Aneutronic fusion is any form of

fusion power Fusion power is a proposed form of power generation that would generate electricity by using heat from nuclear fusion reactions. In a fusion process, two lighter atomic nuclei combine to form a heavier nucleus, while releasing energy. Devices de ...

in which very little of the

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...

released is carried by

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s. While the lowest-threshold

nuclear fusion reactions Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles (neutrons or protons). The difference in mass between the reactants and products is manifeste ...

release up to 80% of their energy in the form of neutrons, aneutronic reactions release energy in the form of

charged particle In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary pa ...

s, typically

protons A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...

alpha particles Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be prod ...

. Successful aneutronic fusion would greatly reduce problems associated with

neutron radiation Neutron radiation is a form of ionizing radiation that presents as free neutrons. Typical phenomena are nuclear fission or nuclear fusion causing the release of free neutrons, which then react with nuclei of other atoms to form new isotopes— ...

such as damaging ionizing radiation,

neutron activation Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emit ...

, reactor maintenance, and requirements for biological shielding, remote handling and safety. Since it is simpler to convert the energy of charged particles into electrical power than it is to convert energy from uncharged particles, an aneutronic reaction would be attractive for power systems. Some proponents see a potential for dramatic cost reductions by converting energy directly to electricity, as well as in eliminating the radiation from neutrons, which are difficult to shield against. However, the conditions required to harness aneutronic fusion are much more extreme than those required for deuterium-tritium fusion such as at ITER or

Wendelstein 7-X The Wendelstein 7-X (abbreviated W7-X) reactor is an experimental stellarator built in Greifswald, Germany, by the Max Planck Institute for Plasma Physics (IPP), and completed in October 2015.Richard F. Post at Lawrence Livermore. He proposed to capture the

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acc ...

of charged particles as they were exhausted from a fusion reactor and convert this into voltage to drive current. Post helped develop the theoretical underpinnings of direct conversion, later demonstrated by Barr and Moir. They demonstrated a 48 percent energy capture efficiency on the

Tandem Mirror Experiment The Tandem Mirror Experiment (TMX and TMX-U) was a magnetic mirror machine operated from 1979 to 1987 at the Lawrence Livermore National Laboratory. It was the first large-scale machine to test the "tandem mirror" concept in which two mirrors trap ...

in 1981.

Polywell The polywell is a proposed design for a fusion reactor using an electric field to heat ions to fusion conditions. The design is related to the fusor, the high beta fusion reactor, the magnetic mirror, and the biconic cusp. A set of electromagn ...

fusion was pioneered by the late

Robert W. Bussard Robert W. Bussard (August 11, 1928 – October 6, 2007) was an American physicist who worked primarily in nuclear fusion energy research. He was the recipient of the Schreiber-Spence Achievement Award for STAIF-2004. He was also a fellow of th ...

in 1995 and funded by the

US Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

. Polywell uses

inertial electrostatic confinement Inertial electrostatic confinement, or IEC, is a class of fusion power devices that use electric fields to confine the plasma rather than the more common approach using magnetic fields found in magnetic fusion energy (MFE) designs. Most IEC devi ...

. He founded EMC2 to continue polywell research. A picosecond pulse of a 10-terawatt laser produced hydrogen–boron aneutronic fusions for a Russian team in 2005. However, the number of the resulting α particles (around 10³ per laser pulse) was low. In 2006, the

Z-machine The Z-machine is a virtual machine that was developed by Joel Berez and Marc Blank in 1979 and used by Infocom for its text adventure games. Infocom compiled game code to files containing Z-machine instructions (called story files or Z-code ...

Sandia National Laboratory Sandia National Laboratories (SNL), also known as Sandia, is one of three research and development laboratories of the United States Department of Energy's National Nuclear Security Administration (NNSA). Headquartered in Kirtland Air Force Bas ...

