Volume 12, Issue 11, November 2005
Index of content:
 LETTERS


The phasespaceresolved twopoint correlation function of ion density fluctuations
View Description Hide DescriptionIon density fluctuations are resolved in space, time, and parallel velocity using laserinduced fluorescence and a twopoint correlation technique. These measurements reveal two parallel wavelengths of distinct correlation lengths, one of which is explained by fluid theory. The other wavelength (referred to as the kinetic component) is ion velocity dependent. The effect of collisions on the fluctuation spectrum is reported. Ion velocitydependent measurements of the fluctuationinduced transport rate are also presented.

Highdensity and high fuel assembly for fastignition inertial confinement fusion
View Description Hide DescriptionScaling relations to optimize implosion parameters for fastignition inertial confinement fusion are derived and used to design highgain fastignition targets. A method to assemble thermonuclear fuel at high densities, high , and with a smallsize hot spot is presented. Massive cryogenic shells can be imploded with a low implosion velocity on a low adiabat using the relaxationpulse technique. While the low yields a small hot spot, the low leads to large peak values of the density and areal density. It is shown that a laser can assemble fuel with , , , , and a hotspot volume of less than 10% of the compressed core. If fully ignited, this fuel assembly can produce high gains of interest to inertial fusion energy applications.

Broadrange neutron spectra identification in ultraintense laser interactions with carbondeuterated plasma
View Description Hide DescriptionDetailed neutron energy spectra produced from a CD2 target irradiated by a , , laser at an intensity of have been studied. Wideranging neutronspectra were observed from two different observation angles 20° and 70° relative to the rearside target normal. The experiment and numerically calculated spectra, by a threedimensional Monte Carlo code, indicate that the range of the measured spectra is larger than that produced by the reaction. An interpretation for the measured spectra is introduced by considering the and reactions. In addition, the study revealed that the neutronspectra produced by the – and –reactions can overlap that produced by the –reaction, and due to their high cross sections, comparing to the –reaction, both of them effectively participate in the neutron yield.

Stochastization as a possible cause of fast reconnection in the frequently interrupted regime of neoclassical tearing modes
View Description Hide DescriptionThe role of stochastization of magnetic field lines in fast reconnection phenomena occurring in magnetized fusion plasma is analyzed. A mapping technique is applied to trace the field lines of toroidally confined plasma where the perturbation parameter is expressed in terms of experimental perturbation amplitudes determined from the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak[S. Günter, C. Angioni, M. Apostoliceanu et al. , Nucl. Fusion45, S98 (2005)]. It is found that fast reconnection observed during amplitude drops of the neoclassical tearing mode instability in the frequently interrupted regime can be related to stochastization.

Observation of core electron temperature rise in response to an edge cooling in toroidal helical plasmas
View Description Hide DescriptionThe first observation of a significant rise of core electron temperature in response to edge cooling in a helical plasma has been made on the Large Helical Device [O. Motojima et al. , Phys. Plasmas6, 1843 (1999)]. When the phenomenon occurs, the electron heat diffusivity in the core region is reduced abruptly without changing local parameters in the region of interest. Therefore the phenomenon can be regarded as a socalled “nonlocal” electron temperature rise observed so far only in many tokamaks.

 ARTICLES


Basic Plasma Phenomena, Waves, Instabilities

Nonlinear hydromagnetic waves in twoionspecies plasmas
View Description Hide DescriptionA theoretical study has been carried out to see the modification of lowdensity electronion plasma dynamics in the presence of heavy thirdion species with positive as well as negative charges. It has been shown that the behavior of such a plasma differs from the behavior of a singleionspecies plasma. The stationary state nonlinear equations were solved in different species with respect to charges and masses, and different plasma profiles have been displayed. The weakly nonlinear case has been discussed to demonstrate the lowamplitude solitons.

