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Dec 2011

Volume 18, Issue 12, Articles (12xxxx)

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Phys. Plasmas 18, 122108 (2011); http://dx.doi.org/10.1063/1.3662430 (9 pages)

Seiji Zenitani, Michael Hesse, Alex Klimas, Carrie Black, and Masha Kuznetsova

 

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Measurements and simulations of shock wave generated plasma-vacuum interface

D. Kaganovich, M. H. Helle, D. F. Gordon, and A. Ting

Phys. Plasmas 18, 120701 (2011); http://dx.doi.org/10.1063/1.3671963 (4 pages)

Online Publication Date: 21 December 2011

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A controlled gradient gas jet was designed, constructed, and tested at the Naval Research Laboratory for the generation of high density and sharp gradient plasma regions. The gas jet uses a laser-generated shock wave to control the density gradient at the vacuum and neutral gas interface. The length scale of the laser produced plasma density gradient is fully controlled by the strength of the shock wave and can be varied continuously from100 μm for a weak shock to under 20 μm in case of strong shock wave as verified by the experimental results and simulations.
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52.35.Tc Shock waves and discontinuities
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.65.-y Plasma simulation

Bounce-free spherical hydrodynamic implosion

Grigory Kagan, Xian-Zhu Tang, Scott C. Hsu, and Thomas J. Awe

Phys. Plasmas 18, 120702 (2011); http://dx.doi.org/10.1063/1.3671949 (4 pages)

Online Publication Date: 27 December 2011

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In a bounce-free spherical hydrodynamic implosion, the post-stagnation hot core plasma does not expand against the imploding flow. Such an implosion scheme has the advantage of improving the dwell time of the burning fuel, resulting in a higher fusion burn-up fraction. The existence of bounce-free spherical implosions is demonstrated by explicitly constructing a family of self-similar solutions to the spherically symmetric ideal hydrodynamic equations. When applied to a specific example of plasma liner driven magneto-inertial fusion, the bounce-free solution is found to produce at least a factor of four improvement in dwell time and fusion energy gain.
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52.50.Lp Plasma production and heating by shock waves and compression
52.58.Qv Electrostatic and high-frequency confinement
52.30.-q Plasma dynamics and flow
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back to top Basic Plasma Phenomena, Waves, Instabilities

Rotating-filaments-pairs in a hexagonal superlattice state in dielectric barrier discharge

Lifang Dong, Yujie Yang, Ben Li, Weili Fan, and Qian Song

Phys. Plasmas 18, 122101 (2011); http://dx.doi.org/10.1063/1.3662443 (6 pages)

Online Publication Date: 7 December 2011

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Rotating-filaments-pairs in a hexagonal superlattice state (HSS) are studied in a dielectric barrier discharge system. The evolution and phase diagrams of HSS are given. The wavelength of HSS and the mean diameter of the two rotating filaments all decrease with the increase of applied voltage. The instantaneous orientations of rotating-filaments-pairs are equal probability approximately. There is a larger peak and a smaller one in both the probability density functions of the rotation speed (ω) of rotating filaments and that of the distance (D) between two rotating filaments. According to the fitting curves of lnω2 vs. lnD, ω2 is inversely proportional to D7. The rotation of filaments is discussed theoretically by the force among surface charges.
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52.30.-q Plasma dynamics and flow
52.80.-s Electric discharges

Investigation of tearing instability using GeFi particle simulation model

X. Y. Wang, Y. Lin, L. Chen, X. Lu, and W. Kong

Phys. Plasmas 18, 122102 (2011); http://dx.doi.org/10.1063/1.3662435 (12 pages)