, a

z-pinch In fusion power research, the Z-pinch (zeta pinch) is a type of plasma confinement system that uses an electric current in the plasma to generate a magnetic field that compresses it (see pinch). These systems were originally referred to simp ...

device, reached 2 billion kelvins and 300 keV. In 2011,

Lawrenceville Plasma Physics Eric J. Lerner (born May 31, 1947) is an American popular science writer, and independent plasma researcher. He wrote the 1991 book ''The Big Bang Never Happened'', which advocates Hannes Alfvén's plasma cosmology instead of the Big Bang th ...

published initial results and outlined a theory and experimental program for aneutronic fusion with the

Dense Plasma Focus A dense plasma focus (DPF) is a type of plasma generating system originally developed as a fusion power device starting in the early 1960s. The system demonstrated scaling laws that suggested it would not be useful in the commercial power role, a ...

(DPF). The effort was initially funded by NASA's

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, United States. Founded in the 1930s by Caltech researchers, JPL is owned by NASA an ...

. Support for other DPF aneutronic fusion investigations came from the

Air Force Research Laboratory The Air Force Research Laboratory (AFRL) is a scientific research organization operated by the United States Air Force Materiel Command dedicated to leading the discovery, development, and integration of aerospace warfighting technologies, pl ...

. A French research team fused protons and boron-11 nuclei using a laser-accelerated proton beam and high-intensity laser pulse. In October 2013 they reported an estimated 80 million fusion reactions during a 1.5 nanosecond laser pulse. In 2016, a team at the Shanghai

Chinese Academy of Sciences The Chinese Academy of Sciences (CAS); ), known by Academia Sinica in English until the 1980s, is the national academy of the People's Republic of China for natural sciences. It has historical origins in the Academia Sinica during the Republi ...

produced a laser pulse of 5.3 petawatts with the ''Superintense Ultrafast Laser Facility'' (SULF) and expected to reach 10 petawatts with the same equipment. In 2021,

TAE Technologies TAE Technologies, formerly Tri Alpha Energy, is an American company based in Foothill Ranch, California developing aneutronic fusion power. The company's design relies on an advanced beam-driven field-reversed configuration (FRC), which combin ...

field-Reversed Configuration A field-reversed configuration (FRC) is a type of plasma device studied as a means of producing nuclear fusion. It confines a plasma on closed magnetic field lines without a central penetration. In an FRC, the plasma has the form of a self-stabl ...

announced that its Norman device was regularly producing a stable plasma at temperatures over 50 million degrees. In 2021, a Russian team reported experimental results in a miniature device with electrodynamic (oscillatory) plasma confinement. It used a ~1–2 J nanosecond vacuum discharge with a virtual cathode. Its field accelerates ions to ~100–300 keV under oscillating ions' collisions. α particles of about /4π (~10 α particles/ns) were obtained within a total of 4 μs of the voltage applications. HB11 Energy is an Australian spin-off company created in September 2017. HB11 holds the patents of UNSW's theoretical physicist Heinrich Hora. Its device uses two petawatt-class, chirped pulse lasers to drive low-temperature proton-boron fusion using an "in-target" approach. One laser drives hydrogen atoms via target normal sheath acceleration towards a boron plasma confined by a kilotesla magnetic field powered by the other laser. The resulting He⁺ ions are converted to electricity without a thermal conversion step with its associated thermal losses. The pico-second laser produces an avalanche reaction that offers a billion time increased fusion yield improvement compared to other ICF systems. In 2022, they claimed to be the first commercial company to demonstrate fusion. It demonstrated fusion, yielding an alpha particle flux of 10¹⁰/ sr, one order of magnitude higher than its earlier results, but still 4 orders of magnitude away from net energy gain.

Definition

Fusion reactions can be categorized according to their neutronicity: the fraction of the fusion energy released as energetic neutrons. The State of

New Jersey New Jersey is a state in the Mid-Atlantic and Northeastern regions of the United States. It is bordered on the north and east by the state of New York; on the east, southeast, and south by the Atlantic Ocean; on the west by the Delaware ...

defined an aneutronic reaction as one in which neutrons carry no more than 1% of the total released energy, although many papers on the subjectJ. Reece Roth (1989)
"Space Applications of Fusion Energy"
''Fusion Technology, Volume 15'', no. 3. Retrieved 2019-05-07. include reactions that do not meet this criterion.

