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  • richardmitnick 7:43 am on May 21, 2019 Permalink | Reply
    Tags: Advanced Radiographic Capability (ARC), , , , , NIF,   

    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

    1
    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.

    3
    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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    LLNL Campus

    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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  • richardmitnick 7:25 am on July 6, 2016 Permalink | Reply
    Tags: , , , NIF, Nucleosynthesis   

    From LLNL: “NIF experiments study how ‘starstuff’ is made” 


    Lawrence Livermore National Laboratory

    1
    Shot-time image from a June 1 NIF experiment simulating stellar nucleosynthesis fusion reactions

    When the renowned cosmologist Carl Sagan declared that “we are made of starstuff,” he wasn’t speaking metaphorically. As Sagan said in the TV series Cosmos, many of the elements in our bodies – “the nitrogen in our DNA, the calcium in our teeth, the iron in our blood” – were forged in the interiors of stars, in a process called stellar nucleosynthesis (element formation). Lighter elements, such as hydrogen and helium, were created in the Big Bang when the universe began.

    How these elements are assembled, or synthesized, is the subject of a new series of National Ignition Facility (NIF) discovery science (DS) experiments, which began May 30.

    LLNL/NIF
    LLNL/NIF

    By fusing elements such as tritium (a form of hydrogen) and helium in the NIF target chamber, a multi-institutional team of researchers hopes to gain new insights into the processes that kick-started and have sustained the universe.

    “All of the stellar nucleosynthesis reactions – fusion reactions that happen inside stars – produce the elements, but we can’t really see inside a star to tell how those reactions are proceeding,” said plasma physicist Alex Zylstra of Los Alamos National Laboratory (LANL). “Models of the production of nuclei in the cosmos depend on having accurate data to inform those models. And studying those reactions in conditions that are actually applicable to the interior of stars or to the universe during the Big Bang is very challenging. This experimental campaign is working toward doing that at relevant conditions that can only be achieved at NIF.”

    “And (the experiments) answer questions about stellar evolution and elemental abundance – it’s really fundamental science,” added nuclear physicist Maria Gatu Johnson of the Plasma Science and Fusion Center at the Massachusetts Institute of Technology (MIT), the campaign’s principal investigator. “The conditions we create in one of these implosions are very similar in density and very similar in temperature to the interior of a star.”

    The first three experiments in the campaign focused on the “proton-proton 1” chain of nuclear reactions, at the beginning of the stellar nucleosynthesis cycle. Nuclear fusion converts hydrogen into helium, and a small amount of matter is turned into energy in the process.

    “It starts with just the protons in the nucleus of regular hydrogen atoms,” Gatu Johnson said. “They fuse to form deuterium (as one of the protons is converted to a neutron), and then deuterium can fuse with a proton to form helium-3. The helium-3 particles, once produced, fuse to form helium-4 (also known as an alpha particle), and generate two protons that will go through the cycle again.

    “This is the most significant energy-producing step in the sun, so it’s very critical to know the rate of that reaction.”

    2
    Members of the stellar and Big Bang nucleosynthesis experimental team (from left): Charles Yeamans (LLNL), Daniel Sayre (LLNL), Matthias Hohenberger (Laboratory for Laser Energetics, University of Rochester), Maria Gatu Johnson (MIT), Daniel Casey (LLNL), Alex Zylstra (LANL), Bruce Remington (LLNL), and Hong Sio (MIT). Indiana University and Ohio University also are participating in the campaign.

    The NIF experiments build on previous studies of the 3He+3He reaction on the OMEGA Laser at the University of Rochester.

    U Rochester Omega Laser
    U Rochester Omega Laser

    The OMEGA and NIF experiments are the first to study stellar nucleosynthesis using high energy density (HED) plasmas (freely moving ions and free electrons). Most previous nucleosynthesis studies were done on particle accelerators.

    “In accelerator experiments you have a solid, cold target that’s hit by a beam of ions (charged particles),” Zylstra said, “and that’s a totally different scenario from what happens in a star or in the universe during the Big Bang. Those are plasma systems; those reactions happen in a plasma in the universe.”

    “And we actually manage to create this kind of environment in the plasma that’s created on NIF and OMEGA,” Gatu Johnson added. “So it’s really much more similar to the stellar conditions compared to other methods.”

    Compared to OMEGA, NIF’s higher laser power and energy and larger HED plasmas allow quantitative studies of the reactions at lower “Gamow-peak” energy – conditions more directly relevant to stellar nucleosynthesis.

    The Gamow peak, named for Russian-American physicist George Gamow, is the energy region – not too high and not too low – where the reaction is most likely to take place. “At OMEGA,” Zylstra said, “you can have a very small volume of plasma that’s hot, and you can see the (reaction) products. Using NIF we can generate a larger volume of plasma at lower temperature, producing a comparable number of reaction products, to get closer to stellar conditions.”

    Along with the 3He+3He reaction, the first set of experiments also studied the complementary tritium-tritium and tritium-helium-3 reactions. The shots used a target called a polar direct-drive exploding pusher target; in exploding pusher shots, the NIF beams heat thin, glass-walled targets, driving strong shocks into the target and fusing the material inside.

    Gatu Johnson said the first experiment, which studied the tritium-tritium reaction, produced enough neutrons for the T-T neutron spectrum to be measured by NIF’s neutron diagnostics. “We got some really good data from that,” she said. Data from the T+3He and 3He+3He experiments weren’t immediately available.

