Showing papers in "Journal of Quantitative Spectroscopy & Radiative Transfer in 2001"

HULLAC, an integrated computer package for atomic processes in plasmas

Abstract: We describe H ULLAC , an integrated code for calculating atomic structure and cross sections for collisional and radiative atomic processes. This code evolved and has been used over the years, but so far, there was no coherent, comprehensive, and in-depth presentation of it. It is based on relativistic quantum mechanical calculations including configuration interaction. The collisional cross sections are calculated in the distorted wave approximation. The theory and code are presented, emphasizing the various novel methods that has been developed to obtain accurate results very efficiently. In particular we describe the parametric potential method used for both bound and free orbitals, the factorization–interpolation method applied in the derivation of collisional rates, the phase amplitude approach for calculating the continuum orbitals and the N JGRAF graphical method used in the calculation of the angular momentum part of the matrix elements. Special effort has been made to insure the simplicity of use, which is demonstrated in an example.

A linearized discrete ordinate radiative transfer model for atmospheric remote-sensing retrieval

Abstract: The radiative transfer forward model simulation of intensities and associated parameter derivatives (weighting functions) is a vital part of the retrieval of earth atmospheric constituent information from measurements of backscattered light. The discrete ordinate method is the most commonly used approach for the determination of solutions to the radiative transfer equation. In this paper, we carry out an internal perturbation analysis of the complete discrete ordinate solution in a plane parallel multi-layered multiple-scattering atmosphere. Perturbations in layer atmospheric quantities will translate into small changes in the single-scatter albedos and optical depth values. In addition, we consider perturbations in layer thermal emission source terms and in the surface albedo. It is shown that the solution of the boundary value problem for the perturbed intensity "eld leads in a natural way to the weighting function associated with the parameter causing the perturbation. We have developed a numerical model LIDORT (linearized discrete ordinate radiative transfer) for the simultaneous generation of backscatter intensities and weighting function output at arbitrary elevation angles, for a user-de"ned set of atmospheric variations. Results for a 5-layer test atmosphere with two scatterers and thermal emission terms are shown. Intensities are validated against benchmark discrete ordinate results, while weighting functions are checked for consistency against "nite di!erence results based on external perturbations. A second example is presented for a 60-layer terrestrial atmosphere with molecular and aerosol scattering and ozone trace gas absorption in the UV spectral range; weighting functions are shown to correspond closely with results derived from another radiative transfer model. Published by Elsevier Science Ltd.

High-temperature (1000–7000 K) collision-induced absorption of H2 pairs computed from the first principles, with application to cool and dense stellar atmospheres

Abstract: The collision-induced absorption (CIA) spectra of H2–H2 and H2–He are known to play an important role for modelling of low-metallicity cool and dense stellar atmospheres. In this paper we present collision-induced absorption spectra of H2–H2 complexes in the rototranslational (Δv=0), the fundamental (Δv=1), the first (Δv=2) and the second (Δv=3) overtone bands in the temperature range from 1000 to 7000 K, and in the frequency region from 0 to 20 000 cm−1. The translational spectral density functions are computed quantum mechanically, based on: (1) the newly developed ab initio collision-induced H2–H2 dipole functions of Zheng (Computational study of collision induced dipole moments and absorption spectra of H2–H2. Ph.D. thesis, Michigan Technological University, 1997), which account for the short-range H2–H2 intermolecular distances (as small as 2.5 a.u.) and for larger H2 internuclear distances (as large as 2.15 a.u.); (2) semiempirical isotropic H2–H2 potential (Ross et al, J Chem Phys 1983;79(3):1487) suitable for high temperatures. We include the collision-induced absorption coefficient of the vibrational transitions as v1,v2,v′1,v′2≤3 which we computed rigorously. We also give our estimate for the collision-induced absorption coefficients of single vibrational transitions such as v i v′ i >3 in the first and second overtone bands. The dependence of CIA spectra on rotational states of H2 molecules is accounted for in our computations. We have previously (Borysow et al, Astronom Astrophys. 1997;324:185–95) studied the effect of CIA for stars of a wide range of fundamental stellar parameters (effective temperature, gravity, and chemical composition), and determined for which combinations of these parameters it is necessary to include CIA in the model and spectrum computation. These calculations showed that CIA from H2–H2 plays an important, and often even a dominating role for stellar atmospheres of a wide range of stars. The approximate character of the estimates of the H2–H2 absorption coefficient we used in our previous work combined with the large effect CIA had on the stellar atmospheres, were the main inspirations to initiate the more accurate computations of the absorption coefficient we present here. The absorption coefficient we compute in the present analysis is in qualitative agreement with our preliminary estimates (Borysow et al, Astronom Astrophys 1997;324:185–95), but in some spectral regions of high importance for the stellar structure, our computed absorption coefficient is up to a factor of 3 larger than our preliminary estimates. We therefore fully confirm our previous suspicion that H2–H2 CIA will have a pronounced effect on the atmosphere for a wide range of stars. In this paper we therefore quantify the effect the new data have on a typical cool dense stellar atmosphere, and compare our new results with our previous estimates.

