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[[File:Sun in X-Ray.png|thumb|300px|The [[Sun]] is a natural fusion reactor.]]
'''Fusion power''' is the [[power (physics)|power]] generated by [[nuclear fusion]] processes. In fusion reactions, two light [[atomic nucleus|atomic nuclei]] fuse to form a heavier nucleus (in contrast with [[fission power]]). In doing so they release a comparatively large amount of energy arising from the [[binding energy]] due to the [[strong nuclear force]] which is manifested as an increase in [[temperature]] of the reactants. Fusion power is a primary area of research in [[plasma physics]].


The term is commonly used to refer to potential commercial production of net usable power from a fusion source, similar to the usage of the term "[[steam engine|steam power]]". The leading designs for controlled fusion research use [[magnetic confinement|magnetic]] ([[tokamak]] design) or  [[inertial confinement|inertial]] ([[laser]]) confinement of a [[plasma (physics)|plasma]].  Both approaches are still under development and are years away from commercial operation in which [[heat]] from the fusion reaction is used to operate a [[steam turbine]] which drives [[electrical generator]]s, as in existing [[fossil fuel]] and [[fission power|nuclear fission]] [[power station]]s.


== Mechanism ==
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{{main|Nuclear fusion}}
[[File:Binding energy curve - common isotopes.svg|thumb|300px|Binding energy for different atoms.  Iron-56 has the highest, it is the most stable. Atoms to the left like fuse. Atoms to the right likely spilt]]
 
Fusion happens when two (or more) nuclei come close together. Here, the [[strong interaction|strong nuclear force]] takes over and pulls them together.  The process takes two light nuclei forms a heaver one. In light fusion reactions, some of the mass is lost in this process. It is converted into energy through  [[Albert Einstein]]'s [[mass-energy equivalence]] formula ''E''&nbsp;=&nbsp;''mc''<sup>2</sup>. The most stable nucleus is is [[iron]]-56. Atoms heavier than iron-56, are more likely to spilt apart by [[nuclear fission]]. While atoms lighter than this are more likely to fuse together by [[fusion]].<ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2|title=Fission and fusion can yield energy}}</ref>
 
To fuse atoms you must overcome the repulsive Coulomb force. This is an [[electrostatic force]] caused by two positive nuclei (containing [[protons]]) coming together. To overcome this "[[Coulomb barrier]]", the atoms must slam together, at high speeds, with high [[kinetic energy]]. The easiest way to do this is to heat the atoms. Heating can be done a variety of ways. Once an is heated above its [[ionization]] energy, its electrons are stripped away, leaving just the bare nuclei: the [[ion]]. Most fusion experiments use a hot cloud [[ion]]s and [[electron]]s. This cloud is known as a [[plasma (physics)|plasma]].
 
Theoretically, any atom could be fused, if enough pressure and temperature was applied.<ref>http://www.newton.dep.anl.gov/askasci/phy00/phy00018.htm</ref> However, mankind has yet, only been able to fuse the light elements. Because its small nuclear charge, [[hydrogen]] is the easiest atom to fuse and this reaction produces [[helium]].
 
=== Cross Section ===
 
[[Image:fusion rxnrate.svg|right|300px|thumb|The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The deuterium-tritium fusion rate peaks at a lower temperature (about 70&nbsp;keV, or 800 million kelvin) and at a higher value than other reactions commonly considered for fusion energy.]]
 
A reactions' '''[[cross section (physics)|cross section]]''', (denoted σ) is the measure of how likely a fusion reaction will happen. It is a probability. It depends on the velocity of the two nuclei, when they slam together. If the atoms move faster, fusion is more likely.  If the atoms hit head on, fusion is more likely.  Cross sections for many different fusion reactions were measured mainly in the seventies, using particle beams.<ref>http://www.osti.gov/scitech/biblio/4014032</ref>  A beam of species A was fired at species B at different speeds, and the amount of neutrons coming off was measured. Neutrons are a key product of fusion reactions. These nuclei are flying around in a hot cloud, with some distribution of velocities.  If the plasma is thermalized, then the distribution looks like a [[bell curve]], or maxwellian distribution.  In this case, it is useful to take the average cross section over the velocity distribution. This is entered into the volumetric fusion rate:.<ref name = "Lawson">"Some Criteria for a Power producing thermonuclear reactor" John Lawson, Atomic Energy Research Establishment, Hanvell, Berks, 2nd November 1956</ref>
 
:<math>P_\text{fusion} = n_A n_B \langle \sigma v_{A,B} \rangle E_\text{fusion}</math>
 
where:
* <math>P_\text{fusion}</math> is the energy made by fusion, per time and volume
* ''n'' is the number density of species A or B, the particles in the volume
* <math>\langle \sigma v_{A,B} \rangle</math> is the cross section of that reaction, average over all the velocities of the two species ''v''
* <math>E_\text{fusion}</math> is the energy released by a that fusion reaction.
 
This equation shows that energy varies with the temperature, density, speed of collision, and fuel used. To reach net power, fusion reactions have to occur fast enough to make up for energy losses. Any power plant using fusion will hold in this hot cloud. Plasma clouds lose energy through [[Thermal conduction|conduction]] and [[radiation]].<ref name = "Lawson"/> Conduction is when [[ion]]s, [[electron]]s or [[neutral particle|neutrals]] touch a surface and leak out. Energy is lost with the particle. Radiation is when energy leaves the cloud as light. Radiation increases as the temperature rises. To get net power from fusion, you must overcome these losses. This leads to an equation for power output.
 
:<math>P_\text{out} = \eta_\text{capture}\left(P_\text{fusion} - P_\text{conduction} - P_\text{radiation}\right)</math>
 
where:
* ''η'', is the efficiency with which the plant captures energy
 
John Lawson used this equation to estimate some conditions for net power <ref name = "Lawson"/> based on a [[Maxwell–Boltzmann distribution|Maxwellian]] cloud.<ref name = "Lawson"/>  This is the [[Lawson criterion]].
 
==History of research==
 
===Development of fusion concept and theory===
Research into nuclear fusion started in the early part of the 20th century. In 1920 the British physicist [[Francis William Aston]] discovered that the total mass of four [[hydrogen atom]]s are heavier than the total mass of one [[helium atom]] ([[Helium-4|He-4]]), which implied that net energy can be released by combining hydrogen atoms together to form helium, and provided the first hints of a mechanism by which stars could produce energy in the quantities being measured. Through the 1920s, [[Arthur Stanley Eddington]] became a major proponent of the [[proton–proton chain reaction]] (PP reaction) as the primary system running the [[Sun]], a theory later verified after [[Hans Bethe]] finally showed in 1939 that [[beta decay]] and [[quantum tunneling]] in the [[solar core|Sun's core]] might convert one of the protons into a [[neutron]] and thereby producing [[deuterium]] rather than a diproton. The deuterium would then fuse through other reactions to further increase the energy output. For this work, Bethe won the 1967 [[Nobel Prize in Physics]].
 
===Uncontrolled fusion and weapons development===
Fusion reactions - sometimes known as 'uncontrolled' fusion since the fusion reaction is intended to escalate out of control - were first made practical in the form of [[nuclear weapon]]s, with the creation of the [[hydrogen bomb]] in 1951. In these devices, the energy released by a fission weapon is used to compress and heat fusion fuel, beginning a fusion reaction that releases a large amount of neutrons. These neutrons interact with a surrounding blanket of fission fuel, causing it to undergo fission much more rapidly than natural fission reactions would - almost instantly in comparison. One side effect is that, while larger fission weapons tend to blow themselves apart before all their fuel can react and therefore tend to have an upper size limit, fusion weapons tend to undergo fusion much more rapidly and efficiently, and therefore do not have this practical upper limit to their power. From 1942 nuclear fusion research was subsumed into the [[Manhattan Project]] and the science became obscured by the [[secrecy]] surrounding the field. The first successful fusion device was the [[boosted fission weapon]] tested in 1951 in the [[Greenhouse Item]] test. This was followed by true fusion weapons in 1952's [[Ivy Mike]], and the first practical examples in 1954's [[Castle Bravo]].
 
===Early efforts at controlled fusion and power production===
Attempts at controlling fusion for power production had already started by this point. Registration of the first patent related to a fusion reactor dates back to 1946<ref>{{cite web|url=http://v3.espacenet.com/textdoc?DB=EPODOC&IDX=GB817681&F=0 |title=British Patent 817681 |publisher=V3.espacenet.com |date= |accessdate=2013-06-22}}</ref> by the [[United Kingdom Atomic Energy Authority]], the inventors being [[George Paget Thomson|Sir George Paget Thomson]] and [[Moses Blackman]]. This was the first detailed examination of the [[Z-pinch]] concept, and small efforts to experiment with it started at several sites in the UK.
 
Around the same time, an expatriate German [[Ronald Richter]] proposed the [[Huemul Project]] in Argentina, announcing positive results in 1951. Although these results turned out to be false, it sparked off intense interest around the world. The UK pinch programs were greatly expanded, culminating in the [[ZETA (fusion reactor)|ZETA]] and [[Sceptre (fusion reactor)|Sceptre]] devices. In the US, pinch experiments like those in the UK started at the [[Los Alamos National Laboratory]]. Similar devices were built in the USSR after data on the UK program was passed to them by [[Klaus Fuchs]]. At [[Princeton University]] a new approach developed as the [[stellarator]], and the research establishment formed there continues to this day as the [[Princeton Plasma Physics Laboratory]]. Not to be outdone, [[Lawrence Livermore National Laboratory]] entered the field with their own variation, the [[magnetic mirror]]. These three groups have remained primary developers of fusion research in the US to this day.
 