Online Publication Date: 9 December 2011

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The gyrokinetic (GK) electron and fully kinetic ion (GeFi) simulation model of Lin et al. [Plasmas Phys. Controlled Fusion 53, 054013 (2011)] has been thoroughly benchmarked and validated for a two-dimensional (2D) Harris current sheet with a finite guide field. First, a gyrokinetic eigenmode theory for the collisionless tearing mode in the small Larmor radius limit is presented. The linear eigenmode structure and growth rate of the tearing mode obtained from the GeFi simulation are benchmarked against those from the GK eigenmode analysis in the limit of L≫ρi>ρe, where L is the current sheet half-width, ρi is ion Larmor radius, and ρe is electron Larmor radius. Second, to valid the GeFi model, both the linear and nonlinear tearing instabilities obtained from the GeFi simulations are compared with the Darwin particle-in-cell (PIC) simulation. The validation of the GeFi model for laboratory and space plasmas is also discussed. Meanwhile, the GeFi simulation is carried out to investigate both the linear and nonlinear tearing instabilities for cases with a broad range of L and guide magnetic field BG. It is found that in a wide current sheet with L > 4.5ρeK, the nonlinear saturation level of the island half-width is ws ≃ 3ρeK, where ρeK = ρeB0/Bx0, B0 is the strength of the asymptotic magnetic field, and Bx0 is the antiparallel field. On the other hand, in a thin current sheet with L < 2.5ρeK, ws ≃ 2.2 L. In addition, a high frequency electrostatic drift mode is found to coexist with the tearing mode.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Kt Drift waves
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Nonlinear stability of the ideal magnetohydrodynamic interchange mode at marginal conditions in a transverse magnetic field

Jupiter Bagaipo, P. N. Guzdar, and A. B. Hassam

Phys. Plasmas 18, 122103 (2011); http://dx.doi.org/10.1063/1.3662437 (9 pages)

Online Publication Date: 9 December 2011

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The stability of the ideal magnetohydrodynamic (MHD) interchange mode at marginal conditions is studied. A sufficiently strong constant magnetic field component transverse to the direction of mode symmetry provides the marginality conditions. A systematic perturbation analysis in the smallness parameter, |b2/Bc|1/2, is carried out, where Bc is the critical transverse magnetic field for the zero-frequency ideal mode and b2 is the deviation from Bc. The calculation is carried out to third order including nonlinear terms. It is shown that the system is nonlinearly unstable in the short wavelength limit, i.e., a large enough perturbation results in instability even if b2/Bc > 0 (linearly stable). The normalized amplitude for instability is shown to scale as |b2/Bc|1/2. A nonlinear, compressible, MHD simulation is done to check the analytic result. Good agreement is found, including the critical amplitude scaling.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Kj Magnetohydrodynamic and fluid equation

The dynamics of ion with electrostatic waves in a sheared magnetic field

Limin Yu, Xianmei Zhang, and Zheng-Mao Sheng

Phys. Plasmas 18, 122104 (2011); http://dx.doi.org/10.1063/1.3662438 (11 pages)

Online Publication Date: 13 December 2011

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The interaction between an ion and multiple electrostatic waves propagating perpendicularly to an ambient magnetic field with shear is investigated. Based on the Lie transformation method, with the wave amplitude and the magnetic shear both as the perturbation parameters, the analytical formulas for the reduced Hamiltonian is derived and results are compared with numerical calculations of the complete equations of motion for the case of two on-resonance waves and the case of two off-resonance waves, respectively. It is found that the effect of magnetic shear drastically prevents the acceleration of an ion in both cases. This result will help us to understand the behaviors of ions in a magnetic sheared device, such as tokamak.
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52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
02.10.Ud Linear algebra
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Current-free double layers: A review

Nagendra Singh

Phys. Plasmas 18, 122105 (2011); http://dx.doi.org/10.1063/1.3664321 (24 pages) | Cited 1 time

Online Publication Date: 14 December 2011

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During the last decade, there has been an upsurge in the research on current-free DLs (CFDLs). Research includes theory, laboratory measurements, and various applications of CFDLs ranging from plasma thrusters to acceleration of charged particles in space and astrophysical plasmas. The purpose of this review is to present a unified understanding of the basic plasma processes, which lead to the formation of CFDLs. The review starts with the discussion on early research on electric fields and double layers (DLs) and ion acceleration in planar plasma expansion. The review continues with the formation of DLs and rarefaction shocks (RFS) in expanding plasma with two electron populations with different temperatures. The basic theory mitigating the formation of a CFDL by two-electron temperature population is reviewed; we refer to such CFDLs as double layers structures formation by two-temperature electron populations (TET-CFDLs). Application of TET-CFDLS to ion acceleration in laboratory and space plasmas was discussed including the formation of stationary steady-state DLs. A quite different type of CFDLs forms in a helicon plasma device (HPD), in which plasma abruptly expands from a narrow plasma source tube into a wide diffusion tube with abruptly diverging magnetic fields. The formation mechanism of the CFDL in HPD, referred here as current free double layer structure in helicon plasma device (HPD-CFDL), and its applications are reviewed. The formation of a TET-CFDL is due to the self-consistent separation of the two electron populations parallel to the ambient magnetic field. In contrast, a HPD-CFDL forms due to self-consistent separation of electrons and ion perpendicular to the abruptly diverging magnetic field in conjunction with the conducting wall of the expansion chamber in the HPD. One-dimensional theoretical models of CFDLs based on steady-state solution of Vlasov-Poisson system of equations are briefly discussed. Applications of CFDLs ranging from helicon double-layer thrusters (HDLTs) to the accelerations of ions in space and astrophysical plasmas are summarized.
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52.40.Kh Plasma sheaths
52.25.Fi Transport properties
52.35.Tc Shock waves and discontinuities
52.72.+v Laboratory studies of space- and astrophysical-plasma processes
52.50.Dg Plasma sources

