bluuebunny/crossref_metadata_2025 huggingface dataset:
Anti-controlling Neimark-Sacker bifurcation of a three-degree-of-freedom vibration system with clearance
Authors: Wu Xin  , Wen Gui-Lin  , Xu Hui-Dong  , He Li-Ping  ,   State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China;,   School of Mechanical Engineering, Hunan Institute of Engineering, Xiangtan 411104, China
Published: December 2020

Anti-control of bifurcation, as an inverse problem of conventional bifurcation analysis, is aimed at creating a certain bifurcation with desired dynamic properties at a pre-specified system parameter location via control. The main purpose of this paper is to address the problem of anti-control of Neimark-Sacker bifurcation of a three-degree-of-freedom vibro-impact system with clearance (i.e., the second Hopf bifurcation of the original system), which may be viewed as a design approach to creating a quasi-periodic impact motion (or torus solution) at a specified system parameter location via control. Firstly, in the premise of no change of periodic solutions of the original system, when the difficulties that are brought about by the implicit Poincaré map of the vibro-impact system are considered, a linear feedback controller is added to the original system and a six-dimensional Poincaré map of the close-loop control system is established. In order to design a desired bifurcation solution by control, the multiple control gains are used to tune the existence of this bifurcation based on the corresponding critical criterion. However, for six-dimensional map of the vibro-impact system in the paper, the analytical expressions of all eigenvalues of Jacobi matrix with respect to parameters are unavailable. This implies that when the classical critical criterion described by the properties of eigenvalues is used, we have to numerically compute eigenvalues point by point and check their properties to search for the control gains. So, the numerical calculation is a laborious job in the process of determining the control gains. To overcome the difficulty originating from the classical bifurcation criterion, the explicit critical criterion without using eigenvalue calculation of high-dimensional map is used to obtain the controlling parameters area when quasi-periodic impact motion occurs. Then, the stability of quasi-periodic bifurcation solution is analyzed by utilizing the center manifold and normal formal theory. Finally the numerical experiments verify that the stable quasi-periodic impact motion can be generated at a designated system parameter point by the proposed control.
Picosecond resolving detection method and experiment for ultrafast X-ray by modulation of an optical probe
Authors: Wang Bo  , Bai Yong-Lin  , Cao Wei-Wei  , Xu Peng  , Liu Bai-Yu  , Gou Yong-Sheng  , Zhu Bing-Li  , Hou Xun  ,   State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, Xi'an 710119, China;,   University of Chinese Academy of Sciences, Beijing 100049, China;,   Key Laboratory of Ultrafast Diagnostic Technique, Xi'an Institute of Optics and Precision Mechanics of Chinese Academy of Sciences, Xi'an 710119, China
Published: December 2020

Diagnostic measurement of single picosecond event in high energy density physics, laser fusion, plasma radiation, and combustion, is of great importance. However, the measuring of the shape of the single X-ray pulse and the synchronization of X-ray and the laser pulse in picosecond resolution is still a great challenge. Restricted by the transit time of electrons, the time-resolution limit of a conventional framing camera based on the microchannel plate is 40 ps. Centered on the full-optical modulation effect of the light-probe, a novel method for X-ray detection of picoseconds temporal resolution based on low temperature GaAs is proposed in this work. The basic physical mechanism of the detector can be explained in both macroscopical and microcosmic ways. In the macroscopical way, the X-ray radiation absorption in the sensor material produces a transient, non-equilibrium electron-hole pair distribution that results in a transient differential change of the local refractive index, which is then sensed by the reflectivity changes of the optical probe beam. In the microcosmic way, X-ray absorption creates photoelectrons and the core level holes are subsequently filled through Auger or fluorescence processes. These excitations ultimately increase conduction and valence band carriers that perturb optical reflectivity.#br#To verify the proposed X-ray detection method, a Fabry-Perot detector is designed, which consists of a 5 μm thick GaAs layer surrounded by a GaAs/AlAs distributed Bragg reflector. The test is carried out on a femtosecond laser facility, where the X-ray source is produced by focusing the 56 fs Ti: Sapphire facility laser, with a central wavelength of 800 nm, onto an aluminum foil. Then the X-ray pulse induces a transient optical reflectivity change in GaAs, which is a powerful tool for establishing the high-speed X-ray detection.#br#The experimental results indicate that this technology can be used to provide X-ray detectors with a temporal resolution of tens of picoseconds. By optimizing the material, the temporal resolution can be enhanced to be less than 1 ps. Through further development, this X-ray detector could provide an insight into previously unmeasurable phenomena in many fields. Future work will focus on developing much faster devices characterizing both the rise and fall time and imaging array technology.
Propagation characteristics of partially coherent decentred annular beams propagating through oceanic turbulence
Authors: Yang Ting  , Ji Xiao-Ling  , Li Xiao-Qing  ,   School of Physics, Sichuan Normal University, Chengdu 610068, China
Published: December 2020

