From DOE’s Lawrence Livermore National Laboratory (US) : “Lawrence Livermore researchers focus on fast flows in thermonuclear fusion” 

From DOE’s Lawrence Livermore National Laboratory (US)

9.24.21

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

1
Multi-part figure showing measured and simulated flows within an imploding ICF hot spot. (a) Time-resolved x-ray emission is used to track the bright “tracer” particle during an implosion. (b) Horizontal and (c) vertical flow velocity for three asymmetry drives: Upward (▲) and downward (▼) driven implosions show strong large vertical flows. (d) Streamline data of internal flows from downward (▼) drive, overlaid on flow field from 2D HYDRA simulation at tbang + 65 ps.

Imagine having a balloon between both hands and trying to squeeze it with the same force on all sides so that it uniformly shrinks down. However, if you push on one side harder than the other the balloon won’t compress uniformly and will, in fact, move away from the hand that is pushing harder.

The same thing happens when the drive pushing on an inertial confinement fusion (ICF) capsule is imbalanced — if it pushes harder on the top than on the bottom the capsule will move downward. This motion detracts from the energy heating the capsule and generating fusion. A short leap is to imagine two pistons compressing this gas instead of hands.

That is how Dave Schlossberg, Lawrence Livermore National Laboratory (LLNL) staff scientist, explains the effect of laser drive asymmetry. Schlossberg is the lead author in a recently published paper in in Physical Review LettersImagine having a balloon between both hands and trying to squeeze it with the same force on all sides so that it uniformly shrinks down. However, if you push on one side harder than the other the balloon won’t compress uniformly and will, in fact, move away from the hand that is pushing harder.

The same thing happens when the drive pushing on an inertial confinement fusion (ICF) capsule is imbalanced — if it pushes harder on the top than on the bottom the capsule will move downward. This motion detracts from the energy heating the capsule and generating fusion. A short leap is to imagine two pistons compressing this gas instead of hands [Physics of Plasmas].

That is how Dave Schlossberg, Lawrence Livermore National Laboratory (LLNL) staff scientist, explains the effect of laser drive asymmetry. Schlossberg is the lead author in a recently published paper in Physical Review Letters.

The team conducted experiments at the National Ignition Facility [below] to investigate a “low-mode” laser asymmetry that was significantly degrading performance. The results from the work led to a detailed understanding of this degradation from the very small-scale up to the largest scale.

“With this knowledge it’s possible to reduce asymmetries and increase performance — which was recently accomplished,” he said, adding that this is one in a series of experiments over the last several years remediating degradations from radiative losses [Physical Review Letters], engineering features [Physical Review Letters] and ablator asymmetries[Physical Review Letters].

Characterizing measurements

There are four key findings from this work that include: measured signatures of asymmetric laser drive; agreement between simulation and experiment; quantification of Doppler-broadening in apparent ion temperature with increased bulk plasma motion; and relating observed, driven hot spot flows to macroscopic input parameters.

“In experimental science we only know what we can measure — so first we needed to characterize the measurements that show these implosions suffered from laser drive asymmetry,” he explained. “The natural next step was to compare these measurements with models and see if they agreed, and they did.”

One product of deuterium-tritium fusion is a neutron traveling 51,234 km/s in the center-of-mass frame — that’s ~17 percent the speed of light. If the plasma producing these neutrons also is moving with some velocity, then that velocity is added to the neutron. The team showed that small variances in this neutron velocity directly related to broadening of the measured, time-of-flight neutron spectrum.

“Think of it as measuring the arrival time of a bullet fired from a gun, where a bullet represents a neutron,” he proposed. “If you’re a precision sharpshooter standing absolutely still every time you fire a bullet, it will arrive at the target at exactly the same time. But, now say you’re running and firing, then some bullets arrive sooner and some later depending how fast you’re moving each time you fire the gun.”

Schlossberg said the same thing is true in the imploding, fusing plasma that’s producing neutrons while it’s moving. The neutron time-of-flight diagnostic precisely measures neutron arrival times and relates them to the plasma’s internal energy. If there’s additional spread in the arrival times because the plasma is moving, that’s an important consideration when inferring the plasma thermal temperature. This work characterized how the apparent ion temperature increases due to variance in the deuterium-tritium velocities.

Mapping flows in fusing plasma

The final finding of this work is direct measurement of the flowing deuterium-tritium ions within the fusing hot spot.

“Here we got a bit lucky, since some of the tungsten used to dope the capsule was injected into the hot plasma and lit up brightly in the X-ray range,” Schlossberg acknowledged.

It served as a tracer particle for these internal flows. By tracking the motion of this tracer particle over time, the team mapped out a flow line while the plasma was fusing. This is important since these flows are the cause of the increased apparent temperature, and it showed consistency between both measurements.

The team used this measurement to connect flows within the microscopic hot spot to asymmetries in the macroscopic laser-drive. When they balanced the laser drive these flows disappeared (see Bal. trace ● in figure). These findings combine to provide a comprehensive understanding of the effects of laser drive asymmetry on implosion performance, and shows agreement across experiment, simulation and theory. This provides confidence for future work to identify and reduce these asymmetries in laser drive, leading to overall improved performance.

“When we saw preliminary time-resolved, X-ray imaging soon after the first shot we were immediately intrigued — something spectacular was showing up, which ended up being the time-resolved motion of the tracer particle traveling through the hot spot,” Schlossberg said. “This material was traveling ~0.1 percent the speed of light through material ~10 times denser than solid material.”

“It’s truly a team effort, and I’m thrilled and humbled to be part of such a great group of people,” Schlossberg expressed, adding that work was done by a team of NIF scientists and engineers spanning across groups that handle data analysis, target fabrication, operations and diagnostics.

In addition to Schlossberg, co-authors include: Gary Grim, Dan Casey, Alastair Moore, Ryan Nora, Ben Bachmann, Laura Robin Benedetti, Richard Bionta, Mark Eckart, John Field, David Fittinghoff, Edward Hartouni, Robert Hatarik, Warren Hsing, Leonard Charles Jarrott, Shahab Khan, Otto Landen, Brian MacGowan, Andrew Mackinnon, David Munro, Sabrina Nagel, Art Pak, Prav Patel, Brian Spears, and Chris Young from LLNL; Maria Gatu-Johnson from The Massachusetts Institute of Technology (US); Verena Geppert-Kleinrath and Kevin Meaney from DOE’s Los Alamos National Laboratory (US); and Joseph Kilkenny from General Atomics (US).

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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