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Dec 1997

Volume 4, Issue 12, pp. 4189-4450

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Effect of induced spatial incoherence on flow induced laser beam deflection: Analytic theory

Sandip Ghosal and Harvey A. Rose

Phys. Plasmas 4, 4189 (1997); http://dx.doi.org/10.1063/1.872609 (3 pages) | Cited 2 times

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Analytic results are presented for the laser beam deflection rate in the case of spatial and temporal smoothing by induced spatial incoherence (ISI). It is shown that for flow perpendicular to the beam propagation direction with Mach number M, temporal smoothing decreases the beam deflection rate for M>1, but may increase it for M<1 and weak acoustic wave damping. © 1997 American Institute of Physics.  
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Stability thresholds for ballooning modes driven by high β internal kinks

Hinrich Lütjens and Jean-François Luciani

Phys. Plasmas 4, 4192 (1997); http://dx.doi.org/10.1063/1.872610 (3 pages) | Cited 5 times

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The nonlinear destabilization of pressure driven (ballooning) modes by internal kink modes during sawtooth cycles was found in the Tokamak Fusion Test Reactor (TFTR) [Park et al., Phys. Rev. Lett. 75, 1763 (1995)]. It is shown by a numerical parameter study, including aspect ratio and magnetic shear profile effects, that in ideal magneto-hydrodynamics (MHD), this nonlinear destabilization occurs in plasma equilibria close to marginal stability to linear sub-harmonics of the internal kink with toroidal mode number n ≥ 3. © 1997 American Institute of Physics.
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52.55.Fa Tokamaks, spherical tokamaks
52.30.-q Plasma dynamics and flow
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Kn Thermodynamics of plasmas

Surface-wave–particle interactions in a cylindrical plasma submitted to a static magnetic field

A. Dengra

Phys. Plasmas 4, 4195 (1997); http://dx.doi.org/10.1063/1.872579 (6 pages)

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A new theoretical model for the study of the surface-wave–particle interactions in a plasma column in the presence of a constant external magnetic field has been developed. The model is based on the linear resolution of the Vlasov equation by the method of characteristics, with the specular reflection hypothesis at the wall. The expression obtained for the rate of increase of kinetic energy per electron permits the analysis of the influence of the critical parameters in this transference process. © 1997 American Institute of Physics.
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52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
52.25.Fi Transport properties

Velocity shear-induced effects on electrostatic ion perturbations

Andria D. Rogava, George D. Chagelishvili, and Vazha I. Berezhiani

Phys. Plasmas 4, 4201 (1997); http://dx.doi.org/10.1063/1.872580 (4 pages) | Cited 8 times

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Linear evolution of electrostatic perturbations in an unmagnetized electron–ion plasma shear flow is studied. New physical effects, arising due to the non-normality of linear dynamics are disclosed. A new class of nonperiodic collective mode with vortical motion of ions, characterized by intense energy exchange with the mean flow, is found. It is also shown that the velocity shear induces extraction of the mean flow energy by ion-sound waves and that during the shear-induced evolution the ion-sound waves turn eventually into ion plasma oscillations. © 1997 American Institute of Physics.
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52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.30.-q Plasma dynamics and flow

Transverse electron susceptibility and the electromagnetic wave absorption in weakly collisional plasmas

V. Yu. Bychenkov, V. T. Tikhonchuk, and W. Rozmus

Phys. Plasmas 4, 4205 (1997); http://dx.doi.org/10.1063/1.872581 (5 pages) | Cited 9 times

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A theory of plasma response to electromagnetic perturbations has been developed based on the full solution to the Fokker–Planck equation in high Z plasmas without electron–electron collisions. For the first time the transverse electron susceptibility has been calculated for the entire range of frequencies and wave numbers including the quantitative description of the weakly collisional regime where the wave frequency is comparable to the electron–ion collision frequency and the wave number is comparable to the inverse electron mean free path. The results have been compared to approximate expressions for the electron conductivity based on the Drude model showing discrepancy by a factor of few in regions where the spatial dispersion is important. The theory is applied to the calculation of laser light absorption in solid density plasmas. © 1997 American Institute of Physics.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.25.Fi Transport properties

Nonlinear dynamics of a charged dust grain in a plasma

Yves Elskens, David P. Resendes, and J. T. Mendonça

Phys. Plasmas 4, 4210 (1997); http://dx.doi.org/10.1063/1.872582 (8 pages) | Cited 9 times

