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Oct 2007

Volume 14, Issue 10, Articles (10xxxx)

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Turbulent acceleration of superthermal electrons

C.-M. Ryu, T. Rhee, T. Umeda, P. H. Yoon, and Y. Omura

Phys. Plasmas 14, 100701 (2007); http://dx.doi.org/10.1063/1.2779282 (4 pages) | Cited 10 times

Online Publication Date: 18 October 2007

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In a recent paper it was suggested on the basis of weak turbulence theory that the collisionality of a plasma, coupled with nonlinear wave-particle interaction, is crucial for the acceleration of electrons by Langmuir turbulence to a superthermal energy level. In this Letter, fully nonlinear Vlasov and particle-in-cell (PIC) simulation techniques are employed to further verify this potentially important finding. The previous conclusion is fully confirmed by observing the expected difference between the Vlasov and PIC simulation results in the weak beam regime. However, in the strong beam regime, both the Vlasov and PIC simulations are found to produce a high-energy tail population, which indicates that there may be other mechanisms in the high beam speed situation, that are responsible for the generation the superthermal electrons.
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52.35.Ra Plasma turbulence
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Mj Particle beam interactions in plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.65.Ff Fokker-Planck and Vlasov equation
52.65.Rr Particle-in-cell method

Fundamental emission via wave advection from a collapsing wave packet in electromagnetic strong plasma turbulence

F. A. Jenet, A. Melatos, and P. A. Robinson

Phys. Plasmas 14, 100702 (2007); http://dx.doi.org/10.1063/1.2787500 (4 pages) | Cited 5 times

Online Publication Date: 19 October 2007

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Zakharov simulations of nonlinear wave collapse in continuously driven two-dimensional, electromagnetic strong plasma turbulence with electron thermal speeds v ≥ 0.01c show that for v≲0.1c, dipole radiation occurs near the plasma frequency, mainly near arrest, but for v≳0.1c, a new mechanism applies in which energy oscillates between trapped Langmuir and transverse modes until collapse is arrested, after which trapped transverse waves are advected into incoherent interpacket turbulence by an expanding annular density well, where they detrap. The multipole structure, Poynting flux, source current, and radiation angular momentum are computed.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Ra Plasma turbulence
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.)

Instability of current sheets and formation of plasmoid chains

N. F. Loureiro, A. A. Schekochihin, and S. C. Cowley

Phys. Plasmas 14, 100703 (2007); http://dx.doi.org/10.1063/1.2783986 (4 pages) | Cited 54 times

Online Publication Date: 29 October 2007

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Current sheets formed in magnetic reconnection events are found to be unstable to high-wavenumber perturbations. The instability is very fast: its maximum growth rate scales as S1/4vA/LCS, where LCS is the length of the sheet, vA the Alfvén speed, and S the Lundquist number. As a result, a chain of plasmoids (secondary islands) is formed, whose number scales as S3/8.
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52.35.Vd Magnetic reconnection
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
94.30.-d Physics of the magnetosphere
94.30.cp Magnetic reconnection
96.60.Iv Magnetic reconnection

Scale-up of spherical tokamak solenoid-free startup by coaxial helicity injection

X. Z. Tang and A. H. Boozer

Phys. Plasmas 14, 100704 (2007); http://dx.doi.org/10.1063/1.2798052 (4 pages) | Cited 2 times

Online Publication Date: 31 October 2007

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Current multiplication and flux amplification are two critical measures in assessing the usefulness of magnetic helicity injection for forming the plasma confining magnetic field in laboratory spheromak and spherical tokamak (ST) experiments. While the two concepts are closely related for spheromaks, they are independent for a Taylor-relaxed ST plasma, and negatively correlated for a more realistic, partially relaxed ST plasma. An important application of this understanding leads to the so-called relaxed transient coaxial helicity injection scheme for solenoid-free ST startup, which can deliver reactor-grade high current multiplication and flux amplification.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Ip Spheromaks
52.55.Wq Current drive; helicity injection
52.25.Fi Transport properties
28.52.Cx Fueling, heating and ignition
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back to top Basic Plasma Phenomena, Waves, Instabilities

