Increase In Pulse Length Research Articles

On the concrete breakage by pulsed water jet impact: Fracture characteristic, stress and damage evolution laws

To reveal the dynamic response characteristic and damage-fracture evolution laws of concrete under pulse water jet impact, the numerical simulations on pulse water jets impinging concrete for different operating parameters were established based on SPH-FEM coupling algorithm and verified in this paper. The concrete-breaking effects of pulse water jet and continuous water jet were compared, and the effects of key jet parameters and concrete strength on the damage-fracture characteristics of concrete were investigated. And the damage and stress evolution laws inner concrete were revealed. The results show that there is a better concrete-breaking effect for pulse water jet by repeatedly utilizing the extremely high water-hammer pressure and effectively weakening the water-cushion effect, compared with continuous water jet. The broken pit area and the total length of main cracks decrease with the increase of pulse length, and there is an optimal pulse spacing for pulse water jet to improve the concrete-breaking effect. The broken pit area and the total length of main cracks both linearly increase with the increasing jet velocity and exponentially increase with the increasing jet diameter. As the concrete strength increases, the broken pit area shows an S-shaped curve decreasing trend and the total length of main cracks exponentially decreases. The broken pit width first rapidly increases and tends to stabilize with the increase of jet velocity and concrete strength, and changes linearly with jet diameter and is less affected by pulse length and spacing. The farther away from the jet impact center, the longer the damage cumulative period of concrete due to the jet impact energy attenuation. In horizontal direction, concrete breakage range is approximately 4–5 times of jet diameter; in vertical direction, the previous pulses will cause concrete to be destroyed or damaged, and the subsequent pulses will make the damaged concrete suffer greater stress, causing more serve damage. And the induced stress peak decreases in form of Power function with the increasing distance. The research results could provide theoretical guidance for the parameters selection of pulse water jet efficiently breaking concrete.

Effect of gas pressure and pulse duration on dry reforming of methane in nanosecond spark discharge

This paper presents the results of experiments on dry reforming of methane under the action of a nanosecond spark discharge at medium pressures up to 5 atm. High-voltage pulse generators with pulse length of 1 to 15 ns and voltage pulse amplitude of 50 to 200 kV were used to power the discharge. It was demonstrated that the spark discharge efficiency significantly exceeds the corona and diffuse discharge efficiency. With increasing pressure, the methane conversion value decreases, and the specific energy consumption increases. As the pulse length increases, the conversion value increases due to the increase in the energy deposition in the gas. The lowest energy consumption for the conversion of a methane molecule was 20 eV per molecule at pressure of 1 atm and discharge pulse length of 15 ns.

Proton beam emittance growth in multipicosecond laser-solid interactions

High intensity laser-solid interactions can accelerate high energy, low emittance proton beams via the target normal sheath acceleration (TNSA) mechanism. Such beams are useful for a number of applications, including time-resolved proton radiography for basic plasma and high energy density physics studies. In experiments using the OMEGA EP laser system, we perform the first measurements of TNSA proton beams generated by up to 100 ps, kilojoule-class laser pulses with relativistic intensities. By systematically varying the laser pulse duration, we measure degradation of the accelerated proton beam quality as the pulse length increases. Two dimensional particle-in-cell simulations and simple scaling arguments suggest that ion motion during the rise time of the longer pulses leads to extended preformed plasma expansion from the rear target surface and strong filamentary field structures which can deflect ions away from uniform trajectories and therefore lead to large emittance growth.

Surface Nanostructuring of Steel in Electrospark Alloying with WC-Based Electrodes

The change in cathode mass and anode erosion during electrospark alloying is investigated, with different process kinetics. With increase in pulse length, the efficiency of deep electrospark alloying increases up to some limit determined by the electrode composition.