A new mode and its interaction through ponderomotive force in electron-positron-ion plasmas

Q. Haque, N. L. Tsintsadze, and W. Masood

Phys. Plasmas 18, 122106 (2011); http://dx.doi.org/10.1063/1.3662058 (6 pages)

Online Publication Date: 14 December 2011

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A new mode is found in e-p-i plasma in the presence of density and temperature difference of lighter particles. The electron beam induced Cherenkov instability condition for the excitation of positron sound wave is obtained for the system under consideration. Zakharov’s equation with sign modification due to negative Ponderomotive pressure is obtained. Nonlinear Schrödinger wave equation for the envelope type solitary waves is derived. Both stationary and nonstationary solutions are found and the subsonic and supersonic limits are also discussed. In the stationary case, rarefactive type solitary solution is obtained, whereas the nonstationary case yields the ion acoustic shock like structure solution which is very interesting. The importance of the study with relevance to both laboratory and astrophysical plasmas is pointed out.
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52.35.Sb Solitons; BGK modes
52.35.Tc Shock waves and discontinuities
02.30.Jr Partial differential equations
52.25.Fi Transport properties
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Decays of electron Bernstein waves near plasma edge

Nong Xiang and John R. Cary

Phys. Plasmas 18, 122107 (2011); http://dx.doi.org/10.1063/1.3662102 (9 pages)

Online Publication Date: 14 December 2011

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Nonlinear wave-wave couplings near the upper hybrid resonance are studied via particle-in-cell simulations. It is found that the decay of an electron Bernstein wave (EBW) depends on the ratio of the incident frequency and electron cyclotron frequency. For ratios less than two, parametric decay into a lower hybrid wave (or an ion Bernstein wave) and EBWs at a lower frequency is observed. For ratios larger than two, the daughter waves could be an electron cyclotron quasi-mode and another EBW or an ion wave and EBW. For sufficiently high incident power, the former process may dominate. Because of the electron cyclotron quasi-mode, electrons can be strongly heated by nonlinear Landau damping. As a result, the bulk of the incident power can be absorbed near plasma edge at high power. The increase in number of decay channels with frequency implies that the allowable power into the plasma must decrease with frequency.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Hf Plasma-material interactions; boundary layer effects
52.65.Rr Particle-in-cell method
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The inner structure of collisionless magnetic reconnection: The electron-frame dissipation measure and Hall fields

Seiji Zenitani, Michael Hesse, Alex Klimas, Carrie Black, and Masha Kuznetsova

Phys. Plasmas 18, 122108 (2011); http://dx.doi.org/10.1063/1.3662430 (9 pages) | Cited 1 time

Online Publication Date: 14 December 2011

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It was recently proposed that the electron-frame dissipation measure, the energy transfer from the electromagnetic field to plasmas in the electron’s rest frame, identifies the dissipation region of collisionless magnetic reconnection [Zenitani et al., Phys. Rev. Lett. 106, 195003 (2011)]. The measure is further applied to the electron-scale structures of antiparallel reconnection, by using two-dimensional particle-in-cell simulations. The size of the central dissipation region is controlled by the electron-ion mass ratio, suggesting that electron physics is essential. A narrow electron jet extends along the outflow direction until it reaches an electron shock. The jet region appears to be anti-dissipative. At the shock, electron heating is relevant to a magnetic cavity signature. The results are summarized to a unified picture of the single dissipation region in a Hall magnetic geometry.
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95.30.Qd Magnetohydrodynamics and plasmas
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Tc Shock waves and discontinuities
52.65.Rr Particle-in-cell method

