Phys. Plasmas (1994-Present) - Physics of Plasmas

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Realizing steady-state tokamak operation for fusion energy

T. C. Luce

Phys. Plasmas 18, 030501 (2011); http://dx.doi.org/10.1063/1.3551571 (27 pages) | Cited 9 times

Online Publication Date: 25 March 2011

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Continuous operation of a tokamak for fusion energy has clear engineering advantages but requires conditions beyond those sufficient for a burning plasma. The fusion reactions and external sources must support both the pressure and the current equilibrium without inductive current drive, leading to demands on stability, confinement, current drive, and plasma-wall interactions that exceed those for pulsed tokamaks. These conditions have been met individually, and significant progress has been made in the past decade to realize scenarios where the required conditions are obtained simultaneously. Tokamaks are operated routinely without disruptions near pressure limits, as needed for steady-state operation. Fully noninductive sustainment with more than half of the current from intrinsic currents has been obtained for a resistive time with normalized pressure and confinement approaching those needed for steady-state conditions. One remaining challenge is handling the heat and particle fluxes expected in a steady-state tokamak without compromising the core plasma performance.

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89.30.Jj	Nuclear fusion power
28.52.Av	Theory, design, and computerized simulation
52.55.Fa	Tokamaks, spherical tokamaks
52.25.Xz	Magnetized plasmas
52.55.Wq	Current drive; helicity injection

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Effects of hyperbolic rotation in Minkowski space on the modeling of plasma accelerators in a Lorentz boosted frame

J.-L. Vay, C. G. R. Geddes, E. Cormier-Michel, and D. P. Grote

Phys. Plasmas 18, 030701 (2011); http://dx.doi.org/10.1063/1.3559483 (4 pages) | Cited 6 times

Online Publication Date: 16 March 2011

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The effects of hyperbolic rotation in Minkowski space resulting from the use of Lorentz boosted frames of calculation on laser propagation in plasmas are analyzed. Selection of a boost frame at the laser group velocity is shown to alter the laser spectrum, allowing the use of higher boost velocities. The technique is applied to simulations of laser driven plasma wakefield accelerators, which promise much smaller machines and whose development requires detailed simulations that challenge or exceed current capabilities. Speedups approaching the theoretical optima are demonstrated, producing the first direct simulations of stages up to 1 TeV. This is made possible by a million times speedup thanks to a frame boost with a relativistic factor γ_b as high as 1300, taking advantage of the rotation to mitigate an instability that limited previous work.

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03.30.+p	Special relativity
52.38.Kd	Laser-plasma acceleration of electrons and ions
29.20.Ej	Linear accelerators
52.65.Rr	Particle-in-cell method

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Heating of ions by low-frequency Alfvén waves in partially ionized plasmas

Chuanfei Dong and Carol S. Paty

Phys. Plasmas 18, 030702 (2011); http://dx.doi.org/10.1063/1.3555532 (4 pages) | Cited 7 times

Online Publication Date: 16 March 2011

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In the solar atmosphere, the chromospheric and coronal plasmas are much hotter than the visible photosphere. The heating of the solar atmosphere, including the partially ionized chromosphere and corona, remains largely unknown. In this letter, we demonstrate that the ions can be substantially heated by Alfvén waves with very low frequencies in partially ionized low-beta plasmas. This differs from other Alfvén wave related heating mechanisms such as ion-neutral collisional damping of Alfvén waves and heating described by previous work on resonant Alfvén wave heating. We find that the nonresonant Alfvén wave heating is less efficient in partially ionized plasmas than when there are no ion-neutral collisions, and the heating efficiency depends on the ratio of the ion-neutral collision frequency to the ion gyrofrequency.

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52.50.-b	Plasma production and heating
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
96.20.Br	Origin and evolution

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Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

T. Fülöp and S. Moradi

Phys. Plasmas 18, 030703 (2011); http://dx.doi.org/10.1063/1.3569841 (4 pages) | Cited 12 times

Online Publication Date: 29 March 2011

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The effect of poloidal asymmetry of impurities on impurity transport driven by electrostatic turbulence in tokamak plasmas is analyzed. It is found that if the density of the impurity ions is poloidally asymmetric then the zero-flux impurity density gradient is significantly reduced and even a sign change in the impurity flux may occur if the asymmetry is sufficiently large. This effect is most effective in low shear plasmas with the impurity density peaking on the inboard side and may be a contributing factor to the observed outward convection of impurities in the presence of radio frequency heating.