    Two more rounds of experiments are scheduled in the campaign. “The primary goal of this set of shots is to get a really good measurement of the 3He+3He proton spectrum and rate,” Gatu Johnson said. “Depending on what we learn from this first round of shots, we’ll fine-tune the implosions to get better data. The resulting data from this effort should greatly improve our knowledge of these reactions in HED plasmas.”

    “This set of shots to study reactions relevant to stellar nucleosynthesis is an important step forward for the Discovery Science Program,” said Bruce Remington, the NIF DS program leader. “We have now broadened the science regimes accessible to NIF to include stellar nuclear physics.”

    See the full article here .

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  • richardmitnick 3:16 pm on June 17, 2016 Permalink | Reply
    Tags: , , NIF,   

    From Physics Today: “NIF may never ignite, DOE admits” 

    Physics Today bloc

    Physics Today

    17 June 2016
    David Kramer

    More than three years after the deadline passed for obtaining a sustained, high-energy-yield nuclear fusion reaction at the National Ignition Facility (NIF), the US Department of Energy is still unsure whether the $3.5 billion laser can ever attain that milestone.

    NIF Bloc
    LLNL/NIF
    NIF

    Much as it did in 2012, the agency has established a new, less ambitious goal for NIF several years hence: to determine whether the machine can ever achieve its eponymous goal, and if not, why not.

    “The question is if the NIF will be able to reach ignition in its current configuration and not when it will occur,” states a May report prepared by DOE’s National Nuclear Security Administration (NNSA).

    NNSA

    The reassessment of progress toward ignition at the Lawrence Livermore National Laboratory facility was conducted three years after the NNSA suspended its formal two-year-long ignition campaign in September 2012. Ignition, the threshold at which more energy results from a fusion reaction than is required to spark it, is an essential determinant in whether inertial confinement fusion (ICF) could ever become a source of fusion power.

    Despite the report’s assurances that much progress has been made toward ignition since 2012, the NNSA appears no closer to committing to ignition on NIF than it was then. In a December 2012 report to Congress, the agency found “no compelling information suggesting that the [NIF’s] indirect-drive approach cannot achieve ignition.” Still, then-NNSA administrator Tom D’Agostino said it was “too early to say whether or not ignition can be achieved at the NIF.”

    In a new plan for the ICF program, the NNSA establishes a goal, with a deadline of 2020, to “determine the efficacy of reaching ignition on NIF.” That contrasts sharply with the virtual assurances of ignition that were made by proponents in 2009, when NIF began operating. Although ignition experiments continue at NIF, they have been interspersed with experiments designed to deepen understanding of other nuclear weapons–related phenomena, such as the behavior of materials under extreme pressures and densities.

    Since 2012 NIF’s 1.8 MJ laser has nearly doubled the frequency of shots, the machine’s diagnostics have been improved, and progress has been made on identifying key impediments to ignition, the new report states. NIF’s indirect-drive approach focuses 192 beams on a cylinder, or hohlraum, containing a tiny capsule of fusion fuel. The hohlraum converts the light to x rays, which implode the capsule.

    In the meantime, the University of Rochester’s Omega laser and Sandia National Laboratories’ Z machine—both also supported by the NNSA’s inertial confinement fusion program—continue research on alternative approaches to ignition.

    U Rochester Omega Laser
    U Rochester Omega Laser

    Sandia Z machine
    Sandia Z machine

    Omega, a glass laser like NIF, uses direct drive, which brings beams to impinge directly on targets; Z uses electromagnetic fields to produce implosions.

    The NNSA review says computer models and codes predicting that NIF would attain ignition conditions “are not capturing the necessary physics to make such predictions with confidence. A lack of appreciation for this, combined with a failed approach to scientific program management, led to the failures” in the ignition campaign.

    Although the performance of NIF’s targets containing fusion fuel continues to improve, “currently, there is no known configuration, specific target design, or approach that will guarantee ignition on the NIF,” says the review.

    Stephen Bodner, a former director of the plasma physics program at the US Naval Research Laboratory (NRL), has been a vocal critic of NIF since before its construction began. In a 1995 paper published in Plasma Physics and Controlled Fusion, Bodner predicted that the highly intense NIF laser would create instabilities in the plasma. That, plus the formation of unpredictable magnetic fields, would prevent the symmetrical implosions required for ignition.

    “Basically [the report] is confirmation of what I predicted in 1995,” Bodner says. “It took the community 21 years, and many billions of dollars, to vindicate my predictions. So sad.”

    Regardless of whether ignition is achieved, there are other compelling nuclear weapons stewardship questions concerning the properties of thermonuclear plasmas with multi-megajoule yields, the NNSA report says. Planned Russian and Chinese laser facilities may surpass NIF’s capabilities, it warns, and in an era without nuclear testing, a source capable of producing 500 MJ of fusion energy “will be essential for the health of the [weapons] program.” Such energy yields are unlikely to be achieved within the next decade but should be considered an ultimate goal, the report says.

    Bodner argues, however, that NIF’s regimes of temperature, ionization, pressure, density, and radiation spectrum are fundamentally different from those that occur in a nuclear weapon. “To extrapolate the regime in the laboratory that they’re using to anything in nuclear weapons would be outrageously irresponsible,” he says. “They should not be using any of that science in the nuclear weapons program.”

    David Crandall, a former NNSA scientist who helped oversee NIF, disagrees. He says the realization that the codes predicting ignition were wrong has instilled a new level of caution among weapons scientists about extrapolating from data sets of nuclear tests. “That piece of reality was extremely important to the weapons program,” he says. Further, Crandall says, new methods have been developed for using NIF-generated fast neutrons to test weapons codes. For those techniques, neutron yield is more important than ignition. Also, he explains, experiments at NIF have already provided important new information about the behavior of plutonium at high pressures.

