Volume 19, Issue 7, July 2012

Barrier discharges at atmospheric pressure in nitrogenoxygen mixture powered by high voltage pulses of widths between 10 μs and 200 ns were investigated. The development of the microdischarges on rising and falling slopes was recorded by streak and intensified CCD cameras simultaneously. The breakdown on the falling slope strongly depends on the pulse width. As a result of pulse width variation the starting point of ignition changes and positive and negative streamers occur simultaneously in the falling slope. The observed effect is caused by the electric field rearrangement in the gap due to the different positive ion densities related to their gap crossing times.
 LETTERS


Barrier discharges driven by submicrosecond pulses at atmospheric pressure: Breakdown manipulation by pulse width
View Description Hide DescriptionBarrier discharges at atmospheric pressure in nitrogenoxygen mixture powered by high voltage pulses of widths between 10 μs and 200 ns were investigated. The development of the microdischarges on rising and falling slopes was recorded by streak and intensified CCD cameras simultaneously. The breakdown on the falling slope strongly depends on the pulse width. As a result of pulse width variation the starting point of ignition changes and positive and negative streamers occur simultaneously in the falling slope. The observed effect is caused by the electric field rearrangement in the gap due to the different positive ion densities related to their gap crossing times.

Generation of magnetized collisionless shocks by a novel, laserdriven magnetic piston
View Description Hide DescriptionWe present experiments on the Trident laser facility at Los Alamos National Laboratory which demonstrate key elements in the production of laserdriven, magnetized, laboratoryscaled astrophysical collisionless shocks. These include the creation of a novel magnetic piston to couple laser energy to a background plasma and the generation of a collisionless shock precursor. We also observe evidence of decoupling between a laserdriven fast ion population and a background plasma, in contrast to the coupling of laserablated slow ions with background ions through the magnetic piston. 2D hybrid simulations further support these developments and show the coupling of the slow to ambient ions, the formation of a magnetic and density compression pulses consistent with a collisionless shock, and the decoupling of the fast ions.

Fieldaligned accelerations by plasma shocks propagating through interstellar magnetic fields
View Description Hide DescriptionA kinetic model of particle acceleration by plasma shocks is analyzed theoretically and with numerical calculations. The shocks are propagating through weakly magnetized background plasmas, namely interstellar magnetic fields(IMFs). Particles located at the shock front are accelerated parallel to the magnetic field of the shock; this is defined as the fieldaligned acceleration (FAA). The cross angle between IMF and the magnetic field of the shock plays an important role in creating the magnetic neutral sheet at the shock front. A test particle trapped by the neutral sheet obtains enormous energy due to the FAA. A reasonable formula for the highest energy gain is derived from theoretical analysis of the relativistic equations of motion. A possible configuration of the electric and magnetic fields in supernova remnants is also proposed by way of example.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

A different approach to obtain Mayer’s extension to stationary single particle Wigner distribution
View Description Hide DescriptionIt is shown that the stationary collisionless singleparticle Wigner equation in one dimension containing quantum corrections at the lowest order is satisfied by a distribution function that is similar in form to the Maxwellian distribution with an effective mass and a generalized potential. The distribution is used to study quantum corrections to electron hole solutions.

Suppression of drift waves in a linear magnetized plasma column
View Description Hide DescriptionIn magnetically confined fusion plasmas, drift wave driven turbulence can lead to enhanced particle transport from the plasma. Because of this, a significant research emphasis has been placed on the suppression of drift waves in the plasma edge. However, the combination of the toroidal geometry and short plasma lifetimes can make it difficult to fully characterize the properties of these instabilities. Because linear magnetized plasma devices offer a combination of simpler geometry and steady state plasma generation, it is possible to perform detailed studies of many types of plasmainstabilities—includingdrift waves. This paper reports on a recent experiment in which low frequency instabilities (ω ≤ ω_{ci} ) in the Auburn Linear EXperiment for Instability Studies plasma device were characterized as drift waves and through changes in the parallel current, it is shown that it is possible to suppress these instabilities.

