Coherent rainbows come from liquids
When the highly-intensive and highly-directional white light from pulsed fiber laser is focused into water, many directional and colorful rings, “coherent rainbows”, are observed. The laser generates bubbles with similar sizes in the water, which serve as scatters. The intensive light leads to spatial self-phase modulation and thus generates the coherent rainbows. Such a phenomenon has been observed in many kinds of liquids.
Effects of adding Ti on microstructure and properties of Al-Bi immiscible alloy
Immiscible alloy, as a kind of special metallurgy characteristic alloy, has been investigated for decades. The fabrication of immiscible alloy with a homogeneous microstructure remains a challenge due to the liquid-liquid phase separation. The microstructure and the properties of Al-Bi immiscible alloy with an addition of Ti are investigated, and the effect of adding Ti on mechanical behavior for self-lubricating performance is measured. The pure Al and Ti are first melted in graphite crucible under argon gas protection. An appropriate amount of Bi is added into the melt. After melting and homogenizing the immiscible alloy, the melt is maintained at 1150 ℃ for 10 min, and then it is quenched. The scanning electron microscope analysis results show that the addition of Ti leads to a significant reduction of Bi-rich droplet size and an increase of particle number. The Bi-rich droplets of the ternary Al-Bi-Ti alloy are more homogeneously distributed throughout the Al matrix than the microstructure of binary Al-Bi alloy. The results from X-ray diffraction and energy disspersive spectrometer indicate that Al3Ti compounds, which are the transformation products between Al and Ti elements, disperse in the Al matrix. The needle-like Al3Ti compounds suspend in Al-Bi melt and impede the Bi phase in the liquid miscibility gap from being segregated. This is conducible to refining the microstructure of Al-Bi alloy. The Al3Ti compounds form before the initial nucleation of the Bi phase in the Al matrix, and impede the Bi phase from being segregated. Al-Bi immiscible alloy is effectively fabricated with dispersed fine second phase droplets by the addition of Ti. For the Al-Bi alloy, the coarse and non-uniform distribution of Bi-rich droplets can be easily broken. The improvement in the wear resistance of Al-Bi immiscible alloy by adding Ti can be attributed not only to the dispersion and size of the Bi soft phase but also to the in-situ formation of Al3Ti compounds. The addition of Ti is effective for refining the microstructure and improving the wear properties, which simultaneously improves the practical applications of self-lubrication bearing material with low coefficient of friction i.e., reducing the energy loss.
Influence of asymmetrical angle on crystal lattice strain analysis using Voigt-function method
The Voigt function provides a rapid and easy method of explaining the breadths of diffraction profiles, and it defines two main broadening types: the domain size and strain component. The latter is caused by lattice imperfection (dislocation and different defects). Thus, diffraction can be used to measure crystal strain with very high precision and accuracy. However, each of all the crystals used in the present study has asymmetrical angle α due to the processes of cutting grinding and polishing. This deviation angle α is the angle between the considered lattice plane and crystal surface. The crystal with asymmetrical angle also satisfies Bragg's law but with different incident angle and reflected one. In the following, we investigate the crystal strain as a function of asymmetrical angle to evaluate the lattice distortion in detail. The single crystal silicon samples with different asymmetrical angles (in a range from 0.008° to 5.306°) are prepared in this experiment. The lattice plane is (111). After grinding and polishing, the surface and subsurface damage are almost wiped off to remove internal stress which comes from cracks and grain refinement. Only broadening from lattice strain depends on the nature of imperfection, and the shape of crystallite can be left. It is convenient to acquire the full width at half maximum (FWHM) and integral breadth of diffraction curve by high resolution X-ray diffraction technique. Using the Voigt function method, diffraction line is characterized by all three parameters of the half-width integral breadth and form factor. The crystal lattice strains are calculated by analyzing the experimental line profile composed of Cauchy and Gaussian parts. Simulation of coherence diffraction of asymmetric crystal silicon is achieved by ray tracing code SHADOW. Both the theoretical calculation and experimental results show that if asymmetrical angle reaches 0.749°, the half-width and integral breadth of diffraction curve change obviously compared with the situation where asymmetrical angle reaches 0.008°. This is why the calculation error of crystal strain will be beyond 5% by the Voigt function method no matter whether we use theoretical value or experimental data. It is shown that the precise crystal cut is extremely important for device application. And this conclusion will also be helpful in other crystal studies by using X-ray diffraction parameters.
Low-energy ion beam modified surface property and mechanism of high temperature superconductor YBa2Cu3O7-δ thin film
The interaction between ion beam and solid target is widely used in material modification. For the high temperature superconducting thin film modification, however, earlier experiments show that the samples are accompanied by the degradation in superconducting properties due to the structural damage of materials. In order to improve surface morphologies and superconducting properties of YBa2Cu3O7-δ (YBCO) thin films, we introduce a new ion beam structure modification (ISM) method. Although the ion bombardment time parameter effect is not clear, the related mechanism should be clarified. In this paper, the bombardment processes with duration times of 8 min, 10 min and 12 min are investigated in a vacuum chamber with an Ar+ Kaufman ion source, and the direction between the incident ion beam and the normal of sample is fixed at a certain angle. Surface morphologies and the microstructures of YBCO samples are characterized by scanning electron micrographs and X-ray diffraction patterns, respectively. In the respect of superconducting properties, the critical current density Jc is measured by Jc-scanning test. The results indicate that the needle-like a-axis grains and pores disappear gradually with the increase of the ion bombardment time. In order to characterize the effects of ion beam bombardment time on the internal strain in YBCO thin films, the relationship between the full width at half maximum and the Bragg diffraction angle of YBCO (00l) peak is studied by the William-Hall equation. The results show that the internal strain in YBCO thin film increases with increasing the ion beam bombardment time. At the same time, the critical current density Jc value of the sample after ISM processing increases, which is more than 2.2 times higher than that of the initial sample. The main reason for the increases of critical current density Jc in YBCO thin film is due to the drastic shrink of Cu–O bond caused by the increasing internal strain. Based on the bond contraction pair theory, the shrink of Cu–O bond improves the energy to break Cooper-pairs, and then increases the current carrying capacity of high temperature superconducting YBCO thin film, especially in copper-oxygen (CuO2) plane. The ISM process might be a useful method of markedly improving the surface morphology, meanwhile, the critical current density Jc value also increases in high temperature superconducting YBCO thin film.
