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International audience; In this paper, the nonlinear interaction between kinetic instabilities driven by multiple ion beams and magnetized electrons is investigated. Electron diffusion across magnetic field lines is enhanced by the coupling of plasma instabilities. A two-dimensional collisionless particle-in-cell simulation is performed accounting for singly and doubly charged ions in a cross-field configuration. Consistent with prior linear kinetic theory analysis and observations from coherent Thomson scattering experiments, the present simulations identify an ion-ion two-stream instability due to multiply charged ions (flowing in the direction parallel to the applied electric field) which coexists with the electron cyclotron drift instability (propagating perpendicular to the applied electric field and parallel to the E×B drift). Small-scale fluctuations due to the coupling of these naturally driven kinetic modes are found to be a mechanism that can enhance cross-field electron transport and contribute to the broadening of the ion velocity distribution functions

Intense electromagnetic fields are created in the quark-gluon plasma by the external ultra-relativistic valence charges. The time-evolution and the strength of this field are strongly affected by the electrical conductivity of the plasma. Yet, it has recently been observed that the effect of the magnetic field on the plasma flow is small. We compute the effect of plasma flow on magnetic field and demonstrate that it is less than 10\%. These observations indicate that the plasma hydrodynamics and the dynamics of electromagnetic field decouple. Thus, it is a very good approximation, on the one hand, to study QGP in the background electromagnetic field generated by external sources and, on the other hand, to investigate the dynamics of magnetic field in the background plasma. We also argue that the wake induced by the magnetic field in plasma is negligible., Comment: 16 pages, 5 figures

The Richtmyer-Meshkov instability (RMI) mixing flow induced by a planar shock wave of Mach 1.6 is investigated using direct numerical simulation method. Interfacial perturbations of different scales between air and sulfur hexafluoride are introduced to study the effect of the initial conditions. Focus is placed on the analysis of the scale-to-scale transfer of kinetic energy in both Fourier and physical spaces. The kinetic energy injected from the perturbation scales is transferred to both larger and smaller scales in an average sense within the inner mixing zone (IMZ) at early times and is mainly passed down into smaller scales at the late stage. The physical-space energy flux due to the subgrid-scale (SGS) stress is studied using a filtering approach in order to shed light on the physical origin of the scale-to-scale kinetic energy transfer. It is found that the pointwise SGS energy flux is highly correlated with the local spike and bubble structures in the IMZ. Moreover, it turns out that the mean SGS energy flux is mainly ascribed to the component in the direction of shock wave propagation. An analysis using the method of conditional averaging manifests that the generation of local SGS energy flux is associated with the property of the surrounding flow induced by quadrupolar or dipolar vortex structures.

Electron-scale surface waves are shown to be unstable in the transverse plane of a sheared flow in an initially unmagnetized collisionless plasma, not captured by (magneto) hydrodynamics. It is found that these unstable modes have a higher growth rate than the closely related electron-scale Kelvin-Helmholtz instability in relativistic shears. Multidimensional particle-in-cell simulations verify the analytic results and further reveal the emergence of mushroomlike electron density structures in the nonlinear phase of the instability, similar to those observed in the Rayleigh Taylor instability despite the great disparity in scales and different underlying physics. This transverse electron-scale instability may play an important role in relativistic and supersonic sheared flow scenarios, which are stable at the (magneto) hydrodynamic level. Macroscopic (>> c/omega(pe)) fields are shown to be generated by this microscopic shear instability, which are relevant for particle acceleration, radiation emission, and to seed magnetohydrodynamic processes at long time scales. info:eu-repo/semantics/publishedVersion

Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the “low-T/|W| instability”), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T/|W| instability, we find that they do so only in a small range of field strengths. Below 4×10^(13) G, poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5×10^(14) G, magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T/|W| instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.

The wakefield, or ion focus, formed by ions streaming past dust particles trapped in the plasma sheath leads to two types of instabilities: the Schweigert instability in multilayer systems and the mode-coupling instability that already appears in single-layer dust systems. Here, a model is presented that treats both types of instability in a common description. The parameter space for the onset of the instabilities is determined. A new variant of the mode-coupling instability is found to arise from the interaction among the layers. For weak confinement, all instabilities continuously merge into each other. For stronger confinement of the dust mainly the Schweigert type of instability is observed.

