Volume 13, Issue 7, July 2006
 LETTERS


Can microbunch instability on solar flare accelerated electron beams account for bright broadband coherent synchrotron microwaves?
View Description Hide DescriptionThe physical processes producing bright broadband coherent synchrotron radiation (CSR) bursts in laboratory accelerators are proposed to happen also in solar flares, bringing a plausible explanation to serious interpretation constraints raised by the discovery of a solar flare submmwave spectral emission component peaking in the terahertz (THz) range simultaneous to the wellknown microwaves component. The THz component is due to incoherent synchrotron radiation (ISR) produced by a beam of ultrarelativistic electrons.Beam density perturbations, on a scale of the order of or smaller than the emitting wavelength, sets a microbunch instability producing the intense CSR at lower frequencies. Hard xray/ray emissions may include a significant synchrotron emission component from the same ISR spectrum, bringing a new possibility to explain the so called “solar flare electron number paradox.”
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Fully kinetic simulations of undriven magnetic reconnection with open boundary conditions
View Description Hide DescriptionKinetic simulations of magnetic reconnection typically employ periodic boundary conditions that limit the duration in which the results are physically meaningful. To address this issue, a new model is proposed that is open with respect to particles, magnetic flux, and electromagnetic radiation. The model is used to examine undriven reconnection in a neutral sheet initialized with a single xpoint. While at early times the results are in excellent agreement with previous periodic studies, the evolution over longer intervals is entirely different. In particular, the length of the electron diffusion region is observed to increase with time resulting in the formation of an extended electron current sheet. As a consequence, the electron diffusion region forms a bottleneck and the reconnection rate is substantially reduced. Periodically, the electron layer becomes unstable and produces a secondary island, breaking the diffusion region into two shorter segments. After growing for some period, the island is ejected and the diffusion region again expands until a new island is formed. Fast reconnection may still be possible provided that the generation of secondary islands remains sufficiently robust. These results indicate that reconnection in a neutral sheet may be inherently unsteady and raise serious questions regarding the standard model of Hall mediated reconnection.

A selfsimilar magnetohydrodynamic model for ball lightnings
View Description Hide DescriptionBall lightning is modeled by magnetohydrodynamic(MHD)equations in twodimensional spherical geometry with azimuthal symmetry. Dynamic evolutions in the radial direction are described by the selfsimilar evolution function . The plasma pressure, mass density, and magnetic fields are solved in terms of the radial label . This model gives spherical MHD plasmoids with axisymmetric forcefree magnetic field, and spherically symmetric plasma pressure and mass density, which selfconsistently determine the polytropic index . The spatially oscillating nature of the radial and meridional field structures indicate embedded regions of closed field lines. These regions are named secondary plasmoids, whereas the overall selfsimilar spherical structure is named the primary plasmoid. According to this model, the time evolution function allows the primary plasmoid expand outward in two modes. The corresponding ejection of the embedded secondary plasmoids results in ball lightning offering an answer as how they come into being. The first is an accelerated expanding mode. This mode appears to fit plasmoids ejected from thundercloud tops with acceleration to ionosphere seen in high altitude atmospheric observations of sprites and blue jets. It also appears to account for midair highspeed ball lightning overtaking airplanes, and ground level highspeed energetic ball lightning. The second is a decelerated expanding mode, and it appears to be compatible to slowly moving ball lightning seen near ground level. The inverse of this second mode corresponds to an accelerated inward collapse, which could bring ball lightning to an end sometimes with a cracking sound.

A specific property of electromagnetic waves interacting with dustladen plasma
View Description Hide DescriptionThe propagation pattern of electromagnetic waves (EMWs) in dusty plasmas is quite different from that in electronion plasmas. For instance, here the ponderomotive force acts on dust grains as a negative pressure, and a nonlinear Schrödinger equation with an additional nonlinear term is obtained. Based on this equation, the modulation instability is examined and it is shown that the growth rate becomes maximum when that additional term compensates the diffraction term. The main part of this work is devoted to the localization of the grains by the EMW. Considering both subsonic and supersonic regimes, it has been shown that under certain conditions the grains are localized and the ions circumnavigate the grains, whereas the electrons escape from the region of localization. Further, the localization of grains by the EMW is found to be shapedependent of the pulse. Comparing pancake and light bullet shaped pulses in the supersonic regime, and it is shown that only the light bullet shape leads to the compression of grains. Finally, investigating nonstationary solution, it is shown that for some parameters, the nonlinear wave breaking and the formation of a shock wave can take place.

