Vol. 67, No. 8 (2018)
2018, 67 (8): 080501. doi: 10.7498/aps.67.20171952
We use a discrete Boltzmann model (DBM) to simulate the multi-mode Rayleigh-Taylor instability (RTI) in a compressible flow.This DBM is physically equivalent to a Navier-Stokes model supplemented by a coarse-grained model for thermodynamic nonequilibrium behavior.The validity of the model is verified by comparing simulation results of Riemann problems,Sod shock tube,collision between two strong shock waves,and thermal Couette flow with analytical solutions.Grid independence is verified.The DBM is utilized to simulate the nonlinear evolution of the RTI from multi-mode initial perturbation with discontinuous interface.We obtain the basic process of the initial disturbance interface which develops into mushroom graphs.The evolution of the system is relatively slow at the beginning,and the interface moves down on a whole.This is mainly due to the fact that the heat transfer plays a leading role,and the exchange of internal energy occurs near the interface of fluid.The overlying fluid absorbs heat,which causes the volume to expand,and the underlying fluid releases heat,which causes the volume to shrink,consequently the fluid interface moves downward.Meanwhile,due to the effects of viscosity and thermal conduction,the perturbed interface is smoothed.The evolution rate is slow at the initial stage.As the modes couple with each other,the evolution begins to grow faster.As the interface evolves gradually into the gravity dominated stage,the overlying and underlying fluids begin to exchange the gravitational potentials via nonlinear evolution.Lately,the two parts of fluid permeate each other near the interface.The system goes through the nonlinear disturbance and irregular nonlinear stages,then develops into the typical “mushroom” stage.Afterwards,the system evolves into the turbulent mixing stage.Owing to the coupling and development of perturbation modes,and the transformation among the gravitational potential energy,compression energy and kinetic energy,the system first approaches to a transient local thermodynamic equilibrium,then deviates from it and the perturbation grows linearly.After that,at the beginning,the fluid system tends to approach to an equilibrium state,which is caused by the adjustment of the system,and the disturbance of the multi-mode initial interface moves toward a process of the eigenmode stage.Then,the system deviates from the equilibrium state linearly,which is due to the flattening of the system interface and the conversing of the compression energy into internal energy.Moreover, the system tends to approach to the equilibrium state again,and this is because the modes couple and the disturbance interface is further “screened”.The system is in a relatively stable state.Furthermore,the system is farther away from the equilibrium state because of the gravitational potential energy of the fluid system transformation.The compression energy of the system is released further,and the kinetic energy is further increased.After that,the nonequilibrium intensity decreases,and then the system is slowly away from thermodynamic equilibrium.The interface becomes more and more complicated,and the nonequilibrium modes also become more and more abundant.
2018, 67 (8): 080503. doi: 10.7498/aps.67.20172718
For matching lattice parameters, AlGaAs alloy is usually grown on a GaAs (001) substrate. The AlGaAs/GaAs multilayer structure has been widely used to manufacture various photoelectric and electronic devices. The practical importance of atomic flat surfaces lies in improving the performances of modern optoelectronic devices based on AlGaAs/GaAs multilayer structure. The influence of temperature on the flatness of the film has not been analyzed in detail, so it is very important to prepare the surface at an atomic level by adjusting annealing temperature. In this paper, 15 ML Al0.17Ga0.83As are deposited on an n-doped GaAs (001) substrate by the molecular beam epitaxy (MBE) technique. We study the effects of various annealing temperatures (520℃, 530℃, 540℃) on the flattening of Al0.17Ga0.83As/GaAs (001) surface under the same condition of arsenic BEP about 1.210-3 Pa, annealing time 60 min and growth rate (0.17 ML/s). The (1000 nm1000 nm) scanning tunneling microscope (STM) images and Fourier transform graphs are obtained to show the evolution of surface morphology. In a temperature range of 520-530℃, island is ripening, the coverage of the island increases, the pit also begins to merge into a larger pit; when the temperature exceeds 530℃, the increasing of ripening rate leads to a big island and the pit turns into terrace, while the coverage of island and the pit gradually decreases. In the annealing process, the area of terrace increases and gradually approaches to 100%. By quantitatively analysing the coverage of pit (island, terrace) and root mean square (RMS) roughness varying with the annealing temperature, a 545℃ (1℃) better annealing temperature is proposed by fitting the curve of RMS roughness variation. At the same time, the film annealing model is analyzed in this paper. Comparing the results in the literature with our experimental data, it is found that the change of annealing temperature can influence the number of active atoms, in which the ratio of annealing atoms contributing to surface flattening () should be proportional to the annealing temperature. According to the experimental results, Al0.17Ga0.83As surface basically presents the flat morphology with 60 min annealing at 540℃ when 0.20 0.25. When the annealing temperature reaches 545℃, we can also speculate that the annealing time is about 55-60 min. This is consistent with our previous conclusion. It should be pointed out that our experiment avoids metallizing the film surface caused by the anti-evaporation of the atoms and the metal gallium atoms climbing on the surface of the film when the annealing temperature is too high. The experimental results are applicable to the Al0.17 Ga0.83As thin film growth and annealing.
Chaotic time series prediction based on brain emotional learning model and self-adaptive genetic algorithm
2018, 67 (8): 080502. doi: 10.7498/aps.67.20172104
Chaos phenomenon is one of the most important physical phenomena, which has significant effects on one's production and life. Therefore, it is indispensable to find out the regularity of chaotic time series from a chaotic system for weather forecasting, space missions, alarm systems, etc. Although various models and learning algorithms have been developed to predict chaotic time series, many traditional methods suffer drawbacks of high computational complexity, slow convergence speed, and low prediction accuracy, due to extremely complex dynamic characteristics of chaotic systems. In this paper, a brain-inspired prediction model, i.e., brain emotional learning (BEL) model combined with self-adaptive genetic algorithm (AGA) is proposed. The establishment of BEL model is inspired by the neurobiology research, which has been put forward by mimicking the high-speed emotional learning mechanism between amygdala and orbitofrontal cortex in mammalian brain, it has advantages of lowcomputational complexity and fast learning. The BEL model employs reward-based reinforcement learning to adjust the weights of amygdala and orbitofrontal cortex. However, the reward-based method is modelsensitive and hard to generalize to other issues. To improve the performance of BEL model, AGA-BEL is proposed for chaotic prediction, in which the AGA is employed for parameter optimization. Firstly, weights and biases of orbitofrontal cortex and amygdala in BEL model are distributed to chromosomal gene sequence for optimization. Secondly, fitness function is employed to adjust the weights of amygdale and orbitofrontal cortex by self-adaptive crossover and mutation operations Therefore, the parameter optimization problem is transformed into a function optimization problem in the search space. Finally, the best chromosome that represents the best combination of weights and biases for BEL model is chosen, which is used for chaotic prediction. Prediction experiments on the benchmark Lorenz chaotic time series and a real-world chaotic time series of geomagnetic activity Dst index are performed. The experimental results and numerical analysis show that the proposed AGA-BEL prediction model achieves lower mean absolute deviation, mean square error, mean absolute percentage error, and higher correlation coefficient than the original BEL, levenberg marquardt-back propagation (LM-BP) and multilayer perceptron-back propagation (MLP-BP). Meanwhile, the BEL-based models take less computational time than the traditional BP-based models. Therefore, the proposed AGA-BEL model possesses the advantages of fast learning and low computational complexity of BEL model as well as the global optimum solution of AGA. It is superior to other traditional methods in terms of prediction precision, execution speed, and stability, and it is suited for online prediction in fast-varying environments.
