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Antikainen A.,University of Rochester | Arteaga-Sierra F.R.,University of Rochester | Agrawal G.P.,University of Rochester | Agrawal G.P.,Laboratory for Laser Energetics
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2017

We show that temporal reflections off a moving refractive index barrier play a major role in the spectral broadening of a dual-wavelength input inside a highly nonlinear, dispersion-decreasing fiber. We also find that a recently developed linear theory of temporal reflections works well in predicting the reflected frequencies. Successive temporal reflections from multiple closely spaced solitons create a blueshifted spectral band, while continuous narrowing of solitons inside the dispersion-decreasing fiber enhances Raman-induced redshifts, leading to supercontinuum generation at relatively low pump powers. We also show how dispersive wave emission can be considered a special case of the more general process of temporal reflections. Hence our findings have implications on all systems able to support solitons. © 2017 American Physical Society.


This mosaic image of the Crab Nebula was taken by NASA's Hubble Space Telescope. Features of this nebula and other astrophysical phenomena are being studied at MIT's Plasma Science and Fusion Center. Credit: NASA / ESA / J. Hester / Arizona State University Senior research scientist Chikang Li wants to experiment with the stars. Intrigued by a curious "kink" phenomenon observed in the Crab Nebula, an interstellar cloud of gas and dust that formed in the wake of a supernova explosion, he has been looking for answers. Images from the Chandra X-ray observatory show that a jet of plasma pouring straight out from the neutron star at the center of the nebula appears to change direction every few years, without changing its structure. Why? Scientists have hypothesized that magnetic fields with the right properties could explain this behavior, but Li wanted proof. "How do you design an experiment on Earth to explain mysteries that are happening 6,500 light years away, and stretching over 13 light years of space?" he asks. "Traditional astrophysics is based on observation. Typically after you make an observation, you build a theoretical model, you do some numerical simulations. But that's it. How can you go there and measure anything? How can you do an experiment to test this model?" Li has been part of MIT's Plasma Science and Fusion Center (PSFC) since becoming a graduate student in 1987. As a co-founder and associate head of the PSFC's High-Energy-Density Physics (HEDP) Division, Li has collaborated regularly with the National Ignition Facility and the University of Rochester's Laboratory for Laser Energetics on inertial confinement fusion and laboratory-astrophysical experiments. He decided to see if he could also use the lab's OMEGA laser to mimic the conditions in the Crab Nebula, and prove the hypothesis that magnetic fields were responsible for the "kink in the crab". Instead of training OMEGA's multiple laser beams on a single pellet of hydrogen fuel, as he would for a fusion experiment, Li bounced lasers off two 3 x 3 mm foils hinged together at a 60-degree angle. Using two laser beams to heat each side, he generated plasma bubbles, or plumes. Li knew that because they are very dense and hot, these plumes would immediately expand, colliding in the middle plane between the two foils to form a jet. Li notes that even though laboratory-generated jets and astrophysical jets have very different size scales, the fundamental physics can be the same because critical dimensionless parameters are similar. As a result, they share enough physical properties to allow Li to scale his laboratory experiments, as one would do from a wind tunnel to an airplane, to conditions in the crab nebula. While the kink in the nebula jet occurs over a period of a few years, the laboratory experiment creates a jet in one nanosecond (billionth of a second), which then propagates for five to six nanoseconds. Li laughs as he considers the speed of the experiments: "You have to generate that, diagnose that, characterize that, quantify that in this period of time!" To measure the magnetic fields generated by the experiment, Li used a monoenergetic proton radiography (MPR) diagnostic invented by his division in 2005, allowing him, through the deflection of the protons, to make a radiograph of the fields. With the quantitative measurements in hand, he has been able to prove that the nebula jet behavior is governed by weak magnetic fields along the jet, which keep its structure largely straight, and other magnetic fields circling around the jet, which create the instability responsible for the directional change. The results were recently published in Nature Communications. HEDP division head Richard Petrasso noted the importance of Li's work: "Through his understanding of instabilities and his development of the MPR diagnostic to map transient magnetic fields in the laboratory, Chikang has been able to explore and explicate, for the first time, such puzzling phenomena as the jetting in the Crab Nebula." More information: C. K. Li et al. Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet, Nature Communications (2016). DOI: 10.1038/ncomms13081


