Dispersion properties of a plasma produced by a short X-ray pulse

A collisionless plasma produced by a short ionizing pulse from an X-ray laser is characterized by an anisotropic monoenergetic electron distribution governed by the classical photoeffect. The dispersion properties of such a photoionized plasma are studied. The spectra of high-frequency plasma waves and their damping, as well as the parameters of the aperiodic instability of a photoionized plasma, are described. The relationship between the electrostatic and magnetic perturbations generated by this instability is investigated, and an analysis is made of how the instability transforms into a purely longitudinal (two-stream-like) instability and into a purely transverse (Weibel-like) instability, depending on the absolute value and direction of the wave vector.     

Nonlinear regimes of Farley-Buneman instability

The dynamic suppression of the instability of a quasi-monochromatic wave by nonlinear wave-wave interaction is considered. It is shown that, near the threshold of linear instability, the process of decay into two strongly damped waves leads to the onset of a quasi-periodic or a stochastic nonlinear stabilization regime involving a small number of modes. A case study is made of the Farley-Buneman instability in an isothermal magnetized current-carrying plasma in which particle collisions play an important role. Typical characteristic features of different stabilization regimes are analyzed as functions of current and other plasma parameters.     

Magnetorotational instability of a weakly ionized accretion disk with vertical and azimuthal magnetic fields

Magnetorotational instability of a weakly ionized accretion disk with an admixture of charged dust grains in a magnetic field with the axial and toroidal components is analyzed. The dispersion relation for perturbations perpendicular to the disk plane is derived with allowance for both the Hall current and the finite transverse plasma conductivity. It is shown that dust grains play an important role in the disk magnetic dynamics. Due to the effect of dust grains, the Hall current can reverse its direction as compared to the case of electron-ion plasma. As a result, the instability threshold shifts toward the short-wavelength range. Under certain conditions, electromagnetic fluctuations of any length can become unstable. It is established that the instability criterion for waves of any scale length is satisfied within a finite interval of the density ratio between the dust and electron plasma components. The width of this interval and the instability growth rate as functions of the plasma parameters and the configuration of the magnetic field in the disk are analyzed.     

Electromagnetic instability of an anisotropic relativistic plasma in the context of the evolution of galactic and intergalactic high-energy particle sources

The electromagnetic instability of a relativistic space plasma is considered. The instability manifests itself during transverse oscillations in an anisotropic plasma either when the wavelength is sufficiently long and the velocity distribution is fixed or when the plasma is strongly anisotropic and the wavelength is fixed. The critical wavenumber is estimated for a velocity distribution in the form of an oblate ellipsoid of revolution.     

High-throughput analysis of vitamin C in human plasma with the use of HPLC with monolithic column and UV-detection

Vitamin C plays a central role in the body. One of its important functions is its role as an antioxidant, and accurate measurements are important for interpretations of this role. However, its reactive nature and instability complicates the assessment, especially in biological samples. A high-throughput chromatographic method using monolithic column and UV-detection was developed for the assessment of plasma ascorbic acid and total ascorbic acid. The method showed excellent analytical sensitivity, specificity, precision, recovery and linearity during the validation study. The method was used for the assessment of ascorbic acid and total ascorbic acid during several clinical studies.     

Ionization instability and dusty plasma structurization

In the presence of ionization processes, a homogeneous equilibrium dust distribution often appears as a balance between plasma generation by ionization and plasma absorption by dust particles. It is shown that such equilibrium, often present in laboratory plasmas, is generally unstable against the formation of dust clumps separated by dust-free regions (dust voids). The driving force that separates an initially homogeneous dusty plasma into dust clumps and dust voids is the drag force produced by ions flowing out from the regions with reduced dust density. The lower the dust density, the lower the electron absorption by dust particles and the larger the ionization rate proportional to the electron density. An increase in the ion drag force leads to a further decrease in the dust density and, thus, drives the instability. In the nonlinear stage, the instability creates structures—dust clouds separated by dust voids. The dependence of the instability growth rate on the wavenumber (or, in other words, on the size of the dust-free and dust-containing regions) is investigated. It is shown that, for sufficiently small wavenumbers, a homogeneous distribution is always unstable. An analogy with a gravitational-like instability related to shadowing of the plasma flux by dust particles is pointed out. This effect, which is due to collective shadowing of the plasma flux, dominates the shadowing by individual dust particles discussed previously. Similar to the usual gravitational instability, perturbations with the largest scales are always unstable. Contrary to the usual gravitational instability, the largest growth rate corresponds not to the largest possible scale but to the size close to the mean free path of plasma particles colliding with dust particles. A special investigation is undertaken to determine the influence of the ion-neutral collisions on the growth rate of the instability.     

