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Dec 2005

Volume 12, Issue 12, Articles (12xxxx)

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back to top Basic Plasma Phenomena, Waves, Instabilities

Magnetic dynamics of simple collective modes in a two-sphere plasma model

Hanno Essén

Phys. Plasmas 12, 122101 (2005); http://dx.doi.org/10.1063/1.2149349 (7 pages) | Cited 3 times

Online Publication Date: 23 December 2005

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A plasma blob is modeled as consisting of two homogeneous spheres of equal radius and equal but opposite charge densities that can move relative to each other. Relative translational and rotational motion are considered separately. Magnetic effects from the current density caused by the relative motion are included. Magnetic interaction is seen to cause an inductive inertia. In the relative translation case the Coulomb attraction, approximately a linear force for small amplitudes, causes an oscillation. For a large number of particles, the corresponding oscillation frequency will not be the Langmuir plasma frequency, because of the large inductive inertia. For rotation an external magnetic field is included and the energy and diamagnetism of the plasma in the model is calculated. Finally, it is noted how the neglect of resistivity is motivated by the results.
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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.)
52.25.Fi Transport properties
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Confinement of Coulomb balls

O. Arp, D. Block, M. Klindworth, and A. Piel

Phys. Plasmas 12, 122102 (2005); http://dx.doi.org/10.1063/1.2147000 (9 pages) | Cited 42 times

Online Publication Date: 23 December 2005

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A model for the confinement of the recently discovered Coulomb balls is proposed. These spherical three-dimensional plasma crystals are trapped inside a rf discharge under gravity conditions and show an unusual structural order in complex plasmas. Measurements of the thermophoretic force acting on the trapped dust particles and simulations of the plasma properties of the discharge are presented. The proposed model of confinement considers thermophoretic, ion-drag, and electric field forces, and shows excellent agreement with the observations. The findings suggest that self-confinement does not significantly contribute to the structural properties of Coulomb balls.
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52.58.Qv Electrostatic and high-frequency confinement
52.27.Lw Dusty or complex plasmas; plasma crystals
52.80.Pi High-frequency and RF discharges
52.65.-y Plasma simulation
back to top Nonlinear Phenomena, Turbulence, Transport

Vortex formation in an electron plasma with a sheared flow

J. O. Hall and P. K. Shukla

Phys. Plasmas 12, 122301 (2005); http://dx.doi.org/10.1063/1.2039547 (8 pages) | Cited 1 time

Online Publication Date: 2 December 2005

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The formation of vortex structures in an electron plasma with a sheared flow is investigated. The electron fluid is drifting in a self-electric field generated by an unshielded electron population. This setting is linearly unstable and an instability of diocotron (slipping-stream) type occurs. The time scale of the dynamics is assumed to be much shorter than the ion plasma and ion gyroperiods. Consequently, the ions do not respond to the wave potential and serve only as a neutralizing background. An equation determining the nonlinear evolution of the electrostatic potential in a plane perpendicular to an external magnetic field is derived within the drift approximation. The governing equation is then analyzed for the case with a localized shear in the electron fluid velocity. Possible final states of the diocotron instability are investigated analytically and solutions in the form of a tripolar vortex, a zonal flow, and a vortex street are found. The nonlinear time evolution of the diocotron instability is investigated by solving the governing equation numerically. In particular, the dynamics of nonlinearly saturated states and the formation of such states are discussed. Numerical solutions show a vortex street structure in a saturated state. The relevance of our investigation for space and laboratory plasmas is discussed.
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52.35.We Plasma vorticity
52.30.-q Plasma dynamics and flow
52.35.Kt Drift waves
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.25.Xz Magnetized plasmas
52.27.Jt Nonneutral plasmas
02.60.Cb Numerical simulation; solution of equations

Bifurcation theory of the transition to collisionless ion-temperature-gradient-driven plasma turbulence

R. A. Kolesnikov and J. A. Krommes

Phys. Plasmas 12, 122302 (2005); http://dx.doi.org/10.1063/1.2116887 (25 pages) | Cited 5 times

