Volume 19, Issue 11, November 2012

We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Kármán type satisfying the steadystate NavierStokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment, whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds numberRe. It is found that the overall lowest is achieved at for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range , where is the second critical magnetic Reynolds number, above which the dynamo is quenched. Samples of the optimized flows and the corresponding dynamo fields are presented.
 LETTERS


Dielectric properties in microwave remote plasma sustained in argon: Expanding plasma conditions
View Description Hide DescriptionThis work is devoted to the study of the relative permittivity in argon expanding plasma produced below a microwave discharge sustained in a quartz tube and working at 2.45 GHz. We discuss results and explain the microwave propagation within the reactor, outside the quartz tube. It is shown that at low pressures (133 Pa) and at powers ranging from 100 W to 400 W, the wave frequency remains lower than the plasma frequency anywhere in the expanding plasma. Under these conditions, the real part of the relative permittivity is negative and the wave is reflected. Surprisingly, in these conditions, the plasma is produced inside and outside the quartz tube, below the wave launcher. This effect can be explained considering a surface wave propagating at the surface of the quartz tube then into the reactor, on the external surface of the expanding plasma below the quartz tube.

Electron cyclotron maser emission mode coupling to the zmode on a longitudinal density gradient in the context of solar type III bursts
View Description Hide DescriptionA beam of superthermal, hot electrons was injected into maxwellianplasma with a density gradient along a magnetic field line. 1.5D particleincell simulations were carried out which established that the EM emission is produced by the perpendicular component of the beam injection momentum. The beam has a positive slope in the distribution function in perpendicular momentum phase space, which is the characteristic feature of a cyclotron maser. The cyclotron maser in the overdense plasma generates emission at the electron cyclotron frequency. The frequencies of generated waves were too low to propagate away from the injection region, hence the wavelet transform shows a pulsating wave generation and decay process. The intensity pulsation frequency is twice the relativistic cyclotron frequency. Eventually, a stable wave packet formed and could mode couple on the density gradient to reach frequencies of the order of the plasma frequency that allowed for propagation. The emitted wave is likely to be a zmode wave. The total electromagnetic energy generated is of the order of 0.1% of the initial beam kinetic energy. The proposed mechanism is of relevance to solar type III radio bursts, as well as other situations, when the injected electron beam has a nonzero perpendicular momentum, e.g., magnetron.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Effect of selfmagnetic fields on the nonlinear dynamics of relativistic electron beam with virtual cathode
View Description Hide DescriptionThe report is devoted to the results of the numerical study of the virtual cathode (VC) formation conditions in the relativistic electron beam (REB) under the influence of the selfmagnetic and external axial magnetic fields. The azimuthal instability of the relativistic electron beam leading to the formation of the vortex electron structure in the system was found out. This instability is determined by the influence of the selfmagnetic fields of the relativistic electron beam, and it leads to the decrease of the critical value of the electron beamcurrent(current when the nonstationary virtual cathode is formed in the drift space). The typical dependencies of the critical current on the external uniform magnetic field value were discovered. The effect of the beam thickness on the virtual cathode formation conditions was also analyzed.

Omnidirectional photonic band gap enlarged by onedimensional ternary unmagnetized plasma photonic crystals based on a new Fibonacci quasiperiodic structure
View Description Hide DescriptionIn this paper, an omnidirectional photonic band gap realized by onedimensional ternary unmagnetized plasmaphotonic crystals based on a new Fibonacci quasiperiodic structure, which is composed of homogeneous unmagnetized plasma and two kinds of isotropic dielectric, is theoretically studied by the transfer matrix method. It has been shown that such an omnidirectional photonic band gap originates from Bragg gap in contrast to gap or single negative (negative permittivity or negative permeability) gap, and it is insensitive to the incidence angle and the polarization of electromagnetic wave. From the numerical results, the frequency range and central frequency of omnidirectional photonic band gap can be tuned by the thickness and density of the plasma but cease to change with increasing Fibonacci order. The bandwidth of omnidirectional photonic band gap can be notably enlarged. Moreover, the plasma collision frequency has no effect on the bandwidth of omnidirectional photonic band gap. It is shown that such new structure Fibonacci quasiperiodic onedimensional ternary plasmaphotonic crystals have a superior feature in the enhancement of frequency range of omnidirectional photonic band gap compared with the conventional ternary and conventional Fibonacci quasiperiodic ternary plasmaphotonic crystals.

