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Ultrafast and nonlinear optical characterization of optical limiting processes in fullerenes

Description: The authors present recent results of broadband femotosecond (fs) transient absorption (TA) and broadband nanosecond (ns) optical limiting (OL) studies of C{sub 60} and derivatized C{sub 60}. Improvements in measurement techniques for fs TA spectra provide sensitivity to 10{sup {minus}5} in differential transmission, allowing detailed comparison of excited-state spectra with established energy level diagrams, as well as comparison of the ratio of triplet to singlet excited-state absorption cross sections from TA spectra with those obtained by modeling time transients at different wavelengths. For derivatized fullerenes, which provide enhanced solubility and a ground-state absorption extended into the infrared compared with C{sub 60} there is no spectral region where the triplet absorption cross section dominates the singlet as strongly as demonstrating broadband limiting in all 6, 6 mono-adducts and neat C{sub 60}. The authors report new approaches to processing sol-gel encapsulated fullerenes to improve the OL performance of solid-state materials to approach the response of solution limiters.
Date: October 1, 1997
Creator: Kohlman, R.; Klimov, V. & Shi, X.
Partner: UNT Libraries Government Documents Department

Self-Consistent Multiscale Theory of Internal Wave, Mean-Flow Interactions

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The research reported here produced new effective ways to solve multiscale problems in nonlinear fluid dynamics, such as turbulent flow and global ocean circulation. This was accomplished by first developing new methods for averaging over random or rapidly varying phases in nonlinear systems at multiple scales. We then used these methods to derive new equations for analyzing the mean behavior of fluctuation processes coupled self consistently to nonlinear fluid dynamics. This project extends a technology base relevant to a variety of multiscale problems in fluid dynamics of interest to the Laboratory and applies this technology to those problems. The project's theoretical and mathematical developments also help advance our understanding of the scientific principles underlying the control of complex behavior in fluid dynamical systems with strong spatial and temporal internal variability.
Date: June 3, 1999
Creator: Holm, D.D.; Aceves, A.; Allen, J.S.; Alber, M.; Camassa, R.; Cendra, H. et al.
Partner: UNT Libraries Government Documents Department

Unified treatment of collective instabilities and nonlinear beam dynamics

Description: Nonlinear dynamics deals with parametric resonances and diffusion, which are usually beam-intensity independent and rely on a particle Hamiltonian. Collective instabilities deal with beam coherent motion, where the Vlasov equa-tion is frequently used in conjunction with a beam-intensity dependent Hamiltonian. We address the questions: Are the two descriptions the same? Are collective instabilities the results of encountering parametric resonances whose driv-ing force is intensity dependent? The space-charge domi-nated beam governed by the Kapchinskij-Vladimirskij (K-V) envelope equation [1] is used as an example.
Date: April 19, 1999
Creator: Lee, K.Y. Ng and S.Y.
Partner: UNT Libraries Government Documents Department

Nonlinear Dynamics of Parametrically Excited Gyroscopic Systems

Description: The primary objective of this project is to determine how some of the powerful geometric methods of dynamical systems can be applied to study nonlinear gyroscopic systems. We proposed to develop techniques to predict local and global behavior and instability mechanisms and to analyze the interactions between noise, stability, and nonlinearities inherent in gyroscopic systems. In order to obtain these results we use the method of normal forms, global bifurcation techniques, and various other dynamical systems tools.
Date: June 1, 2001
Creator: Namachchivaya, N. S.
Partner: UNT Libraries Government Documents Department

Simulational studies of the Farley-Buneman in the equatorial electrojet

Description: The Farley-Buneman instability in the equatorial electrojet current system in the E-region of the ionosphere has been identified as the cause of the observed Type I electron density irregularities. The goal of this work was to study the instability in the equatorial region.
Date: July 1, 1995
Creator: Otani, N.; Seyler, C. & Kelley, M.
Partner: UNT Libraries Government Documents Department

