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May 2008

Volume 15, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Phys. Plasmas 15, 052701 (2008); http://dx.doi.org/10.1063/1.2912457 (6 pages)

B. I. Cho, G. M. Dyer, S. Kneip, S. Pikuz, D. R. Symes, A. C. Bernstein, Y. Sentoku, N. Renard-Le Galloudec, T. E. Cowan, and T. Ditmire
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Interaction between turbulence and a nonlinear tearing mode in the low β regime

F. Militello, F. L. Waelbroeck, R. Fitzpatrick, and W. Horton

Phys. Plasmas 15, 050701 (2008); http://dx.doi.org/10.1063/1.2917915 (4 pages) | Cited 18 times

Online Publication Date: 22 May 2008

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The interaction between turbulence and a nonlinear tearing mode is investigated numerically using a 2D electrostatic model. Turbulence is found to cause transitions between the different roots for the propagation velocity of the mode. The transitions take the mode towards roots with slower propagation that are characterized by a locally flattened density profile. For sufficiently large islands the transition reduces the drive for the tearing mode.
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52.35.Ra Plasma turbulence
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Growth mechanism for spherical carbon particles in a dc methane plasma

Tatsuzo Nagai, Zongbao Feng, Akihiko Kono, and Fumiya Shoji

Phys. Plasmas 15, 050702 (2008); http://dx.doi.org/10.1063/1.2917912 (4 pages) | Cited 6 times

Online Publication Date: 23 May 2008

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The growth mechanism for spherical carbon particles of micron sizes observed in a vertically excited CH4/Ar columnar plasma [ F. Shoji, Z. Feng, A. Kono, and T. Nagai, Appl. Phys. Lett. 89, 171504 (2006) ] is investigated theoretically, based on a model in which the particles are negatively charged in the plasma sheath region where they grow by capturing graphite onions with a diameter of ca. 10 nm and a positive charge. A balance of gravity and electric force keeps the particles in the sheath region during their growth. It is found that the particle radius initially increases linearly with time and then approaches a saturation radius, and that the center of gravity of the particle executes a simple harmonic oscillation about its balance position with a characteristic frequency of the order of 10 Hz determined by its specific charge, gravity, and sheath structure.
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52.40.Kh Plasma sheaths

Production of cumulative jets by ablatively-driven implosion of hollow cones and wedges

S. P. Nikitin, J. Grun, Y. Aglitskiy, C. Manka, D. Zabetakis, A. L. Velikovich, and C. Miller

Phys. Plasmas 15, 050703 (2008); http://dx.doi.org/10.1063/1.2917917 (4 pages) | Cited 1 time

Online Publication Date: 28 May 2008

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Cumulative plasma jets formed by hollow cones imploded via laser ablation of their outer surfaces were observed. The velocity, shape, and density of the jets are measured with monochromatic 0.65 keV x-ray imaging. Depending on cone geometry, cumulative jets with ion density ∼ 2×1020 cm−3 and propagation velocities >10 km/s are formed. Similar results are observed when jets are formed by imploding wedges. Such jets can be used to simulate hydrodynamics of astrophysical jets interacting with stellar or interstellar matter.
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52.75.-d Plasma devices
52.38.Mf Laser ablation
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back to top Basic Plasma Phenomena, Waves, Instabilities

Dynamics of the excitation of an upper hybrid wave by a rippled laser beam in magnetoplasma

Gunjan Purohit, P. K. Chauhan, and R. P. Sharma

Phys. Plasmas 15, 052101 (2008); http://dx.doi.org/10.1063/1.2908353 (10 pages) | Cited 2 times

Online Publication Date: 1 May 2008

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This paper presents the effects of a laser spike (superimposed on an intense laser beam) and a static magnetic field on the excitation of the upper hybrid wave (UHW) in a hot collisionless magnetoplasma, taking into account the relativistic nonlinearity. The laser beam is propagating perpendicular to the static magnetic field and has its electric vector polarized along the direction of the static magnetic field (ordinary mode). Analytical expressions for the growth rate of the ripple, the beam width of the rippled laser beam, and the UHW have been obtained. It is found that the coupling among the main laser beam, ripple, and UHW is strong. The ripple gets focused when the initial power of the laser beam is greater than the critical power for focusing. It has been shown that the presence of a laser spike affects significantly the growth rate and the dynamics of the UHW. In addition, it has been seen that the effect of changing the strength of the static magnetic field on the nonlinear coupling and on the dynamics of the excitation of the UHW is significant. The results are presented for typical laser plasma parameters.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.38.-r Laser-plasma interactions
52.25.Xz Magnetized plasmas

