Volume 18, Issue 12, December 2011
Index of content:

It was recently proposed that the electronframe dissipation measure, the energy transfer from the electromagnetic field to plasmas in the electron’s rest frame, identifies the dissipation region of collisionless magnetic reconnection [Zenitani et al., Phys. Rev. Lett. 106, 195003 (2011)]. The measure is further applied to the electronscale structures of antiparallel reconnection, by using twodimensional particleincell simulations. The size of the central dissipation region is controlled by the electronion mass ratio, suggesting that electron physics is essential. A narrow electron jet extends along the outflow direction until it reaches an electron shock. The jet region appears to be antidissipative. At the shock, electron heating is relevant to a magnetic cavity signature. The results are summarized to a unified picture of the single dissipation region in a Hall magnetic geometry.
 LETTERS


Measurements and simulations of shock wave generated plasmavacuum interface
View Description Hide DescriptionA controlled gradient gas jet was designed, constructed, and tested at the Naval Research Laboratory for the generation of high density and sharp gradient plasma regions. The gas jet uses a lasergenerated shock wave to control the density gradient at the vacuum and neutral gas interface. The length scale of the laser produced plasma density gradient is fully controlled by the strength of the shock wave and can be varied continuously from100 μm for a weak shock to under 20 μm in case of strong shock wave as verified by the experimental results and simulations.

Bouncefree spherical hydrodynamic implosion
View Description Hide DescriptionIn a bouncefree spherical hydrodynamic implosion, the poststagnation hot core plasma does not expand against the imploding flow. Such an implosion scheme has the advantage of improving the dwell time of the burning fuel, resulting in a higher fusion burnup fraction. The existence of bouncefree spherical implosions is demonstrated by explicitly constructing a family of selfsimilar solutions to the spherically symmetric ideal hydrodynamic equations. When applied to a specific example of plasma liner driven magnetoinertial fusion, the bouncefree solution is found to produce at least a factor of four improvement in dwell time and fusion energy gain.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Rotatingfilamentspairs in a hexagonal superlattice state in dielectric barrier discharge
View Description Hide DescriptionRotatingfilamentspairs in a hexagonal superlattice state (HSS) are studied in a dielectric barrier discharge system. The evolution and phase diagrams of HSS are given. The wavelength of HSS and the mean diameter of the two rotating filaments all decrease with the increase of applied voltage. The instantaneous orientations of rotatingfilamentspairs are equal probability approximately. There is a larger peak and a smaller one in both the probability density functions of the rotation speed (ω) of rotating filaments and that of the distance (D) between two rotating filaments. According to the fitting curves of lnω ^{2} vs. lnD, ω ^{2} is inversely proportional to D ^{7}. The rotation of filaments is discussed theoretically by the force among surface charges.

Investigation of tearing instability using GeFi particle simulation model
View Description Hide DescriptionThe gyrokinetic (GK) electron and fully kinetic ion (GeFi) simulation model of Lin et al. [Plasmas Phys. Controlled Fusion 53, 054013 (2011)] has been thoroughly benchmarked and validated for a twodimensional (2D) Harris current sheet with a finite guide field. First, a gyrokineticeigenmodetheory for the collisionless tearing mode in the small Larmor radius limit is presented. The linear eigenmode structure and growth rate of the tearing mode obtained from the GeFi simulation are benchmarked against those from the GK eigenmode analysis in the limit of , where L is the current sheet halfwidth, ρ_{i} is ion Larmor radius, and ρ_{e} is electron Larmor radius. Second, to valid the GeFi model, both the linear and nonlinear tearing instabilities obtained from the GeFi simulations are compared with the Darwin particleincell(PIC) simulation. The validation of the GeFi model for laboratory and space plasmas is also discussed. Meanwhile, the GeFi simulation is carried out to investigate both the linear and nonlinear tearing instabilities for cases with a broad range of L and guide magnetic fieldB_{G} . It is found that in a wide current sheet with L > 4.5ρ_{eK} , the nonlinear saturation level of the island halfwidth is w_{s} ≃ 3ρ_{eK} , where ρ_{eK} = ρ_{e}B _{0}/B_{x} _{0}, B _{0} is the strength of the asymptotic magnetic field, and B_{x} _{0} is the antiparallel field. On the other hand, in a thin current sheet with L < 2.5ρ_{eK} , w_{s} ≃ 2.2 L. In addition, a high frequency electrostatic drift mode is found to coexist with the tearing mode.

