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Feb 2013

Volume 20, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

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

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
back to top Magnetically Confined Plasmas, Heating, Confinement

The Hamiltonian structure and Euler-Poincaré formulation of the Vlasov-Maxwell and gyrokinetic systems

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

Phys. Plasmas 20, 022501 (2013); http://dx.doi.org/10.1063/1.4791664 (14 pages) | Cited 1 time

Online Publication Date: 13 February 2013

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We present a new variational principle for the gyrokinetic system, similar to the Maxwell-Vlasov action presented in H. Cendra et al., [J. Math. Phys. 39, 3138 (1998)]. The variational principle is in the Eulerian frame and based on constrained variations of the phase space fluid velocity and particle distribution function. Using a Legendre transform, we explicitly derive the field theoretic Hamiltonian structure of the system. This is carried out with a modified Dirac theory of constraints, which is used to construct meaningful brackets from those obtained directly from Euler-Poincaré theory. Possible applications of these formulations include continuum geometric integration techniques, large-eddy simulation models, and Casimir type stability methods.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.25.Dg Plasma kinetic equations

On the toroidal plasma rotations induced by lower hybrid waves

Xiaoyin Guan, Hong Qin, Jian Liu, and Nathaniel J. Fisch

Phys. Plasmas 20, 022502 (2013); http://dx.doi.org/10.1063/1.4791666 (11 pages)

Online Publication Date: 13 February 2013

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A theoretical model is developed to explain the plasma rotations induced by lower hybrid waves in Alcator C-Mod. In this model, torodial rotations are driven by the Lorentz force on the bulk-electron flow across flux surfaces, which is a response of the plasma to the resonant-electron flow across flux surfaces induced by the lower hybrid waves. The flow across flux surfaces of the resonant electrons and the bulk electrons are coupled through the radial electric field initiated by the resonant electrons, and the friction between ions and electrons transfers the toroidal momentum to ions from electrons. An improved quasilinear theory with gyrophase dependent distribution function is developed to calculate the perpendicular resonant-electron flow. Toroidal rotations are determined using a set of fluid equations for bulk electrons and ions, which are solved numerically by a finite-difference method. Numerical results agree well with the experimental observations in terms of flow profile and amplitude. The model explains the strong correlation between torodial flow and internal inductance observed experimentally, and predicts both counter-current and co-current flows, depending on the perpendicular wave vectors of the lower hybrid waves.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.55.Fa Tokamaks, spherical tokamaks
02.70.Bf Finite-difference methods
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Development of an identification method of pressure anisotropy based on equilibrium analysis and magnetics

Y. Asahi, Y. Suzuki, K. Y. Watanabe, and W. A. Cooper

Phys. Plasmas 20, 022503 (2013); http://dx.doi.org/10.1063/1.4791665 (7 pages)

Online Publication Date: 14 February 2013

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We evaluate the fluxes measured by the magnetic flux loops installed in LHD by using a three dimensional MHD equilibrium analysis code, ANIMEC, which enable us to directly determine the calibration function between the anisotropic pressure and the measured fluxes for the non-axisymmetric plasmas for the first time. The result indicates that the diamagnetic flux represents a nearly single-valued function of the beta perpendicular with respect to the field, and the saddle loop flux represents a nearly single-valued function of an equally weighted average of the beta values parallel and perpendicular to the field, regardless of the pressure anisotropy or the amount of energetic trapped particles. The values of the beta perpendicular to the field and the equal weighting averaged beta estimated by the single-valued functions (calibration functions) are investigated in order to clarify the magnitude of deviation from those original values, and the range of anisotropy where the beta value evaluated by the magnetic flux measurement is calculated within a 10% error.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Jd Magnetic mirrors, gas dynamic traps

Lower hybrid instability driven by mono-energy α-particles with finite pitch angle spread in a plasma

Pawan Kumar, Vishwesh Singh, and V. K. Tripathi

Phys. Plasmas 20, 022504 (2013); http://dx.doi.org/10.1063/1.4792262 (4 pages)

