Phys. Plasmas (1994-Present) - Physics of Plasmas

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Electron kappa distribution and steady-state Langmuir turbulence

Peter H. Yoon

Phys. Plasmas 19, 052301 (2012); http://dx.doi.org/10.1063/1.4710515 (6 pages)

Online Publication Date: 4 May 2012

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In a recent pair of papers, the present author discussed a self-consistent theory of asymptotically steady-state electron distribution function and Langmuir turbulence intensity in one [P. H. Yoon, Phys. Plasmas 18, 122303 (2011)] and three [P. H. Yoon, Phys. Plasmas 19, 012304 (2012)] dimensions. The resulting electron distribution function is a type of kappa distribution that features a non-Maxwellian energetic tail component. However, while the one-dimensional solution is rigorously correct, the three-dimensional solution, which was obtained using the cylindrical coordinate representation, contains two features that may be inconsistent for field-free plasmas. One is the assumption that the resonance condition can be approximated by ω-k·v ≈ ω-k_∥v_∥. Needless to say, this is not the most general condition. The second inconsistency is that while the electron distribution is isotropic in velocity, the Langmuir turbulence intensity depends on the wave propagation direction. While these features may not be too unrealistic in the presence of an implicit ambient magnetic field, they certainly cannot be correct if the plasma is genuinely unmagnetized. In the present paper, we rectify such shortcomings by properly reformulating the problem using a spherical coordinate system in a truly free-field plasma.

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52.25.Fi	Transport properties
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Ra	Plasma turbulence

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Effect of nonthermal electrons on oblique electrostatic excitations in a magnetized electron-positron-ion plasma

H. Alinejad

Phys. Plasmas 19, 052302 (2012); http://dx.doi.org/10.1063/1.4714609 (6 pages)

Online Publication Date: 14 May 2012

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The linear and nonlinear propagation of ion-acoustic waves are investigated in a magnetized electron-positron-ion (e-p-i) plasma with nonthermal electrons. In the linear regime, the propagation of two possible modes and their evolution are studied via a dispersion relation. In the cases of parallel and perpendicular propagation, it is shown that these two possible modes are always stable. Then, the Korteweg-de Vries equation describing the dynamics of ion-acoustic solitary waves is derived from a weakly nonlinear analysis. The influence on the solitary wave characteristics of relevant physical parameters such as nonthermal electrons, magnetic field, obliqueness, positron concentration, and temperature ratio is examined. It is observed that the increasing nonthermal electrons parameter makes the solitary structures much taller and narrower. Also, it is revealed that the magnetic field strength makes the solitary waves more spiky. The present investigation contributes to the physics of the nonlinear electrostatic ion-acoustic waves in space and laboratory e-p-i plasmas in which wave damping produces an electron tail.

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52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Sb	Solitons; BGK modes
02.10.-v	Logic, set theory, and algebra
52.25.-b	Plasma properties

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Gauge properties of the guiding center variational symplectic integrator

J. Squire, H. Qin, and W. M. Tang

Phys. Plasmas 19, 052501 (2012); http://dx.doi.org/10.1063/1.4714608 (7 pages)

Online Publication Date: 14 May 2012

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Variational symplectic algorithms have recently been developed for carrying out long-time simulation of charged particles in magnetic fields [H. Qin and X. Guan, Phys. Rev. Lett. 100, 035006 (2008); H. Qin, X. Guan, and W. Tang, Phys. Plasmas (2009); J. Li, H. Qin, Z. Pu, L. Xie, and S. Fu, Phys. Plasmas 18, 052902 (2011)]. As a direct consequence of their derivation from a discrete variational principle, these algorithms have very good long-time energy conservation, as well as exactly preserving discrete momenta. We present stability results for these algorithms, focusing on understanding how explicit variational integrators can be designed for this type of system. It is found that for explicit algorithms, an instability arises because the discrete symplectic structure does not become the continuous structure in the t→0 limit. We examine how a generalized gauge transformation can be used to put the Lagrangian in the “antisymmetric discretization gauge,” in which the discrete symplectic structure has the correct form, thus eliminating the numerical instability. Finally, it is noted that the variational guiding center algorithms are not electromagnetically gauge invariant. By designing a model discrete Lagrangian, we show that the algorithms are approximately gauge invariant as long as A and φ are relatively smooth. A gauge invariant discrete Lagrangian is very important in a variational particle-in-cell algorithm where it ensures current continuity and preservation of Gauss’s law [J. Squire, H. Qin, and W. Tang (to be published)].