Theoretical analyses of gaseous spontaneous Rayleigh-Brillouin scattering and pressure retrieving
The gas pressure is an important parameter describing the status of system and relating to many properties of physics and chemistry. The traditional intrusive method for pressure measurement has some effects on the gas status and the measurement accuracy. Therefore, it is desired to develop a non-intrusive method. The spontaneous Rayleigh-Brillouin scattering (SRBS) is a potential tool for accurate, remote, and non-intrusive pressure measurement. In this paper, the SRBS spectra are simulated using the Tenti S6 model convolved with the instrument function of the measurement system at a 90° scattering angle and pressures of 2, 4, and 6 atm (1 atm = 1.01325×105 Pa). In order to eliminate the effect of the instrument function of the measurement system, we propose a deconvolution method by comparing the traditional convolved SRBS method in this paper. According to the principle of the Wiener filter and the truncated singular value decomposition method, the Wiener filtering factor can be obtained. And the deconvolved spectra are obtained by convolving the stimulated spectra with the Wiener filtering factor. We find that the deconvolved spectra are coincident well with those from the Tenti S6 model without convolving with system transmission function. In order to compare the accuracy of the convolution method with that of the deconvolution method in experiment, the SRBS spectra of N2 mixed with aerosols are measured at a 90° scattering angle and pressures of 2, 4, and 6 atm respectively. The experimentally obtained raw spectra are fitted with the theoretical spectra, which are obtained by convolving the Tenti S6 model with the instrument function of the measurement system. The relative errors of retrieved pressure are all less than 6.0%, and the normalized root-mean-square deviation is calculated and found to be less than 6.5%. On the other hand, the deconvolved spectra are obtained by convolving the experimentally obtained raw spectra with the Wiener filtering factor and then fitted with theoretical calculated spectra from Tenti S6 model without convolving with system transmission function. The relative errors of retrieved pressure are all less than 5.0%, and the normalized root-mean-square error is less than 6.0%. By comparing the two methods, it can be found that the deconvolution method can eliminate the effect of instrument function of the measurement system and improve the resolution of Rayleigh-Brillouin scattering spectrum. The performance of fitting and the accuracy of pressure retrieving show that the deconvolution method is better than the convolution method under lower pressure (<2 atm), but worse than the convolution method under higher pressure (>2 atm). The comparison result demonstrates that the deconvolution based on the Wiener filter is likely to be directly applied to the exploring of the properties of the combustor in aero engine, such as pressure profile retrieval or temperature measurements.
Theoretical study of coupling double-bubbles ultrasonic cavitation characteristics
When the distances between bubbles are small enough, the pressure acting on the bubble is not the same as the external driving pressure, because of the radiation pressure wave of the neighboring bubbles. The force between two bubbles due to the bubble-radiated pressure waves by the neighboring bubbles is called the secondary Bjerknes force. Considering the bubble-radiated pressure waves and using the modified Keller-Miksis equation and van der Waals equation, the changes of the radius, the secondary Bjerknes force and the temperature of the double bubbles, which have different sizes, interspaces in between, and noble gases, in the process of ultrasonic cavitation are calculated. The calculations are based on the assumption that the locations of double bubbles stay unchanged in the oscillation process and their shapes always keep spherical. The double bubbles can also oscillate synchronously under the influence of the driving ultrasonic field. Because the sound propagation speed in water extremely fast, the time-delay effect on the secondary Bjerknes force is neglected. From the calculated results, the following conclusions can be drawn: when the sizes of double bubbles are different, the smaller bubble is more restrained and the temperature change is larger. When the sizes of double bubbles are the same, the Bjerknes force is negative, indicating that the coupled double bubbles are attracted to each other during the oscillation and the Bjerknes force has two radial oscillations in one driving period. As the interspace between double bubbles increases from 100 μm to 1 cm, the secondary Bjerknes force decreases from 10-4 N to 10-8 N, indicating that the interaction between double bubbles increases with the decreasing of the distance between the bubbles. The coupling double bubbles with different noble gases have only a small difference in maximum radius in the stage of expansion, but have different oscillation patterns clearly in the stage of rebound. This is because the bubble expansion process can be seen as an isothermal process, the effective polytropic exponent γ is approximately equal to 1. The collapse process can be regarded as an adiabatic process, so the effective polytropic exponent γ of noble gas with large molecules changes rapidly, and the influence of the oscillation of the bubbles becomes large. Our work provides a theoretical basis for establishing the acoustic cavitation model of different-number bubbles, and calculating the interaction force between different-number bubbles.
Influence of secondary electron yield of material on two-sided multipactor discharge in cavity
The influences of secondary electron yield (SEY) of material on the transient and saturation characteristics of two-sided multipactor discharge in cavity are numerically investigated by using particle-in-cell and Monte-Carlo methods. The numerical results indicate that as the SEY increases, the rate of electron number increases and the average value and magnitude of steady electron number also increase. The oscillation start-time of discharge current is shortened, and the steady value of discharge current increases and tends to be saturated. Both the average value and magnitude of steady discharge power increase and tend to be saturated. Both the time-delay and pulse width of deposited power waveform increase and also tend to be saturated. Under the circumstances of higher and lower value of SEY, the physical images of electron phase space, charge density, average impact energy, average SEY, electron number and discharge current are in detail shown in particle-in-cell simulation. The results can be concluded as follows. Under the circumstances of lower value of SEY, the saturation characteristics is determined by both “debunching” and “reverse field” of space charge effects. But under the circumstances of higher value of SEY, the multipactor mechanism tends to be one-sided mode in the steady stage which can be obviously determined by “reverse-field” of space charge effect.
Suppressing second electron yield based on porous anodic alumina
The multipactor effect is a resonant vacuum electron discharge that can occur in microwave and millimeter-wave subsystems,such as filters,multiplexers,and radio-frequency satellite payloads.In a high-power microwave device,multipator discharge can cause the device to break down,and thus degrading its performance.Fortunately,the multipactor effect can be mitigated by reducing the secondary electron yield (SEY) of the material which a microwave device is made from.Therefore,how to reduce the SEY of material is an important matter.In view of this problem,a new method to reduce the SEY is presented in this paper.This method is based on the fact that when aluminum sheet is treated with anodizing,many porous structures with high height-to-width ratios can be formed on the surface of sheet.These porous structures are conducive to reducing SEY.However,the alumina film covers these porous structures.Because alumina has poor performance in conductivity,the loss of high-power microwave device will increase if the microwave device is anodized.In consequence,the performances of the microwave device will deteriorate.In order to avoid this problem, silver film is chosen,and is electroplated on the anodized aluminum sheet.Although silver film is electroplated on the aluminum sheet,there are still many porous structures on the surface.In order to validate the method in this paper, some aluminum samples are anodized.And then,the SEYs of these samples are obtained by the SEY measurement system.The results show that this method is efficient for reducing the SEY.Compared with the non-anodized sample, the uncleaned sample on whose surface there exists the adsorption or contamination shows that the value of the first energy crossing point of the measured curve of emission coefficient of secondary electrons,E1,increases from 45 eV to 77 eV,and the maximum value of SEY (SEYmax) decreases from 2.68 to 1.52;when the samples are all cleaned (in order to obtain ideal surface by wiping off adsorption or contamination),the value of E1 increases from 40 eV to 211 eV, and the value of SEYmax decreases from 2.55 to 1.36.Furthermore,the multipactor threshold of an X-band impedance transformer is simulated with using these SEY data to validate this method.And it is concluded that compared with the threshold of the original design,the multipactor threshold of the impedance transformer which is treated with the method increases from 7000 W to 125000 W.Therefore,it can be seen that the method presented in this paper is helpful in solving the problem of the multipactor in high-power microwave device for space.Meanwhile,as a usual method,the method can also be used to push forward the researches of vacuum electron devices and accelerators.
Comparative studies of irradiation effects in borosilicate glass and fused silica irradiated by energetic Xe ions
As the first safety barrier of high level radioactive waste, the tolerance to radiation of vitrification is critical. Vitrification is a kind of specialized glass used as the package of high-level radioactive waste in nuclear power industry. Because of its structural consistency with the main structure of vitrification (silicon-oxygen tetrahedron), fused silica is used to study the irradiation effect on network of vitrification in the present study. Borosilicate glass, a simplified version of vitrification, is studied under the same conditions for comparison. Hardness values, moduli and refractive indexes of fused silica and borosilicate glass are measured before and after irradiation with nanoindentation technology and elliptic polarization instrument. It is shown in this study that the hardness values of fused silica and borosilicate glass decrease with increasing dose. On the other hand, with dose increasing, the modulus of borosilicate glass decreases but the modulus of silica increases. Change in modulus might be attributed to the change of density, which is consistent with results from the refractive index.
