From DOE’s Lawrence Livermore National Laboratory (US) via phys.org : “New research looks at process of magnetic flux generation in ICF implosions”

From DOE’s Lawrence Livermore National Laboratory (US)

via

phys.org

October 27, 2021

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Hot-spot density and (overlaid) magnetic field profiles for capsules varying the perturbation size (a) and mode number (b). For a given mode number, larger amplitude perturbations generate more magnetic flux. For a given amplitude, higher mode numbers generate more flux. Credit: Lawrence Livermore National Laboratory.

Lawrence Livermore National Laboratory (LLNL) researchers now have a better understanding on how strong the magnetic fields are in an inertial confinement fusion (ICF) implosion at the National Ignition Facility (NIF) [below], the world’s most energetic laser.

The researchers described their findings a paper published in Physics of Plasmas with LLNL scientists Chris Walsh serving as lead author and Dan Clark as co-author.

The primary findings in the paper show that researchers can expect bigger field strengths for hotter implosions, implosions with bigger asymmetries and implosions with short wavelength asymmetries.

Walsh explains that the process of magnetic field generation in the ICF experiments—the so-called Biermann battery effect—is the same process that was the seed for magnetic fields throughout the universe.

“These seed magnetic fields are then amplified by dynamo effects to give us the magnetic field strengths we observe in the universe today,” Walsh said. “The expected magnetic field strengths in our NIF implosions (10,000T) are 200 million times larger than the magnetic field on the surface of the Earth.”

The fields also are 100 million times larger than the sun’s magnetic field strength. These field strengths are large enough to modify the path of electrons in the plasma, which change how heat is transported.

The paper shows that there will be short wavelength magnetic field loops within the hot-spot that can alter the performance of the implosions. The hot-spots are shown to be dominated by fields generated during stagnation when the temperature and density gradients are largest. A scaling of hot-spot magnetic flux is derived and compared with simulations. This reveals that perturbations with both larger amplitudes and higher mode numbers generate more magnetic flux.

The model described in the paper allows for greater understanding of which target designs will be susceptible to magnetohydrodynamic effects. The model can be used to ascertain the time when most magnetic flux is generated. If generation is weighted more toward early times, then more high-mode magnetic field loops will be present. A hot-spot with no high-mode perturbations at time of peak neutron production can still contain significant magnetic flux on those scales. By assuming that magnetic flux is transported with the heat-flow, the model can be used to post-process radiation-hydrodynamics data to estimate magnetic field strengths and magnetizations.

“Magnetic fields exist in our NIF experiments, but we do not usually account for them in designing or interpreting implosions,” Walsh said. “The magnetic fields are expected to change how the fuel cools down, which is a crucial process for achieving significant fusion reactions. The magnetic fields also can increase the perturbation growth.”

The magnetic field generation mechanism is like how a battery works and was discovered by Ludwig Biermann. In the case of ICF implosions the huge pressure gradients that we create act as the electric potential that drives a current like a battery.

The work was conducted on Livermore’s Quartz computing system.

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Livermore’s Quartz computing system.

“We’ve known for years that ICF implosions can be affected by self-generated magnetic fields, but quantifying the effect and predicting it in modeling has been very challenging,” said Clark, who oversaw the work as ICF program capsule modeling working group lead. “For years, Chris has been blazing the trail on modeling these effects in ICF, from his time as a graduate student and postdoc at Imperial College, London, and now continues to do so here at the Lab.”

Clark said it is a great asset to the Lab to have Walsh working directly with us.

“As the latest implosion experiments have moved to the threshold of ignition with record hot-spot temperatures, self-generated fields could become more important than they have been in the past experiments,” Clark said. “Having a means to quickly and efficiently assess how large the self-generated fields could be in a particular implosion, as Chris’s new model does, is very valuable to the program.”

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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