Volume 10, Issue 12, December 2003
 LETTERS


Transient ionization in plasmas produced by pointlike irradiation of solid Al targets
View Description Hide DescriptionTimeresolvedxray spectroscopy has been used to investigate ionizationdynamics of a micrometersized nanosecond laserplasma during the plasma startup phase. Experimental results are modeled using twodimensional hydrodynamic simulations and timedependent collisionalradiative calculations. The study clearly shows that, due to the rapid expansion cooling, xray emission originates predominantly from a welllocalized plasma region characterized by rapidly evolving hydrodynamic conditions. In this region, ionizationdynamics is found to depart substantially from the steadystate regime. The measurements provide clear evidence of this transient ionization regime showing good agreement with the timedependent calculations.

Electron bunch acceleration and trapping by the ponderomotive force of an intense shortpulse laser
View Description Hide DescriptionBy utilizing a pulsed laserbeam of mode, it was found numerically for the first time that an electron bunch can be effectively trapped by the transverse ponderomotive force in the transverse direction and at the same time accelerated by the longitudinal ponderomotive force to about 378 MeV at the laser peak intensity of In addition, the electron bunch size is preferably small: at this laser intensity the electron bunch thickness is in the longitudinal direction and the bunch radius is about 625λ in the transverse direction.

Electrostatic ioncyclotron waves in a currentless, anisotropic plasma with inhomogeneous flow
View Description Hide DescriptionThe linearized dispersion relation describing waves in a plasma having a uniform magnetic field, uniform density, inhomogeneous parallel (to the magnetic field) flow, and thermal anisotropy is used to determine the threshold condition for growth of an electrostatic ion cyclotron wave and its harmonics. The inclusion of moderate ion thermal anisotropy and parallelflow shear values typical of many space and laboratory plasmas, reduces the critical current necessary for instability growth to nearly zero. That an electrostaticinstability conventionally associated with strong field aligned currents can be excited in the absence of any field aligned current and only by moderate parallelflow shear suggests that ion cyclotron instabilities may play a larger role in many plasma environments than previously believed. The decrease in the instability threshold for increasing thermal anisotropy suggests that ion heating due to ion cyclotron waves may result in a positive feedback process absent in homogeneous plasmas.

Threedimensional isotropic magnetohydrodynamic turbulence and thermal velocity of the solar wind ions
View Description Hide DescriptionIt is shown that fluctuations of the solar wind ions velocity (as measured on the kinetic level by the SWICSdetector on board the Advanced Composition Explorer) exhibit statistical properties which are in very good agreement with predictions of the threedimensional (3D) isotropic magnetohydrodynamics (MHD)turbulencetheory [Biskamp and Müller, Phys. Plasmas 7, 4889 (2000)]. The question is: How can 3D isotropic MHD be used to describe the interaction of the solar wind magnetic field with the kinetic (collisionless) ions?
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

Lowfrequency waves in collisional complex plasmas with an ion drift
View Description Hide DescriptionA selfconsistent model of lowfrequency linear waves in collisional complex (dusty) plasmas with an ion drift is presented. Plasma conditions relevant to recent wave experiments under microgravity conditions are considered. Ionneutral, iondust, and neutraldust collisions, as well as external forces acting on the grains and grain charge variations in the presence of the wave are taken into account. A linear dispersion relation is obtained and some limiting cases are analyzed. Comparison of the obtained theoretical results with the experiments under microgravity conditions is presented.

Excitation of nonreciprocal electromagnetic surface waves in semibounded magnetized plasmas by an electron beam
View Description Hide DescriptionThe dispersion relation of nonreciprocal electromagneticsurface waves propagating on the magnetized plasma–vacuum interface is obtained. Furthermore, the dependency of penetration depth on the magnetic field strength and its directivity is investigated. Finally, it will be shown that by an electron beam flowing on the plasma surface, aforementioned waves can be excited.