Mode properties analysis of graphene-coated asymmetric parallel dielectric nanowire waveguides
In this paper, the mode properties of graphene-coated asymmetric parallel dielectric nanowire waveguides are analyzed by the multipole expansion method. First, the surface plasmon modes supported by the waveguides are classified. Then, the influences of frequency, geometry parameters and graphene Fermi energy on the effective refractive index and propagation length of the seven low order modes are studied in detail. The seven low order modes can be divided into two categories: cos mode and sin mode. The cos mode includes modes 0, 2, 4 and 6, while sin mode includes modes 1, 3 and 5. The results show that the characteristics of the modes can be adjusted in a wide range by changing the frequency, geometrical parameters and the Fermi energy of graphene. When the frequency increases from 10 THz to 50 THz, the number of graphene surface plasmon modes increases and the effective refractive index of each mode increases monotonically. Moreover, with the increase of frequency, the propagation length of cos mode decreases monotonically, and the propagation length of sin mode shows the trend of first increasing and then decreasing. As the distance between the two dielectric nanowires increases, the mode properties of modes 0 and 1 change drastically, while the effective refractive indexes and propagation lengths of other modes vary very little. As the radius of one of the dielectric nanowires increases, the number of modes increases in the calculated range, while the effective refractive index and propagation length of each mode are less affected. In the process of increasing the Fermi energy of graphene from 0.3 eV to 0.7 eV, the effective refractive index and propagation length of each mode vary greatly. Moreover, the effective refractive index of each mode decreases monotonically, while the propagation length increases. It is also found that the compositions of the low order modes vary with the size of the two nanowires for this asymmetric structure. The comparison with the finite element method shows that the semi-analytical results based on multipole method are in good agreement with the numerical results from the finite element method. The present work may provide a theoretical basis for designing and fabricating the asymmetric parallel dielectric nanowires coated with graphene.
Theoretical simulations of deformation coupling with phase separation of two-component charged vesicles in a two-dimensional plane
The real bio-membranes are of multi-component, and they usually carry a certain quantity of charges. Therefore, it is of great biological significance to study charged multicomponent vesicles. However, the charged multi-component vesicles have been not yet systematically studied due mainly to the following two reasons: first, there are too many factors that will influence the behaviors of charged multi-component vesicles; second, theoretically it is difficult to deal with the phase separation of the multiple components from the Coulomb interaction of charged components at the same time. This work shows that the combination of the discrete-spatial variational method and dissipative dynamics can be used to address the above issues. For simplicity, we will consider only the deformation coupled with the phase separation of two-component charged vesicles in a two-dimensional plane rather than in three-dimensional space, which can present us more systematic research results. Besides, we have not considered the screening effects of counter ions or salts in this work, or equivalently we consider only the case where the screening length is relatively big. The charged vesicle is composed of two components A and B, where component A is negatively charged while component B is neutral. In particular, the charges on the vesicle can freely move in the membrane, which may be described by a time-dependent Ginzburg Landau equation. Initially, the two components are uniformly distributed on the vesicle.In this work, we specially focus on the influence of the electrostatic interaction on the compatibility of different components. It is found that introduction of charges will promote the apparent miscibility between different components. This could explain that the electrostatic interactions may contribute to the increase of the compatibility of different biomolecules in biological system. When temperature is relatively high, the electrostatic interaction will completely inhibit the phase separation which actually prevents the same component from being clustered. When temperature is relatively low, the electrostatic interaction will increase the number of phase domains, which actually turns the original macro phase separation into the micro one, thus reducing the cluster size.
In this work, we also systematically study the influences of other factors, such as temperature, charge density of charged components, and the averaged concentration of charged component, on the final configuration of charged multicomponent vesicle. In particular, a phase diagram of the temperature and the averaged concentration of the charged component is obtained, and it is found that the number of phase domains will increase with the increase of charge density of component A. These conclusions are also qualitatively applicable to three-dimensional two-component charged vesicles.
Progress of detecting orbital angular momentum states of optical vortices through diffraction gratings
Optical vortices are a new kind of laser beam and receiving more and more attention currently.The complex amplitudes of optical vortices comprise a helical term exp (ilφ),with l being the topological charge and φ the azimuthal angle.Each photon in optical vortices carries the orbital angular momentum (OAM) with a value of lħ,where ħ is the Planck's constant divided by 2π.The topological charge l is the eigenvalue of optical vortices,and determines the helical wavefront distribution,thus also known as OAM state.Moreover,such an OAM state can be an infinite integer state. And vortices with various OAM states are orthogonal to each other,making it possible to be employed in high capacity data-transmission system.In addition,the above unique features contribute to their widely applications in lots of areas such as optical tweezers and spanners,rotation detection,quantum entanglement,etc.In these applications,detecting OAM states is basic,and greatly significant.Recently researchers have developed lots of approaches to detecting the OAM states,including the methods of interference,diffraction gratings,metasurface,etc.Of such approaches,the scheme of diffraction gratings is the simplest and most widely used,where one or more diffraction gratings are employed. When optical vortices propagate through such gratings,the OAM states are acquired immediately through capturing and analyzing the distinct OAM-related diffraction patterns.In this review,we focus on the techniques of detecting OAM states through diffraction gratings,which have been demonstrated by our group and other researchers.Some of the main detection gratings,including double-slit,triangle aperture and slit,angular slit,cylindrical lens,graduallychanging-period grating,annular grating,are introduced.In addition,schemes like composite fork grating,Dammann vortex grating and integrated Dammann grating,are presented to detect the OAM state for coaxial multiplexed vortices. Besides diagnosing OAM state,measuring the intensity proportion of each OAM channel,known as OAM spectrum, in multiplexed vortices is also necessary in some cases.Therefore we also introduce the ways to measure the OAM spectrum,e.g.the OAM mode sorter,the gray-scale algorithm.
Interface adhesion property between graphene film and surface of nanometric microstructure
The performance of graphene can be influenced by its surface mophology, while the surface morphology of graphene is closely related to the substrate. The adsorption and peeling process of graphene on a corrugated surface can provide a theoretical basis for the functional preparation and transfer of graphene. In this work, the adhesion properties and peeling process of graphene on nanostructured substrate are investigated by using molecular dynamics (MD) simulation. As an effective tool of atomic collision theory, MD simulation can provide detailed information about the adsorption configuration and peeling properties of graphene on the nanostructure surface, making up for the deficiency of experiment. The results indicate that graphene can conformably coat on the surface, partially adhere to or remain flat on the top of the stepped substrate. We find that the continuous transition occurs in the adsorption configuration of graphene on the stepped substrate, but the repeated process appears in the transition from partial adherence to conformable coating. When graphene coats on the nanostructured substrate conformably, the adsorption energy can reach its peak value. The adsorption configuration of graphene can change from suspension to partial adhesion after the adsorption energy has exceeded 360 eV · Å-2. It is also shown that the average peeling force fluctuates periodically when the absorption configuration of graphene is conformably coated or suspended on the stepped substrate. Two kinds of behaviors can be noticed in the peeling process. The graphene can directly slide over the bottom while it is fully coated on the surface. The graphene is separated directly from the corrugated surface while it suspends or partially adheres to the surface. If the absorption configuration of graphene is in the suspension state, the average peeling force appears to change drastically within a section of peeling distance and then decreases immediately below zero. Although the flexural stiffness of graphene can be overcome, the interfacial friction between graphene and the substrate is also an essential factor affecting the final adsorption configuration. In this paper, we propose a theoretical formula for the average peeling force according to the changes of size parameters on the nanostructured substrate. The theoretical formula is validated by the simulation results. In addition, with the increase of peeling angle, the average peeling force first increases and then becomes smaller. As a result, a larger average peeling force can be found when the graphene with Stone-Wales defect structure is peeled from the flat substrate. With the increase of double vacancy defect, the maximum peeling force decreases in a certain range, whereas it increases beyond this range. This work can provide a theoretical reference for exploring the peeling property and the adhesion mechanism of graphene on nanostructure surface.
