Volume 15, Issue 5, May 2008

Xray production from the interaction of femtosecond laser pulses focused to relativistic intensity into reentrant targets etched into silicon has been investigated. and hard xray yields were compared when the laser was focused into pyramidal shaped cone targets and wedge shaped targets. Hot electron production is highest in the wedge targets irradiated with transverse polarization, though is maximized with wedge targets and parallel polarization. These results are explained with particleincell simulations.
 LETTERS


Interaction between turbulence and a nonlinear tearing mode in the low regime
View Description Hide DescriptionThe interaction between turbulence and a nonlinear tearing mode is investigated numerically using a 2D electrostaticmodel.Turbulence is found to cause transitions between the different roots for the propagation velocity of the mode. The transitions take the mode towards roots with slower propagation that are characterized by a locally flattened density profile. For sufficiently large islands the transition reduces the drive for the tearing mode.

Growth mechanism for spherical carbon particles in a dc methane plasma
View Description Hide DescriptionThe growth mechanism for spherical carbon particles of micron sizes observed in a vertically excited columnar plasma [F. Shoji, Z. Feng, A. Kono, and T. Nagai, Appl. Phys. Lett.89, 171504 (2006)] is investigated theoretically, based on a model in which the particles are negatively charged in the plasma sheath region where they grow by capturing graphite onions with a diameter of ca. and a positive charge. A balance of gravity and electric force keeps the particles in the sheath region during their growth. It is found that the particle radius initially increases linearly with time and then approaches a saturation radius, and that the center of gravity of the particle executes a simple harmonic oscillation about its balance position with a characteristic frequency of the order of determined by its specific charge, gravity, and sheath structure.

Production of cumulative jets by ablativelydriven implosion of hollow cones and wedges
View Description Hide DescriptionCumulative plasma jets formed by hollow cones imploded via laser ablation of their outer surfaces were observed. The velocity, shape, and density of the jets are measured with monochromatic xray imaging. Depending on cone geometry, cumulative jets with ion density and propagation velocities are formed. Similar results are observed when jets are formed by imploding wedges. Such jets can be used to simulate hydrodynamics of astrophysical jets interacting with stellar or interstellar matter.
 Top

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Dynamics of the excitation of an upper hybrid wave by a rippled laser beam in magnetoplasma
View Description Hide DescriptionThis paper presents the effects of a laser spike (superimposed on an intense laser beam) and a static magnetic field on the excitation of the upper hybrid wave (UHW) in a hot collisionless magnetoplasma, taking into account the relativistic nonlinearity. The laser beam is propagating perpendicular to the static magnetic field and has its electric vector polarized along the direction of the static magnetic field (ordinary mode). Analytical expressions for the growth rate of the ripple, the beam width of the rippled laser beam, and the UHW have been obtained. It is found that the coupling among the main laser beam, ripple, and UHW is strong. The ripple gets focused when the initial power of the laser beam is greater than the critical power for focusing. It has been shown that the presence of a laser spike affects significantly the growth rate and the dynamics of the UHW. In addition, it has been seen that the effect of changing the strength of the static magnetic field on the nonlinear coupling and on the dynamics of the excitation of the UHW is significant. The results are presented for typical laser plasma parameters.

Exceptional points in linear gyrokinetics
View Description Hide DescriptionWhen performing linear gyrokinetic simulations, it is found that various types of microinstabilities, which are usually considered as strictly separated, can actually be transformed into each other via continuous variations of the plasma parameters. This behavior can be explained in terms of socalled exceptional points, which have their origin in the nonHermiticity of the linear gyrokinetic operator and also occur in many other branches of physics. As a consequence, in large regions of parameter space, the designation of unstable modes should be done very carefully or even be avoided altogether.

