Techniques developed with the Laser Energy Laboratory’s OMEGA laser system hold promise for flash fusion on larger scales.
Scientists from the University of Rochester’s Laboratory for Laser Energy (LLE) led experiments to demonstrate an effective “candle” for direct motion inertial confinement fusion (ICF) methods. In two studies published in Nature Physicsthe authors discuss their results and describe how they can be applied on a larger scale with the hope of eventually producing fusion in a future structure.
LLE is the US Department of Energy’s largest university-based program and hosts the OMEGA laser system, which is the largest academic laser in the world but still barely 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 tiny capsules filled with deuterium and tritium fuel, causing the capsules to explode and produce a plasma hot enough to launch fusion reactions between fuel nuclei. The experiments caused fusion reactions that produced more energy than the amount of energy in the hot central plasma.
The OMEGA experiments use direct laser illumination of the capsule and differ from the indirect motion approach used at NIF. When using the indirect motion approach, the laser light is converted into X-rays which in turn cause the capsule to explode. NIF used an indirect machine to irradiate an X-ray capsule using about 2,000 kilojoules of laser energy. This led to a 2022 breakthrough at NIF in achieving fusion ignition—a fusion reaction that creates a net energy gain from the target.
“Generating more fusion energy than the internal energy content of the fusion site is an important threshold,” says lead study author Connor Williams ’23 PhD (physics and astronomy), now a staff scientist at Sandia National Labs in the field of radiation. and ICF target design. “This is a necessary requirement for anything you want to achieve later, like burning plasma or achieving ignition.”
Having shown that they can achieve this level of implosion performance with only 28 kilojoules of laser energy, the Rochester team is excited by the prospect of applying direct steering methods to higher-energy lasers. While demonstrating a spark plug is an important step, 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 many characteristics that are favorable for fusion power compared to indirect drive,” says Varchas Gopalaswamy ’21 PhD (mechanical engineering), the LLE scientist who led the second . study exploring the implications of using the direct steering approach in megajoule-class lasers similar in size to the NIF. “After scaling the OMEGA results to a few megajoules of laser energy, the fusion reactions are predicted to become self-sustaining, a state called ‘burning plasma.’
Gopalaswamy says that direct-drive ICF is a promising approach for achieving thermonuclear ignition and net energy in laser fusion.
“A key 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 principal scientist and professor Robert L. McCrory to the Deptt. of Mechanical Engineering and in the Department of Physics and Astronomy. “These predictive models allow us to narrow down the pool of promising candidate models before conducting valid experiments.”
The Rochester experiments required a highly coordinated effort among a large number of scientists, engineers, and technical staff to operate the complex laser facility. They collaborated with researchers from the MIT Plasma Science and Fusion Center and General Atomics to conduct the experiments. These experiments were funded by the US Department of Energy’s National Nuclear Security Administration. The target design work resulted from machine learning applications funded by the DOE Fusion Energy Sciences program.