Distribution Of Radiation Intensity Research Articles

Study of the Filamentation Phenomenon of Femtosecond Laser Radiation

The filamentation phenomenon during propagation of femtosecond laser radiation in transparent solid media is experimentally studied. An experimental scheme for measuring the filament parameters using spherical and cylindrical optics is developed and assembled. The sample is structured in the filamentation mode, which makes it possible to obtain a specified distribution of laser radiation intensity.

Formation of a Thin Luminescent Layer in LiF Crystals under Glow Discharge Radiation

The formation of thin layers of luminescent defects on the faces of planar lithium fluoride crystals located in the positive column and Faraday dark space of a glow gas discharge was studied by time-resolved confocal scanning luminescent microscopy and time-correlated single photon counting. The formation of aggregated color centers in the surface layers of crystals was established using the spectral and kinetic characteristics of luminescence appearing after irradiation. The role of gas discharge electrons, ions, and photons in the defect formation mechanism was considered. The defects were shown to be formed under the influence of vacuum ultraviolet (VUV) photons. The VUV radiation intensity distribution in the discharge gap was measured by the method of thermostimulated luminescence. The main source of this radiation was the anodic and cathodic voltage drop regions in a glow discharge.

Investigation of the expansion dynamics of silicon plasmas generated by double nanosecond laser pulses

A systematic investigation of the expansion dynamics of plasma plumes generated by two Q-switched Nd:YAG lasers at 1064 nm wavelength operating on a silicon target was undertaken for the inter-pulse delay times of 0, 100, 200, and 400 ns using a technique involving fast-gated intensified charge-coupled device imaging. Our results indicate that the plasmas exhibit free expansion in a vacuum environment at an inter-pulse delay time of 0 ns. With increasing inter-pulse delay time, the plasma front becomes sharpened and an interaction boundary is formed. Moreover, using the radiation intensity distribution along the plasma axis of symmetry, the formation and evolution mechanism of the plasmas generated by the double pulses was analyzed at different inter-pulse delay times. Finally, the experimental results of the expansion of the plasma core and front were compared with a radiation hydrodynamics model and a drag model, and were found to be generally in good agreement.

Radiation Absorption in a Particle Curtain Exposed to Direct High-Flux Solar Irradiation

Radiation absorption is investigated in a particle curtain formed in a solar free-falling particle receiver. An Eulerian–Eulerian granular two-phase model is used to solve the two-dimensional mass and momentum equations by employing computational fluid dynamics (CFD) to find particle distribution in the curtain. The radiative transfer equation (RTE) is subsequently solved by the Monte Carlo (MC) ray-tracing technique to obtain the radiation intensity distribution in the particle curtain. The predicted opacity is validated with the experimental results reported in the literature for 280 and 697 μm sintered bauxite particles. The particle curtain is found to absorb the solar radiation most efficiently at flowrates upper-bounded at approximately 20 kg s−1 m−1. In comparison, 280 μm particles have higher average absorptance than 697 μm particles (due to higher radiation extinction characteristics) at similar particle flowrates. However, as the absorption of solar radiation becomes more efficient, nonuniform radiation absorption across the particle curtain and hydrodynamic instability in the receiver are more probable.

Modification of Photoconductivity Spectra in ZnO–CdSe Quantum- Dot Composites upon Exposure to Additional Photoexcitation

The incorporation of CdS nanocrystals into a porous ZnO matrix results in sensitization of the composite in the visible spectral region. Studies of the photoconductivity spectra upon variable external illumination show that the spectra undergo reversible transformations. It is shown that the shape of the peak and the position of the local photoconductivity maximum corresponding to nanocrystals depend on the wavelength distribution of the incident radiation intensity. The mechanisms responsible for the process are discussed.

