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

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Transient ionization in plasmas produced by point-like irradiation of solid Al targets

L. A. Gizzi, C. A. Cecchetti, M. Galimberti, A. Giulietti, D. Giulietti, L. Labate, S. Laville, and P. Tomassini

Phys. Plasmas 10, 4601 (2003); http://dx.doi.org/10.1063/1.1624603 (4 pages) | Cited 2 times

Online Publication Date: 20 November 2003

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Time-resolved x-ray spectroscopy has been used to investigate ionization dynamics of a micrometer-sized nanosecond laser-plasma during the plasma start-up phase. Experimental results are modeled using two-dimensional hydrodynamic simulations and time-dependent collisional-radiative calculations. The study clearly shows that, due to the rapid expansion cooling, x-ray emission originates predominantly from a well-localized plasma region characterized by rapidly evolving hydrodynamic conditions. In this region, ionization dynamics is found to depart substantially from the steady-state regime. The measurements provide clear evidence of this transient ionization regime showing good agreement with the time-dependent calculations. © 2003 American Institute of Physics.

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52.38.-r	Laser-plasma interactions
52.38.Ph	X-ray, γ-ray, and particle generation
52.70.La	X-ray and γ-ray measurements
52.25.Jm	Ionization of plasmas

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Electron bunch acceleration and trapping by the ponderomotive force of an intense short-pulse laser

Q. Kong, S. Miyazaki, S. Kawata, K. Miyauchi, K. Nakajima, S. Masuda, N. Miyanaga, and Y. K. Ho

Phys. Plasmas 10, 4605 (2003); http://dx.doi.org/10.1063/1.1622952 (4 pages) | Cited 22 times

Online Publication Date: 20 November 2003

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By utilizing a pulsed laser beam of TEM(1,0)+TEM(0,1) mode, it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity of I ∼ 5.48×10¹⁸ W/cm². In addition, the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is ∼ 10λ in the longitudinal direction and the bunch radius is about 625λ in the transverse direction. © 2003 American Institute of Physics.

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29.27.Eg	Beam handling; beam transport
41.75.Fr	Electron and positron beams
07.77.Ka	Charged-particle beam sources and detectors

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Electrostatic ion-cyclotron waves in a currentless, anisotropic plasma with inhomogeneous flow

Earl E. Scime, Ryan Murphy, Gurudas I. Ganguli, and Eric Edlund

Phys. Plasmas 10, 4609 (2003); http://dx.doi.org/10.1063/1.1624604 (4 pages) | Cited 4 times

Online Publication Date: 20 November 2003

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The linearized dispersion relation describing waves in a plasma having a uniform magnetic field, uniform density, inhomogeneous parallel (to the magnetic field) flow, and thermal anisotropy (T_i⊥/T_i∥) is used to determine the threshold condition for growth of an electrostatic ion cyclotron wave and its harmonics. The inclusion of moderate ion thermal anisotropy (T_i⊥/T_i∥ ∼ 3) and parallel-flow shear (dV_d/dx ∼ 0.05Ω_ci), values typical of many space and laboratory plasmas, reduces the critical current necessary for instability growth to nearly zero. That an electrostatic instability conventionally associated with strong field aligned currents can be excited in the absence of any field aligned current and only by moderate parallel-flow shear suggests that ion cyclotron instabilities may play a larger role in many plasma environments than previously believed. The decrease in the instability threshold for increasing thermal anisotropy suggests that ion heating due to ion cyclotron waves may result in a positive feedback process absent in homogeneous plasmas. © 2003 American Institute of Physics.

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52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)

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Three-dimensional isotropic magnetohydrodynamic turbulence and thermal velocity of the solar wind ions

A. Bershadskii

Phys. Plasmas 10, 4613 (2003); http://dx.doi.org/10.1063/1.1627765 (3 pages) | Cited 3 times

Online Publication Date: 20 November 2003

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It is shown that fluctuations of the solar wind ions velocity (as measured on the kinetic level by the SWICS-detector on board the Advanced Composition Explorer) exhibit statistical properties which are in very good agreement with predictions of the three-dimensional (3D) isotropic magnetohydrodynamics (MHD) turbulence theory [Biskamp and Müller, Phys. Plasmas 7, 4889 (2000)]. The question is: How can 3D isotropic MHD be used to describe the interaction of the solar wind magnetic field with the kinetic (collisionless) ions? © 2003 American Institute of Physics.

