Hydrogen Bomb / Fusion Weapons The process of combining nuclei (the protons and neutrons inside an atomic nucleus) together with a release of kinetic energy is called fusion. This process powers the Sun, it contributes to the world stockpile of weapons of mass destruction and may one day generate safe, clean electrical power. This powerful but complex weapon uses the fusion of heavy isotopes of hydrogen, deuterium, and tritium to release large numbers of neutrons when the fusile (sometimes termed "fusionable") material is compressed by the energy released by a fission device called a primary. Fusion (or ‘‘thermonuclear’ weapons derive a significant amount of their total energy from fusion reactions. The intense temperatures and pressures generated by a fission explosion overcome the strong electrical repulsion that would otherwise keep the positively charged nuclei of the fusion fuel from reacting. The first thermonuclear devices used liquid fuel, such as deuterium, which required significant developments in cryogenics to keep the fuel below its boiling point of –250°C. Later devices used lithium deuteride fuel, in solid form, which breeds tritium when exposed to neutrons. It is inconvenient to carry deuterium and tritium as gases in a thermonuclear weapon, and certainly impractical to carry them as liquefied gases, which requires high pressures and cryogenic temperatures. Instead, one can make a “dry” device in which 6Li is combined with deuterium to form the compound 6Li D (lithium-6 deuteride). Neutrons from a fission “primary” device bombard the 6 Li in the compound, liberating tritium, which quickly fuses with the nearby deuterium. The particles, being electrically charged and at high temperatures, contribute directly to forming the nuclear fireball. The neutrons can bombard additional 6Li nuclei or cause the remaining uranium and plutonium in the weapon to undergo fission. This two-stage thermonuclear weapon has explosive yields far greater than can be achieved with one point safe designs of pure fission weapons, and thermonuclear fusion stages can be ignited in sequence to deliver any desired yield. Such bombs, in theory, can be designed with arbitrarily large yields: the Soviet Union once tested a device with a yield of about 59 megatons. In a relatively crude sense, 6 Li can be thought of as consisting of an alpha particle ( 4He) and a deuteron ( 2H) bound together. When bombarded by neutrons, 6 Li disintegrates into a triton ( 3 H) and an alpha: 6 Li + Neutron = 3 H + 3 He + Energy. This is the key to its importance in nuclear weapons physics. The nuclear fusion reaction which ignites most readily is 2 H + 3 H = 4 He + n + 17.6 MeV, or, phrased in other terms, deuterium plus tritium produces 4He plus a neutron plus 17.6 MeV of free energy: D + T = 4 He + n + 17.6 MeV. Lithium-7 also contributes to the production of tritium in a thermonuclear secondary, albeit at a lower rate than 6Li. The fusion reactions derived from tritium produced from 7 Li contributed many unexpected neutrons (and hence far more energy release than planned) to the final stage of the infamous 1953 Castle/BRAVO atmospheric test, nearly doubling its expected yield. History The ultimate success of the United States thermonuclear program rested on five factors. First, was the discovery of a method to overcome the fundamental problem that thermonuclear systems lose as much energy as they create. Second, Los Alamos had to significantly increase the size of its scientific staff. The hydrogen bomb problem required complex interactions among physicists, chemists, and metallurgists. Third, to start a thermonuclear fire, smaller and more efficient fission bombs were needed. Fourth, computational ability had to be greatly enhanced. Fifth, the political decision had to be made to marshal the resources necessary to accomplish the task. The idea for a hydrogen bomb came from the thermonuclear study of stars conducted in the 1930s by Hans Bethe. Unlike fission weapons, which derive their energy from splitting atoms of the heavy elements uranium and plutonium, hydrogen bombs derive their power from fusing atoms of the light element hydrogen. Since fusion can only be achieved with stellar temperatures, hydrogen bombs were not possible until such a heat source (fission bombs) became available. By the end of the 1940s, American scientists began to acknowledge the feasibility of a thermonuclear weapon. Though the technical challenges were daunting, few doubted they could be overcome. However, an even more fundamental question arose: even if hydrogen bombs could be built, should they be? A debate ensued, which included world-renowned scientists, politicians, civil servants, and eventually the president himself. Pressure to build it seemed to mount with the discovery that Manhattan Project scientist Klaus Fuchs had passed nuclear secrets-including concepts for a hydrogen bomb-to the Soviets. Fuchs left Los Alamos on June 15, 1946. By January 1949 suspicion of Fuchs’s involvement in espionage had grown. Fuchs soon confessed to his part in the theft of atomic secrets. On March 1, 1950, Fuchs was found guilty of communicating information to the Soviets concerning atomic research. But the theoretical work of 1950 had shown that every important point of the 1946 thermonuclear program had been wrong. If the Russians started a thermonuclear program on the basis of the information received from Fuchs, Bethe argued that it must have led to the same failure. Teller later claimed that radiation-implosion -- the key concept behind the successful hydrogen bomb -- had also been discussed at the Los Alamos meeting. Bethe disagreed, and the question remained unresolved. Indeed, the Russian account of matters gives Fuchs credit for radiation implosion. "In the spring of 1946, another concept, whose paramount importance became evident afterwards, was suggested during work on the `classical Super.' Klaus Fuchs, with the participation of John von Neumann, proposed a new triggering device. It included an additional secondary unit with liquid D± T mixture that would be heated, compressed, and, as a result, ignited by radiation from the primary nuclear bomb. ... Fuchs's configuration was the first physical scheme using radiation implosion and a precursor of Teller ± Ulam's configuration proposed later. Fuchs's proposal, remarkable for its wealth of novel ideas, was well ahead of its time and could not be developed, given the current state of the mathematical modeling of complex physical processes. On May 28, 1946, Fuchs and von Neumann filed a joint patent application for the invention of the new design of the triggering system for the `classical Super' using radiation implosion." None of this is attested by American accounts of these matters. The "Mike" test of Operation Ivy, 1 November 1952, was the first explosion of a true two-stage thermonuclear device. Some were convinced that there was another spy still at large in the US weapons program, and that the most likely candidate was Oppenheimer. But the American atmospheric tests of 1954 provided the scientific information necessary for the Soviets to deduce and confirm key features about its design, leading them to develop their own bomb in a short time. Information about the new powerful explosion conducted by the USA team on March 1, 1954, renewed the drive of Soviet researchers to invent an efficient design of a high-yield thermonuclear bomb. It became clear to the Soviets that there was an efficient design technique, which had been invented by the American team. The only configuration left was a two-stage gadget. A new mechanism for compression of the secondary thermonuclear core by radiation from the primary nuclear charge had been discovered finally. This happened in March and April 1954. Design Details The main unknowns to the public are the design of the casing, and the shape and size of the secondary, relative to the primary. Whether the hot plastic does the pushing or transmits its heat to a designated ablator which does the pushing a matter of continuing discussion. It would seem to be difficult to shape the secondary like a cylinder, and get a compression wave traveling just before fast neutrons from the sparkplug cause fission - although not impossible. Another problem with the cylindrical shape is that compressing from the sides is like squeezing a tube of toothpaste. If the compression is not fast enough, the material will squirt out the ends. The early secondary were cylindrical, because the original goal was to make the largest possible multi-megaton explosion with a device whose diameter was more tightly constrained than its length, in order to be dropped from a bomber. But when the goal became to fit a warhead in the nosecone of the Polaris missile, length and diameter were of comparable dimensions. The Polaris warhead, the W47, which was tested in 1958 and deployed in the 1960s, contained the first spherical secondary, an arrangement that was soon to become the standard design. The advantage of a spherical secondary is higher compression.
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