Pulse Shape Research Articles

Computational Prediction of Dielectric Breakdown Strength of a Transformer Paper in Oil with Uncertainty Quantification

Abstract The determination of the dielectric breakdown strengths of microstructurally heterogeneous materials has been a primarily experimental endeavor. We report the development of a microstructure-level model for computationally predicting the breakdown strength and analyzing the interactions between electromagnetic pulses (EMP) and the constituents in a composite of cellulose-based paper and mineral oil found in electrical transformers. The model allows explicit simulation of the material breakdown process by tracking the transition of dielectric constituents from non-conductive to conductive states. The focus is on the electric fields induced in the materials and the overall conditions for dielectric breakdown (defined as the onset of avalanche) caused by the electric field induced in the composite. Responses to three distinct pulse shapes, i.e., Steep Front (SF), Lightning (L), and AC with spectra spanning 60 – 9 × 105 Hz are considered. It is found that the breakdown strength of the material is significantly affected by microstructure heterogeneities, the spatial variations of the constituent properties, and the pulse shapes. A probabilistic characterization of the breakdown strength is computationally obtained and compared with experimental measurements. Although one particular material is analyzed, the model and approach are applicable to other heterogeneous materials as well.

Replication of Radial Pulses Using Magneto-Rheological Fluids

The radial pulse is a critical health marker with expanding applications in wearable technology. To improve these applications, developing a pulse generator that consistently produces realistic pulses is crucial for validation and training. The goal of this study was to design and test a cost-effective pulse simulator that can accurately replicate a wide range of age-dependent radial pulses with simplicity and precision. To this end, this study incorporated a magneto-rheological (MR) fluid device into a cam-based pulse simulator. The MR device, as a key component, enables pulse shaping without the need for additional cams, substantially reducing the cost and complexity of control compared with existing pulse simulators. To evaluate the performance of the MR pulse simulator, the root-mean-square (RMS) error criterion (less than 5%) was used to compare the experimentally obtained pulse waveform with the in vivo pulse waveform for specific age groups. After demonstrating that the MR simulator could produce three representative in vivo pulses, a parametric study was conducted to show the feasibility of the slope-based pulse-shaping method for the MR pulse simulator to continuously generate a range of age-related pulses.

Method to determine the optimal impedance profile of nonuniform transmission lines used for pulsed power accelerators

Nonuniform transmission lines (NTLs) are widely used in pulsed power accelerators because they provide an efficient way to achieve impedance matching and pulse shaping. Since designing and constructing these accelerators typically demands substantial effort, finding the optimal impedance profile to maximize the power transmission efficiencies of the NTLs is important. In this paper, a convenient numerical method to determine the optimal impedance profile is proposed. First, the output of the NTL with arbitrary parameters is theoretically analyzed under arbitrary input conditions. It was found that only four factors affect the power transmission efficiency: the ratio of output impedance to input impedance, the ratio of input pulse width to the NTL’s one-way transit time, the normalized impedance profile, and the normalized input pulse. Based on these findings, a method designed to minimize the reflected component within the working frequency range is proposed. Using this method, an impedance profile demonstrating superior power transmission efficiency compared to the traditional exponential profile is identified. This work can provide a rapid and effective method to determine the impedance profile of the NTL, undoubtedly benefiting the design process of pulsed power accelerators. Published by the American Physical Society 2024

Formation of High Areal Density Core using an Efficient and Robust Implosion method for Fast Ignition

Abstract A new fuel compression method for a fast ignition scheme is discussed. To form a high areal density fuel plasma for the ignition condition, homogenous isentropic compression with solid spherical target is effective. We improve a multi-step pulse shape method that uses progressive shockwaves and reflected shockwaves for the compression, where a precisely controlled step-pulse laser drives the shockwaves to compress the fuel and suppress entropy increase. Another advantage of this approach is the relatively smooth high dense fuel is distributed at maximum compression time, compared to our previous design based on Kidder’s homogenous isentropic compression method. In addition, we insert a power dip as a preconditioning before the last pulse step to reduce the electron and ion temperature near critical density. As a result, an optimum implosion is designed using 245 kJ of implosion laser energy to meet the ignition condition.

