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Jun 2008

Volume 15, Issue 6, Articles (06xxxx)

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Hard x-ray radiography for density measurement in shock compressed matter

A. Ravasio, M. Koenig, S. Le Pape, A. Benuzzi-Mounaix, H. S. Park, C. Cecchetti, P. Patel, A. Schiavi, N. Ozaki, A. Mackinnon, B. Loupias, D. Batani, T. Boehly, M. Borghesi, R. Dezulian, et al.

Phys. Plasmas 15, 060701 (2008); http://dx.doi.org/10.1063/1.2928156 (4 pages) | Cited 7 times

Online Publication Date: 10 June 2008

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In this letter we report on the direct density measurement in a shock compressed aluminum target using hard x-ray radiography. Experimental data employing a molybdenum Kα source at 17.5 keV, generated with a short pulse laser are presented. High spatial resolution was obtained thanks to a new design for the backlighter geometry. Density values deduced from radiography are compared to predictions from hydrodynamic simulations, which have been calibrated in order to reproduce shock velocities measured from a rear-side self-emission diagnostic. Our results reveal the great potential of this technique as a diagnostic tool for direct density measurements in dense high-Z opaque materials.
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64.30.-t Equations of state of specific substances
52.35.Tc Shock waves and discontinuities
62.50.-p High-pressure effects in solids and liquids
52.50.Lp Plasma production and heating by shock waves and compression
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back to top Basic Plasma Phenomena, Waves, Instabilities
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Time reversal duality of magnetohydrodynamic shocks

J. P. Goedbloed

Phys. Plasmas 15, 062101 (2008); http://dx.doi.org/10.1063/1.2919795 (19 pages) | Cited 3 times

Online Publication Date: 3 June 2008

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The shock conditions in magnetohydrodynamics (MHD) are reduced to their most concise, three-parameter, distilled form by consistent use of the scale independence of the MHD equations and of the de Hoffmann–Teller transformation. They then exhibit a distinct time reversal duality between entropy-allowed shocks and entropy-forbidden jumps. This yields a new classification of MHD shocks by means of the monotonicity properties with respect to upstream and downstream Alfvén Mach numbers, it exhibits the central role of intermediate discontinuities, and permits straightforward construction of all relevant dimensionless quantities of the shocks. An exhaustive overview is presented of solutions in the different parameter regimes.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Tc Shock waves and discontinuities
52.30.-q Plasma dynamics and flow
47.65.-d Magnetohydrodynamics and electrohydrodynamics

Relativistic electron beam driven instabilities in the presence of an arbitrarily oriented magnetic field

A. Bret and M. E. Dieckmann

Phys. Plasmas 15, 062102 (2008); http://dx.doi.org/10.1063/1.2926634 (9 pages) | Cited 7 times

Online Publication Date: 3 June 2008

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The electromagnetic instabilities driven by a relativistic electron beam, which moves through a magnetized plasma, are analyzed with a cold two-fluid model. It allows for any angle θB between the beam velocity vector and the magnetic field vector and considers any orientation of the wavevector in the two-dimensional plane spanned by these two vectors. If the magnetic field is strong, the two-stream instability dominates if θB = 0 and the oblique modes grow faster at larger θB. A weaker magnetic field replaces the two-stream modes with oblique modes as the fastest-growing waves. The threshold value separating both magnetic regimes is estimated. A further dimensionless parameter is identified, which determines whether or not the wavevector of the most unstable wave is changed continuously, as θB is varied from 0 to π/2. The fastest growing modes are always found for a transverse propagation of the beam with θB = π/2, irrespective of the magnetic field strength.
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52.40.Mj Particle beam interactions in plasmas
52.27.Ny Relativistic plasmas
52.25.Xz Magnetized plasmas

Resonant reduction in microwave reflectivity from an overdense plasma with the employment of a parallel metal grating

C. S. Liu, V. K. Tripathi, and R. Annou

Phys. Plasmas 15, 062103 (2008); http://dx.doi.org/10.1063/1.2918663 (4 pages) | Cited 1 time