===Emergence of new concepts===
In the time since these early experiments, two new approaches developed that have since come to dominate fusion research. The first was the [[tokamak]] approach developed in the Soviet Union, which combined features of the stellarator and the pinch to produce a device that dramatically outperformed either. The majority of magnetic fusion research to this day has followed the tokamak approach.
 
In the late 1960s the concept of "mechanical" fusion through the use of [[laser]]s was developed in the USA, and Lawrence Livermore switched their attention from mirrors to lasers over time. In this system, the fuel is rapidly compressed to high densities, about ten times that of [[lead]], which causes it to rapidly heat to fusion conditions. By comparison, the tokamak and other approaches are "steady state" machines, operating at much lower pressures and densities, but over much longer periods of time.
 
Civilian applications are still being developed. Although it took less than ten years for fission to go from military applications to civilian fission energy production,<ref>The first A-bomb shot dates back to July 16, 1945 in [[Alamogordo]] ([[New Mexico]] desert), while the first civilian fission plant was connected to the electric power network on June 27, 1954 in [[Obninsk]] ([[Russia]]).</ref> it has been very different in the fusion energy field; more than fifty years have already passed since the first fusion reaction took place<ref>The first H-bomb, [[Ivy Mike]], was detonated on [[Enewetak|Eniwetok]], an atoll of the [[Pacific Ocean]], on November 1, 1952 (local time).</ref> and sixty years since the first attempts to produce controlled fusion power, without any commercial fusion energy production plant coming into operation.
 
===Magnetic containment===
 
====Pinch devices====
A major area of study in early fusion power research is the "[[Pinch (plasma physics)|pinch]]" concept. Pinch is based on the fact that plasmas are electrically conducting. By running a current through the plasma, a magnetic field will be generated around the plasma. This field will, according to [[Lenz's law]], create an inward directed force that causes the plasma to collapse inward, raising its density. Denser plasmas generate denser magnetic fields, increasing the inward force, leading to a [[chain reaction]]. If the conditions are correct, this can lead to the densities and temperatures needed for fusion. The difficulty is getting the current into the plasma, which would normally melt any sort of mechanical [[electrode]]. A solution emerges again due to the conducting nature of the plasma; by placing the plasma in the middle of an [[electromagnet]], [[Electromagnetic induction|induction]] can be used to generate the current.
 
Pinch was first developed in the UK in the immediate post-war era. Starting in 1947 small experiments were carried out and plans were laid to build a much larger machine. Two teams were quickly formed and began a series of ever-larger experiments. When the Huemul results hit the news, [[James L. Tuck]], a UK physicist working at Los Alamos, introduced the pinch concept in the US and produced a series of machines known as the [[Perhapsatron]]. In the Soviet Union, unbeknownst to the west, a series of similar machines were being built. All of these devices quickly demonstrated a series of instabilities when the pinch was applied, which broke up the plasma column long before it reached the densities and temperatures required for fusion. In 1953 Tuck and others suggested a number of solutions to these problems.
 
The largest "classic" pinch device was the [[ZETA (fusion reactor)|ZETA]], including all of these suggested upgrades, starting operations in the UK in 1957. In early 1958 [[John Cockcroft]] announced that fusion had been achieved in the ZETA, an announcement that made headlines around the world. When physicists in the US expressed concerns about the claims they were initially dismissed. US experiments soon demonstrated the same neutrons, although temperature measurements suggested these could not be from fusion reactions. The neutrons seen in the UK 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, and the entire field was tainted 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 an experimental run 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 since.
 
====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.
 
====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.<ref>Nathaniel Fisch, [http://books.google.com/books?id=9i9bgMLVjWsC&pg=PA118 "Edward Teller Centennial Symposium"], pg 118</ref>
 
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.
 
====The tokamak====
A new approach was outlined in the theoretical works fulfilled in 1950–1951 by [[Igor Tamm|I.E. Tamm]] and [[Andrei Sakharov|A.D. Sakharov]] in the [[Soviet Union]], which first discussed a [[tokamak]]-like approach. Experimental research on those 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 orbited within the reactor a particular number of times, today known as the "[[Safety factor (plasma physics)|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 (tokamak)|T-3]] and its larger version [[T-4 (tokamak)|T-4]]. T-4 was tested in 1968 in [[Novosibirsk]], producing the first quasistationary thermonuclear fusion reaction ever.<ref>[[Great Soviet Encyclopedia]], 3rd edition, entry on "Токамак", available online here [http://slovari.yandex.ru/art.xml?art=bse/00079/49400.htm]{{dead link|date=August 2012}}</ref> The tokamak was dramatically more efficient than the other approaches of that era, by a factor 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 after measuring 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&nbsp;— 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 plasma, 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 [[Fusion energy gain factor|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, code named Project [[DEMO]], 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 if these goals are 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."
 
===Inertial (laser) containment===
[[File:Nif hohlraum.jpg|150px|right|thumb|Mockup of a gold-plated hohlraum designed for use in the [[National Ignition Facility]]]]
 
Laser fusion, formally known as [[inertial confinement fusion]], involves [[Implosion (mechanical process)|imploding]] a target a using [[laser]] beams.  There are two ways to do this: indirect drive and direct drive.  In direct drive, the laser blasts a pellet of fuel.  In indirect drive, the lasers blast a structure around the fuel.  This makes [[x-rays]] which squeeze the fuel.  Both methods compress the fuel so that fusion can take place.
 
==== 1960's====
Laser fusion was suggested in 1962 by scientists at [[Lawrence Livermore National Laboratory]], shortly after the invention of the laser itself in 1960.  At the time, Lasers were low power machines, but low-level research began as early as 1965.  In 1972, John Nuckolls' outlined the idea of ignition<ref>Nuckolls, John; Wood, Lowell; Thiessen, Albert; Zimmerman, George (15 September 1972). "Laser Compression of Matter to Super-High Densities: Thermonuclear (CTR) Applications". Nature 239: 139–142. doi:10.1038/239139a0</ref>.  This is a fusion, chain reaction.  Hot helium made during fusion, reheats the fuel and starts more reactions.  John argued that ignition would require lasers of about 1 kJ.  This turned out to be wrong.  Nuckolls paper started a major development effort.
 
==== 1970's and 1980's====
 
Several laser systems were built at LLNL.  These included the [[Argus_laser| argus]], the [[Cyclops laser| Cyclops]] the [[Janus_laser| Janus]], the [[Long path laser | long path]], the [[Shiva laser]] and the [[Nova (laser)| Nova]] in 1984.  Early efforts focused on either fast delivery or beam smoothness. Both tried to deliver the energy uniformly to implode the target.  One early problem was that the light in the [[infrared]] wavelength, lost lots of energy before hitting the fuel. Breakthroughs were also made at the [[Laboratory for Laser Energetics]] at the [[University of Rochester]].  Rochester scientists used frequency-tripling crystals to transform the infrared laser beams into ultraviolet beams.  In 1985, Donna Strickland<ref> "Dr. Donna Strickland: Packing a laser punch" The University of Waterloo , Personal Profiles, accessed 1-11-2014, https://uwaterloo.ca/science/magnet-talent-more-stories/dr-donna-strickland
</ref> and [[Gérard Mourou]] invented a method to amplify lasers pulses by "chirping". This method changes a single wavelength into a full spectrum. The system then amplifies the laser at each wavelength and then reconstitutes the beam into one color. Chirp pulsed amplification became instrumental in building the National Ignition Facility and the Omega EP system.  Most research into ICF was towards weapons research, because the implosion is relevant to nuclear weapons.
 
==== 1990's and 2000's ====
 
However the field was turned around after more recent work demonstrated significant savings in the required laser energy were 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 once 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 laser]]s 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 allow the fast ignition concept to outpace the magnetic approaches in further development.
 
With these obstacles overcome, the laser-based concept also had other potential 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 performing maintenance on a laser-based device would be much easier, such as core replacement. Additionally, the lack of strong magnetic fields allows for a wider variety of low-[[Neutron activation|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.
 
===Other approaches===
Over the years there have been a wide variety of other 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 confinement, 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.
 
====Technically viable approaches====
[[Philo T. Farnsworth]], the inventor of the first [[History of television|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. A voltage applied between the two grids causes the fuel to ionize around them, from which 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 L. Hirsch|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 a neutron flux in the order of one billion neutrons per second. Although the [[Energy conversion efficiency|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 electrons 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).<ref name="IAC2006">[http://www.askmar.com/ConferenceNotes/2006-9%20IAC%20Paper.pdf  "The Advent of Clean Nuclear Fusion: Super-performance Space Power and Propulsion"], Robert W. Bussard, Ph.D., 57th International Astronautical Congress, October 2–6, 2006</ref>
 
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).<ref name="IAC2006" />
 
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 10<sup>9</sup> per second running D-D fusion reactions at only 12.5 kV based on [[neutron detection|detecting]] a total of nine neutrons in five tests. Bussard claimed that a scaled-up version of 2.5–3 m in diameter, would operate at over 100 MW net power since fusion power scales as the fourth power of the B field and the cube of the size.<ref name="IAC2006" />
 
A recent<ref name="IFSA2007">[http://iopscience.iop.org/1742-6596/112/4/042084/pdf/jpconf8_112_042084.pdf  "Status of the U. S. program in magneto-inertial fusion"], Y. C. Francis Thio Ph.D., Program Manager, U. S. Department of Energy, Office of Fusion Energy Sciences, Germantown, MD,
USA</ref> 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]],<ref name="PB11">[http://arxiv.org/ftp/arxiv/papers/0710/0710.3149.pdf "ADVANCES TOWARDS PB11 FUSION WITH THE DENSE PLASMA FOCUS"], Eric Lerner, Lawrenceville Plasma Physics, 2008</ref> which typically consists of two coaxial cylindrical electrodes made from copper or beryllium and housed in a vacuum chamber containing a low-pressure fusible gas. An electrical pulse is applied across the electrodes, heating the gas into a plasma. The current forms into a minuscule vortex along the axis of the machine, which then kinks into a cage of current with an associated magnetic field. The cage of current and magnetic field entrapped plasma is called a plasmoid. The acceleration of the electrons about the magnetic field lines heats the nuclei within the plasmoid to fusion temperatures. This will, in principle, yield more energy in the beams than was input to form them. A 2012 paper published in [[Physics of Plasmas]] demonstrated that Lerner's team had achieved temperatures of 1.8 billion degrees C, sufficient for boron fusion, and that fusion reactions were occurring primarily within the contained plasmoid, a necessary condition for net power.
 
