Lawrence Livermore National Laboratory



May 5, 2021

Photo of a target capsuleThis photo shows a representative of two target capsule designs, named Orange and Cutie, that  were among the best performers in polar direct drive (PDD) experiments. Credit: Lane Carlsen/General Atomics.

Scientists from LLNL and the Laboratory for Laser Energetics (LLE) at the University of Rochester are working to improve polar direct drive (PDD) neutron sources on NIF, the world’s most energetic laser.

PDD neutron sources are capsules filled with deuterium-tritium (DT) gas at ambient temperature and shot with robust laser pulses that do not require stringent laser power contrast control or power accuracy. These sources are more time and resource efficient to field on NIF than conventional indirect drive sources that require high-quality cryogenic layers of DT ice. In addition, a lower generated target debris load allows neutron radiation effects experiments to position much closer to the target, creating a stronger neutron radiation field for testing.

The team substantially enhanced the total fusion output and laser-to-fusion energy conversion efficiency for PDD. The team also developed a PDD exploding pusher, or PDXP, platform that has enabled radiation effects testing of recoverable samples at record 14 MeV (million electron volt) neutron fluence levels.

“For over a year and a half after the initial experimental success, this design of PDD was the most efficient way in existence to convert laser energy input into fusion output,” said Charles Yeamans, team lead and first author of a recent paper in Nuclear Fusion. Co-authors include Elijah Kemp, Zach Walters, Heather Whitley, and Brent Blue from LLNL and Steve Craxton, Patrick McKenty, Emma Garcia, and Yujia Yang from LLE.

Photo of Emma Garcia
Emma Garcia co-authored the paper. Credit: Eugene Kowaluk/Laboratory for Laser Energetics. (Click to expand)

“Shooting really big lasers at stuff can stimulate fusion reactions like what happens in the sun and other stars, and terrestrially in the core of a nuclear detonation,” Yeamans said. “We want to study how the intense radiation fields generated from fusion affect materials, electronics, and engineered systems like satellites and airplanes. At NIF, we are able to control and position our test objects close to that source.”

Additionally, similar direct drive capsule platforms have many applications on NIF. With different gas fills, they can be used for studies of nuclear reactions of interest to astrophysics and as a source of protons for point backlighting. They also have been used to produce short pulses of high-brightness continuum x rays for extended x-ray absorption fine structure (EXAFS) studies and for opacity measurements. They also have been used to make large, compressed plasmas for studies of electron-ion energy transfer.

“Overall, a better NIF neutron source design allows us to conduct better radiation effects tests in greater numbers than if we were to rely solely on the mainstream NIF experiments,” Yeamans said.

The work developed a valuable addition to the overall radiation effects experimental test capability for the Lab, Yeamans added. “It also developed the modeling and simulation capability to understand and improve the neutron source design,” he said. “With this work, we are better able to fulfill this responsibility now and in the future.”

The work was conducted by a team of designers—scientists who run computer codes that do complicated physics calculations—and experimentalists—engineers who understand and operate the world’s biggest laser. The team determined the best way to test in practice what works in the simulation.

Photo of Yujia Yang
Yujia Yang is another co-author. Credit: Eugene Kowaluk/Laboratory for Laser Energetics.

Several team members worked in both roles, and others specialized as either designer or experimentalist based on what the research team needed. Sixteen days of NIF experimental time spread over more than five years were included in the source development effort, with the three best-performing designs, each conducted during a shot day in 2019, selected for detailed discussion in the publication.

Heather Whitley, associate program director for High Energy Density Science at LLNL, developed the initial design for a large diameter PDD capsule with Craxton and Garcia from LLE and Warren Garbett from the U.K.’s Atomic Weapons Establishment.

“This platform is important because it provides high neutron fluences and enables the close positioning of samples near the source for survivability experiments,” Whitley said. “The polar direct drive configuration also provides excellent diagnostic access for other high temperature plasma physics experiments.”

Craxton from LLE helped lead the work of undergraduate students Garcia and Yang and said that the participation of the students has been important to this work. Each student was responsible for calculating the optimized laser beam pointing to achieve uniform implosion of a specific capsule diameter. This optimization is complicated by the NIF beam entry angles being optimized to drive a cylindrical hohlraum target. McKenty worked closely with Craxton and the rest of the team to determine the ideal laser pulse shape.

“We went through a whole series of experiments over many years, first to produce neutrons to test NIF neutron diagnostics while NIF was being commissioned,” Craxton said. “These experiments evolved to meet the needs of a wide variety of applications, with the largest targets producing the high yields required for the effects experiments.”

Critical to the success of this effort was fabricating and developing proper testing protocols to obtain key data for prescribing safe fielding pressures of these large (2-5 millimeters in diameter), thin wall (approximately 10-30 micrometers) capsules, which are more susceptible to bursting.

This was done mainly by the target fabrication team at General Atomics (GA) in San Diego, working closely with LLNL’s target fabrication team and the above-mentioned physics team. Claudia Shuldberg and her team led the work at GA, while Bill Saied and Kelly Youngblood led the target fabrication engineering effort at LLNL.

More Information

NIF’s National Security Applications Program

“Energetic Laser Helps Test Weapon Survivability,” Science & Technology Review, May, 2019

Testing Neutron Effects on Electronics,” NIF & Photon Science News, April, 2016

—Michael Padilla

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