The analytical expressions for the average intensity and the centroid position of partially coherent decentred annular beams propagating through oceanic turbulence are derived, and the propagation equation of the position of the maximum intensity is also given. Changes of the average intensity, the centroid position and the position of the maximum intensity of partially coherent decentred annular beams during propagation are studied in detail. It is shown that both in free space and in oceanic turbulence, the position of the maximum intensity moves to the propagation z-axis with increasing the propagation distance, and is kept unchanged when the propagation distance is large enough. Furthermore, in free space the position of the maximum intensity is closer to the propagation z-axis than to the centroid position when the propagation distance is large enough. The position of the maximum intensity is closer to the propagation z-axis with increasing the correlation parameter, and far from the propagation z-axis with increasing the decentered parameter and the obscure ratio. However, in oceanic turbulence the position of the maximum intensity is close to the centroid position when the propagation distance is large enough, and the evolution is speeded with increasing the strength of oceanic turbulence. The influence of the beam coherence on propagation characteristics decreases due to oceanic turbulence. On the other hand, the centroid position is independent of the beam coherence, the propagation distance and the oceanic turbulence. The centroid position is far from the propagation z-axis with increasing the decentered parameter and the obscure ratio. In addition, the hollow core of partially coherent decentred annular beams is filled up as the propagation distance increases, and the evolution is speeded with increasing the strength of oceanic turbulence. The results obtained in this paper are very useful for applications of partially coherent decentred annular beams in oceanic turbulence.
Design and experimental study of mode selective all-fiber fused mode coupler based on few mode fiber
Authors: Xiao Ya-Ling  , Liu Yan-Ge  , Wang Zhi  , Liu Xiao-Qi  , Luo Ming-Ming  ,   Institute of Modern Optics, Nankai University, Tianjin 300071, China
Published: December 2020

Three types of all-fiber mode-selection couplers based on fused few-mode fibers (FMFs) are proposed and demonstrated. The specific mode conversions are achieved with appropriate parameters, keeping to the coupling mode theory. LP01 mode is selectively converted into the LP11, LP21, LP02 mode via a 2×2 fused fiber coupler composed of single-mode fiber (SMF) and FMF. By changing the preset parameters in fabrication, mode conversions are also realized between the LP01 and the mixed high order modes. Moreover, conversions from the LP01 mode to other higher order modes are implemented as well in a 3×3 fiber coupler comprising FMF-SMF-FMF structures. Besides, different modes are simultaneously obtained in separated channels to reduce model crosstalk. Distinguished from other techniques, symmetric and asymmetric fused biconical taper are employed in this paper. The 2×2 fiber coupler achieves the conversion from LP01 mode to a single higher order mode such as LP11 or LP21 mode over a broadband spectral range from 1530 nm to 1560 nm. Meanwhile, the mode conversion efficiency exceeding 80% is recorded in experiment, while the insertion loss remains as low as 0.8 dB. Through the comparison with all-fiber mode-selection couplers reported, the relationship between fusion-degree and conversion efficiency is further studied. The experimental results are consistent with the numerical simulations. In addition, the coupler based mode-selection with lower insertion loss and higher conversion efficiency shows potential applications in mode-division multiplexing and sensing systems.
Cavitation of two kinds of bubble mixtures
Authors: Miao Bo-Ya  , An Yu  ,   Department of Physics, Tsinghua University, Beijing 100084, China
Published: December 2020

By solving the nonlinear wave equation coupled with the modified Rayleigh-Plesset equation, the characteristics of the acoustic field and bubble motion in cavitation environment can be described. In general, the cavitation cloud consists of many kinds of bubbles with different ambient radii. For simplicity, in this work the cavitation process of the mixture of two kinds of bubbles with different ambient radii is numerically simulated, and the ratio of the mixture is adjustable. Suppose that the cavitation in water contained in a cylindrical container is stimulated by ultrasonic horn. The dissipative absorption of the container wall is taken into account, which plays an important role in forming the stationary standing wave field, otherwise, the beat signal of acoustic pressure will appear which is absent in the observation. Based on the stationary acoustic wave field, for the case of the mixed-bubble cavitation, the interactions between bubbles and acoustic field, bubbles and bubbles, as well as the spectrum of acoustic signal are analyzed. We choose the cases that the ratio of two kinds of bubble species is varying, but the total density of bubble number is fixed to be 1/mm3, and find that those results are very different. For the case that the ambient radii of two bubble species are both a few micron, revealing that the interaction between bubbles and acoustic field is usually weak. As the proportion of bigger bubble increases, the change of the acoustic pressure and the averaged radius of bubble behave regularly; for the case that the ambient radius of one of bubble specie is relatively big, for example, the ambient radius is about a few tens of microns, the interactions between bubbles and acoustic field become stronger, and the nonlinearity is more apparent. We can observe the similar trends from the frequency spectrum. For the bubble of a few microns in size, the base frequency is dominant; in contrast, for the bubble of a few tens of microns in size, the components of harmonic frequencies are far beyond the base frequency component. The interesting phenomenon is that there is the cut off frequency and the cut of frequencies for different mixture of bubbles are almost the same.
Comparative study on constructal optimizations of rectangular fins heat sink based on entransy dissipation rate minimization and maximum thermal resistance minimization
Authors: Yang Ai-Bo  , Chen Lin-Gen  , Xie Zhi-Hui  , Sun Feng-Rui  ,   Institute of Thermal Science and Power Engineering, Naval University of Engineering, Wuhan 430033, China;,   Military Key Laboratory for Naval Ship Power Engineering, Naval University of Engineering, Wuhan 430033, China;,   College of Power Engineering, Naval University of Engineering, Wuhan 430033, China
Published: December 2020