Chaotic time series prediction based on robust extreme learning machine
Chaos is seemingly irregular and analogous to random movement happening in a determinative system in nature,and more and more types and numbers of time series with chaotic characteristics are obtained from the actual systems,such as atmospheric circulation,temperature,rainfall,sunspots,and the Yellow River flow.The chaotic time series prediction has become a research hotspot in recent years.Because neural network can be strongly approximated nonlinearly,it has better prediction performance in the chaotic time series modeling.Extreme learning machine is a kind of neural network, and it is widely used due to its simple structure,high learning efficiency and having global optimal solution.Extreme learning machine initializes the input weight randomly and just adjusts the output weight in the training process,in order to be able to obtain the global optimal solution,so it has faster convergence speed and can overcome the disadvantage of gradient vanishing.Due to the above advantages,in recent years,the improved algorithms of the extreme learning machine have been developed rapidly.However,the traditional training methods of extreme learning machine have very poor robustness and can be affected easily by noise and outliers.And in practical applications,the time series are often contaminated by noise and outliers,so it is important to improve the forecasting model robustness and reduce the influence of noise and abnormal points to obtain better prediction accuracy.In this paper,a robust extreme learning machine is proposed in a Bayesian framework to solve the problem that outliers exist in the training data set.Firstly,the input samples are mapped onto the high-dimensional space,and the output weight of the extreme learning machine is used as the parameter to be estimated,then the proposed model utilizes the more robust Gaussian mixture distribution as the likelihood function of the model output.The marginal likelihood of the model output is analytically intractable for the Gaussian mixture distribution,so a variational procedure is introduced to realize the parameter estimation.In the cases of different noise levels and the different numbers of outliers,the proposed model is compared with the other prediction models.The experimental results of Lorenz,Rossler and Sunspot-Runoff in the Yellow River time series with outliers and noise demonstrate that the proposed robust extreme learning machine model could obtain a better prediction accuracy.The proposed robust extreme learning machine not only has the strong capability of the nonlinear approximation but also can learn the model parameters automatically and has strong robustness.At the same time,the time complexities of different models are compared and the convergence of the proposed model is analyzed at the end of the paper.
Spontaneous generation of spiral wave in the array of Hindmarsh-Rose neurons
Spiral waves have been reported to be existent in the neocortex, during pharmacologically induced oscillations and sleep-like states. In the last decades, theoretical studies have demonstrated an underlying mechanism of the generation of spiral waves in a heart system. Nevertheless, how can a neural system produce spontaneous spiral wave and whether this behavior is sensitive to the dynamics of isolated neurons have not been systematically studied yet. In this paper we propose a modified Hindmarsh-Rose (HR) neuron model to study whether spiral wave can occur spontaneously in a two-dimensional array of HR neurons, which evolves from the initial state with a random phase distribution. The simulation results show that whether spiral wave can occur spontaneously in the system depends on the state of the single HR neuron, initial state of system and coupling strength. Especially, the state of the single HR neuron plays a central role. When the single HR neuron is in the state of period 1 spike, multiple spiral waves and spiral pairs can be generated spontaneously in the system for a certain range of coupling strength. In this case, the formations of spiral waves are completely independent of the initial state of the system, and as long as choosing an appropriate coupling strength, a single spiral wave can be found in the system. Furthermore, when the coupling strength exceeds a certain threshold value, the system will exhibit three kinds of dynamical behaviors, and correspond to three kinds of the different initial states, respectively. When system evolves from the first kind of initial state, the single spiral wave can be found occasionally in the system. When the system evolves from the second or third kind of initial state, the oscillation with intermittently global synchronization and oscillation death can be observed in the system, respectively. When a single HR neuron is in the state of period 2 spike, the spiral wave can appear spontaneously in the system only when the phase distribution of the initial state approaches to a uniform distribution. Moreover, the range of coupling strength on the generation of spiral wave is smaller than that of period 1 spike. When the single HR neuron is in a higher periodic state, it is difficult to generate spontaneously spiral wave in the system. These results are useful in understanding the spontaneous generation of spiral waves in the neocortex.
Technical realization and system simulation of ultraviolet multi-mode high-spectral-resolution lidar for measuring atmospheric aerosols
Multi-mode high-spectral-resolution lidar is a new concept of high-spectral-resolution lidar, which uses the multiple-longitudinal-mode pulsed laser rather than the single frequency laser. In this paper, we analyze the multiple longitudinal mode and its spectral distribution of a typical Nd:YAG laser, and calculate its corresponding Mie scattering and Rayleigh scattering spectra, which are a convolution between the spectral distribution of multiple-longitudinal-mode laser pulse and that of the Mie and Rayleigh scattering excited by a single frequency laser pulse. According to the spectral analyses of the elastic lidar returns, we design an ultraviolet multi-mode high-spectral-resolution lidar, in which a high-power non-seeded Nd:YAG pulsed laser at the third harmonic 355 nm wavelength is used as a transmitter, and a Cassegrain telescope serves as a receiver. In the polychromator, a narrow band interfering filter is selected to block the solar background, and a tunable Mach-Zehnder interferometer (MZI) is designed to separate the aerosol Mie scattering signals from the molecular Rayleigh scattering signals excited by the multi-mode pulsed laser. The MZI is composed of a roof mirror mounted on a piezoelectric ceramic and two beam splitters. The optical path difference of the MZI can be adjusted by the piezoelectric ceramic, while its optimum value should make the correspondence between the free spectral range of MZI and the interval between longitudinal modes of Nd:YAG pulsed laser. The photomultiplier tube is selected as a detector, whose output is the convolution between the transmission function of MZI and the Mie and Rayleigh signals excited by the multi-longitudinal mode laser pulse. In the practical experiment, the optimal optical path difference of MZI can be determined by using envelope analysis. For the transmitter laser, when one channel has a maximum output signal and the other has a minimum output, the center wavelength of each longitudinal mode of laser is locked in the optimal optical path difference. The channel of MZI with the maximum output is to pass the Mie scattering signal, while the channel with the minimum output is to block the Mie scattering signal. The aerosol optical characteristics are retrievable by using the complementary properties of the two output channels of MZI. In order to verify the feasibility of the multi-mode high spectral resolution lidar, the system simulation is carried out by using the real atmospheric model and the designed lidar system parameters. The simulation results show that the designed ultraviolet multi-mode high-spectral-resolution lidar can realize the accurate measurement of aerosol within a height of 10 km.
