Volume 21, Issue 4, April 2014
Index of content:
 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Enhanced relativistic selffocusing of HermitecoshGaussian laser beam in plasma under density transition
View Description Hide DescriptionEnhanced and early relativistic selffocusing of HermitecoshGaussian (HChG) beam in the plasmas under density transition has been investigated theoretically using WentzelKramersBrillouin and paraxial ray approximation for mode indices . The variation of beam width parameter with normalized propagation distance for is reported, and it is observed that strong selffocusing occurs as the HChG beam propagates deeper inside the nonlinear medium as spot size shrinks due to highly dense plasmas and the results are presented graphically. A comparative study between selffocusing of HChG beam in the presence and absence of plasmas density transition is reported. The dependency of beam width parameter on the normalized propagation distance for different values of decentered parameter “b” has also been presented graphically. For strong selffocusing is reported for , and for beam gets diffracted. The results obtained indicate the dependency of the selffocusing of the HChG beam on the selected values of decentered parameter. Moreover, proper selection of decentered parameter results strong selffocusing of HChG beam. Stronger selffocusing of laser beam is observed due to the presence of plasma density transition which might be very useful in the applications like the generation of inertial fusion energy driven by lasers, laser driven accelerators, etc.

A framework for moment equations for magnetized plasmas
View Description Hide DescriptionMathematical formalism to solve a system of general moment equations [J.Y. Ji and E. D. Held, Phys. Plasmas 13, 102103 (2006); 16, 102108 (2009)] for magnetized plasmas is presented. A series of ordered moment equations are written using a perturbative expansion based on large cyclotron frequency. For the most general solution, formulas for homogeneous and particular solutions are obtained. These formulas generalize the CGL [G. F. Chew et al., Proc. R. Soc. London, Ser. A 236, 112 (1956)] and nonCGL [C. T. Hsu et al., Phys. Fluids 29, 1480 (1986)] tensors, respectively, from rank2 to arbitrary rank. The parallel moment equations to determine parallel moments in the homogeneous solution are derived. The formalism can be applied to plasmas of general collisionality and magnetic geometry with accurate collision operators provided.

Revision of the Coulomb logarithm in the ideal plasma
View Description Hide DescriptionThe standard picture of the Coulomb logarithm in the ideal plasma is controversial, the arguments for the lower cut off need revision. The two cases of far subthermal and of far superthermal electron drift motions are accessible to a rigorous analytical treatment. We show that the lower cut off is a function of symmetry and shape of the shielding cloud, it is not universal. In the subthermal case, shielding is spherical and is to be identified with the de Broglie wavelength; at superthermal drift the shielding cloud exhibits cylindrical (axial) symmetry and is the classical parameter of perpendicular deflection. In both situations, the cut offs are determined by the electronion encounters at large collision parameters. This is in net contrast to the governing standard interpretation that attributes to the Coulomb singularity at vanishing collision parameters b and, consequently, assigns it universal validity. The origin of the contradictions in the traditional picture is analyzed.

Ion plasma wave and its instability in interpenetrating plasmas
View Description Hide DescriptionSome essential features of the ion plasma wave in both kinetic and fluid descriptions are presented. The wave develops at wavelengths shorter than the electron Debye radius. Thermal motion of electrons at this scale is such that they overshoot the electrostatic potential perturbation caused by ion bunching, which consequently propagates as an unshielded wave, completely unaffected by electron dynamics. So in the simplest fluid description, the electrons can be taken as a fixed background. However, in the presence of magnetic field and for the electron gyroradius shorter than the Debye radius, electrons can participate in the wave and can increase its damping rate. This is determined by the ratio of the electron gyroradius and the Debye radius. In interpenetrating plasmas (when one plasma drifts through another), the ion plasma wave can easily become growing and this growth rate is quantitatively presented for the case of an argon plasma.

Evaporative capillary instability for flow in porous media under the influence of axial electric field
View Description Hide DescriptionWe study the linear analysis of electrohydrodynamic capillary instability of the interface between two viscous, incompressible and electrically conducting fluids in a fully saturated porous medium, when the phases are enclosed between two horizontal cylindrical surfaces coaxial with the interface and, when there is mass and heat transfer across the interface. The fluids are subjected to a constant electric field in the axial direction. Here, we use an irrotational theory in which the motion and pressure are irrotational and the viscosity enters through the jump in the viscous normal stress in the normal stress balance at the interface. A quadratic dispersion relation that accounts for the growth of axisymmetric waves is obtained and stability criterion is given in terms of a critical value of wave number as well as electric field. It is observed that heat transfer has stabilizing effect on the stability of the considered system while medium porosity destabilizes the interface. The axial electric field has dual effect on the stability analysis.

