Volume 18, Issue 4, April 2011
Index of content:

An extended model is proposed to describe the magnetic topology during appearance of edge localized modes(ELMs). It is applied to an ELMing Hmode in a lower single null discharge at DIIID [J. L. Luxon, Nucl. Fusion 42, 614 (2002)]. The process of flux tube formation is discussed based on a previously proposed twostep model. Large currents, as found in measurements in DIIID, are assumed running through newly formed large flux tubes. Two different realizations of the current distribution within the tubes are compared, namely a single filament in each tube and a scenario where the current in each tube is split into subfilaments. The latter scenario is shown to be the more realistic distribution because it leads to much better agreement with infrared camera observations. It is demonstrated that stripe patterns in the divertor heat flux produced by an ELM in the DIIID tokamak can be reproduced numerically by taking into account the magnetic perturbation caused by the thermoelectric current subfilaments.
 LETTERS


Alfvén cascades with downward frequency sweeping
View Description Hide DescriptionIt is suggested that relatively rare, but challenging for the existing theory Alfvén cascades with downward frequency sweeping are actually the infernal Alfvén eigenmodes (IAEs). Such modes exist in discharges with flat or weakly reversed qprofile in the broad central region, when the value of the safety factor in this region is slightly above the integer or loworder rational. Similar to the toroidal Alfvén eigenmode, but in contrast to the “conventional” Alfvén cascade with upward frequency sweeping, the spectrum of IAE is almost degenerate with respect to the mode numbers. Both features mentioned above are consistent with experimental observations.

Laserion acceleration through controlled surface contamination
View Description Hide DescriptionIn laserplasma ion accelerators, control of target contamination layers can lead to selection of accelerated ion species and enhancement of acceleration. To demonstrate this, deuterons up to 75 keV are accelerated from an intense laser interaction with a glass target simply by placing 1 ml of heavy water inside the experimental chamber prior to pumping to generate a deuterated contamination layer on the target. Using the same technique with a deuteratedpolystyrenecoated target also enhances deuteron yield by a factor of 3 to 5, while increasing the maximum energy of the generated deuterons to 140 keV.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Ionacoustic waves in a nonstationary ultracold neutral plasma
View Description Hide DescriptionWe consider the excitation and dispersion of electrostatic ionacoustic (IA) waves in a nonstationary ultracold neutral plasma (UCNP). This can be seen as an extension of timerefraction models of photons and plasmons to the case of lowfrequency IA waves in the UCNP. It is shown that temporal changes in the medium lead to a frequencyshift of the IA wave, and to the emission of the IA waves propagating in a direction opposite to each other. We consider an arbitrary temporal variation of the background plasma density, and determine the transmission and reflection coefficients. We also consider the influence of a fast ionization process, assumed inhomogeneous in volume and show that it excites a welldefined spectrum of ionacoustic waves, which agree very well with a recent experimental observation.

Stability of twodimensional ionacoustic wave packets in quantum plasmas
View Description Hide DescriptionThe nonlinear propagation of twodimensional (2D) quantum ionacoustic waves (QIAWs) is studied in a quantum electron–ion plasma. By using a 2D quantum hydrodynamic model and the method of multiple scales, a new set of coupled nonlinear partial differential equations is derived which governs the slow modulation of the 2D QIAW packets. The oblique modulational instability (MI) is then studied by means of a corresponding nonlinear Schrödinger equation derived from the coupled nonlinear partial differential equations. It is shown that the quantum parameter H (ratio of the plasmon energy density to Fermi energy) shifts the MI domains around the plane, where k is the carrier wave number and is the angle of modulation. In particular, the ionacoustic wave(IAW), previously known to be stable under parallel modulation in classical plasmas, is shown to be unstable in quantum plasmas. The growth rate of the MI is found to be quenched by the obliqueness of modulation. The modulation of 2D QIAW packets along the wave vector is shown to be described by a set of Davey–Stewartsonlike equations. The latter can be studied for the 2D wave collapse in dense plasmas. The predicted results, which could be important to look for stable wave propagation in laboratory experiments as well as in dense astrophysical plasmas, thus generalize the theory of MI of IAW propagations both in classical and quantum electron–ion plasmas.

