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  • richardmitnick 2:01 pm on January 3, 2019 Permalink | Reply
    Tags: "Nuno Loureiro: Understanding turbulence in plasmas", , , , Plasma turbulence,   

    From MIT News: “Nuno Loureiro: Understanding turbulence in plasmas” 

    MIT News
    MIT Widget

    From MIT News

    January 3, 2019
    Peter Dunn

    1
    “When we stimulate theoretically inclined minds by framing plasma physics and fusion challenges as beautiful theoretical physics problems, we bring into the game incredibly brilliant students,” says associate professor of nuclear science and engineering Nuno Loureiro. Photo: Gretchen Ertl

    Theoretical physicist’s focus on the complexity of plasma turbulence could pay dividends in fusion energy.

    Difficult problems with big payoffs are the life blood of MIT, so it’s appropriate that plasma turbulence has been an important focus for theoretical physicist Nuno Loureiro in his two years at the Institute, first as a assistant professor and now as an associate professor of nuclear science and engineering.

    New turbulence-related publications by Loureiro’s research group are contributing to the quest to develop nuclear fusion as a practical energy source, and to emerging astrophysical research that delves into the fundamental mechanisms of the universe.

    Turbulence is around us every day, when smoke rises through air, or milk is poured into coffee. While engineers can draw on substantial empirical knowledge of how it behaves, turbulence’s fundamental principles remain a mystery. Decades ago, Nobel laureate Richard Feynman ’39 referred to it as “the most important unsolved problem of classical physics” — and that still holds true today.

    But turbulence in air or coffee is a simple proposition compared to turbulence in plasma. Ordinary gases and liquids can be modeled as neutral fluids, but plasmas are electromagnetic media. Their turbulent behavior involves both the particles in the plasma (typically electrons and ions, but also electrons and positrons in so-called pair plasmas) and pervading electrical and magnetic fields. In addition, plasmas are often rarefied media where collisions are rare, creating an even more intricate dynamic.

    “There are several additional layers of complexity [in plasma turbulence] over neutral fluid turbulence,” Loureiro says.

    This lack of first-principles understanding is hindering the adaption of fusion for generating electricity. Tokamak-style fusion devices, like the Alcator C-Mod developed at MIT’s Plasma Science and Fusion Center (PSFC), where Loureiro’s research group is based, are a promising approach, and recent the spinout company Commonwealth Fusion Systems (CFS) is working to commercialize the concept. But fusion devices have yet to achieve net energy gain, in large part because of turbulence.

    Alcator C-Mod tokamak at MIT, no longer in operation

    Loureiro and his student Rogério Jorge, with co-author Professor Paolo Ricci from the École Polytechnique Fédérale de Lausanne, Switzerland, recently helped advance thinking in this area in a new paper, “Theory of the Drift-Wave Instability at Arbitrary Collisionality,” published in the journal Physical Review Letters.

    “This was amazing work by a fantastic student — a very complicated calculation that represents a qualitative advancement to the field,” Loureiro says.

    He explains that turbulence in tokamaks changes “flavor” depending on “where you are — at the periphery or near the core.”

    “Both are important, but periphery turbulence has important engineering implications because it determines how much heat reaches the plasma-facing components of the device,” Loureiro says. Preventing heat damage to materials, and maximizing operational life, are key priorities for tokamak developers.

    The paper offers a novel and more-robust description of turbulence in the tokamak periphery caused by low-frequency drift waves, which are a key source of that turbulence and regulators of plasma transport across magnetic fields. And because the computational framework is especially efficient, the approach can be easily extended to other applications. “I think it’s going to be an important piece of work for the fusion concepts that PSFC and CFS are trying to develop,” he says.

    A separate paper, “Turbulence in Magnetized Pair Plasmas,” which Loureiro co-authored with Professor Stanislav Boldyrev of the University of Wisconsin at Madison, puts forward the first theory of turbulence in pair plasmas. The work, published in The Astrophysical Journal Letters, was driven in part by last year’s unprecedented observations of a binary neutron star merger and other discoveries in astrophysics that suggest pair plasmas may be abundant in space — though none has been successfully created on Earth.

    “A variety of astrophysical environments are probably pair-plasma dominated, and turbulent,” notes Loureiro. “Pair plasmas are quite different from regular plasmas. In a normal electron-ion plasma, the ion is about 2,000 times heavier than the electron. But electrons and positrons have exactly the same mass, so there’s a whole range of behaviors that aren’t possible in a normal plasma and vice-versa.”

    Because computational calculations involving equal-weight particles are much more efficient, researchers often run pair-plasma numerical simulations and try to extrapolate findings to electron-ion plasmas.

    “But if you don’t understand how they’re the same or different from a theoretical point of view, it’s very hard to make that connection,” Loureiro points out. “By providing that theory we can help tell which characteristics are intrinsic to pair plasmas and which are shared. Looking at the building blocks may impact electron-ion plasma research too.”

