Teller-Ulam design

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The Teller–Ulam design is a nuclear weapon design which is used in megaton-range thermonuclear weapons, and is more colloquially referred to as "the secret of the hydrogen bomb". It is named after two of its chief contributors, Hungarian born physicist Edward Teller and Polish born mathematician Stanisław Ulam, who developed the design in 1951. The idea is thought to pertain specifically to the use of a fission bomb "trigger" placed near an amount of fusion fuel, known as "staging", and the use of "radiation implosion" to compress the fusion fuel before igniting it. There are a number of other additions and variations to this idea posited by different sources.

The first device to be based on this principle was detonated by the United States in the "Ivy Mike" nuclear test in 1952. In the Soviet Union, this design was known as Andrei Sakharov's "Third Idea". Similar devices were developed by the United Kingdom, France and China though no specific codenames are known for their designs. The most powerful ever tested was the 50 to 58 megaton Soviet Tsar Bomba tes

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The basics of the Teller–Ulam configuration: a fission bomb uses radiation to compress and heat a separate section of fusion fuel.

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Public body of knowledge concerning nuclear weapon design

Detailed knowledge of actual fission and fusion weapons is classified to some degree in virtually every industrialized nation. In the United States, such "knowledge" can by default be classified as Restricted Data even if it is created by persons who are not government employees or associated with weapons programs, in a legal doctrine known as "born secret" (though the constitutional standing of the doctrine has been at times called into question, see United States v. The Progressive, et al.). Born-secret is rarely invoked for cases of private speculation. The official policy of the United States Department of Energy has been not to acknowledge the leaking of design information, as such acknowledgment would potentially validate the information as accurate. In a small number of prior cases, though (see prior restraint), the U.S. government has attempted to censor weapons information in the public press, with limited success.

Though large quantities of vague data have been officially released, and larger quantities of vague data have been unofficially leaked by ex-bomb designers, most public descriptions of nuclear weapon design details rely to some degree on speculation, reverse engineering from known information, or comparison with similar fields of physics (inertial confinement fusion is the primary example). Such processes have resulted in a body of unclassified knowledge about nuclear bombs which is generally consistent with official unclassified information releases, related physics, and is thought to be internally consistent, though there are some points of interpretation which are still considered open. The state of public knowledge about the Teller–Ulam design has been most reliably shaped from a few specific incidences outlined in a section below.

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Basic principle

The basic principle of the Teller–Ulam configuration is based upon the idea that different parts of a thermonuclear weapon can be chained together in "stages" which allow for the full detonation of each. At a bare minimum, this implies a primary section which consists of a fission bomb (a "trigger"), and a secondary section which consists of fusion fuel. Because of the staged design, it is thought that a tertiary section, again of fusion fuel, could be added as well, based on the same principle of the secondary. The energy released by the primary compresses the secondary through the concept of "radiation implosion", at which point it is heated and undergoes nuclear fusion.

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One possible version of the Teller–Ulam configuration.

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Surrounding the other components is a hohlraum or radiation case, a container which traps the first stage or primary's energy inside temporarily. The outside of this radiation case, which is also normally the outside casing of the bomb, is the only direct visual evidence publicly available of any thermonuclear bomb component's configuration. Numerous photographs of various thermonuclear bomb exteriors have been declassified.

The primary is thought to be a standard implosion method fission bomb, though likely with a core boosted by small amounts of fusion fuel for extra efficiency; the fusion fuel releases excess neutrons when heated and compressed, inducing additional fission. Generally, an entity with the capacity to create a thermonuclear bomb has already mastered the ability to engineer boosted fission. When fired, the plutonium-239 (Pu-239) and/or uranium-235 (U-235) core would be compressed to a smaller sphere by special layers of conventional high explosives arranged around it in a lens pattern, initiating the nuclear chain reaction that powers the conventional "atomic bomb".

The secondary is usually shown as a column of fusion fuel and other components wrapped in many layers. Around the column is first a "pusher-tamper", a heavy layer of unenriched uranium-238 (U-238) or lead which serves to help compress the fusion fuel (and, in the case of uranium, may eventually undergo fission itself). Inside this is the fusion fuel itself, usually a form of lithium deuteride, which is used because it is easier to weaponize than liquified tritium/deuterium gas (compare the success of the cryogenic deuterium-based Ivy Mike experiment to the (over)success of the lithium deuteride-based Castle Bravo experiment). This dry fuel, when bombarded by neutrons, produces tritium, a heavy isotope of hydrogen which can undergo nuclear fusion, along with the deuterium present in the mixture. (See the article on nuclear fusion for a more detailed technical discussion of fusion reactions.) Inside the layer of fuel is the "spark plug", a hollow column of fissile material (plutonium-239 or uranium-235) which, when compressed, can itself undergo nuclear fission (because of the shape, it is not a critical mass without compression). The tertiary, if one is present, would be set below the secondary and probably be made up of the same materials.