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52.25.Fi	Transport properties
52.25.Vy	Impurities in plasmas
52.55.Fa	Tokamaks, spherical tokamaks

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Steady plasma channel formation and particle acceleration in an interaction of an ultraintense laser with near-critical density plasma

Y. J. Gu, Q. Kong, Y. Y. Li, H. Y. Ban, Z. Zhu, and S. Kawata

Phys. Plasmas 18, 030704 (2011); http://dx.doi.org/10.1063/1.3559463 (4 pages) | Cited 3 times

Online Publication Date: 31 March 2011

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A well-defined near-steady plasma channel is successfully formed in an interaction of an intense short-pulse laser with a near-critical underdense plasma. The laser is well self-focused in the plasma channel. Our 2.5-dimensional particle-in-cell simulations also demonstrate a violent electron acceleration with a effective gradient of several tens of GeV/cm by a strong longitudinal charge-separated field. At the same time, a strong ion acceleration appears at the rear plasma boundary. The results present a regime for the plasma channel formation and the particle acceleration by ultrashort laser-plasma interaction.

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52.35.Mw

Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

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Anomalous self-generated electrostatic fields in nanosecond laser-plasma interaction

L. Lancia, M. Grech, S. Weber, J.-R. Marquès, L. Romagnani, M. Nakatsutsumi, P. Antici, A. Bellue, N. Bourgeois, J.-L. Feugeas, T. Grismayer, T. Lin, Ph. Nicolaï, B. Nkonga, P. Audebert, et al.

Phys. Plasmas 18, 030705 (2011); http://dx.doi.org/10.1063/1.3555522 (4 pages)

Online Publication Date: 31 March 2011

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Electrostatic (E) fields associated with the interaction of a well-controlled, high-power, nanosecond laser pulse with an underdense plasma are diagnosed by proton radiography. Using a current three-dimensional wave propagation code equipped with nonlinear and nonlocal hydrodynamics, we can model the measured E-fields that are driven by the laser ponderomotive force in the region where the laser undergoes filamentation. However, strong fields of up to 110 MV/m measured in the first millimeter of propagation cannot be reproduced in the simulations. This could point to the presence of unexpected strong thermal electron pressure gradients possibly linked to ion acoustic turbulence, thus emphasizing the need for the development of full kinetic collisional simulations in order to properly model laser-plasma interaction in these strongly nonlinear conditions.

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52.38.Kd	Laser-plasma acceleration of electrons and ions
52.38.Hb	Self-focussing, channeling, and filamentation in plasmas
52.70.Ds	Electric and magnetic measurements
52.50.Jm	Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.Nc	Particle measurements

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Stabilizing effect of a nonresonant radio frequency drive on the m = 1 diocotron instability

G. Maero, B. Paroli, R. Pozzoli, and M. Romé

Phys. Plasmas 18, 032101 (2011); http://dx.doi.org/10.1063/1.3558374 (6 pages)

Online Publication Date: 1 March 2011

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It has been experimentally shown that the rotation radius of a non-neutral plasma column around the longitudinal axis of a Malmberg–Penning trap experiences a growth in amplitude (m = 1 diocotron instability), leading to the loss of the plasma on the surface of the confining electrodes. A new stabilization mechanism has been investigated with the help of systematic experiments in the ELTRAP (ELectron TRAP) device where a high-frequency, low-amplitude drive has been applied on an azimuthally sectored electrode. An effective confining force is created, which reduces the offset of the column from the center. This interpretation and its theoretical analysis show a qualitative agreement with the experimental findings, where a net confinement effect is present for a wide range of drive amplitudes and frequencies.

