Showing papers on "Field (physics) published in 1986"

The statistics of peaks of Gaussian random fields

Abstract: A set of new mathematical results on the theory of Gaussian random fields is presented, and the application of such calculations in cosmology to treat questions of structure formation from small-amplitude initial density fluctuations is addressed. The point process equation is discussed, giving the general formula for the average number density of peaks. The problem of the proper conditional probability constraints appropriate to maxima are examined using a one-dimensional illustration. The average density of maxima of a general three-dimensional Gaussian field is calculated as a function of heights of the maxima, and the average density of 'upcrossing' points on density contour surfaces is computed. The number density of peaks subject to the constraint that the large-scale density field be fixed is determined and used to discuss the segregation of high peaks from the underlying mass distribution. The machinery to calculate n-point peak-peak correlation functions is determined, as are the shapes of the profiles about maxima.

Separation of large DNA molecules by contour-clamped homogeneous electric fields

Abstract: Electric fields can be manipulated by a method in which multiple electrodes are arranged along a closed contour and clamped to predetermined electric potentials. This method may be applied to a broad range of problems in the separation of macromolecules by gel electrophoresis. DNA molecules as large as 2 megabases can be well separated with a contour-clamped homogeneous electric field alternating between two orientations 120 degrees apart. The pattern of separation is independent of position in the gel, which is an advantage over previous methods. DNA less than 50 kilobases can be separated without distortion even at high voltage with a nonalternating contour-clamped homogeneous field. Decreased band broadening in DNA less than 200 bases can be achieved with a contour-clamped inhomogeneous field.

The self-consistent-field approach to polyatomic vibrations

Abstract: on the Born-Oppenheimer approximation which separates the motion of the electrons from that of the nuclei. The electronic motion produces an effective potential which holds molecules together and governs their vibrational motion. The complexity of this potential gives rise to the richness of much of chemistry. Thus, a theoretical picture of the vibrations of molecules is at the heart of many chemical questions. The traditional view of vibrational motion is based on an harmonic approximation to the full nuclear potential. This very simple approximation gives rise to

Energy and action flow through the internal wave field: An eikonal approach

Abstract: The energy and action flow through the small-scale part of the oceanic internal wave field is modeled by use of the eikonal technique, which is not subject to a weak interaction assumption. Both Monte Carlo calculations and a simplified model are presented and found to agree. It is found that the action flows toward slightly higher frequency (and thus the waves gain energy), in striking contrast to weak interaction predictions of a strong frequency decrease. The energy dissipation scales with depth as N2 cosh−1 (N/f), in agreement with measurements. The overall level is, however, a factor of 4 smaller than measurements. Possible sources of this discrepancy are discussed. A comparison is made with previous theoretical approaches for the depth dependence of dissipation.

Systems with Small Dissipation

Abstract: Electromagnetic and mechanical oscillators are crucial in such diverse fields as electrical engineering, microwave technology, optical technology, and experimental physics. For example, such oscillators are the key elements in instruments for detecting extremely weak mechanical forces and electromagnetic signals are essential to highly stable standards of time and frequency. The central problem in developing such instruments is to construct oscillators that are as perfectly simple harmonic as possible; the largest obstacle is the oscillator's dissipation and the fluctuating forces associated with it. This book, first published in Russian in 1981 and updated with new data for this English edition, is a treatise on the sources of dissipation and other defects in mechanical and electromagnetic oscillators and on practical techniques for minimizing such defects. Written by a team of researchers from Moscow State University who are leading experts in the field, the book is a virtual encyclopedia of theoretical formulas, experimental techniques, and practical lore derived from twenty-five years of experience. Intended for the experimenter who wishes to construct near-perfect instrumentation, the book provides information on everything from the role of phonon-phonon scattering as a fundamental source of dissipation to the effectiveness of a thin film of pork fat in reducing the friction between a support wire and a mechanically oscillating sapphire crystal. The researchers that V. B. Braginsky has led since the mid-1960s are best known in the West for their contributions to the technology of gravitational-wave detection, their experimental search for quarks, their test of the equivalency principle, and their invention of new experimental techniques for high-precision measurement, including "quantum nondemolition movements." Here, for the first time, they provide a thorough overview of the practical knowledge and experimental methods that have earned them a worldwide reputation for ingenuity, talent, and successful technique.

