Editing Helium-3 (section)

==Uses==
===Helium-3 spin echo===
Helium-3 can be used to do [[Helium-3 surface spin echo|spin echo experiments of surface dynamics]], which are underway at the Surface Physics Group at [[Cavendish Laboratory|the Cavendish Laboratory]] in Cambridge and in the Chemistry Department at [[Swansea University]].

===Neutron detection===
Helium-3 is an important isotope in instrumentation for [[neutron detection]]. It has a high absorption cross section for thermal [[neutron radiation|neutron]] beams and is used as a converter gas in neutron detectors. The neutron is converted through the nuclear reaction
:n + <sup>3</sup>He → <sup>3</sup>H + <sup>1</sup>H + 0.764 MeV
into charged particles [[tritium]] ions (T, <sup>3</sup>H) and [[Hydrogen ions]], or protons (p, <sup>1</sup>H) which then are detected by creating a charge cloud in the stopping gas of a [[proportional counter]] or a [[Geiger–Müller tube]].<ref>[http://www.lanl.gov/quarterly/q_sum03/neutron_detect.shtml A Modular Neutron Detector | Summer 2003| Los Alamos National Laboratory] {{Webarchive|url=https://web.archive.org/web/20080503051236/http://www.lanl.gov/quarterly/q_sum03/neutron_detect.shtml |date=2008-05-03 }}. Lanl.gov. Retrieved on 2011-11-08.</ref>

Furthermore, the absorption process is strongly [[Spin (physics)|spin]]-dependent, which allows a [[Spin polarization|spin-polarized]] helium-3 volume to transmit neutrons with one spin component while absorbing the other. This effect is employed in [[Polarized neutron scattering|neutron polarization analysis]], a technique which probes for magnetic properties of matter.<ref>[http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm NCNR Neutron Spin Filters] {{Webarchive|url=https://web.archive.org/web/20070520100431/http://www.ncnr.nist.gov/AnnualReport/FY2002_html/pages/neutron_spin.htm |date=2007-05-20 }}. Ncnr.nist.gov (2004-04-28). Retrieved on 2011-11-08.</ref><ref>[http://www.ill.eu/science-technology/neutron-technology-at-ill/optics/<sup>3</sup>He-spin-filters/ ILL <sup>3</sup>He spin filters]{{Dead link|date=September 2023 |bot=InternetArchiveBot |fix-attempted=yes }}. Ill.eu (2010-10-22). Retrieved on 2011-11-08.</ref><ref>{{cite journal|url= http://www.ncnr.nist.gov/staff/hammouda/publications/2000_gentile_j_appl_cryst.pdf|title= SANS polarization analysis with nuclear spin-polarized <sup>3</sup>He|doi= 10.1107/S0021889800099817|journal= J. Appl. Crystallogr.|date= 2000|volume= 33|issue= 3|pages= 771–774|last1= Gentile|first1= T.R.|last2= Jones|first2= G.L.|last3= Thompson|first3= A.K.|last4= Barker|first4= J.|last5= Glinka|first5= C.J.|last6= Hammouda|first6= B.|last7= Lynn|first7= J.W.|bibcode= 2000JApCr..33..771G|access-date= 2011-11-08|archive-date= 2012-04-02|archive-url= https://web.archive.org/web/20120402004050/http://www.ncnr.nist.gov/staff/hammouda/publications/2000_gentile_j_appl_cryst.pdf|url-status= live}}</ref><ref>[http://www.ncnr.nist.gov/equipment/he3nsf/index.html Neutron Spin Filters: Polarized <sup>3</sup>He] {{Webarchive|url=https://web.archive.org/web/20111016142515/http://ncnr.nist.gov/equipment/he3nsf/index.html |date=2011-10-16 }}. NIST.gov</ref>