Vlasov simulation in multiple spatial dimensions

Harvey A. Rose and William Daughton

Phys. Plasmas 18, 122109 (2011); http://dx.doi.org/10.1063/1.3662112 (10 pages)

Online Publication Date: 20 December 2011

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A long-standing challenge encountered in modeling plasma dynamics is achieving practical Vlasov equation simulation in multiple spatial dimensions over large length and time scales. While direct multi-dimension Vlasov simulation methods using adaptive mesh methods [M. Gutnic et al., Comput. Phys. Commun. 164, 214 (2004)] have recently shown promising results in two dimensions (2D) [J. W. Banks et al., Phys. Plasmas 18, 052102 (2011); B. I. Cohen et al., November 10, 2010, http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142], in this paper, we present an alternative, the Vlasov multi dimensional (VMD) model, that is specifically designed to take advantage of solution properties in regimes when plasma waves are confined to a narrow cone, as may be the case for stimulated Raman scatter in large optic f# laser beams. Perpendicular grid spacing large compared to a Debye length is then possible without instability or loss of accuracy, enabling an order 10 decrease in required computational resources compared to standard particle in cell (PIC) methods in 2D, with another reduction of that order in 3D. Further advantage compared to PIC methods accrues in regimes where particle noise is an issue. VMD and PIC results in a 2D model of localized Langmuir waves are in qualitative agreement.
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52.65.Ff Fokker-Planck and Vlasov equation
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.38.Bv Rayleigh scattering; stimulated Brillouin and Raman scattering

Eigenmodes of quasi-static magnetic islands in current sheet

Yi Li, Xiaohui Cai, Lihui Chai, Huinan Zheng, Chao Shen, and Shui Wang

Phys. Plasmas 18, 122110 (2011); http://dx.doi.org/10.1063/1.3672053 (3 pages)

Online Publication Date: 29 December 2011

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As observation have shown, magnetic islands often appear before and/or after the onset of magnetic reconnections in the current sheets, and they also appear in the current sheets in the solar corona, Earth’s magnetotail, and Earth’s magnetopause. Thus, the existence of magnetic islands can affect the initial conditions in magnetic reconnection. In this paper, we propose a model of quasi-static magnetic island eigenmodes in the current sheet. This model analytically describes the magnetic field structures in the quasi-static case, which will provide a possible approach to reconstructing the magnetic structures in the current sheet via observation data. This model is self-consistent in the kinetic theory. Also, the distribution function of charged particles in the magnetic island can be calculated.
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52.35.Vd Magnetic reconnection
52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations

Nonlinear oscillations and waves in an arbitrary mass ratio cold plasma

Prabal Singh Verma

Phys. Plasmas 18, 122111 (2011); http://dx.doi.org/10.1063/1.3672517 (6 pages) | Cited 1 time

Online Publication Date: 30 December 2011

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It is well known that nonlinear standing oscillations in an arbitrary mass ratio cold plasma always phase mix away. However, there exist nonlinear electron-ion traveling wave solutions, which do not exhibit phase mixing because they have zero ponderomotive force. The existence of these waves has been demonstrated using a perturbation method. Moreover, it is shown that cold plasma BGK waves [Albritton et al., Nucl. Fusion 15, 1199 (1975)] phase mix away if ions are allowed to move and the scaling of phase mixing is found to be different from earlier work [Sengupta et al., Phys. Rev. Lett. 82, 1867 (1999)]. Phase mixing of these waves has been further verified in 1-D particle in cell simulation.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.65.Rr Particle-in-cell method
52.27.Ny Relativistic plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Large amplitude solitary waves in ion-beam plasmas with charged dust impurities

A. P. Misra and N. C. Adhikary

Phys. Plasmas 18, 122112 (2011); http://dx.doi.org/10.1063/1.3671951 (7 pages)