One-dimensional magneto-hydrodynamic simulation of the magnetic drive isentropic compression experiments on primary test stand
The 10 MA primary test stand (PTS), the most powerful pulse power generator in China, is used to obtain isentropic compression of Al samples under a pressure of about 100 GPa. The high performance of laser-triggered gas switches enables the precise synchronization of the 24 modules according to the required timing sequence. This advantage makes the PTS a very good platform for dynamic material compression with fundamental capability of pulse shaping. Tens of isentropic compression experiments have been conducted on the PTS, among which two distinct loading profiles were designed and used to obtain distinct compression processes. The first current, which is used to obtain a shockless compression, has a relatively smooth rise, and the rise-rate keeps almost constant during the 400 ns-long compression. The second current shape has a mild rise but a sharp ends, which is designed to make an artificial “turn-point” in the velocity history, which is helpful for the numerical code verification. The current profile, as well as the sample thickness, is optimized by a one-dimensional magneto-hydrodynamic (1D MHD) code MADE1D coupled with a full circuit model for the PTS. The equation of state and conductivity model used here have a wide coverage in the density, temperature and pressure range. The strength of material and its constitution model are also taken into consideration to simulate the elastic and plastic flow of metal at relatively low pressure and temperature. Compared with the experimental results, the simulated velocity at the sample/window interface is found to agree well with the measurement for most of the cases. This suggests that the MHD simulations with the circuit model are able to reflect the main process of the loading history, and help to analyze and elucidate the phenomena contributing to the compression. It shows that the current waveform is one of the most important factors that affect the loading process. For the PTS and strip-line electrodes it uses, a current rise ratio less than 15 kA/ns helps to obtain a smooth off-Hugoniot pressure rise. The temperature rise due to the pdV work is very small, and most of the sample material, except those in the skin layer where current passes through, keeps solid during the compression. However, for a current rises at 40 kA/ns or more, the ramp loading wave could be sharpened into a shock within the sample thicker than 1.2 mm. Based on the PTS flexibility of pulse shaping, a wide range of desired load processes can be gained by designing and controlling the load current and sample thickness precisely.
Analysis and research of polarization aberration in rapid space angle measuring system
The precise angle measurement and transmission technology have been widely used in the precision measurement, aerospace, military, biomedicine and other devices, which are based on the polarized light and magneto-optical modulation. This method has the characteristics of no rigid connection required, long distance transmission, high precision, etc. However, the azimuth information measurement method needs the assistance of complex servo tracking system according to the orthogonal extinction principle of polarization prism, meanwhile, the measurement time is longer, which reduces reliability and reaction sensitivity of the system. In order to improve the measurement accuracy and fast response capability of the system, a fast space goniometry method is proposed through the Wollaston prism polarizing beam splitter, with which the azimuth is directly calculated according to the two light intensities. The measurement time can be shortened, and the accuracy is improved by the use of magneto-optical modulation technology. The rapid space angle measuring system needs to realize the measurement function in a certain translation range, which requires the beam to have a certain coverage area in the receiving unit. However, the system is limited by size and volume of the device; we can only choose to expand the incident beam. Therefore, when the beam is incident onto the receiving unit, some incident angle and azimuth, that is, non-vertical incidence will be produced. However, the polarization of the non-vertically incident light passing through the system will change and polarization aberration exists, which will lead to measurement error. In this paper, the beam passes through the polarizing prism in a certain range of azimuths and incident angles, and the polarized light tracing method and the boundary condition of the electromagnetic field are used to study and simulate the polarization change and distribution of the outgoing beam. The changes of different incident azimuths and angles can be simulated through the translation of receiving unit, and the azimuths can be measured indirectly by using self-collimation theodolite and right angle prism. By comparing the measured azimuths under the translational and centering conditions, the influence of polarization aberration on the angle measuring system and the correctness of the theoretical analysis are verified. It is concluded that when the azimuth angle is 0°, the measurement error is small; when the azimuth is 90°, the measurement error is largest, meanwhile the measurement error will increase with the translation distance becoming longer (i.e., the incident angle). According to the comparison between the experimental data and the simulation results, the existing problems are pointed out, and the corresponding improvement measures are proposed. The results of this work have some significance in guiding the optimization of the system structure, and the further improvement inthe performance of the system.
Characteristics of interaction between single-layer graphene on copper substrate and groove
The two-dimensional material graphene is usually required to be transferred on the target substrate for some special applications, thus it is important to understand the adsorption properties in the graphene transferring and stripping processes. In this paper, the adsorption properties of a single-layered graphene on the grooved copper substrate are investigated using molecular dynamics simulations. The influence of geometric characteristic size of the groove on the adsorption force of the graphene deriving from the substrate is explored. For the fixed boundary conditions of the graphene, the adsorption force increases up to maximum and then decreases with reducing the distance between the graphene and substrate in the adsorbing process. The maximum adsorption force increases with groove depth increasing, with the groove width kept constant. Nevertheless, as the groove depth increases continuously, the adsorption force decreases greatly until the graphene cannot be adsorbed into the groove. In the graphene stripping process, the critical force that can strip the graphene completely from the substrate increases first and then decreases with the increase of the groove depth, which is also dependent on the steady adsorbing configuration of the system before stripping. With the groove depth kept constant, the magnitude of the adsorption force between the graphene and substrate is determined by the steady adsorbing configuration of the graphene in the groove region. The adsorption force versus the distance between the graphene and the grooved substrate can be divided into two groups according to whether the graphene can be adsorbed into the groove. In both adsorbing and stripping processes, the adsorption force for the graphene adsorbed into the groove is obviously larger than that for the graphene covered on the groove. Moreover, the influence of the boundary condition of the graphene on the adsorption properties in the groove region on the substrate is considered preliminarily. It indicates that the tensile plane stress within the graphene sheet induced by the fixed boundaries can hinder the graphene from being adsorbed into the groove. The findings may be helpful for the graphene-based fabrication of nano-apparatus and functionalized surface modification.
Theoretical investigation of atomic low-order harmonics under irradiation of two high frequency laser pulses
With the development of laser technology,the extreme ultraviolet and X-ray light sources can be obtained by utilizing the high-order harmonic radiation and the free electron laser.When an atom is irradiated by an intense highfrequency laser,many nonlinear phenomena can be observed,such as high-order harmonic emission,threshold ionization and ionization stability of atom,etc.
The emission spectra with some new features appear when the atom is irradiated by a high-frequency laser pulse. The harmonic spectra with a clear cut-off plateau do not appear,and the three-step model is no longer valid for explaining the results.In addition to the odd-order harmonic radiation observed in the emission spectra,many super-Raman lines can be seen clearly.These radiations are generated from the transition between the dressed eigenstates of the atom. When the incident high-frequency laser pulse is strong enough,the peak of the harmonic splits into many sub-peaks. The generation of the sub-peaks of harmonic is due to the contributions from the rising and falling parts in the pulse.
With the development of free electron laser technology,one can obtain a combined pulse with different frequencies. Many new two-color schemes are proposed for the experiment,such as the realization of two-photon spectrometer, pump-probe spectrometer.In this work,we investigate the optical radiation of the atom irradiated by a combined laser pulse,whose energies are higher than the ionization energy of the atom.It is found that the odd harmonics of the two high frequencies are shown in the emission spectra,and many satellite peaks appear in the vicinity of these odd harmonics.Furthermore,the intensities of the satellite peaks are enhanced exponentially with the increase of the incident laser intensity,and the frequency difference between the two adjacent peaks is the frequency difference between the two incident laser pulses.We study the time-frequency profile of the harmonic emission by analyzing the wavelets.With the two-color scheme one can achieve coherent soft X-ray and produce short coherent pulse.
