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Jul 2009

Volume 16, Issue 7, Articles (07xxxx)

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Phys. Plasmas 16, 072502 (2009); http://dx.doi.org/10.1063/1.3166137 (7 pages)

J. C. Wright, P. T. Bonoli, A. E. Schmidt, C. K. Phillips, E. J. Valeo, R. W. Harvey, and M. A. Brambilla
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Experimental study of a fourth-harmonic gyromultiplier

I. V. Bandurkin, V. L. Bratman, A. V. Savilov, S. V. Samsonov, and A. B. Volkov

Phys. Plasmas 16, 070701 (2009); http://dx.doi.org/10.1063/1.3179805 (3 pages) | Cited 5 times

Online Publication Date: 14 July 2009

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Simultaneous generation at the second and fourth cyclotron harmonics has been obtained from a single-cavity self-excited gyromultiplier. Output power of the short-wavelength radiation amounts to 100 W at a frequency of 75 GHz. The proposed scheme seems to be promising for the terahertz frequency range.
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52.75.-d Plasma devices
84.40.Ik Masers; gyrotrons (cyclotron-resonance masers)

Discharge ignition characteristics of pulsed radio-frequency glow discharges in atmospheric helium

Jianjun Shi, Yeqing Cai, Jie Zhang, Ke Ding, and Jing Zhang

Phys. Plasmas 16, 070702 (2009); http://dx.doi.org/10.1063/1.3184824 (4 pages) | Cited 4 times

Online Publication Date: 17 July 2009

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An experimental study of radio-frequency (15 MHz) glow discharges in atmospheric helium modulated by pulses with repetition frequency of 500 kHz and duty cycle of 6% and 8% is presented in this paper. In each discharge burst, the discharge is restricted to operate in ignition phase with duration of one or two radio-frequency cycles. The ignition characteristics in terms of spatial-temporal evolution of discharge interelectrode structure and optical emission intensity are investigated by time resolved imaging. Optical emission intensities at lines of 706 and 777 nm are used to capture clearly the temporal evolution of energetic electrons and active specie of atom oxygen generated in discharge.
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52.80.Hc Glow; corona
52.80.Pi High-frequency and RF discharges
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
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back to top Basic Plasma Phenomena, Waves, Instabilities

Jeans instability in quantum magnetoplasma with resistive effects

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

Phys. Plasmas 16, 072101 (2009); http://dx.doi.org/10.1063/1.3168612 (5 pages) | Cited 8 times

Online Publication Date: 2 July 2009

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See Also: RETRACTION

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The Jeans instability in dense quantum plasmas is investigated in the presence of two dimensional magnetic fields and resistive effects. The resistive effects are shown to introduce instability whether the perturbation is stable or not in the ideal magnetohydrodynamic model. The analytical expressions of the growth rate of Jeans instability are obtained for both the finite and remarkable resistive effects cases. The results are relevant to dense astrophysical objects, e.g., neutron stars and the interior of white dwarfs, as well as low-temperature laboratory plasmas.
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52.25.Xz Magnetized plasmas
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Tn Ideal and resistive MHD modes; kinetic modes

Generation of a dressed soliton in a four-component dusty plasma with nonthermal ions

Prasanta Chatterjee, Ganesh Mondal, Kaushik Roy, S. V. Muniandy, S. L. Yap, and C. S. Wong

Phys. Plasmas 16, 072102 (2009); http://dx.doi.org/10.1063/1.3159865 (8 pages) | Cited 9 times

