were later demonstrated to be from different versions of the same instability processes that plagued earlier machines. Cockcroft was forced to retract the fusion claims, which tainted the entire field for years. ZETA ended its experiments in 1968, and most other pinch experiments ended shortly after. In 1974 a study of the ZETA results demonstrated an interesting side-effect; after the experimental runs ended, the plasma would enter a short period of stability. This led to the reversed field pinch concept which has seen some level of development ever since. Recent work on the basic concept started as a result of the appearance of the "wires array" concept in the 1980s, which allowed a more efficient use of this technique. The Sandia National Laboratory runs a continuing wire-array research program with the Zpinch machine. In addition, the University of Washington's ZaP Lab has shown quiescent periods of stability hundreds of times longer than expected for plasma in a Z-pinch configuration, giving promise to the confinement technique. In 1995, the staged Z-pinch concept was introduced by a team of scientist from University of California Irvine (UCI). This scheme can control one of the most dangerous instability that normally disintegrate conventional Z-pinch before the final implosion. The concept is based on a complex load of radiative liner plasma embedded with a target plasma. During implosion the outer surface of the liner plasma becomes unstable but the target plasma remains remarkably stable, up until the final implosion, generating a very high energy density stable target plasma. The heating mechanisms are shock heating, adiabatic compression and trapping of charge particles produced in fusion reaction due to a very strong magnetic field, which develops between the liner and the target. Details of this concept are shown in various publications available on the web page of MIFTI [2]. [edit] Early magnetic approaches The U.S. fusion program began in 1951 when Lyman Spitzer began work on a stellarator under the code name Project Matterhorn. His work led to the creation of the Princeton Plasma Physics Laboratory, where magnetically confined plasmas are still studied. Spitzer planned an aggressive development project of four machines, A, B, C, and D. A and B were small research devices, C would be the prototype of a power-producing machine, and D would be the prototype of a commercial device. A worked without issue, but even by the time B was being used it was clear the stellarator was also suffering from instabilities and plasma leakage. Progress on C slowed as attempts were made to correct for these problems. At Lawrence Livermore, the magnetic mirror was the preferred approach. The mirror consisted of two large magnets arranged so they had strong fields within them, and a weaker, but connected, field between them. Plasma introduced in the area between the two magnets would "bounce back" from the stronger fields in the middle. Although the design would leak plasma through the mirrors, the rate of leakage would be low enough that a useful fusion rate could be maintained. The simplicity of the design was supposed to make up for its lower performance. In practice the mirror also suffered from mysterious leakage problems, and never reached the expected performance. [edit] Gun Club, MHD, instability; progress slows By the mid-1950s it was clear that the simple theoretical tools being used to calculate the performance of all fusion machines were simply not predicting their actual behaviour. Machines invariably leaked their plasma from their confinement area at rates far higher than predicted. In 1954, Edward Teller held a gathering of fusion researchers at the Princeton Gun Club, near the Project Matterhorn (now known as Project Sherwood) grounds. Teller started by pointing out the problems that everyone was having, and suggested that any system where the plasma was confined within concave fields was doomed to fail. Attendees remember him saying something to the effect that the fields were like rubber bands, and they would attempt to snap back to a straight configuration whenever the power was increased, ejecting the plasma. He went on to say that it appeared the only way to confine the plasma in a stable configuration would be to use convex fields, a "cusp" configuration.[12] When the meeting concluded, most of the researchers quickly turned out papers saying why Teller's concerns did not apply to their particular device. The pinch machines did not use magnetic fields in this way at all, while the mirror and stellarator seemed to have various ways out. However, this was soon followed by a paper by Martin David Kruskal and Martin Schwarzschild discussing pinch machines, which demonstrated instabilities in those devices were inherent to the design. A series of similar studies followed, abandoning the simplistic theories previously used and introducing a full consideration of magnetohydrodynamics with a partially-resistive plasma. These concepts developed quickly, and by the early 1960s it was clear that small devices simply would not work. A series of much larger and more complex devices followed as researchers attempted to add field upon field in order to provide the required field strength without reaching the unstable regimes. As cost and complexity climbed, the initial optimism of the fusion field faded. [edit] The tokamak is announced A new approach was outlined in the theoretical works fulfilled in 1950–1951 by I.E. Tamm and A.D. Sakharov in the Soviet Union, which first discussed a tokamak-like approach. Experimental research on these designs began in 1956 at the Kurchatov Institute in Moscow by a group of Soviet scientists led by Lev Artsimovich. The tokamak essentially combined a low-power pinch device with a low-power simple stellarator. The key was to combine the fields in such a way that the particles wound around the reactor a particular number of times, today known as the "safety factor". The combination of these fields dramatically improved confinement times and densities, resulting in huge improvements over existing devices. The group constructed the first tokamaks, the most successful being the T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, producing the first quasistationary thermonuclear fusion reaction ever.[13] The tokamak was dramatically more efficient than the other approaches of that era, on the order of 10 to 100 times. When they were first announced the international community was highly skeptical. However, a British team was invited to see T-3, and having measured it in depth they released their results that confirmed the Soviet claims. A burst of activity followed as many planned devices were abandoned and new tokamaks were introduced in their place - the C model stellarator, then under construction after many redesigns, was quickly converted to the Symmetrical Tokamak and the stellarator was abandoned. Through the 1970s and 80s great strides in understanding the tokamak system were made. A number of improvements to the design are now part of the "advanced tokamak" concept, which includes non-circular plasmas, internal diverters and limiters, often superconducting magnets, and operate in the so-called "H-mode" island of increased stability. Two other designs have also become fairly well studied; the compact tokamak is wired with the magnets on the inside of the vacuum chamber, while the spherical tokamak reduces its cross section as much as possible. The tokamak dominates modern research, where very large devices like ITER are expected to pass several milestones toward commercial power production, including a burning plasma with long burn times, high power output, and online fueling. There are no guarantees that the project will be successful; previous generations of tokamak machines have uncovered new problems many times. But the entire field of high temperature plasmas is much better understood now than formerly, and there is considerable optimism that ITER will meet its goals. If successful, ITER would be followed by a "commercial demonstrator" system, similar in purpose to the very earliest power-producing fission reactors built in the era before wide-scale commercial deployment of larger machines started in the 1960s and 1970s. Even with these goals met, there are a number of major engineering problems remaining, notably finding suitable "low activity" materials for reactor construction, demonstrating secondary systems including practical tritium extraction, and building reactor designs that allow their reactor core to be removed when its materials becomes embrittled due to the neutron flux. Practical commercial generators based on the tokamak concept are far in the future. The public at large has been disappointed, as the initial outlook for practical fusion power plants was much rosier; a pamphlet from the 1970s printed by General Atomic stated that "Several commercial fusion reactors are expected to be online by the year 2000." [edit] Inertial (laser) containment The technique of implosion of a microcapsule irradiated by laser beams, the basis of laser inertial confinement, was first suggested in 1962 by scientists at Lawrence Livermore National Laboratory, shortly after the invention of the laser itself in 1960. Lasers of the era were very low powered, but low-level research using them nevertheless started as early as 1965. A great advance in the field was John Nuckolls' 1972 paper that ignition would require lasers of about 1 kJ, and efficient burn around 1 MJ. kJ lasers were just beyond the state of the art at the time, and his paper sparked off a tremendous development effort to produce devices of the needed power. Early machines used a variety of approaches to attack one of two problems - some focused on fast delivery of energy, while others were more interested in beam smoothness. Both were attempts to ensure the energy delivery would be smooth enough to cause an even implosion. However, these experiments demonstrated a serious problem; laser wavelengths in the infrared area lost a tremendous amount of energy before compressing the fuel. Important breakthroughs in this laser technology were made at the Laboratory for Laser Energetics at the University of Rochester, where scientists used frequency-tripling crystals to transform the infrared laser beams into ultraviolet beams. By the late 1970s great strides had been made in laser power, but with each increase new problems were found in the implosion technique that suggested even more power would be required. By the 1980s these increases were so large that using the concept for generating net energy seemed remote. Most research in this field turned to weapons research, always a second line of research, as the implosion concept is somewhat similar to hydrogen bomb operation. Work on very large versions continued as a result, with the very large National Ignition Facility in the US and Laser Mégajoule in France supporting these research programs. More recent work had demonstrated that significant savings in the required laser energy are possible using a technique known as "fast ignition". The savings are so dramatic that the concept appears to be a useful technique for energy production again, so much so that it is a serious contender for pre-commercial development. There are proposals to build an experimental facility dedicated to the fast ignition approach, known as HiPER. At the same time, advances in solid state lasers appear to improve the "driver" systems' efficiency by about ten times (to 10- 20%), savings that make even the large "traditional" machines almost practical, and might make the fast ignition concept outpace the magnetic approaches in further development. The laser-based concept has other advantages. The reactor core is mostly exposed, as opposed to being wrapped in a huge magnet as in the tokamak. This makes the problem of removing energy from the system somewhat simpler, and should mean that a laser-based device would be much easier to perform maintenance on, such as core replacement. Additionally, the lack of strong magnetic fields allows for a wider variety of low-activation materials, including carbon fiber, which would reduce both the frequency of such neutron activations and the rate of irradiation to the core. In other ways the program has many of the same problems as the tokamak; practical methods of energy removal and tritium recycling need to be demonstrated. [edit] Other approaches Over the years there have been a wide variety of fusion concepts. In general they fall into three groups - those that attempt to reach high temperature/density for brief times (pinch, inertial confinement), those that operate at a steady state (magnetic confinement) or those that try neither and instead attempt to produce low quantities of fusion but do so at an extremely low cost. The latter group has largely disappeared, as the difficulties of achieving fusion have demonstrated that any low-energy device is unlikely to produce net gain. This leaves the two major approaches, magnetic and laser inertial, as the leading systems for development funding. However, alternate approaches continue to be developed, and alternate non-power fusion devices have been successfully developed as well. [edit] Technically viable Philo T. Farnsworth, the inventor of the first all-electronic television system in 1927, patented his first Fusor design in 1968, a device that uses inertial electrostatic confinement. This system consists largely of two concentric spherical electrical grids inside a vacuum chamber into which a small amount of fusion fuel is introduced. Voltage across the grids causes the fuel to ionize around them, and positively charged ions are accelerated towards the center of the chamber. Those ions may collide and fuse with ions coming from the other direction, may scatter without fusing, or may pass directly through. In the latter two cases, the ions will tend to be stopped by the electric field and re-accelerated toward the center. Fusors can also use ion guns rather than electric grids. Towards the end of the 1960s, Robert Hirsch designed a variant of the Farnsworth Fusor known as the Hirsch-Meeks fusor. This variant is a considerable improvement over the Farnsworth design, and is able to generate neutron flux in the order of one billion neutrons per second. Although the efficiency was very low at first, there were hopes the device could be scaled up, but continued development demonstrated that this approach would be impractical for large machines. Nevertheless, fusion could be achieved using a "lab bench top" type set up for the first time, at minimal cost. This type of fusor found its first application as a portable neutron generator in the late 1990s. An automated sealed reaction chamber version of this device, commercially named Fusionstar was developed by EADS but abandoned in 2001. Its successor is the NSD-Fusion neutron generator. Robert W. Bussard's Polywell concept is roughly similar to that of the Fusor, but replaces the problematic grid with a magnetically contained electron cloud, which holds the ions in position and provides an accelerating potential. The polywell consists of electromagnet coils arranged in a polyhedral configuration and positively charged to between several tens and low hundreds of kilovolts. This charged magnetic polyhedron is called a MaGrid (Magnetic Grid). Electrons are introduced outside the "quasi-spherical" MaGrid and are accelerated into the MaGrid due to the electric field, similar to a magnetic bottle. Within the MaGrid, magnetic fields confine most of the electrons and those that escape are retained by the electric field. This configuration traps the electrons in the middle of the device, focusing them near the center to produce a virtual cathode (negative electric potential). The virtual cathode accelerates and confines the ions to be fused which, except for minimal losses, never reach the physical structure of the MaGrid. Bussard had reported a fusion rate of 109 per second running D-D fusion reactions at only 12.5 kV (based on detecting a total of nine neutrons in five tests. Bussard claimed a scaled-up version of 2.5–3 m in diameter, would operated at over 100 MW net power (fusion power scales as the fourth power of the B field and the cube of the size)[14] A recent[when?] area of study is the magneto-inertial fusion (MIF) concept, which combines some form of external inertial compression (like lasers) with further compression through an external magnet (like pinch devices). The magnetic field traps heat within the inertial core, causing a variety of effects that improves fusion rates. These improvements are relatively minor, however the magnetic drivers themselves are inexpensive compared to lasers or other systems. There is hope for a sweet spot that allows the combination of features from these devices to create low-density but also low-cost fusion devices. A similar concept is the magnetized target fusion device, which uses a magnetic field in an external metal shell to achieve the same basic goals. According to Eric Lerner, Focus fusion takes place in a dense plasma focus,[15] which typically consists of two coaxial cylindrical electrodes made from copper or beryllium and housed in a vacuum chamber containing a low-pressure gas, which is used as the reactor fuel. An electrical pulse is applied across the electrodes, producing heating and a magnetic field. The current forms the hot gas into many minuscule vortices perpendicular to the surfaces of the electrodes, which then migrate to the end of the inner electrode to pinch-and-twist off as tiny balls of plasma called plasmoids. The electron beam collides with the plasmoid, heating it to fusion temperatures. This will, in principle, yield more energy in the beams than was input to form them.