XFEL Beamline Optical Instrumentation for Ultrafast Science.

Free electron lasers operating in the soft and hard X-ray regime provide capabilities for ultrafast science in many areas, including X-ray spectroscopy, diffractive imaging, solution and material scattering, and X-ray crystallography. Ultrafast time-resolved applications in the picosecond, femtosecond, and attosecond regimes are often possible using single-shot experimental configurations. Aside from X-ray pump and X-ray probe measurements, all other types of ultrafast experiments require the synchronized operation of pulsed laser excitation for resonant or nonresonant pumping. This Perspective focuses on the opportunities for the optical control of structural dynamics by applying techniques from nonlinear spectroscopy to ultrafast X-ray experiments. This typically requires the synthesis of two or more optical pulses with full control of pulse and interpulse parameters. To this end, full characterization of the femtosecond optical pulses is also highly desirable. It has recently been shown that two-color and two-pulse femtosecond excitation of fluorescent protein crystals allowed a Tannor-Rice coherent control experiment, performed under characterized conditions. Pulse shaping and the ability to synthesize multicolor and multipulse conditions are highly desirable and would enable XFEL facilities to offer capabilities for structural dynamics. This Perspective will give a summary of examples of the types of experiments that could be achieved, and it will additionally summarize the laser, pulse shaping, and characterization that would be recommended as standard equipment for time-resolved XFEL beamlines, with an emphasis on ultrafast time-resolved serial femtosecond crystallography.

Design solutions for the hodoscope of the magnetic proton recoil neutron spectrometer of the SPARC tokamak.

A new 14MeV neutron spectrometer utilizing the magnetic proton recoil (MPR) technique is under development for the SPARC tokamak. This instrument measures neutrons by converting them into protons, whose momenta are subsequently analyzed using a series of magnets before detection by an array of scintillators known as the hodoscope. In this work, we explore various solutions for the hodoscope detectors through laboratory tests with radioactive sources and simulations. We present findings on light collection and pulse shape discrimination based on detector types, as well as optimal solutions for photo-detectors studying the differences between SiPM and PMT. Our results also led to the determination of a better optimized design for the hodoscope detectors, consisting of a 0.7cm width and a 13cm length EJ276D scintillation rod.

Electrohydrodynamic Printing of Biodegradable PLGA Micro‐Patterns on 3D Polymer Structures

AbstractBiodegradable polymers such as polylactic‐co‐glycolic acids (PLGA) are used for various implantable devices such as tissue scaffolds, drug delivery devices, and biosensors in different forms. However, high‐resolution patterning of biodegradable polymers on implantable devices has not been explored much yet. While electrohydrodynamic printing (EHD) can achieve high‐resolution printing compared to other printing methods, EHD printing of PLGA solutions is rarely attempted due to unstable printing. Such printing instability originates from the volatile nature of PLGA inks, and it causes nozzle clogging or change of ink conditions during printing. Here, PLGA ink formulation and a voltage input profile are studied for stable and high‐resolution EHD printing. Addition of glycerol at an optimal ratio as well as the control of voltage pulse shape strongly influenced both the stability and resolution of EHD printing of PLGA patterns. With the optimized inks and voltage inputs, stable printing of PLGA micropatterns down to 5 µm is achieved on both conductive and insulating surfaces for controlled drug release. Furthermore, use of a ring type electrode allows for EHD printing of PLGA micropatterns on 3D surfaces of PLLA tubes and stent struts.