Constructal optimization of a rectangular fin heat sink with two-dimensional heat transfer model is carried out through using numerical simulation by finite element method, in which the minimized maximum thermal resistance and the minimized equivalent thermal resistance based on entransy dissipation are taken as the optimization objectives, respectively. The optimal constructs based on the two objectives are compared. The influences of a global parameter (a) which integrates convective heat transfer coefficient, overall area occupied by fin and its thermal conductivity, and the volume fraction (φ), on the minimized maximum thermal resistance, the minimized equivalent thermal resistances and their corresponding optimal constructs are analyzed. The results show that there does not exist optimal thickness of fins for the two objectives when the shape of the heat sink is fixed, and the optimal constructs based on the two objectives are different when the shape of the heat sinks can be changed freely. Besides, the global parameter has no influence on the optimal constructs based on the two objectives, but the volume fraction does. The increases of the global parameter and the volume fraction reduce the minimum values of the maximum thermal resistance and the equivalent thermal resistance, but the degrees are different. The reduce degree of the global parameter to the minimized equivalent thermal resistance is larger than that to the minimized maximum thermal resistance. The minimized equivalent thermal resistance and the minimized maximum thermal resistance are reduced by 40.03% and 41.42% for a= 0.5, respectively, compared with those for a = 0.3. However, the reduce degree of the volume fraction to the minimized maximum thermal resistance is larger than that to the minimized equivalent thermal resistance. The minimized equivalent thermal resistance and the minimized maximum thermal resistance are reduced by 59.69% and 32.80% for φ= 0.4, respectively, compared with those for φ= 0.3. As a whole, adjusting the parameters of the heat sink to make the equivalent thermal resistance minimum can make the local limit performance good enough at the same time; however, the overall average heat dissipation performance of the heat sink becomes worse when the parameters of the heat sink are adjusted to make the maximum thermal resistance minimum. Thus, it is more reasonable to take the equivalent thermal resistance minimization as the optimization objective when the heat sink is optimized.
Backscattering measurements of plasma coated target in high-enthalpy wind tunnel
Authors: Jin Ming  , Wei Xiao  , Wu Yang  , Zhang Yu-Huai  , Yu Xi-Long  ,   Science and Technology on Electromagnetic Scattering Laboratory, Beijing 100854, China;,   Key Laboratory of High Temperature Gas Dynamics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

When high-speed vehicles enter into the atmosphere, plasma sheath may be excited around due to aerodynamic heating, resulting in difficulties in communicating and changes of electromagnetic scattering properties. Those facts have received lots of attention due to their influences on the aerospace communication and radio telemetry applications. While analytic and numerical studies have been carried out by many native institutions on the electromagnetic radiation/scattering problems in the presence of plasma sheath, there remains the lack of measurement data to support and verify those researches. This work reports the backscattering measurements for the target surrounded by plasma sheath in the ground high-enthalpy shock tunnel facility. Using the step frequency sweeping mode of a commercial instrument, i.e., vector network analyzer, we conduct the experiments in the JF-10 high-enthalpy shock tunnel. The dynamic electromagnetic scattering measurement must be completed on a time scale of ms while the shock tunnel is running. The implementation details are demonstrated in this work, including the experimental configurations, data processing procedures, timing synchronization, and discussion on the relationships between the air flow status and measured target scattering signals. The influences of the plasma sheath on the target RCS (radar cross section) in the C band are successfully and clearly observed. The influence of the air flow status on the measured data can be concluded as follows: the front section of high-speed air flow lasting about 0.5-1 ms will change the measured signal dramatically, which should be avoided in observation due to its instability; the effective plasma sheath lasts only about 2 ms, resulting in an overall reduction on the target RCS by about 2 dB in the measurements. Afterwards, the effects by the plasma sheath on the target scattering vanish quickly.
Recent progress in Ge and GeSn light emission on Si
Authors: He Chao  , Zhang Xu  , Liu Zhi  , Cheng Bu-Wen  ,   State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
Published: December 2020

