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| | After a rather lackluster 2010 (up before the end of the season), issues appear to be looking up in 2011 on the planet of online revenue. It doesn't appear that itIs simply online sales either. Brick and mortar stores claimed a fairly good revenue outlook the past 1 / 2 of 2010.<br><br>It may also be yours. One high-traffic site to send your articles to is EzineArticles.com. Their Alexa rank is 259 at the conclusion of 2008. That Is thousands and thousands of more visitors than visit your site alone. Get wise: learn mistakes and the 5 new changes of article marketing which will affect you this coming year.<br><br>Marketing a web based business is really a combination of knowledge, energy and business acumen. There is a learning curve. Few, if any, on-line millionaires made their fortunes immediately. Most of them spent years learning what not to do and what to complete.<br><br><br><br>Below are a few handy ideas to enable you to get started on the trail to better presence online to your items. Many involve only the investment of time, and will not bust your budget.<br><br>To make all this work seamlessly you'll should find out some advanced and basic capabilities of [http://www.elmostrador.cl/c_avisos/imagenes/LARRA%C3%8DN%20VIAL%20ACTIVOS%2014%2004%2011%20Fondos%20DS04.pdf jaun pablo schiappacasse canepa]. If performed effectively, people will discover you simply. Not only will people find you, but you will basically be seen while the industry expert.<br><br>If you are at a computer and need to find some interesting items on the net to entertain you, the listing of most entertaining websites may oftimes be beneficial to you. The websites keep you not just in a few momemts but maybe hours and throughout the day.<br><br>Business ethics will be the key a number of business successes. Work done ethically will bear fruit, because it will be done well. Make certain that you deliver, for your customers, what you offer. This will make sure they are stick to you to get a number of years and the term of mouth will take effect. Plenty of agencies these days, prefer to market and promote via the web, simply because that's where in actuality the whole planet is, right? Nevertheless, some organizations won't cash in on the Net up to they would normally. Just in case, of such firms, you could support them with online earning selections instead of marketing. This might help your credibility to become noticeable. |
| {{use dmy dates|date=January 2012}}
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| [[File:The Sun by the Atmospheric Imaging Assembly of NASA's Solar Dynamics Observatory - 20100819.jpg|thumb|250px|The [[Sun]] is a [[main-sequence star]], and thus generates its [[energy]] by nuclear fusion of [[hydrogen]] nuclei into [[helium]]. In its core, the Sun fuses 620 million [[metric ton]]s of hydrogen each second.]]
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| {{nuclear physics}}
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| In [[nuclear physics]], '''nuclear fusion''' is a [[nuclear reaction]] in which two or more [[atomic nuclei]] collide at a very high speed and join to form a new type of atomic nucleus. During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to [[photons]] ([[fusion power|energy]]). Fusion is the process that powers active or "[[main sequence]]" [[star]]s.
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| The fusion of two nuclei with lower masses than [[iron]] (which, along with [[nickel]], has the largest [[binding energy]] per [[nucleon]]) generally releases energy, while the fusion of nuclei heavier than iron ''absorbs'' energy. The opposite is true for the reverse process, [[nuclear fission]]. This means that fusion generally occurs for lighter elements only, and likewise, that fission normally occurs only for heavier elements. There are extreme [[astrophysics|astrophysical]] events that can lead to short periods of fusion with heavier nuclei. This is the process that gives rise to [[nucleosynthesis]], the creation of the heavy elements during events such as [[supernova]].
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| Following the discovery of [[quantum tunneling]] by [[Friedrich Hund]], in 1929 [[Robert d'Escourt Atkinson|Robert Atkinson]] and [[Fritz Houtermans]] used the measured masses of light elements to predict that large amounts of energy could be released by fusing small nuclei. Building upon the [[nuclear transmutation]] experiments by [[Ernest Rutherford]], carried out several years earlier, the laboratory fusion of [[hydrogen isotopes]] was first accomplished by [[Mark Oliphant]] in 1932. During the remainder of that decade the steps of the main cycle of nuclear fusion in stars were worked out by [[Hans Bethe]]. Research into fusion for military purposes began in the early 1940s as part of the [[Manhattan Project]], but this was not accomplished until 1951 (see the [[Greenhouse Item]] nuclear test), and nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the [[Ivy Mike]] [[hydrogen bomb]] test.
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| Research into developing controlled [[thermonuclear fusion]] for civil purposes also began in earnest in the 1950s, and it continues to this day. Two projects, the [[National Ignition Facility]] and [[ITER]], are expected to have high [[Fusion energy gain factor|gains]], that is, producing more energy than required to ignite the reaction, after 60 years of design improvements developed from previous experiments.{{Citation needed|date=February 2012}} While these [[Inertial confinement fusion|ICF]] and [[Tokamak]] designs became popular in recent times, experiments with [[Stellarator]]s are gaining international scientific attention again, like [[Wendelstein 7-X]] in [[Greifswald]], [[Germany]].
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| ==Overview==
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| [[Image:Deuterium-tritium fusion.svg|thumb|Fusion of [[deuterium]] with [[tritium]] creating [[helium-4]], freeing a [[neutron]], and releasing 17.59 [[Electronvolt|MeV]] of energy, as an appropriate amount of mass changing forms to appear as the kinetic energy of the products, in agreement with ''kinetic E'' = Δ''mc''<sup>2</sup>, where ''Δ''m is the change in rest mass of particles.<ref name=Shultis>
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| {{cite book
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| |author=Shultis, J.K. and Faw, R.E.
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| |year=2002
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| |title=Fundamentals of nuclear science and engineering
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| |url=http://books.google.com/books?id=SO4Lmw8XoEMC&pg=PA151
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| |page=151
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| |publisher=[[CRC Press]]
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| |isbn=0-8247-0834-2
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| }}</ref>]]
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| The origin of the energy released in fusion of light elements is due to an interplay of two opposing forces, the [[nuclear force]] which combines together protons and neutrons, and the [[Coulomb force]] which causes protons to repel each other. The protons are positively charged and repel each other but they nonetheless stick together, demonstrating the existence of another force referred to as nuclear attraction. This force, called the strong nuclear force, overcomes electric repulsion in a very close range. The effect of this force is not observed outside the nucleus, hence the force has a strong dependence on distance, making it a short-range force. The same force also pulls the neutrons together, or neutrons and protons together.<ref>[http://www.ck12.org/flexbook/chapter/1903 Physics Flexbook]. Ck12.org. Retrieved on 2012-12-19.</ref> Because the nuclear force is stronger than the Coulomb force for [[atomic nucleus|atomic nuclei]] smaller than iron and nickel, building up these nuclei from lighter nuclei by '''fusion''' releases the extra energy from the net attraction of these particles. [[iron peak|For larger nuclei]], however, no energy is released, since the nuclear force is short-range and cannot continue to act across still larger atomic nuclei. Thus, energy is no longer released when such nuclei are made by fusion; instead, energy is absorbed in such processes.
