Volume 20, Issue 12, December 2013
Index of content:

Selfsustained oscillations in a dc glow discharge with a semiconductor layer at atmospheric pressure were investigated by means of a onedimensional fluid model. It is found that the dc glow discharge initially becomes unstable in the subnormal glow region and gives rise to oscillations of plasma parameters. A variety of oscillations with one or more frequencies have been observed under different conditions. The discharge oscillates between the glow discharge mode and the Townsend discharge mode in the oscillations with large amplitude while operates in the subnormal glow discharge mode all the while in the oscillations with small amplitude. Fourier Transform spectra of oscillations reveal the transition mechanism between different oscillations. The effects of semiconductor conductivity on the oscillation frequency of the dominant mode, gas voltage, as well as the discharge current have also been analyzed.
 LETTERS


Electronic excitation as a mode of heat dissipation in laserdriven cluster plasmas
View Description Hide DescriptionElectrons streaming out of laser plasma are known for nonlocal heat transport and energy deposition by the ionization wave. At 100 eV electron temperature, since the electronic excitation cross section is comparable to that of ionization for Ar and CO2, a nonlocal excitation wave akin to the ionization wave is envisaged where energy deposition in excitations forms a excited cluster sheath beyond the laser focus. Here, we show that nanocluster systems have the right parameters to form such an exciton sheath and experimentally demonstrate this via charge transfer reactions.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Stimulated Raman scattering of laser in a plasma in the presence of a copropagating electron beam
View Description Hide DescriptionA relativistic electron beam copropagating with a high power laser in plasma is shown to add to the growth of the stimulated Raman back scattering of the laser. The growth rate is sensitive to phase matching of electron beam with the plasma wave. In the case of phase mismatch, the growth rate drops by an order. The energy spread of the electron beam significantly reduces the effectiveness of the beam on the stimulated Raman process.

Rescaling of microwave breakdown theory for monatomic gases by particleincell/Monte Carlo simulations
View Description Hide DescriptionA particleincell/Monte Carlo code is developed to rescale the microwave breakdown theory which is put forward by Vyskrebentsev and Raizer. The results of simulations show that there is a distinct error in this theory when the high energy tail of electron energy distribution function increases. A rescaling factor is proposed to modify this theory, and the change rule of the rescaling factor is presented.

Linear mode conversion of Langmuir/zmode waves to radiation in plasmas with various magnetic field strength
View Description Hide DescriptionLinear mode conversion of Langmuir/z waves to electromagnetic radiation near the plasma and upper hybrid frequency in the presence of density gradients is potentially relevant to type II and III solar radio bursts, ionospheric radar experiments, pulsars, and continuum radiation for planetary magnetospheres. Here, we study mode conversion in warm, magnetized plasmas using a numerical electron fluid simulation code when the density gradient has a wide range of angle, δ, to the ambient magnetic field, B 0, for a range of incident Langmuir/z wavevectors. Our results include: (1) Lefthanded polarized ordinary (oL) and righthanded polarized extraordinary (xR) mode waves are produced in various ranges of δ for Ω0 = (ωL/c)^{1} ^{/} ^{3}(ωc e/ω) < 1.5, where ωc e is the (angular) electron cyclotron frequency, ω is the angular wave frequency, L is the length scale of the (linear) density gradient, and c is the speed of light; (2) the xR mode is produced most strongly in the range, 40° < δ < 60°, for intermediately magnetized plasmas with Ω0 = 1.0 and 1.5, while it is produced over a wider range, 0° ≤ δ ≤ 90°, for weakly magnetized plasmas with Ω0 = 0.1 and 0.7; (3) the maximum total conversion efficiencies for wave power from the Langmuir/z mode to radiation are of order 50%–99% and the corresponding energy conversion efficiencies are 5%–14% (depending on the adiabatic index γ and β = T e/m e c ^{2}, where T e is the electron temperature and m e is the electron) for various Ω0; (4) the mode conversion window becomes wider as Ω0 and δ increase. Hence, the results in this paper confirm that linear mode conversion under these conditions can explain the weak total circular polarization of interplanetary type II and III solar radio bursts because a strong xR mode can be generated via linear mode conversion near δ ∼ 45°.