Evolution of nonlinear waves in compressing plasma
View Description Hide DescriptionThrough particleincell simulations, the evolution of nonlinear plasma waves is examined in onedimensional collisionless plasma undergoing mechanical compression. Unlike linear waves, whose wavelength decreases proportionally to the system length L(t), nonlinear waves, such as solitary electron holes, conserve their characteristic size during slow compression. This leads to a substantially stronger adiabatic amplification as well as rapid collisionless damping when L approaches . On the other hand, cessation of compression halts the wave evolution, yielding a stable mode.

Magnetic reconnection in a compressible MHD plasma
View Description Hide DescriptionUsing steadystate resistiveMHD,magnetic reconnection is reinvestigated for conditions of high resistivity/low magnetic Reynolds number, when the thickness of the diffusion region is no longer small compared to its length. Implicit expressions for the reconnection rate and other reconnection parameters are derived based on the requirements of mass, momentum, and energy conservation. These expressions are solved via simple iterative procedures. Implications specifically for low Reynolds number/high resistivity are being discussed.

Magnetic reconnection with radiative cooling. I. Optically thin regime
View Description Hide DescriptionMagnetic reconnection processes in many highenergydensity astrophysical and laboratory plasma systems are significantly affected by radiation; hence traditional, nonradiative reconnection models are not applicable to these systems. Motivated by this observation, the present paper develops a Sweet–Parkerlike theory of resistivemagnetic reconnection with strong radiative cooling. It is found that, in the case with zero guide field, intense radiative cooling leads to a strong plasma compression, resulting in a higher reconnection rate. The compression ratio and the reconnection layer temperature are determined by the balance between ohmic heating and radiative cooling. The lower temperature in a radiatively cooled layer leads to a higher Spitzer resistivity and, hence, a higher reconnection rate. Several specific radiative processes (bremsstrahlung, cyclotron, and inverse Compton) in the optically thin regime are considered for both the zero and strongguidefield cases, and concrete expressions for the reconnection parameters are derived, along with the applicability conditions.

A flowing plasma model to describe drift waves in a cylindrical helicon discharge
View Description Hide DescriptionA twofluid model developed originally to describe waveoscillations in the vacuum arc centrifuge, a cylindrical, rapidly rotating, low temperature, and confined plasma column, is applied to interpret plasma oscillations in a RF generated linear magnetized plasma [WOMBAT (waves on magnetized beams and turbulence)], with similar density and field strength. Compared to typical centrifuge plasmas, WOMBAT plasmas have slower normalized rotation frequency, lower temperature, and lower axial velocity. Despite these differences, the twofluid model provides a consistent description of the WOMBAT plasma configuration and yields qualitative agreement between measured and predicted waveoscillation frequencies with axial field strength. In addition, the radial profile of the density perturbation predicted by this model is consistent with the data. Parameter scans show that the dispersion curve is sensitive to the axial field strength and the electron temperature, and the dependence of oscillation frequency with electron temperature matches the experiment. These results consolidate earlier claims that the density and floating potential oscillations are a resistive drift mode, driven by the density gradient. To our knowledge, this is the first detailed physics model of flowing plasmas in the diffusion region away from the RF source. Possible extensions to the model, including temperature nonuniformity and magnetic fieldoscillations, are also discussed.

Global limits on kinetic Alfvénon speed in quasineutral plasmas
View Description Hide DescriptionLargeamplitude kinetic Alfvénon (exact Alfvén soliton) matching condition is investigated in quasineutral electronion and electronpositronion plasmas immersed in a uniform magnetic field. Using the standard pseudopotential method, the magnetohydrodynamics equations are exactly solved, and a global allowed matching condition for propagation of kinetic solitary waves is derived. It is remarked that, depending on the plasma parameters, the kinetic solitons can be subAlfvénic or superAlfvénic, in general. It is further revealed that, either upper or lower soliton speedlimit is independent of fractional plasma parameters. Furthermore, the soliton propagation angle with respect to that of the uniform magnetic field is found to play a fundamental role in controlling the soliton matching speedrange.

