Observation of nonlinear coupling between drift and ion-acoustic oscillations in low-frequency plasma turbulence

It is shown experimentally that the characteristics of structural ion-acoustic turbulence in a plasma are governed primarily by the development of density gradient-driven drift oscillations. The cyclicity of appearance and disappearance of drift wave packets and ensembles of ion-acoustic solitons in a steady-state turbulent plasma, as well as the correlation between them, is determined.     

Resonance oscillations in the human arterial system]

The paper summarizes the results of the experiments aimed at obtaining sphygmograms of peripheral and carotid arteries with due regard to the values of longitudinal dimensions of body and extremities in healthy subjects. Mathematical equations expressing the fact that dicrotic waves recorded on sphygmograms are the reflections of blood eigentones coinciding with resonance oscillations have been derived. It is proved that at least two partial vibration systems oscillating with different own frequencies are present in human arteries. Conditions under which the resonance of constituent frequencies of pulsatile pressure waves and output waves in arteries occurs have been determined. From this point of view a new explanation for the well-known phenomenon of the pulsatile wave amplitude increase from the heart towards peripheric regions is proposed.     

Magnetosonic oscillations of thin plasma columns

Oscillations of a plasma column in a longitudinal magnetic field are considered. It is found that eigenmodes with frequencies close to the ion cyclotron frequency can be excited in columns the radii of which are smaller than the characteristic wavelength of magnetosonic oscillations predicted by the theory of homogeneous plasma. The eigenmodes have the form of waves running around the column axis in the direction of electron gyration in the magnetic field. Magnetosonic oscillations can be excited as a side effect when using helical antennas for ion cyclotron resonance heating of plasma. These oscillations should enhance electron heating in the plasma core, as well as both electron and ion heating at the periphery of the plasma column. The spectrum of eigenmodes of inhomogeneous plasma columns includes oscillations of different nature. Comparative analysis of their properties performed in the present paper is useful for understanding the full picture of the physical processes occurring during ion cyclotron resonance heating and clarifying the characteristic features of the magnetosonic oscillations under study.     

Breaking of nonlinear cylindrical plasma oscillations

Nonlinear axisymmetric cylindrical plasma oscillations are investigated analytically and numerically. It is shown that the breaking of strongly nonlinear oscillations is attributed to a singularity in the electron density and occurs several periods after the onset of an off-axis density maximum. For weakly nonlinear conditions, an analytic dependence of the breaking time of the oscillations on their amplitude is obtained based on the effect of intersection of electron trajectories and is shown to agree with numerical results.     

Resonance microwave volume plasma source

A conceptual design of a microwave gas-discharge plasma source is described. The possibility is considered of creating conditions under which microwave energy in the plasma resonance region would be efficiently converted into the energy of thermal and accelerated (fast) electrons. Results are presented from interferometric and probe measurements of the plasma density in a coaxial microwave plasmatron, as well as the data from probe measurements of the plasma potential and electron temperature. The dynamics of plasma radiation was recorded using a streak camera and a collimated photomultiplier. The experimental results indicate that, at relatively low pressures of the working gas, the nonlinear interaction between the microwave field and the inhomogeneous plasma in the resonance region of the plasmatron substantially affects the parameters of the ionized gas in the reactor volume.     

Group-theory classification of oscillations of plasma clusters

Normal oscillations of three-dimensional configurations of dust grains trapped in a spherically symmetric potential well are studied. All possible oscillations of a system consisting of 7–13 grains are classified in terms of group theory. For a Coulomb interaction potential, all the oscillation frequencies are calculated. The frequencies and polarizations of some oscillation modes are obtained for an arbitrary interaction potential.     

Nonlinear oscillations of a semiconductor plasma with a nonrelativistic electron beam

Nonlinear oscillations of a semiconductor plasma with a low-density electron beam in the absence of an external magnetic field are studied in the hydrodynamic approximation. The beam is assumed to be nonrelativistic and monoenergetic. Cases are studied in which the Langmuir frequency of the electron oscillations in a semiconductor is much higher or much lower than the electron momentum relaxation rate. The self-similar solution obtained for the first case describes the damping of the nonlinear oscillations of the wave potential. Numerical analysis of the second case shows that the electric field distribution in the beam may correspond to that in a shock wave.     