The United States [[Department of Homeland Security]] had hoped to deploy detectors to spot smuggled plutonium in shipping containers by their neutron emissions, but the worldwide shortage of helium-3 following the drawdown in nuclear weapons production since the [[Cold War]] has to some extent prevented this.<ref>{{Cite news |last=Wald |first=Matthew L. |date=2009-11-22 |title=Shortage Slows a Program to Detect Nuclear Bombs |url=https://www.nytimes.com/2009/11/23/us/23helium.html |access-date=2025-05-11 |work=The New York Times |language=en-US |issn=0362-4331}}</ref>  As of 2012, DHS determined the commercial supply of [[boron-10]] would support converting its neutron detection infrastructure to that technology.<ref>{{Cite web |url=http://science.energy.gov/~/media/np/pdf/research/idpra/workshop-on-isotope-federal-supply-and-demand/presentations/Slovik_He3_Alternative_Isotopes_DOE_IP_Workshop_Jan_11_2012.pdf |title=Office of Science |access-date=2014-07-18 |archive-url=https://web.archive.org/web/20140726214826/http://science.energy.gov/~/media/np/pdf/research/idpra/workshop-on-isotope-federal-supply-and-demand/presentations/Slovik_He3_Alternative_Isotopes_DOE_IP_Workshop_Jan_11_2012.pdf |archive-date=2014-07-26 |url-status=dead }}</ref>

===Cryogenics===
Helium-3 refrigerators are devices used in experimental physics for obtaining temperatures down to about 0.2 [[kelvin]].<ref name="liquidseas">{{cite web |title=Workings of a 3He refrigerator |website=Harvard X-Ray Group Home Page |url=https://liquids.seas.harvard.edu/penanen/workings.html |access-date=2025-04-17}}</ref> By [[evaporative cooling]] of helium-4, a [[1-K pot]] liquefies a small amount of [[helium-3]] in a small vessel called a helium-3 pot. Evaporative cooling at low pressure of the liquid helium-3, usually driven by [[adsorption]] since due to its high price the helium-3 is usually contained in a closed system to avoid losses, cools the helium-3 pot to a fraction of a kelvin.

A [[dilution refrigerator]] uses a mixture of helium-3 and helium-4 to reach [[cryogenics|cryogenic]] temperatures as low as a few thousandths of a kelvin.<ref name="cerndilution">{{cite web |title=Dilution Refrigeration |website=na47sun05.cern.ch |date=2009-02-07 |url=http://na47sun05.cern.ch/target/outline/dilref.html |archive-url=https://web.archive.org/web/20100208194054/http://na47sun05.cern.ch/target/outline/dilref.html |archive-date=2010-02-08 |url-status=dead |access-date=2025-04-17}}</ref>

===Nuclear magnetic resonance===
Helium-3 nuclei have an intrinsic [[nuclear spin]] of {{frac|1|2}}, and a relatively high [[gyromagnetic ratio]]. Because of this, it is possible to use [[Nuclear magnetic resonance]] (NMR) to observe Helium-3. This analytical technique, usually called <sup>3</sup>He-NMR, can be  used to identify helium-containing compounds. It is however limited by the low abundance of helium-3 in comparison to helium-4, which is itself not NMR-active.

Helium-3 can be [[Hyperpolarization (physics)|hyperpolarized]] using non-equilibrium means such as spin-exchange optical pumping.<ref>{{cite journal|title = Hyperpolarized <sup>3</sup>He Gas Production and MR Imaging of the Lung|first1 = Jason C.|last1 = Leawoods|first2 = Dmitriy A.|last2 = Yablonskiy|first3 = Brian|last3 = Saam|first4 = David S.|last4 = Gierada|first5 = Mark S.|last5 = Conradi|date =2001|journal =Concepts in Magnetic Resonance|volume =13|issue = 5|pages =277–293|doi=10.1002/cmr.1014|citeseerx = 10.1.1.492.8128}}</ref> During this process, [[circular polarization|circularly polarized]] infrared laser light, tuned to the appropriate wavelength, is used to excite electrons in an [[alkali metal]], such as [[caesium]] or [[rubidium]] inside a sealed glass vessel. The [[angular momentum]] is transferred from the alkali metal electrons to the noble gas nuclei through collisions. In essence, this process effectively aligns the nuclear spins with the magnetic field in order to enhance the NMR signal.