Online Publication Date: 14 February 2013

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A kinetic formalism of lower hybrid wave instability, driven by mono-energy α-particles with finite pitch angle spread, is developed. The instability arises through cyclotron resonance interaction with high cyclotron harmonics of α-particles. The α-particles produced in D-T fusion reactions have huge Larmor radii (∼10 cm) as compared to the wavelength of the lower hybrid wave, whereas their speed is an order of magnitude smaller than the speed of light in vacuum. As a result, large parallel phase velocity lower hybrid waves, suitable for current drive in tokamak, are driven unstable via coupling to high cyclotron harmonics. The growth rate decreases with increase in pitch angle spread of the beam. At typical electron density of ∼1019 m−3, magnetic field ∼4 Tesla and α-particle concentration ∼0.1%, the large parallel phase velocity lower hybrid wave grows on the time scale of 20 ion cyclotron periods. The growth rate decreases with plasma density.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.25.Dg Plasma kinetic equations
52.55.Fa Tokamaks, spherical tokamaks
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)

Toroidal modeling of interaction between resistive wall mode and plasma flow

Yueqiang Liu and Youwen Sun

Phys. Plasmas 20, 022505 (2013); http://dx.doi.org/10.1063/1.4793449 (14 pages) | Cited 1 time

Online Publication Date: 21 February 2013

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The non-linear interplay between the resistive wall mode (RWM) and the toroidal plasma flow is numerically investigated in a full toroidal geometry, by simultaneously solving the initial value problems for the n = 1 RWM and the n = 0 toroidal force balance equation. Here, n is the toroidal mode number. The neoclassical toroidal viscous torque is identified as the major momentum sink that brakes the toroidal plasma flow during the non-linear evolution of the RWM. This holds for a mode that is initially either unstable or stable. For an initially stable RWM, the braking of the flow, and hence the eventual growth of the mode, depends critically on the initial perturbation amplitude.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.55.Tn Ideal and resistive MHD modes; kinetic modes
52.65.-y Plasma simulation
02.30.-f Function theory, analysis
02.60.-x Numerical approximation and analysis
FREE

Local thermodynamics of a magnetized, anisotropic plasma

R. D. Hazeltine, S. M. Mahajan, and P. J. Morrison

Phys. Plasmas 20, 022506 (2013); http://dx.doi.org/10.1063/1.4793735 (8 pages)

Online Publication Date: 26 February 2013

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An expression for the internal energy of a fluid element in a weakly coupled, magnetized, anisotropic plasma is derived from first principles. The result is a function of entropy, particle density and magnetic field, and as such plays the role of a thermodynamic potential: it determines in principle all thermodynamic properties of the fluid element. In particular it provides equations of state for the magnetized plasma. The derivation uses familiar fluid equations, a few elements of kinetic theory, the MHD version of Faraday's law, and certain familiar stability and regularity conditions.
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52.25.Kn Thermodynamics of plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
64.30.Jk Equations of state of nonmetals
65.20.Jk Studies of thermodynamic properties of specific liquids
52.25.Dg Plasma kinetic equations

Studies of the fast ion energy spectra in TJ-II

A. Bustos, J. M. Fontdecaba, F. Castejón, J. L. Velasco, M. Tereshchenko, and J. Arévalo

Phys. Plasmas 20, 022507 (2013); http://dx.doi.org/10.1063/1.4793731 (7 pages) | Cited 1 time

Online Publication Date: 27 February 2013

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The dynamics of the neutral beam injection fast ions in the TJ-II stellarator is studied in this paper from both the theoretical and experimental points of view. The code Integrator of Stochastic Differential Equations for Plasmas (ISDEP) is used to estimate the fast ion distribution function in 3D:1D in real space and 2D in velocity space, considering the 3D structure of TJ-II, the electrostatic potential, non turbulent collisional transport, and charge exchange losses. The results of ISDEP are compared with the experimental data from the compact neutral particle analyzer, which measures the outgoing neutral flux spectra in the energy range E∈(1−45)  keV.
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52.50.Gj Plasma heating by particle beams
52.55.Jd Magnetic mirrors, gas dynamic traps
52.70.Nc Particle measurements
52.75.-d Plasma devices
52.65.Pp Monte Carlo methods
back to top Inertially Confined Plasmas, High Energy Density Plasma Science, Warm Dense Matter