Exceptional points in linear gyrokinetics

M. Kammerer, F. Merz, and F. Jenko

Phys. Plasmas 15, 052102 (2008); http://dx.doi.org/10.1063/1.2909618 (7 pages) | Cited 14 times

Online Publication Date: 1 May 2008

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When performing linear gyrokinetic simulations, it is found that various types of microinstabilities, which are usually considered as strictly separated, can actually be transformed into each other via continuous variations of the plasma parameters. This behavior can be explained in terms of so-called exceptional points, which have their origin in the non-Hermiticity of the linear gyrokinetic operator and also occur in many other branches of physics. As a consequence, in large regions of parameter space, the designation of unstable modes should be done very carefully or even be avoided altogether.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Dg Plasma kinetic equations

Nonaxisymmetric magnetorotational instability in ideal and viscous plasmas

A. B. Mikhailovskii, J. G. Lominadze, R. M. O. Galvão, A. P. Churikov, N. N. Erokhin, A. I. Smolyakov, and V. S. Tsypin

Phys. Plasmas 15, 052103 (2008); http://dx.doi.org/10.1063/1.2907788 (10 pages) | Cited 8 times

Online Publication Date: 2 May 2008

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The excitation of magnetorotational instability (MRI) in rotating laboratory plasmas is investigated. In contrast to astrophysical plasmas, in which gravitation plays an important role, in laboratory plasmas it can be neglected and the plasma rotation is equilibrated by the pressure gradient. The analysis is restricted to the simple model of a magnetic confinement configuration with cylindrical symmetry, in which nonaxisymmetric perturbations are investigated using the local approximation. Starting from the simplest case of an ideal plasma, the corresponding dispersion relations are derived for more complicated models including the physical effects of parallel and perpendicular viscosities. The Friemann–Rotenberg approach used for ideal plasmas is generalized for the viscous model and an analytical expression for the instability boundary is obtained. It is shown that, in addition to the standard effect of radial derivative of the rotation frequency (the Velikhov effect), which can be destabilizing or stabilizing depending on the sign of this derivative in the ideal plasma, there is a destabilizing effect proportional to the fourth power of the rotation frequency, or, what is the same, to the square of the plasma pressure gradient, and to the square of the azimuthal mode number of the perturbations. It is shown that the instability boundary also depends on the product of the plasma pressure and density gradients, which has a destabilizing effect when it is negative. In the case of parallel viscosity, the MRI looks like an ideal instability independent of viscosity, while, in the case of strong perpendicular viscosity, it is a dissipative instability with the growth rate inversely proportional to the characteristic viscous decay rate. We point out, however, that the modes of the continuous range of the magnetohydrodynamics spectrum are not taken into account in this paper, and they can be more dangerous than those that are considered.
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52.30.−q
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Saturation of tearing modes in reversed field pinches with locally linear force-free magnetic fields and its application to quasi-single-helicity states

E. Tassi, F. Militello, F. Porcelli, and R. J. Hastie

Phys. Plasmas 15, 052104 (2008); http://dx.doi.org/10.1063/1.2913263 (11 pages)

Online Publication Date: 9 May 2008

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A simple formula for predicting the width of a saturated island, formed as a consequence of tearing perturbation of linear force-free fields in cylindrical geometry, is derived. The formula makes it possible to calculate the saturated island width in terms of the values of parameters characterizing the initial force-free equilibrium and can be applied to equilibria of interest for reversed field pinches. In particular it is applied, in this paper, to force-free equilibria with piecewise constant radial profile of the pinch parameter, which have been recently suggested to be relevant for the formation of quasi-single-helicity states. The main result is that the island width becomes larger as a parameter, that quantifies the departure from a relaxed Taylor state, increases.
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52.55.Ez Theta pinch
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Modulational instability of magnetosonic waves in a spin 1/2 quantum plasma

A. P. Misra and P. K. Shukla

Phys. Plasmas 15, 052105 (2008); http://dx.doi.org/10.1063/1.2913265 (10 pages) | Cited 9 times

