A novel algorithm to solve the differential equation describing the non-interactive multiple-trap system model in thermoluminescence

The analysis of glow curves in thermoluminescence requires finding a closed expression for the light emitted as a function of the temperature and the parameters characterizing trap and recombination centers. Since it is not possible to derive a closed expression from the set of coupled differential equations describing the thermoluminescence phenomenon, approximations are made even for the simplest models. All of them resort to an approximation known as the quasi-equilibrium approximation (QE), and to further approximations. In this article, an algorithm is reported that permit the derivation of a closed expression for the emitted light for the model known as the non-interactive multi-trap system (NMTS) model by resorting only to the QE approximation. It is shown that the integration of the first order differential equation related to the NMTS model can be replaced by finding the roots of an analytical expression.     

Delta function approximations in level set methods by distance function extension

In [A.-K. Tornberg, B. Engquist, Numerical approximations of singular source terms in differential equations, J. Comput. Phys. 200 (2004) 462–488], it was shown for simple examples that the then most common way to regularize delta functions in connection to level set methods produces inconsistent approximations with errors that are not reduced with grid refinement. Since then, several clever approximations have been derived to overcome this problem. However, the great appeal of the old method was its simplicity. In this paper it is shown that the old method – a one-dimensional delta function approximation extended to higher dimensions by a distance function – can be made accurate with a different class of one-dimensional delta function approximations. The prize to pay is a wider support of the resulting delta function approximations.     

Illustrations of a dynamical theory of the ether

The Schrödinger and Klein-Gordon equations for free, structureless particles are derived classically from two different continuum approximations to a Boltzmann equation for the trace component of a mixture. The majority component is designated as the ether. Deviations from these continuum approximations (rarefied ether) yield deviations from the Schrödinger and Klein-Gordon equations which are shown explicitly.     

Possibility of complex eigenvalues in approximate solutions of the homogeneus Faddeev equations and their implication for the general n-body case

It is shown that use of approximations similar to those encountered in atomic, molecular and nuclear scattering and also in shell model (bound state) calculations may give rise to complex energy eigenvalues for the Faddeev 3-body equations. This is in contradistinction to results based on exact solutions or on use of separable approximations. Some implications for the general many-body case are noted.     

The hilbert expansion to the Boltzmann equation for steady flow

The Hilbert expansion to the Boltzmann equation is carried out for steady flow. It is shown that the first term in the Hubert series for the distribution function is a local Maxwellian leading to the steady Euler equations. The steady field equations that follow from the solution of the second term in the series are derived. The formulas for thermal conductivity and for viscosity of Hilbert that appear in the steady field equations of the second approximation are shown to be precisely the same as those obtained by Chapman and Enskog. The procedure to obtain higher approximations by Hubert's method is summarized.     

Closure Approximations for Passive Scalar Turbulence: A Comparative Study on an Exactly Solvable Model with Complex Features

Some standard closure approximations used in turbulence theory are analyzed by examining systematically the predictions these approximations produce for a passive scalar advection model consisting of a shear flow with a fluctuating cross sweep. This model has a general geometric structure of a jet flow with transverse disturbances, which occur in a number of contexts, and it encompasses a wide variety of possible spatio-temporal statistical structures for the velocity field, including strong long-range correlations. Even though the Eulerian and Lagrangian velocity statistics are not equal and the passive scalar statistics exhibit broader-than-Gaussian intermittency, this model is nevertheless simple enough so that many passive scalar statistics can be computed exactly and compared systematically with the predictions of the closure approximations. Our comparative study illustrates the strength and weaknesses of the closure approximations and points out the physical phenomena that these approximations are able or not able to describe properly. In particular it is shown that the direct interaction approximation (DIA), one of the most sophisticated closure approximations available, fails to reproduce adequately the statistical features of the scalar and may even lead to absdurd predictions, even though the equations it produces are rather complicated and difficult to analyze. Two alternative closure approximations, the Modified DIA (MDIA) and the Renormalized Lagrangian Approximation (RLA), with different levels of sophistication, both are simpler to use than the DIA and perform better. In particular, it is shown that both closure approximations always reproduce exactly the second order statistics for the scalar and that the MDIA is even able to capture intermittency effects.     

