Laser Mégajoule
Encyclopedia
Laser Mégajoule is an experimental inertial confinement fusion
(ICF) device being built near Bordeaux
, in France
by the French nuclear science directorate, CEA
. Laser Mégajoule plans to deliver about 1.8 MJ of laser energy to its targets, making it about as powerful as its US counterpart, the National Ignition Facility
(NIF). Laser Mégajoule is the largest ICF experiment to be built outside the US, where ICF research has been strongly related to nuclear weapon
s research. Likewise, one of Laser Mégajoule's primary tasks will be refining fusion
calculations for France's own nuclear weapons industry.
Laser Mégajoule uses a series of 240 laser beamlines, grouped into eight groups of 30. Each beamline contains two main glass amplifiers, which are optically pumped
using xenon flashlamps. A "feeder" laser beam is fed via optical fiber
into each of the beamlines where it travels through the two amplifiers. In order to extract more power from the amplifiers, which are not particularly efficient in transmitting power to the beam, the laser pulse is sent through the amplifiers twice by an optical switch
in front of a mirror. At the other end of the beamline a deformable mirror is used to remove imperfections in the wavefront.
The target chamber lies in a large experiment room in the middle of the building, with the beamlines arranged on either side. After being switched into the main room, the beams are first aimed towards the target chamber by mirrors, and then travel through an optical frequency multiplier to boost the frequency into the ultraviolet
. The mirrors are arranged in order to have the pulse impinge in the middle of the chamber from all sides.
Like NIF, LMJ intends to use the "indirect drive" approach, where the laser light is used to heat a high-Z
cylinder made of some heavy metal (often gold
) known as a "hohlraum
". The hohlraum then gives off x-ray
s, which are used to heat a small fuel pellet containing a deuterium
-tritium
(DT) fusion fuel. Although considerable laser energy is lost to heating the hohlraum, x-rays are much more efficient at heating the fuel pellet, making the indirect drive method applicable to nuclear weapons research. The x-rays rapidly heat the outer layer of the pellet so quickly that it explodes outward, causing the remainder of the pellet to be forced inward and causes a shock wave to travel in through the pellet to the middle. When the shock wave converges from all directions and meets in the middle, the density and temperature briefly reach the Lawson criterion
and start fusion reactions. If the rate of reactions is high enough the heat generated by these reactions will be enough to cause surrounding fuel to fuse as well, this process continuing until the majority of the fuel in the pellet is consumed. This process is known as "ignition", and has long been a goal of fusion researchers.
Construction on the Laser Mégajoule started with a single set of eight beamlines known as the Ligne d'Integration Laser (Laser Integration Line
), or LIL, powered by a 450 MJ energy bank. When problems are worked out in the LIL, construction will continue with the construction of four more similar units and the installation of the additional beamlines. LIL was completed in 2002. Currently the entire LMJ system is expected to be completed in 2012.
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) device being built near Bordeaux
Bordeaux
Bordeaux is a port city on the Garonne River in the Gironde department in southwestern France.The Bordeaux-Arcachon-Libourne metropolitan area, has a population of 1,010,000 and constitutes the sixth-largest urban area in France. It is the capital of the Aquitaine region, as well as the prefecture...
, in France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
by the French nuclear science directorate, CEA
Commissariat à l'Énergie Atomique
The Commissariat à l'énergie atomique et aux énergies alternatives or CEA, is a French “public establishment related to industrial and commercial activities” whose mission is to develop all applications of nuclear power, both civilian and military...
. Laser Mégajoule plans to deliver about 1.8 MJ of laser energy to its targets, making it about as powerful as its US counterpart, the National Ignition Facility
National Ignition Facility
The National Ignition Facility, or NIF is a large, laser-based inertial confinement fusion research device located at the Lawrence Livermore National Laboratory in Livermore, California. NIF uses powerful lasers to heat and compress a small amount of hydrogen fuel to the point where nuclear fusion...