Online Publication Date: 12 May 2008

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The modulational instability (MI) of magnetosonic waves (MSWs) is analyzed, by using a two-fluid quantum magnetohydrodynamic model that includes the effects of the electron-1/2 spin and the plasma resistivity. The envelope modulation is then studied by deriving the corresponding nonlinear Schrödinger equation from the governing equations. The plasma resistivity is shown to play a dissipative role for the onset of MI. In the absence of resistivity, the microscopic spin properties of electrons can also lead to MI. In such a situation, the dominant spin contribution corresponds to a dense quantum plasma with the particle number density, n0≳1028 m−3. Also, in such a dissipative (absorbing) medium, where the group velocity vector is usually complex for real values of the wave vector, the role of the real group velocity in the propagation of one-dimensional MSW packets in a homogeneous absorbing medium is reported. The effects of quantum spin on the stability/instability conditions of the magnetosonic envelope are obtained and examined numerically. From the nonlinear dispersion relation of the modulated wave packet it is found that the effect of the spin (plasma resistivity) is to decrease (increase) the instability growth rate provided the normalized Zeeman energy does not exceed a critical value. The theoretical results may have relevance to astrophysical (e.g., magnetars) as well as to ultracold laboratory plasmas (e.g., Rydberg plasmas).
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties

Calculation of resistive magnetohydrodynamics and two-fluid tearing modes by example of reversed-field-pinch-like plasma

V. A. Svidzinski and H. Li

Phys. Plasmas 15, 052106 (2008); http://dx.doi.org/10.1063/1.2917916 (11 pages) | Cited 4 times

Online Publication Date: 15 May 2008

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An algorithm suitable for numerical solution of linear eigenmode problems in resistive magnetohydrodynamics (MHD) and two-fluid MHD models without prior approximations is presented. For these plasma models, sets of equations suitable for numerical solution are derived and the details of the algorithm of this solution are given. The algorithm is general and is suitable for solution of boundary (eigenmode) problems for different plasma configurations. It is most effective, however, in one-dimensional models since the grid size has to be sufficiently small in order to resolve the tearing layer together with the scale of the size of the plasma. The technique is applied for solving for tearing eigenmodes in reversed field pinch (RFP) -like plasma in plane geometry. Results of resistive MHD and two-fluid models are compared in this case, showing that the two-fluid effects on tearing modes in RFPs are sizable.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Kj Magnetohydrodynamic and fluid equation

Parametric conversion of a lower hybrid wave into a whistler in a plasma

Pawan Kumar and V. K. Tripathi

Phys. Plasmas 15, 052107 (2008); http://dx.doi.org/10.1063/1.2918343 (4 pages) | Cited 4 times

Online Publication Date: 27 May 2008

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A large amplitude lower hybrid wave parametrically decays into a whistler wave and a low frequency lower hybrid wave in a plasma. The density perturbation due to the low frequency wave couples with the oscillatory velocity due to the pump wave to produce a nonlinear current, driving the whistler. The pump and whistler exert a low frequency ponderomotive force on electrons, driving the lower hybrid decay wave. The growth rate of the parametric instability scales linearly with the amplitude of the pump wave. It decreases with the electron cyclotron frequency. The process is relevant to beam plasma systems where lower hybrid waves are excited with greater ease and the whistler sideband wave can be seen outside the plasma as electromagnetic emission.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
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)
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The dispersive Alfvén wave in the time-stationary limit with a focus on collisional and warm-plasma effects

S. M. Finnegan, M. E. Koepke, and D. J. Knudsen

Phys. Plasmas 15, 052108 (2008); http://dx.doi.org/10.1063/1.2890774 (13 pages)