Online Publication Date: 4 June 2008

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See Also: Publisher's Note

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A microwave normally incident on an overdense plasma is totally reflected. However, the presence of a thin metal grating parallel to the plasma surface greatly reduces the reflectivity when the grating wave number equals the wave number of the surface plasma wave on the plasma-free space interface at microwave frequency. The microwave induces an oscillatory current in the free electrons of the grating at the frequency of the microwave, ω, and wave number of the grating, q. This current resonantly excites a surface plasma wave (SPW) on the plasma surface. The diversion of microwave power into SPW severely reduces the microwave reflectivity.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.40.Hf Plasma-material interactions; boundary layer effects
41.20.Jb Electromagnetic wave propagation; radiowave propagation

Blowup of certain analytic solutions of the Hall magnetohydrodynamic equations

Manuel Núñez, Jorge Álvarez, and Jesús Rojo

Phys. Plasmas 15, 062104 (2008); http://dx.doi.org/10.1063/1.2930471 (6 pages)

Online Publication Date: 9 June 2008

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A recent analytic solution of the Hall magnetohydrodynamics equations is analyzed. It is shown that its evolution in time depends upon a certain set of inequalities upon the initial values of the velocity and the magnetic field. For most of the cases, both magnitudes will blow up in a finite time. This shows that for keeping the original structure of the solution, energy must be introduced into the system until eventually it cannot hold any longer.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg Plasma kinetic equations
02.30.Sa Functional analysis

The relativistic kinetic Weibel instability: Comparison of different distribution functions

U. Schaefer-Rolffs and R. C. Tautz

Phys. Plasmas 15, 062105 (2008); http://dx.doi.org/10.1063/1.2932106 (9 pages) | Cited 9 times

Online Publication Date: 10 June 2008

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Investigations of the relativistic Weibel instability have burgeoned in the last few years because of their potential use in various astrophysical scenarios. In this article, the parameters for the growth rates of well-known distribution functions are provided, based on a recently developed general description. The four distributions to be dealt with are the monochromatic, waterbag, bi-Maxwellian and the κ distribution. The advantages of this treatment are: (i) One has to solve only one integral to obtain the growth rates, thus (ii) one may compare the different distributions easily. Numerical illustrations of the growth rates for each distribution are given. The growth rates can be classified due to the ansatz of the distributions functions. In addition, some formulas of a previous paper are corrected.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.27.Ny Relativistic plasmas

Parametric instabilities of Alfvén waves in a multispecies plasma: Kinetic effects

K. Kauffmann and J. A. Araneda

Phys. Plasmas 15, 062106 (2008); http://dx.doi.org/10.1063/1.2932113 (8 pages) | Cited 5 times

Online Publication Date: 10 June 2008

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Parametric instabilities of a circularly polarized Alfvén wave in a multispecies magnetized plasma are considered. An analytic kinetic description and hybrid simulations for the linear behavior of the instabilities are given. It is found that, even for low-β regimes, both the kinetic effects and the presence of heavy ions substantially modify the characteristics of parametric instabilities as compared to the fluid model. The decay instability can be severely quenched in a plasma composed of massless electrons, protons, and alpha particles when the alphas are slightly hotter than the protons. These results could be important in describing the heating processes of heavy ions in the solar corona.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Streaming instabilities of intense charged particle beams propagating along a solenoidal magnetic field in a background plasma

Edward A. Startsev, Ronald C. Davidson, and Mikhail Dorf

Phys. Plasmas 15, 062107 (2008); http://dx.doi.org/10.1063/1.2918673 (9 pages) | Cited 2 times