Formal study of a chemical reaction by Grad expansion of the Boltzmann equation. II

In a previous communication the Boltzmann equations for a bimolecular chemical reaction were transformed into an infinite system of quadratic differential equations. It is shown here that the reaction rate may be calculated approximately by using the first differential equation only, which is a generalization of the phenomenological law of chemical kinetics. Then, a general method for solving the expanded Boltzmann equations by successive approximations is proposed and studied to first order.     

Calculation of light pulses with chirp in passively mode-locked lasers taking into account the phase memory of absorber and amplifier

Starting from the complete set of Maxwell's equations and density matrix equations for the atomic systems, the change of amplitude and phase of light pulses in passing through absorbing and amplifying samples has been calculated by using certain approximations. It is shown that the phase modulation originates from saturation and phase memory in off-resonance interaction. The use of the simple rate-equation approximations.is only justified if saturation dominates. The full cavity round-trip equation has been established and solved for steady-state pulses under different conditions. For the case of pulses being outside resonance with the media we take into account a linear optical element for intracavity chirp compensation in order to describe the regime, where in experiment the shortest pulses have been found.     

Why and how to avoid standard kinetic equations I. Principles and band conduction problem

A review of ideas leading to full rejection of any finite or partially-infinite order kinetic equation linearized in external field is given on grounds of the time-convolution Generalized Master Equations (GME). By two examples (two-level and band conduction problem), it is shown how standard kinetic equations result from GME in the lowest order approximations which obscure, however, a direct correspondence with the Kubo linear response theory. Without approximations, on the other hand, the rigorous approach is shown to be fully equivalent with the Kubo results. It is argued and illustrated that usual technical simplicity and seeming physical lucidity of standard theories (connected with the presence of field-independent transfer or scattering rates in the fielddependent linearized theory) are just owing to structural features which are solely due to the lowest order approximations involved. These features (i.e. also the possibility of standard physical interpretation of kinetic phenomena) are proved to disappear completely as far as the theory goes properly to higher orders.     

Kinetic theory in the weak-coupling approximation: I. Formal theory and application to classical open systems

A general formalism, where irreversible processes are related to singularities of the resolvent of the Liouville operator, is applied to classical open systems. For a system weakly coupled to a thermal reservoir, a kinetic equation is derived. It is shown that the method leads to equations defining a positivity-preserving semigroup with the Maxwell-Boltzmann distribution as a stationary solution and obeying an H-theory. It is pointed out that these properties are not always shared by irreversible equations obtained as asymptotic approximations of the so-called generalized master equation.     

Quasi steady state and partial equilibrium approximations: their relation and their validity

The quasi steady state and partial equilibrium approximations are analysed in the context of a system of nonlinear differential equations exhibiting multiscale behaviour. Considering systems in the most general and dimensional form , it is shown that both approximations are limiting cases of leading-order asymptotics. Algorithmic conditions are established which guarantee that the accuracy and stability delivered by the two approximations are equivalent to those obtained with leading-order asymptotics. It is shown that the quasi steady state approximation is a limiting case of the partial equilibrium approximation. Algorithms are reported for the identification of the variables in quasi steady state and/or of the processes in partial equilibrium.     

柱等离子体中广义磁流体方程组的简化形式与本征模解

本文从含有广义欧姆定律的磁流体力学方程组出发,仅作符合加热托卡马克等离子体具体实验条件的近似,把广义磁流体方程组简化成4元一阶微分方程组。对均匀密度等离子体柱情况求出解析色散关系(本征模解),并证明Appert理论是本文的极限情况。 关键词：     

The physical and mathematical foundations of the intranuclear cascade model algorithm

The physical and mathematical approximations underlying the intranuclear cascade model (INC) algorithm are considered here. The INC algorithm is shown to be a version of Monte-Carlo calculations for multiple integrals which provide the solutions of the system of coupled equations for many-body distribution functions. The conditions under which these equations are derived from the Schrödinger many-body equations are analysed, with special emphasis on the finite size of the nuclear system.     