[citation needed] [edit] Non-power generating approaches A more subtle technique is to use more unusual particles to catalyse fusion. The best known of these is Muon-catalyzed fusion which uses muons, which behave somewhat like electrons and replace the electrons around the atoms. These muons allow atoms to get much closer and thus reduce the kinetic energy required to initiate fusion. Muons require more energy to produce than can be obtained from muon-catalysed fusion, making this approach impractical for the generation of power. In April 2005, a team from UCLA announced it had devised a way of producing fusion using a machine that "fits on a lab bench", using lithium tantalate to generate enough voltage to smash deuterium atoms together. However, the process does not generate net power. See Pyroelectric fusion. Such a device would be useful in the same sort of roles as the fusor. [edit] Abandoned and discredited approaches Some scientists reported excess heat, neutrons, tritium, helium and other nuclear effects in so-called cold fusion systems, which for a time gained interest as showing promise. Hopes fell when replication failures were weighed in view of several reasons cold fusion is not likely to occur, the discovery of possible sources of experimental error, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts.[16] By late 1989, most scientists considered cold fusion claims dead,[17] and cold fusion subsequently gained a reputation as pathological science.[18] However, a small community of researchers continues to investigate cold fusion[17][19][20][21] claiming to replicate Fleishmann and Pons' results including nuclear reaction byproducts.[22][23] Claims related to cold fusion are largely disbelieved in the mainstream scientific community.[24] In 1989, the majority of a review panel organized by the US Department of Energy (DOE) found that the evidence for the discovery of a new nuclear process was not persuasive. A second DOE review, convened in 2004 to look at new research, reached conclusions similar to the first.[25] Research into sonoluminescence induced fusion, sometimes known as "bubble fusion", also continues, although it is met with as much skepticism as cold fusion is by most of the scientific community. [edit] Key design areas [edit] Confinement Parameter space occupied by inertial fusion energy and magnetic fusion energy devices as of the mid 1990s. The regime allowing thermonuclear ignition with high gain lies near the upper right corner of the plot. Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion: * Equilibrium: There must be no net forces on any part of the plasma, otherwise it will rapidly disassemble. The exception, of course, is inertial confinement, where the relevant physics must occur faster than the disassembly time. * Stability: The plasma must be so constructed that small deviations are restored to the initial state, otherwise some unavoidable disturbance will occur and grow exponentially until the plasma is destroyed. * Transport: The loss of particles and heat in all channels must be sufficiently slow. The word "confinement" is often used in the restricted sense of "energy confinement". The first human-made, large-scale fusion reaction was the test of the hydrogen bomb, Ivy Mike, in 1952. As part of the PACER project, it was once proposed to use hydrogen bombs as a source of power by detonating them in underground caverns and then generating electricity from the heat produced, but such a power plant is unlikely ever to be constructed, for a variety of reasons.[citation needed] Controlled thermonuclear fusion (CTF) refers to the alternative of continuous power production, or at least the use of explosions that are so small that they do not destroy a significant portion of the machine that produces them.[citation needed] To produce self-sustaining fusion, the energy released by the reaction (or at least a fraction of it) must be used to heat new reactant nuclei and keep them hot long enough that they also undergo fusion reactions. Retaining the heat is called energy confinement and may be accomplished in a number of ways.[citation needed] The hydrogen bomb really has no confinement at all. The fuel is simply allowed to fly apart, but it takes a certain length of time to do this, and during this time fusion can occur. This approach is called inertial confinement. If more than milligram quantities of fuel are used (and efficiently fused), the explosion would destroy the machine, so theoretically, controlled thermonuclear fusion using inertial confinement would be done using tiny pellets of fuel which explode several times a second. To induce the explosion, the pellet must be compressed to about 30 times solid density with energetic beams. If the beams are focused directly on the pellet, it is called direct drive, which can in principle be very efficient, but in practice it is difficult to obtain the needed uniformity.[citation needed] An alternative approach is indirect drive, in which the beams heat a shell, and the shell radiates x-rays, which then implode the pellet. The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been investigated.[citation needed] Inertial confinement produces plasmas with impressively high densities and temperatures, and appears to be best suited to weapons research, X-ray generation, very small reactors, and perhaps in the distant future, spaceflight.[citation needed] They require fuel pellets with close to a perfect shape in order to generate a symmetrical inward shock wave to produce the high-density plasma, and in practice these have proven difficult to produce. A recent development in the field of laser induced ICF is the use of ultrashort pulse multi-petawatt lasers to heat the plasma of an imploding pellet at exactly the moment of greatest density after it is imploded conventionally using terawatt scale lasers. This research will be carried out on the (currently being built) OMEGA EP petawatt and OMEGA lasers at the University of Rochester and at the GEKKO XII laser at the institute for laser engineering in Osaka Japan, which if fruitful, may have the effect of greatly reducing the cost of a laser fusion based power source.[citation needed] At the temperatures required for fusion, the fuel is in the form of a plasma with very good electrical conductivity. This opens the possibility to confine the fuel and the energy with magnetic fields, an idea known as magnetic confinement. The Lorenz force works only perpendicular to the magnetic field, so that the first problem is how to prevent the plasma from leaking out the ends of the field lines. There are basically two solutions.[citation needed] The first is to use the magnetic mirror effect. If particles following a field line encounter a region of higher field strength, then some of the particles will be stopped and reflected. Advantages of a magnetic mirror power plant would be simplified construction and maintenance due to a linear topology and the potential to apply direct conversion in a natural way, but the confinement achieved in the experiments was so poor that this approach has been essentially abandoned.[citation needed] The second possibility to prevent end losses is to bend the field lines back on themselves, either in circles or more commonly in nested toroidal surfaces. The most highly developed system of this type is the tokamak, with the stellarator being next most advanced, followed by the Reversed field pinch. Compact toroids, especially the Field-Reversed Configuration and the spheromak, attempt to combine the advantages of toroidal magnetic surfaces with those of a simply connected (non-toroidal) machine, resulting in a mechanically simpler and smaller confinement area. Compact toroids still have some enthusiastic supporters but are not backed as readily by the majority of the fusion community.[citation needed] Finally, there are also electrostatic confinement fusion systems, in which ions in the reaction chamber are confined and held at the center of the device by electrostatic forces, as in the Farnsworth-Hirsch Fusor, which is not believed to be able to be developed into a power plant. The Polywell, an advanced variant of the fusor,[citation needed] has shown a degree of research interest as of late; however, the technology is relatively immature,[citation needed] and major scientific and engineering questions remain which researchers under the auspices of the U.S. Office of Naval Research hope to further investigate.[citation needed] [edit] Materials Main article: International Fusion Materials Irradiation Facility Developing materials for fusion reactors has long been recognized as a problem nearly as difficult and important as that of plasma confinement, but it has received only a fraction of the attention. The neutron flux in a fusion reactor is expected to be about 100 times that in existing pressurized water reactors (PWR). Each atom in the blanket of a fusion reactor is expected to be hit by a neutron and displaced about a hundred times before the material is replaced. Furthermore the high-energy neutrons will produce hydrogen and helium in various nuclear reactions that tends to form bubbles at grain boundaries and result in swelling, blistering or embrittlement. One also wishes to choose materials whose primary components and impurities do not result in long-lived radioactive wastes. Finally, the mechanical forces and temperatures are large, and there may be frequent cycling of both. The problem is exacerbated because realistic material tests must expose samples to neutron fluxes of a similar level for a similar length of time as those expected in a fusion power plant. Such a neutron source is nearly as complicated and expensive as a fusion reactor itself would be. Proper materials testing will not be possible in ITER, and a proposed materials testing facility, IFMIF, was still at the design stage in 2005. The material of the plasma facing components (PFC) is a special problem. The PFC do not have to withstand large mechanical loads, so neutron damage is much less of an issue. They do have to withstand extremely large thermal loads, up to 10 MW/m², which is a difficult but solvable problem. Regardless of the material chosen, the heat flux can only be accommodated without melting if the distance from the front surface to the coolant is not more than a centimeter or two. The primary issue is the interaction with the plasma. One can choose either a low-Z material, typified by graphite although for some purposes beryllium might be chosen, or a high-Z material, usually tungsten with molybdenum as a second choice. Use of liquid metals (lithium, gallium, tin) has also been proposed, e.g., by injection of 1–5 mm thick streams flowing at 10 m/s on solid substrates. If graphite is used, the gross erosion rates due to physical and chemical sputtering would be many meters per year, so one must rely on redeposition of the sputtered material. The location of the redeposition will not exactly coincide with the location of the sputtering, so one is still left with erosion rates that may be prohibitive. An even larger problem is the tritium co-deposited with the redeposited graphite. The tritium inventory in graphite layers and dust in a reactor could quickly build up to many kilograms, representing a waste of resources and a serious radiological hazard in case of an accident. The consensus of the fusion community seems to be that graphite, although a very attractive material for fusion experiments, cannot be the primary PFC material in a commercial reactor. The sputtering rate of tungsten can be orders of magnitude smaller than that of carbon, and tritium is not so easily incorporated into redeposited tungsten, making this a more attractive choice. On the other hand, tungsten impurities in a plasma are much more damaging than carbon impurities, and self-sputtering of tungsten can be high, so it will be necessary to ensure that the plasma in contact with the tungsten is not too hot (a few tens of eV rather than hundreds of eV). Tungsten also has disadvantages in terms of eddy currents and melting in off-normal events, as well as some radiological issues. [edit] Subsystems In fusion research, achieving a fusion energy gain factor Q = 1 is called breakeven and is considered a significant although somewhat artificial milestone. Ignition refers to an infinite Q, that is, a self-sustaining plasma where the losses are made up for by fusion power without any external input. In a practical fusion reactor, some external power will always be required for things like current drive, refueling, profile control, and burn control. A value on the order of Q = 20 will be required if the plant is to deliver much more energy than it uses internally. Despite many differences between possible designs of power plant, there are several systems that are common to most. A fusion power plant, like a fission power plant, is customarily divided into the nuclear island and the balance of plant. The balance of plant converts heat into electricity via steam turbines; it is a conventional design area and in principle similar to any other power station that relies on heat generation, whether fusion, fission or fossil fuel based. The nuclear island has a plasma chamber with an associated vacuum system, surrounded by plasma-facing components (first wall and divertor) maintaining the vacuum boundary and absorbing the thermal radiation coming from the plasma, surrounded in turn by a blanket where the neutrons are absorbed to breed tritium and heat a working fluid that transfers the power to the balance of plant. If magnetic confinement is used, a magnet system, using primarily cryogenic superconducting magnets, is needed, and usually systems for heating and refueling the plasma and for driving current. In inertial confinement, a driver (laser or accelerator) and a focusing system are needed, as well as a means for forming and positioning the pellets. Inertial confinement fusion implosion on the Nova laser creates "microsun" conditions of tremendously high density and temperature.were