Two-dimensional simulations of beam-driven turbulence in the auroral ionosphere have shown the formation and instability of phase-space tubes. These tubes are a generalization of electron phase-space holes in a one-dimensional plasma. In a strongly magnetized plasma, such tubes vibrate at frequencies below the bounce frequency of the trapping potential. A theory for these vibrations yields quantitative agreement with kinetic simulations. Furthermore, the theory predicts that the vibrations can become unstable when resonantly coupled to electrostatic whistlers -- also in agreement with simulations.

We present the results of the first reduced model simulations of the nonlinear development of the two-plasmon decay instability in an inhomogeneous plasma, including properties of the 3/2 harmonic emission. A sharp increase in radiation and Langmuir turbulence fluctuation levels occurs above a threshold laser intensity that depends on initial fluctuation levels. We study the competition between the linear propagation of Langmuir waves in the density gradient and the nonlinear saturation due to the Langmuir decay instability. The secondary decay Langmuir waves can provide the dominant source of the radiation and are essential to explain experiments.

Experimental evidence is reported of an internal kink instability driven by a new mechanism: barely trapped suprathermal electrons produced by off-axis electron cyclotron heating on the DIII-D tokamak. It occurs in plasmas with an evolving safety factor profile $q(r)$ when ${q}_{\mathrm{min}}$ approaches 1. This instability is most active when ECCD is applied on the high field side of the flux surface. It has a bursting behavior with poloidal/toroidal mode $\mathrm{number}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}m/n\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/1$. In positive magnetic shear plasmas, this mode becomes the fishbone instability. This observation can be qualitatively explained by the drift reversal of the barely trapped suprathermal electrons.

This report presents a discussion of the wave number selection mechanism in the Faraday instability, which arises when standing waves form on the free surface of a liquid subject to vertical sinusoidal oscillation. The focus is on the case where the viscous effects are strong, i.e., when the wavelength is of the same order of magnitude as the boundary layer thickness at the free surface. We investigate the relationship between the Faraday instability and the Rayleigh-Taylor instability by performing linear stability calculations. In the deep water limit (wavelength << liquid depth), our results indicate that the preferred wave number in the Faraday instability is primarily determined through a Rayleigh-Taylor instability. In the case of shallow water (wavelength approximately liquid depth), the agreement between the Rayleigh-Taylor and Faraday wave numbers does not appear to be as good, probably due to the interaction between the oscillatory motion of the standing waves and the bottom boundary.

Core plasma rotation is observed to change from counter direction to co-current direction during the transition from low (L) to high (H) confinement mode, in Alcator C-Mod plasmas that are heated purely Ohmically and, hence, have no momentum input. The changes of the toroidal velocities, deduced independently from impurity Doppler measurements and from magnetic perturbations associated with sawteeth, agree. The magnitude of the change is consistent with the previously documented scaling for rotation in ion cyclotron rf--heated H modes. The rotation in this Ohmic experiment is obviously not an rf effect but demonstrates unequivocally a transport effect accelerating the plasma.

The theory for a Penning-Malmberg trap predicts $m\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ diocotron stability. However, experiments with hollow profiles show robust exponential growth. We propose a new mechanism of destabilization of this mode, involving parallel compression due to end curvature. The results are in good agreement with the experiments. The resulting modified drift-Poisson equations are analogous to the geophysical shallow water equations, and conservation of line integrated density corresponds to that of potential vorticity. This analogy predicts Rossby waves in non-neutral plasmas and an $m\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$ instability in fluids.

If the current sheet between adjacent twisted magnetic flux tubes is sufficiently thin, the electron flow velocity becomes comparable to the Alfvén velocity and can destabilize collisionless Alfvén waves. The threshold for instability in force-free plasmas is calculated for both inertial and kinetic Alfvén-wave regimes. When there is strong magnetic shear, unstable kinetic Alfvén waves can resonantly accelerate ions to energies much higher than the electron temperature.

The collisionless Landau damping is analyzed for a homogeneous plasma with a sinusoidal density perturbation. The exact damping values are obtained by numerically solving the kinetic dispersion relation of the electron plasma mode. Quantitative results are provided on the influence of the amplitude and the wave vector of the density modulation. Within a simplified fluid framework, a fit is given to these data.