Online Publication Date: 11 June 2008

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Streaming instabilities of intense charged particle beams propagating along a solenoidal magnetic field in a background plasma are studied analytically and numerically. It is shown that the growth rate of the electromagnetic Weibel instability is modified by a relatively weak solenoidal magnetic field such that ωce>βbωpe, where ωce is the electron gyrofrequency, ωpe is the electron plasma frequency, and βb is the ion-beam velocity relative to the speed of light. Moreover, the Weibel instability is limited to very small propagation angles and long longitudinal wavelengths satisfying k∥2⪡k⊥2 and c2k∥2⪡ωpb2ωpi2/(ωpb2+ωpi2), where ωpb and ωpi are the plasma frequencies of the beam ions and the background plasma ions, respectively. For shorter longitudinal wavelengths, the electrostatic lower-hybrid instability becomes dominant. In this paper, the growth rates of various electrostatic beam-plasma instabilities and the electromagnetic Weibel instability are compared, and the space-time development of the modified two-stream instability is studied in detail and compared with numerical simulations.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.40.Mj Particle beam interactions in plasmas

Self-gravitating rotating anisotropic pressure plasma in presence of Hall current and electrical resistivity using generalized polytrope laws

R. P. Prajapati, G. D. Soni, and R. K. Chhajlani

Phys. Plasmas 15, 062108 (2008); http://dx.doi.org/10.1063/1.2930472 (13 pages) | Cited 4 times

Online Publication Date: 11 June 2008

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The effects of uniform rotation, finite electrical resistivity, electron inertia, and Hall current on the self-gravitational instability of anisotropic pressure plasma with generalized polytrope laws have been studied. A general dispersion relation is obtained with the help of the relevant linearized perturbed magnetohydrodynamic (MHD) equations incorporating the relevant contributions of various effects of the problem using the method of normal mode analysis. The general dispersion relation is further reduced for the special cases of rotation; i.e., parallel and perpendicular to the direction of the magnetic field. The longitudinal and transverse modes of propagation are discussed separately for investigation of condition of instability. The effects of rotation, Hall current, finite electron inertia, and polytropic indices are discussed on the gravitational, “firehose,” and “mirror” instabilities. The numerical calculations have been performed to obtain the dependence of the growth rate of the gravitational unstable mode on the various physical parameters involved. The finite electrical resistivity, rotation, and Hall current have a stabilizing influence on the growth rate of the unstable mode of wave propagation. The finite electrical resistivity removes the effect of magnetic field and polytropic index from the condition of instability in the transverse mode of propagation for both the cases of rotation. It is also found that the Jeans criterion of gravitational instability depends upon rotation, electron inertia, and polytropic indices. In the case of transverse mode of propagation with the axis of rotation parallel to the magnetic field, it is observed that the region of instability and the value of the critical Jeans wavenumber are larger for the Chew–Goldberger–Low set of equations in comparison with the MHD set of equations. The stability of the system is discussed by applying Routh–Hurwitz criterion. The inclusion of rotation or Hall current or both together depresses the growth rate of mirror instability. We also note that the condition of mirror instability depends upon polytropic indices.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Fi Transport properties
95.30.Qd Magnetohydrodynamics and plasmas
95.30.Sf Relativity and gravitation
97.10.Bt Star formation

Nonlinear electron magnetohydrodynamics physics. IV. Whistler instabilities

J. M. Urrutia, R. L. Stenzel, and K. D. Strohmaier

Phys. Plasmas 15, 062109 (2008); http://dx.doi.org/10.1063/1.2934680 (8 pages) | Cited 6 times

Online Publication Date: 19 June 2008

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A very large low-frequency whistler mode is excited with magnetic loop antennas in a uniform laboratory plasma. The wave magnetic field exceeds the ambient field causing in one polarity a field reversal, and a magnetic topology resembling that of spheromaks in the other polarity. These propagating “whistler spheromaks” strongly accelerate the electrons and create non-Maxwellian distributions in their toroidal current ring. It is observed that the locally energized electrons in the current ring excite new electromagnetic instabilities and emit whistler modes with frequencies unrelated to the applied frequency. Emissions are also observed from electrons excited in X-type neutral lines around the antenna. The properties of the excited waves such as amplitudes, frequency spectra, field topologies, propagation, polarization, growth, and damping have been investigated. The waves remain linear (Bwave⪡B0) and convert a small part of the electron kinetic energy into wave magnetic energy (Bwave2/2μ0⪡nkTe).
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.40.Fd Plasma interactions with antennas; plasma-filled waveguides
52.55.Ip Spheromaks
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma

