Pulse Characteristics Research Articles

Time-of-flight spectroscopy for laser-driven proton beam monitoring

Application experiments with laser plasma-based accelerators (LPA) for protons have to cope with the inherent fluctuations of the proton source. This creates a demand for non-destructive and online spectral characterization of the proton pulses, which are for application experiments mostly spectrally filtered and transported by a beamline. Here, we present a scintillator-based time-of-flight (ToF) beam monitoring system (BMS) for the recording of single-pulse proton energy spectra. The setup’s capabilities are showcased by characterizing the spectral stability for the transport of LPA protons for two beamline application cases. For the two beamline settings monitored, data of 122 and 144 proton pulses collected over multiple days were evaluated, respectively. A relative energy uncertainty of 5.5% (1upsigma) is reached for the ToF BMS, allowing for a Monte-Carlo based prediction of depth dose distributions, also used for the calibration of the device. Finally, online spectral monitoring combined with the prediction of the corresponding depth dose distribution in the irradiated samples is demonstrated to enhance applicability of plasma sources in dose-critical scenarios.

Operating region-dependent characteristics of weight updates in synaptic In–Ga–Zn–O thin-film transistors

We present a study on characteristics of operating region-dependent weight updates in a synaptic thin-film transistor (Syn-TFT) with an amorphous In–Ga–Zn–O (IGZO) channel layer. For a synaptic behavior (e.g. a memory phenomenon) of the IGZO TFT, a defective oxide (e.g. SiO2) is intentionally used for a charge trapping due to programming pulses to the gate terminal. Based on this synaptic behavior, a conductance of the Syn-TFT is modulated depending on the programming pulses, thus weight updates. This weight update characteristics of the Syn-TFT is analyzed in terms of a dynamic ratio (drw) for two operating regions (i.e. the above-threshold and sub-threshold regimes). Here, the operating region is chosen depending on the level of the gate read-voltage relative to the threshold voltage of the Syn-TFT. To verify these, the static and pulsed characteristics of the fabricated Syn-TFT are monitored experimentally. As experimental results, it is found that the drw of the sub-threshold regime is larger compared to the above-threshold regime. In addition, the weight linearity in the sub-threshold regime is observed to be better compared to the above-threshold regime. Since it is expected that either the drw or weight linearity can affect performances (e.g. a classification accuracy) of an analog accelerator (AA) constructed with the Syn-TFTs, the AA simulation is performed to check this with a crossbar simulator.

Complete characterization of picosecond optical pulses by the offset frequency intensity modulation.

We present a method for characterizing the intensity waveform, spectrum, frequency chirp, and spectral phase of picosecond pulses at a moderate repetition rate of ∼100 MHz. The proposed method exploits the intensity modulation at ∼10 GHz, which is slightly offset from the integer multiple of the repetition rate of the pulses. The modulated pulses are split into two, and one is measured by an optical spectrum analyzer, whose output is detected by a lock-in amplifier, while the other is directly detected by a photodiode and its output is used as a reference signal of the lock-in amplifier. In the experiment, we demonstrate the measurement of picosecond Ti:sapphire laser pulses to investigate frequency chirp induced by self-phase modulation. We anticipate that the proposed method will be useful for the characterization of various types of picosecond pulses.

Microscopic analysis of Trichel's pulse characteristics and the effect of air pressure and temperature

The negative direct current corona discharge experiment under the needle-plate electrode is conducted, and the typical Trichel pulse waveform is obtained. Furthermore, to analyze the microscopic process of corona discharge, a plasma-fluid discharge simulation model, including air chemical reactions, is established, and a Trichel pulse waveform is obtained, which is consistent with the experimentally obtained waveform. Thus, the chemical reaction that plays a dominant role in the generation and disappearance of charged particles as well as the spatiotemporal distribution and quantitative variation of charged particles during the discharge process are clarified. On this basis, the variation of the microscopic and macroscopic parameters in the discharge process under different air pressure and temperature conditions is studied. The results show that microscopic parameters, such as charge density and electric field intensity, are negatively associated with air pressure and positively associated with temperature, while macroscopic parameters, such as Trichel pulse average current, average peak current, and discharge frequency, are positively associated with the microscopic parameters.