Online Publication Date: 28 May 2008

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A nonlinear, collisional, two-fluid model of uniform plasma convection across a field-aligned current (FAC) sheet, describing the stationary Alfvén (StA) wave, is presented. In a previous work, Knudsen showed that, for cold, collisionless plasma [ D. J. Knudsen, J. Geophys. Res. 101, 10761 (1996) ], the stationary inertial Alfvén (StIA) wave can accelerate electrons parallel to a background magnetic field and cause large, time-independent plasma-density variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. Knudsen suggested that these fundamental properties of the StIA wave may play a role in the formation of discrete auroral arcs. Here, Knudsen’s model has been generalized for warm, collisional plasma. From this generalization, it is shown that nonzero ion-neutral and electron-ion collisional resistivity significantly alters the perpendicular ac and dc structure of magnetic-field-aligned electron drift, and can either dissipate or enhance the field-aligned electron energy depending on the initial value of field-aligned electron drift velocity. It is also shown that nonzero values of plasma pressure increase the dominant Fourier component of perpendicular wavenumber.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
94.05.Pt Wave/wave, wave/particle interactions
94.20.Bb Wave propagation
94.20.W- Ionospheric dynamics and interactions
94.20.wf Plasma waves and instabilities
94.20.wj Wave/particle interactions
94.20.wc Plasma motion; plasma convection; particle acceleration

Nonlocal magnetorotational instability

A. B. Mikhailovskii, J. G. Lominadze, R. M. O. Galvão, A. P. Churikov, O. A. Kharshiladze, N. N. Erokhin, and C. H. S. Amador

Phys. Plasmas 15, 052109 (2008); http://dx.doi.org/10.1063/1.2913613 (8 pages) | Cited 7 times

Online Publication Date: 29 May 2008

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An analytical theory of the nonlocal magnetorotational instability (MRI) is developed for the simplest astrophysical plasma model. It is assumed that the rotation frequency profile has a steplike character, so that there are two regions in which it has constant different values, separated by a narrow transition layer. The surface wave approach is employed to investigate the MRI in this configuration. It is shown that the main regularities of the nonlocal MRI are similar to those of the local instability and that driving the nonaxisymmetric MRI is less effective than the axisymmetric one, also for the case of the nonlocal instability. The existence of nonlocal instabilities in nonmagnetized plasma is predicted.
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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)
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
95.30.Qd Magnetohydrodynamics and plasmas
back to top Nonlinear Phenomena, Turbulence, Transport

Effects of flow shear and Alfvén waves on two-dimensional magnetohydrodynamic turbulence

Jamie Douglas, Eun-jin Kim, and A. Thyagaraja

Phys. Plasmas 15, 052301 (2008); http://dx.doi.org/10.1063/1.2913270 (9 pages) | Cited 2 times

Online Publication Date: 15 May 2008

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The suppression of turbulent transport by large scale mean shear flows and uniform magnetic fields is investigated in two-dimensional magnetohydrodynamic turbulence driven by a small-scale forcing with finite correlation time. By numerical integration the turbulent magnetic diffusivity DT is shown to be significantly quenched, with a scaling DT∝B−2Ω0−5/4, which is much more severe than in the case of a short or delta correlated forcing typified by white noise, studied in E. Kim and B. Dubrulle [Phys. Plasmas 8, 813 (2001)] . Here B and Ω0 are magnetic field strength and flow shear rate, respectively. The forcing with finite correlation time also leads to much stronger suppression of momentum transport through the cancellation of the Reynolds stress by the Maxwell stress with a positive small value of turbulent viscosity, νT>0. While fluctuating kinetic and magnetic energies are unaffected by the magnetic field just as in the case of a delta correlated forcing, they are much more severely quenched by flow shear than in that of a delta correlated forcing. Underlying physical mechanisms for the reduction of turbulent transport and turbulence level by flow shear and magnetic field are discussed.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
52.35.Ra Plasma turbulence

Selective formation of turbulent structures in magnetized cylindrical plasmas

Naohiro Kasuya, Masatoshi Yagi, Kimitaka Itoh, and Sanae-I Itoh

Phys. Plasmas 15, 052302 (2008); http://dx.doi.org/10.1063/1.2912461 (10 pages) | Cited 14 times

Online Publication Date: 16 May 2008

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The mechanism of nonlinear structural formation has been studied with a three-field reduced fluid model, which is extended to describe the resistive drift wave turbulence in magnetized cylindrical plasmas. In this model, ion-neutral collisions strongly stabilize the resistive drift wave, and the formed structure depends on the collision frequency. If the collision frequency is small, modulational coupling of unstable modes generates a zonal flow. On the other hand, if the collision frequency is large, a streamer, which is a localized vortex in the azimuthal direction, is formed. The structure is generated by nonlinear wave coupling and is sustained for a much longer duration than the drift wave oscillation period. This is a minimal model for analyzing the turbulent structural formation mechanism by mode coupling in cylindrical plasmas, and the competitive nature of structural formation is revealed. These turbulent structures affect particle transport.
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Kt Drift waves