Nonlinear electron magnetohydrodynamics physics. V. Triggered whistler emissions

R. L. Stenzel, K. D. Strohmaier, and J. M. Urrutia

Phys. Plasmas 15, 062110 (2008); http://dx.doi.org/10.1063/1.2934699 (9 pages) | Cited 2 times

Online Publication Date: 19 June 2008

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Laboratory experiments on whistler instabilities in the presence of small trigger waves have been performed. The instabilities arise from energizing electrons in magnetic null lines with time-varying magnetic fields. Such fields are created with loop antennas carrying large oscillating currents in the low-frequency whistler branch. X-type and O-type magnetic nulls are produced with electric fields along the toroidal separator. The magnetic field convects in the form of whistler spheromaks and whistler mirrors. Counterpropagating spheromaks merge and form field-reversed configurations (FRCs). Counterpropagating mirrors colliding with an FRC also energize electrons and produce high-frequency whistler emissions. The possibility that these emissions are triggered by incident waves from other null lines in the plasma has been investigated. A controlled experiment on triggered emissions where a test wave has been created with an independent antenna and propagated into the source region to investigate its amplification has also been performed. It is observed that the test wave does not grow but triggers a much larger instability in a spheromak. The enhanced emission has a different magnetic topology and a slightly different frequency from that of the test wave. Space-time measurements in the source region show both convective wave amplification occurs as well as an absolute instability in the current ring.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.55.Ip Spheromaks
52.55.Lf Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps

Theory of current-free double layers in plasmas

K. S. Goswami, K. Saharia, and H. Schamel

Phys. Plasmas 15, 062111 (2008); http://dx.doi.org/10.1063/1.2937153 (6 pages) | Cited 4 times

Online Publication Date: 26 June 2008

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The existence of current-free double layers in unmagnetized plasma is studied by means of the quasipotential method applied to the Vlasov–Poisson system. Crucial for its existence are trapped particle populations that are characterized by notches (dips) in the velocity distribution functions at resonant velocity becoming flat at large amplitude limit. The potential drop across the double layer, or its amplitude ψ, can be arbitrarily strong covering the whole range 0<ψ<∞. Both the small and large amplitude limit are worked out explicitly, inclusively effective kinetic temperatures and pressures. It is, hence, the effective electron (ion) temperature increase (decrease) with increasing potential, caused by the trapped particles, which is responsible for the existence of this two-parameter family of solutions.
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52.40.Kh Plasma sheaths
52.25.Dg Plasma kinetic equations

Fluid model for relativistic, magnetized plasmas

J. M. TenBarge, R. D. Hazeltine, and S. M. Mahajan

Phys. Plasmas 15, 062112 (2008); http://dx.doi.org/10.1063/1.2937123 (12 pages)

Online Publication Date: 27 June 2008

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Many astrophysical plasmas and some laboratory plasmas are relativistic: Either the thermal speed or the local bulk flow in some frame approaches the speed of light. Often, such plasmas are magnetized in the sense that the Larmor radius is smaller than any gradient scale length of interest. Conventionally, relativistic magnetohydrodynamics (MHD) is employed to treat relativistic, magnetized plasmas. However, MHD requires the collision time to be shorter than any other time scale in the system. Thus, MHD employs the thermodynamic equilibrium form of the stress tensor, neglecting pressure anisotropy and heat flow parallel to the magnetic field. Recent work has attempted to remedy these shortcomings. This paper re-examines the closure question and finds a more complete theory, which yields a more physical and self-consistent closure. Beginning with exact moments of the kinetic equation, we derive a closed set of Lorentz-covariant fluid equations for a magnetized plasma allowing for pressure and heat flow anisotropy. Basic predictions of the model, especially of the dispersion relation’s dependence upon relativistic temperature, are examined.
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52.27.Ny Relativistic plasmas
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.Dg Plasma kinetic equations
52.25.Kn Thermodynamics of plasmas
back to top Nonlinear Phenomena, Turbulence, Transport

