Ignition in laser fusion

Ignition in laser fusion

Nuclear fusion by laser bombardment. © Damien Jemison/ LLNL

Nuclear fusion is considered to be the potential energy of the future, but there is still a long way to go before efficiently functioning fusion power plants. However, physicists in the USA have now taken an important step in this direction: in a laser fusion experiment, for the first time they have generated more energy through the fusion of deuterium and tritium nuclei than previously had to be used to heat up the fusion fuel. It is the first time such a net energy gain has been achieved in a fusion reactor. In the fusion chamber of the National Ignition Facility, they also achieved ignition of the fusion plasma and thus a self-sustaining fusion reaction – albeit only for fractions of a second.

It makes stars shine like our sun: Nuclear fusion releases enormous amounts of energy in the form of heat and radiation. It has therefore long been regarded as a promising way of generating electricity and heat in an efficient and climate-friendly manner. In order to bring atomic nuclei to fuse, however, enormous temperatures and pressures are necessary, which are difficult to generate in terrestrial facilities. Various technologies are currently being researched and tested for this purpose. Test reactors such as Wendelstein-7X, Jet and the large reactor ITER under construction confine and heat up a large amount of hydrogen or deuterium-tritium plasma using strong magnetic fields. Another approach is inertial confinement fusion, in which a small amount of fusion fuel is heated and compressed by intense laser bombardment. This fusion technology is being tested at the National Ignition Facility (NIF) of the Lawrence Livermore National Laboratory (LLN) in California, among others.

192 lasers and a tiny target

The physicists at the NIF have now made an important breakthrough in laser fusion: In an experiment on December 5, 2022, the facility generated fusion energy of 3.15 megajoules for the first time – and thus not only fusion in the tiny pellet of the fuel, but also more Energy released than the 2.05 megajoules previously introduced by laser bombardment of deuterium-tritium fusion fuel. This means that nuclear fusion has achieved a net energy gain for the first time – this is considered a milestone for fusion research. “Initiating nuclear fusion is one of the greatest scientific challenges mankind has ever tackled,” said Kim Budil, director of the Lawrence Livermore Laboratory. “Crossing that threshold was the vision that drove 60 years of research.”

To achieve this breakthrough, the physicists used 192 powerful lasers, whose concentrated beams were converted into high-energy UV pulses via dozens of meter-long optics and amplifiers. This UV light is directed onto a gold-coated cylinder about one centimeter long, the so-called cavity. There, through interaction with the inner walls, the laser beams generate strong X-rays, which hit the fuel capsule, which is a few millimeters in size. It contains a mixture of the hydrogen isotopes deuterium and tritium and is heated and compressed by the intense radiation. This creates high pressure and a temperature of more than 120 million degrees Celsius in the center of the fusion fuel. This in turn triggers the fusion of hydrogen into helium. In this fusion reaction, energy is released in the form of particles and heat.

Long way to the fusion power plant

In the current experiment, the combination of the energy introduced by the lasers and the fusion energy was sufficient to ignite the tiny amount of plasma in the fuel capsule. A self-sustaining fusion reaction took place for a few fractions of a second. This advance compared to previous experiments was achieved, among other things, by adapting the shape of the cavity, which made it possible to irradiate the fuel capsule more evenly and intensively. Despite this important breakthrough, the fusion plant of the National Ignition Facility is still a long way from generating usable electricity with nuclear fusion. Because in order to bring the 2.05 megajoules of heating energy to the fusion fuel using a laser, more than 500 megajoules of electrical energy had to be applied to operate the laser. Overall, the energy generated by nuclear fusion is therefore still considerably less than the total energy required for the plant. In addition: The NIF system is usually ignited once a day, but a fusion power plant based on this design principle would have to ignite 10 to 20 times per second with high efficiency.

Source: Lawrence Livermore National Laboratory

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