Radiation Trapping Research Articles

Comparing impact of multi-factor planning layouts in residential areas on summer thermal comfort based on orthogonal design of experiments (ODOE)

The previous studies of outdoor thermal comfort mainly focused on the micro-climate changes caused by urban morphological factors such as street aspect ratio, plot ratio, sky view factor (SVF) et al. As an essential routine to mitigate urban heat island (UHI), vegetation was also discussed by researchers to explore the optimal distribution. However, under the same determined quantity, there is still a blank of the study in how significant each layout factor is to thermal comfort, including the building layout, the pavement layout, and the vegetation layout. This paper followed an orthogonal design of experiment (ODEO) to create various multi-factor layouts in the case of northern China, thoroughly analyzing simulated results based on Physiological Equivalent Temperature (PET) to explain the internal mechanism. Results show that in high-rise residential areas, the layout significance ranking affecting thermal comfort in summer is: buildings, vegetation, pavement. The optimal arrangement is a layout combination of the scattered-form buildings, the grid-form pavement, and the southern vegetation, which can reduce PET of 6.3% maximally during the day. We also found that wind maps are highly consistent with PET maps due to its heat dissipation capability. Another reason for the high PET value is the solar radiation trapping effect, which should be considered with building shade when discussing the relationship between multi-layouts and thermal comfort. The arrangement method can provide a new research direction of urban micro-climate in the future and thus can be employed by decision-makers in the early planning stage.

A model for radiative heating of a high-Z pusher

Several approaches to Inertial Confinement Fusion (ICF), including double-shell, pushered-single-shell, and the Revolver designs, have fuel surrounded by pushers made from high-Z materials. An advantage of these designs is that radiation emitted by the hot fuel will be absorbed and re-radiated into the fuel to reduce cooling. This process is referred to as radiation-trapping, and it lowers the fuel temperature required for ignition. To elucidate the physics of radiation trapping, a simplified model comprising a set of coupled differential equations has been developed to model the arbitrary time-dependent wall temperature from the Hammer and Rosen solution [J. H. Hammer and M. D. Rosen, Phys. Plasmas 10, 1829 (2003)] to a Marshak wave. The derivation of the model and a set of analytical power-law solutions will be presented. A discussion on numerical implementation of the differential equations into a model for burn in ICF capsules is also included.

Development of High-Performance Cascade Arc Discharge Device for Applications to Plasma Window

We fabricated a cascade arc discharge source with a channel diameter of 8 mm for application to plasma window as an alternative differential pumping. Helium arc plasmas up to a discharge current of 100 A were generated, and the performance as the plasma window was investigated. The pressure ratio of high-pressure discharge section to low-pressure vacuum side was about 200 at a gas flow rate of 0.05 L/min. In order to characterize the plasma parameters around the anode electrode, vacuum UV and visible emissions of He I were measured. The electron density was determined to be 4 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for a discharge current 100 A and a gas flow rate of 0.05 L/min, while the electron temperature was 0.11 eV. Moreover, the forbidden transition of He I 2s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> S- <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> P was observed. The effect of radiation trapping of He I Ly-α was studied by the intensity ratio of Lyα to forbidden line and the collisional radiative (CR) model. As a result, it was found that the plasma generated was optically thick with an optical escape factor of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> .

Optical Radiation Trapping by Current in Gyrotropic Liquid Metacrystals

It is shown that direct electric current passing through a suspension of gyrotropic nanoparticles with residual magnetization (gyrotropic liquid metacrystal) forms a unidirectional waveguide for optical radiation so that trapped light can propagate only in the direction opposite to the direction of current. The localization of electromagnetic radiation is associated with the emergence of nonuniform gyrotropy of the medium as a result of reorientation of magnetic nanoparticles in the nonuniform magnetic field of the current. By way of examples, we consider the trapping of the radiation by a plane current sheet and by a cylindrical current-carrying filament. The dispersion equations of trapped modes are derived and analyzed. The analogy with topologically protected edge photon states is considered.

Modeling DC electrical breakdown using a truncated emission spectrum for trapped radiation

Spontaneously emitted radiation from excited atoms can be of principle importance in certain modes of electrical breakdown, especially positive streamers and some regimes of Townsend breakdown. The electrostatic particle-in-cell code Aleph utilizes the direct simulation Monte Carlo method to compute radiation transport. When there is strong radiation trapping, this approach is limited in that it must resolve the timescale associated with self-absorption. This renders many cases computationally intractable as sub-femtosecond time steps can be required to compute solutions for phenomena that occur over nanoseconds or microseconds. For two specific cases which exhibit strong radiation trapping, we find that spontaneous emissions having a frequency near the line center are inactive in the breakdown process and can be neglected. This enables larger time steps and a computational speedup of up to two orders of magnitude is observed. Some considerations for determining the validity of making such an approximation for Townsend breakdown problems and positive ionization wave problems are presented.

