Showing papers on "Thermal equilibrium published in 1990"

Particle Production During Out-of-equilibrium Phase Transitions

Abstract: Techniques are developed to calculate the energy production in quantum fields which obtain a mass through the spontaneous symmetry breaking of a second field which is undergoing a phase transition. All fields are assumed to be out of thermal equilibrium and weakly coupled. The energy produced in a field, which is initially in its ground state, is computed for two generic types of time-dependent masses: a roughly monotonic turn on of the mass and an oscillatory mass. The formalism is applied to the questions of particle production and reheating in inflationary universe models. Requirements are found which the couplings in new-inflation-type models must satisfy for efficient reheating to occur.

Unitarity limits on the mass and radius of dark-matter particles

Abstract: Using partial wave unitarity and the observed density of the Universe, it is show that a stable elementary particle which was once in thermal equilibrium cannot have a mass greater than 340 TeV. An extended object which was once in thermal equilibrium cannot have a radius less than 7.5 x 10(exp -7) fm. A lower limit to the relic abundance of such particles is also found.

Charged black hole in a grand canonical ensemble.

Abstract: A spherical charged black hole in thermal equilibrium is considered from the perspective of a grand canonical ensemble in which the electrostatic potential, temperature, and surface area are specified at a finite boundary. A correspondence is established between the boundary-value data of a well-posed problem in a finite region of Euclidean spacetime and the freely chosen thermodynamic data specifying the ensemble. The Hamiltonian and Gauss's-law constraints are solved and eliminated from the Einstein-Maxwell action, producing a "reduced action" that depends upon two remaining degrees of freedom (two free parameters), as well as on the thermodynamic data. The black-hole temperature, entropy, and corresponding electrostatic potential then follow from relations holding at the stationary points of the reduced action with respect to variation of the free parameters. Investigation of an appropriate eigenvalue problem shows that the criteria for local dynamical and thermodynamical stability are the same. The ensemble can be either stable or unstable, depending upon a certain relation involving mean charge, gravitational radius, and boundary radius. The role of the reduced action in determining the grand partition function, the thermodynamics of charged black holes, and the density of states is discussed.

Analysis of Energy and Momentum Transport for Fluid Flow Through a Porous Bed

Abstract: This paper presents an analysis for the forced convective flow of a gas through a packed bed of spherical solid particles, and the associated heat transport processes. Ergun's correlation was used as the vapor phase momentum equation in order to account for the inertia effects as well as the viscous effects. No local thermal equilibrium was assumed between the solid and the vapor phases. A thorough discussion of the thermal interactions between the solid and vapor phases and their effect on the fluid flow as well as the pressure and density fields is presented. The analysis shows that the local thermal equilibrium condition was very sensitive to the particle Reynolds number (Re{sub p}) and the Darcy number (Da) while thermophysical properties did not have a very significant effect on this condition. On the other hand, two-dimensional behavior of certain variables was found to be very sensitive to thermophysical parameters but insensitive to Re{sub p} and Da.

On the thermoionization in hot cavities

Abstract: Thermally equilibrated gases of low pressure are characterized by a high degree of ionization at temperatures easily reached with laboratory devices. This property is generally believed to be the principle behind ion sources of the hot-cavity-type. The necessary condition of thermal equilibrium for the cavity plasma is, however, often very unlikely, though in general tacitly assumed. It is shown that especially in the most widespread application - low pressure operation for isotope separation on-line - the cavity plasma is far off thermal equilibrium. The consequence is that the influence of the cavity parameters (work function, temperature, plasma density and pressure of the neutral particles) on the ionization efficiency is decisively different from what is expected for a thermalized system.

Nonaxisymmetric thermal equilibria of a cylindrically bounded guiding‐center plasma or discrete vortex system

Abstract: The thermal equilibria of a two‐dimensional guiding‐center model for a single‐species plasma bounded by a cylindrical conductor are considered in the microcanonical ensemble. The same description applies to identical point vortices in a two‐dimensional, ideal fluid surrounded by a circular streamline. The statistically dominant configurations are displaced asymmetrically from the axis, for sufficiently large energies at specified canonical angular momentum. The transition between symmetric and asymmetric states resembles a second‐order phase transition, and occurs at negative temperatures. It is related to a bifurcation in the mean‐field (Vlasov) description. The theory is compared with Monte Carlo simulations of microcanonical ensembles of guiding centers.