A Fully Integrated Counter Flow Energy Reservoir for Peak Power Delivery in Small Form-Factor Sensor Systems

We present a fully integrated energy reservoir unit using a counter flow method for peak power delivery in space-constrained sensor systems. Recent advances in circuits have enabled significant reduction in the size of wireless systems such as implantable biomedical devices. As a consequence, the batteries integrated in these systems have also shrunk, resulting in high internal resistances ( $\sim 10 ~\text{k}\Omega$ ). However, the peak current requirement of power-hungry components such as radios remains in the milliwatt range and hence cannot directly be supplied from the battery. Therefore, an energy reservoir with high output power but small size is required. We present an efficient energy reservoir that dynamically reconfigures a storage capacitor array using a so-called counter flow approach. By creating a voltage gradient on capacitor arrays and moving the capacitors along the slope of the gradient, the supply voltage can be maintained while the energy stored in the reservoir is delivered efficiently to the load. The counter flow energy reservoir delivers 65% of stored energy before recharging is needed which allows up to a $12 \times$ reduction in overall capacitor size compared with our implementation of the previous method. The design supplies up to 13.6-mW output power for $1 ~\mu \text{s}$ . We demonstrate the proposed concept with a pulsed radio, showing an $11.5 \times$ increase in pulse length compared with the previous method.

Short-Circuit Degradation of 10-kV 10-A SiC MOSFET

The short-circuit behavior of power devices is highly relevant for converter design and fault protection. In this paper, the degradation during short circuit of a 10-kV 10-A 4H-SiC MOSFET is investigated at 6 kV dc-link voltage. The study aims to present the behavior of the device during short-circuit transients as it sustains increasing short-circuit pulses during its lifetime. As the short-circuit pulse length increases, degradation of the device can be observed in periodically performed characterizations. The initial degradation seems to be associated with the channel region, and continuous stressing leads to an overall increase in device on-state resistance at the end of the degradation study. Thermal simulation shows that the surface aluminum metalization reached its melting temperature and the top part of the device reaches temperatures above the rated junction temperature. Scanning electron microscope investigation shows aluminum reconstruction and cavities at the contact interface between the aluminum surface metalization and source contacts.

Catastrophic optical bulk damage in InP 7xx emitting quantum dot diode lasers

Catastrophic optical bulk damage occurs in broad-area MOCVD-grown InP/(Al)GaInP 7xx emitting quantum dot diode lasers operated at high power with single, high-current pulses of 5–20 A, in contrast with quantum well devices, which undergo catastrophic optical mirror damage at similar power densities at the facet. After damage the quantum dot devices show a reduction in the nearfield emission intensity across the width of the facet. Panchromatic cathodoluminescence of the active region shows a decrease in intensity and reveals dark, non-radiative spots which enlarge as pulse length increases. The areal density of the spots, also present in unprocessed material from similar structures, is of the order of 106 cm−2, five orders of magnitude lower than the dot populations at 1011 cm−2. Increasing pump current pulse length to 500 ns and beyond did not increase the dark spot size, which depended on the duration of the lasing action, but decreased the cathodoluminescence intensity of both the spots and background regions inside the emitter stripe, while reference regions outside the stripe remained unchanged. These features can be used to characterize improvements in structures and growth in order to improve the performance of InP based quantum dot structures at high powers.

Experimental results and recent developments on the EU 2 MW 170 GHz coaxial cavity gyrotron for ITER

The European Gyrotron Consortium (EGYC) is responsible for developing one set of 170 GHz mm-wave sources, in support of Europe’s contribution to ITER. The original plan of targeting a 2 MW coaxial gyrotron is currently under discussion, in view of essential delays and damages. This paper reports on the latest results and plans with regard to the two 2 MW gyrotron prototypes, the industrial prototype at CRPP’s CW test stand and a modular pre-prototype at KIT. The industrial prototype was delivered to CRPP end of September 2011 and reached an output power of 2 MW at an efficiency of 45 % and with good RF beam pattern, in only four days of short pulse RF test. These results validated all design changes made in reaction to the results of the experiments in 2008. On the fifth experimental day, an internal absorber broke, terminating any further experiment with this tube. In parallel, design and experimental activities at KIT went on, in particular featuring reduced stray radiation down to 4% of the RF power. Next years’ plans for the 2 MW modular pre-prototype foresee a stepwise increase of pulse length.

