Volume 22, Issue 6, June 2015
Index of content:

We study the basic physical properties of composite nonlinear structure induced by the headon collision of magnetosonic solitons. Solitary waves are assumed to propagate in a quantum electronion magnetoplasma with spin1/2 degenerate electrons. The main interest of the present work is to investigate the time evolution of the merged composite structure during a specific time interval of the wave interaction process. We consider three cases of collidingsituation, namely, compressiverarefactive solitons interaction, compressivecompressive solitons interaction, and rarefactiverarefactive solitons interaction, respectively. Compared with the last two colliding cases, the changing process of the composite structure is more complex for the first situation. Moreover, it is found that they are obviously different for the last two colliding cases.
 LETTERS


The tokamak density limit: A thermoresistive disruption mechanism
View Description Hide DescriptionThe behavior of magnetic islands with 3D electron temperature and the corresponding 3D resistivity effects on growth are examined for islands with nearzero net heating in the island interior. We refer to the resulting class of nonlinearities as thermoresistive effects. In particular, the effects of varying impurity mix on the previously proposed local island onset threshold [Gates and DelgadoAparicio, Phys. Rev. Lett. 108, 165004 (2012)] are examined and shown to be consistent with the well established experimental scalings for tokamaks at the density limit. A surprisingly simple semianalytic theory is developed which imposes the effects of heating/cooling in the island interior as well as the effects of island geometry. For the class of current profiles considered, it is found that a new term that accounts for the thermal effects of island asymmetry is required in the modified Rutherford equation. The resultant model is shown to exhibit a robust onset of a rapidly growing tearing mode—consistent with the disruption mechanism observed at the density limit in tokamaks. A fully nonlinear 3D cylindrical calculation is performed that simulates the effect of net island heating/cooling by raising/suppressing the temperature in the core of the island. In both the analytic theory and the numerical simulation, the sudden threshold for rapid growth is found to be due to an interaction between three distinct thermal nonlinearities which affect the island resistivity, thereby modifying the growth dynamics.

First measurements of Hiro currents in vertical displacement event in tokamaks
View Description Hide DescriptionSpecially designed tiles were setup in the 2012 campaign of the Experimental Advanced Superconducting Tokamak (EAST), to directly measure the toroidal surface currents during the disruptions. Hiro currents with direction opposite to the plasma currents have been observed, confirming the sign prediction by the Wall Touching Vertical Mode (WTVM) theory and numerical simulations. During the initial phase of the disruption, when the plasma begins to touch the wall, the surface currents can be excited by WTVM along the plasma facing tile surface, varying with the mode magnitude. The currents are not observed in the cases when the plasma moves away from the tile surface. This discovery addresses the importance of the plasma motion into the wall in vertical disruptions. WTVM, acting as a current generator, forces the Hiro currents to flow through the gaps between tiles. This effect, being overlooked so far in disruption analysis, may damage the edges of the tiles and is important for the ITER device.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Composite nonlinear structure within the magnetosonic soliton interactions in a spin1/2 degenerate quantum plasma
View Description Hide DescriptionWe study the basic physical properties of composite nonlinear structure induced by the headon collision of magnetosonic solitons. Solitary waves are assumed to propagate in a quantum electronion magnetoplasma with spin1/2 degenerate electrons. The main interest of the present work is to investigate the time evolution of the merged composite structure during a specific time interval of the wave interaction process. We consider three cases of collidingsituation, namely, compressiverarefactive solitons interaction, compressivecompressive solitons interaction, and rarefactiverarefactive solitons interaction, respectively. Compared with the last two colliding cases, the changing process of the composite structure is more complex for the first situation. Moreover, it is found that they are obviously different for the last two colliding cases.