Online Publication Date: 6 July 2009

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Dust acoustic solitary waves are studied in a four-component dusty plasma. Positively and negatively charged mobile dust and Boltzmann-distributed electrons are considered. The ion distribution is taken as nonthermal. The Korteweg–de Vries equation is derived using reductive perturbation technique. We are able to reproduce the results obtained by Sayed and Mamun [Phys. Plasmas 14, 014501 (2007) ] provided the Boltzmann distribution is considered for the ions. Higher order inhomogeneous differential equation is obtained for the dressed soliton. By using the renormalization method of Kodama and Taniuti [J. Phys. Soc. Jpn. 45, 298 (1978) ], we derived the expression for the dressed soliton.
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52.35.Sb Solitons; BGK modes
52.35.Dm Sound waves
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.27.Lw Dusty or complex plasmas; plasma crystals
52.25.Fi Transport properties
52.30.Ex Two-fluid and multi-fluid plasmas

Nonlinear positron acoustic solitary waves

Mouloud Tribeche, Kamel Aoutou, Smain Younsi, and Rabia Amour

Phys. Plasmas 16, 072103 (2009); http://dx.doi.org/10.1063/1.3160619 (5 pages) | Cited 3 times

Online Publication Date: 7 July 2009

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The problem of nonlinear positron acoustic solitary waves involving the dynamics of mobile cold positrons is addressed. A theoretical work is presented to show their existence and possible realization in a simple four-component plasma model. The results should be useful for the understanding of the localized structures that may occur in space and laboratory plasmas as new sources of cold positrons are now well developed.
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52.35.Sb Solitons; BGK modes
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.)

Simplified model of nonlinear Landau damping

N. A. Yampolsky and N. J. Fisch

Phys. Plasmas 16, 072104 (2009); http://dx.doi.org/10.1063/1.3160604 (8 pages) | Cited 15 times

Online Publication Date: 8 July 2009

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The nonlinear interaction of a plasma wave with resonant electrons results in a plateau in the electron distribution function close to the phase velocity of the plasma wave. As a result, Landau damping of the plasma wave vanishes and the resonant frequency of the plasma wave downshifts. However, this simple picture is invalid when the external driving force changes the plasma wave fast enough so that the plateau cannot be fully developed. A new model to describe amplification of the plasma wave including the saturation of Landau damping and the nonlinear frequency shift is proposed. The proposed model takes into account the change of the plasma wave amplitude and describes saturation of the Landau damping rate in terms of a single fluid equation, which simplifies the description of the inherently kinetic nature of Landau damping. A proposed fluid model, incorporating these simplifications, is verified numerically using a kinetic Vlasov code.
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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.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Effect of nonlinear Landau damping in plasma-based backward Raman amplifier

N. A. Yampolsky and N. J. Fisch

Phys. Plasmas 16, 072105 (2009); http://dx.doi.org/10.1063/1.3160606 (9 pages) | Cited 9 times

Online Publication Date: 8 July 2009

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A plasma wave can mediate laser coupling in a plasma-based resonant backward Raman amplifier for high power amplification of short laser pulses. The resonant nature of amplification requires a long lifetime of the plasma wave. However, the plasma wave can be heavily Landau damped in warm plasma. On the other hand, Landau damping can be saturated in the presence of a strong plasma wave. We study backward Raman amplifier in the nonlinear regime of Landau damping using a simplified fluid model. We find the regime in which initially high linear Landau damping can be significantly saturated. Because of the saturation effect, higher temperatures can be tolerated in achieving efficient amplification. The plasma temperature can be as much as 50% larger compared to the case of unsaturated Landau damping.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering
42.65.Lm Parametric down conversion and production of entangled photons
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

Nonlinear interaction between ions and multiple electrostatic waves

Zheng-Mao Sheng, Limin Yu, Guangzhou Hao, and Roscoe White

Phys. Plasmas 16, 072106 (2009); http://dx.doi.org/10.1063/1.3157245 (7 pages) | Cited 3 times