From the Langmuir probe measurements in low-power Ohmically heated or electron cyclotron heated plasma discharges and for several types of confinement devices, we have concluded that the electrostatic potential and density fluctuations at the plasma edge are self-similar over a broad range of time scales [1,2]. The selfsimilarity range is, in general, for time scales longer than the turbulence decorrelation times up to times of the order of confinement time, the mesoscale range. The upper bound of the self-similarity range is difficult to determine because of the lengths of time records available and the varying plasma conditions on these longer time scales. It was found that for fluctuation measurements within the plasma confinement region the self-similarity parameter [3] varies between H 0.62 and H 0.75, a relatively small range of variation given the diversity of plasma confinement devices considered. To find out whether these properties of the plasma fluctuations have any bearing on the dynamics of plasma transport, we must investigate the properties of their induced fluxes. The relative phase between density and potential fluctuations could be such that the induced particle flux does not share the self-similarity properties. The problem with studying fluxes is the scarcity of experimental measurements. Flux measurements for core plasmas are practically nonexistent. At the plasma edge, the turbulence-induced particle flux can be inferred from the simultaneous measurement of the density and potential fluctuations. By measuring the density fluctuations at one point in the plasma, ˜ n1 ˜

Phenomenologic description of the two-stream instability in two parallel two-dimensional (2D) metals is derived. The frequency and rate of growth of an instable mode is expressed in terms of frequencies and damping of plasma waves in isolated 2D metals. These input parameters are experimentally accessible, e.g., by the far-infrared spectroscopy or by the Raman scattering. The presented theory indicates that a system with one metal driven and the second one kept under the zero current condition is more promising for the instability than two metals driven in the opposite directions.

A self-excited vertical oscillation of dust particles in a Coulomb crystal has been observed. It occurs near the plasma-sheath boundary of a dc plasma operated at a low plasma density and gas pressure. The excitation of this spontaneous oscillation is attributed to the finite charging time of particles. As particles move, their charge varies with the local plasma conditions, but with a delay due to the charging time. As particles traverse the vertical electric potential gradient in the sheath, they gain energy. A model of this mechanism is developed to predict the instability threshold and the energy gain.

The nonlinear final state of short-pulse lasers is examined using fully explicit particle-in-cell simulations. A new long-wavelength hosing instability is found to be dominant after a few Rayleigh lengths of propagation. This instability causes self-trapped electrons to be displaced off axis; we find that ion motion is important at the highest densities studied. A possible explanation for this instability is given based on a new variational principle analysis for short-pulse lasers propagating in underdense plasma.

Intense laser pulses propagating in gases undergoing ionization are subject to a scattering instability due to the dependence of the ionization rate on the laser electric field. The instability is convective, and growth is limited for a pulse of finite extent by propagation out of the unstable region. In the nonlinear regime, where the scattered wave amplitude becomes large, the scattering instability saturates at a level that gives rise to full modulation of both the plasma density and laser pulse amplitude.

We present an extension of the paraxial wave equation, taking finite-pulse-length effects into account. A pair of coupled envelope-power equations having finite pulse length as well as nonlinear focusing effects is derived and analyzed in a plasma channel. In plasma channels, pulses can undergo an envelope modulation which is damped in the front and initially grows in the back of the pulse. Finite-pulse-length effects also significantly increase the nonlinear focusing. {copyright} {ital 1999} {ital The American Physical Society }

A new method is proposed for measuring the number density of a plasma using spectral measurements of plasma wave. The method is based on predictions that the ratio electric to magnetic fields and the electric intensity is significantly enhanced in a narrow frequency band without the electron plasma frequency for z-mode waves propagating at small angles to the back-ground magnetic field in cold relatively strongly magnetized plasmas. The method is applied to emissions observed in space over Earth's polar cap, allowing identification of the plasma frequency and the mode consistent with the data available. This method should be useful for other cold, strongly magnetized plasmas in space and in the laboratory.

The measured spectra of fast particle driven Alfv{acute e}n eigenmodes in the JET tokamak plasma are interpreted on the basis of a nonlinear model of near-threshold kinetic instabilities. The observed mode splitting is a manifestation of the general nonlinear phenomena due to the combination of resonant wave-particle interaction and collisionlike relaxation of the resonant particles. The instability growth rate and the effective collision frequency of the resonant particles are determined. {copyright} {ital 1998} {ital The American Physical Society }

The perturbation transfer between the rear and front surface (feedout), secular distortion, and Rayleigh-Taylor (R-T) instability growth is studied for long wavelength modes in accelerated flat foils. A simple formula is derived, relating the R-T growth factor to the amplitude of the rear surface perturbation. Comparisons with two-dimensional simulations confirm the validity of the theory. {copyright} {ital 1998} {ital The American Physical Society }

Using the standard reductive perturbation technique, a nonlinear Schr\"odinger equation is derived to study the modulational instability of finite amplitude dust-acoustic (DA) and dust-ion-acoustic (DIA) waves against oblique perturbations (with respect to the propagation direction of the carrier waves) in an unmagnetized dusty plasma. It is shown that both the DA and DIA waves are modulationally unstable. Possible stationary states of the wave packets can appear as envelope solitons.