Online Publication Date: 2 December 2005

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The collisionless limit of the transition to ion-temperature-gradient-driven plasma turbulence is considered with a dynamical-systems approach. The importance of systematic analysis for understanding the differences in the bifurcations and dynamics of linearly damped and undamped systems is emphasized. A model with ten degrees of freedom is studied as a concrete example. A four-dimensional center manifold (CM) is analyzed, and fixed points of its dynamics are identified and used to predict a “Dimits shift” of the threshold for turbulence due to the excitation of zonal flows. The exact value of that shift in terms of physical parameters is established for the model; the effects of higher-order truncations on the dynamics are noted. Multiple-scale analysis of the CM equations is used to discuss possible effects of modulational instability on scenarios for the transition to turbulence in both collisional and collisionless cases.
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52.35.Ra Plasma turbulence
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.)
52.20.-j Elementary processes in plasmas
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Fi Transport properties
05.45.-a Nonlinear dynamics and chaos

Effects of varying the step particle distribution on a probabilistic transport model

S. Bouzat and R. Farengo

Phys. Plasmas 12, 122303 (2005); http://dx.doi.org/10.1063/1.2125507 (7 pages) | Cited 1 time

Online Publication Date: 2 December 2005

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The consequences of varying the step particle distribution on a probabilistic transport model, which captures the basic features of transport in plasmas and was recently introduced in Ref. 1 [ B. Ph. van Milligen et al., Phys. Plasmas 11, 2272 (2004) ], are studied. Different superdiffusive transport mechanisms generated by a family of distributions with algebraic decays (Tsallis distributions) are considered. It is observed that the possibility of changing the superdiffusive transport mechanism improves the flexibility of the model for describing different situations. The use of the model to describe the low (L) and high (H) confinement modes is also analyzed.
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52.25.Fi Transport properties
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
02.50.Cw Probability theory

Spectral transfers and zonal flow dynamics in the generalized Charney-Hasegawa-Mima model

C. N. Lashmore-Davies, A. Thyagaraja, and D. R. McCarthy

Phys. Plasmas 12, 122304 (2005); http://dx.doi.org/10.1063/1.2139973 (12 pages) | Cited 4 times

Online Publication Date: 2 December 2005

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The mechanism of four nonlinearly interacting drift or Rossby waves is used as the basic process underlying the turbulent evolution of both the Charney-Hasegawa-Mima-equation (CHME) and its generalized modification (GCHME). Hasegawa and Kodama’s concept of equivalent action (or quanta) is applied to the four-wave system and shown to control the distribution of energy and enstrophy between the modes. A numerical study of the GCHME is described in which the initial state contains a single finite-amplitude drift wave (the pump wave), and all the modulationally unstable modes are present at the same low level (10−6 times the pump amplitude). The simulation shows that at first the fastest-growing modulationally unstable modes dominate but reveals that at a later time, before pump depletion occurs, long- and short-wavelength modes, driven by pairs of fast-growing modes, grow at 2γmax. The numerical simulation illustrates the development of a spectrum of turbulent modes from a finite-amplitude pump wave.
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52.30.-q Plasma dynamics and flow
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Kt Drift waves
52.35.Ra Plasma turbulence
52.35.We Plasma vorticity
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.65.Kj Magnetohydrodynamic and fluid equation

Discrete particle noise in particle-in-cell simulations of plasma microturbulence

W. M. Nevins, G. W. Hammett, A. M. Dimits, W. Dorland, and D. E. Shumaker

Phys. Plasmas 12, 122305 (2005); http://dx.doi.org/10.1063/1.2118729 (16 pages) | Cited 36 times