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52.65.-y	Plasma simulation
02.30.Xx	Calculus of variations
02.60.Jh	Numerical differentiation and integration

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Kinetic damping of resistive wall modes in ITER

I. T. Chapman, Y. Q. Liu, O. Asunta, J. P. Graves, T. Johnson, and M. Jucker

Phys. Plasmas 19, 052502 (2012); http://dx.doi.org/10.1063/1.4714877 (10 pages)

Online Publication Date: 15 May 2012

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Full drift kinetic modelling including finite orbit width effects has been used to assess the passive stabilisation of the resistive wall mode (RWM) that can be expected in the ITER advanced scenario. At realistic plasma rotation frequency, the thermal ions have a stabilising effect on the RWM, but the stability limit remains below the target plasma pressure to achieve Q = 5. However, the inclusion of damping arising from the fusion-born alpha particles, the NBI ions, and ICRH fast ions extends the RWM stability limit above the target β for the advanced scenario. The fast ion damping arises primarily from finite orbit width effects and is not due to resonance between the particle frequencies and the instability.

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52.55.Fa	Tokamaks, spherical tokamaks
52.25.Dg	Plasma kinetic equations
52.25.Fi	Transport properties
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.50.Gj	Plasma heating by particle beams
52.50.Qt	Plasma heating by radio-frequency fields; ICR, ICP, helicons

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The quantum equations of state of plasma under the influence of a weak magnetic field

N. A. Hussein, D. A. Eisa, and M. G. Eldin

Phys. Plasmas 19, 052701 (2012); http://dx.doi.org/10.1063/1.4704794 (6 pages)

Online Publication Date: 9 May 2012

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The aim of this paper is to calculate the magnetic quantum equations of state of plasma, the calculation is based on the magnetic binary Slater sum in the case of low density. We consider only the thermal equilibrium plasma in the case of nλab3≪1, where λab2 = math

is the thermal De Broglie wave length between two particles. The formulas contain the contributions of the magnetic field effects. Using these results we compute the magnetization and the magnetic susceptibility. Our equation of state is compared with others.

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52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
05.70.Ce	Thermodynamic functions and equations of state

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Tendency of spherically imploding plasma liners formed by merging plasma jets to evolve toward spherical symmetry

J. T. Cassibry, M. Stanic, S. C. Hsu, F. D. Witherspoon, and S. I. Abarzhi

Phys. Plasmas 19, 052702 (2012); http://dx.doi.org/10.1063/1.4714606 (9 pages)

Online Publication Date: 14 May 2012

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We have performed three-dimensional (3D) simulations using smoothed particle hydrodynamics (SPH) in order to study the effects of discrete plasma jets on the processes of plasma liner formation, implosion on vacuum, and expansion. It was found that the pressure histories of the inner portion of the liner from 3D SPH simulations with a uniform liner and with 30 discrete plasma jets were qualitatively and quantitatively similar from peak compression through the complete stagnation of the liner. The 3D simulations with a uniform liner were first benchmarked against results from one-dimensional radiation-hydrodynamic simulations [T. J. Awe et al., Phys. Plasmas 18, 072705 (2011)]. Two-dimensional plots of the pressure field show that the discrete jet SPH case evolves towards a profile that is almost indistinguishable from the SPH case with a uniform liner, thus indicating that non-uniformities due to discrete jets are smeared out by late stages of the implosion. The processes of plasma liner formation and implosion on vacuum were shown to be robust against Rayleigh-Taylor instability growth. Finally, interparticle mixing for a liner imploding on vacuum was investigated. The mixing rate was found to be very small until after the peak compression for the 30 jet simulations.

