Volume 12, Issue 7, July 2005
 LETTERS


Vlasov–Maxwell equilibrium solutions for Harris sheet magnetic field with Kappa velocity distribution
View Description Hide DescriptionAn exact solution of the steadystate, onedimensional Vlasov–Maxwell equations for a plasma current sheet with oppositely directed magnetic field was found by Harris in 1962. The socalled Harris magnetic field model assumes Maxwellian velocity distributions for oppositely drifting ions and electrons and has been widely used for plasma stability studies. This paper extends Harris solutions by using more general distribution functions that incorporate Maxwellian distribution in the limit of . A new functional form for the plasma pressure as a function of the magnetic vector potential is found and the magnetic field is a modified function. In the extended solutions the effective temperature is no longer spatially uniform like in the Harris model and the thickness of the current layer decreases with decreasing .

Observation of persistent edge current driven by coaxial helicity Injection
View Description Hide DescriptionCoaxial Helicity Injection (CHI) has been used on the National Spherical Torus Experiment [Ono et al., Nucl. Fusion40, 557 (2000)], the Helicity Injected Torus (HIT) [Nelson et al., Phys. Rev. Lett.72, 3666 (1994)] and HITII [Jarboe et al., Phys. Plasmas5, 1807 (1998)] to initiate plasma and to drive up to 400 kA of toroidalcurrent. On HITII, CHI initiated discharges have been successfully coupled to Ohmic sustainment [Raman et al., Phys. Plasmas11, 2565 (2004)]. This paper presents the first results on the use of CHI to demonstrate the persistence of edge current drive in a preestablished single null diverted inductive discharge. Edge current drive has the potential to improve plasma stability limits [Menard et al., Nucl. Fusion37, 595 (1997)]. While most current drive methods drive current in the interior of the plasma, CHI is well suited for driving current in the edge plasma.

Cylindrical nebulons, symbolic computation and Bäcklund transformation for the cosmic dust acoustic waves
View Description Hide DescriptionIn a cosmic dusty plasma, the dustacousticwave propagation may be described by a cylindrical KadomtsevPetviashvili equation. In this Letter, for such modeling of environments like supernova shells, Saturn’s Fring, etc., cylindrical nebulons and an autoBäcklund transformation are presented via symbolic computation. Nebulon structures are discussed, and possibly observable effects are proposed for cosmic plasmas.

THz instability by streaming carriers in high mobility solidstate plasmas
View Description Hide DescriptionIt is described how the interaction of streaming electron plasma waves with lattice waves may lead to a highly unstable coupled plasmonphonon excitation in the THz range, in high mobility semiconductor materials. The underlying physical mechanism is related to the ubiquitous beamplasma instability in electrostatically coupled fluids. Continuous amplification in transit, rather than localized amplification at the system boundary reflections, yields high growth rates and may lead to efficient THz generation.

Attractive force on like charges in a complex plasma
View Description Hide DescriptionElectrostatic attractive force between dust particles in a complex plasma with ion flow is studied. It is shown that there is an attractive force between a pair of dust particles along the ion flow as well as perpendicular to the ion flow. The attractive force perpendicular to the flow results from a release of thermodynamic free energy in charged fine particles, while an attractive force associated with the wake potential acts on a pair of dust particles aligned with the ion flow. Recent experimental observation of the sharp boundary of a void in a complex plasma is interpreted as a result of the attractive force.

Magnetic reconnection in nontoroidal plasmas
View Description Hide DescriptionMagnetic reconnection is a major issue in solar and astrophysical plasmas. The mathematical result that the evolution of a magnetic field with only point nulls is always locally ideal limits the nature of reconnection in nontoroidal plasmas. Here it is shown that the exponentially increasing separation of neighboring magnetic field lines, which is generic, tends to produce rapid magnetic reconnection if the length of the field lines is greater than about 20 times the exponentiation, or Lyapunov, length.