Parameter Measurement Method of PD Radar Based on Non-coherent Accumulation

With the improvement in the anti-reconnaissance performance of the new system radar, the signals sent by the radar are more and more difficult to be detected, which brings new challenges to electronic reconnaissance. In order to find the enemy earlier, the receiver usually needs high sensitivity to detect weak signals. Therefore, according to the characteristics of airborne PD radar pulses, a method of PD radar parameter measurement based on non-coherent accumulation is proposed, which is of great significance to improve the receiver sensitivity. In this paper, the main peaks’ solution of PD Radar spectrum after non-coherent accumulation and the method of pulse parameter measurement are introduced in detail. Finally, the effectiveness of this method is verified by simulation experiments.

Theoretical Analysis of Self-Heating and Cooling Processes in the Pulsed Field Emission From ZnO Nanowire for Achieving High Emission Current

Quasi-one-dimensional (Q1D) nanowire field emitter arrays (FEAs) have important applications in vacuum microelectronics. Investigation on its pulsed field emission characteristics is important for not only avoiding self-heating-induced vacuum breakdown but also achieving high emission current. In this work, a model for quantitative calculation of the transient emission current and temperature distribution of individual nanowire during pulsed field emission has been established by considering the time-dependent thermal conduction equation. Taking ZnO nanowire as an example, the influences of the pulsewidth, the nanowire length, the nanowire radius, and the electrical properties of nanowire on its maximum transient emission current have been investigated. Besides, the cooling process is found to be mainly related on the nanowire length when the nanowire temperature is below 1000 K. By calculating the average emission current under different pulsewidths and duty ratios, it is found that its maximum is determined by the maximum emission current in the steady state that the nanowire is self-heating steadily to the critical temperature below the melting point. Finally, discussions for improving the emission current in different potential applications of FEAs are given. The results provide guidelines for designing Q1D nanowire FEAs.

On the critical pressure pulse width for ignition of hydrogen–air and dimethyl ether–air mixtures

Under certain situations, such as a water hammer event, a volume of gas can undergo a rapid compression immediately followed by a rapid expansion, which could lead to auto-ignition under certain conditions. The goal of the present study was to investigate under which critical conditions such a pressure pulse could lead to an ignition event. To simulate the pressure pulse, a simplified Gaussian pressure pulse model was implemented in Cantera. Hydrogen–air and dimethyl ether–air mixtures were employed to study the ignition dynamics of chemical systems with a single and a double step of heat release. The numerical simulations were performed for three peak pressures of 101.3, 1013.2, and 5066 kPa, for a range of compression ratios between 5 and 45, and various pulse widths, which determine the characteristics of the pressure pulse. Depending on the width of the Gaussian pulse, either ignition or chemical quenching were observed. Critical ignition, just at the limit between ignition and quenching, is the result of the competition between chemical reaction and volumetric expansion due to the imposed specific volume changes. This competition can be summarized by the concept of critical pressure pulse width, which is an extension of the critical decay rate concept. To further investigate the mechanisms responsible for critical ignition and quenching, quantitative thermo-chemical analyses were performed for sub-critical, critical and super-critical conditions. A Damköhler number was defined as the ratio of characteristic time of expansion to ignition delay time to characterize the competition between ignition and quenching. A simplified non-dimensional pressure pulse model for which the derivative of the naperian logarithm of density was assumed to follow a linear law was established. It can be employed to derive the critical conditions for which ignition fails. The evolution of non-dimensional temperature was obtained numerically and showed the existence of a critical pulse width for successful ignition and that the location of ignition delay time moves from the expansion phase to the compression phase as the non-dimensional pulse width is increased.

Simulation of Electromagnetic Implosion of Metal Shells to Obtain Supercritical Fluids

This study analyzes the conditions for creating the energy density necessary to obtain supercritical fluids of substances with parameters (temperature T > 1 eV, density N > 1022 cm−3, specific energy density ε > 100 kJ/g). The calculations are carried out on the basis of the one-dimensional (1D) two-temperature (2T) magneto hydrodynamic radiation model, which takes into account the physical processes occurring in the energy storage, switching system and the pulsed plasma load-a cylindrical compressible conductive shell. Developing a mathematical model, we assumed that physical processes were self-consistent. The simulation results were presented as time dependences of the main process parameters. Calculations showed that it becomes possible to sharpen the radiation pulse and pressure in the shock wave. As a result, we formulated the requirements for a laboratory energy source to establish the characteristics of a current pulse flowing through a conductive cylindrical shell and its dimensions (radius and thickness) necessary to achieve the goal.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