Optical frequency comb active filtering and amplification for second cooling laser of strontium optical clock
2018, 67 (8): 080601. doi: 10.7498/aps.67.20172733
In this paper, we propose an optical frequency comb active filtering and amplification method combined with injection-locking technique to select and amplify a single mode from a femtosecond mode-locked laser. The key concept is to optically inject an optical frequency comb into a single mode grating external cavity semiconductor laser. The optical frequency comb based on a femtosecond mode-locked laser with a narrow mode spacing of 250 MHz is used as a master laser. The center wavelength of the optical frequency comb is 689 nm with a 10 nm spectral width. A single mode grating external cavity semiconductor laser with a grating of 1800 lines/mm is used as a slave laser, and the external-cavity length from the diode surface to the grating is approximately 50 mm. The master laser is injected into the slave laser, and in order to select a single comb mode, we adjust the power of the master laser to control the locking range of the slave laser whose linewidth is smaller than the optical frequency comb repetition rate (250 MHz). While the operating current of the slave laser is set to be 55 mA and a seeding power is adopted to be 240 W, a single longitudinal mode is selected and amplified from 2.5104 longitudinal modes of the femtosecond optical comb despite the low power of the single mode. By tuning the optical frequency comb repetition frequency, the single longitudinal mode follows the teeth of the femtosecond optical comb, indicating the success in the optical frequency comb active filtering and amplification. The locking range is measured to be about 20 MHz. Meanwhile, the repetition frequency of the optical frequency comb is locked to a narrow linewidth 698 nm laser system (Hz level), thus the slave laser inherits the spectral characteristics of the 698 nm laser system. The linewidth is measured to be 280 Hz which is limited by the test beating laser. Then a continuous-wave narrow linewidth 689 nm laser source with a power of 12 mW and a side-mode suppression ratio of 100 is achieved. This narrow linewidth laser is used as a second-stage cooling laser source in the 88Sr optical clock, the cold atoms with a temperature of 3 K and a number of 5106 are obtained. This method can also be used to obtain other laser sources for atomic optical clock, and thus enabling the integrating and miniaturizing of a clock system.
One-dimensional magneto-hydrodynamics simulation of magnetically driven solid liner implosions on FP-1 facility
2018, 67 (8): 080701. doi: 10.7498/aps.67.20172300
As an important cylindrical-convergent drive technology, magnetically driven solid liner implosion has been widely used in the high energy density physics (HEDP) experiments for different researches, such as the properties of condensed matter at an extreme pressure, the hydrodynamic behaviors of imploding systems, and the properties and behaviors of dense plasmas. On the 2.2 MA FP-1 facility (with a rise time of 7 s), implosions of aluminum liners and their impact on target liners are studied experimentally for exploring the applications of instability and ejecta mixing. A one-dimensional Lagrangian codeMADE1D is developed to study liner implosions numerically, which is based on magneto-hydrodynamics model with material strength, wide-range equation of state, Lee-More conductivity, and SCG (Steinberg, Cochran and Guinan) constitutive model. The code is based on the finite difference method. The finite difference equations are written in the covariant form for both Cartesian and cylindrical coordinates which enables the accurate simulation of different load geometries. Numerical results, such as the simulated velocity and radius at inner surface of the liner and target, agree well with the measurements. It shows that FP-1 has the ability to accelerate a 0.5 mm thick aluminum liner with an initial radius of 1.5 mm to a speed of more than 1.1 km/s, and the corresponding velocity of inner surface is more than 1.5 km/s due to the cylindrical convergence effect. In our calculation, most of the liner keeps solid throughout the implosion, though its outer surface is melted due to the Ohmic heating. A cylindrical converging shock about 8-10 GPa can be obtained by setting a target with an initial radius of 8-11 mm inside the liner coaxially. The numerical results show that since the imploding liner is fully magnetized when it impacts the target, the shock and the corresponding reflect release wave run faster than in the unmagnetized target. This means that the target will spall near the liner-target interface, though they are impedance-matched acoustically. The movement of the shocked target can be affected by the pre-filled gas inside. Increasing the gas pressure makes the target lose its velocity quickly, and the rebound radius increases as well. By adjusting the load design and gas pressure appropriately, we can obtain the right implosion process to meet the study requirement.
ELECTROMAGNETISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
2018, 67 (8): 084203. doi: 10.7498/aps.67.20171894
Rare earth doped upconverting micro/nanoparticles with controlled size and structure,which are excited by near-infrared light and emit the visible light,possess many applications especially in the areas of biomedicine and photonics devices.There is no universally favored spectral profile in a variety of specific applications.We expect upconversion (UC) nanoparticles with the tunable spectral behavior to meet the demand for actual applications.Although the UC emission wavelengths are strictly limited by the electronic structure of the dopant,the spectral profile could be varied by many factors such as the structure,size,and crystallization. Varying matrix host is the most convenient approach to dynamically tuning UC that is essential for a variety of studies.However,this approach suffers a significant constraint due to insensitive response of most dopant luminescence centers to matrix host.In this paper,a facile EDTA-assisted hydrothermal approach is developed to the shape-selective synthesis of fluoride microcrystals including NaYF4 rods,LiYF4 octahedrons,and YF3 cuboid brick,by only tuning the pH of the mother liquid.The UC spectra of a series of Yb3+/Er3+-doped fluoride particles with the different shapes and phases are investigated in detail under a near-infrared co-focused laser excitation.The effects of matrix hosts on UC luminescence attributed to the 4f-4f transitions of the Er3+ ions in a single particle are amplified through elevating Yb3+ concentration.The associated tuning mechanisms are explored by using the power dependent UC luminescence and the temporal evolutions of up/down-conversion emission spectra. Mechanistic investigation reveals that the sensitive response of Er3+ UC emission to matrix host stems from maximal use of the various channels populated luminescence levels.It is well known that the population and depopulation of the luminescence levels strongly depend on the excitation power density,the energy level structure of electron,the ratio of the population ions between the two levels,maximum phonon energy and phonon density.The matrix plays the most important role in both the population and depopulation of the luminescence levels mediated by modifying the radiation relaxation probability and non-radiation relaxation probability via varying lattice symmetry and phonon energy.However,the fine modification of the matrix by doping is not always effective to luminescence tuning.In the current study,comparing with LiYF4 and YF3 matrixes,it is interestingly found that NaYF4 matrix can effectively tune the intensity ratio of red to green luminescence from 0.48 to 6.11 by varying Yb3+ concentration from 0 to 98% particle.The result indicates that the multiple aspects in the UC process could be influenced by Yb3+ doping NaYF4 matrix structure.We believe that Yb3+/Er3+ codoped NaYF4 matrixes with various Yb3+ concentrations will result in applications in displays,biological imaging,chemical sensing and anticounterfeiting.
2018, 67 (8): 084301. doi: 10.7498/aps.67.20171963
It can be a difficult problem to precisely predict the sound field radiated from a finite elastic structure in shallow water channel because of its strong coupling with up-down boundaries and the fluid medium, whose sound field cannot be calculated directly by current methods, such as Ray theory, normal mode theory and other different methods, which are adaptable to sound fields from idealized point sources in waveguide. So far, there is no reliable prediction method to solve this kind of problem. A combined wave superposition method is proposed for such a problem, which combines the traditional wave superposition method with the transfer function in shallow water channel and the multi-physics field coupling numerical model. This method mainly consists of three sections:1) obtaining the normal velocity on the elastic structure surface in shallow water channel by the finite element method (FEM), whose FEM model includes the up-down boundaries and the completely absorbent sound boundaries in the horizontal direction; 2) getting the equivalent point source strength by traditional wave superposition method; 3) calculating the total sound field by adding up each point source field which is obtained by normal mode method. This method is verified by numerical simulation and theoretical analysis by using an imaginary and elastic spherical sound source respectively, and the results demonstrate that the method is valid and has high precision and calculating efficiency. The acoustic radiation characteristics from elastic cylindrical shells is investigated for different acoustic radiation sources, ocean environments and measurements. The cylindrical shell material is steel, whose radius and length are 3 m and 30 m respectively. The shallow water channel is an ideal waveguide with 50 m in depth, at the upper boundary, i.e., the free surface, the lower boundary is the Neumann boundary, i.e., the normal derivative of the acoustic pressure should be zero. The analysis frequency range is from 30 Hz to 200 Hz. The results show that due to a significant coupling effect of up-down direction boundaries on the sound field, the elastic structure can be equivalent to the point source only in low frequency and far field. The spatial field directivity distribution is more obvious at high frequency. The acoustic power measured by vertical line arrayis greatly influenced by ocean boundary and the depth of target.
