Accepted
, , Received Date: 2020-10-18
Abstract +
Frequency-resolved optical gating (FROG) is a common technique of ultrashort pulse measurement. It reconstructs the test pulse by an iterative two-dimensional phase retrieval algorithm from an FROG trace. Now the most widely used FROG algorithm is principal component generalized projection (PCGP), yet its accuracy of pulse retrieval drops obviously under noise condition, and its iterative speed slows down significantly as the size of FROG trace increases. Actually, most of ultrashort pulses delivered from ultrafast oscillators and amplifiers as well as created in most scientific experiments are of smooth spectral phases, so that they can be determined by a few of dispersion coefficients. In this paper, we propose an FROG algorithm based on seeker optimization algorithm (SOA). After recording the spectrum of the test pulse, several main dispersion coefficients of the spectral phase of the pulse are searched directly by the SOA algorithm to fit the corresponding FROG trace. Then the shape of the test pulse can be uniquely reconstructed. Since this algorithm mainly operates in a spectral domain and its routine of iteration is much simpler than PCGP’s, the faster speed and higher accuracy of pulse retrieval can be expected. In order to prove the advantages of SOA-FROG algorithm, numeral simulations are performed for test pulses with varying dispersions, in the cases without noise and with 1%, 5%, 10%, 20% noise levels, respectively. The simulation results show that the new algorithm performs much better than the PCGP in accuracy and iteration speed. In the case without noise, 97% test pulses reach the condition of rigid convergence (FROG error G ≤ 10–4) after 1500 iteration circles by using the SOA, with an average FROG error G < 10–5. So the accuracy of pulse reconstruction by SOA is at least three orders of magnitude higher than by the PCGP. In cases with different noise levels, the accuracy of pulse reconstruction by SOA is also much higher than by PCGP. By means of background-subtraction and filtering on the FROG traces, the retrieved pulse profiles almost accord with reality. Typically for a 256 × 256 FROG trace, SOA-FROG iterates 100.8 circles per second, about 5 times faster than the PCGP. After 300 iteration circles by SOA in about 3 s, most of test pulses can finish their routines of reconstruction and reach high accuracy. Besides SHG-FROG, the SOA-FROG algorithm can also be utilized in other FROG techniques based on the 3rd order nonlinear optical effects. In summary, the SOA-FROG is expected to be suitable to the real-time pulse measurement with high accuracy in most of application cases. Yet some measures of improvement should be taken to reconstruct complex pulses with rough spectral phases or/and broken spectra.
, , Received Date: 2020-10-20
Abstract +
The working temperature of the crystalline silicon photovoltaic (CSPV) module seriously restricts the cell efficiency and the module lifetime. Therefore, it is of great significance to investigate the cooling effects of PV modules. Recently, introducing nanostructures into polymer backsheets to obtain direct-cooling effects due to enhanced heat conduction and radiation characteristics, has become a new trend for PV cooling technology. In this paper, we study the backside thermal properties of the PV module by combining the energy balance equation and optical simulations. The thermal power and cooling effect are calculated and compared between the standard backsheet and three types of direct-cooling backsheets for three typical ambient temperatures. The structure parameters and encapsulating mode of mainstream commercial silicon cells are adopted in the simulations and calculations. The influences of thermal parameters, i.e, the heat transfer coefficient and the emissivity, on the thermal process and the operating temperature are discussed in detail. We hope that this study may provide a certain reference for the future design of PV-direct-cooling backsheets.
, , Received Date: 2020-10-17
Abstract +
In the insulation system of power equipment, the partial discharge (PD) of short period does not cause the insulation to produce the penetrating breakdown, however the long-term PD of is one of the important causes of local deterioration, and even breakdown in dielectric. Therefore, it is very important to study the location of PD source and the calibration of discharge intensity. To achieve this, in this paper we take the needle-plate discharge model for example and go through the following steps respectively. Firstly, combined with the positive correlation between the ultrasonic signal and the apparent discharge magnitude in the process of PD, the ultrasonic method to detect partial discharge can be implemented. Then, based on the principle of time difference of arrival method (TDOAM), the accuracy of location is analyzed by using quantum genetic algorithm (QGA), genetic algorithm (GA), simulated annealing algorithm (SAA), particle swarm optimization (PSO) and generalized cross correlation method (GCC), respectively. And thus, starting from the study of the attenuation effect of sound pressure caused by the propagation loss, reflection and refraction of acoustic wave, the calibration model of PD intensity is established for the first time after determining the location of PD source with high precision. Some important findings are extracted from simulations and experimental results. First, the localization algorithm of PD source with high precision is observed. The localization of PD source by means of QGA is the most accurate, with maximum deviation of (0.27 ± 0.13) cm. Comparing with GA, SAA, PSO and GCC, the accuracy of location is improved by 33.57%, 41.51%, 32.11% and 87.26%, respectively. Second, due to the attenuation effect of sound pressure, when the measured voltage amplitude of ultrasonic signal is the same, the apparent discharge magnitude of PD source gradually increases with the test distanceincreasing. When the test distance is 37.80 cm, the apparent discharge magnitude of PD source is 633.83 pC, which increases by 28.51% compared with 7.00 cm. Moreover, simulation results and measurement results are compared with each other and they are well consistent. The discharge curve almost coincides with the calibration fitting curve of PD source when the test distance is 7.00 cm. Finally, it is concluded that the discharge intensity calibration model of PD source is accurate, which is of great significance in evaluating the extent of insulation damage.