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A more simplified explanation of the above would be as follows:

1.   An implosion assembly type of fission bomb is exploded. This is the primary stage. If a small amount of tritium gas is placed near the primary explosion, it will be compressed and a fusion reaction will occur; the released neutrons from this fusion reaction will induce further fission in the plutonium-239 or uranium-235 used in the primary stage. The use of fusion fuel to enhance the efficiency of a fission reaction is called boosting. Without boosting, a large portion of the fissile material will remain unreacted; the Little Boy and Fat Man bombs had an efficiency of only 1.4% and 14%, respectively, because they were unboosted.
2.   Energy released in the primary stage is transferred to the secondary (or fusion) stage. The exact mechanism whereby this happens is unknown (see speculation regarding this below). This energy heats and compresses the fusion fuel, which is necessary to induce fusion; the fusion reaction releases neutrons as a product. Generally, increasing the kinetic energy of gas molecules contained in a limited volume will increase both temperature and pressure (see gas laws).
3.   The fusion fuel of the secondary stage may be surrounded by depleted uranium, which is normally completely stable and atomically unreactive. However, when bombarded by the neutrons released in the secondary stage, the U-238 atoms begin splitting and undergo a fission reaction.
Actual designs of thermonuclear weapons may vary. For example, they may or may not use a boosted primary stage, use different types of fusion fuel, and may surround the fusion fuel with beryllium (or another neutron reflecting material) instead of depleted uranium to prevent further fission from occurring.
The basic idea of the Teller–Ulam configuration is that each "stage" would undergo fission or fusion (or both) and release energy, much of which would be transferred to another stage to trigger it. How exactly the energy is "transported" from the primary to the secondary has been the subject of some disagreement, but is thought to be transmitted through the x-rays which are emitted from the fissioning primary. This energy is then used to compress the secondary. There are five proposed theories:

•   Neutron pressure from the primary explosion. This was allegedly Ulam's first concept and was abandoned as unworkable.
•   Blast wave from the primary explosion. This was allegedly Ulam's second concept and was abandoned as unworkable.
•   Radiation pressure exerted by the x-rays. This was the first idea put forth by Howard Morland in the article in The Progressive.
•   X-rays creating a plasma in the radiation case's filler (a polystyrene plastic foam). This was a second idea put forward by Chuck Hansen and later by Howard Morland.
•   Tamper/Pusher ablation. This is currently believed to be the actual mechanism.

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Radiation pressure

The radiation pressure exerted by the large quantity of x-ray photons inside the closed casing might be enough to compress the secondary. For two thermonuclear bombs for which the general size and primary characteristics are well understood, the Ivy Mike test bomb and the modern W-80 cruise missile warhead variant of the W-61 design, the radiation pressure was calculated to be 73 million bar (atmospheres) (7.3 TPa) for the Ivy Mike design and 1,400 million bar (140 TPa) for the W-80.[6]
Foam plasma pressure
Foam plasma pressure is the concept which Chuck Hansen introduced during the Progressive case, based on research which located declassified documents listing special foams as liner components within the radiation case of thermonuclear weapons.
The sequence of firing the weapon (with the foam) would be as follows:
1.   The high explosives surrounding the core of the primary fire, compressing the fissile material into a supercritical state and beginning the fission chain reaction.
2.   The fissioning primary emits x-rays at the speed of light, which "reflect" along the inside of the casing, irradiating the polystyrene foam (see below for a note on what "reflection" means in this context).
3.   The irradiated foam undergoes a phase transition, becoming a hot plasma, pushing against the tamper of the secondary, compressing it tightly, and beginning the fission reaction in the spark plug.
4.   Pushed from both sides (from the primary and the spark plug), the lithium deuteride fuel is highly compressed and heated to thermonuclear temperatures, and begins a fusion reaction.
5.   The fuel undergoing the fusion reaction emits a large flux of neutrons, which irradiates the uranium-238 tamper (or the uranium-238 bomb casing), begins to itself undergo a fission reaction, providing about half of the total energy.
This would complete the fission-fusion-fission sequence. Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive products or large amounts of nuclear fallout. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout. If the last fission stage is omitted, by replacing the uranium tamper with one made of lead, for example, the overall explosive force is reduced by approximately half but the amount of fallout is relatively low.

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Current technical criticisms of the foam plasma pressure focus on unclassified analysis from similar high energy physics fields which indicate that the pressure produced by such a plasma would only be a small multiplier of the basic photon pressure within the radiation case, and that the foam materials intrinsically have a very low absorption efficiency of the gamma and x-ray radiation from the primary. Most of the energy produced would be absorbed by the walls of the radiation case, and the tamper around the secondary. Analyzing the effects of that absorbed energy led to the third mechanism: ablation.