Nonaxisymmetric magnetorotational instability in ideal and viscous plasmas
View Description Hide DescriptionThe excitation of magnetorotational instability(MRI) in rotating laboratory plasmas is investigated. In contrast to astrophysical plasmas, in which gravitation plays an important role, in laboratory plasmas it can be neglected and the plasma rotation is equilibrated by the pressure gradient. The analysis is restricted to the simple model of a magnetic confinement configuration with cylindrical symmetry, in which nonaxisymmetric perturbations are investigated using the local approximation. Starting from the simplest case of an ideal plasma, the corresponding dispersion relations are derived for more complicated models including the physical effects of parallel and perpendicular viscosities. The Friemann–Rotenberg approach used for ideal plasmas is generalized for the viscous model and an analytical expression for the instability boundary is obtained. It is shown that, in addition to the standard effect of radial derivative of the rotation frequency (the Velikhov effect), which can be destabilizing or stabilizing depending on the sign of this derivative in the ideal plasma, there is a destabilizing effect proportional to the fourth power of the rotation frequency, or, what is the same, to the square of the plasma pressure gradient, and to the square of the azimuthal mode number of the perturbations. It is shown that the instability boundary also depends on the product of the plasma pressure and density gradients, which has a destabilizing effect when it is negative. In the case of parallel viscosity, the MRI looks like an ideal instability independent of viscosity, while, in the case of strong perpendicular viscosity, it is a dissipative instability with the growth rate inversely proportional to the characteristic viscous decay rate. We point out, however, that the modes of the continuous range of the magnetohydrodynamics spectrum are not taken into account in this paper, and they can be more dangerous than those that are considered.

Saturation of tearing modes in reversed field pinches with locally linear forcefree magnetic fields and its application to quasisinglehelicity states
View Description Hide DescriptionA simple formula for predicting the width of a saturated island, formed as a consequence of tearing perturbation of linear forcefree fields in cylindrical geometry, is derived. The formula makes it possible to calculate the saturated island width in terms of the values of parameters characterizing the initial forcefree equilibrium and can be applied to equilibria of interest for reversed field pinches. In particular it is applied, in this paper, to forcefree equilibria with piecewise constant radial profile of the pinch parameter, which have been recently suggested to be relevant for the formation of quasisinglehelicity states. The main result is that the island width becomes larger as a parameter, that quantifies the departure from a relaxed Taylor state, increases.

Modulational instability of magnetosonic waves in a spin quantum plasma
View Description Hide DescriptionThe modulational instability (MI) of magnetosonic waves (MSWs) is analyzed, by using a twofluid quantum magnetohydrodynamic model that includes the effects of the electron spin and the plasmaresistivity. The envelope modulation is then studied by deriving the corresponding nonlinear Schrödinger equation from the governing equations. The plasmaresistivity is shown to play a dissipative role for the onset of MI. In the absence of resistivity, the microscopic spin properties of electrons can also lead to MI. In such a situation, the dominant spin contribution corresponds to a dense quantum plasma with the particle number density,. Also, in such a dissipative (absorbing) medium, where the group velocity vector is usually complex for real values of the wave vector, the role of the real group velocity in the propagation of onedimensional MSW packets in a homogeneous absorbing medium is reported. The effects of quantum spin on the stability/instability conditions of the magnetosonic envelope are obtained and examined numerically. From the nonlinear dispersion relation of the modulated wave packet it is found that the effect of the spin (plasmaresistivity) is to decrease (increase) the instability growth rate provided the normalized Zeeman energy does not exceed a critical value. The theoretical results may have relevance to astrophysical (e.g., magnetars) as well as to ultracold laboratory plasmas (e.g., Rydberg plasmas).

Calculation of resistive magnetohydrodynamics and twofluid tearing modes by example of reversedfieldpinchlike plasma
View Description Hide DescriptionAn algorithm suitable for numerical solution of linear eigenmode problems in resistive magnetohydrodynamics(MHD) and twofluid MHDmodels without prior approximations is presented. For these plasmamodels, sets of equations suitable for numerical solution are derived and the details of the algorithm of this solution are given. The algorithm is general and is suitable for solution of boundary (eigenmode) problems for different plasma configurations. It is most effective, however, in onedimensional models since the grid size has to be sufficiently small in order to resolve the tearing layer together with the scale of the size of the plasma. The technique is applied for solving for tearing eigenmodes in reversed field pinch(RFP) like plasma in plane geometry. Results of resistive MHD and twofluid models are compared in this case, showing that the twofluid effects on tearing modes in RFPs are sizable.

