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Exactly one year ago to the day, researchers at the National Ignition Facility achieved an historic milestone in the field of nuclear fusion: They reached the ignition threshold (or “ignition threshold”), or the point at which the fusion reaction is energetic enough to be self-sustaining. For a year they studied the experimental conditions that allowed them to achieve this result. Their analyzes show that the NIF did indeed meet Lawson’s famous criterion.
The National Ignition Facility (NIF) uses 192 high-power laser beams (up to 1.9 megajoules) to heat and compress a small capsule containing a mixture of deuterium and tritium, triggering nuclear fusion reactions. This approach is known as inertial confinement fusion (to be distinguished from magnetic confinement fusion implemented in tokamaks). On August 8, 2021, the experiment unleashed a record energy of 1.3 megajoules — equivalent to 10 quadrillion watts of power for 100 trillionths of a second — bringing researchers to the cusp of fusion ignition.
” The record was a major scientific breakthrough in fusion research, proving that laboratory fusion ignition at the NIF is possible said Omar Hurricane, chief scientist for the inertial confinement fusion program at Lawrence Livermore National Laboratory (LLNL). However, the team never managed to repeat this feat. A year after this historical achievement, the results of the experiment are published in detail in three scientific articles in the journal Physical Verification Letters and Physical Check E.
Lawson’s criterion met for the first time
The articles describe the design, the improvements made to the installation, and then the experimental measurements obtained. It all started in early 2021, when the NIF team showed that the facility can produce hot plasma – a hot, ionized gas in which the fusion reactions themselves are the main source of heat for the fuel (rather than the laser pulses). But the laser still had to supply energy to sustain the fusion reaction.
However, for nuclear fusion to one day be used as an energy source, the reaction must be self-sustaining. The challenge, therefore, is to keep the plasma at temperatures above 100 million degrees long enough for the rate of energy production by fusion to exceed the rate of energy loss to the environment (by conduction and by radiation).
To reach this state, the reaction must locally produce more energy than it loses: the excess energy can then be used to heat other parts of the fuel until it triggers another fusion reaction, and so now. This threshold to be reached was first described in 1966 by the physicist John Lawson. Lawson’s criterion states that ignition occurs when the product of the plasma density and the confinement time is above a certain threshold.
To meet this criterion, the researchers made several improvements to their experiment, including physical design and target quality. You have in particular
The tube that fills the capsule with hydrogen and reduced the size of the laser entry holes of the cavity containing the capsule – which required less energy to heat up in the initial phase – improved the geometry of the capsule and improved the stability of the 192 laser beams .
A particularly sensitive experimental diet
Analysis of the experimental data proves that the Lawson criterion (and thus the ignition threshold) was indeed reached on August 8, 2021, boding well for nuclear fusion research and development. But despite many attempts, the team has not yet managed to achieve the same performance. Annie Kritcher, LLNL physicist and first author of the article describing the experiment’s design, points out that many variables come into play and that each of them can affect the outcome of the experiment.
” The 192 laser beams do not behave exactly the same from shot to shot, target quality varies and the ice layer develops with a different roughness on each target “, she explains. Tiny variations in the material structure of the fuel capsules or in the intensity of the lasers can be enough to affect the final energy output.
Experiments conducted over the past year have nevertheless provided yields on the order of 430 to 700 kJ, the team notes. The data collected will allow the project team to better understand the fundamental processes of ignition and the variability inherent in this particularly sensitive new experimental regime. ” We work in a regime that no researcher has had access to since nuclear testing ended, and this is an incredible opportunity to expand our knowledge as we continue to advance. inspires Omar Hurricane.
They can then continue to improve their setup to reproduce last year’s record yield and most importantly, make the experiment more resilient to small errors so it’s easier to reproduce. The ultimate goal will then be to produce at least as much energy as the lasers deliver (the yield of the August 8, 2021 experiment was about 72%). This is the condition indispensable requirement so that nuclear fusion power plants will one day become a reality.