    John Edwards, associate director for the NIF’s ICF program, declines to say whether he’s optimistic or pessimistic about ignition at NIF. Progress since 2012 includes the first ever laboratory demonstration of the alpha heating process, in which thermal energy is supplied by the helium nuclei that result from fusion. “But there are obstacles which we are quite open about,” Edwards acknowledges. Researchers think they can overcome the instabilities inside the hohlraums by making the cylinders larger; the question is whether NIF’s energy is sufficient to drive the larger targets, he says.

    Reconfiguring NIF to perform direct-drive experiments is being evaluated by a University of Rochester–led team. But that will require a major revamp costing several hundred million dollars.

    Bodner argues that solid-state lasers like NIF and Omega won’t work; he says the krypton fluoride gas laser—two of which he helped build while at the NRL—is the best option for an ignition driver. The KrF laser in a direct-drive mode produces a broader bandwidth beam that can be “smoothed” to eliminate asymmetric hot spots, he says. But the NNSA’s plan doesn’t include KrF lasers among its driver candidates.

    Stephen Dean, president of the nonprofit Fusion Power Associates, says DOE’s justification for NIF has shifted from the energy-relevant milestone of achieving ignition to a focus mainly on weapons research. “They don’t want to be held to ignition,” he says. Dean sees a parallel with DOE’s magnetic fusion program. In 1980 the project was sold as an energy program with a 2000 deadline for construction of a working fusion power plant; today it’s classified as a science program.

    “You have people working who were goal-oriented,” Dean says. “And when the program doesn’t accomplish those goals, there’s a scramble to do something to save it.”

    See the full article here .

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    The mission of Physics Today is to be a unifying influence for the diverse areas of physics and the physics-related sciences.

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  • richardmitnick 8:53 am on June 14, 2016 Permalink | Reply
    Tags: , Diagnostic provides top-down view of neutrons, , , NIF   

    From LLNL: “Diagnostic provides top-down view of neutrons” 


    Lawrence Livermore National Laboratory

    Jun. 13, 2016
    Jeremy Thomas
    thomas244@llnl.gov

    1
    Target Diagnostics engineer Francisco Barbosa completes installation of the North Pole neutron Time of Flight (nTOF) diagnostic on the roof of the National Ignition Facility.

    A new diagnostic built on the National Ignition Facility’s (NIF) roof is giving researchers a clearer picture of the neutrons released during laser-driven implosions of target capsules containing deuterium or deuterium and tritium (DT).

    NIF Bloc
    LLNL/NIF
    LLNL/NIF

    Instruments inside the North Pole neutron Time of Flight (nTOF) enclosure, a structure slightly larger than an industrial shipping container, detect and record neutron arrival times, providing researchers with much-needed data in the northern hemisphere of the NIF Target Chamber.

    “It will give us a view of what we’ve been missing, and we believe a quieter one (with less interference) than what we’ve gotten so far,” said NIF Co-Target Diagnostic Manager Mark Jackson.

    The north pole system joins four other nTOF detectors — one located in the basement of the Target Bay (south pole), two on the Target Chamber equator, and one in the neutron alcove. The new nTOF is on the roof to provide a view of neutrons almost directly opposite from the south pole detector, enabling researchers to determine if the neutron source is moving coherently.

    “The detectors in the nTOF measure the time of flight of the thermonuclear neutrons from the implosion to the detector, and so their velocity,” said Joe Kilkenny, chief NIF experimentalist for measurements. “The north pole nTOF is close to opposite to NIF’s south pole nTOF. Importantly, this allows the Doppler shift of the neutrons due to motion of the compressed plasma — like the pitch of a train’s whistle changing as it approaches you and moves away from you—to be measured from the difference in the arrival times at the north and the south pole nTOFs.”

    As neutrons produced in the target chamber pass through the bibenzyl crystal housed in the North Pole nTOF enclosure, they induce scintillation light. The crystal’s scintillation light is collected by a fast photo-detector which in turn produces a voltage in proportion to the number of passing neutrons. This voltage signal is then digitized every 100 picoseconds (a tenth of a billionth of a second), producing a high-fidelity record of the neutron flux passing through the detector.

    The data collected by the devices will help researchers determine the shot’s yield, as well as interesting properties of the plasma producing the neutrons, including its temperature and velocity. Understanding those properties motivated constructing the north pole system opposite the existing south pole system.

    Construction crews began work on the enclosure last fall, cutting a 40-foot by 20-foot hole in the top of the NIF building down to the inner concrete ceiling of the facility. The precise alignment and the tricky angle of the three-inch-diameter line-of-sight flight path from target chamber center (TCC), projecting through a small hole in the floor of the structure, required an impressive feat of engineering. It also presented challenges from wind and seismic load, facility modification, and temperature control, Jackson said. About 30 LLNL employees worked on the project over the last year, about half of them from the Lab’s engineering staff.

    Behind the scintillators in the nTOF enclosure, preventing the neutrons from traveling further upward and outside the enclosure, is a three-foot thick concrete and steel block. The block is capable of fully stopping neutrons and photons, ensuring that any emissions are within the facility’s safety limits and reducing the risk of outside radiation exposure to a negligible level.

    The line-of-sight path is defined by a set of precision collimators that allow researchers to look at only the neutrons coming from the area around TCC.

    To allow for more efficient operation, and to minimize the time NIF employees will have to be on the roof, the detector can be operated by remote control.