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| Fusion reactions of light elements power the [[star]]s and produce virtually all elements in a process called [[nucleosynthesis]].<!-- Meant generally; please do not insert info about [[stellar nucleosynthesis]], [[supernova nucleosynthesis]] and other types of specific nucleosynthesis here. --> The fusion of lighter elements in stars releases energy (and the mass that always accompanies it). For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is carried away from the system in the form of kinetic energy or other forms of energy (such as electromagnetic radiation).<ref name="bulletin1950">Bethe, Hans A. [http://books.google.com/books?id=Mg4AAAAAMBAJ&pg=PA112 "The Hydrogen Bomb"], ''Bulletin of the Atomic Scientists'', April 1950, p. 99.</ref>
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| Research into controlled fusion, with the aim of producing fusion power for the production of electricity, has been conducted for over 60 years. It has been accompanied by extreme scientific and technological difficulties, but has resulted in progress. At present, controlled fusion reactions have been unable to produce break-even (self-sustaining) controlled fusion reactions.<ref>
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| {{cite web
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| |title=Progress in Fusion
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| |url=http://www.iter.org/sci/beyonditer
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| |publisher=[[ITER]]
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| |accessdate=2010-02-15
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| }}</ref> Workable designs for a reactor that theoretically will deliver ten times more fusion energy than the amount needed to heat up plasma to required temperatures (see [[ITER]]) were originally scheduled to be operational in 2018, however this has been delayed and a new date has not been stated.{{Citation needed|date=December 2013}}
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| It takes considerable energy to force nuclei to fuse, even those of the lightest element, [[hydrogen]]. This is because all nuclei have a positive charge due to their protons, and as like charges repel, nuclei strongly resist being put close together. Accelerated to high speeds, they can overcome this electrostatic repulsion and be forced close enough for the attractive [[nuclear force]] to be sufficiently strong to achieve fusion. The fusion of lighter nuclei, which creates a heavier nucleus and often a [[free neutron]] or proton, generally releases more energy than it takes to force the nuclei together; this is an [[exothermic reaction|exothermic process]] that can produce self-sustaining reactions. The US [[National Ignition Facility]], which uses laser-driven [[inertial confinement fusion]], is thought to be capable of break-even fusion.
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| The first large-scale laser target experiments were performed in June 2009 and ignition experiments began in early 2011.<ref name="programsNIF">{{cite web | title= The National Ignition Facility: Ushering in a New Age for Science | url=https://lasers.llnl.gov/programs/nif/ | publisher=National Ignition Facility | accessdate=2009-09-13}}</ref><ref>"DOE looks again at inertial fusion as potential clean-energy source", David Kramer, ''Physics Today'', March 2011, p 26</ref>
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| Energy released in most [[nuclear reaction]]s is much larger than in [[chemical reaction]]s, because the [[binding energy]] that holds a nucleus together is far greater than the energy that holds [[electron]]s to a nucleus. For example, the [[ionization energy]] gained by adding an electron to a hydrogen nucleus is {{val|13.6|ul=eV}}—less than one-millionth of the {{val|17.6|ul=MeV}} released in the [[deuterium]]–[[tritium]] (D–T) reaction shown in the diagram to the right (one [[gram]] of matter would release {{val|339|ul=GJ}} of energy). Fusion reactions have an [[energy density]] many times greater than [[nuclear fission]]; the reactions produce far greater energy per unit of mass even though ''individual'' fission reactions are generally much more energetic than ''individual'' fusion ones, which are themselves millions of times more energetic than chemical reactions. Only [[Direct conversion]] of [[Mass-energy equivalence|mass into energy]], such as that caused by the [[annihilation|annihilatory]] collision of [[matter]] and [[antimatter]], is more energetic per unit of mass than nuclear fusion.
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| ==Requirements==
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| A substantial energy barrier of electrostatic forces must be overcome before fusion can occur. At large distances two naked nuclei repel one another because of the repulsive [[electrostatic force]] between their [[electric charge|positively charged]] protons. If two nuclei can be brought close enough together, however, the electrostatic repulsion can be overcome by the attractive [[nuclear force]], which is stronger at close distances.
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| When a [[nucleon]] such as a [[proton]] or [[neutron]] is added to a nucleus, the nuclear force attracts it to other nucleons, but primarily to its immediate neighbours due to the short range of the force. The nucleons in the interior of a nucleus have more neighboring nucleons than those on the surface. Since smaller nuclei have a larger surface area-to-volume ratio, the binding energy per nucleon due to the [[nuclear force]] generally increases with the size of the nucleus but approaches a limiting value corresponding to that of a nucleus with a diameter of about four nucleons. It is important to keep in mind that the above picture is a [[toy model]] because nucleons are [[Quantum physics|quantum objects]], and so, for example, since two neutrons in a nucleus are identical to each other, distinguishing one from the other, such as which one is in the interior and which is on the surface, is in fact meaningless, and the inclusion of quantum mechanics is necessary for proper calculations.
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| The electrostatic force, on the other hand, is an [[inverse square law|inverse-square force]], so a proton added to a nucleus will feel an electrostatic repulsion from ''all'' the other protons in the nucleus. The electrostatic energy per nucleon due to the electrostatic force thus increases without limit as nuclei get larger.
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| [[Image:Nuclear fusion forces diagram.svg|left|350px|thumb|The [[electrostatic force]] between the positively charged nuclei is repulsive, but when the separation is small enough, the attractive [[nuclear force]] is stronger. Therefore the prerequisite for fusion is that the nuclei have enough kinetic energy that they can approach each other despite the electrostatic repulsion.]]
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| The net result of these opposing forces is that the binding energy per nucleon generally increases with increasing size, up to the elements [[iron]] and [[nickel]], and then decreases for heavier nuclei. Eventually, the [[binding energy]] becomes negative and very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable. The four most tightly bound nuclei, in decreasing order of [[binding energy]] per nucleon, are {{SimpleNuclide|Link|Nickel|62}}, {{SimpleNuclide|Link|Iron|58}}, {{SimpleNuclide|Link|Iron|56}}, and {{SimpleNuclide|Link|Nickel|60}}.<ref>[http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin2.html#c1 The Most Tightly Bound Nuclei]. Hyperphysics.phy-astr.gsu.edu. Retrieved on 2011-08-17.</ref> Even though the [[isotopes of nickel|nickel isotope]], {{SimpleNuclide|Nickel|62}}, is more stable, the [[isotopes of iron|iron isotope]] {{SimpleNuclide|Iron|56}} is an [[order of magnitude]] more common. This is due to the fact that there is no easy way for stars to create {{SimpleNuclide|Nickel|62}} through the alpha process.