Dust clusters with nonHamiltonian particle dynamics
View Description Hide DescriptionThe modes of clusters formed by two or three charged dust particles in a plasma are analyzed. The nonHamiltonian dynamics of the particles is taken into account, which includes (i) nonreciprocal interaction forces due to wake effects and (ii) spatial variations of the particle charge and shielding parameters. It is shown that these effects can trigger an oscillatory instability under realistic experimental conditions. An experiment is suggested to observe this instability.

Relativistic plasma dielectric tensor evaluation based on the exact plasma dispersion functions concept
View Description Hide DescriptionThe fully relativistic plasmadielectrictensor for any wave and plasma parameter is estimated on the basis of the exact plasma dispersion functions concept. The inclusion of this concept allows one to write the tensor in a closed and compact form and to reduce the tensor evaluation to the calculation of those functions. The main analytical properties of these functions are studied and two methods are given for their evaluation. The comparison between the exact dielectrictensor with the weakly relativistic approximation, widely used presently in plasma waves calculations, is given as well as the range of plasma temperature,harmonic number, and propagation angle in which the weakly relativistic approximation is valid.

Transportation of radiation through opaque magnetoactive plasmas by the means of parametrically induced transparency
View Description Hide DescriptionThe concept of “parametrically induced transparency” (PIT) is introduced: a variant of electromagnetically induced transparency (EIT) in inhomogeneous media, in which the EIT regime takes place in only a limited spatial region in which the signal wave is scattered into a transporting wave by a high power drive wave. It is shown how PIT can be employed for the extraction of radiation from the core of overdense toroidalplasmas. Numerical examples are given for the TEXTOR (Torus Experiment for Technology Oriented Research) tokamak [U. Samm, Fusion Sci. Technol.47, 73 (2005), Special Issue on TEXTOR], which use the existing gyrotron as the high power drive source.

Energy loss of charged projectiles in a selfgravitating Lorentzian dusty plasma
View Description Hide DescriptionThe energy loss of a pair of test charge projectiles passing through a multicomponent, selfgravitating, dusty plasma with a generalized Lorentzian distribution is presented. Analytical and numerical results are obtained for the shielded potential and for the slowing down of a pair of test charge projectiles. The correlation and interference effects of two collinear and noncollinear projectiles on the shielded potential as well as on the energy loss are presented. An interference contribution of these projectiles to the potential and energy loss is observed that depends upon their orientation and separation distance. It is found that the energy loss of collinear projectiles decreases with the increase of the dust Jeans frequency for any separation distance. On the other hand, with small values of (the generalized Lorentzian distribution case), the test charge projectile gains energy instead of losing for projectile velocity greater than the thermal velocity of the dust, and this gain is increased as we increase the dust Jeans frequency. These results are helpful to understand the energy loss mechanism, which plays an important role in explaining the coagulation of dust particles in molecular clouds as well as in dust plasma crystal formation.

Relativistic electron distribution function of a plasma in a nearcritical electric field
View Description Hide DescriptionA corrected relativistic collision operator is used to derive a FokkerPlanck equation for the distribution function of relativistic suprathermal electrons in a weakly relativistic plasma, which is then solved by a procedure similar to that employed in Connor and Hastie [Nucl. Fusion15, 415 (1975)]. Analytical expressions are derived for the electron distribution function in plasmas with the electric field close to critical, which is typical of plasmas with grassy sawteeth on the Joint European Torus. A numerical solution is used for determining the normalization constant, which matches the relativistic region onto the weakly relativistic region. It is found that the scaling of the runaway rate with the electric field obtained by Connor and Hastie is a good approximation in spite of their use of an incomplete form of the collision operator not conserving number of particles. The present analysis determines the proportionality constant and introduces corrections to the earlier scaling of the runaway rate with respect to the electric field. The results obtained for the electron distribution function constitute a basis for studies of experimentally observed phenomena in nearthreshold electric fieldplasmas with a significant suprathermal electron population.

