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Nuclear fusion: breakthrough in laser fusion?

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Nuclear fusion: breakthrough in laser fusion?

Nuclear fusion: breakthrough in laser fusion?

Scientists at Lawrence Livermore National Laboratory (LLNL) say they have restarted a fusion reaction that released more energy than was previously pumped in laser energy. They therefore regard the experiment as an important milestone on the way to “break even”, the point at which the fusion reaction yields more net energy than it took to generate it. Once that has been achieved, one wants to work on a practically usable fusion reactor.

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This time, with 3.5 megajoules, even more energy was released than in the first successful attempt of this kind in December 2022, reports the Financial Times, citing “three people who are familiar with the preliminary results”. The LLNL confirmed a successful attempt to the FT, but wants to evaluate the measurement results first. An official announcement by the institute is not yet available.

So far, the practical use of nuclear fusion has been considered an enticing but very distant vision of energy production – even if around 30 private companies have now bet a total of around 2.5 billion dollars on the feasibility of this technology. However, the vast majority of state and private projects have so far relied on fusion plasma that is enclosed with magnetic fields (magnetic fusion). Until recently, laser fusion was considered an exotic outsider in an exotic research field. This is changing, as can be seen not least from the decision by the Ministry of Research to also promote laser fusion in the future. What’s up with the idea? How is this supposed to work?

Successful nuclear fusion of light elements releases an enormous amount of energy – just like fission of heavy atomic nuclei. A – halfway – clear explanation for this is provided by a curve that shows the binding energy per nucleon – i.e. proton or neutron – as a function of the number of nucleons in the nucleus. The binding energy is the energy that is released when the core components bind. The curve initially rises steeply for small numbers of nucleons, has a maximum at 30 to 40 and then falls flat (How this can be explained physically is another question that we can ignore here for the time being). The sum of the binding energy per nucleon of two deuterium nuclei is smaller than that of a helium nucleus, so their fusion releases energy – analogous to the heat release in a chemical bond.

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However, protons repel each other. And fusion can only be achieved when two atomic nuclei come close enough for the strong nuclear force to overcome electrostatic repulsion. This is achieved by heating the fuel for fusion – usually deuterium and tritium – to extreme temperatures and compressing the resulting plasma. The conditions for a “burning plasma” are reached when the product of temperature, density and energy confinement time exceeds a critical value – experts call it the “Lawson criterion”.

The so-called “inertial fusion” starts with the “density” factor of the plasma. The idea is to extremely compress the fusion fuel by supplying energy from outside – with the gas heating up at the same time. The start-up General Fusion actually wants to use shock waves for this purpose. The “National Ignition Facility” (NIF) at LLNL uses a huge laser for this purpose. A target filled with fusion fuel is fired with the laser. The envelope vaporizes and the inward pressure wave compresses and heats the fuel. Until the fusion reaction occurs.

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However, the primary goal of fusion research at LLNL was never a fusion reactor. These experiments were primarily designed to investigate the conditions prevailing when a nuclear bomb explodes, in order to study the physics of thermonuclear explosions without nuclear weapons tests.

The researchers do this by not shooting at the target directly, but by heating it up via an intermediate target. The approach is therefore also called “indirect drive”: The laser fires onto a gold tube, the so-called cavity. The bombardment produces intense bremsstrahlung, X-rays, that vaporize the bead’s shell, which in turn creates a pressure wave that compresses the hydrogen extremely quickly. The conditions are similar to those of radiation compression in a hydrogen bomb – which also makes the readings useful for the military.

The first experiments at the NIF took place in October 2010 – 13 years after the project started. 192 laser beams with 1.8 megajoules of energy generated an X-ray pulse with a power of 21 kJ within 20 nanoseconds. However, ten years and 3,000 shots later, the NIF researchers still had not ignited a plasma. When asked by the magazine “Nature”, project manager Siegfried Glenzer explained the reason for the problem: “We were overly reliant on simulations”.

The NIF researchers mainly had to contend with two difficulties: First, they did not succeed in coupling as much of the laser energy into the plasma as calculated in the simulations. Above all, however, they had to contend with instabilities. Even the smallest irregularities in the geometry of the fuel sphere cause the burning plasma to fly apart too quickly.

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The researchers only reported a breakthrough in 2021: on August 8, 2021, they succeeded in generating 1.39 megajoules of energy with one shot – and thus 70 percent of the energy that they had previously put into it. The analysis paper on the breakthrough from August 2021 – published in January 2022 – gives two reasons for improving the energy yield: A larger fuel pellet and an adjustment of the shape of the laser pulse in order to better maintain the symmetry when coupling the energy.

The researchers have now proceeded in a similarly groping manner. Since the simulations do not fully capture the process and the measurement results are subject to interpretation, they ran through various scenarios.

Of course, the indirect approach is not the only way to generate laser fusion. The start-up Focused Energy is convinced that firing directly at the target works better.

The team around Markus Roth from the TU Darmstadt has been working together with the LLNL for many years in the field of the civil use of the fusion. In doing so, however, they follow the principle of “direct drive”, in which the capsule is driven directly by laser beams – and is not located in a cavity. In addition, Focused Energy relies on a specially designed target, into which the laser energy is to be better coupled, and on several fast laser pulses in succession: The fuel, which has already been compressed by the first pulse, is again bombarded with a short-pulse petawatt laser. This could simplify the ignition process and start a fusion reaction more quickly.

The Munich start-up Marvel Fusion, which recently announced a private-public partnership with Colorado State University, wants to use laser bombardment to ignite a boron-proton fusion. This is actually even more difficult than the fusion of deuterium and hydrogen because it requires higher temperatures, but it has technical advantages when using the released fusion energy.

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An extremely powerful laser fires a very, very short pulse – focused on a spot only a few microns in diameter. The pellet is “nanostructured” to better couple the energy. Physically, what exactly happens inside the pellet is not fully understood. According to current computer simulations, the script looks something like this: if the laser pulse is fast and intense enough, its electric field accelerates electrons in the pellet. This charge transfer in turn creates a strong electric field that pushes protons inwards from the pellet surface, while the boron nuclei hardly move because of their high mass. If the intensity of the laser pulse is high enough, the protons get enough energy to overcome the repulsion of the boron nuclei and fuse with them.

In fact, as early as 2005, Russian physicists succeeded for the first time in igniting a proton-boron fusion using lasers in the laboratory. A team of researchers led by Vadim Belyaev from the Central Research Institute for Mechanical Engineering in Korolev fired laser pulses at boron-containing polyethylene pellets that lasted a little over a trillionth of a second. Since then, other researchers have fused protons and boron on a handful of occasions, increasing the yield of alpha particles each time. The difficulty, however, lies in keeping the fusion reaction going and making sure that as much of the fuel as possible is actually being used for fusion, rather than just flying apart.

The problem of further increasing the yield of fusion, keeping the first spark alive and turning it into a fire is something all laser fusion projects have in common. Focused Energy and Marvel Fusion must first build new high-power, short-pulse lasers. And the LLNL continues to experiment with pulse shape, wind speed and size of the pellets, but still has a long way to go. In order to achieve a real break even, the researchers would have to get a hundred times more energy out of the fusion than before. In order to use the whole thing practically, they would also have to drastically increase the number of shots per day: Currently, the large laser can fire once a day.

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