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96.20.Br	Origin and evolution
95.30.Qd	Magnetohydrodynamics and plasmas
52.65.Kj	Magnetohydrodynamic and fluid equation

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Low-frequency waves in collisional complex plasmas with an ion drift

S. A. Khrapak and V. V. Yaroshenko

Phys. Plasmas 10, 4616 (2003); http://dx.doi.org/10.1063/1.1621398 (6 pages) | Cited 12 times

Online Publication Date: 20 November 2003

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A self-consistent model of low-frequency linear waves in collisional complex (dusty) plasmas with an ion drift is presented. Plasma conditions relevant to recent wave experiments under microgravity conditions are considered. Ion-neutral, ion-dust, and neutral-dust collisions, as well as external forces acting on the grains and grain charge variations in the presence of the wave are taken into account. A linear dispersion relation is obtained and some limiting cases are analyzed. Comparison of the obtained theoretical results with the experiments under microgravity conditions is presented. © 2003 American Institute of Physics.

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52.27.Lw	Dusty or complex plasmas; plasma crystals
52.35.-g	Waves, oscillations, and instabilities in plasmas and intense beams

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Excitation of nonreciprocal electromagnetic surface waves in semibounded magnetized plasmas by an electron beam

B. Shokri and B. Jazi

Phys. Plasmas 10, 4622 (2003); http://dx.doi.org/10.1063/1.1623765 (5 pages) | Cited 15 times

Online Publication Date: 20 November 2003

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The dispersion relation of nonreciprocal electromagnetic surface waves propagating on the magnetized plasma–vacuum interface is obtained. Furthermore, the dependency of penetration depth on the magnetic field strength and its directivity is investigated. Finally, it will be shown that by an electron beam flowing on the plasma surface, aforementioned waves can be excited. © 2003 American Institute of Physics.

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52.25.-b

Plasma properties

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Resonance cones in a dusty magnetized plasma

Thomas Trottenberg, Björn Brede, Dietmar Block, and Alexander Piel

Phys. Plasmas 10, 4627 (2003); http://dx.doi.org/10.1063/1.1624834 (6 pages) | Cited 3 times

Online Publication Date: 20 November 2003

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A new diagnostic method for magnetized dusty plasmas, the excitation of lower hybrid resonance cones, is investigated experimentally. The resonance cone is excited with a small antenna, and the angular distribution of the wave field with respect to the magnetic field shows a resonant enhancement, which shifts according to the free electron density. It is demonstrated that dust reduces the free electron density in agreement with Langmuir probe results. Wave damping by scattering effects is found negligible. © 2003 American Institute of Physics.

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52.25.Xz	Magnetized plasmas
52.27.Lw	Dusty or complex plasmas; plasma crystals
52.35.Hr	Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.70.Gw	Radio-frequency and microwave measurements

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Linear theory of nonlocal transport in a magnetized plasma

A. V. Brantov, V. Yu. Bychenkov, W. Rozmus, C. E. Capjack, and R. Sydora

Phys. Plasmas 10, 4633 (2003); http://dx.doi.org/10.1063/1.1624249 (12 pages) | Cited 6 times

Online Publication Date: 20 November 2003

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A system of nonlocal electron-transport equations for small perturbations in a magnetized plasma is derived using the systematic closure procedure of Bychenkov et al. [Phys. Rev. Lett. 75, 4405 (1995)]. Solution to the linearized kinetic equation with a Landau collision operator is obtained in the diffusive approximation. The Fourier components of the longitudinal, oblique, and transversal electron fluxes are found in an explicit form for quasistatic conditions in terms of the generalized forces: the gradients of density and temperature, and the electric field. The full set of nonlocal transport coefficients is given and discussed. Nonlocality of transport enhances electron fluxes across magnetic field above the values given by strongly collisional local theory. Dispersion and damping of magnetohydrodynamic waves in weakly collisional plasmas is discussed. Nonlocal transport theory is applied to the problem of temperature relaxation across the magnetic field in a laser hot spot. © 2003 American Institute of Physics.