Generation of coherent THz Cherenkov radiation by electron bunch tilt control

We propose a new method that generates coherent Cherenkov radiation by controlling the tilt of the electron bunch. In this scheme, the wavelength band of coherent radiation depends on the transverse size of the electron bunch. This study demonstrates a coherent Cherenkov radiation (CChR) pulse in the THz region corresponding to the beam size. To generate the CChR, we used a 4.8-MeV electron bunch from a photocathode radio-frequency (rf) gun and controlled the angle through an rf transverse deflecting cavity. Our new method increased the pulse intensity by a factor of 7.3 compared to the case without tilting and the THz pulse characteristics well agreed with the prediction.

Random quasi-phase-matching for pulse characterization from the near to the long wavelength infrared.

Experiments requiring ultrafast laser pulses require a full characterization of the electric field to glean meaning from the experimental data. Such characterization typically requires a separate parametric optical process. As the central wavelength range of new sources continues to increase so too does the need for nonlinear crystals suited for characterizing these wavelengths. Here we report on the use of poly-crystalline zinc selenide as a universal nonlinear crystal in the frequency resolved optical gating characterization technique from the near to long-wavelength infrared. Due to its property of random quasi-phase-matching it's capable of phase matching second-harmonic and sum-frequency generation of ultra-broadband pulses in the near and long wavelength infrared, while being crystal orientation independent. With the majority of ultra-fast laser sources being in this span of wavelengths, this work demonstrates a greatly simplified approach towards ultra-fast pulse characterization spanning from the near to the long-wavelength infrared. To our knowledge there is no single optical technique capable of such flexible capabilities.

Parameter study on characteristic pulse diagram of polycystic ovary syndrome based on logistic regression analysis

This study aimed to explore the parameters of the independent predictive characteristic pulse diagram of polycystic ovary syndrome (PCOS) by analysing the pulse characteristics between healthy women and the PCOS group. A total of 278 women were recruited for this study. Pulse wave parameters were collected by the pulse spectrum analyser. The single-factor analysis of the pulse diagram parameters was used to identify significant indicators, and the logistic regression analysis was carried out on the above indicators with statistical differences to obtain independent predictors. According to the single-factor and multi-factor analyses, h1, h5, h3/h1, t, t1 and t5 were independent predictors of PCOS diagnosis. The results showed that PCOS patients had a faster heart rate, decreased left ventricular systolic function and decreased aortic compliance compared to healthy individuals. These findings suggested that the characteristic pulse parameters screened out are valuable for the diagnosis of PCOS. IMPACT STATEMENT What is already known on this subject? Polycystic ovary syndrome (PCOS) is a common gynecological reproductive endocrine and metabolic disease, which is significant for screening and early intervention in the disease. However, due to the lack of pulse’s diagnostic evidence of PCOS, there is still an unknown area in the research on the correlation between PCOS and pulse diagram parameters. What do the results of this study add? This study fills the gap between the research on PCOS and pulse wave. The study also shows that the pulse characteristic parameters h1, h5, h3/h1, t, t1, and t5 are independent predictors of PCOS, suggesting that the patients have a higher heart rate, lower ventricular systolic function, and aortic compliance than healthy individuals. What are the implications of these findings for clinical practice and/or further research? Prominent risk factors for pulse parameters associated with the occurrence of PCOS facilitate early screening and diagnosis of the disease. The objectification of pulse diagnosis helps to establish a health management model, which can be used for the accurate assessment and treatment of PCOS by traditional Chinese medicine (TCM). It provides a clinical reference for the study of pulse diagnosis objectification.

Realization of harmonic mode-locking in thulium-doped fiber laser using In2(SO4)3-based saturable absorber

Harmonic mode-locking (HML) is realized in a thulium-doped fiber laser (TDFL) using indium sulfate (In2(SO4)3) as a saturable absorber (SA). The In2(SO4)3 has nonlinear saturable absorption properties with the nonlinear optical modulation depth of 13.3 % and the saturation power of 0.11 kW at 1930 nm. The fundamentally mode-locked pulses operate at 1964.44 nm have the 3-dB bandwidth of 4.70 nm, the pulse width of 1.266 ps, and the pulse repetition rate of 25.37 MHz. The maximum resonance frequency of 228 MHz is achieved, which corresponds to the 9th harmonic. The supermode suppression level (SMSL) up to 63 dB is realized. The spectral and temporal characteristics of the output pulses are investigated as a function of harmonic order. The experimental results demonstrated that In2(SO4)3 can be a promising mode-locker for generating harmonic ultrafast pulses in 2 μm waveband.