later demonstrated to be from different versions of the same instability processes that plagued earlier machines. Cockcroft was forced to retract the fusion claims, which tainted the entire field for years. ZETA ended its experiments in 1968, and most other pinch experiments ended shortly after. In 1974 a study of the ZETA results demonstrated an interesting side-effect; after the experimental runs ended, the plasma would enter a short period of stability. This led to the reversed field pinch concept which has seen some level of development ever since. Recent work on the basic concept started as a result of the appearance of the "wires array" concept in the 1980s, which allowed a more efficient use of this technique. The Sandia National Laboratory runs a continuing wire-array research program with the Zpinch machine. In addition, the University of Washington's ZaP Lab has shown quiescent periods of stability hundreds of times longer than expected for plasma in a Z-pinch configuration, giving promise to the confinement technique. In 1995, the staged Z-pinch concept was introduced by a team of scientist from University of California Irvine (UCI). This scheme can control one of the most dangerous instability that normally disintegrate conventional Z-pinch before the final implosion. The concept is based on a complex load of radiative liner plasma embedded with a target plasma. During implosion the outer surface of the liner plasma becomes unstable but the target plasma remains remarkably stable, up until the final implosion, generating a very high energy density stable target plasma. The heating mechanisms are shock heating, adiabatic compression and trapping of charge particles produced in fusion reaction due to a very strong magnetic field, which develops between the liner and the target. Details of this concept are shown in various publications available on the web page of MIFTI [2]. [edit] Early magnetic approaches The U.S. fusion program began in 1951 when Lyman Spitzer began work on a stellarator under the code name Project Matterhorn. His work led to the creation of the Princeton Plasma Physics Laboratory, where magnetically confined plasmas are still studied. Spitzer planned an aggressive development project of four machines, A, B, C, and D. A and B were small research devices, C would be the prototype of a power-producing machine, and D would be the prototype of a commercial device. A worked without issue, but even by the time B was being used it was clear the stellarator was also suffering from instabilities and plasma leakage. Progress on C slowed as attempts were made to correct for these problems. At Lawrence Livermore, the magnetic mirror was the preferred approach. The mirror consisted of two large magnets arranged so they had strong fields within them, and a weaker, but connected, field between them. Plasma introduced in the area between the two magnets would "bounce back" from the stronger fields in the middle. Although the design would leak plasma through the mirrors, the rate of leakage would be low enough that a useful fusion rate could be maintained. The simplicity of the design was supposed to make up for its lower performance. In practice the mirror also suffered from mysterious leakage problems, and never reached the expected performance. [edit] Gun Club, MHD, instability; progress slows By the mid-1950s it was clear that the simple theoretical tools being used to calculate the performance of all fusion machines were simply not predicting their actual behaviour. Machines invariably leaked their plasma from their confinement area at rates far higher than predicted. In 1954, Edward Teller held a gathering of fusion researchers at the Princeton Gun Club, near the Project Matterhorn (now known as Project Sherwood) grounds. Teller started by pointing out the problems that everyone was having, and suggested that any system where the plasma was confined within concave fields was doomed to fail. Attendees remember him saying something to the effect that the fields were like rubber bands, and they would attempt to snap back to a straight configuration whenever the power was increased, ejecting the plasma. He went on to say that it appeared the only way to confine the plasma in a stable configuration would be to use convex fields, a "cusp" configuration.[12] When the meeting concluded, most of the researchers quickly turned out papers saying why Teller's concerns did not apply to their particular device. The pinch machines did not use magnetic fields in this way at all, while the mirror and stellarator seemed to have various ways out. However, this was soon followed by a paper by Martin David Kruskal and Martin Schwarzschild discussing pinch machines, which demonstrated instabilities in those devices were inherent to the design. A series of similar studies followed, abandoning the simplistic theories previously used and introducing a full consideration of magnetohydrodynamics with a partially-resistive plasma. These concepts developed quickly, and by the early 1960s it was clear that small devices simply would not work. A series of much larger and more complex devices followed as researchers attempted to add field upon field in order to provide the required field strength without reaching the unstable regimes. As cost and complexity climbed, the initial optimism of the fusion field faded. [edit] The tokamak is announced A new approach was outlined in the theoretical works fulfilled in 1950–1951 by I.E. Tamm and A.D. Sakharov in the Soviet Union, which first discussed a tokamak-like approach. Experimental research on these designs began in 1956 at the Kurchatov Institute in Moscow by a group of Soviet scientists led by Lev Artsimovich. The tokamak essentially combined a low-power pinch device with a low-power simple stellarator. The key was to combine the fields in such a way that the particles wound around the reactor a particular number of times, today known as the "safety factor". The combination of these fields dramatically improved confinement times and densities, resulting in huge improvements over existing devices. The group constructed the first tokamaks, the most successful being the T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, producing the first quasistationary thermonuclear fusion reaction ever.[13] The tokamak was dramatically more efficient than the other approaches of that era, on the order of 10 to 100 times. When they were first announced the international community was highly skeptical. However, a British team was invited to see T-3, and having measured it in depth they released their results that confirmed the Soviet claims. A burst of activity followed as many planned devices were abandoned and new tokamaks were introduced in their place - the C model stellarator, then under construction after many redesigns, was quickly converted to the Symmetrical Tokamak and the stellarator was abandoned. Through the 1970s and 80s great strides in understanding the tokamak system were made. A number of improvements to the design are now part of the "advanced tokamak" concept, which includes non-circular plasmas, internal diverters and limiters, often superconducting magnets, and operate in the so-called "H-mode" island of increased stability. Two other designs have also become fairly well studied; the compact tokamak is wired with the magnets on the inside of the vacuum chamber, while the spherical tokamak reduces its cross section as much as possible. The tokamak dominates modern research, where very large devices like ITER are expected to pass several milestones toward commercial power production, including a burning plasma with long burn times, high power output, and online fueling. There are no guarantees that the project will be successful; previous generations of tokamak machines have uncovered new problems many times. But the entire field of high temperature plasmas is much better understood now than formerly, and there is considerable optimism that ITER will meet its goals. If successful, ITER would be followed by a "commercial demonstrator" system, similar in purpose to the very earliest power-producing fission reactors built in the era before wide-scale commercial deployment of larger machines started in the 1960s and 1970s. Even with these goals met, there are a number of major engineering problems remaining, notably finding suitable "low activity" materials for reactor construction, demonstrating secondary systems including practical tritium extraction, and building reactor designs that allow their reactor core to be removed when its materials becomes embrittled due to the neutron flux. Practical commercial generators based on the tokamak concept are far in the future. The public at large has been disappointed, as the initial outlook for practical fusion power plants was much rosier; a pamphlet from the 1970s printed by General Atomic stated that "Several commercial fusion reactors are expected to be online by the year 2000." [edit] Inertial (laser) containment The technique of implosion of a microcapsule irradiated by laser beams, the basis of laser inertial confinement, was first suggested in 1962 by scientists at Lawrence Livermore National Laboratory, shortly after the invention of the laser itself in 1960. Lasers of the era were very low powered, but low-level research using them nevertheless started as early as 1965. A great advance in the field was John Nuckolls' 1972 paper that ignition would require lasers of about 1 kJ, and efficient burn around 1 MJ. kJ lasers were just beyond the state of the art at the time, and his paper sparked off a tremendous development effort to produce devices of the needed power. Early machines used a variety of approaches to attack one of two problems - some focused on fast delivery of energy, while others were more interested in beam smoothness. Both were attempts to ensure the energy delivery would be smooth enough to cause an even implosion. However, these experiments demonstrated a serious problem; laser wavelengths in the infrared area lost a tremendous amount of energy before compressing the fuel. Important breakthroughs in this laser technology were made at the Laboratory for Laser Energetics at the University of Rochester, where scientists used frequency-tripling crystals to transform the infrared laser beams into ultraviolet beams. By the late 1970s great strides had been made in laser power, but with each increase new problems were found in the implosion technique that suggested even more power would be required. By the 1980s these increases were so large that using the concept for generating net energy seemed remote. Most research in this field turned to weapons research, always a second line of research, as the implosion concept is somewhat similar to hydrogen bomb operation. Work on very large versions continued as a result, with the very large National Ignition Facility in the US and Laser Mégajoule in France supporting these research programs. More recent work had demonstrated that significant savings in the required laser energy are possible using a technique known as "fast ignition". The savings are so dramatic that the concept appears to be a useful technique for energy production again, so much so that it is a serious contender for pre-commercial development. There are proposals to build an experimental facility dedicated to the fast ignition approach, known as HiPER. At the same time, advances in solid state lasers appear to improve the "driver" systems' efficiency by about ten times (to 10- 20%), savings that make even the large "traditional" machines almost practical, and might make the fast ignition concept outpace the magnetic approaches in further development. The laser-based concept has other advantages. The reactor core is mostly exposed, as opposed to being wrapped in a huge magnet as in the tokamak. This makes the problem of removing energy from the system somewhat simpler, and should mean that a laser-based device would be much easier to perform maintenance on, such as core replacement. Additionally, the lack of strong magnetic fields allows for a wider variety of low-activation materials, including carbon fiber, which would reduce both the frequency of such neutron activations and the rate of irradiation to the core. In other ways the program has many of the same problems as the tokamak; practical methods of energy removal and tritium recycling need to be demonstrated. [edit] Other approaches Over the years there have been a wide variety of fusion concepts. In general they fall into three groups - those that attempt to reach high temperature/density for brief times (pinch, inertial confinement), those that operate at a steady state (magnetic confinement) or those that try neither and instead attempt to produce low quantities of fusion but do so at an extremely low cost. The latter group has largely disappeared, as the difficulties of achieving fusion have demonstrated that any low-energy device is unlikely to produce net gain. This leaves the two major approaches, magnetic and laser inertial, as the leading systems for development funding. However, alternate approaches continue to be developed, and alternate non-power fusion devices have been successfully developed as well. [edit] Technically viable Philo T. Farnsworth, the inventor of the first all-electronic television system in 1927, patented his first Fusor design in 1968, a device that uses inertial electrostatic confinement. This system consists largely of two concentric spherical electrical grids inside a vacuum chamber into which a small amount of fusion fuel is introduced. Voltage across the grids causes the fuel to ionize around them, and positively charged ions are accelerated towards the center of the chamber. Those ions may collide and fuse with ions coming from the other direction, may scatter without fusing, or may pass directly through. In the latter two cases, the ions will tend to be stopped by the electric field and re-accelerated toward the center. Fusors can also use ion guns rather than electric grids. Towards the end of the 1960s, Robert Hirsch designed a variant of the Farnsworth Fusor known as the Hirsch-Meeks fusor. This variant is a considerable improvement over the Farnsworth design, and is able to generate neutron flux in the order of one billion neutrons per second. Although the efficiency was very low at first, there were hopes the device could be scaled up, but continued development demonstrated that this approach would be impractical for large machines. Nevertheless, fusion could be achieved using a "lab bench top" type set up for the first time, at minimal cost. This type of fusor found its first application as a portable neutron generator in the late 1990s. An automated sealed reaction chamber version of this device, commercially named Fusionstar was developed by EADS but abandoned in 2001. Its successor is the NSD-Fusion neutron generator. Robert W. Bussard's Polywell concept is roughly similar to that of the Fusor, but replaces the problematic grid with a magnetically contained electron cloud, which holds the ions in position and provides an accelerating potential. The polywell consists of electromagnet coils arranged in a polyhedral configuration and positively charged to between several tens and low hundreds of kilovolts. This charged magnetic polyhedron is called a MaGrid (Magnetic Grid). Electrons are introduced outside the "quasi-spherical" MaGrid and are accelerated into the MaGrid due to the electric field, similar to a magnetic bottle. Within the MaGrid, magnetic fields confine most of the electrons and those that escape are retained by the electric field. This configuration traps the electrons in the middle of the device, focusing them near the center to produce a virtual cathode (negative electric potential). The virtual cathode accelerates and confines the ions to be fused which, except for minimal losses, never reach the physical structure of the MaGrid. Bussard had reported a fusion rate of 109 per second running D-D fusion reactions at only 12.5 kV (based on detecting a total of nine neutrons in five tests. Bussard claimed a scaled-up version of 2.5–3 m in diameter, would operated at over 100 MW net power (fusion power scales as the fourth power of the B field and the cube of the size)[14] A recent[when?] area of study is the magneto-inertial fusion (MIF) concept, which combines some form of external inertial compression (like lasers) with further compression through an external magnet (like pinch devices). The magnetic field traps heat within the inertial core, causing a variety of effects that improves fusion rates. These improvements are relatively minor, however the magnetic drivers themselves are inexpensive compared to lasers or other systems. There is hope for a sweet spot that allows the combination of features from these devices to create low-density but also low-cost fusion devices. A similar concept is the magnetized target fusion device, which uses a magnetic field in an external metal shell to achieve the same basic goals. According to Eric Lerner, Focus fusion takes place in a dense plasma focus,[15] which typically consists of two coaxial cylindrical electrodes made from copper or beryllium and housed in a vacuum chamber containing a low-pressure gas, which is used as the reactor fuel. An electrical pulse is applied across the electrodes, producing heating and a magnetic field. The current forms the hot gas into many minuscule vortices perpendicular to the surfaces of the electrodes, which then migrate to the end of the inner electrode to pinch-and-twist off as tiny balls of plasma called plasmoids. The electron beam collides with the plasmoid, heating it to fusion temperatures. This will, in principle, yield more energy in the beams than was input to form them.