Semi-analytical research on acoustic-structure coupling calculation of partially submerged cylindrical shell
2018, 67 (8): 084302. doi: 10.7498/aps.67.20172681
Vibroacoustic analysis of a partially submerged cylindrical shell-liquid coupling system is a typical acoustic-structure interaction problem in an acoustic half-space. Generally, the vibration and acoustic solutions of this problem almost depend on numerical method in previous studies. However, whether from the aspect of verifying the numerical methods or from the aspect of revealing the vibroacoustic mechanism of the acoustic-structure interaction system, the development of an analytical or semi-analytical method is indispensable. In this study, a semi-analytical method is proposed to address the vibroacoustic response of a horizontal cylindrical shell partially immersed in water The acoustic coordinate system is established on the free surface, and the sine series function is introduced to express the sound pressure to meet the pressure release boundary condition on the free surface automatically. Then based on the two-dimensional Flgge shell theory, the motion equation of the shell-liquid coupling system is established with using the center of the shell as the origin of the coordinate system. By using the Galerkin method, the velocity continuous condition on the fluid-structure interface is established. Then the relation matrix between the acoustic pressure amplitude and the shell displacement amplitude is derived after converting the sound pressure from different coordinate systems into that in the same coordinate system by using the geometry relationship of two coordinate systems. Then the vibration and sound radiation of this coupling system are predicted by solving the coupled matrix equations, and the coupled free vibration can also be addressed by solving the characteristic equation after setting the determinant of the coefficient matrix to be zero. To verify the accuracy and reliability of the present method, the coupled natural frequencies, the vibration response and the distribution of the sound pressure obtained from the present method are compared with those obtained from the finite element method, showing that they are in good agreement with each other. Then the shapes of the first four order modes for the coupled system are presented and compared with those of the system submerged in unbounded fluid field, and the couple between different modes, and the couple between the symmetric and antisymmetric modes are observed due to the effect of the free surface. The characteristics of the root mean square velocity of the shell with different immersed depths are discussed, which reveals that the peaks of response curves shift to the lower frequencies with increasing immersed depth. The characteristics of far field sound pressure directivity are presented and explained by the image method in detail. This study provides a novel method to analytically predict the vibroacoustic response of an elastic structure partially coupling with the fluid field when bounded in a sound half space.
2018, 67 (8): 084701. doi: 10.7498/aps.67.20172706
Molecular dynamics method is used to investigate gas flows in nanoscale channels. A set of Couette gas flows with the same Knudsen number but different channel heights and densities is simulated to study the dimensional effects on dynamically similar flow conditions. Results show that the gas flow in the channels is divided into two regions:near wall region affected by a wall force field and bulk flow region affected by no wall force field. The flow characteristics in the bulk flow region are in good accordance with the kinetic theory predictions, which are characterized by constant density, normal stress, shear stress and linear velocity distribution while within the near wall region, the velocity, density, normal stress and shear stress distributions exhibit deviations from the kinetic theory predictions. The density and velocity sharply increase, accompanied with a single peak appearing. The normal stress which is dominated by the surface virial is anisotropic and changes drastically. Shear stress value is constant in bulk flow region and part of the near wall region, while the surface virial induces variation at a place about one atom diameter far from the wall. In the near wall region, the normalized density, velocity and stress tensor are constant under different channel heights and densities, which indicates that the gas flow characteristics in this area are determined by the wall force field. Besides, the tangential momentum accommodation coefficient (TMAC) values for different cases can be obtained through the relationship between TAMC and shear stress. It is found that under the same Knudsen number, the TMAC remains constant no matter what the height and density are. Furthermore, another set of Couette gas flows with different gas-surface potential strength ratios but the same channel height and density is simulated to study the gas-surface interaction effects on nanoscale gas flow. The results show that the gas density and velocity in the near wall region increase with increasing potential strength ratio between wall atoms and gas molecules. Large potential strength ratio cases (C 3.0) result in velocity sticking on the surface, which is induced by the gas molecule accumulation and surface adsorption. Using the same approach, the TMAC values for various potential strength ratios are calculated, varying from 0.63 to 0.96 for different cases (C=0.5-4.0), which indicates that the stronger the potential energy acting on the gas molecules, the more easily the gas molecules generate the diffuse reflection on the walls
Fast solution of near-field time reversal electromagnetic field of sub-wavelength perfect conducting ball arrays
2018, 67 (8): 084101. doi: 10.7498/aps.67.20172508
To solve the near-field time reversal electromagnetic fields of sub-wavelength perfect conducting ball arrays rapidly, an analytical formulation is presented based on the equivalent dipole model. As is well known, the efficient use of evanescent information is the key to the realization of sub-wavelength focusing and imaging. However, evanescent components always suffer exponential decays with the increase of propagating distance. Therefore, in order to effectively control the evanescent waves, some measures should be taken in the near field region of the scatters before their amplitudes are reduced to an undetectable level. Since small perfect conducting ball is the basic component of large scatter, the first step should be to study the scattering properties of small perfect conducting ball. The far-field scattering fields of perfect conducting balls have been analyzed for plane waves. However, for spherical waves, the analytical results are not convenient to extend to multi-ball situation since they are all expressed by series. In this paper, the analytical solution to scattering field of the small perfect conducting balls irradiated by spherical radiative waves is analyzed. The result shows that the scattering fields can be approximately equivalent to the superposition of the radiation fields of electrical and magnetic dipoles in some restrictive conditions. The intensity of the equivalent dipole is proportional to the magnitude of the original excitation source dipole. Therefore all the equivalent dipole moments can be calculated easily by setting up the coupling equations between different equivalent dipoles and source dipole. Then, the forward dyadic Green's function can be obtained by combining the vacuum electrical and magnetic Green's function. At the same time, the time reversal dyadic Green's function can be derived through the time reversal cavity theory. Afterwards, the near-field time reversal electromagnetic field of the perfect conductive ball arrays can be calculated directly by the time reversal dyadic Green's function. The results obtained from the proposed method and a numerical software are compared, which shows that a coincidence extent reaches more than 0.95. This confirms the correctness and high efficiency of the proposed method. After that, an imaging experiment is implemented and the result shows that an imaging resolution of 0.3 can be obtained by loading small conducting balls in the near field. All these experiments show that combined with near-field loading of sub-wavelength scatterer arrays, the time reversal technique has the potential to realize super-resolution focusing and imaging.
2018, 67 (8): 084102. doi: 10.7498/aps.67.20172413
China Academy of Engineering Physics terahertz free electron laser (CAEP THz FEL,CTFEL) is the first THz FEL oscillator in China,which is jointly built by CAEP,Peking University and Tsinghua University.It is designed as a high-repetition-rate and high-duty-cycle linac-based FEL facility. This THz FEL mainly consists of a gallium arsenide (GaAs) photocathode high-voltage direct current (DC) gun,a superconducting radio frequency (RF) linac,a planar undulator,and a quasi-concentric optical resonator. The DC gun provides a high-brightness electron beam with the bunch charge of about 100 pC and the repetition rate of 54.167~MHz.The normalized emittance of the electron beam is less than 10m,and the energy spread is less than 0.75%.A 24-cell superconducting RF accelerator provides an effective field gradient of about 10 MV/m and energizes the electron beam to 6-8~MeV.The beam then goes through the undulator and generates the spontaneous radiation,which is reflected back and forth in the optical resonator and then stimulated by the electron beam. The first stimulated saturation of CTFEL in the macro-pulse mode was obtained in August,2017.In this paper,the THz spectrum is measured by a Fourier spectrometer (Bruker VERTEX 80 V).The macro-pulse energy is measured by an absolute energy meter from Thomas Keating Instruments.The longitudinal beam length is preliminarily calculated by the auto-correlation curve from the time-domain signal of the spectrometer.The macro-pulse duration is captured by a GeGa cryogenic detector from QMC Instrument.The measurement results indicate that the terahertz laser frequency is continuously adjustable from 2 THz to 3 THz.The macro-pulse average power is more than 10 W and the micro-pulse power is more than 0.3 MW.The single-pass gain is larger than 2.5%. This facility is now working in macro-pulse mode in the first step,also called step one.The minimum macro-pulse duration is about 50s and the maximum is about 2 ms.The macro-pulse repetition is 1 Hz or 5 Hz.The typical pulse duration and repetition rate are 1 ms and 1 Hz,respectively.In the middle of 2018,the duty cycle will upgrade to more than 10% as step two.And the continuous wave (CW) operation will be obtained in step three by the end of 2018.The spectrum adjustment range will also be expanded to cover from 1 THz to 4 THz by then. Some application experiments have been carried out on the platform of CTFEL.This facility will greatly promote the development of THz science and its applications in material science,chemistry science,biomedicine science and many other cutting-edge areas in general.