    This theme of theoretical integration characterizes much of Loureiro’s work, and led to his being invited to present at a recent interdisciplinary event for plasma physicists and astrophysicists at New York City’s Flatiron Institute Center for Computational Astrophysics, an arm of a foundation created by billionaire James Simons ’58. It is also central to his role as a theorist within the MIT NSE ecosystem, especially on extremely complex challenges like fusion development.

    “There are people who are driven by technology and engineering, and others who are driven by fundamental mathematics and physics. We need both,” he explains. “When we stimulate theoretically inclined minds by framing plasma physics and fusion challenges as beautiful theoretical physics problems, we bring into the game incredibly brilliant students, people who we want to attract to fusion development but who wouldn’t have an engineer’s excitement about new advances in technology.

    “And they will stay on because they see not just the applicability of fusion but also the intellectual challenge,” he says. “That’s key.”

    See the full article here .


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  • richardmitnick 9:14 pm on November 5, 2018 Permalink | Reply
    Tags: , , , , Plasma turbulence, , Space is permeated by magnetic fields with a wide range of strengths, Turbulence in space might solve outstanding astrophysical mystery   

    From PPPL: “Turbulence in space might solve outstanding astrophysical mystery” 


    From PPPL

    November 5, 2018
    Raphael Rosen

    1
    PPPL graduate student Denis St-Onge. (Photo by Elle Starkman)

    Contrary to what many people believe, outer space is not empty. In addition to an electrically charged soup of ions and electrons known as plasma, space is permeated by magnetic fields with a wide range of strengths. Astrophysicists have long wondered how those fields are produced, sustained, and magnified. Now, scientists at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have shown that plasma turbulence might be responsible, providing a possible answer to what has been called one of the most important unsolved problems in plasma astrophysics.

    The researchers used powerful computers at the Princeton Institute for Computational Science and Engineering (PICSciE) and the National Energy Research Scientific Computing Center (NERSC) at the DOE’s Lawrence Berkeley National Laboratory to simulate how the turbulence could intensify magnetic fields through what is known as the dynamo effect, in which the magnetic fields become stronger as the magnetic field lines twist and turn.

    Tiger Dell Linux supercomputer at Princeton University

    NERSC

    NERSC Cray Cori II supercomputer at NERSC at LBNL, named after Gerty Cori, the first American woman to win a Nobel Prize in science


    LBL NERSC Cray XC30 Edison supercomputer


    The Genepool system is a cluster dedicated to the DOE Joint Genome Institute’s computing needs. Denovo is a smaller test system for Genepool that is primarily used by NERSC staff to test new system configurations and software.

    NERSC PDSF


    PDSF is a networked distributed computing cluster designed primarily to meet the detector simulation and data analysis requirements of physics, astrophysics and nuclear science collaborations.

    “This work constitutes an important step toward answering for the first time the question of whether turbulence can amplify magnetic fields to dynamical strengths in a hot, dilute plasma, such as that residing within clusters of galaxies,” said Matthew Kunz, an astrophysics professor at Princeton University and an author of the paper, which was published in The Astrophysical Journal Letters.

    Past research has focused on dynamos as they might occur in so-called collisional plasmas, in which particles collectively behave as a fluid. But intergalactic plasmas are collisionless, so past experiments are not necessarily relevant. This new research is meant to address that gap. “We wanted to see how the dynamo would behave in the collisionless regime,” said Denis St-Onge, graduate student in the Princeton Program in Plasma Physics at PPPL and lead author of the paper.

    St-Onge and Kunz focused on the ways in which the velocities and magnetic fields of individual particles within collisionless plasma are directly linked. This linkage — if one quantity increases or decreases, the other must, too — would seem to rule out the existence of a dynamo. “If this were the whole story, it would be disastrous for the dynamo,” said St-Onge. “To match what we observe in space, the dynamo would have to increase the strength of the seed magnetic field by at least a factor of one trillion, but the energy of the particles would also have to increase, and there’s just not enough available energy in the dynamo for that to happen.”

    To produce the strength of magnetic fields observed in space, the tie that binds particle energy to magnetism must be severed. This is just what St-Onge and Kunz observed in the computer simulations: that types of plasma turbulence known as mirror and firehose instabilities caused the plasma particles to scatter, and scattering broke the link between particle energy and magnetism and allowed the amplitudes of the magnetic fields to grow closer to what is observed in nature.

    Future research, St-Onge notes, will focus on why this turbulent scattering occurs. “In addition, we would like to investigate the specifics of particle scattering,” St-Onge said. “How exactly do the instabilities cause the particles to scatter, how often does the scattering occur, and can the scattering lead to sudden, dramatic growth of a magnetic field? The last idea is a notion proposed by PPPL Director Steven Cowley years ago. We would like to investigate whether this is true.”

    See the full article here .


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    PPPL campus

    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.

     
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