Observation of parametric instabilities in the quarter critical density region driven by the Nike KrF laser

J. L. Weaver, J. Oh, L. Phillips, B. Afeyan, J. Seely, D. Kehne, C. M. Brown, S. P. Obenschain, V. Serlin, A. J. Schmitt, U. Feldman, R. H. Lehmberg, E. Mclean, and C. Manka

Phys. Plasmas 20, 022701 (2013); http://dx.doi.org/10.1063/1.4789379 (5 pages)

Online Publication Date: 4 February 2013

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The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (λ = 248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (λ = 351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns ≤ τ ≤ 1.25 ns) and intensities (up to 2×1015 W/cm2). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for instabilities in the quarter critical density region.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.57.Fg Implosion symmetry and hydrodynamic instability (Rayleigh-Taylor, Richtmyer-Meshkov, imprint, etc.)
52.80.Qj Explosions; exploding wires
52.70.Nc Particle measurements

Comparison for non-local hydrodynamic thermal conduction models

A. Marocchino, M. Tzoufras, S. Atzeni, A. Schiavi, Ph. D. Nicolaï, J. Mallet, V. Tikhonchuk, and J.-L. Feugeas

Phys. Plasmas 20, 022702 (2013); http://dx.doi.org/10.1063/1.4789878 (9 pages) | Cited 1 time

Online Publication Date: 4 February 2013

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Inertial confinement fusion and specifically shock ignition involve temperatures and temperature gradients for which the classical Spitzer-Härm thermal conduction breaks down and a non-local operator is required. In this article, two non-local electron thermal conduction models are tested against kinetic Vlasov-Fokker-Planck simulations. Both models are shown to reproduce the main features of thermal heat front propagation at kinetic timescales. The reduction of the thermal conductivity as a function of the scalelength of the temperature gradient is also recovered. Comparisons at nanosecond timescales show that the models agree on the propagation velocity of the heat front, but major differences appear in the thermal precursor.
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52.58.-c Other confinement methods
52.65.Ff Fokker-Planck and Vlasov equation
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties
52.35.Tc Shock waves and discontinuities
FREE

Ablation dynamics in coiled wire-array Z-pinches

G. N. Hall, S. V. Lebedev, F. Suzuki-Vidal, G. Swadling, J. P. Chittenden, S. N. Bland, A. Harvey-Thompson, P. F. Knapp, I. C. Blesener, R. D. McBride, D. A. Chalenski, K. S. Blesener, J. B. Greenly, S. A. Pikuz, T. A. Shelkovenko, et al.

Phys. Plasmas 20, 022703 (2013); http://dx.doi.org/10.1063/1.4789851 (14 pages)

Online Publication Date: 6 February 2013

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Experiments to study the ablation dynamics of coiled wire arrays were performed on the MAGPIE generator (1 MA, 240 ns) at Imperial College, and on the COBRA generator at Cornell University's Laboratory of Plasma Studies (1 MA, 100 ns). The MAGPIE generator was used to drive coiled wires in an inverse array configuration to study the distribution of ablated plasma. Using interferometry to study the plasma distribution during the ablation phase, absolute quantitative measurements of electron line density demonstrated very high density contrasts between coiled ablation streams and inter-stream regions many millimetres from the wire. The measured density contrasts for a coiled array were many times greater than that observed for a conventional array with straight wires, indicating that a much greater axial modulation of the ablated plasma may be responsible for the unique implosion dynamics of coiled arrays. Experiments on the COBRA generator were used to study the complex redirection of plasma around a coiled wire that gives rise to the ablation structure exhibited by coiled arrays. Observations of this complex 3D plasma structure were used to validate the current model of coiled array ablation dynamics [Hall et al., Phys. Rev. Lett. 100, 065003 (2008)], demonstrating irrefutably that plasma flow from the wires behaves as predicted. Coiled wires were observed to ablate and implode in the same manner on both machines, indicating that current rise time should not be an issue for the scaling of coiled arrays to larger machines with fast current rise times.
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52.38.Mf Laser ablation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.80.Qj Explosions; exploding wires
52.30.-q Plasma dynamics and flow

