X-ray propagation in tapered waveguides: Simulation and optimization

We use the parabolic wave equation to study the propagation of X-rays in tapered waveguides by numerical simulation and optimization. The goal of the study is to elucidate how beam concentration can be best achieved in X-ray optical nanostructures. Such optimized waveguides can e.g. be used to investigate single biomolecules. Here, we compare tapering geometries, which can be parametrized by linear and third-order (Bézier-type) functions and can be fabricated using standard e-beam lithography units. These geometries can be described in two and four-dimensional parameter spaces, respectively. In both geometries, we observe a rugged structure of the optimization problem’s “gain landscape”. Thus, the optimization of X-ray nanostructures in general will be a highly nontrivial optimization problem.     

A tubular segmented-in-series solid oxide fuel cell with metallic interconnect films: A performance study through mathematical simulations

In a segmented-in-series solid oxide fuel cell (SIS-SOFC), an interconnect (IC) provides electrical contact and sealing between the anode of one cell and the cathode of the next. A metallic silver-glass composite (SGC) is considered a promising alternative to ceramic IC materials in SIS-SOFCs. In this work, a simulation study is performed on a tubular SIS-SOFC to assess the effectiveness of the SGC-IC design and to predict the SOFC performance characteristics for various IC geometries and conductivities. The developed model provides detailed information on cell behavior, such as the internal resistance, the potential/current distribution, and the local gas species concentration. The results demonstrate that the SGC material greatly reduces a potential drop across the IC film. Thus, it provides the following substantial advantages over conventional ceramic IC materials: (i) increased power density and (ii) a larger degree of flexibility in the cell design. Moreover, the validation test, i.e., comparison of the simulated results with the experimental data, indicates that the model could serve as a valuable tool for design optimization to achieve the required SOFC performance.     

Fast Optimization of a Biplanar Gradient Coil Set

This work presents an approach for fast optimization of gradient coils, using the simulated annealing method. The shielding condition derived from a target field method and the analytical evaluation of the fields produced by simple geometries were used to reduce the computing time. This method is applied to the optimization of a shielded biplanar gradient coil set. Efficiency, inductance, and homogeneity of the gradient fields produced by the optimized geometries were studied as a function of the number of wires, for the longitudinal and transverse gradient coils. A prototype of the gradient set was made to test the proposed design method. The resulting experimental values of coil efficiency, inductance, field linearity, and shielding performance exhibit good agreement between theory and experiment.     

Transport enhancement and efficiency optimization in two heat reservoir ratchets

We study a Brownian motor moving in a sawtooth potential in the presence of an external driving force and two heat reservoirs. Based on the corresponding Fokker--Planck equation, the analytical expressions of the current and efficiency in the quasi-steady-state limit are obtained. The effects of temperature difference and the amplitude of the external driving force on the current and efficiency are discussed, respectively. The following is our findings. (i) The current increases with both δ and A. In other words, δ and A enhance the transport of the Brownian motor. (ii) The competition between the temperature difference and the amplitude of the external driving force can lead to efficiency optimization. The efficiency is a peaked function of temperature, i.e., δ>0 and a lower amplitude value of the external driving force is necessary for efficiency optimization. (iii) The efficiency increases with δ, and decreases with A. δ and A play opposite roles with respect to the efficiency, which indicates that δ enhances the efficiency of energy transformation while A weakens it.     

C20分子异构体的电子态

使用量子化学从头计算方法优化了C₂₀分子三种异构体（环形、碗形和笼形）的单重和三重电子态几何结构。从优化的几何结构出发，由二阶微扰（MP2）方法得到的单点能给出了一个不符合Hund法则的结果，也就是三种异构体的能量都是单重电子态的最低。使用UHF波函数给出了三种异构体的总电荷密度和静电势，结果显示在笼形和碗形结构的中心区域存在负电荷。最后，本文分析了环形结构的成键特征。     

Thin-disc piezoceramic ultrasonic motor. Part II: system construction and control