Conceptual Modeling of Contaminated Solute Transport Based on Stream Tube Model

In this study, we performed a conceptual modeling on solute transport based on theoretical stream tube model (STM) with various travel time distributions assuming a pure convective flow through each tube in order to investigate how the lengths and distributions of solute travel time through STM affect the breakthrough curves at the end mixing surface. The conceptual modeling revealed that 1) the shape of breakthrough curve (BTC) at the mixing surface was determined by not only input travel time distributions but also solute injection mode such as sampling time and pulse lengths; 2) the increase of pulse length resulted in the linear increase of the first time moment (mean travel time) and quadratic increase of the second time moment (variance of travel time) leading to more spreading of solute, however, the second time moment was not affected by travel time distributions and 3) for a given input distributions the increase in travel distance resulted in more dispersion with the quadratic increase of travel time variance. This indicates that stream tube model obeying strictly pure convective flow follows the concept of convective-lognormal transport (CLT) model regardless the input travel time distributions.

Transport and confinement studies in the RFX-mod reversed-field pinch experiment

In the modified RFX experiment (RFX-mod) external magnetic field coils and a close fitting thin conductive shell control radial magnetic fields. In the so-called virtual shell (VS) operation, radial field zeroing at the thin shell radius is stationary provided by the feedback-controlled coils. First experiments on RFX-mod proved the capability of the active scheme to steadily reduce the radial magnetic field. Furthermore it has been found that such edge magnetic field control extends its beneficial effects to the whole plasma. With respect to the old RFX, where magnetohydrodynamic modes amplitude was controlled by the use of a passive thick conductive shell, a stationary 2- to 3-fold reduction of the Br field amplitude in the core is obtained. The reduction of field fluctuations positively reflects on confinement. In fact, a strong reduction of the loop voltage is observed and correspondingly a 3-fold increase in pulse length is achieved by using the same poloidal flux swing. Temperature and particle measurements confirm the improved confinement properties of the VS operation. With a lower ohmic input power, higher electron temperature and lower particle influx are measured. Particle and heat transport have been studied by means of a 1D code. Local power balance was used to compute the heat conductivity profile: for the VS discharges a lower conductivity over a significant region of the plasma is found. The improved properties of RFX-mod VS operation provide a better confinement scaling in terms of plasma current. The results show that compared with the thick shell configuration, a significant confinement improvement can be obtained under stationary conditions by actively controlling the plasma magnetic boundary.

Active MHD control experiments in RFX-mod

The RFX reversed field pinch experiment has been modified (RFX-mod) to address specific issues of active control of MHD instabilities. A thin shell (τBv∼50 ms) has replaced the old thick one (τBv∼500 ms) and 192 (4 poloidal × 48 toroidal) independently powered saddle coils surround the thin shell forming a cage completely covering the torus. This paper reports the results obtained during the first year of operation. The system has been used with various control scenarios including experiments on local radial field cancellation over the entire torus surface to mimic an ideal wall (‘virtual shell’) and on single and multiple mode feedback control. Successful virtual shell operation has been achieved leading to: a 3-fold increase in pulse length and well controlled 300 ms pulses(∼6 shell times) up to ∼1 MA plasma current; one order of magnitude reduction of the dominant radial field perturbations at the plasma edge and correspondingly 100% increase in global energy confinement time. Robust feedback stabilization of resistive wall modes has been demonstrated in conditions where rotation does not play a role and multiple unstable modes are present.

Two-photon ionization ofH2+by short laser pulses

We present a theoretical study of dissociative multiphoton ionization of the $\mathrm{H}_{2}{}^{+}$ molecular ion in perturbative and nonperturbative regimes including both electronic and nuclear degrees of freedom. Differential (in proton and electron energy) ionization cross sections have been evaluated for various photon energies, laser intensities, and pulse lengths. We have found that the proton energy distribution is modulated by vertical Franck-Condon transitions but also by vibrational resonances associated with intermediate electronic states. We have also found that, as expected, nonperturbative results tend to the time-independent perturbative ones when both the pulse length increases and the laser intensity decreases. No divergence near intermediate-state resonances is found in the perturbative results when the nuclear motion is properly taken into account in the calculations.