Radiative gravitational fields in general relativity I. general structure of the field outside the source

Abstract: We present a well-defined formal framework, together with appropriate mathematical tools, which allow us to implement in a constructive way, and to investigate in full mathematical details, the Bonnor-Thorne approach to gravitational radiation theory. We show how to construct, within this framework, the general radiative (formal) solution of the Einstein vacuum equations, in harmonic coordinates, which is both past-stationary and past-asymptotically Minkowskian. We investigate the structure of the latter general radiative metric (including all tails and nonlinear effects) both in the near zone and in the far zone. As a side result it is proven that post-Newtonian expansions must be done by using the gauge functions (lg c)^p/c^n (p, n = positive integers).

Relation between electroabsorption in bulk semiconductors and in quantum wells: The quantum-confined Franz-Keldysh effect.

Abstract: We evaluate the interband optical absorption of a semiconductor quantum well in the presence of a uniform electric field perpendicular to the layer and neglecting excitonic effects. We show that this formally becomes the Franz-Keldysh effect in the limit of an infinitely thick layer. When the potential drop across the layer is small compared to the confinement energy we obtain behavior qualitatively different from the bulk Franz-Keldysh effect and we explain this in terms of a quantum-confined Franz-Keldysh effect; with increasing field we demonstrate numerically for a GaAs-like semiconductor that we recover Franz-Keldysh-like behavior, once the originally ``forbidden'' quantum-well transitions become strong. Our discussion gives an alternative physical picture for the Franz-Keldysh effect, including a simple explanation of the Franz-Keldysh oscillations.

Time evolution of Bloch electrons in a homogeneous electric field

Abstract: The theory of a Bloch electron moving in the presence of a homogeneous electric field is reviewed and objections to the conventional derivations are discussed. A new derivation of the time development of a Bloch electron moving in a homogeneous, but time-dependent, electric field is presented using a vector potential to describe the field rather than the usual scalar potential. This new treatment avoids all the basic assumptions of the conventional derivations and demonstrates that a Bloch electron will oscillate in a single band with the Bloch period if a homogeneous electric field is abruptly turned on, with a tunneling probability into other bands given by the conventional expression. It is also shown that the calculated optical absorption will have the same ladderlike structure that would be obtained if Wannier-Stark quantized energy levels are assumed, although the present calculation makes no such assumption. The previous objections to the existence of Bloch oscillations for electrons in a perfect periodic potential are examined and found to be irrelevant provided the tunneling probability per Bloch oscillation period is much less than one, a condition that is generally satisfied for typical elemental and compound semiconductors for electric fields smaller than ${10}^{6}$ V/cm.

Magnetostatic equilibria and analogous Euler flows of arbitrarily complex topology. Part 2. Stability considerations

Abstract: 0 The stability (i) of fully three-dimensional magnetostatic equilibria of arbitrarily complex topology, and (ii) of the analogous steady solutions of the Euler equations of incompressible inviscid flow, are investigated through construction of the second variations S2M and S2K of the magnetic energy and kinetic energy with respect to a virtual displacement field q(x) about the equilibrium configuration. The expressions for S2M and S2K differ because in case (i) the magnetic lines of force are frozen in the fluid as it undergoes displacement, whereas in case (ii) the vortex lines are frozen, so that the analogy between magnetic field and velocity field on which the existence of steady flows is based does not extend to the perturbed states. It is shown that the stability condition S2M > 0 for all q(x) for the magnetostatic case can be converted to a form that does not involve the arbitrary displacement q(x), whereas the condition S2K > 0 for all q for the stability of the analogous Euler flow cannot in general be so transformed. Nevertheless it is shown that, if S2M and S2K are evaluated for the same basic equilibrium field, then quite generally S2M+S2K > 0 (all non-trivial q). A number of special cases are treated in detail. In particular, it is shown that the space-periodic Beltrami field BE = (B, cosaz+B, sinuy, B, cosax+ B, sinuz, B, cosay+ B, sinux) is stable (i.e. S2M > 0 for all q) and that the medium responds in an elastic manner to perturbations on a scale large compared with a-l. By contrast, it is shown that S2K is indefinite in sign for the analogous Euler flow, and it is argued that the flow is unstable to certain large-scale helical perturbations having the same sign of helicity as the unperturbed flow. It is conjectured that all topologically non-trivial Euler flows are similarly unstable. 0