Resonance and selective communication via bursts in neurons having subthreshold oscillations

Revealing the role of bursts of action potentials is an important step toward understanding how the neurons communicate. The dominant point of view is that bursts are needed to increase the reliability of communication between neurons [Trends Neurosci. 20 (1997) 38]. In this paper we present an alternative but complementary hypothesis. We consider the effect of a short burst on a model postsynaptic cell having damped oscillation of its membrane potential. The oscillation frequency (eigenfrequency) plays a crucial role. Due to the subthreshold membrane resonance and frequency preference, the responses (i.e. voltage oscillations) of such a cell are amplified when the intra-burst frequency equals the cell's eigenfrequency. Responses are negligible, however, if the intra-burst frequency is twice the eigenfrequency. Thus, the same burst could be effective for one cell and ineffective for another depending on their eigenfrequencies. This theoretical observation suggests that, in addition to coping with unreliable synapses, bursts of action potentials may provide effective mechanisms for selective communication between neurons.     

Nonlinear dynamics of drift structures in a magnetized dissipative plasma

A study is made of the nonlinear dynamics of solitary vortex structures in an inhomogeneous magnetized dissipative plasma. A nonlinear transport equation for long-wavelength drift wave structures is derived with allowance for the nonuniformity of the plasma density and temperature equilibria, as well as the magnetic and collisional viscosity of the medium and its friction. The dynamic equation describes two types of nonlinearity: scalar (due to the temperature inhomogeneity) and vector (due to the convectively polarized motion of the particles of the medium). The equation is fourth order in the spatial derivatives, in contrast to the second-order Hasegawa-Mima equations. An analytic steady solution to the nonlinear equation is obtained that describes a new type of solitary dipole vortex. The nonlinear dynamic equation is integrated numerically. A new algorithm and a new finite difference scheme for solving the equation are proposed, and it is proved that the solution so obtained is unique. The equation is used to investigate how the initially steady dipole vortex constructed here behaves unsteadily under the action of the factors just mentioned. Numerical simulations revealed that the role of the vector nonlinearity is twofold: it helps the dispersion or the scalar nonlinearity (depending on their magnitude) to ensure the mutual equilibrium and, thereby, promote self-organization of the vortical structures. It is shown that dispersion breaks the initial dipole vortex into a set of tightly packed, smaller scale, less intense monopole vortices-alternating cyclones and anticyclones. When the dispersion of the evolving initial dipole vortex is weak, the scalar nonlinearity symmetrically breaks a cyclone-anticyclone pair into a cyclone and an anticyclone, which are independent of one another and have essentially the same intensity, shape, and size. The stronger the dispersion, the more anisotropic the process whereby the structures break: the anticyclone is more intense and localized, while the cyclone is less intense and has a larger size. In the course of further evolution, the cyclone persists for a relatively longer time, while the anticyclone breaks into small-scale vortices and dissipation hastens this process. It is found that the relaxation of the vortex by viscous dissipation differs in character from that by the frictional force. The time scale on which the vortex is damped depends strongly on its typical size: larger scale vortices are longer lived structures. It is shown that, as the instability develops, the initial vortex is amplified and the lifetime of the dipole pair components-cyclone and anticyclone-becomes longer. As time elapses, small-scale noise is generated in the system, and the spatial structure of the perturbation potential becomes irregular. The pattern of interaction of solitary vortex structures among themselves and with the medium shows that they can take part in strong drift turbulence and anomalous transport of heat and matter in an inhomogeneous magnetized plasma.     

Electromagnetic oscillations near the critical surface in a magnetized plasma

It is shown that, in a plasma whose density varies across the magnetic field lines, electromagnetic oscillations that are localized near the critical surface can exist. Such oscillations can be excited spontaneously in a nonequilibrium plasma of closed magnetic confinement systems.     

Evolution of the electron Langmuir oscillations in an inhomogeneous magnetized plasma

The conditions under which the energy of the electron Langmuir oscillations can escape from the plasma into vacuum are determined in the simplest model of a plane slab of an inhomogeneous cold magnetized plasma in a uniform magnetic field.     

Generation of plasma oscillations in a low-pressure microwave discharge

The resonant excitation of plasma (Langmuir) oscillations during the microwave breakdown of a low-pressure gas is studied both analytically and numerically using the simplest uniform model. It is shown that, because of a significant delay in electron heating and cooling, this effect ensures that the plasma density increases at a high (resonant) rate, even after exceeding a critical value, and can reach a very high (overcritical) level.     

Calculation of poloidal velocity in the tokamak plasma with allowance for density inhomogeneity and diamagnetic drift of ions

A one-dimensional evolution equation for the angle-averaged poloidal momentum of the tokamak plasma is derived in the framework of reduced magnetohydrodynamics with allowance for density inhomogeneity and diamagnetic drift of ions. In addition to fluctuations of the E × B drift velocity, the resulting turbulent Reynolds stress tensor includes fluctuations of the ion density and ion pressure, as well as turbulent radial fluxes of particles and heat. It is demonstrated numerically by using a particular example that the poloidal velocity calculated using the refined one-dimensional evolution equation differs substantially from that provided by the simplified model. When passing to the new model, both the turbulent Reynolds force and the Stringer-Winsor force increase, which leads to an increase in the amplitude of the ion poloidal velocity. This, in turn, leads to a decrease in turbulent fluxes of particles and heat due to the effect of shear decorrelation.     

