Volume 18, Issue 5, May 2011

One of the fundamental waves in magnetized plasmas is the shear Alfvén wave. This wave is responsible for rearranging current systems and, in fact all low frequency currents in magnetized plasmas are shear waves. It has become apparent that Alfvén waves are important in a wide variety of physical environments. Shear waves of various forms have been a topic of experimental research for more than fifteen years in the large plasma device (LAPD) at UCLA. The waves were first studied in both the kinetic and inertial regimes when excited by fluctuating currents with transverse dimension on the order of the collisionless skin depth. Theory and experiment on wave propagation in these regimes is presented, and the morphology of the wave is illustrated to be dependent on the generation mechanism. Threedimensional currents associated with the waves have been mapped. The ion motion, which closes the current across the magnetic field, has been studied using laser induced fluorescence. The wave propagation in inhomogeneous magnetic fields and density gradients is presented as well as effects of collisions and reflections from boundaries. Reflections may result in Alfvénic field line resonances and in the right conditions maser action. The waves occur spontaneously on temperature and density gradients as hybrids with drift waves. These have been seen to affect crossfield heat and plasma transport. Although the waves are easily launched with antennas, they may also be generated by secondary processes, such as Cherenkov radiation. This is the case when intense shear Alfvén waves in a background magnetoplasma are produced by an exploding laserproduced plasma. Time varying magnetic flux ropes can be considered to be low frequency shear waves. Studies of the interaction of multiple ropes and the link between magnetic field line reconnection and rope dynamics are revealed. This manuscript gives us an overview of the major results from these experiments and provides a modern prospective for the earlier studies of shear Alfvén waves.
 LETTERS


Selfsuppression of double tearing modes via Alfvén resonance in rotating tokamak plasmas
View Description Hide DescriptionReversed magnetic shear configuration, a key method for improving plasma confinement in advanced tokamaks, is prone to exciting double tearing modes (DTMs) that can severely degrade the plasma confinement. In this letter, we reveal a new mechanism of suppressing the DTMinstability due to the selfinduced Alfvén resonance in rotating tokamakplasmas. The linear growth rate is reduced from of the fast DTM regime to of the slow single tearing mode regime, where is magnetic Reynolds number. Instead of generating magnetic islands at the inner rational surface that can greatly enhance plasma transport in the core region, the formation of current sheets at resonance layers not only prevents the fast nonlinear DTM reconnection phase but also contributes to plasma heating.

Application of tomographic particle image velocimetry to studies of transport in complex (dusty) plasma
View Description Hide DescriptionOver the past twelve years, twodimensional and stereoscopic particle imagevelocimetry(PIV) techniques have been used to obtain detailed measurements of the thermal and transport properties of the microparticle component of dusty plasma systems. This letter reports on an extension of these techniques to obtain a volumetric, threedimensional velocity vector measurement using tomographic PIV. Initial measurements using the tomographic PIVdiagnostic are presented.

Fluctuation driven transport and stationary profiles
View Description Hide DescriptionTransport equations for particles and energy can be derived when the fluctuations conserve adiabatic invariants. The transport equations determine both stationary density and pressure profiles and the direction of the turbulencedriven fluxes which can be inward or outward. An inward turbulent pinch is predicted which creates stationary profiles and reverses direction depending on the density and temperature gradients. The transport fluxes are independent of the underlying drive that leads to plasma turbulence. For low frequency turbulence, the formulation remains correct when the collisional time scale is faster than the confinement time scale.

