Volume 13, Issue 6, June 2006
 LETTERS


Excitation of the geodesic acoustic mode during ion cyclotron resonance heating
View Description Hide DescriptionIt is shown that poloidal polarization of the plasma during ion cyclotron resonance heating (ICRH) can provide the source of free energy for excitation of the geodesic acoustic mode (GAM). A rough estimate for the threshold rf electric field amplitude necessary for GAM instability is given by , where is the rate of ICRHinduced quasilinear diffusion in velocity space, is the ion collision rate, and is the safety factor.

Scalings of steady state Hall magnetohydrodynamic reconnection in highbeta plasmas
View Description Hide DescriptionScalings of Hall magnetohydrodynamics reconnection in high plasmas has been studied in steady states. It again confirms previous temporal evolution reconnection results that while the width of the reconnection layer is scaled by , where is the Alfvén frequency, is the ion gyrofrequency, is the typical system length scale, and is the ion inertial length, the length of the layer should be scaled by [X. Wang and A. Bhattacharjee, Phys. Rev. Lett. 70, 1627 (1993)], to yield the fast reconnection rate of with as the Alfvén velocity [X. Wang, A. Bhattacharjee, and Z. Ma, Phys. Rev. Lett. 87, 265003 (2001)]. It is also shown that the reconnection rate is proportional to the perturbed boundary flow. Furthermore it is found that in the high plasmas, the reconnection keeps constant in the regime , and decays as for .
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Transition scale at quasiperpendicular collisionless shocks: Full particle electromagnetic simulations
View Description Hide DescriptionOnedimensional full particle simulations of almost perpendicular supercritical collisionless shocks over a wide Alfvén Mach number range are presented. The physical ion to electron mass ratio has been used; however, due to computer time limitations a value of the ratio of the electron plasma frequency to the electron gyrofrequency of 4 has been assumed. The shock structure in the density and magnetic field consists of a foot, formed by reflected ions, and a steeper ramp leading to an overshoot. It is shown that the shock ramp scale in units of the upstream ion inertial length is more or less constant and close to 1 over the Mach number regime investigated, i.e., up to . Further, the convective ion gyroradius in units of the upstream ion inertial length is also constant with the Mach number when the gyroradius is evaluated with the magnetic field strength in the overshoot. Thus the shock transition also scales with the convected gyroradius. When a hyperbolic tangent function is fitted to the density profile the neglect of the overshoot essentially results, for high Mach number shocks, in a fit of the foot and not of the ramp, i.e., the shock transition scale is grossly overestimated. The simulations suggest that in a regime above the critical Mach number the nonlinear steepening is balanced by gyroviscosity of the reflected ions as the shock ramp scale is given by the convected gyroradius in the overshoot. At higher Mach numbers the shock becomes unsteady the ramp scale can become as small as several electron inertial length during a part of the reformation cycle. At still higher Mach numbermicroinstabilities in the foot may have growth times much shorter than the inverse ion gyrofrequency so that they can lead to ion heating, and a steady resistive shock will result.

Gyrokinetic linear theory of the entropy mode in a Z pinch
View Description Hide DescriptionThe linear gyrokinetictheory of the entropymode is presented in a Zpinch configuration in the regime of plasma, focusing primarily on the parameter regime in which the ideal interchange mode is stable. The entropymode is a smallscale, nonmagnetohydrodynamic mode that typically has peak growth rates at , with magnitudes comparable to those of ideal modes. The properties of this mode are studied as a function of the density and temperature gradients, plasma collisionality, and electron to ion temperature ratio.

Laserinducedfluorescence observation of ion velocity distribution functions in a plasma sheath
View Description Hide DescriptionExperimental results obtained by laserinducedfluorescence on metastable ion velocity distribution functions (MIVDF) in electrostatic presheaths and sheaths in argon plasmas produced by the thermoionic effect in a multipolar dc discharge are presented. The shape of the measured MIVDF are in qualitative agreement, for the presheath, with Emmert’s model and exhibit: (1) a Maxwellian profile at the center of the device where the potential is zero; (2) a distribution function’s shape made of three distinct parts at the entrance of the presheath. Inside the sheath the recorded MIVDF recovers a Maxwellian profile with a width unexpectedly related to the background neutral pressure. The velocity and potential profiles that can be deduced from the measured MIVDF show a strong influence of the primary electrons emitted by the filaments.