Lowfrequency linearmode regimes in the tokamak scrapeoff layer
View Description Hide DescriptionMotivated by the wide range of physical parameters characterizing the scrapeoff layer (SOL) of existing tokamaks, the regimes of lowfrequency linear instabilities in the SOL are identified by numerical and analytical calculations based on the linear, driftreduced Braginskii equations, with cold ions. The focus is put on ballooning modes and drift wave instabilities, i.e., their resistive, inertial, and ideal branches. A systematic study of each instability is performed, and the parameter space region where they dominate is identified. It is found that the drift waves dominate at high , while the ballooning modes at low ; the relative influence of resistive and inertial effects is discussed. Electromagnetic effects suppress the drift waves and, when the threshold for ideal stability is overcome, the ideal ballooning mode develops. Our analysis is a first stage tool for the understanding of turbulence in the tokamakSOL, necessary to interpret the results of nonlinear simulations.

Wave resonances and the partition of energy in ideal compressible magnetohydrodynamic fluids
View Description Hide DescriptionPhase mixing and resonant absorption are two processes that have been under scrutiny for some time because of their role in wave damping and in providing a mechanism for heating space and laboratory plasmas. The accumulation or absorption of energy that develops within a resonant layer is usually attributed to a logarithmic singularity, but it will be shown that this build up of energy is inextricably tied to a discontinuity in the fluid displacement at the resonant point. This change in the dynamics of the problem will be examined by establishing a partition of energy that identifies and isolates the individual resonances within the fluid. The partition is based on a variational description of the Fourier transformed equations and is guided by an electrical model of the MHD system that not only illustrates the resonant structure threading the fluid but also exposes the mechanism driving the resonant absorption process. A simplified version of this model is then constructed to help determine the approximate rate of energy absorption.

Particle deconfinement in a bent magnetic mirror
View Description Hide DescriptionCoils misalignment in a magnetic mirror can produce additional particle transport. The magnetic field non axisymmetry is responsible for radial and longitudinal drifts in a way much similar to the neoclassical transport in a tandem mirror cell distorted by end plugs. Accordingly, a regime exhibiting large radial displacements––similar to the resonant regime in tandem mirrors––can be obtained by confining ions azimuthally, for example by means of a properly tuned radial electric field. Because of the mass dependence of the magnetic field nonhomogeneity drift velocities, the azimuthal trapping is mass specific, allowing, in principle, the filtering of a specific species based on its mass.

Optimized boundary driven flows for dynamos in a sphere
View Description Hide DescriptionWe perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Kármán type satisfying the steadystate NavierStokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment, whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds numberRe. It is found that the overall lowest is achieved at for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range , where is the second critical magnetic Reynolds number, above which the dynamo is quenched. Samples of the optimized flows and the corresponding dynamo fields are presented.

Surface plasmon polaritons in a semibounded degenerate plasma: Role of spatial dispersion and collisions
View Description Hide DescriptionSurface plasmonpolaritons (SPPs) in a semibounded degenerate plasma (e.g., a metal) are studied using the quasiclassical meanfield kinetic model, taking into account the spatialdispersion of the plasma (due to quantum degeneracy of electrons) and electronion (electronlattice, for metals) collisions. SPP dispersion and damping are obtained in both retarded () and nonretarded () regions, as well as in between. It is shown that the plasma spatialdispersion significantly affects the properties of SPPs, especially at short wavelengths (less than the collisionless skin depth, ). Namely, the collisionless (Landau) damping of SPPs (due to spatialdispersion) is comparable to the purely collisional (Ohmic) damping (due to electronlattice collisions) in a wide range of SPP wavelengths, e.g., from to for SPP in gold at T = 293 K and from to for SPPs in gold at T = 100 K. The spatialdispersion is also shown to affect, in a qualitative way, the dispersion of SPPs at short wavelengths .

