Volume 16, Issue 4, April 2009

It has become very clear that the evolution of structure during supernovae is centrally dependent on the preexisting structure in the star. Modeling of the preexisting structure has advanced significantly, leading to improved understanding and to a physically based assessment of the structure that will be present when a star explodes. It remains an open question whether lowmode asymmetries in the explosion process can produce the observed effects or whether the explosion mechanism somehow produces jets of material. In any event, the workhorse processes that produce structure in an exploding star are blastwave driven instabilities. Laboratory experiments have explored these blastwavedriven instabilities and specifically their dependence on initial conditions. Theoretical work has shown that the relative importance of Richtmyer–Meshkov and Rayleigh–Taylor instabilities varies with the initial conditions and does so in ways that can make sense of a range of astrophysical observations.
 LETTERS


Anomalous energy dissipation of electron current pulses propagating through an inhomogeneous collisionless plasma medium
View Description Hide DescriptionThe evolution of fast rising electron current pulses propagating through an inhomogeneous plasma has been studied through electron magnetohydrodynamic fluid simulations. A novel process of anomalous energy dissipation and stopping of the electron pulse in the presence of plasma density inhomogeneity is demonstrated. The electron current essentially dissipates its energy through the process of electromagnetic shock formation in the presence of density inhomogeneity. A direct relevance of this rapid energy dissipation process to the fast ignition concept of laser fusion is shown.

On the physical interpretation of Malyshkin’s (2008) model of resistive Hall magnetohydrodynamic reconnection
View Description Hide DescriptionA simple Sweet–Parkerlike model for the electron current layer in resistive Hall magnetohydrodynamicreconnection is presented, with the focus on the smallresistivity limit. The derivation readily recovers the main results obtained recently by Malyshkin [Phys. Rev. Lett.101, 225001 (2008)], but is much quicker and more physically transparent. In particular, it highlights the role of resistive drag in determining the electron outflow velocity. The principal limitations of any such approach are discussed.
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 SPECIAL TOPIC: HIGH ENERGY DENSITY LABORATORY ASTROPHYSICS: SUMMARIES OF PAPERS GIVEN DURING A SPECIAL SESSION AT THE AMERICAN PHYSICAL SOCIETY 2008 APRIL MEETING, ST. LOUIS, MISSOURI


Accretion disk dynamics, photoionized plasmas, and stellar opacities
View Description Hide DescriptionWe present a brief review on the atomic kinetics, modeling and interpretation of astrophysical observations, and laboratory astrophysics experiments. The emphasis is on benchmarking of opacity calculations relevant for solar structure models, photoionized plasmas research, the magnetohydrodynamic numerical simulation of accretion disk dynamics, and a connection between radiation transport effects and plasma source geometry details. Specific cases of application are discussed with relevance to recent and proposed laboratory astrophysics experiments as well as Chandra and Xray MultiMirror Mission Newton observations.

Intense laserplasma interactions: New frontiers in high energy density physics
View Description Hide DescriptionA review is presented here of a number of invited papers presented at the 2008 American Physical Society April meeting [held jointly with High Energy Density Physics/High Energy Density Laboratory Astrophysics (HEDP/HEDLA) Conference] devoted to intense lasermatter interactions. They include new insights gained from wavekinetic theory into laserwakefield accelerators and drift wave turbulence interacting with zonal flows in magnetized plasmas; interactions with cluster media for the generation of radiative blast waves; fast electron energy transport in conewire targets; numerical investigations into Weibel instability in electronpositronion plasmas and the generation of gigabar pressures with thin foil interactions.

Frontiers of the physics of dense plasmas and planetary interiors: Experiments, theory, and applications
View Description Hide DescriptionRecent developments of dynamic xray characterization experiments of dense matter are reviewed, with particular emphasis on conditions relevant to interiors of terrestrial and gas giant planets. These studies include characterization of compressed states of matter in light elements by xray scattering and imaging of shocked iron by radiography. Several applications of this work are examined. These include the structure of massive “superEarth” terrestrial planets around other stars, the 40 known extrasolar gas giants with measured masses and radii, and Jupiter itself, which serves as the benchmark for giant planets.