Fabrication of spatial transientdensity structures as highfield plasma photonic devices
View Description Hide DescriptionFabrication of periodic transientdensity structures in a gas jet with a boundary scale length approaching was demonstrated. This was achieved by passing an ultrashort highintensity laser pulse through a patterned mask and imaging the mask onto the target plane. Gas/plasma density at the laserirradiated regions drops as a result of hydrodynamicexpansion following ionization and heating by the laser pulse. The fabrication of gas/plasma density structures with such a scheme is an essential step in the development of plasmaphotonicdevices for applications in highfield physics.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Covariant kinetic dispersion theory of linear waves in anisotropic plasmas. III. Counterstreaming plasmas
View Description Hide DescriptionThe properties of longitudinal and transverse oscillations in unmagnetized counterstreaming Maxwellianplasmas of arbitrary composition for waves propagating parallel to the stream direction are investigated on the basis of Maxwell equations and the relativistic Vlasov equation. These dispersion relations describe the linear response of the system to the initial perturbations and thus define all existing linear parallel propagating plasma modes in the system. By analytic continuation the dispersion relations in the whole complex frequency plane are constructed. The dispersion relations hold for any values of the plasma temperatures, streaming velocity, intensities of the two streams, and for all wave number and frequency values of the oscillations. In the limit of nonrelativistic plasma temperatures these general dispersion relations can be expressed in terms of the welldocumented Fried and Conte plasmadispersion function. Only in the case of infinitely large speed of light they agree with the standard nonrelativistic results.

Measurement of ion motion in a shear Alfvén wave
View Description Hide DescriptionIn this study, the technique of laserinduced fluorescence(LIF) has been used to meausure and the and polarization drifts of shear Alfvén waves in the Large Plasma Device at UCLA [W. Gekelman, H. Pfister, Z. Lucky, J. Bamber, D. Leneman, and J. Maggs, Rev. Sci. Instrum.62, 2875 (1991)]. The waves were launched by an antenna located at the end of the device and were observed to propagate along the axis of a 9 m long, 40 cm diameter cylindrical argon plasma in the kinetic regime , with . Care was taken to record the measurements from various diagnostics at the same spatial positions on four crosssectional planes along the length of the plasma. Twodimensional LIFmeasurements of the ion drifts perpendicular to were undertaken. Ion drifts were observed to be as large as 14% of the ion thermal speed. The ion polarization and drifts were distinguished by their phase relation to . The measured drifts are compared to kinetic theory. (the transverse component of ) was computed from the drift velocities, and was estimated from .

Laserinducedfluorescence characterization of velocity shear in a magnetized plasma column produced by a segmented, machine source
View Description Hide DescriptionControl over the radial profile of one component of ion drift speed , either perpendicular or parallel to the magnetic field, is demonstrated in the magnetized plasma column for two configurations of a doubleended machine in which one of the hot plates is segmented into a central disk and two concentric annuli. Each hot plate can be heated to , and the segmented hot plate is bombarded with barium vapor, resulting in quasineutral, magnetized, bariumion plasma. Since the electric potential in the plasma magnetically maps to the segments, the radial profile of plasma potential, and hence the flow, can be made inhomogeneous. This configuration is referred to as the perpendicular configuration. A negatively biased, crosssectional mesh, inserted between the two ends of the plasma column, reflects electrons from each source causing the radial profile of potential on the nonsegmented source side of the mesh to become uniform. In traversing the mesh sheath from the segmented source side, where the radial potential profile is inhomogeneous, to the nonsegmented source side, where the radial potential profile is homogeneous, the ions experience an axial acceleration that has an inhomogeneous radial profile. This configuration with the mesh is referred to as the parallel configuration. Laserinducedfluorescence is used to measure the ionvelocitydistribution function on the nonsegmentedsource side with and without the mesh inserted. Measurements show that the above configurations result in ion temperatureanisotropy,, and radially nonuniform ion flows with a maximum in the shear profile as large as and as large as where is the ion gyrofrequency.