    Not only is the newest detector a technological improvement over the others, said Perry Bell, NIF Co-Target Diagnostics manager, but the line of sight it affords is the best yet for researchers because there is limited mass around it to interfere with the measurement.

    NIF has more than 70 diagnostics in all, and Bell said two more diagnostics similar to the North Pole nTOF will be built to extend the facility’s diagnostic reach even further.

    “These detectors will provide additional insight into fusion ignition and will represent key diagnostics for the NIF program,” Bell added.

    The U.S. Department of Energy funded the construction and design of the diagnostic.

    See the full article here .

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  • richardmitnick 11:20 am on May 2, 2016 Permalink | Reply
    Tags: , , , , NIF   

    From COSMOS: “Getting primed for fusion power” 

    Cosmos Magazine bloc

    COSMOS

    26 Apr 2016
    Cathal O’Connell

    Cracking fusion power would be one of the great technological achievements of the 21st century, providing almost limitless power with few drawbacks. With global efforts getting bigger and badder every year, Cathal O’Connell provides a primer to the basic technology.

    ITER Tokamak
    ITER Tokamak

    Fusion power is such a huge, potentially game-changing technology that it’s easy to get swept up in its utopian promise. Equally, it’s easy to dismiss the whole shebang as a wild fantasy that will never come to pass.

    Here’s what you need to know to help keep pace with developments in this global quest.

    What is nuclear fusion?

    Atoms are the really small bits from which we are all made. Inside each atom, when you strip away the shells of electrons, is an even smaller bit at the core – the nucleus.

    It turns out that when you join two small nuclei to make a bigger one, an enormous amount of energy is released – about 10 million times more energy than the puny chemical reactions that power most of our technology, such as burning oil, coal or the gasoline in your car.

    We know fusion works because it goes on in the core of the Sun. All the Sun’s heat and light are powered by fusion. The most important reaction is the fusion of two nuclei of hydrogen, which is the lightest atom, combine to form helium, the second lightest.

    How does it work?

    Igniting nuclear fusion is not as simple as starting a fire. It takes a lot of energy to get it going and typically, that means a temperature of millions of degrees Celsius.

    This is because atomic nuclei have a love-hate relationship. Each nucleus has a strong positive charge so they repel one another. To kickstart fusion, you have to overcome this repulsive barrier by ramming two nuclei together incredibly hard. That’s what happens in the core of the Sun, where the temperature is about 15 million °C and pressures are similarly insane.

    When the nuclei get close enough to touch, the nuclear strong force takes over – the strongest force in nature – and it’s the source of fusion energy.

    Fusion energy?

    The idea of fusion energy is to build power plants that generate energy by recreating the core of the Sun. Hundreds of research scale reactors have been built around the world.

    They are usually engineering marvels designed to containment hydrogen nuclei at a 100 million °C, or implode a nuclear pellet using massive lasers (see below).
    But I thought we already had nuclear power

    The nuclear power plants we have so far are based on a different process – nuclear fission, where you derive energy by splitting one big atom into two smaller atoms. That’s a much easier process and so fission reactors have been pumping power into the grid since the 1950s.

    Fusion reactors are much safer than traditional fission reactors because there is no chance of a runaway explosion and when the reaction is done, there’s no long-lived radioactive waste.

    Per kilogram of fuel, fusion releases four times more energy than fission and 10 million times more than coal.

    What’s the fuel?

    The first generations of fusion reactors will likely use two forms of hydrogen for the fuel – deuterium and tritium – because the fusion of these two nuclei is the easiest to achieve.

    Regular hydrogen is the smallest atom – just one electron orbiting a proton nucleus. Deuterium is a fatter version of hydrogen, where the nucleus contains a neutron as well as a proton. And tritium is the fattest hydrogen of all – its nucleus contains a proton and two neutrons.

    Deuterium is easily found in seawater, while tritium can be generated from lithium.

    The long-term goal is to switch to a deuterium-deuterium reaction, meaning all the world’s energy supply could one day be found in seawater.

    Has fusion ever been achieved?

    Humans first managed nuclear fusion on 1 November 1952 when the US exploded the first fusion bomb. Fusion bombs (also known as hydrogen bombs) are the most destructive weapons ever made. They typically use a fission-based atomic bomb to trigger a fusion reaction in the second stage.

    The challenge now is achieving fusion in a controlled manner.

    For more than 70 years researchers have been trialling different designs for containing the fusion reaction. Some of these designs (see below) have achieved fusion. The problem is releasing more energy than is put in, and doing it long enough to be useful. Nobody’s been able to do that yet.
    What’s been holding us back?

    Ah. The problem is the temperature. You have to heat the fuel to such a high temperature (100 million °C or so) that no material vessel could possibly contain it.

    The basic physics behind fusion has been known for decades. It’s the engineering that still needs to be worked out.

    What do fusion reactors look like?

    Fusion reactors come in all sorts of shapes and sizes.

    Most research has looked at containing the reaction within a sort of magnetic bottle. At the extreme temperatures of fusion, all of the electrons are stripped off the deuterium and tritium atoms, and what’s left is called a plasma.

    1
    How a tokamak, or toroidal magnetic confinement system, works.Credit: Encyclopaedia Britannica/UIG Via Getty Images

    The most common design is the toroidal chamber-magnetic (tokamak), which looks a bit like the inside of the Deathstar. Tokamaks form twisting donut-shapes called a torus. The plasma runs in rings and never touches the walls of the torus because it’s contained by the magnetic fields.