(NIF). Laser Mégajoule is the largest ICF experiment to be built outside the US, where ICF research has been strongly related to nuclear weapon
Nuclear weapon
A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion. Both reactions release vast quantities of energy from relatively small amounts of matter. The first fission bomb test released the same amount...
s research. Likewise, one of Laser Mégajoule's primary tasks will be refining fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
calculations for France's own nuclear weapons industry.
Laser Mégajoule uses a series of 240 laser beamlines, grouped into eight groups of 30. Each beamline contains two main glass amplifiers, which are optically pumped
Optical pumping
Optical pumping is a process in which light is used to raise electrons from a lower energy level in an atom or molecule to a higher one. It is commonly used in laser construction, to pump the active laser medium so as to achieve population inversion...
using xenon flashlamps. A "feeder" laser beam is fed via optical fiber
Optical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
into each of the beamlines where it travels through the two amplifiers. In order to extract more power from the amplifiers, which are not particularly efficient in transmitting power to the beam, the laser pulse is sent through the amplifiers twice by an optical switch
Optical switch
In telecommunication, an optical switch is a switch that enables signals in optical fibers or integrated optical circuits to be selectively switched from one circuit to another....
in front of a mirror. At the other end of the beamline a deformable mirror is used to remove imperfections in the wavefront.
The target chamber lies in a large experiment room in the middle of the building, with the beamlines arranged on either side. After being switched into the main room, the beams are first aimed towards the target chamber by mirrors, and then travel through an optical frequency multiplier to boost the frequency into the ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
. The mirrors are arranged in order to have the pulse impinge in the middle of the chamber from all sides.
High-Z
The term "high-Z" is used to refer to:*Chemical elements with a high atomic number of protons in the nucleus*A high impedance electronic signal.*Stars with a high redshift...
cylinder made of some heavy metal (often gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
) known as 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...
". The hohlraum then gives off x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, which are used to heat a small fuel pellet containing a deuterium
Deuterium
Deuterium, also called heavy hydrogen, is one of two stable isotopes of hydrogen. It has a natural abundance in Earth's oceans of about one atom in of hydrogen . Deuterium accounts for approximately 0.0156% of all naturally occurring hydrogen in Earth's oceans, while the most common isotope ...
-tritium
Tritium
Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...
(DT) fusion fuel. Although considerable laser energy is lost to heating the hohlraum, x-rays are much more efficient at heating the fuel pellet, making the indirect drive method applicable to nuclear weapons research. The x-rays rapidly heat the outer layer of the pellet so quickly that it explodes outward, causing the remainder of the pellet to be forced inward and causes a shock wave to travel in through the pellet to the middle. When the shock wave converges from all directions and meets in the middle, the density and temperature briefly reach the Lawson criterion
Lawson criterion
In nuclear fusion research, the Lawson criterion, first derived on fusion reactors by John D. Lawson in 1955 and published in 1957, is an important general measure of a system that defines the conditions needed for a fusion reactor to reach ignition, that is, that the heating of the plasma by the...
and start fusion reactions. If the rate of reactions is high enough the heat generated by these reactions will be enough to cause surrounding fuel to fuse as well, this process continuing until the majority of the fuel in the pellet is consumed. This process is known as "ignition", and has long been a goal of fusion researchers.
Construction on the Laser Mégajoule started with a single set of eight beamlines known as the Ligne d'Integration Laser (Laser Integration Line
Laser Integration Line
The laser integration line is a sub-piece use in the Laser Mégajoule....
), or LIL, powered by a 450 MJ energy bank. When problems are worked out in the LIL, construction will continue with the construction of four more similar units and the installation of the additional beamlines. LIL was completed in 2002. Currently the entire LMJ system is expected to be completed in 2012.
External links
- Optical systems in the LMJ
- Laser Mégajoule (in FrenchFrench languageFrench is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...
)