Antenna excitation of drift wave in a toroidal plasma

A. Diallo, P. Ricci, A. Fasoli, I. Furno, B. Labit, S. H. Müller, M. Podestà, F. M. Poli, and F. Skiff

Phys. Plasmas 14, 102101 (2007); http://dx.doi.org/10.1063/1.2784464 (8 pages)

Online Publication Date: 10 October 2007

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In a magnetized toroidal plasma, an antenna tunable in vertical wave number is used to excite density perturbations. Coherent detection is performed by means of Langmuir probes to directly determine both the wave vector and the plasma response induced by the antenna. Comparison between the theoretical density response predicted by the generalized Hasegawa-Wakatani model, and the experimentally determined density response enables us the identification of one peak of the plasma response as a drift wave.
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52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.35.Kt Drift waves
52.55.Jd Magnetic mirrors, gas dynamic traps
52.25.Xz Magnetized plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.70.Ds Electric and magnetic measurements

Spontaneous emission of Weibel fluctuations by anisotropic distributions

R. C. Tautz and R. Schlickeiser

Phys. Plasmas 14, 102102 (2007); http://dx.doi.org/10.1063/1.2783261 (7 pages) | Cited 5 times

Online Publication Date: 12 October 2007

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Recently [ Yoon, Phys. Plasmas 14, 064504 (2007) ], the spontaneous emission of magnetic field fluctuations in isotropic particle distribution functions was investigated. Here, the question is addressed as to how these fluctuations develop for an anisotropic distribution function that supports the Weibel instability. It is shown that, depending on the parameters, either electromagnetic or aperiodic magnetic fluctuations are dominant.
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52.25.Gj Fluctuation and chaos phenomena
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Excitation of the l = 2 azimuthal mode in a pure electron plasma

G. Bettega, F. Cavaliere, B. Paroli, M. Cavenago, R. Pozzoli, and M. Romé

Phys. Plasmas 14, 102103 (2007); http://dx.doi.org/10.1063/1.2789985 (6 pages) | Cited 5 times

Online Publication Date: 16 October 2007

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The effect of an externally imposed time-varying quadrupole electrostatic perturbation on a pure electron plasma trapped in a Malmberg-Penning trap is investigated experimentally. A resonance occurs when the frequency of the drive matches the frequency of the azimuthal l = 2, kz = 0 mode. Very good agreement is found between the experimentally determined resonance frequency and the real part of the frequency calculated from a two-dimensional linear model for a top-hat density profile, while nonlinear deviations for the mode amplitude are found especially close to the resonance.
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52.55.-s Magnetic confinement and equilibrium

The relative importance of fluid and kinetic frequency shifts of an electron plasma wave

B. J. Winjum, J. Fahlen, and W. B. Mori

Phys. Plasmas 14, 102104 (2007); http://dx.doi.org/10.1063/1.2790385 (4 pages) | Cited 9 times

Online Publication Date: 18 October 2007

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The total nonlinear frequency shift of a plasma wave including both fluid and kinetic effects is estimated when the phase velocity of the wave is much less than the speed of light. Using a waterbag or fluid model, the nonlinear frequency shift due to harmonic generation is calculated for an arbitrary shift in the wavenumber. In the limit where the wavenumber does not shift, the result is in agreement with previously published work [ R. L. Dewar and J. Lindl, Phys. Fluids 15, 820 (1972) ; T. P. Coffey, Phys. Fluids 14, 1402 (1971) ]. This shift is compared to the kinetic shift of Morales and O’Neil [ G. J. Morales and T. M. O’Neil, Phys. Rev. Lett. 28, 417 (1972) ] for wave amplitudes and values of kλD of interest to Raman backscatter of a laser driver in inertial confinement fusion.
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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.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Finite electron temperature effects on interferometric and polarimetric measurements in fusion plasmas

V. V. Mirnov, W. X. Ding, D. L. Brower, M. A. Van Zeeland, and T. N. Carlstrom

Phys. Plasmas 14, 102105 (2007); http://dx.doi.org/10.1063/1.2790886 (12 pages) | Cited 7 times