We also calculate the high-order harmonic spectrum of hydrogen in the two-color laser pulse,the multi-peak structure in the emission spectra can also be found,and the positions and intensity distribution of the emission peaks are consistent well with those from the one-dimensional calculation.In our two-color scheme,by changing the peak intensity and frequency of one of the combined laser pulses,the multi-plateau structure can be shown in the harmonic spectra.Taking advantage of the harmonic plateau,the soft X-ray radiation and ultra-short attosecond pulse chain can be generated.
Theoretical study on the structure and stability of neutral and cationic butanone clusters
The molecular clusters have attracted increasing attention in recent years due to their applications in areas of laser, synchrotron radiation, molecular beam and time-of-flight mass spectrometry. The cluster structures can be speculated by the mass spectrum measurement and predicted by the quantum chemical methods. It is very important for understanding the solvation kinetics and nucleation to explore the formation and growth of clusters. Meanwhile, it is also beneficial to understanding the atomic or intermolecular interactions in the clusters.
The molecular clusters have been studied in our previous work. The acetone clusters (CH3COCH3)n (n ≤ 12) were observed by 355 nm pumping laser. The structures of (CH3COCH3)n with n=2-7 were calculated by density functional theory, and some structures of clusters with low energy were given. Subsequently, several butanone cluster fragment ions and protonated butanone (CH3COC2H5, which is formed by taking a methyl change into ethyl from acetone CH3COCH3) clusters were observed by measuring the mass spectra under the irradiations of 355 nm and 118 nm laser lights, respectively.
It is important to determine the stable cluster structures and explain the dynamics of the clusters by theoretical calculation. The stable geometric structures of neutral and cationic butanone clusters are optimized at B3LYP/6-31G* and B3LYP/6-311+G** levels based on the density functional theory. The structural characteristics and stabilities of butanone clusters with various sizes are also analyzed. The average binding energy, first-order difference energy, HOMO-LUMO gap and ionized energy are further discussed systematically in the present work. The results show that the structures of (CH3COC2H5)n and (CH3COC2H5)n+ have similar characteristics, single-ring structure is the most stable for them when n=3-7, and the structures also occur in some hydrogen bonded clusters, such as (H2O)n (n=3-6), (NH3)n (n=3-6), (CH3OH)n (n=3-6), and (HCHO)n (n=3-8). Moreover, the stability of double ring structure rises with cluster size increasing. The (CH3COC2H5)3 has the best stability in neutral clusters (CH3COC2H5)n with n=2-7, and it corresponds to the strongest peak in experiment. In addition, the (CH3COC2H5)4+ is the most stable in the cationic clusters with corresponding sizes. Furthermore, the vertical ionization energy of butanone molecule is 9.535 eV via theoretical calculation, which is in agreement with the experimental data. At the same time, the structures of (CH3COC2H5)2+ and (CH3COC2H5)2 are proved to be different by the ionization energy. The results provide a theoretical basis for the formation mechanism of butanone cluster fragment ions in experiment, and it is beneficial to the further study of growing the ketone clusters.
A time-varying Doppler spectrum model of radar sea clutter based on different scattering mechanisms
The Doppler spectrum modeling of the sea clutter is of great significance for using the Doppler radar to suppress the sea clutter and detect the target on sea surface. Taking into account the characteristics of the Doppler spectrum components corresponding to three scattering mechanisms, the Bragg, whitecap, and breaking-wave, we propose a time-varying Doppler spectrum model of radar sea clutter in this paper. The Doppler spectrum shifts and bandwidths of the three scattering mechanisms are considered respectively, and a Doppler shift of an added wave speed is introduced into this model. Because the spectrum intensity is defined as a random variable being a function of the measuring time of the sea clutter series, the model has the ability to model both the long-time averaged Doppler spectra and the short-time Doppler spectra. By modeling the short-time Doppler spectra and the long-time averaged Doppler spectra of the shore-based radar sea clutter measured in the China Yellow Sea at P and S bands, it is indicated that the proposed model has a higher precision than the conventional model, and especially, the modeling errors decrease significantly in the cases of the short-time Doppler spectra with long measuring time and the long-time averaged Doppler spectra with complex shapes.
Two-body entanglement in a dilute gas of Rydberg atoms
Since the establishment of quantum mechanics, quantum entanglement has become one of the most important realms in quantum physics. On the one hand, it reflects some of the most fascinating features, such as quantum coherence, probability and non-locality and so on. On the other hand, it proves to be an indispensable resource of quantum information processing and quantum computation, which is considered to greatly promote the development of human science and technology. In the past decades, inspired by advances in quantum information theory and quantum physics, people have been searching for suitable systems with great enthusiasm to prepare the robust and manipulable quantum entanglement. Recently, Rydberg atoms have been considered to be a good candidate for many quantum information and quantum computation tasks. Compared with general neutral atoms, Rydberg atoms with large principal quantum number have several advantages in the quantum information and computation service. Firstly, they have finite lifetimes much larger than general neutral atoms, which indicates that the long-time entanglement between Rydberg atoms can be achieved. Secondly, due to the high-excitation level, Rydberg-excitation atoms have long-ranged dipole-dipole interaction much stronger than ground state atoms. This strong atomic interaction leads to the so-called blockade effect: when one atom is excited to Rydberg level, the excitation of the neighboring atoms will be strictly suppressed due to the energy shift induced by the strong atomic interaction. On the contrast, if the energy shift is compensated for by the detuning between the energy levels and the driven laser field, these atoms can be excited with higher probability simultaneously. These effects imply that Rydberg atoms provide an excellent platform for investigating the quantum information and quantum computation process, and many important achievements based on them have been achieved. Encouraged by these researches on entanglement and Rydberg atoms, in this paper, we study the steady-state and transient dynamical properties of two-body entanglement and the Rydberg-excitation properties in a dilute gas of Rydberg atoms, which can be represented by a tetrahedrally arranged interacting four-atom model. By solving numerically the master equation of four atoms involving Rydberg level, we investigate the higher-order Rydberg excitations and bipartite entanglement, which is estimated by concurrence. Our results show that the bipartite entanglement can only achieve its maximal value in the strongest dipole blockade regime rather than anti-blockade one (the high-order Rydberg excitations). Furthermore, the physical essence of quantum entanglement is analyzed theoretically in relevant regimes. Our work can naturally extend to more complicated atomic space structures, and might be treated as a good platform for fulfilling many quantum information tasks by employing the quantum entanglement.
