Studies confirm that NIF has already reached the ignition threshold

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About a year ago today, researchers at the National Ignition Facility have reached a historic nuclear fusion milestone: They’ve reached the ignition threshold (or “ignition” threshold), or the point at which a fusion reaction is active enough to be self-sustaining. For a year, they examined the experimental conditions that allowed them to obtain this result. Their analyzes show that the NIF has already met Lawson’s famous criterion.

The National Ignition Facility (NIF) uses 192 high-powered laser beams (up to 1.9 megajoules) to heat and compress a small capsule containing a mixture of deuterium and tritium and thus stimulate nuclear fusion reactions. This approach is known as inertial confinement fusion (to be distinguished from magnetic confinement fusion, which is performed in a tokamak). On August 8, 2021, the experiment released 1.3 megajoules of energy — the equivalent of 10 quadrillion watts of energy for 100 trillion milliseconds — putting the researchers on the threshold of fusion ignition.

The record was a major scientific achievement in fusion research, which proves that in vitro fusion ignition is possible in NIF. said Omar Hurrican, chief scientist for the self-confinement fusion program at Lawrence Livermore National Laboratory (LLNL). However, the team was unable to replicate this feat. A year after this landmark achievement, the results of the experiment are detailed in three scientific articles published in the journal physical review messages And the physical review.

Lawson criterion verified for the first time

The articles describe the design, improvements made to the installation, and then experimental measurements obtained. It all started in early 2021, when the NIF team demonstrated that the facility could produce hot plasma – a hot ionized gas in which fusion reactions are the main source of heating the fuel (not laser pulses). But the laser still has to supply energy to keep the fusion reaction going.

Technical imaging of the NIF laser beam entering the chamber containing the target. © TIN

However, in order to someday use nuclear fusion as an energy source, the reaction must be self-sustaining. The challenge, therefore, is to keep the plasma at temperatures of more than 100 million degrees for long enough that the rate of energy production by fusion exceeds the rate of energy loss to the environment (by thermal conductivity and radiation).

Pour atteindre cette condition, la reaction doit produire localement plus d’énergie qu’elle n’en perd : l’énergie excédentaire peut alors servir à chauffer d’autres parties du combustible jusqu’à y initier une autre reaction ainsi fusion, et Consecutive. This threshold was first described in 1966 by physicist John Lawson. The Lawson criterion states that ignition occurs when the product of the plasma density and the time of confinement is above a certain threshold.

(a) Sectional view of the target, which consists of a gold-lined depleted uranium holorum (about 1 cm high), enclosing the capsule with a diameter of 2 mm. The laser beams enter the target through the upper and lower apertures. (b) Total laser power (in blue) as a function of time and radiation temperature for the hohlraum simulation of the August 8, 2021 experiment. Imaging data (c) are used to reconstruct the hotspot volume, which is necessary to infer the pressure and other properties of the plasma. © LLNL

In an effort to meet this standard, the researchers made several improvements to their experiment, including physical design and target quality. They have in particular
Reducing the tube filling the capsule with hydrogen, as well as the size of the capsule’s laser inlet holes containing the capsule – which require less energy to heat up in the initial stage – improved the geometry of the capsule and improved stability of 192 laser beams.

A particularly sensitive experimental diet

Analysis of the experimental data proves that the Lawson criterion (and thus the ignition threshold) was already reached on August 8, 2021, which bodes well for research and development in the field of nuclear fusion. But despite many attempts, the team has not yet succeeded in achieving the same performance. Annie Kretcher, a physicist at LLNL and first author of the paper describing experiment design, points out that many variables play a role and that each can affect the outcome of an experiment.

The 192 lasers do not behave exactly the same from one shot to the next, the quality of the target varies, and the layer of ice develops with a different roughness on each target ‘, she explained. Slight differences in the material structure of the fuel capsules or in the intensity of the laser could be enough to affect the final energy yield.

The team notes that experiments conducted over the past year have provided yields of 430 to 700 kilojoules. The data collected will allow the project team to understand the basic processes of ignition and the versatility inherent in this particularly sensitive new experimental system. ” We work under a system that no researcher has had access to since the nuclear tests ended, and this is a great opportunity to expand our knowledge as we continue to make progress. Hurricane Omar excited.

They will then be able to improve their installation to reproduce the record yield obtained last year and above all, to make the experiment more resistant to small errors so that it is more easily repeatable. The ultimate goal then would be to produce at least as much power as the laser (the August 8, 2021 trial yield was about 72%). This is the condition indispensable condition Until nuclear fusion power plants become a reality one day.

Source: H. Abu-Shawareb et al., Physical Review Letters

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