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| Fig. 1: A deuterium-tritium fusion reaction. (Source: Wikimedia Commons) |
In December 2022, scientists at the Lawrence Livermore National Laboratory (LLNL) achieved fusion ignition, which has been widely considered as a breakthrough in nuclear fusion science. [1] As depicted in Fig. 1, nuclear fusion is the process in which two light atomic nuclei (typically deuterium and tritium) merge to form a single heavier nucleus, which in turn releases a very large amount of energy. [2] Ignition is the phenomenon in which the output energy of this process exceeds its input energy, which LLNL was able to achieve by firing 192 laser beams at a small capsule filled with hydrogen isotopes. [1] When the laser beam was fired at the capsule, it caused the isotopes to fuse.
Why has ignition only been achieved recently? Creating a fusion reaction in the first place has remained very challenging throughout the decades because it has required scientists to address a slew of difficult problems, including maintaining plasma stability, advancing magnetic confinement systems, and developing adequate plasma injection and pumping techniques. [3] Furthermore, nuclear fusion experiments require conditions similar to those in the core of stars, including extremely high temperatures (millions of degrees Celsius) and pressures. In 1920, Arthur Eddington showed that the fusion of light nuclei would release energy, and work began in the late 1940s to begin searching for ways to pursue nuclear fusion as an energy source - the hydrogen bombs served as proof of the massive amounts of energy that it could release. [4]
As the decades went on, scientists began to gravitate towards stellarators and tokamaks as the answer to achieving sustainable fusion energy, which are large devices that use powerful magnetic fields to confine plasma in the shape of a torus. [4] However, even in present day, these devices are far away from achieving ignition. Today's leading tokamak device, the International Thermonuclear Experimental Reactor (ITER), isnt projected to achieve ignition until the late 2030s at the earliest. [5] LLNLs ignition experiment was never meant to be an answer for sustainable fusion energy - rather, its main purpose was to show that ignition could be achieved in the first place, even if only for a fraction of a second.
A study conducted in 2013 focuses on previous ignition experiments conducted at the NIF, using the same method of cryogenic deuterium-tritium layers. Using neutron yield detectors, the researchers were able to uncover data including neutron yield, ion temperature, x-ray image shape, and neutron image shape and size. [7] The study attempted to understand the discrepancy between the measured capsule yield and the yield predicted by 2D radiation-hydrodynamics simulations, which researchers achieved by conducting detailed post-simulation shots using HYDRA. [7] However, despite these efforts, there remained a substantial yield discrepancy between the experiment and simulation, which contributes to the difficulty of reproducing results on a consistent basis. [7]
In the experiment, the 192 lasers fired consumed a total of 322 MJ of electrical energy. This resulted in 2.05 MJ of energy entering the capsule, a pea-sized cylinder containing a frozen pellet of deuterium and tritium, and a total output energy of 3.15 MJ. [5,6] While ignition was still achieved, the overall process is still quite inefficient, as only 0.9% of the total energy consumed was actually converted into output energy. In acknowledgement of this problem, the researchers note that the NIF is not a fusion-energy device, and was not necessarily designed to be efficient. [6]
The December 2022 experiment reflects just one of many experiments that the NIF has conducted using the same techniques, variables, and constraints. While the total neutron yield of the December 2022 experiment is not publicly available, a similar experiment that was conducted by the NIF reflected in a paper published in June 2018 reveals a total neutron yield of 1.9 × 1016, which corresponds to a fusion energy output of 54 kJ. [8] The article also reveals data of previous shot attempts, including their neutron yield, fusion yield, and other data points, as shown in Table 1.
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| Table 1: Previous shot attempts in the June 2018 paper. [8] |
Similarly to the methods the NIF researchers employed, we can assess the yield of the December 2022 experiment by counting the neutrons and then multiplying by 17.6 MeV per neutron:
| 3.15 × 106 J 17.6 × 106 eV neutron-1 × 1.602 × 10-19 J eV-1 |
= | 1.12 × 1018 neutrons |
There is much work to be done before humanity will be able to achieve sustainable fusion energy on a global scale - it will take many years before break-even will be achieved on a tokamak (or any other device invented in the future for the purpose of harboring electric power from fusion energy). Furthermore, the results from the NIF experiments have yet to be recreated with perfect consistency. Regardless, the ignition experiments show that humanity is making progress towards implementing fusion energy as a power source.
© Apollo Lee. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Tollefson, "US Nuclear-Fusion Lab Enters New Era: Achieving 'Ignition' Over and Over," Nature 625, 11 (2023).
[2] R. Kembleton, "Nuclear Fusion: What of the Future?" in Managing Global Warming, ed. by T. M. Letcher (Academic Press, 2018).
[3] D. Clery, "Explosion Marks Laser Fusion Breakthrough," Science 378, 1154 (2022).
[4] D. J. Rose, "On the Feasibility of Power By Nuclear Fusion," Oak Ridge National Laboratory, ORNL-TM-2204, May 1968.
[5] R. J. Bickerton, "History of the Approach to Ignition," Phil.Trans. R. Soc. A 357, 397 (1999).
[6] J. Tollefson and E. Gibney, "Nuclear-Fusion Lab Achieves 'Ignition': What Does It Mean?" Nature 612, 597 (2022).
[7] D. S. Clark et al., "Detailed Implosion Modeling of Deuterium-Tritium Layered Experiments on the National Ignition Facility," Phys. Plasmas 20, 056318 (2013).
[8] S. Le Pape et al., "Fusion Energy Output Greater than the Kinetic Energy of an Imploding Shell at the National Ignition Facility," Phys. Rev. Lett. 120, 245003 (2018).