Online Publication Date: 12 December 2005

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Recent gyrokinetic simulations of electron temperature gradient (ETG) turbulence with the global particle-in-cell (PIC) code GTC [ Z. Lin et al., Proceedings of the 20th Fusion Energy Conference, Vilamoura, Portugal, 2004 (IAEA, Vienna, 2005) ] yielded different results from earlier flux-tube continuum code simulations [ F. Jenko and W. Dorland, Phys. Rev. Lett. 89, 225001 (2002) ] despite similar plasma parameters. Differences between the simulation results were attributed to insufficient phase-space resolution and novel physics associated with global simulation models. The results of the global PIC code are reproduced here using the flux-tube PIC code PG3EQ [ A. M. Dimits et al., Phys. Rev. Lett. 77, 71 (1996) ], thereby eliminating global effects as the cause of the discrepancy. The late-time decay of the ETG turbulence and the steady-state heat transport observed in these PIC simulations are shown to result from discrete particle noise. Discrete particle noise is a numerical artifact, so both these PG3EQ simulations and, by inference, the GTC simulations that they reproduced have little to say about steady-state ETG turbulence and the associated anomalous heat transport. In the course of this work several diagnostics are developed to retrospectively test whether a particular PIC simulation is dominated by discrete particle noise.
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52.35.Ra Plasma turbulence
52.65.Rr Particle-in-cell method
52.25.Fi Transport properties
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Turbulence spreading, anomalous transport, and pinch effect

V. Naulin, A. H. Nielsen, and J. Juul Rasmussen

Phys. Plasmas 12, 122306 (2005); http://dx.doi.org/10.1063/1.2141396 (9 pages) | Cited 22 times

Online Publication Date: 12 December 2005

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A paradigmatic model describing transport and turbulence spreading as coupled processes is proposed, trying to unify the approaches of penetrative overshoot and overshoot. As a natural consequence asymmetric radial spreading of the turbulence, up-gradient transport of the particle density, and front propagation are observed. The model accounts for the interaction between the microscale of the turbulence and the meso-, respectively, system scale on which profile modifications occur. Comparison with direct numerical simulations of two-dimensional interchange turbulence shows qualitatively good agreement with the proposed transport model. Key transport features are reproduced even in the presence of coherent bloblike structures. Features of density pulse dynamics are also investigated.
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52.25.Gj Fluctuation and chaos phenomena
52.35.Ra Plasma turbulence
52.65.Kj Magnetohydrodynamic and fluid equation

Nonlinear instability and saturation of linearly stable current-carrying pair plasmas

A. Luque, H. Schamel, B. Eliasson, and P. K. Shukla

Phys. Plasmas 12, 122307 (2005); http://dx.doi.org/10.1063/1.2140228 (6 pages) | Cited 16 times

Online Publication Date: 20 December 2005

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The nonlinear instability of current-carrying pair plasmas is investigated with a Vlasov–Poisson model for the two-particle species. It is shown that linearly stable configurations are unstable against small incoherent perturbations of the particle distribution functions. The instability gives rise to a self-acceleration and growth of phase-space holes. After the growth of the phase-space holes, the instability reaches a chaotic saturation where the finite-amplitude holes interact and merge, and after a long time, the system attains a stable equilibrium state with a smaller drift and a larger temperature than the initial one, and where a few stable phase-space holes are present.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Sb Solitons; BGK modes
52.27.Ep Electron-positron plasmas
81.05.ub Fullerenes and related materials

Electromagnetic fluid drift turbulence in static ergodic magnetic fields

D. Reiser and B. Scott

Phys. Plasmas 12, 122308 (2005); http://dx.doi.org/10.1063/1.2141928 (18 pages) | Cited 11 times

Online Publication Date: 20 December 2005

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Numerical simulations of three-dimensional nonlinear electromagnetic fluid drift turbulence in a tokamak plasma with externally applied stochastic magnetic-field perturbations are presented. The contributions to the radial particle transport due to nonlinearities arising from E×B advection and magnetic flutter are investigated for perturbation fields of varying strengths in the cases of low and high collisionalities. The perturbation strength is varied to study the physics for Chirikov parameters above 1. In all the cases considered a significant increase of E×B transport is found. A static contribution in the density and velocity perturbations contributes significantly to the total radial E×B transport. For low collisionality, the external perturbation leads to enhanced density and velocity fluctuations over a broad range in the toroidal wave-number spectrum, resulting in an enhanced turbulent flux. For high collisionality, the density fluctuations stay roughly the same and the velocity fluctuations are increased in an intermediate range of the toroidal wave number spectrum, separated from the maximum of the density fluctuations, thus leaving the turbulent flux almost unchanged.
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52.35.Ra Plasma turbulence
52.35.Kt Drift waves
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.20.-j Elementary processes in plasmas
52.25.Gj Fluctuation and chaos phenomena