Si-based optical interconnection is expected to solve the problems caused by electric interconnection with increasing the density of integrated circuits, due to its merits of high speed, high bandwidth, and low consumption. So far, all of the key components except light source of Si-based optical interconnection have been demonstrated. Therefore, the light source has been considered as one of the most important components. Ge and GeSn based on Si have emerged as very promising candidates because of their high compatibility with Si CMOS processing, and the pseudo direct-bandgap characteristic. The energy difference between the direct and indirect bandgap of Ge is only 136 meV at room temperature. Under tensile strain or incorporation with Sn, the energy difference becomes smaller, and even less than zero, which means that Ge or GeSn changes into direct bandgap material. What is more, using large n-type doping to increase the fraction of electrons in valley, we can further increase the luminous efficiency of Ge or GeSn. In this paper, we briefly overview the recent progress that has been reported in the study of Ge and GeSn light emitters for silicon photonics, including theoretical models for calculating the optical gain and loss, several common methods of introducing tensile strain into Ge, methods of increasing the n-type doping density, and the method of fabricating luminescent devices of Ge and GeSn. Finally, we discuss the challenges facing us and the development prospects, in order to have a further understanding of Ge and GeSn light sources. Several breakthroughs have been made in past years, especially in the realizing of lasing from GeSn by optically pumping and Ge by optically and electrically pumping, which makes it possible to fabricate a practical laser used in silicon photonics and CMOS technology.
Electrical and optical behaviors of La-doped BaSnO3 thin film
Authors: Fei Xiao  , Luo Bing-Cheng  , Jin Ke-Xin  , Chen Chang-Le  ,   Shaanxi Key Laboratory of Condensed Matter Structures and Properties, School of Science, Northwestern Polytechnical University, Xi'an 710072, China
Published: December 2020

La-doped BaSnO3 is regarded as a very essential material to construct transparent perovskite devices due to its super high electrical mobility in perovskite transparent conducting oxides. For understanding the high electrical mobility, the effective mass of the carrier in La-doped BaSnO3 is a critical factor and should be determined. In this work, the performances of epitaxial La-doped BaSnO3 thin films grown on (LaAlO3)0.3 (SrAl0.5Ta0.5O3)0.7 (001) substrate by radio-frequency (RF) magnetron sputtering technique are investigated. The electrical properties (resistivity, carrier density, mobility and Seebeck coefficient) and the optical transmittance are analyzed. In addition, it is proved from both the Hall effect and thermoelectric power measurements that the La-doped BaSnO3 thin films are n-type degenerate semiconductor. At 300 K, the resistivity, carrier density, mobility and Seebeck coefficient are 0.987 mΩ·cm, 2.584×1020 cm-3, 24.49 cm2·V-1·s-1 and 45.71 μV/K, respectively. The electron effective mass ～ 0.31m0 (m0, the free electron mass) is extracted by combining the Seebeck coefficient and carrier density. Ba0.99La0.01SnO3 (BLSO) thin film exhibits a high optical transmittance of 73% in the visible spectral region. In order to derive the band-gap energy, the complex dielectric constant, and the film thickness, the transmittance spectrum is simulated based on the dielectric model comprising the band-gap transition (O'Leary-Johnson-Lim model) and free electron excitation (Drude-Lorentz model). The band-gap energy, exponential band tail and thickness of the BLSO thin film are 3.43 eV, 0.27 eV and 781.2 nm, respectively. Wavelength-dependence of complex dielectric function of the BLSO thin film is also obtained from the fitted line. Additionally, the parameters (optical carrier density and mobility) resulting from the optical measurement are in agreement with the results from the electrical measurement, which supports the calculated electron effective mass aforementioned.
Positron annihilation studied defects and their influence on thermal conductivity of chemically synthesized Bi2Te3 nanocrystal
Authors: He Hui-Fang  , Chen Zhi-Quan  ,   Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, China
Published: December 2020

Bismuth telluride (Bi2Te3) and its alloys are regarded as the best thermoelectric materials available nowadays at room temperature and can be well prepared by using existing technology. In this paper, Bi2Te3 nanocrystals are prepared by hydrothermal method and then treated by a spark plasma sintering (SPS) process at five temperatures of 300, 350, 400, 450 and 500 ℃ each for 5 min under a pressure of 20 MPa. X-ray diffraction (XRD) and positron annihilation spectroscopy are used to study the microstructures of the samples after SPS treatment at different temperatures. According to the XRD patterns, the diffraction peaks of the as-grown powder are consistent with those indicated in the standard card for Bi2Te3, which confirms successful synthesis of Bi2Te3 powders. Scanning electron microscope images show that the particles of all the samples take on flake-like structures, and the particle sizes increase from about 100 nm to a few m with the sintering temperature increasing from 350 to 500 ℃. This suggests significant reorganization of nanograins in sintering process, and some grains are agglomerated into larger particles. However, the grain sizes estimated from the X-ray diffraction peaks show little change in all the samples sintered at temperatures between 300-500 ℃. And most of the grains have sizes around 30 nm. Positron lifetime spectra are measured for Bi2Te3 samples sintered at different temperatures. The measurements reveal vacancy defects existing in all the sintered samples. With the increase of sintering temperature, there appears no significant change in trapped positron lifetime (2). This suggests that the defect size has no change during sintering. However, intensity I2 decreases monotonically with increasing sintering temperature, which indicates the lowering of vacancy concentration. The average positron lifetime shows a monotonous decrease with increasing sintering temperature, which indicates the recovery of vacancy defects at higher sintering temperatures. The thermal conductivity of the sample increases from 0.3 Wm-1K-1 to about 2.4 Wm-1K-1 with the sintering temperature increasing from 300 to 500 ℃. Since the lattice thermal conductivity dominates the total thermal conductivity, it can be inferred that sintering at higher temperature leads to the increase of lattice thermal conductivity. According to the positron annihilation lifetime result, the vacancy defects in the interface region gradually recover after sintering at higher temperatures. This shows good correlation with the increase of lattice thermal conductivity, indicating that vacancy-type defects are effective phonon scattering centers for Bi2Te3.
Discriminability of node influence in flower fractal scale-free networks
Authors: Shu Pan-Pan  , Wang Wei  , Tang Ming  , Shang Ming-Sheng  ,   Web Sciences Center, University of Electronic Science and Technology of China, Chengdu 610054, China
Published: December 2020