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52.25.Fi	Transport properties
52.35.Bj	Magnetohydrodynamic waves (e.g., Alfven waves)
52.38.Fz	Laser-induced magnetic fields in plasmas

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The concept of collision strength and a unified kinetic calculation for hard-sphere interactions and inverse square force law interactions

Yongbin Chang

Phys. Plasmas 10, 4645 (2003); http://dx.doi.org/10.1063/1.1625647 (16 pages) | Cited 4 times

Online Publication Date: 20 November 2003

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With a concept of collision strength and other associated definitions, a unified kinetic theory for both hard-sphere interactions and inverse square force law interactions is developed. Collision frequencies that associate with many kinds of physical terms are calculated and expressed by a series special function Υ_j(α,x). Among them are arbitrary higher order linear Fokker–Planck coefficients, collision frequency, and energy exchange frequency. In case of a two-temperature system, the total collision rate, energy exchange rate, and collision strength rate are calculated and expressed in a uniform expression. A primitive form of Coulomb logarithm ½Γ(0,h_min) is found by comparing the exact form of equilibration time with Spitzer’s result. Many unifications are found from the unified expression. The threshold value of collision strength has unified activation energy in chemical reaction rate theory and ionization energy in Thomson’s classical ionization theory. An incomplete gamma function has unified Arrhenius exponential coefficient in chemical reaction rate theory and Coulomb logarithm in plasma physics. © 2003 American Institute of Physics.

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52.25.Dg	Plasma kinetic equations
98.10.+z	Stellar dynamics and kinematics
51.10.+y	Kinetic and transport theory of gases
82.40.-g	Chemical kinetics and reactions: special regimes and techniques

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The two-dimensional magnetohydrodynamic Kelvin–Helmholtz instability: Compressibility and large-scale coalescence effects

H. Baty, R. Keppens, and P. Comte

Phys. Plasmas 10, 4661 (2003); http://dx.doi.org/10.1063/1.1624076 (14 pages) | Cited 15 times

Online Publication Date: 20 November 2003

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The Kelvin–Helmholtz (KH) instability occurring in a single shear flow configuration that is embedded in a uniform flow-aligned magnetic field, is revisited by means of high resolution two-dimensional magnetohydrodynamic simulations. First, the calculations extend previous studies of magnetized shear flows to a higher compressibility regime. The nonlinear evolution of an isolated KH billow emerging from the fastest growing linear mode for a convective sonic Mach number M_cs = 0.7 layer is in many respects similar to its less compressible counterpart (Mach M_cs = 0.5). In particular, the disruptive regime where locally amplified, initially weak magnetic fields, control the nonlinear saturation process is found for Alfvén Mach numbers 4≲M_A≲30. The most notable difference between M_cs = 0.7 vs M_cs = 0.5 layers is that higher density contrasts and fast magnetosonic shocklet structures are observed. Second, the use of adaptive mesh refinement allows to parametrically explore much larger computational domains, including up to 22 wavelengths of the linearly dominant mode. A strong process of large-scale coalescence is found, whatever the magnetic field regime. It proceeds through continuous pairing/merging events between adjacent vortices up to the point where the final large-scale vortical structure reaches the domain dimensions. This pairing/merging process is attributed to the growth of subharmonic modes and is mainly controlled by relative phase differences between them. These grid-adaptive simulations demonstrate that even in very weak magnetic field regimes (M_A ≃ 30), the large-scale KH coalescence process can trigger tearing-type reconnection events previously identified in cospatial current–vortex sheets. © 2003 American Institute of Physics.

