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Feb 2013

Volume 20, Issue 2, Articles (02xxxx)

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Phys. Plasmas 20, 022303 (2013); http://dx.doi.org/10.1063/1.4790639 (12 pages)

Julio J. Martinell and Diego del-Castillo-Negrete
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back to top Nonlinear Phenomena, Turbulence, Transport

Study of nonlinear oscillations in a glow discharge plasma using empirical mode decomposition and Hilbert Huang transform

A. M. Wharton, A. N. Sekar Iyengar, and M. S. Janaki

Phys. Plasmas 20, 022301 (2013); http://dx.doi.org/10.1063/1.4789853 (8 pages)

Online Publication Date: 1 February 2013

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Hilbert Huang transform (HHT) based time series analysis was carried out on nonlinear floating potential fluctuations obtained from hollow cathode glow discharge plasma in the presence of anode glow. HHT was used to obtain contour plots and the presence of nonlinearity was studied. Frequency shift with time, which is a typical nonlinear behaviour, was detected from the contour plots. Various plasma parameters were measured and the concepts of correlation coefficients and the physical contribution of each intrinsic mode function have been discussed. Physically important quantities such as instantaneous energy and their uses in studying physical phenomena such as intermittency and non-stationary data have also been discussed.
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52.80.Hc Glow; corona
02.30.Uu Integral transforms
52.25.Gj Fluctuation and chaos phenomena
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.)

On numerical turbulence generation for test-particle simulations

R. C. Tautz and A. Dosch

Phys. Plasmas 20, 022302 (2013); http://dx.doi.org/10.1063/1.4789861 (8 pages)

Online Publication Date: 4 February 2013

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A modified method is presented to generate artificial magnetic turbulence that is used for test-particle simulations. Such turbulent fields are obtained from the superposition of a set of wave modes with random polarizations and random directions of propagation. First, it is shown that the new method simultaneously fulfils requirements of isotropy, equal mean amplitude and variance for all field components, and vanishing divergence. Second, the number of wave modes required for a stochastic particle behavior is investigated by using a Lyapunov approach. For the special case of slab turbulence, it is shown that already for 16 wave modes the particle behavior agrees with that shown for considerably larger numbers of wave modes.
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52.35.Ra Plasma turbulence
52.65.-y Plasma simulation
02.30.Yy Control theory
02.50.Ey Stochastic processes
02.60.-x Numerical approximation and analysis
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Gyroaverage effects on chaotic transport by drift waves in zonal flows

Julio J. Martinell and Diego del-Castillo-Negrete

Phys. Plasmas 20, 022303 (2013); http://dx.doi.org/10.1063/1.4790639 (12 pages)

Online Publication Date: 8 February 2013

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Finite Larmor radius (FLR) effects on E × B test particle chaotic transport in the presence of zonal flows is studied. The FLR effects are introduced by the gyro-average of a simplified E × B guiding center model consisting of the linear superposition of a non-monotonic zonal flow and drift waves. Non-monotonic zonal flows play a critical role on transport because they exhibit robust barriers to chaotic transport in the region(s) where the shear vanishes. In addition, the non-monotonicity gives rise to nontrivial changes in the topology of the orbits of the E × B Hamiltonian due to separatrix reconnection. The present study focuses on the role of FLR effects on these two signatures of non-monotonic zonal flows: shearless transport barriers and separatrix reconnection. It is shown that, as the Larmor radius increases, the effective zonal flow profile bifurcates and multiple shearless regions are created. As a result, the topology of the gyro-averaged Hamiltonian exhibits very complex separatrix reconnection bifurcations. It is also shown that FLR effects tend to reduce chaotic transport. In particular, the restoration of destroyed transport barriers is observed as the Larmor radius increases. A detailed numerical study is presented on the onset of global chaotic transport as function of the amplitude of the drift waves and the Larmor radius. For a given amplitude, the threshold for the destruction of the shearless transport barrier, as function of the Larmor radius, exhibits a fractal-like structure. The FLR effects on a thermal distribution of test particles are also studied. In particular, the fraction of confined particles with a Maxwellian distribution of gyroradii is computed, and an effective transport suppression is found for high enough temperatures.
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52.35.Kt Drift waves
52.35.Vd Magnetic reconnection
02.60.-x Numerical approximation and analysis
52.25.Fi Transport properties
52.25.Gj Fluctuation and chaos phenomena
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Ion-acoustic K-dV and mK-dV solitons in a degenerate electron-ion dense plasma

