Inertial confinement fusion
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In inertial confinement fusion (ICF), a nuclear fusion reaction is ignited by compressing a target – a pellet that contains deuterium and tritium – by the use of intense laser or ion beams. The beams explosively detonate the outer layers of the target, sending a shock wave into the center. If the shock wave is powerful enough some of the fuel will be heated enough to cause fusion reactions, releasing tremendous amounts of energy. That energy can then heat the surrounding fuel to cause it to fuse as well, creating a chain reaction that burns the fuel load. If the reaction completes efficiently, a small amount of fuel about the size of a pinhead releases the energy equivalent to a barrel of oil.
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Basic fusion
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 wave 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.
ICF design
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 prohibitively expensive to produce, but there is a minimum size that 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. This did not stop efforts to design such a system however, leading to the PACER 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 joules 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-pinch 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.
Brief history
ICF experiments started in earnest in the mid-1970s, when lasers of the required power were first designed. This was long after the successful design of magnetic confinement fusion systems, and even the particularly successful tokamak design that was introduced in the early 1970s. Nevertheless, high funding during the energy crisis made for rapid gains in performance, and inertial designs were soon reaching the same sort of "below breakeven" conditions of the best magnetic systems.
One of the earliest serious attempts at an ICF design was Shiva, a 20-armed neodynium laser system built at the Lawrence Livermore National Laboratory (LLNL) that started operation in 1978. Shiva was a "proof of concept" design, followed by the NOVA design with 10 times the power. Funding for fusion research was severely constrained in the 80's, but NOVA nevertheless successfully gathered enough information for a next generation machine whose goal was ignition. Such a condition is considered nessessary for a practical power system.
The resulting design, now known as the National Ignition Facility, started construction at LLNL in 1997. Originally intended to start construction in the early 1990s, the NIF is now six years behind schedule and massively overbudget to the tune of over $1.4 billion. If this is to be typical of the development of such systems, it is unlikely they will ever be a commercial power source. Nevertheless many of the problems appear to be due to the "big lab" mentality and shifting the focus from pure ICF research to the nuclear stuardship program, LLNLs traditional bombmaking role. NIF is now scheduled to "burn" in 2005, when the remaining lasers in the 192-beam array are finally installed.
Inertial Fusion Energy
Practical power plants built using ICF are now a serious area of study, known as inertial fusion energy, or IFE. IFE plants would deliver a continuous stream of targets to the reaction chamber, several a second typically, and capture the resulting heat to drive a conventional steam turbine.
ICF systems face some of the same problems as magnetic systems in generating useful power from their reactions. One of the primary concerns is how to successfully remove heat from the reaction chamber without interfering with the targets and driver beams, another is that the huge number of neutrons released in the fusion reactions react with the plant, causing them to become radioactive themselves, as well as mechanically weakening metals and slowly "wearing out" the plant.
One current concept in dealing with both of these problems, as shown in the HYLIFE-II baseline design, is to use a "waterfall" of flibe, a molten mix of fluorine, lithium and beryllium salts, which both protect the chamber from neutrons, as well as carrying away heat. The flibe is then passed into a heat exchanger where it heats water for use in the turbines. Another, Sombrero, uses a reaction chamber built of carbon fibre which has a very low neutron cross section. Cooling is provided by a molten ceramic, chosen because of its ability to stop the neutrons from travelling any further, while at the same time being an efficient heat transfer agent.
As a power source, even the best IFE reactors would be hard-pressed to deliver the same economics as coal. Coal can simply be dug up and burned for little cost, the main concern being the shipping costs. The technical needs of an IFE plant and its continued target production appear to be on the same order, with most baseline designs operating at about the same price point. HYLIFE-II claims to be about 40% less expensive than a coal plant of the same size, but considering the problems with NIF, it is simply too early to tell if this is realistic or not.
See also: plasma physics, magnetic fusion energy
