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Nov 2008

Volume 15, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

Phys. Plasmas 15, 112701 (2008); http://dx.doi.org/10.1063/1.3008046 (8 pages)

R. E. Madden, S. C. Bott, D. Haas, Y. Eshaq, U. Ueda, G. Collins, and F. N. Beg
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Existence of weakly damped kinetic Alfvén eigenmodes in reversed shear tokamak plasmas

N. N. Gorelenkov

Phys. Plasmas 15, 110701 (2008); http://dx.doi.org/10.1063/1.3027512 (4 pages) | Cited 12 times

Online Publication Date: 21 November 2008

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A kinetic theory of weakly damped Alfvén eigenmode solutions strongly interacting with the continuum is developed for tokamak plasmas with reversed magnetic shear. It is shown that finite Larmor radius (FLR) effects are required for global eigenmode solutions. FLR effects induce multiple kinetic subeigenmodes and collisionless radiative damping. The theory explains the existence of experimentally observed Alfvénic instabilities with frequencies sweeping down and reaching their minimum (bottom).
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Fa Tokamaks, spherical tokamaks
52.25.Dg Plasma kinetic equations
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.)
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back to top Basic Plasma Phenomena, Waves, Instabilities
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Consistent solution for space-charge-limited current in the relativistic regime for monoenergetic initial velocities

Y. Feng and J. P. Verboncoeur

Phys. Plasmas 15, 112101 (2008); http://dx.doi.org/10.1063/1.3003071 (6 pages) | Cited 2 times

Online Publication Date: 6 November 2008

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The solution for space-charge-limited (SCL) currents in electron vacuum diodes with monoenergetic initial velocity is extended to the relativistic regime. Two types of solutions are found: Type I corresponds to zero steady state surface electric field (field emission with high enhancement factor), and Type II corresponds to a finite steady state surface electric field (other emission mechanisms). Our solution compares well to the classical space-charge-limited currents with initial energy and relativistic space-charge limited currents without initial energy in the appropriate limits. The scaling law between the true SCL and the applied voltage is discussed and the two solution types are verified by particle-in-cell simulation.
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52.25.Fi Transport properties
52.65.Rr Particle-in-cell method
41.75.Ht Relativistic electron and positron beams
84.47.+w Vacuum tubes
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The structure of the electron outflow jet in collisionless magnetic reconnection

Michael Hesse, Seiji Zenitani, and Alex Klimas

Phys. Plasmas 15, 112102 (2008); http://dx.doi.org/10.1063/1.3006341 (5 pages) | Cited 6 times

Online Publication Date: 10 November 2008

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Particle-in-cell simulations and analytic theory are applied to the study of the electron outflow jet in collisionless magnetic reconnection. In these jets, which have also been identified in spacecraft observations, electron flow speeds in thin layers exceed the E×B drift, suggesting that electrons are unmagnetized. In this study, we find the surprising result that the electron flow jets can be explained by a combination of E×B drifts and of diamagnetic effects through the combination of the gradients of particle pressure and of the magnetic field. In a suitably rotated coordinate system, the electron motion is readily decomposed into E×B drift and the motion to support the required current density, consistent with electron gyrotropy. This process appears to be nondissipative.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.75.-d Plasma devices
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Theory and simulation of lower-hybrid drift instability for current sheet with guide field

P. H. Yoon, Y. Lin, X. Y. Wang, and A. T. Y. Lui

Phys. Plasmas 15, 112103 (2008); http://dx.doi.org/10.1063/1.3013451 (7 pages) | Cited 6 times

Online Publication Date: 12 November 2008

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The stability of a thin current sheet with a finite guide field is investigated in the weak guide-field limit by means of linear theory and simulation. The emphasis is placed on the lower-hybrid drift instability (LHDI) propagating along the current flow direction. Linear theory is compared against the two-dimensional linear simulation based on the gyrokinetic electron/fully kinetic ion code. LHDI is a flute mode characterized by k⋅Btotal = 0; hence, it is stabilized by a finite guide field if one is confined to k vector strictly parallel to the cross-field current. Comparison of the theory and simulation shows qualitatively good agreement.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.65.Tt Gyrofluid and gyrokinetic simulations
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
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Ion-acoustic solitary waves and their interaction in a weakly relativistic two-dimensional thermal plasma

