Science

For the first time, a nuclear fusion reaction is approaching the ignition threshold.

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While the climate challenge puts pressure on research and development for cleaner energy, recent advances in the field of nuclear fusion are very encouraging. In this effort, researchers at the National Ignition Facility (NIF) claim to have recently obtained a first “cost-effective” energy fusion reaction: the energy produced by plasma fusion would have been greater than that absorbed by the fuel, even close to it. ignition threshold (the moment when a nuclear reaction begins to become self-sustaining). A world first, a key milestone towards the design of the first viable fusion reactors.

A small reminder is in order: unlike nuclear fission, a physical phenomenon exploited by current power plants, nuclear fusion consists of causing the assembly of atoms, as occurs in the heart of stars like the Sun, to generate a large size . amount of energy. Regarding fission, the energy obtained can be calculated with Einstein’s formula E = mc².

Compared to fission reactors, it is intended that the performance of a viable fusion reactor is much higher, and above all: the exploited phenomenon is much more true, because it cannot “get carried away” in a chain reaction such as . Produce with nuclear fission. Not to mention that the resulting nuclear waste is not radioactive and does not pose major problems in terms of disposal, as is the case with used nuclear fuel, which is still radioactive.

First fusion reaction with positive yield

“This result is a historic advance for inertial confinement fusion research. […] It is also a testament to the innovation, ingenuity, commitment and courage of this team and of the many researchers in this field who, for decades, have tirelessly pursued this goal, ”stated Kim Budil, Director of Lawrence Livermore National Laboratory ( United States).

At the 63rd annual meeting of the APS Plasma Physics Division, a team of researchers led by Lawrence Livermore National Laboratory presented impressive results achieved at the National Ignition Facility (NIF): a fusion reaction they obtained recently would have reached 1.3 MJ. , exceeding the energy absorbed by the fuel used to trigger it and thus marks the first time that such a reaction has shown positive performance.

“One of the scientific milestones of ignition pathway fusion research is the creation of ‘hot plasma.’ We speak of burnt plasma when the energy deposited by the alpha particles produced by fusion is the dominant source of plasma heating, it is a necessary step to achieve ignition “, we can read in the press release of the presentation.

The researchers say they have achieved several key milestones regarding ignition: “First, fuel gain, where the efficiency of neutrons exceeds the energy of deuterium-tritium fuel. This is followed by ‘alpha heating’, where the efficiency of the neutrons is doubled due to the additional energy deposited in the fuel by quenching the alpha particles. We have now reached the state of burning plasma. We will review new designs and experiments and compare the results with the criteria and parameters for burning plasma, “they write in their statement. The article reporting on these latest developments is already available on the arXiv pre-publication server and is pending publication in the journal Nature.

The biggest achievement, however, is the fact that it would have approached the ignition threshold. An element of the announcement somewhat controversial at the moment, but accepted by many experts who defend the result: “As regards most of the people who work in the field, the scientific demonstration of the ignition process has been achieved”, Jeremy Chittenden says. from Imperial College London.

Recreate a “tiny sun”

Nuclear fusion consists of recreating a small star. It all starts with a fuel-filled capsule made up of deuterium and tritium, the heavier isotopes of hydrogen. This fuel capsule is placed in a hollow gold chamber, the size of a pencil eraser, called a hohlraum.

Illustration of lasers radiating into a golden hohlraum containing an aluminum fuel capsule. © Jacob Long

Once the fuel is in place, no fewer than 192 high-powered laser beams are projected onto the hohlraum, where they are converted into X-rays. These X-rays implode the fuel capsule, heating and compressing it under conditions comparable to those in the center of a star, at temperatures of several million degrees Celsius and pressures billions of times higher than those of the Earth’s atmosphere, transforming the fuel capsule into a tiny ball of plasma.

And, just as hydrogen fuses into heavier elements in the core of a main sequence star, so do deuterium and tritium in the fuel capsule. The entire process takes just a few billionths of a second. The goal is to reach the ignition threshold, that is, the moment when a nuclear reaction begins to be self-sufficient. After this stage, no energy input is necessary.

In an experiment conducted on August 8, 2021, the researchers of this new study claim to have achieved a record fusion at the NIF, with momentary fusion powers exceeding 1PW, and for the first time they reached the plasma burning regime, where the alpha heating The amount of fusion of the fuel exceeds the energy supplied to the fuel by compression. Therefore, the ignition threshold would have been very close (and nothing more), but according to some experts, this result is enough to prove it and demonstrate that it is a realistic goal in the short term.

experience 2021 nif fusion nucleaire

a) The two experimental configurations of hohlraum used in the experiments. The lasers are grouped into four cones at the corners. Half of the rays enter the hohlraum through an entry hole at the top of the target and half from the bottom. b) Cuneiform diagram of the capsule, showing the characteristics as a function of the radius. c) Two representative laser beam pulse shapes (solid lines) are shown for the H&E and I-Raum experiments and compared to the measured radiation temperatures (dashed lines). © James Ross et al.

This feat would be the result of long-term work to improve the very design of the hohlraum and the fuel capsule. Improvements in laser precision, new diagnostic tools, and general architecture changes to increase the capsule’s implosion rate are also part of this.

“Reaching the ignition threshold in the laboratory remains one of the great scientific challenges of our time and this result is a considerable step towards achieving this goal,” said physicist Johan Frenje of Plasma Science and Fusion in August. MIT Center.

arXiv

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