    Other variations confined the plasma in different geometries, such as the stellerator design which adds a twist with a different configuration of magnets. Other tokamak designs are spherical.
    But if fuel is squeezed in a magnetic field, how do you get the energy out?

    When deuterium fuses with tritium, an extra neutron is kicked out and receives a huge kick of energy. The neutron is a neutral particle, and so is not affected by the magnetic field – it can fly through the magnetic bottle and smash into a lithium blanket just inside the donut.

    The neutron collisions heat up the lithium. This heat is used to convert water into steam which drives turbines to generate electricity, just like in any other electric power-plant.
    What’s this about using lasers?

    Instead of using a magnetic field to contain a plasma, another idea is to ignite small fusion explosions by firing a powerful laser at a pellet.

    LLNL NIF
    LLNL/NIF

    At the National Ignition Facility at Lawrence Livermore National Laboratory in California, the world’s biggest laser (made of 192 laser beams) are fired simultaneously at a pellet of deuterium/tritium about the size of a pea.

    National Ignition Facility researchers have achieved fusion using this design, but the challenge is extracting more energy than is used to power the lasers. Their biggest problem is in constructing the pellet and its plastic container in the shape to absorb all the laser energy.

    And cold fusion?

    This is the idea to make a fusion reactor that works at close to room temperature. In 1989, British and American scientists seemed to achieve this a running a strong current through a platinum electrode in a thermos of heavy water (water where the hydrogen atoms are partially or completely replaced by deuterium) – but the experiment turned out to be flawed.

    Nowadays research into cold fusion is seen as an example of “pathological science”, like trying to build a perpetual motion machine.

    Best forget about this altogether. It’s not going to happen.

    What’s next for fusion?

    Despite the difficulties, progress in fusion power has actually been very rapid. Power output has increased by a factor of more than a million in 30 years.

    Much of the hope is centred on the ITER (Latin for “the way”) tokamak to be constructed in southern France by 2019.

    3
    ITER, the International Thermonuclear Experimental Reactor, is being designed to test the principles surrounding the generation of power from nuclear fusion, the energy source of stars. It comprises a toroidal chamber in which a plasma (pink) is contained by strong magnetic fields.Credit: MIKKEL JUUL JENSEN / SCIENCE PHOTO LIBRARY/Getty Images

    This design has been in the pipeline for two decades and is designed to be the first fusion reactor to produce more energy (about 10 times more) than it puts in. However, this 500-megawatt reactor is still only a proof-of-concept design, and no electricity will be generated.

    If ITER is successful, the next step is DEMO – which is designed to be the world’s first nuclear power plant to generate electricity, to be constructed by 2033.

    See the full article here .

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  • richardmitnick 3:05 pm on February 2, 2016 Permalink | Reply
    Tags: "Shock/shear” platform, , , , NIF   

    From LLNL: “NIF experiments shed light on turbulent mix” 


    Lawrence Livermore National Laboratory

    NIF Bloc
    LLNL NIF
    NIF

    LLNL NIF target on the National Ignition Facility (NIF) target positioner
    Cryogenics operator John Cagle mounts a target on the National Ignition Facility (NIF) target positioner for an experiment. An area backlighter disc is seen on-edge on the right of the assembly. The front of the target is covered with a gold shield with a diagnostic slit.

    Scientists from Los Alamos National Laboratory (LANL) are leading an experimental campaign on the National Ignition Facility (NIF) designed to further understand turbulent mix models used in both high energy density (HED) and inertial confinement fusion (ICF) experiments. NIF is the only facility with the energy and shot-to-shot reproducibility needed for the experiments.

    During shots using what’s known as the “shock/shear” platform, NIF fires 300 kilojoules of laser energy at each end of a target comprised of two half-hohlraums to produce shock waves from opposite ends of a foam-filled shock tube. These waves turn the foam into plasma and allow the shocks to travel and create a counter-propagating shear mixing effect across a metal foil.

    The target has evolved over time — different experiments have used titanium, copper, aluminum and roughened aluminum, and more materials are to come — but they all have one thing in common: each experiment enhances understanding of turbulent mix models in the HED regime. These models, developed and calibrated by LANL using hydrodynamic test data from the 1980s through the present, are now being examined through the lens of the shock/shear HED experiments to see how the data matches up to more extreme conditions.

    “We have created a system that reproduces instability features similar to those of traditional hydro experiments that have not previously been seen in HED experiments,” said LANL scientist Kirk Flippo, the lead experimental investigator. “This kind of experiment is rapidly evolving our understanding and we’ve discovered a lot of behaviors that we didn’t expect.”

    This enhanced understanding and refined data is vital for ICF. According to Flippo, it has become increasingly clear that ICF capsules experience some kind of mix as they are imploding.

    “Some of the outstanding issues in ICF are how does the capsule mix, how does this play into the degradation of the yield and how does it affect ignition,” he said. “It’s important for us to make sure that when we run a code to model an ICF implosion, we get all of the details correct. These experiments will help us quantify precisely how much of an effect this type of shear mixing has.”

    Shock/shear experiments initially were fielded on the OMEGA Laser at the University of Rochester’s Laboratory for Laser Energetics, but due to the limited volume that could be driven, the experiments experienced boundary effects. The LANL project manager, scientist John Kline, believed the platform was mature enough to be deployed on NIF and pushed hard for its implementation. Kline knew that by scaling the experiments up to NIF energies, the researchers would be able to take advantage of larger volumes to eliminate the edge effects and do the experiments they wanted to do.

    “We cannot do experiments in this way anywhere but at NIF,” Flippo said. “In the regimes that we are in at NIF, the experiment behaves much more like a traditional hydro experiment and scales like a hydro experiment would scale.”