Jiu-ning Han, Sheng-lin Du, and Wen-shan Duan

Phys. Plasmas 15, 112104 (2008); http://dx.doi.org/10.1063/1.3027509 (8 pages) | Cited 14 times

Online Publication Date: 24 November 2008

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This paper discusses the existence of ion-acoustic solitary waves and their interaction in a weakly relativistic two-dimensional thermal plasma. Two Korteweg–de Vries equations for small but finite amplitude solitary waves in both ξ and η directions are derived. The phase shifts and trajectories of two solitary waves after the collision with an arbitrary angle α are also obtained. The effects of parameters of the normalized ion temperature σ, the ratio of heat capacity δ, the relativistic factor Fγ, and the colliding angle α on the amplitudes, the widths and the phase shifts of both the colliding solitary waves are studied. The effects of these parameters on the new nonlinear wave created by the collision between two solitary waves are examined as well. The results suggest that these parameters can significantly influence the amplitude, the width of the newly formed nonlinear wave and the colliding solitary waves. The phase shifts of the colliding solitary waves strongly depend on the colliding angle α. Moreover, there are compressive solitary waves in such a system.
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52.27.Ny Relativistic plasmas
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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Lower-hybrid drift and Buneman instabilities in current sheets with guide field

P. H. Yoon and A. T. Y. Lui

Phys. Plasmas 15, 112105 (2008); http://dx.doi.org/10.1063/1.2996115 (5 pages) | Cited 4 times

Online Publication Date: 24 November 2008

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Lower-hybrid drift and Buneman instabilities operate in current sheets with or without the guide field. The lower-hybrid drift instability is a universal instability in that it operates for all parameters. In contrast, the excitation of Buneman instability requires sufficiently thin current sheet. That is, the relative electron-ion drift speed must exceed the threshold in order for Buneman instability to operate. Traditionally, the two instabilities were treated separately with different mathematical formalisms. In a recent paper, an improved electrostatic dispersion relation was derived that is valid for both unstable modes [ P. H. Yoon and A. T. Y. Lui, Phys. Plasmas 15, 072101 (2008) ]. However, the actual numerical analysis was restricted to a one-dimensional situation. The present paper generalizes the previous analysis and investigates the two-dimensional nature of both instabilities. It is found that the lower-hybrid drift instability is a flute mode satisfying k⋅B = 0 and k⋅∇n = 0, where k represents the wave number for the most unstable mode, B stands for the total local magnetic field, and ∇n is the density gradient. This finding is not totally unexpected. However, a somewhat surprising finding is that the Buneman instability is a field-aligned mode characterized by k×B = 0 and k⋅∇n = 0, rather than being a beam-aligned instability.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
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Global particle-in-cell simulations of Alfvénic modes

Alexey Mishchenko, Roman Hatzky, and Axel Könies

Phys. Plasmas 15, 112106 (2008); http://dx.doi.org/10.1063/1.3021453 (11 pages) | Cited 9 times

Online Publication Date: 25 November 2008

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Global linear gyrokinetic particle-in-cell (PIC) simulations of electromagnetic modes in pinch and tokamak geometries are reported. The global Alfvén eigenmode, the mirror Alfvén eigenmode, the toroidal Alfvén eigenmode, and the kinetic ballooning modes have been simulated. All plasma species have been treated kinetically (i.e., no hybrid fluid-kinetic or reduced-kinetic model has been applied). The main intention of the paper is to demonstrate that the global Alfvén modes can be treated with the gyrokinetic PIC method.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.65.Rr Particle-in-cell method
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Dg Plasma kinetic equations
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
back to top Nonlinear Phenomena, Turbulence, Transport
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On the nature of transport in near-critical dissipative-trapped-electron-mode turbulence: Effect of a subdominant diffusive channel

