Volume 19, Issue 6, June 2012
Index of content:

Penning traps that are used for particular applications, such as in ion pump technology, Larmor, bouncing, and diocotron frequencies, can be of the same order of magnitude. The paper deals with the dynamics of electrons confined in such devices starting from the study of the properties of the trajectories. In cases of interest, in which electronneutral collision frequency is much smaller with respect to the characteristic frequencies of the motion, suitable time averages of the trajectories are introduced in order to simplify the analysis of the problem. In the work, time averages have been calculated in a simple way by using an approximate r–z decoupling of the effective potential. Results obtained with the method are presented and discussed in both linear and nonlinear regimes.
 LETTERS


A coarsegrained kinetic equation for neutral particles in turbulent fusion plasmas
View Description Hide DescriptionA coarsegrained kinetic equation for neutral particles (atoms, molecules) in magnetized fusion plasmas, valid on time scales large compared to the turbulence correlation time, is presented. This equation includes the effects of plasma density fluctuations, described by gamma statistics, on the transport of neutral particles. These effects have so far been neglected in plasma edge modeling, in spite of the fact that the amplitude of fluctuations can be of order unity. Density fluctuations are shown to have a marked effect on the screening of neutrals and on the spatial localization of the ionization source, in particular at high density. The coarsegrained equations obtained in this work are readily implemented in edge code suites currently used for fusion plasma analysis and future divertor design (ITER, DEMO).

Electric field enhanced conductivity in strongly coupled dense metal plasma
View Description Hide DescriptionExperimentation with dense metal plasma has shown that nonnegligible increases in plasmaconductivity are induced when a relatively low electric field (∼6 kV/cm) is applied. Existing conductivitymodels assume that atoms, electrons, and ions all exist in thermal equilibrium. This assumption is invalidated by the application of an appreciable electric field, where electrons are accelerated to energies comparable to the ionization potential of the surrounding atoms. Experimental data obtained from electrically exploded silver wire is compared with a finite difference hydrodynamic model that makes use of the SESAME equationofstate database. Free electrongeneration through both thermal and electric field excitations, and their effect on plasmaconductivity are applied and discussed.

Mechanism of heating of preformed plasma electrons in relativistic lasermatter interaction
View Description Hide DescriptionThe role of the longitudinal ambipolar electric field, present inside a preformed plasma, in electron heating and beamgeneration is investigated by analyzing single electron motion in the presence of one electromagnetic plane wave and “V” shaped potential well (constant electric field) in a one dimensional slab approximation. It is shown that for the electron confined in an infinite potential well, its motion becomes stochastic when the ratio of normalized laser electric field, to normalized longitudinal electric field, exceeds unity, i.e., . For a more realistic potential well of finite depth, present inside the preformed plasma, the condition for stochastic heating of electrons gets modified to , where L is the normalized length of the potential well. The energy of electron beam leaving such a potential well and entering the solid scales , which can exceed the laser ponderomotive energy () in the stochastic regime.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Necessary conditions of the equivalence of canonical and grand canonical ensembles in Coulomb system thermodynamics
View Description Hide DescriptionIt was found that the equivalence of the grand canonical and canonical ensembles for the Coulomb systems is possible only when charged particles of different types in calculating the physical quantities are considered as formally “independent” ones, and the quasineutrality condition is used in the final stage of calculations. The phase equilibrium condition is obtained and the expression is derived for the isothermal compressibility of matter as a twocomponent Coulomb system, which corresponds to the known limit relations for static structure factors. On this basis, it is demonstrated that the critical point of matter, considering as the Coulomb system is determined from the condition of vanishing mean square of fluctuations of the total charge per unit volume.

Geometrical tunability of plasmon excitations of double concentric metallic nanotubes
View Description Hide DescriptionThe plasmon frequencies of a general double concentric metallic nanotube (NT) are obtained by using the plasmonhybridization method. Theoretical calculations indicate that there are four distinct plasmon modes for the system. It is shown that these two alternating layers of dielectric and metal have a greater geometrical tunability than the single metallic nanotubes of similar dimensions.

Potential formulation of the dispersion relation for a uniform, magnetized plasma with stationary ions in terms of a vector phasor
View Description Hide DescriptionThe derivation of the helicon dispersion relation for a uniform plasma with stationary ions subject to a constant background magnetic field is reexamined in terms of the potential formulation of electrodynamics. Under the same conditions considered by the standard derivation, the nonlinear selfcoupling between the perturbed electron flow and the potential it generates is addressed. The plane wave solution for general propagation vector is determined for all frequencies and expressed in terms of a vector phasor. The behavior of the solution as described in vacuum units depends upon the ratio of conductivity to the magnitude of the background field. Only at low conductivity and below, the cyclotron frequency can significant propagation occur as determined by the ratio of skin depth to wavelength.