In addition to the Polywell and Fusor concepts, [[Lockheed Martin]]'s [[Skunk Works]] is currently working on an [[High beta fusion reactor|electrostatic confinement based reactor]] of classified nature. In a recent press release on Google's [[Solve for X]] science think tank web series, the team stated that within 5 years a demonstration reactor creating 100MW of electricity could be complete, and be smaller than a flatbed truck.<ref name="SolveforX">[http://www.youtube.com/watch?v=JAsRFVbcyUY "Solve for X - Skunkworks conference"], Charles Chase, Locheed Martin Skunkworks, 2013</ref>
 
====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]] that 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.
 
The mass of a muon is 207 times the mass of an electron. Muons, deuterons and tritons form "heavy" hydrogen molecules. Due to the increased mass, the Coulomb barrier decreases. These fusion reactions may occur at temperatures as low as room temperature and so they are also known as cold fusion reactions, but they should not be confused with Fleischmann and Pons "cold fusion".<ref>{{Citation |last=Huizenga|first=John R. |title=Cold Fusion: The Scientific Fiasco of the Century |edition=2nd |location=Oxford and New York |publisher=Oxford University Press |year=1993 |page=112 |isbn=0-19-855817-1}}</ref>
 
In April 2005, a team from [[UCLA]] announced<ref>{{cite web|url=http://web.archive.org/web/20061021025402/https://discover.com/issues/jan-06/features/physics/ |title=The Year in Science: Physics |publisher=Web.archive.org |date=2006-10-21 |accessdate=2013-06-22}}</ref> 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.
 
[[Nuclear fusion-fission hybrid|Hybrid nuclear fusion-fission (hybrid nuclear power)]] is a proposed means of generating [[Electrical power industry|power]]  by use of a combination of nuclear fusion and [[Nuclear fission|fission]] processes. The concept dates to the 1950s, and was briefly advocated by [[Hans Bethe]] during the 1970s, but largely remained unexplored until a revival of interest in 2009, due to the delays in the realization of pure fusion.<ref name="hybrid">{{cite journal | author = Gerstner, E. | title = Nuclear energy: The hybrid returns | year = 2009 | journal = [[Nature (journal)|Nature]] | volume = 460 | issue = 7251| pages = 25–8 | pmid = 19571861|doi=10.1038/460025a}}</ref>
[[Project PACER]], carried out at [[Los Alamos National Laboratory]] (LANL) in the mid-1970s, explored the possibility of a fusion power system that would involve exploding small [[H-bomb|hydrogen bomb]]s (fusion bombs) inside an underground cavity. As an energy source, the system is the only fusion power system that could be demonstrated to work using existing technology. However it would also require a large, continuous supply of nuclear bombs, making the economics of such a system rather questionable.
 
====Unconventional, unproven 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.<ref>{{harvnb|Browne|1989}}, {{harvnb|Close|1992}}, {{harvnb|Huizenga|1993}}, {{harvnb|Taubes|1993}}</ref> By late 1989, most scientists considered cold fusion claims dead,<ref name="Browne_1989">{{harvnb|Browne|1989}}</ref> and cold fusion subsequently gained a reputation as [[pathological science]].<ref name="nytdoe">
{{Cite news
|author=
|date=2004-03-25
|title=US will give cold fusion a second look
|url=http://query.nytimes.com/gst/fullpage.html?res=9C01E0DC1530F936A15750C0A9629C8B63
|publisher=The New York Times
|accessdate=2009-02-08
| first=Kenneth
| last=Chang
}}</ref>
 
However, a small community of researchers continues to investigate cold fusion<ref name="Browne_1989"/><ref>{{harvnb|Voss|1999}}, {{harvnb|Platt|1998}}, {{harvnb|Goodstein|1994}}, {{harvnb|Van Noorden|2007}}, {{harvnb|Beaudette|2002}}, {{harvnb|Feder|2005}}, {{harvnb|Hutchinson|2006}}, {{harvnb|Kruglinksi|2006}}, {{harvnb|Adam|2005}}</ref><ref name="nytscorn">
{{Cite news
| author = William J. Broad
| date = 31 October 1989
| title = Despite Scorn, Team in Utah Still Seeks Cold-Fusion Clues
| url = http://query.nytimes.com/gst/fullpage.html?res=950DE6DA1331F932A05753C1A96F948260&pagewanted=all
| work = [[The New York Times]]
| pages = C1
}}</ref><ref name="wired march 2009">{{harvnb|Alfred|2009}}</ref> claiming to replicate Fleishmann and Pons' results including nuclear reaction byproducts.<ref name="ACS Press Release">
{{cite press
|url=http://www.eurekalert.org/pub_releases/2009-03/acs-fr031709.php
|title='Cold fusion' rebirth? New evidence for existence of controversial energy source
|publisher=[[American Chemical Society]]
}}</ref><ref name="Hagelstein et al. 2004">{{harvnb|Hagelstein et al.|2004|ref=CITEREFDOE2004}}</ref> Claims related to cold fusion are largely disbelieved in the mainstream scientific community.<ref name="Feder 2005">{{harvnb|Feder|2005}}</ref>  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.<ref>{{harvnb|Choi|2005}}, {{harvnb|Feder|2005}}, {{harvnb|US DOE|2004|ref=CITEREFDOE2004r}}</ref>
 
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.
 
===Current status and recent successes===
{{Update|section|inaccurate=yes|date=October 2011}}
{{As of|2010|07|}}, the largest experiment by means of magnetic confinement has been the [[Joint European Torus]] (JET). In 1997, JET produced a peak of 16.1MW of fusion power (65% of input power), with fusion power of over 10MW sustained for over 0.5&nbsp;sec. Its successor, the International Thermonuclear Experimental Reactor ([[ITER]]), was officially announced as part of a seven-party consortium (six countries and the EU).<ref name="announcement">[http://www.pppl.gov/polImage.cfm?doc_Id=603&size_code=Doc ITER and the Promise of Fusion Energy].</ref> [[ITER]] is designed to produce ten times more fusion power than the power put into the [[Plasma (physics)#Artificial plasmas|plasma]]. [[ITER]] is currently under construction in [[Cadarache]], France.
 
Inertial (laser) confinement is being developed at the [[United States]] [[National Ignition Facility]] (NIF) based at [[Lawrence Livermore National Laboratory]] in [[California]], the [[France|French]] [[Laser Mégajoule]], and the planned [[European Union]] [[HiPER|High Power laser Energy Research]] (HiPER) facility. NIF reached initial operational status in 2010 and has been in the process of increasing the power and energy of its "shots", with fusion ignition tests to follow.<ref name="Oct8_10">{{cite web |  title = First successful integrated experiment at National Ignition Facility announced | work = General Physics | publisher = PhysOrg.com  | date = October 8, 2010 | url = http://www.physorg.com/news205740709.html  | accessdate = 2010-10-09 }}</ref> A three year goal announced in 2009 to produce net energy from fusion by 2012 was missed; however in September 2013 the facility announced a significant milestone from an August 2013 test which produced more energy from the fusion reaction than had been provided to the fuel pellet. This was reported as the first time this had been accomplished in fusion power research. The facility reported that their next step involved improving the system to prevent the hohlraum breaking up asymmetrically or too soon.<ref>[http://www.bbc.co.uk/news/science-environment-24429621 Nuclear fusion milestone passed at US lab]</ref><ref>http://phys.org/news/2013-09-fusion-weve.html</ref><ref>http://phys.org/news/2013-08-laser-fusion-yields-energy.html#inlRlv</ref>
 
Fusion powered electricity generation was initially believed to be readily achievable, as fission power had been. However, the extreme requirements for continuous reactions and [[plasma containment]] led to projections being extended by several decades. In 2010, more than 60 years after the first attempts, commercial power production was still believed to be unlikely before 2050.<ref name="ITERorg">{{cite web |work=The ITER Project |title=Beyond ITER |publisher=Information Services, Princeton Plasma Physics Laboratory |url=http://www.iter.org/Future-beyond.htm |accessdate=5 February 2011 |archiveurl=http://web.archive.org/web/20061107220145/http://www.iter.org/Future-beyond.htm |archivedate=7 November 2006 }} - Projected fusion power timeline</ref>
 
===Current status===
Despite optimism dating back to the 1950s about the wide-scale harnessing of fusion power, there are still significant barriers standing between current scientific understanding and technological capabilities and the practical realization of fusion as an energy source.  Research, while making steady progress, has also continually thrown up new difficulties.  Therefore it remains unclear whether an economically viable fusion plant is possible.<ref>[http://www.scientificamerican.com/article.cfm?id=fusions-false-dawn Fusion’s False Dawn] by Michael Moyer ''[[Scientific American]]'' March 2010</ref><ref name="newscientist">{{cite web|url=http://www.newscientist.com/channel/opinion/mg19025543.300-editorial-nuclear-fusion-must-be-worth-the-gamble.html |title=Editorial: Nuclear fusion must be worth the gamble |publisher=''[[New Scientist]]'' |date=7 June 2006}}</ref> A 2006 editorial in ''[[New Scientist]]'' magazine opined that "if commercial fusion is viable, it may well be a century away."<ref name="newscientist" />
 
Several fusion D-T burning tokamak test devices have been built ([[TFTR]], [[Joint European Torus|JET]]); it was hoped that TFTR would be able to produce more thermal energy than electrical energy consumed but this was not achieved though a record output power of 65% input power was achieved.
 