Resonance cones in a dusty magnetized plasma
View Description Hide DescriptionA new diagnostic method for magnetized dusty plasmas, the excitation of lower hybridresonance cones, is investigated experimentally. The resonance cone is excited with a small antenna, and the angular distribution of the wave field with respect to the magnetic field shows a resonant enhancement, which shifts according to the free electron density. It is demonstrated that dust reduces the free electron density in agreement with Langmuir probe results. Wave damping by scattering effects is found negligible.
 Nonlinear Phenomena, Turbulence, Transport

Linear theory of nonlocal transport in a magnetized plasma
View Description Hide DescriptionA system of nonlocal electrontransport equations for small perturbations in a magnetized plasma is derived using the systematic closure procedure of Bychenkov et al. [Phys. Rev. Lett. 75, 4405 (1995)]. Solution to the linearized kinetic equation with a Landau collision operator is obtained in the diffusive approximation. The Fourier components of the longitudinal, oblique, and transversal electron fluxes are found in an explicit form for quasistatic conditions in terms of the generalized forces: the gradients of density and temperature, and the electric field. The full set of nonlocal transport coefficients is given and discussed. Nonlocality of transport enhances electron fluxes across magnetic field above the values given by strongly collisional local theory. Dispersion and damping of magnetohydrodynamic waves in weakly collisional plasmas is discussed. Nonlocal transporttheory is applied to the problem of temperature relaxation across the magnetic field in a laser hot spot.

The concept of collision strength and a unified kinetic calculation for hardsphere interactions and inverse square force law interactions
View Description Hide DescriptionWith a concept of collision strength and other associated definitions, a unified kinetic theory for both hardsphere interactions and inverse square force law interactions is developed. Collision frequencies that associate with many kinds of physical terms are calculated and expressed by a series special function Among them are arbitrary higher order linear Fokker–Planck coefficients, collision frequency, and energy exchange frequency. In case of a twotemperature system, the total collision rate, energy exchange rate, and collision strength rate are calculated and expressed in a uniform expression. A primitive form of Coulomb logarithm is found by comparing the exact form of equilibration time with Spitzer’s result. Many unifications are found from the unified expression. The threshold value of collision strength has unified activation energy in chemical reaction rate theory and ionizationenergy in Thomson’s classical ionizationtheory. An incomplete gamma function has unified Arrhenius exponential coefficient in chemical reaction rate theory and Coulomb logarithm in plasma physics.

The twodimensional magnetohydrodynamic Kelvin–Helmholtz instability: Compressibility and largescale coalescence effects
View Description Hide DescriptionThe Kelvin–Helmholtz (KH)instability occurring in a single shear flow configuration that is embedded in a uniform flowaligned magnetic field, is revisited by means of high resolution twodimensional magnetohydrodynamic simulations. First, the calculations extend previous studies of magnetized shear flows to a higher compressibility regime. The nonlinear evolution of an isolated KH billow emerging from the fastest growing linear mode for a convective sonic Mach number layer is in many respects similar to its less compressible counterpart (Mach In particular, the disruptive regime where locally amplified, initially weak magnetic fields,control the nonlinear saturation process is found for Alfvén Mach numbers The most notable difference between vs layers is that higher density contrasts and fast magnetosonic shocklet structures are observed. Second, the use of adaptive mesh refinement allows to parametrically explore much larger computational domains, including up to 22 wavelengths of the linearly dominant mode. A strong process of largescale coalescence is found, whatever the magnetic field regime. It proceeds through continuous pairing/merging events between adjacent vortices up to the point where the final largescale vortical structure reaches the domain dimensions. This pairing/merging process is attributed to the growth of subharmonic modes and is mainly controlled by relative phase differences between them. These gridadaptive simulations demonstrate that even in very weak magnetic field regimes the largescale KH coalescence process can trigger tearingtype reconnection events previously identified in cospatial current–vortex sheets.

Electrostatic instabilities and nonlinear structures of lowfrequency waves in nonuniform electron–positron–ion plasmas with shear flow
View Description Hide DescriptionIt is found that the lowfrequency ion acoustic and electrostaticdrift waves can become unstable in uniform electron–ion and electron–positron–ion plasmas due to the ion shear flow. In a collisional plasma a driftdissipative instability can also take place. In the presence of collisions the temporal behavior of nonlinear driftdissipative mode can be represented in the form of wellknown Lorenz and Stenflo type equations that admit chaotic trajectories. On the other hand, a quasistationary solution of the mode coupling equations can be represented in the form of monopolar vortex. The results of the present investigation can be helpful in understanding electrostatic turbulence and wave phenomena in laboratory and astrophysical plasmas.