[citation needed] [edit] Non-power generating approaches A more subtle technique is to use more unusual particles to catalyse fusion. The best known of these is Muon-catalyzed fusion which uses muons, which behave somewhat like electrons and replace the electrons around the atoms. These muons allow atoms to get much closer and thus reduce the kinetic energy required to initiate fusion. Muons require more energy to produce than can be obtained from muon-catalysed fusion, making this approach impractical for the generation of power. In April 2005, a team from UCLA announced it had devised a way of producing fusion using a machine that "fits on a lab bench", using lithium tantalate to generate enough voltage to smash deuterium atoms together. However, the process does not generate net power. See Pyroelectric fusion. Such a device would be useful in the same sort of roles as the fusor. [edit] Abandoned and discredited approaches Some scientists reported excess heat, neutrons, tritium, helium and other nuclear effects in so-called cold fusion systems, which for a time gained interest as showing promise. Hopes fell when replication failures were weighed in view of several reasons cold fusion is not likely to occur, the discovery of possible sources of experimental error, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts.[16] By late 1989, most scientists considered cold fusion claims dead,[17] and cold fusion subsequently gained a reputation as pathological science.[18] However, a small community of researchers continues to investigate cold fusion[17][19][20][21] claiming to replicate Fleishmann and Pons' results including nuclear reaction byproducts.[22][23] Claims related to cold fusion are largely disbelieved in the mainstream scientific community.[24] In 1989, the majority of a review panel organized by the US Department of Energy (DOE) found that the evidence for the discovery of a new nuclear process was not persuasive. A second DOE review, convened in 2004 to look at new research, reached conclusions similar to the first.[25] Research into sonoluminescence induced fusion, sometimes known as "bubble fusion", also continues, although it is met with as much skepticism as cold fusion is by most of the scientific community. [edit] Key design areas [edit] Confinement Parameter space occupied by inertial fusion energy and magnetic fusion energy devices as of the mid 1990s. The regime allowing thermonuclear ignition with high gain lies near the upper right corner of the plot. Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion: * Equilibrium: There must be no net forces on any part of the plasma, otherwise it will rapidly disassemble. The exception, of course, is inertial confinement, where the relevant physics must occur faster than the disassembly time. * Stability: The plasma must be so constructed that small deviations are restored to the initial state, otherwise some unavoidable disturbance will occur and grow exponentially until the plasma is destroyed. * Transport: The loss of particles and heat in all channels must be sufficiently slow. The word "confinement" is often used in the restricted sense of "energy confinement". The first human-made, large-scale fusion reaction was the test of the hydrogen bomb, Ivy Mike, in 1952. As part of the PACER project, it was once proposed to use hydrogen bombs as a source of power by detonating them in underground caverns and then generating electricity from the heat produced, but such a power plant is unlikely ever to be constructed, for a variety of reasons.[citation needed] Controlled thermonuclear fusion (CTF) refers to the alternative of continuous power production, or at least the use of explosions that are so small that they do not destroy a significant portion of the machine that produces them.[citation needed] To produce self-sustaining fusion, the energy released by the reaction (or at least a fraction of it) must be used to heat new reactant nuclei and keep them hot long enough that they also undergo fusion reactions. Retaining the heat is called energy confinement and may be accomplished in a number of ways.[citation needed] The hydrogen bomb really has no confinement at all. The fuel is simply allowed to fly apart, but it takes a certain length of time to do this, and during this time fusion can occur. This approach is called inertial confinement. If more than milligram quantities of fuel are used (and efficiently fused), the explosion would destroy the machine, so theoretically, controlled thermonuclear fusion using inertial confinement would be done using tiny pellets of fuel which explode several times a second. To induce the explosion, the pellet must be compressed to about 30 times solid density with energetic beams. If the beams are focused directly on the pellet, it is called direct drive, which can in principle be very efficient, but in practice it is difficult to obtain the needed uniformity.[citation needed] An alternative approach is indirect drive, in which the beams heat a shell, and the shell radiates x-rays, which then implode the pellet. The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been investigated.[citation needed] Inertial confinement produces plasmas with impressively high densities and temperatures, and appears to be best suited to weapons research, X-ray generation, very small reactors, and perhaps in the distant future, spaceflight.[citation needed] They require fuel pellets with close to a perfect shape in order to generate a symmetrical inward shock wave to produce the high-density plasma, and in practice these have proven difficult to produce. A recent development in the field of laser induced ICF is the use of ultrashort pulse multi-petawatt lasers to heat the plasma of an imploding pellet at exactly the moment of greatest density after it is imploded conventionally using terawatt scale lasers. This research will be carried out on the (currently being built) OMEGA EP petawatt and OMEGA lasers at the University of Rochester and at the GEKKO XII laser at the institute for laser engineering in Osaka Japan, which if fruitful, may have the effect of greatly reducing the cost of a laser fusion based power source.[citation needed] At the temperatures required for fusion, the fuel is in the form of a plasma with very good electrical conductivity. This opens the possibility to confine the fuel and the energy with magnetic fields, an idea known as magnetic confinement. The Lorenz force works only perpendicular to the magnetic field, so that the first problem is how to prevent the plasma from leaking out the ends of the field lines. There are basically two solutions.[citation needed] The first is to use the magnetic mirror effect. If particles following a field line encounter a region of higher field strength, then some of the particles will be stopped and reflected. Advantages of a magnetic mirror power plant would be simplified construction and maintenance due to a linear topology and the potential to apply direct conversion in a natural way, but the confinement achieved in the experiments was so poor that this approach has been essentially abandoned.[citation needed] The second possibility to prevent end losses is to bend the field lines back on themselves, either in circles or more commonly in nested toroidal surfaces. The most highly developed system of this type is the tokamak, with the stellarator being next most advanced, followed by the Reversed field pinch. Compact toroids, especially the Field-Reversed Configuration and the spheromak, attempt to combine the advantages of toroidal magnetic surfaces with those of a simply connected (non-toroidal) machine, resulting in a mechanically simpler and smaller confinement area. Compact toroids still have some enthusiastic supporters but are not backed as readily by the majority of the fusion community.[citation needed] Finally, there are also electrostatic confinement fusion systems, in which ions in the reaction chamber are confined and held at the center of the device by electrostatic forces, as in the Farnsworth-Hirsch Fusor, which is not believed to be able to be developed into a power plant. The Polywell, an advanced variant of the fusor,[citation needed] has shown a degree of research interest as of late; however, the technology is relatively immature,[citation needed] and major scientific and engineering questions remain which researchers under the auspices of the U.S. Office of Naval Research hope to further investigate.[citation needed] [edit] Materials Main article: International Fusion Materials Irradiation Facility Developing materials for fusion reactors has long been recognized as a problem nearly as difficult and important as that of plasma confinement, but it has received only a fraction of the attention. The neutron flux in a fusion reactor is expected to be about 100 times that in existing pressurized water reactors (PWR). Each atom in the blanket of a fusion reactor is expected to be hit by a neutron and displaced about a hundred times before the material is replaced. Furthermore the high-energy neutrons will produce hydrogen and helium in various nuclear reactions that tends to form bubbles at grain boundaries and result in swelling, blistering or embrittlement. One also wishes to choose materials whose primary components and impurities do not result in long-lived radioactive wastes. Finally, the mechanical forces and temperatures are large, and there may be frequent cycling of both. The problem is exacerbated because realistic material tests must expose samples to neutron fluxes of a similar level for a similar length of time as those expected in a fusion power plant. Such a neutron source is nearly as complicated and expensive as a fusion reactor itself would be. Proper materials testing will not be possible in ITER, and a proposed materials testing facility, IFMIF, was still at the design stage in 2005. The material of the plasma facing components (PFC) is a special problem. The PFC do not have to withstand large mechanical loads, so neutron damage is much less of an issue. They do have to withstand extremely large thermal loads, up to 10 MW/m², which is a difficult but solvable problem. Regardless of the material chosen, the heat flux can only be accommodated without melting if the distance from the front surface to the coolant is not more than a centimeter or two. The primary issue is the interaction with the plasma. One can choose either a low-Z material, typified by graphite although for some purposes beryllium might be chosen, or a high-Z material, usually tungsten with molybdenum as a second choice. Use of liquid metals (lithium, gallium, tin) has also been proposed, e.g., by injection of 1–5 mm thick streams flowing at 10 m/s on solid substrates. If graphite is used, the gross erosion rates due to physical and chemical sputtering would be many meters per year, so one must rely on redeposition of the sputtered material. The location of the redeposition will not exactly coincide with the location of the sputtering, so one is still left with erosion rates that may be prohibitive. An even larger problem is the tritium co-deposited with the redeposited graphite. The tritium inventory in graphite layers and dust in a reactor could quickly build up to many kilograms, representing a waste of resources and a serious radiological hazard in case of an accident. The consensus of the fusion community seems to be that graphite, although a very attractive material for fusion experiments, cannot be the primary PFC material in a commercial reactor. The sputtering rate of tungsten can be orders of magnitude smaller than that of carbon, and tritium is not so easily incorporated into redeposited tungsten, making this a more attractive choice. On the other hand, tungsten impurities in a plasma are much more damaging than carbon impurities, and self-sputtering of tungsten can be high, so it will be necessary to ensure that the plasma in contact with the tungsten is not too hot (a few tens of eV rather than hundreds of eV). Tungsten also has disadvantages in terms of eddy currents and melting in off-normal events, as well as some radiological issues. [edit] Subsystems In fusion research, achieving a fusion energy gain factor Q = 1 is called breakeven and is considered a significant although somewhat artificial milestone. Ignition refers to an infinite Q, that is, a self-sustaining plasma where the losses are made up for by fusion power without any external input. In a practical fusion reactor, some external power will always be required for things like current drive, refueling, profile control, and burn control. A value on the order of Q = 20 will be required if the plant is to deliver much more energy than it uses internally. Despite many differences between possible designs of power plant, there are several systems that are common to most. A fusion power plant, like a fission power plant, is customarily divided into the nuclear island and the balance of plant. The balance of plant converts heat into electricity via steam turbines; it is a conventional design area and in principle similar to any other power station that relies on heat generation, whether fusion, fission or fossil fuel based. The nuclear island has a plasma chamber with an associated vacuum system, surrounded by plasma-facing components (first wall and divertor) maintaining the vacuum boundary and absorbing the thermal radiation coming from the plasma, surrounded in turn by a blanket where the neutrons are absorbed to breed tritium and heat a working fluid that transfers the power to the balance of plant. If magnetic confinement is used, a magnet system, using primarily cryogenic superconducting magnets, is needed, and usually systems for heating and refueling the plasma and for driving current. In inertial confinement, a driver (laser or accelerator) and a focusing system are needed, as well as a means for forming and positioning the pellets. Inertial confinement fusion implosion on the Nova laser creates "microsun" conditions of tremendously high density and temperature.

