The Laboratory for Laser Energetics (LLE; Rochester, NY) is home to two extremely powerful lasers—OMEGA and OMEGA EP—and researchers are using them to explore laser-driven inertial confinement fusion (ICF; see video).
ICF involves compressing a small amount of fuel consisting of hydrogen isotopes, deuterium (D), and tritium (T), and heating it to temperatures greater than the center of stars.
“When these conditions are reached, the fuel undergoes fusion—releasing enormous energy that can be used for research relevant to the National Nuclear Security Administration’s (NNSA) Stockpile Stewardship Program (SSP) and to drive carbon-free power plants,” explains Valeri Goncharov, distinguished scientist and director of the Theory Division at LLE.
LLE was established at the University of Rochester in 1970 and is the largest U.S. Department of Energy university-based research program in the nation, supported by the National Nuclear Security Administration as part of its Stockpile Stewardship Program (SSP).
“As a center for exploring the interaction of intense radiation with matter, LLE is a unique national resource for research and education in science and technology,” says Goncharov. “Our current research includes exploring fusion for the SSP program and as a future source of energy, developing new laser and materials technologies, and pursuing a better understanding of high-energy-density phenomena.”
Laser-driven inertial confinement fusion
ICF experiments are a key focus area at LLE, and OMEGA (see Figs. 1 and 2) conducts approximately 10 experiments per day—compared to 1 or 2 per day on the National Ignition Facility (NIF) at Lawrence Livermore National Lab.
The total amount of energy delivered by the laser for a single experiment isn’t enormous: 30 kJ on OMEGA or 2 MJ on NIF. “With 30 kJ of energy, you can only heat 3 ounces of water from room temperature up to boiling point,” says Goncharov. “But in ICF, space and time are shrunk so much that even this modest amount of energy leads to creating exotic conditions of matter.”
An ICF implosion lasts only a few nanoseconds and, for a successful ignition experiment, plasma needs to be kept at ignition conditions for a tenth of a nanosecond. To help visualize it: light travels only a foot during a nanosecond.
Laser energy is also delivered to a tiny space, and the targets on OMEGA are less than a millimeter in size (see Fig. 3). “Laser energy is delivered to a very small volume during an extremely short time and puts the matter into a very high-energy-density regime (similar to high mass density, if mass is squeezed into a very tiny volume),” Goncharov says. “Under HED conditions, we create pressures of above 1 million atmospheric pressure (atm). For comparison, pressure inside the Earth is 3 million atm and diamonds are formed under 0.5 million atm. Material under HED conditions is relevant to planetary science, the conditions achieved inside the stars.”
During an ICF implosion, when the laser interacts with target material, pressures of 100s million atm are created by the same mechanism as the flow of burnt fuel that generates thrust in a rocket: a thin layer of target material, heated by laser, ablates off and its blow-off flow creates a push that accelerates the rest of the target inward. As the target implodes, spherical convergence amplifies target pressure to 100s billion atm and creates fusion conditions—at a temperature of several hundred million degrees and several hundred billion atm of pressure.
The quest for ignition
On December 5, 2022, NIF achieved ignition for the first time in a laboratory—by generating more fusion energy (3.15 MJ) than laser energy (2.05 MJ) used during an experiment to compress DT fuel.
“This milestone demonstrates our understanding of how matter behaves at HED conditions and, although not complete, has advanced to the point where we can design such complex experiments and achieve fusion conditions that require high precision and accuracy in modeling, laser energy delivery, target fabrication, and data acquisition and analysis,” says Goncharov.
From Goncharov’s point of view, “there’s always a question of whether our knowledge of ICF implosions is complete enough to use lasers to create 100 million atm of pressure to compress DT fuel— reducing its volume by a factor of 10,000—and not introduce any excessive deviations from spherical symmetry (implosion needs to be highly spherically symmetric for effective fuel compression) during the process of compression, and keep the fuel cold enough so its density is maximized,” he says.
It’s no secret NIF was struggling to achieve ignition, and many people who work within the field of HED/ICF had begun to question whether lab ignition is possible at all.
“We were desperately searching for the answers, exploring how physics—not included within the current modeling—could explain the reasons for ignition failure,” says Goncharov. “Based on the data, one thing was clear: energy coupled to the fuel in earlier implosions wasn’t enough to create a ‘fusion spark’ at the target center to ignite the rest of the fuel. Hard work and the ingenuity of LLNL scientists—with many contributions from the entire HED community—led to design modifications that led to higher fuel energy and, ultimately, to ignition.”
LLE and NIF use similar targets—a layer of frozen DT overcoated with a layer of plastic—but differ in the way laser energy is delivered to the targets.
“In NIF implosions, the target is placed inside a high-Z (gold) enclosure called a hohlraum,” explains Goncharov. “A laser is pointed on the inside walls, which get heated and emit x-rays. It’s like an oven where you bake a casserole by heat emitted from the oven’s walls. In the same way, a target surface is heated inside a hohlraum by x-rays.”
