From Lawrence Livermore National Laboratory: “ARC experiments exceed expectations”

From Lawrence Livermore National Laboratory

May 17, 2019
Breanna Bishop
bishop33@llnl.gov
925-423-9802

– Charlie Osolin

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At left: A schematic of the National Ignition Facility’s (NIF) target chamber with 192 NIF long-pulse beams shown in blue and two of the NIF beams “picked off” for ARC shown in red (upper right). The two long-pulse beams are split to form two rectangular beamlets each, giving a total of four beamlets that are compressed to picosecond-pulse lengths. Lower right: The modeled ellipsoidal focal spot for one of the four beamlets at target chamber center.

The first proton-acceleration experiments using the National Ignition Facility’s (NIF) Advanced Radiographic Capability (ARC) short-pulse laser have produced protons with energies about 10 times higher than previous experience would have predicted (see “A Powerful New Source of High-Energy Protons”).

Beams of high-energy protons can be precisely targeted and are able to quickly heat materials before they can expand. Ultrafast heating of matter will enable opacity and equation-of-state measurements at unprecedented energy densities and could open the door to new ways of studying extreme states of matter, such as stellar and planetary interiors. Proton acceleration also promises to enable a variety of other applications in high energy density (HED) and inertial confinement fusion (ICF) research.

In a recently published Physics of Plasmas paper, an international team of researchers reported that the maximum proton energies created in the February 2018 experiments — from 14 to 18 MeV (million electron volts) — are “indicative of (an)…electron acceleration mechanism that sustains acceleration over long (multi-picosecond) time-scales and allows for proton energies to be achieved far beyond what the well-established scalings of proton acceleration (at ARC-level intensities) would predict.

“Coupled with the NIF,” the researchers said, “developing ARC laser-driven ion acceleration capabilities will enable multiple exciting applications. For example, the NIF can deliver 1.8 MJ (million joules) of laser light to drive an experiment and with an energetic proton beam, we could begin to diagnose electromagnetic fields in these experiments by using proton radiography.”

LLNL engineering physicist Derek Mariscal, lead author of the paper, said the surprise results at ARC’s quasi-relativistic, or “modest” laser intensities — about a quintillion (1018) watts per square centimeter — “forced us to try to understand the source of these particles, and we ultimately found that a different mechanism for accelerating particles to MeV electrons was necessary to explain the results.

“While we haven’t completely explained this mechanism,” he said, “we’ve been able to start discounting mechanisms that have been identified in previous short-pulse work to start honing in on how we could get such unexpected electron and subsequent proton energies.

“These results are really encouraging not only for ARC-driven proton beams,” he added, “but for particle acceleration in what’s referred to as the quasi-relativistic laser regime.”

ARC is a petawatt (quadrillion watt)-class short-pulse laser created by splitting two of NIF’s 192 long-pulse beams into four rectangular beamlets. Using a 2018 Nobel Prize-winning process called chirped-pulse amplification, the beamlets are stretched in time to reduce their peak intensity, then amplified at intensities below the optics damage threshold in the laser amplifiers and finally compressed to picosecond (trillionth of a second) pulse lengths and highest peak power in large compressor vessels, as shown in this video.

In the experiments, which are supported by LLNL’s Laboratory Directed Research and Development (LDRD) and NIF’s Discovery Science programs, two ARC shots were fired onto 1.5×1.5-millimeter-square, 33-micron-thick titanium foils. About 2.6 kilojoules of energy were delivered in a 9.6-picosecond pulse and 1.1 kJ were fired in a 1.6-ps pulse. A Target Normal Sheath Acceleration (TNSA) field, first observed on LLNL’s Nova petawatt laser two decades ago, accelerated high-energy protons and ions from the contamination layer of proton-rich hydrocarbons and water coating the target’s surface.

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Illustration of the titanium target foil, ARC beamlet pointing, and images of the proton-acceleration data captured by radiochromic film stacks placed at the front of the primary diagnostics, the NIF Electron Positron Proton Spectrometer (NEPPS) magnetic spectrometers.