Effect of two-temperature trapped electrons to nonlinear dust-ion-acoustic solitons

Waleed M. Moslem and W. F. El-Taibany

Phys. Plasmas 12, 122309 (2005); http://dx.doi.org/10.1063/1.2146940 (7 pages) | Cited 28 times

Online Publication Date: 20 December 2005

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Propagation of three-dimensional dust-ion-acoustic solitons is investigated in a dusty plasma consisting of positive ions, negatively variable-charged dust particles, and two-temperature trapped electrons. We use the reductive perturbation theory to reduce the basic set of fluid equations to one evolution equation called damped modified Kadontsev-Petviashivili equation. Exact solution of this equation is not possible, so we obtain the time evolution solitary wave form approximate solution. It is found that only compressive soliton can propagate in this system. We develop a theoretical estimate condition under which the solitons can propagate. It is found that this condition is satisfied for Saturn’s F ring. It is found also that low electron temperature has a role on the behavior of the soliton width, i.e., for lower (higher) range of low electron temperature the soliton width decreases (increases). However, high electron temperature decreases the width. The trapped electrons have no effect on the soliton width. The ratio of free low (high) to trapped low (high) electron temperatures increases the soliton amplitude. Also, the amplitude increases with free low and free high electron temperatures. To investigate the stabilty of the waves, we used a method based on energy consideration to obtain a condition for stable solitons. It is found that this condition depends on dust charge variation, streaming velocity, directional cosine of the wave vector k along the x axis, and temperatures of dust particles, ions, and free electrons.
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52.35.Sb Solitons; BGK modes
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.27.Lw Dusty or complex plasmas; plasma crystals
52.35.Dm Sound waves
52.25.Fi Transport properties
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Driven dissipative whistler wave turbulence

Dastgeer Shaikh and Gary P. Zank

Phys. Plasmas 12, 122310 (2005); http://dx.doi.org/10.1063/1.2146957 (7 pages) | Cited 17 times

Online Publication Date: 20 December 2005

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Driven dissipative whistler wave turbulence in two-dimensional electron magnetohydrodynamics is investigated using very high resolution nonlinear fluid simulations. It is shown that a dual cascade phenomenon of mean magnetic potential and energy invariants is in agreement with predictions based on statistical ensemble and Kolmogorov’s theories. Turbulent length scales larger than the electron skin depth (whistler wave regime) exhibit a spectral break in the vicinity of the forcing wave number that separates the inverse and forward cascade regimes. On the other hand, length scales smaller than the electron skin depth behave like hydrodynamic eddies in which both small and large scale regimes exhibit identical turbulent spectra. In both cases, however, turbulent fluctuations follow an exact Kolmogorov-type spectra. While wave effects are strong in the whistler wave regime, they are absent entirely in the hydrodynamics regime of the driven electron magnetohydrodynamic turbulence.
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52.35.Ra Plasma turbulence
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.25.Gj Fluctuation and chaos phenomena

Drift wave driven zonal flows in plasmas

T. D. Kaladze, D. J. Wu, O. A. Pokhotelov, R. Z. Sagdeev, L. Stenflo, and P. K. Shukla

Phys. Plasmas 12, 122311 (2005); http://dx.doi.org/10.1063/1.2151108 (6 pages) | Cited 20 times

Online Publication Date: 23 December 2005

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The generation of large-scale zonal flows by small-scale electrostatic drift waves in a plasma is considered. The generation mechanism is based on the parametric excitation of convective cells by finite amplitude drift waves. To describe this process a generalized Hasegawa–Mima equation containing both vector and scalar nonlinearities is used. The drift waves are supposed to have arbitrary wavelengths (as compared with the ion Larmor radius). A set of coupled equations describing the nonlinear interaction of drift waves and zonal flows is deduced. The generation of zonal flows turns out to be due to Reynolds stresses produced by finite amplitude drift waves. It is found that the wave vector of the fastest growing mode is perpendicular to that of the drift pump wave. Explicit expressions for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are obtained. A comparison with previous results is carried out. The present theory can be used for interpretations of drift wave observations in laboratory plasmas.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Kt Drift waves
52.35.Ra Plasma turbulence

