Volume 18, Issue 7, July 2011

This review addresses fieldreversed configurations (FRCs), which are compacttoroidal magnetic systems with little or no toroidal field and very high β (ratio of plasma pressure to magnetic pressure). Although enthusiasm for the FRC has primarily been driven by its potential for an attractive fusion reactor, this review focuses on the physics rather than on technological or engineering aspects. Major advances in both theory and experiment have taken place since the previous comprehensive FRC review in 1988. Even so many questions remain. In particular, even though FRC experiments have exhibited remarkable stability, how well this extrapolates to larger systems remains unresolved. The review considers FRCs under familiar topical categories: equilibrium, global stability, selforganization, transport, formation, and sustainment.
 REVIEW ARTICLE


Review of fieldreversed configurations
View Description Hide DescriptionThis review addresses fieldreversed configurations (FRCs), which are compacttoroidal magnetic systems with little or no toroidal field and very high β (ratio of plasma pressure to magnetic pressure). Although enthusiasm for the FRC has primarily been driven by its potential for an attractive fusion reactor, this review focuses on the physics rather than on technological or engineering aspects. Major advances in both theory and experiment have taken place since the previous comprehensive FRC review in 1988. Even so many questions remain. In particular, even though FRC experiments have exhibited remarkable stability, how well this extrapolates to larger systems remains unresolved. The review considers FRCs under familiar topical categories: equilibrium, global stability, selforganization, transport, formation, and sustainment.
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 LETTERS


NonGaussian properties of global momentum and particle fluxes in a cylindrical laboratory plasma
View Description Hide DescriptionNonGaussian statistical properties of the azimuthally averaged momentum and particle fluxes driven by turbulence have been simultaneously observed in inhomogeneous magnetized plasmas for the first time. We identified the stretched Gaussian distribution of the both fluxes and the transition from the pointwise distribution to averaged ones was confirmed. The change of the particle flux precedes that of the momentum flux, demonstrating that the momentum flux is induced by the relaxation of density gradient.

Lyapunov stability of flowing magnetohydrodynamic plasmas surrounded by resistive walls
View Description Hide DescriptionA general stability condition for plasmavacuum systems with resistive walls is derived by using the Frieman Rotenberg Lagrangian stability formulation [Rev. Mod. Phys. 32, 898 (1960)]. It is shown that the Lyapunov stability limit for external modes does not depend upon the gyroscopic term but upon the sign of the perturbed potential energy only. In the absence of dissipation in the plasma such as viscosity, it is expected that the flow cannot stabilize the system.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Particle acceleration and energy conservation in particle in cell simulations
View Description Hide DescriptionParticle acceleration is a process of great importance in all areas of plasma physics. In most cases, kinetic effects are dominant and require a full kinetic treatment, such as the particle in cell(PIC) method. PIC methods are widely used in all aspects of plasma physics, proving to be a precious and irreplaceable tool. Yet all methods in use and published conserve energy to a good approximation, but not exactly. A well known property of PIC methods, documented extensively in all textbooks, is that energy is not conserved exactly. In fact, the particle noise is a unphysical source of energy that, when insufficient resolution is used, can make the simulations go unstable. In the present paper, we apply a new exactly energy conserving scheme and demonstrate that indeed exact energy conservation plays a key role in determining the correct spectrum of the accelerated particles.

Investigation of effect of excitation frequency on electron energy distribution functions in low pressure radio frequency bounded plasmas
View Description Hide DescriptionParticle in cell (PIC) simulations are employed to investigate the effect of excitation frequency ω on electron energy distribution functions(EEDFs) in a low pressure radio frequency (rf)discharge. The discharge is maintained over a length of 0.10 m, bounded by two infinite parallel plates, with the coherent heating field localized at the center of the discharge over a distance of 0.05 m and applied perpendicularly along the y and z directions. On varying the excitation frequency f (=ω/2π) in the range 0.01–50 MHz, it is observed that for f ≤ 5 MHz the EEDF shows a trend toward a convex (Druyvesteynlike) distribution. For f > 5 MHz, the distribution resembles more like a Maxwellian with the familiar break energy visible in most of the distributions. A prominent “hot tail” is observed at f ≥ 20 MHz and the temperature of the tail is seen to decrease with further increase in frequency (e.g., at 30 MHz and 50 MHz). The mechanism for the generation of the “hot tail” is considered to be due to preferential transit time heating of energetic electrons as a function of ω, in the antenna heating field. There exists an optimum frequency for which high energy electrons are maximally heated. The occurrence of the Druyvesteynlike distributions at lower ω may be explained by a balance between the heating of the electrons in the effective electric field and elastic cooling due to electron neutral collision frequency ν_{en} ; the transition being dictated by ω ∼ 2πν_{en} .