Coulomb barrier

The

Coulomb barrier The Coulomb barrier, named after Coulomb's law, which is in turn named after physicist Charles-Augustin de Coulomb, is the energy barrier due to electrostatic interaction that two nuclei need to overcome so they can get close enough to undergo a ...

is the minimum energy required for the nuclei in a fusion reaction to overcome their mutual

electrostatic repulsion Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for a ...

. This is a function of the total electrical charge of the fuel ions, and is thus minimized for those ions with the fewest

. Countering electrostatic repulsion is the

nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nucle ...

, which increases with the number of

nucleons In physics and chemistry, a nucleon is either a proton or a neutron, considered in its role as a component of an atomic nucleus. The number of nucleons in a nucleus defines the atom's mass number (nucleon number). Until the 1960s, nucleons we ...

. In most fusion concepts, the energy needed to overcome the Coulomb barrier is provided by collisions with other fuel ions. In a thermalized fluid like a plasma, the

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer. Thermometers are calibrated in various Conversion of units of temperature, temp ...

corresponds to an energy spectrum according to the

Maxwell–Boltzmann distribution In physics (in particular in statistical mechanics), the Maxwell–Boltzmann distribution, or Maxwell(ian) distribution, is a particular probability distribution named after James Clerk Maxwell and Ludwig Boltzmann. It was first defined and use ...

. Gases in this state have some particles with high energy even if the average energy is much lower. Fusion devices rely on this distribution; even at bulk temperatures far below the Coulomb barrier energy, the energy released by the reactions is great enough that capturing some of that can supply sufficient high-energy ions to keep the reaction going. Thus, steady operation of the reactor is based on a balance between the rate that energy is added to the fuel by fusion reactions and the rate energy is lost to the surroundings. This concept is best expressed as the

fusion triple product The Lawson criterion is a figure of merit used in nuclear fusion research. It compares the rate of energy being generated by fusion reactions within the fusion fuel to the rate of energy losses to the environment. When the rate of production is ...

, the product of the temperature, density and "confinement time", the amount of time energy remains in the fuel before escaping to the environment. The product of temperature and density gives the reaction rate for any given fuel. The rate of reaction is proportional to the

nuclear cross section The nuclear cross section of a nucleus is used to describe the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a large ...

(σ).Rainer Feldbacher and Manfred Heindler (1 August 1988). "Basic cross section data for aneutronic reactor", ''Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 271,'' No 1, pp 55-64
DOI: 10.1016/0168-9002(88)91125-4
Any given device can sustain some maximum plasma pressure. An efficient device would continuously operate near this maximum. Given this pressure, the largest fusion output is obtained when the temperature is such that <σv>/T² is a maximum. This is also the temperature at which the value of the triple product ''nT''τ required for ignition is a minimum, since that required value is inversely proportional to <σv>/T². A plasma is "ignited" if the fusion reactions produce enough power to maintain the temperature without external heating. Because the Coulomb barrier is a product of the number of nucleons in the fuel ions, varieties of heavy hydrogen,

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

and

tritium Tritium ( or , ) or hydrogen-3 (symbol T or H) is a rare and radioactive isotope of hydrogen with half-life about 12 years. The nucleus of tritium (t, sometimes called a ''triton'') contains one proton and two neutrons, whereas the nucleus of ...

(D-T), give the fuel with the lowest total Coulomb barrier. All other potential fuels have higher Coulomb barriers, and thus require higher operational temperatures. Additionally, D-T fuels have the highest nuclear cross-sections, which means the reaction rates are higher than any other fuel. This makes D-T fusion the easiest to achieve. Comparing the potential of other fuels to the D-T reaction. The table below shows the ignition temperature and cross-section for three of the candidate aneutronic reactions, compared to D-T: As can be seen, the easiest to ignite of the aneutronic reactions, D-³He, has an ignition temperature over four times as high as that of the D-T reaction, and correspondingly lower cross-sections, while the p-¹¹B reaction is nearly ten times more difficult to ignite.

