March 1, 2024

Scientists create effective ‘spark plug’ for direct-drive inertial confinement fusion experiments

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A view of the interior of the OMEGA target chamber during a direct-drive inertial fusion experiment at the University of Rochester’s Laser Energy Laboratory. Scientists fired 28 kilojoules of laser energy into small capsules filled with deuterium and tritium fuel, causing the capsules to implode and produce a plasma hot enough to initiate fusion reactions between the fuel nuclei. Temperatures reached at the center of these implosions reach 100 million degrees Celsius (180 million degrees Fahrenheit). The speed at which implosion occurs is typically between 500 and 600 kilometers per second (1.1 to 1.35 million miles per hour). Pressures in the core are up to 80 billion times greater than atmospheric pressure. Credit: University of Rochester Laser Energy Laboratory Photo / Eugene Kowaluk

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A view of the interior of the OMEGA target chamber during a direct-drive inertial fusion experiment at the University of Rochester’s Laser Energy Laboratory. Scientists fired 28 kilojoules of laser energy into small capsules filled with deuterium and tritium fuel, causing the capsules to implode and produce a plasma hot enough to initiate fusion reactions between the fuel nuclei. Temperatures reached at the center of these implosions reach 100 million degrees Celsius (180 million degrees Fahrenheit). The speed at which implosion occurs is typically between 500 and 600 kilometers per second (1.1 to 1.35 million miles per hour). Pressures in the core are up to 80 billion times greater than atmospheric pressure. Credit: University of Rochester Laser Energy Laboratory Photo / Eugene Kowaluk

Scientists at the University of Rochester’s Laboratory for Laser Energetics (LLE) conducted experiments to demonstrate an effective “spark plug” for inertial confinement fusion (ICF) direct drive methods. In two studies published in Nature PhysicsThe authors discuss their results and describe how they can be applied at larger scales, in hopes of eventually producing fusion in a future installation.

LLE is the U.S. Department of Energy’s largest university program and hosts the OMEGA laser system, which is the largest academic laser in the world but still has nearly one-hundredth the power of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, in California. .

With OMEGA, Rochester scientists completed several successful attempts to fire 28 kilojoules of laser energy into small capsules filled with deuterium and tritium fuel, causing the capsules to implode and produce a plasma hot enough to initiate fusion reactions. between the fuel cores. The experiments caused fusion reactions that produced more energy than the amount of energy in the central hot plasma.

The OMEGA experiments use direct laser illumination of the capsule and differ from the indirect drive approach used in NIF. When using the indirect drive approach, laser light is converted into X-rays which, in turn, drive the implosion of the capsule. The NIF used indirect thrust to irradiate a capsule with X-rays using about 2,000 kilojoules of laser energy. This led to a breakthrough in 2022 at NIF in achieving fusion ignition – a fusion reaction that creates a net gain in target energy.

“Generating more fusion energy than the internal energy content of the site where fusion occurs is an important limit,” says first paper lead author Connor Williams ’23 Ph.D. (physics and astronomy), now staff scientist from Sandia National Labs in radiation and ICF target design. “This is a necessary requirement for anything you want to accomplish later, like burning plasmas or achieving ignition.”

By showing that they can achieve this level of implosion performance with just 28 kilojoules of laser energy, the Rochester team is excited about the prospect of applying direct drive methods to higher energy lasers. Demonstrating a spark plug is an important step, however, the OMEGA is too small to compress enough fuel to achieve ignition.

“If you can create the spark plug and compress the fuel, direct drive will have many characteristics that are favorable for fusion energy compared to indirect drive,” says Varchas Gopalaswamy ’21 Ph.D. (mechanical engineering ), the LLE scientist who led the second study exploring the implications of using the direct drive approach on megajoule-class lasers, similar to the size of the NIF. “After scaling the OMEGA results to a few megajoules of laser energies, fusion reactions are predicted to become self-sustaining, a condition called ‘flaming plasmas’.”

Gopalaswamy says direct-drive ICF is a promising approach to achieving thermonuclear ignition and net energy in laser fusion.

“An important factor contributing to the success of these recent experiments is the development of a new implosion design method based on statistical predictions and validated by machine learning algorithms,” says Riccardo Betti, LLE Chief Scientist and Professor Robert L .McCrory in the Department. of Mechanical Engineering and the Department of Physics and Astronomy. “These predictive models allow us to narrow down the set of promising candidate designs before carrying out valuable experiments.”

The Rochester experiments required a highly coordinated effort among a large number of scientists, engineers, and technical personnel to operate the complex laser facility. They collaborated with researchers at the MIT Plasma Science and Fusion Center and General Atomics to conduct the experiments.

More information:
CA Williams et al, Demonstration of hot-spot fuel gain exceeding unity in direct-drive inertial confinement fusion implosions, Nature Physics (2024). DOI: 10.1038/s41567-023-02363-2

V. Gopalaswamy et al, Demonstration of a Hydrodynamically Equivalent Burning Plasma in Direct Drive Inertial Confinement Fusion, Nature Physics (2024). DOI: 10.1038/s41567-023-02361-4

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