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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
52.30.-q	Plasma dynamics and flow
95.30.Qd	Magnetohydrodynamics and plasmas

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Electrostatic instabilities and nonlinear structures of low-frequency waves in nonuniform electron–positron–ion plasmas with shear flow

Arshad M. Mirza, Asma Hasan, M. Azeem, and H. Saleem

Phys. Plasmas 10, 4675 (2003); http://dx.doi.org/10.1063/1.1620998 (5 pages) | Cited 6 times

Online Publication Date: 20 November 2003

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It is found that the low-frequency ion acoustic and electrostatic drift waves can become unstable in uniform electron–ion and electron–positron–ion plasmas due to the ion shear flow. In a collisional plasma a drift-dissipative instability can also take place. In the presence of collisions the temporal behavior of nonlinear drift-dissipative mode can be represented in the form of well-known Lorenz and Stenflo type equations that admit chaotic trajectories. On the other hand, a quasi-stationary solution of the mode coupling equations can be represented in the form of monopolar vortex. The results of the present investigation can be helpful in understanding electrostatic turbulence and wave phenomena in laboratory and astrophysical plasmas. © 2003 American Institute of Physics.

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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.)
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Mw	Nonlinear phenomena: waves, wave propagation, and other interactions (including parametric effects, mode coupling, ponderomotive effects, etc.)
52.30.-q	Plasma dynamics and flow
52.35.Ra	Plasma turbulence

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Nonlinear slow shear Alfvén wave in electron–positron–ion plasmas

S. Mahmood and H. Saleem

Phys. Plasmas 10, 4680 (2003); http://dx.doi.org/10.1063/1.1622953 (5 pages) | Cited 21 times

Online Publication Date: 20 November 2003

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Nonlinear solitary structures of an arbitrary amplitude slow shear Alfvén wave (SSAW) in ideal electron–positron–ion (e–p–i) plasmas are studied. It is found that the electron density dips of SSAW are formed in the super Alfvénic region. The amplitude and the width of the nonlinear shear Alfvén wave reduces with the increase in the concentration of positrons in electron–ion (e–i) plasmas. The width of the soliton also depends upon the direction of propagation of the perturbation in both e–i and e–p–i plasmas. The numerical results for several different cases have also been presented for illustrative purposes. © 2003 American Institute of Physics.

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52.35.Bj	Magnetohydrodynamic waves (e.g., Alfven waves)
52.35.Sb	Solitons; BGK modes
52.25.Kn	Thermodynamics of plasmas

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Dust acoustic solitary waves and double layers in a dusty plasma with trapped electrons

S. K. El-Labany and W. F. El-Taibany

Phys. Plasmas 10, 4685 (2003); http://dx.doi.org/10.1063/1.1623764 (11 pages) | Cited 25 times

Online Publication Date: 20 November 2003

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The effect of variable dust charge, dust temperature, and trapped electrons on small amplitude dust acoustic waves is investigated. It is found that both compressive and rarefactive solitons as well as double layers exist depending on the nonisothermality parameter. A modified Korteweg–de Vries is derived. At critical density, the Korteweg–de Vries equation is obtained. Employing quasipotential analysis, the Sagdeev potential equation with the inclusion of different new effects has been derived. Because of the presence of free and trapped electrons, the plasma acoustic wave has gained features of various solitary waves. The Sagdeev potential equation, at a small amplitude, shows that the ordering of nonisothermality plays a unique role. In the case of a plasma with first-order nonisothermality, the Sagdeev potential equation shows the compressive solitary wave propagation, while for plasma with higher-order nonisothermality, the solution of this equation reveals the coexistence of both compressive and rarefactive solitary waves. In addition, for certain plasma parameters, the solitary wave disappears and a double layer is expected. Again, with the better approximation in the Sagdeev potential equation, more features of solitary waves, e.g., spiky and explosive, along with the double layers, are also highlighted. The findings of this investigation may be useful in understanding laboratory plasma phenomena and astrophysical situations. © 2003 American Institute of Physics.