Candidate reactions

Several fusion reactions produce no neutrons on any of their branches. Those with the largest cross sections are

Candidate fuels

³He

The ³He–D reaction has been studied as an alternative fusion plasma because it has the lowest energy threshold. The p–⁶Li, ³He–⁶Li, and ³He–³He reaction rates are not particularly high in a thermal plasma. When treated as a chain, however, they offer the possibility of enhanced reactivity due to a non-thermal distribution. The product ³He from the p–⁶Li reaction could participate in the second reaction before thermalizing, and the product p from ³He–⁶Li could participate in the former before thermalizing. Detailed analyses, however, do not show sufficient reactivity enhancement to overcome the inherently low cross section. The ³He reaction suffers from a ³He availability problem. ³He occurs in only minuscule amounts on Earth, so it would either have to be bred from neutron reactions (counteracting the potential advantage of aneutronic fusion) or mined from extraterrestrial sources. The amount of ³He needed for large-scale applications can also be described in terms of total consumption: according to the

US Energy Information Administration The U.S. Energy Information Administration (EIA) is a principal agency of the U.S. Federal Statistical System responsible for collecting, analyzing, and disseminating energy information to promote sound policymaking, efficient markets, and publ ...

, "Electricity consumption by 107 million U.S. households in 2001 totaled 1,140 billion kW·h" (). Again assuming 100% conversion efficiency, 6.7 tonnes per year of ³He would be required for that segment of the energy demand of the United States, 15 to 20 tonnes per year given a more realistic end-to-end conversion efficiency. Extracting that amount of pure ³He would entail processing 2 billion tonnes of lunar material per year, even assuming a recovery rate of 100%. In their own promotional materials,

Helion Energy Helion Energy, Inc. is an American fusion research company, located in Everett, Washington. They are developing a magneto-inertial fusion technology to produce helium-3 and fusion power via aneutronic fusion, which could produce low-cost clean e ...

claims that their 7th fusion prototype (Polaris; fully funded and under construction as of September 2022) will demonstrate "net electricity from fusion", and will demonstrate "helium-3 production through deuterium-deuterium fusion" by means of a "patented high-efficiency closed-fuel cycle".

Deuterium

Although the deuterium reactions (deuterium + ³He and deuterium + ⁶lithium) do not in themselves release neutrons, in a fusion reactor the plasma would also produce D-D side reactions that result in reaction product of ³He plus a neutron. Although neutron production can be minimized by running a plasma reaction hot and deuterium-lean, the fraction of energy released as neutrons is probably several percent, so that these fuel cycles, although neutron-poor, do not meet the 1% threshold. See ³He. The D-³He reaction also suffers from the ³He fuel availability problem, as discussed above.

Lithium

Fusion reactions involving lithium are well studied due to the use of lithium for breeding tritium in

thermonuclear weapon A thermonuclear weapon, fusion weapon or hydrogen bomb (H bomb) is a second-generation nuclear weapon design. Its greater sophistication affords it vastly greater destructive power than first-generation nuclear bombs, a more compact size, a low ...

s. They are intermediate in ignition difficulty between the reactions involving lower atomic-number species, H and He, and the ¹¹B reaction. The p–⁷Li reaction, although highly energetic, releases neutrons because of the high cross section for the alternate neutron-producing reaction ¹p + ⁷Li → ⁷Be + n

Boron

Many studies of aneutronic fusion concentrate on the p–¹¹B reaction, which uses easily available fuel. The fusion of the boron nucleus with a proton produces energetic alpha particles (helium nuclei). Since igniting the p–¹¹B reaction is much more difficult than D-T, alternatives to the usual

tokamak A tokamak (; russian: токамáк; otk, 𐱃𐰸𐰢𐰴, Toḳamaḳ) is a device which uses a powerful magnetic field to confine plasma in the shape of a torus. The tokamak is one of several types of magnetic confinement devices being ...

fusion reactors are usually proposed, such as

inertial confinement fusion Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with thermonuclear fuel. In modern machines, the targets are small spherical pellets about the size of ...

