强磁场下的输运过程与Liouville方程的两种定态解

本文中讨论了在外场中Liouville方程的两种定态解,指出在用Kubo公式计算输运系数时的关键之处是:在完成Kubo公式中的时间积分之前,必须把关联函数的时间行为表示成趋向平衡的形式。注意到这一点就能得到正确的输运系数表示式。用这种办法,我们重新计算了文献[1]中曾讨论过的强磁场下的电导率,得到了正确的非零的结果。 关键词：     

Recovering doping profiles in semiconductor devices with the Boltzmann–Poisson model

We investigate numerically an inverse problem related to the Boltzmann–Poisson system of equations for transport of electrons in semiconductor devices. The objective of the (ill-posed) inverse problem is to recover the doping profile of a device, presented as a source function in the mathematical model, from its current–voltage characteristics. To reduce the degree of ill-posedness of the inverse problem, we proposed to parameterize the unknown doping profile function to limit the number of unknowns in the inverse problem. We showed by numerical examples that the reconstruction of a few low moments of the doping profile is possible when relatively accurate time-dependent or time-independent measurements are available, even though the later reconstruction is less accurate than the former. We also compare reconstructions from the Boltzmann–Poisson (BP) model to those from the classical drift–diffusion-Poisson (DDP) model, assuming that measurements are generated with the BP model. We show that the two type of reconstructions can be significantly different in regimes where drift–diffusion-Poisson equation fails to model the physics accurately. However, when noise presented in measured data is high, no difference in the reconstructions can be observed.     

Maximum likelihood estimates in correlation acoustic thermotomography

Informative capabilities of active—passive acoustic thermotomography that allows a separate reconstruction of the local values of intrinsic and background temperature, absorption coefficient, and ultrasonic velocity are discussed. An iterative procedure based on the maximum likelihood method that provides unbiasedness of estimates is proposed for the correlation processing of a large body of experimental data. The sensitivity of a thermotomographic system is connected to the limitations imposed on the accumulation factor by actual experimental conditions. A sensitivity enhancement is possible at the cost of loosening the requirements on the resolution of the image under reconstruction. Results of numerical simulation of a thermotomographic experiment in narrowband and broadband modes of operation are presented.     

高能闪光照相密度测量的LEPM方法

研究了利用高能闪光照相方法实现密度分布测量的技术.根据闪光照相投影成像原理,将成像过程中的非线性成像因素看成一个综合性的非线性算子,利用先验测量的方法该非线性算子的响应曲线,利用该曲线校正成像图像得到待测客体的光程分布图像.采用以最小误差为重建准则的代数重建法,从光程分布图像中重建出照相客体的线吸收系数分布.最后利用相对等效质量吸收系数以及客体总质量不变的约束条件,获得照相客体的空间密度分布.在高能闪光照相实验平台上进行了静态客体的密度测量实验,测量的均方根误差约为10％.     

Influence of turbulent fluctuation on reconstruction of temperature profile in axisymmetric free flames

An iteration method is extended to analyze the influences of the turbulent fluctuation on the reconstruction of Reynolds time-averaged temperature in turbulent axisymmetric free flames when the temperature profiles are retrieved by the low time-resolution data of outgoing emission and transmission radiation intensities. A simplified probability density function is used to simulate the turbulent fluctuation of temperature and absorption coefficient. The effects of turbulent fluctuating intensities on the estimation of the Reynolds time-averaged temperature and absorption coefficient are examined. The results show that the effects of turbulent fluctuation on the reconstruction of time-averaged absorption coefficient are not significant. In the case of weak turbulent fluctuation, the influences of turbulent fluctuation on the estimation of time-averaged temperature profiles are small. But in the case of strong turbulent fluctuation, the influences of turbulent fluctuation on the estimation of time-averaged temperature profiles are significant.     

Modelling of diffusion and conductivity relaxation of oxide ceramics

A two-dimensional square grain model has been applied to simulate simultaneously the diffusion process and relaxation of the dc conduction of polycrystalline oxide materials due to a sudden change of the oxygen partial pressure of the surrounding gas phase. The numerical calculations are performed by employing the finite element approach. The grains are squares of equal side length (average grain size) and the grain boundaries may consist of thin slabs of uniform thickness. An additional (space charge) layer adjacent to the grain boundary cores (thin slabs) either blocking (depletion layer) or highly conductive for electronic charge carriers may surround the grains. The electronic transport number of the mixed ionic-electronic conducting oxide ceramics may be close to unity (predominant electronic conduction). If the chemical diffusion coefficient of the neutral mobile component (oxygen) of the grain boundary core regions is assumed to be higher by many orders of magnitude than that in the bulk, the simulated relaxation curves for mass transport (diffusion) and dc conduction can deviate remarkably from each other. Deviations between the relaxation of mass transport and dc conduction are found in the case of considerably different electronic conductivities of grain boundary core regions, space charge layers, and bulk. On the contrary, the relaxation curves of mass transport and electronic conductivity are in perfect coincidence, when either effective medium diffusion occurs or the effective conductivity is unaffected by the individual conductivities of core regions and possible space charge layers, i.e. the grain boundary resistivity is negligible.     