Nonlinear evolution of electromagnetic ion cyclotron waves
View Description Hide DescriptionHybrid Vlasov–Fourier modeling is used to investigate the nonlinear evolution of electromagnetic ion cyclotron (EMIC) waves driven by protontemperatureanisotropy in plasmas with a population of ions and a cold proton background. In the pure proton–electron plasma, most of the free energy is converted into highamplitude waves and currents. In the nonlinear stage, within a few hundred proton gyroperiods after the saturation, the wave spectrum shifts toward lower wave numbers and frequencies, from to below . In the presence of even a small population of ions almost all of the free energy is used in heating. The wave activity in the saturated state moves from the linearly unstable upper branch to the linearly stable lower one. In the presence of a background of cold protons, the waves can propagate in the frequency stopband. Our results demonstrate that linear stability theory cannot be used to estimate the characteristics of the expected saturated wave spectra in the terrestrial magnetosphere. Significantly, our nonlinear simulations produce wave spectra which are in close agreement with the EMIC waves observed in situ by satellites as well as by groundbased magnetometers positioned at the ends of the magnetic field lines.

Diffractioncontrolled backscattering threshold and application to Raman gap
View Description Hide DescriptionIn most classic analytical models of linear stimulated scatter,light diffraction is omitted, a priori. However, modern laser optic typically includes a variant of the random phase plate [Y. Kato et al., Phys. Rev. Lett. 53, 1057 (1984)], resulting in diffraction limited laser intensity fluctuations—or localized speckles—which may result in explosive reflectivity growth as the average laser intensity approaches a critical value [H. A. Rose and D. F. DuBois, Phys. Rev. Lett. 72, 2883 (1994)]. Among the differences between stimulated Raman scatter(SRS) and stimulated Brillouin scatter is that the SRSscattered light diffracts more strongly than the laser light with increase of electron density. This weakens the tendency of the SRS light to closely follow the most amplified paths, diminishing gain. Let be the onedimensional power gain exponent of the stimulated scatter. In this paper we show that differential diffraction gives rise to an increase of at the SRS physical threshold with increase of electron density up to a drastic disruption of SRS as electron density approaches one fourth of its critical value from below. For three wave interaction lengths not small compared to a speckle length, this is a physically robust Raman gap mechanism.
 Nonlinear Phenomena, Turbulence, Transport

Reversal of Hugoniot locus for strong shocks due to radiation
View Description Hide DescriptionShock Hugoniot can be used to express the response of a material to shocks, and the compression ratio of the shock can be determined by the Hugoiot locus. When the shock is strong, it will become radiating, and the radiation will affect the Hugoniot. The role of radiation on the Hugoniot condition is studied in the paper. For the radiative fluxdominated shocks, the radiative flux if large enough may render the structure of the shock Hugoniot locus totally different with the case for the pure hydrodynamic shock: the two branches with one in quadrant I and the other in quadrant III are reversed into two in quadrants IV and II, respectively, correspondingly the compression ratio may be larger than the limiting value for ideal gases with index . For the radiative shock in which the radiative heat wave propagates supersonically, a threshold value for the net radiative flux to the preshock is also defined which determines whether the Hugoniot locus is reversed and the compression ratio exceeds the limiting value. Numerical results also verify the reversal of the Hugoniot locus of the shocks if the net radiative flux to the preshock exceeds the threshold value.