 SPECIAL TOPIC: PLANS FOR THE NATIONAL IGNITION CAMPAIGN (NIC) ON THE NIF: ON THE THRESHOLD OF INITIATING IGNITION EXPERIMENTS



Point design targets, specifications, and requirements for the 2010 ignition campaign on the National Ignition Facility
View Description Hide DescriptionPoint design targets have been specified for the initial ignition campaign on the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Opt. Eng. 443, 2841 (2004)]. The targets contain DT fusion fuel in an ablator of either CH with Ge doping, or Be with Cu. These shells are imploded in a U or Auhohlraum with a peak radiation temperature set between 270 and 300 eV. Considerations determining the point design include laserplasma interactions, hydrodynamic instabilities, laser operations, and target fabrication. Simulations were used to evaluate choices, and to define requirements and specifications. Simulation techniques and their experimental validation are summarized. Simulations were used to estimate the sensitivity of target performance to uncertainties and variations in experimental conditions. A formalism is described that evaluates margin for ignition, summarized in a parameter the Ignition Threshold Factor (ITF). Uncertainty and shottoshot variability in ITF are evaluated, and sensitivity of the margin to characteristics of the experiment. The formalism is used to estimate probability of ignition. The ignition experiment will be preceded with an experimental campaign that determines features of the design that cannot be defined with simulations alone. The requirements for this campaign are summarized. Requirements are summarized for the laser and target fabrication.

Capsule implosion optimization during the indirectdrive National Ignition Campaign
View Description Hide DescriptionCapsule performance optimization campaigns will be conducted at the National Ignition Facility [G. H. Miller, E. I. Moses, and C. R. Wuest, Nucl. Fusion44, 228 (2004)] to substantially increase the probability of ignition. The campaigns will experimentally correct for residual uncertainties in the implosion and hohlraum physics used in our radiationhydrodynamic computational models using a variety of ignition capsule surrogates before proceeding to cryogeniclayered implosions and ignition experiments. The quantitative goals and technique options and down selections for the tuning campaigns are first explained. The computationally derived sensitivities to key laser and target parameters are compared to simple analytic models to gain further insight into the physics of the tuning techniques. The results of the validation of the tuning techniques at the OMEGA facility [J. M. Soures et al., Phys. Plasmas3, 2108 (1996)] under scaled hohlraum and capsule conditions relevant to the ignition design are shown to meet the required sensitivity and accuracy. A rollup of all expected random and systematic uncertainties in setting the key ignition laser and target parameters due to residual measurement, calibration, crosscoupling, surrogacy, and scaleup errors has been derived that meets the required budget. Finally, we show how the tuning precision will be improved after a number of shots and iterations to meet an acceptable level of residual uncertainty.

The experimental plan for cryogenic layered target implosions on the National Ignition Facility—The inertial confinement approach to fusion
View Description Hide DescriptionIgnition requires precisely controlled, high convergence implosions to assemble a dense shell of deuteriumtritium (DT) fuel with ρR>∼1 g/cm^{2} surrounding a 10 keV hot spot with ρR ∼ 0.3 g/cm^{2}. A working definition of ignition has been a yield of ∼1 MJ. At this yield the αparticle energy deposited in the fuel would have been ∼200 kJ, which is already ∼10 × more than the kinetic energy of a typical implosion. The National Ignition Campaign includes low yield implosions with dudded fuel layers to study and optimize the hydrodynamic assembly of the fuel in a diagnostics rich environment. The fuel is a mixture of tritiumhydrogendeuterium (THD) with a density equivalent to DT. The fraction of D can be adjusted to control the neutron yield. Yields of ∼10^{14−15} 14 MeV (primary) neutrons are adequate to diagnose the hot spot as well as the dense fuel properties via down scattering of the primary neutrons.Xray imaging diagnostics can function in this low yield environment providing additional information about the assembled fuel either by imaging the photons emitted by the hot central plasma, or by active probing of the dense shell by a separate high energy short pulse flash. The planned use of these targets and diagnostics to assess and optimize the assembly of the fuel and how this relates to the predicted performance of DT targets is described. It is found that a good predictor of DT target performance is the THD measurable parameter, Experimental Ignition Threshold Factor, ITFX ∼ Y × dsf ^{2.3}, where Y is the measured neutron yield between 13 and 15 MeV, and dsf is the down scatteredneutron fraction defined as the ratio of neutrons between 10 and 12 MeV and those between 13 and 15 MeV.

