Almost a year ago, researchers at the National Ignition Facility reached a 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 energized. Self-sustaining. For a year, they tested the experimental conditions that allowed them to achieve this result. Their analyzes show that the NIF indeed meets Lawson’s famous criterion.

The National Ignition Facility (NIF) uses 192 high-power laser beams (delivering up to 1.9 megajoules) to heat and compress a small capsule containing a mixture of deuterium and tritium, thereby inducing a nuclear fusion reaction. This approach is known as inertial confinement fusion (to distinguish it from the magnetic confinement fusion implemented in tokamaks). The experiment was released on August 8, 2021 A record energy of 1.3 megajoules — 10 quadrillion watts, equivalent to 100 trillion watts of electricity per second — puts researchers at the limit of fusion ignition.

« This record was a major scientific breakthrough in fusion research, establishing that laboratory fusion ignition was possible in NIF. », Omar Hurricane said, chief scientist of the Inertial Confusion Fusion Program at Lawrence Livermore National Laboratory (LLNL). But the team could not repeat this feat. A year after this historic achievement, the results of the experiment are detailed in three scientific articles published in the journal. Physical Review Letters etc Physical review e.

Lawson’s criteria were met for the first time

The articles describe the design, improvements made to the installation, and subsequent experimental measurements obtained. In early 2021, the NIF team showed that the facility can produce hot plasma – hot, ionized gas in which fusion reactions are the main source of fuel heating (no longer laser pulses). But the laser still had to provide energy to sustain the fusion reaction.

However, in order to use nuclear fusion as a power source for a day, the reaction must be self-sustaining. The challenge is keeping the plasma at temperatures above 100 million degrees. The rate of energy production through fusion exceeds the rate of energy loss to the environment (through heat conduction and radiation).

To reach this state, the reaction must produce more energy locally than is lost: the excess energy can be used to heat other parts of the fuel until another fusion reaction starts, and so on. This limit to be reached was first described by physicist John Lawson in 1966. The “Lawson criterion” states that combustion occurs when the product of plasma concentration and residence time exceeds a certain threshold.

In an effort to meet this standard, the researchers made several improvements to their experiment, including physical design and target quality. They have especially
The capsule’s hydrogen-filling tube was shortened, as well as the size of the entrance holes for Holram’s laser, which contained the capsule – requiring less energy to heat the initial stage – and the capsule’s geometry was improved. 192 stability of laser beams.

Especially sensitive experimental diet

Analysis of experimental data demonstrates that the Lawson criterion (and hence the ignition threshold) is reached on August 8, 2021, which bodes well for research and development of nuclear fusion. But despite many attempts, the team has not yet been able to achieve the same performance. Anne Critcher, LLNL physicist and first author Article describing the design of the experimentIt is emphasized that many variables come into play and each of them can influence the outcome of the experiment.

« 192 Laser beams do not perform identically from shot to shot, the quality of targets varies, and the ice layer develops with different roughness on each target. », She explains. Small variations in the material composition of the fuel capsules or the intensity of the lasers are sufficient to influence the final energy yield.

The team notes that experiments conducted last year have yielded yields of the order of 430 to 700 kJ. The data collected will allow the project team to better understand the fundamental processes of combustion and the variability inherent in this particularly sensitive new test regime. ” We are working in a regime where no researcher has had access since the end of nuclear testing, and this is an incredible opportunity to expand our knowledge as we continue to make progress. Omar makes the hurricane exciting.

They will be able to reproduce the record yield obtained last year and above all to further improve their installation, making the experiment more resistant to small errors so that it can be reproduced more easily. The ultimate goal would be to produce at least as much energy as a laser (the yield of the August 8, 2021 test was about 72%). This is the situation Sine qua non So nuclear fusion power plants will become a reality one day.

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