Detection of nitrous oxide by resonant photoacoustic spectroscopy based on mid infrared quantum cascade laser
2018, 67 (8): 084201. doi: 10.7498/aps.67.20172696
Atmospheric greenhouse gases have great influence on the climate forcing, which is important to human being and also for natural systems. Nitrous oxide (N2O), such as carbon dioxide and methane, is an important greenhouse gas. It plays an important role in the atmospheric environment. Therefore, sensitive measurement of N2O concentration is of significance for studying the atmospheric environment. In this paper, a photoacoustic spectroscopy (PAS) system based on 7.6 m mid infrared quantum cascade laser combined with resonant PAS technique is established for the sensitive detection of N2O concentration. The PAS has been regarded as a highly sensitive and selective technique to measure trace gases. Compared with laser absorption spectroscopy, the PAS offers several intrinsic attractive features including ultra-compact size and no cross-response of light scattering. In addition, the signal of PAS is recorded with low-cost wavelength-independent acoustic transducer. The performance of the developed system is optimized and improved based on the traditional photoacoustic spectroscopic detection. Dual beam enhancement method is used to increase the effective optical power which effectively improves the detection sensitivity of the system. The N2O absorption line at 1307.66 cm-1 is chosen as the target line, and an operation pressure of 50 kPa is selected for reducing cross-talking from H2O absorption line. By detecting the photoacoustic signals of a certain concentration of N2O at different modulation frequencies and modulation amplitudes, the optimal modulation frequency and modulation amplitude of the system are determined to be 800 Hz and 90 mV, respectively. Different concentrations of N2O gas are detected under the optimized parameters, and calibration curve of the system, that is, the curve of photoacoustic signal versus concentration of N2O is obtained, which shows good linearity. The experimental results show that the minimum detection limit of the system is 150 ppb at a pressure of 50 kPa with an integration time of 30 ms. The system noise can be further reduced by increasing the averaging time. A minimum detection limit of 37 ppb is achieved by averaging signals 100 times, and the signal of N2O in the atmosphere is obtained.
2018, 67 (8): 084202. doi: 10.7498/aps.67.20172262
In order to verify the feasibility of three-dimensional (3D) printing technology in preparing the metamaterial absorbers with complex structure, a three-layer broadband absorbing metamaterial is designed and fabricated by 3D printing technology. The surface layer and middle layer of the metamaterial are composed of periodic arrays with different unit dimensions and the bottom layer of a slab structure. The optimized thickness of the metamaterial is 4.7 mm. A composite absorbent which consists of carbonyl iron powder and nylon is used to fabricate the absorber. In experiment, the obtained absorber is vertically irradiated by an electromagnetic (EM) wave. Two strong absorption peaks at 5.3 GHz and 14.1 GHz are achieved, with the reflection losses of -15.1 dB and -12.5 dB, respectively. The superposition of the two absorption peaks results in a reflection loss below -10 dB in a range from 4 to 18 GHz. The effective EM parameters of the surface layer and the middle layer are calculated by the S parameter inversion method. An effective model of the three-layer structure absorber is proposed and its reflectivity is calculated by using a multilayer structure reflectivity formula. The calculated reflectivity agrees well with the measured one. The absorbing and resonance mechanisms of the two absorption peaks are investigated by analyzing the dynamic distributions of power density loss, electric field and magnetic field. It can be clearly confirmed that the reflection losses at 5.3 GHz and 14.1 GHz are primarily concentrated on the bottom layer and surface layer, and the broadband absorption performance can be derived from the superposition of broadband absorptions of the three absorbing layers. Meanwhile, the strong electric coupling effect between the adjacent units in the surface layer is demonstrated by analyzing the electric-field distributions, which indicates that the strong reflection loss at 14.1 GHz is mainly caused by the electric response. The multiple scattering effects among the three layers are also considered according to the magnetic field distributions at two resonance frequencies. It is shown that there are two magnetic responses at 5.3 GHz and 14.1 GHz, respectively, and the multiple scattering contributes to increasing the EM wave propagation distance and enhancing the power loss. The designed absorbing metamaterials in this paper achieve good broadband absorption performances, particularly in the low frequency band. Combined with 3D printing rapid technology, a promising route to constructing 3D absorbing metamaterials with complex structures is proposed, which would be of great significance and broad practical prospect.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2018, 67 (8): 085203. doi: 10.7498/aps.67.20180029
CsI photocathode is widely applied to high energy X-ray detection. And the spectral response is an important character of CsI photocathode. In this paper, the interaction process of high energy X-ray with CsI is analyzed and the spectral response of CsI photocathode is calculated in a 10-100 keV range. The influences of Compton scattering, X-ray fluorescence radiation and Auger emission on the spectral response are analyzed in accordance with the physical process of high energy X-ray interaction with CsI photocathode. These influences prove to be negligible in comparison with photo-ionization influence. Thus only the photoelectric transition is taken into account in calculation. According to the analyses of the processes of the photoelectron creation, transition and escaping, the formula for CsI spectral response is deduced as a function of secondary electron mean escape depth and photocathode thickness. The formula of secondary electron mean escape depth is then deduced as a function of X-ray energy. These formulae indicate that the mean escape depth of the secondary electrons increases markedly with the rise of X-ray energy and has a remarkable influence on the CsI spectral response. The spectral responses for different CsI thickness values are then calculated in a range of 10-100 keV. The results show that 1000 nm CsI has the best response under 20 keV, while 10000 nm CsI has a higher response over 60 keV. Then the calculation data are compared with experimental data of Hara's and Khan's hard X-ray streak camera measurements. These data agree well with each other and prove that our calculation of CsI spectral response for high energy X-ray is reliable. The spectral responses to CsI thickness for 17.5 keV and 60 keV are also calculated and shown in figures. These calculation data match experimental data of Frumkin and Monte-Carlo simulation data of Gibrekhterman. The measurement error of Frumkin's experiment and the uncertainty of the secondary electron mean escape depth are considered to be the reasons for the deviations of calculation and experimental data. The figures of spectral responses to CsI thickness also reveal the optimal thickness values of CsI for different X-ray photon energies. It is shown that 1 m is the optimal thickness for 17.5 keV X-ray detection, and 10 m is optimal for 60 keV. Finally the spectral response of CsI photocathode in a 10-100 keV range is calculated and the formulae prove to be reliable. According to these formulae and calculations, the optimal thickness of CsI photocathode can thus be given for designing and optimizing the high energy X-ray imaging detectors.
2018, 67 (8): 085201. doi: 10.7498/aps.67.20171795
The interaction between light and plasma is one of the key problems in an inertial confinement fusion system. Some instability processes will occur when the energy of laser is absorbed by plasma. Because reducing the coherence of laser can significantly restrain the instability of the plasma, in practice, a continuous phase plate (CPP) is often used to generate the speckle and thereby to restrain the nonlinear effect in plasma. To clarify the working mechanism of CPP, the propagation characteristics of speckle field are studied in this paper. Since there are two different kinds of media in the light path, the statistical optics theory and the matrix optics method are combined to analyze the propagation characteristics of the speckle field in plasma. The ABCD matrix of the plasma is deduced. And then intensity distribution properties of the speckle filed in the plasma are calculated. Meanwhile, the autocorrelation length of the speckle field is calculated and the mechanism of the nonlinear restraint is explained. The results show that the speckle field is a paseudorandom field. It will bring a random phase disturbance to the wavefront in the propagation direction. It is very different form the ordinary Gaussian beam, the speckle filed has a limited longitudinal autocorrelation length. Though the propagation rule of the speckle field in plasma is similar to that in air, when the laser transmits into plasma, the coherence of the laser speckle weakens rapidly. The autocorrelation length of the speckle field in the plasma is shorter than that in air. Therefore, many kinds of nonlinear effects can be restrained when the speckle transmits into plasma. Specially, the autocorrelation length of the speckle is much shorter in the high density plasma. So the result of suppressing the nonlinear effect is better in plasma with high density than that with low density. This characteristic is very helpful in restraining the different nonlinear effects in plasma.