Ultrafast reverse saturable absorption and all-optical computing with boron dipyrromethene (BODIPY) chromophore derivatives

Ultrafast reverse saturable absorption (RSA) in chromophore-based borondipyrro-methenes (BODIPYs) derivatives, 1,7-Diphenyl-3,5-bis(9,9-dimethyl-9H-fluoren-2-yl)-boron-diuoride-azadipyrromethene (ZL-61) and 1,7-Diphenyl-3,5-bis(4-(1,2,2-triphenyl-vinyl)phenyl)-boron-diuoride azadipyrromethene (ZL-22), has been theoretically analyzed with femtosecond (fs) laser pulses at 800 nm and 850 nm. The results are in good agreement with the reported experimental results. The effect of input pulse intensity, pulse width, nonlinear absorption (NLA) coefficients, sample thickness and laser pulse shaping on transmittance has been studied and optimized for high contrast and low-power operation. All-optical fs NOT and the universal NOR and NAND Boolean logic gates have been designed with ZL-61 and ZL-22. The logic gates with top-hat input pulse profile have a sharp switch ON/OFF time compared to the Gaussian input laser profile. Ultrafast operation at relatively low pump intensities opens up exciting prospects for the applicability of BODIPY derivatives for ultrafast low-power all-optical computing and information processing.

Temporal reflection from short pump pulses inside a dispersive nonlinear medium: the impact of pump parameters

We have studied, through a series of experiments and numerical simulations, how temporal reflection from an intense pump pulse inside a photonic crystal fiber is affected by parameters of the pump pulse used to form a moving high-index boundary. We used femtosecond pump pulses, which slow down inside the fiber as their spectrum red-shifts because of intrapulse Raman scattering. Temporal reflection of probe pulses occurs from such decelerating pump pulses. We changed the width and chirp of our pump pulses with a 4f pulse shaper capable of providing both spectral filtering and frequency chirping. We found that temporal refection exhibited novel features, to our knowledge, when pump pulses were made wider or chirped. In both cases, two or more reflected pulses were produced at different wavelengths in a specific range of the initial pump-probe delays. Numerical simulations reveal that the origin of such novel features is related to the complex nonlinear evolution of pump pulses inside optical fibers.

Three phase spectral interferometry for recording high dynamic range optical waveforms with <1 ps resolution over >2 ns records applied to closed-loop pulse shaping

We demonstrate, to our knowledge, a novel spectral interferometry technique that simultaneously captures three spectral interferograms of a signal waveform with a reference pulse. The measured performance is robust to nonidealities and ambient drifts by implementing a precisely calibrated 3×3 polarization maintaining (PM) splitter that provides three phase shift differences nominally spaced 120° apart. The system can achieve long record length by implementing three, high resolution, virtually imaged phase array (VIPA) spectrometers. Here, we experimentally implement this technique and demonstrate the measurement of waveforms with >2ns of record while maintaining <1ps resolution.

Design and Application of an Onboard Particle Identification Platform Based on Convolutional Neural Networks

Space radiation particle detection plays a crucial role in scientific research and engineering practice, especially in particle species identification. Currently, commonly used in-orbit particle identification techniques include telescope methods, electrostatic analysis time of flight (ESA × TOF), time-of-flight energy (TOF × E), and pulse shape discrimination (PSD). However, these methods usually fail to utilize the full waveform information containing rich features, and their particle identification results may be affected by the random rise and fall of particle deposition and noise interference. In this study, a low-latency and lightweight onboard FPGA real-time particle identification platform based on full waveform information was developed utilizing the superior target classification, robustness, and generalization capabilities of convolutional neural networks (CNNs). The platform constructs diversified input datasets based on the physical features of waveforms and uses Optuna and Pytorch software architectures for model training. The hardware platform is responsible for the real-time inference of waveform data and the dynamic expansion of the dataset. The platform was utilized for deep learning training and the testing of the historical waveform data of neutron and gamma rays, and the inference time of a single waveform takes 4.9 microseconds, with an accuracy rate of over 97%. The classification expectation FOM (figure-of-merit) value of this CNN model is 133, which is better than the traditional pulse shape discrimination (PSD) algorithm’s FOM value of 0.8. The development of this platform not only improves the accuracy and efficiency of space particle discrimination but also provides an advanced tool for future space environment monitoring, which is of great value for engineering applications.