. One proposed method uses one laser to create a boron-11 plasma and another to create a stream of protons that smash into the plasma. The proton beam produces a tenfold increase of fusion because protons and boron nuclei collide directly. Earlier methods used a solid boron target, "protected" by its electrons, which reduced the fusion rate. Experiments suggest that a petawatt-scale laser pulse could launch an 'avalanche' fusion reaction, although this remains controversial. The plasma lasts about one

nanosecond A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds. The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...

, requiring the picosecond pulse of protons to be precisely synchronized. Unlike conventional methods, this approach does not require a magnetically confined plasma. The proton beam is preceded by an electron beam, generated by the same laser, that strips electrons in the boron plasma, increasing the protons chance to collide with the boron nuclei and fuse.

Residual radiation

Calculations show that at least 0.1% of the reactions in a thermal p–¹¹B plasma produce neutrons, although their energy accounts for less than 0.2% of the total energy released. These neutrons come primarily from the reaction: :¹¹B + α → ¹⁴N + n + 157 keV The reaction itself produces only 157 keV, but the neutron carries a large fraction of the alpha energy, close to ''E''_fusion/3 = 2.9

MeV In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacu ...

. Another significant source of neutrons is: :¹¹B + p → ¹¹C + n − 2.8 MeV. These neutrons are less energetic, with an energy comparable to the fuel temperature. In addition, ¹¹C itself is radioactive, but quickly decays to ¹¹B with a half life of only 20 minutes. Since these reactions involve the reactants and products of the primary reaction, it is difficult to lower the neutron production by a significant fraction. A clever magnetic confinement scheme could in principle suppress the first reaction by extracting the alphas as they are created, but then their energy would not be available to keep the plasma hot. The second reaction could in principle be suppressed relative to the desired fusion by removing the high energy tail of the ion distribution, but this would probably be prohibited by the power required to prevent the distribution from thermalizing. In addition to neutrons, large quantities of hard

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s are produced by

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...

, and 4, 12, and 16 MeV

gamma ray A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

s are produced by the fusion reaction :¹¹B + p → ¹²C + γ + 16.0 MeV with a branching probability relative to the primary fusion reaction of about 10⁻⁴.As with the neutron dose, shielding is essential with this level of gamma radiation. The neutron calculation in the previous note would apply if the production rate is decreased a factor of ten and the quality factor is reduced from 20 to 1. Without shielding, the occupational dose from a small (30 kW) reactor would still be reached in about an hour. The hydrogen must be isotopically pure and the influx of impurities into the plasma must be controlled to prevent neutron-producing side reactions such as: :¹¹B + d → ¹²C + n + 13.7 MeV :d + d → ³He + n + 3.27 MeV The shielding design reduces the occupational dose of both neutron and gamma radiation to a negligible level. The primary components are water (to moderate the fast neutrons), boron (to absorb the moderated neutrons) and metal (to absorb X-rays). The total thickness is estimated to be about one meter, mostly water.

Energy capture

Aneutronic fusion produces energy in the form of charged particles instead of

s. This means that energy from aneutronic fusion could be captured using direct conversion instead of thermally. Direct conversion can either be inductive, based on changes in magnetic fields, electrostatic, based on pitting charged particles against an electric field, or photoelectric, in which light energy is captured in a pulsed mode. Electrostatic direct conversion uses the motion of charged particles to create

voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...

. This voltage drives electricity in a wire which becomes electrical power. It is the reverse of phenomena that use a voltage to put a particle in motion. It has been described as a

linear accelerator A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear ...

running backwards. Aneutronic fusion loses much of its energy as light. This energy results from the acceleration and deceleration of charged particles. These speed changes can be caused by Bremsstrahlung radiation or

cyclotron radiation Cyclotron radiation is electromagnetic radiation emitted by non-relativistic accelerating charged particles deflected by a magnetic field. The Lorentz force on the particles acts perpendicular to both the magnetic field lines and the particles' mot ...

or synchrotron radiation or electric field interactions. The radiation can be estimated using the

Larmor formula In electrodynamics, the Larmor formula is used to calculate the total power radiated by a nonrelativistic point charge as it accelerates. It was first derived by J. J. Larmor in 1897, in the context of the wave theory of light. When any charged ...

and comes in the X-ray, IR, UV and visible spectra. Some of the energy radiated as X-rays may be converted directly to electricity. Because of the

photoelectric effect The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid sta ...