Electronion collisional effect on Weibel instability in a Kappa distributed unmagnetized plasma
View Description Hide DescriptionWeibel instability has been investigated in the presence of electronion collisions by using standard VlasovMaxwell equations. The presence of suprathermal electrons has been included here by using Kappa distribution for the particles. The growth rate γ of Weibel instability has been calculated for different values of spectral index κ, collision frequency , and temperature anisotropy parameter β. A comparative study between plasma obeying Kappa distribution and that obeying Maxwellian distribution shows that the growth of instability is higher for the Maxwellian particles. However, in the presence of collisions, the suprathermal particles result in lower damping of Weibel mode.

New insights into the decay of ion waves to turbulence, ion heating, and soliton generation
View Description Hide DescriptionThe decay of a singlefrequency, propagating ion acoustic wave (IAW) via twoion wave decay to a continuum of IAW modes is found to result in a highly turbulent plasma, ion soliton production, and rapid ion heating. Instability growth rates, thresholds, and sensitivities to plasma conditions are studied via fully kinetic Vlasov simulations. The decay rate of IAWs is found to scale linearly with the fundamental IAW potential amplitude for , beyond which the instability is shown to scale with a higher power of , where Z is the ion charge number and Te (Ti ) is the electron (ion) thermal temperature. The threshold for instability is found to be smaller by an order of magnitude than linear theory estimates. Achieving a better understanding of the saturation of stimulated Brillouin scatter levels observed in laserplasma interaction experiments is part of the motivation for this study.

Particleincell simulations of velocity scattering of an anisotropic electron beam by electrostatic and electromagnetic instabilities
View Description Hide DescriptionThe velocity space scattering of an anisotropic electron beam ( ) flowing along a background magnetic field B 0 through a cold plasma is investigated using both linear theory and 2D particleincell simulations. Here, ⊥ and represent the directions perpendicular and parallel to B 0, respectively. In this scenario, we find that two primary instabilities contribute to the scattering in electron pitch angle: an electrostatic electron beam instability and a predominantly parallelpropagating electromagnetic whistler anisotropy instability. Our results show that at relative beam densities and beam temperature anisotropies , the electrostatic beam instability grows much faster than the whistler instabilities for a reasonably fast hot beam. The enhanced fluctuating fields from the beam instability scatter the beam electrons, slowing their average speed and increasing their parallel temperature, thereby increasing their pitch angles. In an inhomogeneous magnetic field, such as the geomagnetic field, this could result in beam electrons scattered out of the loss cone. After saturation of the electrostatic instability, the parallelpropagating whistler anisotropy instability shows appreciable growth, provided that the beam density and latetime anisotropy are sufficiently large. Although the whistler anisotropy instability acts to pitchangle scatter the electrons, reducing perpendicular energy in favor of parallel energy, these changes are weak compared to the pitchangle increases resulting from the deceleration of the beam due to the electrostatic instability.

Laboratory observation of magnetic field growth driven by shear flow
View Description Hide DescriptionTwo magnetic flux ropes that collide and bounce have been characterized in the laboratory. We find screw pinch profiles that include ion flow , magnetic field , current density , and plasma pressure. The electron flow can be inferred, allowing the evaluation of the Hall term in a two fluid magnetohydrodynamic Ohm's Law. Flux ropes that are initially cylindrical are mutually attracted and compress each other, which distorts the cylindrical symmetry. Magnetic field is created via the induction term in Ohm's Law where inplane (perpendicular) shear of parallel flow (along the flux rope) is the dominant feature, along with some dissipation and magnetic reconnection. We predict and measure the growth of a quadrupole outofplane magnetic field δBz . This is a simple and coherent example of a shear flow driven dynamo. There is some similarity with two dimensional reconnection scenarios, which induce a current sheet and thus outofplane flow in the third dimension, despite the customary picture that considers flows only in the reconnection plane. These data illustrate a general and deterministic mechanism for large scale sheared flows to acquire smaller scale magnetic features, disordered structure, and possibly turbulence.

Microturbulence in DIIID tokamak pedestal. I. Electrostatic instabilities
View Description Hide DescriptionGyrokinetic simulations of electrostatic driftwave instabilities in a tokamak edge have been carried out to study the turbulent transport in the pedestal of an Hmode plasma. The simulations use annulus geometry and focus on two radial regions of a DIIID experiment: the pedestal top with a mild pressure gradient and the middle of the pedestal with a steep pressure gradient. A reactive trapped electron instability with a typical ballooning mode structure is excited by trapped electrons in the pedestal top. In the middle of the pedestal, the electrostatic instability exhibits an unusual mode structure, which peaks at the poloidal angle . The simulations find that this unusual mode structure is due to the steep pressure gradients in the pedestal but not due to the particular DIIID magnetic geometry. Realistic DIIID geometry appears to have a stabilizing effect on the instability when compared to a simple circular tokamak geometry.