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The sequence depicted is:

1.   Bomb before detonation. The nested spheres at the top are the fission primary; the cylinders below are the fusion secondary device.
2.   Fission primary's explosives have detonated and collapsed the primary's fissile pit.
3.   The primary's fission reaction has run to completion, and the primary is now at several million degrees and radiating gamma and hard x-rays, heating up the inside of the hohlraum and the shield and secondary's tamper.
4.   The primary's reaction is over and it has expanded. The surface of the pusher for the secondary is now so hot that it is also ablating or expanding away, pushing the rest of the secondary (tamper, fusion fuel, and fissile spark plug) inwards. The spark plug starts to fission. Not depicted: the radiation case is also ablating and expanding outwards (omitted for clarity of diagram).
5.   The secondary's fuel has started the fusion reaction and shortly will burn up, and then blow the remaining components of the bomb apart.
Rough calculations for the basic ablation effect are relatively simple: the energy from the primary is distributed evenly onto all of the surfaces within the outer radiation case, with the components coming to a thermal equilibrium, and the effects of that thermal energy are then analyzed. The energy is mostly deposited within about one x-ray optical thickness of the tamper/pusher outer surface, and the temperature of that layer can then be calculated. The velocity at which the surface then expands outwards is calculated and, from a basic Newtonian momentum balance, the velocity at which the rest of the tamper implodes inwards.
Applying the more detailed form of those calculations to the Ivy Mike device yields vaporized pusher gas expansion velocity of 290 kilometers per second and an implosion velocity of perhaps 400 kilometers per second if 3/4 of the total tamper/pusher mass is ablated off, the most energy efficient proportion. For the W-80 the gas expansion velocity is roughly 410 kilometers per second and the implosion velocity 570 kilometers per second. The pressure due to the ablating material is calculated to be 5.3 billion bar (530 TPa) in the Ivy Mike device and 64 billion bar (6.4 PPa) in the W-80 device.

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Comparing the implosion mechanisms
Comparing the three mechanisms proposed, it can be seen that:

•   Radiation pressure:
o   Ivy Mike: 73 million bar (7.3 TPa)
o   W-80: 1.4 billion bar (140 TPa)
•   Plasma pressure:
o   Ivy Mike: (est.) 350 million bar (35 TPa)
o   W-80: (est.) 7.5 billion bar (750 TPa)
•   Ablation pressure:
o   Ivy Mike: 5.3 billion bar (530 TPa)
o   W-80: 64 billion bar (6400 TPa)
The calculated ablation pressure is one order of magnitude greater than the higher proposed plasma pressures and nearly two orders of magnitude greater than calculated radiation pressure. No mechanism to avoid the absorption of energy into the radiation case wall and the secondary tamper has been suggested, making ablation apparently unavoidable. The other mechanisms appear to be unneeded.
United States Department of Defense official declassification reports indicate that foamed plastic materials are or may be used in radiation case liners, and despite the low direct plasma pressure they may be of use in delaying the ablation until energy has distributed evenly and a sufficient fraction has reached the secondary's tamper/pusher.

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Proposed design variations

A number of possible variations to the weapon design have been proposed:
•   Either the tamper or the casing have been proposed as being made of uranium-238 for the final fission stage.
•   In some descriptions, additional internal structures exist to protect the secondary from receiving excessive neutrons from the primary.
•   The inside of the casing may or may not be specially machined to "reflect" the x-rays. X-ray "reflection" is not like light reflecting off of a mirror, but rather the reflector material is heated by the x-rays, causing the material itself to emit x-rays, which then travel to the secondary.
Two special variations exist which will be discussed in a further section: 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. Most bombs do not apparently have tertiary stages — the U.S. is only thought to have produced one such model, the massive 25 Mt B41 nuclear bomb,[8] and the Soviet Union is thought to have used multiple stages in their 50 megaton Tsar Bomba. 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, with the possible exception of the Sloika design discussed below.
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, resulting in a fission explosion many times more powerful than that which chemical explosives could achieve alone. Second, the radiation from the fissioning of the primary would be used to compress and ignite the secondary, resulting in a fusion explosion many times more powerful than the fission explosion alone. This chain of compression could then be continued with an arbitrary number of secondaries, and would end with the fissioning of the natural uranium tamper, something which could not normally be achieved without the neutron flux provided by the fusion reactions in the secondary. Such a design can be scaled up to an arbitrary strength, potentially to the level of a doomsday device, though usually such weapons are not more than a dozen megatons, which is generally considered enough to destroy even the largest practical targets.