Abstract:Under the dynamic equilibrium among diffraction, dispersion, Kerr effect and multiple photons ionization, a self-guiding filament formed by femtosecond laser with long propagation distance can be constructed, which is accompanied by a narrow plasma channel. In order to control the propagation features of filamentation explicitly in air, a way to effectively regulate filamentation propagation modes and spatiotemporal distributions has become a research hotspot. On the basis of introducing the physical model of filamentation briefly, a general overview of controlling filamentation divided by two parts was presented in this review: temporal modulations and spatial modulations. In particular, the latter consists of modulations designed by phase, amplitude and special beams. Our findings suggest that unique filaments controlled by initial modulations can offer advantages and fundamentals for the realization of numerous potential applications.

Abstract:In order to design high-power narrow linewidth 4.3 μm laser, the narrow linewidth 4.3 μm laser was obtained by ZnGeP₂ OPO (optical parametric oscillation) pumped by a 2.7 μm laser. Experimental results were analyzed in detail. The pump source was a 2.7 μm KTiOPO₄ OPO laser pumped by a 1064 nm Nd:YAG laser, and the KTiOPO₄ OPO was singly resonated. Two identical KTiOPO₄ crystals were placed with opposite optical axis to compensate the walk off effect. The KTiOPO₄ crystals were cut asΦ=0°,θ=62° to generate 2.7 μm wave band laser. To utilize the large effective nonlinear coefficient, the phase matching was based on type Ⅱ(B) (o→o+e). The ZnGeP₂ OPO was singly resonated too. To output narrow linewidth laser, the phase matching is based on typeⅡ(B) (o→o+e).The ZnGeP₂ crystal was cut as Φ=0°,θ=68° to generate 4.3 μm laser. The energy of the 2.7 μm laser is 7.5 mJ with a pulse of 8.6 ns and line-width 12 nm.The pulse energy of 2.12 mJ at 4.26 μm with an optical-optical conversion efficiency of 28.3% and a slop efficiency of 32.6% is obtained, and the pulse width is 8.7 ns, the line-width is 30 nm. The beam qualities M² of horizontal and vertical directions are 6.2 and 13.5, respectively.

Abstract:In order to further reveal the transient evolution characteristics of silicon ablated by femtosecond laser, the theoretical model was established, and the numerical simulation was carried out. The results show that the femtosecond laser can excite a large amount of electrons with its density exceeding the damage threshold in the pulse duration. At that time, the lattice remains in a “cold” state until it reaches the melting point temperature as long as 1 ns order of magnitude. The temperature of electrons dramatically increases to 10⁴ K level in the laser pulse irradiation moment. And then the energy is released slowly to lattice and the thermal equilibrium is reached until 10 ns order of magnitude. The electron has two rapid warming processes. The first one starts from the free-carrier absorption, and stops at the electron-lattice energy coupling; the second one starts from the single photon and two photon absorption, and ends at the end of the pulse. It is found that the higher the temperature and density of electron are, the bigger the pulse energy is and the shorter the pulse duration is.

Abstract:In order to solve the contradiction between high repetition rate of fundamental wave and high average power of second-harmonic wave, a new 1064 nm/532 nm dual-wavelength composite jamming laser with high-repetitive frequency was designed. The composite output characteristics of the laser were obtained by the static analysis of the pump source′s efficiency, heat and force distribution of crystal, and by the Gauss mode of laser. The influence on the dualwavelength composite output was analyzed by changing the Q-switching frequency, the length of the second harmonic generation crystal and by simulating the thermal effect of second harmonic generation crystal. It is found that the high frequency of fundamental wave should be satisfied first, and the output power of secondharmonic wave can be improved by optimizing the length of the second harmonic generation crystal and by controlling the temperature of second harmonic generation crystal under the condition of the constant pumping power and the structure of the resonator. The simulation conclusion was verified by an experiment. The experimental results show that when the length of KTiOPO₄ crystal is chosen as 12 mm, the transmissivity of the output mirror is fixed to 10%, and the Q-switching frequency is selected as 50 kHz, the average output power of the fundamental wave is 18.98 W, and the average power of the output of secondharmonic wave is 2.22 W. 