 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

The magnetic Rayleigh–Taylor instability for inviscid and viscous fluids
View Description Hide DescriptionThe Rayleigh–Taylor instability arises whenever two fluids with different densities are arranged such that the heavier fluid sits above the lighter fluid, with a sharp interface in between. The magnetic Rayleigh–Taylor instability has the further complication due to the presence of a magnetic field throughout both media. The two fluids in question may also have differing magnetic properties, such as the magnetic permeability. When the fluids in consideration are in fact plasmas comprised of charged particles, induced currents, magnetic fields, and Lorentz forces can all act in ways that will affect the stability of the system. Stable base flows exist for the 2D case, and small sinusoidal disturbances to the base flow will grow in the unstable scenario. The numerical method described in this paper calculates the growth of the interface in the nonlinear regime, since closed form solutions are obtained only in the linear approximation. Through the analysis of both the fluid and magnetic vorticities and streamfunctions, the simulated results can be explained from the principles of magnetohydrodynamics. A range of simulations is presented, looking at cases with different initial conditions, cases with strong and weak magnetic fields, and cases with magnetic fields oriented at different angles relative to the interface of the two fluids. It is shown in particular how different initial conditions give rise to outcomes that are very different in terms of the geometry of the interface between the two fluids, primarily the differences between a single mode disturbance and a multimode disturbance to the interface at time t = 0.

Twodimensional Vlasov simulation of electron plasma wave trapping, wavefront bowing, selffocusing, and sideloss^{a)}
View Description Hide DescriptionTwodimensional Vlasov simulations of nonlinear electron plasma waves are presented, in which the interplay of linear and nonlinear kinetic effects is evident. The plasma wave is created with an external traveling wave potential with a transverse envelope of width such that thermal electrons transit the wave in a “sideloss” time, . Here, v_{e} is the electron thermal velocity. The quasisteady distribution of trapped electrons and its selfconsistent plasma wave are studied after the external field is turned off. In cases of particular interest, the bounce frequency, , satisfies the trapping condition such that the wave frequency is nonlinearly downshifted by an amount proportional to the number of trapped electrons. Here, k is the wavenumber of the plasma wave and is its electric potential. For sufficiently short times, the magnitude of the negative frequency shift is a local function of . Because the trapping frequency shift is negative, the phase of the wave on axis lags the offaxis phase if the trapping nonlinearity dominates linear wave diffraction. In this case, the phasefronts are curved in a focusing sense. In the opposite limit, the phasefronts are curved in a defocusing sense. Analysis and simulations in which the wave amplitude and transverse width are varied establish criteria for the development of each type of wavefront. The damping and trappedelectroninduced focusing of the finiteamplitude electron plasma wave are also simulated. The damping rate of the field energy of the wave is found to be about the sideloss rate, . For large wave amplitudes or widths , a trappinginduced selffocusing of the wave is demonstrated.

Electron shearflowdriven instability in magnetized plasmas with magnetic field gradient
View Description Hide DescriptionIt is found that the zeroorder current associated with electron shear flow produces a drift wave in magnetized plasmas, which can become unstable under certain conditions. This wave will be particularly important in low density and low temperature plasmas of heavy ions. As an example, numerical estimates are presented for a barium plasma with parameters compatible with experiments.

Electron streams formation and secondary two stream instability onset in the postsaturation regime of the classical Weibel instability
View Description Hide DescriptionThe electrostatic activity in the postsaturation regime of the velocity anisotropy driven Weibel instability is investigated by means of 1D 3V particle in cell simulations. Two different initial simulation configurations have been chosen to characterize the electrostatic activity in the postsaturation stage. A secondary two stream instability arises in both cases. However, significant differences occur in the thickness of the electron streams, in their initial locations, and in their effects on the bulk electron phase space distribution. An Hamiltonian description of particle motion in a 1D setting explains these differences in terms of the effective potential experienced by particles as a function of their initial perpendicular velocity. The different roles of the longitudinal electric field and the Lorentz force in the formation of electron streams are discussed.