Phase space structure of the electron diffusion region in reconnection with weak guide fields
View Description Hide DescriptionKinetic simulations of magnetic reconnection provide detailed information about the electric and magnetic structure throughout the simulation domain, as well as high resolution profiles of the essential fluid parameters including the electron and ion densities, flows, and pressuretensors. However, the electron distribution function,f(v), within the electron diffusion region becomes highly structured in the three dimensional velocity space and is not well resolved by the data available from the particleincell(PIC) simulations. Here, we reconstruct the electron distribution function within the diffusion region at enhanced resolution. This is achieved by tracing electron orbits in the fields taken from PIC simulations back to the inflow region where an analytic form of the magnetized electron distribution is known. For antiparallel reconnection, the analysis reveals the highly structured nature of f(v), with striations corresponding to the number of times electrons have been reflected within the reconnection current layer, and exposes the origin of gradients in the electron pressuretensor important for momentum balance. The structure of the reconnection region is strongly tied to the pressureanisotropy that develops in the electrons upstream of the reconnection region. The addition of a guide field changes the nature of the electron distributions, and the differences are accounted for by studying the motion of single particles in the field geometry. Finally, the geometry of small guide field reconnection is shown to be highly sensitive to the ion/electron mass ratio applied in the simulation.

On stability of collisional coupling between relativistic electrons and ions in hot plasmas
View Description Hide DescriptionThe collisional coupling of relativistic electrons and nonrelativistic ions in hot plasmas has been analysed. It is found that relativistic effects produce a new feature: while the condition guarantees a stable collisional coupling between electrons and ions in lowtemperature plasmas,relativistic effects shift the upper boundary of stability to higher values. Moreover, for sufficiently high temperatures, , collisional decoupling between relativistic electrons and ions becomes impossible.

Finitetemperature corrections to the timedomain equations of motion for perpendicular propagation in nonuniform magnetized plasmas
View Description Hide DescriptionIn this paper we extend the new techniques of W. Tierens and D. D. Zutter, J. Comput. Phys. 231, 5144 (2012) to include finite Larmor radius effects up to second order in the Larmor radius. We limit ourselves to the case of propagation perpendicular to the background magnetic field . We show that our timedomain technique is able to produce the lowestorder Bernstein wave (a wave believed to be useful for heating fusion devices [H. P. Laqua, Plasma Phys. Controlled Fusion 49, R1 (2007)]). The discrete equations retain many of the favourable properties described in W. Tierens and D. D. Zutter, J. Comput. Phys. 231, 5144 (2012), i.e., unconditional stability and a straightforward relation between the secondorder accurate continuous dispersion relation and the dispersion relation of the discretized problem. The theory is illustrated by a placeindependent and a placedependent temperature numerical example.

Charged electret deposition for the manipulation of high power microwave flashover delay times
View Description Hide DescriptionA quasipermanent charged electret is embedded into the radiation window of a high power microwave system. It was experimentally observed that the additional electrostatic field introduced by the electret alters the delay times associated with the development of plasma at the window surface, resulting from high power microwave excitation. The magnitudes of both the statistical and formative delay times are investigated in detail for different pressures. Experimental observations are related to calculated discharge parameters using known E/p dependent properties.

Landau damping effects on the scattering spinasymmetry and channel preference in electionhole plasmas
View Description Hide DescriptionThe scattering spinasymmetry and spin channel preference in electron collisions are investigated in electronhole plasmas. It is found that the Landau damping effect enhances the scattering channel preference for the spinsinglet electron scattering and suppresses the spintriplet electron scattering except the scattering angle . It is also found that the spinsinglet electron scattering decreases with an increase of the quantum shielding distance. In addition, the Landau damping effect on the spinasymmetry parameter is found to be more significant in the intermediate domain of the quantum Debye length.

Kinetic description of electron plasma waves with orbital angular momentum
View Description Hide DescriptionWe describe the kinetic theory of electron plasma waves with orbital angular momentum or twisted plasmons. The conditions for a twisted Landau resonance to exist are established, and this concept is introduced for the first time. Expressions for the kinetic dispersion relation and for the electron Landau damping are derived. The particular case of a Maxwellian plasma is examined in detail. The new contributions to wavedispersion and damping due the orbital angular momentum are discussed. It is shown that twisted plasmons can be excited by rotating electron beams.