代数迭代法用于傅里叶变换计算层析成像光谱仪数据重建

主要介绍代数迭代法在傅里叶变换计算层析成像光谱仪数据处理中的应用。根据4种不同投影数进行了光谱切片重建,并对光谱相似性进行了比较。针对ART算法可应用少数投影重建的特点,专门在45个投影数时,采用不同迭代系数进行了重建,并进行了比较。最后,对实际场景进行了光谱切片重建。     

一种光学层析图像的多准则重建方法

光学层析成像是一个病态重建问题,为克服重建过程的病态性,提出将多准则优化理论引入到图像重建中。利用了三个用于光学层析图像重建的准则:平方误差函数、图像熵函数和局部平滑函数。采用向量优化方法将多准则优化问题转化为单准则优化问题求解。为了确定各个目标函数间的权重系数,提出一种动态权重系数求解方法。重建过程目标函数关于光学参量的梯度计算是关键,因此提出一种基于梯度树的计算方法。实验过程中对多准则重建结果和基于平方误差函数的单准则重建结果做了比较,证明该方法能够克服传统的偏重单一目标的单准则重建的不足,有效地重建光学层析图像,提高图像重建质量。     

Relationships among coefficients in deterministic and stochastic transient diffusion

Systems are studied in which transport is possible due to large extensions with open boundaries in certain directions, but the particles responsible for transport can disappear from it by leaving it in other directions, by chemical reaction or by adsorption. The connection of the total escape rate, the rate of the disappearance, and the diffusion coefficient is investigated. It leads to the observation that the diffusion coefficient defined by is in general different from the one present in the effective Fokker-Planck equation. The result makes it possible to generalize the Gaspard-Nicolis formula [Phys. Rev. Lett. 65, 1693 (1990)] to this transient case in deterministic systems.     

Nonstationary two-flux model of radiation transport for optical tomography of scattering media

A nonstationary two-flux model is formulated for the transport of radiation in an inhomogeneous scattering medium and is applied to the situation where such a medium is irradiated by the narrow beam of a pulsed laser. It is shown that when the time distribution of the transmitted photons is measured it is possible simultaneously to reconstruct the two spatial functions (the coefficients of absorption coefficient and of scattering of the radiation by the medium) by means of an inverse Radon transformation and the solution of a system of nonlinear differential equations on the projection lines. An analytic solution is obtained in quadratures for these differential equations. The results constitute a method of solving problems in optical tomography in an inhomogeneous scattering medium Zh. Tekh. Fiz. 67, 61–65 (May 1997)     

光学层析重建算法改进的研究

光学层析技术在温度、密度等流场的非接触测试中有着广泛的应用前景，然而光学层析的重建是非完全数据的投影重建，通常采用的代数迭代法不能很好地解决重建精度这一难题。为此提出了一种新的光学层析技术的代数迭代重建算法，在算法中引入了包含先验知识的属性矩阵，并摒弃了通常所采用的对超松弛系数人为的确定取法，采用了变超松弛系数。实验计算结果表明，引入属性矩阵和变超松弛系数的代数迭代法能够较好的重建非完全数据投影的待测场，极大地提高重建精度，较好地与实测结果吻合。     

A flux-form version of the conservative semi-Lagrangian multi-tracer transport scheme (CSLAM) on the cubed sphere grid

A conservative semi-Lagrangian cell-integrated transport scheme (CSLAM) was recently introduced, which ensures global mass conservation and allows long timesteps, multi-tracer efficiency, and shape preservation through the use of reconstruction filtering. This method is fully two-dimensional so that it may be easily implemented on non-cartesian grids such as the cubed-sphere grid. We present a flux-form implementation, FF-CSLAM, which retains the advantages of CSLAM while also allowing the use of flux-limited monotonicity and positivity preservation and efficient tracer sub-cycling. The methods are equivalent in the absence of flux limiting or reconstruction filtering.FF-CSLAM was found to be third-order accurate when an appropriately smooth initial mass distribution and flow field (with at least a continuous second derivative) was used. This was true even when using highly deformational flows and when the distribution is advected over the singularities in the cubed sphere, the latter a consequence of the full two-dimensionality of the method. Flux-limited monotonicity preservation, which is only available in a flux-form method, was found to be both less diffusive and more efficient than the monotone reconstruction filtering available to CSLAM. Despite the additional overhead of computing fluxes compared to CSLAM’s cell integrations, the non-monotone FF-CSLAM was found to be at most only 40% slower than CSLAM for Courant numbers less than one, with greater overhead for successively larger Courant numbers.     

Reconstruction of spatial distributions of sound velocity and absorption in soft biological tissues using model ultrasonic tomographic data

A two-step algorithm is used to reconstruct the spatial distributions of the acoustic characteristics of soft biological tissues-the sound velocity and absorption coefficient. Knowing these distributions is urgent for early detection of benign and malignant neoplasms in biological tissues, primarily in the breast. At the first stage, large-scale distributions are estimated; at the second step, they are refined with a high resolution. Results of reconstruction on the base of model initial data are presented. The principal necessity of preliminary reconstruction of large-scale distributions followed by their being taken into account at the second step is illustrated. The use of CUDA technology for processing makes it possible to obtain final images of 1024 × 1024 samples in only a few minutes.     

