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Omelchenko Y.A.,SciberQuest, Inc. | Karimabadi H.,SciberQuest, Inc.
Physical Review Letters | Year: 2012

By conducting two-dimensional hybrid simulations of an infinitely long field-reversed θ-pinch discharge we discover a new type of plasma rotation, which rapidly develops at the plasma edge in the ion diamagnetic direction due to the self-consistent generation of a Hall-driven radial electric field. This effect is different from the previously identified end-shorting and particle-loss mechanisms. We also demonstrate flutelike perturbations frequently inferred in experiments and show that in the absence of axial contraction effects they may quickly alter the toroidal symmetry of the plasma. © 2012 American Physical Society.


Liu Y.-H.,Los Alamos National Laboratory | Daughton W.,Los Alamos National Laboratory | Karimabadi H.,SciberQuest, Inc. | Li H.,Los Alamos National Laboratory | Roytershteyn V.,SciberQuest, Inc.
Physical Review Letters | Year: 2013

Three-dimensional kinetic simulations of magnetic reconnection reveal that the electron diffusion region is composed of two or more current sheets in regimes with weak magnetic shear angles φ 80. This new morphology is explained by oblique tearing modes which produce flux ropes while simultaneously driving enhanced current at multiple resonance surfaces. This physics persists into the nonlinear regime leading to multiple electron layers embedded within a larger Alfvénic inflow and outflow. Surprisingly, the thickness of these layers and the reconnection rate both remain comparable to two-dimensional models. The parallel electric fields are supported predominantly by the electron pressure tensor and electron inertia, while turbulent dissipation remains small. © 2013 American Physical Society.


Le A.,Massachusetts Institute of Technology | Egedal J.,Massachusetts Institute of Technology | Ohia O.,Massachusetts Institute of Technology | Daughton W.,Los Alamos National Laboratory | And 2 more authors.
Physical Review Letters | Year: 2013

The electron diffusion region during magnetic reconnection lies in different regimes depending on the pressure anisotropy, which is regulated by the properties of thermal electron orbits. In kinetic simulations at the weakest guide fields, pitch angle mixing in velocity space causes the outflow electron pressure to become nearly isotropic. Above a threshold guide field that depends on a range of parameters, including the normalized electron pressure and the ion-to-electron mass ratio, electron pressure anisotropy develops in the exhaust and supports extended current layers. This new regime with electron current sheets extending to the system size is also reproduced by fluid simulations with an anisotropic closure for the electron pressure. It offers an explanation for recent spacecraft observations. © 2013 American Physical Society.


Omelchenko Y.A.,SciberQuest, Inc. | Karimabadi H.,SciberQuest, Inc.
Journal of Computational Physics | Year: 2012

Treatment of disparate timescales remains a major challenge in computational science. Previously we introduced a new asynchronous approach to the explicit time integration of multiscale numerical systems based on partial differential equations and particle techniques - self-adaptive discrete-event simulation (DES). In DES time increments for updates of numerical variables (events) are predicted by imposing small but finite bounds to their changes, and event synchronization requirements are defined with physical rules. The feasibility and superior metrics of DES were demonstrated for several different physical problems in one dimension. Here we extend DES to multiple dimensions by introducing a unidimensional infrastructure for asynchronous simulations on logically uniform meshes. As the first example of this infrastructure we present a new event-driven electromagnetic hybrid code, HYPERS (HYbrid Particle Event-Resolved Simulator). This code is validated in two dimensions against a state-of-the-art time-stepping hybrid code on a numerically challenging problem which describes the interaction between the magnetized plasma flow and a magnetic dipole obstacle. We find that HYPERS achieves significant speedups and remains physically accurate in a broad mesh resolution range, including coarser resolutions where the time-driven code produces numerical artifacts. © 2011 Elsevier Inc.


