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. 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
- 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
- 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. 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.
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.