, X-rays passing through an array of conducting foils transfer some of their energy to electrons, which can then be captured electrostatically. Since X-rays can go through far greater material thickness than electrons, many hundreds or thousands of layers are needed to absorb them.

Technical challenges

Many challenges confront the commercialization of aneutronic fusion.

Temperature

The large majority of fusion research has gone toward D-T fusion, which is the easiest to achieve. Fusion experiments typically use

deuterium-deuterium fusion Deuterium fusion, also called deuterium burning, is a nuclear fusion reaction that occurs in stars and some substellar objects, in which a deuterium nucleus and a proton combine to form a helium-3 nucleus. It occurs as the second stage of the prot ...

(D-D) because deuterium is cheap and easy to handle, being non-radioactive. Experimenting with D-T fusion is more difficult because tritium is expensive and radioactive, requiring additional environmental protection and safety measures. The combination of lower cross-section and higher loss rates in D-³He fusion is offset to a degree because the reactants are mainly charged particles that deposit their energy in the plasma. This combination of offsetting features demands an operating temperature about four times that of a D-T system. However, due to the high loss rates and consequent rapid cycling of energy, the confinement time of a working reactor needs to be about fifty times higher than D-T, and the energy density about 80 times higher. This requires significant advances in plasma physics. Proton–boron fusion requires ion energies, and thus plasma temperatures, some nine times higher than those for D-T fusion. For any given density of the reacting nuclei, the reaction rate for proton-boron achieves its peak rate at around 600

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(6.6 billion degrees Celsius or 6.6 gigakelvins) while D-T has a peak at around 66 keV (765 million degrees Celsius, or 0.765 gigakelvin). For pressure-limited confinement concepts, optimum operating temperatures are about 5 times lower, but the ratio is still roughly ten-to-one.

Power balance

The peak reaction rate of p–¹¹B is only one third that for D-T, requiring better plasma confinement. Confinement is usually characterized by the time τ the energy is retained so that the power released exceeds that required to heat the plasma. Various requirements can be derived, most commonly the Lawson criterion, the product of the density, ''n''τ, and the product with the pressure ''nT''τ. The ''n''τ required for p–¹¹B is 45 times higher than that for D-T. The ''nT''τ required is 500 times higher.Both figures assume the electrons have the same temperature as the ions. If operation with cold electrons is possible, as discussed below, the relative disadvantage of p–¹¹B would be a factor of three smaller, as calculated

here Here is an adverb that means "in, on, or at this place". It may also refer to: Software * Here Technologies, a mapping company * Here WeGo (formerly Here Maps), a mobile app and map website by Here Technologies, Here Television * Here TV (form ...

. Since the confinement properties of conventional fusion approaches, such as the

and laser pellet fusion are marginal, most aneutronic proposals use radically different confinement concepts. In most fusion plasmas,

radiation is a major energy loss channel. (See also bremsstrahlung losses in quasineutral, isotropic plasmas.) For the p–¹¹B reaction, some calculations indicate that the bremsstrahlung power will be at least 1.74 times larger than the fusion power. The corresponding ratio for the ³He-³He reaction is only slightly more favorable at 1.39. This is not applicable to non-neutral plasmas, and different in anisotropic plasmas. In conventional reactor designs, whether based on magnetic or inertial confinement, the bremsstrahlung can easily escape the plasma and is considered a pure energy loss term. The outlook would be more favorable if the plasma could reabsorb the radiation. Absorption occurs primarily via

Thomson scattering Thomson scattering is the elastic scattering of electromagnetic radiation by a free charged particle, as described by classical electromagnetism. It is the low-energy limit of Compton scattering: the particle's kinetic energy and photon frequen ...