On the structure of plasma liners for plasma jet induced magnetoinertial fusion

Hyoungkeun Kim, Lina Zhang, Roman Samulyak, and Paul Parks

Phys. Plasmas 20, 022704 (2013); http://dx.doi.org/10.1063/1.4789887 (10 pages) | Cited 2 times

Online Publication Date: 8 February 2013

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The internal structure and self-collapse properties of plasma liners, formed by the merger of argon plasma jets, have been studied via 3-dimensional numerical simulations using the FronTier code. We have shown that the jets merger process is accomplished through a cascade of oblique shock waves that heat the liner and reduce its Mach number. Oblique shock waves and the adiabatic compression heating have led to the 10 times reduction of the self-collapse pressure of a 3-dimensional argon liner compared to a spherically symmetric liner with the same pressure and density profiles at the merging radius. We have also observed a factor of 10 variations of pressure and density in the leading edge of the liner along spherical surfaces close to the interaction with potential plasma targets. Such a non-uniformity of imploding plasma liners presents problems for the stability of targets during compression.
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52.75.-d Plasma devices
02.60.Cb Numerical simulation; solution of equations
52.35.Tc Shock waves and discontinuities
52.50.Lp Plasma production and heating by shock waves and compression
52.65.-y Plasma simulation

Oblique shock structures formed during the ablation phase of aluminium wire array z-pinches

G. F. Swadling, S. V. Lebedev, N. Niasse, J. P. Chittenden, G. N. Hall, F. Suzuki-Vidal, G. Burdiak, A. J. Harvey-Thompson, S. N. Bland, P. De Grouchy, E. Khoory, L. Pickworth, J. Skidmore, and L. Suttle

Phys. Plasmas 20, 022705 (2013); http://dx.doi.org/10.1063/1.4790520 (11 pages)

Online Publication Date: 8 February 2013

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A series of experiments has been conducted in order to investigate the azimuthal structures formed by the interactions of cylindrically converging plasma flows during the ablation phase of aluminium wire array Z pinch implosions. These experiments were carried out using the 1.4 MA, 240 ns MAGPIE generator at Imperial College London. The main diagnostic used in this study was a two-colour, end-on, Mach-Zehnder imaging interferometer, sensitive to the axially integrated electron density of the plasma. The data collected in these experiments reveal the strongly collisional dynamics of the aluminium ablation streams. The structure of the flows is dominated by a dense network of oblique shock fronts, formed by supersonic collisions between adjacent ablation streams. An estimate for the range of the flow Mach number (M = 6.2-9.2) has been made based on an analysis of the observed shock geometry. Combining this measurement with previously published Thomson Scattering measurements of the plasma flow velocity by Harvey-Thompson et al. [Physics of Plasmas 19, 056303 (2012)] allowed us to place limits on the range of the mathTe of the plasma. The detailed and quantitative nature of the dataset lends itself well as a source for model validation and code verification exercises, as the exact shock geometry is sensitive to many of the plasma parameters. Comparison of electron density data produced through numerical modelling with the Gorgon 3D MHD code demonstrates that the code is able to reproduce the collisional dynamics observed in aluminium arrays reasonably well.
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52.59.Qy Wire array Z-pinches
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.20.Fs Electron collisions
52.35.Tc Shock waves and discontinuities
52.58.Lq Z-pinches, plasma focus, and other pinch devices

Theoretical treatments of the bound-free contribution and experimental best practice in X-ray Thomson scattering from warm dense matter

Brian A. Mattern and Gerald T. Seidler

Phys. Plasmas 20, 022706 (2013); http://dx.doi.org/10.1063/1.4790659 (9 pages)