were later demonstrated to be from different versions of the same instability processes that plagued earlier machines. Cockcroft was forced to retract the fusion claims, which tainted the entire field for years. ZETA ended its experiments in 1968, and most other pinch experiments ended shortly after. In 1974 a study of the ZETA results demonstrated an interesting side-effect; after the experimental runs ended, the plasma would enter a short period of stability. This led to the reversed field pinch concept which has seen some level of development ever since. Recent work on the basic concept started as a result of the appearance of the "wires array" concept in the 1980s, which allowed a more efficient use of this technique. The Sandia National Laboratory runs a continuing wire-array research program with the Zpinch machine. In addition, the University of Washington's ZaP Lab has shown quiescent periods of stability hundreds of times longer than expected for plasma in a Z-pinch configuration, giving promise to the confinement technique.
In 1995, the staged Z-pinch concept was introduced by a team of scientist from University of California Irvine (UCI). This scheme can control one of the most dangerous instability that normally disintegrate conventional Z-pinch before the final implosion. The concept is based on a complex load of radiative liner plasma embedded with a target plasma. During implosion the outer surface of the liner plasma becomes unstable but the target plasma remains remarkably stable, up until the final implosion, generating a very high energy density stable target plasma. The heating mechanisms are shock heating, adiabatic compression and trapping of charge particles produced in fusion reaction due to a very strong magnetic field, which develops between the liner and the target. Details of this concept are shown in various publications available on the web page of MIFTI [2].