A paper published in January 2009 and part of the IAEA Fusion Conference Proceedings at Geneva last October claims that small 50 MW Tokamak style reactors are feasible.<ref>{{cite web|last=Andrews |first=Dave |url=http://www.claverton-energy.com/physics-and-engineering-basis-of-multi-functional-compact-tokamak.html |title=physics and engineering basis of multi-functional compact tokamak |publisher=Claverton-energy.com |date= |accessdate=2013-06-22}}</ref>
 
In 2009, a high-energy laser system, the [[National Ignition Facility]], was created in the US, which can heat hydrogen atoms to temperatures only existing in nature in the cores of stars.  The new laser is expected to have the ability to produce, for the first time, more energy from controlled, inertially confined nuclear fusion than was required to initiate the reaction.<ref>{{cite web|url=http://web.archive.org/web/20101224224354/http://www.breitbart.com/article.php?id=CNG.12fab6f6c00a65e15e6fb5e305aacbb7.41&show_article=1 |title="US lab debuts super laser", Breitbart news site |publisher=Web.archive.org |date=2010-12-24 |accessdate=2013-06-22}}</ref>
<!---
On 28 January 2010, the LLNL announced tests using all 192 laser beams, although with lower laser energies, smaller [[hohlraum]] targets, and substitutes for the fusion fuel capsules.<ref>{{Cite news|url=http://news.bbc.co.uk/2/hi/science/nature/8485669.stm|title=Laser fusion test results raise energy hopes|publisher=BBC News|accessdate=2010-01-29 | date=January 28, 2010}}</ref><ref>{{cite web|url=https://publicaffairs.llnl.gov/news/news_releases/2010/NR-10-01-06.html|title=Initial NIF experiments meet requirements for fusion ignition|publisher=Lawrence Livermore National Laboratory|accessdate=2010-01-29}}</ref> More than one megajoule of ultraviolet energy was fired into the hohlraum, beating the previous world record by a factor of more than 30. The results gave the scientists confidence that they will be able to achieve ignition in more realistic tests scheduled to begin in the summer of 2010.<ref>[http://news.bbc.co.uk/2/hi/science/nature/8485669.stm "BBC:Laser fusion test results raise energy hopes"]</ref>
--->
In 2010, NIF researchers were conducting a series of "tuning" shots to determine the optimal target design and laser parameters for high-energy ignition experiments with fusion fuel in the following months.<ref>{{cite web|author=Seaver, Lynda L |url=https://www.llnl.gov/news/newsreleases/2010/Nov/NR-10-11-02.html |title=World's largest laser sets records for neutron yield and laser energy |publisher=Llnl.gov |date=2010-10-01 |accessdate=2013-06-22}}</ref> Two firing tests were performed on 31 October 2010 and 2 November 2010. In early 2012, NIF director Mike Dunne expected the laser system to generate fusion with net energy gain by the end of 2012.<ref>{{cite web|author=SPIE Europe Ltd |url=http://optics.org/news/3/1/37 |title=PW 2012: fusion laser on track for 2012 burn |publisher=Optics.org |date= |accessdate=2013-06-22}}</ref>  However, it was not achieved by that date due to delays.
 
In September 2013 the NIF was widely claimed to have achieved a milestone in controlled fusion, by successfully initiating a reaction that resulted in the release of more energy than the fuel absorbed. However, reports shortly after<ref>[http://news.sciencemag.org/physics/2013/10/fusion-breakthrough-nif-uh-not-really-%E2%80%A6 Sciencemagazine 10 October 2013, "Fusion "Breakthrough" at NIF? Uh, Not Really …" ]</ref> indicated it was still far short of energy breakeven. The process will need to be made more efficient to yield commercially viable amounts of energy.<ref>[http://www.bbc.co.uk/news/science-environment-24429621 "BBC:Nuclear fusion milestone passed at US lab"]</ref>
 
The [[high beta fusion reactor]] design under development at [[Lockheed Martin]]'s [[Skunk Works]] is expected by the company to yield a functioning 100 megawatt prototype by 2017 and to be ready for regular operation by 2022.<ref>[http://www.fusenet.eu/node/400 FuseNet: The European Fusion Education Network]</ref><ref>[http://www.popsci.com/technology/article/2013-02/fusion-power-could-happen-sooner-you-think Fusion Power Could Happen Sooner Than You Think]</ref>
 
==Fuels ==
 
Using particle beams, many fusion reactions have been tested.  However, the fuels considered for power have all been light elements like the isotopes of hydrogen.  These are: [[deuterium]] and [[tritium]]. Other reactions like the deuterium and Helium<sup>3</sup> reaction or the Helium<sup>3</sup> and Helium<sup>3</sup> reactions, would require a supply of Helium<sup>3</sup>.  This can either come from other nuclear reactions or from extraterrestrial sources.  Finally, researchers hope to do the p-{{SimpleNuclide|Boron|11}} reaction, because it does not directly produce neutrons, though side reactions can.<ref>{{cite book|last=Atzeni|first=Stefano|title=The Physics of Inertial Fusion|year=2009|publisher=Oxford Science Publications|location=USA|isbn=978-0-19-956801-7|pages=12–13}}</ref>
 
===deuterium tritium===
[[Image:Deuterium-tritium fusion.svg|thumb|Diagram of the D-T reaction]]
The easiest nuclear reaction, at the lowest energy, is:
 
:{{Nuclide|Deuterium|link=yes}} + {{Nuclide|Tritium|link=yes}} → {{Nuclide|Helium|link=yes}} + {{SubatomicParticle|10neutron|link=yes}}
 
This reaction is common in research.  [[Deuterium]] is a naturally occurring [[isotope]] of hydrogen and is commonly available. The large mass ratio of the hydrogen isotopes makes their separation easy compared to the difficult [[uranium enrichment]] process.  [[Tritium]] is a natural isotope of hydrogen, but due to its tiny [[half-life]] of 12.32 years, it hard to find, store, produce and is expensive.  Consequently, the deuterium-tritium fuel cycle requires the [[breeder reactor|breeding]] of [[tritium]] from [[lithium]] using one of the following reactions:
 
:{{SubatomicParticle|10neutron}} + {{Nuclide|Lithium|6}} → {{Nuclide|Tritium}} + {{Nuclide|Helium}}
:{{SubatomicParticle|10neutron}} + {{Nuclide|Lithium|7}} → {{Nuclide|Tritium}} + {{Nuclide|Helium}} + {{SubatomicParticle|10neutron}}
 
The reactant neutron is supplied by the D-T fusion reaction shown above, and the one that has the greatest yield of energy. The reaction with <sup>6</sup>Li is [[exothermic reaction|exothermic]], providing a small energy gain for the reactor. The reaction with <sup>7</sup>Li is [[endothermic reaction|endothermic]] but does not consume the neutron. At least some <sup>7</sup>Li reactions are required to replace the neutrons lost to absorption by other elements. Most reactor designs use the naturally occurring mix of lithium isotopes.
 
Several drawbacks are commonly attributed to D-T fusion power:
# It produces substantial amounts of neutrons that result in the [[neutron activation]] of the reactor materials.<ref name="Thinkquest">{{cite web| publisher= | url=http://library.thinkquest.org/17940/texts/fusion_dt/fusion_dt.html | title= Thinkquest: D-T reaction | first= | last= | date= | accessdate=12 June 2010 | postscript= <!--None-->}}</ref>
# Only about 20% of the fusion energy yield appears in the form of charged particles with the remainder carried off by neutrons, which limits the extent to which direct energy conversion techniques might be applied.<ref>{{cite journal|last=Iiyoshi|first=A|coauthors=H. Momota, O Motojima, et al.|title=Innovative Energy Production in Fusion Reactors|journal=National Institute for Fusion Science NIFS|date=October 1993|pages=2–3|url=http://www.nifs.ac.jp/report/nifs250.html|accessdate=14 February 2012}}</ref>
# It requires the handling of the radioisotope tritium. Similar to hydrogen, tritium is difficult to contain and may leak from reactors in some quantity. Some estimates suggest that this would represent a fairly large environmental release of radioactivity.<ref>{{cite web|url=http://www.world-nuclear.org/info/inf66.html|title=Nuclear Fusion Power, Assessing fusion power}}</ref>
 
The [[neutron flux]] expected in a commercial D-T fusion reactor is about 100 times that of current fission power reactors, posing problems for [[plasma facing material|material design]]. After a series of D-T tests at [[Joint European Torus|JET]], the vacuum vessel was sufficiently radioactive that remote handling was required for the year following the tests.<ref>{{cite journal|last=Rolfe|first=A. C.|title=Remote Handling JET Experience|journal=Nuclear Energy|year=1999|volume=38|issue=5|page=6|url=http://www.iop.org/Jet/fulltext/JETP99028.pdf|accessdate=10 April 2012|issn=0140-4067}}</ref>
 
In a production setting, the neutrons would be used to react with [[lithium]] in order to create more tritium. This also deposits the energy of the neutrons in the lithium, which would then be transferred to drive electrical production. The lithium neutron absorption reaction protects the outer portions of the reactor from the neutron flux. Newer designs, the advanced tokamak in particular, also use lithium inside the reactor core as a key element of the design. The plasma interacts directly with the lithium, preventing a problem known as "recycling". The advantage of this design was demonstrated in the [[Lithium Tokamak Experiment]].
 