Nonlinear stability of the ideal magnetohydrodynamic interchange mode at marginal conditions in a transverse magnetic field
View Description Hide DescriptionThe stability of the ideal magnetohydrodynamic(MHD) interchange mode at marginal conditions is studied. A sufficiently strong constant magnetic field component transverse to the direction of mode symmetry provides the marginality conditions. A systematic perturbation analysis in the smallness parameter, b _{2}/B_{c} ^{1/2}, is carried out, where B_{c} is the critical transverse magnetic field for the zerofrequency ideal mode and b _{2} is the deviation from B_{c} . The calculation is carried out to third order including nonlinear terms. It is shown that the system is nonlinearly unstable in the short wavelength limit, i.e., a large enough perturbation results in instability even if b _{2}/B_{c} > 0 (linearly stable). The normalized amplitude for instability is shown to scale as b _{2}/B_{c} ^{1/2}. A nonlinear, compressible, MHD simulation is done to check the analytic result. Good agreement is found, including the critical amplitude scaling.

The dynamics of ion with electrostatic waves in a sheared magnetic field
View Description Hide DescriptionThe interaction between an ion and multiple electrostatic waves propagating perpendicularly to an ambient magnetic field with shear is investigated. Based on the Lie transformation method, with the wave amplitude and the magnetic shear both as the perturbation parameters, the analytical formulas for the reduced Hamiltonian is derived and results are compared with numerical calculations of the complete equations of motion for the case of two onresonance waves and the case of two offresonance waves, respectively. It is found that the effect of magnetic shear drastically prevents the acceleration of an ion in both cases. This result will help us to understand the behaviors of ions in a magnetic sheared device, such as tokamak.

Currentfree double layers: A review
View Description Hide DescriptionDuring the last decade, there has been an upsurge in the research on currentfree DLs (CFDLs). Research includes theory, laboratory measurements, and various applications of CFDLs ranging from plasma thrusters to acceleration of charged particles in space and astrophysical plasmas. The purpose of this review is to present a unified understanding of the basic plasma processes, which lead to the formation of CFDLs. The review starts with the discussion on early research on electric fields and double layers (DLs) and ion acceleration in planar plasma expansion. The review continues with the formation of DLs and rarefaction shocks (RFS) in expanding plasma with two electron populations with different temperatures. The basic theory mitigating the formation of a CFDL by twoelectron temperature population is reviewed; we refer to such CFDLs as double layers structures formation by twotemperature electron populations (TETCFDLs). Application of TETCFDLS to ion acceleration in laboratory and space plasmas was discussed including the formation of stationary steadystate DLs. A quite different type of CFDLs forms in a helicon plasma device (HPD), in which plasma abruptly expands from a narrow plasma source tube into a wide diffusion tube with abruptly diverging magnetic fields. The formation mechanism of the CFDL in HPD, referred here as current free double layer structure in helicon plasma device (HPDCFDL), and its applications are reviewed. The formation of a TETCFDL is due to the selfconsistent separation of the two electron populations parallel to the ambient magnetic field. In contrast, a HPDCFDL forms due to selfconsistent separation of electrons and ion perpendicular to the abruptly diverging magnetic field in conjunction with the conducting wall of the expansion chamber in the HPD. Onedimensional theoretical models of CFDLs based on steadystate solution of VlasovPoisson system of equations are briefly discussed. Applications of CFDLs ranging from helicon doublelayer thrusters (HDLTs) to the accelerations of ions in space and astrophysical plasmas are summarized.

A new mode and its interaction through ponderomotive force in electronpositronion plasmas
View Description Hide DescriptionA new mode is found in epi plasma in the presence of density and temperature difference of lighter particles. The electron beam induced Cherenkov instability condition for the excitation of positron sound wave is obtained for the system under consideration. Zakharov’s equation with sign modification due to negative Ponderomotive pressure is obtained. Nonlinear Schrödinger wave equation for the envelope type solitary waves is derived. Both stationary and nonstationary solutions are found and the subsonic and supersonic limits are also discussed. In the stationary case, rarefactive type solitary solution is obtained, whereas the nonstationary case yields the ion acoustic shock like structure solution which is very interesting. The importance of the study with relevance to both laboratory and astrophysical plasmas is pointed out.