Effects of a random spatial variation of the plasma density on the mode conversion in cold, unmagnetized, and stratified plasmas
View Description Hide DescriptionWe study the effects of a random spatial variation of the plasma density on the mode conversion of electromagnetic waves into electrostatic oscillations in cold, unmagnetized, and stratified plasmas. Using the invariant imbedding method, we calculate precisely the electromagnetic field distribution and the mode conversion coefficient, which is defined to be the fraction of the incident wave power converted into electrostatic oscillations, for the configuration where a numerically generated random density variation is added to the background linear density profile. We repeat similar calculations for a large number of random configurations and take an average of the results. We obtain a peculiar nonmonotonic dependence of the mode conversion coefficient on the strength of randomness. As the disorder increases from zero, the maximum value of the mode conversion coefficient decreases initially, then increases to a maximum, and finally decreases towards zero. The range of the incident angle in which mode conversion occurs increases monotonically as the disorder increases. We present numerical results suggesting that the decrease of mode conversion mainly results from the increased reflection due to the Anderson localization effect originating from disorder, whereas the increase of mode conversion of the intermediate disorder regime comes from the appearance of many resonance points and the enhanced tunneling between the resonance points and the cutoff point. We also find a very large local enhancement of the magnetic field intensity for particular random configurations. In order to obtain high mode conversion efficiency, it is desirable to restrict the randomness close to the resonance region.

The relation between reconnected flux, the parallel electric field, and the reconnection rate in a threedimensional kinetic simulation of magnetic reconnection
View Description Hide DescriptionWe investigate the distribution of parallel electric fields and their relationship to the location and rate of magnetic reconnection in a large particleincell simulation of 3D turbulent magnetic reconnection with open boundary conditions. The simulation's guide field geometry inhibits the formation of simple topological features such as null points. Therefore, we derive the location of potential changes in magnetic connectivity by finding the field lines that experience a large relative change between their endpoints, i.e., the quasiseparatrix layer. We find a good correspondence between the locus of changes in magnetic connectivity or the quasiseparatrix layer and the map of large gradients in the integrated parallel electric field (or quasipotential). Furthermore, we investigate the distribution of the parallel electric field along the reconnecting field lines. We find the reconnection rate is controlled by only the lowamplitude, zeroth and first–order trends in the parallel electric field while the contribution from fluctuations of the parallel electric field, such as electron holes, is negligible. The results impact the determination of reconnection sites and reconnection rates in models and in situ spacecraft observations of 3D turbulent reconnection. It is difficult through direct observation to isolate the loci of the reconnection parallel electric field amidst the large amplitude fluctuations. However, we demonstrate that a positive slope of the running sum of the parallel electric field along the field line as a function of field line length indicates where reconnection is occurring along the field line.

Propagation of surface waves on a semibounded quantum magnetized collisional plasma
View Description Hide DescriptionThe propagation of surface waves on a semibounded quantum plasma in the presence of the external magnetic field and collisional effects is investigated by using quantum magnetohydrodynamics model. A general analytical expression for the dispersion relation of surface waves is obtained by considering the boundary conditions. It is shown that, in some special cases, the obtained dispersion relation reduces to the results reported in previous works. It is also indicated that the quantum, external magnetic field and collisional effects can facilitate the propagation of surface waves on a semibounded plasma. In addition, it is found that the growth rate of the surface wave instability is enhanced by increasing the collision frequency and plasmonic parameter.

Thermal fluctuations and critical behavior in a magnetized, anisotropic plasma
View Description Hide DescriptionThermal fluctuations in a magnetized, anisotropic plasma are studied by applying standard methods, based on the Einstein rule, to the known thermodynamic potential of the system. It is found in particular that magnetic fluctuations become critical when the anisotropy changes sign. By examining the critical region, additional insight on the equations of state for nearcritical anisotropic plasma is obtained.

On the propagation of vorticity in multispecies plasmas
View Description Hide DescriptionThe evolution of plasmas formed by several species is governed by one fluid equation for each species, all of them linked by an electromagnetic forcing and collisional terms, and the Maxwell equations. It is found that in the collisionless case, the field lines of a combination of fluid vorticity and magnetic field are transported by the flow as material points. In consequence, the vorticity propagates at the same velocity as the magnetic field. This is studied in depth for a number of simple configurations, showing that the vorticity travels at a certain fraction of the speed of light, depending on the size of the spatial mode.

Renormalization shielding and eikonal analysis on the atomic collision in dense partially ionized hydrogen plasmas
View Description Hide DescriptionThe renormalization plasma screening effects on the electronion collision are investigated in dense partially ionized hydrogen plasmas. The HamiltonJacobi and eikonal methods with the effective interaction potential are employed to obtain the eikonal scattering phase shift and eikonal cross section for the electronion collision. It is found that the influence of renormalization screening strongly suppresses the eikonal scattering phase shift as well as the eikonal cross section, especially, for small impact parameter regions. In addition, the renormalization screening effect reduces the total eikonal cross section in all energy domains. The variation of the renormalization effects on the electronion collision in dense partially ionized hydrogen plasmas is also discussed.