J. A. Mier, R. Sánchez, L. García, D. E. Newman, and B. A. Carreras

Phys. Plasmas 15, 112301 (2008); http://dx.doi.org/10.1063/1.3006088 (9 pages) | Cited 6 times

Online Publication Date: 3 November 2008

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The change in nature of radial transport in numerical simulations of near-critical dissipative-trapped-electron-mode turbulence is characterized as the relative strength of an additional diffusive transport channel (subdominant to turbulence) is increased from zero. In its absence, radial transport exhibits the lack of spatial and temporal scales characteristic of self-organized-critical systems. This dynamical regime survives up to diffusivity values which, for the system investigated here, greatly exceeds the expected neoclassical value. These results, obtained using a novel Lagrangian method, complete and extend previous works based instead on the use of techniques imported from the study of cellular automata [ J. A. Mier et al., Phys. Plasmas 13, 102308 (2006) ]. They also shed further light on why some features of self-organized criticality seem to be observed in magnetically confined plasmas in spite of the presence of mechanisms which apparently violate the conditions needed for its establishment.
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52.35.Ra Plasma turbulence
52.55.-s Magnetic confinement and equilibrium
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Nonlinear stationary solutions of the Wigner and Wigner–Poisson equations

F. Haas and P. K. Shukla

Phys. Plasmas 15, 112302 (2008); http://dx.doi.org/10.1063/1.3008047 (6 pages)

Online Publication Date: 5 November 2008

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Exact nonlinear stationary solutions of the one-dimensional Wigner and Wigner–Poisson equations in the terms of the Wigner functions that depend not only on the energy but also on position are presented. In this way, the Bernstein–Greene–Kruskal modes of the classical plasma are adapted for the quantum formalism in the phase space. The solutions are constructed for the case of a quartic oscillator potential, as well as for the self-consistent Wigner–Poisson case. Conditions for well-behaved physically meaningful equilibrium Wigner functions are discussed.
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03.65.-w Quantum mechanics
52.25.Dg Plasma kinetic equations
52.35.Sb Solitons; BGK modes
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Characterization of self-similarity properties of turbulence in magnetized plasmas

A. Scipioni, P. Rischette, G. Bonhomme, and P. Devynck

Phys. Plasmas 15, 112303 (2008); http://dx.doi.org/10.1063/1.3006075 (10 pages)

Online Publication Date: 7 November 2008

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The understanding of turbulence in magnetized plasmas and its role in the cross field transport is still greatly incomplete. Several previous works reported on evidences of long-time correlations compatible with an avalanche-type of radial transport. Persistence properties in time records have been deduced from high values of the Hurst exponent obtained with the rescaled range R/S analysis applied to experimental probe data acquired in the edge of tokamaks. In this paper the limitations of this R/S method, in particular when applied to signals having mixed statistics are investigated, and the great advantages of the wavelets decomposition as a tool to characterize the self-similarity properties of experimental signals are highlighted. Furthermore the analysis of modified simulated fractional Brownian motions (fBm) and fractional Gaussian noises (fGn) allows us to discuss the relationship between high values of the Hurst exponent and long range correlations. It is shown that for such simulated signals with mixed statistics persistence at large time scales can still reflect the self-similarity properties of the original fBm and do not imply the existence of long range correlations, which are destroyed. It is thus questionable to assert the existence of long range correlations for experimental signals with non-Gaussian and mixed statistics just from high values of the Hurst exponent.
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.Fi Transport properties
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Reduction of chaotic particle transport driven by drift waves in sheared flows

F. A. Marcus, I. L. Caldas, Z. O. Guimarães-Filho, P. J. Morrison, W. Horton, Yu. K. Kuznetsov, and I. C. Nascimento

Phys. Plasmas 15, 112304 (2008); http://dx.doi.org/10.1063/1.3009532 (9 pages) | Cited 9 times