Collisionless expansion of a Gaussian plasma into a vacuum
View Description Hide DescriptionThe collisionless expansion into a vacuum of a onedimensional Gaussian plasma bunch of size much larger than the Debye length is studied both analytically and with a hybrid Lagrangian code. Both the case of an isothermal plasma fed by an external energy source and the case of an energyconserving plasma are studied. It is shown that chargeseparation effects are responsible for deviations with respect to known selfsimilar solutions. In particular, wave breaking occurs because the inner parts of the expanding plasma acquire faster velocities than the outer parts. As a result an ion front appears at the edge of the plasma. In the isothermal case the selfsimilar solution progressively applies to the whole plasma, while the fastest ion’s velocity asymptotically reaches the same values as in the sharp boundary case. In the energyconserving case the selfsimilar description only applies to a part of the plasma, which approximately corresponds to the part which was initially quasineutral, and the final velocity of the fastest ions is significantly smaller than in the sharp boundary case.

Effects of ionization on the collisional streaming instability
View Description Hide DescriptionThe influence of inelastic collisions on the collisional streaming instability is studied. The dispersion equation of the system is derived in the regime of strongly magnetized electrons and magnetized ions. The linear stability of the system is investigated. The numerical results are applied to a laboratory argon plasma.

Linear and nonlinear dynamics of a dust bicrystal consisting of positive and negative dust particles
View Description Hide DescriptionA dusty plasma crystalline configuration consisting of charged dust grains of alternating charge sign and mass is considered. Both charge and mass of each dust species are taken to be constant. Considering the equations of longitudinal motion, a dispersion relation for linear longitudinal vibrations is derived from first principles and then analyzed. Two harmonic modes are obtained, namely, an acoustic mode and an inversedispersive opticlike one. The nonlinear aspects of acoustic longitudinal dust grain motion are addressed via a generalized Boussinesq (and, alternatively, a generalized Korteweg–de Vries) description.

Resistive and viscous dynamics of finiteamplitude shear waves at a magnetic point
View Description Hide DescriptionThe dynamics and dissipation of axial shear waves, superposed on a planar magnetic point in a resistiveviscous incompressible plasma, are analyzed numerically and analytically. The interplay of viscous and resistive effects is demonstrated by deriving solutions for various values of the scalar coefficients of viscosity and resistivity. These solutions show that viscousresistive coupling can dramatically affect the global energy dissipation. When either viscosity or resistivity vanishes, the solutions are characterized by oscillatory decaying eigenmodes that maintain equipartition between the magnetic and kinetic energies. This behavior persists when resistivity is the dominant dissipation mechanism. When viscosity is the dominant dissipation mechanism, initial oscillations are followed by exponential decay at sufficiently long times. The applicability of the results to flares in solar active regions, where the viscous Reynolds number can be much smaller than the resistive one, is discussed.

Collisionless reversed magnetic shear trapped electron instability and contribution of sidebands to anomalous transport
View Description Hide DescriptionBy keeping account of the trapped electron and curvature drifts, it is found that the spatial decay of the collisionless electron drift wave is governed either by the trapped electron response or by the resonant interaction of ions with the sidebands of the primary oscillation. In the former case, pairs of spatially bounded unstable and damped solutions are obtained for negative magnetic shear if, as usual, ; there are no bounded solutions if . In the latter case, there is either a set of bounded damped solutions if or a set of bounded unstable solutions if . The unstable modes have a radiating character and the growth rates are ( is the Hermite polynomial solution index, the safety factor, the magnetic shear parameter, the major radius, the electron diamagnetic frequency, , and ).The sidebands are responsible for unusually large ratios , where and are the anomalous electron energy flux and the particle flux. These results may explain the boxtype profile observed in lower hybrid current drive reversed magnetic shear plasmas on the Japan Atomic Energy Research Institute Tokamak 60 Upgrade (JT60U) [H. Ninomiya and the JT60U Team, Phys. Fluids B4, 2070 (1992)]. It is finally demonstrated that the ballooning hypothesis generally leads to conflicting requirements: it is thus hardly relevant for the electron drift branch! The “radiating” boundary condition that has formerly been imposed on the slab solution is finally discussed.