“We plan to take this platform in several directions,” Mariscal said. “One of the most obvious directions is for probing electromagnetic field structures generated during experiments driven by the NIF long-pulse beams, which has been a standard use for these proton beams since their discovery here at LLNL around 20 years ago on the Nova petawatt laser.

“In addition to using proton beams as a diagnostic tool,” he said, “we plan to continue to use these beams to create high-energy-density conditions. Since we’re able to generate around 50 joules of proton beam energy, if we can deposit it over a 10-picosecond timescale we can generate plasmas at near solid density with temperatures over 100 eV, which is a truly exotic state of matter known as hot dense matter.”

he researchers also are exploring new target designs that could enhance ARC’s laser intensity to achieve even higher proton energy, enabling probes of ICF experiments. And by varying the length of ARC pulses, they hope to create shaped short pulses using ARC laser beams.

“Pulse shaping with nanosecond pulses allows for driving precision shocks in materials for studying material equations of state, but we plan to use this idea at the sub-picosecond level to manipulate particle acceleration physics,” Mariscal said. “We’ve tried this scheme on the OMEGA EP laser at the University of Rochester’s Laboratory for Laser Energetics and saw greatly enhanced laser coupling to high-energy particles over single short pulses.”
Two-proton-beam experiments

Additional NIF shots will use a 10-picosecond ARC beam to drive a beam of protons intended to rapidly heat a solid sample to more than 50 eV. Concurrently, a higher-intensity one-picosecond ARC beam will be used to generate a second proton beam that will probe the electromagnetic field structures of the heating experiment. “That will ultimately help us to understand how particles are being accelerated to MeV energies with a 10-picosecond pulse,” Mariscal said.

Mariscal credited the “fantastic” suite of diagnostics at the ARC diagnostics table and modeling support from the NIF ARC laser team with enabling the researchers to learn “some very interesting fundamental short-pulse-driven particle acceleration physics in this new regime provided by ARC.

“We’re given a new level of confidence in our interpretations due to the high-quality characterization of delivered ARC laser pulses,” he said. “This allows our physics team to accurately model the laser conditions of the experiment and maximize our understanding from the limited overall number of ARC laser experiments.”

Joining Mariscal on the paper were LLNL colleagues Tammy Ma, Scott Wilks, Andreas Kemp, G. Jackson Williams, Pierre Michel, Hui Chen, Prav Patel, Bruce Remington, Mark Bowers, Lawrence Pelz, Mark Hermann, Warren Hsing, David Martinez, Ron Sigurdsson, Matt Prantil, Alan Conder, Janice Lawson, Matt Hamamoto, Pascal Di Nicola, Clay Widmayer, Doug Homoelle, Roger Lowe-Webb, Sandrine Herriot, Wade Williams, David Alessi, Dan Kalantar, Rich Zacharias, Constantin Haefner, Nathaniel Thompson, Thomas Zobrist, Dawn Lord, Nicholas Hash, Arthur Pak, Nuno Lemos and Max Tabak, along with collaborators from the University of California at San Diego, General Atomics, the University of Oxford and the Central Laser Facility at the STFC Rutherford Appleton Laboratory in the UK, the Institute of Laser Engineering at Osaka University in Japan and Los Alamos National Laboratory.

See the full article here .


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Operated by Lawrence Livermore National Security, LLC, for the Department of Energy’s National Nuclear Security Administration
Lawrence Livermore National Laboratory (LLNL) is an American federal research facility in Livermore, California, United States, founded by the University of California, Berkeley 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. 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 km2) 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,[2] 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 Los Alamos 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 Berkeley 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.[3]

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.[4] The jury found that LLNS breached a contractual obligation to terminate the employees only for “reasonable cause.”[5] 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.[7] The May 2008 layoff was the first layoff at the laboratory in nearly 40 years.[6]

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 km2) 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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