Online Publication Date: 7 November 2008

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Investigations of chaotic particle transport by drift waves propagating in the edge plasma of tokamaks with poloidal zonal flow are described. For large aspect ratio tokamaks, the influence of radial electric field profiles on convective cells and transport barriers, created by the nonlinear interaction between the poloidal flow and resonant waves, is investigated. For equilibria with edge shear flow, particle transport is seen to be reduced when the electric field shear is reversed. The transport reduction is attributed to the robust invariant tori that occur in nontwist Hamiltonian systems. This mechanism is proposed as an explanation for the transport reduction in Tokamak Chauffage Alfvén Brésilien [ R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001) ] for discharges with a biased electrode at the plasma edge.
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52.25.Fi Transport properties
52.35.Kt Drift waves
52.30.-q Plasma dynamics and flow
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Gj Fluctuation and chaos phenomena
52.55.Fa Tokamaks, spherical tokamaks
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Effects of the parallel electron dynamics and finite ion temperature on the plasma blob propagation in the scrape-off layer

D. Jovanović, P. K. Shukla, and F. Pegoraro

Phys. Plasmas 15, 112305 (2008); http://dx.doi.org/10.1063/1.3008050 (9 pages) | Cited 2 times

Online Publication Date: 11 November 2008

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A new three-dimensional model for the warm-ion turbulence at the tokamak edge plasma and in the scrape-off layer is proposed, and used to study the dynamics of plasma blobs in the scrape-off layer. The model is based on the nonlinear interchange mode, coupled with the nonlinear resistive drift mode, in the presence of the magnetic curvature drive, the density inhomogeneity, the electron dynamics along the open magnetic field lines, and the electron-ion and electron-neutral collisions. Within the present model, the effect of the sheath resistivity decreases with the distance from the wall, resulting in the bending and the break up of the plasma blob structure. Numerical solutions exhibit the coupling of interchange modes with nonlinear drift modes, causing the collapse of the blob in the lateral direction, followed by a clockwise rotation and radial propagation. The symmetry breaking, caused both by the parallel resistivity and the finite ion temperature, introduces a poloidal component in the plasma blob propagation, while the overall stability properties and the speed are not affected qualitatively.
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52.35.Ra Plasma turbulence
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Fi Transport properties
FREE

Cutoff criteria of electronic partition functions and transport properties of atomic hydrogen thermal plasmas

D. Bruno, M. Capitelli, C. Catalfamo, and A. Laricchiuta

Phys. Plasmas 15, 112306 (2008); http://dx.doi.org/10.1063/1.3012566 (7 pages) | Cited 7 times

Online Publication Date: 12 November 2008

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Transport coefficients of equilibrium hydrogen plasma have been calculated by using different cutoffs of electronic partition functions and different sets of transport cross sections of electronically excited states. The selection of both the cutoff criterion and transport cross sections deeply affects the transport coefficients of the H, H+, e plasma mixture in the temperature range of 10 000–50 000 K and in the pressure interval of 1–1000 atm.
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52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas
52.20.-j Elementary processes in plasmas
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Mechanisms and scalings of energetic ion transport via tokamak microturbulence

T. Hauff and F. Jenko

Phys. Plasmas 15, 112307 (2008); http://dx.doi.org/10.1063/1.3013453 (12 pages) | Cited 17 times