A multi-stage optimization technique for simultaneous reconstruction of infrared optical and thermophysical parameters in semitransparent media

A multi-stage optimization technique is proposed to simultaneously reconstruct the infrared optical and thermophysical parameters in semitransparent media. The coupled radiative-conductive heat transfer in two-dimensional absorbing, scattering and emitting medium is solved by the discrete ordinate method combined with finite volume method. The exit radiative intensity and temperature distribution on the boundary are served as input for the inverse analysis, and the sequential quadratic programming is used as the inverse technique. Since the measurement signals are much more sensitive to the infrared absorption and scattering coefficients than to the thermal conductivity of medium, the thermophysical property cannot be accurately reconstructed by the conventional method. The multi-stage optimization technique is developed to solve the inverse estimation tasks, through which the optical and thermophysical parameters are reconstructed in different stages based on different objective functions. All the retrieval results demonstrate that the multi-stage optimization technique is robust and effective in simultaneous estimation of absorption coefficient, scattering coefficient and thermal conductivity. The optical and thermophysical parameters can be reconstructed accurately even with measurement errors.

The influences of welding parameters on the metal vapor plume in fiber laser welding based on 3D reconstruction

Fiber laser has significant advantages in the welding of moderately thick stainless steel plates. However, the influences of welding parameters on interaction behavior are still not fully understood. The metal vapor, as process product of fiber laser welding, carries abundant information of interaction behavior. A novel three-dimensional reconstruction technique was used to establish spatial structure of metal vapor during fiber laser welding. The spatial field of temperature and electron density were computed based on the 3D distribution of radiation intensity, as well as the relative spectral intensity method. Experimental results showed that the temperature of the metal vapor was generally at about 4000–5500 K. Defocusing amount had a significant influence on metal vapor and its energy interaction process in fiber laser welding.

Determination of Opaque Material Temperature from the Spectral Distribution of Emitted Radiation Intensity with Unknown Emissivity

The thermodynamic temperature of an opaque material with unknown emissivity was determined from the recorded distribution of spectral intensities of the emitted radiation. The initial system of equations was derived in the logarithmic form in accordance with the Planck formula. At the first stage, the spectral dependence of the material emissivity was investigated with a special function—relative emissivity. Based on the analysis of spectral dependences of this function at different reference temperatures, a parametric model for the material emissivity was chosen. At the second stage, the desired parameters of the chosen model of spectral emissivity and the thermodynamic temperature of the material were determined. An example of the determination of a temperature of a tungsten sample from the spectral distribution of emitted radiation intensities is given. A method for narrowing the range, which includes the thermodynamic temperature of the material, is presented.

Enhancing the x-ray output of a single-wire explosion with a gas-puff based plasma opening switch

We present experiments performed on the 1 MA COBRA generator using a low density, annular, gas-puff z-pinch implosion as an opening switch to rapidly transfer a current pulse into a single metal wire on axis. This gas-puff on axial wire configuration was studied for its promise as an opening switch and as a means of enhancing the x-ray output of the wire. We demonstrate that current can be switched from the gas-puff plasma into the wire, and that the timing of the switch can be controlled by the gas-puff plenum backing pressure. X-ray detector measurements indicate that for low plenum pressure Kr or Xe shots with a copper wire, this configuration can offer a significant enhancement in the peak intensity and temporal distribution of radiation in the 1–10 keV range.

Optical Sectioning Tomographic Reconstruction of Three-Dimensional Flame Temperature Distribution Using Single Light Field Camera

The temperature measurement has great significance for the combustion diagnostics and the study of flame characteristics. In this paper, an optical sectioning tomographic (OST) technique for 3-D flame temperature measurement combined with light field imaging is proposed. First, the fundamental principle of light field OST is introduced in detail. Then, to determine the refocused depth, the point spread functions of the light field camera, and the relationship between the radiation intensity of flame and image gray value, the calibration works are carried out. Furthermore, the images of different sections of flame are acquired by the digital refocusing technology, and then the high-precision and real-time radiation intensity distribution of each section can be reconstructed using a tomographic reconstruction algorithm. Finally, in combination with the relationship between the image gray value and radiation intensity of flame, the 3-D flame temperature distribution can be calculated. The flame experiments of a candle and a single biomass particle in a fluidized bed under oxy-combustion conditions are performed to evaluate the light field OST system. Results show that the reconstructed results are in good agreement with the temperature distribution of the combustion flame. Compared with the traditional optical sectioning tomographic techniques, the light field OST has the advantages of simplicity of measurement system and fast response, and being suitable for 3-D temperature measurement of turbulent flame.