, , Received Date: 2020-08-17
Abstract +
Ultra-short pulse (picosecond) anti-Stokes laser can be obtained by using Raman frequency converter in a crystal medium by the coherent anti-Stokes Raman scattering effect. The crystalline Raman frequency converter based on the pump-probe method can realize the collinear interaction of coherent anti-Stokes Raman scattering, thus effectively improving the conversion efficiency of the anti-Stokes light. Theoretical simulation is an important means to study laser operation. Coupled wave equation is widely used to study the characteristics of Raman laser and anti-Stokes laser. Although the coupling wave theory of anti-Stokes Raman frequency shifter reported previously can reflect the operation law of the frequency shifter, the optimization of the frequency shifter and the influence of the frequency shifter parameters on the output characteristics of anti-Stokes laser have not been reported so far. In this paper, the picosecond anti-Stokes Raman frequency converter based on the pump-probe method is studied theoretically. Considering the generation of the first Stokes light in the probe channel and the second Stokes light in the pump channel, the coupled wave equation of the collinear picosecond anti-Stokes Raman frequency converter is established under the plane wave approximation. Without loss of generality, four dimensionless comprehensive parameters are introduced to normalize the equations. A set of universal theoretical curves describing the operation of the Raman frequency converter is obtained. The numerical solutions of the equations show that the performance of the Raman frequency converter mainly depends on three parameters: the normalized phase mismatch parameter ΔK, the normalized Raman gain coefficient G, and the energy ratio of the probe light to the fundamental light rprobe. The reasonable values of normalized variables are determined when the high efficiency anti-Stokes conversion is realized. Experimental data are used to verify the correctness of the theoretical model. The theoretical value of the anti-Stokes conversion efficiency is basically consistent with the literature data. The normalized coupled wave theory proposed in this paper is helpful in understanding the operation law of the picosecond anti-Stokes Raman frequency shifter, and has guiding significance for the design of the frequency converter.
, , Received Date: 2020-10-10
Abstract +
The rapid development of social economy leads the output of solid waste to increase rapidly. The traditional treatment methods, such as landfilling, incineration and composting, are not only inefficient, but also have many limitations, such as secondary pollution and waste of resources. Therefore, it is urgent to explore new solid waste treatment technology. Due to its high efficiency, environmental protection and high energy conversion, the plasma gasification technology has been applied to the harmless treatment of solid waste. This article introduces the background and significance of plasma gasification technology in solid waste treatment, and summarizes the application of plasma gasification technology to different solid waste treatments, the technical level and research progress of plasma gasification of solid waste in the world are described in detail, and the existing problems in the current application of plasma gasification of solid waste are emphatically analyzed. It is pointed out that plasma gasification technology is an effective way to treat solid waste.