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52.50.Qt	Plasma heating by radio-frequency fields; ICR, ICP, helicons
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.70.Kz	Optical (ultraviolet, visible, infrared) measurements
52.55.Lf	Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps
52.40.Hf	Plasma-material interactions; boundary layer effects
52.27.Jt	Nonneutral plasmas

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Numerical study on rectangular microhollow cathode discharge

Shoujie He, Jiting Ouyang, Feng He, and Shang Li

Phys. Plasmas 18, 032102 (2011); http://dx.doi.org/10.1063/1.3555528 (6 pages) | Cited 2 times

Online Publication Date: 1 March 2011

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Rectangular microhollow cathode discharge in argon is investigated by using two-dimensional time-dependent self-consistent fluid model. The electric potential, electric field, particle density, and mean electron energy are calculated. The results show that hollow cathode effect can be onset in the present configuration, with strong electric field and high mean electron energy in the cathode fall while high density and quasineutral plasma in the negative glow. The potential well and electric filed reversal are formed in the negative glow region. It is suggested that the presence of large electron diffusion flux necessitates the field reversal and potential well.

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52.80.Hc	Glow; corona
52.25.Fi	Transport properties
52.65.-y	Plasma simulation

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Generalized matching criterion for electrostatic ion solitary propagations in quasineutral magnetized plasmas

M. Akbari-Moghanjoughi

Phys. Plasmas 18, 032103 (2011); http://dx.doi.org/10.1063/1.3561779 (7 pages) | Cited 3 times

Online Publication Date: 3 March 2011

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Based on the magnetohydrodynamics model, an exact arbitrary-amplitude general solution is presented for oblique propagation of solitary excitations in two- and three-component quasineutral magnetoplasmas, adopting the standard pseudopotential approach. It is revealed that the necessary matching criterion of existence of such oblique nonlinear propagations in two- and three-fluid magnetoplasmas possesses global features. These features are examined for the cases of electron-ion and electron-positron-ion magnetoplasmas with diverse equations of state. This study also reveals that for electron-ion magnetoplasmas with plasma frequencies larger than the cyclotron frequency (B₀<0.137 math

) a critical angle of β_cr = arccos[B₀/(0.137 math

)] exists at which propagation of solitary excitation is not possible. The Coriolis effect on allowed soliton matching condition in rotating magnetoplasmas is also considered as an extension to this work. Current investigation can have important implications for nonlinear wave dynamics in astrophysical as well as laboratory magnetoplasmas.

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52.30.Ex	Two-fluid and multi-fluid plasmas
52.35.-g	Waves, oscillations, and instabilities in plasmas and intense beams
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

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Transverse instability and magnetic structures associated with electron phase space holes

Aimin Du, Mingyu Wu, Quanming Lu, Can Huang, and Shui Wang

Phys. Plasmas 18, 032104 (2011); http://dx.doi.org/10.1063/1.3561796 (4 pages) | Cited 2 times

Online Publication Date: 8 March 2011

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Electron phase space holes (electron holes) are found to be unstable to the transverse instability. Two-dimensional (2D) electromagnetic particle-in-cell simulations are performed to investigate the structures of the fluctuating magnetic field associated with electron holes. The combined actions between the transverse instability and the stabilization by the background magnetic field (B₀ = B₀ math

_x) lead a one-dimensional electron hole into several 2D electron holes which are isolated in both the x and y directions. The electrons trapped in these 2D electron holes suffer the electric field drift v_E = E×B₀/B02 due to the existence of the perpendicular electric field E_y, which generates the current along the z direction. Then, the unipolar and bipolar structures are formed for the parallel cut of the fluctuating magnetic field along the x and y directions, respectively. At the same time, these 2D electron holes move along the x direction, and the unipolar structures are formed for the parallel cut of the fluctuating magnetic field along the z direction.

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52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi	Transport properties
52.25.Gj	Fluctuation and chaos phenomena
52.65.Rr	Particle-in-cell method

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Multipactor theory for multicarrier signals

S. Anza, M. Mattes, C. Vicente, J. Gil, D. Raboso, V. E. Boria, and B. Gimeno

Phys. Plasmas 18, 032105 (2011); http://dx.doi.org/10.1063/1.3561821 (12 pages) | Cited 1 time