Модификация спектров фотопроводимости в композитных структурах ZnO-квантовые точки CdSe под действием дополнительного фотовозбуждения

AbstractThe incorporation of CdS nanocrystals into a porous ZnO matrix results in sensitization of the composite in the visible spectral region. Studies of the photoconductivity spectra upon variable external illumination show that the spectra undergo reversible transformations. It is shown that the shape of the peak and the position of the local photoconductivity maximum corresponding to nanocrystals depend on the wavelength distribution of the incident radiation intensity. The mechanisms responsible for the process are discussed.

Formation of doughnut and super-Gaussian intensity distributions of laser radiation in the far field using a bimorph mirror

We consider the methods for modelling doughnut and super-Gaussian intensity distributions in the far field by means of deformable bimorph mirrors. A method for the rapid formation of a specified intensity distribution using a Shack – Hartmann sensor is proposed, and the results of the modelling of doughnut and super-Gaussian intensity distributions are presented.

Формирование тонкого люминесцирующего слоя в кристаллах LiF под действием излучения тлеющего разряда

AbstractThe formation of thin layers of luminescent defects on the faces of planar lithium fluoride crystals located in the positive column and Faraday dark space of a glow gas discharge was studied by time-resolved confocal scanning luminescent microscopy and time-correlated single photon counting. The formation of aggregated color centers in the surface layers of crystals was established using the spectral and kinetic characteristics of luminescence appearing after irradiation. The role of gas discharge electrons, ions, and photons in the defect formation mechanism was considered. The defects were shown to be formed under the influence of vacuum ultraviolet (VUV) photons. The VUV radiation intensity distribution in the discharge gap was measured by the method of thermostimulated luminescence. The main source of this radiation was the anodic and cathodic voltage drop regions in a glow discharge.

Linear Reconstruction Methods for Large Thick Aperture Imaging

Large thick aperture imaging method is proposed to measure the radiation intensity distribution of radiation source whose size is several centimetres. The new method contains two steps which are coded imaging and image reconstruction. Geant4 is used to simulate the coded imaging process. Wiener filter, Lucy-Richardson, Regularized filter, blind deconvolution and CT reconstruction method are realized using Matlab software. Wiener filter and Regularized filter produce artifacts. CT reconstruction method can’t reveal the right size of60Co-γ source because of the overlapping of knife edge images. Both L-R and blind deconvolution are effective. If the aperture is moved 0 mm, 2 mm, 4 mm, 6 mm along x-axis, the reconstruction result using L-R method has a larger correlation coefficient with source image. However, L-R method gets bad result if the aperture rotates more than 1.4°. Under this condition, blind deconvolution is the better choice.

基于Fluent的飞行器流场建模与红外辐射特性分析

Taking a fighter aircraft as the research object, the flow field calculation model of the aircraft was established by the three-dimensional modeling and meshing process, and the numerical simulation of the flow field characteristics of the aircraft was carried out based on the commercial CFD software ANSYS Fluent 16. The effect of solar radiation on the temperature field of the fuselage was considered by using the solar ray tracing algorithm, the discrete ordinates method for DO radiation model was used to calculate the iterative calculation of the radiation transfer equation, simulating the after-combusting tail flame injection process with component transport model without chemical reaction (Species Transport Model), and the temperature, concentration and component distribution data of the outflow field of the aircraft were obtained. The influence of solar radiation on the whole temperature field distribution, the influence of the Mach number on the infrared radiation of the flow field and the distribution of tail flame flow field were simulated and analyzed. Analysis shows that the effect of solar radiation on heating of the skin is only about 5 K. With the increase of the number of Mach aircraft fuselage and the difference of infrared radiation intensity is highest in the abdomen, abdominal radiation intensity is about 2 times the maximum radiation intensity. Under the shock wave, there will be the core high temperature region with the highest two 450 K and 580 K discontinuity at the rear of the tail flame. The infrared radiation intensity distribution of the plume conforms to the pear shape distribution trend.

Processing materials in the mode of multiple filamentation of femtosecond laser radiation

Results are presented from experimental studies of the filamentation of femtosecond laser radiation in a transparent medium. Schemes for registering plasma channels of filaments, conical emission, and the spatial distribution of radiation intensity are provided. The results from laser action on a target of stainless steel in the multiple filamentation mode.