Optical emission spectroscopy of helium helps deduce plasma electron density and temperature

Applying a collisional radiative model to line emissions provide estimates that agree with Thompson scattering when radiation trapping is accounted for.

Urban canopy meteorological forcing and its impact on ozone and PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt;: role of vertical turbulent transport

Abstract. It is well known that the urban canopy (UC) layer, i.e., the layer of air corresponding to the assemblage of the buildings, roads, park, trees and other objects typical to cities, is characterized by specific meteorological conditions at city scales generally differing from those over rural surroundings. We refer to the forcing that acts on the meteorological variables over urbanized areas as the urban canopy meteorological forcing (UCMF). UCMF has multiple aspects, while one of the most studied is the generation of the urban heat island (UHI) as an excess of heat due to increased absorption and trapping of radiation in street canyons. However, enhanced drag plays important role too, reducing mean wind speeds and increasing vertical eddy mixing of pollutants. As air quality is strongly tied to meteorological conditions, the UCMF leads to modifications of air chemistry and transport of pollutants. Although it has been recognized in the last decade that the enhanced vertical mixing has a dominant role in the impact of the UCMF on air quality, very little is known about the uncertainty of vertical eddy diffusion arising from different representation in numerical models and how this uncertainty propagates to the final species concentrations as well as to the changes due to the UCMF. To bridge this knowledge gap, we set up the Regional Climate Model version 4 (RegCM4) coupled to the Comprehensive Air Quality Model with Extensions (CAMx) chemistry transport model over central Europe and designed a series of simulations to study how UC affects the vertical turbulent transport of selected pollutants through modifications of the vertical eddy diffusion coefficient (Kv) using six different methods for Kv calculation. The mean concentrations of ozone and PM2.5 in selected city canopies are analyzed. These are secondary pollutants or having secondary components, upon which turbulence acts in a much more complicated way than in the case of primary pollutants by influencing their concentrations not only directly but indirectly via precursors too. Calculations are performed over cascading domains (of 27, 9, and 3 km horizontal resolutions), which further enables to analyze the sensitivity of the numerical model to grid resolution. A number of model simulations are carried out where either urban canopies are considered or replaced by rural ones in order to isolate the UC meteorological forcing. Apart from the well-pronounced and expected impact on temperature (increases up to 2 ∘C) and wind (decreases by up to 2 ms−1), there is a strong impact on vertical eddy diffusion in all of the six Kv methods. The Kv enhancement ranges from less than 1 up to 30 m2 s−1 at the surface and from 1 to 100 m2 s−1 at higher levels depending on the methods. The largest impact is obtained for the turbulent kinetic energy (TKE)-based methods. The range of impact on the vertical eddy diffusion coefficient propagates to a range of ozone (O3) increase of 0.4 to 4 ppbv in both summer and winter (5 %–10 % relative change). In the case of PM2.5, we obtained decreases of up to 1 µg m−3 in summer and up to 2 µg m−3 in winter (up to 30 %–40 % relative change). Comparing these results to the “total-impact”, i.e., to the impact of all meteorological modifications due to UCMF, we can conclude that much of UCMF is explained by the enhanced vertical eddy diffusion, which counterbalances the opposing effects of other components of this forcing (temperature, humidity and wind). The results further show that this conclusion holds regardless of the resolution chosen and in both the warm and cold parts of the year.

Emission spectroscopy of He lines in high-density plasmas in Magnum-PSI

Helium (He) line emissions have been utilized to measure the electron density (ne) and temperature (Te), and validity checks have been conducted in various linear devices. In this study, we performed optical emission spectroscopy (OES) of He line emissions in the linear plasma device Magnum-PSI, where the used density range was 1–8 × 1020 m−3, which was much higher than those used until now. We observed nine line emissions in the wavelength range of 388–728 nm and deduced ne and Te based on comparisons with a collisional radiative model. From the variation of the difference between the experiments and calculations, the joint probability distribution of ne and Te was deduced. We will discuss the effect of radiation trapping, in particular, based on comparisons between OES measurement results and Thomson scattering measurements.