Design and performance evaluation of an ultrasonic motor was discussed in [Wen et al., Thin-disc piezoelectric ultrasonic motor. Part I: design and performance evaluation, Ultrasonics]. Higher precision position control of piezoceramic ultrasonic motor depends on mechanical design and servo control of a very precise and adequate metrology. This paper proposes the design of a driving circuit and controller to deal with non-linearities behavior in the model of piezoceramic-driving ultrasonic motor. The performance of the driver and the effectiveness of the proposed controller are demonstrated by command inputs of sinusoidal and step signals. For comparison purpose, the ultrasonic motor is controlled using two methods: i.e., proportional-integral-derivative (PID) and sliding-mode control (SMC). It was proven that SMC would compensate automatically for unmodeled behaviors such as piezoceramic non-linearities and mechanical stick-slip phenomena. Furthermore, SMC scheme has been successfully applied to position tracking to demonstrate the excellent robust performance in noise rejection.     

Modeling and Design Optimization of Photonic Devices and ICs by Space-Mapping Techniques (Invited paper)

Space-mapping (SM) technique is applied for the modeling and design optimization of photonic devices. A mapping is established between the parameter spaces of the fast but approximate coarse model and the time-intensive yet accurate fine model. Two different applications of the SM techniques, i.e., design optimization and design modeling of photonic devices, are illustrated by way of examples.     

Macroscopic cell experiments and three-dimensionalmodelling on electrode-shaping plasma display cell

A macroscopic cell and three-dimensional fluid model have been used to investigate the discharge characteristics in ac plasma display panel cells of electrode-shaping configurations. Four kinds of non-standard geometries (i.e. $D$-, ${\it\del}$-, $W$- and $U$-shape electrodes) have been considered. The characteristics of the discharge current, the operating voltage and the discharge efficiency of different configurations have been discussed. It is found that the discharge efficiency can be improved by about 10\%--30\% compared with the standard geometry, while the operating voltage increases slightly in the non-standard geometries. There is a trade-off between improving the discharge efficiency and lowering the sustaining voltage in design of plasma display cells by electrode shaping.     

Toward low-loss photonic crystal waveguides in InP/InGaAsP heterostructures

Line-defect photonic crystal waveguides exhibit severe propagation losses if they are implemented in semiconductor heterostructures with a weak refractive index contrast. We present, for what we believe is the first time, experimental structures for which we have evidence that fabrication imperfections are not the limiting factor in terms of propagation losses. We demonstrate a loss figure of 335±5 dB/cm, which is an improvement by a factor of about 2 with respect to state-of-the-art values. Simulations show that even lower losses can be obtained with different waveguide geometries. In other words, the dominant loss mechanism is related to the waveguide design, and losses are not expected to decrease upon further optimization of the fabrication process.     

某光电吊舱内方位电机组件低温摩擦特性分析研究

为提升产品可靠性和环境适应性，需对某光电吊舱内方位电机组件在-50℃的摩擦特性进行改善。分析了可能影响电机组件摩擦特性的因素，并针对各因素提出相应的改善措施，借助仿真与试验相结合的研究方法，对改善措施的效果进行了验证。经研究:-50℃时，电机组件摩擦力矩的增大既有低温单独作用的结果，也有低温和电机组件潜在因素共同作用的结果。电机组件在低温下的微变形不足以引起摩擦力矩的增大，通过增加电机保护罩和在电机与电机座之间增加厚度为1 mm的软磁合金薄片，能使电机的摩擦力矩降低到0.063 N·m。该改善措施适用于光电吊舱电机组件的设计及类似产品的开发研制。     