Kinetics of positive ions and electrically neutral active particles in afterglow in neon at low pressure
View Description Hide DescriptionKinetics of positive ions and electrically neutral active particles formed during breakdown and successive discharge in neonfilled tube at 6.6 millibars pressure had been analyzed. This analysis was performed on the basis of mean value of electrical breakdown time delay dependence on afterglow period (memory curve). It was shown that positive ions are present in the 1 < < 30 ms interval, which is manifested through slow increase with the increase of . A rapid increase in the 30 ms < < 3 s interval is a consequence of significant decrease of positive ions concentration and dominant role in breakdown initiation have ground state nitrogen atoms, which further release secondary electrons from the cathode by catalytic recombination process. These atoms are formed during discharge by dissociation of ground state nitrogen molecules that are present as impurities in neon. For > 3 s, breakdown is initiated by cosmic rays and natural radioactivity. The increase of discharge current leads to decrease of due to the increase of positive ions concentration in inter electrode gap. The increase of applied voltage also decreases for τ > 30 ms due to the increase of the probability for initial electron to initiate breakdown. The presence of UV radiation leads to the decrease of due to the increased electron yield caused by photoelectrons. The influence of photoelectrons on breakdown initiation can be noticed for > 0.1 ms, while they dominantly determine for > 30 ms.

Plasmonic modes and extinction properties of a random nanocomposite cylinder
View Description Hide DescriptionWe study the properties of surface plasmonpolariton waves of a random metaldielectric nanocomposite cylinder, consisting of bulk metal embedded with dielectric nanoparticles. We use the MaxwellGarnett formulation to model the effective dielectric function of the composite medium and show that there exist two surface mode bands. We investigate the extinction properties of the system, and obtain the dependence of the extinction spectrum on the nanoparticles’ shape and concentration as well as the cylinder radius and the incidence angle for both TE and TM polarization.

Localization of linear kinetic Alfvén wave in an inhomogeneous plasma and generation of turbulence
View Description Hide DescriptionThis paper presents a model for the propagation of Kinetic Alfvén waves (KAWs) in inhomogeneous plasma when the inhomogeneity is transverse to the background magnetic field. The semianalytical technique and numerical simulations have been performed to study the KAW dynamics when plasma inhomogeneity is incorporated in the dynamics. The model equations are solved in order to study the localization of KAW and their magnetic power spectrum which indicates the direct transfer of energy from lower to higher wave numbers. The inhomogeneity scale length plays a very important role in the turbulence generation and its level. The relevance of these investigations to space and laboratory plasmas has also been pointed out.

Ballooning modes localized near the null point of a divertor
View Description Hide DescriptionThe stability of ballooning modes localized to the null point in both the standard and snowflake divertors is considered. Ideal magnetohydrodynamics is used. A series expansion of the flux function is performed in the vicinity of the null point with the lowest, nonvanishing term retained for each divertor configuration. The energy principle is used with a trial function to determine a sufficient instability threshold. It is shown that this threshold depends on the orientation of the flux surfaces with respect to the major radius with a critical angle appearing due to the convergence of the field lines away from the null point. When the angle the major radius forms with respect to the flux surfaces exceeds this critical angle, the system is stabilized. Further, the scaling of the instability threshold with the aspect ratio and the ratio of the scrapeofflayer width to the major radius is shown. It is concluded that ballooning modes are not a likely candidate for driving convection in the vicinity of the null for parameters relevant to existing machines. However, the results place a lower bound on the width of the heat flux in the private flux region. To explain convective mixing in the vicinity of the null point, new consideration should be given to an axisymmetric mixing mode [W. A. Farmer and D. D. Ryutov, Phys. Plasmas 20, 092117 (2013)] as a possible candidate to explain current experimental results.

Collisional relaxation of a strongly magnetized twospecies pure ion plasma
View Description Hide DescriptionThe collisional relaxation of a strongly magnetized pure ion plasma that is composed of two species with slightly different masses is discussed. We have in mind two isotopes of the same singly ionized atom. Parameters are assumed to be ordered as and , where Ω1 and Ω2 are two cyclotron frequencies, is the relative parallel thermal velocity characterizing collisions between particles of species i and j, and is the classical distance of closest approach for such collisions, and is the characteristic cyclotron radius for particles of species j. Here, μ ij is the reduced mass for the two particles, and and are temperatures that characterize velocity components parallel and perpendicular to the magnetic field. For this ordering, the total cyclotron action for the two species, and are adiabatic invariants that constrain the collisional dynamics. On the timescale of a few collisions, entropy is maximized subject to the constancy of the total Hamiltonian H and the two actions and , yielding a modified Gibbs distribution of the form . Here, the ’s are related to and through . Collisional relaxation to the usual Gibbs distribution, , takes place on two timescales. On a timescale longer than the collisional timescale by a factor of , the two species share action so that α1 and α2 relax to a common value α. On an even longer timescale, longer than the collisional timescale by a factor of the order , the total action ceases to be a good constant of the motion and α relaxes to zero.