on the

electrons The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

, which has a total cross section of σ_T = . In a 50–50 D-T mixture this corresponds to a range of . This is considerably higher than the Lawson criterion of ρ''R'' > 1 g/cm², which is already difficult to attain, but might be achievable in inertial confinement systems. In megatesla magnetic fields a

quantum mechanical Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, qua ...

effect might suppress energy transfer from the ions to the electrons. According to one calculation, bremsstrahlung losses could be reduced to half the fusion power or less. In a strong magnetic field

is even larger than the bremsstrahlung. In a megatesla field, an electron would lose its energy to cyclotron radiation in a few picoseconds if the radiation could escape. However, in a sufficiently dense plasma (''n''_e > , a density greater than that of a solid), the

cyclotron frequency Cyclotron resonance describes the interaction of external forces with charged particles experiencing a magnetic field, thus already moving on a circular path. It is named after the cyclotron, a cyclic particle accelerator that utilizes an oscillati ...

is less than twice the

plasma frequency Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...

. In this well-known case, the cyclotron radiation is trapped inside the plasmoid and cannot escape, except from a very thin surface layer. While megatesla fields have not yet been achieved, fields of 0.3 megatesla have been produced with high intensity lasers, and fields of 0.02–0.04 megatesla have been observed with the

dense plasma focus A dense plasma focus (DPF) is a type of plasma generating system originally developed as a fusion power device starting in the early 1960s. The system demonstrated scaling laws that suggested it would not be useful in the commercial power role, a ...

device. At much higher densities (''n''_e > ), the electrons will be Fermi degenerate, which suppresses bremsstrahlung losses, both directly and by reducing energy transfer from the ions to the electrons. If necessary conditions can be attained, net energy production from p–¹¹B or D–³He fuel may be possible. The probability of a feasible reactor based solely on this effect remains low, however, because the gain is predicted to be less than 20, while more than 200 is usually considered to be necessary.

Power density

In every published fusion power plant design, the part of the plant that produces the fusion reactions is much more expensive than the part that converts the nuclear power to electricity. In that case, as indeed in most power systems, power density is an important characteristic.Comparing two different types of power systems involves many factors in addition to the power density. Two of the most important are the volume in which energy is produced in comparison to the total volume of the device, and the cost and complexity of the device. In contrast, the comparison of two different fuel cycles in the same type of machine is generally much more robust. Doubling power density at least halves the cost of electricity. In addition, the confinement time required depends on the power density. It is, however, not trivial to compare the power density produced by different fusion fuel cycles. The case most favorable to p–¹¹B relative to D-T fuel is a (hypothetical) confinement device that only works well at ion temperatures above about 400 keV, in which the reaction rate parameter <σ''v''> is equal for the two fuels, and that runs with low electron temperature. p–¹¹B does not require as long a confinement time because the energy of its charged products is two and a half times higher than that for D-T. However, relaxing these assumptions, for example by considering hot electrons, by allowing the D-T reaction to run at a lower temperature or by including the energy of the neutrons in the calculation shifts the power density advantage to D-T. The most common assumption is to compare power densities at the same pressure, choosing the ion temperature for each reaction to maximize power density, and with the electron temperature equal to the ion temperature. Although confinement schemes can be and sometimes are limited by other factors, most well-investigated schemes have some kind of pressure limit. Under these assumptions, the power density for p–¹¹B is about 2,100 times smaller than that for D-T. Using cold electrons lowers the ratio to about 700. These numbers are another indication that aneutronic fusion power is not possible with mainline confinement concepts.

Notes

References

External links

Focus Fusion Society

Proton-boron Fusion Prototype

Aneutronic fusion in a degenerate plasma

Lasers trigger cleaner fusion
(news@nature.com, 26 August 2005)
Observation of neutronless fusion reactions in picosecond laser plasmas
(Physical Review E 72, 2005)
New Opportunities for Fusion in the 21st Century – Advanced Fuels
, G.L. Kulcinski and J.F.Santarius, 14th Topical Meeting on the Technology of Fusion Energy, Oct 15–19, 2000, {{Nuclear Technology Fusion power Nuclear fusion reactions