Abstract:The study of the saturation mechanism of multialkali photocathode is of great significance for improving its electron emission performance. According to the threestep electron emission model, the electron emission process of a multi-alkali photocathode was studied by Monte-Carlo method. Simulation results were compared with the experimental data in the related literatures, which shows that when the photon energy is near the photoemission threshold of the photocathode, the obtained quantum efficiency curves and electron energy distribution curves were in good agreement with experimental data. Based on the above model, the photo-voltage effect and the space charge effect on the electron emission characteristics of multialkali photocathode were studied. Results show that the threshold laser energy density of the photocathode is about 8 μJ/cm² under the photovoltage effect, and 2.23 μJ/cm² under the space charge effect. An experimental investigation of the saturation effect of multi-alkali photocathode irradiated with 266 nm laser is conducted, and the saturation threshold laser energy density is about 2 μJ/cm², which indicates that the space charge effect is the main factor limiting its photoelectron emission capability. 

Abstract:The transverse acceleration property of Airy beams makes it possible for beams to propagate along the curved trajectories in free space. The transverse acceleration propagation property of the new-type Airy beams was studied by utilizing the one-dimensional paraxial wave equation. The simulation results show that the transverse acceleration of Airy beam is relevant to the propagation distance and arbitrary transverse scale when the wavelength keeps constant. The transverse acceleration will become larger when the propagation distance becomes longer and the transverse scale becomes smaller. The Poynting vector was utilized to analyze the mechanism of accelerating property of Airy beams. Moreover, the twodimensional finite Airy beam was generated by utilizing the universal liquid crystal spatial light modulator, and the experimental study on the transverse acceleration property of Airy beam was also carried out. The experimental results coincide well with the simulation results.

Abstract:Strong electromagnetic pulse may cause serious damage once it is coupled into the interior of the electronic system. As a kind of special electromagnetic medium, the plasma has the ability of shielding strong electromagnetic pulse. Therefore, electromagnetic pulse protection technology based on the plasma is of practical significance. The electric field distribution in work space of the nuclear electromagnetic pulse simulator was simulated and analyzed by the CST software. The interfering and damaging effect of the singlechip microcomputer under irradiation of the nuclear electromagnetic pulse was studied, the results showing that the interfering and damaging thresholds of the nuclear electromagnetic pulse to MF-51-1 MCU are 10 kV/m and 18 kV/m respectively. The protection performance of the onelayer plasma array to the nuclear electromagnetic pulse was experimentally studied, and the energy attenuation value can reach up to 10 dB. The research results show that the plasma has the protection ability against strong electromagnetic pulse.

Abstract:The interaction between the plasma and the high power microwave was studied with the wave equation, the electron transport equation and the heavy species transport equation. The change process of the electron density and the electric field in the plasma was calculated, and the dispersion effect of the incident frequency in the interactions was especially analyzed. Result shows that the electron density and the electric field intensity will change suddenly due to the interactions of HPM(high power microwave) and plasma, in which the mean electron density will increase from 1×10⁹ m^-3 to 1×10¹⁹m^-3, and the average electric field strength will decrease from the original value to zero abruptly in a very short time. Moreover, the threshold field strength and minimum generation time in the changing process are considered to study the dispersion effects. As a result, the threshold strength and generation time are vary with the frequency. In addition, it can be admitted that the threshold strength increases linearly and the generation time grows nonlinearly with the frequency of incident wave with further research.