, , Received Date: 2020-08-10
Abstract +
Quantum entanglement is an essential quantum resource. With the development of quantum information science, quantum network consisting of quantum nodes and quantum channels has attracted extensive attention. The development of quantum information network requires the capability of generating, storing and distributing quantum entanglement among multiple quantum nodes. It is significant to construct the quantum information, and it has very important applications in the distributed quantum computation and quantum internet. Here we propose a simple and feasible scheme to deterministically entangle three distant atomic ensembles via the interference and feedforward network of the light-atom mixed entanglement. Firstly, three atomic ensembles placed at three remote nodes in a quantum network are prepared into the mixed entangled state of light and atomic ensembles via the spontaneous Raman scattering (SRS) process. Then, the first and second Stokes optical field are interfered on an R1∶T1 optical beam splitter (BS1), and one of the output optical fields from the first optical beam splitter is interfered with the third Stokes field on the second R2∶T2 optical beam splitter (BS2). The quantum fluctuations of the amplitude and phase quadratures of these three output optical fields from BS1 and BS2 are detected by three sets of balanced homodyne detectors, respectively. Finally, the detected signals of the amplitude and phase quadratures are fed to the three atomic ensembles via the radio frequency coils to establish the entanglement among three remote atomic ensembles. At the user-controlled time, three read optical pulses can be applied to these three atomic ensembles to convert the stored entangled state from the atomic spin waves into the anti-Stokes optical fields via the SRS process. According to the tripartite inseparability criterion, the correlation variance combinations of these three anti-Stokes optical fields can be used to verify the performance of entanglement of three atomic ensembles. This scheme can be extended to larger-scale quantum information network with different physical systems and more atomic nodes. Moreover, the entanglement distillation can be combined with this scheme to realize the entanglement among longer distance quantum nodes.
, , Received Date: 2020-08-09
Abstract +
Cardiac arrhythmias can be caused by the occurrence of electrical spiral waves and spatiotemporal chaos in the cardiac tissues, as well as by the topological changes of cardiac tissues resulting from the electrical coupling of cardiomyocytes to fibroblasts (M-F coupling). How to control the arrhythmia induced by spiral wave or spatiotemporal chaos is the problem which has attracted much attention of scientists. In this paper, a two-layer composite medium is constructed by using cardiomyocytes and fibroblasts. Luo-Rudy phase I cardiac model and passive model of fibroblast are used to study the effects of the M-F coupling on the formation of spiral wave and the control of spiral wave and spatiotemporal chaos in a two-layer composite medium. A control scheme that the spiral waves and spatiotemporal chaos are controlled by increasing the coupling strength between cells is proposed. The numerical results show that the M-F coupling has an important influence on the dynamics of spiral wave. With the increase of the density of fibroblasts, the M-F coupling may result in spiral wave meandering and spiral wave breaking into spatiotemporal chaos, and even induce the transition from spatiotemporal chaos (or spiral wave) to no wave. The eliminating spiral wave and spatiotemporal chaos in the composite medium by increasing the coupling strength between cells is only effective in most of cases, depending on the role played by fibroblasts. When fibroblasts act as current sinks for the cardiomyocyte, the spiral wave and spatiotemporal chaos are effectively eliminated only in most of cases by increasing the coupling strength between cells, and the controlled area is small. When fibroblasts act as a current source for the cardiomyocyte, increasing the coupling strength between cells to a value higher than a critical value can effectively terminate spiral wave and spatiotemporal chaos, and the controllable area is greatly increased compared with the former. Increasing the coupling strength between cardiomyocytes is a key factor in controlling the spiral waves and spatiotemporal chaos.
, , Received Date: 2020-08-03
Abstract +
In this paper, the effects of a Gaussian white noise excitation on the one-dimensional Frenkel-Kontorova (FK) model are studied by the stochastic Runge-Kutta method under two different types of substrate cases, i.e. incommensurate case and commensurate case. The noise excitation is considered through the inclusion of a stochastic force via a Langevin molecular dynamics approach, and we uncover the mechanism of nano-friction phenomenon in the FK model driven by the stochastic force. The relationship between the noise intensity and the nano-friction phenomenon, such as hysteresis, maximum static friction force, and the super-lubricity, is investigated by using the stochastic Runge-Kutta algorithm. It is shown that with the increase of noise intensity, the area of the hysteresis becomes smaller and the maximum static friction force tends to decrease, which can promote the generation of super-lubricity. Similar results are obtained from the two cases, in which the ratios of the atomic distance to the period of the substrate potential field are incommensurate and commensurate, respectively. In particular, a suitable noise density gives rise to super-lubricity where the maximum static friction force vanishes. Hence, the noise excitation in this sense is beneficial to the decrease of the hysteresis and the maximum static friction force. Meanwhile, with the appropriate external driving force, the introduction of a noise excitation can accelerate the motion of the system, making the atoms escape from the substrate potential well more easily. But when the chain mobility reaches a saturation state (B = 1), it is no longer affected by the stochastic excitation. Furthermore, the difference between the two circumstances lies in the fact that for the commensurate interface, the influence of the noise is much stronger and more beneficial to triggering the motion of the FK model than for the incommensurate interface since the atoms in the former case are coupled and entrapped more strongly by the substrate potential.