Collision integrals and high temperature transport properties for N-N, O-O, and N-O

Abstract: Accurate collision integrals for the interactions of N(4S0) + N(4S0), O(3P) + O(3P), and N(4S0) + O(3P) are reported. These are computed from a semiclassical formulation of the scattering using the best available representations of all of the potential energy curves needed to describe the collisions. Spectroscopic curves and other accurate measured data are used where available; the results of accurate ab initio electronic structure calculations are used to determine the remaining potential curves. The high-lying states are found to give the largest contributions to the collision cross sections. The nine collision integrals needed to determine transport properties to second order are tabulated for translational temperatures in the range 250-100,000 K. The viscosity, thermal conductivity, diffusion coefficient, and thermal diffusion factor for a gas composed of nitrogen and oxygen atoms in thermal equilibrium have been calculated. It is found that the second-order contribution to the transport properties is small. Graphs of these transport properties for various mixture ratios are presented for temperatures in the range 5000-15,000 K.

Thermal ionization of excitons in GaAs/AlGaAs quantum well structures

Abstract: Photoluminescence and photoluminescence excitation spectra have been performed on GaAs/AlGaAs quantum well structures in the temperature range 4–300 K. Sharp exciton resonances are present up to room temperature and can be ascribed to localized excitons for T≤50–70 K and to free excitons at higher values of T. Nevertheless, a line‐shape analysis of the PL spectra clearly shows the presence of band‐to‐band recombination. A fit based on a simple statistical model reproduces with high accuracy the photoluminescence spectrum line shape and allows to evaluate the relative densities of excitons and free carriers generated by the excitondissociation. We find that the ratios of the relative densities can be interpreted on the basis of the law of mass action for describing the thermal equilibrium between excitons,electrons, and holes.

Steady evaporation and condensation of isolated clouds in hot plasma

Abstract: The evaporation and condensation of an isolated cloud embedded in a thermally stable hot gas is studied under the assumption that the ambient hot gas is in thermal equilibrium at constant temperature and that the cloud is large enough that classical conduction is valid. It is found that the cloud will evaporate provided that it is smaller than the Field length, which gives the maximum range of thermal conduction. A large cloud can condense only if the pressure exceeds the saturated vapor pressure p(sat). The pressure required for condensation increases as the cloud radius decreases, until condensation becomes impossible. Since p(sat) is close to the maximum pressure at which the hot gas can exist in equilibrium, condensation is likely to occur under unsteady conditions. The Compton-bremsstrahlung case is treated as an example.

A computer simulation study of transport coefficients in alkali halides

Abstract: The authors report the first application of molecular dynamics simulation to the calculation of the thermal conductivity of ionic solids. Results are presented for the thermal conductivity of solid NaCl and KCl at a range of temperatures between room temperature and the melting point, and also for a number of transport coefficients in the liquid state. The calculations are based on simulations in full thermal equilibrium, and make use of the Green-Kubo relations. The simulations reproduce the experimental measurements with satisfactory accuracy (generally to within 10%), except for the solids near the melting point.

An investigation of a latent heat storage porous bed and condensing flow through it

Abstract: In this work the transient analysis of the behavior of a packed bed of encapsulated phase change material (PCM) and the condensing flow through it is presented. The rigorous model used assumes no local thermal equilibrium between the bed particles and working fluid, and incorporates the inertia effects int eh momentum transport by the use of the Ergun-Forchheimer equation. Condensation in the working fluid is investigated. Thermal charging of the packed bed is analyzed and compared for a sensible heat storage material as well as for different latent heat storage materials (PCMs).