Online Publication Date: 30 December 2011

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The nonlinear propagation of large amplitude dust ion-acoustic (DIA) solitary waves (SWs) in an ion-beam plasma with stationary charged dusts is investigated. For typical plasma parameters relevant for experiments [Y. Nakamura and K. Komatsuda, J. Plasma Phys. 60, 69 (1998)], when the beam speed is larger than the DIA speed (νb0 ≳ 1.7cs), three stable waves, namely, the “fast” and “slow” ion-beam modes and the plasma DIA wave, are shown to exist. These modes can propagate as SWs in the beam plasmas. However, in the other regime (cs < νb0 < 1.7cs), one of the beam modes when coupled to the DIA mode may become unstable. The SWs with positive (negative) potential may exist when the difference of the nonlinear wave speed (M) and the beam speed is such that 1.2 ≲ M − νb0 ≲ 1.6 (M − νb0 ≳ 1.6). Furthermore, for real density perturbations, the wave potential ( > 0) is found to be limited by a critical value which typically depends on M, νb0 as well as the ion/beam temperature. The conditions for the existence of DIA solitons are obtained, and their properties are analyzed numerically in terms of the system parameters. While the system supports both the compressive and rarefactive large amplitude SWs, the small amplitude solitons exist only of the compressive type. The theoretical results may be useful for observation of soliton excitations in laboratory ion-beam driven plasmas as well as in space plasmas where the charged dusts play as impurities.
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52.35.Sb Solitons; BGK modes
52.25.Vy Impurities in plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
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.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
back to top Nonlinear Phenomena, Turbulence, Transport

Simulating gyrokinetic microinstabilities in stellarator geometry with GS2

J. A. Baumgaertel, E. A. Belli, W. Dorland, W. Guttenfelder, G. W. Hammett, D. R. Mikkelsen, G. Rewoldt, W. M. Tang, and P. Xanthopoulos

Phys. Plasmas 18, 122301 (2011); http://dx.doi.org/10.1063/1.3662064 (7 pages)

Online Publication Date: 5 December 2011

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The nonlinear gyrokinetic code GS2 has been extended to treat non-axisymmetric stellarator geometry. Electromagnetic perturbations and multiple trapped particle regions are allowed. Here, linear, collisionless, electrostatic simulations of the quasi-axisymmetric, three-field period national compact stellarator experiment (NCSX) design QAS3-C82 have been successfully benchmarked against the eigenvalue code FULL. Quantitatively, the linear stability calculations of GS2 and FULL agree to within ∼10%.
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52.55.Jd Magnetic mirrors, gas dynamic traps
52.65.Tt Gyrofluid and gyrokinetic simulations
52.65.Vv Perturbative methods
28.52.Av Theory, design, and computerized simulation
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Kadomtsev-Petviashvili solitons propagation in a plasma system with superthermal and weakly relativistic effects

Hafeez-Ur-Rehman, Asif Shah, S. Mahmood, and Q. Haque

Phys. Plasmas 18, 122302 (2011); http://dx.doi.org/10.1063/1.3662101 (6 pages) | Cited 1 time

Online Publication Date: 13 December 2011

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Two dimensional (2D) solitons are studied in a plasma system comprising of relativistically streaming ions, kappa distributed electrons, and positrons. Kadomtsev-Petviashvili (KP) equation is derived through the reductive perturbation technique. Analytical solution of the KP equation has been studied numerically and graphically. It is noticed that kappa parameters of electrons and positrons as well as the ions relativistic streaming factor have an emphatic influence on the structural as well as propagation characteristics of two dimensional solitons in the considered plasma system. Our results may be helpful in the understanding of soliton propagation in astrophysical and laboratory plasmas, specifically the interaction of pulsar relativistic wind with supernova ejecta and the transfer of energy to plasma by intense electric field of laser beams producing highly energetic superthermal and relativistic particles [L. Arons, Astrophys. Space Sci. Lib. 357, 373 (2009); P. Blasi and E. Amato, Astrophys. Space Sci. Proc. 2011, 623; and A. Shah and R. Saeed, Plasma Phys. Controlled Fusion 53, 095006 (2011)].
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52.35.Sb Solitons; BGK modes
52.25.Fi Transport properties
52.27.Ny Relativistic plasmas

Asymptotic equilibrium between Langmuir turbulence and suprathermal electrons

Peter H. Yoon

Phys. Plasmas 18, 122303 (2011); http://dx.doi.org/10.1063/1.3662105 (8 pages) | Cited 2 times