, , Received Date: 2020-07-17
Abstract +
After high pressure shock, the shock wave in the metal is unloaded at the metal-gas interface, and micro spallation occurs when the metal melts. When the micro spallation develops to a certain extent, the high pressure gas penetrates the zero pressure vacuum gap between the metal melt droplets. In this paper, the phenomenon of gas penetrating metal micro spallation zone is analyzed theoretically. Based on the regular hexahedron periodic arrangement of metal droplets, the calculation formulas of the maximum penetration depth, the sealing time of the penetration channel and the maximum mass of the gas penetrating the metal micro spallation zone are given through theoretical analysis under the quasi-static and semi-dynamic conditions. The quasi-static process is considered to be the gas penetration process that can be approximated as the escape process of gas into the vacuum, and the gap in the metal micro spallation zone will be filled with gas. The semi-dynamic analysis is based on two basic assumptions: one is the equal droplet size and spacing in the micro spallation zone and the other is the critical sealing condition of gas penetration. In the process of semi-dynamic analysis it is demonstrated that the initial critical sealing distance is independent of the shape factor of the droplet single control volume. The semi-dynamic analysis can give various critical sealing information when the gas stops penetrating the metal micro spallation zone. The results of quasi-static analysis can be used as the upper limit of gas penetration, and the semi-dynamic analysis results can be used as the lower limit of gas penetration. From the sensitivity analysis, it can be seen that the change law of physical phenomena given by theoretical analysis accords with the basic physical understanding of the problem. Through this study, the upper and lower limit of the mixed state of gas penetrating the metal micro spallation zone can be estimated, which can provide more accurate initial metal-gas mixed state for subsequent research of the evolution of mixed state. The theoretical analyses given in this paper are based on a lot of uncertain assumptions, and the in-depth study of this phenomenon is still needed based on the law summary and mutual confirmation of experiment and simulation.
,
Abstract +
Understanding and quantifying the main factors that affect power conversion efficiency is crucial for perovskite solar cells. At present, the three loss mechanisms generally recognized are the optical loss, Ohmic loss, and non-radiative recombination loss. Including trap-assisted bulk recombination and surface recombination, non-radiative recombination proves to be the dominant recombination mechanism that prohibits the increase of efficiency. Taking all the above loss factors into consideration, an equivalent circuit model is proposed to describe the current density-voltage characteristics of perovskite solar cells. Furthermore, by comparing the results from drift-diffusion model with the experimental results, the reliability of the proposed model is verified, and the relative fitting error is less than 2%. According to the model, the dominant recombination type can be clearly identified. Through retrieving the physical parameters corresponding to different loss mechanisms from experimental current density-voltage curves, the model is capable of giving corresponding voltage evolutions in the scanning process, which intuitively reveals the physical principles of efficiency loss. Meanwhile, through analyzing the influences of different loss mechanisms on characteristics of solar cells, the model offers a guideline in effectively approaching the efficiency limit from a circuit-level perpesctive. Overall, this model is a comprehensive simulation and analysis tool for perovskite solar cell.
, , Received Date: 2020-07-09
Abstract +
According to the luminous spectrum characteristics of white light emission diode (WLED) light emission spectrum, through the analysis of the intersection (the trough point of the whole spectrum) of blue light spectrum and yellow light spectrum generated by blue light excited yellow phosphor, in this paper we design an LED steady-state thermal resistance measurement system based on the spectroscopic method by using the conventional spectrometer, and we also use the normal driving current to fit the whole spectrum trough through a certain function algorithm. According to the temperature rise curve, we can calculate the temperature rise of the LED junction temperature relative to the substrate under any working condition, and combine the heat dissipation power of the LED to get the steady-state thermal resistance of the LED. This method avoids the limitation of a similar forward voltage drop method which uses the minimum current calibration and requires the modules of high-speed data acquisition and high-speed sampling conversion, thus making the equipment expensive. Therefore it is necessary to reduce its cost. Finally, the system designed in this paper and the T3ster instrument of Mentor Graphics Corporation in the United States are both used to measure various LEDs and their results are compared with each other. The results show that the maximum deviation of steady-state thermal resistance is only 3.64%. It indicates that the system and method designed in this paper can achieve the same precision as T3ster instrument of Mentor Graphics Corporation, demonstrating that the system and method designed in this paper can achieve the same precision as the T3STER instrument of Mentor Graphics, under the condition without needing expensive equipment, Moreover, this method uses non-traditional spectral method to measure the junction temperature of LED, which has the characteristics of remote real-time online detection of LED junction temperature, low cost, and no restrictions on the LED packaging structure. Therefore, this method has a wider application range than the voltage method adopted by Mentor Graphics T3ster equipment, and has a certain practical value.