L. Nahar, M. S. Zobaer, N. Roy, and A. A. Mamun

Phys. Plasmas 20, 022304 (2013); http://dx.doi.org/10.1063/1.4790519 (5 pages)

Online Publication Date: 11 February 2013

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A theoretical investigation has been made of the nonlinear propagation of ion-acoustic waves associated with a dense plasma system consisting degenerate electron and ion fluids. This fluid model, which is valid for both the non-relativistic and ultra-relativistic limits, has been employed with the reductive perturbation method. The K-dV and modified K-dV (mK-dV) equations have been derived and numerically analyzed. The basic features of solitons have been observed. It has been shown that the plasma system under consideration supports the propagation of solitons (electrostatic solitary structures) obtained from the solutions of K-dV and mK-dV equations. The implications of our results obtained from this investigation in compact astrophysical objects have been briefly discussed.
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52.35.Sb Solitons; BGK modes
95.30.Qd Magnetohydrodynamics and plasmas
02.30.Hq Ordinary differential equations
02.60.Lj Ordinary and partial differential equations; boundary value problems
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.)

Comparing linear ion-temperature-gradient-driven mode stability of the National Compact Stellarator Experiment and a shaped tokamak

J. A. Baumgaertel, G. W. Hammett, and D. R. Mikkelsen

Phys. Plasmas 20, 022305 (2013); http://dx.doi.org/10.1063/1.4791657 (8 pages)

Online Publication Date: 11 February 2013

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One metric for comparing confinement properties of different magnetic fusion energy configurations is the linear critical gradient of drift wave modes. The critical gradient scale length determines the ratio of the core to pedestal temperature when a plasma is limited to marginal stability in the plasma core. The gyrokinetic turbulence code GS2 was used to calculate critical temperature gradients for the linear, collisionless ion temperature gradient (ITG) mode in the National Compact Stellarator Experiment (NCSX) and a prototypical shaped tokamak, based on the profiles of a JET H-mode shot and the stronger shaping of ARIES-AT. While a concern was that the narrow cross section of NCSX at some toroidal locations would result in steep gradients that drive instabilities more easily, it is found that other stabilizing effects of the stellarator configuration offset this so that the normalized critical gradients for NCSX are competitive with or even better than for the tokamak. For the adiabatic ITG mode, NCSX and the tokamak had similar adiabatic ITG mode critical gradients, although beyond marginal stability, NCSX had larger growth rates. However, for the kinetic ITG mode, NCSX had a higher critical gradient and lower growth rates until a/LT ≈ 1.5 a/LT,crit, when it surpassed the tokamak's. A discussion of the results presented with respect to a/LT vs. R/LT is included.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Jd Magnetic mirrors, gas dynamic traps
52.65.Tt Gyrofluid and gyrokinetic simulations
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Kt Drift waves
52.35.Ra Plasma turbulence

Strong nonlinear electron multiplication without impact ionization in dielectric nanoparticles embedded in optical materials

Guillaume Duchateau

Phys. Plasmas 20, 022306 (2013); http://dx.doi.org/10.1063/1.4791662 (12 pages)

Online Publication Date: 13 February 2013

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The interaction of a dielectric nano-particle or nano-defect, embedded in the bulk of an optical material, with an intense and short laser pulse is addressed. Due to the finite size of the target and the possible large production of electrons in the conduction band, large electric field enhancement or surintensity may be induced inside the particle. Since ionization rates also depend on the instantaneous electric field, a strong time-dependent connection between electron production and surintensity may take place. Such a connection is shown to possibly lead to a nonlinear temporal increase in the free electron density relevant from an avalanche process, called optical avalanche, similar to the one induced by electron impact ionization. However, the present build-up in the electron density clearly exhibits more nonlinear features than traditional collisional avalanche, which is shown to induce an exponential growth of the density: when the optical avalanche is engaged, the temporal electron evolution exhibits an explosive behavior. That leads to a nanometric plasma at solid density whose subsequent laser heating may lead locally to matter under extreme conditions. Furthermore, we show that the defect induces a change in the ionization mechanism in the course of interaction: a transition from multiphoton to tunnel ionization may take place.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.-b Plasma properties
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)