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52.27.Lw	Dusty or complex plasmas; plasma crystals
52.35.Fp	Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Sb	Solitons; BGK modes

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Confinement and dynamical regulation in two-dimensional convective turbulence

N. H. Bian and O. E. Garcia

Phys. Plasmas 10, 4696 (2003); http://dx.doi.org/10.1063/1.1625941 (12 pages) | Cited 13 times

Online Publication Date: 20 November 2003

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In this work the nature of confinement improvement implied by the self-consistent generation of mean flows in two-dimensional convective turbulence is studied. The confinement variations are linked to two distinct regulation mechanisms which are also shown to be at the origin of low-frequency bursting in the fluctuation level and the convective heat flux integral, both resulting in a state of large-scale intermittency. The first one involves the control of convective transport by sheared mean flows. This regulation relies on the conservative transfer of kinetic energy from tilted fluctuations to the mean component of the flow. Bursting can also result from the quasi-linear modification of the linear instability drive which is the mean pressure gradient. For each bursting process the relevant zero-dimensional model equations are given. These are finally coupled in a minimal model of convection in fluids and plasmas. The results of the modeling are used to discuss confinement scaling and intermittency, and in a heuristic way, more complex issues such as criticality and transport avalanches. © 2003 American Institute of Physics.

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52.35.Ra

Plasma turbulence

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Unified form for parallel ion viscous stress in magnetized plasmas

E. D. Held

Phys. Plasmas 10, 4708 (2003); http://dx.doi.org/10.1063/1.1626681 (8 pages) | Cited 5 times

Online Publication Date: 20 November 2003

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In this work a unified form for the parallel ion viscous stress in a magnetized plasma is presented. Approximately valid for arbitrary collisionality, the integral nature of this generalized closure results from assuming the maximal ordering between collisional pitch-angle scattering and free streaming effects and from taking a Chapman–Enskog-type approach which includes the parallel ion viscous stress itself as a drive. The ion drift kinetic equation is solved in a sheared slab using an expansion in eigenfunctions of the Lorentz scattering operator. Integrating the coefficient equations in space and taking the proper velocity space moments couples the parallel viscous stress closure to an integral momentum restoring term, thus generalizing the concept of momentum conservation for simplified Coulomb collision operators. The integral closure involves following ions along magnetic field lines which are the ideal, time-independent characteristics of the perturbed distribution function. The fact that the viscous stress and momentum restoring term appear in the kernels of these field line integrals means that in general the closure has the form of coupled Volterra equations with an inhomogeneous term supplied by the traditional flow gradient drive. It is shown that the unified closure agrees both qualitatively and quantitatively with previous results and hence represents a generalized physical form for the parallel ion viscous stress in magnetized plasmas. © 2003 American Institute of Physics.

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52.25.Dg

Plasma kinetic equations

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Marginal stability boundaries for infinite-n ballooning modes in a quasiaxisymmetric stellarator

S. R. Hudson and C. C. Hegna

Phys. Plasmas 10, 4716 (2003); http://dx.doi.org/10.1063/1.1622669 (12 pages) | Cited 7 times

Online Publication Date: 20 November 2003

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A method for computing the ideal magnetohydrodynamic (MHD) stability boundaries in three-dimensional equilibria is employed. Following Hegna and Nakajima [Phys. Plasmas 5, 1336 (1998)], a two-dimensional family of equilibria is constructed by perturbing the pressure and rotational-transform profiles in the vicinity of a flux surface for a given stellarator equilibrium. The perturbations are constrained to preserve the MHD equilibrium condition. For each perturbed equilibrium, the infinite-n ballooning stability is calculated. Marginal stability diagrams are thus constructed that are analogous to (s,α) diagrams for axisymmetric configurations. A quasiaxisymmetric stellarator is considered. Calculations of stability boundaries generally show regions of instability can occur for either sign of the average magnetic shear. Additionally, regions of second-stability are present. © 2003 American Institute of Physics.