Cretin—a radiative transfer capability for laboratory plasmas

Abstract: This paper describes the multi-dimensional non-local thermodynamic equilibrium simulation code Cretin. Cretin self-consistently combines atomic kinetics and radiation transport in a framework suitable for modeling laboratory plasmas. This paper describes the formulations present in the code, illustrating their use in several applications.

A two-layer canopy reflectance model

Abstract: A computationally efficient canopy reflectance model is developed. A typical two-layer canopy of forest understory communities is addressed in the model where a geometrically thin layer of vegetation of different structure and/or optical properties is under the main layer of canopy. The model allows to calculate reflectance spectrum in every given direction for the spectral range 400–2500 nm . The model calculations show that the use of effective canopy parameters in a homogeneous canopy reflectance model may cause significant biases in estimated canopy reflectance.

Accelerated recombination due to resonant deexcitation of metastable states

Abstract: In a recombining plasma the metastable states are known to accumulate population thereby slowing down the recombination process. We show that an account of the doubly excited autoionizing states, formed due to collisional recombination of metastable ions, results in a significant acceleration of recombination. A fully time-dependent collisional-radiative (CR) modeling for stripped ions of carbon recombining in a cold dense plasma demonstrates an order of magnitude faster recombination of He-like ions. The CR model used in calculations is discussed in detail.

A generalized multiparticle Mie-solution: further experimental verification

Abstract: We further test our electromagnetic multisphere-scattering solution developed earlier by comparing theoretical predictions from the theory with a set of laboratory measurements of microwave analog to light scattering by aggregated spheres. This solution is an extension of Mie theory to the multisphere case, generally applicable to an arbitrary aggregate of spherical and/or nonspherical particles. It is demonstrated once again that the theory is in a uniform agreement with experiment, convincingly confirming the veracity of the multiparticle-scattering formulation. The computer code for the calculation of the scattering by an aggregate of spheres in a fixed orientation and the experimental data havebeen made publically available.

Numerical simulation of the light field in the atmosphere–ocean system using the matrix-operator method

Abstract: A computer code to calculate the light field in the stratified atmosphere–ocean system is described and validated. The code is based on the matrix-operator method and includes multiple scattering as well as the effects at the flat or rough sea surface and the ocean ground. Special emphasis is put on the methods employed to ensure numerical accuracy and energy conservation. The code is validated by comparing model predictions with the analytical solution of the radiative transfer equation for a semi-infinite Rayleigh scattering atmosphere and by a model intercomparison for selected problems of the radiative transfer in the atmosphere–ocean system. The observed deviations from the analytical solution are smaller than 0.1% for solar and observation zenith angles

Baseline subtraction using robust local regression estimation

Abstract: A technique entitled robust baseline estimation is introduced, which uses techniques of robust local regression to estimate baselines in spectra that consist of sharp features superimposed upon a continuous, slowly varying baseline. The technique is applied to synthetic spectra, to evaluate its capabilities, and to laser-induced fluorescence spectra of OH (produced from the reaction of ozone with hydrogen atoms). The latter example is a particularly challenging case for baseline estimation because the experimental noise varies as a function of frequency.

A numerical radiative transfer model for a spherical planetary atmosphere: combined differential-integral approach involving the Picard iterative approximation

Abstract: A new radiative transfer model suitable to calculate the radiation field in a spherical planetary atmosphere has been developed. The suggested approach involves the Picard iterative approximation to solve the radiative transfer equation in its integral form. The radiation field calculated by solving the integro-differential radiative transfer equation in a pseudo-spherical atmosphere is used as an initial guess for the iterative scheme. The approach has the same advantages as the Monte-Carlo method, but is much more computationally efficient. The comparisons between the spherical model presented in this paper and a Monte-Carlo radiative transfer model for radiances at the top of the atmosphere show differences less than 1% for most situations. The accuracy of the recently developed CDI approach, which was intended to perform fast and accurate radiance computations for non-limb viewing geometry, has been estimated for limb viewing geometry.