Online Publication Date: 9 July 2009

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The nonlinear interaction of ions with multiple electrostatic waves propagating perpendicularly across a uniform magnetic field is investigated both analytically and numerically. Applying a multiscale expansion method with the wave amplitude as the perturbation parameter, a general nonlinear resonance condition is analytically derived. Under this condition, it is confirmed that multiple waves even below the cyclotron frequency and small amplitude are capable of effectively producing acceleration or stochastic heating by numerical simulation. Compared to the single wave situation, the stochastic threshold for heating by multiple waves with frequencies satisfied with a nonlinear resonance condition is significantly reduced because the nonlinear interaction of ions with multiple waves leads more easily to overlapping of islands and spreading of the stochastic layer in phase space. The above result is helpful to understand the energization mechanism of ions in the solar corona.
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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)
52.50.-b Plasma production and heating
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
95.30.Qd Magnetohydrodynamics and plasmas
96.60.P- Corona

Linearized model Fokker–Planck collision operators for gyrokinetic simulations. II. Numerical implementation and tests

M. Barnes, I. G. Abel, W. Dorland, D. R. Ernst, G. W. Hammett, P. Ricci, B. N. Rogers, A. A. Schekochihin, and T. Tatsuno

Phys. Plasmas 16, 072107 (2009); http://dx.doi.org/10.1063/1.3155085 (13 pages) | Cited 13 times

Online Publication Date: 14 July 2009

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A set of key properties for an ideal dissipation scheme in gyrokinetic simulations is proposed, and implementation of a model collision operator satisfying these properties is described. This operator is based on the exact linearized test-particle collision operator, with approximations to the field-particle terms that preserve conservation laws and an H-theorem. It includes energy diffusion, pitch-angle scattering, and finite Larmor radius effects corresponding to classical (real-space) diffusion. The numerical implementation in the continuum gyrokinetic code GS2 [ Kotschenreuther et al., Comput. Phys. Comm. 88, 128 (1995) ] is fully implicit and guarantees exact satisfaction of conservation properties. Numerical results are presented showing that the correct physics is captured over the entire range of collisionalities, from the collisionless to the strongly collisional regimes, without recourse to artificial dissipation.
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52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.-y Plasma simulation

Influence of self-fields on coupled waves in free electron laser with ion-channel guiding

L. Masoudnia, B. Maraghechi, and T. Mohsenpour

Phys. Plasmas 16, 072108 (2009); http://dx.doi.org/10.1063/1.3170901 (10 pages) | Cited 2 times

Online Publication Date: 15 July 2009

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In this study, the equilibrium orbits and their stability, under the influence of self-electric and self-magnetic fields, are analyzed. A dispersion relation for the Raman regime free electron laser with a helical wiggler magnetic field and ion-channel guiding is derived and analyzed, taking into account self-field effects of the electron beam. This dispersion relation is solved numerically to study unstable couplings between all wave modes. New unstable orbits, in the first part of the group I orbits and in the resonance region of the group II orbits, are found. It was found that self-fields reduce the growth rate of the group I orbits and increase it in the group II orbits.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

Evolution of the bounded magnetized jet and comparison with Helimak experiments

R. B. Dahlburg, W. Horton, W. L. Rowan, C. Correa, and J. C. Perez

Phys. Plasmas 16, 072109 (2009); http://dx.doi.org/10.1063/1.3166598 (14 pages) | Cited 1 time

Online Publication Date: 15 July 2009

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Magnetized jets are important features of many systems of physical interest. To date, most interest has focused on solar and space physics and astrophysical applications, and hence the unbounded magnetized jet, and its cousin, the unbounded magnetized wake, have received the most attention. This work presents calculations of a bounded, magnetized jet for a laboratory experiments in the Helimak device [ K. W. Gentle and H. He, Plasma Sci. Technol. 10, 284 (2008) ]. The Helimak device has a toroidal magnetic field with a controlled velocity flow that represents jets in bounded systems. Experimental and theoretical features include three spatial dimensions, the inclusion of resistivity and viscosity, and the presence of no-slip walls. The results of the linearized model are computed with a Chebyshev-τ algorithm. The bounding walls stabilize the ideal varicose mode found in unbounded magnetized jets. The ideal sinuous mode persists in the bounded system. A comparison theorem is proved showing that two-dimensional modes are more unstable than the corresponding three-dimensional modes for any given set of system parameters. This result is a generalization of the hydrodynamic Squires theorem. An energy-stress theorem indicates that the Maxwell stress is crucial for the growth of the instability. The results of the analysis are consistent with the observed plasma fluctuations with in the limits of using a simple model for the more complex measured jet velocity flow profile. The working gas is singly ionized argon and the jet velocity profile is accurately measured with Doppler shift spectroscopy.
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52.55.Jd Magnetic mirrors, gas dynamic traps
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Gj Fluctuation and chaos phenomena
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Solitary waves and double layers in dense magnetoplasma