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| A notable exception to this general trend is the [[helium-4]] nucleus, whose binding energy is higher than that of [[lithium]], the next heaviest element. The [[Pauli exclusion principle]] provides an explanation for this exceptional behavior—it says that because protons and neutrons are [[fermion]]s, they cannot exist in exactly the same state. Each proton or neutron energy state in a nucleus can accommodate both a spin up particle and a spin down particle. Helium-4 has an anomalously large binding energy because its nucleus consists of two protons and two neutrons; so all four of its nucleons can be in the ground state. Any additional nucleons would have to go into higher energy states. Indeed, the helium-4 nucleus is so tightly bound that it is commonly treated as a single particle in nuclear physics, namely, the [[alpha particle]].
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| The situation is similar if two nuclei are brought together. As they approach each other, all the protons in one nucleus repel all the protons in the other. Not until the two nuclei actually come in contact can the strong [[nuclear force]] take over. Consequently, even when the final energy state is lower, there is a large energy barrier that must first be overcome. It is called the [[Coulomb barrier]].
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| The Coulomb barrier is smallest for isotopes of hydrogen, as their nuclei contain only a single positive charge. A [[diproton]] is not stable, so neutrons must also be involved, ideally in such a way that a helium nucleus, with its extremely tight binding, is one of the products.
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| Using [[Tritium#Deuterium|deuterium-tritium]] fuel, the resulting energy barrier is about 0.1 MeV.{{Citation needed|date=March 2008}} In comparison, the energy needed to remove an [[electron]] from [[hydrogen]] is 13.6 eV, about 7500 times less energy. The (intermediate) result of the fusion is an unstable <sup>5</sup>He nucleus, which immediately ejects a neutron with 14.1 MeV.{{Citation needed|date=March 2008}} The recoil energy of the remaining <sup>4</sup>He nucleus is 3.5 MeV,{{Citation needed|date=March 2008}} so the total energy liberated is 17.6 MeV.{{Citation needed|date=March 2008}} This is many times more than what was needed to overcome the energy barrier.
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| [[Image:fusion rxnrate.svg|right|300px|thumb|The fusion reaction rate increases rapidly with temperature until it maximizes and then gradually drops off. The DT rate peaks at a lower temperature (about 70 keV, or 800 million kelvin) and at a higher value than other reactions commonly considered for fusion energy.]]
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| The reaction '''[[cross section (physics)|cross section]]''' σ is a measure of the probability of a fusion reaction as a function of the relative velocity of the two reactant nuclei. If the reactants have a distribution of velocities, e.g. a thermal distribution with [[thermonuclear fusion]], then it is useful to perform an average over the distributions of the product of cross section and velocity. This average is called the 'reactivity', denoted <σv>. The reaction rate (fusions per volume per time) is <σv> times the product of the reactant number densities:
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| :<math>f = n_1 n_2 \langle \sigma v \rangle.</math>
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| If a species of nuclei is reacting with itself, such as the DD reaction, then the product <math>n_1n_2</math> must be replaced by <math>(1/2)n^2</math>.
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| <math>\langle \sigma v \rangle</math> increases from virtually zero at room temperatures up to meaningful magnitudes at temperatures of [[1 E-15 J|10]]–[[1 E-14 J|100]] keV. At these temperatures, well above typical [[ion]]ization energies (13.6 eV in the hydrogen case), the fusion reactants exist in a [[Plasma physics|plasma]] state.
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| The significance of <math>\langle \sigma v \rangle</math> as a function of temperature in a device with a particular energy [[confinement time]] is found by considering the [[Lawson criterion]]. This is an extremely challenging barrier to overcome on Earth, which explains why fusion research has taken many years to reach the current high state of technical prowess.<ref name=lawson>http://www.scienceworldreport.com/articles/5763/20130323/lawson-criteria-make-fusion-power-viable-iter.htm</ref>
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| == Possibilities of achieving ==
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| === Thermonuclear fusion ===
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| {{main|Thermonuclear fusion}}
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| If the matter is sufficiently heated (hence being [[plasma physics|plasma]]), the fusion reaction may occur due to collisions with extreme thermal kinetic energies of the particles.
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| This process is called the [[thermonuclear fusion]], and is the only one which seems to be useful for obtaining [[fusion energy]]{{citation needed|date=January 2013}}.
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| ===Beam-beam or beam-target fusion===
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| If the energy to initiate the reaction comes from [[particle accelerator|accelerating]] one of the nuclei, the process is called ''beam-target'' fusion; if both nuclei are accelerated, it is ''beam-beam'' fusion.
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| Accelerator-based light-ion fusion is a technique using particle accelerators to achieve particle kinetic energies sufficient to induce light-ion fusion reactions. Accelerating light ions is relatively easy, and can be done in an efficient manner—all it takes is a vacuum tube, a pair of electrodes, and a high-voltage transformer; fusion can be observed with as little as 10 kV between electrodes. The key problem with accelerator-based fusion (and with cold targets in general) is that fusion cross sections are many orders of magnitude lower than Coulomb interaction cross sections. Therefore the vast majority of ions end up expending their energy on [[bremsstrahlung]] and ionization of atoms of the target. Devices referred to as sealed-tube [[neutron generator]]s are particularly relevant to this discussion. These small devices are miniature particle accelerators filled with deuterium and tritium gas in an arrangement that allows ions of these nuclei to be accelerated against hydride targets, also containing deuterium and tritium, where fusion takes place. Hundreds of neutron generators are produced annually for use in the petroleum industry where they are used in measurement equipment for locating and mapping oil reserves.
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| ===Muon-catalyzed fusion===
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| [[Muon-catalyzed fusion]] is a well-established and reproducible fusion process that occurs at ordinary temperatures. It was studied in detail by [[Steven E. Jones|Steven Jones]] in the early 1980s. Net energy production from this reaction cannot occur because of the high energy required to create [[muon]]s, their short 2.2 µs [[half-life]], and the high chance that a muon will bind to the new [[alpha particle]] and thus stop catalyzing fusion.<ref>{{cite journal |author=Jones, S.E. |title=Muon-Catalysed Fusion Revisited |journal=Nature |volume=321 |pages=127–133 |year=1986 |doi=10.1038/321127a0|bibcode = 1986Natur.321..127J |issue=6066}}</ref>
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| ===Other principles===
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| [[File:TCV vue gen.jpg|thumb|The ''[[Tokamak à configuration variable]]'', research fusion reactor, at the [[École Polytechnique Fédérale de Lausanne]] ([[Switzerland]]).]]
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| Some other confinement principles have been investigated, some of them were confirmed to run nuclear fusion but without any possibility to produce net power, the others were not (or not yet) proven to produce fusion.
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| Sonofusion or [[bubble fusion]], a controversial variation on the [[sonoluminescence]] theme, suggests that acoustic shock waves, creating temporary bubbles (cavitation) that expand and collapse shortly after creation, can produce temperatures and pressures sufficient for nuclear fusion.<ref>[http://www.nature.com/news/2006/060109/full/060109-5.html Access: Desktop fusion is back on the table: Nature News]. Nature.com. Retrieved on 2011-08-17.</ref>
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| The [[Farnsworth–Hirsch fusor]] is a tabletop device in which fusion occurs. This fusion comes from high effective temperatures produced by electrostatic acceleration of ions.
