应用方位反向技术提取反射镜零重力面形

为了预估碳化硅反射镜在空间零重力环境下的面形精度,本文开展了在地面环境下利用方位反向技术提取碳化硅反射镜零重力面形的研究。首先,介绍了方位反向技术提取零重力面形的理论依据;其次,利用有限元分析软件,分析了方位反向对反射镜面形的影响;然后,按照试验流程,先后检测了反射镜在0°和180°状态的面形精度,计算两次检测数据的平均值,得到了反射镜零重力面形。结果表明:反射镜地面零重力面形误差RMS值为12.3 nm,能够满足设计指标要求。最后,对数据可信度进行了分析,确认了试验数据真实可信。该结果预示了反射镜在空间零重力环境下的面形精度,对反射镜光学加工与装调有重要的指导意义。     

修约间隔规则与回归系数的有效位数

从微小标准偏差准则导出独立测量结果修约间隔的确定规则.考虑回归系数之间强相关性,系数不确定度的有效位数常常需要保留到两位以上.引入有效修约间隔,导出回归系数修约间隔的确定公式,结合实验物理中的多种不同回归类型例子说明了回归系数的有效位数确定方法.     

Bragg diffraction of finite beams by thick gratings: Two rival theories

Two rival techniques used in the analysis of Bragg diffraction of two-dimensional light beams by thick unslanted phase gratings, namely Fourier plane wave decomposition and 2-D coupled-wave theory, are compared. The advantages and regions of applicability of each are discussed, and the conditions found under which they yield identical results. In particular, the results of plane-wave decomposition are used to provide quantitative conditions for validity of 2-D coupled-wave theory. These conditions are not easily derived using any other technique, and set quantitative limits to how narrow or fast-varying the incident distribution may be before 2-D coupled-wave theory fails. It is also shown that 2-D coupled-wave theory is inadequate when the parameterv ₀, due to Kogelnik (1969), which is proportional to the product of coupling rate and grating thickness, is very large.     

Acoustic and perceptual evaluation of Mandarin tone productions before and after perceptual training

Training American listeners to perceive Mandarin tones has been shown to be effective, with trainees' identification improving by 21%. Improvement also generalized to new stimuli and new talkers, and was retained when tested six months after training [Y. Wang et al., J. Acoust. Soc. Am. 106, 3649-3658 (1999)]. The present study investigates whether the tone contrasts gained perceptually transferred to production. Before their perception pretest and after their post-test, the trainees were recorded producing a list of Mandarin words. Their productions were first judged by native Mandarin listeners in an identification task. Identification of trainees' post-test tone productions improved by 18% relative to their pretest productions, indicating significant tone production improvement after perceptual training. Acoustic analyses of the pre- and post-training productions further reveal the nature of the improvement, showing that post-training tone contours approximate native norms to a greater degree than pretraining tone contours. Furthermore, pitch height and pitch contour are not mastered in parallel, with the former being more resistant to improvement than the latter. These results are discussed in terms of the relationship between non-native tone perception and production as well as learning at the suprasegmental level.     

Interactions of bubbles in acoustic Lichtenberg figure

The evolution of acoustic Lichtenberg figure (ALF) in ultrasound fields is studied using high-speed photography. It is observed that bubbles travel along the branch to the aggregation region of an ALF, promoting the possibility of large bubble or small cluster formation. Large bubbles move away from the aggregation region while surrounding bubbles are attracted into this structure, and a bubble transportation cycle arises in the cavitation field. A simplified model consisting of a spherical cluster and a chain of bubbles is developed to explain this phenomenon. The interaction of the two units is analyzed using a modified expression for the secondary Bjerknes force in this system. The model reveals that clusters can attract bubbles on the chain within a distance of 2 mm, leading to a bubble transportation process from the chain to the bubble cluster. Many factors can affect this process, including the acoustic pressure, frequency, bubble density, and separation distance. The larger the bubble in the cluster, the broader the attraction region. Therefore, the presence of large bubbles might enhance the process in this system. Local disturbances in bubble density could destroy the ALF structure. The predictions of the model are in good agreement with the experimental phenomena.     

