Effect Of Pulse Shape Research Articles

High-speed 2D Raman imaging at elevated pressures.

Two-dimensional (2D) Raman scattering at 10kHz in non-reacting flow mixtures is demonstrated by employing a burst-mode laser with a long-duration pulse of about 70ns and pulse energy of about 750mJ at 532nm. To avoid optical breakdown, the pulse width of the laser was varied in the range of 10-1000ns. The effects of pulse shape, pulse energy, and harmonic conversion on 2D measurements are also studied. The applications of high-speed, single-shot, 2D imaging of CH4 and H2 jets in N2 at elevated pressures are demonstrated. In addition, the scalar dissipation rate of CH4 in N2 at 20bar is determined, and multi-dimensional, multi-species, high-speed imaging of flows at elevated pressures is demonstrated.

Pulse shape effects on the electron-positron pair production in strong laser fields

The pair-production process in the presence of strong linearly polarized laser fields with a subcycle structure is considered. Laser pulses with different envelope shapes are examined by means of a nonperturbative numerical technique. We analyze two different "flat" envelope shapes and two shapes without a plateau for their various parameters including the carrier-envelope phase. The resonant Rabi oscillations, momentum distribution of particles created, and total number of pairs are studied. It is demonstrated that all these characteristics are very sensitive to the pulse shape.

A pulse-shaping technique to investigate the behaviour of brittle materials subjected to plate-impact tests.

Owing to their significant hardness and compressive strengths, ceramic materials are widely employed for use with protective systems subjected to high-velocity impact loadings. Therefore, their mechanical behaviour along with damage mechanisms need to be significantly investigated as a function of loading rates. However, the classical plate-impact testing procedures produce shock loadings in the brittle sample material which cause unrealistic levels of loading rates. Additionally, high-pulsed power techniques and/or functionally graded materials used as flyer plates to smooth the loading pulse remain costly, and are generally difficult to implement. In this study, a shockless plate-impact technique based on the use of either a wavy-machined flyer plate or buffer plate that can be produced by chip-forming is proposed. A series of numerical simulations using an explicit transient dynamic finite-element code have been performed to design and validate the experimental testing configuration. The calculations, conducted in two-dimensional (2D) plane-strain or in 2D axisymmetric modes, prove that the 'wavy' contact surface will produce a pulse-shaping effect, whereas the buffer plate will produce a homogenizing effect of the stress field along the transverse direction of the sample. In addition, 'wavy-shape' geometries of different sizes provide an easy way to change the level of loading rate and rise time in an experimentally tested ceramic specimen. Finally, when a shockless compression loading method is applied to the sample, a Lagrangian analysis of data is made possible by considering an assemblage of ceramic plates of different thicknesses in the target, so the axial stress-strain response of the brittle sample material can be provided.This article is part of the themed issue 'Experimental testing and modelling of brittle materials at high strain rates'.

A Model of Electrically Stimulated Auditory Nerve Fiber Responses with Peripheral and Central Sites of Spike Generation

A computational model of cat auditory nerve fiber (ANF) responses to electrical stimulation is presented. The model assumes that (1) there exist at least two sites of spike generation along the ANF and (2) both an anodic (positive) and a cathodic (negative) charge in isolation can evoke a spike. A single ANF is modeled as a network of two exponential integrate-and-fire point-neuron models, referred to as peripheral and central axons of the ANF. The peripheral axon is excited by the cathodic charge, inhibited by the anodic charge, and exhibits longer spike latencies than the central axon; the central axon is excited by the anodic charge, inhibited by the cathodic charge, and exhibits shorter spike latencies than the peripheral axon. The model also includes subthreshold and suprathreshold adaptive feedback loops which continuously modify the membrane potential and can account for effects of facilitation, accommodation, refractoriness, and spike-rate adaptation in ANF. Although the model is parameterized using data for either single or paired pulse stimulation with monophasic rectangular pulses, it correctly predicts effects of various stimulus pulse shapes, stimulation pulse rates, and level on the neural response statistics. The model may serve as a framework to explore the effects of different stimulus parameters on psychophysical performance measured in cochlear implant listeners.