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For an arbitrary monotonic charging function, the dynamics of a dust grain is dissipative and energy is a Liapunov function. In an arbitrary external potential two types of equilibria exist. The first type, with uncharged grain, is always unstable. The second type of equilibrium, admitting states of both positive and negative charge, can be marginally stable; stability depends on the local potential. Under spatially uniform (constant or time-dependent) potentials, motion is free while the charge adapts to the potential. For a spatially oscillating potential, the phase space is that of the simple pendulum with one additional degree of freedom, the charge. Dissipation in the charging process forbids periodic behavior and ensures the existence of attractors: A grain is at stable equilibrium only when charged positively and trapped in a potential well, or when charged negatively on top of a hill. The small oscillations near a stable equilibrium decay weakly, and the grain charge oscillates at twice the oscillation frequency. © 1997 American Institute of Physics.
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52.25.Vy Impurities in plasmas
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
41.20.Cv Electrostatics; Poisson and Laplace equations, boundary-value problems

Ion transport in a partially ionized impure edge plasma

P. Helander, F. Wising, Peter J. Catto, and R. D. Hazeltine

Phys. Plasmas 4, 4218 (1997); http://dx.doi.org/10.1063/1.872583 (9 pages) | Cited 2 times

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The kinetic theory of transport in an impure, partially ionized, edge plasma is developed. It is found that the thermal force between bulk ions and impurities is enhanced by the presence of neutral atoms, but the dynamical friction is not affected by the neutrals. When the neutral viscosity is large, an additional force on the impurities also arises. This force is parallel to the magnetic field, and is proportional to the shear of the parallel plasma velocity and the perpendicular ion density and temperature gradients. © 1997 American Institute of Physics.
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52.27.Jt Nonneutral plasmas
52.25.Vy Impurities in plasmas
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations

Near-forward stimulated Brillouin scattering

C. J. McKinstrie, J. S. Li, and A. V. Kanaev

Phys. Plasmas 4, 4227 (1997); http://dx.doi.org/10.1063/1.872584 (5 pages) | Cited 6 times

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The spatiotemporal evolution of near-forward stimulated Brillouin scattering (SBS) is studied in detail. For large scattering angles SBS grows and saturates as a three-wave instability. For small scattering angles SBS begins to grow as a three-wave instability, then continues to grow and saturates as a four-wave instability. Expressions for the saturation time and steady-state gain exponent of SBS are derived for large and small scattering angles. © 1997 American Institute of Physics.
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42.65.Es Stimulated Brillouin and Rayleigh scattering
52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Effects of ion and electron drifts on large amplitude solitary waves in a relativistic plasma

Rajkumar Roychoudhury, S. K. Venkatesan, and Chandra Das

Phys. Plasmas 4, 4232 (1997); http://dx.doi.org/10.1063/1.872585 (4 pages) | Cited 6 times

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The effect of ion and electron drifts on the existence of arbitrary amplitude solitary waves is studied using Sagdeev’s pseudopotential method. It is found that if the electron drift velocity u0 is finite, solitary waves may exist for relatively large values of v0/c, where v0 is the ion drift velocity and c is the velocity of light. © 1997 American Institute of Physics.
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52.35.Sb Solitons; BGK modes
52.27.Ny Relativistic plasmas
52.25.Fi Transport properties

A new mathematical approach for shock-wave solution in a dusty plasma

G. C. Das, C. B. Dwivedi, Madhuri Talukdar, and Jnanjyoti Sarma

Phys. Plasmas 4, 4236 (1997); http://dx.doi.org/10.1063/1.872586 (4 pages) | Cited 16 times

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The problem of nonlinear Burger equation in a plasma contaminated with heavy dust grains has been revisited. As discussed earlier [C. B. Dwivedi and B. P. Pandey, Phys. Plasmas 2, 9 (1995)], the Burger equation originates due to dust charge fluctuation dynamics. A new alternate mathematical approach based on a simple traveling wave formalism has been applied to find out the solution of the derived Burger equation, and the method recovers the known shock-wave solution. This technique, although having its own limitation, predicts successfully the salient features of the weak shock-wave structure in a dusty plasma with dust charge fluctuation dynamics. It is emphasized that this approach of the traveling wave formalism is being applied for the first time to solve the nonlinear wave equation in plasmas. © 1997 American Institute of Physics.
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52.35.Tc Shock waves and discontinuities
52.25.Vy Impurities in plasmas
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