Extensive studies have shown that the fractal scaling exists widely in real complex systems, and the fractal structure significantly affects the spreading dynamics on the networks. Although node influence in spreading dynamics of complex networks has attracted more and more attention, systematical studies about the node influence of fractal networks are still lacking. Based on the flower model, node influences of the fractal scale-free structures are studied in this paper. Firstly, the node influences of different fractal dimensions are compared. The results indicate that when the fractal dimension is very low, the discriminability of node influences almost does not vary with node degree, thus it is difficult to distinguish the influences of different nodes. With the increase of fractal dimension, it is easy to recognize the super-spreader from both the global and local viewpoints. In addition, the network noise is introduced by randomly rewiring the links of the original fractal networks, and the effect of network noise on the discriminability of node influence is analyzed. The results show that in fractal network with low dimension, it becomes easier to distinguish the influences of different nodes after adding network noises. In the fractal networks of infinite dimensions, the existence of network noises makes it possible to recognize the influences of medium nodes. However it is difficult to recognize the influences of central nodes from either the global or local perspective.
Transmission protocol and its performance analysis of quantum communication network based on packet switching
Authors: Nie Min  , Wang Lin-Fei  , Yang Guang  , Zhang Mei-Ling  , Pei Chang-Xing  ,   School of Communication and Information Engineering, Xi'an University of Posts and Telecommunication, Xi'an 710121, China;,   School of Electronics and Information, Northwestern Polytechnical University, Xi'an 710072, China;,   State Key Laboratory of Integrated Service Networks, Xi'an University of Electronic Science and Technology, Xi'an 710071, China
Published: December 2020

Quantum entanglement swapping can be used to establish reliable quantum remote transmission channel so as to realize transmission of quantum states. However, the highly stable quantum network is required in the quantum channels when using quantum entanglement swapping, otherwise it will waste a lot of entanglement resources. In order to save entanglement resources, we have to put forward a kind of quantum communication network transmission protocol based on packet switching, according to the theory of quantum teleportation. Firstly, the principle of packet switching in computer network is introduced. Next we describe the implementation process of quantum network transmission protocols which are based on entanglement swapping and packet switching. We then analyze the reliability, security and utilization rate of entanglement of the protocol we have proposed. And after that the quantitative relationship about the number of entanglement quantum states, the number of routers and link error rates are calculated. Finally, we compare these two transmission protocols. Simulation results show that the number of entanglement for these two protocols is equal without consideration of the link errors. When taking them into account, the packet switching transmission protocol can save numbers of entanglement resources obviously. In addition, with the increase of number of routers and the rise in link error rates, the quantum communication network transmission protocol based on packet switching will need less entanglement resources than that based on entanglement swapping. Therefore, when the quantum transmission network is not stable, the packet switching transmission protocol has a better transmission performance, and it can be applied to the future construction of quantum network.
Numerical simulations of hexagonal grid state patterns
Authors: Bai Zhan-Guo  , Liu Fu-Cheng  , Dong Li-Fang  ,   College of Sciences, Hebei University of Science and Technology, Shijiazhuang 050018, China;,   College of Physics Science and Technology, Hebei University, Baoding 071002, China
Published: December 2020

The four hexagonal grid state patterns and a variety of non-grid states are obtained by changing the values of intensity ratio between two Turing modes in the two-layer coupled Lengel-Epstein model system. Results of numerical investigation show that those grid states in reaction diffusion are interleaving structures of three sets of different sublattices, which result from the interaction of both the wave number ratio and intensity ratio between Turing modes in the two subsystems; and the specific expressions of three-wave resonance in physical space are governed by the mode intensity ratio. Furthermore, the value of intensity ratio between the two Turing modes in the grid state patterns is greater than that of non-grid state structures, and the type of pattern selected by the system changes from complex to simple pattern with the increase of mode intensity ratio. Finally, it is found that these four hexagonal grid states correspond to different number pair (a, b) having different stability, and the grid state with the number pair (3, 2) is the most stable structure.
Study of erosion and deposition characteristics of Li during liquid Li limiter experiment in HT-7
Authors: Meng Xian-Cai  , Zuo Gui-Zhong  , Ren Jun  , Sun Zhen  , Xu Wei  , Huang Ming  , Li Mei-Heng  , Deng Hui-Qiu  , Hu Jian-Sheng  , Hu Wang-Yu  ,   Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, China;,   Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China
Published: December 2020