    Data from the NIF experiments already has been used by the campaign’s principal investigator, LANL scientist Forrest Doss, to refine the way the model is implemented in the code — producing a direct, immediate impact. But the work isn’t complete just yet.

    “Now that this platform is available, and has been shown to produce really nice data, we can start modifying it by changing the shock velocities, changing the materials or foams and using different shocks,” Flippo said. “This platform has infinite variation and infinite complexity.”

    See the full article here .

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  • richardmitnick 1:53 pm on January 26, 2016 Permalink | Reply
    Tags: , , , NIF, ,   

    From PPPL: “PPPL team wins 80 million processor hours on nation’s fastest supercomputer” 


    PPPL

    January 26, 2016
    John Greenwald

    The U.S Department of Energy (DOE) has awarded a total of 80 million processor hours on the fastest supercomputer in the nation to an astrophysical project based at the DOE’s Princeton Plasma Physics Laboratory (PPPL). The grants will enable researchers led by Amitava Bhattacharjee, head of the Theory Department at PPPL, and physicist Will Fox to study the dynamics of magnetic fields in the high-energy density plasmas that lasers create. Such plasmas can closely approximate those that occur in some astrophysical objects.

    The awards consist of 35 million hours from the INCITE (Innovative and Novel Impact on Computational Theory and Experiment) program, and 45 million hours from the ALCC, (ASCR — Advanced Scientific Computing Research — Leadership Computing Challenge.) Both will be carried out on the Titan Cray XK7 supercomputer at Oak Ridge National Laboratory. This work is supported by the DOE Office of Science.

    ORNL Titan Supercomputer
    Titan Cray XK7 supercomputer

    The combined research will shed light on large-scale magnetic behavior in space and will help design three days of experiments in 2016 and 2017 on the world’s most powerful high-intensity lasers at the National Ignition Facility (NIF) at the DOE’s Lawrence Livermore National Laboratory.

    Livermore NIF Banner
    Livermore NIF

    “This will enable us to do experiments in a regime not yet accessible with any other laboratory plasma device,” Bhattacharjee said.

    The supercomputer modeling, which is already under way, will investigate puzzles including:

    Magnetic field formation. The research will study Weibel instabilities, the process by which non-magnetic plasmas merge in space to produce magnetic fields. Understanding this phenomena, which takes place throughout the universe but has proven difficult to observe, can provide insight into the creation of magnetic fields in stars and galaxies.

    Magnetic field growth. Another mystery is how small-scale fields can evolve into large ones. The team will model a process called the Biermann battery, which amplifies the small fields through an unknown mechanism, and will attempt to decipher it.

    Explosive magnetic reconnection. The simulations will study still another process called plasmoid instabilities that have been widely theorized. These instabilities are believed to play an important role in producing super high-energy plasma particles when magnetic field lines that have separated violently reconnect.

    The NIF experiments will test these models and build upon the team’s work at the Laboratory for Laser Energetics at the University of Rochester. Researchers there have used high-intensity lasers at the university’s OMEGA EP facility to produce high-energy density plasmas and their magnetic fields.

    At NIF, the lasers will have 100 times the power of the Rochester facility and will produce plasmas that more closely match those that occur in space. The PPPL experiments will therefore focus on how reconnection proceeds in such large regimes.

    Joining Bhattacharjee and Fox on the INCITE award will be astrophysicists Kai Germaschewksi of the University of New Hampshire and Yi-Min Huang of PPPL. The same team is conducting the ALCC research with the addition of Jonathan Ng of Princeton University. Researchers on the NIF experiments, for which Fox is principal investigator, will include Bhattacharjee and collaborators from PPPL, Princeton, the universities of Rochester, Michigan and Colorado-Boulder, and NIF and the Lawrence Livermore National Laboratory.

    See the full article here .

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    Princeton Plasma Physics Laboratory is a U.S. Department of Energy national laboratory managed by Princeton University. PPPL, on Princeton University’s Forrestal Campus in Plainsboro, N.J., is devoted to creating new knowledge about the physics of plasmas — ultra-hot, charged gases — and to developing practical solutions for the creation of fusion energy. Results of PPPL research have ranged from a portable nuclear materials detector for anti-terrorist use to universally employed computer codes for analyzing and predicting the outcome of fusion experiments. The Laboratory is managed by the University for the U.S. Department of Energy’s Office of Science, which is the largest single supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

     
  • richardmitnick 7:25 pm on August 18, 2015 Permalink | Reply
    Tags: , , , NIF   

    From LLNL: “National Ignition Facility fires 300th shot in FY15” 


    Lawrence Livermore National Laboratory

    NIF Bloc

    Aug. 18, 2015
    Breanna Bishop
    bishop33@llnl.gov
    925-423-9802

    1
    NIF’s target chamber is where the magic happens – temperatures of 100 million degrees and pressures extreme enough to compress the target to densities up to 100 times the density of lead are created there. Photo by Damien Jemison/LLNL

    Last week, the National Ignition Facility (NIF) fired its 300th laser target shot in fiscal year (FY) 2015, meeting the year’s goal more than six weeks early. In comparison, the facility completed 191 target shots in FY 2014. Located at Lawrence Livermore National Laboratory (LLNL), the NIF is the world’s most energetic laser.

    Increasing the shot rate has been a top priority for the Inertial Confinement Fusion (ICF) Program and in particular the NIF team at LLNL. The greater than 50 percent increase in NIF shots from FY 2014 to FY 2015 is a direct result of the implementation of an efficiency study conducted in FY 2014 for the NIF.