Energy loss for the assemblies of charged projectiles in a dusty plasma
View Description Hide DescriptionExpressions for the electrostatic potential and the energy loss are derived for a variety of arrays of dust grain projectiles, arranged at different orientations and separations and moving with a constant velocity along the axis in an unmagnetized collisionless dusty plasma, using linear dielectric theory. By employing the dielectric constant of dust acoustic wave, the Debye and wake potentials are calculated for the projectiles. It is found that a projectile moving with high speed forms a negative wake behind and a shock in front of it. Further, for particular values of interspacing, the three different assemblies exhibit almost similar behavior of the energy loss and thus permit the possibility of all of them to move in unison to establish a dynamical equilibrium in a hexagonal geometry.

Numerical investigation of threedimensional singlespecies plasma equilibria on magnetic surfaces
View Description Hide DescriptionPresented for the first time are numerical solutions to the threedimensional nonlinear equilibrium equation for singlespecies plasmas confined on magnetic surfaces and surrounded by an equipotential boundary. The majorradial shift of such plasmas is found to be outward, qualitatively similar to the Shafranov shift of quasineutral plasmas confined on magnetic surfaces. However, this is the opposite of what occurs in the pure toroidal field equilibria of nonneutral plasmas (i.e., in the absence of magnetic surfaces). The effect of varying the number of Debye lengths in the plasma for the threedimensional (3D) model is in agreement with previous 2D calculations: the potential varies significantly on magnetic surfaces for plasmas with few Debye lengths , and tends to be constant on surfaces when many Debye lengths are present . For the case of a conducting boundary that does not conform to the outer magnetic surface, the plasma is shifted towards the conductor and the potential varies significantly on magnetic surfaces near the plasma edge. Debye shielding effects are clearly demonstrated when a nonuniform bias is applied to the boundary. Computed equilibrium profiles are presented for the Columbia NonNeutral Torus [T. S. Pedersen, A. H. Boozer, J. P. Kermer, R. Lefrancois, F. Dahlgren, N. Pomphrey, W. Reiersen, and W. Dorland, Fusion Sci. Technol.46, 200 (2004)], a stellarator designed to confine nonneutral plasmas.

Nonlinear Landau damping in nonextensive statistics
View Description Hide DescriptionThe evolution of electrostatic waves, in unmagnetized collisionless plasmas, is numerically investigated by using a semiLagrangian VlasovPoisson code, in the fully nonlinear regime and in the context of the nonextensive statistics proposed by Tsallis [C. Tsallis, J. Stat. Phys.52, 479 (1988)]. The effect of the Landau damping saturation, due to the nonlinear waveparticle interaction, is analyzed as a function of different values of the nonextensive parameter , which quantifies the degree of nonextensivity of the system. A preliminary linear study is performed in order to compare the analytical results for the frequency and the damping rate of the electric oscillations, with the quantities obtained from the numerical simulations. In the nonlinear regime, the time evolution of the electric field amplitude shows how the nonMaxwellian shape of the equilibrium distribution function drastically modifies the energy exchange between wave and resonant particles and determines the saturation level of the electric field amplitude, in the longtime oscillating regime. A broad spectrum for the electrostaticoscillations is obtained in the case of the initial distribution functions with , while in the case just a monochromatic component is visible.

Kinetic theory of lowfrequency instability of discharge plasma
View Description Hide DescriptionUsing the kinetic equation with BGK (Bhatnagar–Gross–Krook) collision integral and taking into account the relaxation of electron density and temperature, the lowfrequency instability of a constantcurrent carrying discharge is investigated in a weakly ionized and nonmagnetized plasma. It is shown that this instability is caused by the stimulated scattering of drifting electrons from the relaxation oscillations of the electron temperature.

A relativistic beamplasma system with electromagnetic waves
View Description Hide DescriptionA nonlinear multiwave model that describes the interaction of an electron beam, plasma waves, and electromagnetic waves in a cold plasma is derived and studied. The derivation, which is based on slow amplitude and phase change approximations, begins with the electromagneticLagrangian coupled to an electron beam, a background plasma, and electrostatic and electromagnetic waves. The model obtained is finite dimensional, and allows for efficient computational and analytical study. Numerical computations demonstrate that with the inclusion of an electromagnetic wave with the plasma wave, the beamplasma instability is suppressed. If two electromagnetic waves that satisfy a beatwave matching condition are included, the plasma wave is seen to be amplified provided the beatwave amplitude exceeds a certain threshold.

