Electron Beam-plasma System Research Articles

Observations of Subharmonic Waves in an Electron Beam Plasma System

Subharmonic waves of electron plasma waves are observed in an electron beam plasma system and the parametric dependences of the waves are examined in detail. The excitation mechanism is also discussed.

Nonlinear Landau Electron Heating by Electrostatic Waves in an Electron Beam-Plasma System

Electron heating due to nonlinear Landau damping of electrostatic waves in an electron beam-plasma system is studied theoretically. The fast and slow space charge waves of an electron beam and the Trivelpiece-Gould modes interact nonlinearly with plasma or beam electrons and cause an effective electron heating and a beam relaxation which leads to a saturation of electron heating. For nonlinear interaction with plasma electrons, the virtual (beat) wave produced by the two electrostatic waves can heat plasma electrons by its electron cyclotron damping. On the other hand, for nonlinear interaction with beam electrons, the Trivelpiece-Gould mode enhanced by nonlinear Landau damping can heat plasma electrons by its electron Landau damping. For both cases, the conservation of total energy flux and the existence of the energy transport are proved quantitatively.

Microwave emission and beam propagation measurements in a high-power relativistic electron beam-plasma system

Microwave emission was measured from a system consisting of an unmagnetized plasma and a propagating electron beam. A 93-cm/sup 2/ velvet cathode, with an anode-cathode gap of 5.9 cm, injects the electron current into the plasma through an aluminized Mylar anode. Measurements were made of the diode voltage and current in the 6- mu V water dielectric accelerator and net current through the beam-plasma system. The unmagnetized plasma is produced by a 90- mu s, 90-AA current pulse, emitted from a thermionic LaB/sub 6/ electron source, that preionizes argon fill in a 1-m-long, 15-cm-diameter Lucite tube. A microwave spectrometer detects the radio-frequency output in the 2-18, 18-26, and 26-47 GHz bands, filters, and then separates into narrower subbands. The emission takes place in two distinct phases. The 2-GHz output rises promptly with the current pulse and then decays. At 6-GHz and above, a low-level microwave prepulse appears simultaneously with the 2-6 GHz output. This output rises sharply 25 ns after the current pulse begins and includes frequencies out to and beyond 40 GHz. The radio-frequency output falls off before the current pulse ends. The microwave intensity decays monotonically with frequency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Nonlinear Behaviors of a Bounded Electron Beam-Plasma System

Nonlinear developments of a bounded electron beam-plasma system including stationary electrons are investigated experimentally. A stable double layer is formed as a result of ion trapping in a growing negative potential dip induced by the Pierce instability above the current regime of the Buneman instability. In the in-between regime of the Buneman and Pierce instabilities, energetic ions are observed. This effective ion heating is caused by ion detrapping due to double-layer disruption, being consistent with computer simulation.

Electromagnetic radiation and nonlinear energy flow in an electron beam-plasma system

The mechanism by which unstable electron plasma waves are converted into electromagnetic waves in a uniform plasma is investigated. Electromagnetic radiation is generated upon injection of an electron beam (500 eV) into a collisionless quiescent magnetoplasma (ne ≲1012 cm−3, kTe ≊2 eV). The emission (ω0) is observed to peak near the plasma frequency (ωp) which is well above the cyclotron frequency (ωc ≪ωp≲ω0). It is shown that electromagnetic waves (ω0, k0) are produced by the scattering of electrostatic plasma waves (ωe, ke) off self-consistently produced ion-acoustic waves (ωi, ki). At low beam intensities the frequency and wave vector matching conditions are experimentally verified (ωe=ωi+ω0, ke=ki+k0≂ki). The emission is found to be polarized, negligible in intensity at ω0=2ωp, and its source is localized. The space-time evolution of the three-wave interaction is presented. Besides these nonlinear wave–wave interactions the wave–particle interactions are investigated. It is found that the strong Langmuir turbulence exhibits a three-dimensional character. Cross-correlation surfaces are measured, and their characteristic scale lengths are found to decrease with increasing wave intensity toward the Debye length (λD ≂20 μm) although the resolution is probe limited (Lmin ≂1 mm). The beam electrons are diagnosed using a novel directional velocity analyzer which is capable of resolving the true three-dimensional distribution function. After interacting with the intense Langmuir waves, the beam electrons are scattered in velocity space, both parallel and perpendicular to their injection velocity. The background electron distribution is observed to develop an energetic anisotropic tail (E≲50 eV, kTe ≂1 eV). The energization of the background electrons coincides with the strong damping of the Langmuir waves. The nonlinear damping is seen as an anomalous ac resistivity produced by large amplitude ion-density fluctuations (δni/ni ≂5%) in the electron beam region.

