Radiation implosion
Encyclopedia
The term radiation implosion describes the process behind a class of devices which use high levels of electromagnetic radiation
to compress a target. The major use for this technology is in fusion bombs and inertial confinement fusion
research.
and John von Neumann
in the United States, as part of their work on the original "Classical Super" hydrogen bomb design. Their work resulted in a secret patent filed in 1946, and later given to the USSR by Fuchs as part of his nuclear espionage
. However, their scheme was not the same as used in the final hydrogen bomb design, and neither the American nor the Soviet programs were able to make use of it directly in developing the hydrogen bomb (its value would only become apparent after the fact). A modified version of the Fuchs-von Neumann scheme was incorporated into the "George" shot of Operation Greenhouse
.
In 1951, Stanislaw Ulam had the idea to use hydrodynamic shock of a fission weapon to compress more fissionable material to incredible densities in order to make megaton-range, two-stage fission bombs. He then realized that this approach might be useful for starting a thermonuclear reaction. He presented the idea to Edward Teller
, who realized that radiation compression would be both faster and more efficient than mechanical shock. This combination of ideas, along with a fission "sparkplug" embedded inside of the fusion fuel, became what is known as the Teller-Ulam design
for the hydrogen bomb.
at a temperature of 50,000,000 kelvin
s (a little more than three times the temperature of the Sun
's core). The amplitude can be modeled as a trapezoidal pulse with a one microsecond rise time, one microsecond plateau, and one microsecond fall time. For a 30 kiloton fission bomb, the total x-ray output would be 100 terajoules.
, and its outer layers are a material which is opaque to x-rays, such as lead
or uranium-238
.
In order to get the x-rays from the surface of the primary, the fission bomb, to the surface of the secondary, a system of "x-ray reflectors" is used.
The reflector is typically a cylinder made of a material such as uranium. The primary is located at one end of the cylinder and the secondary is located at the other end. The interior of the cylinder is commonly filled with a foam which is mostly transparent to x-rays, such as polystyrene
.
The term reflector is misleading, since it gives the reader an idea that the device works like a mirror
. Some of the x-rays are diffused or scattered, but the majority of the energy transport happens by a two-step process: the x-ray reflector is heated to a high temperature by the flux from the primary, and then it emits x-rays which travel to the secondary. Various classified methods are used to improve the performance of the reflection process.
Some chinese documents showed that chinese scientists used a different method to make radiation implosion. According to these documents, X-ray lens but not reflector should be used to transfer the energy from primary to secondary during the making of the first chinese H-Bomb. （most of these chinese documents are in chinese language, for example, www.math.mun.ca/~xzhao/chinesearticles/ZhangSC2011paper.doc）
, and calculations show that this pressure is very large. In fact, what happens is that the outer layers of the secondary become so hot that they vaporize and fly off the surface at high speeds. The recoil from this surface layer ejection produces pressures which are an order of magnitude stronger than the simple radiation pressure. The so-called radiation implosion is therefore really a radiation-powered ablation
-drive implosion.
s to ignite small amounts of fusion material. This process is known as inertial confinement fusion
(ICF). As part of that research, much information on radiation implosion technology has been declassified.
When using optical lasers, there is a distinction made between "direct drive" and "indirect drive" systems. In a direct drive system, the laser beam(s) are directed onto the target, and the rise time of the laser system determines what kind of compression profile will be achieved.
In an indirect drive system, the target is surrounded by a shell (called a Hohlraum
) of some intermediate-Z material, such as selenium
. The laser heats this shell to a temperature such that it emits x-rays, and these x-rays are then transported onto the fusion target. Indirect drive has various advantages, including better control over the spectrum of the radiation, smaller system size (the secondary radiation typically has a wavelength 100 times smaller than the driver laser), and more precise control over the compression profile.
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
to compress a target. The major use for this technology is in fusion bombs and inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
research.
History
Radiation implosion was first developed by Klaus FuchsKlaus Fuchs
Klaus Emil Julius Fuchs was a German theoretical physicist and atomic spy who in 1950 was convicted of supplying information from the American, British and Canadian atomic bomb research to the USSR during and shortly after World War II...
and John von Neumann
John von Neumann
John von Neumann was a Hungarian-American mathematician and polymath who made major contributions to a vast number of fields, including set theory, functional analysis, quantum mechanics, ergodic theory, geometry, fluid dynamics, economics and game theory, computer science, numerical analysis,...
in the United States, as part of their work on the original "Classical Super" hydrogen bomb design. Their work resulted in a secret patent filed in 1946, and later given to the USSR by Fuchs as part of his nuclear espionage
Nuclear espionage
Nuclear espionage is the purposeful giving of state secrets regarding nuclear weapons to other states without authorization . During the history of nuclear weapons there have been many cases of known nuclear espionage, and also many cases of suspected or alleged espionage...
. However, their scheme was not the same as used in the final hydrogen bomb design, and neither the American nor the Soviet programs were able to make use of it directly in developing the hydrogen bomb (its value would only become apparent after the fact). A modified version of the Fuchs-von Neumann scheme was incorporated into the "George" shot of Operation Greenhouse
Operation Greenhouse
Operation Greenhouse was the fifth American nuclear test series, the second conducted in 1951 and the first to test principles that would lead to developing thermonuclear weapons . Conducted at the new Pacific Proving Ground, all of the devices were mounted in large steel towers, to simulate air...