Scudder J.D.,University of Iowa | Karimabadi H.,SciberQuest, Inc.
Astrophysical Journal | Year: 2013

This paper outlines the rather narrow conditions on a radiatively decoupled plasma where a Maxwell-Boltzmann (MB) distribution can be assumed with confidence. The complementary non-thermal distribution with non-perturbative kurtosis is argued to have a much broader purview than has previously been accepted. These conditions are expressed in terms of the electron Knudsen number, Ke , the ratio of the electron mean free path to the scale length of electron pressure. Rather generally, f(v < v 2(K e )) will be Gaussian, so that MB atomic or wave particle effects controlled by speeds v < v 2 ≡ w(15/8Ke ) 1/4 will remain defensible, where w is the most probable speed. The sufficient condition for Spitzer-Braginskii plasma fluid closure at the energy equation requires globally Ke (s) ≤ 0.01; this global condition pertains to the maximum value of Ke along the arc length s of the magnetic field (to its extremities) provided that contiguous plasma remains uncoupled from the radiation field. The non-thermal regime Ke > 0.01 is common in all main-sequence stellar atmospheres above approximately 0.05 stellar radii from the surface. The entire solar corona and wind are included in this regime where non-thermal distributions with kurtosis are shown to be ubiquitous, heat flux is not well modeled by Spitzer-Braginskii closure, and fluid modeling is qualitative at best. © 2013. The American Astronomical Society. All rights reserved..


Grant
Agency: NSF | Branch: Standard Grant | Program: | Phase: MAGNETOSPHERIC PHYSICS | Award Amount: 80.00K | Year: 2011

This project will investigate the possibility that a new approach to deriving a linear theory of the tearing mode instability in space plasmas can be successfully developed. Such a linear theory would then form the basis of a full non-linear theory of magnetic reconnection in a collisonless plasma. Previous analytical work on the linear tearing mode relied on using an asymptotic approach to make the equations tractable. However, fullly kinetic, particle simulations have indicated that the asymptotic approach breakes down in most realistic geometries. The purpose of this exploratory resarch is threefold:
1) Determine why the asymptotic appoach breaks down and develop a new approach using an exact linear Vlasov solver to determine the conditions where a direct transition from linear theory to non-linear turbulence occurs; 2) Determine whether or not the formation of secondary tearing island formation is due to a linear instability; 3) Examine the controversial question of whether linear mode properties have a significant influence on the resulting reconnection rate in large systems.

This is a project that has the potential to transform the way magnetic reconnection is treated in plasma theory and simulations. It could affect our understanding of phenomena on the sun, in the solar wind, in planetary magnetospheres and in astrophysical plasmas.


Grant
Agency: National Aeronautics and Space Administration | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 99.90K | Year: 2011

With the information age has come a dramatic increase in our ability to generate and collect data while the traditional techniques to analyze this tsunami of data have proven woefully inadequate. The amount of data collected and stored electronically is doubling every three years. Many researchers are simply overwhelmed by the amount of data. Scientific visualization remains the dominant form of data exploration but current tools are focused on image creation and lack the much needed embedded analysis capabilities. To address this urgent need, we propose to develop and introduce a first-of-its kind tool kit called SciViz2. SciViz2 brings together key innovations in three separate fields of scientific visualization, data mining, and computer vision to offer an integrated solution for physics mining of the most complex of data sets; multi-dimensional, multi-variate data sets. SciViz2 bridges the existing gap in software technology by providing a scalable knowledge discovery tool that combines the power of machine learning techniques with scientific visualization.Our Phase I goal is to develop a prototype and demonstrate the viability of the solution by applying it to one of the most challenging data analysis problem, that is the data from peta-scale particle simulations. The underlying physics involves complex interaction of multi-variates and the outputinvolves both grid-based and particle phase space data with nearly trillion particles and over 200 TB of data from a single run. Phase II activity would involve full development of SciViz2 and its dissemination to the wider community. Our close collaboration with NASA's HPC group will be heavilyleveraged to test and release the software for open use.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: SOLAR-TERRESTRIAL | Award Amount: 190.00K | Year: 2012

The Principal Investigator (PI) will perform a comprehensive theoretical study of the poorly explored and important regime of weakly collisional reconnection in plasmas, where Coulomb collisions play a role, but where reconnection electric fields are comparable to, or larger than, the Dreicer limit for runaway electrons. These results will have direct scientific application to describing reconnection processes in the solar corona and transition region. The PI intends to quantify the practical consequences of such reconnection, such as bulk plasma heating and particle acceleration, using self-consistent modeling and state-of-the-art, fully kinetic, particle-in-cell simulations with a Monte-Carlo treatment of the Coulomb collisions, in order to provide a first-principles description of the relevant physics.

