Volume 13, Issue 1, January 2006
Index of content:
 LETTERS


Intermittent turbulence and turbulent structures in a linear magnetized plasma
View Description Hide DescriptionStrongly intermittent turbulence is observed in the shadow of a limiter in the large plasma device at UCLA [W. Gekelman et al., Rev. Sci. Instrum.62, 2875 (1991)]. The amplitude probability distribution function of the turbulence is strongly skewed, with density depletion events (or “holes”) dominant in the highdensity region and densityenhancement events (or “blobs”) dominant in the lowdensity region. Twodimensional crossconditional averaging shows that the blobs are detached, outwardpropagating filamentary structures with a clear dipolar potential, while the holes appear to be part of a more extended turbulent structure. A statistical study of the blobs reveals a typical size of ten times the ion sound gyroradius and a typical velocity of onetenth the sound speed.

Temperature and electron density measurements on laser driven radiative shocks
View Description Hide DescriptionIn this paper, new results on radiative shocks generated by a high power laser in a xenon gas cell are presented. Several shock parameters were measured: temperature, radial expansion and velocity, as well as the electron density in the radiative precursor and its velocity. Multiple laser shot allowed the investigation of physical trends changing initial conditions (laser energy and initial gas pressure). Results are compared with one and twodimensional radiative hydrodynamic simulations. The experiments were carried out at the LULI Laboratory.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Statistics of polarization and Stokes parameters: Multiple orthonormal wave populations
View Description Hide DescriptionA stochastic analysis is developed for the superposition of multiple, fully polarized, electricfield vectors. Each vector is described by a polarization ellipse, with fixed axial ratio and polarization angle, and probability distribution functions (pdfs) for the field strength and phase. These wave populations are then superposed in orthonormal modes of polarization, representing the normal modes of a medium. Central results of this work include analytic and Monte Carlo methods to calculate the pdfs of the measurable Stokes parameters , , , and , and degrees of polarization, of the superposed waves. Predictions are computed for the superposition of some characteristicwave populations, and several striking and counterintuitive results produced. These include nonzero probabilities for , , and at , , and , irrespective of the constituent wavepolarizations and field distributions. For wave populations with identical polarization ellipses, a powerlaw enhancement of the pdf of the intensity at low is found, which is independent of the constituent electricfield distributions. Generation of elliptically polarized light from components which each have an opposite sense of polarization is shown to be possible. A description of the asymptotic limits of the pdfs of the Stokes parameters is obtained, and the appearance of fine structure in the pdfs of the degrees of polarization is demonstrated. Together, these results demonstrate the necessity of systematic analysis when predicting pdfs for the Stokes parameters and degrees of polarization: qualitative results cannot be correctly inferred from intuition alone.

Dielectric tensor for a plasma with a losscone kappaMaxwellian velocity distribution
View Description Hide DescriptionComponents of the dielectrictensor are obtained for a kappaMaxwellian velocity distribution with losscone feature for a hot, infinite, homogeneous plasma immersed in a uniform magnetic field.Expansions valid for small propagation angle (angle between the wave vector and the ambient magnetic field) are given for the elements of the dielectrictensor.

Lowfrequency sheath instability stimulated by an energetic ion component
View Description Hide DescriptionSpontaneous lowfrequency oscillations have been observed in the circuit of a positively biased electrode immersed in a nonMaxwellian laboratory plasma containing an energetic ion component produced by the resonant absorption of a short microwave pulse in a nonuniform plasma column. The oscillations are found to be due to an instability of the electronrich sheath. The instability with its characteristic frequency below the ion plasma frequency is driven by the energetic ion component reflected in the sheath area. A qualitative model of the instability is suggested.

Shearwave Mach cones in a strongly coupled dusty plasma
View Description Hide DescriptionShearwave Mach cones excited in a strongly coupled dusty plasma in the fluid regime are studied using the generalized hydrodynamic model. The Mach cones are excited by a laser beam that is modeled to sweep the threedimensional dusty plasma with a velocity that is supersonic with respect to the phase velocity of the transverse shear waves. The formation of single Mach cone structures in vorticity maps reveals that they are formed due to shear motion. It is found that an asymmetry in the wake excitation technique gives rise to certain asymmetries in the Mach cone patterns.