Spin effect on parametric interactions of waves in magnetoplasmas
View Description Hide DescriptionThe parametric decay instability of upper hybrid wave into lowfrequency electromagnetic Shear Alfvén wave and Ordinary mode radiation (Omode) has been investigated in an electronion plasma immersed in the uniform external magnetic field. Incorporating quantum effect due to electron spin, the fluid model has been used to investigate the linear and nonlinear response of the plasma species for threewave coupling in a magnetoplasma. It is shown that the spin of electrons has considerable effect on the parametric decay of upper hybrid wave into Ordinary mode radiation (Omode) and Shear Alfvén wave even in classical regime.

Nonlinear, stationary electrostatic ion cyclotron waves: Exact solutions for solitons, periodic waves, and wedge shaped waveforms
View Description Hide DescriptionThe theory of fully nonlinear stationary electrostatic ion cyclotron waves is further developed. The existence of two fundamental constants of motion; namely, momentum flux density parallel to the background magnetic field and energy density, facilitates the reduction of the wave structure equation to a first order differential equation. For subsonic waves propagating sufficiently obliquely to the magnetic field,soliton solutions can be constructed. Importantly, analytic expressions for the amplitude of the soliton show that it increases with decreasing waveMach number and with increasing obliquity to the magnetic field. In the subsonic, quasiparallel case, periodic waves exist whose compressive and rarefactive amplitudes are asymmetric about the “initial” point. A critical “driver” field exists that gives rise to a solitonlike structure which corresponds to infinite wavelength. If the wave speed is supersonic, periodic waves may also be constructed. The aforementioned asymmetry in the waveform arises from the flow being driven towards the local sonic point in the compressive phase and away from it in the rarefactive phase. As the initial driver field approaches the critical value, the end point of the compressive phase becomes sonic and the waveform develops a wedge shape. This feature and the amplitudes of the compressive and rarefactive portions of the periodic waves are illustrated through new analytic expressions that follow from the equilibrium points of a wave structure equation which includes a driver field. These expressions are illustrated with figures that illuminate the nature of the solitons. The presently described wedgeshaped waveforms also occur in water waves, for similar “transonic” reasons, when a Coriolis force is included.

Raman shifted third harmonic generation of upper hybrid radiation in a plasma
View Description Hide DescriptionRaman shifted third harmonic generation of upper hybrid radiation is proposed and studied. In the presence of ambient magnetic field, the plasma wave present in the system produces electron density ripple (perturbation) which couples with the velocity imparted by the nonlinear ponderomotive force at twice the laser frequency producing the Raman shifted third harmonic field. The wave vector of the plasma wave provides the uncompensated momentum necessary for phase matching condition. The applied magnetic field can be adjusted to have the phase matching for the given plasma frequency. The energy conversion ratio from pump to the Raman shifted third harmonic generation of upper hybrid radiation is analyzed.
 Nonlinear Phenomena, Turbulence, Transport

Asymmetric chiral alignment in magnetized plasma turbulence
View Description Hide DescriptionMulti species turbulence in inhomogeneous magnetised plasmas is found to exhibit symmetry breaking in the dynamical alignment of a third species with the fluctuating electron density and vorticity with respect to the magnetic field direction and the species’ relative background gradients. The possibility of truly chiralaggregation of charged molecules in magnetized space plasma turbulence is discussed.

Interaction of dustion acoustic solitary waves in nonplanar geometry with electrons featuring Tsallis distribution
View Description Hide DescriptionThe headon collisions between nonplanar dustion acoustic solitary waves are dealt with by an extended version of PoincaréLighthillKuo perturbation method, for a plasma having stationary dustgrains, inertial ions, and nonextensive electrons. The nonplanar geometry modified analytical phaseshift after a headon collision is derived. It is found that as the nonextensive character of the electrons becomes important, the phaseshift decreases monotonically before levellingoff at a constant value. This leads us to think that nonextensivity may have a stabilizing effect on the phaseshift.