Online Publication Date: 8 March 2011

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This work presents a new theory of multipactor under multicarrier signals for parallel-plate geometries, assuming a homogeneous electric field and one-dimensional electron motion. It is the generalization of the nonstationary multipactor theory for single-carrier signals [ S. Anza et al.,Phys. Plasmas 17, 062110 (2010) ]. It is valid for multicarrier signals with an arbitrary number of carriers with different amplitude, arbitrary frequency, and phase conditions and for any material coating. This new theory is able to model the real dynamics of the electrons during the multipactor discharge for both single and double surface interactions. Among other parameters of the discharge, it calculates the evolution in time of the charge growth, electron absorption, and creation rates as well as the instantaneous secondary emission yield and order. An extensive set of numerical tests with particle-in-cell software has been carried out in order to validate the theory under many different conditions. This theoretical development constitutes the first multipactor theory which completely characterizes the multipactor discharge for arbitrary multicarrier signals, setting the first step for further investigations in the field.

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52.80.Pi	High-frequency and RF discharges
52.40.Hf	Plasma-material interactions; boundary layer effects
52.25.Fi	Transport properties
52.65.Rr	Particle-in-cell method
79.20.Hx	Electron impact: secondary emission

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Anisotropic dissipative effects on the buoyancy instability with background heat flux

Haijun Ren, Jintao Cao, Zhengwei Wu, and Paul K. Chu

Phys. Plasmas 18, 032106 (2011); http://dx.doi.org/10.1063/1.3563593 (9 pages)

Online Publication Date: 8 March 2011

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The linear buoyancy instability in magnetized plasmas is investigated in the presence of anisotropic resistivity and viscosity by taking into account the background heat flux. The magnetic field is assumed to be homogeneous and has both horizontal and vertical components. The heat is primarily transported along the magnetic force lines when the gyro radius is much less than the mean collision free path. The Hall term is examined first and shows a damping effect on the magnetothermal instability. The heat-flux-driven buoyancy instability (HBI) is then investigated by taking into account the parallel resistivity (PR), cross-field resistivity (CR), and the anisotropic viscosity. The general dispersion relation (DR) is derived and discussed in several special cases. We show that only the CR and viscosity exert effects on the DR in the first case. The critical condition for the occurrence of HBI is modified by the CR coupled with the viscosity and the value of the instability growth rate is diminished by them. The effects due to the PR (resp. viscosity) on the HBI are examined next. The PR (resp. viscosity) is shown to alter not only the growth rate but also the instability criterion. There exists an unstable mode when the temperature decreases in the direction of gravity while this case is proven to be magnetothermally stable in the ideal magnetohydrodynamic limit. A new unstable mode is solely induced by the presence of PR (resp. viscosity). When the PR and CR are both taken into account, the resistivity is shown to induce a damping mode rather than an instability. Finally, considering the PR and viscosity simultaneously, it is found that a new unstable mode is excited when the PR is not equal to the viscosity, or else, dissipation effects do not alter the instability criterion and just cut down the growth rate.

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52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
47.55.P-	Buoyancy-driven flows; convection
52.25.Kn	Thermodynamics of plasmas

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Ionization-induced dynamics of laser-matter interaction in a focused laser pulse: A comparative analysis

E. S. Efimenko, A. V. Kim, and M. Quiroga-Teixeiro

Phys. Plasmas 18, 032107 (2011); http://dx.doi.org/10.1063/1.3558726 (11 pages)

Online Publication Date: 11 March 2011

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We study the laser-matter interaction via optical-field ionization of a medium in a focused ultrashort laser pulse by means of the finite-difference-time-domain modeling of Maxwell’s equations. General aspects of the ionization-induced dynamics with TE- and TM-polarized laser pulses are analyzed. It is shown that there are two qualitatively different regimes of the interaction depending on the angle of the laser beam focusing. At comparatively low angles plasma distributions are smooth; however, due to departure from the quasioptical behavior of light rays in self-generated plasma, lateral large-scaled plasma structures can also be produced, leading to additional beam focusing and correspondingly to higher electron densities. At tight focusing, small-scaled plasma structures are generated that strongly influence field distribution, energy deposition, and scattering characteristics. The influence of electron collisions and the Kerr effect are also analyzed.