Characterization of EJ-270 and Ce-doped LiCAF scintillators for the development of high-rate neutron reflectometer detectors

The Second Target Station of the Spallation Neutron Source at Oak Ridge National Laboratory is anticipated to provide a neutron source with ∼20 times increase in peak brightness than the First Target Station. The neutron reflectometers currently in operation at the First Target Station need to be upgraded due to the increased neutron flux. A prototype neutron detector module based upon a pixelated scintillator array readout by silicon photomultipliers is being developed to address the high-rate challenge faced with future neutron reflectometer instruments at the Second Target Station. Two types of scintillator materials were considered for this detector development, i.e., 6Li-loaded EJ-270 plastic scintillator and Ce-doped LiCAF single crystal. This paper reports the scintillator characterization results, including light yield, pulse shape discrimination performance, capability to detect thermal neutrons in a high γ-ray field, and γ-ray sensitivity. The number of photons produced per neutron capture by EJ-270 and LiCAF:Ce was measured to be 2176 ± 91 and 2651 ± 108, respectively. EJ-270 demonstrated a good capability to discriminate between neutrons and γ-rays by employing the commonly used charge comparison method (figure-of-merit: 1.13 ± 0.01 for an energy cut of 292–426 keVee) and a reasonable performance when using the time-over-threshold techniques; however, no discrimination was observed from LiCAF:Ce regardless of the pulse shape discrimination approaches utilized, making pulse height discrimination necessary for LiCAF:Ce to differentiate between neutrons and γ-rays. Both EJ-270 and LiCAF:Ce exhibited an acceptable capacity to detect thermal neutrons at high exposure rates up to approximately 584 mR/h. The γ-ray sensitivities measured with a60Co source at an exposure rate of around 1145 mR/h were determined to be (6.11 ± 0.87) × 10−6 and (7.64 ± 1.08) × 10−7 for EJ-270 and LiCAF:Ce, respectively.

Programmable generation of counterrotating bicircular light pulses in the multi-terahertz frequency range

The manipulation of solid states using intense infrared or terahertz light fields is a pivotal area in contemporary ultrafast photonics research. While conventional circular polarization has been well explored, the potential of counterrotating bicircular light remains widely underexplored, despite growing interest in theory. In the mid-infrared or multi-terahertz region, experimental challenges lie in difficulties in stabilizing the relative phase between two-color lights and the lack of available polarization elements. Here, we successfully generated phase-stable counterrotating bicircular light pulses in the 14–39 THz frequency range circumventing the above problems. Employing spectral broadening, polarization pulse shaping with a spatial light modulator, and intra-pulse difference frequency generation leveraging a distinctive angular-momentum selection rule within the nonlinear crystal, we achieved direct conversion from near-infrared pulses into the designed counterrotating bicircular multi-terahertz pulses. Use of the spatial light modulator enables programmable control over the shape, orientation, rotational symmetry, and helicity of the bicircular light field trajectory. This advancement provides a novel pathway for the programmable manipulation of light fields, and marks a significant step toward understanding and harnessing the impact of tailored light fields on matter, particularly in the context of topological semimetals.

Design and Development of a Portable Neutron Detection System Based on a 6Li-Doped ZnS(Ag) Scintillator

In this study, the design and development of a portable neutron detection system based on a 6LiF + ZnS(Ag) scintillator was carried out. The detector system was first modeled using the Monte Carlo simulation code MCNP. As a result of the detector simulation, the materials and photomultiplier tubes (PMTs) were supplied for the system, and the material properties and thicknesses were optimized for the best scintillator, with the EJ-420 (Eljen Technology), consisting of 95% 6Li dispersed in ZnS(Ag), chosen because it has a distinctive feature with regard to high thermal neutron detection efficiency. Since this material has been shown to offer a significant advantage in favor of neutron detection in mixed radiation fields with gammas and neutrons, the pulse shape discrimination method was employed. To achieve this, a proper electronic circuit was developed to discriminate the pulses from neutrons and gammas. In the experimental step, the EJ-420 scintillator with a 50-mm diameter was optically coupled with a special fast PMT (Hamamatsu H1949-51 model). In conclusion, this study, which underlines the performed simulations for a neutron detector configuration, gives the obtained experimental results showing discrimination capability in neutron/gamma detection using a 241Am-Be neutron source. The results show that the EJ-420 is a good scintillator due to its highly enriched 6Li transmator, which results in more effective neutron measurements when a portable neutron detector design is chosen.