Advanced feedback control methods in EXTRAP T2R reversed field pinch
View Description Hide DescriptionPrevious experiments in the EXTRAP T2R reversed field pinch device have shown the possibility of suppression of multiple resistive wall modes(RWM). A feedback system has been installed in EXTRAP T2R having 100% coverage of the toroidal surface by the active coil array. Predictions based on theory and the previous experimental results show that the number of active coils should be sufficient for independent stabilization of all unstable RWMs in the EXTRAP T2R. Experiments using different feedback schemes are performed, comparing the intelligent shell, the fake rotating shell, and the mode control with complex feedback gains. Stabilization of all unstable RWMs throughout the discharge duration of is seen using the intelligent shell feedback scheme. Mode rotation and the control of selected Fourier harmonics is obtained simultaneously using the mode control scheme with complex gains. Different sensor signals are studied. A feedback system with toroidalmagnetic field sensors could have an advantage of lower feedback gain needed for the RWM suppression compared to the system with radial magnetic field sensors. In this study, RWM suppression is demonstrated, using also the toroidal field component as a sensor signal in the feedback system.
 Nonlinear Phenomena, Turbulence, Transport

Bispectral analysis of low to highconfinement mode transitions in the National Spherical Torus Experiment
View Description Hide DescriptionThis paper will present an experimental study of the temporal and spatial characteristics of the autobicoherence calculated from light amplitude fluctuationsmeasured in the edge plasma of the National Spherical Torus Experiment (NSTX) [M. Ono et al., Plasma Phys. Controlled Fusion45, A335 (2003)] using data from the gas puff imaging (GPI) diagnostic [R. J. Maqueda et al., Rev. Sci. Instrum.74, 2020 (2003); S. J. Zweben et al., Nucl. Fusion44, 134 (2004)] obtained during a series of thirteen shots in which the NSTX plasma underwent spontaneous low to highconfinement mode (LH) transitions. The autobicoherence calculated from the available GPI chord signals in the region near the magnetic separatrix and just above the outer midplane indicates that there is no significant increase, i.e., outside the rms error, in the amount of nonlinear coupling between low frequency fluctuations and high frequency fluctuations during the before the transition. Limitations of bicoherence analysis are discussed.

Electron acoustic solitons in a relativistic plasma with nonthermal electrons
View Description Hide DescriptionElectron acoustic solitary waves (EASWs) are studied using Sagdeev’s pseudopotential technique for a plasma comprising relativistic ions, cold relativistic electrons, and nonthermal hot electrons. The parametric range considered here is valid for the auroral zone. It is found that the present plasma model supports EASWs having negative potential. It is seen that the relativistic effect significantly restricts the region of existence for solitary waves. The region of existence of solitary waves also depends crucially on , the parameter that determines the population of the energetic nonthermal electrons. For example, for with the soliton velocity 1.05 and , solitary wave solutions will not exist. We also find that for small values of , solitary waves would exist for .

Global spectral investigation of plasma turbulence in gyrokinetic simulations
View Description Hide DescriptionGyrokinetic global particleincell simulations for a small torus with a large aspect ratio indicate a spectrum for electrostatic turbulence. When electrons are treated kinetically, the simulation results fit that grows from about 1 at the plasma core to about 3 at the plasma edge for the flux surface component of the wave vector perpendicular to the magnetic field, while for adiabatic electrons is found for all radii, in agreement with the HasegawaMima model. The relation between spectra and transport is investigated through the formation of an internal transport barrier. The role of flow shear in suppressing turbulence is illustrated by spectral diagnostics. A strong dependence between the presence of small wavenumbers and transport is explicitly observed. The simulated spectra are compared to recent experimental results.

Projected profile similarity in gyrokinetic simulations of Bohm and gyroBohm scaled DIIID L and H modes
View Description Hide DescriptionGlobal gyrokinetic simulations of DIIID [M. A. Mahdavi and J. L. Luxon, in “DIIID Tokamak Special Issue,” Fusion Sci. Technol.48, 2 (2005)] L and Hmode dimensionally similar discharge pairs are treated in detail. The simulations confirm the Bohm scaling of the wellmatched Lmode pair. The paradoxical but experimentally apparent gyroBohm scaling of the Hmode pair at larger relative gyroradius (rhostar) and lower transport levels is due to poor profile similarity. Simulations of projected experimental plasma profiles with perfect similarity show both the L and Hmode pairs to have Bohm scaling. A stabilization rule for predicting the breakdown of gyroBohm scaling from simulations of a single discharge is presented.

