Fusion ignition is the point at which a nuclear fusion reaction becomes self-sustaining. This occurs when the energy being given off by the reaction heats the fuel mass more rapidly than it cools. In other words, fusion ignition is the point at which the increasing self-heating of the nuclear fusion removes the need for external heating.[1] This is quantified by the Lawson criterion.[2] Ignition can also be defined by the fusion energy gain factor.[3]

In the laboratory, fusion ignition defined by the Lawson criterion was first achieved in August 2021,[4] and ignition defined by the energy gain factor was achieved in December 2022,[5][6] both by the U.S. National Ignition Facility.

Research

Schematic of the stages of inertial confinement fusion using lasers. The blue arrows represent radiation; orange is blowoff; yellow is inwardly transported thermal energy.
  1. Laser beams or laser-produced X-rays rapidly heat the surface of the fusion target, forming a surrounding plasma envelope.
  2. Fuel is compressed by the rocket-like blowoff of the hot surface material.
  3. During the final part of the capsule implosion, the fuel core reaches 20 times the density of lead and ignites at 100,000,000 ˚C.
  4. Thermonuclear burn spreads rapidly through the compressed fuel, yielding many times the input energy.

Ignition should not be confused with breakeven, a similar concept that compares the total energy being given off to the energy being used to heat the fuel. The key difference is that breakeven ignores losses to the surroundings, which do not contribute to heating the fuel, and thus are not able to make the reaction self-sustaining. Breakeven is an important goal in the fusion energy field, but ignition is required for a practical energy producing design.[7]

In nature, stars reach ignition at temperatures similar to that of the Sun, around 15 million kelvins (27 million degrees F). Stars are so large that the fusion products will almost always interact with the plasma before their energy can be lost to the environment at the outside of the star. In comparison, man-made reactors are far less dense and much smaller, allowing the fusion products to easily escape the fuel. To offset this, much higher rates of fusion are required, and thus much higher temperatures; most man-made fusion reactors are designed to work at temperatures over 100 million kelvins (180 million degrees F).[8]

Fusion ignition was first achieved by humans in the cores of detonating thermonuclear weapons. A thermonuclear weapon uses a conventional fission (U-235 or Pu-239/241) "sparkplug" to generate high pressures and compress a rod of fusion fuel (usually lithium deuteride). The fuel reaches high enough pressures and densities to ignite, releasing large amounts of energy and neutrons in the process.[9]

The National Ignition Facility at Lawrence Livermore National Laboratory performs laser-driven inertial confinement fusion experiments that achieve fusion ignition. This is similar to a thermonuclear weapon, but the National Ignition Facility uses a 1.8 MJ laser system instead of a fission weapon to compress the fuel, and uses a much smaller amount of fuel (a mixture of deuterium and tritium, which are both isotopes of hydrogen).[10] In January 2012, National Ignition Facility Director Mike Dunne predicted in a Photonics West 2012 plenary talk that ignition would be achieved at NIF by October 2012.[11] By 2022 the NIF had achieved ignition.

Based on the tokamak reactor design, the ITER is intended to sustain fusion mostly by internal fusion heating and yield in its plasma a ten-fold return on power.[12] Construction is expected to be completed in 2025.

Experts believe that achieving fusion ignition is the first step towards electricity generation using fusion power.[13]

2021 and 2022 ignition reports

The National Ignition Facility at the Lawrence Livermore National Laboratory in California reported in 2021[14] that it had triggered ignition in the laboratory on 8 August 2021, for the first time in the over-60-year history of the ICF program.[15][16] The shot yielded 1.3 megajoules of fusion energy, an 8-fold improvement on tests done in spring 2021.[14] NIF estimates that the laser supplied 1.9 megajoules of energy, 230 kilojoules of which reached the fuel capsule. This corresponds to a total scientific energy gain of 0.7 and a capsule energy gain of 6.[14] While the experiment fell short of ignition as defined by the National Academy of Sciences – a total energy gain greater than one – most people working in the field viewed the experiment as the demonstration of ignition as defined by the Lawson criterion.[14]

In August 2022, the results of the experiment were confirmed in three peer-reviewed papers: one in Physical Review Letters and two in Physical Review E.[17] Throughout 2022, the NIF researchers tried and failed to replicate the August result.[18] However, on 13 December 2022, the United States Department of Energy announced via Twitter that an experiment on December 5 had surpassed the August result, achieving a scientific gain of 1.5,[19][20] surpassing the National Academy of Sciences definition of ignition.[3]