The hyperpolarized gas may then be stored at pressures of 10 atm, for up to 100 hours. Following inhalation, gas mixtures containing the hyperpolarized helium-3 gas can be imaged with an MRI scanner to produce anatomical and functional images of lung ventilation. This technique is also able to produce images of the airway tree, locate unventilated defects, measure the [[pulmonary gas pressures|alveolar oxygen partial pressure]], and measure the [[ventilation/perfusion ratio]]. This technique may be critical for the diagnosis and treatment management of chronic respiratory diseases such as [[chronic obstructive pulmonary disease|chronic obstructive pulmonary disease (COPD)]], [[emphysema]], [[cystic fibrosis]], and [[asthma]].<ref>{{cite journal|title = Hyperpolarized Gas Imaging of the Lung|first1 = Talissa|last1 = Altes|first2 = Michael|last2 = Salerno|date =2004|journal =J Thorac Imaging|volume =19|issue = 4|pages =250–258|doi=10.1097/01.rti.0000142837.52729.38|pmid = 15502612}}</ref>

Because a helium atom, or even [[Helium dimer#Cage|two helium atoms]], can be encased in [[fullerene]]-like cages, the NMR spectroscopy of this element can be a sensitive probe for changes of the carbon framework around it.<ref>{{cite journal |title= <sup>3</sup>He NMR: A Powerful New Tool for Following Fullerene Chemistry |first1=  Martin |last1= Saunders |first2= Hugo A. |last2= Jimenez-Vazquez |first3= Benedict W. |last3= Bangerter |first4= R. James |last4= Cross |first5= Stanley |last5= Mroczkowski |first6= Daron I. |last6= Freedberg |first7= Frank A. L. |last7= Anet |journal= Journal of the American Chemical Society |year= 1994 |volume= 116 |issue= 8 |pages= 3621–3622 |doi= 10.1021/ja00087a067 }}</ref><ref>{{cite web |url= http://chem.ch.huji.ac.il/nmr/techniques/1d/row1/he.html |title= (3He) Helium NMR |author= Institute of Chemistry, The [[Hebrew University of Jerusalem]] }}</ref> Using [[carbon-13 NMR]] to analyze fullerenes themselves is complicated by so many subtle differences among the carbons in anything but the simplest, highly-symmetric structures.

