Three-dimensional emission tomography of a scattering plasma

The three-dimensional emission ray tomography of a plasma partially scattering its self-radiation is considered. The spatial distribution of scattering density and its angular distribution are assumed to be known. The model of projection-data acquisition taking into account single and multiple scattering is constructed. In terms of this model, the problem of determining the distribution of local emission coefficients is posed. For solving this problem, the algorithm based on the Neumann-series expansion of the product of the radiative-transfer operator for a scattering medium and the operator for solving the pure emission problem was proposed. The algorithm was investigated in detail using numerical simulation.     

等离子体图像诊断中带吸收校正的轴对称重建

在轴对称等离子体的图像诊断中,根据等离子体在某个方向的X射线强度图像可以重建出辐射系数的空间分布.由于X射线在等离子体的传播过程中存在一定的吸收,需要在重建过程中考虑吸收校正.本文在滤波反投影法(FBP)的基础上,结合X射线在等离子体中的吸收规律,提出了带吸收校正的FBP迭代算法.通过对钠等离子体辐射系数的轴对称重建模拟,发现该算法重建精度高,收敛性好,取得了很好的重建结果.     

Reconstruction of three-dimensional arc-plasma temperature fields by orthographic and double-wave spectral tomography

This paper presents a technique to diagnose the three-dimensional temperature fields of arc plasma with orthographic and double-wave spectral tomography. First, the system with two orthographic directional CCD image real-time cameras is adopted to get the information of spectrum intensity related to the gray scale of arc plasma. Second, the improved computerized tomography iterative technique which accords with the least-square and max-entropy criterions, simultaneously, has been used to reconstruct the distributions of different emission coefficients, which shows superiority to the conventional tomography algorithms. Finally, based on plasma physical theory, the spectrum relative-intensity method has been used to reconstruct the distribution features of three-dimensional temperature fields. The experimental results have shown the effectiveness of this measurement system with high-precision and real-time ability.     

等离子体发射和吸收系数的轴对称重建

在激光聚变靶丸等离子体诊断中,为了同时重建出等离子体发射系数和吸收系数的空间分布,建立了分层结构成像模型,利用LM（Levenberg-Marquardt）非线性最小二乘优化算法,提出了基于该模型的重建算法。数值模拟结果表明,该算法成功地重建出发射系数和吸收系数,而且重建精度高,明显优于不考虑吸收衰减影响的Abel逆变换重建结果。对于连续分布,当噪声标准差为0.01时,发射系数与吸收系数重建误差分别为0.17,0.22。     

Time-resolved laser optoacoustic tomography of inhomogeneous media

The methods of time-resolved laser optoacoustic tomography of inhomogeneous media and related problems are reviewed. Time-resolved laser optoacoustic tomography allows one to measure the distribution of light absorption in turbid media with depth resolution up to several microns in real time. The theory of laser excitation of acoustic waves by absorbing of light in particles, dispersed in transparent, light-absorbing or scattering media, is developed. The distribution of light absorption can be obtained from the temporal course of acoustic pressure. Two schemes of acoustic wave detection — in the medium under testing (direct detection) and in transparent medium, coupled to the investigated one (indirect detection) — are discussed. In both cases the reconstruction of light absorption can be made by simple calculations. Test experiments with homogeneous and layered media confirm the proposed theoretical models and the possibility of using the proposed experimental schemes. Light absorption in homogeneous, inhomogeneous media and in absorbing particles dispersed in turbid media was investigated. The experimental setup allows one to measure the absorption coefficients over the range 1-500 cm^–1 with the depth resolution 10–15 m over the depth 1–1.5 mm.     

Interference heat emission in an absorbing layer in the presence of a two-wave field

The control of the interference heat emission upon the oblique incidence of two counterpropagating (with respect to the transverse component of the wavevector) identically linearly polarized coherent waves on the opposite sides of an absorbing layer is considered. The dependences of the interference heat emission on the angle of incidence and the layer thickness are established for various refractive indices and absorption coefficients in weakly and strongly absorbing media. The conditions for the maximum interference heat emission are determined.     

Radon inversion of plasma emission

An analytical inversion of plasma spectral emission using the Radon transform is discussed for a plasma with a constant coefficient of absorption and a model displaced gaussian as the spatial distribution of its coefficients of emission.     

Studies of Silicon Erosion in Plasma-Target-Interaction from Optical Emission and Absorption Spectroscopy

The densities of eroded silicon from a target in interaction with a cylindrically symmetric plasma are determined from measurements of the transverse radiance and transmittance of Doppler broadened spectral lines with high spectral resolution. The transmission distributions, branching ratios, and the spectral line shapes, which depend on the optical depths of the emitting and absorbing particles along the line of sight, are investigated for particle density and temperature determination. First results on emission and transmission measurements of the Si I resonance multiplet spectral line branching ratios and line profiles (centered at 251 and 288 nm, respectively) for silicon as the erosion product from a C/C-SiC target in a low temperature plasma jet give absorption coefficients, from which erosion rates can be deduced, which are in good agreement with the results from the emission spectroscopy and from the gravimetric measurements.     