In the magnetic confinement fusion device, the first wall as plasma facing components will directly affect the performance of high temperature plasma. And the interaction of plasma and materials also affect the life of the first wall. Liquid lithium first wall receives more and more attention due to the properties of repairing itself and effectively inhibiting boundary particle recycling. So the research of the interaction between liquid lithium wall and plasma is particularly important. Erosion and deposition characteristics of lithium and its influence on the performance of plasma during lithium limiter experiment in HT-7 device are studied in-depth in this paper. Experimental results show that when the interaction between Li and plasma is weak, Li enters into the plasma mainly by weak surface evaporation and sputtering. During this process, Li line emission is strengthened, impurity and hydrogen recycling is decreased resulting in the improvement of plasma performance. When the interaction between Li and plasma becomes extremely strong, it is found so many big scale Li droplets ejected from liquid lithium surface to cause intense Li efflux into plasma, leading to plasma discharge disruption. Li atoms coming from Li limiter are ionized in the scrape-off layer (SOL), and entered into hot plasma column as ions (Li+, Li2+, Li3+) and transported in plasma. After the experiment, it can be found that a lot of white spots distributed in the vacuum chamber wall, with its main composition being Li2CO3 by XPS analysis. Through observing Li spot distribution and analyzing the lithium film thickness by scanning electron microscopy (SEM) in different samples, it is observed that the lithium is primarily deposited around the limiter, but the number of Li spots is more at the low field side than that at the high field side of the device, and the Li film gradually becomes thinner along the toroidal direction of the HT-7 device, leading to the non-uniformity of impurity and hydrogen recycling. The experiment may provide a reference for studying the interaction of plasma and liquid lithium first wall and the application of liquid lithium first wall in future tokamak device.
Theoretical and experimental study on line intensities of CO2 and CO transitions near 1.5 μm at high temperatures
Authors: Wang Min-Rui  , Cai Ting-Dong  ,   Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China
Published: December 2020

Accurate spectroscopic parameters of probed species, especially the line strengths at high temperatures, are important for combustion diagnosis based on tunable diode laser absorption spectroscopy (TDLAS). However, most of the line strengths in databases are measured at normal atmospheric temperature and calculated at high temperatures. For example, the mostly used HITRAN database focuses on atmospheric conditions where the temperature ranging from 200-350 K. The high-temperature parameters in HITRAN database are obtained by calculation and the temperatures are limited to 3000 K. In this paper the line strengths of 30012-00001 transition band of CO2 and 3-0 transition band of CO at normal temperature (300 K) and some high temperatures (400-6000 K) are calculated using our calculated partition function and experimental transition moment squared and Herman-Waills factor coefficients. The total internal partition sums (TIPS) are calculated for CO2 and CO with the product approximation of the vibrational and the rotational partition functions. The vibrational partition function is calculated in harmonic oscillator approximation. For rotational partition sums, the centrifugal distortion corrections are taken into consideration. In order to validate the calculation, a high-temperature measurement system based on TDLAS is developed using a DFB diode laser near 1.573 μm. High-resolution absorption spectra of CO2 and CO can be recorded in a heated cell as a function of temperature and pressure. The 9 lines of CO2 30012-00001 band and 2 lines of CO 3-0 band have been measured by means of direct absorption spectroscopy in the temperature range of 300-800 K. The corresponding line strengths are inferred from the measured direct absorption spectrum. The calculated result and experimental data are compared with the HITRAN values. The calculated and measured data agree well with the existing databases (HITRAN 2012), the discrepancies being less than 3% for most of the probed transitions. All the results confirm the validity of the calculation of total partition function and the line strengths calculated. The variation of the line strength as a function of temperature for a certain transition is also discussed. While the lower state energy determines the equilibrium molecular population in the unabsorbing state as a function of temperature, how the line strength of a particular transition varies with temperature can also be controlled.
Experimental optimization in ion source configuration of a miniature electron cyclotron resonance ion thruster
Authors: Tang Ming-Jie  , Yang Juan  , Jin Yi-Zhou  , Luo Li-Tao  , Feng Bing-Bing  ,   College of Astronautics, Northwestern Polytechnic University, Xi'an 710072, China
Published: December 2020