The scaling of forced collisionless reconnection

Brian P. Sullivan, Barrett N. Rogers, and M. A. Shay

Phys. Plasmas 12, 122312 (2005); http://dx.doi.org/10.1063/1.2146910 (12 pages) | Cited 5 times

Online Publication Date: 23 December 2005

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We present two-fluid simulations of forced magnetic reconnection with finite electron inertia in a collisionless two-dimensional slab geometry. Reconnection in this system is driven by a spatially localized forcing function that is added to the ion momentum equation inside the computational domain. The resulting forced reconnection process is studied as a function of the temporal and spatial structure of the forcing function, the plasma β, and strength of the out-of-plane guide magnetic field component, and the electron to ion mass ratio. Consistent with previous results found in unforced, large Δ′ systems, for sufficiently strong forcing the reconnection process is found to become Alfvénic, i.e., the inflow velocity scales roughly like some small fraction of the Alfvén speed based on the reconnecting component of the magnetic field just upstream of the dissipation region. The magnitude of this field and thus the rate of reconnection is controlled by the behavior of the forcing function. When the forcing strength is below a certain level, fast reconnection is not observed.
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52.35.Vd Magnetic reconnection
52.30.Ex Two-fluid and multi-fluid plasmas
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.Fi Transport properties
52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
back to top Magnetically Confined Plasmas, Heating, Confinement

Kinetic properties of shear Alfvén eigenmodes in tokamak plasmas

Ph. Lauber, S. Günter, and S. D. Pinches

Phys. Plasmas 12, 122501 (2005); http://dx.doi.org/10.1063/1.2135284 (6 pages) | Cited 24 times

Online Publication Date: 2 December 2005

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This work reports on numerical calculations concerning the kinetic properties of low-n, low-m toroidal Alfvén eigenmodes (TAEs) in tokamak plasmas for fusion relevant parameters. The self-consistent and nonperturbative code LIGKA [ Ph. Lauber, Ph.D. thesis, TU München (2003) ] is employed. It is based on a linear gyrokinetic model consisting of the quasineutrality equation and the moment equation for the perturbed current. It is shown that in a certain limit the underlying equations of LIGKA can be simplified to the equations known as the “reduced kinetic model.” An antenna-like version of LIGKA allows one to systematically find all shear-Alfvén-type modes in a given frequency interval, such as kinetic TAEs (KTAEs) and kinetically modified TAEs. The coupling to the kinetic Alfvén wave (KAW) is found in the form of continuum damping and radiative damping. For the cases examined here, no mode conversion in the centre is found. In the case of a large nonideal parameter, damping rates around 0.5%–1% are found, close to experimental measurements.
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52.55.Tn Ideal and resistive MHD modes; kinetic modes
52.65.Tt Gyrofluid and gyrokinetic simulations

Sheared flow effects on ballooning instabilities in three-dimensional equilibria

C. C. Hegna

Phys. Plasmas 12, 122502 (2005); http://dx.doi.org/10.1063/1.2136870 (6 pages) | Cited 1 time

Online Publication Date: 2 December 2005

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The stability of ideal magnetohydrodynamic ballooning modes in the presence of sheared flow is investigated for three-dimensional equilibria. Application of ballooning formalism reduces the problem to a partial differential equation in three dimensions that can be solved in the limit of small flow. Analytic calculations demonstrate the stabilizing effect of shear flow. The derived stability criterion generalizes prior work related to axisymmetric equilibrium with sheared toroidal flow.
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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)
52.55.Tn Ideal and resistive MHD modes; kinetic modes
02.30.Jr Partial differential equations

Gyro center invariant and associated diamagnetic current

O. Ågren, V. Moiseenko, C. Johansson, and N. Savenko

Phys. Plasmas 12, 122503 (2005); http://dx.doi.org/10.1063/1.2139503 (13 pages) | Cited 3 times