Localized electrostatic excitations in a Thomas–Fermi plasma containing degenerate electrons

U. M. Abdelsalam, W. M. Moslem, and P. K. Shukla

Phys. Plasmas 15, 052303 (2008); http://dx.doi.org/10.1063/1.2912963 (5 pages) | Cited 4 times

Online Publication Date: 20 May 2008

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By using the Thomas–Fermi electron density distribution for quantum degenerate electrons, the hydrodynamic equations for ions, and the Poisson equation, planar and nonplanar ion-acoustic solitary waves in an unmagnetized collisionless plasma are investigated. The reductive perturbation method is used to derive cylindrical and spherical Korteweg–de Vries equations. Numerical solutions of the latter are presented. The present results can be useful in understanding the features of small but finite amplitude localized ion-acoustic solitary pulses in a degenerate plasma.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Sb Solitons; BGK modes
52.90.+z Other topics in physics of plasmas and electric discharges (restricted to new topics in section 52)

On particle transport in Weibel generated magnetic fluctuations

R. C. Tautz and A. Shalchi

Phys. Plasmas 15, 052304 (2008); http://dx.doi.org/10.1063/1.2921788 (8 pages) | Cited 2 times

Online Publication Date: 27 May 2008

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The problem of particle scattering in the turbulent magnetic fields generated by the Weibel instability is investigated on the basis of analytical calculations. From the dispersion relation, the growth and damping rate is calculated and inserted into a quasilinear Fokker–Planck coefficient for pitch-angle scattering, from which the scattering parallel mean free path is determined. A weak background magnetic field is included to ensure the validity of the assumption of uncorrelated wave modes, which is essential for quasilinear calculations. It is shown that, for higher values of the temperature and counterstreaming anisotropy, the parallel mean free path is reduced due to the stronger magnetic turbulence generated by the instability. Furthermore, it is shown that for relativistic particles, the parallel mean free path becomes energy independent.
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52.25.Fi Transport properties
52.25.Xz Magnetized plasmas
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Ff Fokker-Planck and Vlasov equation
95.30.Qd Magnetohydrodynamics and plasmas

Ambipolar stochastic particle diffusion and plasma rotation

A. Wingen and K. H. Spatschek

Phys. Plasmas 15, 052305 (2008); http://dx.doi.org/10.1063/1.2912454 (8 pages) | Cited 7 times

Online Publication Date: 28 May 2008

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The motion of electrons and ions in stochastic magnetic fields is considered. The analysis starts from a Hamiltonian formulation of the drift motion including electric fields. For an efficient statistical evaluation of the resulting particle transport, a symplectic mapping technique is applied. Compared to previous considerations, the ion and electron test particle motion are investigated simultaneously, allowing calculations of the ambipolar electric field and its influence on stochastic transport. The predictions based on the relativistic drift model are applied to the magnetic perturbations in the TEXTOR-DED [ A. Wingen et al., Nucl. Fusion 46, 941 (2006) ]. The influence of the magnetic coil arrangement on the poloidal plasma rotation, caused by the generated radial electric field, is discussed.
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52.30.-q Plasma dynamics and flow
05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion
05.60.-k Transport processes
52.35.Kt Drift waves

The momentum flux probability distribution function for ion-temperature-gradient turbulence

Johan Anderson and Eun-jin Kim

Phys. Plasmas 15, 052306 (2008); http://dx.doi.org/10.1063/1.2918338 (7 pages) | Cited 8 times

Online Publication Date: 28 May 2008

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There has been overwhelming evidence that coherent structures play a critical role in determining the overall transport in a variety of systems. We compute the probability distribution function (PDF) tails of momentum flux and heat flux in ion-temperature-gradient turbulence, by taking into account the interaction among modons, which are assumed to be coherent structures responsible for bursty and intermittent events, contributing to the PDF tails. The tail of PDF of momentum flux R = 〈vxvy〉 is shown to be exponential with the form exp{−ξR3/2}, which is broader than a Gaussian, similar to what was found in the previous local studies. An analogous expression with the same functional dependence is found for the PDF tails of heat flux. Furthermore, we present a detailed numerical study of the dependence of the PDF tail on the temperature and density scale lengths and other physical parameters through the coefficient ξ.
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52.25.Fi Transport properties
52.35.Ra Plasma turbulence