Implosion and explosion of electrostatic cylindrical and spherical shocks in asymmetric pairion plasmas
View Description Hide DescriptionNonlinear electrostaticshock waves are studied in unmagnetized, dissipative pairion plasmas. The dissipation in the system is taken into account by considering the effect of kinematicviscosity of both positive and negative ions in plasmas. The system of fluid equations for asymmetric pairion plasma is reduced to Korteweg–deVries–Burgers equation in the limit of small amplitude perturbation. It is observed that the system under consideration admits rarefactive shocks. Keeping in view the practical applications, the nonlinear propagation of both the exploding and imploding shocks is investigated and the differences are expounded in detail. The present study may have relevance in the study of the formation of electrostatic shocks in laserinduced implosion devices, star formation, supernovae explosion, etc.

Firstorder finiteLarmorradius effects on magnetic tearing in pinch configurations
View Description Hide DescriptionThe linear and nonlinear evolution of a singlehelicity tearing mode in a cylindrical, forcefree pinch are investigated using a fluid model with firstorder finiteLarmorradius corrections. Linear results computed with the nimrod [nonideal magnetohydrodynamics(MHD) with rotation, open discussion] code [Sovinec et al., J. Comput. Phys. 195, 355 (2004)] produce a regime at small where the growth rate is reduced relative to resistive MHD, though the Hall term is not significant. The leading order contributions from ion gyroviscosity may be expressed as a drift associated with and poloidal curvature for experimentally relevant , forcefree equilibria. The heuristic analytical dispersion relation, where is the gyroviscous drift frequency, confirms numerical results. The behavior of our cylindrical computations at large corroborates previous analytic slab studies where an enhanced growth rate and radially localized Hall dynamo are predicted. Similar to previous drifttearing results, nonlinear computations with cold ions demonstrate that the Hall dynamo is small when the island width is large in comparison with the scale for electron–ion coupling. The saturation is then determined by the resistive MHD physics. However, with warm ions the gyroviscous stress supplements the nonlinear Lorentz force, and the saturated island width is reduced.

Soliton propagation, reflection, and transmission in an inhomogeneous plasma with trapped electrons
View Description Hide DescriptionTransmission and reflection of solitons from a semitransparent grid in an inhomogeneous plasma in the presence of trapped electrons is studied analytically. Conditions are obtained for the obliqueness of the propagation and the drift velocity of ions for the soliton transmission and reflection. Also, a transmission–reflection conservation law is derived. The contribution of trapped electrons to the solitons’ propagation and their reflection and transmission is examined through energy, amplitude, and width of the solitons, in addition to the effect of temperature and drift of the ions.

Linear and nonlinear coupled drift and ion acoustic waves in collisional pair ion–electron magnetoplasma
View Description Hide DescriptionLinear and nonlinear coupled electrostatic drift and ion acoustic waves are studied in inhomogeneous, collisional pair ion–electron plasma. The Korteweg–de Vries–Burgers (KdVB) equation for a medium where both dispersion and dissipation are present is derived. An attempt is made to obtain exact solution of KdVB equation by using modified tanh–coth method for arbitrary velocity of nonlinear drift wave. Another exact solution for KdVB is obtained, which gives a structure of shock wave. Korteweg–de Vries (KdV) and Burgers equations are derived in limiting cases with solitary and monotonic shock solutions, respectively. Effects of species density, magnetic field, obliqueness, and the acoustic to drift velocity ratio on the solitary and shock solutions are investigated. The results discussed are useful in understanding of low frequency electrostatic waves at laboratory pair ion plasmas.