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52.75.-d	Plasma devices
52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.-y	Plasma simulation

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Orbital ferromagnetism and the Chandrasekhar mass-limit

M. Akbari-Moghanjoughi

Phys. Plasmas 19, 052901 (2012); http://dx.doi.org/10.1063/1.4714611 (7 pages)

Online Publication Date: 15 May 2012

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In this paper, we use quantum magnetohydrodynamic as well as magnetohydrostatic (MHS) models for a zero-temperature Fermi-Dirac plasma to show the fundamental role of Landau orbital ferromagnetism (LOFER) on the magnetohydrostatic stability of compact stars. It is revealed that the generalized flux-conserved equation of state of form B = βρ^2s/3 only with conditions 0 ≤ s ≤ 1 and 0 ≤ β< math

can lead to a stable compact stellar configuration. The distinct critical value β_cr = math

is shown to affect the magnetohydrostatic stability of the LOFER (s = 1) state and the magnetic field strength limit on the compact stellar configuration. Furthermore, the value of the parameter β is remarked to fundamentally alter the Chandrasekhar mass-radius relation and the known mass-limit on white dwarfs when the star is in LOFER state. Current findings can help to understand the role of flux-frozen ferromagnetism and its fundamental role on hydrostatic stability of relativistically degenerate super-dense plasmas such as white dwarfs.

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52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
95.30.Qd	Magnetohydrodynamics and plasmas
97.10.Ld	Magnetic and electric fields; polarization of starlight
97.20.Rp	Faint blue stars (including blue stragglers), white dwarfs, degenerate stars, nuclei of planetary nebulae
52.27.Ny	Relativistic plasmas

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Quasi-matched propagation of ultra-short, intense laser pulses in plasma channels

C. Benedetti, C. B. Schroeder, E. Esarey, and W. P. Leemans

Phys. Plasmas 19, 053101 (2012); http://dx.doi.org/10.1063/1.4707393 (8 pages)

Online Publication Date: 4 May 2012

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The propagation of an ultrashort and relativistically intense laser pulse in a preformed plasma channel is investigated. The nonlinear paraxial wave equation describing the laser propagation in the plasma is solved both analytically and numerically. For any arbitrary temporal laser pulse profile with a given power (less then the critical power for self-focusing) and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse ensuring quasi-matched propagation, neglecting non-paraxial effects. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes laser evolution (e.g., minimizes spot size oscillations). The analytical and semi-analytical results obtained for both cases in the weakly relativistic regime are presented and validated through comparison with numerical simulations.

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52.38.Hb	Self-focussing, channeling, and filamentation in plasmas
52.27.Ny	Relativistic plasmas
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.65.-y	Plasma simulation
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
02.60.Cb	Numerical simulation; solution of equations

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Frequency-selective plasmonic wave propagation through the overmoded waveguide with photonic-band-gap slab arrays

Young-Min Shin

Phys. Plasmas 19, 053102 (2012); http://dx.doi.org/10.1063/1.4707394 (5 pages)

Online Publication Date: 10 May 2012

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Confined propagation of guided waves through the periodically corrugated channel sandwiched between two staggered dielectric photonic-band-gap slab arrays is investigated with the band-response analysis. Numerical simulations show that longitudinally polarized evanescent waves within the band gap propagate with insertion loss of ∼−0.2 to 1 dB (−0.05 to 0.4 dB/mm at G-band) in the hybrid band filter. This structure significantly suppresses low energy modes and higher-order-modes beyond the band-gap, including background noises, down to ∼−45 dB. This would enable the single-mode propagation in the heavily over-moded waveguide (TEM-type), minimizing abnormal excitation probability of trapped modes. This band filter could be integrated with active and passive RF components for electron beam and optoelectronic devices.

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52.40.Fd	Plasma interactions with antennas; plasma-filled waveguides
52.65.-y	Plasma simulation
52.25.Mq	Dielectric properties
42.70.Qs	Photonic bandgap materials
52.25.Os	Emission, absorption, and scattering of electromagnetic radiation
02.60.Cb	Numerical simulation; solution of equations

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Effect of pulse profile and chirp on a laser wakefield generation

Xiaomei Zhang, Baifei Shen, Liangliang Ji, Wenpeng Wang, Jiancai Xu, Yahong Yu, Longqing Yi, Xiaofeng Wang, Nasr A. M. Hafz, and V. Kulagin

Phys. Plasmas 19, 053103 (2012); http://dx.doi.org/10.1063/1.4714610 (7 pages)