Transport of radial heat flux and second sound in fusion plasmas

Ö. D. Gürcan, P. H. Diamond, X. Garbet, V. Berionni, G. Dif-Pradalier, P. Hennequin, P. Morel, Y. Kosuga, and L. Vermare

Phys. Plasmas 20, 022307 (2013); http://dx.doi.org/10.1063/1.4792161 (8 pages) | Cited 2 times

Online Publication Date: 15 February 2013

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Simple flux-gradient relations that involve time delay and radial coupling are discussed. Such a formulation leads to a rather simple description of avalanches and may explain breaking of gyroBohm transport scaling. The generalization of the flux-gradient relation (i.e., constitutive relation), which involve both time delay and spatial coupling, is derived from drift-kinetic equation, leading to kinetic definitions of constitutive elements such as the flux of radial heat flux. This allows numerical simulations to compute these cubic quantities directly. The formulation introduced here can be viewed as an extension of turbulence spreading to include the effect of spreading of cross-phase as well as turbulence intensity, combined in such a way to give the flux. The link between turbulence spreading and entropy production is highlighted. An extension of this formulation to general quasi-linear theory for the distribution function in the phase space of radial position and parallel velocity is also discussed.
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52.25.Fi Transport properties
52.35.Ra Plasma turbulence
02.60.Cb Numerical simulation; solution of equations
52.25.Dg Plasma kinetic equations

Self-generated magnetic fields in q-distributed plasmas

Ding-Guo Li, San-Qiu Liu, and Xiao-Qing Li

Phys. Plasmas 20, 022308 (2013); http://dx.doi.org/10.1063/1.4793455 (11 pages)

Online Publication Date: 22 February 2013

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A quasi-steady magnetic field can be generated with high-frequency electromagnetic radiation through wave–wave and wave–particle interactions in astrophysical plasmas and laser-produced plasmas. Nonlinear coupling equations of self-generated magnetic fields are obtained in nonextensive distribution frame, as a generalization for the standard Maxwellian distribution frame. The numerical results show that self-generated magnetic fields may collapse and lead to various turbulent patterns with different index q.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Ra Plasma turbulence
95.30.Qd Magnetohydrodynamics and plasmas
02.60.-x Numerical approximation and analysis
52.25.Fi Transport properties

Simulations of material mixing in laser-driven reshock experiments

Brian M. Haines, Fernando F. Grinstein, Leslie Welser-Sherrill, and James R. Fincke

Phys. Plasmas 20, 022309 (2013); http://dx.doi.org/10.1063/1.4793443 (14 pages)

Online Publication Date: 26 February 2013

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We perform simulations of a laser-driven reshock experiment [Welser-Sherrill et al., High Energy Density Phys. (unpublished)] in the strong-shock high energy-density regime to better understand material mixing driven by the Richtmyer–Meshkov instability. Validation of the simulations is based on direct comparison of simulation and radiographic data. Simulations are also compared with published direct numerical simulation and the theory of homogeneous isotropic turbulence. Despite the fact that the flow is neither homogeneous, isotropic nor fully turbulent, there are local regions in which the flow demonstrates characteristics of homogeneous isotropic turbulence. We identify and isolate these regions by the presence of high levels of turbulent kinetic energy (TKE) and vorticity. After reshock, our analysis shows characteristics consistent with those of incompressible isotropic turbulence. Self-similarity and effective Reynolds number assessments suggest that the results are reasonably converged at the finest resolution. Our results show that in shock-driven transitional flows, turbulent features such as self-similarity and isotropy only fully develop once de-correlation, characteristic vorticity distributions, and integrated TKE, have decayed significantly. Finally, we use three-dimensional simulation results to test the performance of two-dimensional Reynolds-averaged Navier-Stokes simulations. In this context, we also test a presumed probability density function turbulent mixing model extensively used in combustion applications.
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52.35.Tc Shock waves and discontinuities
52.38.Kd Laser-plasma acceleration of electrons and ions
52.65.Kj Magnetohydrodynamic and fluid equation
52.30.-q Plasma dynamics and flow
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Ra Plasma turbulence