Nonlinear slow shear Alfvén wave in electron–positron–ion plasmas
View Description Hide DescriptionNonlinear solitary structures of an arbitrary amplitude slow shear Alfvén wave (SSAW) in ideal electron–positron–ion plasmas are studied. It is found that the electron density dips of SSAW are formed in the super Alfvénic region. The amplitude and the width of the nonlinear shear Alfvén wave reduces with the increase in the concentration of positrons in electron–ion plasmas. The width of the soliton also depends upon the direction of propagation of the perturbation in both and plasmas. The numerical results for several different cases have also been presented for illustrative purposes.

Dust acoustic solitary waves and double layers in a dusty plasma with trapped electrons
View Description Hide DescriptionThe effect of variable dust charge, dust temperature, and trapped electrons on small amplitude dust acoustic waves is investigated. It is found that both compressive and rarefactive solitons as well as double layers exist depending on the nonisothermality parameter. A modified Korteweg–de Vries is derived. At critical density, the Korteweg–de Vries equation is obtained. Employing quasipotential analysis, the Sagdeev potential equation with the inclusion of different new effects has been derived. Because of the presence of free and trapped electrons, the plasma acoustic wave has gained features of various solitary waves. The Sagdeev potential equation, at a small amplitude, shows that the ordering of nonisothermality plays a unique role. In the case of a plasma with firstorder nonisothermality, the Sagdeev potential equation shows the compressive solitary wave propagation, while for plasma with higherorder nonisothermality, the solution of this equation reveals the coexistence of both compressive and rarefactive solitary waves. In addition, for certain plasma parameters, the solitary wave disappears and a double layer is expected. Again, with the better approximation in the Sagdeev potential equation, more features of solitary waves, e.g., spiky and explosive, along with the double layers, are also highlighted. The findings of this investigation may be useful in understanding laboratory plasma phenomena and astrophysical situations.

Confinement and dynamical regulation in twodimensional convective turbulence
View Description Hide DescriptionIn this work the nature of confinement improvement implied by the selfconsistent generation of mean flows in twodimensional convective turbulence is studied. The confinement variations are linked to two distinct regulation mechanisms which are also shown to be at the origin of lowfrequency bursting in the fluctuation level and the convective heat flux integral, both resulting in a state of largescale intermittency. The first one involves the control of convective transport by sheared mean flows. This regulation relies on the conservative transfer of kinetic energy from tilted fluctuations to the mean component of the flow. Bursting can also result from the quasilinear modification of the linear instability drive which is the mean pressure gradient. For each bursting process the relevant zerodimensional model equations are given. These are finally coupled in a minimal model of convection in fluids and plasmas. The results of the modeling are used to discuss confinement scaling and intermittency, and in a heuristic way, more complex issues such as criticality and transport avalanches.

Unified form for parallel ion viscous stress in magnetized plasmas
View Description Hide DescriptionIn this work a unified form for the parallel ion viscous stress in a magnetized plasma is presented. Approximately valid for arbitrary collisionality, the integral nature of this generalized closure results from assuming the maximal ordering between collisional pitchangle scattering and free streaming effects and from taking a Chapman–Enskogtype approach which includes the parallel ion viscous stress itself as a drive. The ion drift kinetic equation is solved in a sheared slab using an expansion in eigenfunctions of the Lorentzscattering operator. Integrating the coefficient equations in space and taking the proper velocity space moments couples the parallel viscous stress closure to an integral momentum restoring term, thus generalizing the concept of momentum conservation for simplified Coulomb collision operators. The integral closure involves following ions along magnetic field lines which are the ideal, timeindependent characteristics of the perturbed distribution function. The fact that the viscous stress and momentum restoring term appear in the kernels of these field line integrals means that in general the closure has the form of coupled Volterra equations with an inhomogeneous term supplied by the traditional flow gradient drive. It is shown that the unified closure agrees both qualitatively and quantitatively with previous results and hence represents a generalized physical form for the parallel ion viscous stress in magnetized plasmas.
 Magnetically Confined Plasmas, Heating, Confinement