One-dimensional Riemann solvers and the maximum entropy closure

Abstract: The maximum entropy closure for the two-moment approximation of the neutron transport equation is presented. We use a robust Roe-type Riemann solver to solve the resulting moment equations. We also present three boundary conditions to use with this method. The ghost cell method effectively implements the Mark boundary condition by placing phantom cells just outside the physical system. This method is extremely simple to implement and gives reasonable results. The boundary Eddington factor method implements the Marshak boundary condition. While it yields good results at boundaries with incoming neutrons, it does not do so well at vacuum boundaries. The partial numerical flux method is an extension of the Marshak boundary condition, allowing us to specify extra angular information about the incoming neutron distribution. The neutron flux calculations with this method are generally the best out of the three boundary conditions presented here. Several simple steady-state and time-dependent problems illustrate the qualities, both good and bad, of the maximum entropy closure.

Fast and accurate discrete ordinates methods for multidimensional radiative transfer. Part I, basic methods

Abstract: The ability to solve the radiative transfer equation in a fast and accurate fashion is central to several important applications in combustion physics, controlled thermonuclear fusion and astrophysics. Most practitioners see the value of using discrete ordinates methods for such applications. However, previous efforts at designing discrete ordinates methods that are both fast and accurate have met with limited success. This is especially so when parts of the application satisfy the free streaming limit in which case most solution strategies become unacceptably diffusive or when parts of the application have high absorption or scattering opacities in which case most solution strategies converge poorly. Designing a single solution strategy that retains second-order accuracy and converges with optimal efficiency in the free streaming limit as well as the optically thick limit is a challenge. Recent results also indicate that schemes that are less than second-order accurate will not retrieve the radiation diffusion limit. In this paper we analyze several of the challenges involved in doing multidimensional numerical radiative transfer. It is realized that genuinely multidimensional discretizations of the radiative transfer equation that are second-order accurate exist. Because such discretizations are more faithful to the physics of the problem they help minimize the diffusion in the free streaming limit. Because they have a more compact stencil, they have superior convergence properties. The ability of the absorption and scattering terms to couple strongly to the advection terms is examined. Based on that we find that operator splitting of the scattering and advection terms damages the convergence in several situations. Newton–Krylov methods are shown to provide a natural way to incorporate the effects of nonlinearity as well as strong coupling in a way that avoids operator splitting. Used by themselves, Newton–Krylov methods converge slowly. However, when the Newton–Krylov methods are used as smoothers within a full approximation scheme multigrid method, the convergence is vastly improved. The combination of a genuinely multidimensional, nonlinearly positive scheme that uses Full Approximation Scheme multigrid in conjunction with the Newton–Krylov method is shown to result in a discrete ordinates method for radiative transfer that is highly accurate and converges very rapidly in all circumstances. Several convergence studies are carried out which show that the resultant method has excellent convergence properties. Moreover, this excellent convergence is retained in the free streaming limit as well as in the limit of high optical depth. The presence of strong scattering terms does not slow down the convergence rate for our method. In fact it is shown that without operator splitting, the presence of a strong scattering opacity enhances the convergence rate in quite the same way that the convergence is enhanced when a high absorption opacity is present! We show that the use of differentiable limiters results in substantial improvement in the convergence rate of the method. By carrying out an accuracy analysis on meshes with increasing resolution it is further shown that the accuracy that one obtains seems rather close to the designed second-order accuracy and does not depend on the specific choice of limiter. The methods for multidimensional radiative transfer that are presented here should improve the accuracy of several radiative transfer calculations while at the same time improving their convergence properties. Because the methods presented here are similar to those used for simulating neutron transport problems and problems involving rarefied gases, those fields should also see improvements in their numerical capabilities by assimilation of the methods presented here.