Linear and nonlinear quantum ion-acoustic waves in dense magnetized electron-positron-ion plasmas

S. A. Khan and W. Masood

Phys. Plasmas 15, 062301 (2008); http://dx.doi.org/10.1063/1.2920273 (6 pages) | Cited 19 times

Online Publication Date: 3 June 2008

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The linear and nonlinear quantum ion-acoustic waves propagating obliquely in two dimensions in superdense, magnetized electron-positron-ion quantum plasma are investigated on the basis of quantum hydrodynamic model. It is found in linear analysis that the quantum corrections of diffraction are important in the very short wavelength regime that may be found in dense astrophysical plasmas. To investigate the solitary waves, the Zakharov-Kuznetsov equation is derived and the solution is presented in the small amplitude limit. By numerical analysis, it is found that the soliton structure of the ion acoustic wave depends upon quantum pressure, concentration of positrons, strength of magnetic field, and the propagation angle.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
95.30.Qd Magnetohydrodynamics and plasmas

Filamentation of dispersive Alfvén waves in density channels: Hall magnetohydrodynamics description

D. Borgogno, D. Laveder, T. Passot, C. Sulem, and P. L. Sulem

Phys. Plasmas 15, 062302 (2008); http://dx.doi.org/10.1063/1.2930470 (12 pages) | Cited 2 times

Online Publication Date: 3 June 2008

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Filamentation of dispersive Alfvén waves initiated by low or high density channels (depending on the plasma beta) is simulated numerically in the framework of ideal Hall magnetohydrodynamics, and asymptotically modeled with a two-dimensional nonlinear Schrödinger equation including a linear attracting potential. Compared with the dynamics in a homogeneous plasma, the phenomenon is accelerated and occurs for a broader range of parameters. In the case of an isolated channel with a width comparable to the pump wavelength, the transverse wave collapse can be replaced by a moderate amplification. In many cases, a relatively complex dynamics takes place, characterized by an oscillation between magnetic filaments and magnetic ribbons, leading to the formation of small scales at which dissipative effects could become relevant. Alfvén vortices, governed by the equations of the reduced magnetohydrodynamics, are also identified in the simulations, in spite of their small amplitude relative to the wave. The formation of structures under the effect of periodic or random distributions of low and high density channels is also discussed.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
52.65.Kj Magnetohydrodynamic and fluid equation
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.)
52.35.Ra Plasma turbulence

Analytic, nonlinearly exact solutions for an rf confined plasma

Kushal Shah and Harishankar Ramachandran

Phys. Plasmas 15, 062303 (2008); http://dx.doi.org/10.1063/1.2926632 (13 pages) | Cited 3 times

Online Publication Date: 6 June 2008

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RF confined electron plasmas are of importance in Paul traps [ W. Paul, Rev. Mod. Phys. 62, 531 (1990) ]. The stability of such plasmas is unclear and statistical heating arguments have been advanced to explain the observed heating in such plasmas [ I. Siemers et al., Phys. Rev. A 38, 5121 (1988) ]. This study investigates the nature of a one-dimensional collisionless electron plasma that is confined by an rf field of the form [−B+A cos(ωt)]x, where x is the space coordinate and ω is the rf frequency. Nonlinearly exact solutions are obtained. The distribution function and the plasma density are obtained in closed form and have constant shapes with time varying oscillations. These oscillations are at the rf frequency and its harmonics, modulated by a low frequency related to the electron bounce time. The linear limit of weak fields is recovered. Analytic expressions are obtained for the required external field to make it consistent with prescribed distribution functions. These solutions remain valid even in the presence of collisions. Solutions involving multiple species are also obtained, though only for collisionless traps. It is found that the ponderomotive force response needs to be corrected to account for the temperature fluctuations. No stochastic heating is observed in this field configuration.
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52.58.Qv Electrostatic and high-frequency confinement
52.25.-b Plasma properties