Prasanta Chatterjee, Taraknath Saha, Sithi V. Muniandy, S. L. Yap, and C. S. Wong

Phys. Plasmas 16, 072110 (2009); http://dx.doi.org/10.1063/1.3179759 (8 pages) | Cited 1 time

Online Publication Date: 16 July 2009

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Using Sagdeev’s pseudopotential technique, ion acoustic solitary waves and double layers are studied subject to an external magnetic field in a two-component dense magnetoplasma consisting of ions and degenerate electrons. The ions are described by the hydrodynamic equations, and the electrons are assumed to follow the Thomas–Fermi density distribution. The pseudopotential is derived directly from Poisson’s equation without assuming the quasineutrality condition. The ranges of parameters for which solitary waves and double layers exist are studied in detail using Sagdeev’s technique.
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52.35.Sb Solitons; BGK modes
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.40.Kh Plasma sheaths
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.-b Plasma properties

A nonextensive approach for the instability of current-driven ion-acoustic waves in space plasmas

Zhipeng Liu, Liyan Liu, and Jiulin Du

Phys. Plasmas 16, 072111 (2009); http://dx.doi.org/10.1063/1.3176516 (5 pages) | Cited 26 times

Online Publication Date: 16 July 2009

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The instability of current-driven ion-acoustic waves in the collisionless magnetic-field-free space plasma is investigated by using a nonextensive approach. The ions and the electrons are thought of in the power-law distributions that can be described by the generalized q-Maxwellian velocity distribution and are considered with the different nonextensive q-parameters. The generalized q-wave frequency and the generalized instability q-growth rate for the ion-acoustic waves are derived. The numerical results show that the nonextensive effects on the ion-acoustic waves are not apparent when the electron temperature is much more than the ion temperature, but they are salient when the electron temperature is not much more than the ion temperature. As compared to the electrons, the ions play a dominant role in the nonextensive effects.
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05.90.+m Other topics in statistical physics, thermodynamics, and nonlinear dynamical systems (restricted to new topics in section 05)
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)

On reflection of solitary waves in a magnetized multicomponent plasma with nonisothermal electrons

Hitendra K. Malik, Dhananjay K. Singh, and Yasushi Nishida

Phys. Plasmas 16, 072112 (2009); http://dx.doi.org/10.1063/1.3177446 (7 pages) | Cited 3 times

Online Publication Date: 20 July 2009

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Reflection of ion acoustic solitary waves is studied in a magnetized inhomogeneous multicomponent plasma with positive ions, negative ions, and two temperature nonisothermal electrons. Relevant modified Korteweg–de Vries (mKdV) equations corresponding to two oppositely moving solitary waves are coupled to obtain a variable coefficient coupled mKdV equation, which is solved by finding some transformations to investigate the characteristics of the reflected soliton. The reflection of the soliton is possible only when the magnetic field is applied at an angle θ that remains below certain critical value, which shows strong dependence on the concentration and temperature of nonisothermal electrons in addition to the negative ion density. The present plasma model supports compressive solitons only, which reflect strongly in the presence of low temperature electron component in the plasma and are found to downshift after the reflection.
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52.35.Sb Solitons; BGK modes
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.-b Plasma properties
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