Online Publication Date: 19 October 2007

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Finite electron temperature effects on interferometry and polarimetry measurements for burning plasma are considered with particular focus on analytically understanding the role of weakly relativistic effects. Development of a new iterative technique, in the limit when the probing wave frequency is much higher than the electron cyclotron frequency, yields the dispersion relation to lowest (linear) order in Te/mec2⪡1. Perturbative treatment of the wave phase and polarization is presented in a form suitable for interpretation of experimental data. Previous analysis of the problem included nonrelativistic calculations only. Herein, it is shown that relativistic effects are equally important. Theoretical results are in agreement with computations and can be used for benchmarking of ray tracing codes. The implication of finite temperature effects on future burning plasma interferometer diagnostics is discussed.
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52.25.-b Plasma properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
52.27.Ny Relativistic plasmas
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.25.Dg Plasma kinetic equations

Propagation of axisymmetric Trivelpiece-Gould mode along vortex columns generated by diocotron instability

Y. Kawai, Y. Kiwamoto, Y. Soga, and J. Aoki

Phys. Plasmas 14, 102106 (2007); http://dx.doi.org/10.1063/1.2793731 (5 pages) | Cited 2 times

Online Publication Date: 23 October 2007

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A high-frequency (HF) oscillation has been observed to propagate along magnetic field lines with an axisymmetric profile extending over multiple columns of magnetized pure electron plasma which undergo two-dimensional vortex motions. The observed frequency is a few hundred times higher than the rotation frequency of the vortices and varies in time as the density distribution of electrons deforms from an unstable hollow profile into vortex columns and approaches a single-peaked distribution. Experimental examinations reveal that the HF oscillation is excited by the gate pulse applied to prepare the initial profile and that the oscillation does not influence the subsequent dynamics of vortex columns. Further examinations including detailed observations and theoretical analyses indicate that the HF oscillation represents a profile-dependent eigenfunction of the Trivelpiece-Gould mode wave that consists of a dominant axisymmetric component and small fractions of higher azimuthal modes associated with the density distribution separated into columns.
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47.32.-y Vortex dynamics; rotating fluids
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Absorption of high-frequency guided waves in a plasma-loaded waveguide

A. Ganguli, Kamran Akhtar, and R. D. Tarey

Phys. Plasmas 14, 102107 (2007); http://dx.doi.org/10.1063/1.2799161 (10 pages)

Online Publication Date: 24 October 2007

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A kinetic theory model for the absorption of high-frequency guided plasma waves is presented for a warm, inhomogeneous, magnetized plasma column loaded inside a waveguide. Electron cyclotron resonance (ECR) absorption and Landau damping terms, derived as the anti-Hermitian part of the susceptibility tensor, are included in the model for a loss-free plasma-loaded waveguide developed earlier [ A. Ganguli et al., Phys. Plasmas 5, 1178, (1998) ]. In this formulation, the imaginary part of the complex propagation constant (in the presence of absorption) is obtained using a perturbation technique treating the anti-Hermitian part of the dielectric tensor as small in comparison to the Hermitian part, for the loss-free plasma. In this paper, we present the formulation for the inclusion of ECR absorption and Landau damping along with numerical results describing the role of a small population of warm electrons in wave damping in such a discharge. Numerical results are presented in the form of dispersion curves (plots of Vphase versus ω/Ωe) and damping curves (plots of ∣Im kz/kv∣ versus ω/Ωe). It is seen that although the warm electrons have a marginal effect on wave dispersion, their presence produces a marked increase in the damping rates away from the ECR region. It is also shown that damping occurs primarily through Doppler-shifted ECR resonance and not Landau damping, even well away from ECR. Power absorption calculations are also presented for two magnetic field profiles.
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52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.25.Dg Plasma kinetic equations
52.25.Mq Dielectric properties
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.80.Pi High-frequency and RF discharges
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

Nonlinear absorption of surface plasmons and emission of electrons from metallic targets

D. B. Singh, Gagan Kumar, and V. K. Tripathi

Phys. Plasmas 14, 102108 (2007); http://dx.doi.org/10.1063/1.2799173 (4 pages)