Effect of laser polarization on the electron dynamics and photon emission in near-critical-density plasmas

When the laser intensity approaches 1022 W cm−2, the recoil force from high-energy photons radiated by relativistic electrons becomes important and may be comparable with the ponderomotive force of the laser pulse. As a result, the electron dynamics are modified greatly. By using particle-in-cell simulations, we show that the polarization of laser pulse has significant influence on the electron behaviors and accompanying radiation in near-critical-density plasmas. Due to the RR force and the pinching effect of self-generated magnetic field, it turns out that much more electrons are trapped in the center of a circularly-polarized (CP) laser pulse than that of a linearly-polarized (LP) laser pulse. Meanwhile, the γ−photon emission is characterized by a two-peak structure in the LP case, while it is dominated in the forward direction in the CP case where electrons keep moving forward along the laser propagating direction. These may serve as a potential signature for radiation trapping effect in forthcoming experiments to be carried out on petawatt laser facilities.

Canyon effects in urban configurations: tropical context study

The increase of the Urban Heat Island (UHI) is a continuing phenomenon in dense cities. Technically, the building of heat gain generated from modified solar radiation in tropical urban spaces requires the scientific solution. The investigation on the behavior of urban microclimate, especially solar radiation in the urban canyon, was comprehensively investigated by early urban energy studies. However, the solar radiation trapping effect as the behavior of solar radiation in different type of urban configurations is still unclear. This study, therefore, aims to investigate the impact of urban configurations on the solar radiation performance in Kuala Lumpur tropical context. By using ENVI-met V3.1, this study simulates four urban configurations situated in two scenarios of canyon directions; East-West and South-North. The results recorded that the short wave and long wave solar radiation varies in a different type of urban configurations. It concludes that besides the Sky View Factor (SVF) and Height to Width (H/W) aspect ratio, the direction of urban canyon towards the exposure of the source of solar radiation significantly influences the solar radiation trapping effect. This finding will be a significant reference to strategies for the urban configuration to improve the microclimate in the urban spaces and mitigating climate change.

2-D Simulation of Two-Chamber Photoplasma for Conversion of Light Radiation to Electrical Energy

A self-consistent study of two-chamber photoplasma has been carried out to quantitatively investigate the conversion of light radiation to electrical energy. The main idea is to perform photoionization of alkali metal atoms in the first chamber by light irradiation, and then creating a potential difference due to the significant electron density gradient between two joint chambers. A 2-D simulation using COMSOL for a two-chamber cell with a Na–Ar mixture has been fulfilled and the basic photoplasma parameters (charged and excited particle densities, electron temperatures, and electric potentials) have been obtained. Plasma chemistry included the photoexcitation of sodium atoms, radiation trapping, energy pooling, penning and associative ionization, electron–atom impact reactions, stepwise ionization, and dissociative and collision–radiative recombination. Strong space nonuniformity for the electron density in this cell has been found. The possibility to obtain an electromotive force (EMF) (electric potential difference between two chambers) with resonant radiation pumping has been established quantitatively. For cylindrical geometry, where both chambers have equal radii ${R} _{{1}} = {R} _{{2}} = 0.5$ cm but different lengths ${L} _{{1}} =1$ cm and ${L} _{{2}} =$ (0.5–3) cm, the EMF could reach 0.59 V at ${L} _{{2}} =3$ cm for gas pressures ${P} _{{Ar}}= 1$ Torr and ${P} _{{Na}}= 0.02$ Torr with a homogeneous spatial profile in the first chamber of the photoexcitation rate of resonant sodium atoms. The effect of changing the second chamber length has been investigated. The obtained results can be used for the design of renewable ecological sources of electrical energy in solar photovoltaics.

Many-body cooperative effects in an ensemble of pointlike impurity centers near a charged conductive surface

On the basis of a quantum microscopic approach we study the cooperative effects induced by the dipole-dipole interaction in an ensemble of point-like impurity centers located near a charged perfectly conducting surface. We analyze the simultaneous influence of the modified spatial structure of field modes near the conductive surface and the electric field on the transition spectrum of an excited atom inside an ensemble and on the radiation trapping. We show that the electric field modifies the cooperative Lamb shift, as well as the character of sub- and superradiant decay. We also demonstrate that these modifications differ from those taking place in the case of atomic ensembles in free space, without conducting surface.