Online Publication Date: 13 December 2011

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In both laboratory and natural environment such as the solar wind, suprathermal, or non-Maxwellian electron distributions are frequently observed. Electron velocity distribution functions containing non-Maxwellian, power-law energetic tail component are often modeled by the so-called kappa distribution, but their physical origin is not clearly understood. In a series of publications, the present author and his colleagues discussed the self-consistent formation of kappa-like distributions as a result of electron-Langmuir turbulence interaction process. However, these discussions were either based upon numerical initial value solution of the weak turbulence equation or by direct particle-in-cell simulation method. It was not evident that the formation of kappa-like state, which was demonstrated during the long-time evolution of the system, did indeed correspond to the genuine asymptotically steady-state solution or not in a mathematical sense. The present paper presents the self-consistent asymptotic solution of the electrons-Langmuir turbulence system and shows that the non-Maxwellian kappa-like state does indeed correspond to a rigorous solution.
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52.35.Ra Plasma turbulence
52.65.Rr Particle-in-cell method
52.25.Fi Transport properties

Cross-correlation based time delay estimation for turbulent flow velocity measurements: Statistical considerations

Balazs Tal, Attila Bencze, Sándor Zoletnik, Gabor Veres, and Gabor Por

Phys. Plasmas 18, 122304 (2011); http://dx.doi.org/10.1063/1.3662432 (15 pages)

Online Publication Date: 14 December 2011

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Time delay estimation methods (TDE) are well-known techniques to investigate poloidal flows in hot magnetized plasmas through the propagation properties of turbulent structures in the medium. One of these methods is based on the estimation of the time lag at which the cross-correlation function (CCF) estimation reaches its maximum value. The uncertainty of the peak location refers to the smallest determinable flow velocity modulation, and therefore the standard deviation of the time delay imposes important limitation to the measurements. In this article, the relative standard deviation of the CCF estimation and the standard deviation of its peak location are calculated analytically using a simple model of turbulent signals. This model assumes independent (non interacting) overlapping events (coherent structures) with randomly distributed spatio-temporal origins moving with background flow. The result of our calculations is the derivation of a general formula for the CCF variance, which is valid not exclusively in the high event density limit, but also for arbitrary event densities. Our formula reproduces the well known expression for high event densities previously published in the literature. In this paper we also present a derivation of the variance of time delay estimation that turns out to be inversely proportional to the applied time window. The derived formulas were tested in real plasma measurements. The calculations are an extension of the earlier work of Bencze and Zoletnik [Phys. Plasmas 12, 052323 (2005)] where the autocorrelation-width technique was developed. Additionally, we show that velocities calculated by a TDE method possess a broadband noise which originates from this variance, its power spectral density cannot be decreased by worsening the time resolution and can be coherent with noises of other velocity measurements where the same turbulent structures are used. This noise should not be confused with the impact of zero mean frequency zonal flow modulations and can be the explanation for the TEXTOR velocity spectra measured by beam emission spectroscopy.
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52.35.Ra Plasma turbulence
52.70.-m Plasma diagnostic techniques and instrumentation
02.70.Rr General statistical methods
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

On relaxation and transport in gyrokinetic drift wave turbulence with zonal flow

Y. Kosuga and P. H. Diamond

Phys. Plasmas 18, 122305 (2011); http://dx.doi.org/10.1063/1.3662428 (16 pages)

Online Publication Date: 20 December 2011

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We present a theory for relaxation and transport in phase space for gyrokinetic drift wave turbulence with zonal flow. The interaction between phase space eddys and zonal flows is considered in two different limits, namely for K>>1 and K ≃ 1 where K is the Kubo number. For K>>1, the growth of an isolated coherent phase space structure is calculated, including the associated zonal flow dynamics. For K ≃ 1, mean field relaxation dynamics is considered in the presence of phase space granulations and zonal flows. In both limits, it is shown that the evolution equations for phase space structures are structurally similar to a corresponding Charney-Drazin theorem for zonal momentum balance in a potential vorticity conserving, quasi-geostrophic system. The transport flux in phase space is calculated in the presence of phase space density granulations and zonal flows. The zonal flow exerts a dynamical friction on ion phase space density evolution, which is a fundamentally new zonal flow effect.
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52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Kt Drift waves
52.35.Ra Plasma turbulence
52.35.We Plasma vorticity

Electrostatic solitary structures in presence of non-thermal electrons and a warm electron beam on the auroral field lines

S. V. Singh, G. S. Lakhina, R. Bharuthram, and S. R. Pillay

Phys. Plasmas 18, 122306 (2011); http://dx.doi.org/10.1063/1.3671955 (7 pages)

