Short Rise Time Pulses Research Articles

Voltage endurance testing

The medium-voltage ASD stator insulation systems operate under greater stress than those using the sine-wave power. The repetitive short rise-time pulses generated by most ASDs cause an additional stress. The pulses are often increased by the effect of the cable between the ASD and motor. It is incumbent on the user and ASD manufacturer to account for the magnitude, rate of rise, and repetition rate of these pulses. The medium-voltage machines powered by ASDs operate with some level of PD. The machine manufacturer must ensure that the insulation system will provide adequate life in the PD environment. VE testing is a well-understood and accepted method to qualify insulation systems. It is a reliable means to compare new insulation systems with the documented existing ones. The TS describes several qualification procedures based on the knowledge of the converter waveform. These will assure that all parts of the machine insulation system will present acceptable test life.

Particle-in-cell simulation of an electron shock wave in a rapid rise time plasma immersion ion implantation process

A one-dimensional Monte Carlo collision‐particle-in-cell plasma computer code was used to simulate plasma immersion ion implantation by applying a negative voltage pulse to the substrate while the reactor wall is grounded. The results presented here show the effect of short rise time pulses: for rise times shorter than the electron plasma period stypically 5 ns/ kVd, an electron shock wave is observed where a rapidly expanding sheath heats the electrons up to high energies. Many of these fast electrons are expelled from the plasma leading to a high plasma potential and thus to a high surface electric field on the earthed electrode which could give rise to non-negligible electron field emission. © 2005 American Institute of Physics . fDOI: 10.1063/1.1872894g

Detection and Localization of Defects in Shielded Cables by Time-Domain Measurements With UWB Pulse Injection and Clean Algorithm Postprocessing

An experimental procedure to detect and localize defects in shielded cables is presented. First, time-domain measurements are carried out by injecting a short rise time pulse in the input section of the shielded cable. Then, the clean algorithm is applied to the measurement results to identify possible damages in the cable line. The localization of the cable section with defects is finally obtained in a very simple way due to the adopted method of measurement in time domain using a ultrawide-band pulser with a very fast rise time. The proposed method is validated by detecting and localizing known defects purposely introduced in test cables.

Dynamic compression testing of particle-reinforced polymer roll cover materials

Abstract Two polymer composite roll cover materials were studied using dynamic compression tests. The main focus was determination of viscoelastic properties of the materials and development of mathematical models to describe the behavior of these materials in the contact area between two rolls. The compression tests were conducted using short rise time pulses with different durations in servohydraulic materials testing machines. In the modeling, combinations of standard elastic and viscoelastic elements were used together with the Boltzmann superposition principle. A simple spring-dashpot model was found to fit sufficiently to the experimental data with relaxation (retardation) times ranging from a few milliseconds in transient loading tests to tens of hours in static compression tests

Demonstration of microwave generation by a ferroelectric-cathode tube

A ferroelectric cathode is employed in a cyclotron-resonance maser (CRM). The CRM oscillator device operates at ∼7 GHz, near the cutoff frequency of a hollow cylindrical cavity. The cathode is made of a PLZT 12/65/35 ceramic with high-dielectric constant (εr∼4000). Electrons are extracted from the plasma excited on the cathode surface by ∼1 kV short rise-time pulses. The use of ferroelectric cathodes may advance the microwave tube technology for various applications.

A physical model of the effect of a shallow weak layer on strong ground motion for strike-slip ruptures

AbstractWe report results of foam rubber modeling of the effect of a shallow weak layer on ground motion from strike-slip ruptures. Computer modeling of strong ground motion from strike-slip earthquakes has in some cases involved somewhat arbitrary assumptions about the nature of slip along the shallow part of the fault (e.g., fixing the slip to be zero along the upper 2 km of the fault plane). Fault-slip inversion studies indicate that the high-frequency radiation from the shallow part of strike-slip faults is typically less than that for the deeper parts of the fault. In many cases, faults (1) may be weak along the upper few kilometers of the fault zone and may not be able to maintain high levels of shear strain required for high dynamic energy release during earthquakes and (2) may have different constitutive relations for fault slip, for example, slip strengthening. The object of this article is to present results of physical modeling using a shallow weak layer, in order to support the physical basis for assuming a long rise time and a reduced high-frequency pulse for the slip on the shallow part of faults. A weak zone was modeled by inserting weak plastic layers of a few inches in width into the foam rubber model. The long-term strength of the weak layer is about an order of magnitude less than the rest of the model. The transient strength is velocity strengthening with the strength estimated to be about three times higher at slip velocities typical of dynamic slip events. It appears a 2-km-deep, weak zone along strike-slip faults could indeed reduce the high-frequency energy radiated from shallow slip and that this effect can best be represented by superimposing a small-amplitude, short rise-time pulse at the onset of a much longer rise-time slip. For the 15-cm weak zone, the average pulse amplitude is reduced by a factor of about 0.4. The reduction factor for the 20-cm case is about 0.2. For the 30-cm case, it is about 0.1. From these results, we can see that the thicker the weak layer, the more difficult it is for a short rise-time acceleration pulse to push its way through the weak layer to the surface. The velocity strengthening property of the weak layer further damps the slip motion and increases the rise time. These results support reducing the high-frequency radiation from shallower parts of strike-slip faults in modeling studies if it is known that the shallow part of the fault is weak or has not stored up a large shear stress.

