ulam design" is not definitively known in the public domain—the degree of credit assigned to Teller by his contemporaries is almost exactly commensurate with how well they thought of Teller in general. In an interview with Scientific American from 1999, Teller told the reporter:

I contributed; Ulam did not. I'm sorry I had to answer it in this abrupt way. Ulam was rightly dissatisfied with an old approach. He came to me with a part of an idea which I already had worked out and difficulty getting people to listen to. He was willing to sign a paper. When it then came to defending that paper and really putting work into it, he refused. He said, "I don't believe in it."

The issue is controversial. Bethe in his “Memorandum on the History of the Thermonuclear Program” (1952) cited Teller as the discoverer of an “entirely new approach to thermonuclear reactions”, which “was a matter of inspiration” and was “therefore, unpredictable” and “largely accidental.” At the Oppenheimer hearing, in 1954, Bethe spoke of Teller's “stroke of genius” in the invention of the H-bomb. And finally in 1997 Bethe stated that “the crucial invention was made in 1951, by Teller.”

Other scientists (antagonistic to Teller, such as J. Carson Mark) have claimed that Teller would have never gotten any closer without the idea of Ulam. The nuclear weapons designer Ted Taylor was clear about assigning credit for the basic staging and compression ideas to Ulam, while giving Teller the credit for recognizing the critical role of radiation as opposed to hydrodynamic pressure.

Priscilla Johnson McMillan in her book The Ruin of J. Robert Oppenheimer: And the Birth of the Modern Arms Race, writes that Teller sought to "conceal the role" of Ulam, and that only "radiation implosion" was Teller's idea. Teller went as far as refusing to sign the patent application because it would need Ulam's signature. Thomas Powers writes that "of course the bomb designers all knew the truth, and many considered Teller the lowest, most contemptible kind of offender in the world of science, a stealer of credit".

Teller became known in the press as the "father of the hydrogen bomb", a title which he did not seek to discourage. Many of Teller's colleagues were irritated that he seemed to enjoy taking full credit for something he had only a part in, and in response, with encouragement from Enrico Fermi, Teller authored an article titled "The Work of Many People", which appeared in Science magazine in February 1955, emphasizing that he was not alone in the weapon's development (he would later write in his memoirs that he had told a "white lie" in the 1955 article, and would imply that he should receive full credit for the weapon's invention). Hans Bethe, who also participated in the hydrogen bomb project, once said, "For the sake of history, I think it is more precise to say that Ulam is the father, because he provided the seed, and Teller is the mother, because he remained with the child. As for me, I guess I am the midwife."

The Teller–Ulam breakthrough—the details of which are still classified—was apparently the separation of the fission and fusion components of the weapons, and to use the radiation produced by the fission bomb to first compress the fusion fuel before igniting it. Some sources have suggested that Ulam initially proposed compressing the secondary through the shock waves generated by the primary and that it was Teller who then realized that the radiation from the primary would be able to accomplish the task (hence "radiation implosion"). However, compression alone would not have been enough and the other crucial idea, staging the bomb by separating the primary and secondary, seems to have been exclusively contributed by Ulam. The elegance of the design impressed many scientists, to the point that some who previously wondered if it were feasible suddenly believed it was inevitable and that it would be created by both the US and the Soviet Union. Even Oppenheimer, who was originally opposed to the project, called the idea "technically sweet." The "George" shot of Operation Greenhouse in 1951 tested the basic concept for the first time on a very small scale (and the next shot in the series, "Item," was the first boosted fission weapon), raising expectations to a near certainty that the concept would work.

On November 1, 1952, the Teller–Ulam configuration was tested in the "Ivy Mike" shot at an island in the Enewetak atoll, with a yield of 10.4 megatons of TNT (44 PJ) (over 450 times more powerful than the bomb dropped on Nagasaki during World War II). The device, dubbed the Sausage, used an extra-large fission bomb as a "trigger" and liquid deuterium, kept in its liquid state by 20 short tons (18 tonnes) of cryogenic equipment, as its fusion fuel, and it had a mass of around 80 short tons (73 tonnes) altogether. An initial press blackout was attempted, but it was soon announced that the US had detonated a megaton-range hydrogen bomb.

The elaborate refrigeration plant necessary to keep its fusion fuel in a liquid state meant that the "Ivy Mike" device was too heavy and too complex to be of practical use. The first deployable Teller–Ulam weapon in the US was developed in 1954, when the liquid deuterium fuel of the "Ivy Mike" device was replaced with a dry fuel of lithium deuteride and tested in the "Castle Bravo" shot (the device was codenamed the Shrimp). The solid lithium deuteride performed much better than expected, and the "Castle Bravo" device that was detonated in 1954 had a yield two-and-a-half times greater than had been expected (at 15 Mt (63 PJ), it was also the most powerful bomb ever detonated by the United States). Because much of the yield came from the final fission stage of its ²³⁸
U tamper, it generated severe nuclear fallout, which caused one of the worst nuclear accidents in US history after unforeseen weather patterns blew it over populated areas of the atoll and Japanese fishermen on board the Daigo Fukuryu Maru.