Online Publication Date: 10 May 2012

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A laser wakefield driven by an asymmetric laser pulse with/without chirp is investigated analytically and through two-dimensional particle-in-cell simulations. For a laser pulse with an appropriate pulse length compared with the plasma wavelength, the wakefield amplitude can be enhanced by using an asymmetric un-chirped laser pulse with a fast rise time; however, the growth is small. On the other hand, the wakefield can be greatly enhanced for both positively chirped laser pulse having a fast rise time and negatively chirped laser pulse having a slow rise time. Simulations show that at the early laser-plasma interaction stage, due to the influence of the fast rise time the wakefield driven by the positively chirped laser pulse is more intense than that driven by the negatively chirped laser pulse, which is in good agreement with analytical results. At a later time, since the laser pulse with positive chirp exhibits opposite evolution to the one with negative chirp when propagating in plasma, the wakefield in the latter case grows more intensely. These effects should be useful in laser wakefield acceleration experiments operating at low plasma densities.

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52.38.Kd	Laser-plasma acceleration of electrons and ions
52.65.Rr	Particle-in-cell method
52.25.Fi	Transport properties

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A study of fast electron energy transport in relativistically intense laser-plasma interactions with large density scalelengths

R. H. H. Scott, F. Perez, J. J. Santos, C. P. Ridgers, J. R. Davies, K. L. Lancaster, S. D. Baton, Ph. Nicolai, R. M. G. M. Trines, A. R. Bell, S. Hulin, M. Tzoufras, S. J. Rose, and P. A. Norreys

Phys. Plasmas 19, 053104 (2012); http://dx.doi.org/10.1063/1.4714615 (13 pages)

Online Publication Date: 15 May 2012

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A systematic experimental and computational investigation of the effects of three well characterized density scalelengths on fast electron energy transport in ultra-intense laser-solid interactions has been performed. Experimental evidence is presented which shows that, when the density scalelength is sufficiently large, the fast electron beam entering the solid-density plasma is best described by two distinct populations: those accelerated within the coronal plasma (the fast electron pre-beam) and those accelerated near or at the critical density surface (the fast electron main-beam). The former has considerably lower divergence and higher temperature than that of the main-beam with a half-angle of ∼20°. It contains up to 30% of the total fast electron energy absorbed into the target. The number, kinetic energy, and total energy of the fast electrons in the pre-beam are increased by an increase in density scalelength. With larger density scalelengths, the fast electrons heat a smaller cross sectional area of the target, causing the thinnest targets to reach significantly higher rear surface temperatures. Modelling indicates that the enhanced fast electron pre-beam associated with the large density scalelength interaction generates a magnetic field within the target of sufficient magnitude to partially collimate the subsequent, more divergent, fast electron main-beam.

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52.25.Fi	Transport properties
52.27.Ny	Relativistic plasmas
52.40.Hf	Plasma-material interactions; boundary layer effects
52.50.Gj	Plasma heating by particle beams
52.70.Kz	Optical (ultraviolet, visible, infrared) measurements

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Highly efficient accelerator of dense matter using laser-induced cavity pressure acceleration

J. Badziak, S. Jabłoński, T. Pisarczyk, P. Rączka, E. Krousky, R. Liska, M. Kucharik, T. Chodukowski, Z. Kalinowska, P. Parys, M. Rosiński, S. Borodziuk, and J. Ullschmied

Phys. Plasmas 19, 053105 (2012); http://dx.doi.org/10.1063/1.4714660 (8 pages)

Online Publication Date: 15 May 2012

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Acceleration of dense matter to high velocities is of high importance for high energy density physics, inertial confinement fusion, or space research. The acceleration schemes employed so far are capable of accelerating dense microprojectiles to velocities approaching 1000 km/s; however, the energetic efficiency of acceleration is low. Here, we propose and demonstrate a highly efficient scheme of acceleration of dense matter in which a projectile placed in a cavity is irradiated by a laser beam introduced into the cavity through a hole and then accelerated in a guiding channel by the pressure of a hot plasma produced in the cavity by the laser beam or by the photon pressure of the ultra-intense laser radiation trapped in the cavity. We show that the acceleration efficiency in this scheme can be much higher than that achieved so far and that sub-relativisitic projectile velocities are feasible in the radiation pressure regime.

