It separates the primary

It separates the primary from the secondary and performs the same function as the previous reflector. There are about six neutron guns (seen here from Sandia National Laboratories^[9]) each poking through the outer edge of the reflector with one end in each section; Sony VAIO PCG-8152M Battery

all are clamped to the carriage and arranged more or less evenly around the casing's circumference. The neutron guns are tilted so the neutron emitting end of each gun end is pointed towards the central axis of the bomb. Neutrons from each neutron gun pass through and are focussed by the neutron focus lens towards the centre of primary in order to boost the initial fissioning of the plutonium. Sony VAIO PCG-31111M Battery

A "Polystyrene Polarizer/Plasma Source" is also shown (see below).

The first U.S. government document to mention the interstage was only recently released to the public promoting the 2004 initiation of the Reliable Replacement Warhead Program. Sony VAIO PCG-61111M Battery

A graphic includes blurbs describing the potential advantage of a RRW on a part by part level, with the interstage blurb saying a new design would replace "toxic, brittle material" and "expensive 'special' material... [which require] unique facilities".^[10] Sony VAIO PCG-51112M Battery

The "toxic, brittle material" is widely assumed to be beryllium, which fits that description and would also moderate the neutron flux from the primary. Some material to absorb and re-radiate the X-rays in a particular manner may also be used.^[11]

The "special material" is thought to be a substance called "FOGBANK",Sony VAIO PCG-51211M Battery

an unclassified codename, though it is often referred to as "THE fogbank" (or "A Fogbank") as if it were a subassembly instead of a material. Its composition is classified, though aerogel has been suggested as a possibility. Manufacture stopped for many years; however, the Life Extension Program required it to start up again – Sony VAIO PCG-51212M Battery

Y-12 currently being the sole producer (the "unique facility" referenced). Manufacture involves the moderately toxic and moderately volatile solvent called acetonitrile, which presents a hazard for workers (causing three evacuations in March 2006 alone).

A simplified summary of the above explanation would be: Sony VAIO PCG-41111M Battery

An implosion assembly type of fission bomb is exploded. This is the primary stage. If a small amount of deuterium/tritium gas is placed inside the primary's core, it will be compressed during the explosion and a nuclear fusion reaction will occur; Sony VAIO PCG-41111V Battery

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; Sony VAIO PCG-61412V Battery

1. the Little Boy and Fat Man bombs had anefficiency of only 1.4% and 17%, respectively, because they were unboosted.

Energy released in the primary stage is transferred to the secondary (or fusion) stage. The exact mechanism whereby this happens is unknown. This energy compresses the fusion fuel and sparkplug; Sony VAIO PCG-71112M Battery

the compressed sparkplug becomes critical and undergoes a fission chain reaction, further heating the compressed fusion fuel to a high enough temperature to induce fusion, and also supplying neutrons that react with lithium to create tritium for fusion. Generally, increasing the kinetic energy of gas molecules contained in a limited volume will increase both temperature and pressure (see gas laws). Sony VAIO PCG-81111V Battery

The fusion fuel of the secondary stage may be surrounded by depleted uranium or natural uranium, whose U-238 is not fissile and cannot sustain a chain reaction, but which is fissionable when bombarded by the high-energy neutrons released by fusion in the secondary stage. Sony VAIO PCG-81111V Battery

This process provides considerable energy yield (as much as half of the total yield in large devices), but is not considered a tertiary "stage". Tertiary stages are further fusion stages (see below), which have been only rarely used, and then only in the most powerful bombs ever made. Sony VAIO PCG-81212M Battery

Thermonuclear weapons 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. Sony VAIO PCG-81212V Battery

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 in the open press, Sony VAIO PCG-51111M Battery

but is thought to be transmitted through the X-rays which are emitted from the fissioning primary. This energy is then used to compress thesecondary. The crucial detail of how the X-rays create the pressure is the main remaining disputed point in the unclassified press. There are five proposed theories: Sony VAIO VPCS13X9E/B battery

• 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. Sony VAIO VPCS12V9E/B battery

• 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 or "FOGBANK" plastic foam). This was a second idea put forward by Chuck Hansen and later by Howard Morland.