Selfsimilar expansion of finitesize nonquasineutral plasmas into vacuum: Relation to the problem of ion acceleration
View Description Hide DescriptionA new selfsimilar solution is presented which describes nonrelativistic expansion of a finite plasma mass into vacuum with a full account of charge separation effects. The solution exists only when the ratio of the plasma scale length to the Debye length is invariant, i.e., under the condition , where , 2, and 3 corresponds, respectively, to the planar, cylindrical, and spherical geometries. For the position of the ion front and the maximum energy of accelerated ions are calculated analytically: in particular, for one finds , where is the initial electron temperature, is the ion charge, and is the Lambert function. It is argued that, when properly formulated, the results for can be applied more generally than the selfsimilar solution itself. Generalization to a twotemperature electron system reveals the conditions under which the highenergy tail of accelerated ions is determined solely by the hotelectron population.

Simulations of dusty plasmas using a specialpurpose computer system designed for gravitational body problems
View Description Hide DescriptionSimulations of dusty plasmas were performed using GRAPE6, a specialpurpose computer designed for gravitational body problems. The collective behavior of dust particles, which are injected into the plasma, was studied by means of threedimensional computer simulations. As an example of a dusty plasma simulation, experiments on Coulomb crystals in plasmas are simulated. Formation of a quasitwodimensional Coulomb crystal has been observed under typical laboratory conditions. Another example was to simulate movement of dust particles in plasmas under microgravity conditions. Fully threedimensional spherical structures of dust clouds have been observed. For the simulation of a dusty plasma in microgravity with particles, GRAPE6 can perform the whole operation 1000 times faster than by using a Pentium 4 1.6 GHz processor.

The relativistic kinetic Weibel instability: General arguments and specific illustrations
View Description Hide DescriptionA general description is developed of the kinetic Weibel [Phys. Rev. Lett.2, 83 (1959)] instability in relativistic plasmas for arbitrary plasmadistribution functions. Then general conditions for the existence of the relativistic instability are given as well as the influence of limits on the wavenumber range and so on the instability rate. A comparison with previous works on biGaussian distributions is also provided. Furthermore, ultrarelativistic and also weakly anisotropic relativistic distributions are investigated in general. Finally, isolated Weibel modes (in which both the frequency and wave number are fixed) are derived for asymmetric relativistic plasma distributions and constraints on these isolated modes are presented.

Kinetic theory of the stability of the inhomogeneous plasmas with transverse inhomogeneous electric field
View Description Hide DescriptionThe stability of a magnetized inhomogeneous plasma with transverse inhomogeneous electric field is analyzed by using a kinetic formalism. It is found that for a smooth profile of the flow velocity, drift and ioncyclotron waves are damped due to resonant interaction with ions. On the other hand, drift and driftcyclotron kinetic instabilities modified by shear flow are excited by inverse electron Landau damping. A renormalized nonlinear dispersion equation, which accounts for random scattering of ions in the presence of the inhomogeneous transverse shear flow, is derived and applied to determine the saturation level of the driftcyclotron instability in a plasma with transverse shear flow.

Equation of state for the “electrostatic pressure” in dusty plasmas
View Description Hide DescriptionThe “electrostatic pressure” is caused by the repulsion between shielded dust particles and arises due to an inhomogeneous distribution of dust within the plasma background. An equation of state, which relates this pressure to the dust density, is obtained and some properties of this equation are discussed. Furthermore, it is shown that for typical experimental parameters, this pressure could be significant and a good fraction of the background plasma pressure. Using this equation of state, the dispersion relation of the sound wave is obtained which shows that it slows down with increasing dust density.

