G. J. Morales

Generation of density cavities and localized electric fields in a nonuniform plasma

A simple model is presented which is capable of describing the detailed space-time evolution of the interaction of long wavelength electromagnetic radiation with an unmagnetized plasma having a nonuniform density profile. The model consists of describing the electric field via the nonlinear Schrodinger equation and the density changes through the ion-acoustic wave equation with the ponderomotive force effects included self-consistently. In the linear regime, this formulation explains the time evolution of the mode-conversion process that leads to the excitation of a short wavelength Langmuir wave in the neighborhood of the resonance layer where the frequency of the external radiation matches the local value of the electron plasma frequency. In the nonlinear regime, the model predicts the generation of density cavities and the associated spatial localization of the electric field.

Structure of Alfvén waves at the skin‐depth scale

This analytical study demonstrates that shear Alfven waves having transverse scale on the order of the electron skin depth exhibit a collisionless divergence determined by propagation cones that emanate from the edges of the exciting structures. Axial current channels are found to spread radially due to the skin effect up to the cone trajectories and at distances of a few wavelengths from the exciter develop radial diffraction patterns. For values of the collision frequency slightly larger than the wave frequency resistive diffusion allows the axial currents to expand beyond the cone trajectories.

Analytic expressions for mode conversion in a plasma with a linear density profile

The transformation of electromagnetic waves into Langmuir oscillations (and vice versa) is explicitly examined in the vicinity where the wave frequency matches the electron plasma frequency in an inhomogeneous plasma. For an unmagnetized plasma with a linear density profile of scale length L, closed‐form, analytic expressions are derived, in terms of Airy functions, for the reflection and mode conversion coefficients of Langmuir and electromagnetic waves utilizing a source approximation that is valid when the electromagnetic field scale length is large compared to that of the electrostatic field. The technique developed to determine the energy flux coefficients and the fields is general enough to apply to a plasma with a profile other than a linear one, and should prove useful in other problems where a scale length separation is valid. The reflection coefficient for the ‘‘direct’’ problem (incident electromagnetic wave) is equal in magnitude to that of the ‘‘inverse’’ problem (incident electrostatic wave)...

Magnetic fluctuations associated with field-aligned striations

In a laboratory experiment modeling conditions in the auroral ionosphere, magnetic and density fluctuations are observed to grow spontaneously within a field aligned density depletion. The plasma has both cross-field pressure gradients and magnetically field-aligned currents. The magnetic fluctuations, identified as shear Alfven waves, are coupled with the density fluctuations depending upon the plasma beta, β. The fluctuations exhibit a coherent eigenmode structure at higher β and evolve towards broadband Alfven turbulence as β is lowered. We identify the cross field pressure gradient as the dominant driving source for the fluctuations.

Sheath structure in a magnetized plasma

The sheath formed between a magnetized plasma and a particle absorbing wall is examined for the case in which the magnetic field intercepts the wall at a small angle 0°<e≲9°, where sin e=B⋅n/‖B‖, and n is the unit normal to the wall. The model is time‐independent and one‐dimensional (1‐D) with all functions varying only in the direction normal to the wall. The ions are modeled by a Maxwellian velocity distribution which is modified by the condition that ions, which would have hit the wall, are absent. For the electrons a fluid description is used, including the effects of electron–neutral collisions. The transport of particles due to turbulent electrostatic fluctuations is modeled by a constant electric field perpendicular to both B and n. It is found that in the range of angles under consideration, there are two distinct regimes of sheath formation. If e≲ν=ν/Ωe (grazing incidence), where ν is the electron–neutral collision frequency and Ωe is the electron cyclotron frequency, then the properties of t...

Spiky turbulence generated by a propagating electrostatic wave of finite spatial extent

The excitation of a new type of plasma turbulence consisting of a collection of neighboring localized electric fields of high intensity is investigated. The individual localized fields that constitute the spiky turbulence oscillate at frequencies close to the electron plasma frequency, and their localization is attributed to wave‐trapping inside the self‐consistent density cavities generated by the total ponderomotive force. Unlike previous investigations of this type of nonlinearity, which have dealt almost exclusively with the effects triggered by electromagnetic radiation, the present work studies the turbulence which is generated by a propagating electrostatic wave of finite spatial extent. This type of pump wave may arise naturally in beam‐plasma experiments or in other situations which lead to a finite amplification region. The spiky turbulence is found to evolve in the nonlinear regime out of the linear parametric instabilities that can be excited by such a pump wave. A detailed linear analysis of ...

Large-scale HF-induced ionospheric modifications: Experiments

Experimental investigations of large-scale modifications created by a high-power HF beam (effective radiated power of ∼100 MW, frequency of 3–5 MHz) in the nighttime ionosphere above Arecibo Observatory are presented. The modifications consist of field-aligned temperature enhancements (δTe/Te0 ∼ 2–4) and density depletions (|δne|/ne0 ∼ 25–50%) and have length scales along the geomagnetic field of tens of kilometers. A two-stage time evolution of the modifications is documented; they start from a broad and symmetric perturbation and develop into a narrow, northward shifted universal steady state on a time scale of 15–30 min. It is found that nonlinear refraction, i.e., the self-consistent bending of the O mode HF beam across the geomagnetic field lines and the reorientation of the HF reflection surface to a geometry parallel to the geomagnetic field, is the key process involved in the generation of the large modifications. Preconditioning results showing the immediate onset of the narrow late-stage behavior provide dramatic confirmation of the underlying nonlinear mechanism sustaining the large modifications. Highlights of quantitative comparisons of these observations with a transport model including nonlinear refraction have been given in a previous publication (Hansen et al., 1990).

Fluctuations associated with a filamentary density depletion

Density and magnetic fluctuations arising spontaneously in a narrow field-aligned density striation in a magnetized discharge He plasma are found to exhibit a radial eigenmode structure. The nature of the fluctuations depends upon the electron plasma beta, βe. For βe greater than the electron to ion mass ratio (βe>m/M) the frequency spectrum exhibits sharply peaked eigenfrequencies with the density and magnetic fluctuations strongly coupled so that the growing mode is identified as the drift-Alfven wave. For βe less than the mass ratio (βe<m/M) the density and magnetic fluctuations separate in frequency and broadband magnetic shear Alfven wave turbulence develops. The driving source for the fluctuations is the cross-field density and temperature gradients in the edge of the striation which have scale lengths on the order of the electron skin depth. The fluctuations associated with the striation are compared to the edge fluctuations of the plasma column which are found to exhibit a universal exponential fr...

ICRF heating and its effect on single-particle confinement in tokamaks

Ion cyclotron resonance heating (ICRH) and its intrinsic effect on confinement in tokamaks is investigated. The stochastic nature of ICRH in the absence of collisions is examined through finite-mapping equations. The threshold RF amplitude for the transition from superadiabatic to stochastic behaviour is found to be exceeded in present-day experiments. The effect of heating on confinement is evaluated by studying the drift orbit changes due to heating with collisions included through a Monte-Carlo method. The results indicate that transport induced by the ion cyclotron range of frequencies (ICRF) could be significant at the higher RF power levels planned for the next generation of experiments.

Behavior of the ponderomotive effect near gyroresonance

A theoretical, numerical, and experimental study is made of the behavior of the ponderomotive effect for rf frequencies close to the particle gyrofrequency. Near gyroresonance the stopping potential is found to be small rather than infinite. The interaction is not adiabatic, and heating results both in the parallel and perpendicular directions. This behavior is explainable in terms of the finite transit time of the particle through the localized rf structure. A perturbation theory that includes finite transit time is found to be in good quantitative agreement with both the numerical study and the experimental results.