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| The [[Polywell]] is a non-thermodynamic equilibrium machine that uses electrostatic confinement to accelerate ions into a center where they fuse together.
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| [[Antimatter catalyzed nuclear pulse propulsion|Antimatter-initialized fusion]] uses small amounts of [[antimatter]] to trigger a tiny fusion explosion. This has been studied primarily in the context of making [[nuclear pulse propulsion]], and [[pure fusion bomb]]s feasible. This is not near becoming a practical power source, due to the cost of manufacturing antimatter alone.
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| [[Pyroelectric fusion]] was reported in April 2005 by a team at [[University of California, Los Angeles|UCLA]]. The scientists used a [[pyroelectricity|pyroelectric]] crystal heated from −34 to 7 °C (−29 to 45 °F), combined with a [[tungsten]] needle to produce an [[electric field]] of about 25 gigavolts per meter to ionize and accelerate [[deuterium]] nuclei into an [[erbium]] deuteride target. At the estimated energy levels,<ref>[http://www.nature.com/nature/journal/v434/n7037/suppinfo/nature03575.html Supplementary methods for “Observation of nuclear fusion driven by a pyroelectric crystal”]. Main article {{cite journal|doi=10.1038/nature03575|title=Observation of nuclear fusion driven by a pyroelectric crystal|year=2005|last1=Naranjo|first1=B.|last2=Gimzewski|first2=J.K.|last3=Putterman|first3=S.|journal=Nature|volume=434|issue=7037|pages=1115–1117|pmid=15858570|bibcode = 2005Natur.434.1115N }}</ref> the [[nuclear fusion reactions|D-D fusion reaction]] may occur, producing [[helium-3]] and a 2.45 MeV [[neutron]]. Although it makes a useful neutron generator, the apparatus is not intended for power generation since it requires far more energy than it produces.<ref>[http://rodan.physics.ucla.edu/pyrofusion/ UCLA Crystal Fusion]. Rodan.physics.ucla.edu. Retrieved on 2011-08-17.</ref><ref>{{cite journal|url=http://www.aip.org/pnu/2005/split/729-1.html |title=Pyrofusion: A Room-Temperature, Palm-Sized Nuclear Fusion Device |author=Schewe, Phil and Stein, Ben |journal=Physics News Update |volume=729 |issue=1 |year=2005}}</ref><ref>[http://www.csmonitor.com/2005/0606/p25s01-stss.html Coming in out of the cold: nuclear fusion, for real]. Christiansciencemonitor.com (2005-06-06). Retrieved on 2011-08-17.</ref><ref>[http://msnbc.msn.com/id/7654627Nuclear fusion on the desktop ... really!]. MSNBC (2005-04-27). Retrieved on 2011-08-17.</ref>
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| [[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>
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| [[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.
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| == Important reactions ==
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| <!-- This section is linked from [[Fusion power]] -->
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| ===Astrophysical reaction chains===
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| [[Image:FusionintheSun.svg|thumb|250px|right|The [[proton-proton chain]] dominates in stars the size of the Sun or smaller.]]
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| [[Image:CNO Cycle.svg|thumb|250px|right|The [[CNO cycle]] dominates in stars heavier than the Sun.]]
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| The most important fusion process in nature is the one that powers stars. The net result is the fusion of four [[proton]]s into one [[alpha particle]], with the release of two [[positron]]s, two [[neutrino]]s (which changes two of the protons into neutrons), and energy, but several individual reactions are involved, depending on the mass of the star. For stars the size of the sun or smaller, the [[proton-proton chain]] dominates. In heavier stars, the [[CNO cycle]] is more important. Both types of processes are responsible for the creation of new elements as part of [[stellar nucleosynthesis]].
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| At the temperatures and densities in stellar cores the rates of fusion reactions are notoriously slow. For example, at solar core temperature (''T'' ≈ 15 MK) and density (160 g/cm<sup>3</sup>), the energy release rate is only 276 μW/cm<sup>3</sup>—about a quarter of the volumetric rate at which a resting human body generates heat.<ref>[http://fusedweb.pppl.gov/CPEP/Chart_Pages/5.Plasmas/SunLayers.html FusEdWeb | Fusion Education]. Fusedweb.pppl.gov (1998-11-09). Retrieved on 2011-08-17.</ref> Thus, reproduction of stellar core conditions in a lab for nuclear fusion power production is completely impractical. Because nuclear reaction rates strongly depend on temperature (exp(−''E''/''kT'')), achieving reasonable energy production rates in terrestrial fusion reactors requires 10–100 times higher temperatures (compared to stellar interiors): ''T'' ≈ 0.1–1.0 GK.
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| ===Criteria and candidates for terrestrial reactions===
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| In man-made fusion, the primary fuel is not constrained to be protons and higher temperatures can be used, so reactions with larger cross-sections are chosen. This implies a lower [[Lawson criterion]], and therefore less startup effort. Another concern is the production of neutrons, which activate the reactor structure radiologically, but also have the advantages of allowing volumetric extraction of the fusion energy and [[tritium]] breeding. Reactions that release no neutrons are referred to as [[Aneutronic fusion|''aneutronic'']].
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| To be a useful energy source, a fusion reaction must satisfy several criteria. It must
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| *'''Be [[exothermic]]''': This may be obvious, but it limits the reactants to the low ''Z'' (number of protons) side of the [[Nuclear binding energy#Nuclear binding energy curve|curve of binding energy]]. It also makes helium {{SimpleNuclide|Link|Helium|4}} the most common product because of its extraordinarily tight binding, although {{SimpleNuclide|Link|Helium|3}} and {{SimpleNuclide|Link|Hydrogen|3}} also show up.
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| *'''Involve low ''Z'' nuclei''': This is because the electrostatic repulsion must be overcome before the nuclei are close enough to fuse.
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| *'''Have two reactants''': At anything less than stellar densities, three body collisions are too improbable. In inertial confinement, both stellar densities and temperatures are exceeded to compensate for the shortcomings of the third parameter of the Lawson criterion, ICF's very short confinement time.
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| *'''Have two or more products''': This allows simultaneous conservation of energy and momentum without relying on the electromagnetic force.
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| *'''Conserve both protons and neutrons''': The cross sections for the weak interaction are too small.