Laboratory studies of hydrodynamic effects driven by a flowing, expanding plasma of high-energy density and high Mach number are reported. The flowing plasma is the ejecta from matter accelerated and heated by an ablative shock. X-ray backlighting diagnoses the structure produced when this plasma impacts low-density foam. We observe the forward shock driven into the foam and the stagnated ejecta which drives a reverse shock into the flow.

During high-intensity laser-plasma interactions the ponderomotive forces associated with focused laser radiation can produce large-amplitude plasma waves in the ‘‘wake’’ of the laser pulse as it propagates through an underdense plasma. Since these waves have large longitudinal electric fields and phase velocities close to the speed of light, they have been proposed as a means of accelerating an injected electron beam to high energy @1#. Large-amplitude plasma waves for acceleration purposes can be produced either in the standard laser wake-field accelerator ~LWFA !@ 1 #, if the intense laser pulsewidth ~t! is similar to the electron plasma period ~i.e., t pe;2p/vpe!, or in the ‘‘self-modulated’’ ~SM !~ LWFA! regime @2# in which a wake field is generated as a longduration, intense laser pulse undergoes a self-modulation instability as it propagates in higher-density plasma. This instability causes the pulse to break up into beamlets having a duration corresponding to the plasma period and the subsequent resonant interaction with the plasma can produce a large-amplitude wake field. There have been several recent experiments that have measured the temporal evolution of the wake field using both schemes@3‐5# as well as the energy of electrons accelerated by these waves ~up to 100 MeV in the SM-LWFA regime !@ 6,7#. In this paper we present results of recent high-intensity laser-plasma interaction experiments performed at the Naval Research Laboratory in which we find that second-harmonic radiation produced during such interactions can act as a probe of large-amplitude plasma waves created at the front of the laser pulse as it propagates through the plasma. The angular distribution and frequency structure of this scattered radiation reveals information concerning the properties of these waves and may have implications for the feasibility of the SM-LWFA concept. Our results show that at high plasma densities, as a very intense laser pulse undergoes selfmodulation, radial plasma wave structures ~large-amplitude cylindrical oscillations! can be generated and may dominate the coherently Thomson scattered spectrum. These results are distinct from and complementary to other recent experiments that measured the radial structure of plasma oscillations produced in the low-density standard LWFA regime using the technique of frequency domain interferometry @3#. Our experiments were performed by focusing a high

Lower hybrid waves in an inhomogeneous plasma with dust particles of variable charge have been theoretically estimated. An expression for growth rate has been obtained, which shows that the growth of such waves can be controlled by an appropriate selection of number of parameters, viz., the external magnetic field, the density inhomogeneity scale lengths, and charge on dust particles. {copyright} {ital 1997} {ital The American Physical Society}

A detailed analytical and numerical study of the suppression of the transverse head-tail instability by modulating the chromaticity over a synchrotron period is presented. We find that a threshold can be developed, and it can be increased to a value larger than the strong head-tail instability threshold. The stability criterion derived agrees very well with the simulations. The underlying physical mechanisms of the damping scheme are rotation of the head-tail phase such that the instability does not occur, and Landau damping due to the incoherent betatron tune spread generated by the varying chromaticity. {copyright} {ital 1997} {ital The American Physical Society}

