Wave Excitation Process Research Articles

Saturation Properties of Whistler Wave Instability in a Plasma With Two Electron Components

AbstractSaturation properties of whistler wave instability driven by electron temperature anisotropy in a plasma with two electron components are investigated using particle‐in‐cell (PIC) simulations. Most of the previous self‐consistent PIC simulations or quasi‐linear theory used a single bi‐Maxwellian distribution of electrons, which might apply to solar wind or magnetosheath. In the inner magnetosphere, however, there is frequently a cold and dense electron component, coexisting with a hot electron component. In this work, we investigate the relation between temperature anisotropy (A) and parallel plasma beta (β‖) at saturation. Our results agree well with the previous conclusion obtained from linear theory in most cases. However, We show that the inverse relation breaks when β‖ is larger than certain value , beyond which A increases with increasing β‖. We also investigated the dependence of the saturation wave intensity on plasma beta and the initial linear growth rate and show that the saturation amplitude can be modeled as a function of the maximum initial linear growth rate even in a plasma with two electron components. Our results might be useful to couple the microscopic wave excitation process with macroscopic global energetic electron dynamics and to understand whistler wave excitation in the inner magnetosphere.

Excitation mechanisms in 1 mJ picosecond laser induced low pressure He plasma and the resulting spectral quality enhancement

We report in this paper the results of an experimental study on the spectral and dynamical characteristics of plasma emission induced by 1 mJ picoseconds (ps) Nd-YAG laser using spatially resolved imaging and time resolved measurement of the emission intensities of copper sample. This study has provided the experimental evidence concerning the dynamical characteristics of the excitation mechanisms in various stages of the plasma formation, which largely consolidate the basic scenarios of excitation processes commonly accepted so far. However, it is also clearly shown that the duration of the shock wave excitation process induced by ps laser pulses is much shorter than those observed in laser induced breakdown spectroscopy employing nanosecond laser at higher output energy. This allows the detection of atomic emission due exclusively to He assisted excitation in low pressure He plasma by proper gating of the detection time. Furthermore, the triplet excited state associated with He I 587.6 nm is shown to be the one most likely involved in the process responsible for the excellent spectral quality as evidenced by its application to spectrochemical analysis of a number of samples. The use of very low energy laser pulses also leads to minimal destructive effect marked by the resulted craters of merely about 10 μm diameter and only 10 nm deep. It is especially noteworthy that the excellent emission spectrum of deuterium detected from D-doped titanium sample is free of spectral interference from the undesirable ubiquitous water molecules without a precleaning procedure as applied previously and yielding an impressive detection limit of less than 10 μg/g. Finally, the result of this study also shows a promising application to depth profiling of impurity distribution in the sample investigated.

Numerical Investigation of Air-Coupled Generation of Lamb Waves in Isotropic Plates

In the case of non-destructive testing and evaluation (NDT & NDE) of plate materials and structures ultrasonic Lamb waves are used. Very often air-coupled ultrasonic transducers are used for non-contact Lamb wave excitation and reception. This method has a very serious drawback: global insertion losses for an air-coupled system may be 120 dB–160 dB typically. In order to improve this NDT method a better understanding of Lamb wave excitation process is needed. A numerical investigation was performed in order to evaluate air-coupled Lamb wave excitation in an isotropic single layer plate. It is shown how Lamb wave normal displacement amplitude depends on an incidence angle and the distance between the plate and the transducer. DOI: http://dx.doi.org/10.5755/j01.eee.20.1.6162

The role of Helium plasma in improving the sensitivity of Hydrogen detection in laser induced plasma spectroscopy

The experimental studies have been performed on the characteristics of laser induced He plasma that give influence to the sensitivity enhancement and the long lifetime of hydrogen emission line. Under He ambient gas, the emission intensity of spectra of hydrogen and oxygen increase with low background emission. In order to confirm the intensity enhancement, spatial intensity distribution of hydrogen and helium was measured along the plasma expansion direction. The result revealed that the maxima come at the far end of the plasma where the available shock wave generated thermal excitation energy should have been significantly reduced. It implies the time-mismatch effect on the shock wave excitation process and the presence of possible contribution of helium metastable excited state in He plasma.