Buneman Instability in a Bounded Electron Beam-Plasma System

Effects of stationary electrons on the Buneman instability in a bounded electron beam-plasma system are investigated theoretically and experimentally. The Buneman instability is generated in a low-beam-current regime and is coupled with the Pierce instability in the high-beam-current regime. The threshold current of the Buneman instability is considerably increased by introducing a small amount of stationary electrons into the system.

Effects of a cold electron beam on the parametric decay of a Langmuir wave

A formalism for the parametric decay of a Langmuir wave into a backscattered Langmuir wave and an electrostatic ion wave in an electron beam-plasma system is developed. Explicit growth rates and dispersion relations for the ion wave modified by mode coupling and a nearly monoenergetic electron beam are derived. A numerical study of the effects of the beam on the growth rate for this resonant backscatter decay instability is presented.

Effects of a cold electron beam on Brillouin backscattering in a plasma

A kinetic analysis of the stimulated backscattering of an electromagnetic wave modified by the presence of a nearly monoenergetic electron beam is presented. The results of a numerical study of the effects of the beam on the growth rate and wavenumber locking are described. Electron beam-plasma systems are known to exist in the magnetosphere and can be created in fusion and other laboratory devices. Electromagnetic wave scattering may prove useful in detecting and analysing electron beams which occur in plasmas either naturally or by injection. In a recent paper (Willett and Aktas, 1983) the authors have investigated the effects of a warm electron beam on stimulated Brillouin backscattering in a plasma. Within the limits of fluid theory, the beam effects tend to increase with decreasing beam electron temperature. The purpose of this research note is to extend the study, by use of kinetic theory, to the case of a nearly monoenergetic electron beam.

Growing and Damping of Space Charge Waves of an Electron Beam due to Nonlinear Landau Damping

We have observed experimentally damping and growing of fast and slow space charge waves of an electron beam due to nonlinear Landau damping in an electron beam-plasma system by using spectrum analysis. The former with positive energy damps and the latter with negative energy grows by nonlinear interaction with the Trivelpiece-Gould mode and the electron beam. The lowest and higher modes of the Trivelpiece-Gould modes with respect to a radial wavelength grow by this nonlinear wave-particle interaction satisfying the resonant condition.

Collisionless electron shocks in electron-beam–plasma systems

One-dimensional Vlasov simulations show that when an electron beam is suddenly injected into a plasma, a fast-moving monotonic shock forms during the early stage of the transient plasma response. In the shock, the ions are nearly immobile. The shock appears to be an electrostatic electron-beam–plasma mode. The shock evolves from an initial positive potential perturbation, which is supported by an ion burst. The steepening of the perturbation into a shock is characterized by an electron-beam–plasma mode.

On the nonlinear theory of surface wave generation in an electron beam-plasma system

The nonlinear interaction of a relativistic charged beam with the surface wave excited when the beam slides along a plasma layer is considered. The self-consistent nonlinear state of the wave-beam system has been analyzed on the basis of the Maxwell and kinetic equations. The saturation amplitude, wave-phase velocity, the wave generation efficiency, and other quantities have been obtained as a function of the initial parameters of the system.

Observation of the Backward Cyclotron Wave in a Spiral Elebtron Beam-Plasma System

The cyclotron waves of higher order (| n |≧2) are observed in a spiral electron beam-plasma system, in addition to the space charge ( n =0) and the fundamental cyclotron (| n |=1) waves. In the frequency range of ω< n ω c , the observed waves are identified to the backward cyclotron waves, by using Fourier and inverse Fourier transformation methods.

Experimental study on excitation and suppression of a multimode beam-plasma instability in a bounded system

Four or five longitudinal electron waves of axial beam modes are excited simultaneously in a bounded electron beam-plasma system. They are accompanied with an ion-acoustic wave of an axial mode under appropriate plasma conditions. This ion-acoustic wave can not be excited directly by beam-plasma interaction, where vb/vte>or=3. When a beam density is modulated by an external r.f. signal whose frequency is close to any one of frequencies of electron waves excited spontaneously, all of the electron waves and the ion-acoustic wave are suppressed simultaneously. It is verified experimentally that quenching of the ion-acoustic wave is not brought about directly by the beam modulation, but by the damping of the electron wave which constitutes an energy source of the ion-acoustic wave.