Electric-field dependence of linear optical properties in quantum well structures: Waveguide electroabsorption and sum rules

Abstract: We summarize the electric-field dependence of absorption and luminescence in quantum wells for fields perpendicular to the layers, present extended discussion of electroabsorption spectra and devices in waveguide samples, and derive sum rules for electroabsorption. Optical bistability, self-linearized modulation, and optical level shifting are demonstrated in self-electrooptic effect device configurations, with good modulation contrast and polarization-dependent properties. The electroabsorption spectra enable quantitative comparison of theory and experiment for absorption strengths in quantum wells with field. The sum rules enable excitonic effects to be included in the comparison, and good agreement is seen. One sum rule is also more generally applicable to electroabsorption in semiconductors.

Generalized nonlinear harmonic gyrotron theory

Abstract: The nonlinear efficiency for a gyrotron oscillator operating at harmonics of the cyclotron frequency has been calculated and is presented as a function of generalized parameters for the second through fifth harmonics. The numerical results are valid for a wide range of operating conditions, including voltage, current, beam radius, cavity dimensions, and operating mode. Relatively high efficiencies are found even at high harmonics; the maximum transverse efficiencies for harmonics 2, 3, 4, and 5 are 0.72, 0.57, 0.45, and 0.36, respectively. The calculation of the efficiency in terms of generalized parameters allows the straightforward design and optimization of harmonic gyrotrons. The influence of the axial profile of the rf field in the gyrotron cavity on the efficiency is also investigated. Improved efficiency can be achieved with asymmetric field profiles. The implications of these results for the generation of millimeter and submillimeter wave radiation by harmonic emission are discussed.

Dissipation in a fundamental model of quantum optical resonance

Abstract: The fully quantum-electrodynamical model of a two-level atom interacting with a single-cavity mode predicts an atomic evolution whose form is dictated by the discrete nature of the field energy and its statistical distribution. We demonstrate that the revivals of atomic excitation which are the signature of the quantum nature of the evolution are strongly affected by field dissipation even when the damping hardly affects the underlying Rabi oscillations.

Fundamentals concerning wave loading on offshore structures

Abstract: This article is aimed at relating a certain substantial body of established material concerning wave loading on offshore structures to fundamental principles of mechanics of solids and of fluids and to important results by G. I. Taylor (1928a, b). The object is to make some key parts within a rather specialised field accessible to the general fluid-mechanics reader.The article is concerned primarily to develop the ideas which validate a separation of hydrodynamic loadings into vortex-flow forces and potential-flow forces; and to clarify, as Taylor (1928b) first did, the major role played by components of the potential-flow forces which are of the second order in the amplitude of ambient velocity fluctuations. Recent methods for calculating these forces have proved increasingly important for modes of motion of structures (such as tension-leg platforms) of very low natural frequency.

Coherent optical eigenstate memory.