Submillimeter atmospheric transmission measurements on Mauna Kea during extremely dry El Nino conditions: implications for broadband opacity contributions

Abstract: We present broadband atmospheric transmission spectra obtained on Mauna Kea, Hawaii (4100 m. above sea level) on UT April 1st, 1998 and July 1st, 1999 under very similar pressure and temperature conditions. The 1998 measurements occurred under conditions of extremely low atmospheric water vapor, with a ground-level relative humidity of ≈2%. As a result of its dryness the Mauna Kea site allows access to a partially transparent atmosphere up to frequencies exceeding 1000 GHz, where the relative importance of atmospheric continuum-like absorption is much larger than at millimeter wavelengths, and hence easier to measure. As shown in this paper, these conditions have allowed us to measure and separate the submillimeter absorption spectrum into three terms: resonant lines, non-resonant absorption of the dry atmosphere due to collision-induced mechanisms involving electric quadrupoles, and continuum-like water vapor opacity. The spectra presented here were obtained with a Fourier transform spectrometer (FTS) at the Caltech Submillimeter Observatory and cover a continuous frequency range from 350 to 1100 GHz, with a finest spectral resolution of 200 MHz. The 1998 conditions were so exceptionally dry that an atmospheric window centered at 1035 GHz showed up to 35% zenith transmission. The calibration of our data is especially careful and includes corrections for differences between the ground atmospheric temperature and the calibrator temperature, as well as for the tropospheric temperature lapse rate, and the water vapor scale height. This procedure is able to yield transmission spectra calibrated to within 1–2%. A multilayer atmospheric radiative transfer model has been used for data analysis.

Dense matter characterization by X-ray Thomson scattering

Abstract: We discuss the extension of the powerful technique of Thomson scattering to the X-ray regime for providing an independent measure of plasma parameters for dense plasmas. By spectrally resolving the scattering, the coherent (Rayleigh) unshifted scattering component can be separated from the incoherent Thomson component, which is both Compton and Doppler shifted. The free electron density and temperature can then be inferred from the spectral shape of the high-frequency Thomson scattering component. In addition, as the plasma temperature is decreased, the electron velocity distribution as measured by incoherent Thomson scattering will make a transition from the traditional Gaussian Boltzmann distribution to a density-dependent parabolic Fermi distribution. We also present a discussion for a proof-of-principle experiment appropriate for a high-energy laser facility.

Light-scattering properties of fractal aggregates: numerical calculations by a superposition technique and the discrete-dipole approximation

Abstract: Dust particles in space often grow by mutual collisions and appear to be an agglomeration of individual grains, the morphology of which can be described by the concept of fractals. In this paper, we study light scattering by fractal aggregates of identical spheres (monomers) using the superposition technique incorporated into the T -matrix method where the orientationally averaged scattering matrix is analytically obtained. We also apply the discrete-dipole approximation, in which the dipole polarizability of spherical monomers is determined by the first term of the scattering coefficients in the Mie theory. Two cases of the ballistic aggregation process (particle–cluster and cluster–cluster aggregations) are considered to model fractal aggregates consisting of silicate or carbon material. The dependences of light-scattering properties on the monomer sizes, aggregate structures and material compositions are intensively investigated. The light-scattering properties of the fractal aggregates strongly depend on the size parameters of the monomers. The difference in the scattering function between the particle–cluster and cluster–cluster aggregates can be seen in the case of monomers much smaller than the wavelength of incident radiation. When the size parameter of monomers exceeds unity, the material composition of the monomers influences the light-scattering properties of the aggregates, but different morphologies result in similar scattering and polarization patterns. We show that silicate aggregates consisting of submicron-sized monomers, irrespective of the aggregate size and morphology, produce a backscattering enhancement and a negative polarization observed for dust in the solar system.

Identification of cloud phase from PICASSO-CENA lidar depolarization: a multiple scattering sensitivity study

Abstract: A fast Monte Carlo simulation scheme is developed to assess the impact of multiple scattering on space-based lidar backscattering depolarization measurements. The specific application of our methodology is to determine cloud thermodynamic phase from satellite-based lidar depolarization measurements. Model results indicate that multiple scattering significantly depolarizes backscatter return from water clouds. Multiple scattering depolarization is less significant for non-spherical particles. There are sharp contrasts in the depolarization profile between a layer of spherical particles and a layer of non-spherical particles. Although it is not as obvious as ground-based lidar observations, it is likely that we can identify cloud phase not only for a uniform cloud layer, but also for overlapping cloud layers where one layer contains ice and the other water droplets.