Diode laser probing of I*(2P1/2) Doppler profiles: Time evolution of a fast, anisotropic velocity distribution in a thermal bath

Abstract: The relaxation of a nonthermal translational population distribution of fast I*(2P1/2) atoms dilutely dispersed in a gaseous bath at thermal equilibrium is studied by time‐resolved Doppler spectroscopy. The fast, anisotropic velocity distribution of I* atoms is produced by pulsed laser photolysis of n‐perfluoropropyl iodide (n‐C3F7I) at 266 nm. A frequency‐narrowed, GaAsInP diode laser is tuned across the iodine (2P1/2,F=3←2P3/2,F=4) transition at 1315 nm to measure the Doppler gain profile of the I* photofragments. The velocity distribution is expressed as a separable product of a radial speed function and an angular function describing the anisotropy. The collision‐induced time evolution of both the speed and anisotropy components of the nascent velocity population distribution relaxing to form a 300 K Maxwellian equilibrium distribution is determined. The thermalization dynamics of I* are studied for a heavy bath gas (n‐C3F7I) and a light (He) bath gas. In the case of the heavy bath gas the anisotropy ...

Structure and evolution of irradiated accretion disks. I. Static thermal equilibrium structure. II. Dynamical evolution of a thermally unstable torus

Abstract: The thermal equilibrum structure and dynamical behavior of externally irradiated accretion disks are investigated. For radiative disks only the surface layer is heated, while for convective disks the heat penetrates deeply into the disk. For sufficiently strong radiation and given irradiation flux F(irr), the disk is completely stabilized against thermal instabilities of the sort invoked to explain dwarf novae. For moderately strong irradiation there is still an unstable branch in the thermal equilibrium curve. In typical soft X-ray transients, the disk is unstable against the dwarf-nova type instability. Fixed F(irr) on accretion disk annuli reduces the amplitude and the quiescent times and increases the outburst duration of the resultant light curves. Varying F(irr) in proportion to the mass accretion rate at the disks's inner edge results in light curves with a plateau in the decay from outbursts. In the case when irradiation is suddenly switched on, a temperature inversion results which leads to the formation of an accretion corona.

Nonideal J‐V characteristics and interface states of an a‐Si:H Schottky barrier

Abstract: A new theory is developed for nonideal J‐V characteristics of Schottky barriers with an interfacial layer. This theory is based on the model that nonideal characteristics are due to changes of population in the interface states under applied bias and accompanying changes of the barrier height. The population in the interface states is expressed by the Fermi level, which can be determined by analyzing experimental results. The J‐V characteristics are obtained from the flow of carriers into and out of the interface. Tunneling through the interfacial layer constitutes the bottleneck for the carrier flow. Under forward bias, the carrier concentration ns at the interface is proved to be in thermal equilibrium with the bulk. Under reverse bias, ns is in local thermal equilibrium with the interface states. This theory is applied to an undoped a‐Si:H Schottky barrier without introducing any ambiguous quantities. The experimental ideality factor, its dependence on temperature and voltage, and current density are q...

Critical behavior of the driven diffusive lattice gas

Abstract: This paper reviews simulational and theoretical investigations of critical behavior in a stochastic, interacting lattice gas under the influence of a uniform external driving field. By studying this model system one wishes to gain a deeper understanding of steady states far from thermal equilibrium, and their dynamic universality classes. The major result in the case of attractive particle-particle interactions is the emergence of a novel non-equilibrium fixed point, different from the Wilson-Fisher fixed point of the equilibrium system. The fluctuations of internal energy, the structure factor and the two-point correlations all display surprising features associated with the non-equilibrium nature of the system. For repulsive interactions and small driving forces, one finds a continuous, Ising-like transition which turns first order for larger fields until it is completely destroyed.

Response of apparent mass to thermal gradients

Abstract: The change of the apparent mass of 20-g masses and tubes that are not in thermal equilibrium with ambient air has been observed. Buoyancy, adsorption and convection influences are discussed. Quantitative comparisons show that, under such conditions, it is predominantly free convection forces which change the apparent mass.

Formulation and numerical results of the transfer-matrix method for quantum spin chains

Abstract: We give a novel and explicit description of the numerical transfer-matrix method for quantum spin chains in thermal equilibrium, based on a real-space Trotter decomposition of the corresponding statistical operator. We apply the method to a spin- 1 2 chain with a magnetic field and varying anisotropy in the nearest-neighbor exchange interaction. Some of the results are compared with exact ones which are available in limiting cases.