[edit] Early magnetic approaches

The U.S. fusion program began in 1951 when Lyman Spitzer began work on a stellarator under the code name Project Matterhorn. His work led to the creation of the Princeton Plasma Physics Laboratory, where magnetically confined plasmas are still studied. Spitzer planned an aggressive development project of four machines, A, B, C, and D. A and B were small research devices, C would be the prototype of a power-producing machine, and D would be the prototype of a commercial device. A worked without issue, but even by the time B was being used it was clear the stellarator was also suffering from instabilities and plasma leakage. Progress on C slowed as attempts were made to correct for these problems.
At Lawrence Livermore, the magnetic mirror was the preferred approach. The mirror consisted of two large magnets arranged so they had strong fields within them, and a weaker, but connected, field between them. Plasma introduced in the area between the two magnets would "bounce back" from the stronger fields in the middle. Although the design would leak plasma through the mirrors, the rate of leakage would be low enough that a useful fusion rate could be maintained. The simplicity of the design was supposed to make up for its lower performance. In practice the mirror also suffered from mysterious leakage problems, and never reached the expected performance.

[edit] Gun Club, MHD, instability; progress slows

By the mid-1950s it was clear that the simple theoretical tools being used to calculate the performance of all fusion machines were simply not predicting their actual behaviour. Machines invariably leaked their plasma from their confinement area at rates far higher than predicted.
In 1954, Edward Teller held a gathering of fusion researchers at the Princeton Gun Club, near the Project Matterhorn (now known as Project Sherwood) grounds. Teller started by pointing out the problems that everyone was having, and suggested that any system where the plasma was confined within concave fields was doomed to fail. Attendees remember him saying something to the effect that the fields were like rubber bands, and they would attempt to snap back to a straight configuration whenever the power was increased, ejecting the plasma. He went on to say that it appeared the only way to confine the plasma in a stable configuration would be to use convex fields, a "cusp" configuration.^[12]
When the meeting concluded, most of the researchers quickly turned out papers saying why Teller's concerns did not apply to their particular device. The pinch machines did not use magnetic fields in this way at all, while the mirror and stellarator seemed to have various ways out. However, this was soon followed by a paper by Martin David Kruskal and Martin Schwarzschild discussing pinch machines, which demonstrated instabilities in those devices were inherent to the design. A series of similar studies followed, abandoning the simplistic theories previously used and introducing a full consideration of magnetohydrodynamics with a partially-resistive plasma. These concepts developed quickly, and by the early 1960s it was clear that small devices simply would not work. A series of much larger and more complex devices followed as researchers attempted to add field upon field in order to provide the required field strength without reaching the unstable regimes. As cost and complexity climbed, the initial optimism of the fusion field faded.

[edit] The tokamak is announced

A new approach was outlined in the theoretical works fulfilled in 1950–1951 by I.E. Tamm and A.D. Sakharov in the Soviet Union, which first discussed a tokamak-like approach. Experimental research on these designs began in 1956 at the Kurchatov Institute in Moscow by a group of Soviet scientists led by Lev Artsimovich. The tokamak essentially combined a low-power pinch device with a low-power simple stellarator. The key was to combine the fields in such a way that the particles wound around the reactor a particular number of times, today known as the "safety factor". The combination of these fields dramatically improved confinement times and densities, resulting in huge improvements over existing devices.
The group constructed the first tokamaks, the most successful being the T-3 and its larger version T-4. T-4 was tested in 1968 in Novosibirsk, producing the first quasistationary thermonuclear fusion reaction ever.^[13] The tokamak was dramatically more efficient than the other approaches of that era, on the order of 10 to 100 times. When they were first announced the international community was highly skeptical. However, a British team was invited to see T-3, and having measured it in depth they released their results that confirmed the Soviet claims. A burst of activity followed as many planned devices were abandoned and new tokamaks were introduced in their place - the C model stellarator, then under construction after many redesigns, was quickly converted to the Symmetrical Tokamak and the stellarator was abandoned.
Through the 1970s and 80s great strides in understanding the tokamak system were made. A number of improvements to the design are now part of the "advanced tokamak" concept, which includes non-circular plasmas, internal diverters and limiters, often superconducting magnets, and operate in the so-called "H-mode" island of increased stability. Two other designs have also become fairly well studied; the compact tokamak is wired with the magnets on the inside of the vacuum chamber, while the spherical tokamak reduces its cross section as much as possible.
The tokamak dominates modern research, where very large devices like ITER are expected to pass several milestones toward commercial power production, including a burning plasma with long burn times, high power output, and online fueling. There are no guarantees that the project will be successful; previous generations of tokamak machines have uncovered new problems many times. But the entire field of high temperature plasmas is much better understood now than formerly, and there is considerable optimism that ITER will meet its goals. If successful, ITER would be followed by a "commercial demonstrator" system, similar in purpose to the very earliest power-producing fission reactors built in the era before wide-scale commercial deployment of larger machines started in the 1960s and 1970s.
Even with these goals met, there are a number of major engineering problems remaining, notably finding suitable "low activity" materials for reactor construction, demonstrating secondary systems including practical tritium extraction, and building reactor designs that allow their reactor core to be removed when its materials becomes embrittled due to the neutron flux. Practical commercial generators based on the tokamak concept are far in the future. The public at large has been disappointed, as the initial outlook for practical fusion power plants was much rosier; a pamphlet from the 1970s printed by General Atomic stated that "Several commercial fusion reactors are expected to be online by the year 2000."