Anomalous cross-field transport of mass and energy due to low-frequency, electrostatic fluctuations is known to influence the formation of equilibria in magnetically confined plasmas. It has not been generally recognized, however, that anomalous cross-field current plays a similar role in some plasma discharges. Collisionless crossfield currents are also an integral part of many space plasma systems, but in many cases the anomalous mechanism is poorly understood. In this paper we present fluid simulation of a model where the fluctuating ionpolarization drift provides a cross-field current which may flow along the density gradient. The model equations describe quite generally two-dimensional flute dynamics of a very low beta plasma in a nonuniform magnetic field. However, the geometry, boundary conditions, and sinks and sources have been chosen for comparison with experimental results obtained in a steady-state plasma device with a purely toroidal magnetic field [1]. Historically, the interest in this configuration has suffered from the nonexistence of an MHD equilibrium [2]. However, this does not prevent a transport dominated equilibrium to exist. The nonexistence proof is based on the assumption of zero boundary pressure, while a transport dominated equilibrium (balancing interior sources) requires plasma wall contact. The simple torus configuration is interesting as a paradigm for laboratory and space plasmas fulfilling the following criteria: (i) magnetic field curvature and very low beta, (ii) perpendicular transport more important than parallel transport, and (iii) effects of rotational transform and magnetic shear negligible. The modeling and simulations presented here show that radial transport of charge as well as mass in a plasma produced by a hot cathode is associated with the occurrence of a large, coherent dipole vortex rotating poloidally with the mean plasma flow. This flute-mode vortex has plasma density and potential perturbations approximately in antiphase, and is consistent with observations in the BLAAMANN torus at the University of Tromso [3,4]. The theory offers a mechanism for the collisionless radial discharge current and plasma transport observed in these experiments, explains how the potential profile depends on magnetic field, discharge current, and ion mass, and shows that important aspects of the global physics can be adequately described without incorporating drift-wave phenomena. Before we formulate the simulation model, we will discuss the important aspects of the underlying physics, and start with the one-fluid force-balance equation

In a partially stripped plasma the presence of bound electrons can significantly alter the propagation and stability of intense laser pulses. In the presence of both free and bound electrons, an atomic modulation instability develops that can have a growth rate substantially higher than either the conventional relativistic modulational instability or the forward Raman instability. In addition, the filamentation instability can be significantly enhanced by bound electrons while the backward Raman instability is unaffected. {copyright} {ital 1997} {ital The American Physical Society}

Instabilities of dust acoustic waves in a plasma with a significant background pressure of neutrals have been investigated. A long wavelength mode is found to be unstable due to recombination of electrons and ions on the surface of dust particles. At short wavelengths, a dissipative instability driven by relative drift between ions and the dust particles is found to be important. Nonlinearly, the short wavelength modes lead to the formation of K{minus}dV solitons whereas the long wavelength end is dominated by modulational instabilities. {copyright} {ital 1997} {ital The American Physical Society}

It is first demonstrated that a V-shaped dc potential structure is created by a current-driven electrostatic ion-cyclotron instability by means of an open two-dimensional particle simulation model. A positive dc potential difference along magnetic field lines is generated by anomalous resistivity caused by the ion-cyclotron instability. In the direction across the magnetic field lines, the dc potential rises from the high-current region to the low-current region. {copyright} {ital 1997} {ital The American Physical Society}

An analytical and numerical study of the suppression of the transverse head-tail instability by modulating the chromaticity over a synchrotron period is presented. We find that a threshold can be developed, and it can be increased to a value larger than the strong head-tail instability threshold. The stability criterion derived agrees very well with the simulations. The underlying physical mechanisms of the damping scheme are rotation of the head-tail phase such that the instability does not occur, and Landau damping due to the incoherent betatron tune spread generated by the varying chromaticity. {copyright} {ital 1997} {ital The American Physical Society}

The modulational instability of a linearly polarized, strong, electromagnetic wave in a (unmagnetized) positron-electron plasma is analyzed using relativistic two-fluid hydrodynamics to properly account for physical regimes of very high temperatures. The effect of phonon damping is also included in the treatment. The theory can be reduced to a pair of extended Zakharov equations. The envelope modulation is then studied by deriving the corresponding nonlinear Schr\"odinger (NLS) equation, using multiscale perturbation analysis. According to the intensity of the damping three different types of NLS are obtained. The main results are (a) that relativistic temperatures modify the stability result found in the literature for low temperature, zero damping, ${\mathrm{e}}_{+}$ -${\mathrm{e}}_{\mathrm{\ensuremath{-}}}$ plasmas, and (b) that phonon damping also produces substantial changes in the NLS, which then predict unstable envelopes. This work extends previous analyses, showing that if the phonon damping is O(${\mathrm{\ensuremath{\epsilon}}}^{0}$ ) or O(${\mathrm{\ensuremath{\epsilon}}}^{1}$ ) (\ensuremath{\epsilon} is the perturbation parameter), a modulational instability appears in the electron-positron case in all ranges of temperature and wave frequencies. Thus presence of some amount of sound absorption helps to produce an envelope decay. When the phonon damping is very small [O(${\mathrm{\ensuremath{\epsilon}}}^{2}$ )] the self-modulational instability occurs in a finite band near the reduced plasma frequency, for ultrarelativistic temperatures.

