Fast Transient Phenomena Research Articles

A generalized two‐cycle componentwise splitting method for solving three‐dimensional parabolic differential equations with variable coefficients on multilayers

Describes a generalized two‐cycle componentwise splitting method for solving three‐dimensional parabolic differential equations with variable coefficients which has been developed based on the idea of the regularized difference scheme. The method is simple, unconditionally stable and well suited for simulating fast transient phenomena and for computations on fine spatial meshes. A numerical procedure that employs the generalized two‐cycle componentwise splitting scheme was developed to solve three‐dimensional parabolic differential equations with variable coefficients on multilayers. In the procedure, the generalized “divide and conquer” method for solving tridiagonal linear systems is applied in order to overcome the problem with the unknown value at the interface between layers. Numerical results show that the procedure is accurate and efficient.

A generalized Douglas ADI method for solving three‐dimensional parabolic differential equations on multilayers

Develops a generalized Douglas ADI method for solving three‐dimensional parabolic differential equations based on the idea of the regularized difference scheme. The method is simple, unconditionally stable and well suited for either simulating fast transient phenomena or for computations on fine spatial meshes. Numerical procedures that employ the generalized Douglas ADI scheme were developed to solve three‐dimensional parabolic differential equations on multilayers. In these procedures, the generalized “divide and conquer” method for solving tridiagonal linear systems is applied in order to overcome the problem with the unknown value at the interface between layers. Numerical results show that the procedure is accurate and efficient.

In situ fast ellipsometric analysis of repetitive surface phenomena

We present an ellipsometric technique and ellipsometric analysis of repetitive phenomena, based on the experimental arrangement of conventional phase modulated ellipsometers (PME) conceived to study fast surface phenomena in repetitive processes such as periodic and triggered experiments. Phase modulated ellipsometry is a highly sensitive surface characterization technique that is widely used in the real-time study of several processes such as thin film deposition and etching. However, fast transient phenomena cannot be analyzed with this technique because precision requirements limit the data acquisition rate to about 25 Hz. The presented new ellipsometric method allows the study of fast transient phenomena in repetitive processes with a time resolution that is mainly limited by the data acquisition system. As an example, we apply this new method to the study of surface changes during plasma enhanced chemical vapor deposition of amorphous silicon in a modulated radio frequency discharge of SiH4. This study has revealed the evolution of the optical parameters of the film on the millisecond scale during the plasma on and off periods. The presented ellipsometric method extends the capabilities of PME arrangements and permits the analysis of fast surface phenomena that conventional PME cannot achieve.

A Generalized Peaceman–Rachford ADI Scheme for Solving Two-Dimensional Parabolic Differential Equations

A generalized Peaceman–Rachford alternating-direction implicit (ADI) scheme for solving two-dimensional parabolic differential equations has been developed based on the idea of regularized difference scheme. It is to be very well to simulate fast transient phenomena and to efficiently capture steady state solutions of parabolic differential equations. Numerical example is illustrated.

A New ADI Scheme for Solving Three-Dimensional Parabolic Differential Equations

A new alternating-direction implicit (ADI) scheme for solving three-dimensional parabolic differential equations has been developed based on the idea of regularized difference scheme. It is unconditionally stable and second-order accurate. Further, it overcomes the drawback of the Douglas scheme and is to be very well to simulate fast transient phenomena and to efficiently capture steady state solutions of parabolic differential equations. Numerical example is illustrated.

Resonant tunnelling light-emitting diodes

Resonant tunnelling light-emitting diodes (RTLEDS) are p-i-n diodes containing a double-barrier (or multi-barrier) resonant tunnelling structure in the intrinsic region of the diode. The simultaneous intraband tunnelling of electrons and heavy holes gives rise to injection electroluminescence from the quantum well and from the two accumulation layers on either side of the resonant tunnelling structure. The fast transient phenomena in these structures give rise to a fast response of the optical output otherwise only found in semiconductor lasers. Important aspects of the bipolar diode like speed and charge dynamics will be discussed.