Cross sections of charge exchange and ionization in O^{8+} +H collision in Debye plasmas
View Description Hide DescriptionCharge exchange and ionization processes in O^{8+} +H collision system in a Debye plasma are studied using the classical trajectory Monte Carlo (CTMC) method in the collision energy ranging from 1 keV/amu to 500 keV/amu. Total charge exchange and ionization cross sections have been determined in both screening and unscreening environments. In the unscreened case, partial cross sections for transfer into individual n shells of the projectile have also been determined. An interesting and remarkable feature of sudden increase in the ionization cross sections at lower velocities is discussed in terms of the CTMC framework. Results are analyzed in light of available theoretical and experimental results. The cross sections dependencies on Debye screening lengths have been investigated, and plasma screening effect on charge exchange and ionization cross sections has been found throughout the collision energies range, but is particularly pronounced at low projectile collision energies. The sudden rise in the ionization cross sections towards lower energies is explained qualitatively in terms of the multiple encounter model.

The interaction between two planar and nonplanar quantum electron acoustic solitary waves in dense electronion plasmas
View Description Hide DescriptionThe interaction between two planar and nonplanar (cylindrical and spherical) quantum electron acoustic solitary waves (QEASWs) in quantum dense electronion plasmas has been studied. The extended PoincaréLighthillKuo method is used to obtain planar and nonplanar phase shifts after the interaction of the two QEASWs. The change of phase shifts and trajectories for QEASWs due to the effect of the different geometries, the quantum corrections of diffraction, and the cold electrontohot electron number density ratio are discussed. It is shown that the interaction of the QEASWs in planar geometry, cylindrical geometry, and spherical geometry are different. The present investigation may be beneficial to understand the interaction between two planar and nonplanar QEASWs that may occur in the quantum plasmas found in laserproduced plasmas as well as in astrophysical plasmas.

Exact nonlinear excitations in doubledegenerate plasmas
View Description Hide DescriptionIn this work, we use the conventional hydrodynamics formalism and incorporate the ChewGoldbergerLow doubleadiabatic theory to evaluate the nonlinear electrostatic ion excitations in doubledegenerate (electron spinorbit degenerate) magnetized quantum plasmas. Based on the Sagdeev pseudopotential method, an exact general pseudopotential is calculated which leads to the allowed Machnumber range criteria for such localized density structures in an anisotropic magnetizedplasma. We employ the criteria on the Machnumber range for diverse magnetized quantum plasma with different equations of state. It is remarked that various plasma fractional parameters such as the system dimensionality, iontemperature, relativisticdegeneracy, Zeemanenergy, and plasma composition are involved in the stability of an obliquely propagating nonlinear ionacoustic wave in a doubledegenerate quantum plasma. Current study is most appropriate for nonlinear wave analysis in dense astrophysical magnetized plasma environments such as whitedwarfs and neutronstar crusts where the strong magnetic fields can be present.

Nonlinear electrostatic excitations of charged dust in degenerate ultradense quantum dusty plasmas
View Description Hide DescriptionThe linear and nonlinear properties of lowfrequency electrostaticexcitations of charged dust particles (or defects) in a dense collisionless, unmagnetized ThomasFermi plasma are investigated. A fully ionized threecomponent model plasma consisting of electrons, ions, and negatively charged massive dustgrains is considered. Electrons and ions are assumed to be in a degenerate quantum state, obeying the ThomasFermi density distribution, whereas the inertial dust component is described by a set of classical fluid equations. Considering largeamplitude stationary profile travellingwaves in a moving reference frame, the fluid evolution equations are reduced to a pseudoenergybalance equation, involving a Sagdeevtype potential function. The analysis describes the dynamics of supersonic dustacoustic solitary waves in ThomasFermi plasmas, and provides exact predictions for their dynamical characteristics, whose dependence on relevant parameters (namely, the iontoelectron Fermi temperature ratio, and the dust concentration) is investigated. An alternative route is also adopted, by assuming weakly varying smallamplitude disturbances off equilibrium, and then adopting a multiscale perturbation technique to derive a Korteweg–de Vries equation for the electrostatic potential, and finally solving in terms for electric potential pulses (electrostaticsolitons). A critical comparison between the two methods reveals that they agree exactly in the smallamplitude, weakly superacoustic limit. The dust concentration (Havnes) parameter affects the propagation characteristics by modifying the phase speed, as well as the electron/ion Fermi temperatures. Our results aim at elucidating the characteristics of electrostaticexcitations in dustcontaminated dense plasmas, e.g., in metallic electronic devices, and also arguably in supernova environments, where charged dust defects may occur in the quantum plasma regime.