===Radio energy absorber for tokamak plasma experiments===
Both MIT's [[Alcator C-Mod]] tokamak and the [[Joint European Torus]] (JET) have experimented with adding a little helium-3 to a H–D plasma to increase the absorption of radio-frequency (RF) energy to heat the hydrogen and deuterium ions, a "three-ion" effect.<ref>{{Cite web |url=https://www.popularmechanics.com/science/energy/a27961/mit-nuclear-fusion-experiment-increases-efficiency/ |title=''MIT Achieves Breakthrough in Nuclear Fusion'' Aug 2017 |date=28 August 2017 |access-date=2020-07-18 |archive-date=2020-08-01 |archive-url=https://web.archive.org/web/20200801123209/https://www.popularmechanics.com/science/energy/a27961/mit-nuclear-fusion-experiment-increases-efficiency/ |url-status=live }}</ref><ref>{{cite journal|url= https://www.nature.com/articles/nphys4167.epdf?author_access_token=kPYxN3CzZYD2LT4Si32eOdRgN0jAjWel9jnR3ZoTv0MmuIUEPmNcONMVzXjNf2zbVw9w-V0n8MdnZGKP1E4gbnnf8HWpVEg2srMePcKJH7P-Epjuig2d7CKPHhckCLXI|title= Efficient generation of energetic ions in multi-ion plasmas by radio-frequency heating|journal= Nature Physics|date= 19 June 2017|doi= 10.1038/nphys4167|last1= Kazakov|first1= Ye. O.|last2= Ongena|first2= J.|last3= Wright|first3= J. C.|last4= Wukitch|first4= S. J.|last5= Lerche|first5= E.|last6= Mantsinen|first6= M. J.|last7= Van Eester|first7= D.|last8= Craciunescu|first8= T.|last9= Kiptily|first9= V. G.|last10= Lin|first10= Y.|last11= Nocente|first11= M.|last12= Nabais|first12= F.|last13= Nave|first13= M. F. F.|last14= Baranov|first14= Y.|last15= Bielecki|first15= J.|last16= Bilato|first16= R.|last17= Bobkov|first17= V.|last18= Crombé|first18= K.|last19= Czarnecka|first19= A.|last20= Faustin|first20= J. M.|last21= Felton|first21= R.|last22= Fitzgerald|first22= M.|last23= Gallart|first23= D.|last24= Giacomelli|first24= L.|last25= Golfinopoulos|first25= T.|last26= Hubbard|first26= A. E.|last27= Jacquet|first27= Ph.|last28= Johnson|first28= T.|last29= Lennholm|first29= M.|last30= Loarer|first30= T.|volume= 13|issue= 10|pages= 973–978|bibcode= 2017NatPh..13..973K|hdl= 1721.1/114949|s2cid= 106402331|display-authors= 1|hdl-access= free|access-date= 18 July 2020|archive-date= 1 August 2020|archive-url= https://web.archive.org/web/20200801122526/https://www.nature.com/articles/nphys4167.epdf?author_access_token=kPYxN3CzZYD2LT4Si32eOdRgN0jAjWel9jnR3ZoTv0MmuIUEPmNcONMVzXjNf2zbVw9w-V0n8MdnZGKP1E4gbnnf8HWpVEg2srMePcKJH7P-Epjuig2d7CKPHhckCLXI|url-status= live}}</ref>