Particle energization in 3D magnetic reconnection of relativistic pair plasmas
View Description Hide DescriptionWe present large scale 3D particleincell simulations to examine particle energization in magnetic reconnection of relativistic electronpositron (pair) plasmas. The initial configuration is set up as a relativistic Harris equilibrium without a guide field. These simulations are large enough to accommodate a sufficient number of tearing and kink modes. Contrary to the nonrelativistic limit, the linear tearing instability is faster than the linear kink instability, at least in our specific parameters. We find that the magnetic energy dissipation is first facilitated by the tearing instability and followed by the secondary kink instability. Particles are mostly energized inside the magnetic islands during the tearing stage due to the spatially varying electric fields produced by the outflows from reconnection. Secondary kink instability leads to additional particle acceleration. Accelerated particles are, however, observed to be thermalized quickly. The large amplitude of the vertical magnetic field resulting from the tearing modes by the secondary kink modes further help thermalizing the nonthermal particles generated from the secondary kink instability. Implications of these results for astrophysics are briefly discussed.

Coulomb divergence of the highfrequency expansion of the dielectric permittivity
View Description Hide DescriptionBased on accurate results of the linear response theory and the KramersKronig relations, the permittivity of the nonrelativistic Coulomb system is analyzed at arbitrary thermodynamic parameters in the highfrequency limit . It is shown that the expansion of the real part of the permittivity in powers of 1 is limited to only three terms. Limit constraints on the powerlaw behavior of the imaginary part of the permittivity are determined.

Nonlinear beam generated plasma waves as a source for enhanced plasma and ion acoustic lines
View Description Hide DescriptionObservations by, for instance, the EISCAT Svalbard Radar (ESR) demonstrate that the symmetry of the naturally occurring ion line in the polar ionosphere can be broken by an enhanced, nonthermal, level of fluctuations (naturally enhanced ionacoustic lines, NEIALs). It was in many cases found that the entire ion spectrum can be distorted, also with the appearance of a third line, corresponding to a propagation velocity significantly slower than the ion acoustic sound speed. It has been argued that selective decay of beam excited primary Langmuir waves can explain some phenomena similar to those observed. We consider a related model, suggesting that a primary nonlinear process can be an oscillating twostream instability, generating a forced low frequency mode that does not obey any ion sound dispersion relation. At later times, the decay of Langmuir waves can give rise also to enhanced asymmetric ion lines. The analysis is based on numerical results, where the initial Langmuir waves are excited by a cold dilute electron beam. By this numerical approach, we can detect fine details of the physical processes, in particular, demonstrate a strong spacetime intermittency of the electron waves in agreement with observations. Our code solves the full Vlasov equation for electrons and ions, with the dynamics coupled through the electrostatic field derived from Poisson’s equation. The analysis distinguishes the dynamics of the background and beam electrons. This distinction simplifies the analysis for the formulation of the weakly nonlinear analytical model for the oscillating twostream instability. The results have general applications beyond their relevance for the ionospheric observations.

Shear flowdriven electrostatic instabilities in low density and low temperature pairion plasmas with and without electrons
View Description Hide DescriptionThe shear flowdriven electrostaticinstabilities are investigated in ideal low density, low temperature pairionelectron and pure pairion plasmas in several different cases, including homogeneous and inhomogeneous density effects. In uniform pairionelectron plasma, when the shear flow is of the order of the acoustic speed, the purely growing D’Angelo mode can give rise to electrostatic fields. In the case of an inhomogeneous plasma, the drift wave becomes unstable. The presence of negative ions, however, reduces the growth rate. If the positive and negative ions are not in thermal equilibrium with each other, then the shear flow also gives rise to an electrostaticinstability in pure pairion plasma.