Decays of electron Bernstein waves near plasma edge
View Description Hide DescriptionNonlinear wavewave couplings near the upper hybrid resonance are studied via particleincell simulations. It is found that the decay of an electron Bernstein wave(EBW) depends on the ratio of the incident frequency and electron cyclotron frequency. For ratios less than two, parametric decay into a lower hybrid wave (or an ion Bernstein wave) and EBWs at a lower frequency is observed. For ratios larger than two, the daughter waves could be an electron cyclotron quasimode and another EBW or an ion wave and EBW. For sufficiently high incident power, the former process may dominate. Because of the electron cyclotron quasimode, electrons can be strongly heated by nonlinear Landau damping. As a result, the bulk of the incident power can be absorbed near plasma edge at high power. The increase in number of decay channels with frequency implies that the allowable power into the plasma must decrease with frequency.

The inner structure of collisionless magnetic reconnection: The electronframe dissipation measure and Hall fields
View Description Hide DescriptionIt was recently proposed that the electronframe dissipation measure, the energy transfer from the electromagnetic field to plasmas in the electron’s rest frame, identifies the dissipation region of collisionless magnetic reconnection [Zenitani et al., Phys. Rev. Lett. 106, 195003 (2011)]. The measure is further applied to the electronscale structures of antiparallel reconnection, by using twodimensional particleincell simulations. The size of the central dissipation region is controlled by the electronion mass ratio, suggesting that electron physics is essential. A narrow electron jet extends along the outflow direction until it reaches an electron shock. The jet region appears to be antidissipative. At the shock, electron heating is relevant to a magnetic cavity signature. The results are summarized to a unified picture of the single dissipation region in a Hall magnetic geometry.

Vlasov simulation in multiple spatial dimensions
View Description Hide DescriptionA longstanding challenge encountered in modeling plasma dynamics is achieving practical Vlasov equation simulation in multiple spatial dimensions over large length and time scales. While direct multidimension Vlasov simulation methods using adaptive mesh methods [M. Gutnic et al., Comput. Phys. Commun. 164, 214 (2004)] have recently shown promising results in two dimensions (2D) [J. W. Banks et al., Phys. Plasmas 18, 052102 (2011); B. I. Cohen et al., November 10, 2010, http://meetings.aps.org/link/BAPS.2010.DPP.NP9.142], in this paper, we present an alternative, the Vlasov multi dimensional (VMD) model, that is specifically designed to take advantage of solution properties in regimes when plasma waves are confined to a narrow cone, as may be the case for stimulated Raman scatter in large optic f# laser beams. Perpendicular grid spacing large compared to a Debye length is then possible without instability or loss of accuracy, enabling an order 10 decrease in required computational resources compared to standard particle in cell(PIC) methods in 2D, with another reduction of that order in 3D. Further advantage compared to PIC methods accrues in regimes where particle noise is an issue. VMD and PIC results in a 2D model of localized Langmuir waves are in qualitative agreement.

Eigenmodes of quasistatic magnetic islands in current sheet
View Description Hide DescriptionAs observation have shown, magnetic islands often appear before and/or after the onset of magnetic reconnections in the current sheets, and they also appear in the current sheets in the solar corona, Earth’s magnetotail, and Earth’s magnetopause. Thus, the existence of magnetic islands can affect the initial conditions in magnetic reconnection. In this paper, we propose a model of quasistatic magnetic islandeigenmodes in the current sheet. This model analytically describes the magnetic field structures in the quasistatic case, which will provide a possible approach to reconstructing the magnetic structures in the current sheet via observation data. This model is selfconsistent in the kinetic theory. Also, the distribution function of charged particles in the magnetic island can be calculated.