News Article | October 23, 2015
Site: news.mit.edu

Dennis Whyte, professor of nuclear science and engineering, began pursuing fusion energy research at MIT in 2006, attracted in large part by the opportunity to work at the Plasma Science and Fusion Center (PSFC). He says it’s “a special place, with a compelling combination of being at the forefront of an important research area like fusion, while being rooted strongly in education.” As 2015 begins, Whyte is assuming leadership of the PSFC from 20-year veteran professor of physics Miklos Porkolab, who is returning to teaching and research. Whyte plans to steer the center into an era of smaller, faster, and more modular experimentation while maintaining the center’s educational excellence. Whyte refers to his assignment as “humbling,” but expresses confidence in the “really amazing” PSFC team and its ability “to build an even more cohesive case for fusion energy going forward.” His extensive Department of Nuclear Science and Engineering (NSE)/PSFC research contributions include studies of magnetic energy confinement techniques that were honored with the International Atomic Energy Agency’s 2013 Nuclear Fusion Journal Prize. Founded in 1976, the PSFC has about 200 affiliated faculty members, students, researchers, and technicians from units spanning the Institute, including the departments of Physics, Materials Science and Engineering, and Electrical Engineering and Computer Science, as well as NSE. Also on site are several dozen domestic and international collaborators. While relatively large for a university program, the center is small by global fusion-research standards. “We have impact way above our size,” says Whyte. “By any measure we’ve been extremely successful at creating key breakthroughs across broad areas of fusion — magnetic and inertial confinement, diagnostics, plasma-materials interactions, and superconducting magnet technologies. We plan to continue contributing, while ensuring that education of the next generation of scientists is completely integrated into everything we do.” Much of PSFC’s work has leveraged the high-field Alcator C-Mod tokamak fusion reactor, one of just three major tokamak facilities in the U.S. But Whyte says today’s budgetary realities and the changing nature of fusion development require a new strategy. Whyte’s top near-term priority is evaluation of options that will enable “MIT-quality” experimentation at the scientific frontier, while keeping the center’s research staff intact and cohesive. One possible example is the Advanced Divertor Experiment (ADX), a facility that would use “a small, purpose-built experiment to address key challenges in the boundary-plasma interface with materials, and controlling and sustaining electric current in plasmas. That’s what MIT should be doing, and the educational mission is built right in,” he explains. In addition, work in related fields like basic plasma-materials interactions, nuclear detection, and advanced diagnostics will also be nourished, in support of other NSE and Institute-wide efforts, including nuclear security. Indeed, extending and expanding PSFC’s ties to MIT academics is another priority. “Tying into the breadth of activities at MIT helps makes PSFC unique,” says Whyte. “Nurturing our faculty and our academic roots is part of how we’ll continue to attract the best and brightest students.” Looking beyond the Institute, Whyte sees opportunities for continued integration with national and international fusion efforts. He cites PSFC senior scientist Richard Petrasso’s group as a model — “they’ve developed capabilities to make unique measurements for the National Ignition Facility, and the University of Rochester’s Laboratory for Laser Energetics, two of the largest laser/inertial fusion experiments. There’s heavy student involvement, and they’re a vital part of those national teams.” Whyte acknowledges that the international ITER fusion reactor project in France is facing challenges, but said the PSFC — a key contributor to date — will continue its support. Simultaneously, he notes, the nimbler experimental approach could enable fusion development on smaller devices like the PSFC’s conceptual ARC reactor. It leverages new high-temperature, high-field superconducting magnet technology and could produce as much power as ITER’s reactor in just one-tenth the volume. “Smaller and sooner is my mantra,” says Whyte. Disruptive technologies, like the ARC’s superconducting magnets, could create “an extremely exciting pathway that can make fusion happen sooner, while also informing what we do in the near term. We’re advocating for that, and for applying an integrated, multi-disciplinary view of fusion, importantly including nuclear engineering perspectives, to near-term plasma research. “We should study the parameter space required for magnetic fusion energy: high-plasma pressure, high-power density, and steady-state. The compact high-field Alcator experiments, and a focused experiment like ADX, show that this approach can be highly effective for advancing fusion science at small size,” he concludes. While the PSFC will be a different place in five or 10 years, Whyte is confident that its special combination of impactful leading-edge research, international collaboration, and strong student involvement will thrive and evolve.