Raman backscattering in an electron beam-plasma system

The parametric decay of an electromagnetic pump wave into a backscattered electromagnetic wave and an electron electrostatic scatterer wave in a plasma traversed by an electron beam is investigated. A formula for the growth rate of the backscattered and scatterer waves is derived and studied numerically.

Numerical simulation on the nonlinear interaction between a modulated electron beam and a plasma

Concerning the control of wave spectrum and the suppression of the wave due to pre-modulation of the beam, interaction between two waves is investigated numerically for the unbounded and the bounded electron beam-plasma systems. In the case of the unbounded system, deformation of the beam distribution function is occurred in relation to suppression of one wave by the other wave. While, in the case of the bounded system, occurrence of deformation depends on whether reflected beams are present or not. When injected beams are reflected at the other end, two wave interaction occurs under the constant wave energy. On the other hand, when injected beams are absorbed at the other end, suppression of one wave due to the other wave is accompanied with additional broadening of the beam distribution function.

Radio emission from a helical electron beam-plasma system in a twisted magnetic field

The excitation of electromagnetic radiation near the harmonics of electron plasma frequency from a helical electron beam travelling parallel to a helical magnetic field through a stationary inhomogeneous plasma is studied. The motivation behind this study is to explain the observed characteristics of the type III solar radio bursts and thus to predict the nature of the plasma system responsible for the generation of these radio bursts.

Parametric instabilities in an electron beam plasma system

The excitation of low-frequency parametric instabilities by a finite wavelength pump in a system consisting of a warm electron plasma traversed by a warm electron beam is investigated in a fluid dissipationless model. The dispersion relation for the three-dimensional problem in a magnetized plasma with arbitrary directions for the waves is derived, and the one-dimensional case is analyzed numerically. For the one-dimensional back-scattering decay process, it is found that when the plasma-electron Debye length (λDp) is larger than the beam-electron Debye length (λDb), two low-frequency electrostatic instability branches with different growth rates may exist simultaneously. When λDp≃λDb, the large growth rate instability found in the analysis depends strongly on the amplitude of the pump field. For the case λDp&amp;lt;λDb, only one low-frequency instability branch is generally excited.

Behavior of the fast cyclotron wave in a nonuniform magnetic field

The fast cyclotron wave becomes unstable and is excited in a spiral electron beam-plasma system when the perpendicular energy component of the beam is sufficiently large. When a nonuniform magnetic field is applied to the system, the cyclotron frequency as well as the parallel velocity component of the beam vary spatially. It is confirmed experimentally, that the variation affects the excited wave and results in a spatial variation of its wavenumber in the way predicted by the dispersion relation of the fast cyclotron wave.

Competition between Naturally Excited and Externally Applied Waves in the Beam-Plasma Interaction

Variation of a naturally excited electron wave or waves by an externally launched wave is studied in electron beam-plasma systems, where the wave saturates due to a temperature broadening of beam. In a counter stream type, when the external wave suppresses the amplitude of a natural wave, the beam energy distribution function shows no appreciable change, and both waves compete with each other within a range that the total wave energy is not altered much. Equations describing this mode coupling for multimodes are similar to a gas laser system. In the case of single stream type, the energy distribution function is altered more by suppression. An approach is made to explain the suppression phenomenon of travelling waves.

Stability theory of a relativistic electron beam-plasma system with finite geometries

An extensive study of the stability of a relativistic electron beam-plasma system in finite (cylindrical) geometries is performed. The analysis is based on the collisionless fluid theory; it accomodates both the finiteness and relativistic effects by employing the full set of Maxwell’s equation. Finite boundaries of the beam and plasma are found to have a stabilizing effect on the two stream instability under a static magnetic field. The instability tends to have a smaller growth rate when the plasma radius becomes smaller and the magnetic field becomes stronger. In the extreme case of an infinite magnetic field, the marginal beam density for stability is a sharply increasing function of the relativistic factor γ0 and the inverse of the plasma radius, and a mildly decreasing function of the plasma electron density. Surface modes of the filamentation instability in cylindrical geometry for a thin hollow beam are also examined. Possible applications of the present theory to the long solenoid reactor configuration and the runaway electron problem in a low densith regime of a tokamak are discussed.