代数重建技术中投影序列选择次序的研究

在代数重建技术中投影序列的选择次序对重建收敛速度及精度均具有重要影响。针对在360°方向上均匀获取投影数据的场合,提出加权距离正交投影排序方法。该方法将投影序列分成若干组,在每一组内前后选择的投影序列彼此正交,同时相邻组之间投影序列尽可能均匀分布。该方法能提高重建精度及收敛速度,同时利用前后正交投影角度下射线经过像素信息关系,可将重建速度提高1倍。     

基于CUDA的多GPU加速SART迭代重建算法

为解决SART迭代重建算法计算耗时的问题,在单GPU基础上,利用多块GPU 的并行计算能力,提出了一种多GPU加速迭代重建算法。实验结果表明,与CPU重建相比,在不影响重建图像质量的情况下,采用GPU重建速度有明显提高,且增加GPU数量可以进一步提高重建速度。     

Reconstruction of time-averaged temperature of non-axisymmetric turbulent unconfined sooting flame by inverse radiation analysis

A multi-wavelength inversion method is extended to reconstruct the time-averaged temperature distribution in non-axisymmetric turbulent unconfined sooting flame by the multi-wavelength measured data of low time-resolution outgoing emission and transmission radiation intensities. Gaussian, β and uniform distribution probability density functions (PDF) are used to simulate the turbulent fluctuation of temperature, respectively. The reconstruction of time-averaged temperature consists of three steps. First, the time-averaged spectral absorption coefficient is retrieved from the time-averaged transmissivity data by an algebraic reconstruction technique. Then, the time-averaged blackbody spectral radiation intensity is estimated from the outgoing spectral emission radiation intensities. Finally, the time-averaged temperature is approximately reconstructed from the multi-wavelength time-averaged spectral emission radiation data by the least-squares method. Noisy input data have been used to test the performance of the proposed inversion method. The results show that the time-averaged temperature distribution can be estimated with good accuracy, even with noisy input data. The accuracy of the estimation decreases with the increase of turbulent fluctuation intensity of temperature and the effects of assumed PDF on the reconstruction of temperature are small.     

Analysis of algebraic reconstruction technique for accurate imaging of gas temperature and concentration based on tunable diode laser absorption spectroscopy

An improved algebraic reconstruction technique(ART) combined with tunable diode laser absorption spectroscopy(TDLAS) is presented in this paper for determining two-dimensional(2D) distribution of H_2O concentration and temperature in a simulated combustion flame.This work aims to simulate the reconstruction of spectroscopic measurements by a multi-view parallel-beam scanning geometry and analyze the effects of projection rays on reconstruction accuracy.It finally proves that reconstruction quality dramatically increases with the number of projection rays increasing until more than 180 for 20 × 20 grid,and after that point,the number of projection rays has little influence on reconstruction accuracy.It is clear that the temperature reconstruction results are more accurate than the water vapor concentration obtained by the traditional concentration calculation method.In the present study an innovative way to reduce the error of concentration reconstruction and improve the reconstruction quality greatly is also proposed,and the capability of this new method is evaluated by using appropriate assessment parameters.By using this new approach,not only the concentration reconstruction accuracy is greatly improved,but also a suitable parallel-beam arrangement is put forward for high reconstruction accuracy and simplicity of experimental validation.Finally,a bimodal structure of the combustion region is assumed to demonstrate the robustness and universality of the proposed method.Numerical investigation indicates that the proposed TDLAS tomographic algorithm is capable of detecting accurate temperature and concentration profiles.This feasible formula for reconstruction research is expected to resolve several key issues in practical combustion devices.     

基于代数迭代算法的燃烧火焰温度场和气体浓度场重建研究

基于TDLAS(tunable diode laser absorption spectroscopy)技术,以水汽作为目标气体,采用直接吸收的测量方式,探测了甲烷空气预混平焰炉燃烧区域水汽的吸收光谱信号,通过ART(algebraic reconstruction technique)代数迭代算法对燃烧场温度和水汽浓度分布进行了模拟重建和实验研究,模拟重建采取5×5共25个网格的正方形重建区域,假定25个网格的一个温度浓度二维分布,模拟28条激光束从不同的角度方位穿越重建区域,得到模拟射线下的投影值,经ART算法重建,结果显示温度场和水汽浓度场的重建偏差均在1%以内。实验采用分布反馈式激光器作为光源,选取H₂O的7 153.722,7 153.748和7 154.354 cm-1三条吸收线作为测温谱线,其中前两条线不区分作为一条吸收线来处理。使用平移台多方位平行扫描,共获取30路光谱吸收信号,经数据处理、算法重建和双线比值法测温原理得到了圆形平焰炉16个不同区域的温度浓度值,且炉面偏向中心区域温度浓度值较高,边缘较小,结果表明代数迭代算法能够很好地实现燃烧区域温度场和水汽浓度场的反演。     

