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. Bui...
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.
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...
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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 co...
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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.
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...
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.
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 excit...
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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 ?.
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 spec...
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.
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 Ga...
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.
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...
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
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 ...
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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.
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