Kinetic path for a relaxation process of an f.c.c. disordered phase

Abstract: Tetrahedron approximation of the Cluster Variation Method and the Path Probability Method are employed for an f.c.c. disordered system, and the time evolution of spin configuration, which is regarded as the first approximation to an alloy system, is investigated. The convergent values at infinite time exactly coincides with the cluster probabilities independently obtained by the Cluster Variation Method for thermal equilibrium system. Furthermore, the kinetic path is placed on the thermodynamic configurational space spanned by correlation functions.

An explanation of light-curve asymmetries in contact binaries by means of the coriolis effect

Abstract: In this study, it is proved that the isothermal surface is not identical with the isobaric surface in a static model of a contact binary system based on hydrostatic and thermal equilibrium equations. Therefore, there is a baroclinic structure in a contact binary atmosphere, and the development of circumfluence is thus inevitable. It is suggested that a static model should not be adopted in modeling a contact binary atmosphere. The asymmetry (different brightness between the maxima) commonly found among the light curves of W UMa binary systems can be explained in terms of the asymmetry of circumfluence in the contact binary stars due to the Coriolis effect. 8 refs.

Thermal equilibration of fluids near the liquid-vapor critical point: 3 He and 3 He- 4 He mixtures

Abstract: We study the temperature-equilibration process of fluids at constant volume in a thermal conductivity cell, where an initial temperature gradient relaxes to zero. The calculation is performed in the linear approximation for a pure fluid and a binary mixture. Near the critical point of the pure fluid, the adiabatic heating process, which takes place at constant volumeV, causes equilibration to proceed four times faster whenC P /C V ≫1 than for the process at constant pressureP. For the mixtures, the relaxation rate enhancement at constantV compared with constantP is restricted to a temperature region where the coupling between temperature and mass diffusion is small. The predictions are compared with experimental results for3He and for two3He-4He mixtures along their critical isochores. Finally, we discuss the thermal relaxation in the two-phase (liquid-gas) and one-phase (gas) regimes at the critical density, as measured with a conductivity and a calorimetry cell. The contrasting behavior for3He and a3He-4He mixture in these two regimes and under these different constraints is pointed out and discussed.

Euclidean action and the thermodynamics of manifolds without boundary.

Abstract: I consider general, spherically symmetric spacetimes with cosmological and black-hole horizons. I find that a state of thermal equilibrium may exist in classical manifolds with two horizons as long as a matter distribution is present. The Euclidean action is calculated for general (nonclassical) manifolds without boundary. It is found to equal the grand canonical weighting factor for a spacetime with zero mean thermal energy. I also consider how the Euclidean action is modified by the imposition of fixed-temperature shells in the manifold. I find that the mean thermal energy of the cosmological horizon is negative.

Entropy and other thermodynamic properties of classical electromagnetic thermal radiation.

Abstract: Building upon previous work, several new thermodynamic properties are found for classical electromagnetic random radiation in thermal equilibrium with classical electric dipole harmonic oscillators. Entropy is calculated as a function of temperature and as a function of the positions of the dipole oscillators. In the process, a new derivation is obtained for what is often called Wien's displacement law. The original derivation of this law makes a number of implicit assumptions not found in the present derivation, which prevents the original analysis from being sufficiently general to address an important class of thermal radiation spectrum candidates: namely, those that are nonzero at T=0. While leading up to the entropy calculation, a number of other thermodynamic properties are deduced. For example, a natural development is presented for reformulating the St\'efan-Boltzmann law to correspond to experimental observations about changes in thermal radiation energy. Also, the Rayleigh-Jeans spectrum is shown to conflict with basic concepts of thermodynamic processes, and asymptotic limits are found for the spectrum of classical electromagnetic thermal radiation. One asymptotic restriction arises from the demand of finite specific heat for thermal radiation. This restriction is sufficient to ensure that the classical electrodynamic system of dipole oscillators and thermal radiation must obey the third law of thermodynamics. The calculations described here include full nonperturbative evaluations of retarded van der Waals thermodynamic functions.