Nonlinear oscillations and waves in an arbitrary mass ratio cold plasma
View Description Hide DescriptionIt is well known that nonlinear standing oscillations in an arbitrary mass ratio cold plasma always phase mix away. However, there exist nonlinear electronion traveling wavesolutions, which do not exhibit phase mixing because they have zero ponderomotive force. The existence of these waves has been demonstrated using a perturbation method. Moreover, it is shown that cold plasma BGK waves [Albritton et al., Nucl. Fusion 15, 1199 (1975)] phase mix away if ions are allowed to move and the scaling of phase mixing is found to be different from earlier work [Sengupta et al., Phys. Rev. Lett. 82, 1867 (1999)]. Phase mixing of these waves has been further verified in 1D particle in cell simulation.

Large amplitude solitary waves in ionbeam plasmas with charged dust impurities
View Description Hide DescriptionThe nonlinear propagation of large amplitude dust ionacoustic (DIA) solitary waves (SWs) in an ionbeamplasma with stationary charged dusts is investigated. For typical plasma parameters relevant for experiments [Y. Nakamura and K. Komatsuda, J. Plasma Phys. 60, 69 (1998)], when the beam speed is larger than the DIA speed ( _{ b } _{0} ≳ 1.7c _{ s }), three stable waves, namely, the “fast” and “slow” ionbeam modes and the plasma DIA wave, are shown to exist. These modes can propagate as SWs in the beamplasmas. However, in the other regime (c _{ s } < _{ b } _{0} < 1.7c _{ s }), one of the beam modes when coupled to the DIA mode may become unstable. The SWs with positive (negative) potential may exist when the difference of the nonlinear wave speed (M) and the beam speed is such that 1.2 ≲ M − _{ b } _{0} ≲ 1.6 (M − _{ b } _{0} ≳ 1.6). Furthermore, for real density perturbations, the wave potential ( > 0) is found to be limited by a critical value which typically depends on M, _{ b } _{0} as well as the ion/beam temperature. The conditions for the existence of DIA solitons are obtained, and their properties are analyzed numerically in terms of the system parameters. While the system supports both the compressive and rarefactive large amplitude SWs, the small amplitude solitons exist only of the compressive type. The theoretical results may be useful for observation of soliton excitations in laboratory ionbeam driven plasmas as well as in space plasmas where the charged dusts play as impurities.
 Nonlinear Phenomena, Turbulence, Transport

Simulating gyrokinetic microinstabilities in stellarator geometry with GS2
View Description Hide DescriptionThe nonlinear gyrokinetic code GS2 has been extended to treat nonaxisymmetric stellarator geometry. Electromagnetic perturbations and multiple trapped particle regions are allowed. Here, linear, collisionless, electrostatic simulations of the quasiaxisymmetric, threefield period national compact stellaratorexperiment (NCSX) design QAS3C82 have been successfully benchmarked against the eigenvalue code FULL. Quantitatively, the linear stability calculations of GS2 and FULL agree to within ∼10%.

KadomtsevPetviashvili solitons propagation in a plasma system with superthermal and weakly relativistic effects
View Description Hide DescriptionTwo dimensional (2D) solitons are studied in a plasma system comprising of relativistically streaming ions, kappa distributed electrons, and positrons. KadomtsevPetviashvili (KP) equation is derived through the reductive perturbation technique. Analytical solution of the KP equation has been studied numerically and graphically. It is noticed that kappa parameters of electrons and positrons as well as the ions relativistic streaming factor have an emphatic influence on the structural as well as propagation characteristics of two dimensional solitons in the considered plasma system. Our results may be helpful in the understanding of soliton propagation in astrophysical and laboratory plasmas, specifically the interaction of pulsar relativistic wind with supernova ejecta and the transfer of energy to plasma by intense electric field of laser beams producing highly energetic superthermal and relativistic particles [L. Arons, Astrophys. Space Sci. Lib. 357, 373 (2009); P. Blasi and E. Amato, Astrophys. Space Sci. Proc. 2011, 623; and A. Shah and R. Saeed, Plasma Phys. Controlled Fusion 53, 095006 (2011)].