Numerical simulation on particle density and reaction pathways in methane needle-plane discharge plasma at atmospheric pressure
2018, 67 (8): 085202. doi: 10.7498/aps.67.20172192
Methane needle-plane discharge has practical application prospect and scientific research significance since methane conversion heavy oil hydrogenation is formed by coupling methane needle-plane discharge with heavy oil hydrogenation, which can achieve high-efficient heavy oil hydrogenation and increase the yields of high value-added light olefins. In this paper, a two-dimensional fluid model is built up for numerically simulating the methane needle-plane discharge plasma at atmospheric pressure. Spatial and axial distributions of electric intensity, electron temperature and particle densities are obtained. Reaction yields are summarized and crucial pathways to produce various kinds of charged and neutral particles are found out. Simulation results indicate that axial evolutions of CH3+ and CH4+ densities, electric intensity and electron temperature are similar and closely related. The CH5+ and C2H5+ densities first increase and then decrease along the axial direction. The CH3 and H densities have nearly identical spatial and axial distributions. Particle density distributions of CH2, C2H4 and C2H5 are obviously different in the area near the cathode but comparatively resemblant in the positive column region. The CH3+ and CH4+ are produced by electron impact ionizations between electrons and CH4. The CH5+ and C2H5+ are respectively generated by molecular impact dissociations between CH3+ and CH4 and between CH4+ and CH4. Electron impact decomposition between electrons and CH4 is a dominated reaction to produce CH3, CH2, CH and H. The reactions between CH2 and CH4 and between electrons and C2H4 are critical pathways to produce C2H4 and C2H2, respectively. In addition, the yields of electron impact decomposition reactions between electrons and CH4 and reactions between CH2 and CH4 account for 52.15% and 47.85% of total yields of H2 respectively.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
First-principle study on electronic structures, magnetic, and optical properties of different valence Mn ions doped InN
2018, 67 (8): 087501. doi: 10.7498/aps.67.20172504
InN,as an important Ⅲ-nitride,has high electron mobility and low electron effective mass,so it has a wide range of applications in optoelectronic devices,high-frequency high-speed devices,and high-power microwave devices.The Ⅲ-nitrides based dilute magnetic semiconductors (DMSs) can be developed by leveraging the existing fabrication technology for Ⅲ-nitride semiconductor electronic devices,leading to novel semiconductor spintronic devices with a multiplicity of electrical,optical,and magnetic properties.It has been reported that room temperature ferromagnetism exists in InN nanostructures and thin films as well as InN-based DMSs systems.However,the origin mechanism and the formation mechanism of ferromagnetism in these materials have not been fully understood.In Ⅲ-V compound semiconductors,the transition element Mn ions exist mostly in the form of Mn2+ valences while it is also possible for them to emerge in Mn3+ valence states under certain conditions.Although Mn2+ and Mn3+ valance states affect the physical properties of the doped semiconductor differently,there lacks in-depth understanding of such different effects resulting from Mn doping in InN. Under the framework of the density functional theory,in this paper we adopt the generalized gradient approximation (GGA+U) plane wave pseudopotential method to calculate the electronic structure,energy and optical properties of undoped InN and InN doped with three different orderly placeholders of Mn2+ or Mn3+ after geometry optimization.The conducted analysis shows that the system exhibits lower total and formation energies,and improved stability after Mn doping.Manganese doping introduces a spin-polarized impurity band near the Fermi level,and as a result the doped material system has obvious spin polarization.Doping with different valences of Mn ions lead to varying effects on the electronic structure and magnetic property of the material system.The analyses of electronic structure and magnetic property show that both the p-d exchange mechanism and the double exchange mechanism play important roles in the magnetic exchange of the doped system,and Mn3+ doping helps to push the Curie temperature above the room temperature.Comparing with the pure InN,the value of the static dielectric function of the doped system increases significantly.The present analysis concludes that the imaginary part of the dielectric function and the absorption spectrum of the doped system presents strong new peaks in the low-energy region due to the electronic transition associated with the spin-polarized impurity band near the Fermi level. Broadly,this work sheds new light on the microscopic mechanism for the magnetic ordering of Ⅲ-nitride based DMSs,and lays a foundation for developing the novel Ⅲ-nitride based DMSs and devices.
2018, 67 (8): 087901. doi: 10.7498/aps.67.20172740
The multipactor effect has to be dealt with seriously when designing and manufacturing high power microwave devices used in space, as it will cause inreversible damage to devices and hence the whole system fails to work. Lowering the secondary electron yield of device surface is an effective way to suppress multipactor effect, which can be realized by creating trapping structure or depositing nonmetallic materials with low secondary electron yield on the surface. However, these treatments will result in electrical performance changing even to an unacceptable extent in some cases. To solve this problem, the deposited materials with conductivity as good as metals' should be used, besides, they should be chemically inactive in air. Taking the above into account, the method of suppressing the secondary electron yield of silver plated surface of device by magnetron sputtering platinum is proposed and investigated in the present paper. Firstly, platinum film with a thickness of 100 nm is deposited on silver plated aluminum alloy substrates by magnetron sputtering, and secondary electron yields of substrates with and without deposited platinum film are measured with the bias current method. The experimental results indicate that the maximum value of secondary electron yield and the first cross energy of silver plated aluminum alloy sample are 2.40 and 30 eV, respectively. After depositing platinum film on sample, these values change to 1.77 and 70 eV, a reduction of 26% in maximum of secondary electron yield is achieved. Secondly, universal law, Vaughan model, Furman model and Scholtz model are used to fit the experimental data of secondary electron yield, and the results indicate that only Vaughan model accords well with the property of secondary electron yield in an energy range of 40-1500 eV, and corresponding parameters are also obtained. The Chung-Everhart model is used to fit the secondary electron spectrum curve, and the fitted work function is 9.9 eV. Finally, the simulation of multipactor threshold of Ku-band impedance transformer is carried out by using a software with utilizing the experimental data and fitted results of secondary electron emission of samples. The simulation results indicate that the multipactor thresholds by utilizing the data of samples with and without platinum are 7500 W and 36000 W, respectively, which means that the large increase of multipactor threshold of high power microwave device can be achieved by depositing platinum film on the surface. The method proposed and results obtained in the present work provide a reference not only for studying the secondary electron emission of metal, but also for suppressing the multipactor effect of high power microwave device.
2018, 67 (8): 087201. doi: 10.7498/aps.67.20180098
Spin noise spectroscopy (SNS) is a new kind of Faraday rotation technique, which does not need spin injection to generate polarized spin. This method uses a linearly polarized laser to detect the spontaneous spin fluctuation in a thermal equilibrium state. However, the signal of spontaneous spin fluctuation is so weak (~V) in the thermal equilibrium system that a big signal-noise ratio (SNR) is often demanded. Here, we report on the build-up and improvement of a spin noise spectrum measurement system. A home-made field-programmable gate array (FPGA) based data-acquisition card with real-time fast Fourier transform (DAC-FFT) is used to improve the SNR of the SNS measurement system. The reduction of intrinsic noise in the experimental system is discussed in detail. Both the dependence of background noise and the dependence of spin noise on the intensity of probe laser are analyzed. We find that the background noise is proportional to the intensity of the probe laser, while the spin noise signal shows square dependence on probe laser intensity. The spin noise indeed comes from the spontaneous spin fluctuation as experimentally confirmed via an acousto-optic modulator (AOM) inserted in the measurement system. The measurement performances of two FPGA based DAC-FFTs (the 8-bit FFTsDAC1 and the 12-bit FFTsDAC2, respectively) are compared. Several factors are found to affect the SNR of the system, including the measurement efficiency and the acquisition resolution. The FFTsDAC2 has longer single acquisition time and faster data transmission speed (with USB 3.0) than the FFTsDAC1, when the total measurement time is set to be the same, the effective measurement time realized in FFTsDAC2 is longer than in FFTsDAC1. With better measurement efficiency and sampling depth and longer single acquisition time, the FFTsDAC2 has a better SNR and finer frequency resolution with a much narrower full width at half maximum (FWHM) value. Moreover, the simulations of the measurement process show the effect of the single acquisition time on the FWHM of spin noise peak, further clarifying the reason why the spin noise spectrum measured by FFTsDAC2 is more accurate.