Relaxation of an electron beam during the onset of the electromagnetic filamentation instability

Results are presented from three-dimensional particle-in-cell simulations of relaxation of an electron beam in a plasma. When penetrating into the plasma, the electron beam generates the return current carried by the plasma electrons. In a collisionless plasma, the relaxation mechanism is related to the onset of an electromagnetic filamentation instability. The instability leads to the generation of a quasistatic magnetic field, which decays due to the magnetic field reconnection in the final stage of the system evolution.     

Behavior of hard X-ray emission in discharges with current disruptions in the DAMAVAND and TVD tokamaks

Results are presented from studies of the behavior of hard X-ray emission in discharges with current disruptions in the DAMAVAND and TVD tokamaks. The current disruptions are caused by either an MHD instability or the instability related to the vertical displacement of the plasma column. Experiments were conducted at a fixed value of the safety factor at the plasma boundary (q _a ? 2.3). Experimental data show that, during a disruption caused by an MHD instability, hard X-ray emission is suppressed by this instability if the amplitude of the magnetic field fluctuations exceeds a certain level. If the disruption is caused by the instability related to the vertical displacement of the plasma column, then hard X-ray emission is observed at the instant of disruption. The experimental results show that the physical processes resulting in the generation and suppression of runaway electron beams are almost identical in large and small tokamaks.     

On the excess energy of nonequilibrium plasma

The energy that can be released in plasma due to the onset of instability (the excess plasma energy) is estimated. Three potentially unstable plasma states are considered, namely, plasma with an anisotropic Maxwellian velocity distribution of plasma particles, plasma with a two-beam velocity distribution, and an inhomogeneous plasma in a magnetic field with a local Maxwellian velocity distribution. The excess energy can serve as a measure of the degree to which plasma is nonequilibrium. In particular, this quantity can be used to compare plasmas in different nonequilibrium states.     

Reversal of the hall current and amplification of the magnetorotational instability in a weakly ionized plasma

A criterion for the development of a magnetorotational instability in a weakly ionized dusty plasma is considered. A dispersion relation for the wavenumber and the growth rate of an unstable perturbation is derived for an arbitrary angle between the wave vector and magnetic field. It is shown that the presence of dust grains can reverse the direction of the Hall current in the plasma and can shift the instability threshold to shorter wavelengths. Under certain conditions, Alfvén fluctuations of arbitrary scale can be unstable. The Hall current reversal is found to have a strong effect on the development of a magnetorotational instability when the Alfvén resonance frequency in a weakly ionized plasma is close to the rotation frequency of the accretion disk.     

Nonlinear theory of coupled waves exemplified by beam-plasma instability

The nonlinear stage of instability of an annular electron beam spatially separated from an annular plasma is investigated. The equations describing coupled waves for an arbitrary ratio between the beam and plasma densities are derived. It is shown that instability saturates at distances on the order of several inverse spatial growth rates. The saturation is caused by relativistic nonlinearity, generation of the second harmonic, and low-frequency modulation of the electromagnetic field. At larger distances, resonant generation of low-frequency beam oscillations becomes a dominant factor. In the case of a low-density beam, an expression for the maximum power of the generated plasma wave is obtained in an explicit form.     

Role of the Hall flute instability in the interaction of laser and space plasmas with a magnetic field

The interaction of an expanding laser plasma with a uniform external magnetic field is studied over a wide range of experimental parameters (for a plasma energy of up to 300 J and a magnetic induction of up to 8 kG). By analyzing the data from these and other experiments, as well as the results of simulations with the use of a two-fluid Hall plasma model, it was found for the first time that the flute instability of the plasma boundary plays a decisive role in the process of the plasma cloud expansion. It is shown that, when the ion Larmor radius is sufficiently large, this instability can significantly affect the maximum radius of the diamagnetic cavity of the plasma cloud and the deceleration of its front by the magnetic field. A physical model based on the Hall effect is proposed to explain such influence. The model adequately describes data from one-dimensional simulations, as well as from experiments with quasi-spherical laser plasma clouds. The results obtained can be helpful in interpreting the data from active magnetospheric experiments with barium plasma clouds (such as AMPTE) and analyzing the plasma dynamics in future ICF reactors and propulsion systems with a magnetic field for direct conversion of fusion energy into electric energy.     

Kinetic study of the linear stage of thermocurrent instability in the framework of the Boltzmann equation for a model gas

The linear stage of thermocurrent instability is investigated for a model gas in which the integral of inelastic collisions of electrons with gas particles has a divergent form and the frequencies of elastic and inelastic collisions are independent of the electron velocity. The proposed approach consists in the reduction of the Boltzmann equation for electrons in an inhomogeneous plasma to a set of equations for the moments of the electron velocity distribution function. The instability growth rate and the wave phase velocity as functions of the perturbation wavenumber are calculated, the maximum growth rate and the corresponding wavenumber are determined, and the dependence of these quantities on the degree of plasma quasineutrality is examined. It is demonstrated that the model satisfactorily (both qualitatively and quantitatively) describes the linear stage of thermocurrent instability in helium.     