Fully nonlinear solitary waves in a dusty electronegative multispecies plasmas
View Description Hide DescriptionThe formation and dynamics of fully nonlinear ionacoustic solitary waves, which accompany electronegative plasmas composed of positive ions, twonegative ions, isothermal electrons, as well as a fraction of stationary charged (positive or negative) dust impurities are investigated. By using the hydrodynamic and Poisson equations, an energyintegral equation involving a Sagdeev pseudopotential is derived. Using the latter, we have defined precisely the existence regions of the electrostatic localized pulses. The critical total negative ions concentration and critical secondnegative ion density ratio thresholds, which indicate where the solitary pulses set in, have been determined for various regimes. Numerical calculations reveal that only supersonic pulses can exist. The total negative ions concentration, the secondtototal negative ions density ratio, electronstopositive ions temperature ratio, dust impurities concentration, positivetonegative mass ratio, and Mach number have been investigated on the nonlinear wave profile. It is found that the total negative ion concentration as well as the dust particles concentration play the significant role in deciding the polarity of the propagating pulses. The results could be applied to investigate and predict the behavior of the nonlinear solitary structure in future laboratory plasmaexperiment having dusty electronegative multispecies plasmas as referred by Ichiki et al. [Phys. Plasmas8, 4275 (2001)].

Quasilinear theory and simulation of Buneman instability
View Description Hide DescriptionIn a recently developed nonlinear theory of Buneman instability, a simplifying assumption of selfsimilarity was imposed for the electron distribution function, based upon which, a set of moment kinetic equations was derived and solved together with nonlinear wave kinetic equation [P. H. Yoon and T. Umeda, Phys. Plasmas 17, 112317 (2010)]. It was found that the theoretical result compared reasonably against onedimensional electrostatic Vlasov simulation. In spite of this success, however, the simulated distribution deviated appreciably from the assumed selfsimilar form during the late stages of nonlinear evolution. In order to rectify this shortcoming, in this paper, the distribution function is computed on the basis of rigorous velocity space diffusion equation. A novel theoretical scheme is developed so that both the quasilinear particle diffusion equation and the adiabatic dispersion relation can be solved for an arbitrary particle distribution function. Comparison with Vlasov simulation over relatively early quasilinear phase of the instability shows a reasonable agreement, despite the fact that quasilinear theory lacks coherent nonlinear effects as well as mode–mode coupling effects.

Modulational instability and nonlinear evolution of twodimensional electrostatic wave packets in ultrarelativistic degenerate dense plasmas
View Description Hide DescriptionWe consider the nonlinear propagation of electrostatic wave packets in an ultrarelativistic (UR) degenerate dense electron–ion plasma, whose dynamics is governed by the nonlocal twodimensional nonlinear Schrödingerlike equations. The coupled set of equations is then used to study the modulational instability (MI) of a uniform wave train to an infinitesimal perturbation of multidimensional form. The condition for the MI is obtained, and it is shown that the nondimensional parameter, (where is the reduced Compton wavelength and n _{0} is the particle number density) associated with the UR pressure of degenerate electrons, shifts the stable (unstable) regions at to unstable (stable) ones at higher densities, i.e., . It is also found that the higher the values of n _{0}, the lower is the growth rate of MI with cutoffs at lower wave numbers of modulation. Furthermore, the dynamical evolution of the wave packets is studied numerically. We show that either they disperse away or they blowup in a finite time, when the wave action is below or above the threshold. The results could be useful for understanding the properties of modulated wave packets and their multidimensional evolution in UR degenerate dense plasmas, such as those in the interior of white dwarfs and/or preSupernova stars.

Large acoustic solitons and double layers in plasmas with two positive ion species
View Description Hide DescriptionLarge nonlinear acousticwaves are discussed in a plasma made up of cold supersonic and adiabatic subsonic positive ions, in the presence of hot isothermal electrons, with the help of Sagdeev pseudopotential theory. In this model, no solitons are found at the acoustic speed, and no compositional parameter ranges exist where solutions of opposite polarities can coexist. All nonlinear modes are thus superacoustic, but polarity changes are possible. The upper limits on admissible structure velocities come from different physical arguments, in a strict order when the fractional cool ion density is increased: infinite cold ion compression, warm ion sonic point, positive double layers, negative double layers, and finally, positive double layers again. However, not all ranges exist for all mass and temperature ratios. Whereas the cold and warm ion sonic point limitations are always present over a wide range of mass and temperature ratios, and thus positive polarity solutions can easily be obtained, double layers have a more restricted existence range, specially if polarity changes are sought.