Stellar explosions, instabilities, and turbulence
View Description Hide DescriptionIt has become very clear that the evolution of structure during supernovae is centrally dependent on the preexisting structure in the star. Modeling of the preexisting structure has advanced significantly, leading to improved understanding and to a physically based assessment of the structure that will be present when a star explodes. It remains an open question whether lowmode asymmetries in the explosion process can produce the observed effects or whether the explosion mechanism somehow produces jets of material. In any event, the workhorse processes that produce structure in an exploding star are blastwave driven instabilities. Laboratory experiments have explored these blastwavedriven instabilities and specifically their dependence on initial conditions. Theoretical work has shown that the relative importance of Richtmyer–Meshkov and Rayleigh–Taylor instabilities varies with the initial conditions and does so in ways that can make sense of a range of astrophysical observations.

Astrophysical jets: Observations, numerical simulations, and laboratory experiments
View Description Hide DescriptionThis paper provides summaries of ten talks on astrophysical jets given at the HEDP/HEDLA08 International Conference in St. Louis. The talks are topically divided into the areas of observation, numerical modeling, and laboratory experiment. One essential feature of jets, namely, their filamentary (i.e., collimated) nature, can be reproduced in both numerical models and laboratory experiments. Another essential feature of jets, their scalability, is evident from the large number of astrophysical situations where jets occur. This scalability is the reason why laboratory experiments simulating jets are possible and why the same theoretical models can be used for both observed astrophysical jets and laboratory simulations.

The National Ignition Facility: Ushering in a new age for high energy density science
View Description Hide DescriptionThe National Ignition Facility (NIF) [E. I. Moses, J. Phys.: Conf. Ser.112, 012003 (2008); https://lasers.llnl.gov/], completed in March 2009, is the highest energy laser ever constructed. The high temperatures and densities achievable at NIF will enable a number of experiments in inertial confinement fusion and stockpile stewardship, as well as access to new regimes in a variety of experiments relevant to xray astronomy, laserplasma interactions, hydrodynamic instabilities, nuclear astrophysics, and planetary science. The experiments will impact research on black holes and other accreting objects, the understanding of stellar evolution and explosions, nuclear reactions in dense plasmas relevant to stellar nucleosynthesis, properties of warm dense matter in planetary interiors, molecular cloud dynamics and star formation, and fusion energy generation.
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 ARTICLES

 Basic Plasma Phenomena, Waves, Instabilities

The Weibel instability inside the electronpositron Harris sheet
View Description Hide DescriptionRecent fullparticle simulations of electronpositron reconnection revealed that the Weibel instability plays an active role in controlling the dynamics of the current layer and maintaining fast reconnection. A fourbeam model is developed to explore the development of the instability within a narrow current layer characteristic of reconnection. The problem is reduced to two coupled secondorder differential equations, whose growing eigenmodes are obtained via both asymptotic approximations and finite difference methods. Full particle simulations confirm the linear theory and help probe the nonlinear development of the instability. The current layer broadening in the reconnection outflow jet is linked to the scattering of highvelocity streaming particles in the Weibelgenerated, outofplane magnetic field.

Effects of line tying on resistive tearing instability in slab geometry
View Description Hide DescriptionThe effects of line tying on resistivetearing instability in slab geometry are studied within the framework of reduced magnetohydrodynamics [B. B. Kadomtsev and O. P. Pogutse, Sov. Phys. JETP38, 283 (1974); H. R. Strauss, Phys. Fluids19, 134 (1976)]. It is found that line tying has a stabilizing effect. The tearing mode is stabilized when the system length is shorter than a critical length , which is independent of the resistivity. When is not too much longer than , the growth rate is proportional to . When is sufficiently long, the tearing mode scaling is recovered. The transition from to occurs at a transition length .

Fast magnetic reconnection in a kinked current sheet
View Description Hide DescriptionMagnetic reconnection processes in a kinked current sheet are investigated using threedimensional electromagnetic particleincell simulations in a large system where both the tearing and kink modes are able to be captured. The spatial resolution is efficiently enhanced using the adaptive mesh refinement and particle splittingcoalescence method. The kink mode scaled by the current sheet width such as is driven by the ions that are accelerated due to the reconnectionelectric field in the ionscale diffusion region. Although the kink mode deforms the current sheet structure drastically, the gross rate of reconnection is almost identical to the case without the kink mode and fast magnetic reconnection is achieved. The magnetic dissipation mechanism is, however, found very different between the cases with and without the kink mode. The kink mode broadens the current sheet width and reduces the electron flow velocity, so that the electron inertia resistivity is decreased. Nevertheless, anomalous dissipation through the electron thermalization compensates the decrease in the inertia resistivity so as to keep a high reconnection rate. This suggests that the electron dynamics in the electron diffusion region is automatically adjusted so as to generate sufficient dissipation for fast magnetic reconnection. The electron thermalization occurs effectively because the electron meandering scale along the current sheet is comparable to the wavelength of the kink mode. On the other hand, twodimensional simulations in the plane orthogonal to the magnetic field shows that in higher mass ratio cases with the electron thermalization is caused due to a hybridscale mode with wavelength intermediate between the ion and electron inertia lengths rather than the largescale kink mode with , because the electron meandering scale is shortened as the mass ratio increases.