Analysis of the influence of external biasing on Texas Helimak turbulence

D. L. Toufen, Z. O. Guimarães-Filho, I. L. Caldas, J. D. Szezech, S. Lopes, R. L. Viana, and K. W. Gentle

Phys. Plasmas 20, 022310 (2013); http://dx.doi.org/10.1063/1.4793732 (11 pages)

Online Publication Date: 27 February 2013

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We analyze alterations on the electrostatic turbulence in experiments with imposed bias to control the plasma radial electric field in Texas Helimak (K. W. Gentle and H. He, Plasma Sci. Technol. 10, 284 (2008)), a toroidal plasma device with a one-dimensional equilibrium, magnetic curvature, and shear. Comparing discharges from different biased potentials, we identify, in a roughly uniform gradient region, a continuous variation from low turbulence level and narrower frequency spectra, for negative bias, to high turbulence level and broadband spectra for positive bias. Overall, we distinguish two kinds of perturbed turbulence, classified according to their intensity, spectral, statistical, and recurrence properties. When the bias is positive, the turbulence shows enhanced and broadband spectra with non Gaussian probability distribution functions having noticeable long tails (extreme events) similar to the turbulence in tokamak scrape-off layer. On the other hand, negative bias reduces the turbulence level and decreases the spectrum widths. Also for negative bias, we found large frequency widths whenever the coupling between drift waves and the sheared plasma flow is fast enough to allow the enhancement of sidebands modes.
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52.35.Ra Plasma turbulence
52.55.Jd Magnetic mirrors, gas dynamic traps
52.55.Fa Tokamaks, spherical tokamaks
52.80.-s Electric discharges
52.75.-d Plasma devices
52.35.Kt Drift waves

Modelling of radiative divertor operation towards detachment in experimental advanced superconducting tokamak

YiPing Chen, F. Q. Wang, X. J. Zha, L. Q. Hu, H. Y. Guo, Z. W. Wu, X. D. Zhang, B. N. Wan, and J. G. Li

Phys. Plasmas 20, 022311 (2013); http://dx.doi.org/10.1063/1.4791659 (7 pages)

Online Publication Date: 28 February 2013

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In order to actively control power load on the divertor target plates and study the effect of radiative divertor on plasma parameters in divertor plasmas and heat fluxes to the targets, dedicated experiments with Ar impurity seeding have been performed on experimental advanced superconducting tokamak in typical L-mode discharge with single null divertor configuration, ohmic heating power of 0.5 MW, and lower hybrid wave heating power of 1.0 MW. Ar is puffed into the divertor plasma at the outer target plate near the separatrix strike point with the puffing rate 1.26×1020 s−1. The radiative divertor is formed during the Ar puffing. The SOL/divertor plasma in the L-mode discharge with radiative divertor has been modelled by using SOLPS5.2 code package [V. Rozhansky et al., Nucl. Fusion 49, 025007 (2009)]. The modelling shows the cooling of the divertor plasma due to Ar seeding and is compared with the experimental measurement. The changes of peak electron temperature and heat fluxes at the targets with the shot time from the modelling results are similar to the experimental measurement before and during the Ar impurity seeding, but there is a major difference in time scales when Ar affects the plasma in between experiment and modelling.
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52.55.Fa Tokamaks, spherical tokamaks
52.55.Jd Magnetic mirrors, gas dynamic traps
52.80.Tn Other gas discharges
52.25.Vy Impurities in plasmas
52.40.Hf Plasma-material interactions; boundary layer effects
52.50.Nr Plasma heating by DC fields; ohmic heating, arcs
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