Simulations of KelvinHelmholtz modes in partially ionized dusty plasmas comparing different charge numbers, charge polarities, and masses of the dust
View Description Hide DescriptionResults of multifluid simulations of KelvinHelmholtz modes in partially ionized, dusty plasmas are presented assuming different masses and charges of the dust. The results show a stabilizing effect for more massive dust grains and indicate a destabilizing effect for higher charge numbers. No significant dependence on the charge polarity of the dust was found.

Equilibrium, multistability, and chiral asymmetry in rotated mirror plasmas
View Description Hide DescriptionThe Hall term in twofluid magnetohydrodynamics is shown to be necessary to balance the curl of the ion inertial force in a rotating plasma with spatially nonuniform crossed electric and magnetic fields. Twofluid solutions are obtained that qualitatively explain the multistable rotational response observed in magnetoBernoulli experiment, imply chiral symmetry breaking, i.e., handedness, and yield new dynamolike electromotive terms in the effective circuit equation for externally rotated mirrorplasma equilibria.

Speed and shape of dust acoustic solitary waves with variable dust charge and two temperature ions
View Description Hide DescriptionDust acoustic solitary waves are investigated on the nonlinear, unmagnetized homogeneous dust ion electron plasma with variable dust charge and two temperature ions. The Sagdeev’s pseudopotential is determined in terms of , the dust ion speed. It is found that there exists a critical value of , beyond which the solitary waves cease to exist. This critical value of depends on other plasma parameters also.

Propagation of radially localized helicon waves in longitudinally nonuniform plasmas
View Description Hide DescriptionA gradient in the plasma density across the guiding magnetic field can support a lowfrequency radially localized helicon (RLH) wave in a plasma column. If the radial density gradient changes along the magnetic field, this wave can undergo reflection and also excite conventional whistlers. This paper presents calculations of the corresponding reflection coefficient, including the effect of whistler radiation. It is shown that a sharp longitudinal density drop causes a nearly complete reflection of the RLH wave. The longitudinal wavelength of the excited whistlers is much greater than that of the RLH wave, and, as a result, only a small fraction of the RLH wave energy is transferred to the whistlers.

Twostream instability for a longitudinally compressing charged particle beam
View Description Hide DescriptionThe electrostatic twostream instability for a cold, longitudinally compressing charged particle beam propagating through a background plasma has been investigated both analytically and numerically. Smallsignal coupled equations describing the evolution of the perturbations are derived, and the asymptotic solutions are obtained. The results are confirmed by direct numerical solution of the linearized fluid equations. It is found that the longitudinal beam compression strongly modifies the spacetime development of the instability. In particular, the dynamic compression leads to a significant reduction in the growth rate of the twostream instability compared to the case without an initial velocity tilt.

Perpendicularly propagating electromagnetic modes in a strongly magnetized hot plasma with nonMaxwellian distribution function
View Description Hide DescriptionElectromagnetic modes (ordinary and extraordinary) for strongly magnetized plasma are studied and their damping factors and are calculated using nonMaxwellian velocity distribution function. It is observed that for moderate values of the spectral indices and [used in distribution functions], both the damping decrements show substantial change. As the value of the spectral index increases for a fixed value of , the damping increases for the mode but decreases for the mode. In the limiting case of , , the damping factors reduce to the standard Maxwellian values.

Study of twodimensional Debye clusters using Brownian motion
View Description Hide DescriptionA twodimensional Debye cluster is a system of identical particles confined in a parabolic well and interacting through a screened Coulomb (i.e., a DebyeHückel or Yukawa) potential with a Debye length . Experiments were performed for 27 clusters with –63 particles in a capacitively coupled 9 W rf discharge at a neutral argon pressure of . In the strongcoupling regime each particle exhibits small amplitude Brownian motion about its equilibrium position. These motions were projected onto the centerofmass and breathing modes and Fourier analyzed to give resonance curves from which the mode frequencies, amplitudes, and damping rates were determined. The ratio of the breathing frequency to the centerofmass frequency was compared with theory to selfconsistently determine the Debye shielding parameter , Debye length , particle charge , and mode temperatures. It is found that , and decreases weakly with . The particle charge averaged over all measurements is , and decreases slightly with . The two centerofmass modes and the breathing mode are found to have the same temperature, indicating that the clusters are in thermal equilibrium with the neutral gas. The average clustertemperature is .