Asymptotic equilibrium between Langmuir turbulence and suprathermal electrons
View Description Hide DescriptionIn both laboratory and natural environment such as the solar wind, suprathermal, or nonMaxwellian electron distributions are frequently observed. Electron velocity distribution functions containing nonMaxwellian, powerlaw energetic tail component are often modeled by the socalled kappa distribution, but their physical origin is not clearly understood. In a series of publications, the present author and his colleagues discussed the selfconsistent formation of kappalike distributions as a result of electronLangmuir turbulence interaction process. However, these discussions were either based upon numerical initial value solution of the weak turbulenceequation or by direct particleincell simulation method. It was not evident that the formation of kappalike state, which was demonstrated during the longtime evolution of the system, did indeed correspond to the genuine asymptotically steadystate solution or not in a mathematical sense. The present paper presents the selfconsistent asymptotic solution of the electronsLangmuir turbulence system and shows that the nonMaxwellian kappalike state does indeed correspond to a rigorous solution.

Crosscorrelation based time delay estimation for turbulent flow velocity measurements: Statistical considerations
View Description Hide DescriptionTime delay estimation methods (TDE) are wellknown techniques to investigate poloidal flows in hot magnetized plasmas through the propagation properties of turbulent structures in the medium. One of these methods is based on the estimation of the time lag at which the crosscorrelation function (CCF) estimation reaches its maximum value. The uncertainty of the peak location refers to the smallest determinable flowvelocity modulation, and therefore the standard deviation of the time delay imposes important limitation to the measurements. In this article, the relative standard deviation of the CCF estimation and the standard deviation of its peak location are calculated analytically using a simple model of turbulent signals. This model assumes independent (non interacting) overlapping events (coherent structures) with randomly distributed spatiotemporal origins moving with background flow. The result of our calculations is the derivation of a general formula for the CCF variance, which is valid not exclusively in the high event density limit, but also for arbitrary event densities. Our formula reproduces the well known expression for high event densities previously published in the literature. In this paper we also present a derivation of the variance of time delay estimation that turns out to be inversely proportional to the applied time window. The derived formulas were tested in real plasma measurements. The calculations are an extension of the earlier work of Bencze and Zoletnik [Phys. Plasmas12, 052323 (2005)] where the autocorrelationwidth technique was developed. Additionally, we show that velocities calculated by a TDE method possess a broadband noise which originates from this variance, its power spectral density cannot be decreased by worsening the time resolution and can be coherent with noises of other velocity measurements where the same turbulent structures are used. This noise should not be confused with the impact of zero mean frequency zonal flow modulations and can be the explanation for the TEXTORvelocity spectra measured by beam emission spectroscopy.

On relaxation and transport in gyrokinetic drift wave turbulence with zonal flow
View Description Hide DescriptionWe present a theory for relaxation and transport in phase space for gyrokinetic drift wave turbulence with zonal flow. The interaction between phase space eddys and zonal flows is considered in two different limits, namely for and K ≃ 1 where K is the Kubo number. For , the growth of an isolated coherent phase space structure is calculated, including the associated zonal flow dynamics. For K ≃ 1, mean field relaxation dynamics is considered in the presence of phase space granulations and zonal flows. In both limits, it is shown that the evolution equations for phase space structures are structurally similar to a corresponding CharneyDrazin theorem for zonal momentum balance in a potential vorticity conserving, quasigeostrophic system. The transport flux in phase space is calculated in the presence of phase space density granulations and zonal flows. The zonal flow exerts a dynamical friction on ion phase space density evolution, which is a fundamentally new zonal flow effect.

Electrostatic solitary structures in presence of nonthermal electrons and a warm electron beam on the auroral field lines
View Description Hide DescriptionElectrostatic solitary waves (ESWs) have been observed by satellites in the auroral region of the Earth’s magnetosphere. These ESWs are found to be having both positive and negative electrostatic potentials. Using the Sagdeeev psuedopotential technique, arbitrary amplitude electronacoustic solitary waves/double layers are studied in an unmagnetized plasma consisting of nonthermally distributed hot electrons, fluid cold electrons, a warm electron beam, and ions. The inertia of the warm electrons, and not the beam speed, is essential for the existence of positive potential solitary structures. Existence domains for positive as well as negative potential electrostatic solitons/double layers are obtained. For the typical auroral region parameters, the electric field amplitude of the negative potential solitons is found to be in the range ∼(3–30) mV/m and ∼(5–80) mV/m for the positive potential solitons. For the negative potential solitons/double layers, the amplitudes are higher when their widths are smaller. On the other hand, the amplitude of the positive potential structures increase with their widths.