, , Received Date: 2020-09-28
Abstract +
Inductively coupled plasma generator is often used to simulate high enthalpy and high speed plasma sheath, which is one of the core components of near-space high-speed target plasma electromagnetic scientific experimental research device. In order to study the discharge characteristics of inductively coupled plasma generator under high power, low frequency and low pressure, the numerical simulation and experiment are combined to study its internal heat transfer and flow characteristics in this paper. Based on the local thermodynamic equilibrium conditions, the numerical simulation of large-scale low-frequency and low-pressure inductively coupled plasma with a power of 100–400 kW is carried out through the multi-field coupling of flow, electromagnetic and temperature field, and verified by light intensity and spectrum experiment. The results show that the electromagnetic field distribution in the high-power thermal balance inductively coupled plasma generator is similar to that of the small- and medium-sized power plasma generator. The discharge energy dissipation occurs mainly in the area where the induction coil is located. The temperature of the inner wall of the quartz tube is higher at the coil location than in other areas, and the plasma has a ring-shaped high-temperature structure. The outer wall of the quartz tube is set to be the boundary condition of heat flux for simulating the temperature change of the quartz tube under cold blowing. This setting is in coincidence with factual situations. The wall temperature of the quartz tube at the entrance and at the induction coil section are found to be relatively high. When the large-size inductively coupled plasma generator works, an obvious return vortex is generated at the entrance due to the temperature difference and the electromagnetic pumping effect, and the exit velocity increases slightly with the increase of power. At the same time, the discharge experiment under the corresponding conditions shows that there is found a ring structure with bright edges and dark centers in the axial image of the argon discharge. Moreover, the emission spectrum of argon plasma is measured through the spectrum diagnosis system and the spatial distribution of the generator electron temperature is obtained. The light intensity of the discharge image and the electron temperature measured by the spectrum are found to be in comparative coincidence with the COMSOL simulation temperature result, demonstrating the validity of the numerical simulation results under thermodynamic equilibrium conditions. The numerical simulation results in this paper are also applicable to the optimization design and temperature resistance evaluation of the inductively coupled plasma generator.
, , Received Date: 2020-10-23
Abstract +
Because of the high water flux and excellent ion rejection, the pores graphene is considered as a promising candidate material for fabricating the membranes in reverse osmosis (RO) process. Unfortunately, water molecules cannot pass through the perfect graphene, and how to effectively create a large number of nanopores with controllable size remains a challenge, which seriously prevents the practical application and development of graphene membrane for desalination. Recently, the emergence of pillared graphene (PGN) might open a new way for designing the graphene-based membranes, which can compensate for the deficiency of porous graphene membrane. The PGN has been extensively studied in gas storage and separation, and its RO characteristics and mechanism still remain unclear because the limitation of large area preparation in desalination. In this paper, the RO process of seawater through PGN membranes is investigated by molecular dynamics simulations, and the influences of the pressure within feed solution, temperature and the shearing of membrane on the desalination properties are considered. It is found that the water flux increases linearly with the pressure within feed solution increasing, and the PGN membrane with nanopore diameter of 0.8 nm can conduct water molecules but completely rejects high-concentration ions. As the diameter of nanopores increases to 1.2 nm, the rise of temperature can increase the permeability of water molecules, whereas the salt rejection is not appreciably sensitive to the temperature. Particularly, the shearing membrane can improve the salt rejection and hinder the water molecules from permeating at the same time. The designed PGN membrane exhibits excellent performance of water purification, and the ultrahigh water flux obtained in this work reaches 56.15 L·cm–2·day–1·MPa–1 with a salt rejection of 88.9%. Subsequently, the hydrogen bond dynamics is calculated in order to explain the variation of water permeability under different conditions. The result shows that the rise of temperature reduces the stability of hydrogen bonds and leads the water flux to increase, while the increase of shearing speed will enhance the stability of hydrogen bonds and inhibit water seepage. Furthermore, the analysis results of hydrogen bond and ionic hydration structure show that the shear motion on RO membrane will improve the stability of ionic hydration shell, which makes it more difficult for the ions to pass through nanopores by weakening the hydration shell. On the contrary, rising temperature will impair the strength of ionic hydration shell, leading more ions to pass through the RO membrane. The simulation results can provide an in-depth understanding of the desalination performance of PGN membrane under different key conditions, and further demonstrate the promising applications of graphene-based membrane in water desalination.
, , Received Date: 2021-01-14
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