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用密度调制的方法研究了等离子体中粒子输运问题。采用了注入脉冲式补充送气和超声分子束两种不同的密度调制方法。在HL-2A装置常规欧姆放电的情况下,运用有限差分法和Nagashima矩阵技术,求解了粒子平衡方程。计算出了粒子的输运系数(对流速度v和扩散系数D)。研究了粒子输运系数与等离子体线平均密度之间的关系。实验结果表明,在欧姆放电的情况下,等离子体芯部的粒子对流速度方向始终是向内的,并且密度低时,粒子输运系数(粒子扩散系数D和对流速度v)较大;密度高时,粒子输运系数较小。     

Error consideration in contrast-enhanced three-dimensional optical tomography

We present three-dimensional tomographic images of the absorption coefficient that is due to the presence of a fluorophore reconstructed from frequency domain fluence measurements of a tissue phantom containing a single, fluorescence contrast-enhanced inclusion. We show that such a reconstruction may be improved when the importance of measurement error correlations between relative phase shift and amplitude is assessed and when measurements are preprocessed to reduce the magnitude and the bias of system error.     

多目标优化发射层析算法在等离子体场光谱诊断中的应用

提出一种基于多目标优化原理的发射光谱层析(EST)图像重建新算法MCIRT.通过计算机数值 模拟，考察了该算法对非对称发射系数场分布的重建效果.结果表明，与传统层析算法相比 ，MCIRT算法具有收敛快，重建精度高的优势，适合于非完全数据情况下的等离子体发射系 数场重建，并且实时性更好.作为一个应用实例, 运用谱线相对强度法重建了自由电弧等离 子体的三维温度及粒子数密度分布. 关键词： 等离子体诊断 图像重建     

Single-photon emission computed tomography in a proportional scattering medium

The reconstruction of the spatial distribution of radiation sources in a proportional scattering medium has been considered. An exact solution of the inverse tomographic problem has been obtained by analytically solving the radiation transport equation for arbitrary distributions of radiation sources and extinction coefficient and exact boundary conditions. The type and scale of distortions associated with radiation scattering in tomographic reconstructions have been demonstrated.     

A novel wavefront reconstruction algorithm based on interpolation coefficient matrix for radial shearing interferometry

A novel wavefront reconstruction algorithm for radial shearing interferometer (RSI) is proposed in this paper. Based on the shearing relationship of RSI, an interpolation coefficient matrix is established by the radial shearing ratio and the number of discrete points of test wavefront. Accordingly, the expanded wavefront is characterized by the interpolation coefficient matrix and the test wavefront. Consequently the test wavefront can be calculated from the phase difference wavefront. The numerical simulation is conducted to confirm the correctness of the proposed algorithm. Compared with the previous wavefront reconstruction methods, the proposed algorithm is more accurate and stable.     

单层传声器阵列重建相干声场的近场声全息方法研究

针对传声器阵列两侧存在相干声源的非自由声场重建问题,提出基于球面谐波函数扩展近场声全息理论的相干声场重建方法。该方法在已知测量面两侧声源几何位置时,使用单层传声器阵列获取测量面处的声压分布,通过最小二乘法获得与目标声源和干扰噪声源响应对应的最优球波函数扩展项数和最优系数向量,结合测点位置的空间坐标进行声波分解,并分别重建出各声源在测量面上的声压分布。为了验证方法的有效性,分别给出了相干噪声源为球形声源和非球形声源的仿真验证,并在全消声室内对双扬声器产生的相干声场的重建进行了实验验证。结果表明:该方法对球形声源和非球形声源干扰下的声场重建都具有较好的效果,球形声源干扰下的重建精度更高。      

Low-viscosity lattice gases

A class of lattice gas models are studied which are variants of the FCHC model. The aim is to achieve the highest possible Reynolds coefficient (inverse dimensionless viscosity) for efficient simulations of the three-dimensional incompressible Navier-Stokes equations. The models include an arbitrary number of rest particles and violation of semi-detailed balance. Within the framework of the Boltzmann approximation exact expressions are obtained for the Reynolds coefficients. The minimization of the viscosity is done by solving a Hitchcock-type optimization problem for the fine tuning of the collision rules. When the number of rest particles exceeds one, there is a range of densities at which the viscosity takes negative values. Various optimal models with up to 26 bits per node have been implemented on a CRAY-2 and their true transport coefficients have been measured with good accuracy. Fairly large discrepancies with Boltzmann values are observed when semi-detailed balance is violated; in particular, no negative viscosity is obtained. Still, the best model has a Reynolds coefficient of 13.5, twice that of the best previously implemented model, and thus is about 16 times more efficient computationally. Suggestions are made for further improvements. It is proposed to use models with very high Reynolds coefficients for sub-grid-scale modeling of turbulent flows.