Initial studies of a long-pulse relativistic backward-wave oscillator utilizing a disk cathode

One of the major issues in high-power microwave device operation is pulse shortening, which often limits microwave pulses to less than 100 ns. This has been the focus of many studies on the long-pulse backward-wave oscillator (BWO) at the University of New Mexico. Previous diagnostics have indicated that significant plasma is produced by a graphite knife-edge or "cookie cutter" geometry cathode. This plasma caused the beam to expand radially to match the dimensions of the cutoff neck upon entrance into the slow-wave structure. This effect led to an impedance collapse at which point the microwave production ceased. In recent studies by Loza and colleagues, they have produced intense annular relativistic electron beams that maintain a stable cross section for 1-/spl mu/s duration utilizing a disk cathode. Whereas the beams produced by Loza and colleagues were not used in microwave sources, we have incorporated such a disk cathode in a long-pulse relativistic BWO to study its effort on pulse shortening. This simple solution has led to an increase in pulselength and radiated microwave power up to a factor of two as compared to the "cookie cutter" cathode.

LASER HEATING OF SILICON: A KINETIC THEORY APPROACH

The present study considers laser heating of silicon. A three-dimensional form of electron kinetic theory approach is considered when deriving the governing heat transfer equation. The numerical method employing a finite difference scheme is used to discretize the resulting equations. The step input laser pulse with Gaussian spatial distribution is introduced. The present predictions are compared with the results of the Fourier heating model. It is found that the temperature rise in the surface vicinity depends on the electron lattice site atom collisions. As heating progresses, the internal energy gain dominates the conduction losses in this region. The electron kinetic theory predictions and the Fourier theory results become similar as the laser pulse length increases.

Two methods for peak RF power minimization of multiple inversion-band pulses.

Two novel methods to minimize peak RF power for high order longitudinal Hadamard encoding are described and demonstrated experimentally. The first method uses the fact that the choice of a reference phase in an inversion process does not affect the final frequency response. In this method, the different single inversion-band pulses are added together, each with a different reference phase. For a proper phase choice, minimization of the peak RF power is obtained. Scaling laws are defined allowing the use of a given phase-set in multiple cases. In the second method, single inversion-band pulses are added together, each partially shifted in time. This results in a significant reduction in peak power with only a moderate increase in pulse length. Theoretical conditions outlining the optimal addition order are defined. Experimental results verify the theoretical conditions and demonstrate that the frequency response is not affected by the peak power minimization process. With the new low peak RF power, longitudinal Hadamard encoding of 8TH (or 16TH) order can be performed in any clinical setting.

A steady-state heat conduction solution for optical disk recording using constant or periodically modulated laser irradiation

A fast method is presented for accurately calculating just the steady-state temperature in thin-film multilayers heated by continual laser irradiation. The key features are a simple coordinate transformation and a special lattice with which the steady-state portion of the heat conduction solution reported earlier [J. Appl. Phys. 75, 4382 (1994)] may be quickly and efficiently extracted. The earlier solution, expressed in terms of Fourier-transformed Green’s functions, was designed for temperature problems as typically found in optical disk recording situations. In particular, the laser irradiation was assumed to be modulated arbitrarily in time and finite in duration. As the pulse length increases, though, both execution time and memory requirements increase. Thus when long laser pulses are used to model continual laser irradiation, the numerical performance may degrade to unacceptable levels. For the case of periodically modulated laser irradiation (including constant laser irradiation as a special case) which is important for magneto-optical disk recording, the time and memory requirements can be reduced substantially by defining a new coordinate and lattice system in the space spanned by the time and scan directions. In frequency space, the new lattice filters out the frequencies which contribute to the transient part of the solution and retains only the frequencies which contribute to the steady-state part. The temperature formulas themselves remain unchanged; they are merely evaluated over a specially defined lattice which yields just the steady-state temperature more quickly. This paper describes the new coordinate and lattice systems, discusses some numerical issues, and compares the results of the steady-state theory to the results of the earlier original theory which includes the transients.

Effect of undernutrition on pulsatile luteinizing hormone (LH) secretion in castrate and intact male rats using an ultrasensitive immunofluorometric LH assay.