Electromagnetic solitons produced by stimulated Brillouin pulsations in plasmas
View Description Hide DescriptionNumerical simulations and their theoretical interpretation of the formation of standing electromagneticsolitons produced in a regime of pulsating stimulated Brillouin scattering are presented. The localized field structures are stable, longliving entities and their physical properties are in very good agreement with theoretical predictions of electromagneticsolitons in a hot and dense plasma. They are the final outcome of a specific modecoupling regime that can be attained for high laser intensities via the transient excitation of kinetic electrostatic nonlinear electron modes.

Nonlinear Phenomena, Turbulence, Transport

General expression of the gyroviscous force
View Description Hide DescriptionAssuming only small gyromotion periods and Larmor radii compared to any other time and length scales, and retaining the lowest significant order in , the general expression of the ion gyroviscous stress tensor is presented. This expression covers both the “fast dynamics” (or “magnetohydrodynamic”) ordering, where the time derivative and ion gyroviscous stress are first order in relative to the ion gyrofrequency and scalar pressure, respectively, and the “slow dynamics” (or “drift”) ordering, where the time derivative and ion gyroviscous stress are, respectively, second order in . This general stress tensor applies to arbitrary collisionality and does not require the distribution function to be close to a Maxwellian. Its exact divergence (gyroviscous force) is written in a closed vector form, allowing for arbitrary magnetic geometry, parallel gradients, and flow velocities. Considering, in particular, the contribution from the velocity gradient (rate of strain) term, the final form of the momentum conservationequation after the “gyroviscous cancellation” and the “effective renormalization of the perpendicular pressure by the parallel vorticity” is precisely established.

Phase matching for thirdharmonic generation in collisional magnetoplasmas
View Description Hide DescriptionThis paper presents a derivation of the phasematching conditions for generation of the thirdharmonic and combination frequencies , by two fundamental electromagnetic waves of frequencies and , propagating in the extraordinary mode along a magnetic field in a collisional plasma. Expressions for the corresponding optimum distance of propagation for maximum output and the associated values of the phase functions have been derived. In the case of the thirdharmonic generation phase matching occurs when the gyrofrequency of the electrons is twice the wave frequency (for all values of plasma frequency). In the case of the combination frequencies, the phasematching condition is represented by an expression for plasma frequency in terms of the wave frequencies , and the gyrofrequency of electrons. Furthermore, it is seen that in the plane, phase matching occurs in a narrow region; for every point in this region, there is a corresponding value of . The dependence of optimum distance of propagation and the corresponding value of the phase function on electron collision frequency have been graphically illustrated for a specific case. An estimate of the maximum value of the thirdharmonic conversion efficiency has also been made.

Statistical properties of an ensemble of vortices interacting with a turbulent field
View Description Hide DescriptionAn analytical formalism is developed with the purpose to determine the statistical properties of a system consisting of an ensemble of vortices with random position in plane interacting with a turbulent field. The generating functional is calculated by pathintegral methods. The function space is the statistical ensemble composed of two parts, the first one representing the vortices influenced by the turbulence and the second one the turbulent field scattered by the randomly placed vortices.

Direct comparisons of compressible magnetohydrodynamics and reduced magnetohydrodynamics turbulence
View Description Hide DescriptionDirect numerical simulations of low Mach number compressible threedimensional magnetohydrodynamic (CMHD3D) turbulence in the presence of a strong mean magnetic field are compared with simulations of reduced magnetohydrodynamics (RMHD). Periodic boundary conditions in the three spatial coordinates are considered. Different sets of initial conditions are chosen to explore the applicability of RMHD and to study how close the solution remains to the full compressible MHD solution as both freely evolve in time. In a first set, the initial state is prepared to satisfy the conditions assumed in the derivation of RMHD, namely, a strong mean magnetic field and planepolarized fluctuations, varying weakly along the mean magnetic field. In those circumstances, simulations show that RMHD and CMHD3D evolve almost indistinguishably from one another. When some of the conditions are relaxed the agreement worsens but RMHD remains fairly close to CMHD3D, especially when the mean magnetic field is large enough. Moreover, the wellknown spectral anisotropy effect promotes the dynamical attainment of the conditions for RMHD applicability. Global quantities (mean energies, meansquare current, and vorticity) and energy spectra from the two solutions are compared and pointtopoint separation estimations are computed. The specific results shown here give support to the use of RMHD as a valid approximation of compressible MHD with a mean magnetic field under certain but quite practical conditions.