Phase-sensitive OTDR probe pulse shapes robust against modulation-instability fading

Typical phase-sensitive optical time-domain reflectometry (ϕOTDR) schemes rely on the use of coherent rectangular-shaped probe pulses. In these systems, there is a trade-off between the signal-to-noise ratio (SNR), spatial resolution, and operating range of the ϕOTDR system. To increase any of these parameters, an increase in the pulse peak power is usually indispensable. However, as it is well known, there is a limit in the allowable increase in probe power due to the onset of undesired nonlinear effects such as modulation instability. In this Letter, we perform an analysis of the effect of the probe pulse shape on the visibility fading due to modulation instability. In particular, four different temporal profiles are chosen: rectangular, Gaussian, triangular, and super-Gaussian (order 2). Our numerical and experimental analyses reveal that the use of triangular or Gaussian-like pulses can significantly inhibit the visibility fading issues. As such, an increase in the range up to twofold for the same pulse energy (i.e., SNR) and nominal spatial resolution can be achieved, as compared with the results obtained when using rectangular pulses. This is due to a more robust behavior of the Gaussian and triangular pulses against the Fermi-Pasta-Ulam recurrence occurring in modulation instability.

Refilling characteristics of high frequency piezo driven ink jet print heads

A piezo driven ink jet print head is in principle an open microfluidic system, there are no valves that control the direction of the flow. In order to avoid flooding of the nozzle plate at nozzle level a small under-pressure is maintained. In equilibrium the meniscus of the ink is retracted and pinned to the rim of the nozzle. This equilibrium is controlled by the surface tension of the ink and the setting of under-pressure controller. Operated at low frequencies, after droplet formation, there is ample time for the ink to return to the equilibrium state and to be ready for the next firing of a droplet. Driven at higher frequencies, in between droplets, there is no time to return to the equilibrium state and other mechanisms for refilling come into view. When the meniscus has retracted in the nozzle, the next pressure pulse needs to accelerate a relatively small amount of ink. Moreover the viscous drag in a partly filled nozzle is less compared to a completely filled nozzle. With a constant surface tension pressure, the Washburn equation learns that the refilling speed of a partly filled nozzle increases with decreasing filling of the nozzle. Both effects are supposed to cause that a print head driven at high frequency delivers enough fluid to nozzle to maintain droplet emission. In this paper this theoretical framework is extended by taken into account: <list list-type="bullet"> <list-item>• Droplet formation on the dynamics of the fluid motion,</list-item> <list-item>• Change of droplet volume at high frequencies,</list-item> <list-item>• Inertia effects due to the variable mass in the nozzle. A complete non-linear analysis will be outlined including:</list-item> <list-item>• Limitation of the capillary pressure; only close to the nozzle the capillary pressure becomes a linear function of the meniscus displacement, otherwise it is a constant,</list-item> <list-item>• The dependence of the viscous drag on the filling of the nozzle,</list-item> <list-item>• The effects of droplet formation,</list-item> <list-item>• Inertia effects due to the variable mass in the nozzle.</list-item> <list-item>• Calculations will be performed for a sample pump of which the dimensions are representative for a standard print head. Two inks will be investigated one with a viscosity of 0.01 Pa.s and another with a low viscosity equal to 0.002 Pa.s.</list-item> </list>The non-linear analysis will reveal many details of the fluid dynamics of the ink in a print head, including effects of surface tension, viscosity, droplet formation, pulse shape and repeat rate.

Quantum multimode treatment of light scattering by an atom in a waveguide

We present a full multimode treatment of the interaction of the quantized radiation field with a single two-level atom in a one-dimensional waveguide configuration. Starting with an incident pulse consisting of an arbitrary (finite) number of photons in a general initial state, we derive the equations of motion and a closed-form expression for the shape of the pulse after its interaction with the atom. We then specialize our results to the two-photon case where a number of analytical results can be derived, for both unidirectional and bidirectional systems. We study the effects of different pulse shapes, the manifestations of the entangled, so-called bound state of the two photons, single- and two-photon detection probabilities, and provide simple approximate results for the strong-coupling (or adiabatic, or long-pulse) regime. We also discuss the requirements for a true unidirectional setup, and the application of such a setup to photon sorting proposed by Witthaut, Lukin, and S\o{}rensen [Europhys. Lett. 97, 50007 (2012)].