Energy levels of a heavy ion moving in dense plasmas
View Description Hide DescriptionIn this paper, the potential of a slowly moving test particle moving in collisional dense plasmas is studied. It is composed of the Debyeshielding potential, wake potential, and collision term. The Ritz variationalperturbational method is developed for calculating relativistic binding energy levels of a heavy ion moving in dense plasmas. Binding energy levels of a heavy ion moving in plasmas are calculated. The results show that both nonrelativistic energy levels and relativistic energy levels become more negative as the temperature becomes high. They also become more negative as the number density decreasing. Relativistic correction is important for calculating binding energy levels. Both relativistic energy levels and nonrelativistic energy levels vary minutely as the speed of heavy ion varies.

Theory of waves in pairion plasmas: Natural explanation of backward modes
View Description Hide DescriptionBackward waves observed in the experiments by Oohara and Hatakeyama (Phys. Rev. Lett. 91, 205005 (2003)) are identified to be ion cyclotron harmonic waves inherent to the kinetic theory. The derived dispersion equation is based on exact solutions of the characteristic equations of the Vlasov equation in a bounded cylindrical coordinate system; it is different from its counterpart in unbounded plasmas, and it provides all the branches of the dispersion relations observed in the experiment. Positive and negative ions respond to a potential in the same time scale and cooperate to expose kinetic orbital behaviors to the macroscopic propagation characteristics. In addition, the experimental setting of the large Larmor radius makes higher harmonic ion cyclotron backward/forward waves observable. The large Larmor radius effects are naturally treated by a kinetic theory.

Nonlinear electron acoustic waves in presence of shear magnetic field
View Description Hide DescriptionNonlinear electron acoustic waves are studied in a quasineutral plasma in the presence of a variable magnetic field. The fluid model is used to describe the dynamics of two temperature electron species in a stationary positively charged ion background. Linear analysis of the governing equations manifests dispersion relation of electron magneto sonic wave. Whereas, nonlinear wave dynamics is being investigated by introducing Lagrangian variable method in long wavelength limit. It is shown from finite amplitude analysis that the nonlinear wave characteristics are well depicted by KdV equation. The wave dispersion arising in quasineutral plasma is induced by transverse magnetic field component. The results are discussed in the context of plasma of Earth's magnetosphere.

Plasma response to electron energy filter in large volume plasma device
View Description Hide DescriptionAn electron energy filter (EEF) is embedded in the Large Volume Plasma Device plasma for carrying out studies on excitation of plasma turbulence by a gradient in electron temperature (ETG) described in the paper of Mattoo et al. [S. K. Mattoo et al., Phys. Rev. Lett. 108, 255007 (2012)]. In this paper, we report results on the response of the plasma to the EEF. It is shown that inhomogeneity in the magnetic field of the EEF switches on several physical phenomena resulting in plasma regions with different characteristics, including a plasma region free from energetic electrons, suitable for the study of ETG turbulence. Specifically, we report that localized structures of plasma density, potential, electron temperature, and plasma turbulence are excited in the EEF plasma. It is shown that structures of electron temperature and potential are created due to energy dependence of the electron transport in the filter region. On the other hand, although structure of plasma density has origin in the particle transport but two distinct steps of the density structure emerge from dominance of collisionality in the sourceEEF region and of the Bohm diffusion in the EEFtarget region. It is argued and experimental evidence is provided for existence of drift like flute RayleighTaylor in the EEF plasma.

Propagation and oblique collision of ionacoustic solitary waves in a magnetized dusty electronegative plasma
View Description Hide DescriptionThe propagation and oblique collision of ionacoustic (IA) solitary waves in a magnetized dusty electronegative plasma consisting of cold mobile positive ions, Boltzmann negative ions, Boltzmann electrons, and stationary positive/negative dust particles are studied. The extended PoincaréLighthillKuo perturbation method is employed to derive the Kortewegde Vries equations and the corresponding expressions for the phase shifts after collision between two IA solitary waves. It turns out that the angle of collision, the temperature and density of negative ions, and the dust density of opposite polarity have reasonable effects on the phase shift. Clearly, the numerical results demonstrated that the IA solitary waves are delayed after the oblique collision. The current finding of this work is applicable in many plasma environments having negative ion species, such as D and Fregions of the Earth's ionosphere and some laboratory plasma experiments.

High and low frequency instabilities driven by a single electron beam in twoelectron temperature space plasmas
View Description Hide DescriptionIn an attempt to understand the excitation mechanisms of broadband electrostatic noise, beamgenerated electrostatic instabilities are investigated using kinetic theory in a fourcomponent magnetised plasma model composed of beam electrons (magnetic fieldaligned), background hot and cool electrons and ions. All species are fully magnetised and considered to be Maxwellian. The dependence of the instability growth rates and real frequencies on various plasma parameters such as beam speed, particle densities and temperatures, magnetic field strength, wave propagation angle, and temperature anisotropy of the beam are examined. In this study we have found that the electronacoustic, electron beamresonant and ionacoustic instabilities are excited. Our studies have focused on three velocity regimes, namely, the low , intermediate , and high velocity regimes, where vdbz (Ch ) is the electron beam drift speed (thermal speed of the hot electrons). Plasma parameters from satellite measurements are used where applicable to provide realistic predictions.