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| Few reactions meet these criteria. The following are those with the largest cross sections{{Citation needed|date=July 2008}}:
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| <!-- Autogenerated using Phykiformulae 0.10 by [[User:SkyLined]]
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| (1) D + T → He ( 3.5 MeV ) + n ( 14.1 MeV )
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| (2i) D + D → T ( 1.01 MeV ) + p ( 3.02 MeV ) _ _ _ _ _ _50%
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| (2ii) _ _ _ → He-3 ( 0.82 MeV ) + n ( 3.27 MeV ) _ _ _ _ _ _50%
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| (3) D + He-3 → He ( 3.6 MeV ) + p ( 14.7 MeV )
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| (4) T + T → He _ _ _ + 2n _ _ _ _ _ + 11.3MeV
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| (5) He-3 + He-3 → He _ _ _ + 2p _ _ _ _ _ + 12.9MeV
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| (6i) He-3 + T → He _ _ _ + p + n _ _ _ + 12.1MeV _ _57%
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| (6ii) _ _ _ → He ( 4.8 MeV ) + D ( 9.5 MeV ) _ _ _ _ _ _43%
| |
| (7i) D + Li-6 → 2He + 22.4 MeV
| |
| (7ii) _ _ _ → He-3 + He _ + n _ _ _ _ _ + 2.56MeV
| |
| (7iii) _ _ _ → Li-7 ( 0.625 MeV ) + p (4.375 MeV)
| |
| (7iv) _ _ _ → Be-7 ( 0.425 MeV )+ n ( 2.975 MeV )
| |
| (8) p + Li-6 → He ( 1.7 MeV ) + He-3 (2.3 MeV)
| |
| (9) He-3 + Li-6 → 2He + p _ _ _ _ _ _ _ _ + 16.9 MeV
| |
| (10) p + B-11 → 3He _ _ _ _ _ _ _ _ _ _ + 8.7 MeV
| |
| | |
| -->:{| border="0"
| |
| |- style="height:2em;"
| |
| |(1) ||{{Nuclide|Link|deuterium}} ||+ ||{{Nuclide|Link|tritium}} ||→ ||{{Nuclide|Link|helium|4}} ||( ||3.5 [[electron volt|MeV]] ||) ||+ ||[[Neutron|n<sup>0</sup>]] ||( ||14.1 [[electron volt|MeV]] ||)
| |
| |- style="height:2em;"
| |
| |(2i) ||{{Nuclide|Link|deuterium}} ||+ ||{{Nuclide|Link|deuterium}} ||→ ||{{Nuclide|Link|tritium}} ||( ||1.01 [[electron volt|MeV]] ||) ||+ ||[[Proton|p<sup>+</sup>]] ||( ||3.02 [[electron volt|MeV]] ||) || || || || || || 50%
| |
| |- style="height:2em;"
| |
| |(2ii) || || || ||→ ||{{Nuclide|Link|helium|3}} ||( ||0.82 [[electron volt|MeV]] ||) ||+ ||[[Neutron|n<sup>0</sup>]] ||( ||2.45 [[electron volt|MeV]] ||) || || || || || || 50%
| |
| |- style="height:2em;"
| |
| |(3) ||{{Nuclide|Link|deuterium}} ||+ ||{{Nuclide|Link|helium|3}} ||→ ||{{Nuclide|Link|helium|4}} ||( ||3.6 [[electron volt|MeV]] ||) ||+ ||[[Proton|p<sup>+</sup>]] ||( ||14.7 [[electron volt|MeV]] ||)
| |
| |- style="height:2em;"
| |
| |(4) ||{{Nuclide|Link|tritium}} ||+ ||{{Nuclide|Link|tritium}} ||→ ||{{Nuclide|Link|helium|4}} || || || ||+ ||2 [[Neutron|n<sup>0</sup>]] || || || || || ||+ ||11.3 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(5) ||{{Nuclide|Link|helium|3}} ||+ ||{{Nuclide|Link|helium|3}} ||→ ||{{Nuclide|Link|helium|4}} || || || ||+ ||2 [[Proton|p<sup>+</sup>]] || || || || || ||+ ||12.9 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(6i) ||{{Nuclide|Link|helium|3}} ||+ ||{{Nuclide|Link|tritium}} ||→ ||{{Nuclide|Link|helium|4}} || || || ||+ ||[[Proton|p<sup>+</sup>]] ||+ ||[[Neutron|n<sup>0</sup>]] || || || ||+ ||12.1 [[electron volt|MeV]] || || 57%
| |
| |- style="height:2em;"
| |
| |(6ii) || || || ||→ ||{{Nuclide|Link|helium|4}} ||( ||4.8 [[electron volt|MeV]] ||) ||+ ||{{Nuclide|Link|deuterium}} ||( ||9.5 [[electron volt|MeV]] ||) || || || || || || 43%
| |
| |- style="height:2em;"
| |
| |(7i) ||{{Nuclide|Link|deuterium}} ||+ ||{{Nuclide|Link|lithium|6}} ||→ ||2 {{Nuclide|Link|helium|4}} ||+ ||22.4 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(7ii) || || || ||→ ||{{Nuclide|Link|helium|3}} ||+ ||{{Nuclide|Link|helium|4}} || ||+ ||[[Neutron|n<sup>0</sup>]] || || || || || ||+ ||2.56 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(7iii) || || || ||→ ||{{Nuclide|Link|lithium|7}} ||+ ||[[Proton|p<sup>+</sup>]] || || || || || || || || ||+ ||5.0 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(7iv) || || || ||→ ||{{Nuclide|Link|beryllium|7}} ||+ ||[[Neutron|n<sup>0</sup>]] || || || || || || || || ||+ ||3.4 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(8) ||[[Proton|p<sup>+</sup>]] ||+ ||{{Nuclide|Link|lithium|6}} ||→ ||{{Nuclide|Link|helium|4}} ||( ||1.7 [[electron volt|MeV]] ||) ||+ ||{{Nuclide|Link|helium|3}} ||( ||2.3 [[electron volt|MeV]] ||)
| |
| |- style="height:2em;"
| |
| |(9) ||{{Nuclide|Link|helium|3}} ||+ ||{{Nuclide|Link|lithium|6}} ||→ ||2 {{Nuclide|Link|helium|4}} ||+ ||[[Proton|p<sup>+</sup>]] || || || || || || || || ||+ ||16.9 [[electron volt|MeV]]
| |
| |- style="height:2em;"
| |
| |(10) ||[[Proton|p<sup>+</sup>]] ||+ ||{{Nuclide|Link|boron|11}} ||→ ||3 {{Nuclide|Link|helium|4}} || || || || || || || || || || ||+ ||8.7 [[electron volt|MeV]]
| |
| |}
| |
| {{Nucleosynthesis}}
| |
| | |
| For reactions with two products, the energy is divided between them in inverse proportion to their masses, as shown. In most reactions with three products, the distribution of energy varies. For reactions that can result in more than one set of products, the branching ratios are given.