Numerical simulation of a negative ion plasma expansion into vacuum

L. G. Garcia, J. Goedert, H. Figua, E. Fijalkow, and M. R. Feix

Phys. Plasmas 4, 4240 (1997); http://dx.doi.org/10.1063/1.872587 (14 pages) | Cited 16 times

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The expansion into vacuum of a one-dimensional, collisionless, negative ion plasma is investigated in the framework of the Vlasov–Poisson model. The basic equations are written in a “new time space” by use of a rescaling transformation and, subsequently, solved numerically through a fully Eulerian code. As in the case of a two species plasma, the time-asymptotic regime is found to be self-similar with the temperature decreasing as t−2. The numerical results exhibit clearly the physically expected effects produced by the variation of parameters such as initial temperatures, mass ratios and charge of the negative ions. © 1997 American Institute of Physics.
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52.27.Jt Nonneutral plasmas
52.65.-y Plasma simulation

Plasma screening effects on eikonal phase for electron-ion elastic collisions in weakly coupled plasmas

Young-Dae Jung

Phys. Plasmas 4, 4254 (1997); http://dx.doi.org/10.1063/1.872588 (4 pages) | Cited 4 times

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Eikonal approximation is applied to investigate elastic electron-ion collisions in weakly coupled plasmas. Plasma screening effects on eikonal phase are investigated for eikonal differential elastic scattering cross sections. The electron-ion interaction potential in weakly coupled plasmas has been obtained by the introduction of the longitudinal plasma dielectric function. The semiclassical straight-line trajectory method is applied to the path of the projectile electron in order to investigate the variation of the eikonal phase as a function of the impact parameter and the plasma parameters. In the first-order eikonal approximation, the dynamic plasma screening effect is identical to the static screening effect obtained by the Debye–Hückel potential. The eikonal differential elastic cross section substantially decreases with an increase in the projectile energy and increases as the plasma screening effect decreases through the Debye length. The plasma screening effects are more significant for large impact parameters. © 1997 American Institute of Physics.
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52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Classical electron-ion scattering in strongly magnetized plasmas. I. A generalized Coulomb logarithm

David K. Geller and Jon C. Weisheit

Phys. Plasmas 4, 4258 (1997); http://dx.doi.org/10.1063/1.872589 (14 pages) | Cited 12 times

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In a strongly magnetized plasma, where the electron cyclotron radius is less than the Debye length, the Rutherford scattering formula is expected to break down. In this paper, analytic expressions are developed for classical, small-angle scattering of electrons and ions in strong magnetic fields. Numerical evaluation of these expressions shows quantitatively how strong B fields can significantly inhibit electron deflections. The influence of the field on transport phenomena is then explored—in particular, a generalized Coulomb logarithm which includes the effects of a magnetic field is formulated and computed for a wide range of trajectory pitch angles. This generalized Coulomb logarithm is used to illustrate how a strong field influences the effective electron-ion cross section, the electron velocity diffusion coefficient, and the (parallel) electrical and thermal resistivity in a variety of astrophysical and terrestrial plasmas. © 1997 American Institute of Physics.
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52.25.Fi Transport properties
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

The radial electric field dynamics in the neoclassical plasmas

S. V. Novakovskii, C. S. Liu, R. Z. Sagdeev, and M. N. Rosenbluth

Phys. Plasmas 4, 4272 (1997); http://dx.doi.org/10.1063/1.872590 (11 pages) | Cited 61 times

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A numerical simulation and analytical theory of the radial electric field dynamics in low collisional tokamak plasmas are presented. An initial value code “ELECTRIC” has been developed to solve the ion drift kinetic equation with a full collisional operator in the Hirshman–Sigmar–Clarke form together with the Maxwell equations. Different scenarios of relaxation of the radial electric field toward the steady-state in response to sudden and adiabatic changes of the equilibrium temperature gradient are presented. It is shown, that while the relaxation is usually accompanied by the geodesic acoustic oscillations, during the adiabatic change these oscillations are suppressed and only the magnetic pumping remains. Both the collisional damping and the Landau resonance interaction are shown to be important relaxation mechanisms. Scalings of the relaxation rates versus basic plasma parameters are presented. © 1997 American Institute of Physics.
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52.65.-y Plasma simulation
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Ideal magnetohydrodynamic instabilities with low toroidal mode numbers localized near an internal transport barrier in high-βp mode plasmas in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade