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42.25.Bs	Wave propagation, transmission and absorption
52.25.Jm	Ionization of plasmas
52.50.Jm	Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)

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Rotation of a magnetized plasma

B. M. Annaratone, A. Escarguel, T. Lefevre, C. Rebont, N. Claire, and F. Doveil

Phys. Plasmas 18, 032108 (2011); http://dx.doi.org/10.1063/1.3566004 (4 pages) | Cited 3 times

Online Publication Date: 15 March 2011

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The plasma rotation in the axial magnetic field of the linear machine Mistral [ A. Escarguel, Eur. Phys. J. D 56, 209 (2010)] is well described by the assumption that the electrons injected from the source exit radially from the central column and are subject to the Lorentz force. Electrons and ions rotate together by ambipolarity. The solution of the momentum equations foresees correctly the observed radial dependence of the ionic radial velocity measured by laser induced fluorescence. The resolution of these equations is also in good agreement with the measured dependence of the rotation frequency on the applied magnetic field and on the background pressure.

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52.25.Vy	Impurities in plasmas
52.90.+z	Other topics in physics of plasmas and electric discharges (restricted to new topics in section 52)
81.10.Mx	Growth in microgravity environments
82.70.Dd	Colloids

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Methodology for turbulence code validation: Quantification of simulation-experiment agreement and application to the TORPEX experiment

Paolo Ricci, C. Theiler, A. Fasoli, I. Furno, K. Gustafson, D. Iraji, and J. Loizu

Phys. Plasmas 18, 032109 (2011); http://dx.doi.org/10.1063/1.3559436 (11 pages) | Cited 11 times

Online Publication Date: 16 March 2011

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A methodology for plasma turbulence code validation is discussed, focusing on quantitative assessment of the agreement between experiments and simulations. The present work extends the analysis carried out in a previous paper [ P. Ricci et al., Phys. Plasmas 16, 055703 (2009) ] where the validation observables were introduced. Here, it is discussed how to quantify the agreement between experiments and simulations with respect to each observable, how to define a metric to evaluate this agreement globally, and—finally—how to assess the quality of a validation procedure. The methodology is then applied to the simulation of the basic plasma physics experiment TORPEX [ A. Fasoli et al., Phys. Plasmas 13, 055902 (2006) ], considering both two-dimensional and three-dimensional simulation models.

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52.35.Ra	Plasma turbulence
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Gj	Fluctuation and chaos phenomena
52.70.Ds	Electric and magnetic measurements
52.25.Xz	Magnetized plasmas
52.65.Kj	Magnetohydrodynamic and fluid equation

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Numerical simulation of laminar plasma dynamos in a cylindrical von Kármán flow

I. V. Khalzov, B. P. Brown, F. Ebrahimi, D. D. Schnack, and C. B. Forest

Phys. Plasmas 18, 032110 (2011); http://dx.doi.org/10.1063/1.3559472 (9 pages) | Cited 4 times

Online Publication Date: 24 March 2011

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The results of a numerical study of the magnetic dynamo effect in cylindrical von Kármán plasma flow are presented with parameters relevant to the Madison Plasma Couette Experiment. This experiment is designed to investigate a broad class of phenomena in flowing plasmas. In a plasma, the magnetic Prandtl number Pm can be of order unity (i.e., the fluid Reynolds number Re is comparable to the magnetic Reynolds number Rm). This is in contrast to liquid metal experiments, where Pm is small (so, Re⪢Rm) and the flows are always turbulent. We explore dynamo action through simulations using the extended magnetohydrodynamic NIMROD code for an isothermal and compressible plasma model. We also study two-fluid effects in simulations by including the Hall term in Ohm’s law. We find that the counter-rotating von Kármán flow results in sustained dynamo action and the self-generation of magnetic field when the magnetic Reynolds number exceeds a critical value. For the plasma parameters of the experiment, this field saturates at an amplitude corresponding to a new stable equilibrium (a laminar dynamo). We show that compressibility in the plasma results in an increase of the critical magnetic Reynolds number, while inclusion of the Hall term in Ohm’s law changes the amplitude of the saturated dynamo field but not the critical value for the onset of dynamo action.