A modified discrete algebraic reconstruction technique for multiple grey image reconstruction for limited angle range tomography

The `missing wedge', which is due to a restricted rotation range, is a major challenge for quantitative analysis of an object using tomography. With prior knowledge of the grey levels, the discrete algebraic reconstruction technique (DART) is able to reconstruct objects accurately with projections in a limited angle range. However, the quality of the reconstructions declines as the number of grey levels increases. In this paper, a modified DART (MDART) was proposed, in which each independent region of homogeneous material was chosen as a research object, instead of the grey values. The grey values of each discrete region were estimated according to the solution of the linear projection equations. The iterative process of boundary pixels updating and correcting the grey values of each region was executed alternately. Simulation experiments of binary phantoms as well as multiple grey phantoms show that MDART is capable of achieving high‐quality reconstructions with projections in a limited angle range. The interesting advancement of MDART is that neither prior knowledge of the grey values nor the number of grey levels is necessary.     

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。     

悬摆法测量气体推进剂激光推进冲量耦合系数

给出了悬摆法测量激光推进冲量耦合系数的原理,分别以空气、氩气、氮气和氦气为推进剂,用悬摆法测量了抛物形推力器在能量不同的单脉冲TEA CO2激光辐照下的冲量耦合系数。实验结果表明：氩气的冲量耦合系数最高,氦气的冲量耦合系数最低;在实验测试的激光能量范围内,4种推进剂气体的激光推进冲量耦合系数基本上都随着激光能量的增加而线性增大,冲量耦合系数的相对误差为5.4%~6.4%。实验结果与国外相关实验结论一致。     

悬摆法测量气体推进剂激光推进冲量耦合系数

 给出了悬摆法测量激光推进冲量耦合系数的原理,分别以空气、氩气、氮气和氦气为推进剂,用悬摆法测量了抛物形推力器在能量不同的单脉冲TEA CO₂激光辐照下的冲量耦合系数。实验结果表明：氩气的冲量耦合系数最高,氦气的冲量耦合系数最低;在实验测试的激光能量范围内,4种推进剂气体的激光推进冲量耦合系数基本上都随着激光能量的增加而线性增大,冲量耦合系数的相对误差为5.4%~6.4%。实验结果与国外相关实验结论一致。     

可调谐半导体激光吸收光谱技术应用于平面火焰中气体浓度二维分布重建的研究

研究了可调谐半导体激光吸收光谱技术实现气体浓度二维分布测量的方法,探索了重建气体浓度二维分布的模型和算法,并采用数值模拟的方式对重建程序进行了可行性验证。搭建了由24束激光束构成的场参数测量系统,以甲烷/空气预混火焰中H₂O为测量对象,运用重建程序重建了火焰中H₂O浓度的二维分布,实验结果分析表明,重建结果较真实的反应了火焰中H₂O浓度的二维分布状态,为实现优化燃烧控制提供重要数据。     

两种构形喷管的激光推进冲量耦合系数数值研究

提炼了由入射激光能量、能量沉积率、环境气体压强、喷管母线长度及其半顶角组合而成的圆锥形喷管无量纲因子,并将其推广到抛物形喷管的研究中。用辐射流体力学计算程序计算了4种半顶角下圆锥形和抛物形喷管的冲量耦合系数随无量纲因子的变化规律。计算结果表明：两种喷管的冲量耦合系数都存在极大值,极大值对应的无量纲因子约为0.4;无量纲因子相同时,半顶角越小,冲量耦合系数越大;同等条件下抛物形喷管的冲量耦合系数高于圆锥形喷管的冲量耦合系数。     

代数迭代重建算法在折射衬度CT中的应用

X射线折射衬度CT是一种基于相位衬度的断层成像技术,特别适合对由轻元素组成的生物、医学样品进行成像,可以观察到常规吸收衬度CT无法观察到的软组织内部微细结构,是一种具有巨大发展潜力的新成像方法.迭代重建算法和解析重建算法是计算机断层成像技术中并行发展的两种算法,虽然已经提出了几种X射线折射衬度CT的解析重建算法,可是还未见X射线折射衬度CT迭代重建算法的报道.研究了代数迭代重建算法在X射线折射衬度CT中的应用,比较分析了不同的投影数据排列方式对于折射衬度CT重建图像的影响,并对实验数据进行了图像重建,获得了满意的CT图像.研究结果表明,在相位衬度CT中,迭代重建算法相对于解析重建算法而言,能减少投影次数,降低曝光剂量,减少对生物样品的辐射损伤,在生物样品成像和投影数据不完整的情况下具有明显的优势. 关键词： 衍射增强成像 代数迭代算法 CT重建 同步辐射     