Large deformation, damage evolution and failure of ductile structures to pulse-pressure loading

In this paper, a model is developed for an elastic perfectly-plastic structural beam system subjected to general pulse-pressure loadings - this may be either impulsive or non-impulsive - which is capable of capturing large non-linear deformation, ductile damage evolution and its consequential failure. The proposed model is an extension of Schleyer and Hsu (2000) by incorporating interactions between bending, membrane stretch and transverse shear in the fully plastic stress state, and uses damage mechanics to capture the loss of integrity at the supports and the subsequent beam detachment. Predictions by the model were validated against existing experimental data from literature and to three-dimensional finite element models developed in this paper. Parametric studies were performed to elucidate the effects of loading duration on the mode of deformation by the beam and the critical conditions governing their transition. The efficacy of Youngdahl’s (1970; 1971) technique on desensitising pulse shape effects is also investigated using different pressure pulse profiles and it will be shown that the technique is successful only for monotonically decaying pulse-pressures.

Effect of pulse duration and shape on dynamic buckling of stiffened panels

Dynamic buckling of stiffened panels under axial compression loading having the form of finite duration pulse was analyzed by finite element modeling. Welding induced defects modifying the skin plate curvature were incorporated. Material degradation in the heat-affected zone was also taken into account. The Budiansky and Roth criterion was employed to predict the collapse load. Various pulse shapes were investigated. The obtained results have shown that the pulse period and profile have severe effects on the buckling strength. For the considered boundary conditions and load pulses up to 56% reduction of the strength was observed in comparison with static buckling.

Dependence of plasma wake wave amplitude on the shape of Gaussian chirped laser pulse propagating in a plasma channel

The generation of longitudinal electrostatic plasma waves (wakefields) due to the propagation of a chirped laser pulse through a parabolic plasma channel is studied. The wakes generated by a temporally symmetric Gaussian laser pulse are compared with those generated by asymmetric one. The main interest in this paper is to investigate the effects of a laser pulse shape with of sharp rising and slow falling time scales on the excited wakefield amplitude. Moreover, positive, negative and un-chirped laser pulses are employed in numerical codes to evaluate the influence of the initial chirp on wakefield excitation. Numerical results showed that for an appropriate laser pulse length compared with the plasma wavelength, the wakefield amplitude can be enhanced for a positively chirped asymmetric Gaussian laser pulse with a fast rise time.

Pulse shape effect on rotational excitation and 2-D alignment alternation by elliptic laser pulses

We examine theoretically the time-evolution of NAREX (non-adiabatic rotational excitation) and molecular 2-D alignment (2DA) interacting with a pair of elliptically polarized laser pulses. The pulse shapes taken are half-cycle pulse (HCP) and square pulse (SQP). By choosing the proper value of elliptically polarized field parameters, we demonstrate that efficient field-free 2DA alignment can be achieved. It is also shown that NAREX can be controlled by various laser parameters, out of which pulse shape plays the most significant role. The effect of pulse width along with elliptic parameter on probabilities of rotational states is also under concern. The delay time between the two pulses decides the maximum in 2DAs.

A Phenomenological Model of the Electrically Stimulated Auditory Nerve Fiber: Temporal and Biphasic Response Properties.