S. Takeji, Y. Kamada, T. Ozeki, S. Ishida, T. Takizuka, Y. Neyatani, and S. Tokuda

Phys. Plasmas 4, 4283 (1997); http://dx.doi.org/10.1063/1.872591 (9 pages) | Cited 11 times

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Local steep pressure gradient generated near an internal transport barrier drives a radially localized magnetohydrodynamic (MHD) instability with low toroidal mode number (n) in the high-βp mode plasma in the Japan Atomic Energy Research Institute Tokamak-60 Upgrade (JT-60U) [Y. Koide et al., Phys. Plasmas 4, 1623 (1997)]. The instability occurs in the βp regime lower than that for the βp-collapse and its growth rate is of the order of the ideal MHD instability. By a linear analysis of ideal MHD stability, low n modes localized near the internal transport barrier are found to be destabilized in the situation that the bootstrap current driven by the steep pressure gradient reduces the local magnetic shear (s≲0) where the safety factor is right close to an integer value. © 1997 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.)
52.55.Fa Tokamaks, spherical tokamaks
52.25.-b Plasma properties

Observation of vortex-like coherent structures in the edge plasma of the ADITYA tokamak

B. K. Joseph, R. Jha, P. K. Kaw, S. K. Mattoo, C. V. S. Rao, and Y. C. Saxena

Phys. Plasmas 4, 4292 (1997); http://dx.doi.org/10.1063/1.872611 (9 pages) | Cited 40 times

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Vortex-like coherent structures are observed in the edge plasma of ohmically heated ADITYA tokamak [ Phys. Rev. Lett. 69, 1375 (1992)]. The structures are observed on statistical basis when the floating potential fluctuations are analyzed using conditional averaging technique. The structures, which have dipole nature, experience stretching until their radial isolation across the limiter is destroyed. The potential fluctuation also shows non-Gaussian statistics indicating intermittency in broadband turbulence of the edge plasma. © 1997 American Institute of Physics.
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52.40.Hf Plasma-material interactions; boundary layer effects
52.55.Fa Tokamaks, spherical tokamaks
52.25.Gj Fluctuation and chaos phenomena
52.35.Ra Plasma turbulence
52.25.Kn Thermodynamics of plasmas
52.70.-m Plasma diagnostic techniques and instrumentation

Collisionless pitch angle scattering of plasma ions at the edge region of a field-reversed configuration

Toshiki Takahashi, Yukihiro Tomita, Hiromu Momota, and Nikita V. Shabrov

Phys. Plasmas 4, 4301 (1997); http://dx.doi.org/10.1063/1.872592 (8 pages) | Cited 6 times

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The motion of a plasma ion gyrating around the separatrix of a field-reversed configuration is studied. Numerical studies showed that the action integral of a particle changes abruptly when a particle passes through the vicinity of a field null x point. This phenomena is understood as collisionless stochastic scattering of the pitch angle. In the case of a particle with positive canonical angular momentum Pθ, the resultant correlation coefficients of the action integral between before and after the scattering appear to be stochastic in some cases. As the action integral increases for a particle with negative Pθ, its motion tends to be adiabatic. If the negative Pθ of a particle approaches zero, a stochastic motion is observed. © 1997 American Institute of Physics.
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52.25.Fi Transport properties
52.55.Ez Theta pinch
52.40.Hf Plasma-material interactions; boundary layer effects

Two-dimensional magneto-hydrodynamic modeling of carbon fiber Z-pinch experiments

J. P. Chittenden, R. Aliaga Rossel, S. V. Lebedev, I. H. Mitchell, M. Tatarakis, A. R. Bell, and M. G. Haines

Phys. Plasmas 4, 4309 (1997); http://dx.doi.org/10.1063/1.872593 (9 pages) | Cited 24 times