Gyrokinetic calculations of diffusive and convective transport of α particles with a slowing-down distribution function

C. Angioni and A. G. Peeters

Phys. Plasmas 15, 052307 (2008); http://dx.doi.org/10.1063/1.2913610 (15 pages) | Cited 19 times

Online Publication Date: 29 May 2008

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Quasilinear gyrokinetic calculations of the transport of fast α particles with a slowing-down equilibrium distribution function in the trace limit are presented. Diffusive and convective contributions to the total flux are separated and their dependence on the ratio of the fast particle energy to the background plasma temperature is investigated. The results are compared with those obtained in the case an equivalent Maxwellian distribution function is assumed for the fast particles. On the basis of the gyrokinetic results, simple models for α particle transport are proposed for transport modeling purposes.
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52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Fa Tokamaks, spherical tokamaks
52.35.Ra Plasma turbulence

Long global gyrokinetic simulations: Source terms and particle noise control

B. F. McMillan, S. Jolliet, T. M. Tran, L. Villard, A. Bottino, and P. Angelino

Phys. Plasmas 15, 052308 (2008); http://dx.doi.org/10.1063/1.2921792 (10 pages) | Cited 24 times

Online Publication Date: 29 May 2008

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In global gyrokinetic simulations it takes a long time for the turbulence to reach a quasisteady state, and quantitative predictions about the quasisteady state turbulence have been difficult to obtain computationally. In particular, global particle-in-cell gyrokinetic simulations have been inefficient for long simulations due to the accumulation of noise. It is demonstrated that a simple Krook operator can effectively control noise; it also introduces an unphysical dissipation, which damps the zonal flows and can significantly affect simulation results even when the relaxation time is very long. However, it is possible to project out the effects of the Krook operator on the zonal flows. This permits noise accumulation to be controlled while preserving the physics of interest; simulations are then run to determine the level of quasisteady state transport and the variation across the ensemble of turbulent dynamics. Convergence is demonstrated both in the number of computational particles and the unphysical relaxation time.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
52.65.Rr Particle-in-cell method

Dynamics of turbulent transport dominated by the geodesic acoustic mode near the critical gradient regime

Kazuhiro Miki, Yasuaki Kishimoto, Jiquan Li, and Naoaki Miyato

Phys. Plasmas 15, 052309 (2008); http://dx.doi.org/10.1063/1.2908742 (14 pages) | Cited 3 times

Online Publication Date: 30 May 2008

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The effects of geodesic acoustic modes (GAMs) on the toroidal ion temperature gradient turbulence and associated transport near the critical gradient regime in tokamak plasma are investigated based on global Landau-fluid simulations and extended predator-prey modeling analyses. A new type of intermittent dynamics of transport accompanied with the emission and propagation of the GAMs, i.e., GAM intermittency [ K. Miki et al., Phys. Rev. Lett. 99, 145003 (2007)] , has been found. The intermittent bursts are triggered by the onset of spatially propagating GAMs when the turbulent energy exceeds a critical value. The GAMs suffer collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region. The stationary zonal flows gradually increase due to the accumulation of non-damped residual part over many periods of quasi-periodic intermittent bursts and eventually quench the turbulence, leading to a nonlinear upshift of the linear critical gradient; namely, the Dimits shift. This process is categorized as a new class of transient dynamics, referred to as growing intermittency. The Dimits shift is found to be established through this dynamical process. An extended minimal predator-prey model with collisionless damping of the GAMs is proposed, which qualitatively reproduce the main features of the growing intermittency and approximately predict its various time scales observed in the simulations.
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52.30.-q Plasma dynamics and flow
52.35.Ra Plasma turbulence
52.65.Tt Gyrofluid and gyrokinetic simulations

Existence of non-Landau solutions for Langmuir waves

G. Belmont, F. Mottez, T. Chust, and S. Hess

Phys. Plasmas 15, 052310 (2008); http://dx.doi.org/10.1063/1.2921791 (14 pages) | Cited 4 times