Instability of current sheets with a localized accumulation of magnetic flux
View Description Hide DescriptionThe longstanding problem of whether a current sheet with curved magnetic field lines associated with a small “normal” Bz component is stable is investigated using twodimensional electromagnetic particleincell simulations, employing closed boundary conditions analogous to those normally assumed in energy principle calculations. Energy principle arguments [Sitnov and Schindler, Geophys. Res. Lett. 37, L08102 (2010)] have suggested that an accumulation of magnetic flux at the tailward end of a thin current sheet could produce a tearing instability. Two classes of such current sheet configurations are probed: one with a monotonically increasing Bz profile and the other with a localized Bz “hump.” The former is found to be stable (in 2D) over any reasonable time scale, while the latter is prone to an ideallike instability that shifts the hump peak in the direction of the curvature normal and erodes the field on the opposite side. The growth rate of this instability is smaller by an order of magnitude than previous suggestions of an instability in an open system. An example is given that suggests that such an unstable hump configuration is unlikely to be produced by external driving of a current sheet with no Bz accumulation even in the presence of open boundary conditions.

Precise energy eigenvalues of hydrogenlike ion moving in quantum plasmas
View Description Hide DescriptionThe analytic form of the electrostatic potential felt by a slowly moving test charge in quantum plasma is developed. It has been shown that the electrostatic potential is composed of two parts: the DebyeHuckel screening term and the nearfield wake potential. The latter depends on the velocity of the test charge as well as on the number density of the plasma electrons. RayleighRitz variational calculation has been done to estimate precise energy eigenvalues of hydrogenlike carbon ion under such plasma environment. A detailed analysis shows that the energy levels gradually move to the continuum with increasing plasma electron density while the level crossing phenomenon has been observed with the variation of ion velocity.

Reconnection dynamics with secondary tearing instability in compressible Hall plasmas
View Description Hide DescriptionThe dynamics of a secondary tearing instability is systematically investigated based on compressible Hall magnetohydrodynamic. It is found that in the early nonlinear phase of magnetic reconnection before onset of the secondary tearing instability, the geometry of the magnetic field in the reconnection region tends to form a Ytype structure in a weak Hall regime, instead of an Xtype structure in a strong Hall regime. A new scaling law is found that the maximum reconnection rate in the early nonlinear stage is proportional to the square of the ion inertial length ( ) in the weak Hall regime. In the late nonlinear phase, the thin elongated current sheet associated with the Ytype geometry of the magnetic field breaks up to form a magnetic island due to a secondary tearing instability. After the onset of the secondary tearing mode, the reconnection rate is substantially boosted by the formation of the Xtype geometries of magnetic field in the reconnection regions. With a strong Hall effect, the maximum reconnection rate linearly increases with the increase of the ion inertial length ( ).

Enhanced magnetic reconnection in the presence of pressure gradients
View Description Hide DescriptionMagnetic reconnection in the presence of background pressure gradients is studied, with special attention to parallel (compressional) magnetic fluctuations. A process is reported that reconnects fields through coupling of driftwavetype instabilities with current sheets. Its time scale is set not by the reconnecting field but by inhomogeneities of the background density or temperature. The observed features can be attributed to a pressuregradientdriven linear instability which interacts with the reconnecting system but is fundamentally different from microtearing. In particular, this mode relies on parallel magnetic fluctuations and the associated drift. For turbulent reconnection, similar or even stronger enhancements are reported. In the solar corona, this yields a critical pressure gradient scale length of about 200 km below which this new process becomes dominant over the tearing instability.

Stimulated Brillouin sidescattering of the beat wave excited by two counterpropagating Xmode lasers in magnetized plasma
View Description Hide DescriptionThe stimulated Brillouin scattering (SBS) of nonresonant beat mode in the presence of static magnetic field is investigated in a plasma. Two counterpropagating lasers of frequencies ( and ) and wave vectors ( and ) drive a nonresonant space charge beat mode at the phase matching condition of frequency and wave number . The driver wave parametrically excites a pair of ion acoustic wave and a sideband electromagnetic wave . The beat wave couples with the sideband electromagnetic wave to exert a nonlinear ponderomotive force at the frequency of ion acoustic wave. Density perturbations due to ion acoustic wave and ponderomotive force couple with the oscillatory motion of plasma electron due to velocity of beat wave to give rise to a nonlinear current (by feedback mechanism) responsible for the growth of sideband wave at resonance. The growth rate of SBS was reduced (from to ) by applying a transverse static magnetic field T. The present study can be useful for the excitation of fast plasma waves (for the purpose of electron acceleration) by two counterpropagating laser beams.