High-power wideband radio-frequency intensity modulated continuous wave laser
A high-power wideband radio-frequency (RF) intensity modulated continuous wave light source is demonstrated. The high-power dual-frequency light source is obtained via a dual-frequency laser signal seeding fiber power amplifier. A diode laser pumped dual-frequency laser is built as the seed and a diode laser pumped three-stage Yb3 + doped large mode area fiber power amplifier is used to enhance the output power to 50 W. In the dual-frequency seed laser, a coupled cavity composed of the Nd:YAG gain crystal and output coupler is used as the mode selector and enforces single longitude mode to oscillate. Two quarter wave plates are inserted in the laser cavity to lift the frequency degeneration of the two orthogonally polarized modes. By changing the angle between the fast axes of the two quarter wave plates, the frequency difference between the two orthogonally polarized modes can be tuned from 30 MHz to 1.5 GHz. The standard difference of beat frequency is 1.6144 MHz and stability is 1.52% when a frequency difference of output dual-frequency laser is 250 MHz. This stable dual-frequency seed signal is amplified via a diode pumped Yb3 +-doped fiber power amplifier. In order to suppress amplified spontaneous emission and other nonlinear effects, a three-stage fiber amplification system is used. The first stage is a diode pumped fiber (5 m, 6/125 μm, NA = 0.13) power amplifier. The pump power is fixed at 600 mW. The input dual frequency signal is 3.2 mW, and it is amplified to several hundred mW by the first fiber power amplifier. The second fiber amplifier is a diode laser pumped fiber (5 m, 10/125 μm, NA = 0.075/0.46) amplifier. The pump power is fixed at 10 W, and the dual frequency signal is amplified to sub watts after the second fiber amplifier. A 5 m large mode area fiber (25/250 μm, NA=0.065/0.46) is used in the final amplification. A maximum amplified power of 50.2 W is obtained when the pump power is 70 W in the experiment. The signal-to-noise ratio of the beat note increases from 25 dB to 40 dB via amplification. The output power fluctuation of the amplified signal at 50 W is smaller than 0.1 W during 30 min. The RF frequency stability is well maintained during the amplification, and the beat-note frequency instability is 1.777 MHz. This high-power dual-frequency light source with wide beat note frequency bandwidth has potential applications in dual-frequency coherent lidar system for long distance ranging and imaging or underwater detections after the frequency has been doubled to 532 nm.
Heterojunction polarization beam splitter based on self-collimation in photonic crystal
Polarization beam splitter (PBS) is an important device in optical system, in which the optical signal can be separated into two mutually orthogonal polarized light and transmit along different paths. It is difficult for the traditional PBS to meet the needs of the modern optical integrated systems because of its low transmission efficiency and high dependence on the incident angle. Therefore, it is necessary to design more efficient and compact PBSs. In recent years, photonic crystals have attracted more attention due to their ability to manipulate photon motion.
In this paper, a photonic crystal PBS with a non-orthogonal heterojunction structure is proposed, which is based on the self-collimation effect and bandgap properties of photonic crystal. The proposed PBS structure is composed of two square lattice photonic crystals with the same lattice constant and different air hole radii in silicon (Si), in which the beam can be self-collimated and propagate without diffraction, and the polarization separation of and transverse electric (TE) mode from transverse magnetic (TM) mode is realized at the interface. The self-collimation effect can be used to control the transmission of light in order to realize the general light guiding of the waveguide, and it can greatly reduce the difficulty in manufacturing process because of no additional defects introduced. The splitting properties, transmission properties and polarization extinction ratio of the PBS are numerically simulated and analyzed by using Rsoft software combined with the plane wave expansion method and the two finite-difference time-domain method. It is shown that a high efficiency and a large separating angle for TE and TM modes in a wide frequency range 0.275-0.285(a/λ ) can be achieved. The transmission efficiency is above 88% for both TE and TM modes, and the extinction ratios are more than 26.57 dB for TE mode and 17.50 dB for TM mode, respectively. This structure can be applied to the transmission system of terahertz band: a=26 μm, the size is 572 μm×546 μm, and the separation of TE mode from TM mode can be achieved in a wavelength range of 91-95 μm. A PBS for optical communication system can be also designed by using the same structure: n=3.48, a=426.25 nm, and the proposed PBS is only 9.38 μm×8.95 μm in size, which can separate these two polarization beams in a wavelength range of 1511-1579 nm. What is more, the proposed PBS based on photonic crystal is simple and easy to integrate, which has important application value in optical communication technology.
Coupled oscillation and shape instability of bubbles in acoustic field
Based on the Lagrange's equation, the dynamic equations and shape mode equations of two bubbles with nonspherical distortion are obtained. The radial oscillations and shape instabilities of two bubbles with nonspherical distortion in an acoustic field are numerically investigated. The numerical results show that there are two coupled modes between two nonspherical bubbles: shape coupled mode and radial coupled mode. The coupled modes between two nonspherical bubbles depend on the shape modes of two bubbles. When the shape mode of the first bubble is equal to that of the second bubble (n=m), the shape coupled mode and radial coupled mode both exist. The interaction force between bubbles is caused by these two coupled modes. If the two bubbles have different shape mode orders (n≠ m), there is a radially coupled mode between two bubbles. The interaction force between two bubbles is caused by radially coupled mode. The interaction caused by the radial coupling and shape coupling has an influence on the instability of gas bubble. The influencing factors depend on the shape mode, the equilibrium radius of neighboring bubble, and the driving acoustic field. The results demonstrate that the shape coupling can change the severity of the collapse of a gas bubble, and increase the ability of a gas bubble to resist distortion under a certain condition. The nonspherical disturbance of a real bubble in an acoustic field is not a single shape mode, but the coupling of different shape modes, so the shape coupling has an obvious influence on the shape instability of a real bubble. These may be the reason why bubbles can form some stable structures and keep stable oscillations in an acoustic field.
Improvement of field method and its application to integrating motion equation in Riemann-Cartan space
Like the Hamilton-Jacobi method, the Vujanović field method transforms the problem of seeking the particular solution of an ordinary differential equations into the problem of finding the complete solution of a first order quasilinear partial differential equation, which is usually called the basic partial differential equation. Due to no need of the strong restrictive conditions required in the classic Hamilton-Jacobi method, the Vujanović field method may be used in many fields, such as non-conservative systems, nonholonomic systems, Birkhoff systems, controllable mechanical systems, etc. Even so, there is still a fundamental difficulty in the Vujanović field method. That is, for most of dynamical systems, it is hard to find the complete solution of the basic partial differential equation. In this paper, the Vujanović field method is improved into a new field method. The purpose of the improved field method is to find the first integrals of the motion equations, but not the particular solutions of the motion equations. The improved field method points out that for a basic partial differential equation with n independent variables, m (m ≤ n) first integrals of a dynamical system can be found as long as a solution with m arbitrary constants of the basic partial differential equation is found. In particular, if the complete solution (the complete solution is a special case of m=n) of the basic partial differential equation is found, all first integrals of the dynamical system can be found. That means that the motion of the dynamical system is completely determined. The Vujanović field method is just equivalent to this particular case. The improved field method expands the applicability of the field method, and is simpler than the Vujanović field method. Two examples are given to illustrate the effectiveness of the method. In addition, the improved field method is used to integrate the motion equations in Riemann-Cartan space. For a first-order linear homogenous scleronomous nonholonomic system subjected to an active force, its motion equation in its Riemann-Cartan configuration space can be obtained by a first order nonlinear nonholonomic mapping. Since the motion equations in Riemann-Cartan configuration space contain quasi-speeds, they are often considered to be difficult to solve directly. In this paper we give a briefing of how to construct the motion equations of a first order linear nonholonomic constraint system in its Riemann-Cartan configuration space, and how to obtain the first integrals of the motion equations in the Riemann-Cartan configuration space by the improved field method. This is an effective method to study some nonholonomic nonconservative motions.
