Volume 21, Issue 1, January 2014

The transport of impurities by a sawtooth crash is simulated with the XTOR2F code. Impurities are modeled as passive scalars, evolving in the compressible MHD flow inferred from the main MHD plasma. For a peaked impurity density profile, the nonlinear kink flow of the sawtooth crash redistributes the profile efficiently and most of the particles in the peak inside the q = 1 surface are expelled. For an initially hollow impurity density profile, the crash leads to a significant penetration up to the magnetic axis. The results are compared with Kadomtsev's model. Despite essentially different mechanisms, the evolution of the particle content inside the q = 1 surface for Kadomtsev's model and for the nonlinear case are virtually identical for the peaked profile, while the model slightly overestimates penetration for the hollow case.
 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

The effect of magnetic field on bistability in 1D photonic crystal doped by magnetized plasma and coupled nonlinear defects
View Description Hide DescriptionIn this work, we study the defect mode and bistability behavior of 1D photonic band gap structure with magnetized plasma and coupled nonlinear defects. The transfer matrix method has been employed to investigate the magnetic field effect on defect mode frequency and bistability threshold. The obtained results show that the frequency of defect mode and bistability threshold can be altered, without changing the structure of the photonic multilayer. Therefore, the bistability behavior of the subjected structure in the presence of magnetized plasma can be utilized in manufacturing wide frequency range devices.

Study of nonlinear electronacoustic solitary and shock waves in a dissipative, nonplanar space plasma with superthermal hot electrons
View Description Hide DescriptionWith the consideration of the superthermal electron distribution, we present a theoretical investigation about the nonlinear propagation of electronacoustic solitary and shock waves in a dissipative, nonplanar nonMaxwellian plasma comprised of cold electrons, superthermal hot electrons, and stationary ions. The reductive perturbation technique is used to obtain a modified Kortewegde Vries Burgers equation for nonlinear waves in this plasma. We discuss the effects of various plasma parameters on the time evolution of nonplanar solitary waves, the profile of shock waves, and the nonlinear structure induced by the collision between planar solitary waves. It is found that these parameters have significant effects on the properties of nonlinear waves and collisioninduced nonlinear structure.

Current sheets and pressure anisotropy in the reconnection exhaust
View Description Hide DescriptionA particleincell simulation shows that the exhaust during antiparallel reconnection in the collisionless regime contains a current sheet extending 100 inertial lengths from the X line. The current sheet is supported by electron pressure anisotropy near the X line and ion anisotropy farther downstream. Fieldaligned electron currents flowing outside the magnetic separatrices feed the exhaust current sheet and generate the outofplane, or Hall, magnetic field. Existing models based on different mechanisms for each particle species provide good estimates for the levels of pressure anisotropy. The ion anisotropy, which is strong enough to reach the firehose instability threshold, is also important for overall force balance. It reduces the outflow speed of the plasma.

Magnetohydrodynamics simulation study of deflagration mode in coaxial plasma accelerators
View Description Hide DescriptionExperimental studies by Poehlmann et al. [Phys. Plasmas 17(12), 123508 (2010)] on a coaxial electrode magnetohydrodynamic (MHD) plasma accelerator have revealed two modes of operation. A deflagration or stationary mode is observed for lower power settings, while higher input power leads to a detonation or snowplow mode. A numerical modeling study of a coaxial plasma accelerator using the nonideal MHD equations is presented. The effect of plasma conductivity on the axial distribution of radial current is studied and found to agree well with experiments. Lower conductivities lead to the formation of a high current density, stationary region close to the inlet/breech, which is a characteristic of the deflagration mode, while a propagating current sheet like feature is observed at higher conductivities, similar to the detonation mode. Results confirm that plasma resistivity, which determines magnetic field diffusion effects, is fundamentally responsible for the two modes.

Doublyexcited ^{1,3}D^{e} resonance states of twoelectron positive ions Li^{+} and Be^{2+} in Debye plasmas
View Description Hide DescriptionWe investigate the bound ^{1,3}D states and the doublyexcited ^{1,3}D^{e} resonance states of twoelectron positive ions Li^{+} and Be^{2+} by employing correlated exponential wave functions. In the framework of the stabilization method, we are able to extract three series (2pnp, 2snd, 2pnf) of ^{1}D^{e} resonances and two series (2pnp, 2snd) of ^{3}D^{e} resonances below the N = 2 threshold. The ^{1,3}D^{e} resonance parameters (resonance energies and widths) for Li^{+} and Be^{2+} along with the boundexcited 1s3d ^{1,3}D state energies are reported for the first time as functions of the screening parameter. Accurate resonance energies and widths are also reported for Li^{+} and Be^{2+} in vacuum. For freeatomic cases, comparisons are made with the reported results and few resonance states are reported for the first time.