Linear and nonlinear studies of velocity shear driven three dimensional electronmagnetohydrodynamics instability
View Description Hide DescriptionThe study of electron velocity shear driven instability in electron magnetohydrodynamics (EMHD) regime in three dimensions has been carried out. It is well known that the instability is nonlocal in the plane defined by the flow direction and that of the shear, which is the usual KelvinHelmholtz mode, often termed as the sausage mode in the context of EMHD. On the other hand, a local instability with perturbations in the plane defined by the shear and the magnetic field direction exists which is termed as kink mode. The interplay of these two modes for simple sheared flow case as well as that when an external magnetic field exists has been studied extensively in the present manuscript in both linear and nonlinear regimes. Finally, these instability processes have been investigated for the exact 2D dipole solutions of EMHD equations [M. B. Isichenko and A. N. Marnachev, Sov. Phys. JETP 66, 702 (1987)] for which the electron flow velocity is sheared. It has been shown that dipoles are very robust and stable against the sausage mode as the unstable wavelengths are typically longer than the dipole size. However, we observe that they do get destabilized by the local kink mode.

On the anomalous diffusion in nonisothermal plasma
View Description Hide DescriptionIn nonisothermal plasmas at temperaturediffusion plays a decisive role at conditions of smooth inhomogeneity when the inhomogeneity size is much larger than times the Debye radius. When the inhomogeneity is rather abrupt and this condition is violated, then during the spreading process the Maxwellian relaxation of ion charges becomes significant. Here, we consider these two phenomena together and refer to the anomalous character of diffusion, i.e., anomalous diffusion.

Ballooning theory of the second kind—two dimensional tokamak modes
View Description Hide DescriptionThe 2D ballooning transform, devised to study local high toroidal number (n) fluctuations in axisymmetric toroidal system (like tokamaks), yields a welldefined partial differential equation for the linear eigenmodes. In this paper, such a ballooning equation of the second kind is set up for ion temperature gradient driven modes pertinent to a 2D nondissipative fluid plasma; the resulting partial differential equation is numerically solved, to calculate the global eigenvalues, and the 2D mode structure is presented graphically along with analytical companions. The radial localization of the mode results from translational symmetry breaking for growing modes and is a vivid manifestation of spontaneous symmetry breaking in tokamak physics. The eigenmode, poloidally ballooned at , is radially shifted from associated rational surface. The global eigenvalue is found to be very close to the value obtained in 1D parameterized () case. The 2D eigenmode theory is applied to estimate the toroidal seed Reynolds stress [Y. Z. Zhang, Nucl. Fusion Plasma Phys. 30, 193 (2010)]. The solution obtained from the relatively simplified ballooning theory is compared to the solution of the basic equation in original coordinate system (evaluated via FFTs); the agreement is rather good.

High power microwave generation from the lowimpedance transittime oscillator without foils
View Description Hide DescriptionA lowimpedance transittime oscillator without foils (LITTO) has been proposed in our previous work. Recently, the experiment is carried out on an intense relativistic electron beam (IREB) generator, which is capable of producing a 50 ns duration electron beam in the voltage range of 0.41 MV. With a 600 kV, 24 kA electron beam guided by an external magnetic field of 0.5 T, a radiation power of 2.7 GW at 1.64 GHz has been achieved and the corresponding power conversion efficiency is 18.75%. With the similar voltage and current parameters, the experimental results are reexamined and confirmed by the particleincell(PIC) simulation.

A numerical electromagnetic linear dispersion relation for Maxwellian ringbeam velocity distributions
View Description Hide DescriptionA positive slope in a velocity distribution function perpendicular to the ambient magnetic field, such as due to a loss cone or ring velocity distribution, can become a free energy source for the excitation of various plasma waves. Since there exists no analytic expression for integrals of Maxwellian ring velocity distribution functions, their linear properties have previously been studied using several approximations or modeled distributions. In this paper, a numerical method for analyzing the linear dispersion relation for Maxwellian ringbeam velocity distributions is developed. The obtained linear properties are confirmed by direct comparison with full particle simulation results.

