Inertial confinement fusion (ICF) is a concept that intends to initiate nuclear fusion reactions by heating and compressing a fuel target, typically in the form of a pellet that most often contains a mixture of deuterium and tritium.
To compress and heat the fuel, energy is delivered to the outer layer of the target using high-energy beams of laser light, electrons or ions, although for a variety of reasons, almost all ICF devices to date have used lasers. The heated outer layer explodes outward, producing a reaction force against the remainder of the target, accelerating it inwards, compressing the target. This process is designed to create shock waves that travel inward through the target. A sufficiently powerful set of shock waves can compress and heat the fuel at the center so much that fusion reactions occur.
The energy released by these reactions will then heat the surrounding fuel, and if the heating is strong enough this could also begin to undergo fusion. The aim of ICF is to produce a condition known as "ignition", where this heating process causes a chain reaction that burns a significant portion of the fuel. Typical fuel pellets are about the size of a pinhead and contain around 10 milligrams of fuel: in practice, only a small proportion of this fuel will undergo fusion, but if all this fuel were consumed it would release the energy equivalent to burning a barrel of oil.
ICF is one of two major branches of fusion energy research, the other being magnetic confinement fusion. When it was first proposed in the early 1970s, ICF appeared to be a practical approach to fusion power production and the field flourished. Experiments during the 1970s and '80s demonstrated that the efficiency of these devices was much lower than expected, and reaching ignition would not be as easy as expected. Throughout the 1980s and '90s, many experiments were conducted in order to understand the complex interaction of high-intensity laser light and plasma. These led to the design of newer machines, much larger, that would finally reach ignition energies.
The largest operational ICF experiment is the National Ignition Facility (NIF) in the US, designed using all of the decades-long experience of earlier experiments. Like those earlier experiments, however, NIF has failed to reach ignition and is, as of 2013, generating about 1/3rd of the required energy levels. A similar large-scale device in France, Laser Megajoule, has not begun operation.
Basic Fusion
Fusion reactions combine lighter atoms, such as hydrogen, together to form larger ones.
Generally the reactions take place at such high temperatures that the atoms have been ionized, their electrons stripped off by the heat; thus, fusion is typically described in terms of "nuclei" instead of "atoms".
Nuclei are positively charged, and thus repel each other due to the electrostatic force.
Overcoming this repulsion costs a considerable amount of energy, which is known as the Colomb barrier or fusion barrier energy. Generally, less energy will be needed to cause lighter nuclei to fuse, as they have less charge and thus a lower barrier energy, and when they do fuse, more energy will be released. As the mass of the nuclei increase, there is a point where the reaction no longer gives off net energy — the energy needed to overcome the energy barrier is greater than the energy released in the resulting fusion reaction. The crossover point is iron, Fe56.
The best fuel from an energy perspective is a one to one mix of deuterium and tritium; both are heavy isotopes of hydrogen. The D-T (deuterium & tritium) mix has a low barrier because of its high ratio of neutrons to protons. The presence of neutral neutrons in the nuclei helps pull them together via the nuclear force, while the presence of positively charged protons pushes the nuclei apart via electrostatic force. Tritium has one of the highest ratios of neutrons to protons of any stable or moderately unstable nuclide—two neutrons and one proton. Adding protons or removing neutrons increases the energy barrier.
A mix of D-T at standard conditions does not undergo fusion; the nuclei must be forced together before the nuclear force can pull them together into stable collections. Even in the hot, dense center of the sun, the average proton will exist for billions of years before it fuses. For practical fusion power systems, the rate must be dramatically increased; heated to tens of millions of degrees, and/or compressed to immense pressures. The temperature and pressure required for any particular fuel to fuse is known as the Lawson criterion. These conditions have been known since the 1950s when the first H-bombs were built.
ICF Mechanism of Action
In a hydrogen bomb, the fusion fuel is compressed and heated with a separate fission bomb (see Teller-Ulam design). A variety of mechanisms transfers the energy of the fission "trigger"'s explosion into the fusion fuel. The requirement of a fission bomb makes the method impractical for power generation. Not only would the triggers be prohibitively expensive to produce, but there is a minimum size that such a bomb can be built, defined roughly by the critical mass of the plutonium fuel used. Generally it seems difficult to build nuclear devices smaller than about 1 kiloton in size, which would make it a difficult engineering problem to extract power from the resulting explosions. Also the smaller a thermonuclear bomb is, the "dirtier" it is, that is to say, the percentage of energy produced in the explosion by fusion is decreased while the percent produced by fission reactions tends toward unity (100%). This did not stop efforts to design such a system however, leading to the PACER concept.
If some source of compression could be found, other than a nuclear bomb, then the size of the reaction could be scaled down. This idea has been of intense interest to both the bomb-making and fusion energy communities. It was not until the 1970s that a potential solution appeared in the form of very large, very high power, high energy lasers, which were then being built for weapons and other research. The D-T mix in such a system is known as a target, containing much less fuel than in a bomb design (often only micro or milligrams), and leading to a much smaller explosive force.
Generally ICF systems use a single laser, the driver, whose beam is split up into a number of beams which are subsequently individually amplified by a trillion times or more. These are sent into the reaction chamber (called a target chamber) by a number of mirrors, positioned in order to illuminate the target evenly over its whole surface. The heat applied by the driver causes the outer layer of the target to explode, just as the outer layers of an H-bomb's fuel cylinder do when illuminated by the X-rays of the fission device.
The material exploding off the surface causes the remaining material on the inside to be driven inwards with great force, eventually collapsing into a tiny near-spherical ball. In modern ICF devices the density of the resulting fuel mixture is as much as one-hundred times the density of lead, around 1000 g/cm3. This density is not high enough to create any useful rate of fusion on its own. However, during the collapse of the fuel, shock waves also form and travel into the center of the fuel at high speed. When they meet their counterparts moving in from the other sides of the fuel in the center, the density of that spot is raised much further.
Given the correct conditions, the fusion rate in the region highly compressed by the shock wave can give off significant amounts of highly energetic alpha particles. Due to the high density of the surrounding fuel, they move only a short distance before being "thermalised", losing their energy to the fuel as heat. This additional energy will cause additional fusion reactions in the heated fuel, giving off more high-energy particles. This process spreads outward from the centre, leading to a kind of self-sustaining burn known as ignition.
http://en.wikipedia.org/wiki/Laser_fusion
[This Wikipedia article also delves into substantial detail dealing with the difficulties in achieving sustained ignition and substantial net power production using laser fusion.]
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