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

Scientists at the University of Rochester’s Laser Energy Laboratory (LLE) have conducted experiments to demonstrate the effectiveness of a “spark plug” for direct-drive methods of inertial confinement fusion (ICF). In two studies published in Natural physicsThe authors discuss their results and explain how they can be applied on a larger scale in hopes of eventually producing fusion in a future facility.

LLE is the largest university program in the United States Department of Energy and is home to the OMEGA laser system, which is the largest university laser in the world, yet represents nearly one-hundredth of the energy of the National Ignition Facility (NIF) at the Lawrence Livermore National Laboratory in California. .

With OMEGA, Rochester scientists made several successful attempts to fire 28 kilojoules of laser energy at small capsules filled with deuterium and tritium, causing the capsules to implode and producing 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 present in the central hot plasma.

The OMEGA experiments use direct laser illumination of the capsule and differ from the indirect drive approach used on the NIF. When using the indirect drive approach, laser light is converted into x-rays which in turn cause the capsule to implode. The NIF used indirect drive to irradiate a capsule with X-rays using approximately 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 where the fusion is taking place is an important threshold,” says lead author of the first paper, Connor Williams ’23 Ph.D. (physics and astronomy) , now a scientist at Sandia National Labs in radiation target and ICF design. “It’s a necessary condition for anything you want to accomplish later, like burning plasmas or performing an ignition.”

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

“If you can eventually create the spark plug and compress the fuel, direct drive has a lot of favorable characteristics 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 energy, fusion reactions should become self-sustaining, a condition called ‘burning plasmas’.”

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

“A major 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 number of promising candidate models before performing valuable experiments.”

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