The qprofile effect on highorder harmonic q = 1 tearing mode generation during sawtooth crashes
View Description Hide DescriptionThe effect of profiles on the excitation of highorder harmonic tearing modes during sawtooth crashes is investigated by a collisionless fluid model with the electron inertia term in Ohm’s law. It is found that for a flat profile in the core region, the highorder harmonics, such as and/or modes, comparable to or stronger than the component, can be excited during tokamak sawteeth. The stronger the magnetic shear on the surface is, the more unstable the higher modes are. For smoothly monotonously increased profiles, a lower value on the plasma edge tends to easily excite higherm harmonics at the same level as the m = 1 mode simultaneously. The spatial characteristics of the eigenmodes in the cases with the typical profiles are also discussed. In addition, the basic feature of the magnetic island structures in the nonlinear evolution is numerically obtained, which is consistent qualitatively with the experimentally reconstructed phenomenon.

Study of envelope electron acoustic solitary waves under transverse perturbations having kappa distributed hot electrons
View Description Hide DescriptionAs is well known, the envelope electron acoustic (EA) nonlinear waves are expressed by nonlinear Schrödinger equation. In this paper, we find that under transverse perturbations, this kind of nonlinear waves can be described by DaveyStewartson equation. In this work, modulational properties of EA wave and its stability regions in twodimensional plasma have been studied.

Cylindrical ionacoustic solitary waves in electronegative plasmas with superthermal electrons
View Description Hide DescriptionBy using the standard reductive perturbation technique, a threedimensional cylindrical KadomtsevPetviashvili equation (CKPE), which governs the dynamics of ion acoustic solitary waves (IASWs), is derived for small but finite amplitude ionacoustic waves in cylindrical geometry in a collisionless unmagnetized plasma with kappa distributed electrons, thermal positrons, and cold ions. The generalized expansion method is used to solve analytically the CKPE. The existence regions of localized pulses are investigated. It is found that the solution of the CKPE supports only compressive solitary waves. Furthermore, the effects of superthermal electrons, the ratio of the electron temperature to positron temperature, the ratio of the positron density to electron density and direction cosine of the wave propagation on the profiles of the amplitudes, and widths of the solitary structures are examined numerically. It is shown these parameters play a vital role in the formation of ion acoustic solitary waves.

Ion stochastic heating by obliquely propagating magnetosonic waves
View Description Hide DescriptionThe ion motions in obliquely propagating Alfven waves with sufficiently large amplitudes have already been studied by Chen et al. [Phys. Plasmas 8, 4713 (2001)], and it was found that the ion motions are stochastic when the wave frequency is at a fraction of the ion gyrofrequency. In this paper, with test particle simulations, we investigate the ion motions in obliquely propagating magnetosonic waves and find that the ion motions also become stochastic when the amplitude of the magnetosonic waves is sufficiently large due to the resonance at subcyclotron frequencies. Similar to the Alfven wave, the increase of the propagating angle, wave frequency, and the number of the wave modes can lower the stochastic threshold of the ion motions. However, because the magnetosonic waves become more and more compressive with the increase of the propagating angle, the decrease of the stochastic threshold with the increase of the propagating angle is more obvious in the magnetosonic waves than that in the Alfven waves.
 Nonlinear Phenomena, Turbulence, Transport

Nonlinear shear wave in a non Newtonian viscoelastic medium
View Description Hide DescriptionAn analysis of nonlinear transverse shear wave has been carried out on nonNewtonian viscoelastic liquid using generalized hydrodynamic model. The nonlinear viscoelastic behavior is introduced through velocity shear dependence of viscosity coefficient by well known CarreauBird model. The dynamical feature of this shear wave leads to the celebrated FermiPastaUlam problem. Numerical solution has been obtained which shows that initial periodic solutions reoccur after passing through several patterns of periodic waves. A possible explanation for this periodic solution is given by constructing modified Korteweg de Vries equation. This model has application from laboratory to astrophysical plasmas as well as in biological systems.