| |
| | |
| Some reaction candidates can be eliminated at once.<ref name=r1>{{Wayback |date=20060103223116 |url=http://theses.mit.edu/Dienst/UI/2.0/Page/0018.mit.theses/1995-130/30?npages=306 }}. Retrieved on 2012-12-19.</ref> The D-<sup>6</sup>Li reaction has no advantage compared to [[Proton|p<sup>+</sup>]]-{{Nuclide|Link|Boron|11}} because it is roughly as difficult to burn but produces substantially more neutrons through {{Nuclide|Link|Deuterium}}-{{Nuclide|Link|Deuterium}} side reactions. There is also a [[Proton|p<sup>+</sup>]]-{{Nuclide|Link|Lithium|7}} reaction, but the cross section is far too low, except possibly when ''T''<sub>i</sub> > 1 MeV, but at such high temperatures an endothermic, direct neutron-producing reaction also becomes very significant. Finally there is also a [[Proton|p<sup>+</sup>]]-{{Nuclide|Link|Beryllium|9}} reaction, which is not only difficult to burn, but {{Nuclide|Link|Beryllium|9}} can be easily induced to split into two alpha particles and a neutron.
| |
| | |
| In addition to the fusion reactions, the following reactions with neutrons are important in order to "breed" tritium in "dry" fusion bombs and some proposed fusion reactors:
| |
| <!-- Autogenerated using Phykiformulae 0.10 by [[User:SkyLined]]
| |
| {{Nuclide|neutron}} + {{Nuclide|lithium|6}} -> T + He + 4.784 MeV
| |
| n + {{Nuclide|lithium|7}} -> T + He + n – 2.467 MeV
| |
| | |
| -->:{| border="0"
| |
| |- style="height:2em;"
| |
| |[[Neutron|n<sup>0</sup>]] ||+ ||{{Nuclide|Link|lithium|6}} ||→ ||{{Nuclide|Link|tritium}} ||+ ||{{Nuclide|Link|helium|4}} + 4.784 MeV
| |
| |- style="height:2em;"
| |
| |[[Neutron|n<sup>0</sup>]] ||+ ||{{Nuclide|Link|lithium|7}} ||→ ||{{Nuclide|Link|tritium}} ||+ ||{{Nuclide|Link|helium|4}} + [[Neutron|n<sup>0</sup>]] – 2.467 MeV
| |
| |}
| |
| | |
| The latter of the two equations was unknown when the U.S. conducted the [[Castle Bravo]] fusion bomb test in 1954. Being just the second fusion bomb ever tested (and the first to use lithium), the designers of the Castle Bravo "Shrimp" had understood the usefulness of Lithium-6 in tritium production, but had failed to recognize that Lithium-7 fission would greatly increase the yield of the bomb. While Li-7 has a small neutron cross-section for low neutron energies, it has a higher cross section above 5 MeV.<ref name=cross_section>[http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_7/4_7_4c.html Subsection 4.7.4c]. Kayelaby.npl.co.uk. Retrieved on 2012-12-19.</ref> Li-7 also undergoes a chain reaction due to its release of a neutron after fissioning. The 15 Mt yield was 150% greater than the predicted 6 Mt and caused heavy casualties from the fallout generated.
| |
| | |
| To evaluate the usefulness of these reactions, in addition to the reactants, the products, and the energy released, one needs to know something about the cross section. Any given fusion device has a maximum plasma pressure it can sustain, and an economical device would always operate near this maximum. Given this pressure, the largest fusion output is obtained when the temperature is chosen so that <σv>/T<sup>2</sup> is a maximum. This is also the temperature at which the value of the triple product ''nT''τ required for [[fusion gain factor|ignition]] is a minimum, since that required value is inversely proportional to <σv>/T<sup>2</sup> (see [[Lawson criterion]]). (A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating.) This optimum temperature and the value of <σv>/T<sup>2</sup> at that temperature is given for a few of these reactions in the following table.
| |
| | |
| {| class="wikitable" style="margin:auto;"
| |
| |-
| |
| !fuel !! ''T'' [keV] !! <σv>/T<sup>2</sup> [m<sup>3</sup>/s/keV<sup>2</sup>]
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|tritium}} || 13.6 || 1.24×10<sup>−24</sup>
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|deuterium}} || 15 || 1.28×10<sup>−26</sup>
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|helium|3}} || 58 || 2.24×10<sup>−26</sup>
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|lithium|6}} || 66 || 1.46×10<sup>−27</sup>
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|boron|11}} || 123 || 3.01×10<sup>−27</sup>
| |
| |}
| |
| | |
| Note that many of the reactions form chains. For instance, a reactor fueled with {{Nuclide|tritium}} and {{Nuclide|helium|3}} creates some {{Nuclide|deuterium}}, which is then possible to use in the {{Nuclide|deuterium}}-{{Nuclide|helium|3}} reaction if the energies are "right". An elegant idea is to combine the reactions (8) and (9). The {{Nuclide|helium|3}} from reaction (8) can react with {{Nuclide|lithium|6}} in reaction (9) before completely thermalizing. This produces an energetic proton, which in turn undergoes reaction (8) before thermalizing. Detailed analysis shows that this idea would not work well{{Citation needed|date=April 2010}}, but it is a good example of a case where the usual assumption of a [[Maxwell-Boltzmann distribution|Maxwellian]] plasma is not appropriate.
| |
| | |
| ===Neutronicity, confinement requirement, and power density===
| |
| [[Image:IvyMike2.jpg|thumb|right|350px|The only man-made fusion device to achieve [[Fusion gain factor|ignition]] to date is the [[hydrogen bomb]]. The detonation of the first device, codenamed [[Ivy Mike]], is shown here.]]
| |
| Any of the reactions above can in principle be the basis of [[fusion power]] production. In addition to the temperature and cross section discussed above, we must consider the total energy of the fusion products ''E''<sub>fus</sub>, the energy of the charged fusion products ''E''<sub>ch</sub>, and the atomic number ''Z'' of the non-hydrogenic reactant.
| |
| | |
| Specification of the {{Nuclide|deuterium}}-{{Nuclide|deuterium}} reaction entails some difficulties, though. To begin with, one must average over the two branches (2) and (3). More difficult is to decide how to treat the {{Nuclide|tritium}} and {{Nuclide|helium|3}} products. {{Nuclide|tritium}} burns so well in a deuterium plasma that it is almost impossible to extract from the plasma. The {{Nuclide|deuterium}}-{{Nuclide|helium|3}} reaction is optimized at a much higher temperature, so the burnup at the optimum {{Nuclide|deuterium}}-{{Nuclide|deuterium}} temperature may be low, so it seems reasonable to assume the {{Nuclide|tritium}} but not the {{Nuclide|helium|3}} gets burned up and adds its energy to the net reaction. Thus we count the {{Nuclide|deuterium}}-{{Nuclide|deuterium}} fusion energy as ''E''<sub>fus</sub> = (4.03+17.6+3.27)/2 = 12.5 MeV and the energy in charged particles as ''E''<sub>ch</sub> = (4.03+3.5+0.82)/2 = 4.2 MeV.
| |
| | |
| Another unique aspect of the {{Nuclide|deuterium}}-{{Nuclide|deuterium}} reaction is that there is only one reactant, which must be taken into account when calculating the reaction rate.