An Improved Linear Tetrahedral Element for Plasticity

Description: A stabilized, nodally integrated linear tetrahedral is formulated and analyzed. It is well known that linear tetrahedral elements perform poorly in problems with plasticity, nearly incompressible materials, and acute bending. For a variety of reasons, linear tetrahedral elements are preferable to quadratic tetrahedral elements in most nonlinear problems. Whereas, mixed methods work well for linear hexahedral elements, they don't for linear tetrahedrals. On the other hand, automatic mesh generation is typically not feasible for building many 3D hexahedral meshes. A stabilized, nodally integrated linear tetrahedral is developed and shown to perform very well in problems with plasticity, nearly incompressible materials and acute bending. Furthermore, the formulation is analytically and numerically shown to be stable and optimally convergent. The element is demonstrated to perform well in several standard linear and nonlinear benchmarks.
Date: April 25, 2005
Creator: Puso, M
Partner: UNT Libraries Government Documents Department

Use of response surface metamodels for damage identification of a simple nonlinear system.

Description: The need for low order models capable of performing damage identification has become apparent in many structural dynamics applications where structural health monitoring and damage prognosis programs are implemented. These programs require that damage identification routines have low computational requirements and be reliable with some quantifiable degree of accuracy. Response surface metamodels (RSMs) are proposed to fill this need. Popular in the fields of chemical and industrial engineering, RSMs have only recently been applied in the field of structural dynamics and to date there have been no studies which fully demonstrate the potential of these methods. In this thesis, several RSMs are developed in order to demonstrate the potential of the methodology. They are shown to be robust to noise (experimental variability) and have success in solving the damage identification problem, both locating and quantifying damage with some degree of accuracy, for both linear and nonlinear systems. A very important characteristic of the RSMs developed in this thesis is that they require very little information about the system in order to generate relationships between damage indicators and measureable system responses for both linear and nonlinear structures. As such, the potential of these methods for damage identification has been demonstrated and it is recommended that these methods be developed further.
Date: January 1, 2003
Creator: Cundy, A. L. (Amanda L.); Hemez, F. M. (Fran├žois M.); Inman, D. J. & Park, G. H. (Gyu Hae)
Partner: UNT Libraries Government Documents Department

Annual progress report

Description: The nonlinear analysis of plasma instabilities in the threshold regime is discussed. The emphasis remains on carrying out calculations in realistic geometry and making comparison with experiment. Attention has shifted entirely to fully ionized plasma. An analysis was made of the collisional drift-wave instability. Considerable progress was made in explaining the explosive nature of the mirror-flute instability and in understanding the flute instability in the presence of both density and temperature gradients. The application of the threshold method to parametric instabilities in nonuniform plasma is discussed. Details of the results and comparison with experiment are given. (auth)
Date: September 18, 1973
Creator: Simon, A.
Partner: UNT Libraries Government Documents Department

Edge detection by nonlinear dynamics

Description: We demonstrate how the formulation of a nonlinear scale-space filter can be used for edge detection and junction analysis. By casting edge-preserving filtering in terms of maximizing information content subject to an average cost function, the computed cost at each pixel location becomes a local measure of edgeness. This computation depends on a single scale parameter and the given image data. Unlike previous approaches which require careful tuning of the filter kernels for various types of edges, our scheme is general enough to be able to handle different edges, such as lines, step-edges, corners and junctions. Anisotropy in the data is handled automatically by the nonlinear dynamics.
Date: July 1994
Creator: Wong, Yiu-fai
Partner: UNT Libraries Government Documents Department

Analyzing measurements of nonlinear transfer functions with Tschebyshev polynomials