===deuterium deuterium ===
The second easiest fusion reaction is the fusing of deuterium with itself. This reaction has two branches that occur with nearly equal probability:
:{|
|D + D || → T|| + <sup>1</sup>H
|-
|D + D || → <sup>3</sup>He || + n
|}
This reaction is also common in research.  This fuel is commonly used by [[fusor|amateurs]] who fuse. The optimum energy to initiate this reaction is 15&nbsp;keV, only slightly higher than the optimum for the D-T reaction. The first branch does not produce neutrons, but it does produce tritium, so that a D-D reactor will not be completely tritium-free, even though it does not require an input of tritium or lithium. Unless the tritons can be quickly removed, most of the tritium produced would be burned before leaving the reactor, which would reduce the handling of tritium, but would produce more neutrons, some of which are very energetic. The neutron from the second branch has an energy of only {{convert|2.45|MeV|abbr=on}}, whereas the neutron from the D-T reaction has an energy of {{convert|14.1|MeV|abbr=on}}, resulting in a wider range of isotope production and material damage. When the tritons are removed quickly while allowing the 3He to reactor, the fuel cycle is called "tritium suppressed fusion"<ref>M. Sawan, S. Zinkle, and J. Sheffield, ''Fusion Eng Des'' 61-2, 561 (2002).</ref> The removed tritium decays to 3He with a 12.5 year half life. By recycling the 3He produced from the decay of tritium back into the fusion reactor, the fusion reactor does not require materials resistant to fast {{convert|14.1|MeV|abbr=on}} neutrons.
 
Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons would be only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding would not be required. Other advantages are independence from scarce lithium resources and a somewhat softer neutron spectrum. The disadvantage of D-D compared to D-T is that the energy confinement time (at a given pressure) must be 30 times longer and the power produced (at a given pressure and volume) would be 68 times less.
 
Assuming complete removal of tritium and recycling of 3He, only 6% of the fusion energy is carried by neutrons. The tritium-suppressed D-D fusion requires an energy confinement that is 10 times longer compared to D-T and a plasma temperature that is twice as high.<ref>J. Kesner, D. Garnier, A. Hansen, M. Mauel, and L. Bromberg, ''Nucl Fusion'' 44, 193 (2004).</ref>
 
===D-<sup>3</sup>He fuel cycle===
A second-generation approach to controlled fusion power involves combining [[helium-3]] (<sup>3</sup>He) and [[deuterium]] (<sup>2</sup>H):
 
:{|
|D + <sup>3</sup>He || → <sup>4</sup>He || + <sup>1</sup>H
|}
 
This reaction produces a helium-4 nucleus (<sup>4</sup>He) and a high-energy proton. As with the p-<sup>11</sup>B [[aneutronic fusion]] fuel cycle, most of the reaction energy is released as charged particles, reducing [[neutron activation|activation]] of the reactor housing and potentially allowing more efficient energy harvesting (via any of several speculative technologies).{{Citation needed|date=December 2011}} In practice, D-D side reactions produce a significant number of neutrons, resulting in p-<sup>11</sup>B being the preferred cycle for aneutronic fusion.
 
===p-<sup>11</sup>B fuel cycle===
If [[aneutronic fusion]] is the goal, then the most promising candidate may be the Hydrogen-1 (proton)/[[boron]] reaction:
 
:<sup>1</sup>H + <sup>11</sup>B → 3&nbsp;<sup>4</sup>He
 
Under reasonable assumptions, side reactions will result in about 0.1% of the fusion power being carried by neutrons.<ref>Heindler and Kernbichler, ''Proc. 5th Intl. Conf. on Emerging Nuclear Energy Systems'', 1989, pp. 177–82. See also [[Aneutronic fusion#Residual radiation from a p–11B reactor|Residual radiation from a p–11B reactor]]</ref> At 123 keV, the optimum temperature for this reaction is nearly ten times higher than that for the pure hydrogen reactions, the energy confinement must be 500 times better than that required for the D-T reaction, and the [[power density]] will be 2500 times lower than for D-T. Since the confinement properties of conventional approaches to fusion such as the tokamak and laser pellet fusion are marginal, most proposals for aneutronic fusion are based on radically different confinement concepts, such as the [[Polywell]] and the [[Dense Plasma Focus]].
 
==Confinement==
 
[[Image:IFE and MFE parameter space.svg|thumb|right|250px|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.]]
 
=== Unconfined ===
 
The first human-made, large-scale fusion reaction was the test of the [[hydrogen bomb]], [[Ivy Mike]], in 1952. As part of the [[PACER (fusion)|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. {{Citation needed|date=February 2010}} The hydrogen bomb really has no confinement at all.
 
=== General Confinement Principals ===
 
Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion:
 
*[[Mechanical equilibrium|Equilibrium]]: The forces acting on the plasma must be balanced so that it will not rapidly disassemble. The exception, of course, is [[inertial confinement fusion|inertial confinement]], where the relevant physics must occur faster than the disassembly time.
*[[Plasma stability|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".
 
Controlled fusion refers to the continuous power production from fusion, 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.<ref>{{cite book|last=Atzeni|first=Stefano|title=The Physics of Inertial Fusion|year=2009|publisher=Oxford Science Publications|location=USA|isbn=978-0-19-956801-7|page=42}}</ref>  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 energy is called energy confinement and may be accomplished in a number of ways, Material, Gravitational, Electrostatic, Inertial, and Magnetic Confinement.<ref>{{cite book|last=Harms|first=A|title=Principles of Fusion Energy|year=2000|publisher=World Scientific|location=USA|isbn=978-981-238-033-3|pages=47–56|url=http://www.worldscientific.com}}</ref>
 
===Inertial Confinement ===
 
[[inertial confinement fusion|Inertial confinement]] is the use of rapidly imploding shell to heat and confine plasma.  The shell is imploded using a direct laser blast (direct drive) or a secondary x-ray blast (indirect drive). Theoretically, fusion using lasers 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 direct drive is used - the beams are focused directly on the pellet - it can in principle be very efficient, but in practice is difficult to obtain the needed uniformity.<ref>{{cite book|last=Pfalzner|first=Susanne|title=An Introduction to Inertial Confinement Fusion|year=2006|publisher=Taylor & Francis|location=USA|isbn=0-7503-0701-3|pages=19–20}}</ref> The alternative approach, indirect drive, uses beams to heat a shell, and then the shell radiates [[x-rays]], which then implode the pellet. The beams are commonly laser beams, but heavy and light [[ion beam]]s and electron beams have all been investigated.<ref>{{cite book|last=Pfalzner|first=Susanne|title=An Introduction to Inertial Confinement Fusion|year=2006|publisher=Taylor & Francis|location=USA|isbn=0-7503-0701-3|pages=182–193}}</ref>
 
Inertial confinement produces plasmas with very high densities and temperatures making it suitable for weapons research, [[X-ray generation]] and perhaps in the distant future, spaceflight {{Citation needed|date=October 2009}}. ICF implosions require fuel pellets with close to a perfect shape in order to generate an symmetrical inward [[shock wave]] and to produce the high-density plasma.  These are known as targets and, building them has presented its own technical challenges.
 
A recent development in ICF research is '''fast ignition'''.  This is the use of two laser systems to heat a compressed targets.  A conventional laser system compresses the pellet, after which, a second ultrashort laser pulse heats the compressed plasma.  This burst has many [[petawatt]] of energy.  Fast ignition implodes the pellet at the exactly the moment of greatest density.  Research into fast ignition has been carried out at the OMEGA EP petawatt and [[LLE|OMEGA]] lasers at the [[University of Rochester]] and at the GEKKO XII laser at the institute for laser engineering in Osaka Japan. If fruitful, it may have the effect of greatly reducing the cost of a laser fusion based power source.{{Citation needed|date=October 2009}}
 
===Magnetic Confinement ===
 
At the temperatures required for fusion, the fuel is heated to a plasma state.  In this state it has a very good [[electrical conductivity]].  This opens the possibility of confining the plasma with [[magnetic field]]s, an idea known as [[magnetic fusion energy|magnetic confinement]].  This puts a [[Lorenz force]] on the plasma.  The force, works perpendicular to the magnetic fields, so one problem in magnetic confinement is preventing the plasma from leaking out the ends of the field lines.  These ends are known as magnetic cusps.  There are basically two solutions.{{Citation needed|date=October 2009}}
 
The first is to use the [[magnetic mirror]] effect.  If a particle follows the field line and enters a region of higher field strength, the particles can be reflected.  There are several devices which try to use this effect.  The most famous was the magnetic mirror machines, which was a series of large, expensive devices built at the [[Lawrence Livermore National Laboratory]] from the 1960s to mid 1980s.<ref>Booth, William. "Fusion's $372-Million Mothball." Science [New York City] 9 Oct. 1987, Volume 238 ed.: 152-55. Print</ref>  Some other examples include the magnetic bottles and [[Biconic cusp]].<ref>Containment in a cusped Plasma System, Dr. Harold Grad, NYO-9496</ref>  Because the mirror machines were straight, they had some advantages over a ring shape.  First, mirrors would easier to construct and maintain and second [[Direct conversion]] energy capture, was easier to implement.<ref>"Experimental results from a beam direct converter at 100 kV" R. W. MOIR, W. L. BARR, Journal of fusion energy, Volume 2, No 2, 1982</ref>  However the confinement achieved in experiments was poor, that this approach has been essentially abandoned.{{Citation needed|date=October 2009}}
 