Nonlinear finiteLarmorradius driftkinetic equation
View Description Hide DescriptionAn efficient method is described for deriving the driftkinetic equation. A maximal ordering is invoked: the ordering parameter is formally taken to be proportional to , subject to the proviso that the parallel electric field. Electric drifts can be of the order of particle thermal velocities. The driftkinetic equation is derived up to second order in , and is in a form such that the phasespace volume following the particle phasespace trajectories is preserved. The mean density, mean velocity, momentum flow tensor, and the presure tensor are evaluated in terms of the electromagnetic fields and the velocity moments of the driftkinetic distribution function. The moments of the driftkinetic equation reproduce the corresponding moments of the Vlasov equation up to order . A consistent set of fluidkinetic equations is formulated, with the fluidlike perpendicular motion described by the perpendicular component of the momentum equation. The driftkinetic equation describes the parallel motion, and the solution is required to evaluate the velocity moments necessary to close the set of equations.

Nonlinear electromagnetic susceptibilities of unmagnetized plasmas
View Description Hide DescriptionFully electromagneticnonlinear susceptibilities of unmagnetized plasmas are analyzed in detail. Concrete expressions of the secondorder nonlinear susceptibility are found in various forms in the literature, usually in connection with the discussions of various threewave decay processes, but the thirdorder susceptibilities are rarely discussed. The secondorder susceptibility is pertinent to nonlinear wavewave interactions (i.e., the decay/coalescence), whereas the thirdorder susceptibilities affect nonlinear waveparticle interactions (i.e., the induced scattering). In the present article useful approximate analytical expressions of these nonlinear susceptibilities that can be readily utilized in various situations are derived

Dynamic of ion density perturbations observed in a microwaveplasma interaction
View Description Hide DescriptionThe dynamical behavior of ion density perturbations propagated at lowfrequency wave nature is experimentally observed in microwaveplasma interaction. An unmagnetized, inhomogeneous laboratory plasma irradiated by an obliquely incident microwave with maximum power and pulse width approximately ion plasma period is studied. The polarized electricfield component of the interacted microwave of frequency leads to a nonlinear phenomenon driven by the ponderomotive force by the process of resonance absorption at the critical layer where is satisfied. The nonlinear ion density perturbations are created from the resonant layer and propagated to an underdense plasma as an electrostatic wave nature.

Electron and ion kinetic effects in the saturation of a driven ion acoustic wave
View Description Hide DescriptionThe role of ion and electron kinetic effects is investigated in the context of the nonlinear saturation of a driven ion acoustic wave(IAW) and its parametric decay into subharmonics. The simulations are carried out with a full–particleincell (PIC) code, in which both ions and electrons are treated kinetically. The fullPIC results are compared with those obtained from a hybridPIC code (kinetic ions and Boltzmann electrons). It is found that the largest differences between the two kinds of simulations take place when the IAW is driven above the ion wavebreaking limit. In such a case of a strong drive, the hybridPIC simulations lead to a BersteinGreeneKruskallike nonlinear IAW of a large amplitude, while in the fullPIC the IAW amplitude decays to a small level after a transient stage. The electron velocity distribution function is significantly flattened in the domain of small electron velocities. As a result the nonlinear frequency shift due to the electron kinetic effects compensates partly the nonlinear frequency shift due to the ion kinetic effects, allowing then for the parametric decay of the driven IAW into subharmonics. These observations lead to the conclusion that electron kinetic effects become important whenever the nonlinear effects come into play.