Description: Recently, due to advances in computers and data aquisition systems, the following type of measurement has become more common: (1) Impress a given modulation on a device to be tested. (2) Acquire a data stream, usually at equally spaced sample intervals, of the response of the system to the modulation. (3) Fit the data thereby acquired to some nonlinear function set that might (or might not!) describe the response of the device. In this paper it is pointed out that by choosing to modulate the test parameter sinusoidally, and by fast-Fourier transforming the acquired data stream, one unambiguously determines the Tschebyshev expansion of the response function around the working point, potentially yielding quantitative information about high nonlinear orders in the system response. The need for data fitting is thereby eliminated. A detailed example, the analysis of the nonlinear phase-phase transfer function in the Jefferson Lab injector, is presented.
Date: June 1, 1997
Creator: Krafft, G. A.
Partner: UNT Libraries Government Documents Department

Beam stability and nonlinear dynamics. Summary report

Description: A {open_quotes}Beam Stability and Nonlinear Dynamics{close_quotes} Symposium was held October 3-5, 1996 at the Institute for Theoretical Physics (ITP) in Santa Barbara. This was one of the 3 symposia hosted by the ITP and supported by its sponsor, the National Science Foundation, as part of our {open_quotes}New Ideas for Particle Accelerators{close_quotes} program. The symposia was organized and chaired by Dr. Zohreh Parsa of ITP/ Brookhaven National Laboratory. The purpose of this symposium was to deal with some of the fundamental theoretical problems of accelerator physics by bringing together leaders from accelerator physics communities, mathematics, and other fields of physics. The focus was on nonlinear dynamics and beam stability. The symposium began with some defining talks on relevant mathematical topics such as single-particle Hamiltonian dynamics, chaos, and new ideas in symplectic integrators. The physics topics included single-particle and many-particle dynamics. These topics concern circular accelerators in which particles circulate for a very large number of turns as well as linear accelerators where space charge and wakefields induced in accelerating cavities play a strong role. A major question is to determine the best model for numerical simulations in order to accurately reproduce behavior of beams in real accelerators and to predict long-term or long distance stability. Comparison with experiment is recognized as an important tool in improving models.
Date: December 31, 1996
Creator: Parsa, Z.
Partner: UNT Libraries Government Documents Department

Study of microwave instability

Description: The microwave instability is usually described by linearizing Vlasov equation in the angle-action variables I, {phi} and assuming that the interaction of azimuthal harmonics {rho}{sub n}(I) of the distribution function {rho} is weak. The argument implied here is that the Hamiltonian flow smears out particles over invariant tori characterized by the action variables, and the remaining azimuthal dependence of the distribution function is small. Indeed, such an approach successfully describes bunch spectrum and the threshold of the microwave instability. However, recently there have been interesting observations of bunch centroid and bunch shape oscillations above instability threshold at LEP and the damping ring at SLAC. There are also indicates that the oscillations sometimes occur in localized region in the longitudinal coordinate instead of affecting the entire longitudinal distribution as one expects by an action-angle analysis. In this paper the authors describe an alternative approach to the problem of bunch stability using decomposition of the Fokker-Plank equation in the system of nonlinear equations for the moments of the distribution function. In particular, this approach allows them to avoid the conventional action-angle decomposition. The physical quantities they are interested in, the moments, are expressed in the Cartesian z {minus} {delta} phase space. To close the infinite hierarchy of moments equations, the authors assume that higher order correlations are small. Although both the action-angle and the Cartesian languages must be equivalent before truncation, they may have different speed of convergence depending on the problem being studied. It is hoped that Cartesian expansion approach would converge faster for the cases corresponding to those observed recently above threshold. The recent experimental observations made them interested in it again. This note is a progress report of their work.
Date: December 1, 1996
Creator: Heifets, S. & Chao, A.
Partner: UNT Libraries Government Documents Department