The recent development of an ultrasensitive immunofluorometric rat LH assay makes possible evaluation of pulsatile LH secretion in intact male rats under physiological conditions of minimal volume blood sampling without requiring orchidectomy. Specifically, we applied this assay to determine the effect of macronutrient restriction on pulsatile LH secretion in the presence or absence of testes. In testes-intact rats, halving of food intake for 7 days while maintaining micronutrient supply caused a reduction of mean, maximal, and basal LH levels and LH pulse amplitude (all P < 0.05) compared with those in ad libitum fed controls. The loss of body weight was positively correlated with decreases in mean LH level, pulse amplitude, and area under the curve (all P < 0.009). In contrast, the same food restriction in castrated rats caused an increase in pulse length and area under the curve and a decrease in pulse frequency, but did not change mean, maximal, and basal LH levels or LH pulse amplitude compared to castrated ad libitum fed controls (all P < 0.02). The observed positive correlations between body weight and the LH secretion parameters in intact rats were absent or reversed in castrated rats. This study demonstrates qualitatively different effects of macronutrient restriction on pulsatile LH secretion in castrated and intact rats, indicating that it is not necessary valid to extrapolate consequences of undernutrition on LH secretion from castrate to intact male rats. We conclude that undernutrition-induced inhibition of LH secretion involves both an indirect suppression of LH secretion via amplification of endogenous testicular negative feedback as well as more direct suppression of GnRH release.

Effects of pulsed ultrasound on the frog heart: I. Thresholds for changes in cardiac rhythm and aortic pressure

High intensity pulsed ultrasound at 1.2 MHz is shown to change the cardiac rhythm and aortic pressure of frog hearts in vivo. Threshold levels for these effects occur at acoustic pressure amplitudes of the order of 10 MPa for 5 ms pulse lengths. Depending upon the phase of the heart cycle, a pulse of ultrasound either may cause a premature ventricular contraction, a reduction in the strength of contraction as measured by the aortic pressure, or an enhanced relaxation of the heart muscle. There is an increase in the effectiveness of the ultrasound with increase in pulse length in the range from 1 to 5 ms.

A temperature sensitive mutation that reduces mitotic rate inDrosophila melanogaster.

A temperature-sensitive cell autonomous mutation ofDrosophila, l(1)ts-1126 (1-16±2), that affects the rate of cell division is described. When mutant animals are exposed to the restrictive temperature of 29°C during the first and second larval stages, the growth rate of the larvae is retarded. A delay in pupariation occurs during which larvae reach their full size, and the resulting flies are normal. When mutant animals are exposed to restrictive temperature during the third larval stage, growth is also retarded but no delay in pupariation occurs, and the resulting flies are reduced in size. Their small size is due in part to a decreased number of cells and in part to a smaller size of the cells.X-ray induced, marked, homozygousl(1)ts-1126 clones in an otherwise normal animal, are smaller in animals exposed to pulses of restrictive temperature when compared to clones in animals kept at permissive temperature of 22°C. Clone size decreases as pulse length increases. Clones on the wing blade induced 24 h after oviposition are smaller than clones induced at 48 h in animals grown at restrictive temperature. This result is interpreted as an inability of the slower dividingl(1)ts-1126 cells to survive when in competition with wildtype cells. The distribution of survivingl(1)ts-1126 clones in gynandromorphs grown at restrictive temperature supports this conclusion.

Visibility on Cathode-Ray Tube Screens The Effect of Size and Shape of Pip*

The area and contour of a stimulus have been indicated experimentally as factors in its visibility. In order to evaluate the role of such factors in radar presentation, an experiment was conducted to determine the effect upon visibility of changes in the size and shape of a radar pip. A wide range of beam widths (which determine the angular dimension of the pip) and pulse lengths (which determine its radial dimension) was investigated at each of three test field brightnesses. Resuits show that visibility, on a decibel scale, is a linear function of the logarithm of beam width at each of the three brightness levels used. For the dim background visibility is also a linear function of the logarithm of pulse length. For the brighter backgrounds, on the other hand, visibility is shown to be a curvilinear function of the logarithm of pulse length. It is suggested that the reason for the difference between an increase in beam width and an increase in pulse length lies in the time factor. Doubling the beam width, for example, results in a greater increase in the time during which the pip is exposed to the eye than does a corresponding increase in the pulse length.