Online Publication Date: 12 December 2005

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The gyro center radial Clebsch coordinate math0 is an exact invariant in confining fields where the gyro center is restricted to move on a magnetic flux surface, and math0 could also be expected to be a useful approximating invariant in other confining magnetic fields. A radial drift invariant Ir generalizes the invariance of math0 if there are oscillatory gyro center radial displacements off the magnetic surface. Expressions for math0(x,v) and Ir(x,v) are obtained for gyrating particles in the drift ordering. An exact energy integral is proven to exist for the first-order drift motion of the gyro center. The gyro center parallel motion is periodic with respect to a certain curve parameter math‖ (the “proper time” for the parallel motion) that deviates slightly, due to the slow perpendicular drift, from the ordinary time. A modification of the parallel invariant J‖ is derived which leads to an exact (not only adiabatic) invariant to first order. By using math0 in solutions of the Vlasov equation, it is demonstrated that the approximating gyro center invariant math0 determines the perpendicular plasma diamagnetic current. It is also shown that a fourth stationary motional invariant is required to calculate the parallel plasma current. Several systems with four time independent invariants are identified, and the general solution for straight cylindrical Vlasov equilibria with adiabatic particle motion is determined. A set (ε,μ,I‖,Ir) of four invariants is proposed for adiabatic equilibria in general geometry, including systems where single valued flux surfaces may not exist.
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52.25.Fi Transport properties
52.55.Dy General theory and basic studies of plasma lifetime, particle and heat loss, energy balance, field structure, etc.
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Analytic description of high poloidal beta equilibrium with a natural inboard poloidal field null

Bingren Shi

Phys. Plasmas 12, 122504 (2005); http://dx.doi.org/10.1063/1.2140227 (8 pages) | Cited 5 times

Online Publication Date: 12 December 2005

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Analytical high poloidal beta equilibria for toroidally axisymmetric plasmas with arbitrary aspect ratio and elongation are described. These equilibria that can describe a transition from nondivertor to divertor configuration are exact solutions of the Grad-Shafranov equation when the toroidal current density is quasiuniform. Generally, these are high poloidal beta equilibria, limited by the appearance of a natural inboard poloidal field null. Some of their properties, including the nonuniformity of the poloidal magnetic field in the poloidal direction, the safety factor profile and the magnetic shear profile near the separatrix, the parameter dependence of the poloidal beta βp and εβp, as well as the toroidal beta βT on the aspect ratio and the elongation of the magnetic surface, are discussed. Applications to experiments of the Tokamak Fusion Test Reactor (TFTR) [Sabbagh et al., Phys. Fluids B3, 2277 (1991)] are particularly analyzed.
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52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties
52.30.Cv Magnetohydrodynamics (including electron magnetohydrodynamics)

Runaway electron generation in a cooling plasma

H. Smith, P. Helander, L.-G. Eriksson, and T. Fülöp

Phys. Plasmas 12, 122505 (2005); http://dx.doi.org/10.1063/1.2148966 (9 pages) | Cited 13 times

Online Publication Date: 20 December 2005

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The usual calculation of Dreicer [Phys. Rev. 115, 238 (1959) ; 117, 329 (1960)] generation of runaway electrons assumes that the plasma is in a steady state. In a tokamak disruption this is not necessarily true since the plasma cools down quickly and the collision time for electrons at the runaway threshold energy can be comparable to the cooling time. The electron distribution function then acquires a high-energy tail which can easily be converted to a burst of runaways by the rising electric field. This process is investigated and simple criteria for its importance are derived. If no rapid losses of fast electrons occur, this can be a more important source of runaway electrons than ordinary Dreicer generation in tokamak disruptions.
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52.20.Fs Electron collisions
52.55.Fa Tokamaks, spherical tokamaks
52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

Ion heating during magnetic relaxation in the helicity injected torus-II experiment

R. G. O’Neill, A. J. Redd, W. T. Hamp, R. J. Smith, and T. R. Jarboe

Phys. Plasmas 12, 122506 (2005); http://dx.doi.org/10.1063/1.2141932 (6 pages) | Cited 6 times