Buneman instability in a magnetized currentcarrying plasma with velocity shear
View Description Hide DescriptionBuneman instability is often driven in magnetic reconnection. Understanding how velocity shear in the beams driving the Buneman instability affects the growth and saturation of waves is relevant to turbulence, heating, and diffusion in magnetic reconnection. Using a Mathieuequation analysis for weak cosine velocity shear together with Vlasov simulations, the effects of shear on the kinetic Buneman instability are studied in a plasma consisting of strongly magnetized electrons and cold unmagnetized ions. In the linearly unstable phase, shear enhances the coupling between oblique waves and the sheared electron beam, resulting in a wider range of unstable eigenmodes with common lower growth rates. The wave couplings generate new features of the electric fields in space, which can persist into the nonlinear phase when electron holes form. Lower hybrid instabilities simultaneously occur at with a much lower growth rate and are not affected by the velocity shear.

Power loss of a single electron charge distribution confined in a quantum plasma
View Description Hide DescriptionThe dielectrictensor for a quantum plasma is derived by using a linearized quantum hydrodynamic theory. The wave functions for a nanostructure bound system have been investigated. Finally, the power loss for an oscillating charge distribution of a mixed state will be calculated, using the dielectric function formalism.

Reactive instabilities of lower hybridlike waves in regions with parallel currents
View Description Hide DescriptionThe dispersion and reactive instabilities of obliquely propagating waves near the lower hybrid (LH) frequency are studied in plasma carrying a current parallel to the magnetic field. Possible applications of these instabilities include magnetic reconnection regions, where LHlike waves may accelerate and heat both ions and electrons. In plasmas with a bulk drift of electrons relative to the ions at speed v _{ d } along the magnetic field, the forward and backward propagating LH modes are shown to be replaced by four LHlike modes. Reactive instabilities are discovered here for a forward propagating mode with and a backward propagating mode with . Numerical warm, fully electromagnetic, kinetic calculations are compared with cold plasma calculations and agree well, confirming that the discovered instabilities are reactive. In the cold plasma limit, the forward and backward propagating instabilities occur for v _{ d } below and above some thresholds, respectively.

On entropymaximized velocity distributions in circularly polarized finite amplitude Alfvén waves
View Description Hide DescriptionA special solution of the VlasovMaxwell system, which represents a circularly polarized Alfvén wave, is derived as an entropymaximized state. It is shown that Alfvénic correlation between transverse bulk motion and magnetic field given by the entropymaximized distribution is consistent with the equilibrium point of the single particle system. We demonstrate that as far as the monochromatic, circularly polarized magnetic field is concerned, the resultant distribution may be a relaxed state corresponding to one in the Hallmagnetohydrodynamic system. Stability of the distribution function is numerically discussed by using an ionhybrid simulation code. Numerical results suggest that the relaxed states in nonmonochromatic waves are different from those in monochromatic waves.

Nonlinear ionacoustic structures in dusty plasma with superthermal electrons and positrons
View Description Hide DescriptionNonlinear ionacoustic structures are investigated in an unmagnetized, fourcomponent plasma consisting of warm ions, superthermal electrons and positrons, as well as stationary charged dust impurities. The basic set of fluid equations is reduced to modified Kortewegde Vries equation. The latter admits both solitary waves and double layers solutions. Numerical calculations indicate that these nonlinear structures cannot exist for all physical parameters. Therefore, the existence regions for both solitary and double layers excitations have been defined precisely. Furthermore, the effects of temperature ratios of ionstoelectrons and electronstopositrons, positrons and dust concentrations, as well as superthermal parameters on the profiles of the nonlinear structures are investigated. Also, the acceleration and deceleration of plasma species have been highlight. It is emphasized that the present investigation may be helpful in better understanding of nonlinear structures which propagate in astrophysical environments, such as in interstellar medium.