A miniature ion thruster has been proposed in recent years for a small propulsion system applied in space missions such as deep space exploration, precise high-stability attitude and position control. An electron cyclotron resonance (ECR) ion thruster is free from contamination and degradation of electron emission capacity and will offer a potentially longer thruster lifetime than that in the electron bombardment type. The microwave ECR ion source with a 20-mm diameter designed here consists of two annular permanent magnets (SmCo), ring coupling antenna and a grid system including screen and acceleration. For the ion source performance optimization, with a fixed magnetic structure, the antenna position and cavity length in the discharge chamber can be adjusted to strengthen electron ECR heating and increase ion beam extraction. According to the distribution of static magnetic field and the ECR layer measured by Gauss meter, three possible sizes of antenna position (L1) are set; depending on the cut-off characteristics of the discharge chamber and the distribution of microwave electric field calculated by finite element method, six candidate sizes of cavity length (L2) are set. By comparing the difference in plasma discharge and ion beam extraction, the optimal structure of ion source can be obtained. Experimental results show that for a given antenna position, there is a cavity length not too long or too short to extract the maximum ion beam. And the launch of microwave from strong magnetic field near ECR layer is conductive to lossless wave propagation in plasma and highly efficient electron ECR heating. To maintain a plasma in very low power and flow, the size combination of 0.6-mm in L1 and 5-mm in L2 is selected as the preferred structure. The performances of miniature ECR ion source, that is, ion beam current, discharge loss, propellant utilization efficiency, thrust and specific impulse are 5.4 mA, 389 W/A, 15%, 163 μup N and 1051 s, respectively, at an incident power of 2.1 W and argon flow of 14.9 μg/s.
Simulation on surface morphology evolution of metal targets irradiated by intense pulsed electron beam
Authors: Yu Xiao  , Shen Jie  , Zhong Hao-Wen  , Zhang Jie  , Zhang Gao-Long  , Zhang Xiao-Fu  , Yan Sha  , Le Xiao-Yun  ,   School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China;,   Institute of Heavy Ion Physics, Peking University, Beijing 100871, China
Published: December 2020

Based on the review of previous experimental and theoretical studies on the surface processing by a pulsed intense electron beam, the induced temperature field in aluminum and 304 stainless steel is simulated by the finite element method (FEM) to estimate the existing time and depth of molten metal flow field on the irradiated surface. The generation of craters is attributed to the thermal resistance formed by the grain boundaries, and the influence of material properties on the mechanism of crater evolution is also discussed. Two-phase flow field simulation on molten metal is carried out with a combination of level-set method and FEM to estimate the mass transfer behavior at the craters and surface protuberance. It is revealed that the mass transfer effect driven by surface tension is an important factor for the formation and evolution of round-shaped craters on the surface of metals with high melting point, viscosity and surface tension coefficient. However, for metals with low melting point, due to the strong disturbance by ablating gas plume and low surface tension effect, the craters are more likely to have irregular splashing edges.
Angular dependence of proton single event multiple-cell upsets in nanometer SRAM
Authors: Luo Yin-Hong  , Zhang Feng-Qi  , Guo Hong-Xia  , Guo Xiao-Qiang  , Zhao Wen  , Ding Li-Li  , Wang Yuan-Ming  ,   State Key Laboratory of Intense Pulsed Radiation Simulation and Effect Northwest Institute of Nuclear Technology, Xi'an 710024, China
Published: December 2020

Single event multiple-cell upsets (MCU) increase sharply as the feature size of semiconductor devices shrinks. MCU poses a large challenge on present radiation hardening technology and modeling test technique. Experimental study of the influence of proton incidence angle on single event multiple-cell upsets in 90 nm static random access memory (SRAM) for middle and high energy proton is carried out. The result shows that MCU percentage and multiplicity increase with increasing proton energy, and the MCU topological pattern presents a certain track-orientation characteristic along the trajectories of the incidence ion when the incidence proton is tilted along the X-direction or Y-direction. Single event upset (SEU) cross section has no evident angular dependence. There is some difference in proton MCU cross section between normal incidence and tilt angle incidence only for 30 MeV proton. Angular effect of proton MCU is associated with proton energy. Due to the lower efficiency of Monte-Carlo method in calculating proton MCU, a fast calculation method for cross section, which aims at single event MCU induced by proton nuclear reaction, is adopted. The binary cascade model in Geant4 toolkit serves as event generators in middle on high proton nuclear reaction. In terms of double differential scattering cross section of secondary particle from proton-material spallation reaction, proton MCU cross section is calculated through integration over the entire space of memory cells array. Based on the distribution of secondary particles, those spallation products with the highest linear energy transfer (LET) and longest range are revealed to emit preferentially in the forward direction, which is the root cause why the angular effect of proton-induced MCU exists. The angular dependence of single event MCU in nanometer SRAM depends strongly on proton energy and critical charge. The higher the proton energy is, the wider the angular distribution of secondary particle is, the greater the energy and LET value of the lateral scattered secondary particle is; and so the angular enhancement effect in MCU cross section for lower energy protons is greater than the higher energy protons. MCU cross section is more isotropic with the increase of the proton energy. Angular effect in MCU cross section becomes stronger with the increase of the critical charge for the same energy proton.
Study on controlling the stress in flexible Al/PI film by interface alloying
Authors: Jiang Zhao  , Chen Xue-Kang  ,   Science and Technology on Vacuum Technology and Physics Laboratory, Lanzhou Space Technology Institute of Physics, Lanzhou 730010, China
Published: December 2020