We present a phenomenological model of electrically stimulated auditory nerve fibers (ANFs). The model reproduces the probabilistic and temporal properties of the ANF response to both monophasic and biphasic stimuli, in isolation. The main contribution of the model lies in its ability to reproduce statistics of the ANF response (mean latency, jitter, and firing probability) under both monophasic and cathodic-anodic biphasic stimulation, without changing the model's parameters. The response statistics of the model depend on stimulus level and duration of the stimulating pulse, reproducing trends observed in the ANF. In the case of biphasic stimulation, the model reproduces the effects of pseudomonophasic pulse shapes and also the dependence on the interphase gap (IPG) of the stimulus pulse, an effect that is quantitatively reproduced. The model is fitted to ANF data using a procedure that uniquely determines each model parameter. It is thus possible to rapidly parameterize a large population of neurons to reproduce a given set of response statistic distributions. Our work extends the stochastic leaky integrate and fire (SLIF) neuron, a well-studied phenomenological model of the electrically stimulated neuron. We extend the SLIF neuron so as to produce a realistic latency distribution by delaying the moment of spiking. During this delay, spiking may be abolished by anodic current. By this means, the probability of the model neuron responding to a stimulus is reduced when a trailing phase of opposite polarity is introduced. By introducing a minimum wait period that must elapse before a spike may be emitted, the model is able to reproduce the differences in the threshold level observed in the ANF for monophasic and biphasic stimuli. Thus, the ANF response to a large variety of pulse shapes are reproduced correctly by this model.

Study of temporal pulse shape effects on W using simulations and laser heating

Transient heat pulses with triangular, square, and ELM-like temporal shapes are investigated in order to further understand how transient plasma instabilities will affect plasma facing components in tokamaks. A solution to the 1D heat equation for triangular pulses allows the peak surface temperature to be written analytically for arbitrary rise times. The solution as well as ANSYS simulations reveal that a positive ramp (maximum rise time) triangular pulse has a higher peak surface temperature by a factor of compared to that from a negative ramp (rise time = 0) pulse shape with equal energy density, peak power, and pulse width. Translating the results to ITER, an ELM or disruption pulse with the shortest rise time is the most benign compared to other pulse shapes with the same peak heat flux and same energy density.

Effects of Pulse Shape and Polarity on Sensitivity to Cochlear Implant Stimulation: A Chronic Study in Guinea Pigs.

Most cochlear implants (CIs) stimulate the auditory nerve with trains of symmetric biphasic pulses consisting of two phases of opposite polarity. Animal and human studies have shown that both polarities can elicit neural responses. In human CI listeners, studies have shown that at suprathreshold levels, the anodic phase is more effective than the cathodic phase. In contrast, animal studies usually show the opposite trend. Although the reason for this discrepancy remains unclear, computational modelling results have proposed that the degeneration of the peripheral processes of the neurons could lead to a higher efficiency of anodic stimulation. We tested this hypothesis in ten guinea pigs who were deafened with an injection of sysomycin and implanted with a single ball electrode inserted in the first turn of the cochlea. Animals were tested at regular intervals between 1 week after deafening and up to 1 year for some of them. Our hypothesis was that if the effect of polarity is determined by the presence or absence of peripheral processes, the difference in polarity efficiency should change over time because of a progressive neural degeneration. Stimuli consisted of charge-balanced symmetric and asymmetric pulses allowing us to observe the response to each polarity individually. For all stimuli, the inferior colliculus evoked potential was measured. Results show that the cathodic phase was more effective than the anodic phase and that this remained so even several months after deafening. This suggests that neural degeneration cannot entirely account for the higher efficiency of anodic stimulation observed in human CI listeners.

Analytical and Experimental Investigations on Ultrawideband Pulse Width and Shape Effect on the Accuracy of 3-D Localization

This letter presents analytical and experimental investigation of three-dimensional indoor localization accuracy using different subbands of the Ultrawideband (UWB) spectrum. Combined effect of ranging errors of each base station (due to change in pulse width, bandwidth, and UWB source pulse) on 3-D localization using a compact tapered slot antenna is studied. Higher-order Gaussian derivative source pulses (3rd and 7th) are compared to first-order Gaussian derivative theoretically and analyzed in terms of the fidelity of the UWB source pulses; localization accuracy and Cramer–Rao lower bounds have also been calculated to validate the experimental findings. It is observed that 5-GHz bandwidth gives sufficiently high localization accuracy results with an average error of 2–3 cm. Results related to the different UWB source pulse show that the third-order Gaussian derivative signal can be used to enhance (30% increase) the localization accuracy and also preserve a good tradeoff between accuracy and complexity of the UWB system.

