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Nonlinear electrostatic waves in collisionless plasmas

Physical Review E 1995年第3期52卷 3015-3015页

作者： Mark Buchanan J. Dorning Engineering Physics Program and Department of Applied Mathematics University of Virginia Charlottesville Virginia 22903

We report results concerning small amplitude Bernstein-Greene-Kruskal (BGK) waves, which are exact undamped traveling wave solutions of the nonlinear Vlasov-Poisson-Ampère equations for collisionless plasmas. Building upon previous work, we first develop a simple but powerful formalism that facilitates a methodical investigation of the types and properties of small amplitude BGK plasma waves that can exist near a given collisionless plasma equilibrium. Using this formalism, we then show that any physically relevant spatially uniform plasma equilibrium supports nonlinear spatially periodic BGK waves that are described by the Vlasov dispersion relation in the small amplitude limit. We demonstrate also that these equilibria are characterized by a discrete set of critical velocities vc(i), i=1,2,..., at which BGK solitary waves of vanishingly small amplitude can propagate in the plasma. The existence of these exact nonlinear spatially periodic and solitary wave solutions illustrates the fundamental incompleteness of the linear Vlasov-Landau theory of plasma waves since, by virtue of particle trapping, these nonlinear waves neither damp nor grow even when their amplitude is arbitrarily small.

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Effect of young's modulus on bubble formation and pressure waves during pulsed holmium ablation of tissue phantoms 6

Effect of young's modulus on bubble formation and pressure w...

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Laser-Tissue Interaction VI 1995

作者： Jansen, E. Duco Asshauer, Thomas Frenz, Martin Delacretaz, Guy Motamedi, Massoud Welch, A.J. Biomedical Engineering Program University of Texas at Austin TX78712 United States Laboratoire d'Optique Appliquée Swiss Federallnstitute of Technology Lausanne Switzerland Institute of Applied Physics University of Bern Switzerland Biomedical Laser and Spectroscopy Program University of Texas Medical Branch GalvestonTX77555 United States

Mechanical injury during pulsed laser ablation of tissue is caused by rapid bubble expansions and collapse or by laserinduced pressure waves. In this study the effect of material elasticity on the ablation process has been investigated. Polyacrylamide tissue phantoms with various water concentrations (75 -95%) were made. The Young's moduli of the gels were determined by measuring the stress-strain relationship. An optical fiber (200 or 400 μm) was translated into the clear gel and one pulse of holmium:YAG laser radiation was given. The laser was operated in either the Q-switched mode (τp =500 ns, Qp = 14 ± mJ, 200 μm fiber, H0 =446 mJ/mm2) or the free-running mode (τp = 100 μs, Qp = 200 ± 5 mJ, 400 μm fiber, H0 = 1592 mJ:mn2) Bubble formation inside the gels was recorded using a fast flash photography setup while simultaneously recording pressures with a PVDP needle hydrophone (40 ns risetime) positioned in the gel, approximately 2 mm away from the fibertip. A thermo-elastic expansion wave was measured only during Q-switched pulse delivery. The amplitude of this wave (-40 bar at 1 mm from the fiber) did not vary significantly in any of the phantoms investigated. Rapid bubble formation and collapse was observed inside the clear gels. Upon bubble collapse, a pressure transient was emitted;the amplitude of this transient depended strongly on bubble size and geometry. It was found that 1) the bubble was almost spherical for the Q-switched pulse and became more elongated for the free-running pulse, and 2) the maximum bubble size and thus the collapse amplitude decreased with an increase in Young's modulus (from 68 ± 11 bar at 1 mm in 95% water gel to 25 ± 10 bar at 1 mm in 75% water gel). © 1995 SPIE. All rights reserved.