Ion acoustic shock waves in degenerate plasmas
View Description Hide DescriptionKorteweg de Vries Burgers equation for negative ion degenerate dissipative plasma has been derived using reductive perturbation technique. The quantum hydrodynamic model is used to study the quantum ion acoustic shock waves. The effects of different parameters on quantum ion acoustic shock waves are studied. It is found that quantum parameter, electrons Fermi temperature,temperature of positive and negative ions, mass ratio of positive to negative ions, viscosity, and density ratio have significant impact on the shock wave structure in negative ion degenerate plasma.

Dispersion relation for pure dust Bernstein waves in a nonMaxwellian magnetized dusty plasma
View Description Hide DescriptionPure dust Bernstein waves are investigated using nonMaxwellian kappa and (r,q) distribution functions in a collisionless, uniform magnetized dusty plasma.Dispersion relations for both the distributions are derived by considering waves whose frequency is of the order of dust cyclotron frequency, and dispersion curves are plotted. It is observed that the propagation band for dust Bernstein waves is rather narrow as compared with that of the electron Bernstein waves. However, the band width increases for higher harmonics, for both kappa and (r,q) distributions. Effect of dust charge on dispersion curves is also studied, and one observes that with increasing dust charge, the dispersion curves shift toward the lower frequencies. Increasing the dust to ion density ratio causes the dispersion curve to shift toward the higher frequencies. It is also found that for large values of spectral index kappa (κ), the dispersion curves approach to the Maxwellian curves. The (r,q) distribution approaches the kappa distribution for r = 0, whereas for r > 0, the dispersion curves show deviation from the Maxwellian curves as expected. Relevance of this work can be found in astrophysical plasmas, where nonMaxwellian velocity distributions as well as dust particles are commonly observed.

Weibel instability and structures of magnetic island in antiparallel collisionless magnetic reconnection
View Description Hide DescriptionTwodimensional (2D) particleincell simulations are performed to investigate the structures of the outofplane magnetic field in magnetic island, which is produced during antiparallel collisionless magnetic reconnection. Regular structures with alternate positive and negative values of the outofplane magnetic field along the direction are formed in magnetic island. The generation mechanism of such structures is also proposed in this paper, which is due to the Weibel instability excited by the temperature anisotropy in magnetic island.

Theory and simulations of the scaling of magnetic reconnection with symmetric shear flow
View Description Hide DescriptionThe scaling of magnetic reconnection in the presence of an oppositely directed subAlfvénic shear flow parallel to the reconnectingmagnetic field is studied using analytical scaling arguments and twodimensional twofluid numerical simulations of collisionless (Hall) reconnection. Previous studies noted that the reconnection rate falls and the current sheet tilts with increasing flow speed, but no quantitative theory was presented. This study presents a physical model of the effect of shear flow on reconnection, resulting in expressions for the scaling of properties such as the reconnection rate, outflow speed, and thickness and length of the dissipation region, which are verified numerically. Differences between Hall and SweetParker reconnection are pointed out. The tilting of the current sheet is explained physically and a quantitative prediction is presented and verified.

Gyrokinetic particle simulation of driftcompressional modes in dipole geometry
View Description Hide DescriptionGyrokinetic particle simulation of low frequency compressional modes has been developed using flux coordinates in the global magnetic dipole geometry. The compressional component is formulated in a scalar form of the parallel magnetic perturbation, and the gyroaveraging is performed explicitly in the configuration space. A reduced gyrokinetic model, in which the compressional perturbations are decoupled from the shear Alfvén and electrostatic perturbations, has been implemented. Linear simulation results have been verified using a numerical Nyquist analysis of the dispersion relation in the slab limit. Global simulations of unstable driftcompressional modes in the dipole geometry with kinetic ions find that finite Larmor radius (FLR) effects reduce the linear growth rate significantly but change little the real frequency. Global eigenmode structures show that the modes are even along the equilibrium magnetic field and broadened by the FLR effects in the radial direction. Radial propagation away from the region of excitation is observed.

