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. Source
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.. Source
Karimabadi H.,University of California at San Diego |
Karimabadi H.,SciberQuest, Inc. |
Lazarian A.,University of Wisconsin - Madison
Physics of Plasmas | Year: 2013
Magnetic reconnection is an important process that violates flux freezing and induces change of magnetic field topology in conducting fluids and, as a consequence, converts magnetic field energy into particle energy. It is thought to be operative in laboratory, heliophysical, and astrophysical plasmas. These environments exhibit wide variations in collisionality, ranging from collisionless in the Earth's magnetosphere to highly collisional in molecular clouds. A common feature among these plasmas is, however, the presence of turbulence. We review the present understanding of the effects of turbulence on the reconnection rate, discussing both how strong pre-existing turbulence modifies Sweet-Parker reconnection and how turbulence may develop as a result of reconnection itself. In steady state, reconnection rate is proportional to the aspect ratio of the diffusion region. Thus, two general MHD classes of models for fast reconnection have been proposed, differing on whether they keep the aspect ratio finite by increasing the width due to turbulent broadening or shortening the length of the diffusion layer due to plasmoid instability. One of the consequences of the plasmoid instability model is the possibility that the current sheet thins down to collisionless scales where kinetic effects become dominant. As a result, kinetic effects may be of importance for many astrophysical applications which were considered to be in the realm of MHD. Whether pre-existing turbulence can significantly modify the transition to the kinetic regime is not currently known. Although most studies of turbulent reconnection have been based on MHD, recent advances in kinetic simulations are enabling 3D studies of turbulence and reconnection in the collisionless regime. A summary of these recent works, highlighting similarities and differences with the MHD models of turbulent reconnection, as well as comparison with in situ observations in the magnetosphere and in the solar wind, are presented. The paper concludes with a list of important open questions and suggestions for future work. © 2013 AIP Publishing LLC. Source
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.
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. Source