Field emission from awl-shaped diamond-like carbon by using filtered cathodic arc plasma technique on anodic aluminum oxide template

Awl-shaped diamond-like carbon (DLC) was directly grown on anodic aluminum oxide (AAO) template by using filtered cathodic arc plasma (FCAP) technique at room temperature. The awls of DLC were about 250 nm in the height and the diameters of the awls were ∼100 nm at the top. The awl density was estimated to be ∼10⁸ cm⁻². A broad asymmetric band ranging from 1100 to 1800 cm⁻¹ was detected by Raman spectrum. This asymmetric band was characteristic band of DLC. The sp³/(sp³+sp²) ratio of C-C bond of the awl-shaped DLC was measured by X-ray photoelectron spectrum, and it was about 68.3%. Field-emission properties of the awl-shaped DLC were investigated. A low turn-on field of 2.6 V/μm at 10 μA/cm² with an emission area of 3.14 mm² was achieved, and the emission current stability was very good. The results indicated that the electrons were emitted under both the effect of enhanced field because of the geometry and the work function of the DLC sample. Based on Fowler-Nordheim plot, the values of work function for the awl-shaped DLC were estimated in ranges of 0.23-1.08 from a linearity plot.     

Synthesis and characterization of tantalum nitride films prepared by cathodic vacuum arc technique

Tantalum nitride films were deposited on silicon wafer and steel substrates by cathodic vacuum arc in N₂/Ar gas mixtures. The chemical composition, crystalline microstructure and morphology of the films were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), respectively. According to the results, film composition and microstructure depends strongly on the N₂ partial pressure and the applied negative bias (V_s).     

相对论速调管振荡器中两个调制腔耦合系数的计算

用场方法研究了相对论速调管振荡器中两个调制腔的耦合,解析地得到了两腔耦合系数与腔和波导的尺寸、间隙及两腔之间的距离等参数之间的关系。耦合系数的数值与由耦合与否腔的频率测量确定的实验结果在数量级上符合得较好。     

Measurements of electron-phonon coupling factor and interfacial thermal resistance of metallic nano-films using a transient thermoreflectance technique

Using a transient thermoreflectance (TTR) technique,several Au films with different thicknesses on glass and SiC substrates are measured for thermal characterization of metallic nano-films,including the electron-phonon coupling factor G,interfacial thermal resistance R,and thermal conductivity K s of the substrate. The rear heating-front detecting (RF) method is used to ensure the femtosecond temporal resolution. An intense laser beam is focused on the rear surface to heat the film,and another weak laser beam is focused on the very spot of the front surface to detect the change in the electron temperature. By varying the optical path delay between the two beams,a complete electron temperature profile can be scanned. Different from the normally used single-layer model,the double-layer model involving interfacial thermal resistance is studied here. The electron temperature cooling profile can be affected by the electron energy transfer into the substrate or the electron-phonon interactions in the metallic films. For multiple-target optimization,the genetic algorithm (GA) is used to obtain both G and R. The experimental result gives a deep understanding of the mechanism of ultra-fast heat transfer in metals.     

HCl化学激光振转跃迁爱因斯坦自发辐射系数的计算

验证了用Level程序计算爱因斯坦自发辐射系数的可信性,选用精确的RKR势函数和最新的电偶极矩函数,计算得到了振动态量子数小于6的氯化氢激光基频振转跃迁的爱因斯坦自发辐射系数,更新了文献中的数据。结果表明Level是非常适合量子电子学同行使用的一个工具。     

HCl化学激光振转跃迁爱因斯坦自发辐射系数的计算

 验证了用Level程序计算爱因斯坦自发辐射系数的可信性,选用精确的RKR势函数和最新的电偶极矩函数,计算得到了振动态量子数小于6的氯化氢激光基频振转跃迁的爱因斯坦自发辐射系数,更新了文献中的数据。结果表明Level是非常适合量子电子学同行使用的一个工具。     

A concept of equivalent absorption and emission coefficients for gray analysis of radiative heat transfer

Due to the non-gray of gas radiation, the total emissivity differs from the total absorptivity. Therefore, in gray analysis, the equivalent absorption coefficient is not equal to the equivalent emission coefficient. Based on this idea, in this paper, a concept of equivalent absorption coefficient and equivalent emission coefficients is presented for gray analysis of gas radiative heat transfer. The equivalent emission coefficients are calculated by Leckner's formula and the equivalent absorption coefficients are estimated by inverse analysis. A one-dimensional gas radiation is taken as an example to show the efficiency of this concept. The results show that the concept of equivalent absorption and emission coefficients is feasible. It is necessary to use both the equivalent absorption coefficient and the equivalent emission coefficient for gray analysis.