Online Publication Date: 13 November 2008

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The turbulent E×B advection of energetic ions in three-dimensional tokamak geometry is investigated both analytically and numerically. It is shown that orbit averaging (leading to a significant reduction of the diffusivity) is only valid for low magnetic shear. At moderate or high magnetic shear, a rather slow decrease of the diffusivity is found, proportional to (E/Te)−1 or (E/Te)−1.5 for particles with a large or small parallel velocity component, respectively. The decorrelation mechanisms responsible for this behavior are studied and explained in detail. Moreover, it is found that resonances between the toroidal drift of the particles and the diamagnetic drift of the turbulence can lead to an enhancement of the fast ion transport.
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52.25.Fi Transport properties
52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.30.-q Plasma dynamics and flow
52.55.Fa Tokamaks, spherical tokamaks
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Analysis and modeling of edge fluctuations and transport mechanism in the Maryland Centrifugal Experiment

I. U. Uzun-Kaymak, P. N. Guzdar, R. Clary, R. F. Ellis, A. B. Hassam, and C. Teodorescu

Phys. Plasmas 15, 112308 (2008); http://dx.doi.org/10.1063/1.3028312 (8 pages) | Cited 1 time

Online Publication Date: 25 November 2008

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The Maryland Centrifugal Experiment [ R. F. Ellis et al., Phys. Plasmas 12, 055704 (2005) ] is a mirror machine designed to have a plasma axially confined by supersonic rotation and dominantly interchange stable by the radial shear in the azimuthal velocity. Nevertheless, residual fluctuations still persist. To investigate the presence of such fluctuations, an azimuthal array of 16 magnetic pickup coils at the edge region of the plasma has been employed. A comprehensive analysis of the magnetic fluctuations reveals that, under the imposed shear flow, only m = 0 and m = 2 modes are dominant; yet, the observed frequency spectrum is broadband. Using higher order spectral analysis, clear evidence of nonlinear mode coupling is detected. It is also observed that the amplification of magnetic fluctuations leads to enhanced transport consistent with the drop of the plasma density and voltage. As a result, the magnetic fluctuations start to decrease in amplitude as the central plasma pressure drops. In return, the anomalous radial particle and momentum transport are reduced; thus, the plasma confinement improves. As the plasma pressure starts to build up, the plasma voltage increases, destabilizing the m = 2 interchange mode. The cycle of enhanced transport and intermittent fluctuations repeats itself. A two-dimensional magnetohydrodynamics code in slab geometry is employed to investigate the dynamics of the primary interchange instability and to assess the level of transport. For very low sheared rotation, a broad spatial spectrum of unstable modes is obtained. As the sheared rotation is increased, the high mode numbers become stabilized and low mode numbers dominate the spectrum. Both the experimental data obtained from the azimuthal array probes and the simulations in case of parabolic shear flow show clear evidence of nonlinear mode coupling, explaining the broadband frequency spectrum for low mode numbers. A detailed comparison of spatiotemporal dynamics of simulations with the experimental data is presented.
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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.)
28.52.Av Theory, design, and computerized simulation
52.55.-s Magnetic confinement and equilibrium
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Acoustic solitary waves in dusty and/or multi-ion plasmas with cold, adiabatic, and hot constituents

Frank Verheest, Manfred A. Hellberg, and Ioannis Kourakis

Phys. Plasmas 15, 112309 (2008); http://dx.doi.org/10.1063/1.3026716 (11 pages) | Cited 17 times