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52.38.Kd	Laser-plasma acceleration of electrons and ions
52.25.Fi	Transport properties
52.27.Ny	Relativistic plasmas

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X-ray conversion efficiency in vacuum hohlraum experiments at the National Ignition Facility

R. E. Olson, L. J. Suter, J. L. Kline, D. A. Callahan, M. D. Rosen, S. N. Dixit, O. L. Landen, N. B. Meezan, J. D. Moody, C. A. Thomas, A. Warrick, K. Widmann, E. A. Williams, and S. H. Glenzer

Phys. Plasmas 19, 053301 (2012); http://dx.doi.org/10.1063/1.4704795 (9 pages)

Online Publication Date: 7 May 2012

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X-ray fluxes measured in the first 96 and 192 beam vacuum hohlraum experiments at the National Ignition Facility (NIF) were significantly higher than predicted by computational simulations employing XSN average atom atomic physics and highly flux-limited electron heat conduction. For agreement with experimental data, it was found that the coronal plasma emissivity must be simulated with a detailed configuration accounting model that accounts for x-ray emission involving all of the significant ionization states. It was also found that an electron heat conduction flux limit of f = 0.05 is too restrictive, and that a flux limit of f = 0.15 results in a much better match with the NIF vacuum hohlraum experimental data. The combination of increased plasma emissivity and increased electron heat conduction in this new high flux hohlraum model results in a reduction in coronal plasma energy and, hence, an explanation for the high (∼85%-90%) x-ray conversion efficiencies observed in the 235 < T_r < 345 eV NIF vacuum hohlraum experiments.

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52.25.Os	Emission, absorption, and scattering of electromagnetic radiation
52.50.Gj	Plasma heating by particle beams
52.65.-y	Plasma simulation
52.70.La	X-ray and γ-ray measurements
52.25.Fi	Transport properties

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Correction factors for saturation effects in white light and laser absorption spectroscopy for application to low pressure plasmas

S. Briefi, C. Wimmer, and U. Fantz

Phys. Plasmas 19, 053501 (2012); http://dx.doi.org/10.1063/1.4714764 (10 pages)

Online Publication Date: 15 May 2012

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In white light absorption spectroscopy, the broadening of the absorption signal due to the apparatus profile of the spectrometer may lead to an underestimation of the determined density as one measures an apparent optical depth. This is in particular true for high optical depth where saturation effects of the transmitted intensity occur. Provided that the line profile of the absorption line is known, the apparent optical depth effect can be accounted for by introducing a correction factor. The impact of the saturation and the approach of considering the effect are demonstrated for argon and indium lines in low pressure plasmas where correction factors of one order of magnitude or even higher are reached very easily. For the indium line, the hyperfine splitting has been taken into account. In laser absorption, the line profile is resolved. However, the weak but rather broad background emission of the laser diode can cause a saturation signal at the photo diode resulting also in an underestimation of the density obtained from the analysis. It is shown that this can be taken into account by fitting the theoretical line profile to the measured absorption signal which yields also a correction factor. The method is introduced and demonstrated at the example of the cesium resonance line including the hyperfine splitting. Typical correction factors around two are obtained for the cesium ground state density at conditions of a low pressure negative hydrogen ion source in which cesium is evaporated to enhance the negative ion production.

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52.38.Dx	Laser light absorption in plasmas (collisional, parametric, etc.)
52.25.-b	Plasma properties

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About excitation of surface plasma waves by elliptical relativistic electron beam in a magnetized dusty plasma column with elliptical cross section

A. Abdoli-Arani and B. Jazi

Phys. Plasmas 19, 053701 (2012); http://dx.doi.org/10.1063/1.4714607 (8 pages)

Online Publication Date: 14 May 2012

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The surface plasma waves in a magnetized dusty plasma elliptical cylinder driven by elliptic relativistic electron beam propagating inside the elliptical cylinder are studied. The dispersion relation of surface plasma waves has been retrieved from the derived dispersion relation by considering that the beam is absent and there is no dust in the plasma cylinder. Mathematically, it is shown that the beam can interact with the surface plasma waves via Cerenkov interaction and fast cyclotron interaction. The growth rate and phase velocity in every cases are obtained. Finally, the numerical results and graphs are presented.