Non-neural BOLD variability in block and event-related paradigms

Block and event-related stimulus designs are typically used in fMRI studies depending on the importance of detection power or estimation efficiency. The extent of vascular contribution to variability in block and event-related fMRI-BOLD response is not known. With scaling, the extent of vascular variability in the fMRI-BOLD response during block and event-related design tasks was investigated. Blood oxygen level-dependent (BOLD) contrast data from healthy volunteers performing a block design motor task and an event-related memory task requiring performance of a motor response were analyzed from the regions of interest (ROIs) surrounding the primary and supplementary motor cortices. Average BOLD signal change was significantly larger during the block design compared to the event-related design. In each subject, BOLD signal change across voxels in the ROIs had higher variation during the block design task compared to the event-related design task. Scaling using the resting state fluctuation of amplitude (RSFA) and breath-hold (BH), which minimizes BOLD variation due to vascular origins, reduced the within-subject BOLD variability in every subject during both tasks but significantly reduced BOLD variability across subjects only during the block design task. The strong non-neural source of intra- and intersubject variability of BOLD response during the block design compared to event-related task indicates that study designs optimizing for statistical power through enhancement of the BOLD contrast (for, e.g., block design) can be affected by enhancement of non-neural sources of BOLD variability.     

Elastic property of multiphase composites with random microstructures

We propose a computational method with no ad hoc empirical parameters to determine the elastic properties of multiphase composites of complex geometries by numerically solving the stress–strain relationships in heterogeneous materials. First the random microstructure of the multiphase composites is reproduced in our model by the random generation-growth method. Then a high-efficiency lattice Boltzmann method is employed to solve the governing equation on the multiphase microstructures. After validated against a few standard solutions for simple geometries, the present method is used to predict the effective elastic properties of real multiphase composites. The comparisons between the predictions and the existing experimental data have shown that the effects of pores/voids in composites are not negligible despite their seemingly tiny amounts. Ignorance of such effects will lead to over-predictions of the effective elastic properties compared with the experimental measurements. When the pores are taken into account and treated as a separate phase, the predicted Young’s modulus, shear modulus and Poisson’s ratio agree well with the available experimental data. The present method provides an alternative tool for analysis, design and optimization of multiphase composite materials.     

固体火箭发动机振动试验夹具优化分析与研究

通过相同激励条件下对三种典型振动试验夹具进行宽频段随机振动试验分析,发现夹具在试验频段内出现不同程度的共振现象,过试验现象严重。通过三种典型振动试验夹具质量、一阶固有频率、位移变化量及应变变化量等数据对比、分析,得出影响振动试验夹具振动传递特性的主要因素,以影响因素为优化变量,以典型部位平均响应与输入的振动传递比为优化目标,进行拓扑优化设计,最终经过夹具振动试验验证,过试验情况得到有效控制,满足相关规范要求,为固体火箭发动机振动试验控制提供保障。     

Global optimization of high harmonic generation

We investigate the relevance of the absorption limit concept in the optimization of high harmonic generation. Thanks to the first direct observation of the coherence length of the process from high-contrast Maker fringes, we unravel experimental conditions for which the harmonic dipole response is enhanced when phase matching is realized within the absorption limit, leading to record conversion efficiencies in argon. Moreover, we show that harmonic generation in guided or freely propagating geometries are equivalent in the loose focusing regime. This analysis is generalized to other advanced phase-matching schemes, thereby predicting the possibility to boost the conversion efficiencies using light noble gases.     

Pressure gradient tailoring effects on vorticity dynamics in the near-wake of bluff-body premixed flames

We investigate the role of mean streamwise pressure gradients in the development of a bluff-body-stabilized premixed flame in the near-wake of the bluff body. To this end, a triangular prism flame holder is situated in three different channel geometries: a nominal case with straight walls, a nozzle with a stronger mean pressure gradient, and a diffuser with a comparatively weaker mean pressure gradient. All geometries are implemented using embedded boundaries, and adaptive mesh refinement is used to locally resolve all relevant thermal (i.e., flame) and fluid-mechanical (i.e., vorticity) scales. A premixed propane flame, modeled using a 66-step skeletal mechanism, interacts with vorticity in the boundary layer of the triangular bluff body in the presence of each mean pressure gradient. Analysis of flame-related enstrophy budget terms reveals key differences in the behavior of baroclinic torque between cases, the specifics of which are tied to larger variations in the mean flow structure, recirculation zone structure, and confinement effects. Our results show that the baroclinic torque changes significantly among the configurations, with the nozzle exhibiting the largest baroclinic torque production. However, these differences are shown to be only a secondary consequence of the background pressure gradient, with the primary consequence being the change in the recirculation zone length resulting from the different channel configurations. These results are relevant for flame stabilization with bluff bodies, where clear understanding of the sensitivities to global mean pressure gradient is important to engineering design.     