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52.55.Jd	Magnetic mirrors, gas dynamic traps
52.25.Kn	Thermodynamics of plasmas
52.75.-d	Plasma devices
52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.40.Hf	Plasma-material interactions; boundary layer effects

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Plasma flow and confinement in the vicinity of a rotating island in tokamaks

K. C. Shaing

Phys. Plasmas 10, 4728 (2003); http://dx.doi.org/10.1063/1.1623198 (9 pages) | Cited 12 times

Online Publication Date: 20 November 2003

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The theory for the electric field and the plasma confinement in the vicinity of a magnetic island in tokamaks [Phys. Plasmas 9, 3470 (2002)] is extended to the situation where the magnetic island is rotating. The electric field that is parallel to the magnetic field, E_∥, is assumed to vanish. With this assumption, the theory for a nonrotating island is applicable to a rotating island if the radial electric field in the nonrotating theory is replaced by the radial gradient of F. Here, F is the part of the electrostatic potential that is constant on the rotating island magnetic surface. As an application of the theory, the radial electric field, toroidal flow speed, ambipolar particle flux, heat flux, and island rotation frequency in the collisionless regime are also presented. © 2003 American Institute of Physics.

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52.25.Fi	Transport properties
52.55.Fa	Tokamaks, spherical tokamaks

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Role of the m = 0 modes in the edge region of a reversed-field pinch during pulsed poloidal current drive

A. Cravotta, G. Spizzo, P. Zanca, and P. Martin

Phys. Plasmas 10, 4737 (2003); http://dx.doi.org/10.1063/1.1623271 (7 pages) | Cited 4 times

Online Publication Date: 20 November 2003

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Changes in the edge profile of soft x-ray emission during pulsed poloidal current drive (PPCD) experiments are in agreement with the modifications of the plasma surface, as seen from magnetic measurements. In particular, the m = 0 modes resonant at the reversal radius have been analyzed. A comparison with an m = 0 island reconstruction model shows that PPCD reduces the bulging associated with the m = 0 modes by shifting inward the reversal surface, where the m = 0 island is located. Regarding the total m = 0 amplitude, there are evidences that during PPCD, on average, it does not change significantly. Additional data coming from standard discharges support the idea that two competing phenomena act during PPCD: one is the inward shift of the reversal, which destabilizes the m = 0 modes; the other one is the decrease of the m = 1 fluctuations, which acts in the opposite direction. © 2003 American Institute of Physics.

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52.30.Cv	Magnetohydrodynamics (including electron magnetohydrodynamics)
52.55.Lf	Field-reversed configurations, rotamaks, astrons, ion rings, magnetized target fusion, and cusps
52.55.Wq	Current drive; helicity injection
52.55.Tn	Ideal and resistive MHD modes; kinetic modes

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Drift-ordered fluid equations for field-aligned modes in low-β collisional plasma with equilibrium pressure pedestals

Andrei N. Simakov and Peter J. Catto

Phys. Plasmas 10, 4744 (2003); http://dx.doi.org/10.1063/1.1623492 (14 pages) | Cited 25 times

Online Publication Date: 20 November 2003

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Starting from the complete short mean-free path fluid equations describing magnetized plasmas, assuming that plasma pressure is small compared to magnetic pressure, considering field-aligned plasma fluctuations, and adopting an ordering in which the plasma species flow velocities are much smaller than the ion thermal speed, a system of nonlinear equations for plasma density, electron and ion temperatures, parallel ion flow velocity, parallel current, electrostatic potential, perturbed parallel electromagnetic potential, and a perturbed magnetic field is derived. The equations obtained allow sharp equilibrium radial gradients of plasma quantities, and are shown to contain the neoclassical (Pfirsch–Schlüter) results for plasma current, parallel ion flow velocity (with the correct temperature gradient terms), and parallel gradients of equilibrium electron and ion temperatures. Special care is taken to ensure the divergence-free character of perturbed magnetic field and total plasma current, as well as local particle number and total energy conservation. © 2003 American Institute of Physics.