Abstract:The RVE (representative volume element) models were generated by using the equalcircle optimal circular packing scheme to characterize the microstructure of solid propellants. In order to obtain the influence laws of volume fraction and component material on effective modulus and Poisson′s ratio, a numerical analysis method for evaluating effective mechanical properties of solid propellants was developed by combining the VCFEM (Voronoi cell finite element method) and the homogenization method. The displacement result of a symmetric numerical example was compared with the traditional finite element method for validating the effectiveness of this method. The relative error between the two methods is less than 5%. The analysis efficiency is improved by using VCFEM, because it needs fewer elements. Moreover, the RVE models of different microstructures were calculated. It is found that with the increase of particle volume fraction, the enhancement effect of particles is more obvious. And the matrix material has a more significant influence than the inclusion material on the effective mechanical properties. 

Abstract:Due to its superiorities of high exhaust speed, wide frequency band and rapid response, the plasma synthetic jet actuator has shown wide and promising application prospects in high-speed flow control. On account of the weak control authority and small affected area of single actuator, a parallel actuator array was investigated. A parallel microsecond-grade pulse power supply was developed to achieve a three-path discharge. The power supply shows good stability with and without the load at 1000 Hz working frequency. As the capacitor increases, the temperature of the plasma arc increases. The air in the actuator chamber is heated more prominently. So the speed of the jet rises. As the frequency increases, the jet speed decreases due to a lower breakdown voltage. The parallel actuators can be controlled independently by manipulating the trigger signals, which makes the parallel actuators more flexible than the serial actuators. The phase of the actuator was shown to be controlled accurately by the trigger signal.

Abstract:Under the extreme LLL （low light level） conditions (environment illumination less than 2×10^-3lux), the LLL image has the characteristics of low signaltonoise ratio and low contrast, so that the target is difficult to be identified, thus seriously affecting the observation effect. In order to improve the LLL image quality, a convolutional autoencoder deep neural network for image enhancement was designed. In view of the fact that the traditional mean square error loss function cannot meet the human visual perception characteristics, several loss functions including perceptual loss were studied and a novel, differentiable loss function was proposed in combination with the existing full reference image quality evaluation index. Experimental results show that the proposed loss function can improve the detail information of the image while improving the signaltonoise ratio and contrast ratio of the lowlight level image when the network structure does not change. 

Abstract:For the entry vehicle equipped with RCS (reaction control system) and rudder, an optimal control allocation method based on PWPF (pulsewidth pulse-frequency) modulator was proposed. By transforming the discrete control signal of RCS into the continuous quantity, the control allocation problem was constructed as a quadratic programming problem. And the active-set method was used to solve the quadratic programming problem. Then, the on-off signal of RCS was obtained by using the discrete method and PWPF modulator. Compared with the mixed integer programming problem, the quadratic programming problem is easier to be solved, which is favorable for on-board computing. The proposed method has the fault-tolerant ability by reconstituting the quadratic programming problem.

Abstract:In order to solve the fast calculation of the free trajectory considering the effects of the earth non-spherical gravitation, the differential equations of motion under the earth gravitation including the J₂-perturbation were established in the non-orthogonal coordinate system, and the differential equations of motion under disturbing gravity were established in the orbital coordinate system. The acceleration of upward disturbing gravity was equivalent to the mass deviation of the earth, and introduced into the standard elliptical orbit. The differential equations under J₂-perturbation were modified, and the parameters of the position and velocity were calculated under the analytical solution. The simulation results show that the analytic solution has a higher efficiency, and the position error of impact point is within 20 meters. The analytic solution satisfies the requirement of the real-time guidance and orbit forecast.

Abstract:The point target location based on a moving visual platform was studied. After introducing the visual location method, the sight-line restriction of the collinear equation was analyzed. The error impact of view-line angle vector to the point target location was deduced in the view-line associated coordinate system. Multitime measurements were also analyzed in the location error analysis. Based on the minimization of the target location error, the optimal maneuver trajectory of the observer platform was designed. Using a method that is simpler and easier to understand, the results obtained from the proposed method are the same as those from the method by maximizing the determinant of the Fisher information matrix. Simulations and tests were performed, and results show that the target location error analysis and the platform optimal maneuver trajectory design are valid.