Nonlinear current filamentation via magnetically trapped particles

D. Jovanović and A. Simić

Phys. Plasmas 15, 062304 (2008); http://dx.doi.org/10.1063/1.2919168 (8 pages)

Online Publication Date: 9 June 2008

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The nonlinear kinetic theory of the saturated state of collisionless reconnection with a guide magnetic field is presented, associated with kinetic Alfvén waves. Using the drift-kinetic description of electrons, the effects of particle resonances are studied in details, in the presence of self-consistent nonlinear perturbations in the form of moving magnetic islands. The new type of magnetic resonance in velocity space is found. The corresponding magnetically trapped electrons are confined in the real space to the interior of the magnetic islands and at their edge, giving rise to a surface current in the vicinity of the magnetic separatrix. It is shown that the fast reconnection, associated with the near-singularities of the current, can be stabilized by this new nonlinear current.
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52.35.Sb Solitons; BGK modes
52.35.Vd Magnetic reconnection
94.30.cp Magnetic reconnection
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)

Anisotropic spectra of acoustic type turbulence

E. Kuznetsov and V. Krasnoselskikh

Phys. Plasmas 15, 062305 (2008); http://dx.doi.org/10.1063/1.2928160 (5 pages)

Online Publication Date: 10 June 2008

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The problem of spectra for acoustic type of turbulence generated by shocks being randomly distributed in space is considered. It is shown that for turbulence with a weak anisotropy, such spectra have the same dependence in k-space as the Kadomtsev–Petviashvili spectrum: E(k) ∼ k−2. However, the frequency spectrum has always the falling ∼ ω−2, independent of anisotropy. In the strong anisotropic case the energy distribution relative to wave vectors takes anisotropic dependence, forming in the large-k region spectra of the jet type.
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52.35.Ra Plasma turbulence
52.35.Tc Shock waves and discontinuities
47.35.Rs Sound waves
47.35.Jk Wave breaking

The role of plasma elongation on the linear damping of zonal flows

P. Angelino, X. Garbet, L. Villard, A. Bottino, S. Jolliet, Ph. Ghendrih, V. Grandgirard, B. F. McMillan, Y. Sarazin, G. Dif-Pradalier, and T. M. Tran

Phys. Plasmas 15, 062306 (2008); http://dx.doi.org/10.1063/1.2928849 (11 pages) | Cited 10 times

Online Publication Date: 11 June 2008

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Drift wave turbulence is known to self-organize to form axisymmetric macroscopic flows. The basic mechanism for macroscopic flow generation is called inverse energy cascade. Essentially, it is an energy transfer from the short wavelengths to the long wavelengths in the turbulent spectrum due to nonlinear interactions. A class of macroscopic flows, the poloidally symmetric zonal flows, is widely recognized as a key constituent in nearly all cases and regimes of microturbulence, also because of the realization that zonal flows are a critical agent of self-regulation for turbulent transport. In tokamaks and other toroidal magnetic confinement systems, axisymmetric flows exist in two branches, a zero frequency branch and a finite frequency branch, named Geodesic Acoustic Modes (GAMs). The finite frequency is due to the geodesic curvature of the magnetic field. There is a growing body of evidence that suggests strong GAM activity in most devices. Theoretical investigation of the GAMs is still an open field of research. Part of the difficulty of modelling the GAMs stems from the requirement of running global codes. Another issue is that one cannot determine a simple one to one relation between turbulence stabilization and GAM activity. This paper focuses on the study of ion temperature gradient turbulence in realistic tokamak magnetohydrodynamic equilibria. Analytical and numerical analyses are applied to the study of geometrical effects on zonal flows oscillations. Results are shown on the effects of the plasma elongation on the GAM amplitude and frequency and on the zonal flow residual amplitude.
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52.35.Ra Plasma turbulence
52.55.Fa Tokamaks, spherical tokamaks
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.35.Kt Drift waves
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Ion-acoustic solitons in plasmas with two-temperature ions

