Aneutronic fusion is any form of fusion power in which very little of the energy released is carried by neutrons. While the lowest-threshold nuclear fusion reactions release up to 80% of their energy as the kinetic energy of neutrons, aneutronic reactions release energy as the kinetic energy of charged particles, typically protons or alpha particles. Successful aneutronic fusion would greatly reduce problems associated with neutron radiation such as damaging ionizing radiation, neutron activation, 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.Larry T. Cox Jr., Franklin B. Mead Jr. and Chan K. Choi Jr., (1990). "Thermonuclear Reaction Listing with Cross-Section Data for Four Advanced Reactions"], Fusion Technology, Volume 18, no. 2. Retrieved 2019-05-07. However, the conditions required to harness aneutronic fusion are much more extreme than those required for deuterium–tritium (D–T) fusion such as at ITER.
History The first experiments in the field started in 1939, and serious efforts have been continual since the early 1950s.
An early supporter was Richard F. Post at Lawrence Livermore. He proposed to capture the kinetic energy of charged particles as they were exhausted from a fusion reactor and convert this into voltage to drive current."Mirror Systems: Fuel Cycles, Loss Reduction and Energy Recovery" R.F. Post, BNES nuclear Fusion Reactor Conference at Culham Labs, September 1969 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 in 1981."Experimental Results from a beam Direct Converter at 100 kV" W. L. Barr, R. W. Moir and G Hamilton, December 3, 1981, Journal of Fusion Energy Vol 2, No. 2, 1982
Polywell fusion was pioneered by the late Robert W. Bussard in 1995 and funded by the US Navy. Polywell uses inertial electrostatic confinement. He founded EMC2 to continue polywell research.Bussard, R. W. & Jameson L. W., Inertial-Electrostatic-Fusion Propulsion Spectrum: Air-Breathing to Interstellar Flight , Journal of Propulsion and Power Vol. 11, No. 2, March–April 1995
A picosecond pulse of a 10-terawatt laser produced hydrogen–boron aneutronic fusions for a Russian team in 2005., mentioned in [email protected] on August 26, 2005 : Lasers trigger cleaner fusion However, the number of the resulting α particles (around 103 per laser pulse) was low.
In 2006, the Z-machine at Sandia National Laboratory, a z-pinch device, reached 2 billion kelvins and 300 keV.Malcolm Haines et al., Viscous Heating of Ions through Saturated Fine-Scale MHD Instabilities in a Z-Pinch at 200–300 keV Temperature; Phys. Rev. Lett. 96, 075003 (2006)
In 2011, Lawrenceville Plasma Physics published initial results and outlined a theory and experimental program for aneutronic fusion with the dense plasma focus (DPF). The effort was initially funded by NASA's Jet Propulsion Laboratory.JPL Contract 959962, JPL Contract 959962 Support for other DPF aneutronic fusion investigations came from the Air Force Research Laboratory.
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 2021, TAE Technologies field-reversed configuration 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 boron ions and protons to ~ 100–300 keV under oscillating ions' collisions. α-particles of about /4π (~ 10 α-particles/ns) were obtained during the 4 μs of applied voltage.
Australian spin-off company HB11 Energy was created in September 2019. In 2022, they claimed to be the first commercial company to demonstrate fusion.
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 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 is the minimum energy required for the nuclei in a fusion reaction to overcome their mutual electrostatic repulsion. Once reactants are close enough, they become strongly attracted by the nuclear force, which dominates over the electrostatic repulsion.
Repulsive force between a particle with charge Z1 and one with Z2 is proportional to , where r is the distance between them. The Coulomb barrier facing a pair of reacting, charged particles depends both on total charge and on how equally those charges are distributed; the barrier is lowest when a low-Z particle reacts with a high-Z one and highest when the reactants are of roughly equal charge. Barrier energy is thus minimized for those ions with the fewest protons.
When the two nuclei approach to within ~2 femtometres (~3 proton radii), the attractive strong nuclear interaction between their nucleons becomes appreciable. At that range, the nuclear attraction can dominate the remaining Coulomb repulsion and drive the system toward fusion. When the fused system has lower mass, the difference is released as nuclear energy , carried off by the kinetic energy of resultant fragments.
More nucleons can give more nuclear binding, because there are more protons and neutrons for the strong force to bind. But protons also repel other protons, so adding protons also adds electrical repulsion. The neutron/proton ratio matters because nuclei with the wrong ratio are unstable.
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 corresponds to an energy spectrum according to the Maxwell–Boltzmann distribution. 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 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 (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 nTτ 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 proportional to the product of proton counts () of the two reactants, varieties of heavy hydrogen, deuterium and tritium (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: +Candidate reactionsReactionIgnition T [keV]Cross-section σv/T2 [m3/s/keV2]–13.6–58p+–66p+–123 To estimate the temperature in Kelvin, multiply the temperature in keV by ten million. 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: IsotopesReaction–2D +3He→ 4He +1p + 18.3 MeV–2D +6Li→24He + 22.4 MeVp–1p +6Li→4He +3He + 4.0 MeV–3He +6Li→24He +1p + 16.9 MeV–3He +3He→ 4He +2 1p + 12.86 MeVp–1p +7Li→24He + 17.2 MeVp–1p +11B→34He + 8.7 MeVp–1p +15N→ 12C +4He + 5.0 MeV
Candidate fuels 3He
Helium-3 has been investigated as a candidate fuel for aneutronic fusion because several reactions involving 3He predominantly produce charged particles rather than neutrons, potentially reducing neutron-induced material damage and radioactive activation compared with deuterium–tritium fusion (D-T).
A theoretically fully aneutronic, third-generation reaction is 3He–3He fusion:
³₂He + ³₂He → ⁴₂He + 2¹₁p + 12.86 MeV.
While this reaction produces only charged particles, it requires extremely high plasma temperatures and confinement and has not been demonstrated under reactor-relevant conditions, limiting its practical applicability to aneutronic fusion concepts.
Aneutronic fusion research involving 3He has therefore focused primarily on the deuterium–helium-3 (D–³He) reaction,
D + ³He → ⁴He + p + 18.3 MeV,
which yields a helium-4 nucleus and a proton - charged particles that could, in principle, be confined and converted directly into electricity, offering a pathway to reduced neutron shielding requirements and simplified energy conversion. However, D–³He fusion requires substantially higher plasma temperatures and improved confinement compared with D–T, making it more challenging to achieve experimentally. In addition, unavoidable side reactions such as deuterium–deuterium fusion (D–D) inevitably occur in realistic plasmas, producing neutrons and introducing a small but non‑zero neutron flux; while this neutron production is significantly lower than in D–T fusion, it limits the extent to which D–³He systems can be considered strictly aneutronic.