Abstract:Optical non-contact measurement of large-scale scenes has wide application in engineering. The multi-view reconstruction based on a single camera needs to arrange scale datum into the measurement area, and the image acquisition process is complex and requires a high overlap ratio between adjacent images. Therefore, a method of multi-view reconstruction using binocular cameras was presented. This method not only inherits the advantages of large measurement range in multi-view reconstruction, but also does not need to arrange scale datum, and the image acquisition process is simpler. Besides，the double camera constraint was added in the process of data optimization. Specific methods are as follows: firstly the internal parameters and solid connection of two cameras installed on the fixed rod were obtained according to camera calibration; then the binocular cameras were used to acquire the images of the area to be measured and obtain the spatial point cloud by binocular intersection. The transformation relation of the coordinate system of point cloud in adjacent time was calculated by the method of pose estimation, and the camera′s parameters and point cloud′s coordinate of each moment were transformed to the specified coordinate system. The position and pose of the camera and the coordinate of point cloud were optimized by using the bundle adjustment, and the size parameters and the deformation information were calculated from the point cloud coordinates. The test results show that in the range of 5 m, the measurement′s error is 3 mm, the average error is 1 mm. This method is suitable for the measurement of large scale object and scene and is stable and reliable.

Abstract:In practical engineering, it is often necessary to use models to describe and analyze problems. However, uncertainty exists in the models. There are many models that can describe the same phenomenon, such as the multiple fatigue analysis models. In order to solve the problem of fatigue reliability of aircraft structures, a combination forecasting method based on Bayesian model averaging was proposed by considering three kinds of crack propagation models. Three random crack propagation models were established by using the crack propagation data of aircraft structures under three stress levels. Considering the uncertainty of the model parameters, the Bayesian model averaging method was used to combine the three models. The reliability of the structure was analyzed based on the combined model. The combined model was used to analyze the fatigue reliability of aircraft structure, which can improve the robustness of the results. In addition, considering the uncertainty of the model parameters, more reasonable predictive distribution of crack propagation as well as the reliability can be obtained. Examples and analysis results are given to show the feasibility of this method. 

Abstract:The fiber laser beam has wave-front aberration, which restricts the performance and efficiency of optic systems. The aberrations′ character of the super-continuum fiber laser beam was analyzed with a building experiment system, and defocus was the main ingredient. According to the character, a prototype of 3-element unimorph DM (unimorph deformable mirror) was manufactured and tested. The simulation accuracy of the influence functions is more than 77% and the frequency response is 12.1 kHz. The results of closed loop correction simulation show that the 3-element unimorph deformable mirror′s correction accuracy for the actual aberration is 0.77. Meanwhile, the finite element analysis is used to optimize the fitting accuracy and displacement. At last a 9-element unimorph DM with the aperture 7.5 mm can reach the accuracy level of about 0.9. The thickness ratio of the mirror and the piezoelectric disk is optimized about 0.3, which further promotes more than 60% of the initial displacement.

Abstract:To predict the dynamic characteristics of seeker servo mechanism in the process of precision assembly, the quantitative relationship between the micro assembly parameters and the resonance frequency of servo mechanism was built by using the theoretical modeling method. Based on the Hertz contact theory, a five-degree-of-freedom bearing stiffness model was established and the influence of the micro assembly errors on the bearing stiffness matrix was modeled and analyzed. Using the theory of Timoshenko beam, the cell matrix of elastic multi-diameter shaft was derived. The method of solving the eigenvalue and eigenvector of the whole system was determined. An analysis system for resonant frequency of servo mechanism was established through using MATLAB/GUI and the experimental system was established to verify the theoretical model. The analysis and experimental results show that the established analytical method and system can accurately calculate the resonant frequency variation caused by the microassembly errors of bearing. This method solves the problem of dynamic characteristic prediction in the process of precision assembly of servo mechanism.