Online Publication Date: 24 October 2007

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A large-amplitude surface plasma wave (SPW) over a metal-vacuum interface Ohmically heats the electrons and undergoes nonlinear absorption. The attenuation rate increases with the local SPW amplitude. The enhanced electron temperature leads to stronger thermionic emission of electrons. At typical Nd:glass laser intensity IL = 7 GW/cm2, if one takes the amplitude of the SPW to be ≈ 6 times the amplitude of the laser, one obtains the thermionic electron emission current density J = 200 A/cm2. However, the emission current density decreases with propagation distance at a much faster rate than the SPW amplitude and electron temperature.
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79.40.+z Thermionic emission
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Anomalous resistivity due to kink modes in a thin current sheet

Toseo Moritaka, Ritoku Horiuchi, and Hiroaki Ohtani

Phys. Plasmas 14, 102109 (2007); http://dx.doi.org/10.1063/1.2767623 (10 pages) | Cited 6 times

Online Publication Date: 25 October 2007

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The roles of microscopic plasma instabilities on the violation of the frozen-in constraint are investigated by examining the force balance equation based on explicit electromagnetic particle simulation for a thin current sheet. Wave-particle interactions associated with lower hybrid drift instability and drift kink instability (DKI) contribute to the wavy electric force term at the periphery of the current sheet and the wavy magnetic force term at the neutral sheet, respectively. In the linear growing phase of DKI, the wavy magnetic force term balances with the electric force term due to the dc electric field at the neutral sheet. It is concluded that the growth of DKI can create anomalous resistivity and result in the violation of the frozen-in constraint as well as the diffusion of current density.
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52.35.Vd Magnetic reconnection
52.65.-y Plasma simulation

Soliton propagation in an inhomogeneous plasma at critical density of negative ions: Effects of gyratory and thermal motions of ions

Hitendra K. Malik and Shigeo Kawata

Phys. Plasmas 14, 102110 (2007); http://dx.doi.org/10.1063/1.2783259 (8 pages) | Cited 7 times

Online Publication Date: 25 October 2007

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The effects of gyratory and thermal motions of ions on soliton propagation in an inhomogeneous plasma that contains positive ions, negative ions, and electrons are studied at a critical density of negative ions. Since at this critical negative ion density the nonlinear term of the relevant Korteweg–deVries (KdV) equation vanishes, a higher order of nonlinearity is considered by retaining higher-order perturbation terms in the expansion of dependent quantities together with the appropriate set of stretched coordinates. Under this situation, time-dependent perturbation leads to the evolution of modified KdV solitons, which are governed by a modified form of the KdV equation that has an additional term due to the density gradient present in the plasma. On the basis of the solution of this equation and obliquely applied magnetic field, the effects of gyratory and thermal motions of ions are analyzed on the soliton propagation for three cases, nn0<ne0, nn0 = ne0, and nn0>ne0, together with nn0 (ne0) as the density of negative ions (electrons). The role of the negative ions in the evolution of the modes and the solitons is also discussed. Under the limiting cases, our calculations reduce to the ones obtained by other investigators in the past. This substantiates the generality of the present analysis.
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52.35.Sb Solitons; BGK modes

Simulations of plasma confinement in an antihydrogen trap

K. Gomberoff, J. Fajans, A. Friedman, D. Grote, J.-L. Vay, and J. S. Wurtele

Phys. Plasmas 14, 102111 (2007); http://dx.doi.org/10.1063/1.2778420 (9 pages) | Cited 6 times

Online Publication Date: 30 October 2007

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The three-dimensional particle-in-cell (3-D PIC) simulation code WARP is used to study positron confinement in antihydrogen traps. The magnetic geometry is close to that of a UC Berkeley experiment conducted, with electrons, as part of the ALPHA collaboration [ W. Bertsche et al., AIP Conf. Proc. 796, 301 (2005) ]. In order to trap antihydrogen atoms, multipole magnetic fields are added to a conventional Malmberg-Penning trap. These multipole fields must be strong enough to confine the antihydrogen, leading to multipole field strengths at the trap wall comparable to those of the axial magnetic field. Numerical simulations reported here confirm recent experimental measurements of reduced particle confinement when a quadrupole field is added to a Malmberg-Penning trap. It is shown that, for parameters relevant to various antihydrogen experiments, the use of an octupole field significantly reduces the positron losses seen with a quadrupole field. A unique method for obtaining a 3-D equilibrium of the positrons in the trap with a collisionless PIC code was developed especially for the study of the antihydrogen trap; however, it is of practical use for other traps as well.
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52.55.-s Magnetic confinement and equilibrium