Generation of nano-voids inside polylactide using femtosecond laser radiation

The arrangement of nanometer-sized voids, induced by focusing intense laser radiation within transparent material can allow the generation of transparent components with dimensions in the micrometer to nanometre range due to internal contour cut and thus satisfy the progressive miniaturization of products in micro-optics and medical technologies. For further improvements in the precision of those components, a deep understanding of the involved processes during the interaction of laser radiation within the material is necessary. In this work, voids inside bulk polylactide (PLA), a bioabsorbable polymer, were generated using a femtosecond laser (λ = 1030 nm, τH = 180 fs) with single and multiple pulse irradiation. The dependence of the spot size was examined by the use of four microscope objectives with focus radii of 4.9, 3.3, 2 and 1.2 µm. For the experiments, the pulse energy and focusing depth into the material were varied. The dimensions of the voids were experimentally determined as function of the intensity. Differences in the lateral and axial extents of the voids were obtained for different focus radii and focusing depths at same intensities. Furthermore, the intensity distribution of the laser radiation inside the material for the different focus radii and focusing depths, and their dependence on the lateral and axial sizes of the voids was simulated and compared with the experimental results.

Evolution of arbitrary moments of radiant intensity distribution for partially coherent general beams in atmospheric turbulence

The evolution law of arbitrary order moments of the Wigner distribution function, which can be applied to the different spatial power spectra, is obtained for partially coherent general beams propagating in atmospheric turbulence using the extended Huygens–Fresnel principle. A coupling coefficient of radiant intensity distribution (RID) in turbulence is introduced. Analytical expressions of the evolution of the first five-order moments, kurtosis parameter, coupling coefficient of RID for general beams in turbulence are derived, and the formulas are applied to Airy beams. Results show that there exist two types for general beams in turbulence. A larger value of kurtosis parameter for Airy beams also reveals that coupling effect due to turbulence is stronger. Both theoretical analysis and numerical results show that the maximum value of kurtosis parameter for an Airy beam in turbulence is independent of turbulence strength parameter and is only determined by inner scale of turbulence. Relative angular spread, kurtosis and coupling coefficient are less influenced by turbulence for Airy beams with a smaller decay factor and a smaller initial width of the first lobe.

Analysis of radiation heat transfer and temperature distributions of solar thermochemical reactor for syngas production

This paper investigated radiation heat transfer and temperature distributions of solar thermochemical reactor for syngas production using the finite volume discrete ordinate method (fvDOM) and P1 approximation for radiation heat transfer. Different parameters including absorptivity, emissivity, reflection based radiation scattering, and carrier gas flow inlet velocity that would greatly affect the reactor thermal performance were sufficiently investigated. The fvDOM approximation was used to obtain the radiation intensity distribution along the reactor. The drop in the temperature resulted from the radiation scattering was further investigated using the P1 approximation. The results indicated that the reactor temperature difference between the P1 approximation and the fvDOM radiation model was very close under different operating conditions. However, a big temperature difference which increased with an increase in the radiation emissivity due to the thermal non-equilibrium was observed in the radiation inlet region. It was found that the incident radiation flux distribution had a strong impact on the temperature distribution throughout the reactor. This paper revealed that the temperature drop caused by the boundary radiation heat loss should not be neglected for the thermal performance analysis of solar thermochemical reactor.

A method for evaluating photovoltaic potential in China based on GIS platform

Solar photovoltaic systems are commonly utilized in China. However, the associated research is still lack of its resource potential analysis in all regions in China. Based on the existed data about solar radiation and system conversion efficiency data, a new method for distributed photovoltaic potential assessment has been presented. The experiment of three kinds of solar photovoltaic system has been set up for the purpose of analyzing the relationship between conversion efficiency and environmental parameters. This paper fits the relationship between conversion efficiency and solar radiation intensity. This method takes into account the amount of solar radiation that is effectively generated and drives away the weak values. With the spatial analysis function of geographic information system (GIS) platform, frequency distribution of solar radiation intensity and PV potential in China can be derived. Furthermore, analytical results show that monocrystalline-silicon PV generation in the north-western and northern areas have reached a level of more than 200 kWh/(m2.a), making those areas be suitable for the development of PV system. However, the potential for southwest areas reaches a level of only 130 kWh/(m2.a). This paper can provide the baseline reference for solar energy development planning.