Parametric conversion of a lower hybrid wave into a whistler in a plasma
View Description Hide DescriptionA large amplitude lower hybrid wave parametrically decays into a whistler wave and a low frequency lower hybrid wave in a plasma. The density perturbation due to the low frequency wave couples with the oscillatory velocity due to the pump wave to produce a nonlinear current, driving the whistler. The pump and whistler exert a low frequency ponderomotive force on electrons, driving the lower hybrid decay wave. The growth rate of the parametric instability scales linearly with the amplitude of the pump wave. It decreases with the electron cyclotron frequency. The process is relevant to beam plasma systems where lower hybrid waves are excited with greater ease and the whistler sideband wave can be seen outside the plasma as electromagnetic emission.

The dispersive Alfvén wave in the timestationary limit with a focus on collisional and warmplasma effects
View Description Hide DescriptionA nonlinear, collisional, twofluid model of uniform plasma convection across a fieldaligned current(FAC) sheet, describing the stationary Alfvén (StA) wave, is presented. In a previous work, Knudsen showed that, for cold, collisionless plasma [D. J. Knudsen, J. Geophys. Res.101, 10761 (1996)], the stationary inertial Alfvén (StIA) wave can accelerate electrons parallel to a background magnetic field and cause large, timeindependent plasmadensity variations having spatial periodicity in the direction of the convective flow over a broad range of spatial scales and energies. Knudsen suggested that these fundamental properties of the StIA wave may play a role in the formation of discrete auroral arcs. Here, Knudsen’s model has been generalized for warm, collisional plasma. From this generalization, it is shown that nonzero ionneutral and electronion collisional resistivity significantly alters the perpendicular ac and dc structure of magneticfieldaligned electron drift, and can either dissipate or enhance the fieldaligned electron energy depending on the initial value of fieldaligned electron drift velocity. It is also shown that nonzero values of plasma pressure increase the dominant Fourier component of perpendicular wavenumber.

Nonlocal magnetorotational instability
View Description Hide DescriptionAn analytical theory of the nonlocal magnetorotational instability(MRI) is developed for the simplest astrophysical plasma model. It is assumed that the rotation frequency profile has a steplike character, so that there are two regions in which it has constant different values, separated by a narrow transition layer. The surface wave approach is employed to investigate the MRI in this configuration. It is shown that the main regularities of the nonlocal MRI are similar to those of the local instability and that driving the nonaxisymmetric MRI is less effective than the axisymmetric one, also for the case of the nonlocal instability. The existence of nonlocal instabilities in nonmagnetized plasma is predicted.
 Nonlinear Phenomena, Turbulence, Transport

Effects of flow shear and Alfvén waves on twodimensional magnetohydrodynamic turbulence
View Description Hide DescriptionThe suppression of turbulenttransport by large scale mean shear flows and uniform magnetic fields is investigated in twodimensional magnetohydrodynamicturbulence driven by a smallscale forcing with finite correlation time. By numerical integration the turbulent magnetic diffusivity is shown to be significantly quenched, with a scaling , which is much more severe than in the case of a short or delta correlated forcing typified by white noise, studied in E. Kim and B. Dubrulle [Phys. Plasmas8, 813 (2001)]. Here and are magnetic field strength and flow shear rate, respectively. The forcing with finite correlation time also leads to much stronger suppression of momentum transport through the cancellation of the Reynolds stress by the Maxwell stress with a positive small value of turbulentviscosity,. While fluctuating kinetic and magnetic energies are unaffected by the magnetic field just as in the case of a delta correlated forcing, they are much more severely quenched by flow shear than in that of a delta correlated forcing. Underlying physical mechanisms for the reduction of turbulenttransport and turbulence level by flow shear and magnetic field are discussed.

Selective formation of turbulent structures in magnetized cylindrical plasmas
View Description Hide DescriptionThe mechanism of nonlinear structural formation has been studied with a threefield reduced fluid model, which is extended to describe the resistive drift waveturbulence in magnetized cylindrical plasmas. In this model, ionneutral collisions strongly stabilize the resistive drift wave, and the formed structure depends on the collision frequency. If the collision frequency is small, modulational coupling of unstable modes generates a zonal flow. On the other hand, if the collision frequency is large, a streamer, which is a localized vortex in the azimuthal direction, is formed. The structure is generated by nonlinear wave coupling and is sustained for a much longer duration than the drift wave oscillation period. This is a minimal model for analyzing the turbulent structural formation mechanism by mode coupling in cylindrical plasmas, and the competitive nature of structural formation is revealed. These turbulent structures affect particle transport.