Laser shock processing of materials, physical processes involved and examples of applications

Laser shock processing (LSP) consists of irradiating a metallic target with a short (about 20 ns) and intense (>1013W m−2) laser light in order to generate, through a high pressure surface plasma (>1 GPa), a plastic deformation and a surface strengthening within materials. This paper initially reviews the physical processes involved in the analytical modeling of the generation pressure mechanism in a confined plasma regime. Limiting factors such as the dielectric breakdown in the confining medium are also discussed together with current research directions aimed at improving the laser—material coupling such as using short rise time pulses instead of Gaussian ones or shorter wavelengths than the traditional λ = 1.06 μm. Surface mechanical effects are also theoretically and experimentally presented. They consist mainly of compressive residual stresses generated in the first 1–2 mm of depth that are the key to enhanced mechanical properties. The application of LSP to two new areas is presented. These areas are the improvement of the fatigue behavior in aluminum alloys and a thermomechanical treatment using both a CO2 laser for surface quenching and a pulsed Nd–glass laser for mechanical shock strengthening. Finally the attractive and detrimental features of LSP are discussed with the objective of defining an industrially convenient laser shock configuration.

Gyrotropic propagation of vertical Bloch lines

Vertical Block line (BL) gyrotropic propagation was studied experimentally. Single BLs nucleated in an as-grown (YSmBi)/sub 3/(FeGa)/sub 5/O/sub 12/ garnet film were viewed directly using the PADO (polarized anisotropic dark-field observation) technique. Gyrotropic force was provided by an asymmetrical triangular-shaped bias field pulse. A short-risetime pulse moved the line of a straight wall in one direction, whereas a large-risetime pulse moved it backward. This gyrotropic propagation phenomenon is reproducible for varying bias field; however, some scatter is observed in BL displacement. Block wall response to the bias field pulses is also considered. The range of effective drive field inducing good gyrotropic propagation is discussed. >

A high-voltage, short-risetime pulse generator based on a ferrite pulse sharpener

A high-voltage, short-risetime pulse generator is described. The generator consists of a Marx bank, which produces an initial high-voltage pulse, and a ferrite pulse sharpener that reduces the risetime of the pulse. The generator delivers 70-kV, 350-ps risetime pulses into a 50-Ω load.

The effect of pulse pile-up on discrimination between neutrons and gamma rays

Pulse pile-up lengthens the rise-time of pulses. With an organic scintillator such as NE 213, pile-up can cause a short rise-time pulse originating from gamma rays to be interpreted by a rise-time analyser as a neutron. The degradation of pulse shape analyser performance at high count rates is shown to be directly related to pulse pile-up. Using this relationship, the contribution of piled-up gamma rays and neutrons to count rate related errors is calculated for a time-dependent fast neutron energy spectrum measurement. Errors of a few per cent occur even when the probability of a count per burst is as low as 0.01.

FM modulation of mm-wave IMPATT oscillators with PCM baseband pulses

The modulation sensitivity, linearity, and FM noise performance of a mm IMPATT oscillator is described. It is shown that when the oscillator is FM modulated with short rise time pulses, the detected FM pulses show almost no distortion compared to the original modulating baseband pulses.

Formation and duration of shock waves in nonlinear lines

The formation of very short-risetime pulses is considered in nonlinear delay lines employing reverse-biased diodes. Simple expressions for the growth and duration of the shock-wave fronts are obtained, and their validity is confirmed by experimental results.

Generation of high-speed waveforms using nonlinear delay lines

In nonlinear delay lines employing back biased p-n junctions as variable capacitors, the velocity of propagation is a function of signal amplitude. This effect may be used to produce short-risetime pulses which may be of high repetition rate. Two circuit applications are discussed, these being the processing of sinewaves to produce fast-edge triangular pulses, and also the processing of long-risetime pulses to produce short-risetime pulses. The analysis employed is in the time domain, and stems from linear delay-line theory, relying upon certain qualifications and restrictions being imposed when applied to nonlinear delay lines. This approach produces good agreement with experimental results. The questions of `ringing? and overshoot are discussed in some detail, and design criteria are obtained for eliminating these undesirable features from delay lines.

A TACAN-compatible accurate DME for short ranges

DME accuracy is improved greatly by using pulses having a short rise-time (0.2 μs) instead of the 2.5 μs TACAN pulses. The greatest improvement is in the reduction of errors caused by pulse-distorting echoes and by the large variation (80 dB) in the amplitude of interrogations from near and far aircraft. All existing ground transponders are equipped with Ferris discriminators which will not answer interrogations from adjacent channels and therefore will not supply erroneous distance data. Such discriminators can be improved to pass sharply rising pulses without integration while maintaining their near-absolute selectivity. This paper describes characteristics of ground and airborne DME, using sharply rising pulses, which provide very high accuracy at short ranges, especially in collaboration with ILS. The equipment operates within the TACAN-VORTAC system in that “short rise-time” airborne DME is used to interrogate existing en route VORTAG beacons, and existing airborne DME can obtain distance information from the “short rise-time” beacons although with less accuracy than “short rise-time” interrogators. Existing airborne and ground DME were modified to emit and use short rise-time pulses and were flight-tested. Repeated passes over a measured course provided an accuracy of ±85 feet at three miles.