Volume 11, Issue 5, May 2004
 LETTERS


Simple analysis of the nonlinear saturation of the tearing mode
View Description Hide DescriptionA simple nonlinear treatment of the reduced magnetohydrodynamics equations, that describes the saturation of small width magnetic island, is presented. This procedure is an extension of Rutherford’s theory. It is found that the tearing mode saturates at a finite amplitude, which is proportional to the linear stability parameter, The correct value of the proportionality coefficient between and is obtained in this paper, in agreement with numerical simulations.

The generation of microfiducials in laseraccelerated proton flows, their imaging property of surface structures and application for the characterization of the flow
View Description Hide DescriptionA mechanism that has gained great attention in the field of laserion acceleration due to remarkable properties of the accelerated beam is laser acceleration of protons from the rear surface of solid targets. It is shown that these protons are capable of transporting structure information from the rear surface of the target into the detector, which can be used to measure essential properties of the flow like an upper limit of the lateral beam emittance and the source area of the laser acceleratedprotons for a particular energy. In this Letter the essential mechanism responsible for the generation of images of surface structures by laseraccelerated protons is addressed and used to explain the experiment is described in the Letter.

Efficient generation of extended plasma waveguides with the axicon ignitorheater scheme
View Description Hide DescriptionAn efficient method for generating extended plasma waveguides is developed by using the axicon lens in conjunction with the ignitorheater scheme. The shortpulse ignitor generates the seed electrons by multiphoton ionization, and the longpulse heater expands the plasma by inverse bremsstrahlung heating and builds up the plasma density barrier by collisional ionization. A 1.2cmlong plasma waveguide is generated in pure Ar gas with a total energy of only 100 mJ. Evolution of the plasma density profile is measured by timeresolved interferometry to show the waveguide forming process and how it can be optimized.

The local limit of global gyrokinetic simulations
View Description Hide DescriptionGlobal gyrokinetic simulations of turbulence include physical effects that are not retained in local fluxtube simulations. Nevertheless, in the limit of sufficiently small (gyroradius compared to system size) it is expected that a local simulation should agree with a global one (at the local simulation radius) since all effects that are dropped in the local simulations are expected to vanish as In this Letter, global simulations of a wellestablished test case are indeed shown to recover the fluxtube limit at each radius.

Dynamofree plasma in the reversed field pinch
View Description Hide DescriptionTransient application of a poloidal electric field to reversed field pinch(RFP)plasmas has led to a period in which dynamo activity (inherent in standard RFPplasmas) nearly vanishes. Measurements of the plasmaresistivity,current density and electric field profiles show the edgeapplied electric field accommodates Ohm’s law balance without a dynamo term over the entire cross section. Neoclassical theory accurately predicts the resistivity in the RFP, as the predicted resistivity profile (based on measurements of electron temperature, effective ionic charge, and twodimensional equilibrium effects) is in agreement with the ratio of the parallel electric field and current density profiles.
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 ARTICLES

 Nonlinear Phenomena, Turbulence, Transport

Reduction of asymmetry transport in the annular Penning trap
View Description Hide DescriptionIn the Penning trap, there is transport of electrons in the limit of zero gas pressure that arises from asymmetric stray electric fields. In an annular version of the Penning trap, this asymmetry transport is shown to be greatly reduced when the plasmafacing surfaces are coated with colloidalgraphite. In a separate device, an emissive probe is used to examine the space potential a few millimeters above coated and uncoated surfaces. It is found that the rms potential variation is approximately 250 mV for uncoated surfaces and 15 mV for coated surfaces. The characteristic length scale of the inhomogeneities is ∼1 cm. Glowdischarge cleaning, which is easily renewed, is shown to reduce the potential variation to the same level that is obtained with the colloidalgraphite coating.
 Basic Plasma Phenomena, Waves, Instabilities

Dust voids due to dustphasespace vortices in plasmas
View Description Hide DescriptionIt is shown that a dust void in a dusty plasma can be formed due to the presence of positive plasma potential, which is built up owing to a nonisothermal dust particle distribution arising from negatively charged dust particle trapping. It is found that the dust void shrinks with the increase of the dust charge density, while it enlarges with the increase of the dust temperature. The implications of our results to the possibility for the formation of dust voids in space dusty plasmas, particularly in Saturn’s dusty rings, is discussed.
 Nonlinear Phenomena, Turbulence, Transport