Abstract:In order to discuss the dynamic behaviors of the bearingrotor system of high-speed motorized spindles, a rotor dynamic model with centrifugal force, gyroscopic couple and system damp was presented by using finite element method and Timoshenko beam theory. An overall model test on a 120MD60Y6-type motorized spindle was carried out to acquire the natural frequencies with different work conditions. The results indicate that centrifugal force will lead to the bearing softening and spindle shaft softening, and the latter is the main reason of the decrease of the natural frequency of the spindle system. Each natural frequency of the spindle system was divided into the forward and backward modes, and the forward and backward modes had different change regularities with speed. The good agreement between the theoretical results and the experimental data shows that the proposed model is capable of accurately predicting the dynamic behaviors of highspeed motorized spindles.

Abstract:For the problem of the existing spare parts supply optimization method cannot consider the life cycle, the life cycle was merged into network design in stakes and it was optimized based on the dynamic characteristics of the spare parts demand. Firstly, the requirement characteristics of spare parts at different stages of life cycle were deeply analyzed. Secondly, the spare parts supply respond time transfer equation was built, and the programming model with the optimization goal of maximum support effectiveness throughout life cycle was established under constraint conditions. Simulation cases indicate that a combination of a variety of spare parts protection model can improve the effectiveness of the life cycle, and the overall optimization of the life cycle can be implemented with more efficient spare parts supply.

Abstract:On the basis of introducing the working principle of ultrahigh pressure common-rail system, the simulation model of ultrahigh pressure common-rail diesel engine of single cylinder was built by using GT-Power software, and the accuracy of this model was verified via experiment, then the effects of different fuel injection rate, saddle-shaped fuel injection rate coupled with different pilot injection fuel mass and different pilot injection advance angle on the power, economy, combustion and emission characteristics of ultrahigh pressure commonrail-diesel engine were analyzed by this model. The results show that the variable fuel injection rate can be realized by adjusting the opening time of the electric-controlled pressure amplifier solenoid valve and the injector solenoid valve in ultra-high pressure commonrail system. With the fuel injection rate varying from rectangular to saddle-shaped, the cylinder pressure, temperature and NO_x emissions of diesel engine decrease gradually, while the soot emissions rise gradually. The saddle-shaped fuel injection rate can make the diesel engine acquire the maximum power (torque) and the lowest fuel consumption rate. The saddle-shaped fuel injection rate coupled with lesser pilot injection fuel mass can make the diesel engine power, economy and combustion characteristics better, but it also leads to the rise of NO_x emissions. The saddle-shaped fuel injection rate coupled with larger pilot injection advance angle can improve the emission characteristics of diesel engine, but the effect is not obvious due to the limit of pilot injection fuel mass.

Abstract:Carbide with its products is extensively applied in the field of mechanical machining of metal materials. In this study, graphene was employed as a beneficial additive to semisynthetic cutting fluid 3034, and the related frictional characteristic behavior was investigated via experimental on carbide-steel friction pair. In the experiment, a ball specimen was made of the carbide material of YG8, and a disk was made of the steel material of 45#. On the basis of ball-on-disk friction pair, experiments were conducted on a tribometer machine. The mass fraction of graphene in the tested coolant ranges from 0.1% to 0.9%. Surface topologies of the specimen were investigated using a laser confocal microscope. The scanning electron microscope and Raman spectra analysis was used to analyze wear mechanism. Experimental results revealed the improvement of lubrication condition between carbide and steel is substantially improved, demonstrated by the decreasing of averaged friction coefficient by 12.9% to 57.3% as well as the decreasing of wear rate by 33.82%. The resultant cutting force was decreased by 18.58% compared with that of semisynthetic cutting fluid 3034, and the surface roughness was reduced by 7.5% of the machined workpiece.