A new formulation and simplified derivation of the dispersion function for a plasma with a kappa velocity distribution

R. L. Mace and M. A. Hellberg

Phys. Plasmas 16, 072113 (2009); http://dx.doi.org/10.1063/1.3179807 (9 pages) | Cited 11 times

Online Publication Date: 20 July 2009

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A simplified derivation of the relationship between the dispersion function for a plasma with a kappa velocity distribution and the Gauss hypergeometric function is presented. This derivation relies on only a few standard integrals. It naturally leads to a new integral representation for the dispersion function, which readily yields the power and Laurent series for it. The new integral representation is shown to be closely related to the Gordeyev integral for a kappa distribution.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams

Nonlinear electromagnetic wave equations for superdense magnetized plasmas

Nitin Shukla, G. Brodin, M. Marklund, P. K. Shukla, and L. Stenflo

Phys. Plasmas 16, 072114 (2009); http://dx.doi.org/10.1063/1.3184571 (4 pages) | Cited 12 times

Online Publication Date: 21 July 2009

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See Also: Erratum

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By using the quantum hydrodynamic and Maxwell equations, we derive the generalized nonlinear electron magnetohydrodynamic, the generalized nonlinear Hall-MHD (HMHD), and the generalized nonlinear dust HMHD equations in a self-gravitating dense magnetoplasma. Our nonlinear equations include the self-gravitating, the electromagnetic, the quantum statistical electron pressure, as well as the quantum electron tunneling and electron spin forces. They are useful for investigating a number of wave phenomena including linear and nonlinear electromagnetic waves, as well as three-dimensional electromagnetic wave turbulence spectra and structures arising from mode coupling processes at nanoscales in dense quantum magnetoplasmas.
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52.25.Dg Plasma kinetic equations
52.27.Gr Strongly-coupled plasmas
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
71.10.Ca Electron gas, Fermi gas

Mean shear flows generated by nonlinear resonant Alfvén waves

Christopher T. M. Clack and Istvan Ballai

Phys. Plasmas 16, 072115 (2009); http://dx.doi.org/10.1063/1.3194273 (10 pages) | Cited 1 time

Online Publication Date: 28 July 2009

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In the context of resonant absorption, nonlinearity has two different manifestations. The first is the reduction in amplitude of perturbations around the resonant point (wave energy absorption). The second is the generation of mean shear flows outside the dissipative layer surrounding the resonant point. Ruderman et al. [Phys. Plasmas 4, 75 (1997)] studied both these effects at the slow resonance in isotropic plasmas. Clack et al. [Astron. Astrophys. 494, 317 (2009)] investigated nonlinearity at the Alfvén resonance; however, they did not include the generation of mean shear flow. In this present paper, we investigate the mean shear flow, analytically, and study its properties. We find that the flow generated is parallel to the magnetic surfaces and has a characteristic velocity proportional to ϵ1/2, where ϵ is the dimensionless amplitude of perturbations far away from the resonance. This is, qualitatively, similar to the flow generated at the slow resonance. The jumps in the derivatives of the parallel and perpendicular components of mean shear flow across the dissipative layer are derived. We estimate the generated mean shear flow to be of the order of 10 km s−1 in both the solar upper chromosphere and solar corona; however, this value strongly depends on the choice of boundary conditions. It is proposed that the generated mean shear flow can produce a Kelvin–Helmholtz instability at the dissipative layer which can create turbulent motions. This instability would be an additional effect, as a Kelvin–Helmholtz instability may already exist due to the velocity field of the resonant Alfvén waves. This flow can also be superimposed onto existing large scale motions in the solar upper atmosphere.
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95.30.Qd Magnetohydrodynamics and plasmas
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra Plasma turbulence
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
96.60.Na Chromosphere
96.60.P- Corona

Modulational instability and envelope excitation of ion-acoustic waves in quantum electron-positron-ion plasmas