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52.40.Db	Electromagnetic (nonlaser) radiation interactions with plasma
52.25.Fi	Transport properties
52.27.Lw	Dusty or complex plasmas; plasma crystals
52.27.Ny	Relativistic plasmas
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)

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Laser plasma accelerators

V. Malka

Phys. Plasmas 19, 055501 (2012); http://dx.doi.org/10.1063/1.3695389 (11 pages)

Online Publication Date: 5 April 2012

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This review article highlights the tremendous evolution of the research on laser plasma accelerators which has, in record time, led to the production of high quality electron beams at the GeV level, using compact laser systems. I will describe the path we followed to explore different injection schemes and I will present the most significant breakthrough which allowed us to generate stable, high peak current and high quality electron beams, with control of the charge, of the relative energy spread and of the electron energy.

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52.38.Kd	Laser-plasma acceleration of electrons and ions
52.38.Dx	Laser light absorption in plasmas (collisional, parametric, etc.)
52.25.Fi	Transport properties

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Creating and studying ion acoustic waves in ultracold neutral plasmas

T. C. Killian, P. McQuillen, T. M. O’Neil, and J. Castro

Phys. Plasmas 19, 055701 (2012); http://dx.doi.org/10.1063/1.3694654 (9 pages)

Online Publication Date: 23 March 2012

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We excite ion acoustic waves in ultracold neutral plasmas by imprinting density modulations during plasma creation. Laser-induced fluorescence is used to observe the density and velocity perturbations created by the waves. The effect of expansion of the plasma on the evolution of the wave amplitude is described by treating the wave action as an adiabatic invariant. After accounting for this effect, we determine that the waves are weakly damped, but the damping is significantly faster than expected for Landau damping.

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52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.Fi	Transport properties
52.70.Kz	Optical (ultraviolet, visible, infrared) measurements
52.38.Kd	Laser-plasma acceleration of electrons and ions

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Applying alpha-channeling to mirror machines

A. I. Zhmoginov and N. J. Fisch

Phys. Plasmas 19, 055702 (2012); http://dx.doi.org/10.1063/1.3701997 (7 pages)

Online Publication Date: 18 April 2012

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The α-channeling effect entails the use of radio-frequency waves to expel and cool high-energetic α particles born in a fusion reactor; the device reactivity can then be increased even further by redirecting the extracted energy to fuel ions. Originally proposed for tokamaks, this technique has also been shown to benefit open-ended fusion devices. Here, the fundamental theory and practical aspects of α channeling in mirror machines are reviewed, including the influence of magnetic field inhomogeneity and the effect of a finite wave region on the α-channeling mechanism. For practical implementation of the α-channeling effect in mirror geometry, suitable contained weakly damped modes are identified. In addition, the parameter space of candidate waves for implementing the α-channeling effect can be significantly extended through the introduction of a suitable minority ion species that has the catalytic effect of moderating the transfer of power from the α-channeling wave to the fuel ions.

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52.55.Fa	Tokamaks, spherical tokamaks
28.52.Av	Theory, design, and computerized simulation
28.52.Fa	Materials

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Understanding disruptions in tokamaks

Leonid E. Zakharov, Sergei A. Galkin, Sergei N. Gerasimov, and JET-EFDA contributors

Phys. Plasmas 19, 055703 (2012); http://dx.doi.org/10.1063/1.4705694 (13 pages)

Online Publication Date: 1 May 2012

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This paper describes progress achieved since 2007 in understanding disruptions in tokamaks, when the effect of plasma current sharing with the wall was introduced into theory. As a result, the toroidal asymmetry of the plasma current measurements during vertical disruption event (VDE) on the Joint European Torus was explained. A new kind of plasma equilibria and mode coupling was introduced into theory, which can explain the duration of the external kink 1/1 mode during VDE. The paper presents first results of numerical simulations using a free boundary plasma model, relevant to disruptions.

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52.55.Fa	Tokamaks, spherical tokamaks
52.55.Pi	Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.40.Hf	Plasma-material interactions; boundary layer effects
52.70.Ds	Electric and magnetic measurements
52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.-y	Plasma simulation

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Evidence of critical balance in kinetic Alfvén wave turbulence simulations

J. M. TenBarge and G. G. Howes

Phys. Plasmas 19, 055901 (2012); http://dx.doi.org/10.1063/1.3693974 (8 pages)

Online Publication Date: 21 March 2012

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A numerical simulation of kinetic plasma turbulence is performed to assess the applicability of critical balance to kinetic, dissipation scale turbulence. The analysis is performed in the frequency domain to obviate complications inherent in performing a local analysis of turbulence. A theoretical model of dissipation scale critical balance is constructed and compared to simulation results, and excellent agreement is found. This result constitutes the first evidence of critical balance in a kinetic turbulence simulation and provides evidence of an anisotropic turbulence cascade extending into the dissipation range. We also perform an Eulerian frequency analysis of the simulation data and compare it to the results of a previous study of magnetohydrodynamic turbulence simulations.