Online Publication Date: 25 November 2008

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Large nonlinear acoustic waves are discussed in a four-component plasma, made up of two superhot isothermal species, and two species with lower thermal velocities, being, respectively, adiabatic and cold. First a model is considered in which the isothermal species are electrons and ions, while the cooler species are positive and/or negative dust. Using a Sagdeev pseudopotential formalism, large dust-acoustic structures have been studied in a systematic way, to delimit the compositional parameter space in which they can be found, without restrictions on the charges and masses of the dust species and their charge signs. Solitary waves can only occur for nonlinear structure velocities smaller than the adiabatic dust thermal velocity, leading to a novel dust-acoustic-like mode based on the interplay between the two dust species. If the cold and adiabatic dust are oppositely charged, only solitary waves exist, having the polarity of the cold dust, their parameter range being limited by infinite compression of the cold dust. However, when the charges of the cold and adiabatic species have the same sign, solitary structures are limited for increasing Mach numbers successively by infinite cold dust compression, by encountering the adiabatic dust sonic point, and by the occurrence of double layers. The latter have, for smaller Mach numbers, the same polarity as the charged dust, but switch at the high Mach number end to the opposite polarity. Typical Sagdeev pseudopotentials and solitary wave profiles have been presented. Finally, the analysis has nowhere used the assumption that the dust would be much more massive than the ions and hence, one or both dust species can easily be replaced by positive and/or negative ions and the conclusions will apply to that plasma model equally well. This would cover a number of different scenarios, such as, for example, very hot electrons and ions, together with a mix of adiabatic ions and dust (of either polarity) or a very hot electron-positron mix, together with a two-ion mix or together with adiabatic ions and cold dust (both of either charge sign), to name but some of the possible plasma compositions.
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52.27.Lw Dusty or complex plasmas; plasma crystals
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
95.30.Qd Magnetohydrodynamics and plasmas
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Geodesic acoustic modes excited by finite beta drift waves

N. Chakrabarti, P. N. Guzdar, R. G. Kleva, V. Naulin, J. J. Rasmussen, and P. K. Kaw

Phys. Plasmas 15, 112310 (2008); http://dx.doi.org/10.1063/1.3028311 (5 pages) | Cited 6 times

Online Publication Date: 25 November 2008

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Presented in this paper is a mode-coupling analysis for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by finite beta drift waves. The finite beta effects give rise to a strong stabilizing influence on the parametric excitation process. The dominant finite beta effect is the combination of the Maxwell stress, which has a tendency to cancel the primary drive from the Reynolds stress, and the finite beta modification of the drift waves. The zonal magnetic field is also excited at the GAM frequency. However, it does not contribute to the overall stability of the three-wave process for parameters of relevance to the edge region of tokamaks.
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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.Kt Drift waves
52.55.Fa Tokamaks, spherical tokamaks
back to top Magnetically Confined Plasmas, Heating, Confinement
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Anomalous impurity ion heating from Alfvénic cascade in the reversed field pinch

Varun Tangri, P. W. Terry, and Gennady Fiksel

Phys. Plasmas 15, 112501 (2008); http://dx.doi.org/10.1063/1.2998829 (12 pages) | Cited 5 times

Online Publication Date: 3 November 2008

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Anomalous ion and impurity heating in reversed field pinch plasmas is addressed. Previous work [ N. Mattor et al., Comments Plasma Phys. Controlled Fusion 15, 65 (1992) ], which calculated the heating of bulk ions by gyro and Landau resonances with turbulent fluctuations cascading from unstable tearing modes, is extended to impurity species measured in Madison symmetric torus (MST). The heavier mass of impurities allows gyro-resonant heating at lower frequencies where more energy is present in the fluctuations. A 0D transport model is used to examine heating rates under various time-dependent, experimental heating scenarios, such as a sawtooth crash. Impurity heating rates calculated for impurities found in MST are comparable to observed rates inferred in the impurity temperature rise during sawtooth events.
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52.50.-b Plasma production and heating
52.55.Ez Theta pinch
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
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Variational derivation of the dispersion relation of kinetic coherent modes in the acoustic frequency range in tokamaks

C. Nguyen, X. Garbet, and A. I. Smolyakov

Phys. Plasmas 15, 112502 (2008); http://dx.doi.org/10.1063/1.3008048 (15 pages) | Cited 13 times