2018, 67 (8): 087902. doi: 10.7498/aps.67.20180079
Secondary electron emission (SEE), which is a frequent phenomenon in space high power microwave systems, is one of the basic inducement of multipactor in space microwave components. It is already verified that lowering SEE is an effective method to mitigate the undesirable effect. Metal black nanostructures have ever been reported to suppress SEE remarkably, however, the SEE characteristics of the gold nanostructures are rarely investigated. In this work, we use the thermal evaporation to fabricate the gold nanostructures under various evaporation gas pressures, and further analyze their SEE characteristics as well as energy distribution information. Experimental results reveal that the evaporation gas pressure determines the morphology of gold nanostructure, and the morphology dominates the SEE level of the gold nanostructure. To be specific, as the evaporation gas pressure rises, the porosity of the nanostructure increases and the SEE yield decreases. The energy distribution information indicates that the gold nanostructure just suppresses the true secondary electrons (TSEs) effectively. However, the effect of the nanostructure on the back scattered electrons (BSEs) is heavily dependent on the surface morphology. Specifically, the nanostructure fabricated at 70 Pa suppresses the BSEs weakly while the nanostructures fabricated at 40-60 Pa enhance the BSEs to some degree. To theoretically explain the experimental phenomena, we establish an equivalent model, which is made up of the periodical combination of a hemisphere and a composite groove, to imitate the fabricated gold nanostructure and simulate its SEE characteristics based on the SEE phenomenological probability model. Simulation results indicate that the hemisphere induces more TSEs and BSEs while the composite groove suppresses them, besides, the groove suppresses the TSEs much more remarkably than the BSEs. The SEE level of the nanostructure model is determined by the weighted average effect of both the hemisphere and the groove. The simulations qualitatively explain the experimental phenomena. This work in depth reveals the SEE mechanism for the gold nanostructures, and is of considerable significance for developing the low SEE surface on a nanometer scale in a space high power microwave-system.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2018, 67 (8): 088201. doi: 10.7498/aps.67.20180109
Single-stranded DNA binding proteins (SSBs) widely exist in different kinds of creatures. It can bind single-stranded DNA (ssDNA) with high affinity. The binding is sequence independent. SSB can also interact with different kinds of proteins, and thus leading them to work at the special sites. It plays an essential role in cell metabolism. E.coli SSB is a representative of SSB among all kinds of SSBs, it is a homotetramer consisting of four 18.9 kD subunits, the homotetramer is stable under low concentration. E.coli SSB has different binding modes under different salt concentrations (for example NaCl). When NaCl concentration is higher than 200 mM, E.coli SSB can bind 65 nt ssDNA, when NaCl concentration is lower than 20 mM, it can bind 35 nt ssDNA, and when the NaCl concentration is between 20 mM and 200 mM, it can bind 56 nt ssDNA. The characteristics of E.coli SSB are so attractive that a large number of researches have been done to distinguish its binding process. Earlier researchers tried to use stop flow technology to study the interaction between SSB and ssDNA in bulk. However, the high affinity between SSB and ssDNA makes this interaction too rapid to be observed at all, and the dissociate interaction even could not be measured. Single molecule technology which combines with low and accurate force offers researchers another way to achieve this goal. Some researchers observed the unwrapping phenomenon in an optical tweezers pulling experiment. However, they did not find the detailed process of binding or dissociation. In our work, we use a magnetic tweezer to pull the SSB/ssDNA complex and find a special phenomenon like double-state jump. Using the single molecule dynamics to analyse the data, we find that this phenomenon is the combination and dissociation between SSB and ssDNA. After comparing the pulling curve of ssDNA only and SSB/ssDNA complex, we find that the SSB binding process consists of two stages, one is rapid combination/dissociation under the action of a critical force; the other is continuous wrapping following the reduced force. According to Bell formula and SSB/ssDNA complex binding model, we obtain the interaction rate and free energy parameters under 0 pN, and we calibrate the free energy to obtain its continuous wrapping part, so we can obtain the whole free energy landscape and understand the binding process. Our analysis way is also applicable to the case of similar interactions to obtain their interaction details and free energy characteristics.
2018, 67 (8): 088202. doi: 10.7498/aps.67.20172748
Solid oxide fuel cells (SOFCs) have been attracting people's attention for their high energy conversion efficiency, good fuel compatibility, no precious metal catalysts, and pollution-free emissions. However, the high operating temperature (800-1200℃) of the traditional SOFC can reduce the long-term stability and cause the difficulties in either the selecting of material or the sealing of SOFC. Therefore, great efforts have been devoted to developing the intermediate temperature SOFC (IT-SOFC), which works at 600-800℃. In the IT-SOFC, the ionic conductivity of doped CeO2-based electrolyte has a significant advantage relative to that of the conventional yttria-stabilized zirconia (YSZ) electrolyte. For example, at 600℃, the ionic conductivity of Sm-doped CeO2 is 0.02 S/cm much higher than that of the traditional YSZ electrolyte (only 0.0032 S/cm). Therefore, the doped CeO2-based electrolyte is a very promising electrolyte for IT-SOFC.Recently, the co-doping of two different elements into CeO2 has become a hot research topic. The ionic conductivity of Sm3+ and Sr2+ co-doped CeO2 has proved to be nearly twice as high as that of Sm3+ doped CeO2 (SDC). However, the mechanism for the co-doping effect on the conductivity of CeO2 is not clear. In this paper, Sm3+ and Sr2+ co-doped CeO2 is systematically studied using the DFT+U method. The microscopic properties of the Sm3+ and Sr2+ co-doped CeO2 including electronic density of states, band structure, oxygen vacancy formation energy and oxygen vacancy migration energy and so on have been calculated and analyzed by comparing with those of the Sm3+ or Sr2+ single doped CeO2. The calculation results indicate that Sm3+ and Sr2+ co-doping has a synergistic effect on the performance improvement of CeO2-based electrolyte, which can not only suppress the electronic conductivity of doped CeO2 system, but also can reduce the oxygen vacancy formation energy on the basis of single doped CeO2. The existence of Sm3+ can help to reduce the trapping effect of Sr2+ on oxygen vacancies, meanwhile the addition of Sr2+ can further reduce the minimum oxygen vacancy migration energy on the basis of SDC. Calculations by the climbing image nudged elastic band (CINEB) method indicate that the oxygen vacancy migration energy of the co-doped system can reach as low as 0.314/0.295 eV, which is lower than the minimum oxygen vacancy migration energy of SDC. Our research reveals the synergistic mechanism for Sm3+ and Sr2+ co-doping effect on the conductivity of CeO2, which is of great instructive significance for the further research and development of other high-performance co-doped electrolyte materials in IT-SOFC.
2018, 67 (8): 088701. doi: 10.7498/aps.67.20172625
Image quality is seriously degraded when propagating through the turbid atmosphere. It is practical to characterize the degradation process in terms of modulation transfer function (MTF). The MTF can describe the effect of the turbid medium on imaging quantitatively in spatial frequency domain, including attenuation and multiple scattering. It is inherent property of the turbid medium. The whole spatial frequency characteristic of the turbid atmosphere MTF can be acquired through the equivalence principle, i, e., the equivalence between the MTF of a turbid medium and the transmitted radiance from the medium under isotropic diffuse illumination. In practice, the image quality is not only affected by the turbid medium MTF but also related tightly to the background radiation. The influence of scattered background radiation on imaging was almost not considered in the past when dealing with the imaging problem in the turbid atmosphere. In this paper, this issue is considered in detail. The analysis results demonstrate that the scattered background radiation increases the zero frequency component of image in spatial frequency domain. As a result, it degrades the image contrast seriously in spatial domain. Based on the optical model of image degradation in the atmosphere, the theoretical analysis is carried out to study the image quality degradation process in spatial frequency domain. The formalized MTF is proposed, which considers the effects of attenuation, multiple scattering and scattered background radiation by the turbid medium on image quality. The quantitative relation among the formalized MTF, turbid medium MTF and background radiation is confirmed. Image blur simulations show that the results from the formalized MTF are more consistent with actual scenes than results only from turbid medium MTF. Thus, the formalized MTF can describe the image degradation process through atmosphere comprehensively. The image restoration results indicate that the formalized MTF method performs better than dark channel prior method. In order to evaluate different image restoration methods effectively in spatial frequency domain, spectrum area (AS) is proposed. The AS is the area of middle-high frequency information of the region of interest in restored image. So AS can represent the scene details in the restored image. The higher the AS, the better the image quality is, which is demonstrated in this paper. In conclusion, the formalized MTF provides a more effective method for image quality analysis and assessment. Additionally, it also supplies a new standpoint for researching atmospheric degradation mechanism and correction method for imaging in turbid atmosphere. Then, AS can be an effective reference for correction to the method evaluation.
