A. Y. Wong

Stability Limits for Longitudinal Waves In Ion Beam‐Plasma Interaction

The two‐stream instability is examined for the case of an ion beam traversing a plasma. The dispersion equation for linearized, longitudinal waves in a plasma where collisions are negligible is used to find the restrictions on beam velocity, temperature, and density which will lead to growing waves.

Nonlinear evolution of electron-beam-plasma interactions

The nonlinear evolution of the electron‐beam–plasma instability is investigated experimentally in two stages in a large, uniform, and unmagnetized plasma. In the initial stage, the beam‐driven, linearly unstable wave saturates due to changes in the beam distribution. At sufficiently intense wave amplitude, the instability evolves into a second stage where nonlinear effects due to the ponderomotive force dominate and the wave rapidly undergoes spatial collapse and contracts to very small scale lengths in the coexisting density cavity. The transverse contraction of the wave agrees with theories on three‐dimensional Langmuir collapse. This second stage can further be classified according to the beam strength. At low beam density, a single collapsing wave packet is formed and the electron beam decouples from the collapsing wave and propagates relatively unperturbed through the plasma. At higher beam density, multiple field spikes are formed both along and across the beam path and the beam distribution is broa...

ELECTRON PLASMA WAVES AND FREE-STREAMING ELECTRON BURSTS.

The response of electrons in a collisionless plasma to a fast‐rising voltage step is investigated experimentally and theoretically. At small amplitudes the step generates a wave train, the leading edge of which propagates along the column at the long‐wavelength electron plasma wave velocity. At large step amplitudes a fast burst of electrons is observed, with kinetic energy roughly proportional to the step amplitude, followed by a nonlinear electron plasma wave signal which shows solitonlike behavior.

Stabilization of the diocotron instability in an annular plasma

The stabilization of the diocotron instability by an applied radial dc electric field in an annular magnetized pure electron plasma is investigated experimentally. The radial electric field E(r) from the annular plasma gives rise to a radially dependent azimuthal E(r)×Bz drift. The shear in this drift provides the free‐energy source for the instability. A sufficiently large bias applied to a central conductor changes the rotation rates of the plasma and stabilizes the instability. The diocotron instability is experimentally identified by its dispersion relation. Its nonlinear saturation is explained in terms of the broadening of its radial extent and trapping of the azimuthally drifting electrons.

Collisional Drift Waves in the Linear Regime

A method of external excitation of density gradient drift waves in a stable, collision‐dominated plasma is presented whereby the linear theory of drift waves is verified. The procedure permits limitation of the perturbation amplitude such that e∅1/kT < 0.1. This represents a new approach in the study of drift waves since nonlinear saturation effects present in all previous experiments on these waves are avoided. The driven oscillations are coherent, nearly sinusoidal modes on which measurements of ω and k as a function of the plasma parameters are made. In this manner the linear dispersion relation derived from the fluid equations is verified demonstrating the strong stabilizing effect of transverse diffusion resulting from ion viscosity. In connection with this an additional damping process resulting from ion loss at the cathodes is introduced whereby complete stabilization of the plasma to drift waves is possible. Next, the functional dependence of the phase angle between the density and potential pertu...

Interchange stability of an axisymmetric, average minimum‐B magnetic mirror

An axisymmetric magnetic mirror with large diameter and easily variable mirror ratio is described. The magnetic field configuration is a simple mirror with an average minimum‐B field region on the outer surface. Experimental results are given which demonstrate that the surface field region ensures interchange stability. Stability can be maintained with both positive and negative radial plasma pressure gradients provided that the product of the pressure gradient and specific flux volume gradient is positive.

Theory of double resonance parametric excitation in plasmas

Parametric instabilities in a plasma driven by a long wavelength electric field with two “pump” frequencies ω1 and ω2 which lie near the resonant frequency for Langmuir oscillations, their difference Δ = ω1 − ω2 being chosen close to a low frequency resonance, linear or nonlinear, at Ω − i Γ are studied. A general dispersion relation in terms of linear susceptibilities, χ, is derived by retaining, on a selective basis, terms of fourth order in the pump amplitudes. Illustrative calculations are carried out using resonant fluid approximations for the χ. The most interesting cases occur when Δ = Ω or Δ = 2Ω. A lowering of the net power threshold for instability is found in both cases, when the linear damping rate of the electronic wave is large compared with Ω. In addition, a coupling between the “decay” and “oscillating two‐stream” instabilities occurs when Δ = Ω, the threshold for exciting the latter with the ω2 pump being arbitrarily small when the ω1 pump amplitude is near the usual decay instability thr...

Trapping of plasma waves in cavitons

The observed characteristics of electron plasma waves trapped inside a density cavity, a caviton, created by the electrostatic fields associated with the waves verify quantitatively the predictions of the Zakharov equations. A two‐frequency excitation technique was used to demonstrate the existence of eigenmodes for the trapped electron plasma waves in the caviton.

Spectral content of strong Langmuir turbulence in the beam plasma interaction

The detailed spectral content of strong Langmuir turbulence generated by an electron beam is investigated experimentally in an unmagnetized plasma. It is found that the power spectra of the electrostatic waves follows a consistent, reproducible pattern in which the Langmuir energy is concentrated in high frequencies (410 MHz) immediately following a collapse event, transitions to lower frequencies between events, with collapse taking place at the lowest frequencies (350 MHz). Also, after a collapse event, the electrostatic wave intensity falls by about two orders of magnitude in a microsecond, but then continues to fall over the next approximately 10 μs by an additional factor of 10 before beginning to build toward the next collapse. The spectral width and electron saturation current also exhibit reproducible patterns. Measurements are also performed to determine the low-frequency, ion acoustic spectral content. It is found that the peak of the ion acoustic spectrum scales inversely with the average time ...

Turbulence in electrostatic ion acoustic shocks

Three types of collisionless electrostatic ion‐acoustic shocks are investigated using the University of California, Los Angeles, double plasma device: (a) laminar shocks; (b) small amplitude turbulent shocks in which the turbulence is confined to be upstream of the shock potential jump; and (c) large amplitude turbulent shocks in which the wave turbulence occurs throughout the shock transition. The wave turbulence is generated by ions which are reflected from the shock potential; linear theory spatial growth increments agree with experimental values. The experimental relationship between the shock Mach number and the shock potential is shown to be inconsistent with theoretical shock models which assume that the electrons are isothermal. Theoretical calculations which assume a trapped electron equation of a state and a turbulently flattened velocity distribution function for the reflected ions yields a Mach number vs potential relationship in agreement with experiment.