Online Publication Date: 11 November 2008

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In the present paper, we compare two modes with frequencies belonging to the acoustic frequency range: the geodesic acoustic mode (GAM) and the Beta Alfvén eigenmode (BAE). For this, a variational gyrokinetic energy principle coupled to a Fourier sidebands expansion is developed. High order finite Larmor radius and finite orbit width effects are kept. Their impact on the mode structures and on the Alfvén spectrum is calculated and discussed. We show that in a local analysis, the degeneracy of the electrostatic GAM and the BAE dispersion relations is verified to a high order and based in particular on a local poloidal symmetry of the two modes. When a more global point of view is taken, and the full radial structures of the modes are computed, differences appear. The BAE structure is shown to have an enforced localization, and to possibly connect to a large magnetohydrodynamic structure. On the contrary, the GAM is seen to have a wavelike, nonlocalized structure, as long as standard slowly varying monotonic profiles are considered.
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52.55.Fa Tokamaks, spherical tokamaks
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
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Toroidal self-consistent modeling of drift kinetic effects on the resistive wall mode

Yueqiang Liu, M. S. Chu, I. T. Chapman, and T. C. Hender

Phys. Plasmas 15, 112503 (2008); http://dx.doi.org/10.1063/1.3008045 (12 pages) | Cited 29 times

Online Publication Date: 12 November 2008

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A self-consistent kinetic model is developed to study the stability of the resistive wall mode in toroidal plasmas. This model is compared with other models based on perturbative approaches. The degree of the kinetic modification to the stability of the mode depends on the plasma configurations. Both stabilizing and destabilizing kinetic effects are observed. The nonperturbative approach, with a self-consistent inclusion of the eigenfunctions and the eigenvalues of the resistive wall mode, normally finds less stabilization than the perturbative approach.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
28.52.Av Theory, design, and computerized simulation
52.55.Fa Tokamaks, spherical tokamaks
52.55.Tn Ideal and resistive MHD modes; kinetic modes
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Role of secondary long wavelength structures in the saturation of electron temperature gradient driven turbulence

Jiquan Li and Y. Kishimoto

Phys. Plasmas 15, 112504 (2008); http://dx.doi.org/10.1063/1.3012567 (10 pages) | Cited 5 times

Online Publication Date: 13 November 2008

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The dynamics of secondary long wavelength structures (LWSs) in electron temperature gradient (ETG) driven turbulence are investigated by performing gyrofluid simulations and modeling analyses in a slab geometry with an emphasis of the underlying nonlinear interaction processes. It is shown that the back-reaction of the secondary LWS on the ambient fluctuations essentially contributes to saturating ETG instability and limiting the electron transport. The LWS is nonlinearly generated mainly through the beating of the most unstable ETG modes, even a weak modulation instability. The back-reaction is identified as the enhanced stabilization of the ETG modes due to the streamer-type feature of the LWS, which dominantly produces a local poloidal mode coupling among unstable and highly damped spectral components to form a global mode, besides the suppression effect of the LWS due to the radial shearing decorrelation and/or the radial mode coupling. Finally, the correspondence between the LWS in the slab model and the quasimode observed in toroidal ETG simulation [ Z. Lin et al., Phys. Plasmas 12, 056125 (2005) ] and the importance of the nonlinear mode coupling in the multiscale turbulence interaction are discussed.
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52.25.Fi Transport properties
52.35.Ra Plasma turbulence
52.25.Gj Fluctuation and chaos phenomena
52.40.Mj Particle beam interactions in plasmas
52.65.Tt Gyrofluid and gyrokinetic simulations
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Pitch angle scattering and synchrotron radiation of relativistic runaway electrons in tokamak stochastic magnetic fields

J. R. Martín-Solís and R. Sánchez

Phys. Plasmas 15, 112505 (2008); http://dx.doi.org/10.1063/1.3013849 (8 pages) | Cited 2 times

Online Publication Date: 13 November 2008

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In a recent work [ J. R. Martín-Solís and R. Sánchez, Phys. Plasmas 13, 012508 (2006) ], the increase that the presence of stochastic magnetic fields causes on the synchrotron radiation losses of relativistic runaway electrons was quantified using a guiding-center approximation. Here, we complete those studies by considering instead the mechanism which dominates the interaction at the gyromotion level. It is shown that, under typical tokamak conditions, the resonant cyclotron interaction with high enough parallel (to the magnetic field) wave numbers (k∥) modes can create, even for moderate magnetic fluctuation levels, an upper bound on the runaway energy. Implications for disruption-generated runaway electrons will be also discussed.
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52.55.Fa Tokamaks, spherical tokamaks
41.60.-m Radiation by moving charges
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Toroidal equilibria in spherical coordinates