The PI notes that this research will have widespread applications in a variety of sub-fields of plasma physics. This projects educational objectives include the training of a postdoctoral researcher in reconnection physics, petascale computing, and scientific visualization. In addition, the PIs employer, SciberQuest, Inc., has an ongoing partnership with the San Diego Supercomputer Centers internship program entitled Research Experience for High School Students (REHS). As part of this project, the PI will mentor two students per year through the REHS program.


Grant
Agency: Department of Health and Human Services | Branch: | Program: SBIR | Phase: Phase I | Award Amount: 148.66K | Year: 2012

DESCRIPTION (provided by applicant): Adverse events and medical errors result in thousands of accidental deaths and over one million excess injuries each year. To avoid medical errors in radiation cancer treatment, careful attention needs to be made to ensure accurate implementation of the intended treatment plan. We propose a SmartTool to automatically detect and highlight potential errors in a radiotherapy treatment plan, in real time and before its execution. SmartTool will double check all the treatment parameters in the background against a previously built Predictive Model of a Medical Error (PMME) and flag the operator, [post human QA,] if there is a discrepancy in the treatment plan, by stopping execution, highlighting the outlier treatment parameter and prompting human intervention. To build the PMME we will mine the dataset of previously treated cancer patients, by clustering the data in the groups based on treatment parameter similarity, labeling the clusters and using an innovative algorithm to build a highly accurate anomaly detection tool. PMME will also be dynamically updated [to include new treatment data instances coming in to the system, and updating the model should any treatment flags be identified as false positive or false negative]. Thevastly innovative aspect of SmartTool is in the novel use of machine learning techniques to automatically build an anomaly prediction model on unlabeled data (customarily a labeled data is required to build a predictive model) and provide an automatic, real time and unobtrusive intelligent computational treatment checking algorithm. Moreover, having an analytical model of an outlier/anomaly offers the capability to describe the conditions of the outlier being created and is the essential in gaining investigative (and medical) insight in what went wrong and how to improve the process in the future. SmartTool can also be applied in a variety of other medical areas (e.g. predicting errors in pharmacy, laboratory data, and treatment procedure data), to detect anomalies and describe them, offering potential novel medical discoveries and a prospect of saving thousands more lives, with a vast commercialization aspect. PUBLIC HEALTH RELEVANCE: The proposal is aimed at promoting research and development in biomedical computational science and technology that is consistent with the objective of the NIH and NCI to support rapid progress in areas of scientific opportunity in biomedical research, and enhancing the public health. If the project is successfully completed, this proof of concept study will result in a valuable health information technology tool for automatic detection of catastrophic errors in cancer radiotherapy, which adds another safeguard for patient safety.


Grant
Agency: NSF | Branch: Continuing grant | Program: | Phase: PLASMA PHYSICS | Award Amount: 37.18K | Year: 2012

Magnetic reconnection is a ubiquitous phenomenon in magnetized plasma that is thought to play a key role in the dynamics of many systems in nature under a wide range of plasma conditions. Examples of such systems are the Earths magnetosphere, solar corona, laboratory fusion experiments, etc. Significant progress in understanding of magnetic reconnection has been made, but many important basic questions remain open. The aim of this project is to execute an extensive simulation program designed to significantly advance the state of knowledge on reconnection by working in parallel with Magnetic Reconnection eXperiment (MRX), one of leading experimental facilities dedicated to studies of magnetic reconnection. This work targets the following scientific question of great significance: What is the influence of microscopic turbulence originating from instabilities of thin reconnection layers on the dynamics of magnetic reconnection in various parameter regimes? A unique aspect of this collaboration is a close coordination of simulation and experimental efforts, where experiments are used to verify and challenge theoretical analysis, while simulations help interpret and guide experimental campaigns. The work utilizes state-of-the art simulation technologies and makes use of a variety of modern petascale computers.

We expect that this work will have broad implications for the studies of reconnection. The results will be presented at scientific meetings and published in the appropriate scientific literature. The simulation data will be made widely available to the community. This research is based on our ongoing collaboration with MRX team at Princeton Plasma Physics Laboratory. An important element of this work is participation of graduate and undergraduate students from Princeton University in all aspects of the research. Access to simulation data and theoretical expertise provides the students with fundamentally new ways of analyzing their measurements and greatly enhances their expertise. In addition, the students have opportunities to participate in summer internship programs at SciberQuest and at Los Alamos National Laboratory, where they have a chance to get hands on experience with modern simulation techniques.

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