Theoretical models and experiments for the time-space characteristics of internal waves generated by towed bodies
In this paper, we perform experiments on the time-space characteristics of internal waves generated by horizontally towed bodies with three aspect ratios in a stratified fluid with a halocline. By the real-time measurements of conductivity probe arrays which are arranged symmetrically in the transverse section of the stratified fluid tank, it is shown that the transition between the body-generated internal wave and the wake-generated internal wave is related to a critical Froude number Frc, which is linearly dependent on the aspect ratio. For Fr<Frc, the correlation velocities of internal waves are consistent with the towing speeds of the towed bodies, indicating that such internal waves in this range are dominated by the body-forced effect. The heights of such body-generated internal waves first increase with the increase of Fr until Fr reaches a certain value of Frp, which is also linearly dependent on the aspect ratio, and then decrease. For Fr>Frc, the correlation velocities of internal waves are noticeably lower than the towing speeds, indicating that such internal waves in this range are dominated by the wake-forced effect, and that the Froude numbers with respect to the correlation velocities of such internal waves vary in a range from 0.43 to 1.18. The heights of such wake-generated internal waves nearly linearly increase with Fr increasing regardless of the aspect ratio. Moreover, the patterns of body-generated waves are symmetric, while the patterns of wake-generated waves are not symmetric. Based on the experimental results and the equivalent source method which has been proposed to simulate the internal waves generated by a towed sphere, a new equivalent source method is developed to calculate the internal waves generated by towed slender bodies. For the body-generated waves, the method of designing the speed, length and diameter of the equivalent source is proposed. The symmetrical and anti-symmetrical equivalent source and their speed and size are also proposed for the wake-generated waves. The numerical results are in good accordance with the experimental results in the heights and patterns of waves, indicating that such a theoretical method and its parameter settings are reasonable and effective.
Modeling and experimental verification of surface dynamics of magnetic fluid deformable mirror
As a key component of the adaptive optics (AO) system,wavefront corrector plays a crucial role in determining the performance of the AO system.At present,the typical wavefront correctors,including solid deformable mirrors and liquid crystal spatial light modulators,have the common drawbacks of high cost of per actuator channel,and the relatively low stroke deflection (normally less than 50 μm) due to the limitation of material and manufacturing technology.In the face of the growing demand for deformable mirrors with large stroke,low power dissipation and low cost,the magnetic fluid based deformable mirror (MFDM) is proposed in this paper.The magnetic fluid has the characteristic of the fluidity of liquid and can be magnetized by an external magnetic field.Therefore,the surface deflection of the MFDM can be controlled by the surrounding magnetic field generated by an array of electromagnetic coils located underneath the magnetic fluid layer.Compared with the conventional deformable mirrors,the MFDM has the advantages of a continuous and smooth mirror surface,large shape deformation,low manufacture cost,and easy extension.The surface dynamics model of MFDM with a circular geometry has been studied previously in the literature.In the present paper, considering the possible applications in the wavefront control of rectangular laser beams,we study the MFDM with a rectangular array of actuators.
Firstly,based on the governing equations of the magnetic fluid,derived from the principles of conservation of fluid mass and magnetic field,the dynamics model of surface deflection of the rectangular MFDM is analyzed in Cartesian coordinates under the boundary condition of magnetic field and the kinematic conditions of magnetic fluid.The analytical solutions of the surface movement of the mirror subject to the applied currents in the electromagnetic coils are obtained by properly separating the variables with truncated model numbers.Secondly,based on the derived analytical model, the optimal design procedure for the structure and parameters of the MFDM to obtain the required performance,i.e. the largest stroke and inter-actuator stroke of the mirror,as well as the coupling coefficient of the influence function, is presented.The resulting surface response performance of the designed MFDM is validated by the co-simulation in MATLAB,COMSOL Multiphysics and Tracepro software.Finally,a prototype of square MFDM consisting of the square array of miniature electromagnetic coils,a Maxwell coil and the magnetic fluid filled in a rectangular container is fabricated for experimental evaluation.The experimental results of the surface response of the mirror subject to two adjacent active coils are first presented to validate the stroke performance and linear characteristics of the MFDM. A parabolic surface shape is then further produced in the AO setup system with the MFDM subject to the array of coils driven by the currents calculated from the analytical model.The experimental results verify the accuracy of the established dynamics model and show that the proposed MFDM can be used to effectively control the wavefront of laser beam.
Meso-scale transport characteristics and budget diagnoses of water vapor in binary typhoons
In this paper, we simulate a process of binary typhoon and the budget diagnoses of water vapor in this process to analyze the transportation characteristics of water vapor and their influences on the variation of typhoon intensity. The results show that the interactions between typhoon Fitow and surrounding systems, including subtropical high, mid-latitude trough, west of the continent high and Southeast trailing typhoon Danas, change the background wind fields of Fitow, and then adjust the transport channels of moisture. Those surrounding systems, especially the trailing typhoon Danas which can be called the “collection-transfer station” of water vapor, have important effects on the intensity maintenance and the northern strong precipitation in the offshore and landing period of Fitow. The distribution and evolution of water vapor flux convergence band are consistent with those of strong convection band, revealing that the water vapor transport has important influences on the structure and intensity of the inner-core convection band in typhoon. The budget results show that the time series of total water vapor flux and typhoon intensity change synchronously. And the eastern boundary is the main source of water vapor transport, and the southern and northern boundary are also important, while the western boundary makes a negative contribution. The inflow transport channel is mainly located at the bottom of the troposphere, while the outflow transport area of water vapor is located at middle- and low-level troposphere of western boundary. The vertical transportation of water vapor plays an important role in redistributing the internal moisture of typhoon. The duration of sever convection band in typhoon is accompanied by the strong vertical transport of water vapor, which indicates that the vertical transport of water vapor is important for developing the strong convection in “U” and “V” type typhoon.
Simulation analysis of aerosol effect on shortwave infrared remote sensing detection of atmospheric CO2
The research of carbon dioxide (CO2) sources and sinks within the carbon cycle is significant for enhancing our understanding of global climate change. Space based measurement of CO2 concentration in lower atmosphere by reflected sunlight in near infrared (NIR) band has become a hot research topic at present. The global characteristic of atmospheric CO2 retrieval from NIR is studied using the expected measurement performance of Tansat (Tan Satellite) mission. With the development of CO2 retrieval algorithms, the light-path modification due to multiple scattering by aerosol is identified as a major source of error when retrieving CO2 from high resolution near-infrared spectrum. The present study focuses on atmospheric CO2 retrieval sensitivity to aerosol properties such as aerosol types, aerosol modes, and profiles aiming at the demands for retrieval accuracy of CO2 no larger than 0.3%-0.5% on a regional scale. Here, we carry out the aerosol scattering effects analysis on retrieving atmospheric CO2 near 1610 nm using the simulated nadir observation for Tansat based on CALIPSO aerosol profile products and SCIATRAN radiative transfer model. The results show that light path modification due to aerosol scattering is closely related to their types, modes and vertical distributions. For aerosol types, on the one hand, urban aerosol has the most significant influence on the measured radiance, followed by maritime aerosols, and has a much smaller influence for rural aerosol, which will lead to overestimated CO2 concentration for the typical surface albedo. On the other hand, the measured radiance will decrease with the increase of aerosol optical thickness (AOT) for urban and rural aerosols, but exactly the opposite to maritime aerosols. For aerosol modes and vertical distributions, aerosols in accumulation mode, the continental aerosols with multilayer aerosol vertical distribution and maritime aerosols with AOT less than 0.3 will bring about less than 5% of negative radiance changes, and will cause positive changes with the increase of AOT. However, aerosols in coarse mode will always cause negative changes of radiance regardless of aerosol vertical distribution, and thus resulting in an overestimation of CO2. In addition, the higher the aerosol layer distributed, the smaller the negative radiance change is. If aerosol profiles can be successfully retrieved as a state vector, then it can be expected that satellite measurement can lead to tremendous improvement in CO2 retrieval precision. This study provides important information about estimations of the influence of aerosol property on CO2 retrieval algorithm. All these results can contribute to improving the accuracy of CO2 retrieval.