Resonant Excitation of Disk Oscillations in Deformed Disks IV: A New Formulation Studying Stability

The possibility has been suggested that high-frequency quasi-periodic oscillations observed in low-mass X-ray binaries are resonantly excited disk oscillations in deformed (warped or eccentric) relativistic disks (Kato 2004). In this paper we examine this wave excitation process from a viewpoint somewhat different from that of previous studies. We consider how amplitudes of a set of normal mode oscillations change secularly with time by their mutual couplings through disk deformation. As a first step, we consider the case where the number of oscillation modes contributing to the resonance coupling is two. The results show that two prograde oscillations interacting through disk deformation can grow if their wave energies have opposite signs.

Effect of a laser spike on the beat-wave excitation of a plasma wave

This paper presents an investigation on the effect of a laser spike (ripple) on the plasma wave excitation process in laser-produced plasma. The plasma wave is generated on account of beating of two co-axial lasers of frequencies ω1 and ω2, with ω1−ω2≅ωp. One of the main laser beams has intensity spikes. The mechanism of nonlinearity is assumed to be a ponderomotive force, and following the WKB and paraxial-ray approximations, the effect of laser spikes on the beat-wave excitation has been studied. The nonlinearity (due to the ponderomotive force) not only depends on the intensity of one laser beam, but also on the second laser beam. Therefore, the behavior of the first laser beam affects the second laser beam and cross-focusing takes place. The spike grows inside the plasma due to the interaction of the spike and main beams (Purohit et al 2003 Laser Particle Beams 21 567, Purohit et al 2005 Laser Particle Beams 23 69). The laser spikes in the plasma affect the dynamics of the electron plasma wave. It has been observed that the power associated with the excited plasma wave, which depends on the background electron concentration and the intensity of the two beams, becomes drastically modified with the distance of propagation in the presence of the laser spike. The results are presented for typical laser plasma parameters.

Effect of laser ripple on the beat wave excitation and particle acceleration

Abstract.This paper presents the effect of ripple on the plasma wave excitation process and acceleration of electrons in a laser produced plasma. The plasma wave is generated by the beating of two coaxial lasers of frequencies ω1 and ω2, such that ω1-ω2≅ωp. One of the main laser beams also has intensity spikes. The nonlinearity due to the relativistic mass variation depends not only on the intensity of one laser beam but also on the second laser beam. Therefore the behavior of the first laser beam affects the second laser beam, hence cross-focusing takes place. Owing to the interaction of ripple and the main laser beams, the ripple grows inside the plasma. The behavior of the ripple in the plasma affects the excitation of the electron plasma wave as well as the electron acceleration. The amplitude of the electron plasma wave and the electron energy are calculated, in the presence of ripple.

Effects of mass difference on pressure-dependent emission characteristics in laser-induced plasma spectroscopy

A comprehensive experimental study was performed to demonstrate the effects of ambient air pressure on the emission characteristics of analyte elements in a variety of host materials in laser-induced plasma spectroscopy in low ambient air pressures. It was shown that the pressure-dependent characteristics of the emission lines are generally influenced by host elements when significant mass difference exists between the analyte and the host elements. Further investigation on the time profiles of the spatially integrated emission intensities reveal the important interplay among the dynamical factors associated with the mass-difference effect, which effectively influence the shock wave excitation process and hence the related emission intensities. The result of this study also indicates the need of proper control of time delay for the detection of maximum emission intensity in the cases considered.

Electron cyclotron waves in the presence of parallel electric fields in the Earth's auroral plasma

Abstract. The electron cyclotron waves that originate at low altitudes (<0.5 RE) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.

Electron cyclotron waves in the presence of parallel electric fields in the Earth’s auroral plasma

The electron cyclotron waves that originate at low altitudes (<0.5 RE) and observed by ground facilities have been studied in the presence of a weak parallel electric field in auroral magnetoplasma consisting of trapped energetic auroral electrons and cold background electrons of ionospheric origin. The model distribution for auroral trapped electrons is taken as Maxwellian ring distribution. An expression for the growth rate has been obtained in the presence of parallel electric field assuming that the real frequency in the whistler mode is not affected by the presence of the electric field. The results show that waves grow (or damp) in amplitude for a parallel (or antiparallel) electric field. The influence of the electric field is more pronounced at a shorter wavelength spectrum. An increase in population of energetic electrons increases the growth rate and thus, plays a significant role in the wave excitation process in the auroral regions.