A unified approach to the Darwin approximation

Todd B. Krause, A. Apte, and P. J. Morrison

Phys. Plasmas 14, 102112 (2007); http://dx.doi.org/10.1063/1.2799346 (10 pages) | Cited 1 time

Online Publication Date: 30 October 2007

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There are two basic approaches to the Darwin approximation. The first involves solving the Maxwell equations in Coulomb gauge and then approximating the vector potential to remove retardation effects. The second approach approximates the Coulomb gauge equations themselves, then solves these exactly for the vector potential. There is no a priori reason that these should result in the same approximation. Here, the equivalence of these two approaches is investigated and a unified framework is provided in which to view the Darwin approximation. Darwin’s original treatment is variational in nature, but subsequent applications of his ideas in the context of Vlasov's theory are not. We present here action principles for the Darwin approximation in the Vlasov context, and this serves as a consistency check on the use of the approximation in this setting.
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02.30.Mv Approximations and expansions
03.50.De Classical electromagnetism, Maxwell equations

Longitudinal photons in a relativistic magneto-active plasma

N. L. Tsintsadze, Ayesha Rehman, G. Murtaza, and H. A. Shah

Phys. Plasmas 14, 102113 (2007); http://dx.doi.org/10.1063/1.2768511 (6 pages) | Cited 2 times

Online Publication Date: 30 October 2007

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This paper presents some aspects of interaction of superstrong high-frequency electromagnetic waves with strongly magnetized plasmas. The case in which the photon-photon interaction dominates the photon-plasma particle interaction is considered. Strictly speaking, the photon and photon bunch interaction leads to the self-modulation of the photon gas. Assuming that the density of the plasma does not change, the dispersion relation, which includes relativistic self-modulation, is investigated. The existence of longitudinal photons in a strong magnetic field has the well-known Bogoliubov-type energy spectrum. The stability of the photon flow is investigated and an expression for Landau damping of the photons is obtained. Finally, it has been shown that the interaction of even a very strong electromagnetic radiation with a plasma does not always lead to instability, but causes only a change in plasma properties, whereby the plasma remains stable.
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52.27.Ny Relativistic plasmas
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.30.-q Plasma dynamics and flow

Scaling of asymmetric magnetic reconnection: General theory and collisional simulations

P. A. Cassak and M. A. Shay

Phys. Plasmas 14, 102114 (2007); http://dx.doi.org/10.1063/1.2795630 (11 pages) | Cited 46 times

Online Publication Date: 31 October 2007

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A Sweet-Parker-type scaling analysis for asymmetric antiparallel reconnection (in which the reconnecting magnetic field strengths and plasma densities are different on opposite sides of the dissipation region) is performed. Scaling laws for the reconnection rate, outflow speed, the density of the outflow, and the structure of the dissipation region are derived from first principles. These results are independent of the dissipation mechanism. It is shown that a generic feature of asymmetric reconnection is that the X-line and stagnation point are not colocated, leading to a bulk flow of plasma across the X-line. The scaling laws are verified using two-dimensional resistive magnetohydrodynamics numerical simulations for the special case of asymmetric magnetic fields with symmetric density. Observational signatures and applications to reconnection in the magnetosphere are discussed.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.20.-j Elementary processes in plasmas
52.25.-b Plasma properties
52.65.Kj Magnetohydrodynamic and fluid equation
94.30.cp Magnetic reconnection
back to top Nonlinear Phenomena, Turbulence, Transport

Broken symmetries and magnetic dynamos

John V. Shebalin

Phys. Plasmas 14, 102301 (2007); http://dx.doi.org/10.1063/1.2780138 (12 pages) | Cited 7 times

Online Publication Date: 8 October 2007

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Phase space symmetries inherent in the statistical theory of ideal magnetohydrodynamic (MHD) turbulence are known to be broken dynamically to produce large-scale coherent magnetic structure. Here, results of a numerical study of decaying MHD turbulence are presented that show large-scale coherent structure also arises and persists in the presence of dissipation. Dynamically broken symmetries in MHD turbulence may thus play a fundamental role in the dynamo process.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Kinetic theory of hydromagnetic turbulence. I. Formal results for parallel propagation