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A two-dimensional magneto-hydrodynamic simulation incorporating cold start conditions is used to explain the early phase of carbon fiber Z-pinch experiments. The rapid development of large scale, nonlinear m = 0 perturbations in the plasma corona is reproduced. X-ray bright spot formation in the necks of the instability is followed by bright spot bifurcation and fast axial motion. Bright spot bifurcation is found to be due to axial components of the j×B force and occurs off-axis due to the presence of a residual core of unionized carbon. Artificial diagnostic images are generated from the simulations data to allow direct comparison with experimental x-ray imaging and laser probing diagnostics. The accurate reproduction of the experimental images provides confirmation that the experimentally observed features are a repercussion of the non-linear development of the m = 0 instability in an ionizing medium. © 1997 American Institute of Physics.
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52.55.Ez Theta pinch
52.65.Kj Magnetohydrodynamic and fluid equation
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.80.Hc Glow; corona
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.70.La X-ray and γ-ray measurements

Energy description of wave-plasma interaction in the ion cyclotron range of frequency: Application to fast wave absorption and emission in tokamaks

D. Fraboulet and A. Bécoulet

Phys. Plasmas 4, 4318 (1997); http://dx.doi.org/10.1063/1.872594 (13 pages) | Cited 2 times

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An alternative description of the energy transfer between an electromagnetic wave and a tokamak plasma is presented. It involves the evaluation of local coefficients of emission and absorption and leads to rapid numerical procedures. This method allows a coherent description of emission and absorption processes and is thus particularly suited for the determination of the rf electromagnetic energy radiated from the plasma, as is shown by calculating ion cyclotron emission spectra. Furthermore, this “fluid” energy description proves also useful for the evaluation of the fast wave plasma absorption during ion cyclotron range of frequency (ICRF) heating: a fast “fluid” one-dimensional slab numerical code has been developed. It cannot cope with singularities of the wave vector such as resonance or cutoff, but it is well adapted to scenario designs of fast wave direct coupling to electrons and evaluation of partitioning of energy between species in the case of ions high harmonics. © 1997 American Institute of Physics.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.55.Fa Tokamaks, spherical tokamaks
52.55.Jd Magnetic mirrors, gas dynamic traps
52.50.Gj Plasma heating by particle beams

Electron transport fluxes in potato plateau regime

K. C. Shaing and R. D. Hazeltine

Phys. Plasmas 4, 4331 (1997); http://dx.doi.org/10.1063/1.872595 (2 pages) | Cited 5 times

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Electron transport fluxes in the potato plateau regime are calculated from the solutions of the drift kinetic equation and fluid equations. It is found that the bootstrap current density remains finite in the region close to the magnetic axis, although it decreases with increasing collision frequency. This finite amount of the bootstrap current in the relatively collisional regime is important in modeling tokamak startup with 100% bootstrap current. © 1997 American Institute of Physics.
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52.25.Fi Transport properties
52.55.Fa Tokamaks, spherical tokamaks
52.20.Dq Particle orbits

Ion wave response to intense laser beams in underdense plasmas

V. V. Elisseev, I. Ourdev, W. Rozmus, V. T. Tikhonchuk, C. E. Capjack, and P. E. Young

Phys. Plasmas 4, 4333 (1997); http://dx.doi.org/10.1063/1.872596 (14 pages) | Cited 17 times

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Numerical and theoretical studies of laser beam interaction with underdense plasmas involving ion wave instabilities are presented. The theoretical model that is used involves realistic distribution of laser intensity in a focal spot and a non-paraxial electromagnetic wave equation coupled to the ion acoustic wave equation in a two-dimensional geometry. Included among the important results is a weak correlation between backscattered stimulated Brillouin scattering (SBS) reflectivity and filamentation or self-focusing instabilities. The transmitted light demonstrates angular spreading and frequency shifts consistent with near-forward SBS. The role of filamentation and self-focusing on the transmitted light is also discussed. © 1997 American Institute of Physics.
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52.40.Db Electromagnetic (nonlaser) radiation interactions with plasma
52.38.-r Laser-plasma interactions
42.65.Es Stimulated Brillouin and Rayleigh scattering
52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)

Turbulent Richtmyer–Meshkov instability experiments with strong radiatively driven shocks

Guy Dimonte and Marilyn Schneider

Phys. Plasmas 4, 4347 (1997); http://dx.doi.org/10.1063/1.872597 (11 pages) | Cited 23 times