Enhancing resilience of interdependent networks against cascading failures under preferential recovery strategies
2018, 67 (8): 088901. doi: 10.7498/aps.67.20172526
Interdependent networks are extremely fragile because a very small node failure in one network would trigger a cascade of failures in the entire system. Therefore, the resilience of interdependent networks is always a critical issue studied by researchers in different fields. Existing studies mainly focused on protecting several influential nodes for enhancing robustness of interdependent networks before the networks suffer random failures. In reality, it is necessary to repair a failing interdependent network in time to prevent it from suffering total breakdown. Recent investigations introduce a failure-recovery model for studying the concurrent failure process and recovery process of interdependent networks based on a random recovery strategy. This stochastic strategy covers repairing a small fraction of mutual boundary nodes which are the failed neighbors of the giant connected component of each network, with a random probability of recovery . Obviously, the random recovery is simple and straightforward. Here, we analyze the recovery process of interdependent networks with two types of connectivity links, i.e., the first-type connectivity links and the second-type connectivity links, which represent the mutual boundary nodes(being also failed nodes) linked to survival nodes in current giant connected component, and linked to failed nodes out of current giant connected component in networks, respectively. We find that when mutual boundary nodes have more first-type connectivity links, the current giant connected component has higher average degree and immediately makes better interdependent network resilience, on the other hand, more second-type connectivity links generate more candidates during the recovery procedure, and indirectly make better system resilience. In short, two types of connectivity links of mutual boundary nodes both have great effects on the resilience of interdependent networks during the recovery. In this paper, we propose a new recovery strategy (preferential recovery based on connectivity link, or PRCL) to identify the mutual boundary node recovery influence in interdependent networks, based on the failure-recovery model. By defining two indexes that represent the numbers of first-type and links second-type connectivity links, respectively, we calculate the boundary influence with one parameter f by combining together with two indexes. After calculating all boundary nodes in the current process, we obtain a boundary importance index which is more accurate to indicate recovery influence of boundary node for each boundary node in interdependent networks. Our strategy is applied to interdependent networks constructed by ER random network or/and scale-free network with the same average degree. And a dynamical model of random failure based on percolation theory is used to make a comparison of performance between PRCL and other recovery strategies(including random recovery, preferential recovery based on degree, preferential recovery based on local centrality) in terms of four quantitative indices, i.e., probability of existence of the giant connected component, number of iteration steps, recovery robustness and average degree of the steady state of the giant connected component. Experiments on different interdependent networks (ER-ER/SF-SF/ER-SF/SF-ER) demonstrate that with a very small number of mutual boundary node recoveries by PRCL strategy, the resilience and robustness of entire system under the recovery process can be greatly enhanced. Finally, the only parameter f in PRCL strategy is also discussed. Our strategy is meaningful in practice as it can largely enhance interdependent network resilience and contribute to the decrease of system breakdown risk.
2018, 67 (8): 088401. doi: 10.7498/aps.67.20180024
Traveling wave tube amplifiers are one of the most widely used vacuum electronic devices which are employed in various applications, in the areas of such as radar, wireless communication and electronic countermeasures system. Among traveling wave tubes, space-borne helix traveling wave tubes which are of high power, high efficiency, high reliability, long life and radiation hardened, are extensively used in satellite transmitter, data communication system and global positioning system. With the rapid development of the multiphase digital modulation schemes, communication systems are placing greater demands on the output power, electronic efficiency and nonlinear distortion characteristics of space-borne helix traveling wave tubes. However, the nonlinear beam-wave interaction will lead to the generation of harmonics, and thus reduces the output power and electronic efficiency. The harmonics can also act to create beats with the fundamental wave, and thus generate these beat frequencies which are commonly known as intermodulation products. As a result, the bit-error-rate will be increased and the system performance will be compromised. Therefore, the generation of harmonics is of significant current interest in space-borne helix traveling wave tubes. Understanding this effect provides a strong motivation for nonlinear analysis of a helix traveling wave tube. In this paper, a continuous electron phase distribution is obtained by treating the discrete electron beam as a charge fluid based on the Lagrangian theory. Then, to obtain a nonlinear Eulerian theory considering harmonic interaction, the electron phases in Lagrangian theory have been expanded into a series of harmonic components. Considering the 0th component and 1st component of the electron phases only and integrating over the initial phase distribution with the help of the relation of Bessel function, the nonlinear Eulerian theory considering harmonic interaction is established. The nonlinear Eulerian theory considering harmonic interaction is compared to a Lagrangian theory on a set of traveling wave tube parameters which are based on a single section of L-and C-bands traveling wave tubes. It is found that the nonlinear Eulerian theory considering harmonic interaction agrees accords well with the Lagrangian theory before the saturation effect occurs. But, it begins to make a difference near saturation point where the electron overtaking happens. The maximum error in gain between the nonlinear Eulerian theory considering harmonic interaction and the Lagrangian theory is less than 4% at 1 dB gain compression point. So the present nonlinear Eulerian theory considering harmonic interaction can effectively describe harmonic generation at 1 dB gain compression point. The simulation results validate the correctness and effectiveness of our nonlinear Eulerian theory considering harmonic interaction. In futuristic future efforts, it is hoped that the present nonlinear Eulerian theory considering harmonic interaction may provide insights into the behavioral mechanisms of nonlinear effects in space-borne helix traveling wave tubes.
ATOMIC AND MOLECULAR PHYSICS
2018, 67 (8): 083201. doi: 10.7498/aps.67.20172570
Bombarded by slow highly charged ion (SHCI), particles including ions and atoms of metal are excited and ejected from the sample. Optical emission can be observed for the radiative de-excitation of some excited atomic particles. The important information about particle ejection and incident ion neutralization, as well as the nature, the kinetic energy, and the number of the sputtered excited particles can be obtained by studying the optical emission process. The optical emission from the the collisions between slow (V~0.38 VBohr) highly charged Xeq+ (4 q 20) ions and high purity Ni (99.995%) surface is studied. The experiment is carried out at the 320 kV for multi-discipline research with HCIs in the Institute of Modern Physics, Chinese Academy of Sciences. The spectral lines are analyzed by using an Sp-2558 spectrometer equipped with a pattern of 1200 groves/mm blazed at 500 nm and an R955 photomultiplier tube at the exit slit. The target beam current corresponding to the dwell time is recorded, which can be translated into the incident ion current. Based on the formula of Y=N/(t/Ceq), the spectral line intensity is normalized. The normalized spectrum can be obtained from the interaction of 0.38VBohr Xe20+ ions with Ni surface in a wavelength range of 400-510 nm. The species at excited state can be identified by comparing the wavelengths of spectral lines with those in the standard spectroscopic table. Most of the observed spectral lines are identified as being from the electron transitions of Ni I 3d9(2D)4p-3d9(2D5/2)4d, Ni I 3d8(3F)4s4p(3P)-3d84s(4F)5s and Ni Ⅱ 3p63d9-3p63d8(3P)4s, as well as Xe I 5p5(2P3/2)6s-5p5(2P3/2)8p, Xe Ⅱ 5p4(3P2)6p-5p4(3P2)6d and Xe Ⅲ 5s25p3(2D)6s-5s25p3(2D)6p. Compared with the single charged ion, some neutralized incident ions yield Xe I, Xe Ⅱ, Xe Ⅲ spectral lines. The photon yields of spectral lines, such as Xe Ⅱ 410.419, Xe Ⅲ 430.444, Xe Ⅱ 434.200, Xe Ⅱ 486.254, Ni I 498.245, Ni I 501.697, Ni I 503.502, Ni I 505.061 and Ni I 508.293 nm, are presented each as a function of charge state of incident ion. The results show that the photon yield increases with the increase of the charge state, which is consistent with the potential energy of the incident ion. The potential energy is the driving force for photon emission of excited Ni atom. The neutralization of Xeq+ is in good agreement with that indicated by the classical over-the-barrier model.