Linear dispersion properties of ring velocity distribution functions
View Description Hide DescriptionLinear properties of ring velocity distribution functions are investigated. The dispersion tensor in a form similar to the case of a Maxwellian distribution function, but for a general distribution function separable in velocities, is presented. Analytical forms of the dispersion tensor are derived for two cases of ring velocity distribution functions: one obtained from physical arguments and one for the usual, ad hoc ring distribution. The analytical expressions involve generalized hypergeometric, Kampé de Fériet functions of two arguments. For a set of plasma parameters, the two ring distribution functions are compared. At the parallel propagation with respect to the ambient magnetic field, the two ring distributions give the same results identical to the corresponding biMaxwellian distribution. At oblique propagation, the two ring distributions give similar results only for strong instabilities, whereas for weak growth rates their predictions are significantly different; the two ring distributions have different marginal stability conditions.

Reduction of a collisionalradiative mechanism for argon plasma based on principal component analysis
View Description Hide DescriptionThis article considers the development of reduced chemistry models for argon plasmas using Principal Component Analysis (PCA) based methods. Starting from an electronic specific CollisionalRadiative model, a reduction of the variable set (i.e., mass fractions and temperatures) is proposed by projecting the full set on a reduced basis made up of its principal components. Thus, the flow governing equations are only solved for the principal components. The proposed approach originates from the combustion community, where Manifold Generated Principal Component Analysis (MGPCA) has been developed as a successful reduction technique. Applications consider ionizing shock waves in argon. The results obtained show that the use of the MGPCA technique enables for a substantial reduction of the computational time.

Nonlinear evolution of the electromagnetic electroncyclotron instability in biKappa distributed plasma
View Description Hide DescriptionThis paper presents a numerical study of the linear and nonlinear evolution of the electromagnetic electroncyclotron (EMEC) instability in a biKappa distributed plasma. Distributions with high energy tails described by the Kappa powerlaws are often observed in collisionless plasmas (e.g., solar wind and accelerators), where waveparticle interactions control the plasma thermodynamics and keep the particle distributions out of Maxwellian equilibrium. Under certain conditions, the anisotropic biKappa distribution gives rise to plasma instabilities creating lowfrequency EMEC waves in the whistler branch. The instability saturates nonlinearly by reducing the temperature anisotropy until marginal stability is reached. Numerical simulations of the VlasovMaxwell system of equations show excellent agreement with the growthrate and real frequency of the unstable modes predicted by linear theory. The waveamplitude of the EMEC waves at nonlinear saturation is consistent with magnetic trapping of the electrons.

Computational hydrodynamics and optical performance of inductivelycoupled plasma adaptive lenses
View Description Hide DescriptionThis study addresses the optical performance of a plasma adaptive lens for aerooptical applications by using both axisymmetric and threedimensional numerical simulations. Plasma adaptive lenses are based on the effects of free electrons on the phase velocity of incident light, which, in theory, can be used as a phaseconjugation mechanism. A closed cylindrical chamber filled with Argon plasma is used as a model lens into which a beam of light is launched. The plasma is sustained by applying a radiofrequency electric current through a coil that envelops the chamber. Four different operating conditions, ranging from low to high powers and induction frequencies, are employed in the simulations. The numerical simulations reveal complex hydrodynamic phenomena related to buoyant and electromagnetic laminar transport, which generate, respectively, large recirculating cells and wallnormal compression stresses in the form of local stagnationpoint flows. In the axisymmetric simulations, the plasma motion is coupled with nearwall axial striations in the electrondensity field, some of which propagate in the form of lowfrequency traveling disturbances adjacent to vortical quadrupoles that are reminiscent of TaylorGörtler flow structures in centrifugally unstable flows. Although the refractiveindex fields obtained from axisymmetric simulations lead to smooth beam wavefronts, they are found to be unstable to azimuthal disturbances in three of the four threedimensional cases considered. The azimuthal striations are optically detrimental, since they produce highorder angular aberrations that account for most of the beam wavefront error. A fourth case is computed at high input power and high induction frequency, which displays the best optical properties among all the threedimensional simulations considered. In particular, the increase in induction frequency prevents local thermalization and leads to an axisymmetric distribution of electrons even after introduction of spatial disturbances. The results highlight the importance of accounting for spatial effects in the numerical computations when optical analyses of plasma lenses are pursued in this range of operating conditions.