An Investigation on Exhaust Pulse Characteristics of Asymmetric Twin-Scroll Turbocharged Heavy-Duty Diesel Engine

<div>The shape and energy distribution characteristics of exhaust pulse of an asymmetric twin-scroll turbocharged engine have a significant impact on the matching between asymmetric twin-scroll turbines and engines, as well as the matching between asymmetric twin scrolls and turbine wheels. In this article, the exhaust pulse characteristics of an asymmetric twin-scroll turbocharged engine was studied. Experiments were conducted on a turbine test rig and an engine performance stand to determine the operation rules of exhaust pulse strength, turbine flow parameters, turbine isentropic energy, and turbine efficiency. The results showed that the exhaust pulse strength at the inlets of both the small and large scrolls continuously decreased with the increase of engine speed. And the flow parameters at the inlets of the small and large scrolls exhibited a “ring” or “butterfly” shape with the change of expansion ratio depending on the pressure deviation of the extreme points at the troughs on both sides of the “secondary peak” of the exhaust pressure pulse, respectively. Besides, the distribution trend of turbine isentropic power was consistent with the trend of exhaust pressure pulse at the inlets of the small and large scrolls. Furthermore, when opening the balance valve, it caused the appearance of “concave” and “convex” features near the “main peak” and “secondary peak” of the turbine isentropic power pulses, respectively. Finally, as the engine speed increased, the fluctuation of turbine instantaneous efficiency gradually decreased. When calculating the instantaneous efficiency of the turbine, the influence of the rotor’s rotational inertia needs to be considered, otherwise, there may be a false phenomenon of exceeding 100% efficiency.</div>

A discharge plasma regulation method with spike current for electrical discharge machining

Improving material removal rate (MRR) has been one of the most important issues in electrical discharge machining (EDM). A major factor which hinders the improvement of MRR in EDM is the relatively low expulsion ratio of molten material. During a discharge process, a large portion of molten material cannot be expelled sufficiently from the molten pool but re-solidifies, ultimately resulting in a low machining efficiency. In this study, aiming at enhancing the expulsion ratio of molten material in EDM, a discharge plasma regulation method with a spike current is proposed. A novel discharge pulse shape, with a spike current aligned at pulse turn-off stage, is adopted. The transient behaviors of the discharge plasma when a spike current engaged are captured using a high-speed camera, and the morphologies of the corresponding discharge craters are also analyzed. A particle-in-cell with Monte Carlo collision simulation method is used to analyze the changes in particle state inside the plasma. It is found that the spike current applied at the stage when the plasma contracts to a smaller diameter is more conducive to facilitating molten material expulsion. The results of consecutive-pulse machining experiments verify that the proposed discharge plasma regulation method can increase MRR by at least 25.89 % for stainless steel.

Pulse Shape and Voltage-Dependent Synchronization in Spiking Neuron Networks.