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52.25.Gj

Fluctuation and chaos phenomena

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Stability of the ion-temperature-gradient-driven mode with negative magnetic shear

M. Uchida, S. Sen, A. Fukuyama, and D. R. McCarthy

Phys. Plasmas 10, 4758 (2003); http://dx.doi.org/10.1063/1.1616015 (5 pages) | Cited 2 times

Online Publication Date: 20 November 2003

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A model for transition to the enhanced reverse shear or negative central shear mode triggered in tokamaks is proposed. This model takes into account the linear behavior of the ion temperature gradient (ITG) driven perturbation, considered nowadays as the dominant source of anomalous energy losses in the low confinement mode, in the presence of a radially varying parallel velocity. Analytic and numerical studies show that when the magnetic shear has the same sign as the second derivative of the parallel velocity with respect to the radial coordinate, the ITG mode may become more unstable. On the other hand, when the magnetic shear has the opposite sign to the second derivative of the parallel velocity, the linear ITG mode may be completely stabilized. This result is similar to our earlier works on parallel velocity shear instability [S. Sen et al., Phys. Plasmas 7, 1192 (2000); D. R. McCarthy et al., Phys. Plasmas 8, 3645 (2001)]. © 2003 American Institute of Physics.

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52.55.Fa	Tokamaks, spherical tokamaks
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

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Separatrix shapes and internal structures of a field-reversed configuration plasma

Hiroshi Gota, Kayoko Fujimoto, Yasunori Ohkuma, Tsutomu Takahashi, and Yasuyuki Nogi

Phys. Plasmas 10, 4763 (2003); http://dx.doi.org/10.1063/1.1624835 (8 pages) | Cited 9 times

Online Publication Date: 20 November 2003

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The separatrix shape of a field-reversed configuration plasma is determined in comparison with measured fluxes surrounding the plasma and the solution of the Grad–Shafranov equation with an edge-layer plasma in the open field region. A reconnection point of the bias field, an outgoing flow of torn plasmas and a large amplitude ripple on the separatrix surface are clearly observed at the formation phase. A smoothed separatrix shape having definite ends is observed at the quiescent phase. It is also estimated that the beta value at the separatrix and the thickness of the edge-layer plasma are, respectively, β_s = 0.5–0.7 and 4–6 ρ_i (ρ_i: ion gyroradius). The magnetic structure inside the separatrix is investigated by solving the Grad–Shafranov equation with the measured separatrix shape and β_s. It is found that magnetic islands are formed near the magnetic axis not only at the formation phase but also at the quiescent phase. The appearance and coalescence of the islands are repeated during the discharge. © 2003 American Institute of Physics.

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52.55.Ez	Theta pinch
52.58.Lq	Z-pinches, plasma focus, and other pinch devices
52.40.Hf	Plasma-material interactions; boundary layer effects

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Destabilization of fast magnetoacoustic waves by circulating energetic ions in toroidal plasmas

V. S. Belikov, Ya. I. Kolesnichenko, and R. B. White

Phys. Plasmas 10, 4771 (2003); http://dx.doi.org/10.1063/1.1625375 (5 pages) | Cited 4 times

Online Publication Date: 20 November 2003

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An instability of fast magnetoacoustic waves driven by circulating energetic ions in axisymmetric toroidal plasmas and characterized by frequencies below the ion gyrofrequency is considered. An important role of the l = 0 resonance (l is the number of a cyclotron harmonic) in the wave–particle interaction is revealed: It is shown that this resonance considerably extends an unstable region in the space of the pitch-angles of the energetic ions and the wave frequencies. The analysis is carried out for a “slow” instability, which has the growth rate less than the bounce frequency of the energetic ions. Specific examples relevant to the National Spherical Torus Experiment [Spitzer et al., Fusion Technol. 30, 1337 (1996)], where instabilities of this kind were observed, are considered. © 2003 American Institute of Physics.