Stress-energy tensor for a massless spin 1/2 field in static black hole spacetimes

The stress-energy tensor for the massless spin 1/2 field is numerically computed outside and on the event horizons of both charged and uncharged static nonrotating black holes, corresponding to the Schwarzschild, Reissner-Nordstr?m, and extreme Reissner-Nordstr?m solutions of Einstein's equations. The field is assumed to be in a thermal state at the black hole temperature. Comparison is made between the numerical results and previous analytic approximations for the stress-energy tensor in these spacetimes. For the Schwarzschild (charge zero) solution, it is shown that the stress energy differs even in sign from the analytic approximation. For the Reissner-Nordstr?m and extreme Reissner-Nordstr?m solutions, divergences predicted by the analytic approximations are shown not to exist.     

The calculation of Dirac sea effects on finite solitons

The effects of polarization of the Dirac sea on finite solitons in a simple theory in which fermions interact with a single scalar field are studied. The mass shift for a given background scalar field is computed numerically and compared to approximations arising from expansions in inverse powers of the effective fermion mass and in powers of derivatives of the background scalar field. The conditions under which such approximations succeed are discussed. When such approximations work one can derive local equations of motion for the soliton fields which include the effects of polarizing the Dirac sea. These new equations are studied and energy minimization is used to explore the effects of the Dirac sea on the structure of the soliton. Calculations for a typical Friedberg-Lee soliton are presented, and it is shown that, while the approximations do not work well for fields employed to model the quark structure of nucleons, they do provide an upper bound for the mass of the soliton. A scalar field typical of those used to model ¹⁶O in quantum hadrodynamics is also studied, and it is shown that, when the effective potential is supplemented by the next term occurring in a derivative expansion, the renormalized shift in the energy of the Dirac sea is well approximated.     

Atomic density of a harmonically trapped ideal gas near Bose-Einstein transition temperature

We have studied the atomic density of a cloud confined in an isotropic harmonic trap at the vicinity of the Bose-Einstein transition temperature. We show that, for a non-interacting gas and near this temperature, the ground-state density has the same order of magnitude as the excited states density at the centre of the trap. This holds in a range of temperatures where the ground-state population is negligible compared to the total atom number. We compare the exact calculations, available in a harmonic trap, to semi-classical approximations. We show that these latter should include the ground-state contribution to be accurate.     

On the weak-field approximation in the two-body problem of general relativity

We consider the two-body problem of general relativity taking into account the retardation of interactions. The equations of motion are shown to be simplified, and this enables one to neglect the effects of heredity. The problem of validity of the approximations involved is considered in the extended-particle formalism. It is shown that under the correct treatment the self-interaction terms do not lead to unphysical solutions.     

Role of nonlinear refraction in the generation of terahertz field pulses by light fields

The generation of microwave (terahertz) pulses without any envelope in a four-level quasi-resonant medium is considered. Two intense quasi-monochromatic laser fields lead to a partial upper-level population. Microwave field pulses cause the transition between these levels. For appropriately chosen scales, the evolution of the fields is shown to be described by the pseudo-spin evolution equations in a microwave field with the inclusion of nonlinear refraction caused by an adiabatic upper-level population. The evolution of terahertz field pulses is described outside the scope of the slow-envelope approximation. When a number of standard approximations are taken into account, this system of equations is shown to be equivalent to an integrable version of the generalized reduced Maxwell-Bloch equations or to the generalized three-wave mixing equations. The soliton solution found by the inverse scattering transform method is used as an example to show that nonlinear refraction leads to a strong compression of the microwave (terahertz) field soliton.     

Plasmon-exciton self-induced transparency

The possibility of forming stable bound plasmon-polariton states in an extended metallic cylinder surrounded by a two-level medium has been investigated. The dynamics of plasmons is described in the hydrodynamic approximation. It has been shown that the equations of motion of charge-density bunches and the Bloch equations for the two-level medium are reduced in certain approximations to integrable equations for both transverse and longitudinal plasmons. In the former case, the initial system of equations after the application of the slow-envelope approximation is reduced to equations equivalent to the Maxwell-Bloch equations. In the latter case, the equations describe wave dynamics beyond the slow-envelope approximation. In the approximation of unidirectional wave propagation, the initial system of equations is reduced to equations related to the reduced Maxwell-Bloch equations. Soliton and breather-like solutions of the derived equations describe plasmon-exciton self-induced transparency.