Online Publication Date: 29 December 2011

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Electrostatic solitary waves (ESWs) have been observed by satellites in the auroral region of the Earth’s magnetosphere. These ESWs are found to be having both positive and negative electrostatic potentials. Using the Sagdeeev psuedo-potential technique, arbitrary amplitude electron-acoustic solitary waves/double layers are studied in an unmagnetized plasma consisting of non-thermally distributed hot electrons, fluid cold electrons, a warm electron beam, and ions. The inertia of the warm electrons, and not the beam speed, is essential for the existence of positive potential solitary structures. Existence domains for positive as well as negative potential electrostatic solitons/double layers are obtained. For the typical auroral region parameters, the electric field amplitude of the negative potential solitons is found to be in the range ∼(3–30) mV/m and ∼(5–80) mV/m for the positive potential solitons. For the negative potential solitons/double layers, the amplitudes are higher when their widths are smaller. On the other hand, the amplitude of the positive potential structures increase with their widths.
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94.30.Aa Auroral phenomena in magnetosphere
95.30.Qd Magnetohydrodynamics and plasmas
94.05.Fg Solitons and solitary waves
52.35.Sb Solitons; BGK modes
back to top Magnetically Confined Plasmas, Heating, Confinement

Isotope mass and charge effects in tokamak plasmas

I. Pusztai, J. Candy, and P. Gohil

Phys. Plasmas 18, 122501 (2011); http://dx.doi.org/10.1063/1.3663844 (10 pages)

Online Publication Date: 13 December 2011

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The effect of primary ion species of differing charge and mass—specifically, deuterium, hydrogen, and helium—on instabilities and transport is studied in DIII-D plasmas through gyrokinetic simulations with gyro [J. Candy and E. Belli, General Atomics Technical Report No. GA-A26818, 2010]. In linear simulations under imposed similarity of the profiles, there is an isomorphism between the linear growth rates of hydrogen isotopes, but the growth rates are higher for Z > 1 main ions due to the appearance of the charge in the Poisson equation. On ion scales the most significant effect of the different electron-to-ion mass ratio appears through collisions stabilizing trapped electron modes. In nonlinear simulations, significant favorable deviations from pure gyro-Bohm scaling are found due to electron-to-ion mass ratio effects and collisions. The presence of any non-trace impurity species cannot be neglected in a comprehensive simulation of the transport; including carbon impurity in the simulations caused a dramatic reduction of energy fluxes. The transport in the analyzed deuterium and helium discharges could be well reproduced in gyrokinetic and gyrofluid simulations while the significant hydrogen discrepancy is the subject of ongoing investigation.
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52.55.Fa Tokamaks, spherical tokamaks
52.65.Tt Gyrofluid and gyrokinetic simulations
52.25.Fi Transport properties
52.25.Vy Impurities in plasmas
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Mean-field Ohm’s law and coaxial helicity injection in force-free plasmas

R. H. Weening

Phys. Plasmas 18, 122502 (2011); http://dx.doi.org/10.1063/1.3664322 (12 pages)

Online Publication Date: 13 December 2011

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A theoretical analysis of steady-state coaxial helicity injection (CHI) in force-free plasmas is presented using a parallel mean-field Ohm’s law that includes resistivity η and hyper-resistivity Λ terms. Using Boozer coordinates, a partial differential equation is derived for the time evolution of the mean-field poloidal magnetic flux, or magnetic Hamiltonian function, from the parallel mean-field Ohm’s law. A general expression is obtained from the mean-field theory for the efficiency of CHI current drive in force-free plasmas. Inductances of internal energy, magnetic helicity, and poloidal magnetic flux are used to characterize axisymmetric plasma equilibria that have a model current profile. Using the model current profile, a method is suggested to determine the level of magnetohydrodynamic activity at the magnetic axis and the consequent deviation from the completely relaxed Taylor state. The mean-field Ohm’s law model suggests that steady-state CHI can be viewed most simply as a boundary layer problem.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
02.30.Jr Partial differential equations

Analytic model for coaxial helicity injection in tokamak plasmas

R. H. Weening

Phys. Plasmas 18, 122503 (2011); http://dx.doi.org/10.1063/1.3665088 (11 pages)

