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Dahlburg R.B.,U.S. Navy | Einaudi G.,Berkeley Research Associates | Rappazzo A.F.,University of Delaware | Velli M.,Jet Propulsion Laboratory
Astronomy and Astrophysics | Year: 2012

Context. Photospheric motions shuffle the footpoints of the strong axial magnetic field that threads coronal loops, which gives rise to turbulent nonlinear dynamics that are characterized by the continuous formation and dissipation of field-aligned current sheets in which energy is deposited at small-scales and the heating occurs. Previous studies showed that the current sheet thickness is several orders of magnitude smaller than present-day state-of-the-art observational resolution (∼700 km). Aims. To understand coronal heating and correctly interpret observations it is crucial to study the thermodynamics of such a system in which energy is deposited at unresolved small-scales. Methods. Fully compressible three-dimensional magnetohydrodynamic simulations were carried out to understand the thermodynamics of coronal heating in the magnetically confined solar corona. Results. We show that temperature is highly structured at scales below observational resolution. It is also nonhomogeneously distributed so that only a fraction of the coronal mass and volume is heated at each time. Conclusions. This is a multi-thermal system in which hotter and cooler plasma strands are also found next to each other at sub-resolution scales and exhibit a temporal dynamics. © 2012 ESO.

News Article
Site: www.nrl.navy.mil

Dr. David Abe, head of the Electromagnetics Technology Branch at the U.S. Naval Research Laboratory (NRL), has been named Fellow of the Institute of Electrical and Electronics Engineers (IEEE) for leadership and contributions to the development of high power microwave and millimeter wave vacuum electronic devices. As a supervisory research physicist at NRL, Abe leads a 47-person multi-disciplinary group of physicists, materials scientists, engineers, mechanical designers, and technicians involved in research and exploratory development of radio-frequency concepts, materials, devices, components, and circuits in the ultra-high to terahertz frequency range. "One of Dr. Abe's most significant accomplishments is his work with multiple-beam klystrons," states Dr. Baruch Levush, superintendent of the Electronics Science and Technology Division (ES&TD). "His team used an innovative development approach that emphasized simulation-based design to achieve optimized performance without the need for multiple hardware prototyping." Levush adds: "The overall success of his leadership can be seen in the first-pass design successes of three successive multiple-beam klystrons — a class of vacuum electronic amplifier — each more challenging in design and performance than its predecessor: This work also stands as a good example of Dr. Abe's ability to combine individual technical expertise with superb leadership and organizational skills." Abe received a Bachelor of Science in engineering from Harvey Mudd College (1981), a Master of Science in electrical engineering from the University of California, Davis (1988), and a doctorate in electrophysics from the University of Maryland (1992). At the University of Maryland, Abe was a major contributor to several high power microwave (HPM) research projects including experiments with plasma-filled backward-wave oscillators, overmoded backward-wave oscillators, and the first U.S. experiments with multiwave Cherenkov generators. In 1996 he joined the Vacuum Electronics Branch at the NRL. Prior to NRL, Abe worked on interdisciplinary projects in pulsed power, high power microwave generation, and electromagnetic effects at the Lawrence Livermore National Laboratory, Berkeley Research Associates, and the U.S. Army Research Laboratory. In 2006 Abe became a section head of the Devices and Design Section of the Vacuum Electronics Branch, and from 2010 to 2013 he also served acting branch head. In 2013 he became the head of a newly established Electromagnetics Technology Branch in ES&TD. The world's prominent professional association for advancing technology for humanity, the IEEE, through its 400,000 members in 160 countries, is a leading authority on a wide variety of areas ranging from aerospace systems, computers and telecommunications to biomedical engineering, electric power and consumer electronics. The IEEE Board of Directors confers the grade of Fellow upon nominated members that demonstrate an extraordinary record of accomplishments in any of the IEEE fields of interest. 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.

Krall J.,U.S. Navy | Huba J.D.,U.S. Navy | Ossakow S.L.,Berkeley Research Associates | Joyce G.,Icarus Research Inc.
Geophysical Research Letters | Year: 2010

The Naval Research Laboratory (NRL) three-dimensional simulation code SAMI3/ESF is used to study the long time evolution of equatorial spread F (ESF) bubbles. The ESF bubbles are modeled until they stop rising and become-gfossils-h with results analyzed to address previously-untested hypotheses. Specifically, it has been suggested that bubbles stop rising when either the local electron density inside the bubble is equal to that of the nearby background or the fluxtube-integrated electron density inside the bubble is equal to that of the nearby background. It is shown that equatorial bubbles stop rising when the magnetic flux-tube-integrated ion mass density inside the bubble equals that of the surrounding background ionosphere. In the case of a singleion ionosphere this reduces to the condition that the fluxtube-integrated electron densities are in balance, consistent with the hypothesis of Mendillo et al. (2005). Copyright © 2010 by the American Geophysical Union.

Schmitt A.J.,U.S. Navy | Bates J.W.,U.S. Navy | Obenschain S.P.,U.S. Navy | Zalesak S.T.,Berkeley Research Associates | Fyfe D.E.,U.S. Navy
Physics of Plasmas | Year: 2010

Continuing work in the design of shock ignition targets is described. Because of reduced implosion velocity requirements, low target adiabats, and efficient drive by short wavelength lasers, these targets produce high gain (>100) at laser energies well below 1 MJ. Effects of hydrodynamic instabilities such as Rayleigh-Taylor or Richtmyer-Meshkov are greatly reduced in these low-aspect ratio targets. Of particular interest is the optimum ratio of ignitor to compression pulse energy. A simple pellet model and simulation-derived coupling coefficients are used to analyze optimal fuel assembly, and determine that shock ignition allows enough control to create theoretically optimum assemblies. The effects on target design due to constraints on the compression and ignitor pulse intensities are also considered and addressed. Significant sensitivity is observed from low-mode perturbations because of large convergence ratios, but a more powerful ignitor can mitigate this. © 2010 American Institute of Physics.

Rappazzo A.F.,University of Delaware | Velli M.,Jet Propulsion Laboratory | Einaudi G.,Berkeley Research Associates
Astrophysical Journal | Year: 2013

We present simulations modeling closed regions of the solar corona threaded by a strong magnetic field where localized photospheric vortical motions twist the coronal field lines. The linear and nonlinear dynamics are investigated in the reduced magnetohydrodynamic regime in Cartesian geometry. Initially the magnetic field lines get twisted and the system becomes unstable to the internal kink mode, confirming and extending previous results. As typical in this kind of investigations, where initial conditions implement smooth fields and flux-tubes, we have neglected fluctuations and the fields are laminar until the instability sets in. However, previous investigations indicate that fluctuations, excited by photospheric motions and coronal dynamics, are naturally present at all scales in the coronal fields. Thus, in order to understand the effect of a photospheric vortex on a more realistic corona, we continue the simulations after kink instability sets in, when turbulent fluctuations have already developed in the corona. In the nonlinear stage the system never returns to the simple initial state with ordered twisted field lines, and kink instability does not occur again. Nevertheless, field lines get twisted, although in a disordered way, and energy accumulates at large scales through an inverse cascade. This energy can subsequently be released in micro-flares or larger flares, when interaction with neighboring structures occurs or via other mechanisms. The impact on coronal dynamics and coronal mass ejections initiation is discussed. © 2013. The American Astronomical Society. All rights reserved.

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