Waves in the Hallmagnetohydrodynamics model
View Description Hide DescriptionThe three magnetohydrodynamic(MHD)waves are followed as they transition under the influence of an increasingly strong Hall current effect to the characteristic waves of the HallMHD model. Also followed are the wave normal surfaces and the ray surfaces (approximating wave fronts) of these waves. The changes in the nature of the waves are found to be considerable, and are described both analytically and numerically. Most notably, the incompressible MHD shear Alfvén wave becomes a compressible fluiddynamical wave with negligible perturbation of the electromagnetic field, while the two MHD compressible waves become incompressible, the fast wave becoming mostly electromagnetic and the slow wave becoming mostly fluiddynamical.

Melting dynamics of finite clusters in dusty plasmas
View Description Hide DescriptionThe nonequilibrium meltingdynamics of finite twodimensional dust clusters is investigated experimentally. Nonequilibrium melting is induced by placing a single particle in the layer below the actual cluster which leads to unstable oscillations of the lower particle. The dynamic and thermodynamic properties during the twostep melting process have been investigated. Using the novel technique of singular value decomposition, the driving modes have been unambiguously related to the unstable oscillation. These modes are not accessible by normal mode analysis. Further it has been observed that angular melting occurred prior to the radial melting. Also, clusters with smaller number of particles revealed stronger heating than larger. This behavior is attributed to constant energy input by the unstable oscillation. Finally, clusters consisting of smaller particles are found to be more susceptible to fluctuations of the plasma parameters leading to a less pronounced twostep melting.

Quantum electrodynamical effects in dusty plasmas
View Description Hide DescriptionA new nonlinear electromagnetic wave mode in a magnetized dusty plasma is predicted. Its existence depends on the interaction of an intense circularly polarized electromagnetic wave with a dusty plasma, where quantum electrodynamical photonphoton scattering is taken into account. Specifically, we consider a dusty electronpositronion plasma and show that the propagation of the new mode is admitted. It could be of significance for the physics of supernova remnants and in neutron star formation.

Current singularities at finitely compressible threedimensional magnetic null points
View Description Hide DescriptionThe formation of current singularities at linetied two and threedimensional (2D and 3D, respectively) magnetic null points in a nonresistive magnetohydrodynamic environment is explored. It is shown that, despite the different separatrix structures of 2D and 3D null points, current singularities may be initiated in a formally equivalent manner. This is true no matter whether the collapse is triggered by flux imbalance within closed, linetied null points or driven by externally imposed velocity fields in open, incompressible geometries. A Lagrangian numerical code is used to investigate the finite amplitude perturbations that lead to singular current sheets in collapsing 2D and 3D null points. The form of the singular current distribution is analyzed as a function of the spatialanisotropy of the null point, and the effects of finite gas pressure are quantified. It is pointed out that the pressure force, while never stopping the formation of the singularity, significantly alters the morphology of the current distribution as well as dramatically weakening its strength. The impact of these findings on 2D and 3D magnetic reconnection models is discussed.
 Nonlinear Phenomena, Turbulence, Transport

Ion thermal pressure effects on dust ion acoustic solitary waves in a dusty plasma obliquely propagating to an external magnetic field
View Description Hide DescriptionEffects of the isothermal ion pressure on the dust ion acoustic solitary waves (DIASWs) in a dusty plasma, which are obliquely propagating to an external magnetic field, are investigated based on the Sagdeev potential. It is found that as the ion temperature increases the speed of the DIASW increases. The increase is more effective for a higher value of the directional cosine . The small amplitude solutions of the Sagdeev potential expanded up to and , as was done in the previous study [Choi et al., Phys. Plasmas12, 022304 (2005)], exhibit different behaviors with respect to the change in the ion temperature. The width and height of a double layer are found to decrease with the increase in the ion temperature for both the small and the large amplitude solutions.