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52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj	Magnetohydrodynamic and fluid equation
47.65.Md	Plasma dynamos

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Nonlinear collisionless plasma wakes of small particles

I. H. Hutchinson

Phys. Plasmas 18, 032111 (2011); http://dx.doi.org/10.1063/1.3562885 (10 pages) | Cited 9 times

Online Publication Date: 24 March 2011

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The wake behind a spherical particle smaller than the Debye length (λ_De) in flowing plasma is calculated using a particle-in-cell code. The results with different magnitudes of charge reveal substantial nonlinear effects down to values that for a floating particle would correspond to a particle radius ∼ 10⁻²λ_De. The peak potential in the oscillatory wake structure is strongly suppressed by nonlinearity, never exceeding ∼ 0.4 times the unperturbed ion energy. By contrast, the density peak arising from ion focusing can be many times the ambient. Strong heating of the ions occurs in the nonlinear regime. Direct ion absorption by the particle is not important for the far wake unless the radius exceeds 10⁻¹λ_De, and is therefore never significant (for the far wake) in the linear regime. Reasonable agreement with full-scale linear response calculations are obtained in the linear regime. The wake wavelength is confirmed and an explanation, in terms of the conical potential structure, is proposed for experimentally-observed oblique alignment of different-sized grains.

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52.65.-y	Plasma simulation
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

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Gyrokinetic analysis of tearing instabilities in a collisionless plasma

A. K. Sundaram and A. Sen

Phys. Plasmas 18, 032112 (2011); http://dx.doi.org/10.1063/1.3568837 (8 pages)

Online Publication Date: 24 March 2011

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Using a gyrokinetic description, an analytic investigation of tearing instabilities is carried out for a collisionless tokamak plasma, with particular emphasis on delineating the effects associated with Landau and ∇B resonances. The linear characteristics of Δ′-driven tearing modes are studied by including short wavelength variations across the confining magnetic field and long wavelength variations along the field. For the case when electrons are adiabatic and ions are fluidlike, the dispersion relation is solved analytically for mode widths lying between electron and ion excursion lengths. It is shown that electron Landau damping effect can significantly influence the tearing mode growth rate by making it proportional to (Δ′)^1/2 in contrast to earlier kinetic results, which show a linear dependence on Δ′. The growth rate can further slow down when compressional mode coupling effects are taken into account. Likewise, analytic conditions for the growth of the gyrokinetic tearing mode in the presence of electron ∇B resonance effect are obtained for both the Δ′ driven global mode as well as the large Δ′ branch of this instability and expressions for the real frequency and growth rate of the modes are given. Our analytic results, besides providing physical insights into the influence of these ‘resonance’ effects, can also serve as useful benchmark signatures to look for in large scale numerical gyrokinetic simulations.

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52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Fa	Tokamaks, spherical tokamaks

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Generation of shear Alfvén waves by a rotating magnetic field source: Three-dimensional simulations

A. V. Karavaev, N. A. Gumerov, K. Papadopoulos, Xi Shao, A. S. Sharma, W. Gekelman, Y. Wang, B. Van Compernolle, P. Pribyl, and S. Vincena

Phys. Plasmas 18, 032113 (2011); http://dx.doi.org/10.1063/1.3562118 (13 pages) | Cited 1 time

Online Publication Date: 25 March 2011

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multimedia

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The paper discusses the generation of polarized shear Alfvén waves radiated from a rotating magnetic field source created via a phased orthogonal two-loop antenna. A semianalytical three-dimensional cold two-fluid magnetohydrodynamics model was developed and compared with recent experiments in the University of California, Los Angeles large plasma device. Comparison of the simulation results with the experimental measurements and the linear shear Alfvén wave properties, namely, spatiotemporal wave structure, a dispersion relation with nonzero transverse wave number, the magnitude of the wave dependences on the wave frequency, show good agreement. From the simulations it was found that the energy of the Alfvén wave generated by the rotating magnetic field source is distributed between the kinetic energy of ions and electrons and the electromagnetic energy of the wave as: ∼ 1/2 is the energy of the electromagnetic field, ∼ 1/2 is the kinetic energy of the ion fluid, and ∼ 2.5% is the kinetic energy of electron fluid for the experiment. The wave magnetic field power calculated from the experimental data and using a fluid model differ by ∼ 1% and is ∼ 250 W for the experimental parameters. In both the experiment and the three-dimensional two-fluid magnetohydrodynamics simulations the rotating magnetic field source was found to be very efficient for generating shear Alfvén waves.