Nonlinear relativistic singleelectron Thomson scattering power spectrum for incoming laser of arbitrary intensity
View Description Hide DescriptionThe classical nonlinear incoherent Thomson scattering power spectrum from a single relativistic electron with incoming laser radiation of any intensity, investigated numerically by the present authors in a previous publication, displayed both an approximate quadratic behavior in frequency and a redshift of the power spectrum for high intensity incoming radiation. The present work is devoted to justify, in a more general setup, those numerical findings. Those justifications are reinforced by extending suitably analytical approaches, as developed by other authors. Moreover, our analytical treatment exhibits differences between the Dopplerlike frequencies for linear and circular polarization of the incoming radiation. Those differences depend nonlinearly on the laser intensity and on the electron initial velocity and do not appear to have been displayed by previous authors. Those Dopplerlike frequencies and their differences are validated by new Monte Carlo computations beyond our previuos ones and reported here.

Ionacoustic solitons in negative ion plasma with twoelectron temperature distributions
View Description Hide DescriptionIon–acoustic solitons in a warm positive and negative ion species with different masses, concentrations, and charge states with two electron temperature distributions are studied. Using reductive perturbation method, Korteweg deVries (KdV) and modifiedKdV (mKdV) equations are derived for the system. The soliton solution of the KdV and mKdV equations is discussed in detail. It is found that if the ions have finite temperatures, then there exist two types of modes, namely slow and fast ionacoustic modes. It is also investigated that the parameter determining the nature of soliton (i.e., whether the system will support compressive or rarefactive solitons) is different for slow and fast modes. For the slow mode, the parameter is the relative temperature of the two ion species; whereas for the fast mode, it is the relative concentration of the two ion species. At a critical concentration of negative ions, both compressive and rarefactive solitons coexist. The amplitude and width of the solitons are discussed in detail at critical concentration for mKdV solitons. The effect of the relative temperature of the twoelectron and coldelectron concentration on the characteristics of the solitons are also discussed.

Continuum limit of electrostatic gyrokinetic absolute equilibrium
View Description Hide DescriptionElectrostaticgyrokinetic absolute equilibria with continuum velocity field are obtained through the partition function and through the Green function of the functional integral. The new results justify and explain the prescription for quantization/discretization or taking the continuum limit of velocity. The mistakes in the Appendix D of our earlier work [J.Z. Zhu and G. W. Hammett, Phys. Plasmas 17, 122307 (2010)] are explained and corrected. If the lattice spacing for discretizing velocity is big enough, all the invariants could concentrate at the lowest Fourier modes in a negativetemperature state, which might indicate a possible variation of the dual cascade picture in 2D plasma turbulence.

Quasilinear transport modelling at low magnetic shear
View Description Hide DescriptionAccurate and computationally inexpensive transportmodels are vital for routine and robust predictions of tokamak turbulenttransport. To this end, the QuaLiKiz [Bourdelle et al., Phys. Plasmas 14, 112501 (2007)] quasilinear gyrokinetictransportmodel has been recently developed. QuaLiKiz flux predictions have been validated by nonlinear simulations over a wide range in parameter space. However, a discrepancy is found at low magnetic shear, where the quasilinear fluxes are significantly larger than the nonlinear predictions. This discrepancy is found to stem from two distinct sources: the turbulence correlation length in the mixing length rule and an increase in the ratio between the quasilinear and nonlinear transport weights, correlated with increased nonlinear frequency broadening. Significantly closer agreement between the quasilinear and nonlinear predictions is achieved through the development of an improved mixing length rule, whose assumptions are validated by nonlinear simulations.

Suprathermal ion transport in simple magnetized torus configurations
View Description Hide DescriptionInspired by suprathermal ion experiments in the basic plasma experiment TORPEX, the transport of suprathermal ions in ideal interchange mode turbulence is theoretically examined in the simple magnetized torus configuration. We follow ion tracer trajectories as specified by ideal interchange mode turbulence imported from a numerical simulation of driftreduced Braginskii equations. Using the variance of displacements, , we find that depends strongly on suprathermal ion injection energy and the relative magnitude of turbulent fluctuations. The value of also changes significantly as a function of time after injection, through three distinguishable phases: ballistic, interaction, and asymmetric. During the interaction phase, we find the remarkable presence of three regimes of dispersion: superdiffusive, diffusive, and subdiffusive, depending on the energy of the suprathermal ions and the amplitude of the turbulent fluctuations. We contrast these results with those from a “slab” magnetic geometry in which subdiffusion does not occur during the interaction phase. Initial results from TORPEX are consistent with data from a new synthetic diagnostic used to interpret our simulation results. The simplicity of the simple magnetized torus makes the present work of interest to analyses of more complicated contexts ranging from fusion devices to astrophysics and space plasma physics.