Positronacoustic shock waves associated with cold viscous positron fluid in superthermal electronpositronion plasmas
View Description Hide DescriptionA theoretical investigation is made on the positronacoustic (PA) shock waves (SHWs) in an unmagnetized electronpositronion plasma containing immobile positive ions, cold mobile positrons, and hot positrons and electrons following the kappa (κ) distribution. The cold positron kinematic viscosity is taken into account, and the reductive perturbation method is used to derive the Burgers equation. It is found that the viscous force acting on cold mobile positron fluid is a source of dissipation and is responsible for the formation of the PA SHWs. It is also observed that the fundamental properties of the PA SHWs are significantly modified by the effects of different parameters associated with superthermal (κ distributed) hot positrons and electrons.

Effect of anisotropic dust pressure and superthermal electrons on propagation and stability of dust acoustic solitary waves
View Description Hide DescriptionEmploying the reductive perturbation technique, Zakharov–Kuznetzov (ZK) equation is derived for dust acoustic (DA) solitary waves in a magnetized plasma which consists the effects of dust anisotropic pressure, arbitrary charged dust particles, Boltzmann distributed ions, and Kappa distributed superthermal electrons. The ZK solitary wave solution is obtained. Using the smallk expansion method, the stability analysis for DA solitary waves is also discussed. The effects of the dust pressure anisotropy and the electron superthermality on the basic characteristics of DA waves as well as on the threedimensional instability criterion are highlighted. It is found that the DA solitary wave is rarefactive (compressive) for negative (positive) dust. In addition, the growth rate of instability increases rapidly as the superthermal spectral index of electrons increases with either positive or negative dust grains. A brief discussion for possible applications is included.

Hydrodynamic and kinetic models for spin1/2 electronpositron quantum plasmas: Annihilation interaction, helicity conservation, and wave dispersion in magnetized plasmas
View Description Hide DescriptionWe discuss the complete theory of spin1/2 electronpositron quantum plasmas, when electrons and positrons move with velocities mach smaller than the speed of light. We derive a set of two fluid quantum hydrodynamic equations consisting of the continuity, Euler, spin (magnetic moment) evolution equations for each species. We explicitly include the Coulomb, spinspin, Darwin and annihilation interactions. The annihilation interaction is the main topic of the paper. We consider the contribution of the annihilation interaction in the quantum hydrodynamic equations and in the spectrum of waves in magnetized electronpositron plasmas. We consider the propagation of waves parallel and perpendicular to an external magnetic field. We also consider the oblique propagation of longitudinal waves. We derive the set of quantum kinetic equations for electronpositron plasmas with the Darwin and annihilation interactions. We apply the kinetic theory to the linear wave behavior in absence of external fields. We calculate the contribution of the Darwin and annihilation interactions in the Landau damping of the Langmuir waves. We should mention that the annihilation interaction does not change number of particles in the system. It does not related to annihilation itself, but it exists as a result of interaction of an electronpositron pair via conversion of the pair into virtual photon. A pair of the nonlinear Schrodinger equations for the electronpositron plasmas including the Darwin and annihilation interactions is derived. Existence of the conserving helicity in electronpositron quantum plasmas of spinning particles with the Darwin and annihilation interactions is demonstrated. We show that the annihilation interaction plays an important role in the quantum electronpositron plasmas giving the contribution of the same magnitude as the spinspin interaction.

Ion closure theory for high collisionality revisited
View Description Hide DescriptionAccording to analytical calculations of the ion collision operator, the ionelectron collision terms could be larger than the ionion collision terms. In the previous work [J.Y. Ji and E. D. Held, Phys. Plasmas 20, 042114 (2013)], the ionelectron collision effects are diminished by the ion temperature change terms introduced from unlikely assumptions. In this work, the highcollisionality closures for ions are calculated without the temperature change terms. The ionelectron collision terms significantly modify existing closure coefficients.