Covariant kinetic dispersion theory of linear transverse waves parallel propagating in magnetized plasmas with thermal anisotropy
View Description Hide DescriptionThe properties of transverse waves parallel propagating in magnetized plasmas with arbitrary composition and thermally anisotropic, are investigated on the basis of relativistic VlasovMaxwell equations. The transverse dispersion relations for plasmas with arbitrary distribution functions are derived. These dispersion relations describe the linear response of the system to the initial perturbations and thus define all existing linear (transverse) plasma modes in the system. By analytic continuation the dispersion relations in the whole complex frequency plane are constructed. Further analysis is restricted to the important case of anisotropic biMaxwellian equilibrium plasmadistribution functions. Explicit forms of the relativistically correct transverse dispersion relations are derived that hold for any values of the plasma temperatures and the temperatureanisotropy. In the limit of nonrelativistic plasma temperatures the dispersion relations are expressed in terms of plasmadispersion function, however, the dependence on frequency and wave numbers is markedly different from the standard noncovariant nonrelativistic analysis. Only in the strictly unphysical formal limit of an infinitely large speed of light, , does the nonrelativistic dispersion relations reduce to the standard noncovariant dispersion relations.

Effect of electrostatic plasma oscillations on the kinetic energy of a charged macroparticle
View Description Hide DescriptionThe stochastic energy acquired by an isolated charged macroparticle (“dust” particle) due to electrostaticfluctuations of a weakly ionized plasma is investigated. Analytic relations are derived and numerical modeling of the problem for the conditions close to those of typical laboratory experiments in a complex dusty plasma is done. The study demonstrates that the kinetic energy of a dust particle, induced by the considered effect, can significantly exceed the temperature of the background gas. The most important contribution to the energy acquired by the macroparticle is due to the ion plasma component.

Nonlinear Phenomena, Turbulence, Transport

Nonlinear energy principle for model current sheets
View Description Hide DescriptionIt is demonstrated on the basis of exact invariants of nonlinear Vlasov equation and model current sheets that the change in magnetic topology (i.e., reconnection) in a finite closed system leads to the conversion of magneticfield energy to particle energy. It is also shown that the volumeaveraged conversion efficiency diminishes as the spatial average is taken over larger and larger system size, while it increases when the system size becomes smaller. This finding may have an important implication for numerical simulation of reconnection processes under finite geometry.

The failure of the scattering angle cutoff for Coulomb interactions
View Description Hide DescriptionCareful study of the collision frequency for Coulomb interactions indicates that the traditional practice of making cutoff on a scattering angle is a failure. The only complete cutoff should be on velocity change. After making the velocity change as an independent variable, we obtain more satisfactory physical results for both Coulomb interactions and hardsphere interactions. These physical results include collision frequency, dynamical friction coefficient, diffusion coefficients, energy exchange frequency, and arbitrary highorder Fokker–Planck coefficients. New special functions,, are obtained for the expression of the arbitrary order FokkerPlanck coefficients for both Coulomb interactions and hardsphere interactions. The combined results establish a valuable relationship between Coulomb interactions and hardsphere interactions with a parameter . The special functions,, have some interesting properties which make it possible to analyze arbitrary order FokkerPlanck coefficients easily for the first time. A new constant is introduced to replace Couloumb logarithm in the precise calculation of plasma equilibrium time. This new constant is related with Arrhenius exponential constant defined in the collision theory for chemical reaction rate. A physical term is proposed as the accumulative measure of a force to unify the concept of collision strength.

Study of nonMaxwellian trapped electrons by using generalized distribution function and their effects on the dynamics of ion acoustic solitary wave
View Description Hide DescriptionBy using the generalized distribution function, the effect of particle trapping on the linear and nonlinear evolution of an ionacoustic wave in an electronion plasma has been discussed. The spectral indices and contribute to the highenergy tails and flatness on top of the distribution function respectively. The generalized Korteweg–de Vries equations with associated solitary wave solutions for different ranges of parameter are derived by employing a reductive perturbation technique. It is shown that spectral indices and affect the trapping of electrons and subsequently the dynamics of the ion acoustic solitary wave significantly.

Nonlinear propagation of ionacoustic waves in electronpositronion plasma with trapped electrons
View Description Hide DescriptionA theoretical investigation has been made for ionacoustic waves in an unmagnetized electronpositronion plasma. A more realistic situation in which plasma consists of a negatively charged ion fluid, free positrons, and trapped as well as free electrons is considered. The properties of stationary structures are studied by the reductive perturbation method, which is valid for small but finite amplitude limit, and by pseudopotential approach, which is valid for large amplitude. With an appropriate modified form of the electronnumber density, two new equations for the ion dynamics have been found. When deviations from isothermality are finite, the modified KortewegdeVries equation has been found, and for the case that deviations from isothermality are small, calculations lead to a generalized KortewegdeVries equation. It is shown from both weakly and highly nonlinear analysis that the presence of the positrons may allow solitary waves to exist. It is found that the effect of the positron density changes the maximum value of the amplitude and (Mach number) for which solitary waves can exist. The present theory is applicable to analyze arbitrary amplitude ionacoustic waves associated with positrons which may occur in space plasma.