    NIF is funded by the National Nuclear Security Administration (NNSA), the agency charged with ensuring the nation’s nuclear security. The chief mission of NIF is to provide experimental insight and data for NNSA’s science-based Stockpile Stewardship Program in the area of high-energy-density physics, a scientific field of direct relevance to nuclear deterrence and national nuclear security.

    “Demand for experiments at NIF have always exceeded capacity. The impressive work by the team at NIF to produce additional shots has provided important new opportunities for NIF users and increased this unique scientific platform’s contributions to national security,” said Brig. Gen Stephen Davis, USAF, acting deputy administrator for Defense Programs. “I congratulate the NIF team and its many partners for not only meeting, but exceeding the goal.”

    2
    The NIF Control Room preparing for the 300th shot. From left: Shot Director Dean Latray, Operations Manager Bruno Van Wonterghem and Lead Operator Rod Rinnert. Photo by Jason Laurea/LLNL.

    “Achieving 300 shots this year enabled so many critical accomplishments: first-of-a-kind dynamic materials data, more efficiently driven ICF capsules, increased opportunities for academic users, new radiation sources for the Department of Defense and acceleration of new diagnostic development,” said Keith LeChien, director of ICF for NNSA.

    “This is a remarkable achievement for team NIF, whose incredible effort and persistence turned this huge challenge into a reality,” said LLNL Director Bill Goldstein. “Without the support of NNSA and our many partners, this would not have been possible.”

    This 120-day efficiency study was developed in partnership with other NNSA laboratories and drew on best practices at the Z Facility at Sandia National Laboratories and the Omega Laser at the University of Rochester. This study identified more than 80 improvements to equipment and procedures that could lead to reduced time and effort for fielding experiments.

    To date, the NIF team has implemented more than 50 of these improvements and will continue implementing the remainder of the improvements in FY 2016. Improvements include equipment modifications to reduce the time needed to perform critical tasks. Some of the most significant were control system improvements to streamline the shot cycle; process improvements to reduce time needed to align targets and diagnostics; and user interface improvements to make it easier for users to set up and execute experiments.

    See the full article here.

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  • richardmitnick 11:56 am on June 10, 2015 Permalink | Reply
    Tags: , , NIF   

    From LLNL- “Dante: Measuring NIF’s inferno” 


    Lawrence Livermore National Laboratory

    Jun. 9, 2015

    Breanna Bishop
    bishop33@llnl.gov (link sends e-mail)
    925-423-9802

    1
    Target area operator Sky Marshall installs new apertures in the Dante 2 X-ray diodes. Photo by James Pryatel/LLNL

    The smooth blue sphere of the National Ignition Facility’s (NIF) target chamber bristles with diagnostics — nuclear, optical and X-ray instruments that together provide some 300 channels for experimental data. These diagnostics provide vital information to help NIF scientists understand how well an experiment performed.

    LLNL NIF
    NIF

    Two of these diagnostics, known as Dante 1 and Dante 2, are pressed into service for nearly every shot. These broadband, time-resolved X-ray spectrometers measure the time-dependent soft X-ray power produced by the NIF lasers interacting with the hohlraum — the small gold cylinder that holds the NIF target capsule. The X-rays heat and ablate the outer surface of the capsule and drive the capsule’s rocket-like implosion. The resulting data are used to calculate the radiation spectrum and infer the temperature of the radiation field inside the hohlraum. This information can be directly compared to hohlraum simulations to determine if the hohlraum and laser pulse are performing as designed.

    “Dante is one of the workhorse diagnostics of NIF — it participates in almost every shot,” said Alastair Moore, responsible scientist for Dante. “Even when a hohlraum is not used, it is one of the few absolutely calibrated soft X-ray diagnostics that can provide absolute measurements of the conversion efficiency of laser light into X- rays.”

    Each Dante diagnostic measures the voltage produced by 18 filtered X-ray diodes. Each diode is filtered to record the X-ray power in a specific part of the spectrum. Spectral ranges are controlled by filters, metallic mirrors and X-ray diode material. Dante 1 has five channels with mirrors, and Dante 2 has eight mirrored channels.

    Because Dante is an absolutely calibrated system, every component must be calibrated and tracked, making it one of the more challenging diagnostics to maintain and operate. According to Moore, the calibration of each component typically involves approximately 100 measurements.

    “On the two Dante systems, we maintain approximately 10 different filter configurations, each of which contains about 45 calibrated filters,” he said. “The filters are pretty fragile components and debris from shots can damage them over time, requiring a continual replenishing of the stock of components.”

    In addition to the filters, approximately 50 X-ray diodes are calibrated on a cyclical basis and maintained along with about 20 grazing incidence X-ray mirrors. This adds up to an inventory of thousands of components that must be tracked, maintained and verified.

    1
    Target area operator Mike Morris inspects a Dante 1 filter wheel to ensure the delicate filters are intact before a shot. Photo by James Pryatel/LLNL

    The Dante team recently transitioned to a new way of operating the diagnostic filter configurations, introducing standardized sets. This adaptation significantly reduced the overhead costs associated with building filter configurations and also reduced the error bars on the measurements, making better shot-to-shot comparisons possible. The team also is in the process of replacing the 18 oscilloscopes used to record the X-ray diode signals and automate the setup, reducing manual interactions required for each shot.

    Looking to the future, the team is exploring a modification to some of the channels that measure the part of the spectrum containing M-band radiation from the hohlraum, an important measurement for inertial confinement fusion.