Lownoise electromagnetic particleincell simulation of electron Bernstein waves
View Description Hide DescriptionThe conversion of the extraordinary mode to an electron Bernstein wave(EBW) is one way to get rf energy into an overdense plasma. Analysis of this is complex, as the EBW is a fully kinetic wave, and so its linear propagation is described by an intractable integrodifferential equation. Nonlinear effects cannot be calculated within this rubric at all. Full particleincell(PIC) simulations cannot be used for these analyses, as the noise levels for reasonable simulation parameters are much greater than the typical rf amplitudes. It is shown that the deltaf computations are effective for this analysis. In particular, the accuracy of those computations has been verified by comparison with full PIC, cold plasmatheory, and small gyroradius theory. This computational method is then used to analyze mode conversion in different frequency regimes. In particular, reasonable agreement with the theoretical predictions of Ram and Schultz [Phys. Plasmas7, 4084 (2000)] in the linear regime is found, where 100% mode conversion has been obtained when the driving frequency is less than twice the electron gyrofrequency. The results show that coldplasma theory well predicts the mode conversion efficiency, as is consistent with the phasespace picture of mode conversion. From this it can be shown that nearly 100% mode conversion cannot be obtained when the frequency is higher than the electron second harmonic cyclotron frequency.

Ionacoustic dressed solitons in a dusty plasma
View Description Hide DescriptionUsing the reductive perturbation method,equations for ionacoustic waves governing the evolution of first and secondorder potentials in a dusty plasma including the dynamics of charged dust grains have been derived. The renormalization procedure of Kodama and Taniuti is used to obtain a steady state nonsecular solution of these equations. The variation of velocity and width of the Kortewegde Vries (KdV) as well as dressed solitons with amplitude have been studied for different concentrations and charge multiplicity of dust grains. The higherorder perturbation corrections to the KdV soliton description significantly affect the characteristics of the solitons in dusty plasma. It is found that in the presence of positively charged dust grains the system supports only compressive solitons. However, the plasma with negatively charged dust grains can support compressive solitons only up to a certain concentration of dust. Above this critical concentration of negative charge, the dusty plasma can support rarefactive solitons. An expression for the critical concentration of negatively charged dust in terms of charge and mass ratio of dust grains with plasma ions is also derived.
 Nonlinear Phenomena, Turbulence, Transport

On the use of critical gradient models in fusion plasma transport studies
View Description Hide DescriptionTransportmodels for tokamak devices are often based on transport coefficients involving a critical threshold condition. In this paper, it is argued that the validation of such models against experimental data requires special care when the system profiles are close to this threshold (at some locations), due to the contribution of fluctuations to transport. The arguments presented here could have implications for the understanding and modeling of heat transport in tokamaks, since the large stiffness of the temperature profile observed in experimental points to a nearcritical situation over much of the radius. The difficulties are illustrated by means of a simplified transportmodel, and a possible way to ameliorate this issue is proposed.

Modulational instability of dust acoustic waves in dusty plasmas: Modulation obliqueness, background ion nonthermality, and dust charging effects
View Description Hide DescriptionThe oblique modulational instability of dust acoustic (DA) waves in an unmagnetized warm dusty plasma with nonthermal ions, taking into account dust grain charge variation (charging), is investigated. A nonlinear Schrödingertype equation governing the slow modulation of the wave amplitude is derived. The effects of dust temperature, dust charge variation, ion deviation from Maxwellian equilibrium (nonthermality) and constituent species’ concentration on the modulational instability of DA waves are examined. It is found that these parameters modify significantly the oblique modulational instability domain in the plane. Explicit expressions for the instability rate and threshold have been obtained in terms of the dispersion laws of the system. The possibility and conditions for the existence of different types of localized excitations are also discussed. The findings of this investigation may be useful in understanding the stable electrostatic wave packet acceleration mechanisms close to the Moon, and also enhances our knowledge on the occurrence of instability associated to pickup ions around unmagnetized bodies, such as comets, Mars, and Venus.