Compressive and rarefactive ionacoustic solitons in biion plasmas
View Description Hide DescriptionNonlinear propagation of ionacoustic solitary structures in plasmas with an admixture of heavy ions is studied in the wave frame, where they are stationary, using a recently developed gasdynamic approach, as an alternative to the conventional Sagdeev pseudopotential method. This viewpoint brings out the gasdynamic aspects, which then allow a characterization of the solitary wave structures in terms of the species’ sonic points, the global charge neutral points, and critical collective Mach numbers. It is shown that the concepts of a critical density in the Korteweg–de Vries (KdV) treatment, and of a changeover from compressive to rarefactive soliton character, correspond to the formation of a second charge neutral point (outside equilibrium) in the rarefactive regime, at which the electric stresses maximize. It is possible therefore that in certain regions of parameter space compressive and rarefactive solitons can coexist. The compressive solitons are not predicted by a weakly nonlinear KdV treatment, except very close to the changeover region at sufficiently small enough amplitudes, as typified by the “modified” KdV (mKdV) equation. Their prediction requires a fully nonlinear treatment. Existence criteria are deduced and evaluated numerically in the parameter space of solitonMach number and normalized density of the heavy species for compressive and rarefactive ionacoustic solitons in a plasma with negative ion admixture.
 Basic Plasma Phenomena, Waves, Instabilities

Probe induced voids in a dusty plasma
View Description Hide DescriptionAn experimental study of the formation of voids (dustfree regions) around negatively biased probes in a dusty plasma is described. Stable voids are maintained by the balance of electric and ion drag forces on the dust particles. A theoretical model is proposed to explain how the size of the void scales with the probe bias potential.
 Lowtemperature Plasmas, Plasma Applications, Plasma Sources, Sheaths

Selfconsistent nonlinear transmission line model of standing wave effects in a capacitive discharge
View Description Hide DescriptionIt has been shown previously [Lieberman et al., Plasma Sources Sci. Technol. 11, 283 (2002)], using a nonselfconsistent model based on solutions of Maxwell’sequations, that several electromagnetic effects may compromise capacitive discharge uniformity. Among these, the standing wave effect dominates at low and moderate electron densities when the driving frequency is significantly greater than the usual 13.56 MHz. In the present work, two different global discharge models have been coupled to a transmission line model and used to obtain the selfconsistent characteristics of the standing wave effect. An analytical solution for the wavelength λ was derived for the lossless case and compared to the numerical results. For typical plasma etching conditions (pressure 10–100 mTorr), a good approximation of the wavelength is where is the wavelength in vacuum, is the rf voltage magnitude in volts at the discharge center, l is the electrode spacing in meters, and f the driving frequency in hertz.
 Magnetically Confined Plasmas, Heating, Confinement

Nonlinear fluxes and forces from radiofrequency waves with application to driven flows in tokamaks
View Description Hide DescriptionNonlinear rfdriven sheared flows are of interest for turbulence control and basic physics experiments. Shortwavelength slow modes are required for efficient coupling of wave momentum to the plasma, requiring a kinetic hotplasma theory. Here, a guidingcenter formulation is developed which calculates the nonlinear particle and energy fluxes, energy absorption, and nonlinear forces on the plasma using a kinetic moment approach that is valid to first order in the ratio of the gyroradius compared to the wave envelope scale length and the plasma equilibrium scale length. Both the stress tensor and Lorentz force contribute to the net force on a fluid element. The forces driving fluxsurfaceaveraged flows in a tokamak are extracted from the parallel and toroidal components. It is shown that fluxsurfaceaveraged flows are driven by two classes of terms: direct absorption of wave momentum and dissipative stresses. Furthermore, the general kinetic expression for the force is shown to reduce to the standard coldfluid ponderomotive force in an appropriate limit, but in this limit no flows are driven.