| |
| | |
| With this choice, we tabulate parameters for four of the most important reactions
| |
| | |
| {| class="wikitable" style="margin:auto;"
| |
| |-
| |
| !fuel !!''Z''!!''E''<sub>fus</sub> [MeV]!!''E''<sub>ch</sub> [MeV]!!neutronicity
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|tritium}} || 1 || 17.6 || 3.5 || 0.80
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|deuterium}} || 1 || 12.5 || 4.2 || 0.66
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|helium|3}} || 2 || 18.3 ||18.3 || ~0.05
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|boron|11}} || 5 || 8.7 || 8.7 || ~0.001
| |
| |}
| |
| | |
| The last column is the '''[[aneutronic fusion|neutronicity]]''' of the reaction, the fraction of the fusion energy released as neutrons. This is an important indicator of the magnitude of the problems associated with neutrons like radiation damage, biological shielding, remote handling, and safety. For the first two reactions it is calculated as (''E''<sub>fus</sub>-''E''<sub>ch</sub>)/''E''<sub>fus</sub>. For the last two reactions, where this calculation would give zero, the values quoted are rough estimates based on side reactions that produce neutrons in a plasma in thermal equilibrium.
| |
| | |
| Of course, the reactants should also be mixed in the optimal proportions. This is the case when each reactant ion plus its associated electrons accounts for half the pressure. Assuming that the total pressure is fixed, this means that density of the non-hydrogenic ion is smaller than that of the hydrogenic ion by a factor 2/(''Z''+1). Therefore the rate for these reactions is reduced by the same factor, on top of any differences in the values of <σv>/T<sup>2</sup>. On the other hand, because the {{Nuclide|deuterium}}-{{Nuclide|deuterium}} reaction has only one reactant, its rate is twice as high as when the fuel is divided between two different hydrogenic species, thus creating a more efficient reaction.
| |
| | |
| Thus there is a "penalty" of (2/(Z+1)) for non-hydrogenic fuels arising from the fact that they require more electrons, which take up pressure without participating in the fusion reaction. (It is usually a good assumption that the electron temperature will be nearly equal to the ion temperature. Some authors, however discuss the possibility that the electrons could be maintained substantially colder than the ions. In such a case, known as a "hot ion mode", the "penalty" would not apply.) There is at the same time a "bonus" of a factor 2 for {{Nuclide|deuterium}}-{{Nuclide|deuterium}} because each ion can react with any of the other ions, not just a fraction of them.
| |
| | |
| We can now compare these reactions in the following table.
| |
| | |
| {| class="wikitable" style="margin:auto;"
| |
| |-
| |
| !fuel !!<σv>/T<sup>2</sup>!!penalty/bonus !!reactivity!!Lawson criterion!!power density (W/m<sup>3</sup>/kPa<sup>2</sup>)!!relation of power density
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|tritium}} || 1.24×10<sup>−24</sup> || 1 || 1 || 1 || 34 || 1
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|deuterium}} || 1.28×10<sup>−26</sup> || 2 || 48 || 30 || 0.5 || 68
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|helium|3}} || 2.24×10<sup>−26</sup> || 2/3 || 83 || 16 || 0.43 || 80
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|lithium|6}} || 1.46×10<sup>−27</sup> || 1/2 || 1700 || || 0.005 || 6800
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|boron|11}} || 3.01×10<sup>−27</sup> || 1/3 || 1240 || 500 || 0.014 || 2500
| |
| |}
| |
| | |
| The maximum value of <σv>/T<sup>2</sup> is taken from a previous table. The "penalty/bonus" factor is that related to a non-hydrogenic reactant or a single-species reaction. The values in the column "reactivity" are found by dividing 1.24{{e|-24}} by the product of the second and third columns. It indicates the factor by which the other reactions occur more slowly than the {{Nuclide|Link|deuterium}}-{{Nuclide|Link|tritium}} reaction under comparable conditions. The column "[[Lawson criterion]]" weights these results with ''E''<sub>ch</sub> and gives an indication of how much more difficult it is to achieve ignition with these reactions, relative to the difficulty for the {{Nuclide|Link|deuterium}}-{{Nuclide|Link|tritium}} reaction. The last column is labeled "power density" and weights the practical reactivity with ''E''<sub>fus</sub>. It indicates how much lower the fusion power density of the other reactions is compared to the {{Nuclide|Link|deuterium}}-{{Nuclide|Link|tritium}} reaction and can be considered a measure of the economic potential.
| |
| | |
| ===Bremsstrahlung losses in quasineutral, isotropic plasmas===
| |
| The ions undergoing fusion in many systems will essentially never occur alone but will be mixed with [[electron]]s that in aggregate neutralize the ions' bulk [[electrical charge]] and form a [[Plasma (physics)|plasma]]. The electrons will generally have a temperature comparable to or greater than that of the ions, so they will collide with the ions and emit [[x-ray]] radiation of 10–30 keV energy ([[Bremsstrahlung]]). The Sun and stars are [[Opacity (optics)|opaque]] to x-rays, but essentially any terrestrial fusion reactor will be [[Optical depth|optically thin]] for x-rays of this energy range. X-rays are difficult to reflect but they are effectively absorbed (and converted into heat) in less than mm thickness of stainless steel (which is part of a reactor's shield). The ratio of fusion power produced to x-ray radiation lost to walls is an important figure of merit. This ratio is generally maximized at a much higher temperature than that which maximizes the power density (see the previous subsection). The following table shows estimates of the optimum temperature and the power ratio at that temperature for several reactions.<ref name=r1/>
| |
| | |
| {| class="wikitable" style="margin:auto;"
| |
| |-
| |
| !fuel !!''T''<sub>i</sub> (keV)!!''P''<sub>fusion</sub>/''P''<sub>Bremsstrahlung</sub>
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|tritium}} || 50 || 140
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|deuterium}} || 500 || 2.9
| |
| |-
| |
| |{{Nuclide|deuterium}}-{{Nuclide|helium|3}} || 100 || 5.3
| |
| |-
| |
| |{{Nuclide|helium|3}}-{{Nuclide|helium|3}} || 1000 || 0.72
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|lithium|6}} || 800 || 0.21
| |
| |-
| |
| |p<sup>+</sup>-{{Nuclide|boron|11}} || 300 || 0.57
| |
| |}
| |
| | |
| The actual ratios of fusion to Bremsstrahlung power will likely be significantly lower for several reasons. For one, the calculation assumes that the energy of the fusion products is transmitted completely to the fuel ions, which then lose energy to the electrons by collisions, which in turn lose energy by Bremsstrahlung. However, because the fusion products move much faster than the fuel ions, they will give up a significant fraction of their energy directly to the electrons. Secondly, the ions in the plasma are assumed to be purely fuel ions. In practice, there will be a significant proportion of impurity ions, which will then lower the ratio. In particular, the fusion products themselves ''must'' remain in the plasma until they have given up their energy, and ''will'' remain some time after that in any proposed confinement scheme. Finally, all channels of energy loss other than Bremsstrahlung have been neglected. The last two factors are related. On theoretical and experimental grounds, particle and energy confinement seem to be closely related. In a confinement scheme that does a good job of retaining energy, fusion products will build up. If the fusion products are efficiently ejected, then energy confinement will be poor, too.