Pulse-coupled spiking neural networks are a powerful tool to gain mechanistic insights into how neurons self-organize to produce coherent collective behavior. These networks use simple spiking neuron models, such as the θ-neuron or the quadratic integrate-and-fire (QIF) neuron, that replicate the essential features of real neural dynamics. Interactions between neurons are modeled with infinitely narrow pulses, or spikes, rather than the more complex dynamics of real synapses. To make these networks biologically more plausible, it has been proposed that they must also account for the finite width of the pulses, which can have a significant impact on the network dynamics. However, the derivation and interpretation of these pulses are contradictory, and the impact of the pulse shape on the network dynamics is largely unexplored. Here, I take a comprehensive approach to pulse coupling in networks of QIF and θ-neurons. I argue that narrow pulses activate voltage-dependent synaptic conductances and show how to implement them in QIF neurons such that their effect can last through the phase after the spike. Using an exact low-dimensional description for networks of globally coupled spiking neurons, I prove for instantaneous interactions that collective oscillations emerge due to an effective coupling through the mean voltage. I analyze the impact of the pulse shape by means of a family of smooth pulse functions with arbitrary finite width and symmetric or asymmetric shapes. For symmetric pulses, the resulting voltage coupling is not very effective in synchronizing neurons, but pulses that are slightly skewed to the phase after the spike readily generate collective oscillations. The results unveil a voltage-dependent spike synchronization mechanism at the heart of emergent collective behavior, which is facilitated by pulses of finite width and complementary to traditional synaptic transmission in spiking neuron networks.

RMS error analysis of spatial uniformity and energy stability of the laser beam in second harmonic generation

The coupling of the energy stability and spatial uniformity of the laser beam before and after second harmonic generation (SHG) was analyzed. SHG experiments were performed using a Nd:YAG nanosecond laser and LBO crystals, and images, pulse shapes, and energies were measured. The relationship between energy stability and spatial uniformity uses a formula derived from the previous study to analyze changes in energy stability and spatial uniformity of the input beam and converted beam. In addition, the measured input beam shape and energy are compared with the results of applying the SHG converting equation considering pump depletion. Both methods were similar to the experimental results when corrected by empirical factors. Through SHG, it was confirmed that there is an optimal point of energy stability and spatial uniformity of the laser beam near the critical power.

Coherence evolution of multi-pulse pumped supercontinuum generation in all-normal dispersion fibers

We numerically calculate the coherence of supercontinuum (SC) spectrum generated in all-normal dispersion (ANDi) fibers by multiple picosecond pulses. We show that multi-pulse pumping significantly mitigates the phenomenon of coherence degradation and maintains high spectral coherence over a broader wavelength range compared to the single pumping. It is found that inter-pulse four-wave-mixing (FWM) and stimulated Raman scattering (SRS) significantly contribute to the superior coherence characteristics of the SC. We further discuss the impact of key parameters on the multi-pulse pumped SC. Results indicate that a large temporal interval between pulses reduces spectral broadening efficiency, while closely spaced pulses generate a significant number of incoherent photons, severely degrading spectral coherence. Introducing initial chirp effectively mitigates this coherence degradation. A slight increase in the dispersion parameter notably suppresses the spectral broadening efficiency of the multi-pulse generation. Pulse shape primarily affects the SC shape without significantly impacting the spectral broadening efficiency.

Optimization of fast output from SiPM array for pulse shape discrimination techniques at high counting rates

Plastic scintillators with pulse shape discrimination (PSD) capability, when combined with silicon photo-multipliers (SiPMs), enable the development of compact neutron detectors with lower power consumption and insensitivity to magnetic fields. However, the SiPM array has a significantly large pulse width of the output signal, which leads to discrimination difficulties at high counting rates. There are two outputs of a SiPM array: a standard output for energy measurement and a fast output for timing measurement. The pulse width of the standard output signal is in the range of hundreds nanoseconds, while the pulse width of the other one is in the range of tens nanoseconds. Fast output signals have traditionally been considered suitably for fast timing, however utilizing them for PSD has been little explored. In our previous work, we successfully implemented PSD by using fast output signals from a SiPM. Despite the obvious negative overshoot in fast signals, we achieved PSD of fast signals by using longer sampling times. However, this approach was still limited at high counting rates. This study proposes a compensation network for fast output signals, which is composed of a compensation unit consisting of a resistor and a capacitor. This design effectively compensates the overshoot of the fast output and shortens the pulse width from 60 ns to 44 ns. Additionally, it highlights the frequency amplitude differences between the neutron and gamma signals from fast output in the high-frequency component, thus improving the PSD capability of the fast output.