Two special variations

Two special variations exist which will be discussed in a further section: HP Compaq HSTNN-DB05 Battery

the cryogenically cooled liquid deuterium device used for the Ivy Mike test, and the putative design of the W88 nuclear warhead — a small, MIRVed version of the Teller–Ulam configuration with a prolate (egg or watermelon shaped) primary and an elliptical secondary. HP Compaq HSTNN-DB06 Battery

Most bombs do not apparently have tertiary "stages" —that is, third compression stage(s), which are additional fusion stages compressed by a previous fusion stage (the fissioning of the last blanket of uranium, which provides about half the yield in large bombs, does not count as a "stage" in this terminology). HP Compaq HSTNN-DB0E Battery

The U.S. tested three-stage bombs in several explosions (see Operation Redwing) but is only thought to have fielded one such tertiary model, i.e., a bomb in which a fission stage, followed by a fusion stage, finally compresses yet another fusion stage. HP Compaq HSTNN-DB11 Battery

This U.S. design was the heavy but highly efficient (i.e.,nuclear weapon yield per unit bomb weight) 25 Mt B41 nuclear bomb^[15]. The Soviet Union is thought to have used multiple stages (including more than one tertiary fusion stages) in their 50 megaton (100 Mt in intended use) Tsar Bomba HP Compaq HSTNN-DB16 Battery

(however, as with other bombs, the fissionable jacket could be replaced with lead in such a bomb, and in this one, for demonstration, it was). If any hydrogen bombs have been made from configurations other than those based on the Teller–Ulam design, the fact of it is not publicly known. HP Compaq HSTNN-DB28 Battery

(A possible exception to this is the Soviet early Sloika design).

In essence, the Teller–Ulam configuration relies on at least two instances of implosion occurring: first, the conventional (chemical) explosives in the primary would compress the fissile core, HP Compaq HSTNN-DB29 Battery

resulting in a fission explosion many times more powerful than that which chemical explosives could achieve alone (first stage). Second, the radiation from the fissioning of the primary would be used to compress and ignite the secondary fusion stage, resulting in a fusion explosion many times more powerful than the fission explosion alone. HP Compaq HSTNN-DB67 Battery

This chain of compression could then be continued with an arbitrary number of tertiary fision stages. Finally, efficient bombs (but not so-called neutron bombs) end with the fissioning of the final natural uranium tamper, something which could not normally be achieved without the neutron flux provided by the fusion reactions in secondary or tertiary stages. HP Compaq HSTNN-FB05 Battery

Such designs can be scaled up to an arbitrary strength (with apparently as many fusion stages as desired), potentially to the level of a "doomsday device." However, usually such weapons were not more than a dozen megatons, which was generally considered enough to destroy even most hardened practical targets HP Compaq HSTNN-FB18 Battery

(for example, a control facility such as the Cheyenne Mountain Operations Center). Even such large bombs have been replaced by smaller-yield bunker buster type nuclear bombs, see also nuclear bunker buster.

As discussed above, for destruction of cities and non-hardened targets, HP Compaq HSTNN-FB51 Battery

breaking the mass of a single missile payload down into smaller MIRV bombs, in order to spead the energy of the explosions into a "pancake" area, is far more efficient in terms of area-destruction per unit of bomb energy. This also applies to single bombs deliverable by cruise missile or other system, such as a bomber, HP Compaq HSTNN-FB52 Battery

resulting in most operational warheads in the U.S. program having yields of less than 500 kilotons.

The idea of a thermonuclear fusion bomb ignited by a smaller fission bomb was first proposed by Enrico Fermi to his colleague Edward Teller in 1941 at the start of what would become the Manhattan Project. HP Compaq HSTNN-I04C Battery

Teller spent most of the Manhattan Project attempting to figure out how to make the design work, to some degree neglecting his assigned work on the Manhattan Project fission bomb program. His difficult and devil's advocate attitude in discussions led Robert Oppenheimer to sidetrack him and other "problem" physicists into the super program to smooth his way. HP Compaq HSTNN-I12C Battery

Stanislaw Ulam, a coworker of Teller's, made the first key conceptual leaps towards a workable fusion design. Ulam's two innovations which rendered the fusion bomb practical were that compression of the thermonuclear fuel before extreme heating was a practical path towards the conditions needed for fusion, HP Compaq HSTNN-I39C Battery

and the idea of staging or placing a separate thermonuclear component outside a fission primary component, and somehow using the primary to compress the secondary. Teller then realized that the gamma and X-ray radiation produced in the primary could transfer enough energy into the secondary to create a successful implosion and fusion burn, HP Compaq HSTNN-I40C Battery

if the whole assembly was wrapped in a hohlraum or radiation case. Teller and his various proponents and detractors later disputed the degree to which Ulam had contributed to the theories underlying this mechanism. Indeed, shortly before his death, and in a last-ditch effort to discredit Ulam's contributions, HP Compaq HSTNN-I44C Battery

Teller claimed that one of his own "graduate students" had proposed the mechanism.