Effect of the ponderomotive force on the development of beam-plasma instability

The amplitude of the wave generated in a plasma during the development of beam-plasma instability is nonuniform in the longitudinal direction. The ponderomotive force associated with this nonuniformity leads to a redistribution of the plasma density; as a result, the wave amplitude and its spatial distribution change. As the beam current grows, the ponderomotive force plays an increasingly important role and radically changes the mechanism by which the beam-plasma instability saturates. Ion acoustic waves generated by the ponderomotive force propagate in the direction opposite to the propagation direction of the beam, thereby ensuring distributed feedback and giving rise to a strong low-frequency self-modulation of the wave amplitude and phase. Results are presented from experimental investigations of the self-modulation regime of the beam-plasma instability in a magnetized plasma waveguide. Theoretical estimates of the parameters of the low-frequency self-modulation agree well with the experimental data.     

Cherenkov Excitation of Spatial Waves by a Straight Nonrelativistic Electron Beam in a Plasma Waveguide

The problem of the excitation of plasma waves by a thin-walled annular electron beam in a waveguide filled entirely with a plasma is analyzed in the quasistatic approximation. The instability growth rates are derived and are studied as functions of the waveguide parameters. The evolution of different seed perturbations in the nonlinear stage of the instability is investigated.     

Ponderomotive force of the low-frequency field and modulational instability of drift waves

A study is made of the processes that occur in an inhomogeneous nonisothermal plasma in a strong external magnetic field and whose characteristic frequencies are lower than the ion Langmuir frequency but higher than the collision frequency. An expression for the ponderomotive force of the low-frequency field is derived. The excitation of a long-wavelength low-frequency drift wave during the development of the modulational instability of a drift pump wave is investigated. The growth rates of the instability are obtained, and the conditions for its onset are determined. The possible relation of the modulational instability to the formation of structures in the plasma is discussed.     

Numerical simulation of the instability of a nonuniform plasma flow: Nonlinear dynamics of slipping instability

A kinetic model is proposed that describes the nonlinear dynamics of the instabilities of a transversely nonuniform plasma flow. It is shown that, in the linear approximation, the model yields the familiar boundary-value problem for the scalar potential in plasma. The slipping instability in a plane waveguide is considered as an example. The general dispersion relation for a flow with a stepwise uniform density profile and with a tangential discontinuity in its longitudinal velocity is analyzed qualitatively. The dynamics of the slipping instability is investigated numerically for a flow that is detached from the waveguide walls and whose longitudinal velocity obeys a linear, a sinusoidal, or a discontinuous distribution. In the nonlinear stage of the instability, the flow expands in such a way as to come into contact with the walls, the spread in the longitudinal velocities remains smaller than the initial velocity variation, and the longitudinal velocities of different transverse layers in the flow are not completely equalized.     

Classification of the regimes of Cherenkov beam instabilities in plasma waveguides

The regimes of the instabilities of an annular relativistic electron beam in a waveguide with an annular plasma are systematically analyzed and classified. The growth rates of the instabilities are calculated different limiting cases, and the resonance conditions for the development of the instabilities are determined. The fastest growing instability of a high-current relativistic electron beam in a waveguide with a dense plasma is considered. The possible onset of a low-frequency instability of a beam in a waveguide with a low-density plasma is investigated. Typical examples of how the growth rates depend on the perturbation wavenumbers are presented for systems with parameters close to the experimental ones.     

Effect of the dipole-dipole interaction of particles in an active medium on the character of superradiation

The motion of a system of interacting nonlinear charged oscillators is investigated numerically. Because of nonlinearity, the total collective electric field gives rise to a phasing effect—correlations in the phases of the oscillators. The consequence is superradiation—the enhanced spontaneous short-term emission of the energy stored in the oscillators. It is shown that the oscillations of the oscillators become stochastic because of the dipole-dipole interaction between them and their nearest neighbors. As a result, as the density of the oscillators increases, distant collective correlations are suppressed, superradiation ceases to be generated, and radiation is shielded in the medium. The phenomena considered in the present paper can play an important role in cyclotron emission from a plasma and thus should be taken into account in emission calculations. The process whereby the energy of the transverse electron motion in electron cooling devices decreases is analyzed as an example. This process occurs as a result of the development of cyclotron maser instability and has the nature of superradiation. The onset of correlations between individual electrons moving in their Larmor circles is the initial, linear stage of instability developing in the plasma. Superradiation is the final, nonlinear instability stage.