2018, 67 (8): 083202. doi: 10.7498/aps.67.20172340
The pulse deformation and time delay, which appear when the laser propagates in a thick atom vapor, influence the ionization yield and selectivity of atomic multi-step photoionization process directly. In this paper, we study the propagation of laser pulse and atomic photoionization in a thick atom vapor medium according to the atom vapor laser isotope separation. The process of atomic multi-step photoionization and the propagation of laser in a thick medium are described by density matrix equation and Maxwell equations, respectively. The medium consists of target isotope and non-target isotope, which is non-resonantly excited. Through numerical solution of the coupled equations we illustrate the propagation characteristics of laser and the influences of atom vapor parameters and laser parameters on average ionization yield and average selectivity in a thick medium. The important results of calculation are as follows:when the atom vapor medium is rather thick, the average ionization yield increases while average selectivity decreases with the increase of laser power. When the atom vapor is relatively thin, the average ionization yield and average selectivity increase with the decrease of laser power simultaneously. Besides, there is a positive time delay between two laser pulses in which case the average ionization yield of target isotope reaches its maximum value. Moreover, when the parameters of atom vapor are constant, extending the width of laser pulses as great as possible can not only increase average ionization yield and average selectivity simultaneously, but also loosen the control accuracy of time delay between laser pulses.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2018, 67 (8): 086801. doi: 10.7498/aps.67.20180080
Silicene is a close relative of graphene with a honeycomb lattice structure. However, silicene is unlike the strictly two-dimensional graphene and it has a buckled structure, i.e., the A and B atoms form two sublattice planes with a small vertical separation distance in between. Thus a perpendicular electric field applied to silicene can induce a staggered sublattice potential and different onsite energies in the A and B sublattices. As a result, silicene has a large spin-orbit gap compared with graphene. In addition, the mass of Dirac electrons in silicone is controllable by an external electric field, which leads to several controllable polarized transports in the silicene junction, including valley-, spin-and pseudospin-polarization transport. However, in a single silicone junction the manipulations of polarizations are not ideal. In this work, we consider several silicene-based superlattices in order to effectively control the properties of polarization transport. Using the transfer matrix method, we study valley-, spin-and pseudospin-polarization transport in silicene-based electrostatic potential, ferromagnetic and antiferromagnetic superlattices. The effects of ferromagnetic exchange field, antiferromagnetic exchange field and chemical potential on transport properties are analyzed and the roles of electrostatic field in regulating valley-, spin-and pseudospin-polarization are discussed. The ferromagnetic superlattices result in spin-dependent chemical potential in ferromagnetic regime, while Dirac-like mass depends on the antiferromagnetic exchange field and spin. For electrostatic potential superlattice, the pseudospin-polarization occurs and there is no spin-polarixation nor valley-polarization. The peaks of both the pseudospin conductances are completely separated from each other and the pseudospin is completely polarized in the wide range of the zero field for both sides. For ferromagnetic superlattice, the ferromagnetic exchange field and chemical potential lead to the concurrences of spin-and valley-polarizations. The spin-and valley-polarizations can realize a sudden reversal from -1 to +1 by adjusting the electric field. For antiferromagnetic superlattice, the similar properties of spin-and valley-polarizations are observed. Comparing with the ferromagnetic superlattice, only the polarization order is different when the same change is made in the electric field. These results indicate that when the number of lattices in the superlattice is more than 10, the valley-, spin-and pseudospin-polarization reach 100% easily in silicene-based superlattice. The polarization direction can be reversed by adjusting the electric field, which is helpful in manipulating the freedom degrees of valley, spin and pseudospin in silicene superlattice.
2018, 67 (8): 088402. doi: 10.7498/aps.67.20172684
There are many kinds of high power microwave devices.According to the phase and frequency characteristics of the output microwave,they can be divided into the phase and frequency locking high power microwave (HPM) devices and HPM oscillator.Among them,the frequency and phase of HPM devices with locked frequency and phase can be adjusted by the injecting microwave,which has achieved great progress of the HPM research.In this paper,the latest progress of HPM devices with locked frequency and phase which have been developed by the Institute of Applied Electronics,China Academy of Engineering Physics in recent years are reviewed,covering relativistic klystron amplifier (RKA) and relativistic backward-wave oscillator (RBWO) with injection-locked.Aiming at the problems encountered in the research of high power and long pulse RKA,in this paper we briefly introduce the key technologies in design and experiment,including the beam-wave interaction merits,the suppression of multi-frequency oscillation,pulse shortening, high frequency and high power operation,high gain,etc.The performances of RKA,such as power,phase stability and gain,have been improved remarkably.High-power output with stable frequency and phase has been realized by single-annular beam RKA in S-band,whose output power reaches more than 1 GW with a pulse width of 165 ns and phase fluctuation of 18 at a repetitive pulse of 25 Hz/1 s.The high gain RKA also achieves a similar output power and phase stability under the condition of injected microwave power of several kW.In X-band RKA,a structure of coaxial multi-beam has been used to break through the limitations of high frequency and high power capacity,which generates more than 1 GW output power with an input power of 30 kW,the beam-wave conversion efficiency is 34% and the phase fluctuation is 15 with a pulse width of 140 ns.On the basis of an in-depth understanding of RBWO technology, and using the advantages of high efficiency and compact structure,the RBWO research of injection modulated electron beam is proposed and carried out.More than 1 GW output microwave with locked phase is realized by 100 kW seed microwave.These results not only extend scientific and technological research of a large family of HPM devices,but also make it possible for HPM devices to be used in power synthesis,particle acceleration,high-performance radar,etc.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
2018, 67 (8): 089201. doi: 10.7498/aps.67.20172225
To illustrate the formation mechanisms for the Pacific decadal oscillation (PDO) and the North Pacific gyre oscillation (NPGO) as the dominant and less dominant climate patterns of the North Pacific, and correlations between their periods of oscillation and the length of the ocean in the East-West direction, this paper adopts a mid-latitude channel linear quasi-equilibrium ocean model with reduced gravity to seek the analytical solution of the ocean flow field response to zonal wind forcing, with a special focus on resonance. Main findings include that the response pattern of the bounded ocean resembles the PDO and NPGO modes during winter respectively; specifically, to the east of the west coast of the ocean, the former is characterized by a gyre in an oval shape and the latter by two gyres rotating in opposite directions in the north and the south, constituting a gyre couple; across the entire ocean, the former features basin-wide ocean general circulation, while the latter features basin-wide general circulation in the north and the south respectively, which rotate in opposite directions. The above situations can be forced by anomalous positions of mid-latitude westerlies to the north and the south respectively. The frequency (period) of ocean flow field response to zonal wind field forcing is identical to the frequency (period) of zonal wind forcing; the response is observed after zonal wind forcing while the flow field (stream function) of the response is proportional to the zonal wind in scale. When the frequency (period) of zonal wind forcing equals that of the natural frequency (period) of the ocean, resonance will happen, with the observation of the strongest ocean response; while when the two frequencies differ by wide margins, rather small response will be observed. Smaller frictions correlate with stronger resonance along with more resonance occurrences. The length of the bounded ocean in the East-West direction has an obvious effect on the natural frequency (period), namely, the frequency (period) of resonance, and plays a decisive role in determining such a frequency; the distance between two neighboring resonance periods increases as the length is reduced. Different non-linear air-sea interactions lead to the complexity of the oscillation frequencies of a random wind field, ranging from extremely low to extremely high frequencies; through the resonance, resonance period identical or similar to the natural frequency of the ocean can be identified, at which frequency the ocean flow response to wind fields is the strongest, thus determining the periods of the PDO and NPGO. The final conclusion is that such a non-linear interaction, the effect of wind field forcing on flow field, and resonance are three key factors leading to the PDO and NPGO; the analytical solution is in nature a time-varying resonant Rossby wave.