Online Publication Date: 20 December 2005

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Ion doppler spectroscopy (IDS) is applied to the helicity injected torus (HIT-II) spherical torus to measure impurity ion temperature and flows. [ A. J. Redd et al., Phys. Plasmas 9, 2006 (2002) ] The IDS instrument employs a 16-channel photomultiplier and can track temperature and velocity continuously through a discharge. Data for the coaxial helicity injection (CHI), transformer, and combined current drive configurations are presented. Ion temperatures for transformer-driven discharges are typically equal to or somewhat lower than electron temperatures measured by Thomson scattering. Internal reconnection events in transformer-driven discharges cause rapid ion heating. The CHI discharges exhibit anomalously high ion temperatures >250 eV, which are an order of magnitude higher than Thomson measurements, indicating ion heating through magnetic relaxation. The CHI discharges that exhibit current and poloidal flux buildup after bubble burst show sustained ion heating during current drive.
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52.50.Gj Plasma heating by particle beams
52.35.Vd Magnetic reconnection
52.55.Wq Current drive; helicity injection
52.55.Ip Spheromaks
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Vy Impurities in plasmas

Nonlinear effects of lower hybrid wave absorption on location of power deposition in the HL-2A reversed-shear plasma

Qingdi Gao and Jinhua Zhang

Phys. Plasmas 12, 122507 (2005); http://dx.doi.org/10.1063/1.2142246 (7 pages) | Cited 1 time

Online Publication Date: 20 December 2005

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In the simulations of the reversed-shear plasma of the HL-2A tokamak [Q. Gao et al., Nucl. Fusion 40, 1897 (2000)] , the absorption of the lower hybrid (LH) wave is weak and makes many passes through the plasma until the initial launched wave spectrum is sufficiently broadened to be absorbed. As the constraint imposed by the wave propagation condition limits the maximum allowed n‖ upshift, the LH wave absorption is bounded in the region defined by the strong Landau-damping limit and the boundary of the wave propagation domain. This mechanism of the LH wave absorption causes interplay of the distribution of the LH wave driven current with the modification of the plasma configuration, which constitutes nonlinearity in the LH wave deposition. Due to the nonlinear coupling between the LH power deposition and the profiles of both pressure and current, the LH wave deposition position changes spontaneously, generating two distinct quasistationary reversed magnetic shear (RS) configurations in a single discharge without changing the discharge condition. Therefore, the feedback control of the plasma current profile through controlling the externally driven current by the LH wave is a challenge in the RS plasma regimes for steady-state high performance tokamak operations.
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52.35.Mw Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.55.Fa Tokamaks, spherical tokamaks
52.25.Fi Transport properties

Dust-particle transport in tokamak edge plasmas

A. Yu. Pigarov, S. I. Krasheninnikov, T. K. Soboleva, and T. D. Rognlien

Phys. Plasmas 12, 122508 (2005); http://dx.doi.org/10.1063/1.2145157 (15 pages) | Cited 62 times

Online Publication Date: 20 December 2005

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Dust particulates in the size range of 10 nm–100 μm are found in all fusion devices. Such dust can be generated during tokamak operation due to strong plasma∕material-surface interactions. Some recent experiments and theoretical estimates indicate that dust particles can provide an important source of impurities in the tokamak plasma. Moreover, dust can be a serious threat to the safety of next-step fusion devices. In this paper, recent experimental observations on dust in fusion devices are reviewed. A physical model for dust transport simulation and a newly developed code DUSTT are discussed. The DUSTT code incorporates both dust dynamics due to comprehensive dust-plasma interactions as well as the effects of dust heating, charging, and evaporation. The code tracks test dust particles in realistic plasma backgrounds as provided by edge-plasma transport codes. The results are presented for dust transport in current and next-step tokamaks. The effect of dust on divertor plasma profiles and core plasma contamination is examined.
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52.27.Lw Dusty or complex plasmas; plasma crystals
52.25.Fi Transport properties
52.55.Fa Tokamaks, spherical tokamaks
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Vy Impurities in plasmas
28.52.Nh Safety

Nonlinear viscosity and its role in drift-Alfvén modes

V. S. Tsypin, A. B. Mikhailovskii, M. S. Shirokov, E. A. Kovalishen, S. V. Konovalov, and R. M. O. Galvão

Phys. Plasmas 12, 122509 (2005); http://dx.doi.org/10.1063/1.2151169 (9 pages) | Cited 1 time