Tamper/Pusher ablation. Sony VAIO VPCS12V9E/B battery

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, Sony VAIO VPCF13M8E/B battery

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 T Pa) for the Ivy Mike design and 1,400 million bar (140 TPa) for the W-80.^[13] Sony VAIO VPCF13Z0E/B battery

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. Sony VAIO VPCM13M1E/L battery

The sequence of firing the weapon (with the foam) would be as follows:

The high explosives surrounding the core of the primary fire, compressing the fissile material into a supercritical state and beginning the fission chain reaction. Sony VAIO VPCM13M1E/W battery

1. The fissioning primary emits X-rays, which "reflect" along the inside of the casing, irradiating the polystyrene foam.

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. Sony VAIO VPCF22M1E battery

Pushed from both sides (from the primary and the spark plug), the lithium deuteride fuel is highly compressed and heated to thermonuclear temperatures. Also, by being bombarded with neutrons, each lithium-6 atom splits into one tritium atom and one alpha particle. Sony VAIO VPCF11M1E/H battery

1. Then begins a fusion reaction between the tritium and the deuterium, releasing even more neutrons, and a huge amount of energy.

The fuel undergoing the fusion reaction emits a large flux of neutrons, which irradiates the U-238 tamper (or the U-238 bomb casing), causing it to undergo a fission reaction, providing about half of the total energy. Sony VAIO VPCF13M0E/B battery

This would complete the fission-fusion-fission sequence. Fusion, unlike fission, is relatively "clean"—it releases energy but no harmful radioactive productsor large amounts of nuclear fallout. The fission reactions though, especially the last fission reaction, release a tremendous amount of fission products and fallout. Sony VAIO VPCYB2M1E battery

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. The neutron bomb is a hydrogen bomb without the final fission stage. Sony VAIO VPCYB3V1E battery

Current technical criticisms of the idea of "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, Sony VAIO VPCY11M1E battery

and also that the known foam materials intrinsically have a very low absorption efficiency of the gamma ray and X-ray radiation from the primary. Most of the energy produced would be absorbed by either the walls of the radiation case and/or the tamper around the secondary. Sony VAIO VPCS12L9E/B battery

Analyzing the effects of that absorbed energy led to the third mechanism: ablation.

The proposed tamper-pusher ablation mechanism is that the primary compression mechanism for the thermonuclear secondary is that the outer layers of the tamper-pusher, Sony VAIO VPCF11S1E/B battery

or heavy metal casing around the thermonuclear fuel, are heated so much by the X-ray flux from the primary that they ablate away, exploding outwards at such high speed that the rest of the tamper recoils inwards at a tremendous velocity, crushing the fusion fuel and the spark plug. Sony VAIO VPCYB3V1E/R Battery

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. Sony VAIO VPCF23P1E Battery

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, Sony VAIO VPCF23N1E Battery

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 Sony VAIO VPCY21S1E/L Battery

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. Sony VAIO VPCY21S1E/G battery

The pressure due to the ablating material is calculated to be 5.3 billion bar (530 T Pa) in the Ivy Mike device and 64 billion bar (6.4 P Pa) in the W-80 device.

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. Sony VAIO VPCF24M1E battery

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. HP Compaq HSTNN-105C Battery

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.^[14] HP Compaq HSTNN-C12C Battery

Richard Rhodes' book Dark Sun stated that a 1-inch-thick (25 mm) layer of plastic foam was fixed to the lead liner of the inside of the Ivy Mike steel casing using copper nails. Rhodes quotes several designers of that bomb explaining that the plastic foam layer inside the outer case is to delay ablation and thus recoil of the outer case: HP Compaq HSTNN-C66C Battery

if the foam were not there, metal would ablate from the inside of the outer case with a large impulse, causing the casing to recoil outwards rapidly. The purpose of the casing is to contain the explosion for as long as possible, allowing as much X-ray ablation of the metallic surface of the secondary stage as possible, HP Compaq HSTNN-C66C-4 Battery

so it compresses the secondary efficiently, maximizing the fusion yield. Plastic foam has a low density, so causes a smaller impulse when it ablates than metal does.^[14]

A number of possible variations to the weapon design have been proposed: HP Compaq HSTNN-C66C-5 Battery

Either the tamper or the casing have been proposed to be made of uranium-235 (highly enriched uranium) in the final fission jacket. The far more expensive U-235 is also fissionable with fast neutrons like the standard U-238, HP Compaq HSTNN-C67C Battery

but its fission-efficiency is higher than natural uranium, which is almost entirely U-238. Using a final fissionable jacket of U-235 would thus be expected to increase the yield of any Teller-Ulam bomb above a U-238 (depleted uranium) or natural uranium jacket design. HP Compaq HSTNN-C67C-4 Battery

• 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. HP Compaq HSTNN-C67C-5 Battery

• 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.