Dynamic Properties of Non-Autonomous Femtosecond Waves Modeled by the Generalized Derivative NLSE with Variable Coefficients

The primary purpose of this study is to analyze non-autonomous femtosecond waves with various geometrical configurations correlated to the generalized derivative nonlinear Shrödinger equation (NLSE) with variable coefficients. Numerous academic publications, especially in nonlinear optics, material science, semiconductor, chemical engineering, and many other fields, have looked into this model since it is closer to real-world situations and has more complex wave structures than models with constant coefficients. It can serve as a reflection for the slowly altering inhomogeneities, non-uniformities, and forces acting on boundaries. New complex wave solutions in two different categories are proposed: implicit and elliptic (or periodic or hyperbolic) forms are obtained for this model via the unified method. Indeed, the innovative wave solutions that were achieved and reported here are helpful for investigating optical communication applications as well as the transmission characteristics of light pulses.

Ultrashort Pulse Retrieval from Experimental Spectra Transformed in Chalcogenide and Silica Fibers

The characterization of ultrashort optical pulses is a highly requested task. The most popular commercially available hardware/software systems are based on interferometric measurements and second-harmonic generation, leading to some ambiguities and limitations. Here we experimentally test the non-interferometric method of pulse retrieval from three spectra: the fundamental spectrum and two spectra that transformed in an element with Kerr nonlinearity and accumulated different nonlinear phases (different Β-integrals). This method has no ambiguities related to time direction, and allows simple hardware/software implementation. We test a novel simple algorithm for experimental data processing based on the search for a polynomial-approximated spectral phase. Two experimental cases are considered. In the first one, we retrieved 160 fs pulses using a chalcogenide arsenic sulfide glass fiber as a nonlinear Kerr element. In the second case, we retrieved 670 fs pulses with a complex spectrum using a piece of silica-based fiber. The results are confirmed by independent measurements using a standard SHG-FROG technique (Second-Harmonic Generation Frequency-Resolved Optical Gating).

A novel method for small and unbalanced sample pattern recognition of gas insulated switchgear partial discharge using an auxiliary classifier generative adversarial network

AbstractConvolutional neural networks (CNNs) have been widely used for gas‐insulated switchgear (GIS) partial discharge (PD) pattern recognition due to their powerful feature extraction ability. However, there is commonly a scarcity of fault samples due to low insulation failure rate of GIS equipment, which degrades the diagnostic performance of these CNN networks when directly applied to small and unbalanced datasets. Therefore, we propose a novel auxiliary classifier generative adversarial network for GIS PD pattern recognition for small and unbalanced samples. First, we propose using synchrosqueezed wavelet transform to extract time‐frequency characteristics of PD pulses and obtain a time‐frequency image with high energy aggregation and time‐frequency distribution rate. Then, we propose an improved generative adversarial network with an auxiliary classier and self‐attention mechanism, which can generate high‐quality PD samples for situations with few classes. Experiments show that our proposed method can reach 95.75% recognition accuracy for small datasets, which is the highest among several comparable methods. Furthermore, the proposed method has excellent and stable recognition performance for various unbalanced datasets.

Carrier-envelope-phase and helicity control of electron vortices and spirals in photodetachment.

Formation of electron vortices and spirals in photodetachment from the H- anion driven by isolated ultrashort laser pulses of circular polarization or by pairs of such pulses (of either co-rotating or counter-rotating polarizations) are analyzed under the scope of the strong-field approximation. It is demonstrated that the carrier-envelope phase (CEP) and helicity of each individual pulse can be used to actively manipulate and control the vortical and spiral patterns in the probability amplitude of photodetachment. Specifically, we show that the vortical patterns can be rotated in momentum space by the CEP of the driving pulse (or, of two identical pulses); thus, offering a tool of pulse characterization. For co-rotating pulses of arbitrary CEPs, a novel type of structured vortices is discovered. Also, we demonstrate that the momentum spirals are formed when photodetachment is driven by two pulses of time-reversal symmetry, which is accompanied by absolute disappearance of vortical structures. Hence, we attribute the spiral formation to annihilation of vortices with antivortices, which are generated by time-reversed pulses comprising the train. Finally, the CEP and helicity control of spiral structures is demonstrated, leading to their rotation in momentum space.