See also

References

  1. ↑ Chandler, David L. (10 May 2010). "New project aims for fusion ignition". MIT News. MIT. Retrieved 24 February 2012.
  2. ↑ Lawson, J. D. (December 1955). "Some Criteria for a Power Producing Thermonuclear Reactor". Proceedings of the Physical Society, Section B. 70 (1): 6–10. Bibcode:1957PPSB...70....6L. doi:10.1088/0370-1301/70/1/303.
  3. 1 2 Bishop, Breanna (6 February 2023). "Ignition gives U.S. 'unique opportunity' to lead world's IFE research". Lawrence Livermore National Laboratory. Retrieved 26 July 2023. This feat established a scientific energy gain of 1.5, over the gain of 1 used by the National Academy of Sciences to define ignition
  4. ↑ Abu-Shawareb, H.; Acree, R.; Adams, P. (8 August 2022). "Lawson Criterion for Ignition Exceeded in an Inertial Fusion Experiment". Phys. Rev. Lett. 129: 075001. doi:10.1103/PhysRevLett.129.075001. hdl:10044/1/99300. Retrieved 26 July 2023.
  5. ↑ Clery, Daniel (13 December 2022). "With historic explosion, a long sought fusion breakthrough". Science. doi:10.1126/science.adg2803. Retrieved 13 December 2022.
  6. ↑ David Kramer (13 December 2022), "National Ignition Facility surpasses long-awaited fusion milestone", Physics Today, American Institute of Physics, 2022 (2): 1213a, doi:10.1063/PT.6.2.20221213a, S2CID 254663644, The shot at Lawrence Livermore National Laboratory on 5 December is the first-ever controlled fusion reaction to produce an energy gain.
  7. ↑ "The National Ignition Facility: Ushering in a New Age for Science". Lawrence Livermore National Laboratory. Archived from the original on 2 May 2012. Retrieved 26 February 2012.
  8. ↑ Narang, Gaurvi (11 September 2022). "Nuclear fusion reactor reaches 100 Million C for 30 secs. Here's what it means for the future". ThePrint. Retrieved 28 December 2022.
  9. ↑ Hansen, Chuck (1988). U.S. Nuclear Weapons: The Secret History. Crown. ISBN 978-0517567401. LCCN 87021995. OCLC 865554459. OL 2392513M. Retrieved 10 November 2021 – via Internet Archive.
  10. ↑ National Research Council (U.S.). Plasma Committee (20 December 2007). Plasma Science: Advancing Knowledge in the National Interest. The National Academic Press. p. 24. ISBN 978-0-309-16436-8.
  11. ↑ Hatcher, Mike (26 January 2012). "PW 2012: fusion laser on track for 2012 burn". Optics.org. San Francisco. Retrieved 11 January 2019.
  12. ↑ "What is ITER?".
  13. ↑ National Research Council (U.S.). Plasma Committee (20 December 2007). Plasma Science: Advancing Knowledge in the National Interest. The National Academic Press. ISBN 978-0-309-16436-8.
  14. 1 2 3 4 Wright, Katherine (30 November 2021). "Ignition First in a Fusion Reaction". Physics. 14: 168. Bibcode:2021PhyOJ..14..168W. doi:10.1103/Physics.14.168. S2CID 244829710.
  15. ↑ Dunning, Hayley (17 August 2021). "Major nuclear fusion milestone reached as 'ignition' triggered in a lab". Phys.org.
  16. ↑ Bishop, Breanna (18 August 2021). "National Ignition Facility experiment puts researchers at threshold of fusion ignition". Lawrence Livermore National Laboratory.
  17. ↑ Padilla, Michael (8 August 2022). "Three peer-reviewed papers highlight scientific results of National Ignition Facility record yield shot". Lawrence Livermore National Laboratory.
  18. ↑ Kramer, David (3 December 2021). "Lawrence Livermore's latest attempts at ignition fall short". Physics Today. 2021 (2): 1203a. doi:10.1063/PT.6.2.20211203a. S2CID 244935714.
  19. ↑ Davis, Nicola (12 December 2022). "Breakthrough in nuclear fusion could mean 'near-limitless energy'". The Guardian. Retrieved 13 December 2022.
  20. ↑ @energy (13 December 2022). "BREAKING NEWS: This is an announcement that has been decades in the making" (Tweet). Retrieved 14 December 2022 – via Twitter.
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