Micro-eletromechanical system (MEMS) thermal-control shutters for spacecraft are fabricated by using the flexible Al/PI film, because of its light mass, no brittleness and withstanding severe mechanical environment (mechanical environment adaptability) in space. But the stress in the film would be able to bend the shutters too much to fabricate shutter array. Therefore, how to control the thin film stress is an important problem and it is necessary for flat shutters to take some measure to remove or reduce the thin film stress. This internal stress in the thin film formed intricately during the deposition process would make the film exhibit macroscopic compressive stress. So it is difficult to control the thin film stress micro-mechanically, but macro-mechanically. According to the results of the current study, the controlling technology of thin stress is commonly applicable to rigid substrates. In this paper, the flexible Al/PI film may be controlled by interface alloying. We put forward a way of adding Sn layer to the flexible Al/PI film, which makes Al/Sn interface to be alloyed as a measure to control the stress. In the alloy phase, lattice expansion and distortion results in the emergence of transverse shearing stress. The intrinsic compressive stress can be canceled out by the transverse shearing stress and the apparent stress in the films decreases consequently. The Sn atoms diffusion behaviour is proved to form Al-Sn alloying layer by SEM and EDS. This method can be used as a new technology of controlling thin film stress.
In-situ investigation on the growth of Cu-Al intermetallic compounds in Cu wire bonding
Authors: Yang Qing-Ling  , Tan Yik-Yee  , Wu Xing  , Sim Kok Swee  , Sun Li-Tao  ,   SEU-FEI Nano-Pico Center, Key laboratory of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China;,   Multimedia University, Melaka 75450, Malaysia
Published: December 2020

According to Moore's Law, as the feature size of semiconductor devices becoming smaller and smaller, the chip integration degree keeps increasing. In particular, accompanying with the development of high chip integration and unit size reduction, the metal interconnects, i. e. the wire bonding, are becoming a challenging problem. Copper wire is believed to be an excellent metal for wire bonding, instead of gold wire, due to its attractive advantages such as low cost, favorable electrical and thermal conductivities etc. However, the excess Cu/Al intermetallic compounds (IMC) at the interface of copper wire and aluminum pad will increase the contact resistance and reduce bonding strength. This can affect the properties and reliability of devices. Currently, the evolutions of the interfacial microstructures as well as the growth mechanism of Cu/Al IMC at the bonding interface under thermal condition are still unclear.In-situ transmission electron microscope (TEM) has high spatial resolution and strong analysis ability. With fast CCD cameras, TEM can also record the dynamic structure evolution of the sample in real time. Combined with multi-function holders, TEM can also exert diverse fields and loads on the sample and synchronously monitor their structures and component evolutions. Hence, in situ TEM provides an advanced technique to explore the structural evolution and growth mechanism of Cu/Al IMC.In this paper, the growth mechanism of Cu/Al IMC is investigated during the annealing temperature from 50-220 ℃ based on the in-situ high resolution transmission electron microscopy (in-situ HRTEM). Specifically, the dynamic growth and structural evolution of Cu/Al IMC during annealing are recorded in real time. Results show that the isolated Cu/Al IMC is distributed in the bonding interface before annealing. The main component of IMC is Cu9Al4, whereas the minor one of IMC is CuAl2. After annealing at 50-220 ℃ for 24 h, Cu/Al IMC near the Cu layer is Cu9Al4, while Cu-Al IMC apart from the Cu layer is CuAl2. Meanwhile, the reaction rates and the activation energy of Cu/Al IMC at different temperatures are calculated. Furthermore, the more accurate growth equation of Cu/Al IMC is also proposed based on the in-situ experimental results, which will benefit the optimization of bonding process and the reliability of Cu/Al wire bonding.
Phase string effect and mutual Chern-Simons theory of Hubbard model
Authors: Zhang Long  , Weng Zheng-Yu  ,   Institute for Advanced Study, Tsinghua University, Beijing 100084, China
Published: December 2020

The fermion sign plays a dominant role in Fermi liquid theory. However, in Mott insulators, the strong Coulomb interaction suppresses the charge fluctuations and eliminates the fermion signs due to electron permutation. In this article, we first review the phase string theory of the Hubbard model for a bipartite lattice, which unifies the Fermi liquid at weak coupling and the antiferromagnetic Mott insulator at strong coupling. We first derive the exact sign structure of the Hubbard model for an arbitrary Coulomb interaction U. In small U limit, the conventional fermion sign is restored, while at large U limit, it leads to the phase string sign structure of the t-J model. For half filling, we construct an electron fractionalization representation, in which chargons and spinons are coupled to each other via emergent mutual Chern-Simons gauge fields. The corresponding ground state ansatz and low energy effective theory capture the ground state phase diagram of the Hubbard model qualitatively. For weak coupling regime, the Fermi liquid quasiparticle is formed by the bound state of a chargon and a spinon, and the long range phase coherence is determined by the background spin correlation. The Mott transition can be realized either by forming the chargon gap or by condensing the background spinons.