Optimization of One-Way Quantum Computation Measurement Patterns

In one-way quantum computation (1WQC), an initial highly entangled state, called a graph state, is used to perform universal quantum computations by a sequence of adaptive single-qubit measurements and post-measurement Pauli-X and Pauli-Z corrections. 1WQC computation can be represented by a measurement pattern (or simply a pattern). The entanglement operations in a pattern can be shown by a graph which together with the identified set of its input and output qubits is called the geometry of the pattern. Since a pattern is based on quantum measurements, which are fundamentally nondeterministic evolutions, there must be conditions over geometries to guarantee determinism. These conditions are formalized by the notions of flow and generalized flow (gflow). Previously, three optimization methods have been proposed to optimize 1WQC patterns which can be performed using the measurement calculus formalism by rewriting rules. However, the serial implementation of these rules is time consuming due to executing many ineffective commutation rules. To overcome this problem, in this paper, a new scheme is proposed to perform the optimization techniques simultaneously on patterns with flow and only gflow based on their geometries. Furthermore, the proposed scheme obtains the maximally delayed gflow order for geometries with flow. It is shown that the time complexity of the proposed approach is improved over the previous ones.     

The effect of reflector geometry on the acoustic field and bubble dynamics produced by an electrohydraulic shock wave lithotripter

A theoretical model for the propagation of shock wave from an axisymmetric reflector was developed by modifying the initial conditions for the conventional solution of a nonlinear parabolic wave equation (i.e., the Khokhlov-Zabolotskaya-Kuznestsov equation). The ellipsoidal reflector of an HM-3 lithotripter is modeled equivalently as a self-focusing spherically distributed pressure source. The pressure wave form generated by the spark discharge of the HM-3 electrode was measured by a fiber optic probe hydrophone and used as source conditions in the numerical calculation. The simulated pressure wave forms, accounting for the effects of diffraction, nonlinearity, and thermoviscous absorption in wave propagation and focusing, were compared with the measured results and a reasonably good agreement was found. Furthermore, the primary characteristics in the pressure wave forms produced by different reflector geometries, such as that produced by a reflector insert, can also be predicted by this model. It is interesting to note that when the interpulse delay time calculated by linear geometric model is less than about 1.5 micros, two pulses from the reflector insert and the uncovered bottom of the original HM-3 reflector will merge together. Coupling the simulated pressure wave form with the Gilmore model was carried out to evaluate the effect of reflector geometry on resultant bubble dynamics in a lithotripter field. Altogether, the equivalent reflector model was found to provide a useful tool for the prediction of pressure wave form generated in a lithotripter field. This model may be used to guide the design optimization of reflector geometries for improving the performance and safety of clinical lithotripters.     

Rapid four-ring birdcage coil analysis: Design optimization for high efficiency,low interference,and improved body loading tolerance

The Design accuracy of MRI system with dual-tuned four-ring birdcage coils remains challenging due to inter-coil interference that tends to deteriorate signal-to-noise ratio and thus image quality. In this study, we develop an accurate and efficient FEM modeling strategy by optimizing model geometries, meshing density, and boundary conditions in accord with the correlated measurements on test vehicles. By taking full advantages of simulation accuracy and efficiency, we are able to proceed with sensitivity analysis on critical RF characteristics such as resonance pattern, quality factor, coil coupling, and body loading effects, etc., which may further serve as guidelines for coil design optimization.