===Nuclear fuel===
{{See also|Aneutronic fusion|Fusion rocket}}
{| class="wikitable"
|+ Comparison of [[neutronicity]] for different reactions<ref>{{cite web|url=http://members.tm.net/lapointe/IEC_Fusion.html|title=Inertial Electrostatic Confinement Fusion|access-date=2007-05-06|archive-date=2021-01-26|archive-url=https://web.archive.org/web/20210126172744/https://members.tm.net/lapointe/IEC_Fusion.html|url-status=live}}</ref><ref>{{cite web|url=http://www.lancs.ac.uk/ug/suttond1/#fusion|title = Nuclear Fission and Fusion|access-date=2007-05-06|archive-url=https://web.archive.org/web/20070404153838/http://www.lancs.ac.uk/ug/suttond1/#fusion <!-- Bot retrieved archive -->|archive-date=2007-04-04}}</ref><ref>{{cite web|url=http://library.thinkquest.org/28383/nowe_teksty/htmla/2_37a.html|title=The Fusion Reaction|access-date=2007-05-06|archive-date=2013-07-31|archive-url=https://web.archive.org/web/20130731134644/http://library.thinkquest.org/28383/nowe_teksty/htmla/2_37a.html|url-status=dead}}</ref><ref>{{cite web|url=http://fti.neep.wisc.edu/pdf/fdm1291.pdf|title=A Strategy for D – {{SimpleNuclide|Helium|3}} Development|author=John Santarius|date=June 2006|access-date=2007-05-06|archive-date=2007-07-03|archive-url=https://web.archive.org/web/20070703200058/http://fti.neep.wisc.edu/pdf/fdm1291.pdf|url-status=dead}}</ref><ref>{{cite web|url=http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/nucrea.html|title=Nuclear Reactions|access-date=2007-05-06|archive-date=2000-02-01|archive-url=https://web.archive.org/web/20000201210035/http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/nucrea.html|url-status=live}}</ref>
|-
! scope="col" |
! scope="col" | Reactants
! scope="col" | Products
! scope="col" | ''Q''
! scope="col" | n/MeV
|-
! scope="row" rowspan="3" |First-generation fusion fuels
| {{chem2|[[Deuterium|^{2}D]] + ^{2}D}}
| {{chem2|^{3}He}} + {{Physics particle|n|TL=1|BL=0}}
| style="text-align: right;" | 3.268 [[MeV]]
| 0.306
|- 
| {{chem2|^{2}D + ^{2}D}}
| {{chem2|^{3}T|link=Tritium}} + {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" | 4.032 [[MeV]]
| 0
|- 
| {{chem2|^{2}D + ^{3}T}}
| {{chem2|^{4}He}} + {{Physics particle|n|TL=1|BL=0}}
| style="text-align: right;" |17.571 [[MeV]]
| 0.057
|- 
! scope="row" | Second-generation fusion fuel
| {{chem2|^{2}D + ^{3}He}}
| {{chem2|^{4}He}} + {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" |18.354 [[MeV]]
| 0
|- 
! scope="row" | Net result of <sup>2</sup>D burning (sum of first 4 rows)
| {{chem2|6 ^{2}D}}
| class="nowrap" | 2({{chem2|^{4}He}} + n + p)
| class="nowrap" style="text-align: right;" |43.225 [[MeV]]
| 0.046
|- 
! scope="row" rowspan="2" | Third-generation fusion fuels
| class="nowrap" | {{chem2|^{3}He + ^{3}He}}
| {{chem2|^{4}He}} + 2 {{Physics particle|p|TL=1|BL=1}}
| style="text-align: right;" |12.86 [[MeV]]
| 0
|- 
| {{chem2|^{11}B|link=Boron-11}} + {{Physics particle|p|TL=1|BL=1}}
| 3 {{chem2|^{4}He}}
| style="text-align: right;" |8.68 [[MeV]]
| 0
|-
! scope="row" | Current nuclear fuel
| {{chem2|[[Uranium 235|^{235}U]] + n}}
| 2 [[Fission product|FP]]+ 2.5n
| style="text-align: right;" |~200 [[MeV]]
| 0.0075
|}
{{chem2|^{3}He}} can be produced by the low temperature fusion of  {{overset|(D-p)|<sup>2</sup>H + <sup>1</sup>p}}  → {{chem2|^{3}He}} + γ + 4.98 MeV. If the fusion temperature is below that for the helium nuclei to fuse, the reaction produces a high energy alpha particle which quickly acquires an electron producing a stable light helium ion which can be utilized directly as a source of electricity without producing dangerous neutrons.  [[File: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&nbsp;keV, or 800 million kelvins) and at a higher value than other reactions commonly considered for fusion energy.]]

{{chem2|^{3}He}} can be used in fusion reactions by either of the reactions {{chem2|^{2}H + ^{3}He -> ^{4}He + ^{1}p}} + 18.3 [[electronvolt|MeV]], or {{chem2|^{3}He + ^{3}He -> ^{4}He + 2 ^{1}p}} + 12.86 MeV.

The conventional [[deuterium]] + [[tritium]] ("[[deuterium–tritium fusion|D–T]]") fusion process produces energetic neutrons which render reactor components [[radioactive]] with [[activation product]]s. The appeal of helium-3 fusion stems from the [[aneutronic fusion|aneutronic]] nature of its reaction products. Helium-3 itself is non-radioactive. The lone high-energy by-product, the [[proton]], can be contained by means of electric and magnetic fields. The momentum energy of this proton (created in the fusion process) will interact with the containing electromagnetic field, resulting in direct net electricity generation.<ref>{{cite web|url=http://fti.neep.wisc.edu/presentations/jfs_ieee0904.pdf|title=Lunar {{SimpleNuclide|Helium|3}} and Fusion Power|author=John Santarius|date=September 28, 2004|access-date=2007-05-06|archive-date=2007-07-03|archive-url=https://web.archive.org/web/20070703200103/http://fti.neep.wisc.edu/presentations/jfs_ieee0904.pdf|url-status=dead}}</ref>