Application of argon collisional-radiative model for inductive RF discharge research

The technique of optical emission spectroscopy (OES) in combination with plasma emission models for determining the parameters of a gas discharge is presented. Measurements of charge carrier concentration and electron temperature in an inductively coupled RF plasma of argon are carried out under low pressure. The concentration and temperature of electrons were determined by selecting the ratio of the intensities of the spectral lines calculated with the collision-radiative model (CRM) to the intensities of the lines obtained in the experiment. The model describes the kinetics of the first 30 excited states of argon and takes into account the following processes: direct electron impact excitation / relaxation, spontaneous emission, radiation trapping, electron impact ionization and charge loss due to diffusion on the walls. The obtained OES results were compared with the results of Langmuir probe measurements of plasma parameters.

Radiation forces on a Rayleigh particle produced by partially coherent circular Airy beams.

In this work, the radiation force on a Rayleigh dielectric particle induced by the partially coherent circular Airy beam (PCCAB) is investigated. Our numerical results show that the PCCAB can be used to trap and manipulate particles. The radiation force distribution and trapping stability have been analyzed under different coherent lengths. It is found that, with the increase of the spatial coherent length, the radiation force is increased and the particle can be stably trapped at more points. Therefore, the radiation force as well as the depth of potential well can be effectively modulated by controlling the spatial coherent length in optical micromanipulation. The trapping properties of PCCAB have also been studied under other different parameters, including the scale factor and initial radius.

Analysis of radiative opacities for optically thin and thick astrophysical plasmas

The resolution of the radiative transfer equation in radiation-hydrodynamic simulations of astrophysical plasmas require radiative opacities. In this work, an analysis of the monochromatic and multigroup opacities of an astrophysical plasma mixture has been carried out. The study has been made in ranges of electron temperatures and densities of 1−1000 eV and 1011−1020 cm−3, respectively, a wide range of plasma conditions that can be found in several astrophysical scenarios where local and non-local thermodynamic regimes are attained. Collisional-radiative calculations were performed to obtain the plasma level populations and the monochromatic opacities in that range of plasma conditions, covering both thermodynamic regimes. Since the astrophysical mixture includes chemical elements from hydrogen to iron, their contribution to the total opacity will depend on the plasma conditions and we have made a characterization of their contribution as a function of the electron density and temperature and also of the photon frequency. Multigroup and gray approaches are commonly used in the radiative transfer equation in radiation-hydrodynamic calculations. We have analyzed the influence of the number of the groups in the accuracy of the multigroup opacities and we have showed that, for a given plasma condition, the opacity of the multicomponent plasma in the gray approach may be considerably influenced by only some of the contributing elements, due to the influence of the weighting function in the mean, which can lead to great differences with respect to the monochromatic opacity. Finally, since there are situations in which the self-absorption of the plasma radiation becomes relevant due to the dimensions of the plasma, we have performed an analysis of the influence of the radiation trapping in the monochromatic and multigroup opacities in terms of the plasma conditions and the width of the plasma slab, assuming the plasma with planar geometry and we have also studied the departures of the local thermodynamic regime.

On the role of hydrogen radiation absorption in divertor plasma detachment

Transition to the detached divertor regime that allows lowering the peak power loads on the divertor targets to a tolerable level requires low plasma temperature Te ~ 1 eV and high plasma density ne ~ 1021 m−3 in front of the target. Under such conditions, radiation trapping of the Lyman lines of hydrogen isotopes becomes important. It can influence both energy balance and the ionization/recombination rates significantly. Nevertheless, opacity is typically neglected in the 2D edge transport codes used to study the divertor plasma detachment. We report on the first in-depth investigation of radiation opacity effects on transition to detachment. Simulations are performed with the SOLPS 4.3 code package using a DIII-D size tokamak as a particular example. It is found that in pure hydrogen plasma suppression of the neutral hydrogen radiation loss due to the photon absorption makes reaching the detached plasma regime more difficult. A significantly higher average separatrix plasma pressure is required to reach a similar degree of detachment. Adding plasma impurities compensates for the reduced neutral radiation and offsets the effect of opacity. In a carbon device, where the impurity source is due to erosion of the divertor components and is strongly connected to the edge plasma density, the absorbed fraction of the hydrogen radiation is easily compensated with the increasing carbon radiation loss. However, nowadays, in the more relevant case of the full-metal wall and seeded impurity with the feedback-controlled content, the increase of the edge plasma density alone is insufficient to compensate for trapping of the hydrogen radiation. In this case, achievement of the desired degree of detachment requires higher average separatrix plasma pressure or seeded impurity content than those obtained from the transparent plasma model.