Quantum kinetic theory of the filamentation instability
View Description Hide DescriptionThe quantum electromagnetic dielectric tensor for a multispecies plasma is rederived from the gaugeinvariant WignerMaxwell system and presented under a form very similar to the classical one. The resulting expression is then applied to a quantum kinetic theory of the electromagneticfilamentation instability. Comparison is made with the quantum fluid theory including a Bohm pressure term and with the cold classical plasma result. A number of analytical expressions are derived for the cutoff wave vector, the largest growth rate, and the most unstable wave vector.

Onset of fast reconnection in Hall magnetohydrodynamics mediated by the plasmoid instability
View Description Hide DescriptionThe role of a superAlfvénic plasmoid instability in the onset of fast reconnection is studied by means of the largest Hall magnetohydrodynamics simulations to date, with system sizes up to 10^{4} ion skin depths (d_{i} ). It is demonstrated that the plasmoid instability can facilitate the onset of rapid Hall reconnection, in a regime where the onset would otherwise be inaccessible because the Sweet–Parker width is significantly above d_{i} . However, the topology of Hall reconnection is not inevitably a single stable Xpoint. There exists an intermediate regime where the single Xpoint topology itself exhibits instability, causing the system to alternate between a single Xpoint geometry and an extended current sheet with multiple Xpoints produced by the plasmoid instability. Through a series of simulations with various system sizes relative to d_{i} , it is shown that system size affects the accessibility of the intermediate regime. The larger the system size is, the easier it is to realize the intermediate regime. Although our Hall magnetohydrodynamics(MHD) model lacks many important physical effects included in fully kinetic models, the fact that a single Xpoint geometry is not inevitable raises the interesting possibility for the first time that Hall MHD simulations may have the potential to realize reconnection with geometrical features similar to those seen in fully kinetic simulations, namely, extended current sheets and plasmoid formation.

Polarizations of coupling kinetic Alfvén and slow waves
View Description Hide DescriptionKinetic Alfvén waves (KAWs) are dispersive Alfvén waves with short perpendicular wavelengths and have been extensively applied to various energization phenomena of plasma particles. KAWs are coupled to slow magnetosonic waves in the case of a finiteplasma. In this paper, the electromagneticpolarization states of the coupling KAWs and slow waves are investigated. The results show that the polarization states of these waves depend sensitively on the local plasma parameters such as the ionelectron temperature ratio () and the plasma kineticmagnetic pressure ratio () as well as their perpendicular wavenumber (). The polarization states of waves play an important and key role in waveparticle interactions and hence have a great interest of understanding the physics of particle energization phenomena by these waves.

Magnetic field gradient effects on RayleighTaylor instability with continuous magnetic field and density profiles
View Description Hide DescriptionIn this paper, the effects of magnetic field gradient (i.e., the magnetic field transition layer effects) on the RayleighTaylor instability(RTI) with continuous magnetic field and density profiles are investigated analytically. The transition layers of magnetic field and density with two different typical profiles are studied and the analytic expressions of the linear growth rate of the RTI are obtained. It is found that the magnetic fieldeffects strongly reduce the linear growth rate of the RTI, especially when the perturbation wavelength is short. The linear growth rate of the RTI increases with the thickness of the magnetic field transition layer, especially for the case of small thickness of the magnetic field transition layer. When the magnetic field transition layer width is long enough, the linear growth rate of the RTI can be saturated. Thus when one increases the width of the magnetic field transition layer, the linear growth rate of the RTI increases only in a certain range, which depends on the magnetic field strength. The numerical results are compared with the analytic linear growth rates and they agree well with each other.