Nonlinear tearing mode interactions and mode locking in reversed field pinches

Description: The nonlinear interaction of a set of tearing instabilities and plasma flow is studied in a cylindrical plasma. An analytic theory of mode locking is developed which includes the effects of the localized electromagnetic torques, plasma inertia and cross-field viscosity. The calculation is specialized for the case of mode locking on the Madison Symmetric Torus (MST) reversed field pinch. In MST plasmas, a set of m = 1 tearing instabilities become phase locked and form a toroidally localized, rotating magnetic disturbance. An evolution equation for the phase velocity of this magnetic disturbance is derived which accounts for two types of electromagnetic torques. The external torques describe the interaction of the tearing modes with static magnetic perturbations located outside the plasma region. The interior torques describe the nonlinear interaction of three tearing modes which satisfy a wave number resonance condition. For conditions typical of MST, the internal torques dominate the external torques, which suggest the nonlinear interaction of tearing instabilities play a prominent role in the momentum degradation and mode locking.
Date: June 1, 1996
Creator: Hegna, C.C.
Partner: UNT Libraries Government Documents Department

Relationship between k{sub eff} and the fraction of critical mass

Description: It is not universally understood that k{sub eff} and fractional critical mass are related in a non linear fashion. For example, a neutronic system with a k{sub eff} = 0. 95 is NOT at 95% of its critical mass. What is striking is just how non-linear the relationship between k{sub eff} and critical mass really is. This relationship is investigated and documented below for both unfavorable (i.e., very reactive) and favorable (less reactive) geometries. The implications of this non-linearity for criticality safety regulation will also be discussed.
Date: May 1, 1997
Creator: O`Dell, R.D. & Parsons, D.K.
Partner: UNT Libraries Government Documents Department

Low level monitoring and control of nonlinear systems

Description: In this paper, we propose a nonparametric method for monitoring and controlling nonlinear systems whose dynamics is, in general, unknown or only partially known. Our nonparametric method is based on the stochastic linearization of the underlying (unknown) nonlinear system.
Date: December 31, 1994
Creator: Cover, A.; Reneke, J.; Lenhart, S. & Protopopescu, V.
Partner: UNT Libraries Government Documents Department

{open_quotes}Heavy light bullets{close_quotes} in electron-positron plasma

Description: The nonlinear propagation of circularly polarized electromagnetic waves with relativistically strong amplitudes in an unmagnetized hot electron-positron plasma with a small fraction of ions is investigated. The possibility of finding localized solutions in such a plasma is explored. It is shown that these plasmas support the propagation of {open_quotes}heavy light bullets{close_quotes}; nondiffracting and nondispersive electromagnetic (EM) pulses with large density bunching.
Date: March 1, 1995
Creator: Berezhiani, V.I. & Mahajan, S.M.
Partner: UNT Libraries Government Documents Department

Nonlinear whistler wave scattering in space plasmas

Description: In this paper the evolution of nonlinear scattering of whistler mode waves by kinetic Alfven waves (KAW) in time and two spatial dimensions is studied analytically. The authors suggest this nonlinear process as a mechanism of kinetic Alfven wave generation in space plasmas. This mechanism can explain the dependence of Alfven wave generation on whistler waves observed in magnetospheric and ionospheric plasmas. The observational data show a dependence for the generation of long periodic pulsations Pc5 on whistler wave excitation in the auroral and subauroral zone of the magnetosphere. This dependence was first observed by Ondoh T.I. For 79 cases of VLF wave excitation registered by Ondoh at College Observatory (L=64.6 N), 52 of them were followed by Pc5 geomagnetic pulsation generation. Similar results were obtained at the Loparskaia Observatory (L=64 N) for auroral and subauroral zone of the magnetosphere. Thus, in 95% of the cases when VLF wave excitation occurred the generation of long periodic geomagnetic pulsations Pc5 were observed. The observations also show that geomagnetic pulsations Pc5 are excited simultaneously or insignificantly later than VLF waves. In fact these two phenomena are associated genetically: the excitation of VLF waves leads to the generation of geomagnetic pulsations Pc5. The observations show intensive generation of geomagnetic pulsations during thunderstorms. Using an electromagnetic noise monitoring system covering the ULF range (0.01-10 Hz) A.S. Fraser-Smith observed intensive ULF electromagnetic wave during a large thunderstorm near the San-Francisco Bay area on September 23, 1990. According to this data the most significant amplification in ULF wave activity was observed for waves with a frequency of 0.01 Hz and it is entirely possible that stronger enhancements would have been measured at lower frequencies.
Date: April 1, 1997
Creator: Yukhimuk, V. & Roussel-Dupre, R.
Partner: UNT Libraries Government Documents Department