Online Publication Date: 30 May 2008

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The propagation of linear one dimensional (1D) Langmuir waves is reinvestigated using numerical simulations of a new type with very low noise. The dependence of the result on the initial conditions is shown. New solutions are exhibited, with properties different from Landau’s, even in the asymptotic behavior, in particular with regard to the damping rate. These solutions are shown to demand a special preparation of the initial plasma perturbation, but in a way which is quite physical, without any singularity in the electron distribution function, contrary to the classical van Kampen’s solutions. Using an original theoretical calculation, a simple analytical form is derived for the perturbed distribution function, which allows interpreting both the Landau and non-Landau solutions observed numerically. The numerical results presented and their interpretations are potentially important in several respects: 1) They outline that Landau solutions, for the 1D electrostatic problem in collisionless plasmas, are only a few among an infinite amount of others; even if the non-Landau solutions are much less probable, their existence provides a different view on the concept of kinetic damping and may suggest interpretations different from usual for the subsequent nonlinear effects; 2) they show that the shape of the initial perturbation δf(v), and not only its amplitude, is important for the long time wave properties, both linear and nonlinear; 3) the existence of non-Landau solutions makes clear that the classical energy arguments cannot be fully universal as long as they allow deriving the Landau damping rate independently of the initial conditions; 4) the particle signature of Landau damping, different from the usual guess, should imply a change in our understanding of the role of the resonant particles.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.65.-y Plasma simulation

Observation of ion-acoustic shock wave transition due to enhanced Landau damping

H. Bailung, Y. Nakamura, and Y. Saitou

Phys. Plasmas 15, 052311 (2008); http://dx.doi.org/10.1063/1.2918318 (6 pages) | Cited 1 time

Online Publication Date: 30 May 2008

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Ion-acoustic shock waves are observed experimentally introducing strong Landau damping by increasing the ion temperature in a double plasma device. An oscillatory ion-acoustic shock wave undergoes transition with enhanced Landau damping and forms a monotonic shock wave. Numerical results of the Korteweg–de Vries equation with an additional integral term to account for the strength of Landau damping are compared with the experimental findings. Enhancement of Landau damping is found to increase the dissipation of the wave, which manifests in quenching of the oscillatory structure behind the shock front.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Tc Shock waves and discontinuities
52.25.-b Plasma properties
52.75.-d Plasma devices
back to top Magnetically Confined Plasmas, Heating, Confinement

Poloidal motion of trapped particle orbits in real-space coordinates

V. V. Nemov, S. V. Kasilov, W. Kernbichler, and G. O. Leitold

Phys. Plasmas 15, 052501 (2008); http://dx.doi.org/10.1063/1.2912456 (13 pages)

Online Publication Date: 9 May 2008

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The bounce averaged poloidal drift velocity of trapped particles in stellarators is an important quantity in the framework of optimization of stellarators because it allows us to analyze the possibility for closure of contours of the second adiabatic invariant and therefore for improvement of α-particle confinement in such a device. Here, a method is presented to compute such a drift velocity directly in real space coordinates through integration along magnetic field lines. This has the advantage that one is not limited to the usage of magnetic coordinates and can use the magnetic field produced by coil currents and more importantly also results of three-dimensional magnetohydrodynamic finite beta equilibrium codes, such as PIES [ A. H. Reiman and H. S. Greenside, J. Comput. Phys. 75, 423 (1988) ] and HINT [ Y. Suzuki et al., Nucl. Fusion 46, L19 (2006) ].
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52.25.Fi Transport properties
52.20.Dq Particle orbits
52.55.Jd Magnetic mirrors, gas dynamic traps
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation

Lower hybrid instability in a tokamak under neutral beam injection and magnetic shear

Animesh Kuley and V. K. Tripathi

Phys. Plasmas 15, 052502 (2008); http://dx.doi.org/10.1063/1.2912460 (5 pages) | Cited 1 time

Online Publication Date: 22 May 2008

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A slab model is developed to study the excitation of lower hybrid instability triggered by the injection of a transverse neutral beam in a tokamak with magnetic shear. The lower hybrid mode is evanescent in the inner and outer region while propagating waves in the intermediate region. The neutral beam, on getting fully ionized in the plasma, resonantly couples with the lower hybrid wave in the intermediate region, driving the mode unstable. The theory of this process reveals that the growth rate scales as one third power of beam density, and increases significantly with the sheared magnetic field due to modification in the parallel wave number and the mode structure.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.55.Fa Tokamaks, spherical tokamaks
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