Online Publication Date: 23 December 2005

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The moment approach is used to analyze the part of the magnetized plasma viscosity related to the nonlinear character of the Landau collision integral in the Boltzmann kinetic equation (nonlinear viscosity), pointed out by Catto and Simakov [Phys. Plasmas 11, 90 (2004)] . It is shown that the results of these authors, who have used an alternative procedure based on a more detailed analysis of the kinetic equation, correspond to a 15-moment approach. In comparison with the 13-moment approach (density, temperature, velocity, heat flux, and the viscosity tensor) of Grad, the 15-moment approach takes into account two higher-order moments, one of which is the vector-type moment similar to the parallel heat flux and the second is the tensor-type moment similar to the parallel projection of the viscosity tensor. Both these higher-order moments enter into the Braginskii approximation. The nonlinear viscosity calculated in the scope of the 13-moment Grad approach is qualitatively the same as that found by Catto and Simakov. Its role is investigated for drift-Alfvén modes, driven by the combined effect of the dissipative part of perpendicular heat conductivity and the standard collisional viscosity, and it is shown to be essential for the radial transport of these modes. It is shown that the wave packet of drift-Alfvén modes, propagating in the diamagnetic drift direction and driven for reversed temperature gradient, is transported down the pressure gradient. In contrast to this, the wave packet propagating in the electron diamagnetic drift direction and driven for positive temperature gradient is transported up the pressure gradient.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Simulation study of relativistic dynamics of MeV alpha particles in magnetized plasmas for explaining an experimental anomaly

K. R. Chen and T. H. Tsai

Phys. Plasmas 12, 122510 (2005); http://dx.doi.org/10.1063/1.2148912 (9 pages) | Cited 3 times

Online Publication Date: 23 December 2005

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In a Test Fusion Tokamak Reactor [ R. J. Hawryluk et al., Phys. Plasmas 5, 1577 (1998) ] experiment, the measured energy spectrum of the deeply trapped alpha particles is found to be 1 MeV too broad to be explained by classical collisions and the peak energy similarly off by 450 keV. The relativistic effect is proposed as an explanation. Here, we report high-resolution Monte Carlo (MC) and particle-in-cell (PIC) simulation studies in detail, under the assumption of a uniform magnetic field, for the identification of the cause of the observed anomaly. The 3.5 MeV alpha particles produced by thermonuclear fusion reaction are broadened due to Doppler effect. The relativistic alpha particle dynamics are followed with the PIC code. The relativistic ion cyclotron instability grows to saturation on a time scale (10−5 s) much shorter than the experimental time scale of 0.1 s. The MC code is then used to follow, in real time, the collisional slowing down of the gyrobroadened alphas, including the effect of the time delay in diagnostic pellet releasing and flight. Relativistic gyrobroadening is shown to be crucial in shaping the birth and slowed-down spectra. The resultant alpha particle energy spectrum fits well with that of the measurement, with a reduced chi square of unity.
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52.27.Ny Relativistic plasmas
52.55.Pi Fusion products effects (e.g., alpha-particles, etc.), fast particle effects
52.65.Pp Monte Carlo methods
52.65.Rr Particle-in-cell method
52.25.Xz Magnetized plasmas
52.55.Fa Tokamaks, spherical tokamaks

Effect of drift-acoustic waves on magnetic island stability in slab geometry

R. Fitzpatrick and F. L. Waelbroeck

Phys. Plasmas 12, 122511 (2005); http://dx.doi.org/10.1063/1.2146983 (7 pages) | Cited 10 times

Online Publication Date: 23 December 2005

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A mathematical formalism is developed for calculating the ion polarization term in the Rutherford island width evolution equation in the presence of drift-acoustic waves. The calculation is fully nonlinear, includes both ion and electron diamagnetic effects, as well as ion compressibility, but is performed in slab geometry. Magnetic islands propagating in a certain range of phase velocities are found to emit drift-acoustic waves. Wave emission gives rise to rapid oscillations in the ion polarization term as the island phase velocity varies, and also generates a net electromagnetic force acting on the island region. Increasing ion compressibility is found to extend the range of phase velocities over which drift-acoustic wave emission occurs in the electron diamagnetic direction.
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52.30.Ex Two-fluid and multi-fluid plasmas
52.55.Tn Ideal and resistive MHD modes; kinetic modes
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