When the surface material blows off, it creates a rocket effect that drives the rest of the target inward. “Since laser energy gets to the target indirectly, it’s called ‘indirect drive.’ At LLE, we use the direct drive approach, which involves using a laser to heat up the target surface directly,” Goncharov says. “Instead of an oven, it’s akin to setting a pot on a gas stove. The laser heats up the target surface and the rest of the drive process proceeds the same way as indirect drive.”
Indirect drive vs. direct drive
There are pros and cons to both indirect drive and direct drive. Indirect drive is less efficient (less laser energy gets to the fuel), but target compression is more symmetric. LLE has developed several techniques to overcome nonuniform heating by the laser beams—earlier disadvantages of direct drive are largely resolved.
“Direct drive still has a major flaw: when a laser travels through blow-off plasma to reach the target, a high level of electron and ion waves is excited,” says Goncharov. “These waves scatter away laser light before reaching the target region where it can get efficiently absorbed. This was an a-ha! moment on OMEGA, because we were struggling to understand why ablation pressure generated in direct-drive implosions by the rocket effect was below expectations based on model predictions.”
To understand the issue, the researchers measure the color (frequency) of laser light scattered from the target. “Laser light is nearly monochromatic—it only has one wavelength, as opposed to everyday white light that has many different wavelengths visible in rainbows,” says Goncharov. “But when light, as any other wave, reflects from a moving surface, its wavelength shifts due to the Doppler effect. It’s akin to hearing different pitches from the sound of a siren based on whether the emergency vehicle is moving toward or away from you. Laser wavelength also gets slightly longer when it reflects from an imploding target surface—moving away from the source.”
By measuring the wavelength of reflected laser light, they saw a smaller-than-expected shift from a moving target. “The only explanation is that the laser light doesn’t penetrate deeply enough and reflects earlier prior to reaching the target surface,” Goncharov says. “It’s reflected from ion waves that were launched into the plasma by the laser itself. Since two intersecting beams are needed to create a reflective wave pattern, this phenomenon is a cross-beam energy transfer (CBET). It’s responsible for 30 to 40% of light reflection from the target and a significant reduction in rocket effect and ablation pressure.”
This is both good and bad. Good, because the researchers finally figured out why their implosions weren’t efficient. And bad, because it was the nature of the direct drive—the direct interaction of laser light with a target plasma—causing this drive deficiency. What made this discovery even worse is the level of coupling losses scales with the target size.
If a bigger laser facility like NIF performs direct-drive implosions, CBET losses will be larger there and prevent efficient laser energy coupling to DT fuel—and it makes ignition a very difficult task for the direct-drive approach.
“A solution seemed trivial: break a big ion wave into a bunch of smaller waves,” Goncharov says. “As with waves on a water surface, they’ll have a regular structure if the source is regular in time—like if you throw stones into the water at regular intervals.”
When a monochromatic—single wavelength—laser light travels through the plasma, certain regions in the plasma (resonance region) can have regular-pattern ion waves that get excited. “What happens if I use a laser with two slightly different wavelengths? Two different waves with slightly different frequencies will be created—each with smaller amplitude and different locations,” points out Goncharov.
And what happens if you repeat the process of introducing more and more laser wavelengths and break the laser beams into several smaller beams with different frequencies? “I will totally destroy the refractive ion wave pattern and make the wave amplitude so small and spread out in space that these waves won’t be able to scatter the laser light anymore,” Goncharov says.
This is the main idea of fighting CBET, and another recent a-ha! moment that got Goncharov and colleagues very excited about the future of direct drive.
If they can demonstrate CBET is defeated by using broadband lasers (laser light that consists of several frequencies separated by a few present), “the advantage of the laser drive approach will be enormous: nearly 100% of laser light can be coupled to the target and ablation pressure can be increased to several hundred million atm,” Goncharov explains. “With such a highly efficient laser, direct drive is predicted to achieve fusion yields a factor of 50 to 100 more than currently possible with indirect drive on NIF using the same laser energy.”
A broadband laser beam prototype
What’s next? Building a broadband laser beam prototype at LLE to be used on OMEGA to demonstrate CBET is mitigated (see Fig. 4). “We call this laser beam ‘FLUX,’ which is short for Fourth-Generation Laser for Ultra-Broadband Experiments,” says Goncharov.
These experiments will be conducted during 2024 and, if successful, “we’ll put together a proposal for a next-generation broadband implosion facility,” Goncharov adds.
Although much smaller than NIF, the facility’s main goal is to demonstrate that all direct-drive deficiencies due to laser-plasma interactions can be eliminated via a spherical implosion and that laser technology is ready for an at-scale implosion facility to ultimately deliver high fusion yields (>100 MJ) for the Stockpile’s needs and a future inertial fusion energy power plant.
FURTHER READING
1. H. Abu-Shawareb et al., Phys. Rev. Lett., 132, 065102 (Feb. 5, 2024); https://doi.org/10.1103/PhysRevLett.132.065102.
2. R. S. Craxton et al., Phys. Plasmas, 22, 110501 (2015); https://doi.org/10.1063/1.4934714.