关键词： Elastic moduli

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Book reviews

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Bulletin of Mathematical Biology 1995年第1期57卷 169-173页

作者： Bruce West J. M. Cushing Department of Physics University of North Texas Denton USA Department of Mathematics Interdisciplinary Program on Applied Mathematics University of Arizona Tucson USA

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INFLUENCE OF ION-BOMBARDMENT ON A-SI-H FILMS FABRICATED BY PLASMA CHEMICAL-VAPOR-DEPOSITION

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ELECTRONICS AND COMMUNICATIONS IN JAPAN PART II-ELECTRONICS 1995年第2期78卷 70-78页

作者： KATO, I YONEDA, T MATSUSHITA, T Member School of Science and Engineering Waseda University Tokyo Japan 169 Nonmembers Torn Matsushita:received his B.E. degree in 1993 from the Department of Electronic and Communications Waseda University where at present he is in the graduate program engaged in research on microwave plasma chemical vapor deposition. He is a member of the Applied Physics Society.

Plasma parameters of Ar/SiH4 plasma were measured as a function of de bias in a double-tubed coaxial line-type microwave plasma chemical vapor deposition (CVD) apparatus. The results indicate that it is possible to control the ion bombardment energy without affecting either the gas phase reaction or ion flux density incident to the substrate. Hydrogenated amorphous silicon films were deposited as a function of the ion bombardment energy and characteristics of the deposited films were investigated. The results indicate that the ion bombardment improves film density, bonding characteristics of hydrogen, and optical band gap but increases the concentration of dangling bonds due to Ar ion implantation. The ion bombardment not only causes the heating of the films but also induces sputtering and ion implantation.

关键词： PLASMA CVD A-SI, H DC BIAS VOLTAGE SHEATH VOLTAGE ION BOMBARDMENT

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Near-equilibrium multiple-wave plasma states

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Physical Review E 1994年第2期50卷 1465-1465页

作者： Mark Buchanan J. Dorning Engineering Physics Program and Department of Applied Mathematics University of Virginia Charlottesville Virginia 22903-2442

We report results showing that spatially periodic Bernstein-Greene-Kruskal (BGK) waves, which are exact nonlinear traveling wave solutions of the Vlasov-Maxwell equations for collisionless plasmas, satisfy a nonlinear principle of superposition in the small-amplitude limit. For an electric potential consisting of N traveling waves, cphi(x,t)= Ji=1Ncphi(i)(x-νit), where νi is the velocity of the ith wave and each wave amplitude cphi(i) is of order ε which is small, we first derive a set of quantities scrĒ(i)(x,u,t) which are invariants through first order in ε for charged particle motion in this N-wave field. We then use these functions scrĒ(i)(x,u,t) to construct smooth distribution functions for a multispecies plasma which satisfy the Vlasov equation through first order in ε uniformly over the entire x-u phase plane for all time. By integrating these distribution functions to obtain the charge and current densities, we also demonstrate that the Poisson and Ampère equations are satisfied to within errors that are O(ε3/2). Thus the constructed distribution functions and corresponding field describe a self-consistent superimposed N-wave solution that is accurate through first order in ε. The entire analysis explicates the notion of small-amplitude multiple-wave BGK states which, as recent numerical calculations suggest, is crucial in the proper description of the time-asymptotic state of a plasma in which a large-amplitude electrostatic wave undergoes nonlinear Landau damping.

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Two-dimensional versus three-dimensional excitons in wide GaAs quantum wells

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Journal of applied physics 1994年第1期75卷 289-289页

作者： Kusano, Jun-ichi Bauer, Gerrit E. W. Aoyagi, Yoshinobu Frontier Research Program The Institute of Physical and Chemical Research RIKEN Hirosawa 2‐1 Wako‐shi Saitama 351‐01 Japan Faculty of Applied Physics and DIMES Delft University of Technology 2628 CJ Delft The Netherlands The Institute of Physical and Chemical Research RIKEN Hirosawa 2‐1 Wako‐shi Saitama 351‐01 Japan