Suppression of VFT in 1100 kV GIS by adopting resistor-fitted disconnector

With 1000 kV transmission lines planned in Japan, very fast transient (VFT) phenomena will be suppressed by installing a resistor in a disconnector of gas insulated switchgear (GIS). In this paper the VFT overvoltage suppressing effect of the resistor and the duty required of the resistor are clarified. A 1100 kV resistor-fitted disconnector was tested by constructing a charging current interruption test circuit. It has been clarified that the disconnector accepts the required duty. The disconnector tested is used for the field test.

Fast electro-optical system for pulsed ESPI carrier fringe generation

Pulsed Electronic Speckle Pattern Interferometry (ESPI) offers the possibility of analysing fast transient phenomena in experimental mechanics. However, due to the short pulse separation required, addition fringes with very low visibility are generated. Several methods have recently been developed to process these data but they all suffer some limitations. This paper presents an electro-optical system based on the use of a Pockels cell which enables the introduction of carrier fringes between the firing of two consecutive laser pulses. This allows calculation of the wrapped phase distribution from a single fringe pattern using the Spatial Synchronous Detection method and enables the laser pulse separation to be significantly reduced. Preliminary experimental measurements performed using a continuous wave laser are used to illustrate the performance of this approach.

Numerical calculation of transient excitonic Green functions in the presence of phonon scattering

As a demonstration of the growing feasiblity of quantum treatment of complex systems with parallel computing we have performed a transient population dynamics calculation for carriers interacting via Coulomb and electron-phonon interactions. Two-point Keldysh functions are obtained by integrating the Dyson equation in the time domain. Keldysh functions are treated explicitly as matrices with discretized real time as their index in order to be able to deal squarely with fast transient phenomena. The Coulomb interaction is included in the electron self-energy within the Hartree-Fock approximation, while the interaction with localized non-dispersive thermal phonons is taken into account up to the second order in the electron-phonon vertex. The numerical procedure is applied to the case of carrier excitation by an ultrashort laser pulse in a model two-band semiconductor to obtain the transient behaviour of carrier population and the luminescence spectrum shortly after the excitation.

Numerical simulation of impact and fast transient phenomena using AUTODYN™-2D and 3D

Abstract AUTODYN-2D and 3D are interactive, integrated hydrocodes in worldwide usage on personal computers through to supercomputers. They provide a number of fully coupled numerical processors, including Lagrange, Euler, Arbitrary Lagrange Euler and Shell processors, which make them suited to a wide range of non-linear dynamic problems. The codes are particularly suited to the modelling of impact, penetration, blast and explosion events. The results of a number of selected analyses are presented, to highlight typically observed impact and fast transient phenomena. These analyses also illustrate the importance of selecting the appropriate processor or combination of processors to facilitate both an accurate and computationally efficient solution.

Computational study of very fast transients in GIS with special reference is effects of trapped charge and risetime on overvoltage amplitude

Very fast transient phenomena in gas insulated substations (GIS) have been studied by computer simulation. The influence an overvoltage amplitude of trapped charge, risetime and section length of GIS is described. Under identical conditions, the amplitudes of overvoltages caused by very fast transients at the same point in a circuit, but having different trapped charges, are connected by a simple relation due to the linear nature of the circuit. In the case of high predicted overvoltages, the assumption of 1 pu trapped charge causes a large overestimation of the overvoltage which can be in excess of 20%, in comparison with the overvoltage due to 0.4 pu trapped charge, a value which, in practice, is rarely exceeded. For an open ended circuit, the assumption of zero risetime can cause an overestimation of more than 30% in the overvoltage in comparison with the case of 5 ns or more risetime. Variation of the trunk section length has a greater effect on the overvoltages than that of a branch section. The difference in the overvoltages at an open end for various section lengths can be in excess of 50%. In some cases even a small variation of section length causes a very large change in overvoltages.

