Excitation Of Ion-acoustic Waves Research Articles

The excitation of backward propagated ion-acoustic instability in collision-dominated plasmas

The process of spontaneous excitation of ion-acoustic waves (IAW) of growing amplitude by electron drift, in collision-dominated weakly ionized plasmas placed in external d.c. electric field is considered from the standpoint of the linear kinetic theory based on the one-particle distributionfunction formalism. The longitudinal permeability is evaluated, with both e-n and i-n collisions taken into account, by way of the respective collision integral perturbations. The results are derived for the range of frequencies νen ≫ ω ≫ νin and νen ω ≪ k2vte2, provided that the electron drift-to-thermal velocity ratio satisfies u/vte ≪ kvte/νen ≪ 1. It is found that, for certain forms of the steady-state electron distribution function (EDF), some of which are very probable in experimental situations, the criterion for the onset of the ion-acoustic instability is satisfied not for cos θ > 0, but rather for cos θ < 0 (cos θ = u · k/uk), i.e. one can find instances in which the instability is propagated in the direction opposite to that of the electron drift. In particular, this is found to be the case in some weakly ionized gas-discharge argon plasmas, where the IAW of growing amplitude propagated backwards are expected to set in, if the i-n collision frequency is sufficiently small.

Growth of a Gaussian Laser Ripple on a Gaussian Beam in a Collisionless Magnetoplasma and its Effect on the Excitation of Ion-Acoustic Wave

This paper presents an investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless magnetoplasma. Here we have presented the effect of magnetic field and the intensity of the laser beam on the growth of ripple in some detail. The effect of presence of ripple on the excitation of an ion-acoustic wave is also investigated. Coupling of a weak ion-acoustic wave with main laser beam is through modified background density. Interesting feature of the analysis is that the combined effect of increased intensity of the laser beam and magnetic field is observed to suppress the growth of the ripple as well as the excitation of the ion-acoustic wave.

Ion acoustic instabilities excited by injection of an electron beam in space

It is shown by means of numerical simulations that an injection of a tenuous suprathermal electron beam into a fully ionized plasma from spacecraft can lead to an excitation of ion acoustic waves. The return current carried by the ambient electrons has a drift speed that is much slower than the beam speed, so that these electrons can excite the ion waves via wave‐particle interaction. Numerical simulations confirm the excitation of ion acoustic waves and the acceleration of ions near the spacecraft producing a high‐energy tail.

Effects of a weak ion beam on grid-launched ion-acoustic waves

Experimentally observed multiple ion-acoustic wave excitation in a weak ion beam plasma system is described. An additional anomalous signal was observed to form and propagate hundreds of Debye lengths from the grid in a large quiescent plasma. The existence of this signal is interpreted to be caused by the grid excitation mechanism, and it can be used to ascertain the dimension of the sheath surrounding the grid.

Reflection of ion acoustic waves from positive and negative space charge sheaths

Experimental evidence is presented showing that externally excited ion acoustic waves are reflected from positive and negative space charge sheaths around a cylindrical wire.

The Excitation of Ion-acoustic Waves by a Magnetic Pump in Cylindrical Geometry

The modulational, parametric and purely 8rowing mode instabilities of a magnetic pump with a finite radial wavenumber in cylindrical geometry are investigated. The modulational instability is compared with the instability of a parallel propagating pump wave, and the growth rates are found to be similar. The growth rate of a pair of ion-acoustic waves is shown to be zero for a collisionless plasma, in agreement with less general results found previously. The growth rate in the collisional case is found to be nonzero. The purely growing mode growth rate is calculated for excitation of ionacoustic plus torsional Alfven waves.

Excitation of ion acoustic waves by a slow ion beam

The measurement of the power spectrum of ion acoustic waves has been performed in a laboratory plasma. The phase velocities of these waves as well as the relative ion-electron bulk velocity can be deduced from the quasi-periodic structure that they induce in the shape of the spectrum.

Quadrupole Radiation of Ion-Acoustic Waves

An experiment to measure the radiation of ion-acoustic waves from a quadrupole antenna is described. The quadrupole consists of 4 hemispherical grids located at the corners of a square structure. The results are contrasted with those obtained from an antenna with the 4 hemispheres excited in phase and compared with a fluid model prediction. Electrostatically coupled excitation of ion-acoustic waves from 3 hemispheres when only the fourth is excited is also observed in this experiment.

Excitation of ion acoustic waves at the difference frequency of two microwave beams in a semiconductor

This letter presents an investigation of the resonant excitation of the electrostatic ion acoustic wave at the difference frequency of two microwave beams in a semiconductor, viz., n-type InSb. The ponderomotive force at the difference frequency on electrons drives the ion acoustic wave at the difference frequency. The resonance conditions are satisfied over a wide range of semiconductor parameters. For typical plasma parameters of n-InSb and microwave beams of power densities 1 MW cm −2, the power density of the excited ion acoustic wave is ≈ 1.76 kW cm −2.