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52.35.Bj	Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Ra	Plasma turbulence
47.27.E-	Turbulence simulation and modeling

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Numerical simulations of strong incompressible magnetohydrodynamic turbulence

J. Mason, J. C. Perez, S. Boldyrev, and F. Cattaneo

Phys. Plasmas 19, 055902 (2012); http://dx.doi.org/10.1063/1.3694123 (7 pages)

Online Publication Date: 21 March 2012

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Magnetised plasma turbulence pervades the universe and is likely to play an important role in a variety of astrophysical settings. Magnetohydrodynamics (MHD) provides the simplest theoretical framework in which phenomenological models for the turbulent dynamics can be built. Numerical simulations of MHD turbulence are widely used to guide and test the theoretical predictions; however, simulating MHD turbulence and accurately measuring its scaling properties is far from straightforward. Computational power limits the calculations to moderate Reynolds numbers and often simplifying assumptions are made in order that a wider range of scales can be accessed. After describing the theoretical predictions and the numerical approaches that are often employed in studying strong incompressible MHD turbulence, we present the findings of a series of high-resolution direct numerical simulations. We discuss the effects that insufficiencies in the computational approach can have on the solution and its physical interpretation.

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52.35.Ra	Plasma turbulence
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj	Magnetohydrodynamic and fluid equation
95.30.Qd	Magnetohydrodynamics and plasmas

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Impact of resonant magnetic perturbations on nonlinearly driven modes in drift-wave turbulence

M. Leconte and P. H. Diamond

Phys. Plasmas 19, 055903 (2012); http://dx.doi.org/10.1063/1.3694675 (11 pages)

Online Publication Date: 23 March 2012

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In this work, we study the effects of resonant magnetic perturbations (RMPs) on turbulence, flows, and confinement in the framework of resistive drift wave turbulence. We extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large math

. Both the vorticity flux (Reynolds stress) and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolution of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. We find states that are similar to the ZF-dominated state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude, the energy of zonal flows decreases and the turbulence energy increases with math

, corresponding to a damping of zonal flows.

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52.35.Kt	Drift waves
52.35.Ra	Plasma turbulence
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Fi	Transport properties

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Thermal plasma and fast ion transport in electrostatic turbulence in the large plasma device

Shu Zhou, W. W. Heidbrink, H. Boehmer, R. McWilliams, T. A. Carter, S. Vincena, S. K. P. Tripathi, and B. Van Compernolle

Phys. Plasmas 19, 055904 (2012); http://dx.doi.org/10.1063/1.3695341 (8 pages)

Online Publication Date: 28 March 2012

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The transport of thermal plasma and fast ions in electrostatic microturbulence is studied. Strong density and potential fluctuations (δn/n ∼ δφ/kT_e ∼ 0.5, f ∼ 5–50 kHz) are observed in the large plasma device (LAPD) [W. Gekelman, H. Pfister, Z. Lucky et al., Rev. Sci. Instrum. 62, 2875 (1991)] in density gradient regions produced by obstacles with slab or cylindrical geometry. Wave characteristics and the associated plasma transport are modified by driving sheared E × B drift through biasing the obstacle and by modification of the axial magnetic fields (B_z) and the plasma species. Cross-field plasma transport is suppressed with small bias and large B_z and is enhanced with large bias and small B_z. The transition in thermal plasma confinement is well explained by the cross-phase between density and potential fluctuations. Large gyroradius lithium fast ion beam (ρ_fast/ρ_s ∼ 10) orbits through the turbulent region. Scans with a collimated analyzer give detailed profiles of the fast ion spatial-temporal distribution. Fast-ion transport decreases rapidly with increasing fast-ion energy and gyroradius. Background waves with different scale lengths also alter the fast ion transport. Experimental results agree well with gyro-averaging theory. When the fast ion interacts with the wave for most of a wave period, a transition from super-diffusive to sub-diffusive transport is observed, as predicted by diffusion theory. Besides turbulent-wave-induced fast-ion transport, the static radial electric field (E_r) from biasing the obstacle leads to drift of the fast-ion beam centroid. The drift and broadening of the beam due to static E_r are evaluated both analytically and numerically. Simulation results indicate that the E_r induced transport is predominately convective.