A. P. Misra, C. Bhowmik, and P. K. Shukla

Phys. Plasmas 16, 072116 (2009); http://dx.doi.org/10.1063/1.3192762 (7 pages) | Cited 6 times

Online Publication Date: 28 July 2009

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The theoretical study of modulational instability (MI) and localized envelope excitations of finite amplitude ion-acoustic waves (IAWs) is revisited in an unmagnetized quantum electron-positron-ion plasma. For this purpose, a one-dimensional nonlinear Schrödinger equation, which governs the slow modulation of IAW packets, is derived by using the standard reductive perturbations technique. Two parameters, defining the ratio of the electron to ion number density (μ) and the quantum coupling parameter (H) describing the ratio of the “plasmonic energy density” to the Fermi energy density, are shown to play crucial roles in determining the modulational stability/MI domains, as well as for the existence of both bright and dark envelope solitons. It is found that the stability region increases (decreases) with increasing μ(H), whereas the MI region for the IAW mode shifts to larger (smaller) wave number k as the value of μ(H) increases. Moreover, the parameter H is shown to suppress the MI growth rate of the IAWs. The present results may be relevant to dense astrophysical plasmas (e.g., white dwarfs, where the electron-positron annihilation can be important, and where the particle density is of the order of 1034–1035 m−3) as well as to the next generation intense laser solid density plasma experiments.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Sb Solitons; BGK modes

Influence of Coulomb collisions on the structure of reconnection layers

W. Daughton, V. Roytershteyn, B. J. Albright, H. Karimabadi, L. Yin, and Kevin J. Bowers

Phys. Plasmas 16, 072117 (2009); http://dx.doi.org/10.1063/1.3191718 (16 pages) | Cited 13 times

Online Publication Date: 30 July 2009

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The influence of Coulomb collisions on the structure of reconnection layers is examined in neutral sheet geometry using fully kinetic simulations with a Monte Carlo treatment of the Fokker–Planck operator. The algorithm is first carefully benchmarked against key predictions from transport theory, including the parallel and perpendicular resistivities as well as the thermal force. The results demonstrate that the collisionality is accurately specified, thus allowing the initial Lundquist number to be chosen as desired. For modest Lundquist numbers S≲1000, the classic Sweet–Parker solution is recovered. Furthermore, a distinct transition to a faster kinetic regime is observed when the thickness of the resistive layer δSP falls below the ion inertial length di. For higher Lundquist numbers S≳1000, plasmoids (secondary islands) are observed within the elongated resistive layers. These plasmoids give rise to a measurable increase in the reconnection rate and for certain cases induce a transition to kinetic regimes sooner than expected from the δSP ≈ di condition. During this transition, the reconnection electric field exceeds the runaway limit, leading to electron scale current layers in which the nonideal electric field is supported predominantly by off-diagonal components in the electron pressure tensor, along with a residual contribution from electron-ion momentum exchange. These weakly collisional electron layers are also unstable to the formation of new plasmoids.
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52.35.Vd Magnetic reconnection
52.65.-y Plasma simulation
back to top Nonlinear Phenomena, Turbulence, Transport

Plasma relaxation and the turbulent dynamo

John V. Shebalin

Phys. Plasmas 16, 072301 (2009); http://dx.doi.org/10.1063/1.3159866 (14 pages) | Cited 5 times

