prevents it from escaping. At normal pressure, this three-body reaction occurs at a marginal rate. But compression increases the rate, Teller says he eventually realized, “and the greater the density, the more the improvement. Therefore, when we make estimates on the basis of the similarity theorem, we really have been too pessimistic. [Photons] may be absorbed in… three-body collisions.” Absorbing such radiation would deposit its energy in the compressed fuel as heat which otherwise would have been lost, improving the prospects for sustained thermonuclear burning.
“I don't understand this three-body crap,” Carson Mark responds impatiently to Teller's explanation. “Compression reduces the volume and changes the energy needed to overcome the loss due to radiation.” According to standard explanations of thermonuclear fusion processes, compression works in thermonuclear fuels in much the same way it works in fission fuels, squeezing nuclei closer together and therefore improving their chances of interacting. With compression, fusion reactions proceed faster and in a smaller volume, which makes that smaller volume correspondingly hotter. Four-fifths of the energy from the fusion reactions goes into heating neutrons, which are electrically neutral and which therefore escape from the burning fuel, carrying away energy. But the remaining 20 percent of the energy released in the fusion reactions goes into heating alpha particles — the helium nuclei formed from the hydrogen nuclei when they fuse — and since alpha particles are positively charged, they interact electrically with the surrounding fuel, colliding and giving up their heat. This process of alpha-particle heating of the fuel increases proportionally as the fuel density increases. With sufficient compression, the alphas can even be stopped from escaping entirely, more than compensating for the loss of heat from radiation and from neutron loss. Whatever Teller's reason for rejecting compression, Mark comments, “eventually he came to understand that it was wrong.”
Paradoxically, Soviet research may have contributed significantly to Teller's education in the virtues of compression. It may also have informed Ulam's thinking. Arnold Kramish had carried an intelligence report to Los Alamos in April 1950 that concerned deuterium compression. An Austrian scientist named Schintlemeister, who had recently been repatriated from internship in the Soviet Union, had reported that Peter Kapitza was experimenting with magnetic compression of deuterium cylinders. The report may have been accurate or it may have been garbled. Though Peter Kapitza had been placed under virtual house arrest after challenging Lavrenti Beria's authority, he had not been barred from physics research. Kapitza was a specialist not only in cryogenics but also in the generation of powerful magnetic fields. It may well have occurred to him in the late 1940s that such magnetic fields might be used to compress cylinders of deuterium to produce thermonuclear fusion in a controllable rather than an explosive form — that is, to make a fusion reactor. Andrei Sakharov, however, has reported pursuing magnetic-confinement fusion beginning in the summer of 1950 and describes a different (perhaps independent) origin for the idea: a letter from a young sailor, Oleg Lavrentiev, that Beria passed on to Igor Tamm. “My first vague thoughts on magnetic rather than electrostatic confinement occurred to me,” Sakharov writes, “as I read Lavrentiev's letter and wrote my reply.” Sakharov's ideas, developed with Tamm, led to the invention of the magnetic-confinement fusion system known as the tokamak.
However the idea originated, Kramish carried it to Los Alamos, where he briefed Teller, Emil Konopinski, John Wheeler, Teller protege Frederic de Hoffmann and possibly Ulam on April 18, 1950. “I traveled to Los Alamos several times after that,” he recalls, “to bring Teller and de Hoffmann up to date on intelligence indicators. Each time, Edward returned to refinements of the Schintlemeister report. Towards the end of January 1951, at Los Alamos, the idea came up again, Edward having made more calculations of temperature and other parameters. And he had remained infatuated with the relationships of compression, temperature and radiation.” In Kramish's judgment, Schintlemeister's information about Kapitza's work prepared Teller to understand Ulam's innovation.
If the three-body reaction is “crap,” why did Mark and others not pursue compression before Ulam's breakthrough? They did not do so, Mark explains, because chemical explosives were the only known materials with which to generate the necessary pressures. Chemical explosives were inadequate on two grounds: they blew material into the thermonuclear fuel that would probably quench the thermonuclear burn entirely, and they could not generate sufficient compression to make a useful difference in the thermonuclear reaction rates. Ulam's “iterative scheme,” says Mark, “changed all that.”
Ulam understood that compression was the issue by the time he proposed his staged system to Teller late in January 1951; he described his idea a few years later as “an implosion of the main body of the device… [in order to] obtain very high compressions of the thermonuclear part, which then might be made to give a considerable energy yield.” Teller was not immediately convinced, Ulam recalled. “I don't think he had any real animosity toward me for the negative results of the work with Everett [that had been] so damaging to his plans, but our relationship seemed definitely strained.” (Teller and Ulam “knew each other quite well,” Mark comments, “and I knew them both. Each was aware that the other was a pretty bright person — sharp, intelligent. Ulam used to make witty, pointed, scornful, shamefully disreputable remarks about Teller when Teller wasn't there. Once in a while his feelings about Teller couldn't have escaped Edward's notice. Edward reciprocated those feelings generously, so each was talking down the other and that went on for years.”)
“For the first half an hour or so during our conversation,” Ulam continues, “[Teller] did not want to accept this new possibility… ” But “after a few hours,” Teller “took up [Ulam's] suggestions, hesitantly at first,” then “enthusiastically.” Ulam mentions two reasons why Teller warmed to the staging idea: “He had seen… the novel elements” and he had “found a parallel version, an alternative to what I had said, perhaps more convenient and generalized.” The “novel elements” were presumably compression as a way to improve the reaction rates and staging as a way to achieve it. Teller's “parallel version” was a brilliant adaptation of his own; he proposed to use the
“From then on,” Ulam concludes, “pessimism gave way to hope. In the following days I saw Edward several times. We discussed the problem for about half an hour each time. I wrote a first sketch of the proposal. Teller made some changes and additions, and we wrote a joint report quickly. It contained the first engineering sketches of the new possibilities of starting thermonuclear explosions. We wrote about two parallel schemes based on these principles.”
Ulam may have collapsed the time frame in memory. The joint report he and Teller wrote was issued on March 9, 1951. Earlier, Kramish recalls — during the last week in January — “Teller came along and said Ulam has an idea but I have a better one.” The “better idea” was radiation implosion:
Teller assembled de Hoffmann, Max Goldstein and myself to discuss Ulam's new idea. Edward said that Ulam was on track but hadn't gotten it right. Edward suggested a geometry and the four of us began to work out the equations — which turned out to be quite complex. At the end of the afternoon, Edward said he had to go to dinner with family and play the piano. In his polite and forceful way, he said, “That's it. Why don't you three work a little while and we will discuss the meaning of the solution in the morning.”
The “little while” was all night… Working with our Frieden and Marchand calculators, we came up with an approximate solution in the morning. Teller, refreshed, became absolutely joyous, while we three were on the point of collapse.
If Kramish's recollection is accurate (and the all-night session eventuated in a paper, “An Estimate of [deleted] Temperatures,” issued on February 4), then Ulam's “following days” comprised most of a month. He wrote von Neumann about the breakthrough on February 23. “Had the following couple of thoughts (ideas) about bombs,” he noted laconically. He outlined the ideas in a few words, then caricatured Teller's response: “Edward is full of enthusiasm about these possibilities; this is perhaps an indication they will not work.” Teller might think the remark cruel; Ulam had earned the right to make it if anyone had.
The joint report of March 9, crediting “work done by” E. Teller and S. Ulam, was titled “On Heterocatalytic