Cylindrical effects on RichtmyerMeshkov instability for arbitrary Atwood numbers in weakly nonlinear regime
View Description Hide DescriptionWhen an incident shock collides with a corrugated interface separating two fluids of different densities, the interface is prone to RichtmyerMeshkov instability(RMI). Based on the formal perturbation expansion method as well as the potential flow theory, we present a simple method to investigate the cylindrical effects in weakly nonlinear RMI with the transmitted and reflected cylindrical shocks by considering the nonlinear corrections up to fourth order. The cylindrical results associated with the material interface show that the interface expression consists of two parts: the result in the planar system and that from the cylindrical effects. In the limit of the cylindrical radius tending to infinity, the cylindrical results can be reduced to those in the planar system. Our explicit results show that the cylindrical effects exert an inward velocity on the whole perturbed interface, regardless of bubbles or spikes of the interface. On the one hand, outgoing bubbles are constrained and ingoing spikes are accelerated for different Atwood numbers (A) and mode numbers . On the other hand, for ingoing bubbles, when , bubbles are considerably accelerated especially at the small and ; otherwise, bubbles are decelerated. For outgoing spikes, when , spikes are dramatically accelerated especially at large and ; otherwise, spikes are decelerated. Furthermore, the cylindrical effects have a significant influence on the amplitudes of the ingoing spike and bubble for large . Thus, it should be included in applications where the cylindrical effects play a role, such as inertial confinement fusion ignition target design.

Generation of lower hybrid and whistler waves by an ion velocity ring distribution
View Description Hide DescriptionUsing fully kinetic simulations in two and three spatial dimensions, we consider the generation and nonlinear evolution of lower hybridwaves produced by a cold ion ring velocity distribution in a low beta plasma. We show that the initial development of the instability is very similar in two and three dimensions and not significantly modified by electromagnetic effects, consistent with linear theory. At saturation, the level of electric field fluctuations is a small fraction of the background thermal energy; the electric field and corresponding density fluctuations consist of long, fieldaligned striations. Energy extracted from the ring goes primarily into heating the background ions and the electrons at comparable rates. The initial growth and saturation of the magnetic components of the lower hybridwaves are related to the electric field components, consistent with linear theory. As the growing electric field fluctuations saturate, parallel propagating whistler waves develop by the interaction of two lower hybridwaves. At later times, these whistlers are replaced by longer wavelength, parallel propagating whistlers that grow through the decay of the lower hybrid fluctuations. Wave matching conditions demonstrate these conversion processes of lower hybridwaves to whistler waves. The conversion efficiency (=ratio of the whistler wave energy to the energy in the saturated lower hybridwaves) is computed and found to be significant (∼15%) for the parameters of the threedimensional simulation (and even larger in the twodimensional simulation), although when normalized in terms of the initial kinetic energy in the ring ions the overall efficiency is very small (<10^{−4}). The results are compared with relevant linear and nonlinear theory.

Gyrokinetics for highfrequency modes in tokamaks
View Description Hide DescriptionGyrokinetics for highfrequency modes in tokamaks is developed. It is found that the breakdown of the invariants by perturbed electromagnetic fields drives microinstability. The obtained diamagnetic frequency, , is proportional to only the toroidal mode number rather than transverse mode numbers. Therefore, there is no nonadiabatic drive for axisymmetrical modes in gyrokinetics. Meanwhile, the conventional eikonal Ansatz breaks down for the axisymmetrical modes. The ion driftcyclotron instability discovered in a mirror machine is found for the first time in the toroidal system. The growth rates are proportional to , and the slope changes with magnetic curvature. In spherical torus, where magnetic curvature is greater than that of traditional tokamaks,instability poses a potential danger to such devices.

Photonic band structures of onedimensional photonic crystals doped with plasma
View Description Hide DescriptionThe photonicband structures (PBSs) of oblique incidence propagation in onedimensional plasmadoped photonic crystals (PCs) are investigated carefully. When the lattice constant of plasmadoped PCs is less than the incident wavelength, the PC becomes anisotropic. Therefore, the dielectric constant of PC is converted into a complex tensor dielectric constant. This determines the PBSs of PCs. In the present paper, onedimensional PCs are taken as an example to study both normal and absorption PBSs. Using both the effective medium approximation and the transfer matrix method, we can derive the dispersion relation for PCs. The dependence of the plasma filling factor on the effective dielectric constant and PBSs is calculated and discussed.