Online Publication Date: 2 July 2009

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Ideal magnetohydrodynamic (MHD) turbulence may be represented by finite Fourier series whose independent coefficients form a canonical ensemble described by a Gaussian probability density function containing a Hermitian covariance matrix with positive eigenvalues. When the eigenvalues at lowest wave number are very small, a large-scale coherent structure appears: a turbulent dynamo, which is seen in computations. A theoretical explanation is given and contains Taylor’s theory of force-free states. Numerical effects are examined and it is shown that larger grid sizes and smaller time steps provide for better resolution of coherent structure. Ideal hydrodynamic (HD) turbulence is examined and the results are compared and contrasted with those of ideal MHD turbulence. In particular, coherent structure appears in ideal MHD turbulence at the lowest wave number, but can occur in ideal HD turbulence only at the highest wave numbers in a simulation. In the case of real, i.e., dissipative flows, coherent structure and broken ergodicity are expected to occur in MHD turbulence at the largest scale. However, real HD turbulence at all scales and real MHD turbulence at all scales but the largest are expected to be ergodic.
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05.20.Jj Statistical mechanics of classical fluids
47.27.Gs Isotropic turbulence; homogeneous turbulence
91.25.Cw Origins and models of the magnetic field; dynamo theories
95.30.Qd Magnetohydrodynamics and plasmas

Reconnection in semicollisional, low-β plasmas

S. Schmidt, S. Günter, and K. Lackner

Phys. Plasmas 16, 072302 (2009); http://dx.doi.org/10.1063/1.3155453 (10 pages) | Cited 3 times

Online Publication Date: 9 July 2009

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Reconnection of semicollisional, low-β plasmas is studied numerically for two model problems using a two-field description of the plasma including electron pressure effects (and hence kinetic Alfvén-wave dynamics). The tearing unstable Harris sheet, with the global parameters of the Geospace Environment Modeling-challenge case, shows a linear growth of the peak reconnection rate with the drift parameter ρs when this scale is significantly larger than the resistive skin depth, and the island is smaller than the Harris sheet current layer width. As exemplary for a driven, rather than a spontaneous reconnection situation we study as second model system two coalescing islands, starting from a nonequilibrium situation. The peak reconnection rate again increases initially linearly with ρs but saturates and becomes ρs independent for larger values. In this saturated regime, no flux pileup occurs, and the reconnection is limited by the rate of approach of the two coalescing islands. The qualitative differences between spontaneous and driven reconnection cases and their scaling behavior are best understood by considering the reconnection rate as a triple product of outflow Mach number, outflow to inflow channel width ratio, and magnetic energy density at a height ρs above the X point.
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52.35.Vd Magnetic reconnection
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Gyrokinetic simulation tests of quasilinear and tracer transport

R. E. Waltz, A. Casati, and G. M. Staebler

Phys. Plasmas 16, 072303 (2009); http://dx.doi.org/10.1063/1.3167391 (9 pages) | Cited 8 times

Online Publication Date: 13 July 2009

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A nonlinear gyrokinetic simulation code is used to test the quasilinear transport approximation (QLTA) with simulated nonlinear spectral (potential field) intensity. Two common forms of the QLTA are defined. The first uses the linear mode spectrum (mQLTA) and the second uses the complete frequency spectrum (fQLTA) for the nonlinear spectral intensity. The mQLTA is tested via two-step linear then nonlinear simulations convoluting a quasilinear weight with a nonlinear field intensity spectral weight to get the quasilinear transport in comparison with the actual nonlinear transport. The fQLTA is tested via one-step simulations that have ion and electron “plasma species” at full densities and “tracer species” at negligible densities (and making no contribution to the Poisson field solve equation). If the tracer and plasma gyrokinetic equations are identical, then so are their respective energy and particle diffusivities. Comparing tracer and plasma (actual) diffusivities, when the tracer equation nonlinearity is deleted, provides a quantifiable test of the fQLTA form. The mQLTA preserves ambipolarity but the two-step test includes only the leading linear normal modes at each wave number. The one-step test of the fQLTA subsumes all normal modes but precludes ambipolar particle flow. The mQLTA and fQLTA quasilinear weights (per normal mode) are shown to be identical for a commonly used (but unphysical) mode frequency line width model. In successful cases, quasilinear diffusivities are typically 1.4–1.8 (1.2–1.4) larger than actual diffusivities for mQLTA (fQLTA). The QLTA is expected to make best predictions in the ratios of energy and particle flows. Electron to ion energy flow ratios are well approximated but both forms of the QLTA appear to breakdown most evidently for ratios of particle to energy flows in cases with strongly pinched (and impractically large) particle flows. An example of the so-called passive tracer diffusivity, which includes only linear and nonlinear E×B motion, is given for comparison with actual diffusivities.
Show PACS
52.35.Ra Plasma turbulence
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.55.Fa Tokamaks, spherical tokamaks

