An announcement that raises a lot of hope, but also questions. The Lawrence Livermore National Laboratory (LLNL), in California, announced on Tuesday, December 13, that it had taken a symbolic step in the field of nuclear fusion. This physical phenomenon is at work both in the heart of the stars that it makes shine and in thermonuclear bombs. Its mastery on Earth is presented as a source of energy that can substitute for all others, abundant, safe and practically non-emitting CO2 Operating.
Also listen Nuclear fusion, when is it?
This is no doubt the reason why the announcement was made in person by the United States National Secretary of Energy, Jennifer M. Granholm, who welcomed the fact that the National Ignition Facility ( NIF) succeeded in producing, on Monday, December 5, more energy than that which had been brought to the system. And also 2.2 times more energy than a previous milestone reached in August 2021. « This is a historic achievement for the NIF researchers and staff who have dedicated their careers to making this fusion ignition real, and this milestone will undoubtedly spark even more discoveries,” said the Secretary of State.
The NIF is a facility that pursues both civilian objectives – studying molten matter – and military objectives – validating models intended to maintain nuclear weapons without resorting to underground tests. While current power plants operate on the principle of nuclear fission, that is to say the breaking of large nuclei into smaller ones, releasing energy, this laboratory seeks to force the marriage between nuclei light, like deuterium and tritium, cousins of hydrogen. For this, it is necessary in very short times and in a tiny space to heat them to temperatures higher than those of the Sun (several million degrees).
If these conditions are met, two elements merge, then release energy as well as other particles which will maintain the boiler and generate other fusions. The initial “match” is struck by pulses of 192 laser beams lasting only a few billionths of a second. These beams converge inside a cylindrical gold cavity one centimeter high, which thus bombarded emits X-rays within it, which are then used to directly heat a small ball 2 millimeters in diameter containing the deuterium mixture and tritium. The capsule implodes and fusion reactions take place.
“It would take ten shots per second to have an electricity production system”
In August 2021, after several unsuccessful attempts and fears for the very future of the program, a threshold was crossed with a record of energy produced, 1.37 megajoules, while the laser beams had provided 1.92 megajoules. This time, for 2.05 megajoules injected, more than 3.15 megajoules were produced, again in an extremely short time of less than ten billionths of a second. The system therefore produced more energy than was given to it. Victory for clean energy in the future?
Not quite. Technically, the 2021 experiment already had the property that the reactions were self-sustaining and that the losses, by radiation, were less than the gain. The ignition, according to the consecrated expression, was therefore already carried out, as the researchers wrote in a research article published a year later, in August 2022, by the journal Physical Review Letters.
Also, the return is not greater than one, as the numbers might suggest. Indeed, the colossal energy, approximately 300 megajoules, necessary to produce the laser beams considerably reduces the actual energy efficiency.
Finally, there is a long way from this spectacular demonstration to an electricity production reactor. Apart from the fact that the gain would have to be a hundred times greater to allow efficient exploitation of the phenomenon, there is a size limit. The NIF only fires once a day and only produces energy in a very short time. “It would take ten shots per second to have an electricity production system”believes Erik Lefèbvre, head of the Atomic Energy and Alternative Energies Commission (CEA) for plasma-laser experiments at the Laser Mégajoule (LMJ) facility, quite close to the NIF, near Bordeaux, under construction.
But the LMJ, without all these beams, or the NIF, with this small gain, are already useful for military research, because they create plasmas whose behavior resembles those generated by atomic bombs.
The NIF teams can nevertheless rejoice. For several months, they couldn’t do as well as in August 2021. Then they succeeded, “by slightly increasing the energy of the laser beams”according to Eric Lefebvre.
For the specialist, “This announcement will bring comfort to the NIF, but also energize the field of fusion”. Indeed, there is no shortage of projects around this very old idea, which will not be ready to help fight against global warming anyway. In Cadarache, a prototype fusion reactor is under construction, ITER, but which uses a different technique, using strong magnetic fields to trap, concentrate and heat matter. Start-ups have also launched themselves in this very futuristic field, the slow progress of which probably testifies to the difficulty of controlling the phenomenon. In November, a team from LLNL also published a method to improve laser target heating using magnetic fields. In the absence of nuclear fusion, there is at least already a fusion of approaches.