Inertial confinement fusion

In inertial confinement fusion (ICF), nuclear fusion reactions are initiated by heating and compressing a target – a pellet that most often contains deuterium and tritium – by the use of intense laser or ion beams. The beams explosively detonate the outer layers of the target, accelerating the remaining target layers inward and sending a shock wave into the center. If the shock wave is powerful enough and if high enough density at the center is achieved some of the fuel will be heated enough to cause fusion reactions, releasing energy. In a target which has been heated and compressed to the point of thermonuclear ignition, energy can then heat surrounding fuel to cause it to fuse as well, creating a chain reaction that burns the fuel load, potentially releasing tremendous amounts of energy. Theoretically, if the reaction completes with perfect efficiency (a practically impossible feat), a small amount of fuel about the size of a pinhead releases the energy equivalent to a barrel of oil.

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 "nuclei" instead of "atoms".

Fusion reactions on a scale useful for energy production require a very large amount of energy to initiate in order to overcome the so-called Coulomb barrier or fusion barrier energy. Since the positively-charged nuclei 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 nuclei to fuse, and when they do, 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. This point occurs when an iron nuclei is formed and is the result of death of some massive stars, though this phenomenon plays no role in laboratory induced fusion since the energy and temperatures required to form iron nuclei is so phenomenally huge it doesn't occur.

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 one to one mix of deuterium and tritium, both heavy isotopes of hydrogen. This D-T mix has a lower barrier than any other fuel due to the presence of neutral neutrons in the nuclei, 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 combination of the level of temperature and 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 is released and heats the outer layers of a cylinder of fusion fuel. This causes the outer layer to explode outward. Following Newton's Third Law, the outward moving mass forces the remainder of the fuel inward and creates a shock wave that travels into the cylinder, which, when it converges in the center, the Lawson criterion can be met.

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. Also the smaller a thermonuclear bomb is, the "dirtier" it is, that is to say, the percentage of energy produced in the explostion 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 does when illuminated by the X-rays of the nuclear device. This causes extremely rapid heating and inward compression of the fuel inside the capsule and the formation of a shock wave, spherical instead of cylindrical, which further heats the fuel in the very center. In an ignition scale fuel capsule (used in a laser system which delivers enough energy capable to ignite it) the heat released by the reaction initiates fusion in the fuel surrounding it through heating by irradiation by the high energy alpha particles produced by the first fusion reactions at the center of the target, thereby leading to a chain reaction known as ignition.

Issues with the successful achievement of ICF

The primary problems with increasing ICF performance since the early experiments in the 1970s have been of energy delivery to the target and of symmetry of the imploding fuel and formation of a 'tight' shockwave convergence at fuel center. In order to focus the shock wave on the center of the target, the target must be made with extremely high precision and sphericity with abberations of no more than a few micrometres over its surface (inner and outer). Likewise the aiming of the laser beams must be extremely precise and the beams must arrive at the same time at all points on the target. Beam timing is a relatively simple issue and is solved by using delay lines in the beams' optical path to achieve picosecond levels of timing accuracy. Other problems plaguing the achievement of high symmetry and high temperatures/densities of the imploding target are so called "beam-beam" imbalance and beam anisotropy. These problems are, respectively, where the energy delivered by one beam may be higher or lower than other beams impinging on the target and of "hot spots" within a beam diameter hitting a target which induces uneven compression on the target surface, thereby forming rayleigh-taylor instabilities in the fuel, mixing it and reducing heating efficacy. All of these problems have been substantially mitigated in the past two decades of research by using various beam smoothing techniques and beam energy diagnostics to balance beam to beam energy. Target design has also improved tremendously over the years. Modern cryogenic targets tend to freeze a thin layer of the D-T mix just on the inside of a plastic sphere while irradiating it with a low power IR laser to smooth it's inner surface and monitoring it with a microscope equipped camera, thereby allowing the layer to be closely monitored ensuring its "smoothness".

Certain targets are surrounded by a small metal cylinder which is then irradiated by the laser beams instead of the target itself (called indirect drive ICF) (lasers are focused on the inner side of the cylinder almost instantly heating it to a superhot plasma which radiates mosly in X-rays) the X-rays from this plasma are then absorbed by the target surface, imploding it in the same way as if it had been hit with the lasers directly. The absorption of x-rays by the target is more efficient than the direct absorption of laser light, however these hohlraums or "burning chambers" 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. Often indirect drive hohlraum targets are used to simulate thermonuclear weapons tests due to the fact that the fusion fuel in them is also imploded mainly by X-ray radiation.

A variety of ICF drivers are being explored. Lasers have improved dramatically since the 1970s, scaling up in power from a few joules and kilo to gigawatts to megajoules (see NIF laser) and hundreds of terawatts, using mostly frequency doubled or tripled light from neodymium glass amplifiers. Other designs use heavy ion beams, or even imploding vaporized 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, it is very difficult to achieve the very high energies required to implode a target efficiently and most ion-beam systems require the use of a hohlraum surrounding the target, reducing the overall efficiengy of the coupling of the ion beam's energy to that of the imploding target further.

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 neodymium 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. Although net energy can be released even without ignition (the breakeven point), ignition 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 practical 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 stewardship 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 serious concern 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. Fusion plants built of metal would have a fairly short lifetime and the core containment vessels will have to be replaced frequently.

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, one of the main costs being shipping. An IFE plant would likely be similar in cost to a coal-fired one in terms of construction and machinery, but the fuel is considerably more complex while also being much more powerful. It is generally estimated that an IFE plant would have long-term operational costs about the same as coal, discounting development. 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
• nuclear fusion
• nuclear fission
• Muon-catalyzed fusion
• Antimatter catalyzed nuclear pulse propulsion
• Timeline of nuclear fusion
• Laboratory for Laser Energetics

External link

• Inertial Fusion Energy: A Tutorial on the Technology and Economics (http://www.nuc.berkeley.edu/thyd/icf/IFE.html)
• National Ignition Facility Project (http://www.llnl.gov/nif/nif.html)
• Igniting a Fsion Face Off (http://www.abqtrib.com/archives/news01/121801_news_nifa1.shtml)
• Zpinch Home Page (http://zpinch.sandia.gov/)fr:Fusion par confinement inertiel