Linear and nonlinear physics of the magnetoacoustic cyclotron instability of fusionborn ions in relation to ion cyclotron emission
View Description Hide DescriptionThe magnetoacoustic cyclotron instability (MCI) probably underlies observations of ion cyclotron emission (ICE) from energetic ion populations in tokamak plasmas, including fusionborn alphaparticles in JET and TFTR [Dendy et al., Nucl. Fusion 35, 1733 (1995)]. ICE is a potential diagnostic for lost alphaparticles in ITER; furthermore, the MCI is representative of a class of collective instabilities, which may result in the partial channelling of the free energy of energetic ions into radiation, and away from collisional heating of the plasma. Deep understanding of the MCI is thus of substantial practical interest for fusion, and the hybrid approximation for the plasma, where ions are treated as particles and electrons as a neutralising massless fluid, offers an attractive way forward. The hybrid simulations presented here access MCI physics that arises on timescales longer than can be addressed by fully kinetic particleincell simulations and by analytical linear theory, which the present simulations largely corroborate. Our results go further than previous studies by entering into the nonlinear stage of the MCI, which shows novel features. These include stronger drive at low cyclotron harmonics, the reenergisation of the alphaparticle population, selfmodulation of the phase shift between the electrostatic and electromagnetic components, and coupling between low and high frequency modes of the excited electromagnetic field.

Generation of electromagnetic waves in the very low frequency band by velocity gradient
View Description Hide DescriptionIt is shown that a magnetized plasma layer with a velocity gradient in the flow perpendicular to the ambient magnetic field is unstable to waves in the Very Low Frequency band that spans the ion and electron gyrofrequencies. The waves are formally electromagnetic. However, depending on wave vector (normalized by the electron skin depth) and the obliqueness, , where are wave vectors perpendicular and parallel to the magnetic field, the waves are closer to electrostatic in nature when and and electromagnetic otherwise. Inhomogeneous transverse flows are generated in plasma that contains a static electric field perpendicular to the magnetic field, a configuration that may naturally arise in the boundary layer between plasmas of different characteristics.

Stabilization of beamweibel instability by equilibrium density ripples
View Description Hide DescriptionIn this paper, we present an approach to achieve suppression/complete stabilization of the transverse electromagnetic beam Weibel instability in counter streaming electron beams by modifying the background plasma with an equilibrium density ripple, shorter than the skin depth; this weakening is more pronounced when thermal effects are included. On the basis of a linear two stream fluid model, it is shown that the growth rate of transverse electromagnetic instabilities can be reduced to zero value provided certain threshold values for ripple parameters are exceeded. We point out the relevance of the work to recent experimental investigations on sustained (long length) collimation of fast electron beams and integral beam transport for laser induced fast ignition schemes, where beam divergence is suppressed with the assistance of carbon nanotubes.

Experimental observation of 3D, impulsive reconnection events in a laboratory plasma
View Description Hide DescriptionFast, impulsive reconnection is commonly observed in laboratory, space, and astrophysical plasmas. In this work, impulsive, local, 3D reconnection is identified for the first time in a laboratory current sheet. The twofluid, impulsive reconnection events observed on the Magnetic Reconnection Experiment (MRX) [Yamada et al., Phys Plasmas 4, 1936 (1997)] cannot be explained by 2D models and are therefore fundamentally threedimensional. Several signatures of flux ropes are identified with these events; 3D high current density regions with Opoint structure form during a slow buildup period that precedes a fast disruption of the reconnecting current layer. The observed drop in the reconnection current and spike in the reconnection rate during the disruption are due to ejection of these flux ropes from the layer. Underscoring the 3D nature of the events, strong outofplane gradients in both the density and reconnecting magnetic field are found to play a key role in this process. Electromagnetic fluctuations in the lower hybrid frequency range are observed to peak at the disruption time; however, they are not the key physics responsible for the impulsive phenomena observed. Important features of the disruption dynamics cannot be explained by an anomalous resistivity model. An important discrepancy in the layer width and force balance between the collisionless regime of MRX and kinetic simulations is also revisited. The wider layers observed in MRX may be due to the formation of flux ropes with a wide range of sizes; consistent with this hypothesis, flux rope signatures are observed down to the smallest scales resolved by the diagnostics. Finally, a 3D twofluid model is proposed to explain how the observed outofplane variation may lead to a localized region of enhanced reconnection that spreads in the direction of the outofplane electron flow, ejecting flux ropes from the layer in a 3D manner.

