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Fusion reactions combine lightweight 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 being stripped off by the heat, and thus fusion is typically described in terms of "nucleii" instead of "atoms".
Fusion reactions require quite a bit of energy to produce, the so-called fusion barrier energy. Since the positively-charged nucleii are naturally repelling each other, this repulsive force must be overcome by providing some form of external energy. When this occurs, however, the reaction is a rather energetic one. Generally less energy will be needed to cause lighter nucleii to fuse, and when they do, more energy will be released. As the mass of the nucleii 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 key to practical fusion power is to select a fuel that requires the minimum amount of energy to start, that is, the lowest barrier energy. The best fuel from this standpoint is a 50-50 mix of deuterium and tritium, both heavier isotopes of hydrogen. This D-T mix has a lower barrier than any other fuel due to the presence of neutral neutrons in the nucleii, which help pull them together, while still only containing one proton, which is pushing them apart. Adding protons or removing neutrons increases the energy barrier.
While D-T mix has the lowest barrier, it is still very high in real world terms. In order to create the required conditions, the fuel must be heated to tens of millions of degrees, and/or compressed to immense pressures. The amount of temperature or pressure needed for any particular fuel to fuse is known as the Lawson criterion. These conditions have been available since the 1950s when the first H-bombs were built.
In an H-bomb the energy from a small nuclear bomb, notably the X-rays it releases, heats the outer layers of a cylinder of fuel. This causes the outer layer to explode outward, just like dropping water on a hot pan. Following Newton's Third Law, the outward moving mass creates a shock waveFor the vector animation platform, see Macromedia Shockwave. In fluid dynamics, a shock wave is a strong pressure wave. See Rankine-Hugoniot equation. In compressible fluids such as air, disturbances such as pressure changes caused by a solid object movin that travels into the cylinder, and when it converges in the center, the Lawson criterion can be met. In cylinder is typically built of a solid fuel, lithium-deuteride, that turns into deuterium and tritium when heated. Only the very center of the device needs to have an actual D-T mix, a liquid pit known as the trigger.
The use of a nuclear bomb to ignite a fusion reaction makes the concept less than useful as a power source. Not only would the bombs be prohibitavely expensive to produce, but there is a minimum size that a bomb can be built, defined roughly by the critical massFor the car-free environment event, see Critical Mass For the English Anarcho Environmentalist pressure group founded in 1984, see Critical Mass (pressure group This term is also used figuratively, meaning something like "be sufficient to work properly", of the plutoniumPlutonium is a radioactive, metallic, chemical element. It has the symbol Pu and the atomic number 94. Its atomic weight is 244. 06, its density 19,800 kg/m3. It is the element used in most modern nuclear weapons. The most important isotope of plutonium i fuel used. Generally it seems difficult to build nuclear devices smaller than about 1 kiloton in size, which would makes it a difficult engineering problem to extract power from the resulting explosions. This did not stop efforts to design such a system however, leading to the PACERThe PACER project, carried out at Los Alamos National Laboratory in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small H-bombs inside an underground cavity. The proposed system would absorb the energy of th concept.
If some other source of compression could be found, one 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 bombmaking and fusion energy communities. It was not until the 1970s that a solution appeared in the form of very large lasers, which were then being built for weapons research. The D-T mix in such a system is known as a target, containing much less fuel than in a bomb design, and leading to a much smaller explosive force.
Generally ICF systems use a single large laser, the driver, whose beam is split up into a number of beams which are individually amplified. These are sent into the reaction chamber by a number of mirrors, positioned in order to illuminate the target evenly over the 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 nuclear device. This causes a shock wave, often spherical instead of cylindrical, which ignites the fuel in the very center. Once ignited, the heat released by the reaction ignited the fuel around it, leading to a chain reaction known as ignition.
The primary problem with increasing ICF performance since the early experiments in the 1970s has been mechanical - in order to focus the shock wave on the center of the target, the target must be made with extremely high precision. Likewise the aiming of the laser beam must be extremely precise, and the beam must arrive at the same time at all points on the target. This later problem is actually fairly difficult, as the laser is typically situated to one side of the reaction chamber, and the beams would naturally reach that side of the chamber first.
Target design has improved tremendously over the years. Early designs used small beads with liquid D-T inside, often covered with some sort of material designed specifically to burn off and provide the compression. Modern targets tend to freeze a thin layer of the D-T mix just on the inside of a plastic sphere, thereby allowing the layer to be studied to ensure its "smoothness". Some targets also include a thin metal cylinder that acts as an X-ray mirror, reflecting the X-rays created by the laser's interaction with the plastic inside the cylinder back into the target. However these hohlraums also take up considerable energy to heat on their own, and are a debated feature even today; the equally numerous direct-drive designs do not use them.
A variety of drivers are being explored. Lasers have improved dramatically since the 1970s, scaling up in power from a few jouleThe joule (symbol J also called newton metre or coulomb volt is the SI unit of energy and work. The unit is pronounced to rhyme with "tool", and is named in honour of the physicist James Prescott Joule (1818-1889). 1 joule 1 N · 1 m 1 newton · 1 metre 1 ks to megajoules and using several different generation systems. Techniques for amplifying the beams and slowing them so they all arrive at the same time have also been generally "solved". Other designs use heavy ion beams, or even metal wires as in the z-pinchIn fusion power design, Z-pinch or zeta pinch is a type of plasma confinement system that uses an electrical current in the plasma to generate a magnetic field that compresses it. The name refers to the direction of the earliest experimental devices in En design. Ion beams are particularly interesting for commercial generation, as they are easy to create, control and focus. On the downside, most ion-beam systems require a hohlraum, making the fuel pellets themselves more expensive.