Excitons confined in high‐quality GaAs/Al0.19Ga0.81As double heterostructures have been studied experimentally and theoretically with emphasis on phenomena associated with the transition from a two‐dimensional exciton in a quantum well to a three‐dimensional exciton in a thin film. Exciton luminescence and reflectance spectra are obtained for GaAs film widths between 750 and 5200 ?. Exciton polariton luminescence is observed for the 5200 ? sample. Highly resolved exciton peaks are obtained in the free‐exciton luminescence spectra for both the 990 and 2010 ? samples. Magneto‐exciton spectra of the 990 ? sample reveal an enhanced spin splitting of the ground‐level exciton. Experimental results of both the 750 and the 990 ? samples are qualitatively explained by an effective‐mass theory which considers mixing of a large number of quasi‐two‐dimensional excitons. The large spin‐splitting of the 990 ? sample is a distinctive feature of a wide quantum well which can be explained by the well width dependence of exciton mixing. The comparison between the discrete luminescence spectrum of the 2010 ? sample and the reflectance spectrum with an exciton polariton interference pattern is presented. The quantization of the spatial dispersion contributes to the luminescence phenomena of the quasi‐three‐dimensional exciton which means that the exciton coherence length must be larger than 4020 ?.

关键词： QUANTUM WELLS ALUMINIUM ARSENIDES GALLIUM ARSENIDES EXCITONS HETEROSTRUCTURES TERNARY ALLOYS LUMINESCENCE SPECTRAL REFLECTANCE POLARITONS ENERGY?LEVEL SPLITTING EFFECTIVE MASS

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Chapman Conference delves into double-diffusive convection

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Eos, Transactions American Geophysical Union 1994年第45期75卷 524-525页

作者： H. J. S. Fernando Alan Brandt Environmental Fluid Dynamics Program Arizona State University Tempe Ariz. The Johns Hopkins University Applied Physics Laboratory Laurel Md.

A recent AGU Chapman Conference brought together a variety of research groups to review significant advances made in double-diffusive convection (DDC) over the past decade. This phenomenon occurs when two or more species of different molecular diffusivities make opposing contributions to a stable density gradient, causing fluid motions to develop even in the absence of external driving forces. If more than two components are involved, it is called multicomponent convection. In a double-diffusive (DD) system, if the fluid is unstably stratified with respect to the slowly diffusive solute and vice versa, narrow fingerlike cells may appear, within which fluid particles may rise and sink. This is known as finger convection. When the fluid system is unstable with respect to the faster diffusing species, a series of convecting layers separated by stable interfaces is formed; such interfaces are known as diffusive interfaces. In the decade following the discovery of DDC, linear stability theories for the finger and diffusive cases were developed and laboratory experiments and oceanic observations on DDC phenomenon were reported. Applications in areas such as fluid dynamics, astrophysics, geology, oceanography, limnology, and energy technology were soon realized.

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Raman Scattering and X-Ray Diffraction Studies of Gallium Nitride Films Grown on (100) Gallium Arsenide

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MRS Online Proceedings Library 1994年第1期339卷 503-508页

作者： S. W. Brown S. C. Rand C.-H. Hong D. Pavlidis University of Michigan Applied Physics Program Ann Arbor USA Department of Electrical Engineering and Computer Science University of Michigan Solid State Electronics Laboratory Ann Arbor USA

Raman spectroscopy and x-ray diffraction are used to characterize Gallium Nitride (GaN) films grown on (100) Gallium Arsenide (GaAs) substrates. Reflection X-ray diffraction data from (200) planes of GaAs and cubic GaN are presented. The linewidth of the cubic GaN diffraction peak is shown to be a strong function of the growth temperature. Raman spectra are presented for a series of samples grown at different temperatures. Raman scattering is characterized by strong peaks at 560 cm-1 and at 736 cm-1, corresponding to TO and LO phonon modes of cubic GaN, respectively. An additional, unexplained feature at 768 cm-1 is clearly observed in Raman spectra of c-GaN samples grown at lower temperatures. The polarization dependence of the intensity of the GaN LO phonon mode is presented and compared with the GaAs LO phonon mode to establish the relative orientation of the c-GaN epitaxy on GaAs.