[edit] Inertial (laser) containment

The technique of implosion of a microcapsule irradiated by laser beams, the basis of laser inertial confinement, was first suggested in 1962 by scientists at Lawrence Livermore National Laboratory, shortly after the invention of the laser itself in 1960. Lasers of the era were very low powered, but low-level research using them nevertheless started as early as 1965. A great advance in the field was John Nuckolls' 1972 paper that ignition would require lasers of about 1 kJ, and efficient burn around 1 MJ. kJ lasers were just beyond the state of the art at the time, and his paper sparked off a tremendous development effort to produce devices of the needed power.
Early machines used a variety of approaches to attack one of two problems - some focused on fast delivery of energy, while others were more interested in beam smoothness. Both were attempts to ensure the energy delivery would be smooth enough to cause an even implosion. However, these experiments demonstrated a serious problem; laser wavelengths in the infrared area lost a tremendous amount of energy before compressing the fuel. Important breakthroughs in this laser technology were made at the Laboratory for Laser Energetics at the University of Rochester, where scientists used frequency-tripling crystals to transform the infrared laser beams into ultraviolet beams. By the late 1970s great strides had been made in laser power, but with each increase new problems were found in the implosion technique that suggested even more power would be required. By the 1980s these increases were so large that using the concept for generating net energy seemed remote. Most research in this field turned to weapons research, always a second line of research, as the implosion concept is somewhat similar to hydrogen bomb operation. Work on very large versions continued as a result, with the very large National Ignition Facility in the US and Laser Mégajoule in France supporting these research programs.
More recent work had demonstrated that significant savings in the required laser energy are possible using a technique known as "fast ignition". The savings are so dramatic that the concept appears to be a useful technique for energy production again, so much so that it is a serious contender for pre-commercial development. There are proposals to build an experimental facility dedicated to the fast ignition approach, known as HiPER. At the same time, advances in solid state lasers appear to improve the "driver" systems' efficiency by about ten times (to 10- 20%), savings that make even the large "traditional" machines almost practical, and might make the fast ignition concept outpace the magnetic approaches in further development.
The laser-based concept has other advantages. The reactor core is mostly exposed, as opposed to being wrapped in a huge magnet as in the tokamak. This makes the problem of removing energy from the system somewhat simpler, and should mean that a laser-based device would be much easier to perform maintenance on, such as core replacement. Additionally, the lack of strong magnetic fields allows for a wider variety of low-activation materials, including carbon fiber, which would reduce both the frequency of such neutron activations and the rate of irradiation to the core. In other ways the program has many of the same problems as the tokamak; practical methods of energy removal and tritium recycling need to be demonstrated.

[edit] Other approaches

Over the years there have been a wide variety of fusion concepts. In general they fall into three groups - those that attempt to reach high temperature/density for brief times (pinch, inertial confinement), those that operate at a steady state (magnetic confinement) or those that try neither and instead attempt to produce low quantities of fusion but do so at an extremely low cost. The latter group has largely disappeared, as the difficulties of achieving fusion have demonstrated that any low-energy device is unlikely to produce net gain. This leaves the two major approaches, magnetic and laser inertial, as the leading systems for development funding. However, alternate approaches continue to be developed, and alternate non-power fusion devices have been successfully developed as well.

[edit] Technically viable

Philo T. Farnsworth, the inventor of the first all-electronic television system in 1927, patented his first Fusor design in 1968, a device that uses inertial electrostatic confinement. This system consists largely of two concentric spherical electrical grids inside a vacuum chamber into which a small amount of fusion fuel is introduced. Voltage across the grids causes the fuel to ionize around them, and positively charged ions are accelerated towards the center of the chamber. Those ions may collide and fuse with ions coming from the other direction, may scatter without fusing, or may pass directly through. In the latter two cases, the ions will tend to be stopped by the electric field and re-accelerated toward the center. Fusors can also use ion guns rather than electric grids. Towards the end of the 1960s, Robert Hirsch designed a variant of the Farnsworth Fusor known as the Hirsch-Meeks fusor. This variant is a considerable improvement over the Farnsworth design, and is able to generate neutron flux in the order of one billion neutrons per second. Although the efficiency was very low at first, there were hopes the device could be scaled up, but continued development demonstrated that this approach would be impractical for large machines. Nevertheless, fusion could be achieved using a "lab bench top" type set up for the first time, at minimal cost. This type of fusor found its first application as a portable neutron generator in the late 1990s. An automated sealed reaction chamber version of this device, commercially named Fusionstar was developed by EADS but abandoned in 2001. Its successor is the NSD-Fusion neutron generator.
Robert W. Bussard's Polywell concept is roughly similar to that of the Fusor, but replaces the problematic grid with a magnetically contained electron cloud, which holds the ions in position and provides an accelerating potential. The polywell consists of electromagnet coils arranged in a polyhedral configuration and positively charged to between several tens and low hundreds of kilovolts. This charged magnetic polyhedron is called a MaGrid (Magnetic Grid). Electrons are introduced outside the "quasi-spherical" MaGrid and are accelerated into the MaGrid due to the electric field, similar to a magnetic bottle. Within the MaGrid, magnetic fields confine most of the electrons and those that escape are retained by the electric field. This configuration traps the electrons in the middle of the device, focusing them near the center to produce a virtual cathode (negative electric potential). The virtual cathode accelerates and confines the ions to be fused which, except for minimal losses, never reach the physical structure of the MaGrid. Bussard had reported a fusion rate of 10⁹ per second running D-D fusion reactions at only 12.5 kV (based on detecting a total of nine neutrons in five tests. Bussard claimed a scaled-up version of 2.5–3 m in diameter, would operated at over 100 MW net power (fusion power scales as the fourth power of the B field and the cube of the size)^[14]
A recent^[when?] area of study is the magneto-inertial fusion (MIF) concept, which combines some form of external inertial compression (like lasers) with further compression through an external magnet (like pinch devices). The magnetic field traps heat within the inertial core, causing a variety of effects that improves fusion rates. These improvements are relatively minor, however the magnetic drivers themselves are inexpensive compared to lasers or other systems. There is hope for a sweet spot that allows the combination of features from these devices to create low-density but also low-cost fusion devices. A similar concept is the magnetized target fusion device, which uses a magnetic field in an external metal shell to achieve the same basic goals.
According to Eric Lerner, Focus fusion takes place in a dense plasma focus,^[15] which typically consists of two coaxial cylindrical electrodes made from copper or beryllium and housed in a vacuum chamber containing a low-pressure gas, which is used as the reactor fuel. An electrical pulse is applied across the electrodes, producing heating and a magnetic field. The current forms the hot gas into many minuscule vortices perpendicular to the surfaces of the electrodes, which then migrate to the end of the inner electrode to pinch-and-twist off as tiny balls of plasma called plasmoids. The electron beam collides with the plasmoid, heating it to fusion temperatures. This will, in principle, yield more energy in the beams than was input to form them.^{[citation needed]}

[edit] Non-power generating approaches

A more subtle technique is to use more unusual particles to catalyse fusion. The best known of these is Muon-catalyzed fusion which uses muons, which behave somewhat like electrons and replace the electrons around the atoms. These muons allow atoms to get much closer and thus reduce the kinetic energy required to initiate fusion. Muons require more energy to produce than can be obtained from muon-catalysed fusion, making this approach impractical for the generation of power.
In April 2005, a team from UCLA announced it had devised a way of producing fusion using a machine that "fits on a lab bench", using lithium tantalate to generate enough voltage to smash deuterium atoms together. However, the process does not generate net power. See Pyroelectric fusion. Such a device would be useful in the same sort of roles as the fusor.

[edit] Abandoned and discredited approaches

Some scientists reported excess heat, neutrons, tritium, helium and other nuclear effects in so-called cold fusion systems, which for a time gained interest as showing promise. Hopes fell when replication failures were weighed in view of several reasons cold fusion is not likely to occur, the discovery of possible sources of experimental error, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts.^[16] By late 1989, most scientists considered cold fusion claims dead,^[17] and cold fusion subsequently gained a reputation as pathological science.^[18] However, a small community of researchers continues to investigate cold fusion^{[17][19][20][21]} claiming to replicate Fleishmann and Pons' results including nuclear reaction byproducts.^[22][23] Claims related to cold fusion are largely disbelieved in the mainstream scientific community.^[24] In 1989, the majority of a review panel organized by the US Department of Energy (DOE) found that the evidence for the discovery of a new nuclear process was not persuasive. A second DOE review, convened in 2004 to look at new research, reached conclusions similar to the first.^[25]
Research into sonoluminescence induced fusion, sometimes known as "bubble fusion", also continues, although it is met with as much skepticism as cold fusion is by most of the scientific community.