Radon inversion of spectral intensities emitted by an asymmetric and absorbing plasma

An analytical method of inverting plasma emissions using the Radon transform is discussed. The model plasma considered has a constant coefficient of absorption and a displaced Gaussian as the spatial distribution of its emission coefficients.     

A sensitivity function-based conjugate gradient method for optical tomography with the frequency-domain equation of radiative transfer

The Sensitivity Function-based Conjugate Gradient Method (SFCGM) is described. This method is used to solve the inverse problems of function estimation, such as the local maps of absorption and scattering coefficients, as applied to optical tomography for biomedical imaging. A highly scattering, absorbing, non-reflecting, non-emitting medium is considered here and simultaneous reconstructions of absorption and scattering coefficients inside the test medium are achieved with the proposed optimization technique, by using the exit intensity measured at boundary surfaces. The forward problem is solved with a discrete-ordinates finite-difference method on the framework of the frequency-domain full equation of radiative transfer. The modulation frequency is set to 600 MHz and the frequency data, obtained with the source modulation, is used as the input data. The inversion results demonstrate that the SFCGM can retrieve simultaneously the spatial distributions of optical properties inside the medium within a reasonable accuracy, by significantly reducing a cross-talk between inter-parameters. It is also observed that the closer-to-detector objects are better retrieved.     

基于多通道时间分辨光学层析成像系统的差分图像重建

报道了基于32通道时间分辨光学层析成像系统的差分图像重建结果,实验采用两个光参数与生物组织体大致相同的圆柱型模拟体,分别用于模拟新生儿头部和成人手臂,重建结果表明;采用差分测量的方法很好地重建出异质体的位置．尺寸及吸收系数相对于背景的变化;模拟和实验重建验证了所发展的二维,半三维重建算法的可行性;使用平均飞行时间作为数据类型,32个探测通道测量,比用强度作为数据类型或16通道测量更为准确地分出了两个相距20mm的异质体。结果表明目前的系统可以定性地重建出异质体的位置,尺寸,吸收系数的变化,展示该光学层析成像技术将在监测诸如血液含氧量变化,组织体涉氧新陈代谢等生理过程中具有良好应用前景,也对该种成像方法存在的问题及进一步改进的措施进行了讨论。     

定量光声层析成像的研究进展

光声层析(Photoacoustic tomography,PAT)成像结合了超声成像的高分辨率和光学成像的高对比度的优势,是一种新型的生物医学成像模式。PAT成像算法包含两个逆问题,即根据组织产生的光声信号构建初始声压分布图(即图像重建)以及在此基础上估算成像区域的光学特性参数。后者是一个非线性的不适定问题,通常称为定量光声层析(Quantitative photo-acoustic tomography,q PAT)成像。本文在介绍光声成像原理的基础上,对主要的q PAT算法进行综述,讨论各自的优势和不足,并对未来可能的发展方向进行展望。     

电感耦合氩等离子体中的碰撞截面和输运系数

本文计算了电感耦合氩等离子体中各碰撞截面及电导、热导、扩散和粘滞系数。指出热导在能量传递中起着重要作用,双极扩散则会造成冷等离子体区拥有比局部热平衡值大得多的电子密度。更重要的是,计算表明:三体复合和超弹性碰撞会导致冷等离子体区出现大量的高能电子,这种电子速度分布对Maxwell分布的偏离对作为发射光谱光源的等离子体的激发性质有着特殊的重要意义。 关键词：     

Phonon absorbing boundary conditions for molecular dynamics

With the goal of minimizing the domain size for molecular dynamics (MD) simulations, we develop a new class of absorbing boundary conditions (ABCs) that mimic the phonon absorption properties of an unbounded exterior. The proposed MD-ABCs are extensions of perfectly matched discrete layers (PMDLs), originally developed as an absorbing boundary condition for continuous wave propagation problems. Called MD-PMDL, this extension carefully targets the absorption of phonons, the high frequency waves, whose propagation properties are completely different from continuous waves. This paper presents the derivation of MD-PMDL for general lattice systems, followed by explicit application to one-dimensional and two-dimensional square lattice systems. The accuracy of MD-PMDL for phonon absorption is proven by analyzing reflection coefficients, and demonstrated through numerical experiments. Unlike existing MD-ABCs, MD-PMDL is local in both space and time and thus more efficient. Based on their favorable properties, it is concluded that MD-PMDL could provide a more effective alternative to existing MD-ABCs.     