Peter H. Yoon

Phys. Plasmas 14, 102302 (2007); http://dx.doi.org/10.1063/1.2780139 (11 pages) | Cited 10 times

Online Publication Date: 8 October 2007

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Formal weak turbulence kinetic equations for magnetized collisionless plasmas are derived. The kinetic theory of plasma turbulence found in the literature is largely applicable for unmagnetized plasmas, and most of the available literature only deal with electrostatic Langmuir turbulence problem. However, real plasmas in nature and laboratory are usually immersed in magnetic fields. At present there is no practical kinetic theory for turbulence in magnetized plasmas. The present paper and a companion article [ P. H. Yoon and T.-M. Fang, Phys. Plasmas 14, 102303 (2007) ] constitute a first step in the formulation of general kinetic theory for magnetized plasmas. For the sake of simplicity, it is assumed that turbulent fluctuations predominantly propagate along the direction of ambient magnetic field vector, and that the characteristic frequency associated with the fluctuations is much lower than the electron gyrofrequency, i.e., hydromagnetic turbulence. The effects of spontaneous thermal fluctuation and spatial inhomogeneity are also ignored.
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52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Tn Ideal and resistive MHD modes; kinetic modes
52.65.Kj Magnetohydrodynamic and fluid equation

Kinetic theory of hydromagnetic turbulence. II. Susceptibilities

Peter H. Yoon and Ta-Ming Fang

Phys. Plasmas 14, 102303 (2007); http://dx.doi.org/10.1063/1.2780140 (5 pages) | Cited 7 times

Online Publication Date: 8 October 2007

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The present paper augments the previous paper [ P. H. Yoon, Phys. Plasmas 14, 102302 (2007) ] in which a formal kinetic theory of hydromagnetic turbulence propagating back-and-forth along the ambient magnetic field was developed. In the present paper, linear wave properties are discussed in detail, various symmetry properties associated with response functions are derived, alternative forms of the response functions are presented, and limiting and approximate forms of nonlinear susceptibilities are discussed.
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg Plasma kinetic equations
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Generation of coherent wave packets of kinetic Alfvén waves in solar plasmas

H. D. Singh and R. P. Sharma

Phys. Plasmas 14, 102304 (2007); http://dx.doi.org/10.1063/1.2786066 (6 pages) | Cited 6 times

Online Publication Date: 9 October 2007

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This work presents the numerical simulations to study the filamentation of kinetic Alfvén waves in solar plasmas. Using the modified nonlinear Schrödinger equation model, we study the effect of changing the initial perturbation on filament formation and their nonlinear dynamics. The spectral indices of the power spectrum are calculated with different initial conditions of the simulations. The relevance of the present investigation in coronal heating and solar wind acceleration/turbulence is also pointed out.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
96.20.Br Origin and evolution
96.30.Ys Asteroids, meteoroids
96.60.P- Corona

Study of statistical properties of edge turbulence in the National Spherical Torus Experiment with the gas puff imaging diagnostic

M. Agostini, S. J. Zweben, R. Cavazzana, P. Scarin, G. Serianni, R. J. Maqueda, and D. P. Stotler

Phys. Plasmas 14, 102305 (2007); http://dx.doi.org/10.1063/1.2776912 (9 pages) | Cited 20 times

Online Publication Date: 12 October 2007

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An investigation is presented of the edge turbulence in the National Spherical Torus Experiment [ M. Ono, M. G. Bell, R. E. Bell et al., Plasma Phys. Control. Fusion, 45, A335 (2003) ] based on the optical gas puff imaging (GPI) diagnostic. First of all, the edge fluctuations are characterized for the low confinement mode (L-mode) discharges as a function of the radius. The probability distribution function of the fluctuations is shown to be non-Gaussian for all the radial positions studied, but the deviation from the normal distribution is greater outside the separatrix; in this region the area occupied by the edge structures (“blobs”) is greater than inside the separatrix, and this is correlated with the decrease of the logarithmic radial derivative of the pressure gradient. Then the difference between the L-mode and the high confinement mode (H-mode) is studied. With the continuous wavelet transform technique the intense bursts in the GPI signal can be detected; their number decreases in the H-mode with respect to the L-mode, with no significant change in the poloidal velocities. The difference between the two regimes is also observed in the poloidal wavenumber spectra: L-mode and H-mode have two different injection scales for the energy, and different cascades take place. Only in the L-mode the energy flows toward the small wavenumber feeding the bigger blobs.
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52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Gj Fluctuation and chaos phenomena
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
02.50.-r Probability theory, stochastic processes, and statistics
02.30.Uu Integral transforms