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The Richtmyer–Meshkov instability is investigated in the turbulent regime with strong radiatively driven shocks (Mach >10) on the Nova laser [Phys. Rev. Lett. 70, 1806 (1993)]. The targets are planar with near solid density and Atwood number ∼−0.9. The interfacial perturbations are three dimensional (3-D), random, and nonlinear with a broad scale distribution such that they develop into a turbulent mixing zone (TMZ). The TMZ is diagnosed radiographically using x-ray dopants localized to the center of the target to avoid edge effects. Two different diagnostic configurations yield comparable results. The total width of the TMZ is found to increase in time as H ∼ tΘ, with Θ∼0.5±0.1. When compared to shock tube data, this result supports the suggestion [Phys. Fluids 8, 614 (1996)] that Θ decreases with Mach number. The data are used to test turbulence models and to determine the effective drag coefficient Cd ∼ 2.6±1.2 for potential flow mix models in the high compression regime. © 1997 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.Ra Plasma turbulence
52.35.Tc Shock waves and discontinuities
52.70.-m Plasma diagnostic techniques and instrumentation

Quasistatic magnetic field generated by a short laser pulse in an underdense plasma

Leonid M. Gorbunov, Patrick Mora, and Thomas M. Antonsen

Phys. Plasmas 4, 4358 (1997); http://dx.doi.org/10.1063/1.872598 (11 pages) | Cited 28 times

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The quasistatic magnetic field generated by short laser pulse in a uniform rarefied plasma is found analytically and compared to that of two-dimensional particle simulations. It is shown that an axisymmetric laser pulse generates an azimuthal magnetic field the structure of which is rather complicated inside the laser pulse body. Behind the pulse in the wake region the magnetic field contains a component that is homogeneous in the longitudinal direction and a component that is oscillating at the wave number 2kp, where kp is the wave number of the plasma wake. Particle simulations confirm the analytical results and are also used to treat the case of high intense laser pulses. © 1997 American Institute of Physics.
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52.38.-r Laser-plasma interactions
52.65.Rr Particle-in-cell method

Effect of the speckle self-focusing on the stationary stimulated Brillouin scattering reflectivity from a randomized laser beam in an inhomogeneous plasma

V. T. Tikhonchuk, S. Hüller, and Ph. Mounaix

Phys. Plasmas 4, 4369 (1997); http://dx.doi.org/10.1063/1.872599 (13 pages) | Cited 22 times

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The effect of laser light self-focusing (SF) in speckles on stimulated Brillouin scattering (SBS) in an inhomogeneous plasma is studied. It is found that below but near critical power SF dramatically enhances the SBS reflectivity from an individual speckle, while above the critical power the pump power depletion due to SBS prevents strong SF and limits the maximum laser intensity in a speckle. The parameters that control the SBS/SF interplay are the ratio of plasma inhomogeneity scale length to the speckle length and the product of the plasma density and the speckle cross section. The consequences of the SF effect on the averaged SBS reflectivity of the randomized laser beam are revealed and their manifestations in recent SBS experiments with preformed plasmas are discussed. © 1997 American Institute of Physics.
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42.30.Ms Speckle and moiré patterns
42.65.Es Stimulated Brillouin and Rayleigh scattering
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
52.25.Kn Thermodynamics of plasmas
52.38.-r Laser-plasma interactions

Modified magnetohydrodynamic waves in a current sheet in space

M. Yamauchi and A. T. Y. Lui

Phys. Plasmas 4, 4382 (1997); http://dx.doi.org/10.1063/1.872600 (6 pages) | Cited 1 time

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Large-scale coherent motions of a current sheet such as flapping or tearing of the entire current sheet are studied. The basic magnetohydrodynamic (MHD) equations are integrated over the thickness of the current sheet, and linear analysis is applied to obtain the modified dispersion relations for the MHD fast, Alfvén, and the slow waves under non-zero background cross-sheet current. The dispersion relation for the fast and slow modes contains an imaginary part, because energy is exchanged between the wave and the background sheet current. A short-wavelength MHD slow wave propagating against/along the magnetic tension force is unstable/stable, whereas the situation is reversed for the MHD fast waves. For a thin current sheet (long-wavelength limit), the MHD slow wave becomes stagnant and very unstable, whereas the MHD fast wave propagates slowly and its stability depends on the strength of the background current. © 1997 American Institute of Physics.
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52.35.Bj Magnetohydrodynamic waves (e.g., Alfven waves)
47.35.-i Hydrodynamic waves
47.65.-d Magnetohydrodynamics and electrohydrodynamics
52.30.-q Plasma dynamics and flow
52.35.-g Waves, oscillations, and instabilities in plasmas and intense beams
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