Twistingtearing compound mode induced by energetic ions
View Description Hide DescriptionIt is shown that, during the burst of the high frequency twisting mode induced by energetic ions, significant reconnected flux can be transiently generated due to toroidal coupling with tearingparity mode. This forced reconnection is accompanied by a sudden jump of the local plasma rotation frequency, induced by electromagnetic torque, which tends to “lock” the twisting mode to the tearing mode at the coupled rational surface. Results are applied to interpret early experiments on the DIIID tokamak [Luxon et al., Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], where sawtooth inversion has been observed during high frequency intermediate mode number bursting activity [Heidbrink et al., Nucl. Fusion31, 1635 (1991)].

Doublekink fishbone instability caused by circulating energetic ions
View Description Hide DescriptionThe destabilization of double kink modes by the circulating energetic ions in tokamaks with the plasma current having an offaxis maximum is studied. It is shown that the highfrequency fishbone instability [energetic particle mode (EPM)] and the lowfrequency (diamagnetic) fishbones are possible for such an equilibrium, their poloidal and toroidal mode numbers being not necessarily equal to unity. A new kind of the EPM instability, “doublet fishbones,” is predicted. This instability is characterized by two frequencies; it can occur in a plasma with a nonmonotonic radial profile of the energetic ions when the particle orbit width is less than the width of the region where the mode is localized. It is found that the diamagnetic fishbone branch exists even when the orbit width exceeds the mode width; in this case, however, the instability growth rate is relatively small.

Local shear damping of ion and electron temperature gradient modes
View Description Hide DescriptionThe influence of local magnetic fieldline shear on linear toroidal ion and electron temperature gradient drift modes is studied within a local gyrokinetic model. Elementary representations of local shear functions are constructed to resemble generic geometric aspects of lowshear stellarators and divertortokamaks.Modes are damped for all these local shear structures although the amount of reduction of growth rates depends strongly on the form and magnitude of the structure.
 Ionospheric, Solarsystem, and Astrophysical Plasmas

The evolution of magnetic helicity in driven reconnection processes and the flux rope structures at the dayside magnetopause
View Description Hide DescriptionA twodimensional magnetohydrodynamics simulation with three components is carried out to explore the driven reconnection processes at the dayside magnetopause. The numerical results show significant enhancements of the magnetic field strength and the component at the centers of the recurrent plasmoidlike structures. Such features are in line with the signatures of flux transfer events which are envisaged as passages of magnetic flux tubes. The evolution of the topological structure in reconnected flux tubes is investigated in terms of the principle of magnetic helicity. It is found that the total magnetic helicity in the domain increases as time elapses due to the transportation of magnetic helicity flux through the system boundaries and a peak value of the magnetic helicity density always occurs in the central region of the reconnected flux tube. Furthermore, the gradual accumulation of in the current sheet is associated with the accumulated magnetic flux which is carried into this region by the plasma inflow. The concentration of the helicity density in the core of a flux tube arises from the enhancement of the axial field which is associated with the continuous addition of flux to the domain. The present results indicate that the occurrence of the flux rope structure in the vicinity of the magnetopause might be related to the solar wind plasmaflow.

Electron acceleration in the dynamic magnetotail: Test particle orbits in threedimensional magnetohydrodynamic simulation fields
View Description Hide DescriptionElectron acceleration and energetic electron flux increases in the inner tail are investigated on the basis of testparticle orbits in the dynamic fields of a threedimensional magnetohydrodynamic simulation of neutral line formation and dipolarization in the magnetotail. Past models have mostly considered equatorial orbits, using the gyrocenter drift approximation. In this paper, the investigation is extended to include nonequatorial drifts and full orbit integrations in regions where the drift approximation breaks down. Typical acceleration mechanisms consist of betatron acceleration at large pitch angles and Fermi acceleration at small pitch angles, resulting from the dipolarization and shortening of field lines moving earthward from the neutral line. In comparison, acceleration at the nearEarth neutral line plays a negligible role in flux increases observed in the near tail. Energetic electron fluxes appear preferentially enhanced around 90° pitch angle, so that restriction to those pitch angles might lead to an overestimate of flux increases. Otherwise, the results explain the observed limitation of the range of flux enhancements between a few keV and a few hundred keV.
 Lasers, Particle Beams, Accelerators, Radiation Generation