Calculation and optical measurement of laser trapping forces on non-spherical particles

Abstract: Optical trapping, where microscopic particles are trapped and manipulated by light is a powerful and widespread technique, with the single-beam gradient trap (also known as optical tweezers) in use for a large number of biological and other applications The forces and torques acting on a trapped particle result from the transfer of momentum and angular momentum from the trapping beam to the particle Despite the apparent simplicity of a laser trap, with a single particle in a single beam, exact calculation of the optical forces and torques acting on particles is difficult Calculations can be performed using approximate methods, but are only applicable within their ranges of validity, such as for particles much larger than, or much smaller than, the trapping wavelength, and for spherical isotropic particles This leaves unfortunate gaps, since wavelength-scale particles are of great practical interest because they are readily and strongly trapped and are used to probe interesting microscopic and macroscopic phenomena, and non-spherical or anisotropic particles, biological, crystalline, or other, due to their frequent occurance in nature, and the possibility of rotating such objects or controlling or sensing their orientation The systematic application of electromagnetic scattering theory can provide a general theory of laser trapping, and render results missing from existing theory We present here calculations of force and torque on a trapped particle obtained from this theory and discuss the possible applications, including the optical measurement of the force and torque

Extension of T-matrix to scattering of electromagnetic plane waves by non-axisymmetric dielectric particles: application to hexagonal ice cylinders

Abstract: The calculation of scattering by cirrus particles at intermediate size parameters greater than 20 has not as yet been satisfactorily solved with exact theory. This has made it difficult to represent scattering and absorption of ice clouds in remote sensing retrievals and in modelling radiative forcing. The T-matrix approach applied to ice particles has previously been implemented only for axisymmetric particles, but ice clouds consist of particles which are not axisymmetric. In this paper an implementation of T-matrix which provides exact solutions for scattering and absorption from non-axisymmetric particles is presented. Rigorous tests demonstrate the stability and accuracy of the method. Results for finite hexagonal cylinders are presented. The general T-matrix formulation has major conceptual and practical advantages over other existing methods such as the finite difference time domain (FDTD) and the discrete dipole approximation (DDA). These advantages are due largely to its analytic character, which allows exact fulfillment of the radiation condition. Other advantages are the restriction of calculations to the scatterer's surface, and the exploitation of particle symmetries which considerably simplifies computation. The new T-matrix implementation is tested against existing T-matrix results for the circular cylinder and the cube in terms of differential scattering cross-sections, no differences are found between the T-matrix calculations. Comparisons with a recently improved implementation of FDTD for randomly oriented hexagonal ice columns show good agreement with T-matrix in terms of absorption and extinction efficiencies.

Scattering by inhomogeneous particles: microwave analog experiments and comparison to effective medium theories

Abstract: It is common practice to use effective medium theories (EMT) to estimate average, “effective” optical constants of inhomogeneous materials. A variety of EMTs were developed for different internal structures of the medium and for a variety of shapes, size distributions and physical properties of the inhomogeneities. The most popular EMTs (Maxwell Garnett, Bruggeman, Looyenga, etc.) consider inhomogeneities that are much smaller than the wavelength. The so-called extended EMTs were developed to find effective optical constants in the case of inhomogeneities comparable and slightly larger than the wavelength. This paper compares angular distribution and wavelength dependence of intensity and polarization of scattered light obtained from calculations using the most popular EMTs and extended EMTs with the results of microwave analog measurements at the microwave facilities of the University of Florida. We simulated the light scattering by organic grains with silicate inclusions of size parameter x=0.075 (≈0.01 μm ) , 0.60 (≈0.1 μm ) , and 1.24 (≈0.2 μm ). The conclusion is that for inclusions of a small size and for a small volume fraction of them in the mixture all EMTs yield similar results and show reasonable agreement with experimental results. The accuracy is better for the angular dependencies of the intensity and of the polarization of the scattered light than for their wavelength dependencies. For inhomogeneities comparable and larger than the wavelength extended EMTs work better but for smaller inclusions non-extended EMTs show more accurate results. Large volume fractions of the inclusions in the mixture (>10%) essentially reduce the accuracy of the results obtained with EMTs. Based on our study we do not recommend to use EMTs in the back-scattering domain and at the scattering angles 30°

Modelling of plasmas in an average-atom local density approximation: the CASSANDRA code

Abstract: This paper describes the CASSANDRA LDA average-atom model for dense plasmas and liquid metals. Recent equation-of-state and opacity calculations are described by way of illustration.

