From DOE’s Lawrence Livermore National Laboratory (US) : “Scientists develop a new geometry for a neutron source platform for NIF”

From DOE’s Lawrence Livermore National Laboratory (US)

7.8.21

Michael Padilla
padilla37@llnl.gov
925-341-8692

1
In the inverted-corona platform, laser beams are pointed onto the inside walls via laser entrance holes. Graphic provided by Matthias Hohenberger.

The National Ignition Facility (NIF) [below] at Lawrence Livermore National Laboratory (LLNL) has added a new tool to its growing list of capabilities.

A team of scientists has demonstrated a new geometry for a neutron source platform for NIF, called the inverted-corona platform, which does not rely on spherically symmetric laser irradiation.

This new tool has significantly less-stringent laser-symmetry requirements than conventional laser driven neutron sources on NIF. In this technique, laser energy is used to heat the inner surface of a millimeter-scale capsule. The wall material expands and launches a centrally stagnating shock into the gas fill to heat the gas to fusion conditions.

“This platform has relevance to applications in effects testing or forensics,” said Matthias Hohenberger, LLNL staff scientist. “We have an experiment scheduled in 2022 for exploring applications as a neutron backlighter, and as a neutron source for nuclear-cross-section measurements with sample materials attached to the outside of the capsule.”

Hohenberger said there are other potential applications in basic science, and is one-of-a-kind in its geometry flexibility. “It also represents a challenging problem to simulate because of the relatively low plasma density,” he said. “So we’re using it to test mix models in state-of-the-art simulation codes, and to train junior scientists.”

The work, highlighted in a paper in Review of Scientific Instruments, presents a novel neutron-source platform for NIF. Typically, NIF neutron platforms are based on the spherical compression of a capsule filled with deuterium and tritium (DT) fuel, thus achieving the pressures and temperatures necessary for the DT to undergo fusion reactions. This is achieved using either indirect-drive intertial confinement fusion (ICF) platforms or directly-driven exploding pushers. In these platforms, the incident laser results in a pushing action from the outside of the capsule, accelerating the capsule wall inwards — either from the X-rays generated in the hohlraum, or from the laser incident on the capsule itself. That means performance is highly sensitive to drive asymmetries, as they result in an uneven push of the wall, and eventual mixing of fuel and wall material into the hot spot, said Hohenberger, who is the lead author of the paper.

“This can, and does, affect fusion performance,” he said. “It also means that the wall composition must be controlled tightly. Even small impurities in the wall, thickness variations or even surface roughness will affect the performance and neutron yield.”

Pointing lasers onto the inside of capsule wall

Hohenberger said in this new scheme, which was tested on the OMEGA laser and the NIF, the laser beams are pointed through laser entrance holes onto the inside wall of a ~5-millimeter diameter, gas-filled (D2 or DT) capsule.

This causes the wall material to ablate inwards, which then launches a converging shock wave into the gas fill. The shock stagnates on center and heats the gas fill to fusion conditions (similarly to an exploding pusher). However, because the laser beams are incident onto the inside wall, the capsule wall itself is pushed outwards and away from the center, and the fusion performance is dominated by the ablatively-driven shock.

Hohenberger said this work has two key advantages. First, it decouples the wall composition from the neutron source and significantly relaxes requirements on capsule quality such as thickness uniformity, material purity and surface roughness, because the wall does not mix with the hot spot since it is pushed out rather than inwards. Second, the performance is highly insensitive to low-mode asymmetries. That means it is possible to have laser beams incident from only one side, rather than symmetrically distributed around the target, without a reduction in neutron yield.

The platform was successfully demonstrated in experiments on both the OMEGA laser and NIF.

The work was funded through LLNL’s Laboratory Directed Research and Development program.

In addition to Hohenberger, co-authors include Nathan Meezan, Bob Heeter, Rick Heredia, Nino Landen, Andrew MacKinnon and Warren Hsing from LLNL; Will Riedel and Mark Cappelli from Stanford University (US); Neel Kabadi and Richard Petrasso from Massachusetts Institute of Technology (US); Chad Forrest from the Laboratory for Energetics (US) at the University of Rochester (US); Loosineh Aghaian, Mike Farrell and Claudia Shuldberg from General Atomics; and Franziska Treffert and Siegfried Glenzer from DOE’s SLAC National Accelerator Laboratory.

See the full article here .