The second possibility to prevent end losses is to bend the field lines back on themselves, either in circles or more commonly in nested [[torus|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 space|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|date=October 2009}}
 
===Electrostatic Confinement ===
 
There is also [[Inertial electrostatic confinement|electrostatic confinement fusion]] devices.  These devices heats and confines [[ion]]s using electrostatic fields.  The most well know is the [[Fusor]].  This device has an cathode inside a anode wire cage.  Positive ions fly towards the negative inner cage, and are heated by the electric field in the process.  If they miss the inner cage they can collide and fuse.  However, ions typically hit the cathode, creating prohibitory high [[conduction]] losses.  Also, fusion rates in [[fusor]]s are very low due to competing physical effects, such as energy loss in the form of light radiation.<ref>Ion Flow and Fusion Reactivity, Characterization of a Spherically convergent ion Focus. PhD Thesis, Dr. Timothy A Thorson, Wisconsin-Madison 1996.</ref>  Designs have been purposed to avoid the problems associated with the cage, by generating the field using a non-neutral cloud.  These include a plasma oscillating device,<ref>"Stable, thermal equilibrium, large-amplitude, spherical plasma oscillations in electrostatic confinement devices", DC Barnes and Rick Nebel, PHYSICS OF PLASMAS VOLUME 5, NUMBER 7 JULY 1998</ref> a [[penning trap]] and the [[polywell]].<ref>Carr, M.; Khachan, J. (2013). "A biased probe analysis of potential well formation in an electron only, low beta Polywell magnetic field". Physics of Plasmas 20 (5): 052504. Bibcode:2013PhPl...20e2504C. doi:10.1063/1.4804279.</ref>  However, the technology is relatively immature, and many scientific and engineering questions remain.
 
== Other Issues ==
 
===Materials===
{{Main|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 reactor]]s (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 by way of various nuclear reactions that tends to form bubbles at grain boundaries and result in swelling, blistering or embrittlement. There is also a need for 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 as of 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 large thermal loads, up to 10&nbsp;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-[[Atomic number|Z]] material, such as [[graphite]] or [[beryllium]], or a high-[[Atomic number|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&nbsp;mm thick streams flowing at 10&nbsp;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 by the plasma fuel ions is orders of magnitude smaller than that of carbon, and tritium is much less 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.
 
===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 [[nuclear power|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 turbine]]s; 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, itself surrounded 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.
 
[[Image:Fusion target implosion on NOVA laser.jpg|thumb|right|200px|Inertial confinement fusion implosion on the [[Nova laser]] creates "microsun" conditions of tremendously high density and temperature.]]
Although the standard solution for electricity production in fusion power plant designs is conventional steam turbines using the heat deposited by neutrons, there are also designs for direct conversion of the energy of the charged particles into electricity. These are of little value with a D-T fuel cycle, where 80% of the power is in the neutrons, but are indispensable with [[aneutronic fusion]], where less than 1% is. Direct conversion has been most commonly proposed for open-ended magnetic configurations like [[magnetic mirror]]s or [[Field-Reversed Configuration]]s, where charged particles are lost along the magnetic field lines, which are then expanded to convert a large fraction of the random energy of the fusion products into directed motion. The particles are then collected on electrodes at various large electrical potentials. Typically the claimed conversion efficiency is in the range of 80%, but the converter may approach the reactor itself in size and expense.
 
==Safety and the environment==
 
===Accident potential===
There is no possibility of a catastrophic accident in a fusion reactor resulting in major release of radioactivity to the environment or injury to non-staff, unlike modern fission reactors. The primary reason is that the requirements for nuclear fusion differ greatly from nuclear fission - it requires extremely precise and controlled temperature, pressure, and magnetic field parameters for any net energy to be produced, and a far smaller amount of fuel. If the reactor suffered damage or lost even a small degree of required control, fusion reactions and heat generation would rapidly cease.<ref name=afraid>{{cite web |url= http://www.iter.org/newsline/107/1489 |title= Who is afraid of ITER? |first= Krista|last=Dulon |work=iter.org |year=2012 |accessdate=18 August 2012}}</ref>
 
Therefore fusion reactors are considered extremely safe in this sense, making them favorable over fission reactors, which, in contrast, continue to generate heat through [[beta-decay]] for several months after reactor shut-down, meaning that melting of fuel rods is possible even after the reactor has been stopped due to continued accumulation of heat.<ref name="McCrackenStott2012">{{cite book|last1=McCracken|first1=Garry |last2=Stott|first2=Peter |title=Fusion: The Energy of the Universe|url=http://books.google.com/books?id=e6jEZfO2gO4C&pg=PA198|accessdate=18 August 2012|date=8 June 2012|publisher=Academic Press|isbn=978-0-12-384656-3|pages=198–199}}</ref>
 
There is also no risk of a runaway reaction in a fusion reactor. The [[plasma (physics)|plasma]] is burnt at optimal conditions, and any significant change will render it unable to react or to produce excess heat. In fusion reactors the reaction process is so delicate that this level of safety is inherent; no elaborate failsafe mechanism is required.  Although the plasma in a fusion power plant will have a volume of 1000 cubic meters or more, the density of the plasma is extremely low, and the total amount of fusion fuel in the vessel is very small, typically a few grams.<ref name="McCrackenStott2012"/> If the fuel supply is closed, the reaction stops within seconds. In comparison, a fission reactor is typically loaded with enough fuel for several years, and no additional fuel is necessary to keep the reaction going.<ref name="Angelo2004">{{cite book|last=Angelo|first=Joseph A. |title=Nuclear Technology|url=http://books.google.com/books?id=ITfaP-xY3LsC&pg=PA474|accessdate=18 August 2012|date=30 November 2004|publisher=Greenwood Publishing Group|isbn=978-1-57356-336-9|page=474}}</ref>
 
In the magnetic approach, strong fields are developed in coils that are held in place mechanically by the reactor structure. Failure of this structure could release this tension and allow the magnet to "explode" outward. The severity of this event would be similar to any other industrial accident or an [[MRI]] machine [http://www.indyrad.iupui.edu/RadWeb/Portals/0/ContentFiles/Education/Training/MR_Safety_files/frame.htm quench]/explosion, and could be effectively stopped with a [[containment building]] similar to those used in existing (fission) nuclear generators. The laser-driven inertial approach is generally lower-stress. Although failure of the reaction chamber is possible, simply stopping fuel delivery would prevent any sort of catastrophic failure.
 
Most reactor designs rely on the use of liquid [[lithium]] as both a coolant and a method for converting stray neutrons from the reaction into [[tritium]], which is fed back into the reactor as fuel. Lithium is highly flammable, and in the case of a fire it is possible that the lithium stored on-site could be burned up and escape. In this case the tritium contents of the lithium would be released into the atmosphere, posing a radiation risk. However, calculations suggest that at about 1&nbsp;kg the total amount of tritium and other radioactive gases in a typical power plant would be so small that they would have diluted to legally acceptable limits by the time they blew as far as the plant's [[perimeter fence]].<ref name="WorldEnergyCouncil"/>
 
The likelihood of ''small industrial'' accidents including the local release of radioactivity and injury to staff cannot be estimated yet. These would include accidental releases of lithium, tritium, or mis-handling of decommissioned radioactive components of the reactor itself.
 
===Effluents during normal operation===
The natural product of the fusion reaction is a small amount of [[helium]], which is completely harmless to life.  Of more concern is [[tritium]], which, like other isotopes of hydrogen, is difficult to retain completely. During normal operation, some amount of tritium will be continually released. There would be no acute danger, but the cumulative effect on the world's population from a fusion economy could be a matter of concern.{{Citation needed|date=June 2007}}
 
Although tritium is volatile and biologically active, the health risk posed by a release is much lower than that of most radioactive contaminants, due to tritium's short half-life (12.32 years), very low decay energy (~14.95 keV), and the fact that it does not [[bioaccumulation|bioaccumulate]] (instead being cycled out of the body as water, with a [[biological half-life]] of 7 to 14 days).<ref name="nuclearsafety-petrangeli">{{cite book |title=Nuclear Safety |last=Petrangeli |first=Gianni |year=2006 |publisher=Butterworth-Heinemann |isbn=978-0-7506-6723-4 |page=430 |accessdate=}}</ref> Current ITER designs are investigating total containment facilities for any tritium.
 
===Waste management===
The large flux of high-energy neutrons in a reactor will make the structural materials radioactive. The radioactive inventory at shut-down may be comparable to that of a fission reactor, but there are important differences.
 
The half-life of the [[radioisotopes]] produced by fusion tends to be less than those from fission, so that the inventory decreases more rapidly. Unlike fission reactors, whose waste remains radioactive for thousands of years, most of the radioactive material in a fusion reactor would be the reactor core itself, which would be dangerous for about 50 years, and low-level waste another 100.  Although this waste will be considerably more radioactive during those 50 years than fission waste, the very short half-life makes the process very attractive, as the waste management is fairly straightforward.  By 300 years the material would have the same radioactivity as [[coal ash]].<ref name="WorldEnergyCouncil">{{cite web
|url=http://www.worldenergy.org/wec-geis/publications/default/tech_papers/18th_Congress/downloads/ds/ds6/ds6_5.pdf
|format=PDF|title=Fusion as a Future Power Source: Recent Achievements and Prospects
|author=T. Hamacher and A.M. Bradshaw
|publisher=World Energy Council
|date=October 2001
|archiveurl=http://web.archive.org/web/20040506065141/http://www.worldenergy.org/wec-geis/publications/default/tech_papers/18th_Congress/downloads/ds/ds6/ds6_5.pdf
|archivedate=2004-05-06
}}</ref>
 
Additionally, the choice of materials used in a fusion reactor is less constrained than in a fission design, where many materials are required for their specific [[neutron cross-section]]s. This allows a fusion reactor to be designed using materials that are selected specifically to be "low activation", materials that do not easily become radioactive. [[Vanadium]], for example, would become much less radioactive than [[stainless steel]]. [[Carbon fiber]] materials are also low-activation, as well as being strong and light, and are a promising area of study for laser-inertial reactors where a magnetic field is not required.
 