Online Publication Date: 8 February 2013

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By comparison with high-resolution synchrotron x-ray experimental results, we assess several theoretical treatments for the bound-free (core-electron) contribution to x-ray Thomson scattering (i.e., also known as nonresonant inelastic x-ray scattering). We identify an often overlooked source of systematic error in the plane-wave form factor approximation (PWFFA) used in the inference of temperature, ionization state, and free electron density in some laser-driven compression studies of warm dense matter. This error is due to a direct violation of energy conservation in the PWFFA. We propose an improved practice for the bound-free term that will be particularly relevant for XRTS experiments performed with somewhat improved energy resolution at the National Ignition Facility or the Linac Coherent Light Source. Our results raise important questions about the accuracy of state variable determination in XRTS studies, given that the limited information content in low-resolution XRTS spectra does not strongly constrain the models of electronic structure being used to fit the spectra.
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52.70.La X-ray and γ-ray measurements
52.25.-b Plasma properties
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.50.Lp Plasma production and heating by shock waves and compression

Analysis of spatially resolved Z-pinch spectra to investigate the nature of “bright spots”

J. P. Apruzese, J. L. Giuliani, J. W. Thornhill, C. A. Coverdale, B. Jones, and D. J. Ampleford

Phys. Plasmas 20, 022707 (2013); http://dx.doi.org/10.1063/1.4792256 (10 pages)

Online Publication Date: 15 February 2013

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Localized, intensely radiating regions are often observed in Z pinches. High resolution images of such areas have been recorded at least as far back as the 1970s. However, there is as yet no widely accepted consensus on the nature of these “bright spots” or how they are formed. This phenomenon has also been referred to “hot spots” or “micropinches.” To shed further light on this issue, we have analyzed axially resolved K-shell spectra from 4 Z pinches driven by the refurbished Z generator (“ZR”) at Sandia National Laboratories, and the previous version of the Z machine (“Z”). The atomic numbers of the loads varied from 13 to 29. We find that higher spatial K-shell intensity in the Al pinch correlates with density. The K-shell intensity within a copper shot taken on ZR correlates strongly with increased electron temperature, but another, somewhat less well-diagnosed copper shot from Z shows correlation with density. The bright spots in a Ti pinch correlate with neither density nor temperature, but do correlate with the product of density and diameter (proportional to opacity). This opacity correlation is also observed in the other 3 pinches.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.-b Plasma production and heating

Optimal conditions for shock ignition of scaled cryogenic deuterium–tritium targets

M. Lafon, X. Ribeyre, and G. Schurtz

Phys. Plasmas 20, 022708 (2013); http://dx.doi.org/10.1063/1.4792265 (10 pages)

Online Publication Date: 15 February 2013

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Within the framework of the shock-ignition (SI) scheme, ignition conditions are reached following the separation of the compression and heating phases. First, the shell is compressed at a sub-ignition implosion velocity; then an intense laser spike is launched at the end of the main drive, leading to the propagation of a strong shock through the precompressed fuel. The minimal laser energy required for ignition of scaled deuterium–tritium (DT) targets is assessed by calculations. A semi-empiric model describing the ignitor shock generation and propagation in the fuel assembly is defined. The minimal power needed in the laser spike pulse to achieve ignition is derived from the hydrodynamic model. Optimal conditions for ignition of scaled targets are explored in terms of laser intensity, shell-implosion velocity, and target scale range for the SI process. Curves of minimal laser requirements for ignition are plotted in the energy–power diagram. The most economic and reliable conditions for ignition of a millimeter DT target are observed in the 240- to 320-km/s implosion velocity range and for the peak laser intensity ranging from ∼2 × 1015 W/cm2 up to 5 × 1015 W/cm2. These optimal conditions correspond to shock-ignited targets for a laser energy of ∼250 kJ and a laser power of 100 to 200 TW. Large, self-ignited targets are particularly attractive by offering ignition at a lower implosion velocity and a reduced laser intensity than for conventional ignition. The SI scheme allows for the compression and heating phases of the high power laser energy research facility target to be performed at a peak laser intensity below 1016 W/cm2. A better control of parametric and hydrodynamic instabilities within the SI scheme sets it as an optimal and reliable approach to attain ignition of large targets.
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52.35.Tc Shock waves and discontinuities
52.50.Lp Plasma production and heating by shock waves and compression
28.52.Cx Fueling, heating and ignition
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
28.52.Fa Materials
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
back to top Ionospheric, Solar-System, and Astrophysical Plasmas