Experimental Study on Split Hopkinson Pressure Bar Pulse-Shaping Techniques for Concrete

AbstractThe dynamic compressive behavior of concrete is investigated using a Φ74-mm variable cross section straight taper split Hopkinson pressure bar (SHPB) apparatus in this study. In order to reduce and eliminate the waveform dispersion effect, two pulse shaping techniques are adopted to improve the incident wave shape. The two techniques include changing the striker (length, shape) and copper pulse shapers (diameter, thickness, shape). Experimental results show that the pulse shaping effect with the small diameter of copper shaper is better than that of the tapered striker and annular pulse shaper, all of which could reach dynamic stress equilibrium, and reduce the high-frequency oscillation, and achieve constant strain-rate deformation approximately.

Investigation of the characteristic properties of high-order harmonic spectrum in atoms using Bohmian trajectories

We study the electron quantum dynamics of high harmonic generation (HHG) processes of atomic hydrogen under intense near infrared (NIR) laser fields by means of the De Broglie–Bohm’s framework of Bohmian mechanics. The proposed accurate 3D numerical scheme is utilized to discuss the mechanism of the multiple plateau generation and the cut-off extension, as the main characteristic features of HHG spectrum. One-color (1600 nm) and two-color (1600 nm + 800 nm) laser fields with different time delays are used to investigate the effect of pulse shape on electron dynamics and HHG process. The presented results on Bohmian trajectories and their energy content, along with the analysis of the emission time period of different groups of trajectories, provide a comprehensive and fresh electron dynamical picture and uncover novel mechanisms of the HHG processes and power spectra.

The effect of transient temporal pulse shape on surface temperature and tungsten damage

The plasma-facing components (PFCs) in future fusion devices such as ITER will receive intense transient heat fluxes from plasma instabilities, such as edge localized modes (ELMs) and disruptions, which will limit material lifetime. The energy density and transient pulse duration are typically used to characterize the PFC damage threshold. However, these parameters are not sufficient to define a damage threshold, because different transient pulse shapes with the same energy density and same pulse duration produce different peak surface temperatures (and thus stresses, which ultimately determine material damage). The surface temperature and damage of tungsten (the material to be used for the ITER divertor target armor) in the form of surface roughening and melting are investigated using various temporal pulse shapes from an Nd:YAG laser in the PISCES-B facility. The heat flux factor is examined and shown to be an inadequate parameter to characterize the temperature rise except for square temporal pulse shapes. For ELM-like heat pulses, the long tail in the temporal shape results in a lower peak surface temperature and less damage compared to a symmetric triangle pulse with equal energy density.

Pulse-shape effects in strong-field atomic ionization by an XUV pulse

SynopsisAchieving a significant displacement, defined as a time integral of the vector potential taken over the pulse duration, in strong-field atomic ionization critically depends on the envelope function used for the electric field. Due to the sensitivity of theoretical predictions to the pulse details, an experimental realization of the effect appears to be a major challenge.

Pulse-shape effects in ionization of atomic hydrogen by short-pulse XUV intense laser radiation: A sensitivity study

The displacement effect studied in a recent paper [Ivanov et al., Phys. Rev. A 90, 043401 (2014)] in atomic ionization by a short XUV pulse is investigated in more detail. It is shown that achieving a significant displacement critically depends on the assumption of a plateau in the envelope function of the electric field, and that the ramp-on is fine-tuned in such a way that a drift velocity generated during the ramp-on phase can increase this displacement further. Seemingly minor variations in the electric fields defined in slightly different ways cause significant changes in the final results, in particular regarding the angular-momentum distribution of the ejected electron. In light of such a strong sensitivity seen in the predictions made with idealized pulse shapes and the likely difficulties of preparing such pulses experimentally, an experimental realization of the displacement effect will likely be a major challenge.