Kessler T.J.,Laboratory for Laser Energetics
Optics InfoBase Conference Papers | Year: 2010

Diffraction gratings have competing performance requirements when used in high-peak-power chirped-pulse-amplification laser systems. Diffraction efficiency, wavefront quality, and laser-damage threshold are interdependent criteria for multilayer diffraction gratings. Critical fabrication-related artifacts will be discussed. © 2010 Optical Society of America.


Yang J.-H.,Laboratory for Laser Energetics | Craxton R.S.,Laboratory for Laser Energetics | Haines M.G.,Imperial College London
Plasma Physics and Controlled Fusion | Year: 2011

This study examines single-particle electron motions in both a plane electromagnetic wave and a Gaussian focus in vacuum. Exact, explicit analytic expressions for relativistic electron trajectories in a plane wave are obtained, using the proper time as a parameter, in the general case of arbitrary initial positions and velocities. It is shown that previous analyses can be completed using the proper-time parameter. The conditions under which localized oscillatory motions ('figure-of-eight' orbits) occur are derived from the new solutions. The general solutions are also connected with the figure-of-eight orbits by a Lorentz transformation. The analytic solutions for arbitrary initial conditions and an arbitrary initial field phase can be used to determine the ranges of electron ejection angle and emerging electron energy in a vacuum laser accelerator, in which electrons are ejected externally, and provide a basis for explaining the spectrum of nonlinear Thomson scattering radiation. Numerical solutions are used for electron motions in the focus of a Gaussian laser beam, and the mean motion allows one to test a new expression for the relativistic ponderomotive force. It is suggested that plane wave solutions can provide a basis for approximating the orbital motion of particles in Gaussian beams. © 2011 IOP Publishing Ltd.


Marshall K.L.,Laboratory for Laser Energetics | Didovets O.,Laboratory for Laser Energetics | Saulnier D.,Laboratory for Laser Energetics
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2014

The exceptionally high 1054-nm laser-damage resistance of photoalignment materials (approaching that of fused silica) has made it possible to fabricate a wide variety of photoaligned liquid crystal (LC) devices for high-peak-power laser applications. Despite these advances, materials selection and photoalignment exposure conditions are still determined using costly and time-consuming "trial-and-error" methods. The contact angle of a fluid droplet on an alignment layer yields important information about LC-surface physicochemical interactions, and as such, it has potential as a rapid and convenient metric for optimizing photoaligned device quality. To this end, we report on efforts to correlate fluid contact angle with surface energy and azimuthal-anchoring energy to aid in the assessment of alignment quality in photoalignment materials systems. © 2014 SPIE.


Yang J.-H.,Laboratory for Laser Energetics | Craxton R.S.,Laboratory for Laser Energetics
Physics of Plasmas | Year: 2011

This work investigates the capability of ultraintense lasers with irradiance from 1018 to 1021 W cm-2 to produce highly energetic electron beams from a Gaussian focus in a low-density plasma. A simple particle simulation code including a physical model of collective electrostatic effects in relativistic plasmas has been developed. Without electrostatic fields, free electrons escape from the Gaussian focal region of a 10-ps petawatt laser pulse very quickly, well before the laser field reaches its maximum amplitude. However, it has been demonstrated that the electrostatic field generated by the electron flow is able to strongly modify the range and direction of the laser-generated MeV electrons by allowing trapped electrons to experience much higher laser-intensity peaks along their trajectories. This modeling predicts some collimation but not enough to meet the requirements of fast ignition. © 2011 American Institute of Physics.