Generating vorticity and magnetic fields in plasmas in general relativity: Spacetime curvature drive

Felipe A. Asenjo, Swadesh M. Mahajan, and Asghar Qadir

Phys. Plasmas 20, 022901 (2013); http://dx.doi.org/10.1063/1.4792257 (8 pages) | Cited 1 time

Online Publication Date: 14 February 2013

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Using the generally covariant magnetofluid formalism for a hot plasma, a spacetime curvature driven mechanism for generating seed vorticity/magnetic field is presented. The “battery” owes its origin to the interaction between the gravity modified Lorentz factor of the fluid element and the inhomogeneous plasma thermodynamics. The general relativistic drive is evaluated for two simple cases: seed formation in a simplified model of a hot plasma accreting in stable orbits around a Schwarzschild black hole and for particles in free fall near the horizon. Some astrophysical applications are suggested.
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95.30.Qd Magnetohydrodynamics and plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
04.70.Bw Classical black holes
52.35.We Plasma vorticity
52.27.Ny Relativistic plasmas
52.25.Kn Thermodynamics of plasmas

Comparison between hybrid and fully kinetic models of asymmetric magnetic reconnection: Coplanar and guide field configurations

Nicolas Aunai, Michael Hesse, Seiji Zenitani, Maria Kuznetsova, Carrie Black, Rebekah Evans, and Roch Smets

Phys. Plasmas 20, 022902 (2013); http://dx.doi.org/10.1063/1.4792250 (10 pages) | Cited 1 time

Online Publication Date: 15 February 2013

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Magnetic reconnection occurring in collisionless environments is a multi-scale process involving both ion and electron kinetic processes. Because of their small mass, the electron scales are difficult to resolve in numerical and satellite data, it is therefore critical to know whether the overall evolution of the reconnection process is influenced by the kinetic nature of the electrons, or is unchanged when assuming a simpler, fluid, electron model. This paper investigates this issue in the general context of an asymmetric current sheet, where both the magnetic field amplitude and the density vary through the discontinuity. A comparison is made between fully kinetic and hybrid kinetic simulations of magnetic reconnection in coplanar and guide field systems. The models share the initial condition but differ in their electron modeling. It is found that the overall evolution of the system, including the reconnection rate, is very similar between both models. The best agreement is found in the guide field system, which confines particle better than the coplanar one, where the locality of the moments is violated by the electron bounce motion. It is also shown that, contrary to the common understanding, reconnection is much faster in the guide field system than in the coplanar one. Both models show this tendency, indicating that the phenomenon is driven by ion kinetic effects and not electron ones.
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52.65.Kj Magnetohydrodynamic and fluid equation
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg Plasma kinetic equations
52.25.Fi Transport properties

Ponderomotive force in the presence of electric fields

G. V. Khazanov and E. N. Krivorutsky

Phys. Plasmas 20, 022903 (2013); http://dx.doi.org/10.1063/1.4789874 (5 pages)

Online Publication Date: 21 February 2013

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This paper presents averaged equations of particle motion in an electromagnetic wave of arbitrary frequency with its wave vector directed along the ambient magnetic field. The particle is also subjected to an math×math drift and a background electric field slowly changing in space and acting along the magnetic field line. The fields, wave amplitude, and the wave vector depend on the coordinate along the magnetic field line. The derivations of the ponderomotive forces are done by assuming that the drift velocity in the ambient magnetic field is comparable to the particle velocity. Such a scenario leads to new ponderomotive forces, dependent on the wave magnetic field intensity, and, as a result, to the additional energy exchange between the wave and the plasma particles. It is found that the parallel electric field can lead to the change of the particle-wave energy exchange rate comparable to that produced by the previously discussed ponderomotive forces.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.25.Fi Transport properties
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