Abstract:In order to explore the relations between design factors of the launcher and performance of the produced hypervelocity fragment, the AUTODYNTM-employed numerical simulation towards launch process was performed repeatedly under the orthogonal experiment design framework. Design factors include explosive type, diameter ratio of charge, material type, thickness, taper angle and radius of the top close to charge chamber for liner part were considered. Multiple target launcher designs were demonstrated: three of which were capable to provide the red copper fragment with the mass of 1.563 g, the aluminum fragment with the velocity of 11.649 km/s and the aluminum one with the kinetic energy of 856 kJ respectively; two of which can produce compact cylindrical fragments with the mass over 1 g and velocity over 11km/s simultaneously. The associated experimental validation was confirmed as well and a further analysis of the design factors was made which explains how they were related to the mass, velocity and kinetic energy of generated fragment.

Abstract:Aimed at the problem of high processing delay and low quality of services of cloud computing in individual soldier combat system, a distributed individual soldier combat information system supported by wearable technology was proposed. A local computing layer was constructed by using the intelligent equipment of the soldier, which can provide a platform for battlefield data processing at the nearest combat place and supply soldier with the ability of information fusion and processing. The generalized diffusion load balancing algorithm was used to balance the load of equipment for reducing the delay of business processing and the reliability of the system was enhanced by the distributed computing in realistic battlefield environment. The simulation results show that the distributed wearable local network architecture can effectively reduce the processing delay and enhance the reliability of the combat system.

Abstract:Aimed at the problem that the detection performance losses of multi-input multiple-output radar due to the inaccurate prior knowledge of the distribution characteristics of target impulse response and clutter impulse response, a joint optimization algorithm of constant modulus robust waveform and receiver filter was proposed. The optimization problem in which the prior knowledge of the distribution characteristics of target impulse response and clutter impulse response is not accurate was modeled as a maximum minimization problem. During the optimizations, the joint design problem was divided into two steps: the optimization problem of receiver filter weights when the waveform was fixed was modeled as the generalized Rayleigh quotient model, and the corresponding weight vector of the receiver filter was obtained; the waveform optimization problem of fixed weight was solved by using positive semi-definite relaxation technique to obtain the corresponding waveform matrix, and the constant modulus waveform was obtained by Gauss random method according to the waveform matrix obtained. The convergence of the proposed algorithm was proved, and the simulation results show the effectiveness of the proposed algorithm. 

Abstract:As the key technology of reliability evaluation for hull structure in-service, the merits of the reliability assessment theories determines the effectiveness and confidence level of the reliability evaluation result directly. The development course and current research situation of traditional reliability assessment theories and non-probabilistic reliability assessment theory were elaborated in the view of calculation principle, computing of measurement index and engineering application. Simultaneously, the advantages and disadvantages of all current reliability assessment theories that applied in hull structure in-service were analyzed and summarized. The future research outlook was developed and the focus in the study of hull structure reliability was outlined.

Abstract:The ionospheric EDPs (electron density profiles) obtained during 2014 to 2016 from two GNSS(global navigation satellite system) RO (radio occultation) missions, the FY-3C mission and the COSMIC mission, were used to compare the ionospheric peak density NmF2 and the ionospheric peak height hmF2 of two missions under the collocation criterion with the time window of 7.5 min and the space window of 2.5°. The results show that the peak parameters derived from the two RO missions are highly correlated, with the yearly average correlation coefficient of NmF2 and hmF2 being 0.87 and 0.75, respectively. The biases of the peak parameters derived from the two RO missions are very small, with the absolute and the relative bias of NmF2 and hmF2 being less than ±0.25×10⁵ el/cm³and ±15.0%, and ±7.00 km and ±1.80%, respectively. In addition, the stripshaped spatial distributions of the two ionospheric characteristic parameters from both of the two RO missions show that the values of NmF2 and hmF2 are larger in the areas of the equator and low latitudes than in other regions, and that the values of NmF2 and hmF2 decline significantly in 2016 compared with 2014 and 2015. 