Abstract: A coherent optical memory is described The device operates on the principle that an optical resonator employing a holographic grating can have user-prescribed eigenmodes Each information entity (eg, an image of a cat) defines an eigenmode of the resonator The stored information is accessed by injecting partial information (eg, an image of the cat’s ear) into the resonator A gain medium internal to the resonator amplifies the field belonging to the eigenmode that most resembles the injected field; the other eigenmodes are suppressed through a competition for the gain A simple optical memory exhibiting associative recall is demonstrated

Active magnetic screening of coils for static and time-dependent magnetic field generation in NMR imaging

Abstract: A new method is introduced for screening or reducing extraneous magnetic fields outside coil structures The method utilises a set of current-carrying conductors or a discrete wire array to simulate the induced surface currents which occur in superconducting magnetic screens or which occur in high conductivity thick metal screens when placed around coils producing time-dependent fields or field gradients The active screens have the advantage that they work at any frequency including DC Field gradient coil structures utilising the principle can be compact and of low inductance Strong external fields can be made arbitrarily low thus making it feasible to generate large rapidly switched gradients within and in close proximity to a superconductive magnet Such gradients can be especially useful in the implementation of high speed NMR imaging methods

Two-and-One-Half Dimensional In-Plane Wave Propagation.

Abstract: : The purpose of this paper is to collect certain wave propagation results in two-and-one-half dimensions -- defined as three dimensional propagation in a medium that has variations in two dimensions only. The results of interest are for sources and receivers in the plane determined by the two directions of parameter variation. The objective of this work is to reduce the analysis of the in-plane propagation to two dimensional analysis while retaining -- at least asymptotically -- the proper three dimensional geometrical spreading. We do this for the free space Green's function and for the Kirchhoff approximate upward scattered field from a single reflector. In both cases, we carry out a derivation under the assumption of a background velocity with two dimensional -- c(x,z) -- variation; we specialize the results to a constant background velocity and a depth dependent background velocity. For the convenience of the user we have included a glossary and two tables of equation numbers to help in finding specific results. Keywords include: Ray method; Geometrical optics; Wave propagation; Green's function; Jacobian; Travel time; and WKB.

The average magnetic field draping and consistent plasma properties of the Venus magnetotail

Abstract: The detailed average draping pattern of the magnetic field in the deep Venus magnetotail is examined. The variability of the data ordered by spatial location is studied, and the groundwork is laid for developing a coordinate system which measured locations with respect to the tail structures. The reconstruction of the tail in the presence of flapping using a new technique is shown, and the average variations in the field components are examined, including the average field vectors, cross-tail current density distribution, and J x B forces as functions of location across the tail. The average downtail velocity is derived as a function of distance, and a simple model based on the field variations is defined from which the average plasma acceleration is obtained as a function of distance, density, and temperature.

Structure of the temperature field downwind of a line source in grid turbulence

Abstract: A Lagrangian stochastic model is used in conjunction with detailed wind-tunnel measurements to examine the structure and development of the temperature field behind a line source in grid turbulence. It is shown that on the scale of these experiments molecular diffusion and viscosity have an important influence on temperature fluctuations (particularly on the intensity of these fluctuations) and must be explicitly modelled. The model accounts for a wide range of the measured properties of the temperature field and provides a unified treatment of temperature fluctuations through all stages of the development of the temperature field. This development is discussed in terms of a simple physical picture in which the hot plume is initially smooth and is moved about bodily by the turbulence, but gradually develops increasing internal structure or patchiness as it grows with distance downstream until a self-similar state is reached in which this internal structure maintains the temperature fluctuations.

Evolution of Experimental Techniques for the Study of the Electrical Properties of Insulating Materials

Abstract: When an electric field is applied to an insulating material three basic processes can take place: the dipoles tend to rotate, ions migrate and space charge can be injected at the interfaces, depending on such parameters as temperature or applied electric field. For many years, a direct analysis was impossible since the experimentally observable variables gave only an average of what was taking place in the samples under study. Many methods were developed over the years combining available measuring techniques with theoretical hypotheses. They led to phenomenological descriptions but it appeared that a direct measurement of space charge or polarization distributions in the materials would be a way for understanding the physical processes involved. In this paper we will present the evolution of these methods and will describe on which at present seems the most promising. It uses the propagation in the sample of a pressure wave which acts as a virtual probe sensitive to charge, field or potential. A very elegant technique to generate this pressure wave involves the use of a short-duration laser pulse. It will be shown how this can find applications in a very large number of areas. Examples will be given in such different fields as electrets, HV insulation, and transducer materials.