Fluctuations in electronpositron plasmas: Linear theory and implications for turbulence
View Description Hide DescriptionLinear kinetic theory of electromagnetic fluctuations in a homogeneous, magnetized, collisionless electronpositron plasma predicts two lightly damped modes propagate at relatively long wavelengths: an Alfvénlike mode with dispersion and a magnetosoniclike mode with dispersion if . Here is the Alfvén speed in an electronpositron plasma and refers to the direction relative to the background magnetic field . Both modes have phase speeds which monotonically decrease with increasing wavenumber. The Alfvénlike fluctuations are almost incompressible, but the magnetosoniclike fluctuations become strongly compressible at short wavelengths and propagation sufficiently oblique to . Using the linear dispersion properties of these modes, scaling relations are derived which predict that turbulence of both modes should be relatively anisotropic, with fluctuating magnetic energy preferentially cascading in directions perpendicular to . Turbulent spectra in the solar wind show two distinct powerlaw regimes separated by a distinct breakpoint in observed frequency; this characteristic should not be present in electronpositron turbulence because of the absence of whistlerlikedispersion. Linear theory properties of the cyclotron and mirror instabilities driven by either electron or positron temperature anisotropies are generally analogous to those of the corresponding instabilities in electronproton plasmas.

Persistent subplasmafrequency kinetic electrostatic electron nonlinear waves
View Description Hide DescriptionDriving a onedimensional collisionless Maxwellian (Vlasov) plasma with a sufficiently strong longitudinal ponderomotive driver for a sufficiently long time results in a selfsustaining nonsinusoidal wave train with welltrapped electrons even for frequencies well below the plasma frequency, i.e., in the plasma wave spectral gap. Typical phase velocities of these waves are somewhat above the electron thermal velocity. This new nonlinear wave is being termed a kinetic electrostatic electron nonlinear (KEEN) wave. The drive duration must exceed the bounce period of the trapped electrons subject to the drive, as calculated from the drive force and the linear plasma response to the drive. For a given wavenumber a wide range of KEEN wave frequencies can be readily excited. The basic KEEN structure is essentially kinetic, with the trapped electron density variation being almost completely shielded by the free electrons, leaving just enough net charge to support the wave.

Tearing modes with pressure gradient effect in pair plasmas
View Description Hide DescriptionThe general dispersion relation of tearing mode with pressure gradient effect in pair plasmas is derived analytically. If the pressure gradients of positron and electron are not identical in pair plasmas, the pressure gradient has significant influence at tearing mode in both collisionless and collisional regimes. In collisionless regime, the effects of pressure gradient depend on its magnitude. For small pressure gradient, the growth rate of tearing mode is enhanced by pressure gradient. For large pressure gradient, the growth rate is reduced by pressure gradient. The tearing mode can even be stabilized if pressure gradient is large enough. In collisional regime, the growth rate of tearing mode is reduced by the pressure gradient. While the positron and electron have equal pressure gradient, tearing mode is not affected by pressure gradient in pair plasmas.

Lowerhybrid drift instability in a thin current sheet with velocity distribution
View Description Hide DescriptionThe lowerhybrid drift instability (LHDI) in a thin current sheet in the intermediatewavelength (, where , , and are the wave vector and the electron and ion gyroradii, respectively) regime for particles with velocity distribution is studied. The latter is more suitable for describing nonthermal distributions with an enhanced highenergy tail and includes the Maxwellian as a limiting case. It is shown that linear electromagnetic LHDI can be excited near the center of the current sheet. The growth rate decreases, but the electromagnetic component of the LHD mode increases with increase in hot particles.