Manfred A. Hellberg and Frank Verheest

Phys. Plasmas 15, 062307 (2008); http://dx.doi.org/10.1063/1.2930468 (8 pages) | Cited 3 times

Online Publication Date: 23 June 2008

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This paper discusses the existence of ion-acoustic solitons in a plasma consisting of hot and cool ions, and hot electrons, where the words “hot” and “cool” reflect the ratio of thermal effects to inertial effects. For the hot species general polytropic pressure relations are considered, as well as the isothermal limit, γ = 1 (Boltzmann distribution). It is shown that negative potential solitons are not supported. Existence domains in parameter space (effective soliton Mach number and fractional cool ion density) for positive potential solitons are found, the limits being imposed by physical effects such as vanishing densities. Numerical results are given for a typical value of γ>1 and for γ = 1, for three representative values of the hot ion to electron temperature ratio.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.Fi Transport properties
52.25.Kn Thermodynamics of plasmas
52.65.-y Plasma simulation
52.35.Sb Solitons; BGK modes

Propagation of electron magnetohydrodynamic structures in a two-dimensional inhomogeneous plasma

Sharad Kumar Yadav, Amita Das, and Predhiman Kaw

Phys. Plasmas 15, 062308 (2008); http://dx.doi.org/10.1063/1.2943693 (9 pages) | Cited 8 times

Online Publication Date: 30 June 2008

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The fully three-dimensional governing equations in the electron magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density are obtained. These equations in the two-dimensional limit can be cast in terms of the evolution of two coupled scalar fields. The nonlinear simulations for the two-dimensional case are carried out to understand the propagation of EMHD magnetic structures in the presence of inhomogeneity. A novel effect related to the trapping of dipolar magnetic structures in the high density plasma region in the EMHD regime is observed. The interpretation of this phenomena as well as its relevance to the problem of hot spot generation in the context of fast ignition is presented.
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52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.25.-b Plasma properties
52.65.-y Plasma simulation

Nonlinear transport processes in tokamak plasmas. I. The collisional regimes

Giorgio Sonnino and Philippe Peeters

Phys. Plasmas 15, 062309 (2008); http://dx.doi.org/10.1063/1.2939377 (23 pages) | Cited 3 times

Online Publication Date: 30 June 2008

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An application of the thermodynamic field theory (TFT) to transport processes in L-mode tokamak plasmas is presented. The nonlinear corrections to the linear (“Onsager”) transport coefficients in the collisional regimes are derived. A quite encouraging result is the appearance of an asymmetry between the Pfirsch–Schlüter (P-S) ion and electron transport coefficients: the latter presents a nonlinear correction, which is absent for the ions, and makes the radial electron coefficients much larger than the former. Explicit calculations and comparisons between the neoclassical results and the TFT predictions for Joint European Torus (JET) plasmas are also reported. It is found that the nonlinear electron P-S transport coefficients exceed the values provided by neoclassical theory by a factor that may be of the order 102. The nonlinear classical coefficients exceed the neoclassical ones by a factor that may be of order 2. For JET, the discrepancy between experimental and theoretical results for the electron losses is therefore significantly reduced by a factor 102 when the nonlinear contributions are duly taken into account but, there is still a factor of 102 to be explained. This is most likely due to turbulence. The expressions of the ion transport coefficients, determined by the neoclassical theory in these two regimes, remain unaltered. The low-collisional regimes, i.e., the plateau and the banana regimes, are analyzed in the second part of this work.
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52.25.Fi Transport properties
52.20.-j Elementary processes in plasmas
52.55.Fa Tokamaks, spherical tokamaks
52.25.Kn Thermodynamics of plasmas
52.35.Ra Plasma turbulence
back to top Magnetically Confined Plasmas, Heating, Confinement