Because of the higher [[Coulomb barrier]], the temperatures required for {{chem2|^{2}H + ^{3}He}} fusion are much higher than those of conventional [[deuterium–tritium fusion|D–T fusion]]. Moreover, since both reactants need to be mixed together to fuse, reactions between nuclei of the same reactant will occur, and the D–D reaction ({{chem2|^{2}H + ^{2}H}}) does produce a [[neutron]]. Reaction rates vary with temperature, but the D–{{chem2|^{3}He}} reaction rate is never greater than 3.56 times the D–D reaction rate (see graph). Therefore, fusion using D–{{chem2|^{3}He}} fuel at the right temperature and a D-lean fuel mixture, can produce a much lower neutron flux than D–T fusion, but is not clean, negating some of its main attraction.

The second possibility, fusing {{chem2|^{3}He}} with itself ({{chem2|^{3}He + ^{3}He}}), requires even higher temperatures (since now both reactants have a +2 charge), and thus is even more difficult than the D-{{chem2|^{3}He}} reaction. It offers a theoretical reaction that produces no neutrons; the charged protons produced can be contained in electric and magnetic fields, which in turn directly generates electricity. {{chem2|^{3}He + ^{3}He}} fusion is feasible as demonstrated in the laboratory and has immense advantages, but commercial viability is many years in the future.<ref>{{cite journal|url=http://www.technologyreview.com/energy/19296/|title=Mining the Moon: Lab experiments suggest that future fusion reactors could use helium-3 gathered from the moon|author=Mark Williams|journal=MIT Technology Review|date=August 23, 2007|access-date=2011-01-25|archive-date=2010-12-30|archive-url=https://web.archive.org/web/20101230224725/http://www.technologyreview.com/Energy/19296/|url-status=live}}</ref>

The amounts of helium-3 needed as a replacement for [[fossil fuel|conventional fuel]]s are substantial by comparison to amounts currently available. The total amount of energy produced in the {{chem2|^{2}D + ^{3}He}} reaction is 18.4 M[[electronvolt|eV]], which corresponds to some 493 [[watt-hour|megawatt-hour]]s (4.93×10<sup>8</sup> W·h) per three [[gram]]s (one [[mole (chemistry)|mole]]) of {{chem2|^{3}He}}. If the total amount of energy could be converted to electrical power with 100% efficiency (a physical impossibility), it would correspond to about 30 minutes of output of a gigawatt electrical plant per mole of {{chem2|^{3}He}}.  Thus, a year's production (at 6 grams for each operation hour) would require 52.5 kilograms of helium-3. The amount of fuel needed for large-scale applications can also be put in terms of total consumption: electricity consumption by 107 million U.S. households in 2001<ref>Date from the US Energy Information Administration</ref> totaled 1,140 billion kW·h (1.14×10<sup>15</sup> W·h). Again assuming 100% conversion efficiency, 6.7 [[tonne]]s per year of helium-3 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.{{citation needed|date=January 2011}}

A second-generation approach to controlled [[nuclear fusion|fusion]] power involves combining helium-3 and [[deuterium|deuterium, {{chem2|^{2}D}}]]. This reaction produces an [[alpha particle]] and a high-energy [[proton]]. The most important potential advantage of this fusion reaction for power production as well as other applications lies in its compatibility with the use of [[electrostatic]] fields to control fuel [[ion]]s and the fusion protons. High speed protons, as positively charged particles, can have their kinetic energy converted directly into [[electricity]], through use of [[Solid-state chemistry|solid-state]] conversion materials as well as other techniques. Potential conversion efficiencies of 70% may be possible, as there is no need to convert proton energy to heat in order to drive a [[turbine]]-powered [[Electric generator|electrical generator]].{{Citation needed|date=April 2012}}