Field-equation approximations and the dynamics of high-gain lasers.

Semiclassical models for laser-dynamics studies typically incorporate a derivative approximation that reduces the second-order wave equation for the electric field to a first-order equation. It is shown here that this approximation and further frequency approximations are not necessary and may lead to significant errors for some high-gain laser systems. A more exact analysis also reveals a partial decoupling of the electric and magnetic fields that occurs with fast transient phenomena. These ideas are illustrated in terms of well-known dynamical effects.

Design and performance of a surface potentiometer

AbstractThe electrostatic surface potentiometer has the feature that the distance compensation effect is included and the fast transient phenomenon can easily be observed. On the other hand, the noise is increased when a wideband is realized while maintaining a high input resistance. When a low potential is to be measured or the area of the measuring electrode is decreased aiming at a high resolution, a limit of performance is produced. The investigation of this point is not yet sufficient.This paper derives the formula for the bandwidth of the amplifier, which is needed in the design and construction of such a surface potentiometer. Using the angular frequency and the output noise voltage as the parameters, a formula is derived which relates the amplification A to the capacitance between the sample and the sensing electrode. Those results were verified by experiment. The operating region for A and m (which is the ratio between the capacitance between the sample and the sensing electrode, and the input capacitance of the potentiometer) is useful in the design and construction of the potentiometer.

Dynamics of solute-solvent interactions

Abstract

Effect of spacecraft debris on the observation of fast transient phenomena

Effect of spacecraft debris on the observation of fast transient phenomena

Time-Controllable Continuum Photon Emission from a Pulsed Vacuum Spark and Application to Absorption Spectroscopy in the Vacuum Ultraviolet

A high-voltage vacuum spark emitting a continuum in the vacuum ultraviolet was triggered by the particles originating from a dense plasma produced by a laser beam focused onto a nearby solid target. The emitted radiation was used to observe absorption processes in the same laser plasma. Because of the fast transient phenomena occurring in this plasma, the delay-time before spark breakdown had to be minimized. With the particular device used, the minimum delay-time between laser pulse and continuum emission was found to be about 200 nsec with a jitter time of the order of 20 nsec. This delay-time is too long to allow numerous observations of absorption by singly charged ions, but the applicability of the method to absorption studies on neutral species was demonstrated. Examples of absorption spectra from atoms of beryllium and magnesium are discussed.

Laser Interferometry of High Density Shocked Plasma

Laser is a very suitable light source of the interferometer. The advantages of the use of a laser come from the excellent coherency or long coherent length, good monochromaticity and high intensity. We have measured the density of shocked plasmas by using Mach-Zehnder interferometer with He-Ne gas laser. This device seems to be very powerful to investigate fast transient phenomena in high electron density such as plasma shock waves. The precise experiments are going on.

Recording fast transient phenomena with cathode ray oscillograph in free air as well as in high vacuum

In spite of the great improvements which the cathode-ray oscillograph for high-vacuum photography has undergone in the last few years, the problem of photography in free air is most important on account of the great simplifications for general construction and for taking of photographs. An available method for taking photographs outside the vacuum, was found to be the employment of an electron permeable window (Lenard window) of the size of the oscillogram to be made. Ordinary aluminum or cellon foil of approximately 7 or 16 μ, thickness respectively may be used if the common exciting voltages of 66–75 kv. are employed. With such foils, all results are to obtain which hitherto have been feasible only with high-vacuum photography. According to this method, oscillograms in free air were taken which attained recording speeds up to 5000 km. per sec, using an exciting voltage of 75 kv. only. With this recording speed, details in the oscillograms of the duration of one milliardth of a second may be dissolved. As compared with the high-vacuum photography hitherto exclusively used for recording quickest phenomena, the Lenard window oscillograph allows taking photographs in a considerably shorter time with the advantages of the simpler construction and manipulation.