Selfconsistent nonlinear transverse waves in relativistic plasmas
View Description Hide DescriptionPrevious investigations of the relativistic Weibel instability provide motivation to consider the nonlinear domain because, for asymmetric particle distributions, there is only an isolated unstable Weibel mode—reminiscent of nonlinear wavetypes of behavior. From the collisionless Boltzmann equation together with Maxwell’sequations, a nonlinear, selfconsistent wave equation is derived that is solvable for a broad range of distribution functions. For monochromatic electrons the nonlinear equation can be solved exactly, but results in an unphysical behavior of the magnetic field due to the compact support required of the distribution function. The general equation can be solved by asymptotic representation producing physically correct nonlinear wavesolutions over bounded domains with varying internal structure of the electric and magnetic fields that range from nearly Gaussian to “sawtooth” in shape. A lower limit on the nonlinear wave amplitude is required in order that the nonlinear wave be of limited extent and so not represent a sinusoidal disturbance with no bounding domain. Limits for the nonlinear wave maximum magnetic field, and particle number density within the nonlinear wave, are given by considering the constraints on the nonlinear wave due to radiation processes, electron collision effects, and electron degeneracy pressure. The basic physical scale results are depicted mostly conducive for astrophysical applications involving relativistic flows and ray emission, for which detailed investigations will be given elsewhere.

Selfsustaining vortex perturbations in smooth shear flows
View Description Hide DescriptionThe nonlinear dynamics of coherent circular/elliptical cyclonic and anticyclonic vortices in plane flow with constant shear is investigated numerically using a dealiased Fourier pseudospectral code. The flow is asymptotically linearly stable, but is highly nonnormal, allowing perturbations to gain energy transiently from the background shear flow. This linear transient growth interplays with nonlinear processes. In certain cases it is shown that the nonlinear feedback is positive, leading to selfsustaining coherent vortices. Selfsustaining coherent vortices exist where the vorticity is parallel to the mean flowvorticity (cyclonic rotation). The required nonlinear feedback is absent for small amplitude anticyclonic vortices. However, elliptical anticyclonic vortices become selfsustaining if the amplitude exceeds a threshold value. The selfsustaining of coherent vortices is similar to the subcritical, socalled bypass, transition to turbulence in shear flows. The common features are: transient linear growth; positive nonlinear feedback; and anisotropy of the linear and nonlinear phenomena (in contrast to isotropic Kolmogorov turbulence). A plasma laboratory experiment is suggested based on the results of this investigation.

Physical origin of the quadrupole outofplane magnetic field in Hallmagnetohydrodynamic reconnection
View Description Hide DescriptionA quadrupole pattern of the outofplane component of the magnetic field inside a reconnection region is seen as an important signature of the Hallmagnetohydrodynamic regime of reconnection. It has been first observed in numerical simulations and just recently confirmed in the Magnetic Reconnection Experiment [Y. Ren, M. Yamada, S. Gerhardt, H. Ji, R. Kulsrud, and A. Kuritsin, Phys. Rev. Lett.95, 055003 (2005)] and also seen in spacecraft observations of Earth’s magnetosphere. In this study, the physical origin of the quadrupole field is analyzed and traced to a current of electrons that flows along the lines in and out of the inner reconnection region to maintain charge neutrality. The role of the quadrupolemagnetic field in the overall dynamics of the reconnection process is discussed. In addition, the bipolar poloidal electric field is estimated and its effect on ion motions is emphasized.

Modeling of the turbulent magnetohydrodynamic residualenergy equation using a statistical theory
View Description Hide DescriptionThe difference between the kinetic and magnetic energies in a conducting fluid is investigated in the framework of magnetohydrodynamics. The deviation from equipartition is measured by the turbulent residual energy . With the aid of the twoscale directinteraction approximation, a statistical analytical theory for inhomogeneous turbulence, expressions for the correlation tensors appearing in the evolution equation for the residual energy are derived. Using these results, we propose a modelequation for evolution. Examination of the structure of this equation shows that the evolution of the scaled residual energy is related to the cross helicity (velocitymagneticfield correlation) of turbulence coupled with the meanfield shears. An application to the solar wind shows that the scaled can be increased near the outside of the Alfvén point in the inner heliosphere whereas the almost stationary behavior of is suggested in the outer heliosphere. These results are consistent with observations of solarwindturbulence.