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52.35.Bj	Magnetohydrodynamic waves (e.g., Alfven waves)
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg	Plasma kinetic equations
52.65.Kj	Magnetohydrodynamic and fluid equation

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The particle distributions of asymmetric kinetic electrostatic structures

L. Nocera and L. J. Palumbo

Phys. Plasmas 18, 032114 (2011); http://dx.doi.org/10.1063/1.3562875 (5 pages) | Cited 2 times

Online Publication Date: 25 March 2011

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We give the energy distributions of electrons and ions supporting a steady state electrostatic structure in a collisionless plasma. The electric potential of the structure is skew asymmetrically distributed in space. We show that the jump discontinuous, logarithmically singular electron and ion distributions may be reduced to elliptic integrals. We give the coefficients of the logarithmic terms and the jumps at the discontinuities and we show that they are reciprocally proportional. We calculate bounds for the potential skew asymmetry and show that these bounds are regulated by the boundary conditions of the particle distributions. Despite singularities, our treatment reproduces a smooth space distribution of the potential amplitude and electron and ion distributions that are smooth at one of the boundaries of the electrostatic structure.

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52.35.Sb	Solitons; BGK modes
94.05.Fg	Solitons and solitary waves
94.30.Tz	Electromagnetic wave propagation
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.72.+v	Laboratory studies of space- and astrophysical-plasma processes
52.80.-s	Electric discharges
52.65.Ff	Fokker-Planck and Vlasov equation

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Mode signature and stability for a Hamiltonian model of electron temperature gradient turbulence

E. Tassi and P. J. Morrison

Phys. Plasmas 18, 032115 (2011); http://dx.doi.org/10.1063/1.3569850 (13 pages) | Cited 1 time

Online Publication Date: 29 March 2011

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Stability properties and mode signature for equilibria of a model of electron temperature gradient (ETG) driven turbulence are investigated by Hamiltonian techniques. After deriving new infinite families of Casimir invariants, associated with the noncanonical Poisson bracket of the model, a sufficient condition for stability is obtained by means of the Energy-Casimir method. Mode signature is then investigated for linear motions about homogeneous equilibria. Depending on the sign of the equilibrium “translated” pressure gradient, stable equilibria can either be energy stable, i.e., possess definite linearized perturbation energy (Hamiltonian), or spectrally stable with the existence of negative energy modes. The ETG instability is then shown to arise through a Kreĭn-type bifurcation, due to the merging of a positive and a negative energy mode, corresponding to two modified drift waves admitted by the system. The Hamiltonian of the linearized system is then explicitly transformed into normal form, which unambiguously defines mode signature. In particular, the fast mode turns out to always be a positive energy mode, whereas the energy of the slow mode can have either positive or negative sign. A reduced model with stable equilibria shear flow that possess a continuous spectrum is also analyzed and brought to normal form by a special integral transform. In this way it is seen how continuous spectra can have signature as well.

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52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Kt	Drift waves
52.35.Ra	Plasma turbulence
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

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Modulational instability of ion-acoustic waves in plasmas with superthermal electrons

H. Gharaee, S. Afghah, and H. Abbasi

Phys. Plasmas 18, 032116 (2011); http://dx.doi.org/10.1063/1.3566006 (5 pages) | Cited 1 time

Online Publication Date: 31 March 2011

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Using the reductive perturbation technique, the modulational instability of ion- acoustic waves in a plasma containing superthermal electrons is studied. It is found that the presence of superthermal electrons significantly changes the instability domain. A Lorentzian (kappa) velocity distribution function is used to model superthermal electrons. It is shown that the presence of superthermal electrons reduces the critical frequency of the modulational instability of ion-acoustic waves. Besides, due to the presence of the superthermal electrons, ion-acoustic waves are unstable on a vaster region. Moreover, the modulational instability growth rate is larger for a larger population of superthermal electrons.

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94.30.Kq	Electric fields, field-aligned currents and current systems, and ring currents
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Sb	Solitons; BGK modes
94.30.Tz	Electromagnetic wave propagation