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52.25.Fi	Transport properties
52.35.Ra	Plasma turbulence
52.35.Kt	Drift waves
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.65.-y	Plasma simulation
52.25.Gj	Fluctuation and chaos phenomena

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First-order finite-Larmor-radius fluid modeling of tearing and relaxation in a plasma pinch

J. R. King, C. R. Sovinec, and V. V. Mirnov

Phys. Plasmas 19, 055905 (2012); http://dx.doi.org/10.1063/1.3695346 (11 pages)

Online Publication Date: 29 March 2012

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Drift and Hall effects on magnetic tearing, island evolution, and relaxation in pinch configurations are investigated using a non-reduced first-order finite-Larmor-radius (FLR) fluid model with the nonideal magnetohydrodynamics (MHD) with rotation, open discussion (NIMROD) code [C.R. Sovinec and J. R. King, J. Comput. Phys. 229, 5803 (2010)]. An unexpected result with a uniform pressure profile is a drift effect that reduces the growth rate when the ion sound gyroradius (ρ_s) is smaller than the tearing-layer width. This drift is present only with warm-ion FLR modeling, and analytics show that it arises from ∇B and poloidal curvature represented in the Braginskii gyroviscous stress. Nonlinear single-helicity computations with experimentally relevant ρ_s values show that the warm-ion gyroviscous effects reduce saturated-island widths. Computations with multiple nonlinearly interacting tearing fluctuations find that m = 1 core-resonant-fluctuation amplitudes are reduced by a factor of two relative to single-fluid modeling by the warm-ion effects. These reduced core-resonant-fluctuation amplitudes compare favorably to edge coil measurements in the Madison Symmetric Torus (MST) reversed-field pinch [R. N. Dexter et al., Fusion Technol. 19, 131 (1991)]. The computations demonstrate that fluctuations induce both MHD- and Hall-dynamo emfs during relaxation events. The presence of a Hall-dynamo emf implies a fluctuation-induced Maxwell stress, and the simulation results show net transport of parallel momentum. The computed magnitude of force densities from the Maxwell and competing Reynolds stresses, and changes in the parallel flow profile, are qualitatively and semi-quantitatively similar to measurements during relaxation in MST.

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52.58.Lq	Z-pinches, plasma focus, and other pinch devices
52.25.Fi	Transport properties
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.)
52.25.Gj	Fluctuation and chaos phenomena
52.65.Kj	Magnetohydrodynamic and fluid equation

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Dissipation range turbulent cascades in plasmas

P. W. Terry, A. F. Almagri, G. Fiksel, C. B. Forest, D. R. Hatch, F. Jenko, M. D. Nornberg, S. C. Prager, K. Rahbarnia, Y. Ren, and J. S. Sarff

Phys. Plasmas 19, 055906 (2012); http://dx.doi.org/10.1063/1.3698309 (10 pages)

Online Publication Date: 2 April 2012

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Dissipation range cascades in plasma turbulence are described and spectra are formulated from the scaled attenuation in wavenumber space of the spectral energy transfer rate. This yields spectra characterized by the product of a power law and exponential fall-off, applicable to all scales. Spectral indices of the power law and exponential fall-off depend on the scaling of the dissipation, the strength of the nonlinearity, and nonlocal effects when dissipation rates of multiple fluctuation fields are different. The theory is used to derive spectra for MHD turbulence with magnetic Prandtl number greater than unity, extending previous work. The theory is also applied to generic plasma turbulence by considering the spectrum from damping with arbitrary wavenumber scaling. The latter is relevant to ion temperature gradient turbulence modeled by gyrokinetics. The spectrum in this case has an exponential component that becomes weaker at small scale, giving a power law asymptotically. Results from the theory are compared to three very different types of turbulence. These include the magnetic plasma turbulence of the Madison Symmetric Torus, the MHD turbulence of liquid metal in the Madison Dynamo Experiment, and gyrokinetic simulation of ion temperature gradient turbulence.

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