Relativistic and ponderomotive effects on laser plasma interaction dynamics
View Description Hide DescriptionThe combined effect of relativistic and ponderomotive nonlinearities on the propagation characteristics and modulation instability of a laser beam propagating through partially stripped plasma is studied. Under the present approach it is seen that the ponderomotive nonlinearity tends to defocus the laser beam as against the nonlinear relativistic selffocusing phenomenon. Also density perturbation arising due to the ponderomotive nonlinearity when combined with relativistic nonlinearity tends to increase the modulation instability of the laser beam. The peak growth rate is enhanced in comparison to the case where ponderomotive nonlinearity is neglected.
 Ionospheric, Solarsystem, and Astrophysical Plasmas

Electron heating and acceleration in the shock transition region: Background plasma parameter dependence
View Description Hide DescriptionIn addition to the wellknown plasma parameters of Mach number and plasma β, the other plasma parameter of the frequency ratio of the electron plasma to gyration, is emphasized as another important parameter for the plasma heating/acceleration process in the shock transition region. The process of energy conversion from the ion bulk flow into energetic electrons through field activity due to plasma instabilities cannot be described only by such macro averaged parameters as and β. The evolution of plasma instabilities largely depends on the background plasma properties, i.e., magnetic field and density which is represented by Recent studies have shown that electron phase space hole generation due to the nonlinear evolution of Buneman instability between reflected ions and inflow electrons plays an important role in particle energization in the shock transition region. However, even under the same Mach number and plasma β, the formation of electron phase space holes depends on the frequency ratio Moreover, depending on different heating and acceleration properties can be obtained. The effect of the frequency ratio on the electron energization is investigated in the shock transition region by using periodic simulations with realistic mass ratio. When no electron phase space hole is generated and mild heating of the electrons with a nonthermal energy spectrum is observed. In the region of 10 a clear series of electron phase space holes is generated and, after that, ion phase space holes are generated in the resultant hot electrons. These ion phase space holes play an important role in the subsequent strong electron energization.
 Basic Plasma Phenomena, Waves, Instabilities

Effect of secondary electron emission on the propagation of dust acoustic waves in a dusty plasma
View Description Hide DescriptionThe effect of secondary electron emission on dust acoustic wave (DA) propagation has been investigated based on orbit motion limited theory of dust grain charging. The emitted secondaries are assumed to have the same temperature as that of the ambient plasmaelectrons so that the plasma effectively consists of three components: the ions, electrons, and the variable charge dusts. Together with the effect of secondary emission, the effect of ion and electron capture and ionization of neutral atoms and recombination have been included in the ion and electron fluid equations. Small amplitude perturbation is considered about a charge neutral steady state. It is seen that if the dust charge is positive there may occur under certain conditions zero frequency exponentially growing perturbation about the equilibrium. The possibility of the occurrence of such purely growing mode in a dusty plasma was not noted earlier. The frequency and damping decrement of DA waves in dusty plasmas with negatively charged dust and also of DA waves with positively charged dust, when they exist, are determined. Comparisons with corresponding results of DA waves in the absence of secondary emission are exhibited graphically.
 Nonlinear Phenomena, Turbulence, Transport

Propagation of nonplanar dustacoustic envelope solitary waves in a twoiontemperature dusty plasma
View Description Hide DescriptionThe evolution of the cylindrical and spherical dustacoustic envelope solitary wave (DAESW) in an unmagnetized dusty plasma consisting of negatively charged dust fluid and ions of two different temperatures is investigated. By using the reductive perturbation method, the cylindrical and spherical geometrymodified nonlinear Schrödinger equation (GMNLSE) is obtained. The change of the DAESW amplitude due to the cylindrical and spherical geometry effects is deduced analytically. It is shown that there exist two time ranges. On the other hand, the wave amplitude changes with time τ as when the geometry effect is stronger and as when the geometry effect is weaker, where is the initial time, and (2) refers to the cylindrical (spherical) case. The theoretical results are verified by the numerical calculation for the GMNLSE. The modulational instability of dustacoustic waves governed by the GMNLSE is also presented. It is shown that the propagation of the DAESW in cylindrical geometry, spherical geometry, and planar onedimensional geometry are very different. The presence of a second component of ions would modify the nature of the modulation instabilities.