On the Alfvén wave cutoff in partly ionized collisional plasmas
View Description Hide DescriptionThe cutoff of the Alfvén wave, caused by plasma collisions with neutrals in multicomponent partially ionized plasmas, is discussed. Full multicomponent theory is used, and similarities and differences regarding the classic magnetohydrodynamic theory are presented. It is shown that the cutoff in partially ionized plasma, in principle, may remain the same as predicted in classic magnetohydrodynamic works, although multicomponent theory also yields some essential differences. Due to electric field, the ion motion is intrinsically twodimensional and this results in additional forced oscillations of neutrals. One new small parameter, containing the ion inertial length, appears in the multicomponent theory. This new small parameter is missing in the magnetohydrodynamic description, and it turns out that for some parameters it may be greater than the ionstoneutrals density ratio which is the only small parameter in the magnetohydrodynamic description. Due to this the Alfvén wave behavior can become much different as compared to classic magnetohydrodynamic results. It is shown also that in plasmas with unmagnetized ions, Alfvén waves cannot be excited. This by all means applies to the solar photosphere where the ion collision frequency may be far above the ion gyrofrequency.

Trapping of electrons in troughs of self generated electromagnetic standing waves in a bounded plasma column
View Description Hide DescriptionObservations and measurements are reported on electron trapping in troughs of selfgenerated electromagnetic standing waves in a bounded plasma column confined in a minimumB field. The boundaries are smaller than the free space wavelength of the waves. Earlier work of researchers primarily focused upon electron localization effects induced by purely electrostatic perturbation. We demonstrate the possibility in the presence of electromagnetic standing waves generated in the bounded plasma column. The electron trapping is verified with electrostatic measurements of the plasma floating potential, electromagnetic measurements of the wave field profile, and optical intensity measurements of Argon ionic line at 488 nm. The experimental results show a reasonably good agreement with predictions of a Monte Carlo simulation code that takes into account all kinematical and dynamical effects in the plasma in the presence of bounded waves and external fields.

Spatiotemporal characteristics of Trichel pulse at low pressure
View Description Hide DescriptionTrichel pulses are investigated using a needletoplane electrode geometry at low pressure. The evolution of current and voltage, the spatiotemporal discharge images of Trichel pulse are measured. The rising time and duration time in a pulse are about 10 μs and several tens of microseconds, respectively. One period of pulse can be divided into three stages: the stage preceding cathode breakdown, cathode glow formation, and discharge decaying process. Besides a cathode glow and a dark space, an anode glow is also observed. The emission spectra mainly originate from the C^{3}Пu → B^{3}Пg transition for nitrogen. In addition, the capacitances in parallel connected with the discharge cell have important influence on the pulsing frequency.

Influence of gap spacing on the characteristics of Trichel pulse generated in pointtoplane discharge gaps
View Description Hide DescriptionIn this paper, the specific characteristics of the Trichel pulse generated in wide pointtoplane discharge gaps are investigated and compared with those of the currents generated in narrow gaps. A set of empirical formulas are derived to describe the specific characteristics. The influence of the gap spacing both on the current characteristics and on the coefficients of the formulas is studied. Based on the experiment results, an improvement is made to the space charge calculation method proposed by Lama and Gallo [J. Appl. Phys. 45, 103–113 (1974)] and the calculation results are compared to the ones obtained with Lama and Gallo's original method. With the influence of the space charge considered, the modified method obtains more accurate results of the space charge accumulating in the gap and gives a more precise description of the motion of the space charge in the gap. Based on the calculation results, the influence of the space charge on the distribution of the electric field is examined and the influence of the gap spacing on the current characteristics is also studied.

Nonlinearly driven harmonics of Alfvén modes
View Description Hide DescriptionIn order to study the leading order nonlinear magnetohydrodynamic (MHD) harmonic response of a plasma in realistic geometry, the AEGIS code has been generalized to account for inhomogeneous source terms. These source terms are expressed in terms of the quadratic corrections that depend on the functional form of a linear MHD eigenmode, such as the Toroidal Alfvén Eigenmode. The solution of the resultant equation gives the second order harmonic response. Preliminary results are presented here.
 Nonlinear Phenomena, Turbulence, Transport

Steadystate modification of the electron density profile in the resonance region of moving plasma subjected to an alternating electric field
View Description Hide DescriptionSelfconsistent electron density profiles of the plasma stream interacting with the electric field of an ultrashort laser pulse are calculated in the framework of the onedimensional steadystate model with nonlocal polarizability. These profiles are settled down against the background of a given (linear) ion density distribution as a result of the equilibrium between the averaged ponderomotive force, the static spacecharge field, and the electron pressure. The average electron density and the longitudinal (parallel to the density gradient) ac electric field are found to be greatly spacemodulated in the region downstream of the plasma resonance point.