Recent progress in linear and nonlinear studies of toroidal Alfven eigenmode

Description: TAE modes are studied in linear and nonlinear regimes using several kinetic/MHD hybrid models. It is shown that the stability of TAE mode is largely determined by its radial mode structure. The calculated stability thresholds are correlated well with observations, including the recently observed alpha-driven TAE modes in the TFTR DT experiments. In the nonlinear regime, quasilinear simulations with multiple modes show that the saturation level is enhanced by nonlinear wave-particle resonance overlapping when the linear growth rate exceeds a critical value. A fully self-consistent {delta}f noise reduction method for the 3D particle/MHD hybrid model is developed.
Date: May 1, 1997
Creator: Fu, G.Y.; Chen, Y. & Budny, R.
Partner: UNT Libraries Government Documents Department

Multifractal analysis and modeling of one- and two-dimensional data with discrete wavelet transforms, isotropic or not

Description: The authors compare several ways of uncovering multifractal properties of data in 1D and 2D using wavelet transforms. The WTMM or (Continuous) Wavelet Transform Maximum Modulus method has been extensively documented and widely applied by Dr. Alain Arneodo`s (Bordeaux) group, to the point where their successes have overshadowed simpler techniques that use the Discrete WT. What the latter lack in robustness is gained in efficiency, thus enabling virtually real-time multifractal analysis of data as it is collected. Another advantage of DWT-based approaches is that tensor products of dyadic and triadic branching schemes enable a straightforward attack on strong anisotropy in natural and artificial 2D random fields.
Date: December 1998
Creator: Davis, A. B.
Partner: UNT Libraries Government Documents Department

Nonlinear elastic wave interaction in a sandstone bar: A summary of recent pulse-mode experiments

Description: We have performed nonlinear pulse propagation experiments in a 3.8 cm diameter rod of Berea sandstone 1.8 m long at ambient conditions. Unlike earlier studies, we measured acceleration and not displacement. Moreover, we detected 2nd and 3rd harmonic growth at smaller strain amplitudes than were observed previously (10{sup {minus}7}). Harmonic growth at identical strain amplitudes has also been noted in resonance studies using the same rock type. Current measurements are underway with the rod in vacuum where the wave attenuation is less and the conditions can be carefully controlled. Ultimately, we wish to test the validity of current analytic and numerical models for nonlinear propagation in microcracked materials.
Date: March 1, 1995
Creator: Johnson, P.A.; TenCate, J.A.; Cherry, R.; McCall, K.; Van Den Abeele, K.; Kadish, A. et al.
Partner: UNT Libraries Government Documents Department

How Wigner functions transform under symplectic maps

Description: It is shown that, while Wigner and Liouville functions transform in an identical way under linear symplectic maps, in general they do not transform identically for nonlinear symplectic maps. Instead there are quantum corrections whose {Dirac_h} {r_arrow} 0 limit may be very complicated. Examples of the behavior of Wigner functions in the {Dirac_h} {r_arrow} 0 limit are given in order to examine to what extent the corresponding Liouville densities are recovered.
Date: May 16, 1998
Creator: Dragt, A.J. & Habib, S.
Partner: UNT Libraries Government Documents Department

Summary report of the group on single-particle nonlinear dynamics

Description: This report summarizes the research on single-particle nonlinear beam dynamics. It discusses the following topics: analytical and semi-analytical tools; early prediction of the dynamic aperture; how the results are commonly presented; Is the mechanism of the dynamic aperture understand; ripple effects; and beam-beam effects.
Date: October 1, 1996
Creator: Axinescu, S.; Bartolini, R. & Bazzani, A.
Partner: UNT Libraries Government Documents Department