Modulation of dust acoustic waves with a quantum correction
View Description Hide DescriptionThe onedimensional quantum hydrodynamic model is considered in the limit of low phase velocity (compared to the Fermi thermal velocities) as well as the limit of low frequency (compared to the particleneutral collision frequency) to study the amplitude modulation of dustacoustic waves in a threespecies quantum dusty plasma. Using the standard reductive perturbation technique, a nonlinear Schrödinger equation containing the quantum effects is derived. The quantum mechanical effects containing the quantum diffraction and quantum statistics on the modulational instability are studied both analytically and numerically. It is found that quantum effects are to suppress the instability.

Global particleincell simulations of microturbulence with kinetic electrons^{a)}
View Description Hide DescriptionThe effects of nonadiabaticelectrons on ion temperature gradient drift instabilities have been studied in global toroidal geometry using the gyrokinetic particle simulation approach. Compared to the nonlinear global simulations based on only the adiabatic response of the electrons, we have found that the crossfield ion heat transport is two to three times larger in the presence of trapped electrons as compared to the purely adiabatic electron case, and that the zonal component of the electrostatic potential has a shorter wavelength. The numerical methods for calculating both the adiabatic and the nonadiabatic responses for the electrons are presented.

Convergence of ChapmanEnskog calculation of transport coefficients of magnetized argon plasma
View Description Hide DescriptionConvergence properties of the ChapmanEnskog method in the presence of a magnetic field for the calculation of the transport properties of nonequilibrium partially ionized argon have been studied emphasizing the role of the different collision integrals. In particular, the Ramsauer minimum of electronargon cross sections affects the convergence of the ChapmanEnskog method at low temperature, while Coulomb collisions affect the results at higher temperatures. The presence of an applied magnetic field mitigates the slow convergence for the components affected by the field.

Large scale magnetic fields and coherent structures in nonuniform unmagnetized plasma
View Description Hide DescriptionThe properties of streamers and zonal magnetic structures in magnetic electron drift mode turbulence are investigated. The stability of such large scale structures is investigated in the kinetic and the hydrodynamic regime, for which an instability criterion similar to the Lighthill criterion for modulational instability is found. Furthermore, these large scale flows can undergo further nonlinear evolution after initial linear growth, which can lead to the formation of longlived coherent structures consisting of selfbound wave packets between the surfaces of two different flow velocities with an expected modification of the anomalous electron transport properties.
 Magnetically Confined Plasmas, Heating, Confinement

Plasma equilibria for the case of a strong coupling of parallel and flows
View Description Hide DescriptionIt is shown that the plasma of a tokamak can adopt equilibria with large pressure and electric potential variations along a magnetic fluxsurface, due to the strong coupling between parallel and perpendicular flows. Different types of solutions to the equilibrium twofluid equations are found that describe actual plasma states. In particular, one of the equilibria is such that some plasma parameters, like the plasma pressure, can have maximum values in the vicinity of the X point of the separatrix, in agreement with the recent observations in DIIID [J. L. Luxon and L. G. Davis, Fusion Technol.8, 441 (1985)].

Mitigation and control of the particle pinch in the Electric Tokamak
View Description Hide DescriptionThe Electric Tokamak[R. J. Taylor, T. A. Carter, J.L. Gauvreau et al., Nucl. Fusion45, 1634 (2005)] operates at high plasma density (one and a half times the Greenwald limit) due to a strong particle pinch. However, particle accumulation causes several problems. The operation of the machine can suffer several violent disruptions that hinder the study of many plasma phenomena. Plasma motion and large density swings are undesirable because they alter continuous processes, leaving only transient regimes to study. Particle source and local temperature control can defeat the fundamental mechanisms of this ‘‘electric’’ pinch. If edge fueling feedback is not sufficient to induce quiescent behavior, the fast ion loss caused by second harmonic ioncyclotron rf injection functions as a particle sink deep within the outer plasma cross section. By linking these strong effects to the fueling feedback, stable medium density plasmas can be sustained for several seconds. This new regime yields surprisingly long and calm discharges.