Rotational temperature measurements in air and nitrogen plasmas using the first negative system of N2

Abstract: Recent Fourier-transform spectrometry measurements of a wide array of rovibrational lines of the first negative system of N 2 + were used to develop a quantitative line-by-line radiation code that can be used for accurate rotational temperature determinations in nitrogen and air plasmas. The model is applied to the interpretation of spectral measurements obtained in a nonequilibrium nitrogen/argon plasma produced by a 50 kW radio-frequency inductively coupled plasma torch. Two rotational temperature determination techniques are presented that consist either in performing a global fit of the spectral region 3700–3920 A or in comparing the intensity of a single group of isolated lines at 3759.5 A to the intensity of the (0–0) band head. Both techniques yield a rotational temperature of 4850 K with an accuracy of better than 2%.

Application of the statistical narrow-band correlated-k method to non-grey gas radiation in CO2–H2O mixtures: approximate treatments of overlapping bands

Abstract: The statistical narrow-band correlated- k method was employed to calculate narrow-band intensities along a line-of-sight and radiative transfer in a three-dimensional rectangular enclosure containing nonisothermal CO 2 –H 2 O–N 2 mixtures at 1 atm. The correlated treatment of overlapping narrow bands of CO 2 and H 2 O based on the multiplication property of gas transmissivity significantly increases the execution time of this method at each overlapping band. Three new approximate treatments for overlapping bands were proposed to improve the computational efficiency of the statistical narrow-band correlated- k method for gas radiation calculations in media containing two or more radiating gases. The accuracy of the statistical narrow-band correlated- k method using four approximate treatments of overlapping bands in CO 2 –H 2 O–N 2 mixtures was evaluated by comparing their numerical results with those obtained using the correlated treatment of overlapping bands. Such comparisons were made for both spectrally integrated quantities and narrow-band intensities along a line-of-sight. Results of the statistical narrow-band model were also obtained as a reference solution in the evaluation of the results of the statistical narrow-band correlated- k method.

A general non-complex analytical expression for the nth Fourier component of a wavelength-modulated Lorentzian lineshape function

Abstract: A general, analytical expression for the n th Fourier component of a wavelength-modulated Lorentzian lineshape function in terms of a normalized detuning and a normalized modulation amplitude is derived. The expression is cast in a purely real form and is therefore easier to use than the normally used expression, which is given in terms of various combinations (sums, square roots, and powers) of complex expressions and their complex conjugates. Analytical expressions for the nine first Fourier components ( n =0,…, 8 ), clearly showing their dependence on normalized detuning and modulation amplitude, are explicitly given. Simplified expressions for the even harmonics of the Fourier components on resonance, at which they take their maximum value, solely given in terms of the normalized modulation amplitude, are also explicitly given. It is shown that previously published, numerically calculated conditions for maximization of higher-order Fourier components are incorrect. The normalized modulation amplitudes that maximize the four lowest even harmonics of the Fourier components, i.e. for n =2, 4, 6, and 8, are 2.20, 4.12, 6.08, and 8.06, respectively.

Narrowing and broadening parameters of H2O lines perturbed by He, Ne, Ar, Kr and nitrogen in the spectral range 1850–2140 cm−1

Abstract: Collision effects on water vapor at low concentration in mixture with noble gases and nitrogen have been studied by Fourier transform spectroscopy in a pressure range where line narrowing by dynamic confinement (Dicke effect) and collision broadening are observable, i.e. when the Voigt function cannot reproduce the observed profiles. Precise values of the broadening parameter have been obtained for R branch lines of the ν2 band of H2O and narrowing parameter values were derived using the soft and hard collision models. Furthermore, it is shown that, when neglecting the confinement effect, systematic errors on the broadening parameters may be introduced and reach several percents for the narrowest lines corresponding to the highest J values.

Multispectrum fitting technique for data recorded by Fourier transform spectrometer: application to N2O and CH3D

Abstract: To improve the fit of spectroscopic parameters recorded by Fourier transform spectrometer (intensities, broadening coefficients, and pressure shift), we have developed a multispectrum fitting technique, this technique being already described by other research groups (Benner DC, Rinsland CP, Malathy Devi V, Smith MAH, Atkins D. JQSRT 1995;53:705; Carlotti, Appl Opt 1988;27:3250). We describe the algorithm of the software allowing to analyse and visualise several spectra simultaneously. To validate our software, line intensities (for N 2 O and CH 3 D) and self-broadening coefficients were fitted and compared with previous studies. We have also studied the wave numbers of N 2 O and compared them with heterodyne measurements in order to estimate the relative accuracy of our results.