Online Publication Date: 13 December 2011

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Using a partial differential equation for the time evolution of the mean-field poloidal magnetic flux that incorporates resistivity η and hyper-resistivity Λ terms, an exact analytic solution is obtained for steady-state coaxial helicity injection (CHI) in force-free large aspect ratio tokamaks. The analytic mean-field Ohm’s law model allows for calculation of the tokamak CHI current drive efficiency and the plasma inductances at arbitrary levels of magnetic fluctuations, or dynamo activity. The results of the mean-field model suggest that CHI approaching Ohmic efficiency is only possible in tokamaks when the size of the effective current drive boundary layer, δ ≡ (Λ/η)1/2, becomes greater than half the size of the plasma, δ>a/2, with a the plasma minor radius. The electron thermal diffusivity due to magnetic fluctuation induced transport is obtained from the expression χe = Λ/μ0de2, with μ0 the permeability of free space and de the electron skin depth, which for typical tokamak fusion plasma parameters is on the order of a millimeter. Thus, the ratio of the energy confinement time to the resistive diffusion time in a tokamak plasma driven by steady-state CHI approaching Ohmic efficiency is shown to be constrained by the relation τE/τη<(de/a)2 ≈ 10-6. The mean-field model suggests that steady-state CHI can be viewed most simply as a boundary layer of stochastically wandering magnetic field lines.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

X-transport of ions in diverted tokamaks, with application to DIII-D

Weston M. Stacey

Phys. Plasmas 18, 122504 (2011); http://dx.doi.org/10.1063/1.3671910 (9 pages)

Online Publication Date: 29 December 2011

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A calculation model for X-transport due to the radially outward grad-B and curvature drift of ions trapped poloidally in the null-Bθ X-region just inside the X-point in diverted tokamaks is presented. Calculations are presented for two representative DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] shots which indicate that X-transport effects are significant and should be taken into account in calculations of present and future experiments.
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52.55.Fa Tokamaks, spherical tokamaks
28.52.Fa Materials
52.25.Fi Transport properties

Linear and nonlinear verification of gyrokinetic microstability codes

R. V. Bravenec, J. Candy, M. Barnes, and C. Holland

Phys. Plasmas 18, 122505 (2011); http://dx.doi.org/10.1063/1.3671907 (9 pages)

Online Publication Date: 30 December 2011

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Verification of nonlinear microstability codes is a necessary step before comparisons or predictions of turbulent transport in toroidal devices can be justified. By verification we mean demonstrating that a code correctly solves the mathematical model upon which it is based. Some degree of verification can be accomplished indirectly from analytical instability threshold conditions, nonlinear saturation estimates, etc., for relatively simple plasmas. However, verification for experimentally relevant plasma conditions and physics is beyond the realm of analytical treatment and must rely on code-to-code comparisons, i.e., benchmarking. The premise is that the codes are verified for a given problem or set of parameters if they all agree within a specified tolerance. True verification requires comparisons for a number of plasma conditions, e.g., different devices, discharges, times, and radii. Running the codes and keeping track of linear and nonlinear inputs and results for all conditions could be prohibitive unless there was some degree of automation. We have written software to do just this and have formulated a metric for assessing agreement of nonlinear simulations. We present comparisons, both linear and nonlinear, between the gyrokinetic codes GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)] and GS2 [W. Dorland, F. Jenko, M. Kotschenreuther, and B. N. Rogers, Phys. Rev. Lett. 85, 5579 (2000)]. We do so at the mid-radius for the same discharge as in earlier work [C. Holland, A. E. White, G. R. McKee, M. W. Shafer, J. Candy, R. E. Waltz, L. Schmitz, and G. R. Tynan, Phys. Plasmas 16, 052301 (2009)]. The comparisons include electromagnetic fluctuations, passing and trapped electrons, plasma shaping, one kinetic impurity, and finite Debye-length effects. Results neglecting and including electron collisions (Lorentz model) are presented. We find that the linear frequencies with or without collisions agree well between codes, as do the time averages of the nonlinear fluxes without collisions. With collisions, the differences between the time-averaged fluxes are larger than the uncertainties defined as the oscillations of the fluxes, with the GS2 fluxes consistently larger (or more positive) than those from GYRO. However, the electrostatic fluxes are much smaller than those without collisions (the electromagnetic energy flux is negligible in both cases). In fact, except for the electron energy fluxes, the absolute magnitudes of the differences in fluxes with collisions are the same or smaller than those without. None of the fluxes exhibit large absolute differences between codes. Beyond these results, the specific linear and nonlinear benchmarks proposed here, as well as the underlying methodology, provide the basis for a wide variety of future verification efforts.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Ra Plasma turbulence
52.65.Tt Gyrofluid and gyrokinetic simulations
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