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PILOT-STUDY RISK ASSESSMENT FOR SELECTED PROBLEMS AT 3 UNITED-STATES-DEPARTMENT-OF-ENERGY FACILITIES

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ENVIRONMENT INTERNATIONAL 1994年第5期20卷 585-604页

作者： HAMILTON, LD HOLTZMAN, S MEINHOLD, AF MORRIS, SC PARDI, R ROWE, MD SUN, C ANSPAUGH, LR BOGEN, KT DANIELS, JI LAYTON, DW MCKONE, TE STRAUME, T ANDRICEVIC, R JACOBSON, RL Biomedical and Environmental Assessment Group Department of Applied Science Brookhaven National Laboratory Upton NY 11973-5000 USA Radiological Sciences Division Department of Advanced Technology Brookhaven National Laboratory Upton NY 11973-5000 USA Health and Ecological Assessment Division Geosciences and Environmental Research Program Physics Department Lawrence Livermore National Laboratory Livermore CA 94551-9900 USA Desert Research Institute Water Resources Center University and Community College System of Nevada Las Vegas NV 89132-0040 USA

Objective and realistic human health risk assessments were performed for environmental problems at the Savannah River Site (SRS), the Fernald Environmental Management Project (FEMP), and the Nevada Test Site (NTS). At the SRS, cancer mortality risks were analyzed for projected public exposures to H-3 and Cs-137 released into the Savannah River. For annual human exposures to SRS tritium in Savannah River water, calculated incremental individual lifetime risks in two human receptor populations were small (8x10(-7);upper 95 percentile point of the distribution). The 95th percentile point of the distribution for incremental individual lifetime risks from one year's exposure to Cs-137 is less than 10(-8). No deaths are expected in either population as a result of exposures to tritium or cesium released to the Savannah River from the SRS. Routine releases of radon and radon progeny from the K-65 silos at FEMP resulted in individual lifetime risks greater than 1x10(-4) only for onsite workers and fenceline residents. Population risks were less than 1.0 for all identified receptor populations. Assessment of risks from exposure to uranium in ground water released by the FEMP predicted no toxic effects for human receptors. All estimated cancer risks were small. The largest predicted no individual lifetime risk was for a well close to the facility (1.3x10(-5)). For the various above-ground shot sites at the NTS, the highest predicted lifetime cancer risks are for a resident farmer, assuming a loss of institutional control, and exceed 1x10(-4) at the 95th cumulative percentile level. At 50 000 and 100 000 y in the future, the predicted cancer risks are all below 10(-6). In the assessment of exposure to radionuclides in ground water at the NTS, for an individual onsite near the site boundary, the geometric mean of the maximum potential excess lifetime risk of cancer mortality for an individual is 7x10(-3). For an individual using water offsite, the geometric mean of the maximum

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Survival analysis of the gamma-ray burst data

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Journal of the American Statistical Association 1994年第426期89卷 452-452页

作者： Efron, Bradley Petrosian, Vahé Professor/Chairman Statistics Department Stanford University CA 94305-4065 Professor of Health Research and Policy Department of Statistics Stanford University CA 94305-4065 Chairman of Astronomy Program and Professor of Physics and Applied Physics Departments Stanford University CA 94305-4065 We thank the SWiNE SMM and BATSE teams

Gamma-ray bursts are short but powerful pulses of electromagnetic radiation coming from space. Specially equipped satellites have observed the arrival of several hundred bursts during the past two decades. The origin of these bursts, whether from near the earth, across the galaxy, or out of the deep cosmos, has become a major question for astronomers. At the heart of this question lies a statistical puzzle relating to familiar issues in survival analysis Kaplan--Meier-type curves, data truncation, hazard rate analysis, Mantel--Haenszel statistics, and so on. Survival analysis is used here to critically examine two different models that have been proposed for the gamma-ray burst data. The analysis contradicts one of these models.

关键词： Bayesian statistics Climatic change Gaussian random fields

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