[edit] Key design areas

[edit] Confinement

Parameter space occupied by inertial fusion energy and magnetic fusion energy devices as of the mid 1990s. The regime allowing thermonuclear ignition with high gain lies near the upper right corner of the plot.

Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion:

• Equilibrium: There must be no net forces on any part of the plasma, otherwise it will rapidly disassemble. The exception, of course, is inertial confinement, where the relevant physics must occur faster than the disassembly time.
• Stability: The plasma must be so constructed that small deviations are restored to the initial state, otherwise some unavoidable disturbance will occur and grow exponentially until the plasma is destroyed.
• Transport: The loss of particles and heat in all channels must be sufficiently slow. The word "confinement" is often used in the restricted sense of "energy confinement".

The first human-made, large-scale fusion reaction was the test of the hydrogen bomb, Ivy Mike, in 1952. As part of the PACER project, it was once proposed to use hydrogen bombs as a source of power by detonating them in underground caverns and then generating electricity from the heat produced, but such a power plant is unlikely ever to be constructed, for a variety of reasons.^{[citation needed]} Controlled thermonuclear fusion (CTF) refers to the alternative of continuous power production, or at least the use of explosions that are so small that they do not destroy a significant portion of the machine that produces them.^{[citation needed]}
To produce self-sustaining fusion, the energy released by the reaction (or at least a fraction of it) must be used to heat new reactant nuclei and keep them hot long enough that they also undergo fusion reactions. Retaining the heat is called energy confinement and may be accomplished in a number of ways.^{[citation needed]}
The hydrogen bomb really has no confinement at all. The fuel is simply allowed to fly apart, but it takes a certain length of time to do this, and during this time fusion can occur. This approach is called inertial confinement. If more than milligram quantities of fuel are used (and efficiently fused), the explosion would destroy the machine, so theoretically, controlled thermonuclear fusion using inertial confinement would be done using tiny pellets of fuel which explode several times a second. To induce the explosion, the pellet must be compressed to about 30 times solid density with energetic beams. If the beams are focused directly on the pellet, it is called direct drive, which can in principle be very efficient, but in practice it is difficult to obtain the needed uniformity.^{[citation needed]} An alternative approach is indirect drive, in which the beams heat a shell, and the shell radiates x-rays, which then implode the pellet. The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been investigated.^{[citation needed]}
Inertial confinement produces plasmas with impressively high densities and temperatures, and appears to be best suited to weapons research, X-ray generation, very small reactors, and perhaps in the distant future, spaceflight.^{[citation needed]} They require fuel pellets with close to a perfect shape in order to generate a symmetrical inward shock wave to produce the high-density plasma, and in practice these have proven difficult to produce. A recent development in the field of laser induced ICF is the use of ultrashort pulse multi-petawatt lasers to heat the plasma of an imploding pellet at exactly the moment of greatest density after it is imploded conventionally using terawatt scale lasers. This research will be carried out on the (currently being built) OMEGA EP petawatt and OMEGA lasers at the University of Rochester and at the GEKKO XII laser at the institute for laser engineering in Osaka Japan, which if fruitful, may have the effect of greatly reducing the cost of a laser fusion based power source.^{[citation needed]}
At the temperatures required for fusion, the fuel is in the form of a plasma with very good electrical conductivity. This opens the possibility to confine the fuel and the energy with magnetic fields, an idea known as magnetic confinement. The Lorenz force works only perpendicular to the magnetic field, so that the first problem is how to prevent the plasma from leaking out the ends of the field lines. There are basically two solutions.^{[citation needed]}
The first is to use the magnetic mirror effect. If particles following a field line encounter a region of higher field strength, then some of the particles will be stopped and reflected. Advantages of a magnetic mirror power plant would be simplified construction and maintenance due to a linear topology and the potential to apply direct conversion in a natural way, but the confinement achieved in the experiments was so poor that this approach has been essentially abandoned.^{[citation needed]}
The second possibility to prevent end losses is to bend the field lines back on themselves, either in circles or more commonly in nested toroidal surfaces. The most highly developed system of this type is the tokamak, with the stellarator being next most advanced, followed by the Reversed field pinch. Compact toroids, especially the Field-Reversed Configuration and the spheromak, attempt to combine the advantages of toroidal magnetic surfaces with those of a simply connected (non-toroidal) machine, resulting in a mechanically simpler and smaller confinement area. Compact toroids still have some enthusiastic supporters but are not backed as readily by the majority of the fusion community.^{[citation needed]}
Finally, there are also electrostatic confinement fusion systems, in which ions in the reaction chamber are confined and held at the center of the device by electrostatic forces, as in the Farnsworth-Hirsch Fusor, which is not believed to be able to be developed into a power plant. The Polywell, an advanced variant of the fusor,^{[citation needed]} has shown a degree of research interest as of late; however, the technology is relatively immature,^{[citation needed]} and major scientific and engineering questions remain which researchers under the auspices of the U.S. Office of Naval Research hope to further investigate.^{[citation needed]}

[edit] Materials

Main article: International Fusion Materials Irradiation Facility

Developing materials for fusion reactors has long been recognized as a problem nearly as difficult and important as that of plasma confinement, but it has received only a fraction of the attention. The neutron flux in a fusion reactor is expected to be about 100 times that in existing pressurized water reactors (PWR). Each atom in the blanket of a fusion reactor is expected to be hit by a neutron and displaced about a hundred times before the material is replaced. Furthermore the high-energy neutrons will produce hydrogen and helium in various nuclear reactions that tends to form bubbles at grain boundaries and result in swelling, blistering or embrittlement. One also wishes to choose materials whose primary components and impurities do not result in long-lived radioactive wastes. Finally, the mechanical forces and temperatures are large, and there may be frequent cycling of both.
The problem is exacerbated because realistic material tests must expose samples to neutron fluxes of a similar level for a similar length of time as those expected in a fusion power plant. Such a neutron source is nearly as complicated and expensive as a fusion reactor itself would be. Proper materials testing will not be possible in ITER, and a proposed materials testing facility, IFMIF, was still at the design stage in 2005.
The material of the plasma facing components (PFC) is a special problem. The PFC do not have to withstand large mechanical loads, so neutron damage is much less of an issue. They do have to withstand extremely large thermal loads, up to 10 MW/m², which is a difficult but solvable problem. Regardless of the material chosen, the heat flux can only be accommodated without melting if the distance from the front surface to the coolant is not more than a centimeter or two. The primary issue is the interaction with the plasma. One can choose either a low-Z material, typified by graphite although for some purposes beryllium might be chosen, or a high-Z material, usually tungsten with molybdenum as a second choice. Use of liquid metals (lithium, gallium, tin) has also been proposed, e.g., by injection of 1–5 mm thick streams flowing at 10 m/s on solid substrates.
If graphite is used, the gross erosion rates due to physical and chemical sputtering would be many meters per year, so one must rely on redeposition of the sputtered material. The location of the redeposition will not exactly coincide with the location of the sputtering, so one is still left with erosion rates that may be prohibitive. An even larger problem is the tritium co-deposited with the redeposited graphite. The tritium inventory in graphite layers and dust in a reactor could quickly build up to many kilograms, representing a waste of resources and a serious radiological hazard in case of an accident. The consensus of the fusion community seems to be that graphite, although a very attractive material for fusion experiments, cannot be the primary PFC material in a commercial reactor.
The sputtering rate of tungsten can be orders of magnitude smaller than that of carbon, and tritium is not so easily incorporated into redeposited tungsten, making this a more attractive choice. On the other hand, tungsten impurities in a plasma are much more damaging than carbon impurities, and self-sputtering of tungsten can be high, so it will be necessary to ensure that the plasma in contact with the tungsten is not too hot (a few tens of eV rather than hundreds of eV). Tungsten also has disadvantages in terms of eddy currents and melting in off-normal events, as well as some radiological issues.

[edit] Subsystems

In fusion research, achieving a fusion energy gain factor Q = 1 is called breakeven and is considered a significant although somewhat artificial milestone. Ignition refers to an infinite Q, that is, a self-sustaining plasma where the losses are made up for by fusion power without any external input. In a practical fusion reactor, some external power will always be required for things like current drive, refueling, profile control, and burn control. A value on the order of Q = 20 will be required if the plant is to deliver much more energy than it uses internally.
Despite many differences between possible designs of power plant, there are several systems that are common to most. A fusion power plant, like a fission power plant, is customarily divided into the nuclear island and the balance of plant. The balance of plant converts heat into electricity via steam turbines; it is a conventional design area and in principle similar to any other power station that relies on heat generation, whether fusion, fission or fossil fuel based.
The nuclear island has a plasma chamber with an associated vacuum system, surrounded by plasma-facing components (first wall and divertor) maintaining the vacuum boundary and absorbing the thermal radiation coming from the plasma, surrounded in turn by a blanket where the neutrons are absorbed to breed tritium and heat a working fluid that transfers the power to the balance of plant. If magnetic confinement is used, a magnet system, using primarily cryogenic superconducting magnets, is needed, and usually systems for heating and refueling the plasma and for driving current. In inertial confinement, a driver (laser or accelerator) and a focusing system are needed, as well as a means for forming and positioning the pellets.

Inertial confinement fusion implosion on the Nova laser creates "microsun" conditions of tremendously high density and temperature.