Weak and strong regimes of incompressible magnetohydrodynamic turbulence

G. Gogoberidze, S. M. Mahajan, and S. Poedts

Phys. Plasmas 16, 072304 (2009); http://dx.doi.org/10.1063/1.3177455 (4 pages)

Online Publication Date: 13 July 2009

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It is shown that in the framework of the weak turbulence theory, the autocorrelation and cascade time scales are always of the same order of magnitude. This means that, contrary to the general belief, any model of turbulence that implies a large number of collisions among wave packets for an efficient energy cascade (such as the Iroshnikov–Kraichnan model) is not compatible with the weak turbulence theory.
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52.35.Ra Plasma turbulence
47.27.Gs Isotropic turbulence; homogeneous turbulence

Collisional model of quasilinear transport driven by toroidal electrostatic ion temperature gradient modes

I. Pusztai, T. Fülöp, J. Candy, and R. J. Hastie

Phys. Plasmas 16, 072305 (2009); http://dx.doi.org/10.1063/1.3168611 (10 pages) | Cited 4 times

Online Publication Date: 13 July 2009

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The stability of ion temperature gradient (ITG) modes and the quasilinear fluxes driven by them are analyzed in weakly collisional tokamak plasmas using a semianalytical model based on an approximate solution of the gyrokinetic equation, where collisions are modeled by a Lorentz operator. Although the frequencies and growth rates of ITG modes far from threshold are only very weakly sensitive to the collisionality, the a/LTi threshold for stability is affected significantly by electron-ion collisions. The decrease in collisionality destabilizes the ITG mode driving an inward particle flux, which leads to the steepening of the density profile. Closed analytical expressions for the electron and ion density and temperature responses have been derived without expansion in the smallness of the magnetic drift frequencies. The results have been compared with gyrokinetic simulations with GYRO and illustrated by showing the scalings of the eigenvalues and quasilinear fluxes with collisionality, temperature scale length, and magnetic shear.
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52.55.Fa Tokamaks, spherical tokamaks
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Fi Transport properties
52.65.Tt Gyrofluid and gyrokinetic simulations

The effects of nonuniform magnetic field strength on density flux and test particle transport in drift wave turbulence

J. M. Dewhurst, B. Hnat, and R. O. Dendy

Phys. Plasmas 16, 072306 (2009); http://dx.doi.org/10.1063/1.3177382 (8 pages) | Cited 4 times

Online Publication Date: 14 July 2009

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The extended Hasegawa–Wakatani equations generate fully nonlinear self-consistent solutions for coupled density n and vorticity ∇2ϕ, where ϕ is electrostatic potential, in a plasma with background density inhomogeneity κ = −∂ ln n0/∂x and magnetic field strength inhomogeneity C = −∂ ln B/∂x. Finite C introduces interchange effects and ∇B drifts into the framework of drift turbulence through compressibility of the E×B and diamagnetic drifts. This paper addresses the direct computation of the radial E×B density flux Γn = −n∂ϕ/∂y, tracer particle transport, the statistical properties of the turbulent fluctuations that drive Γn and tracer motion, and analytical underpinnings. Systematic trends emerge in the dependence on C of the skewness of the distribution of pointwise Γn and in the relative phase of density-velocity and density-potential pairings. It is shown how these effects, together with conservation of potential vorticity Π = ∇2ϕ−n+(κ−C)x, account for much of the transport phenomenology. Simple analytical arguments yield a Fickian relation Γn = (κ−C)Dx between the radial density flux Γn and the radial tracer diffusivity Dx, which is shown to explain key trends in the simulations.
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52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
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