Enrico Fermi was always turning ideas over in his head. One day in the fall of 1941, walking with Edward Teller and others in New York toward Columbia University’s Pupin Hall (the physics building where the group was working on nuclear fission), Fermi asked Teller if he thought an atomic explosion might be used to produce a thermonuclear reaction—that is, an H bomb.{5} Fermi suggested that if deuterium rather than ordinary hydrogen were used, the chances of making that happen might be improved. Teller tells us in his Memoirs that a week or two later, while the two of them were taking a Sunday-afternoon stroll, he explained to Fermi why it wouldn’t work.{6} Teller had hit on the idea that was in fact destined to dog the classical Super for the next ten years. Too much energy, he reasoned, would be radiated away, escaping the mass of deuterium instead of heating it to a temperature at which thermonuclear burning could be sustained.
The problem bothered Teller, though. A few months later, in the spring of 1942 (not long before Oppenheimer’s Berkeley gathering), Teller, newly arrived in Chicago, sat down with another new arrival there, Emil Konopinski, to see if the two of them could prove mathematically that the process envisioned by Fermi was indeed impossible, as Teller at first supposed, or if there was some flaw in his argument such that a thermonuclear reaction, initiated by a fission bomb, might in fact be possible.{7} They concluded that they could not prove the impossibility of the H bomb, that perhaps with enough tinkering of design, it might be possible. Teller became a convert and never let go of the idea.[52]
Fusion probably more than fission was on Teller’s mind in early July 1942 when he and his wife Mici along with Hans and Rose Bethe boarded the train in Chicago for California. Teller had the opportunity on the train ride to explain his thinking to Bethe and to convince Bethe that the subject of an H bomb deserved more study. As indeed it got. After Oppenheimer’s group reached an upbeat assessment of the prospects for the fission bomb just a few days into the Berkeley meeting, the focus of attention shifted to H bombs, whose prospects were a good deal fuzzier, but which offered wonderful challenges for the physicists’ minds.{8} The word “Super” was coined at that meeting,{8} and remained current although later modified to “classical Super” after the radiation implosion idea was born.[53]
On December 2, 1942, less than half a year after the Berkeley conference and just short of a year after the attack on Pearl Harbor, Fermi’s “pile”—the first nuclear reactor—went critical in Chicago. At that time, plans were already well advanced for far larger reactors in the state of Washington to produce plutonium and for an isotope-separation plant in Tennessee to produce uranium-235. By then, the Manhattan Project had been created with General Leslie Groves at its head,{9}, [54] and the Ranch School in Los Alamos, New Mexico had been selected as the place to design and build a nuclear weapon.{11}
When Project Y got going in Los Alamos in March 1943, Teller and perhaps a few others wanted the lab to pursue both the fission and fusion tracks in a balanced way. As it turned out, fission got a lot of attention and fusion very little during the war years and even afterwards, up to the first Soviet A-bomb explosion in late summer 1949. To use a slang term, thermonuclear weapons were put on the back burner, and they stayed there for more than seven years following the 1942 Berkeley conference. The Los Alamos Lab’s director, Robert Oppenheimer, and the head of its Theory Division, Hans Bethe (holding the T-Division post that Teller had hoped for{12}), realized that the fission bomb posed challenge enough, and that, in any case, no fusion bomb would work without a fissionbomb trigger. Teller was, however, allowed—perhaps even encouraged—to pursue his H-bomb ideas on his own. In his “Many People” article,{13} Teller puts it this way: “In spite of the urgency of the situation, Oppenheimer did not lose sight of the more distant possibilities. He continued to urge me with detailed and helpful advice to keep exploring what lay beyond the immediate aims of the laboratory.” Teller adds that Oppenheimer’s advice was not easy to take, since it would have been easier for him (Teller) to participate in the lab’s central mission. This may be disingenuous, but it’s also quite possible that Teller believed what he was writing when he wrote it. He did make contributions to the fission bomb, but it seems clear that his heart belonged to fusion. He worked at first in T Division, then was transferred to F Division, headed by, and named for, Fermi.{14} F Division was to “consider issues outside the main project.”{15}
In any event, through whatever combination of the lab’s encouragement and its acquiescence, Teller did keep a theoretical H-bomb program alive during World War II. He undoubtedly got Fermi involved in some way. That involvement is clearly shown in Fermi’s 1945 lectures on the Super, to which I alluded near the end of the previous chapter.{16} These lectures provide a beautiful view into Fermi’s characteristic clarity and thoroughness, mixing basic theory, practical application, and numerical examples.
Fermi ruled out an equilibrium Super, as did everyone else for the next five years. Here is what he said: “If thermal equilibrium between particles and radiation were established, it would be impossible to heat deuterium to required temperature. In actual fact there will be no thermal equilibrium and we have to consider rate of energy transfer from electrons to radiation.”[55] In other words, the runaway Super is the only option. Fermi went on to calculate ignition temperatures for deuterium with various admixtures of tritium. He noted that a 50-50 DT mixture would burn if exposed to the temperature of an exploding fission bomb, but that with a much smaller, and more practical, admixture of tritium, the ignition temperature would be out of the range of what could be directly supplied by a fission bomb. Fermi even made a crude drawing of a possible two-stage fission-fusion bomb. These lectures set the whole tone of thermonuclear work until the Teller-Ulam concept of the highly compressed equilibrium Super arrived on the scene in 1951.
The von Neumann-Fuchs invention, proposed not long after Fermi’s lectures (and which I introduced at the end of the previous chapter), involved the use of radiation from a fission bomb to help ignite thermonuclear burning, but the mechanism was indirect, not direct.
They specified a four-stage device, consisting of
• a “detonator” (a fission-bomb trigger)
• a “primer” (a 50-50 D-T mixture, the most easily ignited hydrogen fuel)
• a “booster” (D with a 4-percent admixture of T to facilitate its burning)
• a “main charge” (pure D, with no limit on size, that might burn if suitably ignited with the primer and booster)
52
Teller’s love affair with the H bomb extended into his old age, when he advocated its use for peaceful purposes such as digging canals.
53
The word “Super” was intended to be descriptive. When the term “gadget” was later coined for the fission bomb, or A bomb, it had just the opposite intent,