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DOE’s Lawrence Livermore National Laboratory (LLNL) (US) is an American federal research facility in Livermore, California, United States, founded by the University of California-Berkeley (US) in 1952. A Federally Funded Research and Development Center (FFRDC), it is primarily funded by the U.S. Department of Energy (DOE) and managed and operated by Lawrence Livermore National Security, LLC (LLNS), a partnership of the University of California, Bechtel, BWX Technologies, AECOM, and Battelle Memorial Institute in affiliation with the Texas A&M University System (US). In 2012, the laboratory had the synthetic chemical element livermorium named after it.

LLNL is self-described as “a premier research and development institution for science and technology applied to national security.” Its principal responsibility is ensuring the safety, security and reliability of the nation’s nuclear weapons through the application of advanced science, engineering and technology. The Laboratory also applies its special expertise and multidisciplinary capabilities to preventing the proliferation and use of weapons of mass destruction, bolstering homeland security and solving other nationally important problems, including energy and environmental security, basic science and economic competitiveness.

The Laboratory is located on a one-square-mile (2.6 km^2) site at the eastern edge of Livermore. It also operates a 7,000 acres (28 km2) remote experimental test site, called Site 300, situated about 15 miles (24 km) southeast of the main lab site. LLNL has an annual budget of about $1.5 billion and a staff of roughly 5,800 employees.

LLNL was established in 1952 as the University of California Radiation Laboratory at Livermore, an offshoot of the existing UC Radiation Laboratory at Berkeley. It was intended to spur innovation and provide competition to the nuclear weapon design laboratory at Los Alamos in New Mexico, home of the Manhattan Project that developed the first atomic weapons. Edward Teller and Ernest Lawrence, director of the Radiation Laboratory at Berkeley, are regarded as the co-founders of the Livermore facility.

The new laboratory was sited at a former naval air station of World War II. It was already home to several UC Radiation Laboratory projects that were too large for its location in the Berkeley Hills above the UC campus, including one of the first experiments in the magnetic approach to confined thermonuclear reactions (i.e. fusion). About half an hour southeast of Berkeley, the Livermore site provided much greater security for classified projects than an urban university campus.

Lawrence tapped 32-year-old Herbert York, a former graduate student of his, to run Livermore. Under York, the Lab had four main programs: Project Sherwood (the magnetic-fusion program), Project Whitney (the weapons-design program), diagnostic weapon experiments (both for the DOE’s Los Alamos National Laboratory(US) and Livermore laboratories), and a basic physics program. York and the new lab embraced the Lawrence “big science” approach, tackling challenging projects with physicists, chemists, engineers, and computational scientists working together in multidisciplinary teams. Lawrence died in August 1958 and shortly after, the university’s board of regents named both laboratories for him, as the Lawrence Radiation Laboratory.

Historically, the DOE’s Lawrence Berkeley National Laboratory (US) and Livermore laboratories have had very close relationships on research projects, business operations, and staff. The Livermore Lab was established initially as a branch of the Berkeley laboratory. The Livermore lab was not officially severed administratively from the Berkeley lab until 1971. To this day, in official planning documents and records, Lawrence Berkeley National Laboratory is designated as Site 100, Lawrence Livermore National Lab as Site 200, and LLNL’s remote test location as Site 300.

The laboratory was renamed Lawrence Livermore Laboratory (LLL) in 1971. On October 1, 2007 LLNS assumed management of LLNL from the University of California, which had exclusively managed and operated the Laboratory since its inception 55 years before. The laboratory was honored in 2012 by having the synthetic chemical element livermorium named after it. The LLNS takeover of the laboratory has been controversial. In May 2013, an Alameda County jury awarded over $2.7 million to five former laboratory employees who were among 430 employees LLNS laid off during 2008.The jury found that LLNS breached a contractual obligation to terminate the employees only for “reasonable cause.” The five plaintiffs also have pending age discrimination claims against LLNS, which will be heard by a different jury in a separate trial.[6] There are 125 co-plaintiffs awaiting trial on similar claims against LLNS. The May 2008 layoff was the first layoff at the laboratory in nearly 40 years.

On March 14, 2011, the City of Livermore officially expanded the city’s boundaries to annex LLNL and move it within the city limits. The unanimous vote by the Livermore city council expanded Livermore’s southeastern boundaries to cover 15 land parcels covering 1,057 acres (4.28 km^2) that comprise the LLNL site. The site was formerly an unincorporated area of Alameda County. The LLNL campus continues to be owned by the federal government.

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