The effect of plasma shaping on turbulent transport and E×B shear quenching in nonlinear gyrokinetic simulations

J. E. Kinsey, R. E. Waltz, and J. Candy

Phys. Plasmas 14, 102306 (2007); http://dx.doi.org/10.1063/1.2786857 (13 pages) | Cited 19 times

Online Publication Date: 12 October 2007

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Nonlinear gyrokinetic simulations with kinetic electron dynamics are used to study the effects of plasma shaping on turbulent transport and E×B shear in toroidal geometry including the presence of kinetic electrons using the GYRO code. Over 120 simulations comprised of systematic scans were performed around several reference cases in the local, electrostatic, collisionless limit. Using a parameterized local equilibrium model for shaped geometry, the GYRO simulations show that elongation κ (and its gradient) stabilizes the energy transport from ion temperature gradient (ITG) and trapped electron mode (TEM) instabilities at fixed midplane minor radius. For scans around a reference set of parameters, the GYRO ion energy diffusivity, in gyro-Bohm units, approximately follows a κ−1 scaling which is qualitatively similar to recent experimental energy confinement scalings. Most of the κ scaling is due to the shear in the elongation rather than the local κ itself. The κ scaling for the electrons is found to vary and can be stronger or weaker than κ−1 depending on the wavenumber where the transport peaks. The κ scaling is weaker when the energy diffusivity peaks at low wavenumbers and is stronger when the peak occurs at high wavenumbers. The simulations also demonstrate a nonlinear upshift in the critical temperature gradient as the elongation increases due to an increase in the residual zonal flow amplitude. Triangularity is found to be slightly destabilizing and its effect is strongest for highly elongated plasmas. Finally, we find less E×B shear is needed to quench the transport at high elongation and low aspect ratio. A new linear E×B shear quench rule, valid for shaped tokamak geometry, is presented.
Show PACS
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks

Collisionality and magnetic geometry effects on tokamak edge turbulent transport. II. Many-blob turbulence in the two-region model

D. A. Russell, J. R. Myra, and D. A. D’Ippolito

Phys. Plasmas 14, 102307 (2007); http://dx.doi.org/10.1063/1.2780137 (11 pages) | Cited 12 times

Online Publication Date: 12 October 2007

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A two-region model, coupling the outboard midplane and the X-point region, was proposed in Paper I [ J. R. Myra, D. A. Russell, and D. A. D’Ippolito, Phys. Plasmas 13, 112502 (2006) ] to study the effects of collisionality and magnetic geometry on electrostatic turbulent transport in the edge and scrape-off layer of a diverted tokamak plasma by filamentary coherent structures or “blobs.” Attention was focused on the properties of isolated blobs. That study is extended here to the many-blob, turbulent saturated state driven by a linearly unstable density profile. The evolution of the density profile is included. It is demonstrated that turbulent density transport increases with collisionality but decreases with enhanced magnetic field-line fanning and shear in this model. Field-line shear induces poloidal velocity in isolated blob propagation and de-correlates the electrostatic potentials in the two regions in the turbulent regime. Probability density functions of density flux resemble those of experimental probe data: both are insensitive to magnetic field geometry and collisionality. It is shown that blobs are born where the skewness of density fluctuations vanishes and the logarithmic pressure gradient is maximized. The simulations show increased particle fluxes with increased plasma resistivity, which are due to increases in both blob velocity and creation rate (or spatial “packing fraction”). A wavelet-type Gaussian-fitting analysis is used to study the dependence of blob velocity on blob size. It is found that streamers, which dominate the simulations, move faster than circular blobs when the two regions are electrically disconnected.
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52.20.-j Elementary processes in plasmas
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.35.Ra Plasma turbulence
52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
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