激光诱导Al等离子体中连续辐射波长分布

用Ar作环境气体,压强固定在100 kPa,激光脉冲能量145 mJ/pulse,利 用时空分辨技术,采集激光烧蚀Al靶产生的等离子体辐射的时间分辨谱。分析了Al等离子体 连续辐射特征。根据连续辐射强度的时间分布,简要讨论了激光诱导等离子体连续辐射的产 生机理。根据连续辐射强度的波长分布,提出了原子对激光诱导等离子体连续辐射共振吸收 机理。认为：Al原子吸收能量的主要机制是原子实吸收连续辐射光子,原子实吸收光子的方 式是与价电子构成极性振子对连续辐射共振吸收。     

Localization of inhomogeneities in diffuse optical tomography based on late arriving photons

An algorithm for localizing inhomogeneities in pulsed diffuse optical tomography is proposed and implemented. A distinctive feature of this technique is the formation of an initial approximation to the spatial distributions of the absorption and scattering coefficients in a biomedical object under study based on the angle-dependent homogeneity index, HI(a). The method allows one to determine the approximate optical structure of the object using late arriving photons and thus solve more rapidly the inverse problem. The suggestion that all absorbing and scattering inhomogeneities in an object under study are spherical also simplifies and enhances image reconstruction.     

Global and local hard X‐ray tomography of a centimeter‐size tumor vessel tree

The visualization of the vascular network in tumors down to the smallest vessels requires high spatial resolution and reasonable contrast. Stained corrosion casts of the microvasculature network guarantee superior X‐ray absorption contrast and highest reproduction fidelity. Tomography of a centimeter‐size tumor, however, is unfeasible at the spatial resolution needed to reveal the smallest vessels. Therefore, local tomography has been performed to visualize the smallest capillaries within the region of interest. These three‐dimensional data show the detailed morphology, but the reconstructed absorption coefficients obtained in local tomography differ substantially from the absorption coefficients retrieved from the less detailed global tomography data. This paper deals with the adaptation of local tomography data using the global data and considers two‐parameter histogram matching of the radiographs, sinogram extension, and multi‐parameter cupping correction. It is demonstrated that two‐parameter histogram matching of the radiographs already provides reasonable agreement. The change of the lens in front of the detector's camera, however, significantly affects the obtained local X‐ray absorption coefficients in the tomograms predominantly owing to the dissimilar point‐spread functions of the two configurations used, and much less to the fact that one of the data sets was acquired in a local geometry.     

Determination of the radial distribution of the absorption and emission coefficients in optically dense plasma

A new numerical method is suggested which, proceeding from a unitary algorithm and without using Abel's transformation, enables us to obtain the radial distribution of the absorption and emission coefficients from known values of the transparency and intensity at various cross sections of a plasma column. Two experimental and two hypothetical examples are calculated by this method. The accuracy of the method is 2–4%.     

Absorption of Long Waves by Nonresonant Parametric Microstructures

Using simple acoustical and mechanical models, we consider the conceptual possibility of designing an active absorbing (nonreflecting) coating in the form of a thin layer with small-scale stratification and fast time modulation of parameters. Algorithms for space-time modulation of the controlled-layer structure are studied in detail for a one-dimensional boundary-value problem. These algorithms do not require wave-field measurements, which eliminates the self-excitation problem that is characteristic of active systems. The majority of the considered algorithms of parametric control transform the low-frequency incident wave to high-frequency waves of the technological band for which the waveguiding medium inside the layer is assumed to be opaque (absorbing). The efficient use conditions are found for all the algorithms. It is shown that the absorbing layer can be as thin as desired with respect to minimum spatial scale of the incident wave and ensures efficient absorption in a wide frequency interval (starting from zero frequency) that is bounded from above only by a finite space-time resolution of the parameter-control operations. The structure of a three-dimensional parametric black coating whose efficiency is independent of the angle of incidence of an incoming wave is developed on the basis of the studied one-dimensional problems. The general solution of the problem of diffraction of incident waves from such a coating is obtained. This solution is analyzed in detail for the case of a disk-shaped element.     

Analysis of X-ray spectral data with genetic algorithms

An algorithmic method for the analysis of X-ray line spectra using genetic algorithms is presented. This technique permits the extraction of diagnostic information on the emitting medium from the spectral data. As an example of the method, plasma electron number density and temperature are extracted from the analysis of X-ray spectral data recorded in an Ar-doped inertial-confinement-fusion core. For the study of a sequence of gradually changing spectra, a combination of genetic algorithms and case-based reasoning that learns from experience is used to accelerate the analysis. The technique is general and can be applied to other plasma spectroscopy studies including analysis of spatially and temporally resolved line absorption or emission data.