    “This radiation can preheat the capsule significantly, resulting in an increase in instability growth,” Moore said. “The upgrade will add multi-layer X-ray mirrors to these channels to provide a better constrained X-ray bandpass and a more accurate measurement of the power in this region.”

    The forerunner of the Dante diagnostic originated in the era of underground nuclear weapons testing, where it was developed for the same purpose as it is used today — to measure the absolute X-ray power produced. A multi-institutional team with members from Lawrence Livermore, Los Alamos and Sandia national laboratories, the UK’s Atomic Weapons Establishment, National Securities Technologies and General Atomics contributed to adapting the diagnostic to its current use.

    See the full article here.

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  • richardmitnick 5:25 pm on December 3, 2014 Permalink | Reply
    Tags: , , NIF,   

    From NIF at LLNL: “Measuring NIF’s enormous shocks” 


    Lawrence Livermore National Laboratory

    Nov. 21, 2014

    Breanna Bishop
    bishop33@llnl.gov
    925-423-9802

    LLNL NIF Banner

    LLNL NIF

    NIF experiments generate enormous pressures—many millions of atmospheres—in a short time: just a few billionths of a second. When a pressure source of this type is applied to any material, the pressure wave in the material will quickly evolve into a shock front. One of NIF’s most versatile and frequently-used diagnostics, the Velocity Interferometer System for Any Reflector (VISAR), is used to measure these shocks, providing vital information for future experiment design and calibration.

    LLNL NIF VISAR
    Target Diagnostics Operator Andrew Wong sets up the Velocity Interferometer System for Any Reflector (VISAR) optical diagnostic system for a shock timing shot.

    Invented in the 1970s, VISAR was developed by Sandia National Laboratory scientists to study the motion of samples driven by shocks and other kinds of dynamic pressure loading. It has since become a standard measurement tool in many areas where dynamic pressure loading is applied to materials.

    “It is a big challenge to figure out how to apply these enormous pressures without immediately forming a shock wave,’ said Peter Celliers, responsible scientist for VISAR. “Instead of trying to avoid forming shocks, many NIF experiments use a sequence of increasing shocks as a convenient way of monitoring the performance of the target as the pressure drive is increased—for example, during a (target) capsule implosion.”

    v
    Target Area Operator Mike Visaya aligns a VISAR transport mirror in preparation for an experiment.

    To measure these shocks, VISAR determines the speed of a moving object by measuring the Doppler shift in a reflected light beam. More specifically, it directs a temporally-coherent laser beam at the object, collects a returned reflection, and sends it through a specially-configured interferometer. The interferometer produces an interference pattern containing information about the Doppler shift.

    The Doppler shift provides information on how fast the reflecting part of the target is moving. In most cases the reflector is a shock front, which acts like a mirror moving through a transparent material (for example liquid deuterium, quartz or fused silica). In some cases the moving mirror is a physical surface on the back part of a package (called a free surface) that is accelerated during the experiment. In yet other scenarios, the moving mirror could be a reflecting interface embedded in the target behind a transparent window.

    After the light reflected from the target passes through the interferometers, it forms a fringe pattern. With the NIF VISAR design, this light is collected in the form of a two-dimensional image with an optical image relay system. The fringe pattern is superimposed on the image, then projected on the slit of a streak camera. Because the target image is spatially-resolved across the slit of the streak camera, this type of VISAR is called a line-imaging VISAR. The spatial and temporal arrangement of the fringe pattern in the output streak record reveals how different parts of the target move during the experiment.

    There is a very close connection between the velocities of the moving parts of the target and the pressure driving the motion. If the velocity is measured accurately, a highly accurate picture of the driving pressure can be formed. This information is vital for understanding the details of target performance.

    “Our simulation models are not accurate enough to calculate the timing of the shocks that produces the best performance without some sort of calibration,” Celliers said. “But by monitoring the shocks with the VISAR, we have precise and detailed information that can be used to tune the laser pulse (the pressure drive) to achieve optimal target performance, and to calibrate the simulation codes.”

    Looking to the future, VISAR will see improvements to its streaked optical pyrometer (SOP), an instrument that can be used to infer the temperature of a hot object by measuring the heat radiated from the object in the form of visible light. The SOP is undergoing modifications to improve its imaging performance and to reduce background levels on the streak camera detectors. This will benefit future equation-of-state experiments where accurate thermal emission data is crucial. This upgrade will be complete in early 2015.

    men
    Physicists Dave Farley (left) and Peter Celliers and scientist Curtis Walter watch a live VISAR image as they monitor the deuterium fill of a keyhole capsule in the NIF Control Room during shock-timing experiments.

    Along with Celliers, the VISAR implementation team includes Stephen Azevedo, David Barker, Jeff Baron, Mark Bowers, Aaron Busby, Allen Casey, John Celeste, Hema Chandrasekaran, Kim Christensen, Philip Datte, Jon Eggert, Gene Frieders, Brad Golick, Robin Hibbard, Matthew Hutton, John Jackson , Dan Kalantar, Kenn Knittel, Kerry Krauter, Brandi Lechleiter, Tony Lee, Brendan Lyon, Brian MacGowan, Stacie Manuel, JoAnn Matone, Marius Millot, Jason Neumann, Ed Ng, Brian Pepmeier, Karl Pletcher, Lynn Seppala, Ray Smith, Zack Sober, Doug Speck, Bill Thompson, Gene Vergel de Dios, Abbie Warrick, Phil Watts, Eric Wen, Ziad Zeid and colleagues from National Security Technologies.

    See the full article here.

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