In general terms, fusion reactors would create far less radioactive material than a fission reactor, the material it would create is less damaging biologically, and the radioactivity "burns off" within a time period that is well within existing engineering capabilities for safe long-term waste storage.
 
===Nuclear proliferation===
{{main|Nuclear proliferation}}
Although fusion power uses nuclear technology, the overlap with nuclear weapons technology is limited.
 
A huge amount of [[tritium]] would be produced in fusion power plants. Tritium is used in the trigger of [[hydrogen bomb]]s and in most modern [[boosted fission weapon]]s but it can be also produced by nuclear fission.
 
The energetic neutrons from a fusion reactor could be used to breed weapon usable [[plutonium]] or [[uranium]] for an atomic bomb (for example by transmutation of U<sup>238</sup> to Pu<sup>239</sup>, or Th<sup>232</sup> to U<sup>233</sup>).
 
'''A study conducted 2011 assessed the risk of three scenarios:'''<ref name="ProliferationRisk_Goldston" />
 
====Using a secret small-scale nuclear fusion system====
Due to much higher power consumption, heat dissipation and a more unique design compared to enrichment [[gas centrifuge]]s this choice would be much easier to detect and therefore implausible.<ref name="ProliferationRisk_Goldston" />
 
====Covert modifications to produce weapon-usable material in a declared commercial-size facility====
The production potential is significant. But no fertile or fissile substances necessary for the production of weapon-usable materials needs to be present at a civil fusion system at all.
If not shielded, a detection of these materials can be done by their characteristic gamma radiation. The underlaying redesign could be detected by regular design information verifications. In the (technically more feasible) case of solid breeder blanket modules, it would be necessary for incoming components to be inspected for the presence of fertile material,<ref name="ProliferationRisk_Goldston" /> otherwise plutonium for several weapons could be produced each year.<ref name="StrongNeutronSources" />
 
====Prioritizing a fast production of weapon usable material regardless of secrecy====
The fastest way to produce weapon usable material was seen in modifying a prior civil fusion power plant. Unlike in some nuclear power plants, there is no weapon compatible material during civil use. Even without the need for covert action this modification would still take about 2 months to start the production and at least an additional week to generate a significant amount for weapon production. This was seen as enough time to detect a military use and to react with diplomatic or military means. To stop the production, a military destruction of inevitable parts of the facility leaving out the reactor itself would be sufficient. This, together with the intrinsic safety of fusion power would only bear a low risk of radioactive contamination.<ref name="ProliferationRisk_Goldston" />
 
Another study concludes that "[..]large fusion reactors – even if not designed for fissile material breeding – could easily produce several hundred kg Pu per year with high weapon quality and very low source material requirements." It is empathized that the implementation of features for intrinsic proliferation resistance might only be possible at this early phase of research and development.<ref name="StrongNeutronSources" />
The theoretical and computational tools needed for hydrogen bomb design are closely related to those needed for [[inertial confinement fusion]], but have very little in common with the more scientifically developed [[magnetic confinement fusion]].
 
===As a sustainable energy source===
Large-scale reactors using neutronic fuels (e.g. [[ITER]]) and thermal power production (turbine based) are most comparable to [[nuclear power|fission power]] from an engineering and economics viewpoint. Both fission and fusion power plants involve a relatively compact heat source powering a conventional steam turbine-based power plant, while producing enough neutron radiation to make [[neutron activation|activation]] of the plant materials problematic. The main distinction is that fusion power produces no high-level radioactive waste (though activated plant materials still need to be disposed of). There are some power plant ideas which may significantly lower the cost or size of such plants; however, research in these areas is nowhere near as advanced as in [[tokamak]]s{{Citation needed|date=July 2013}}.
 
Fusion power commonly proposes the use of [[deuterium]], an [[isotope]] of hydrogen, as fuel and in many current designs also use [[lithium]]. Assuming a fusion energy output equal to the 1995 global power output of about 100 [[exa-|E]]J/yr (= 1 × 10<sup>20</sup> J/yr) and that this does not increase in the future, then the known current lithium reserves would last 3000 years, lithium from sea water would last 60 million years, and a more complicated fusion process using only deuterium from sea water would have fuel for 150 billion years.<ref>{{cite web|url=http://www.agci.org/dB/PDFs/03S2_MMauel_SafeFusion%3F.pdf |title=Energy for Future Centuries |format=PDF |date= |accessdate=2013-06-22}}</ref> To put this in context, 150 billion years is close to 30 times the remaining lifespan of the sun,<ref name="sunlife">{{cite web|url=http://helios.gsfc.nasa.gov/qa_sun.html#sunlife|title=Cosmicopia|last=Dr. Eric Christian, Et al.|publisher=NASA|accessdate=2009-03-20}}</ref> and more than 10 times the estimated age of the universe.
 
==Economics==
While fusion power is still in early stages of development, substantial sums have been and continue to be invested in research. In the EU almost {{nowrap|€10 billion}} was spent on fusion research up to the end of the 1990s, and the new [[ITER]] reactor alone is budgeted at {{nowrap|€10 billion}}.
 
It is estimated that up to the point of possible implementation of electricity generation by nuclear fusion, R&D will need further promotion totalling around {{nowrap|€60–80 billion}} over a period of {{nowrap|50 years}} or so (of which {{nowrap|€20–30 billion}} within the EU) based on a report from 2002.<ref>{{cite web| title=The current EU research programme | url=http://www.tab.fzk.de/en/projekt/zusammenfassung/ab75.htm | work=[[Sixth Framework Programme|FP6]] | publisher= | date= | accessdate= }}</ref> Nuclear fusion research receives {{nowrap|€750 million}} (excluding ITER funding), compared with {{nowrap|€810 million}} for all non-nuclear energy research combined,<ref>{{cite web| title=The Sixth Framework Programme in brief | url=http://ec.europa.eu/research/fp6/pdf/fp6-in-brief_en.pdf | work= | publisher= | date= | accessdate= }}</ref> putting research into fusion power well ahead of that of any single rivaling technology.
 
==Advantages==
Fusion power would provide much more energy for a given weight of fuel than any technology currently in use,<ref>{{cite web
|url=http://fusedweb.llnl.gov/FAQ/section2-energy/part2-enviro.txt
|title=Conventional Fusion FAQ Section 2/11 (Energy) Part 2/5 (Environmental)
|author=Robert F. Heeter, et al.}}</ref> and the fuel itself (primarily [[deuterium]]) exists abundantly in the Earth's ocean: about 1 in 6500 hydrogen atoms in seawater is deuterium.<ref>{{cite web
|url=http://presolar.wustl.edu/work/abundances.html
|archiveurl=http://web.archive.org/web/20110720122226/http://presolar.wustl.edu/work/abundances.html
|archivedate=2011-07-20
|title=Relative Abundances of Stable Isotopes
|author=Dr. Frank J. Stadermann
|publisher=Laboratory for Space Sciences, Washington University in St. Louis
}}</ref>  Although this may seem a low proportion (about 0.015%), because nuclear fusion reactions are so much more energetic than chemical combustion and seawater is easier to access and more plentiful than fossil fuels, fusion could potentially supply the world's energy needs for millions of years.<ref>{{cite web
|url=http://www.agci.org/dB/PDFs/03S2_MMauel_SafeFusion%3F.pdf
|format=PDF|title=Energy for Future Centuries
|author=J. Ongena and G. Van Oost
|publisher=Laboratorium voor Plasmafysica– Laboratoire de Physique des Plasmas Koninklijke Militaire School– Ecole Royale Militaire; Laboratorium voor Natuurkunde, Universiteit Gent
|pages=Section III.B. and Table VI
}}</ref><ref>{{cite web
|url=http://www.eps.org/about-us/position-papers/fusion-energy/
|archiveurl=http://web.archive.org/web/20081008001417/http://www.eps.org/about-us/position-papers/fusion-energy/
|archivedate=2008-10-08
|title=The importance of European fusion energy research
|publisher=The European Physical Society
|author=EPS Executive Committee
}}</ref>
 
Despite being technically [[non-renewable energy|non-renewable]], fusion power has many of the benefits of renewable energy sources (such as being a long-term energy supply and emitting no [[greenhouse gas]]es) as well as some of the benefits of the resource-limited energy sources as hydrocarbons and nuclear fission (without [[Nuclear reprocessing|reprocessing]]). Like these currently dominant energy sources, fusion could provide very high power-generation density and uninterrupted power delivery (due to the fact that it is not dependent on the [[weather]], unlike wind and solar power).
 
Another aspect of fusion energy is that the cost of production does not suffer from [[diseconomies of scale]]. The cost of water and wind energy, for example, goes up as the optimal locations are developed first, while further generators must be sited in less ideal conditions. With fusion energy, the production cost will not increase much, even if large numbers of plants are built, because the sites of plants are not resources.
 
Some problems which are expected to be an issue in this century such as [[water resources|fresh water shortages]] can alternatively be regarded as problems of energy supply. For example, in [[desalination]] plants, [[seawater]] can be purified through [[distillation]] or [[reverse osmosis]]. However, these processes are energy intensive. Even if the first fusion plants are not competitive with alternative sources, fusion could still become competitive if large-scale desalination requires more power than the alternatives are able to provide.
 