After an initial period focused on making multi-megaton hydrogen bombs, efforts in the United States shifted towards developing miniaturized Teller–Ulam weapons which could outfit intercontinental ballistic missiles and submarine launched ballistic missiles. The last major design breakthrough in this respect was accomplished by the mid-1970s, when versions of the Teller–Ulam design were created which could fit on the end of a small MIRVed missile.

Soviet research

In the Soviet Union, the scientists working on their own hydrogen bomb project also ran into difficulties in developing a megaton-range fusion weapon. Because Klaus Fuchs had only been at Los Alamos at a very early stage of the hydrogen bomb design (before the Teller–Ulam configuration had been completed), none of his espionage information was of much use, and the Soviet physicists working on the project had to develop their weapon independently.

The first Soviet fusion design, developed by Andrei Sakharov and Vitaly Ginzburg in 1949 (before the Soviet Union had a working fission bomb), was dubbed the Sloika, after a Russian layered puff pastry, and was not of the Teller–Ulam configuration, but rather used alternating layers of fissile material and lithium deuteride fusion fuel spiked with tritium (this was later dubbed Sakharov's "First Idea"). Though nuclear fusion was technically achieved, it did not have the scaling property of a staged weapon, and their first hydrogen bomb test, Joe 4, is considered a hybrid fission/fusion device more similar to a large boosted fission weapon than a Teller–Ulam weapon (though using an order of magnitude more fusion fuel than a boosted weapon). Detonated in 1953 with a yield equivalent to 400 kt (1,700 TJ) (only 15%–20% from fusion), the Sloika device did, however, have the advantage of being a weapon which could actually be delivered to a military target, unlike the "Ivy Mike" device, though it was never widely deployed. Teller had proposed a similar design as early as 1946, dubbed the "Alarm Clock" (meant to "wake up" research into the "Super"), though it was calculated to be ultimately not worth the effort and no prototype was ever developed or tested.

Attempts to use a Sloika design to achieve megaton-range results proved unfeasible in the Soviet Union as it had in the calculations done in the US, but its value as a practical weapon since it was 20 times more powerful than their first fission bomb, should not be underestimated. The Soviet physicists calculated that at best the design might yield a single megaton of energy if it was pushed to its limits. After the US tested the "Ivy Mike" device in 1952, proving that a multimegaton bomb could be created, the Soviet Union searched for an additional design and continued to work on improving the Sloika (the "First Idea"). The "Second Idea", as Sakharov referred to it in his memoirs, was a previous proposal by Ginzburg in November 1948 to use lithium deuteride in the bomb, which would, by the bombardment by neutrons, produce tritium. In late 1953, physicist Viktor Davidenko achieved the first breakthrough, that of keeping the primary and secondary parts of the bombs in separate pieces ("staging"). The next breakthrough was discovered and developed by Sakharov and Yakov Zeldovich, that of using the X-rays from the fission bomb to compress the secondary before fusion ("radiation implosion"), in early 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the Soviet Union, was tested in the shot "RDS-37" in November 1955 with a yield of 1.6 Mt (6.7 PJ).

If the Soviet Union had been able to analyze the fallout data from either the "Ivy Mike" or "Castle Bravo" tests, they could have been able to discern that the fission primary was being kept separate from the fusion secondary, a key part of the Teller–Ulam device, and perhaps that the fusion fuel had been subjected to high amounts of compression before detonation. One of the key Soviet bomb designers, Yuli Khariton, later said:

At that time, Soviet research was not organized on a sufficiently high level, and useful results were not obtained, although radiochemical analyses of samples of fallout could have provided some useful information about the materials used to produce the explosion. The relationship between certain short-lived isotopes formed in the course of thermonuclear reactions could have made it possible to judge the degree of compression of the thermonuclear fuel, but knowing the degree of compression would not have allowed Soviet scientists to conclude exactly how the exploded device had been made, and it would not have revealed its design.

Sakharov stated in his memoirs that though he and Davidenko had fallout dust in cardboard boxes several days after the "Mike" test with the hope of analyzing it for information, a chemist at Arzamas-16 (the Soviet weapons laboratory) had mistakenly poured the concentrate down the drain before it could be analyzed. Only in late 1952 did the Soviet Union set up an organized system for monitoring fallout data. Nonetheless, the memoirs also say that the yield from one of the American tests, which became an international incident involving Japan, told Sakharov that the US design was much better than theirs, and he decided that they must have exploded a separate fission bomb and somehow used its energy to compress the lithium deuteride. He then turned his focus to finding a way for an explosion to one side to be used to compress the ball of fusion fuel within 5% of symmetry, which he realised could be achieved by focusing the X-rays.

The Soviet Union demonstrated the power of the "staging" concept in October 1961 when they detonated the massive and unwieldy Tsar Bomba, a 50 Mt (210 PJ) hydrogen bomb which derived almost 97% of its energy from fusion rather than fission—its uranium tamper was replaced with one of lead for the test, in an effort to prevent excessive nuclear fallout. Had it been fired in its "full" form, it would have yielded at around 100 Mt (420 PJ). The weapon was technically deployable (it was tested by dropping it from a specially modified bomber), but militarily impractical, and was developed and tested primarily as a show of Soviet strength. It is the largest nuclear weapon developed and tested by any country.