News Article | November 10, 2016
Site: phys.org

When Z fires, its huge electromagnetic field crushes pre-warmed fuel, forcing it to fuse. Tritium-enriched fuel should release many more neutrons than previous maximums at Z, already among the highest in the world. "This thing about creating energy where none existed before—we don't yet have a bonfire, but we're squirting starter on the grill," said Mike Cuneo, senior manager of Sandia's Pulsed Power Accelerator Science and Technology group. The output of Z has been used over decades to provide information for computer simulations that test the readiness of America's nuclear stockpile without exploding an actual weapon. It's also used by astrophysicists using the machine's momentarily astounding pressures and temperatures to understand conditions in stars and the cores of planets. And some hope that pressures created mainly by electricity and magnetism one day may reach nuclear fusion conditions suitable for energy production; this condition is called "high yield." The introduction of tritium is of high technical interest because a 50/50 mix of tritium and deuterium—the two isotopes of hydrogen—emits 80 times more neutrons, and 500 times more energy, than deuterium alone. Energy from deuterium—in a manner of speaking, a relatively low-octane fuel—has been the upper limit on output at Z. But it's still early days. A dry run in July, testing containment hardware and instrumentation, preceded Z's first tritium experiment three weeks later, when a fraction of a percent was cautiously introduced into the experiment's fuel. "We're going to crawl before we walk and run," said Cuneo. "We will gradually increase that fraction in contained experiments as we go." Only two other Department of Energy-supported, high-energy-density research sites, at Lawrence Livermore National Laboratory and the Laboratory for Laser Energetics at the University of Rochester, had been approved to use tritium, a potential environmental hazard. The Sandia experiments use electromagnetics to smash Z's more massive target and its entire target support area like they were hit by a sledgehammer. Unlike the laser facilities, the  Z chamber must be entered by personnel after each experiment to refurbish the facility for the next experiment. Under those conditions, introducing tritium into the target requires extreme care and forethought in the design, transport and containment of tritium to meet rigorous safety standards. "Tritium's like sand at the beach, it gets into everything," said Cuneo. "So for now, we can't let it go anywhere." The isotope is a small molecule with a lot of mobility, and the first big hurdle, he says, is to make sure the radioactive material with its 12-year half-life doesn't migrate to the million-gallon pools of water and oil that insulate Z's pulsed power components. "Laser facilities don't have these pools," he pointed out. Tritium also could bond to the metal walls of Z's central area, presenting a potential radioactive hazard where technicians enter daily to scrub after each shot. However, using the same unique design that has contained plutonium on more than a dozen previous Z shots, no tritium was released. Nearly 100 Sandia personnel contributed directly to the effort, funded through Sandia's Laboratory Directed Research and Development program. Also participating were researchers from General Atomics, Los Alamos National Laboratory, the University of New Mexico and Utah State University. Future work will be funded by the National Nuclear Security Administration (NNSA). "There was a high level of integration on facility containment and radiation protection, to do it right," said Brent Jones, facility integration lead. "The Sandia-California gas transfer group, with decades of experience dealing with tritium, developed a method of housing, delivering and containing the material. They built a device that could load a small but defined quantity of tritium; the neutron generator people filled the target with tritium; and the plutonium confinement folks contributed their shot expertise." The team now must evaluate whether tritium can be used safely in uncontained experiments, their ultimate goal. Confined tests can evaluate the compatibility of tritium with Z's materials and pressures, but don't accurately measure fusion outputs. "The use of contained tritium on Z is the first step on this journey," said Cuneo. "There is much more work to do. "Similar to what is done at the laser [fusion] facilities, one idea [for an uncontained experiment] is to purge the tritium immediately after a shot so that it doesn't stick to the walls of the Z chamber. We need to be able to efficiently purge the center section back to a safe level before technicians enter to refurbish it." Uncontained experiments will begin with very small levels of tritium and gradually ramp up in a several-year process. "We hope to find that we will able to safely handle 1-3 percent tritium in uncontained experiments, enough to advance Inertial Confinement Fusion applications, other weapons science applications and neutron effects testing," Cuneo said. It will be at least three years before experiments approach the 50/50 mix of tritium and deuterium, depending on funding and Sandia and NNSA priorities for Z. Explore further: Scientists create method to obtain the most precise data for thermonuclear reactors