Wireless Excitation of Ion Acoustic Wave in a Plasma

Wireless Excitation of Ion Acoustic Wave in a Plasma

Theory of double resonance parametric excitation in plasmas. Part 2

Using a simpler formalism than in the original paper on this subject, we verify the earlier result that, for a pump frequency separation Δ approximately equal to twice the ion acoustic frequency Ω, the use of two long-wavelength pumps can reduce the threshold for parametric excitation of ion-acoustic waves when, andonlywhen, the Langmuir wave damping rate γ is much larger than Ω. The threshold is then reduced by a factor of order Ω/Ω, the optimum value of t being ▵ 2Ω –Γ for equal pump amplitudes, where Γ is the ion-acoustic wave damping rate and Γ ≪ Ω. The analysis presented in a recent paper is shown to be valid only for γ ≪ Ω, where the threshold reduction is quite small, vanishing in the limit of weak ion-acoustic wave damping (Te/Ti≫ 1).

Observation of Parametric Instabilities at Unstable Electrostatic Ion Cyclotron Waves in an Ion Beam-Plasma System

We have observed experimentally the parametric excitation of ion acoustic waves and electrostatic ton cyclotron waves by h f electric fields with frequencies of unstable electrostatic ion cyclotron waves. The resonant condition is satisfied, and the decay waves are amplified exponentially by increasing the amplitude of the pump wave. The growth rate of the decay waves and the threshold of pump h f electric fields measured experimentally is compared with the theory.

Excitation of ion-acoustic waves by two Gaussian laser beams

Excitation of ion-acoustic waves in a hot collisionless isotropic plasma by two Gaussian laser beams is investigated. On account of the non-uniform intensity distribution along the wavefront of the beams, the ponderomotive force becomes finite. This leads to modification of the plasma density profile in the plane transverse to the beam axis and hence also to modification in the propagation characteristics of the waves propagating in the plasma. It is shown that self-focusing of the waves can occur and this considerably influences the beat wave power flux.

Dispersion Theory of Self Excited Ion Acoustic Waves in a Hot Cylindrical Plasma. I. The General Formula

AbstractStarting from the system of the Vlasov‐Poisson equations, a closed equation for the potential of the electrostatic field of ion acoustic waves in a hot cylindrical plasma with drifting electrons is found. For the electrons ideal reflection at the plasma boundary is supposed. From this equation in a first approximation a dispersion relation is derived, which includes dispersion equations presented in the literature for special cases.

Parametric instabilities at unstable electrostatic ion cyclotron waves in an ion beam-plasma system

We have observed experimentally the parametric excitation of ion acoustic waves and electrostatic ion cyclotron waves by hf electric fields with frequencies of unstable electrostatic ion cyclotron waves. We have verified wave number and frequency selection rules.

Interference with, and excitation of ion-acoustic waves by laser light

The effect of laser light having Gaussian spatial and temporal variations upon ion-acoustic waves is investigated analytically. At high power densities this light can interfere with coherent propagating waves in addition to exciting waves streamers which propagate away from the perturbed region at the ion sound velocity.

Excitation of ion acoustic waves with probes

A large cylindrical Langmuir probe (length=30 cm, diameter=1.25 mm) and a probe of similar dimensions consisting of a conductor completely enclosed in an insulator are compared as exciters of ion acoustic waves. When the potential drop across the sheath of the Langmuir probe is increased from −143 to +10 V, the ion wave amplitude increases by a factor of 40. This result can be qualitatively explained with a simple model which relates the wave amplitude to the sheath radius and to the ac sheath impedance. When large negative pulses (≳100 kTe/e) are applied to the probes, the plasma response is similar for the biased and the insulated probe. The advantages of the insulated probe as an exciter of ion waves over ordinary probes are discussed. Ion waves with peak-to-peak amplitudes (δn/n) ≃0.44 can be excited with the insulated probe.

Electron Wave Cavity Formation in the Electron Beam-Plasma Interaction

Nonlinear evolution of the electron-beam instability is examined experimentally in a plasma unstable for ion-acoustic wave excitation. Amplitude modulation of the electron wave is observed, with the threshold for the energy density practically equal to zero. When the beam density is more than a few percents of the plasma density, the electron wave energy grows up to several percents of the thermal energy. The peaks in the amplitude are found to correspond, for the wave, to the trapping in the density depressions executing ion-acoustic oscillation. A striking feature of the present experiment is a coupled relaxation oscillation between the electron wave and the ton-acoustic wave.

Ion Energy Increment by Ion Acoustic Waves

An ion energy increment by externally excited ion acoustic waves has been studied in a simple double plasma device. The energy increment was measurd in detail by means of a small electrostatic energy analyzer whose resolving power was better than 0.1 eV. A small energy increment of about 10% was observed under the condition where ion fluctuations caused by the wave fields were less than 10% and the ratio of the electron-to-ion temperature was about 10. The measured energy increment was approximately proportional to the wave frequency and to the square of the wave potential. The fact is well understood if the wave energy is transferred to the ions through the Landau damping. The electron energy increment, however, was negligible in accordance with the above interpretation.

Excitation of ion-acoustic wave by power-modulated electromagnetic wave in a weakly ionized plasma

It is shown that the electromagnetic ponderomotive force fnl of a locally irradiated microwave beam is mainly responsible for the excitation of an ion-acoustic wave, even when the ionization effect is taken into consideration.