Excitation of low-frequency electrostatic oscillations in a collisionless plasma with an anisotropic ion distribution

The stability of an anisotropic ion distribution with unoccupied regions (holes) in velocity space is studied. Such distributions are expected to form near the neutral plane of the Earth's magnetotail. It is shown that, in such systems, electrostatic waves can be excited. The growth rate and propagation direction of these oscillations are determined by the parameters characterizing the ion hole, as well as by the relation between the electron and ion temperatures. The solution to the quasilinear equation for the waves propagating perpendicular to the current sheet is found, and the energy of the excited oscillations as a function of the parameters of the ion hole is evaluated.     

On the longitudinal distribution of electric field in the acceleration zones of plasma accelerators and thrusters with closed electron drift

The long-term experience in controlling the electric field distribution in the discharge gaps of plasma accelerators and thrusters with closed electron drift and the key ideas determining the concepts of these devices and tendencies of their development are analyzed. It is shown that an electrostatic mechanism of ion acceleration in plasma by an uncompensated space charge of the cloud of magnetized electrons “kept” to the magnetic field takes place in the acceleration zones and that the electric field distribution can be controlled by varying the magnetic field in the discharge gap. The role played by the space charge makes the mechanism of ion acceleration in this type of thrusters is fundamentally different from the acceleration mechanism operating in purely electrostatic thrusters.     

Quantum nature of the damping of Langmuir oscillations and the boson peak in plasma

It is shown that the damping of Langmuir plasma oscillations is quantum in nature and that the damping rate, which is proportional to the fourth power of the electron charge, is caused by thermal electron fluctuations and depends nonanalytically on the Plank constant ℏ at ℏ → 0. At frequencies of ∼T/ℏ, the damping rate has a maximum, which can be identified with a boson peak.     

Analytical and numerical treatment of resistive drift instability in a plasma slab

An analytic approach combining the effect of equilibrium diamagnetic flows and the finite ionsound gyroradius associated with electron?ion decoupling and kinetic Alfvén wave dispersion is derived to study resistive drift instabilities in a plasma slab. Linear numerical computations using the NIMROD code are performed with cold ions and hot electrons in a plasma slab with a doubly periodic box bounded by two perfectly conducting walls. A linearly unstable resistive drift mode is observed in computations with a growth rate that is consistent with the analytic dispersion relation. The resistive drift mode is expected to be suppressed by magnetic shear in unbounded domains, but the mode is observed in numerical computations with and without magnetic shear. In the slab model, the finite slab thickness and the perfectly conducting boundary conditions are likely to account for the lack of suppression.     

Resonance interaction of electrons with a slow transverse wave in a weakly inhomogeneous plasma

Excitation of a circularly polarized slow wave by external sources and its subsequent propagation in a weakly inhomogeneous plasma with a positive density gradient are described in terms of the adiabatic approach. It is shown that the wave dispersion is mainly determined by the ratio between the contributions of trapped and nonresonant untrapped electrons to the total wave current. The relationship between the wave amplitude and its phase velocity and the limiting phase velocity above which the wave is strongly damped are found using the energy balance equation and the dispersion relation.     

Electromagnetic oscillations near the critical surface in a plasma: Methodological note

Agreement between different approaches to studying the propagation of electromagnetic oscillations near the critical surface is elucidated. The propagation of plane waves, electromagnetic rays, and wave beams are analyzed. The results obtained are valid when the angles between the magnetic field and the plasma density gradient are not too small.     

On the nature of global plasma oscillations in a tokamak. Part I: Kinetic properties of oscillations observed at the T-10 tokamak

Large-scale plasma oscillations (so-called MHD oscillations) in the T-10 tokamak are investigated. The central electron cyclotron heating was used to enhance oscillations at the m/n = 1/1 mode with the goal of determining the internal characteristics of the process. Spatially resolved measurements of electron cyclotron emission in a wide frequency range for two polarizations allowed for analyzing a number of effects indicating the kinetic nature of plasma oscillations. The major part of the electron distribution over longitudinal velocities in the plasma core experiences strong periodic oscillations accompanied by synchronous spikes of radiation emitted by high-energy electrons at the edge of the plasma column.