大口径空间反射镜高精度面形检测的支撑技术研究

 由于空间失重状态与地面状态不同,大口径空间反射镜在地面检测时需要通过特殊支撑结构对其重力进行卸载,同时严格控制支撑力引入的反射镜变形,以满足反射镜高精度面形检测的要求。通过对一口径750 mm的空间SiC非球面反射镜进行检测,对比分析检测结果和有限元仿真结果,研究支撑结构对反射镜面形所造成的影响。主要针对吊带支撑和中心支撑进行详细的检测验证,分析其不能实现该反射镜高精度检测的原因;通过改进支撑方案,最后提出背板支撑,改善支撑变形的影响,不同角度下测量重复性达到纳米级,收到较好的检测效果,为该反射镜加工质量和加工效率的提升奠定了基础。     

基于单目视觉的静止目标定位方法

以无人机感知与避障为背景,提出了基于运动的单目视觉测距方法.选择摄像机运动过程中在不同位置对同一目标获取的两幅图像,然后利用尺度不变特征变换算法对所选图像进行特征检测和匹配,通过分析同一目标特征点在两幅图像中不同成像位置的变化,结合无人飞行器自身的运动参数,求解出无人飞行器与障碍目标之间的位置信息.采用该方法对不同位置的目标进行定位实验,结果表明该算法的测量精度和时效性可以满足实际避障的要求.     

Category theoretical construction of the figure of states

We put Mielnik's construction of the convex set of all states of a physical system in the general frame of category theory and give topological details lacking in previous papers on the subject.     

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介绍了EAST真空室内巡视系统(IVVS)系统对EAST真空室第一壁进行巡检操作的工作原理,并基于系统部分材料出气率测试对其真空性能进行了分析.获得了在有效抽速为1064L·s–1的条件下,IVVS系统试验样机所采用的真空容器的极限真空度为3.37×10–3Pa.为优化IVVS真空性能,结合材料放气率试验,将部分材料更换为放气率低的材料,对系统进行烘烤处理,并对其中的相关结构提出了优化方案.优化后系统的真空性能有明显提高,在同等抽速条件下的极限真空度在1×10–4Pa范围内,能够满足EAST接受的真空环境要求.此外,抽气机组增添低温泵可以更进一步提高系统的真空性能.     

Effects of affective picture viewing on postural control

Background  

Emotion theory holds that unpleasant events prime withdrawal actions, whereas pleasant events prime approach actions. Recent studies have suggested that passive viewing of emotion eliciting images results in postural adjustments, which become manifest as changes in body center of pressure (COP) trajectories. From those studies it appears that posture is modulated most when viewing pictures with negative valence. The present experiment was conducted to test the hypothesis that pictures with negative valence have a greater impact on postural control than neutral or positive ones. Thirty-four healthy subjects passively viewed a series of emotion eliciting images, while standing either in a bipedal or unipedal stance on a force plate. The images were adopted from the International Affective Picture System (IAPS). We analysed mean and variability of the COP and the length of the associated sway path as a function of emotion.     

Thermoelectric figure of merit for semiconductor submicron layers

We show that in semiconductors of submicron dimensions, for certain relations between the electronic and phonon boundary conditions a heat flux may occur only through the electronic subsystem. Consequently, the thermoelectric figure of merit z does not involve the total thermal conductivity, but only its electronic component. In the ideal case of isothermal contacts, this leads to a sharp increase in z compared with massive semiconductors.Ya. A. Galan Ternopol' Pedagogical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 68–72, September, 1993.     

Light figure spectroscopy of optically active anisotropic materials

Low order interference curves in optic-axis light figures of the optically active AgGaS₂ and TeO₂ are investigated in the visible spectral range at room temperature. Light figure patterns of AgGaS₂ are found not to be affected by optical activity within the incidence angles of 10° for which, according to our symmetry considerations, the normal component of gyration tensor is small and evolution of light figures with wavelength is overwhelmingly determined by optical anisotropy. The absolute value of this anisotropy is falling down to null when passing through the isotropic point, with the result of a remarkable change of the parameters of light figure patterns. Unlike the case of AgGaS₂ and consistently with our symmetry considerations, light figures of TeO₂ are influenced by optical activity especially strongly in low orders of interference. The obtained results are compared with similar results for optically inactive CaCO₃ and discussed in the frameworks of our recent analyses.     