K. H. Tsui

Phys. Plasmas 15, 112506 (2008); http://dx.doi.org/10.1063/1.3006340 (7 pages) | Cited 4 times

Online Publication Date: 24 November 2008

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The standard Grad–Shafranov equation for axisymmetric toroidal plasma equilibrium is customary expressed in cylindrical coordinates with toroidal contours, and through which benchmark equilibria are solved. An alternative approach to cast the Grad–Shafranov equation in spherical coordinates is presented. This equation, in spherical coordinates, is examined for toroidal solutions to describe low β Solovev and high β plasma equilibria in terms of elementary functions.
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52.55.Jd Magnetic mirrors, gas dynamic traps
back to top Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter
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Cross-point coronal plasma dynamics in two- and four-wire x-pinches

R. E. Madden, S. C. Bott, D. Haas, Y. Eshaq, U. Ueda, G. Collins, and F. N. Beg

Phys. Plasmas 15, 112701 (2008); http://dx.doi.org/10.1063/1.3008046 (8 pages) | Cited 1 time

Online Publication Date: 5 November 2008

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Studies of the late time diode gap formation in two- and four-wire tungsten x-pinches using an 80 kA, 50 ns current pulse are presented. Quantitative measurements of the coronal plasma density are recovered using interferometry simultaneously with laser shadowgraphy. Axial expansion of the gap occurs at ∼ 106 cm/s for both two- and four-wire systems and is likely to be driven by an axial J×B force resulting from radial current flow in the plasma minidiode “electrodes.” Radial density profiles suggest repinching of the low density plasma occurs after the main pinch resulting in secondary x-ray emission peak >10 ns after the first, which is recorded with a pair of pin diodes.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.70.La X-ray and γ-ray measurements
52.80.-s Electric discharges
52.25.Fi Transport properties
52.75.Fk Magnetohydrodynamic generators and thermionic convertors; plasma diodes
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Simulations of electron transport and ignition for direct-drive fast-ignition targets

A. A. Solodov, K. S. Anderson, R. Betti, V. Gotcheva, J. Myatt, J. A. Delettrez, S. Skupsky, W. Theobald, and C. Stoeckl

Phys. Plasmas 15, 112702 (2008); http://dx.doi.org/10.1063/1.3000674 (6 pages) | Cited 9 times

Online Publication Date: 7 November 2008

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The performance of high-gain, fast-ignition fusion targets is investigated using one-dimensional hydrodynamic simulations of implosion and two-dimensional (2D) hybrid fluid-particle simulations of hot-electron transport, ignition, and burn. The 2D/3D hybrid-particle-in-cell code LSP [ D. R. Welch et al., Nucl. Instrum. Methods Phys. Res. A 464, 134 (2001) ] and the 2D fluid code DRACO [ P. B. Radha et al., Phys. Plasmas 12, 056307 (2005) ] are integrated to simulate the hot-electron transport and heating for direct-drive fast-ignition targets. LSP simulates the transport of hot electrons from the place where they are generated to the dense fuel core where their energy is absorbed. DRACO includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. The self-generated resistive magnetic field is found to collimate the hot-electron beam, increase the coupling efficiency of hot electrons with the target, and reduce the minimum energy required for ignition. Resistive filamentation of the hot-electron beam is also observed. The minimum energy required for ignition is found for hot electrons with realistic angular spread and Maxwellian energy-distribution function.
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52.25.Fi Transport properties
52.80.Qj Explosions; exploding wires
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.65.Rr Particle-in-cell method
52.65.Ww Hybrid methods
52.57.Kk Fast ignition of compressed fusion fuels
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