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52.55.Fa	Tokamaks, spherical tokamaks
52.35.Dm	Sound waves

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Analysis of stable resistive wall modes in a rotating plasma

A. M. Garofalo, T. H. Jensen, and E. J. Strait

Phys. Plasmas 10, 4776 (2003); http://dx.doi.org/10.1063/1.1625942 (8 pages) | Cited 37 times

Online Publication Date: 20 November 2003

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Measurements of the resistive wall mode (RWM) response to external resonant field pulses yield complete knowledge of the mode characteristics in the parameter range explored. An ideal magnetohydrodynamics model [Garofalo et al., Phys. Plasmas 9, 4573 (2002)] has been generalized to include the effects of plasma rotation and dissipation, and the new model is found to explain quantitatively the experimental observations. Rotation of the RWM with respect to the wall is often described as an essential feature of the mechanism by which plasma rotation stabilizes the RWM. This interpretation of the rotational stabilization of the RWM appears inconsistent with the measurements from recent DIII–D [Luxon and Davis, Fusion Technol. 8, 441 (1985)] experiments. It is found that the theoretically predicted mode rotation with respect to the wall is not needed for stabilization and is only a consequence of torque balance in the absence of magnetic-field nonaxisymmetries. © 2003 American Institute of Physics.

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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.)
52.35.Qz	Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.55.Fa	Tokamaks, spherical tokamaks

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First observation of density profile in directly laser-driven polystyrene targets for ablative Rayleigh–Taylor instability research

Shinsuke Fujioka, Hiroyuki Shiraga, Masaharu Nishikino, Keisuke Shigemori, Atsushi Sunahara, Mitsuo Nakai, Hiroshi Azechi, Katsunobu Nishihara, and Tatsuhiko Yamanaka

Phys. Plasmas 10, 4784 (2003); http://dx.doi.org/10.1063/1.1622951 (6 pages) | Cited 8 times

Online Publication Date: 20 November 2003

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The temporal evolution of the density profile of a directly laser-driven polystyrene target was observed for the first time using an x-ray penumbral imaging technique coupled with side-on x-ray backlighting at the GEKKO XII [C. Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)]–High Intensity Plasma Experimental Research laser facility (I_L = 0.7×10¹⁴ W/cm², λ_L = 0.35 μm). This density measurement makes it possible to experimentally confirm all physical parameters [γ(k),k,g, math

,ρ_a,L_m] appearing in the modified Takabe formula for the growth rate of the ablative Rayleigh–Taylor instability. The measured density profiles were well reproduced by a one-dimensional hydrodynamic simulation code. The density measurement contributes toward fully understanding the ablative Rayleigh–Taylor instability. © 2003 American Institute of Physics.

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52.35.Py	Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.50.Jm	Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.57.Fg	Implosion symmetry and hydrodynamic instability (Rayleigh-Taylor, Richtmyer-Meshkov, imprint, etc.)
52.70.La	X-ray and γ-ray measurements

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Length scaling of dynamic-hohlraum axial radiation

T. W. L. Sanford, R. C. Mock, S. A. Slutz, and D. L. Peterson

Phys. Plasmas 10, 4790 (2003); http://dx.doi.org/10.1063/1.1625938 (10 pages) | Cited 13 times

Online Publication Date: 20 November 2003

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Radiation generated within a 10-mm-long foam-target DH (dynamic hohlraum) is used for high-temperature (>200 eV) radiation-flow and inertial-confinement-fusion studies [Sanford et al., Phys. Plasmas 9, 3573 (2002)]. The length of this DH is varied from 5 to 20 mm, keeping the mass/unit length constant in an effort to study the scaling of axial radiation power with length, and better understand its production. Measurements show a greater variation in this power with length than would be expected from simple arguments [Slutz et al., Phys. Plasmas 8, 1673 (2001)]. Maximum axial power of ∼ 10 TW is produced with a length of ∼ 7.5 mm, similar to the typical power for the baseline 10 mm DH. The decreasing axial power (at a rate of ∼ 0.65 TW per mm at longer lengths) is bounded by radiation-magnetohydrodynamic simulations [Peterson et al., Phys. Plasmas 6, 2178 (1999)] that include the development of the magnetic Rayleigh–Taylor instability in the r–z plane. The dramatic drop in axial power below 7.5 mm, by contrast, was unanticipated. This decrease suggests the presence of differing mechanisms for limiting power at short and long lengths. © 2003 American Institute of Physics.

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