====He-3 power plants====
There have been many claims about the capabilities of helium-3 power plants. According to proponents, fusion power plants operating on [[deuterium]] and helium-3 would offer lower capital and [[operating cost]]s than their competitors due to less technical complexity, higher conversion efficiency, smaller size, the absence of radioactive fuel, no air or water [[pollution]], and only low-level [[radioactive]] waste disposal requirements. Recent estimates suggest that about $6 billion in [[Investment (macroeconomics)|investment]] [[Capital (economics)|capital]] will be required to develop and construct the first helium-3 fusion [[power plant]]. Financial break even at today's wholesale [[electricity]] prices (5 US cents per [[kilowatt-hour]]) would occur after five 1-[[gigawatt]] plants were on line, replacing old conventional plants or meeting new demand.<ref>{{cite news|url=http://www.popularmechanics.com/science/air_space/1283056.html?page=4|title=Mining The Moon|author=Paul DiMare|date=October 2004|work=Popular Mechanics|access-date=2007-05-06|archive-url=https://web.archive.org/web/20070814162104/http://www.popularmechanics.com/science/air_space/1283056.html?page=4|archive-date=2007-08-14|url-status=dead}}</ref>

The reality is not so clear-cut. The most advanced fusion programs in the world are [[inertial confinement fusion]] (such as [[National Ignition Facility]]) and [[magnetic confinement fusion]] (such as [[ITER]] and [[Wendelstein 7-X]]). In the case of the former, there is no solid roadmap to power generation. In the case of the latter, commercial power generation is not expected until around 2050.<ref>{{cite news|url=http://www.iter.org/proj/Pages/ITERAndBeyond.aspx |title=ITER & Beyond |access-date=2009-08-04 |url-status=dead |archive-url=https://web.archive.org/web/20090520151601/http://www.iter.org/PROJ/Pages/ITERAndBeyond.aspx |archive-date=2009-05-20 }}</ref> In both cases, the type of fusion discussed is the simplest: D–T fusion. The reason for this is the very low [[Coulomb barrier]] for this reaction; for D+<sup>3</sup>He, the barrier is much higher, and it is even higher for <sup>3</sup>He–<sup>3</sup>He. The immense cost of reactors like [[ITER]] and [[National Ignition Facility]] are largely due to their immense size, yet to scale up to higher plasma temperatures would require reactors far larger still. The 14.7 MeV proton and 3.6 MeV alpha particle from D–<sup>3</sup>He fusion, plus the higher conversion efficiency, means that more electricity is obtained per kilogram than with [[deuterium–tritium fusion|D–T fusion]] (17.6 MeV), but not that much more. As a further downside, the rates of reaction for [[Aneutronic fusion#Candidate reactions|helium-3 fusion reactions]] are not particularly high, requiring a reactor that is larger still or more reactors to produce the same amount of electricity.

In 2022, [[Helion Energy]] claimed 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".<ref>{{cite web |url=https://www.helionenergy.com/faq/ |title=Helion FAQ |accessdate=29 September 2022}}</ref>

====Alternatives to He-3====
To attempt to work around this problem of massively large power plants that may not even be economical with D–T fusion, let alone the far more challenging D–<sup>3</sup>He fusion, a number of other reactors have been proposed&nbsp;– the [[Fusor]], [[Polywell]], [[Focus fusion]], and many more, though many of these concepts have fundamental problems with achieving a net energy gain, and generally attempt to achieve fusion in thermal disequilibrium, something that could potentially prove impossible,<ref>{{cite news|title=A general critique of inertial-electrostatic confinement fusion systems|author= Todd Rider|hdl = 1721.1/29869}}</ref> and consequently, these long-shot programs tend to have trouble garnering funding despite their low budgets. Unlike the "big" and "hot" fusion systems, if such systems worked, they could scale to the higher barrier [[aneutronic fusion|aneutronic]] fuels, and so their proponents tend to promote [[Aneutronic fusion#Boron|p-B fusion]], which requires no exotic fuel such as helium-3.