A scenario has been presented of the effect of the commercialization of fusion power on the future of human civilization.<ref>{{cite web
|url=http://www.plasmafocus.net/IPFS/2010%20Papers/LSmankind.pdf|title=Nuclear Fusion Energy-Mankind's Giant Step Forward|author=Sing Lee and Sor Heoh Saw}}</ref> ITER and later Demo are envisioned to bring online the first commercial nuclear fusion energy reactor by 2050. Using this as the starting point and the history of the uptake of nuclear fission reactors as a guide, the scenario depicts a rapid take up of nuclear fusion energy starting after the middle of this century.
 
Fusion power could be used in [[interstellar space]], where solar energy is not available.
 
==See also==
*[[List of fusion experiments]]
* [[FuseNet]]
{{Portal bar|Energy|Sustainable development}}
 
==References==
 
=== Notes ===
{{Reflist|30em
 
<ref name="ProliferationRisk_Goldston">R. J. Goldston, A. Glaser, A. F. Ross: [http://web.mit.edu/fusion-fission/HybridsPubli/Fusion_Proliferation_Risks.pdf "Proliferation Risks of Fusion Energy: Clandestine Production, Covert Production, and Breakout"];''9th  IAEA Technical Meeting on Fusion Power Plant Safety'' (accessible at no cost, 2013) and
 
{{Cite DOI|10.1088/0029-5515/52/4/043004}}</ref>
 
<ref name="StrongNeutronSources">[http://www.ianus.tu-darmstadt.de/media/ianus/pdfs/matthias/Strong_Neutron_Sources_final.pdf "Strong Neutron Sources - How to cope with weapon material production capabilities of fusion and spallation neutron sources?"] Matthias Englert, Giorgio Franceschini, Wolfgang Liebert (2011); ''7th INMM/Esarda Workshop'', Aix­‐en­‐Provence</ref>
}}
 
=== Bibliography ===
{{refbegin|60em}}
* Chen, Francis (2011). ''An Indispensable Truth: How Fusion Power Can Save the Planet''. New York: Springer. ISBN 978-1441978196
* Clery, Daniel (2013). ''A Piece of the Sun''. New York: Overlook. ISBN 978-1468304930
* Dean, Stephen (2013). ''Search for the Ultimate Energy Source: A History of the U.S. Fusion Energy Program''. New York: Springer. ISBN 978-1461460367
* Molina, Andrés de Bustos (2013) ''Kinetic Simulations of Ion Transport in Fusion Devices''. New York: Springer. ISBN 978-3319004211
* {{cite journal |last=Voss|first=David |title=What Ever Happened to Cold Fusion |journal=Physics World |date=March 1, 1999 |url=http://physicsworld.com/cws/article/print/1258|accessdate=1 May 2008 |issn=0953-8585|ref=harv|accessdate=2012-08-18}}
* {{cite journal|last=Kruglinksi|first=Susan |title=Whatever Happened To... Cold Fusion? |journal=Discover Magazine |date=2006-03-03 |url=http://discovermagazine.com/2006/mar/cold-fusion|accessdate = 20 June 2008|issn=0274-7529|ref=harv}}
* {{cite journal |last=Choi|first=Charles |title=Back to Square One |periodical=Scientific American |year=2005 |url=http://www.scientificamerican.com/article.cfm?id=back-to-square-one |accessdate=25 November 2008 |doi=|ref=harv}}
* {{cite journal|last=Feder|first=Toni |title=Cold Fusion Gets Chilly Encore |journal=Physics Today |date=January 2005 |url=http://physicstoday.org/journals/doc/PHTOAD-ft/vol_58/iss_1/31_1.shtml?bypassSSO=1  |doi=10.1063/1.1881896 |volume=58 |page=31|bibcode = 2005PhT....58a..31F |ref=harv}}
* {{Citation |last=Hagelstein|first=Peter L. |authorlink=Peter L. Hagelstein |last2=Michael|first2=McKubre |last3=Nagel|first3=David |last4=Chubb|first4=Talbot |last5=Hekman|first5=Randall |ref=CITEREFDOE2004 |title=New Physical Effects in Metal Deuterides |location=Washington |publisher=US Department of Energy |year=2004 |url=http://web.archive.org/web/20070106185101/www.science.doe.gov/Sub/Newsroom/News_Releases/DOE-SC/2004/low_energy/Appendix_1.pdf  |format=PDF}} (manuscript)
* {{Citation |author=U.S. Department of Energy |ref=CITEREFDOE2004r |year=2004 |title=Report of the Review of Low Energy Nuclear Reactions |location=Washington, DC |publisher=U.S. Department of Energy |url=http://www.science.doe.gov/Sub/Newsroom/News_Releases/DOE-SC/2004/low_energy/CF_Final_120104.pdf |format=PDF |accessdate=2008-07-19 |archiveurl=http://web.archive.org/web/20080226210800/http://www.science.doe.gov/Sub/Newsroom/News_Releases/DOE-SC/2004/low_energy/CF_Final_120104.pdf |archivedate=2008-02-26}}
* {{Citation |last=Goodstein|first=David |title=Whatever happened to cold fusion? |journal=American Scholar |publisher=Phi Beta Kappa Society |volume=63 |issue=4 |year=1994 |pages=527–541|url=http://www.its.caltech.edu/~dg/fusion_art.html |accessdate = 2008-05-25 |issn=0003-0937 |doi=}}
* {{Citation |last=Close|first=Frank E. |authorlink=Frank Close |title=Too Hot to Handle: The Race for Cold Fusion |edition=2 |location=London |publisher=Penguin |year=1992 |isbn=0-14-015926-6}}
* {{Citation |last=Beaudette|first=Charles G. |title=Excess Heat & Why Cold Fusion Research Prevailed |year=2002 |location=South Bristol, Maine |publisher=Oak Grove Press |isbn=0-9678548-3-0}}
* {{Citation |last=Van Noorden|first= R. |title=Cold fusion back on the menu |journal=Chemistry World |date=April 2007 |url=http://www.rsc.org/chemistryworld/News/2007/March/22030701.asp |accessdate=2008-05-25 |issn=1473-7604 |doi= }}
* {{cite book |last=Taubes|first=Gary |authorlink=Gary Taubes|title=[[Bad Science: The Short Life and Weird Times of Cold Fusion]]|location=New York |publisher=Random House |year=1993 |isbn=0-394-58456-2|ref=harv}}
* {{Citation |last=Browne|first=M. |title=Physicists Debunk Claim Of a New Kind of Fusion |newspaper=New York Times |date=May 3, 1989 |url=http://partners.nytimes.com/library/national/science/050399sci-cold-fusion.html |accessdate=2008-05-25}}<!--also http://query.nytimes.com/gst/fullpage.html?res=950DE2D71539F930A35756C0A96F948260&pagewanted=all -->
* {{Citation |last=Adam|first=David |editor-last=Rusbringer|editor-first=Alan |title=In from the cold |newspaper=The Guardian |date=24 March 2005 |url=http://www.guardian.co.uk/education/2005/mar/24/research.highereducation2 |accessdate=2008-05-25 |issn= |doi= | location=London}}
* {{Citation |last=Platt|first=Charles |title=What if Cold Fusion is Real? |periodical=[[Wired Magazine]] |year=1998 |issue=6.11 |url=http://www.wired.com/wired/archive/6.11/coldfusion.html?pg=1&topic=&topic_set= |accessdate=2008-05-25}}
*{{Citation|last=Hutchinson|first=Alex |title=The Year in Science: Physics |periodical=Discover Magazine (online) |date=January 8, 2006 |url=http://discovermagazine.com/2006/jan/physics|accessdate=2008-06-20 |issn=0274-7529 |doi=}}
* {{Citation |last=Adam|first=David |editor-last=Rusbringer|editor-first=Alan |title=In from the cold |newspaper=The Guardian |date=24 March 2005 |url=http://education.guardian.co.uk/higher/research/story/0,9865,1444306,00.html |accessdate=2008-05-25 |issn= |doi=| location=London}}
* {{Cite journal |ref=harv |first=Randy|last=Alfred |title=March 23, 1989: Cold Fusion Gets Cold Shoulder |journal=[[Wired (magazine)|Wired]] |date=2009-03-23 |url=http://www.wired.com/science/discoveries/news/2009/03/dayintech_0323 }}
{{refend}}
 
==External links==
{{external links|date=November 2012}}
* [http://www.iop.org/activity/policy/Publications/file_31695.pdf Fusion as an Energy Source]
* [http://science.energy.gov/fes/ U.S. Fusion Energy Science Program]
* [http://www.fusion.org.uk/ EURATOM/UKAEA Fusion Association]
* [http://www.iter.org/ ITER]
* [http://www.efda.org/ European Fusion Development Agreement]
* {{PDFlink|[http://web.archive.org/web/20090327020927/http://www.fusion.org.uk/techdocs/euromat.pdf "Low Activation Material Candidates For Fusion Power Plants"; C.B.A. Forty and N.P. Taylor]|58.8&nbsp;KB}}
* [http://www.fusionenergy.net.au/ A Central Site for Fusion Energy Links]
* [http://www.plasmafocus.net/ Institute for Plasma Focus Studies]
 
{{Fusion power}}
{{Nuclear Technology}}
{{Emerging technologies}}
 
{{DEFAULTSORT:Fusion Power}}
[[Category:Fusion power|*]]
[[Category:Alternative energy]]
[[Category:Emerging technologies]]
 
[[th:ปฏิกิริยานิวเคลียร์ฟิวชัน]]

Revision as of 22:10, 11 February 2014


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