News Article | April 11, 2016
Site: www.nrl.navy.mil

U.S. Naval Research Laboratory (NRL) plasma physicist, Dr. Alexander L. Velikovich, receives the 2015 IEEE Plasma Science and Applications Award for advancing the theory of plasma shocks and hydrodynamics and magneto-hydrodynamics — enabling many-fold increases in both Z-pinch and laser-plasma experimental performance in radiation and fusion applications. Presented by the Nuclear and Plasma Sciences Society (NPSS), the award recognizes outstanding contributions to the field of plasma science and engineering to include plasma dynamics, thermonuclear fusion, plasma sources, relativistic electron beams, laser plasma interactions, diagnostics and solid-state plasmas. Author and co-author of more than 170 publications, with over 2,500 citations, Velikovich developed the first analytical theory to calculate the time-dependent growth of compressible Richtmyer-Meshkov (RM) instability in the linear regime, as well as, non-linear RM theory explaining reduction of its growth rate for large initial amplitude. Most recently Velikovich developed a theory explaining the effect of shock-generated turbulence — discovered in numerical simulations at NRL over a decade ago — on the Rankine-Hugoniot jump conditions. Derived from the laws of mass, momentum and energy, many physical effects, first observed on the Nike krypton fluoride (KrF) laser at NRL, were recreated on both the Nova laser at the Lawrence Livermore National Laboratory, California, and the Omega laser at the Laboratory for Laser Energetics (LLE) located in New York. Velikovich's research on High Energy Density Physics (HEDP) and Inertial Confinement Fusion (ICF), particularly laser-fusion and Z-pinch-related plasma hydrodynamics, formed the theoretical basis for x-ray generation in Z-pinch plasma radiation. Modeling and interpretation of these results were instrumental in establishing the physical picture of the Rayleigh-Taylor (RT) instability seeding in laser fusion targets caused by the roughness of the front and rear surface of a laser target. Research that later translated into the development of a theory that provided the physical basis for most hydrodynamic experiments performed on the Nike laser over the past decade. Earning a Master of Science equivalent degree in physics from Moscow State University Department of Physics, Moscow, Russia, in 1974, Velikovich completed a Ph.D. equivalent degree in plasma physics and chemistry awarded by Kapitza Institute for Physical Problems, U.S.S.R. Academy of Sciences in 1978. In 1991 he earned an Advanced Degree of Doctor of Science (equivalent of Habilitation in European Union countries) in Electrophysics awarded by High Current Electronics Institute, Russian Academy of Sciences, Tomsk, Russia. Velikovich started at NRL in 1993 as a contractor, transitioning to federal civil service at NRL in 1999. In 2005 Velikovich was elected Fellow of the American Physical Society upon the recommendation of its Division of Plasma Physics for "outstanding contributions to the theories of dynamics and stability of Z-pinch plasmas, Richtmyer-Meshkov instability and related effects of early-time perturbation seeding and evolution in laser plasma targets." In 2010, along with colleagues from Sandia and NRL, Velikovich shared the 2010 Department of Energy (DoE) Defense Programs Award of Excellence for "increased cold x-ray source yields, improved source characterization and debris mitigation techniques to qualify stockpile components on refurbished Z machine." In 2000, 2007, 2010, and 2012, he was the recipient of the NRL Alan Berman Research Publication Award. In 2015 he received the NRL Sigma Xi award for Pure Science. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.


News Article | January 27, 2016
Site: www.scientificcomputing.com

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. 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. “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: 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. PPPL, on Princeton University's Forrestal Campus in Plainsboro, NJ, 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.

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