Marginal stability boundaries for infiniten ballooning modes in a quasiaxisymmetric stellarator
View Description Hide DescriptionA method for computing the ideal magnetohydrodynamic(MHD)stability boundaries in threedimensional equilibria is employed. Following Hegna and Nakajima [Phys. Plasmas 5, 1336 (1998)], a twodimensional family of equilibria is constructed by perturbing the pressure and rotationaltransform profiles in the vicinity of a flux surface for a given stellarator equilibrium. The perturbations are constrained to preserve the MHD equilibrium condition. For each perturbed equilibrium, the infinite ballooning stability is calculated. Marginal stability diagrams are thus constructed that are analogous to diagrams for axisymmetric configurations. A quasiaxisymmetric stellarator is considered. Calculations of stability boundaries generally show regions of instability can occur for either sign of the average magnetic shear. Additionally, regions of secondstability are present.

Plasma flow and confinement in the vicinity of a rotating island in tokamaks
View Description Hide DescriptionThe theory for the electric field and the plasma confinement in the vicinity of a magnetic island in tokamaks [Phys. Plasmas 9, 3470 (2002)] is extended to the situation where the magnetic island is rotating. The electric field that is parallel to the magnetic field, is assumed to vanish. With this assumption, the theory for a nonrotating island is applicable to a rotating island if the radial electric field in the nonrotating theory is replaced by the radial gradient of F. Here, F is the part of the electrostatic potential that is constant on the rotating island magnetic surface. As an application of the theory, the radial electric field,toroidal flow speed, ambipolar particle flux, heat flux, and island rotation frequency in the collisionless regime are also presented.

Role of the modes in the edge region of a reversedfield pinch during pulsed poloidal current drive
View Description Hide DescriptionChanges in the edge profile of soft xray emission during pulsed poloidal current drive (PPCD) experiments are in agreement with the modifications of the plasma surface, as seen from magnetic measurements. In particular, the modes resonant at the reversal radius have been analyzed. A comparison with an islandreconstruction model shows that PPCD reduces the bulging associated with the modes by shifting inward the reversal surface, where the island is located. Regarding the total amplitude, there are evidences that during PPCD, on average, it does not change significantly. Additional data coming from standard discharges support the idea that two competing phenomena act during PPCD: one is the inward shift of the reversal, which destabilizes the modes; the other one is the decrease of the fluctuations, which acts in the opposite direction.

Driftordered fluid equations for fieldaligned modes in lowβ collisional plasma with equilibrium pressure pedestals
View Description Hide DescriptionStarting from the complete short meanfree path fluid equations describing magnetized plasmas, assuming that plasma pressure is small compared to magnetic pressure, considering fieldaligned plasma fluctuations, and adopting an ordering in which the plasma species flow velocities are much smaller than the ion thermal speed, a system of nonlinear equations for plasma density, electron and ion temperatures, parallel ion flow velocity, parallel current, electrostatic potential, perturbed parallel electromagnetic potential, and a perturbed magnetic field is derived. The equations obtained allow sharp equilibrium radial gradients of plasma quantities, and are shown to contain the neoclassical (Pfirsch–Schlüter) results for plasma current, parallel ion flow velocity (with the correct temperature gradient terms), and parallel gradients of equilibrium electron and ion temperatures. Special care is taken to ensure the divergencefree character of perturbed magnetic field and total plasma current, as well as local particle number and total energy conservation.

Stability of the iontemperaturegradientdriven mode with negative magnetic shear
View Description Hide DescriptionA model for transition to the enhanced reverse shear or negative central shear mode triggered in tokamaks is proposed. This model takes into account the linear behavior of the ion temperature gradient(ITG) driven perturbation, considered nowadays as the dominant source of anomalous energy losses in the low confinement mode, in the presence of a radially varying parallel velocity. Analytic and numerical studies show that when the magnetic shear has the same sign as the second derivative of the parallel velocity with respect to the radial coordinate, the ITGmode may become more unstable. On the other hand, when the magnetic shear has the opposite sign to the second derivative of the parallel velocity, the linear ITGmode may be completely stabilized. This result is similar to our earlier works on parallel velocity shear instability [S. Sen et al., Phys. Plasmas 7, 1192 (2000); D. R. McCarthy et al., Phys. Plasmas 8, 3645 (2001)].