Nonlinear convergence, accuracy, and time step control in nonequilibrium radiation diffusion

Abstract: We study the interaction between converging the nonlinearities within a time step and time step control, on the accuracy of nonequilibrium radiation diffusion calculations. Typically, this type of calculation is performed using operator-splitting where the nonlinearities are lagged one time step. This method of integrating the nonlinear system results in an “effective” time-step constraint to obtain accuracy. A time-step control that limits the change in dependent variables (usually energy) per time step is used. We investigate the possibility that converging the nonlinearities within a time step may allow significantly larger time-step sizes and improved accuracy as well. The previously described Jacobian-free Newton–Krylov method (JQSRT 63 (1999) 15) is used to converge all nonlinearities within a time step. In addition, a new time-step control method, based on the hyperbolic model of a thermal wave (J. Comput. Phys. 152 (1999) 790), is employed. The benefits and cost of a second-order accurate time step are considered. It is demonstrated that for a chosen accuracy, significant increases in solution efficiency can be obtained by converging nonlinearities within a time step.

On laboratory-frame radiation hydrodynamics

Abstract: We derive and discuss the equations of radiation hydrodynamics with all radiation quantities expressed in the laboratory frame. Relativistically exact transformations from the comoving to laboratory frame are used. We obtain simple, fully covariant, expressions for the radiation energy and momentum source/sink terms using lab-frame radiation quantities. The mathematical simplicity of having lab-frame radiation quantities in conservative operators on the left-hand side is maintained. The resulting system can be solved efficiently using an Eddington tensor formulation for angular dependences, and an iterative frequency-splitting, in the spirit of the multifrequency-gray method. Use of lab-frame radiation quantities can present numerical difficulties in making an accurate connection to the diffusion limit for extremely optically thick media; but it may have no difficulty for neutron-transport problems where very large scattering-thicknesses are generally not encountered (for non-critical systems).

A generalized speed-dependent line profile combining soft and hard partially correlated Dicke-narrowing collisions

Abstract: We have generalized the combined strong and weak Dicke-narrowed spectral profile of Rautian and Sobel'man (Sov Phys Usp 1967;9:701–16) for speed-dependent broadening and shifts partially correlated with hard and/or soft velocity-changing collisions. The proposed line shape model is tested on Ar-broadened HF spectra.

Radiation force caused by scattering, absorption, and emission of light by nonspherical particles

Abstract: General formulas for computing the radiation force exerted on arbitrarily oriented and arbitrarily shaped nonspherical particles due to scattering, absorption, and emission of electromagnetic radiation are derived. For randomly oriented particles with a plane of symmetry, the formula for the average radiation force caused by the particle response to external illumination reduces to the standard Debye formula derived from the Lorenz-Mie theory, whereas the average radiation force caused by emission vanishes.

Neon photoionization experiments driven by Z-pinch radiation

Abstract: Present-day Z-pinch experiments generate ∼2×10 21 erg / s peak power, ∼6 ns full-width at half-maximum X-ray bursts that provide new possibilities to study radiation-heated matter. This source is being used to investigate the production of plasmas in which photoionization dominates collisional ionization. Spectroscopic measurements of such plasmas can serve to benchmark atomic physics models of the photoionized plasmas. Beyond intrinsic interest in the atomic physics, these models will be applied to the interpretation of data from the new generation of satellite X-ray spectrographs that will promote the understanding of accretion-powered objects such as X-ray binaries and active galactic nuclei. Moreover, this information is needed for X-ray laser research. Our experiments use a 1-cm-scale neon gas cell to expose 10 18 atoms / cm 3 to an X-ray flux of ∼5×10 18 erg / cm 2 / s . Thin mylar ( 1.5 μm ) windows confine the gas and allow the radiation to flow into the cell. The ionization is monitored with absorption spectra recorded with crystal spectrometers, using the pinch as a backlight source. In initial experiments we acquired an absorption spectrum from Li- and He-like Ne, confirming the ability to produce a highly ionized neon plasma.