.
In 1951, Stanislaw Ulam had the idea to use hydrodynamic shock of a fission weapon to compress more fissionable material to incredible densities in order to make megaton-range, two-stage fission bombs. He then realized that this approach might be useful for starting a thermonuclear reaction. He presented the idea to Edward Teller
Edward Teller
Edward Teller was a Hungarian-American theoretical physicist, known colloquially as "the father of the hydrogen bomb," even though he did not care for the title. Teller made numerous contributions to nuclear and molecular physics, spectroscopy , and surface physics...
, who realized that radiation compression would be both faster and more efficient than mechanical shock. This combination of ideas, along with a fission "sparkplug" embedded inside of the fusion fuel, became what is known as the Teller-Ulam design
Teller-Ulam design
The Teller–Ulam design is the nuclear weapon design concept used in most of the world's nuclear weapons. It is colloquially referred to as "the secret of the hydrogen bomb" because it employs hydrogen fusion, though in most applications the bulk of its destructive energy comes from uranium fission,...
for the hydrogen bomb.
Fission bomb radiation source
Most of the energy released by a fission bomb is in the form of x-rays. The spectrum is approximately that of a black bodyBlack body
A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum...
at a temperature of 50,000,000 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s (a little more than three times the temperature of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
's core). The amplitude can be modeled as a trapezoidal pulse with a one microsecond rise time, one microsecond plateau, and one microsecond fall time. For a 30 kiloton fission bomb, the total x-ray output would be 100 terajoules.
Radiation transport
In a Teller-Ulam bomb, the object to be imploded is called the "secondary". It contains fusion material, such as lithium deuterideLithium hydride
Lithium hydride is the inorganic compound with the formula LiH. It is a colorless solid, although commercial samples are gray. Characteristic of a salt-like, or ionic, hydride, it has a high melting point and is not soluble in any solvent with which it does not react...
, and its outer layers are a material which is opaque to x-rays, such as lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
or uranium-238
Uranium-238
Uranium-238 is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239...
.
In order to get the x-rays from the surface of the primary, the fission bomb, to the surface of the secondary, a system of "x-ray reflectors" is used.
The reflector is typically a cylinder made of a material such as uranium. The primary is located at one end of the cylinder and the secondary is located at the other end. The interior of the cylinder is commonly filled with a foam which is mostly transparent to x-rays, such as polystyrene
Polystyrene
Polystyrene ) also known as Thermocole, abbreviated following ISO Standard PS, is an aromatic polymer made from the monomer styrene, a liquid hydrocarbon that is manufactured from petroleum by the chemical industry...
.
The term reflector is misleading, since it gives the reader an idea that the device works like a mirror
Mirror
A mirror is an object that reflects light or sound in a way that preserves much of its original quality prior to its contact with the mirror. Some mirrors also filter out some wavelengths, while preserving other wavelengths in the reflection...
. Some of the x-rays are diffused or scattered, but the majority of the energy transport happens by a two-step process: the x-ray reflector is heated to a high temperature by the flux from the primary, and then it emits x-rays which travel to the secondary. Various classified methods are used to improve the performance of the reflection process.
Some chinese documents showed that chinese scientists used a different method to make radiation implosion. According to these documents, X-ray lens but not reflector should be used to transfer the energy from primary to secondary during the making of the first chinese H-Bomb. （most of these chinese documents are in chinese language, for example, www.math.mun.ca/~xzhao/chinesearticles/ZhangSC2011paper.doc）
The implosion process
The term "radiation implosion" suggests that the secondary is crushed by radiation pressureRadiation pressure
Radiation pressure is the pressure exerted upon any surface exposed to electromagnetic radiation. If absorbed, the pressure is the power flux density divided by the speed of light...
, and calculations show that this pressure is very large. In fact, what happens is that the outer layers of the secondary become so hot that they vaporize and fly off the surface at high speeds. The recoil from this surface layer ejection produces pressures which are an order of magnitude stronger than the simple radiation pressure. The so-called radiation implosion is therefore really a radiation-powered ablation
Ablation
Ablation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
-drive implosion.
Laser radiation implosions
There has been much interest in the use of large laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
s to ignite small amounts of fusion material. This process is known as inertial confinement fusion
Inertial confinement fusion
Inertial confinement fusion is a process where nuclear fusion reactions are initiated by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium....
(ICF). As part of that research, much information on radiation implosion technology has been declassified.
When using optical lasers, there is a distinction made between "direct drive" and "indirect drive" systems. In a direct drive system, the laser beam(s) are directed onto the target, and the rise time of the laser system determines what kind of compression profile will be achieved.
In an indirect drive system, the target is surrounded by a shell (called a Hohlraum
Hohlraum
In radiation thermodynamics, a hohlraum is a cavity whose walls are in radiative equilibrium with the radiant energy within the cavity. This idealized cavity can be approximated in practice by making a small perforation in the wall of a hollow container of any opaque material...
) of some intermediate-Z material, such as selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...
. The laser heats this shell to a temperature such that it emits x-rays, and these x-rays are then transported onto the fusion target. Indirect drive has various advantages, including better control over the spectrum of the radiation, smaller system size (the secondary radiation typically has a wavelength 100 times smaller than the driver laser), and more precise control over the compression profile.