Beat wave excitation of electron plasma wave by relativistic cross focusing of coshGaussian laser beams in plasma
View Description Hide DescriptionA scheme for beat wave excitation of electron plasma wave (EPW) is proposed by relativistic crossfocusing of two coaxial CoshGaussian (ChG) laser beams in an under dense plasma. The plasma wave is generated on account of beating of two coaxial laser beams of frequencies and . The mechanism for laser produced nonlinearity is assumed to be relativistic nonlinearity in electron mass. Following moment theory approach in Wentzel Kramers Brillouin (W.K.B) approximation, the coupled differential equations governing the evolution of spot size of laser beams with distance of propagation have been derived. The relativistic nonlinearity depends not only on the intensity of first laser beam but also on the intensity of second laser beam. Therefore, propagation dynamics of one laser beam affect that of second beam and hence crossfocusing of the two laser beams takes place. Due to non uniform intensity distribution of pump laser beams, the background electron concentration gets modified. The amplitude of EPW, which depends on the background electron concentration, thus gets nonlinearly coupled with the laser beams. The effects of relativistic electron mass nonlinearity and the crossfocusing of pump beams on excitation of EPW have been incorporated. Numerical simulations have been carried out to investigate the effect of laser as well as plasma parameters on crossfocusing of laser beams and further its effect on power of excited EPW.

Effect of dynamical nonneutrality on the modulational instability of laser propagating through hot magnetoplasma
View Description Hide DescriptionIn this work, modulational instability of a laser pulse in a hot magnetized plasma is investigated. Nonlinear relativistic equation, describing the amplitude evolution of a laser with finite longitudinal and transversal structure, is obtained. Taking into account the plasma dynamical nonneutrality caused by the ponderomotive force, the growth rate of the modulational instability is derived. Effect of the pulse length on the instability growth rate is investigated.

Kinetic instability of drift magnetosonic wave in anisotropic low beta plasmas
View Description Hide DescriptionThe kinetic instability of the obliquely propagating drift magnetosonic wave for temperature anisotropic low beta plasmas is studied by using the gyrokinetic model. The interplay between the temperature anisotropy and the density inhomogeneity free energy sources is discussed in order to provide stabilization of drift instability by the temperature anisotropy effect. It is shown that the anisotropy suppresses the growth rate when the anisotropy ratio is greater than unity, whereas it enhances the growth rate for . Comparison of kinetic instability with reactive instability [Naim et al., Phys. Plasmas 21, 102112 (2014)] and the scaling of growth time with the diffusion and the anisotropy relaxation times are presented. Additionally, the stability analysis applicable to a wide range of plasma parameters is also performed.
 Nonlinear Phenomena, Turbulence, Transport

Multispecies density peaking in gyrokinetic turbulence simulations of low collisionality Alcator CMod plasmas
View Description Hide DescriptionPeaked density profiles in lowcollisionality AUG and JET Hmode plasmas are probably caused by a turbulently driven particle pinch, and Alcator CMod experiments confirmed that collisionality is a critical parameter. Density peaking in reactors could produce a number of important effects, some beneficial, such as enhanced fusion power and transport of fuel ions from the edge to the core, while others are undesirable, such as lower beta limits, reduced radiation from the plasma edge, and consequently higher divertor heat loads. Fundamental understanding of the pinch will enable planning to optimize these impacts. We show that density peaking is predicted by nonlinear gyrokinetic turbulence simulations based on measured profile data from low collisionality Hmode plasma in Alcator CMod. Multiple ion species are included to determine whether hydrogenic density peaking has an isotope dependence or is influenced by typical levels of lowZ impurities, and whether impurity density peaking depends on the species. We find that the deuterium density profile is slightly more peaked than that of hydrogen, and that experimentally relevant levels of boron have no appreciable effect on hydrogenic density peaking. The ratio of density at r/a = 0.44 to that at r/a = 0.74 is 1.2 for the majority D and minority H ions (and for electrons), and increases with impurity Z: 1.1 for helium, 1.15 for boron, 1.3 for neon, 1.4 for argon, and 1.5 for molybdenum. The ion temperature profile is varied to match better the predicted heat flux with the experimental transport analysis, but the resulting factor of two change in heat transport has only a weak effect on the predicted density peaking.