Nonlinear seismic response and index correlation of high arch dams under cross-stream oblique incidence of near-fault SV waves based on wavelet decomposition

This paper aims to propose an analysis framework for the response index correlation of high arch dams under cross-stream oblique incidence of near-fault pulse-like and non-pulse seismic waves. The influence of the dynamic pulse effect of near-fault seismic waves on the index correlation at different incidence angles is evaluated. Further, the seismic response through an arbitrary incident angle is quantitatively estimated. Several typical near-fault records of extracted strong velocity pulses are selected, and their decomposed non-pulse ground motions (GMs) are used to ensure the similarity and uniformity of the response spectra. Furthermore, a seismic input model of the oblique incidence of the SV wave is established based on the three-dimensional viscous-spring artificial boundary method, and its accuracy was confirmed through the numerical application. The engineering demand parameters (EDPs) of pulse-like SV waves and non-pulse SV waves, including dam damage, dynamic displacement, and joint opening, with cross-stream incident angles are discussed emphatically. Finally, the correlation between the response indices of near-fault SV waves at different incident angles is quantitatively analyzed. The numerical results indicate that pulse-like seismic waves with different input angles are more destructive than non-pulse seismic waves, and different response indices of the dam may have different most unfavorable incident angles. A prediction model of the dam response considering different input angles of seismic waves is proposed to roughly estimate the response index values of each input angle. The influence of the dynamic pulse characteristics on the correlation degree of response indices under different input angles is mainly divided into three types: no impact, increasing correlation degree, and decreasing correlation degree. In the seismic design and performance evaluation of arch dams, it should be suggested to comprehensively consider the influence of the oblique incident angle of GMs and near-fault pulse characteristics.

Characterization and modeling of drain lag using a modified RC network in the ASM-HEMT framework

Gallium-Nitride High Electron Mobility Transistors (GaN-HEMTs) are susceptible to trapping effects, significantly degrading device performance. The degradation can be seen in the current–voltage characteristics and also manifests in the form of a shift in the dynamic on-resistance and threshold voltage. In this paper, we present pulsed characterization and empirical modeling of a 250 × 10 μm RF GaN HEMT device. To study the impact of drain-induced trapping, pulse I-V characterization is performed at a fixed quiescent gate voltage of −7 V with varying drain quiescent voltage. An empirical RC network approach is proposed to accurately model the trapping in the device. The model effectively captures the impact of trapping and takes into account the self-limiting behavior of traps. The model is implemented in Verilog-A within the Advanced SPICE Model for High Electron Mobility Transistors (ASM-HEMT) framework. Validation of the model is done with six different quiescent conditions of pulse measurement with on-state drain bias varying from 0 V to 20 V.

Electrical and optical characterization of multi-hollow surface dielectric barrier discharge in configuration with the air-exposed electrode

In this paper, multi-hollow surface dielectric barrier discharge generated by a perforated ceramic substrate in a configuration with the air-exposed electrode was investigated. The electrical characteristics (discharge power, peak, and average amplitude of current pulses) and optical characteristics (emission intensity) of the discharge were evaluated under various conditions of applied voltage (peak voltage 3–6 kV, frequency 200–2000 Hz), air flow rate (0.5–2.4 L/min), and air relative humidity (0%–80%). Temperature of ceramic substrate was also monitored. Statistical analysis of current pulses was also performed, and histograms of amplitudes of current pulses were calculated. The results showed that discharge characteristics strictly depend on given working conditions. The analysis of current pulses showed opposite trends in average overall number of positive and negative pulses with an increase of discharge power: number of positive current pulses gradually increased, while number of negative current pulses slightly decreased. The highest peak currents were found at 4 kV (1.8 W). With further increase of peak voltage, peak current decreased and beginning of detection of current pulses upon a rising (declining) slope of applied voltage was slightly shifted toward earlier times. At the highest applied peak voltage, pulses appeared even before polarity of applied voltage reversed. Therefore, we suppose that a residual charge accumulated on dielectric surface plays a crucial role in characteristics of the current pulses. Significant influence on current pulses and discharge emission intensity was also found with a change of air relative humidity, while the effect of air flow rate was found weaker.