Electrostatic turbulence driven by high magnetohydrodynamic activity in Tokamak Chauffage Alfvén Brésilien

Zwinglio O. Guimarães-Filho, Iberê L. Caldas, Ricardo L. Viana, Maria Vittoria A. P. Heller, Ivan C. Nascimento, Yuri K. Kuznetsov, and Roger D. Bengtson

Phys. Plasmas 15, 062501 (2008); http://dx.doi.org/10.1063/1.2920211 (8 pages) | Cited 6 times

Online Publication Date: 3 June 2008

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In Tokamak Chauffage Alfvén Brésilien [ R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001) ], high magnetohydrodynamic (MHD) activity may appear spontaneously or during discharges with a voltage biased electrode inserted at the plasma edge. The turbulent electrostatic fluctuations, measured by Langmuir probes, are modulated by Mirnov oscillations presenting a dominant peak with a common frequency around 10 kHz. We report the occurrence of phase locking of the turbulent potential fluctuations driven by MHD activity at this frequency. Using wavelet cross-spectral analysis, we characterized the phase and frequency synchronization in the plasma edge region. We introduced an order parameter to characterize the radial dependence of the phase-locking intensity.
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52.35.Ra Plasma turbulence
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Fa Tokamaks, spherical tokamaks
52.80.-s Electric discharges
52.40.Hf Plasma-material interactions; boundary layer effects
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Power balance in a high-density field reversed configuration plasma

R. M. Renneke, T. P. Intrator, S. C. Hsu, G. A. Wurden, W. J. Waganaar, E. L. Ruden, and T. C. Grabowski

Phys. Plasmas 15, 062502 (2008); http://dx.doi.org/10.1063/1.2934588 (8 pages) | Cited 1 time

Online Publication Date: 5 June 2008

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A global power balance analysis has been performed for the Field Reversed Experiment with Liner high density (>5×1022 m−3) field reversed configuration (FRC) plasma. The analysis was based on a zero-dimensional power balance model [ D. J. Rey and M. Tuszewski, Phys. Fluids 27, 1514 (1984) ]. The key findings are as follows. First, the percentage of radiative losses relative to total loss is an order of magnitude lower than previous lower density FRC experiments. Second, Ohmic heating was found to correlate with the poloidal flux trapping at FRC formation, suggesting that poloidal flux dissipation is primarily responsible for plasma heating. Third, high density FRCs analyzed in this work reinforce the low-density adiabatic scaling, which shows that particle confinement time and flux confinement time are approximately equal.
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52.55.Lf Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps
52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.50.-b Plasma production and heating

Validation in fusion research: Towards guidelines and best practices

P. W. Terry, M. Greenwald, J.-N. Leboeuf, G. R. McKee, D. R. Mikkelsen, W. M. Nevins, D. E. Newman, D. P. Stotler, Task Group on Verification and Validation, U.S. Burning Plasma Organization, and U.S. Transport Task Force

Phys. Plasmas 15, 062503 (2008); http://dx.doi.org/10.1063/1.2928909 (12 pages) | Cited 26 times

Online Publication Date: 6 June 2008

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Because experiment/model comparisons in magnetic confinement fusion have not yet satisfied the requirements for validation as understood broadly, approaches to validating mathematical models and numerical algorithms are recommended as good practices. Previously identified procedures, such as, verification, qualification, and analysis of errors from uncertainties and deficiencies, remain important. However, particular challenges intrinsic to fusion plasmas and physical measurement therein lead to identification of new or less familiar concepts that are also critical in validation. These include the primacy hierarchy, which tracks the integration of measurable quantities, and sensitivity analysis, which assesses how model output is apportioned to different sources of variation. The use of validation metrics for individual measurements is extended to multiple measurements, with provisions for the primacy hierarchy and sensitivity. This composite validation metric is essential for quantitatively evaluating comparisons with experiments. To mount successful and credible validation in magnetic fusion, a new culture of validation is envisaged.
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52.55.-s Magnetic confinement and equilibrium
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