An Investigation on Radiation Trapping and Quenching in Optically Pumped Atomic Vapor Cell

A model is presented to investigate the effects of quenching gas on atomic polarization in optically pumped atomic vapor cell. For Rb atomic vapor cell, the formula obtained by the present model can well describe the relation between the atomic polarization and the pressure of quenching gas nitrogen or hydrogen, and shows that if we keep the pumping rate and the quenching gas pressure unchanged, the atomic polarization tends to be one definite value as the atomic density increases. In addition, the fitting values of the pumping rate show certain dependance on radiation trapping and a model is presented to investigate it.

Pushered single shell implosions for mix and radiation trapping studies using high-Z layers on National Ignition Facility

Pushered Single Shells (PSSs) are an alternative approach to Inertial Confinement Fusion implosions that employ high-Z materials in the innermost capsule layer (pusher) as a means to enhance radiation trapping and lower core ignition requirements. However, adding high-Z materials can also increase losses due to mix, provide extra tamping, and make the capsule emission opaque to x-ray diagnostics. The first PSS implosions performed on the National Ignition Facility use plastic ablators with a germanium (Ge) dopant as a high-Z surrogate in the pusher to isolate the effects of high-Z mix and radiation trapping without changing tamping. Using a 2-shock laser pulse, the PSS implosions are designed and symmetrized to reach 3.7 keV core temperatures. A low concentration (2.8%) Ge dopant is added to the innermost layer, and the resulting effects on mix and x-ray opacity are observed. The method of separated reactants is used to infer information about mixing between the deuterated plastic pusher and the capsule fill gas (25% tritium) from the resulting nuclear DT reactions. Radiation transport is studied via capsule emission x-ray spectroscopy and imaging. Both nuclear and x-ray data corroborate the hypothesis that the addition of Ge strongly affects the mix region through radiation losses but has a minimal effect on the core and the warm, unmixed regions. Simulations using diffusive and turbulent mix models agree qualitatively with data, but quantitative agreement may require hybrid mix models that can model the transitional regime between turbulence and diffusion. Simulations matching the observables show increased core radiation trapping when Ge is added.

1.5 μm photoluminescence and upconversion photoluminescence in GeGaAsS:Er chalcogenide glass

Abstract The paper reports on ≈1.5 μm Stokes photoluminescence (PL) emission and upconversion photoluminescence (UCPL) emission in the visible and near-infrared spectral region in Er3+-doped Ge25Ga8As2S65 chalcogenide glasses at pumping wavelengths of 980 and 1550 nm. The ≈1.5 μm PL emission spectra are broadened with increasing concentration of Er ions which is discussed in terms of radiation trapping and UCPL dynamics affecting the Er3+: 4 I 13/2 level lifetime. The UCPL emission was observed at ≈530, ≈550, ≈660, ≈810 and ≈990 nm and its overall intensity as well as red-to-green UCPL emission intensity ratio increases with increasing Er concentration. To explore the UCPL dynamics we measured double logarithmic dependency of green (≈550 nm) and red (≈660 nm) UCPL emission versus pump power at pumping wavelength of 975 nm. Moreover, we measured quadrature frequency resolved spectroscopy (QFRS) on green UCPL emission (≈550 nm) using 975 nm pumping wavelength and various excitation powers. The QFRS spectra on green UCPL were analyzed in term of QFRS transfer function for three-level model from which we deduced energy transfer upconversion rate w 11 (s−1) originating from Er3+: 4 I 11/2, 4 I 11/2→4 F 7/2, 4 I 15/2 transitions.

Radiation trapping effect versus superradiance in quantum simulation of light-matter interaction

We propose a realization of two remarkable effects of Dicke physics in quantum simulation of light-matter many-body interactions with artificial quantum systems. These effects are a superradiant decay of an ensemble of qubits and the opposite radiation trapping effect. We show that both phenomena coexist in the crossover regime of a ‘moderately bad’ single-mode cavity coupled to the qubit subsystem. Depending on the type of the initial state and on the presence of multipartite entanglement in it, the dynamical features can be opposite resulting either in the superradiance or in the radiation trapping despite of the fact that the initial state contains the same number of excited qubits. The difference originates from the symmetrical or nonsymmetrical character of the initial wave function of the ensemble, which corresponds to indistinguishable or distinguishable emitters. We argue that a coexistence of both effects can be used in dynamical quantum simulators to demonstrate realization of Dicke physics, effects of multipartite quantum entanglement, as well as quantum interference and thus to deeply probe quantum nature of these artificial quantum systems.