Nonlinear Phenomena, Turbulence, Transport

Direct observations of LIH and HIL transitions with the Xpoint reciprocating probe in ASDEX Upgrade
View Description Hide DescriptionA reciprocating Langmuir probe was used to directly measure the behavior of turbulence and flows in the Xpoint region during transitions between low(L) and highconfinement (H) mode in ASDEX Upgrade. The probe traverses the divertor horizontally in 140 ms, typically 2–5 cm below the Xpoint. Toroidal Mach number, density, floating potential , and electron temperature (Te ) are measured. In the regime accessible to the probe ( , lineintegrated core density ), the LH transition features an intermediate phase (Iphase), characterized by limitcycle oscillations at 0.5–3 kHz [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)]. The probe measurements reveal that this pulsing affects both the density and the toroidal Mach number. It is present in both the low(LFS) and highfield sides (HFS) of the scrapeoff layer, while highamplitude broadband turbulence usually dominates the privateflux region. Profile comparisons between Lmode and Iphase show lower density in pulsing regions and small shifts in Te , directed oppositely on LFS and HFS, which are compensated by shifts in to yield a surprisingly unchanged plasma potential profile. Directly observed LIphase transitions reveal that the onset of the pulsing is preceded by a fast 50% density drop in the HFS Xpoint region. Back transitions to Lmode occur essentially symmetrically, with the pulsing stopping first, followed by a fast recovery to Lmode density levels in the divertor.

Nonlinear simplified model to study localization of kinetic Alfvén wave
View Description Hide DescriptionWe have presented the numerical simulation of the coupled equations governing the dynamics of kinetic Alfvén wave (KAW) and ion acoustic wave in the intermediate β plasma, where β is the ratio of thermal pressure to the background magnetic pressure. We have also developed a simplified model for this nonlinear interaction using the results obtained from the simulation to understand the physics of nonlinear evolution of KAW. Localization of magnetic field intensity of KAW has been studied by means of the simplified model.

Perpendicular ion acceleration in whistler turbulence
View Description Hide DescriptionWhistler turbulence is an important contributor to solar wind turbulence dissipation. This turbulence contains obliquely propagating whistler waves at electron scales, and these waves have electrostatic components perpendicular to the mean magnetic field. In this paper, a full kinetic, twodimensional particleincell simulation shows that whistler turbulence can accelerate ions in the direction perpendicular to the mean magnetic field. When the ions pass through waveparticle resonances region in the phase space during their cyclotron motion, the ions are effectively accelerated in the perpendicular direction. The simulation results suggest that whistler turbulence contributes to the perpendicular heating of ions observed in the solar wind.

Three dimensional electromagnetic wavepackets in a plasma: Spatiotemporal modulational instability
View Description Hide DescriptionThe nonlinear interaction of an intense electromagnetic beam with relativistic collisionless unmagnetized plasma is investigated by invoking the reductive perturbation technique, resting on the model of threedimensional nonlinear Schrödinger (NLS) equation with cubic nonlinearity which incorporates the effects of selffocusing, selfphase modulation, and diffraction on wave propagation. Relying on the derived NLS equation, the occurrence of spatiotemporal modulational instability is investigated in detail.

A new fast twocolor interferometer at Alcator CMod for turbulence measurements and comparison with phase contrast imaging
View Description Hide DescriptionAccurately predicting the turbulent transport properties of magnetically confined plasmas is a major challenge of fusion energy research. Validation of transport models is typically done by applying socalled “synthetic diagnostics” to the output of nonlinear gyrokinetic simulations, and the results are compared to experimental data. As part of the validation process, comparing two independent turbulence measurements to each other provides the opportunity to test the synthetic diagnostics themselves; a step which is rarely possible due to limited availability of redundant fluctuation measurements on magnetic confinement experiments. At Alcator CMod, phasecontrast imaging (PCI) is a commonly used turbulence diagnostic. PCI measures lineintegrated electron density fluctuations with high sensitivity and wavenumber resolution ( ). A new fast twocolor interferometry (FTCI) diagnostic on the Alcator CMod tokamak measures longwavelength ( ) lineintegrated electron density fluctuations. Measurements of coherent and broadband fluctuations made by PCI and FTCI are compared here for the first time. Good quantitative agreement is found between the two measurements. This provides experimental validation of the lowwavenumber region of the PCI calibration, and also helps validate the lowwavenumber portions of the synthetic PCI diagnostic that has been used in gyrokinetic model validation work in the past. We discuss possibilities to upgrade FTCI, so that a similar comparison could be done at higher wavenumbers in the future.