Plasmonic optical switches based on MachZender interferometer
View Description Hide DescriptionElectrooptical switches composed of two MachZehnder interferometerwaveguides filled with 4dimethylaminoNmethyl4stilbazolium tosylate (DAST) have been proposed and investigated. The characteristics of the switch waveguide are analyzed. The finitedifference time domain simulation results reveal that the structure can operate as an optical switch by controlling the voltage V _{1} or V _{2} applied on the region filled with the DAST material with the π phase shift voltage as V _{π} = 2.9 V. The structure has a small dimension of hundreds of nanometers.
 Nonlinear Phenomena, Turbulence, Transport

Gyrokinetic large eddy simulations
View Description Hide DescriptionThe large eddy simulation approach is adapted to the study of plasma microturbulence in a fully threedimensional gyrokinetic system. Ion temperature gradient driven turbulence is studied with the GENE code for both a standard resolution and a reduced resolution with a model for the subgrid scale turbulence. A simple dissipative model for representing the effect of the subgrid scales on the resolved scales is proposed and tested. Once calibrated, the model appears to be able to reproduce most of the features of the free energy spectra for various values of the ion temperature gradient.

Threedimensional electromagnetic strong turbulence. II. Wave packet collapse and structure of wave packets during strong turbulence
View Description Hide DescriptionLargescale simulations of wave packet collapse are performed by numerically solving the threedimensional (3D) electromagnetic Zakharov equations, focusing on individual wave packet collapses and on wave packets that form in continuously driven strong turbulence. The collapse threshold is shown to decrease as the electron thermal speed ν _{ e }/c increases and as the temperature ratio T _{ i }/T _{ e } of ions to electrons decreases. Energy lost during wave packet collapse and dissipation is shown to depend on ν _{ e }/c. The dynamics of density perturbations after collapse are studied in 3D electromagnetic strong turbulence for a range of T _{ i }/T _{ e }. The structures of the Langmuir, transverse, and total electric field components of wave packets during strong turbulence are investigated over a range of ν _{ e }/c. For , strong turbulence is approximately electrostatic and wave packets have very similar structure to purely electrostatic wave packets. For , transverse modes become trapped in density wells and contribute significantly to the structure of the total electric field. At all ν _{ e }/c, the Langmuir energy density contours of wave packets are predominantly oblate (pancake shaped). The transverse energy density contours of wave packets are predominantly prolate (sausage shaped), with the major axis being perpendicular to the major axes of the Langmuir component. This results in the wave packet becoming more nearly spherical as ν _{ e }/c increases, and in turn generates more spherical density wells during collapse. The results obtained are compared with previous 3D electrostatic results and 2D electromagnetic results.

On radial geodesic forcing of zonal modes
View Description Hide DescriptionThe elementary local and global influence of geodesic field line curvature on radial dispersion of zonal modes in magnetised plasmas is analysed with a primitive drift waveturbulencemodel. A net radial geodesic forcing of zonal flows and geodesic acoustic modes can not be expected in any closed toroidal magnetic confinement configuration, since the flux surface average of geodesic curvature identically vanishes. Radial motion of poloidally elongated zonal jets may occur in the presence of geodesic acoustic mode activity. Phenomenologically a radial propagation of zonal modes shows some characteristics of a classical analogon to second sound in quantum condensates.

Nonlocality and the critical Reynolds numbers of the minimum state magnetohydrodynamic turbulence
View Description Hide DescriptionMagnetohydrodynamic(MHD) systems can be strongly nonlinear (turbulent) when their kinetic and magnetic Reynolds numbers are high, as is the case in many astrophysical and space plasma flows. Unfortunately these high Reynolds numbers are typically much greater than those currently attainable in numerical simulations of MHDturbulence. A natural question to ask is how can researchers be sure that their simulations have reproduced all of the most influential physics of the flows and magnetic fields? In this paper, a metric is defined to indicate whether the necessary physics of interest has been captured. It is found that current computing resources will typically not be sufficient to achieve this minimum state metric.

Nonplanar ionacoustic solitary waves with superthermal electrons in warm plasma
View Description Hide DescriptionIn this paper, we consider an unmagnetized plasma consisting of warm adiabatic ions, superthermal electrons, and thermal positrons. Nonlinear cylindrical and spherical modified Korteweg–de Vries (KdV) equations are derived for ion acoustic waves by using reductive perturbation technique. It is observed that an increasing positron concentration decreases the amplitude of the waves. Furthermore, the effects of the superthermal parameter (k) on the ion acoustic waves are found.