The scaling properties of twodimensional compressible magnetohydrodynamic turbulence
View Description Hide DescriptionUnderstanding the phenomenology captured in direct numerical simulation (DNS) of magnetohydrodynamic(MHD)turbulence rests upon models and assumptions concerning the scaling of field variables and dissipation. Here compressible MHDturbulence is simulated in two spatial dimensions by solving the isothermal equations of resistive MHD on a periodic square grid. In these simulations it is found that the energy spectrum decreases more slowly with , and the viscous cutoff length is larger, than would be expected from the 1941 phenomenology of Kolmogorov (K41). Both these effects suggest that the cascade time is modified by the presence of Alfvén waves as in the phenomenology of Iroshnikov and Kraichnan (IK). Motivated by this, these scaling exponents are compared with those of the IKbased model of Politano and Pouquet [Phys. Rev. E52, 636 (1995)], which is an extension of the model of She and Leveque [Phys. Rev. Lett.72, 336 (1994)]. However, the scaling exponents from these simulations are not consistent with the model of Politano and Pouquet, so that neither IK nor K41 models would appear to describe the simulations. The spatial intermittency of turbulent activity in such simulations is central to the observed phenomenology and relates to the geometry of structures that dissipate most intensely via the scaling of the local rate of dissipation. The framework of She and Leveque implies a scaling relation that links the scaling of the local rate of dissipation to the scaling exponents of the pure Elsässer field variables (). This scaling relation is conditioned by the distinct phenomenology of K41 and IK. These distinct scaling relations are directly tested using these simulations and it is found that neither holds. This deviation suggests that additional measures of the character of the dissipation may be required to fully capture the turbulent scaling, for example, pointing towards a refinement of the phenomenological models. It may also explain why previous attempts to predict the scaling exponents of the pure Elsässer fields in twodimensional magnetohydrodynamicturbulence by extending the theory of She and Leveque have proved unsuccessful.

Radially propagating fluctuation structures in the scrapeoff layer of Alcator CMod
View Description Hide DescriptionRadially propagating spatiotemporal fluctuation structures are observed in the scrapeoff layer of Alcator CMod [I. H. Hutchinson et al., Phys. Plasmas1, (1994)] using the combination of electric probes, a radial array of views measuring emission, and twodimensional imaging of emission. For a specific magneticfield configuration the electric probe and the array measuredplasma density and potential fluctuations along the same magneticflux tube. Calculations of the crosscorrelation functions of intensity fluctuations with ion saturation current fluctuations and floating potential fluctuations, respectively, reveal that the potential associated with fluctuation structures is of dipole type, consistent with fundamental models for radial blob propagation. Radial and poloidal velocities of fluctuation structures are obtained by twodimensional spatiotemporal turbulence imaging using an ultrafast framing camera observing the emission intensity in the poloidal plane. In the poloidal direction the fluctuation structures are predominantly propagating in direction of background velocity.Measuredradial velocities are directed outwards with a magnitude of typically 1% of the ion sound speed.

Generation of vortex rings by nonstationary laser wake field
View Description Hide DescriptionA new concept of generating quasistatic magnetic fields,vortex rings, and electron jets in an isotropic homogeneous plasma is presented. The propagation of plasma waves, generated by a relativistically intense short pulse laser, is investigated by using the kinetic model and a novel nonpotential, timedependent ponderomotive force is derived by obtaining a hydrodynamic equation of motion. This force can in turn generate quasistatic magnetic fields,vortex rings, and electron jets. It is also shown that the vortex rings can become a means for accelerating electrons, which are initially in equilibrium. The conservation of canonical momentum circulation and the frozenin condition for the vorticity is discussed. The excitation of the vortexwaves by the modulation of the amplitude of the plasma waves is considered. These vortexwaves, which generate a lower hybrid mode propagating across the generated magnetic field, are also investigated.