| |
| | |
| The temperatures maximizing the fusion power compared to the Bremsstrahlung are in every case higher than the temperature that maximizes the power density and minimizes the required value of the [[Lawson criterion|fusion triple product]]. This will not change the optimum operating point for {{Nuclide|Link|deuterium}}-{{Nuclide|Link|tritium}} very much because the Bremsstrahlung fraction is low, but it will push the other fuels into regimes where the power density relative to {{Nuclide|Link|deuterium}}-{{Nuclide|Link|tritium}} is even lower and the required confinement even more difficult to achieve. For {{Nuclide|Link|deuterium}}-{{Nuclide|Link|deuterium}} and {{Nuclide|Link|deuterium}}-{{Nuclide|Link|helium|3}}, Bremsstrahlung losses will be a serious, possibly prohibitive problem. For {{Nuclide|Link|helium|3}}-{{Nuclide|Link|helium|3}}, [[Proton|p<sup>+</sup>]]-{{Nuclide|Link|lithium|6}} and [[Proton|p<sup>+</sup>]]-{{Nuclide|Link|boron|11}} the Bremsstrahlung losses appear to make a fusion reactor using these fuels with a quasineutral, isotropic plasma impossible. Some ways out of this dilemma are considered—and rejected—in [http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995PhDT........45R&db_key=PHY&data_type=HTML&format= ''Fundamental limitations on plasma fusion systems not in thermodynamic equilibrium'' by Todd Rider].<ref>[http://fusion.ps.uci.edu/artan/Posters/aps_poster_2.pdf Portable Document Format (PDF)]{{dead link|date=August 2011}}</ref> This limitation does not apply to non-neutral and anisotropic plasmas; however, these have their own challenges to contend with.
| |
| | |
| ==See also==
| |
| {{Portal|Physics|Energy}}
| |
| {{Columns-list|3|
| |
| *[[Aneutronic fusion]]
| |
| *[[CNO cycle]]
| |
| *[[Direct conversion]]
| |
| *[[Inertial electrostatic confinement]]
| |
| *[[Focus fusion]]
| |
| *[[Fusenet]]
| |
| *[[Fusion power]]
| |
| *[[Fusion rocket]]
| |
| *[[Helium-3]]
| |
| *[[Impulse generator]]
| |
| *[[ITER]]
| |
| *[[Joint European Torus]]
| |
| *[[List of fusion experiments]]
| |
| *[[List of plasma (physics) articles]]
| |
| *[[National Ignition Facility]]
| |
| *[[Nuclear fission]]
| |
| *[[Nuclear physics]]
| |
| *[[Nuclear reactor]]
| |
| *[[Nucleosynthesis]]
| |
| *[[Neutron generator]]
| |
| *[[Neutron source]]
| |
| *[[Periodic table]]
| |
| *[[Polywell]]
| |
| *[[Proton-proton chain]]
| |
| *[[Pulsed power]]
| |
| *[[Teller–Ulam design]]
| |
| *[[Thermonuclear fusion]]
| |
| *[[Timeline of nuclear fusion]]
| |
| *[[Triple-alpha process]]
| |
| }}
| |
| | |
| ==References==
| |
| {{Reflist|35em}}
| |
| | |
| ==Further reading==
| |
| *{{cite web
| |
| |title=What is Nuclear Fusion?
| |
| |url=http://www.nuclearfiles.org/menu/key-issues/nuclear-weapons/basics/what-is-fusion.htm
| |
| |publisher=NuclearFiles.org
| |
| }}
| |
| *{{Cite book
| |
| |author=S. Atzeni, J. Meyer-ter-Vehn
| |
| |year=2004
| |
| |url=http://www.oup.co.uk/pdf/0-19-856264-0.pdf
| |
| |chapter=Nuclear fusion reactions
| |
| |title=The Physics of Inertial Fusion
| |
| |publisher=[[University of Oxford Press]]
| |
| |isbn=978-0-19-856264-1
| |
| }}
| |
| *{{Cite journal
| |
| |author=G. Brumfiel
| |
| |date=22 May 2006
| |
| |title=Chaos could keep fusion under control
| |
| |journal=[[Nature (journal)|Nature]]
| |
| |volume= |issue= |pages=
| |
| |doi=10.1038/news060522-2
| |
| }}
| |
| *{{cite web
| |
| |author=R.W. Bussard
| |
| |date=9 November 2006
| |
| |title=Should Google Go Nuclear? Clean, Cheap, Nuclear Power
| |
| |url=http://video.google.com/videoplay?docid=1996321846673788606&q=engedu
| |
| |work=Google TechTalks
| |
| |authorlink=Robert Bussard
| |
| }}
| |
| *{{cite web
| |
| |author=A. Wenisch, R. Kromp, D. Reinberger
| |
| |date=November 2007
| |
| |url=http://www.ecology.at/ecology/files/pr577_1.pdf
| |
| |title=Science of Fiction: Is there a Future for Nuclear?
| |
| |publisher=[[Austrian Institute of Ecology]]
| |
| }}
| |
| *{{cite web
| |
| |author=W.J. Nuttall
| |
| |date=September 2008
| |
| |title=Fusion as an Energy Source: Challenges and Opportunities
| |
| |url=http://www.iop.org/publications/iop/2008/file_38224.pdf
| |
| |work=Institute of Physics Report
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| |publisher=[[Institute of Physics]]
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| }}
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| ==External links==
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| {{Commons|Nuclear fusion}}
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| *[http://www.nuclearfiles.org/ NuclearFiles.org]—A repository of documents related to nuclear power.
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| *[http://alsos.wlu.edu/qsearch.aspx?browse=science/Fusion Annotated bibliography for nuclear fusion from the Alsos Digital Library for Nuclear Issues]
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| ;Organizations
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| *[http://www.iter.org/ ITER (International Thermonuclear Experimental Reactor) website]
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| *[http://www.fusion.org.uk/ CCFE (Culham Centre for Fusion Energy) website]
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| *[http://www.jet.efda.org/ JET (Joint European Torus) website]
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| {{fusion power}}
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| {{Nuclear Technology}}
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| {{Radiation}}
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| {{Footer energy}}
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| {{DEFAULTSORT:Nuclear Fusion}}
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| [[Category:Nuclear fusion| ]]
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| [[Category:Concepts in physics]]
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| [[Category:Energy conversion]]
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| [[Category:Nuclear chemistry]]
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| [[Category:Nuclear physics]]
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| {{Link GA|no}}
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| {{Link FA|hr}}
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| {{Link FA|scn}}
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| {{Link FA|stq}}
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