Drift ion acoustic shock waves in an inhomogeneous twodimensional quantum magnetoplasma
View Description Hide DescriptionLinear and nonlinear propagation characteristics of drift ion acoustic waves are investigated in an inhomogeneous quantum plasma with neutrals in the background employing the quantum hydrodynamics (QHD) model. In this regard, a quantum Kadomtsev–Petviashvili–Burgers (KPB) equation is derived for the first time. It is shown that the ion acoustic wave couples with the drift wave if the parallel motion of ions is taken into account. Discrepancies in the earlier works on drift solitons and shocks in inhomogeneous plasmas are also pointed out and a correct theoretical framework is presented to study the onedimensional as well as the twodimensional propagation of shock waves in an inhomogeneous quantum plasma. Furthermore, the solution of KPB equation is presented using the tangent hyperbolic (tanh) method. The variation of the shock profile with the quantum Bohm potential, collision frequency, and ratio of drift to shock velocity in the comoving frame, , are also investigated. It is found that increasing the number density and collision frequency enhances the strength of the shock. It is also shown that the fast drift shock (i.e., ) increases, whereas the slow drift shock (i.e., ) decreases the strength of the shock. The relevance of the present investigation with regard to dense astrophysical environments is also pointed out.

Resonant magnetohydrodynamic waves in highbeta plasmas
View Description Hide DescriptionWhen a global magnetohydrodynamic(MHD)wave propagates in a weakly dissipative inhomogeneous plasma, the resonant interaction of this wave with either local Alfvén or slow MHD waves is possible. This interaction occurs at the resonant position where the phase velocity of the global wave coincides with the phase velocity of either Alfvén or slow MHD waves. As a result of this interaction a dissipative layer embracing the resonant position is formed, its thickness being proportional to , where is the Reynolds number. The wavemotion in the resonant layer is characterized by large amplitudes and large gradients. The presence of large gradients causes strong dissipation of the global wave even in very weakly dissipative plasmas. Very often the global wavemotion is characterized by the presence of both Alfvén and slow resonances. In plasmas with small or moderate plasma beta , the resonance positions corresponding to the Alfvén and slow resonances are well separated, so that the wavemotion in the Alfvén and slow dissipative layers embracing the Alfvén and slow resonant positions, respectively, can be studied separately. However, when , the two resonance positions are so close that the two dissipative layers overlap. In this case, instead of two dissipative layers, there is one mixed Alfvénslow dissipative layer. In this paper the wavemotion in such a mixed dissipative layer is studied. It is shown that this motion is a linear superposition of two motions, one corresponding to the Alfvén and the other to the slow dissipative layer. The jump of normal velocity across the mixed dissipative layer related to the energy dissipation rate is equal to the sum of two jumps, one that occurs across the Alfvén dissipative layer and the other across the slow dissipative layer.
 Nonlinear Phenomena, Turbulence, Transport

Effect of resonant helical magnetic fields on plasma rotation
View Description Hide DescriptionThe effect of a resonant helical magnetic field on plasma rotation is investigated numerically based on the two fluid equations. It is found that depending on the frequency and the direction of the original plasma rotation, a static helical field of a small amplitude can either increase or decrease the rotation speed. With increasing the field amplitude, the plasma rotation frequency approaches the electron diamagnetic drift frequency but rotates in the ion drift direction. These results provide a new understanding of the recent experimental observations of TEXTOR [K. H. Finken et al., Phys. Rev. Lett.94, 015003 (2005)].

The truncation model of the derivative nonlinear Schrödinger equation
View Description Hide DescriptionThe derivative nonlinear Schrödinger (DNLS) equation is explored using a truncation model with three resonant traveling waves. In the conservative case, the system derives from a timeindependent Hamiltonian function with only one degree of freedom and the solutions can be written using elliptic functions. In spite of its low dimensional order, the truncation model preserves some features from the DNLS equation. In particular, the modulational instability criterion fits with the existence of two hyperbolic fixed points joined by a heteroclinic orbit that forces the exchange of energy between the three waves. On the other hand, numerical integrations of the DNLS equation show that the truncation model fails when waveenergy is increased or lefthand polarized modulational unstable modes are present. When dissipative and growth terms are added the system exhibits a very complex dynamics including appearance of several attractors, period doubling bifurcations leading to chaos as well as other nonlinear phenomenon. In this case, the validity of the truncation model depends on the strength of the dissipation and the kind of attractor investigated.