Localized electrostatic excitations in a Thomas–Fermi plasma containing degenerate electrons
View Description Hide DescriptionBy using the Thomas–Fermi electron density distribution for quantum degenerate electrons, the hydrodynamic equations for ions, and the Poisson equation, planar and nonplanar ionacoustic solitary waves in an unmagnetized collisionless plasma are investigated. The reductive perturbation method is used to derive cylindrical and spherical Korteweg–de Vries equations.Numerical solutions of the latter are presented. The present results can be useful in understanding the features of small but finite amplitude localized ionacoustic solitary pulses in a degenerate plasma.

On particle transport in Weibel generated magnetic fluctuations
View Description Hide DescriptionThe problem of particle scattering in the turbulentmagnetic fields generated by the Weibel instability is investigated on the basis of analytical calculations. From the dispersion relation, the growth and damping rate is calculated and inserted into a quasilinear Fokker–Planck coefficient for pitchangle scattering, from which the scattering parallel mean free path is determined. A weak background magnetic field is included to ensure the validity of the assumption of uncorrelated wave modes, which is essential for quasilinear calculations. It is shown that, for higher values of the temperature and counterstreaming anisotropy, the parallel mean free path is reduced due to the stronger magnetic turbulence generated by the instability. Furthermore, it is shown that for relativistic particles, the parallel mean free path becomes energy independent.

Ambipolar stochastic particle diffusion and plasma rotation
View Description Hide DescriptionThe motion of electrons and ions in stochastic magnetic fields is considered. The analysis starts from a Hamiltonian formulation of the drift motion including electric fields. For an efficient statistical evaluation of the resulting particle transport, a symplectic mapping technique is applied. Compared to previous considerations, the ion and electron test particle motion are investigated simultaneously, allowing calculations of the ambipolar electric field and its influence on stochastic transport. The predictions based on the relativistic drift model are applied to the magnetic perturbations in the TEXTORDED [A. Wingen et al., Nucl. Fusion46, 941 (2006)]. The influence of the magnetic coil arrangement on the poloidal plasma rotation, caused by the generated radial electric field, is discussed.

The momentum flux probability distribution function for iontemperaturegradient turbulence
View Description Hide DescriptionThere has been overwhelming evidence that coherent structures play a critical role in determining the overall transport in a variety of systems. We compute the probability distribution function (PDF) tails of momentum flux and heat flux in iontemperaturegradientturbulence, by taking into account the interaction among modons, which are assumed to be coherent structures responsible for bursty and intermittent events, contributing to the PDF tails. The tail of PDF of momentum flux is shown to be exponential with the form , which is broader than a Gaussian, similar to what was found in the previous local studies. An analogous expression with the same functional dependence is found for the PDF tails of heat flux. Furthermore, we present a detailed numerical study of the dependence of the PDF tail on the temperature and density scale lengths and other physical parameters through the coefficient .

Gyrokinetic calculations of diffusive and convective transport of particles with a slowingdown distribution function
View Description Hide DescriptionQuasilinear gyrokinetic calculations of the transport of fast particles with a slowingdown equilibrium distribution function in the trace limit are presented. Diffusive and convective contributions to the total flux are separated and their dependence on the ratio of the fast particle energy to the background plasma temperature is investigated. The results are compared with those obtained in the case an equivalent Maxwelliandistribution function is assumed for the fast particles. On the basis of the gyrokinetic results, simple models for particle transport are proposed for transport modeling purposes.

Long global gyrokinetic simulations: Source terms and particle noise control
View Description Hide DescriptionIn global gyrokinetic simulations it takes a long time for the turbulence to reach a quasisteady state, and quantitative predictions about the quasisteady state turbulence have been difficult to obtain computationally. In particular, global particleincellgyrokinetic simulations have been inefficient for long simulations due to the accumulation of noise. It is demonstrated that a simple Krook operator can effectively control noise; it also introduces an unphysical dissipation, which damps the zonal flows and can significantly affect simulation results even when the relaxation time is very long. However, it is possible to project out the effects of the Krook operator on the zonal flows. This permits noise accumulation to be controlled while preserving the physics of interest; simulations are then run to determine the level of quasisteady state transport and the variation across the ensemble of turbulentdynamics. Convergence is demonstrated both in the number of computational particles and the unphysical relaxation time.