Phenomena of oscillations in atmospheric pressure direct current glow discharges
View Description Hide DescriptionSelfsustained oscillations in a dc glow discharge with a semiconductor layer at atmospheric pressure were investigated by means of a onedimensional fluid model. It is found that the dc glow discharge initially becomes unstable in the subnormal glow region and gives rise to oscillations of plasma parameters. A variety of oscillations with one or more frequencies have been observed under different conditions. The discharge oscillates between the glow discharge mode and the Townsend discharge mode in the oscillations with large amplitude while operates in the subnormal glow discharge mode all the while in the oscillations with small amplitude. Fourier Transform spectra of oscillations reveal the transition mechanism between different oscillations. The effects of semiconductor conductivity on the oscillation frequency of the dominant mode, gas voltage, as well as the discharge current have also been analyzed.

Selffocusing and stimulated Brillouin backscattering of a long intense laser pulse in a finite temperature relativistic plasma
View Description Hide DescriptionThe nonlinear dynamics of electromagnetic waves propagating through a plasma considering the effects of relativistic mass and ponderomotive nonlinearities is investigated. The modified electron density distribution, the dispersion relation, and the spatial profiles of electromagnetic wave amplitude in the plasma are obtained. It is shown that the cutoff frequency decreases, and there is an intensity range in which the ponderomotive selffocusing takes place. In the upper limit of this range, the laser beam is defocused due to the relativistic ponderomotive force. In addition, the stability of electromagnetic waves to stimulated Brillouin scattering is studied, and the backscattered wave resulting from decay of high power electromagnetic beam is resolved in relativistic regime. The study of effects of electron density and temperature on the growth rate of backscattered wave has been shown that by increasing these effects, the growth rate of instability increases.

Magnetohydrodynamic study for threedimensional instability of the Petschek type magnetic reconnection
View Description Hide DescriptionThe 3D instability of the spontaneous fast magnetic reconnection process is studied with magnetohydrodynamics (MHD) simulations, where the 2D model of the spontaneous fast magnetic reconnection is destabilized in three dimension. As well known in many 2D numerical MHD studies, when a 1D current sheet is destabilized with the currentdriven anomalous resistivity, the 2D Petschek type fast magnetic reconnection is established. This paper shows that the 2D Petschek type fast magnetic reconnection can be destabilized in three dimension by an initial resistive disturbance which includes a weak fluctuation in the sheet current direction, i.e., along the magnetic neutral line. The resulting 3D fast magnetic reconnection finally becomes intermittent and random through a 3D instability. In addition, it is also shown that the 3D instability is suppressed by the uniform resistivity. It suggests that the 3D instability is caused in the Petschektype reconnection process which is characterized by a strongly localized magnetic diffusion region and the slow shock acceleration of the plasma jets and is suppressed in the SweetParker type reconnection process.

Nonlinear Phenomena, Turbulence, Transport

Magnetic island evolution in the presence of iontemperature gradientdriven turbulence
View Description Hide DescriptionTurbulence is known to drive and sustain magnetic islands of width equal to multiples of the Larmor radius. The nature of the drive is studied here by means of numerical simulations of a fluid electrostatic model in 2D (single helicity) shearedslab geometry. The electrostatic model eliminates the coalescence of short wavelength islands as a mechanism for sustaining longer wavelength islands. In quiescent islands, the polarization current, which depends on the propagation velocity of the island through the plasma, plays a critical role in determining the growth or decay of island chains. For turbulent islands, the unforced propagation velocity is significantly changed by strong zonal flow. The simulations show, however, that the turbulent fluctuations in the current density are much larger and faster than those in the zonal flow, and that they dominate the steadystate perturbed current density. In order to distinguish the roles of the zonal flow from the direct action of the fluctuations on the islands, a new diagnostic is implemented. This new diagnostic separates the effects of all the sources of parallel current. These are the curvature (which drives PfirschSchlüter currents) and the divergences of the viscous and Reynolds stresses (the latter driving polarization currents). The new diagnostic also enables the contributions from short and long wavelengths to be separated for each term. It shows that in the absence of curvature, the drive is dominated by the contributions to the polarization current from the short wavelength fluctuations, while the longwavelength fluctuations play a stabilizing role. In the presence of unfavorable curvature, by contrast, the effects of the short and longwavelength contributions of the polarization current reverse roles but nearly cancel, leaving the PfirschSchlüter current as the dominant drive.