Noise figure characterization of preamplifiers at NMR frequencies

A method for characterizing the noise figure of preamplifiers at NMR frequencies is presented. The noise figure of preamplifiers as used for NMR and MRI detection varies with source impedance and with the operating frequency. Therefore, to characterize a preamplifier's noise behavior, it is necessary to perform noise measurements at the targeted frequency while varying the source impedance with high accuracy. At high radiofrequencies, such impedance variation is typically achieved with transmission-line tuners, which however are not available for the relatively low range of typical NMR frequencies. To solve this issue, this work describes an alternative approach that relies on lumped-element circuits for impedance manipulation. It is shown that, using a fixed-impedance noise source and suitable ENR correction, this approach permits noise figure characterization for NMR and MRI purposes. The method is demonstrated for two preamplifiers, a generic BF998 MOSFET module and an MRI-dedicated, integrated preamplifier, which were both studied at 128MHz, i.e., at the Larmor frequency of protons at 3 Tesla. Variations in noise figure of 0.01dB or less over repeated measurements reflect high precision even for small noise figures in the order of 0.4dB. For validation, large sets of measured noise figure values are shown to be consistent with the general noise-parameter model of linear two-ports. Finally, the measured noise characteristics of the superior preamplifier are illustrated by SNR measurements in MRI data.     

EAST真空室内巡视系统的真空性能研究分析

介绍了EAST真空室内巡视系统(IVVS)系统对EAST真空室第一壁进行巡检操作的工作原理,并基于系统部分材料出气率测试对其真空性能进行了分析。获得了在有效抽速为1064L•s^–1的条件下,IVVS系统试验样机所采用的真空容器的极限真空度为3.37×10^–3Pa。为优化IVVS真空性能,结合材料放气率试验,将部分材料更换为放气率低的材料,对系统进行烘烤处理,并对其中的相关结构提出了优化方案。优化后系统的真空性能有明显提高,在同等抽速条件下的极限真空度在1×10^–4Pa范围内,能够满足EAST接受的真空环境要求。此外,抽气机组增添低温泵可以更进一步提高系统的真空性能。     

厚针孔射线成像品质因数的研究

定义了一种评价针孔成像性能的品质因数,给出了数学表达式.此品质因数综合反映了针孔成像的总空间分辨和灵敏度因素.基于透射模型,给出了针孔成像中心点等效直径、 点扩散函数均方根半径数学表达式.给出了全视场和有效视场下针孔成像品质因数的计算方法,分析了直孔型、刀口型、船底型和长船底型等4种典型针孔的成像特性.     

Time reversal of ocean noise

It has recently been shown [Roux et al., "Extracting coherent wave fronts from acoustic ambient noise in the ocean," J. Acoust. Soc. Am. 116, 1995-2003 (2004)] that the time-averaged correlation of ocean noise between two points yields a deterministic waveguide arrival structure embedded in the time-domain Green's function. By performing a set of these correlations between a vertical receive array and a single receiver, transfer functions necessary for time reversal can be extracted from ocean noise. Theory and simulation demonstrate this process and data of opportunity from the North Pacific Acoustic Laboratory experiment confirm the expected performance of a noise-based time reversal mirror.     

The logic of time reversal

Active time reversal in the sense of object reversal and passive time reversal in the sense of a frame reversal of time are discussed separately and then together so as to bring out their dual nature. An understanding of that duality makes it unavoidable to contrast symmetry properties of matter with symmetry properties to be assigned to antimatter. Only frame reversal of time can see all conceivable active time reversals relevant to physical objects. Only frame reversal of time can be used for a meaningful extension of the Neumann principle to the time domain.     

Time reversal of Bose-Einstein condensates

Using Gross-Pitaevskii equation, we study the time reversibility of Bose-Einstein condensates (BEC) in kicked optical lattices, showing that in the regime of quantum chaos, the dynamics can be inverted from explosion to collapse. The accuracy of time reversal decreases with the increase of atom interactions in BEC, until it is completely lost. Surprisingly, quantum chaos helps to restore time reversibility. These predictions can be tested with existing experimental setups.