Multicomponent near-surface correction for land VSP data

Multicomponent seismic data collected using directional sources are degraded by the wave excitation process due to inaccurate control of the ground motion, unequal activation strengths or ground couplings between differently oriented sources, and misalignment of the pad. These acquisition uncertainties are exacerbated by the complicated near-surface scattering present in most seismic areas. Neither group of effects should be neglected in multicomponent analyses that make use of relative wavefield attributes derived from compressional and shear waves. These effects prevent analysis of the direct and reflected waves using procedures based on standard scalar techniques or a prima facia interpretation of the vector wavefield properties, even for the seemingly straightforward case of a near-offset vertical seismic profile (VSP). Near-surface correction, using a simple matrix operator designed from the shallowest recordings, alleviates many of these interpretational difficulties in near-offset VSP data. Results from application of this technique to direct waves from a nine-component VSP shot at the Conoco test-site facility, Oklahoma, are encouraging. The technique corrects for unexpected compressional-wave energy from shear-wave vibrators and collapses near-surface multiples, thus facilitating further processing for the upgoing wavefield. The method provides a simple and effective processing step for routine application to near-offset VSP analyses.

Temporal evolution of the HF‐enhanced plasma line in sporadic E

The high‐power, high‐frequency (HF) facility at Arecibo, Puerto Rico, has been used to study the excitation of Langmuir waves in mid‐latitude sporadic E. Measurements of the temporal evolution of the so‐called HF‐enhanced plasma line (HFPL) were made using the Arecibo 430‐MHz radar. After HF turn‐on in the plasma the HFPL exhibits a rapid growth phase followed by a quick overshoot. During periods of strong HFPL excitation the e‐folding growth time of the HFPL power is typically ≲ 20 µs, and the total overshoot period is ∼1 ms. On the basis of the current observations, mode conversion of the HF wave into Langmuir waves near HF reflection appears to be a promising mechanism for the production of Langmuir waves in sporadic E. Caviton formation at the critical layer is expected to accompany this process, and there is some evidence that the 430‐MHz radar is probing the plasma in a region where density cavities of this nature form. While no specific explanation is offered for the HFPL overshoot, it appears that this phenomenon is fundamental to the Langmuir wave excitation process.

On Beat Wave Excitation of Relativistic Plasma Waves

The beat wave excitation process is examined analytically in the Eulerian fluid description. The effects of plasma drifts, harmonics, pump rise time, frequency mismatch, phenomenological damping, plasma inhomogeneities, and two dimensions are discussed. The consistency between the Eulerian and Lagrangian fluid descriptions is verified.

Theory of Parametric Interactions in Semiconductors Under a Simultaneous Action of Two High-Power Electromagnetic Radiation Fields. II. Instability Analysis

The parametric wave excitation process in saturated semiconductors is considered in the two-band model treated in the previous work [1]. Using the expressions for the generalized quantum distribution functions obtained there, the eigenmode problem is solved and the conditions of parametric excitation of certain resonant modes are analyzed. The analytical expression for the instability growth rate is obtained which shows the simultaneous action of two radiation fields on the parametric interaction process in the system. For illustrative purposes the numerical estimation is presented for a sample of GaAs.

Edge waves and beach cusps

Genetically, beach cusps are of at least two types: those linked with incident waves which are surging and mostly reflected (reflective systems) and those generated on beaches where wave breaking and nearshore circulation cells are important (dissipative systems). The spacings of some cusps formed under reflective wave conditions both in the laboratory and in certain selected natural situations are shown to be consistent with models hypothesizing formation by either (1) subharmonic edge waves (period twice that of the incident waves) of zero mode number or (2) synchronous (period equal to that of incident waves) edge waves of low mode. Experiments show that visible subharmonic edge wave generation occurs on nonerodable plane laboratory beaches only when the incident waves are strongly reflected at the beach, and this observation is quantified. Edge wave resonance theory and experiments suggest that synchronous potential edge wave generation can also occur on reflective beaches and is a higher-order, weaker resonance than the subharmonic type. In dissipative systems, modes of longshore periodic motion other than potential edge waves may be important in controlling the longshore scale of circulation cells and beach morphologies. On reflective plane laboratory beaches, initially large subharmonic edge waves rear-rage sand tracers into shapes which resemble natural beach cusps, but the edge wave amplitudes decrease as the cusps grow. Cusp growth is thus limited by negative feedback from the cusps to the edge wave excitation process. Small edge waves can form longshore periodic morphologies by providing destabilizing perturbations on a berm properly located in the swash zone. In this case the retreating incident wave surge is channelized into breeches in the berm caused by the edge waves, and there is an initially positive feedback from the topography to longshore periodic perturbations.