Long wavelength gradient drift instability in Hall plasma devices. I. Fluid theory
View Description Hide DescriptionThe problem of long wavelength instabilities in Hall thruster plasmas is revisited. A fluid model of the instabilities driven by the drift in plasmas with gradients of density, electron temperature, and magnetic field is proposed. It is shown that full account of compressibility of the electron flow in inhomogeneous magnetic field leads to quantitative modifications of earlier obtained instability criteria and characteristics of unstable modes. Modification of the stability criteria due to finite temperaturefluctuations is investigated.

Physics of selfsustained oscillations in the positive glow corona
View Description Hide DescriptionThe physics of selfsustained oscillations in the phenomenon of positive glow corona is presented. The dynamics of chargedparticle oscillation under static electric field has been briefly outlined; and, the resulting selfsustained current oscillations in the electrodes have been compared with the measurements from the positive glow corona experiments. The profile of selfsustained electrode current oscillations predicted by the presented theory qualitatively agrees with the experimental measurements. For instance, the experimentally observed sawtooth shaped electrode current pulses are reproduced by the presented theory. Further, the theory correctly predicts the pulses of radiation accompanying the abrupt rises in the sawtooth shaped current oscillations, as verified from the various glow corona experiments.

Threewave coupling in electronpositronion plasmas
View Description Hide DescriptionThe threewave coupling processes in electronpositronion plasmas are investigated. The nonlinear dispersion relation is derived along with the nonlinear growth rate in both resonant and non resonant processes. It is shown that the inclusion of positron affects the dielectric properties of the plasma as well as the nonlinear growth rates of parametric processes. As one increases the positron density to electron density ratio from 0 to 1, maintaining quasi neutrality of the plasma, the growth rates of stimulated Raman, Brillouin, and Compton scattering processes in an isothermal plasma tend to zero due to the ponderomotive forces acting on electrons and positrons due the pump and scatteredwaves being equal.

Electronic excitation and isentropic coefficients of high temperature planetary atmosphere plasmas
View Description Hide DescriptionIn this paper, we have discussed the effects of electronically excited states of atomic species in affecting the isentropic coefficients of plasmas, focusing on mixtures representing the atmospheres of Jupiter, Mars, and Earth. General behaviors have been rationalized on the basis of simplified approaches. The contribution of the electronically excited states has been evidenced by comparing results obtained considering only the ground state and those obtained using either Fermi or Griem cutoff criteria.

On the existence of Weibel instability in a magnetized plasma. II. Perpendicular wave propagation: The ordinary mode
View Description Hide DescriptionIn a magnetized plasma with a temperatureanisotropy (where and denote directions with respect to the uniform magnetic field), the nonresonant Weibel instability can develop and destabilize purely growing, ordinary plasma modes (). This paper presents a rigorous extended analysis of this instability on the basis of a new threshold , which enables to determine the instability conditions as well as the upper limits of the growth rates. Accurate analytical forms of the threshold conditions are provided here for the first time and for the full physical range of the temperatureanisotropy and the parallel plasma beta. The marginal and threshold conditions for the plasma parameters, which directly lead to an instability of the ordinary mode, are explicitly derived numerically and analytically. The new analytical tools developed here provide premises for a comprehensive investigation of the interplay of this instability with the firehose instability, as they both can develop in the same conditions.

Kinetic Alfven double layer formed by electron viscosity
View Description Hide DescriptionThe effect of the electron viscosity on the kinetic Alfven solitary wave is investigated. It is found that small electron viscosity changes the electron motion along the magnetic field producing a boundary layer, and thus that in a low beta electronion plasma(), an obliquely propagating kinetic solitary Alfven wave can become a double layer. This double layer can exist in the subAlfvenic and superAlfvenic regimes. The length scale of density drop for this double layer is on the order of that of the conventional kinetic solitary Alfven wave, and thus this double layer can accelerate electrons on a very short length scale.