The "George" shot of Operation Greenhouse in 1951 tested the basic concept for the first time on a very small scale, raising expectations to a near certainty that the concept would work. HP Compaq HSTNN-I44C-A Battery

On November 1, 1952, the Teller–Ulam configuration was tested at full scale in the "Ivy Mike" shot at an island in the Enewetak Atoll, with a yield of 10.4 megatons (over 450 times more powerful than the bomb dropped on Nagasaki during World War II). HP Compaq HSTNN-I44C-B Battery

The device, dubbed the Sausage, used an extra-large fission bomb as a "trigger" and liquid deuterium—kept in its liquid state by 20short tons (18 metric tons) of cryogenic equipment—as its fusion fuel, and weighed around 80 short tons (70 metric tons) altogether. HP Compaq HSTNN-I45C Battery

The liquid deuterium fuel of Ivy Mike was impractical for a deployable weapon, and the next advance was to use a solid lithium deuteride fusion fuel instead. In 1954 this was tested in the "Castle Bravo" shot (the device was code-named the Shrimp), which had a yield of 15 megatons (2.5 times higher than expected) and is the largest U.S. bomb ever tested. HP Compaq HSTNN-I45C-A Battery

Efforts in the United States soon shifted towards developing miniaturized Teller–Ulam weapons which could easily outfit intercontinental ballistic missiles andsubmarine-launched ballistic missiles. By 1960, with the W47 warhead^[16] deployed on Polaris ballistic missile submarines, HP Compaq HSTNN-I45C-B Battery

megaton-class warheads were as small as 18 inches (0.5 m) in diameter and 720 pounds (320 kg) in weight. It was later found in live testing that the Polaris warhead did not work reliably and had to be redesigned. Further innovation in miniaturizing warheads was accomplished by the mid-1970s, HP Compaq HSTNN-I48C-A Battery

when versions of the Teller–Ulam design were created which could fit ten or more warheads on the end of a small MIRVed missile (see the section on the W88 below).^[4]

The first Soviet fusion design, developed by Andrei Sakharov and Vitaly Ginzburg in 1949 (before the Soviets had a working fission bomb), HP Compaq HSTNN-I48C-B Battery

was dubbed the Sloika, after a Russian layer cake, and was not of the Teller–Ulam configuration. It used alternating layers of fissile material and lithium deuteride fusion fuel spiked with tritium (this was later dubbed Sakharov's "First Idea"). HP Compaq HSTNN-I49C Battery

Though nuclear fusion might have been technically achievable, it did not have the scaling property of a "staged" weapon. Thus, such a design could not produce thermonuclear weapons whose explosive yields could be made arbitrarily large (unlike U.S. designs at that time). HP Compaq HSTNN-I50C-B Battery

The fusion layer wrapped around the fission core could only moderately multiply the fission energy (modern Teller–Ulam designs can multiply it 30-fold). Additionally, the whole fusion stage had to be imploded by conventional explosives, HP Compaq HSTNN-I54C Battery

along with the fission core, multiplying the bulk of chemical explosives needed substantially.

Their first Sloika design test, RDS-6s, was detonated in 1953 with a yield equivalent to 400 kilotons of TNT (15%–20% from fusion). Attempts to use a Sloikadesign to achieve megaton-range results proved unfeasible. HP Compaq HSTNN-I64C-5 Battery

After the U.S. tested the "Ivy Mike" bomb in November 1952, proving that a multimegaton bomb could be created, the Soviets searched for an additional design. The "Second Idea", as Sakharov referred to it in his memoirs, HP Compaq HSTNN-I65C-5 Battery

was a previous proposal by Ginzburg in November 1948 to use lithium deuteride in the bomb, which would, in the course of being bombarded by neutrons, produce tritium and free deuterium.^[17] 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"). HP Compaq HSTNN-IB05 Battery

The next breakthrough was discovered and developed by Sakharov and Yakov Zel'dovich, that of using the X-rays from the fission bomb to compress the secondarybefore fusion ("radiation implosion"), in early 1954. Sakharov's "Third Idea", as the Teller–Ulam design was known in the USSR, was tested in the shot "RDS-37" in November 1955 with a yield of 1.6 megatons. HP Compaq HSTNN-IB08 Battery