Momentum transport in the vicinity of q_{min} in reverse shear tokamaks due to ion temperature gradient turbulence
View Description Hide DescriptionWe present an analytic study of momentum transport of tokamak plasmas in the vicinity of minimum safety factor (q) position in reversed magnetic shear configuration. Slab ion temperature gradient modes with an equilibrium flow profile are considered in this study. Quasilinear calculations of momentum flux clearly show the novel effects of qcurvature on the generation of intrinsic rotation and mean poloidal flow without invoking reflectional symmetry breaking of parallel wavenumber ( ). This qcurvature effect originates from the inherent asymmetry in populations with respect to a rational surface due to the quadratic proportionality of when qcurvature is taken into account. Discussions are made of possible implications of qcurvature induced plasma flows on internal transport barrier formation in reversed shear tokamaks.

Dissipative solitons in pairion plasmas
View Description Hide DescriptionThe effects of ionneutral collisions on the dynamics of the nonlinear ion acoustic wave in pairion plasma are investigated. The standard perturbative approach leads to a Kortewegde Vries equation with a linear damping term for the dynamics of the finite amplitude wave. The ionneutral collision induced dissipation is responsible for the linear damping. The analytical solution and numerical simulation reveal that the nonlinear wave propagates in the form of a weakly dissipative compressive solitons. Furthermore, the width of the soliton is proportional to the amplitude of the wave for fixed soliton velocity. Results are discussed in the context of the fullerene pairion plasma experiment.

Dynamic behavior of the quantum ZakharovKuznetsov equations in dense quantum magnetoplasmas
View Description Hide DescriptionQuantum ZakharovKuznetsov (qZK) equation is found in a dense quantum magnetoplasma. Via the spectral analysis, we investigate the Hamiltonian and periodicity of the qZK equation. Using the Hirota method, we obtain the bilinear forms and Nsoliton solutions. Asymptotic analysis on the twosoliton solutions shows that the soliton interaction is elastic. Figures are plotted to reveal the propagation characteristics and interaction between the two solitons. We find that the one soliton has a single peak and its amplitude is positively related to He , while the two solitons are parallel when He < 2, otherwise, the one soliton has two peaks and the two solitons interact with each other. Hereby, He is proportional to the ratio of the strength of magnetic field to the electronic Fermi temperature. External periodic force on the qZK equation yields the chaotic motions. Through some phase projections, the process from a sequence of the quasiperiod doubling to chaos can be observed. The chaotic behavior is observed since the power spectra are calculated, and the quasiperiod doubling states of perturbed qZK equation are given. The final chaotic state of the perturbed qZK is obtained.

Presheath density drop induced by ionneutral friction along plasma blobs and implications for blob velocities
View Description Hide DescriptionThe presheath density drop along the magnetic field in fieldaligned, radially propagating plasma blobs is investigated in the TORPEX toroidal experiment [Fasoli et al., Plasma Phys. Controlled Fusion 52, 124020 (2010)]. Using Langmuir probes precisely aligned along the magnetic field, we measure the density nse at a poloidal limiter, where blobs are connected, and the upstream density n 0 at a location half way to the other end of the blobs. The presheath density drop nse /n 0 is then computed and its dependence upon the neutral background gas pressure is studied. At low neutral gas pressures, the presheath density drop is ≈0.4, close to the value of 0.5 expected in the collisionless case. In qualitative agreement with a simple model, this value decreases with increasing gas pressure. No significant dependence of the density drop upon the radial distance into the limiter shadow is observed. The effect of reduced blob density near the limiter on the blob radial velocity is measured and compared with predictions from a blob speedversussize scaling law [Theiler et al., Phys. Rev. Lett. 103, 065001 (2009)].

Transition from thermal to turbulent equilibrium with a resulting electromagnetic spectrum
View Description Hide DescriptionA recent paper [Ziebell et al., Phys. Plasmas 21, 010701 (2014)] discusses a new type of radiation emission process for plasmas in a state of quasiequilibrium between the particles and enhanced Langmuir turbulence. Such a system may be an example of the socalled “turbulent quasiequilibrium.” In the present paper, it is shown on the basis of electromagnetic weak turbulence theory that an initial thermal equilibrium state (i.e., only electrostatic fluctuations and Maxwellian particle distributions) transitions toward the turbulent quasiequilibrium state with enhanced electromagnetic radiation spectrum, thus demonstrating that the turbulent quasiequilibrium discussed in the above paper correctly describes the weakly turbulent plasma dynamically interacting with electromagnetic fluctuations, while maintaining a dynamical steadystate in the average sense.