Experimental evidence for the transient phase of the filamentation instability of a laser beam in a thermal force dominated plasma is presented. When the laser beam is crossed with a weaker degenerate probe beam at a small angle, the interference of the two beams drives a thermally enhanced ion grating which during its transient phase acts to seed the filamentation/forward Brillouin instability. {copyright} {ital 1997} {ital The American Physical Society}

Using the equations of self-consistent charge dynamics for the grain charge fluctuations, electron plasma oscillations in a dusty plasma are studied. It is shown that the instability in longitudinal plasma oscillations disappears if the self-consistent grain charge dyanmics is taken into account. The relation of this result with previous work is discussed. {copyright} {ital 1997} {ital The American Physical Society}

The interaction of an ultraintense laser pulse with an overcritical collisionless plasma at normal incidence is investigated with 1.5 dimensional particle-in-cell simulations. Laser absorption and hot electron energy are reported for a large range of intensities and plasma densities. We observe a strong dependence of the electron temperature on the plasma density profile, and a transition between two different heating mechanisms when the gradient length is varied. For sharp edged profiles, the electron temperature is well below the laser ponderomotive potential. {copyright} {ital 1997} {ital The American Physical Society}

The resonant interaction of a negative-energy wave with a positive-energy wave gives rise to a linear instability. Whereas a single crossing of rays in a nonuniform medium leads to a {ital convectively} saturated instability, we show that a double crossing can yield an {ital absolute} instability, if the two rays are oppositely directed. We obtain expressions for the growth rate and the threshold, and present one application. {copyright} {ital 1996 The American Physical Society.}

Cycloidal electron flows in a gap with crossed electric and magnetic fields are found to be violently unstable when a small ac voltage is imposed across the gap. This instability is electrostatic and one dimensional. It occurs over a wide band of frequencies and is insensitive to the precise level of the injected current density. The physical origin of this instability is the formation of a virtual cathode right in front of the cathode. An analytic theory is presented which accurately predicts the onset of the instability in the low-frequency limit. {copyright} {ital 1996 The American Physical Society.}

A rapidly growing uhf ([gt]250 MHz) wave burst, identified as the Buneman two-stream instability, was detected at the beginning of a high-current plasma discharge. It preceded by nearly 50 ns a parallel electric-field pulse with alternating polarity and amplitude of typically 2.0 kV/cm. A comparison with the nonlinear theory of the Buneman instability showed that the density evacuation mechanism in its nonlinear stage caused a solitary potential pulse within the observed delay time determined by ion dynamics.

We study the nonlinear propagation of electrostatic wave packets in a collisional plasma composed of strongly coupled ions and relativistically degenerate electrons. The equilibrium of ions is maintained by an effective temperature associated with their strong coupling, whereas that of electrons is provided by the relativistic degeneracy pressure. Using a multiple scale technique, a (3+1)-dimensional coupled set of nonlinear Schr\"{o}dinger-like equations with nonlocal nonlinearity is derived from a generalized viscoelastic hydrodynamic model. These coupled equations, which govern the dynamics of wave packets, are used to study the oblique modulational instability of a Stoke's wave train to a small plane wave perturbation. We show that the wave packets, though stable to the parallel modulation, becomes unstable against oblique modulations. In contrast to the long-wavelength carrier modes, the wave packets with short-wavelengths are shown to be stable in the weakly relativistic case, whereas they can be stable or unstable in the ultra-relativistic limit. Numerical simulation of the coupled equations reveals that a steady state solution of the wave amplitude exists together with the formation of a localized structure in (2+1) dimensions. However, in the (3+1)-dimensional evolution, a Gaussian wave beam self-focuses after interaction and blows up in a finite time. The latter is, however, arrested when the dispersion predominates over the nonlinearities. This occurs when the Coulomb coupling strength is higher or a choice of obliqueness of modulation, or a wavelength of excitation is different. Possible application of our results to the interior as well as in an outer mantle of white dwarfs are discussed., Comment: 18 pages, 7 figures; To appear in Phys. Rev. E (2012); The manuscript has been revised with few discussions and citation of some relevant references

A traveling-wave instability at finite critical wave number is found in extrinsic semiconductors at the onset of impact ionization. A condition for this instability is a strong increase of the carrier-generation rate as a function of the electric field, e.g., due to a sufficiently low ionization energy of the impurities. The linear instability of the uniform steady state is discussed for a simple model, and traveling waves are shown to exist by solving numerically the basic partial differential equations.

The nonlinear evolution of the Kelvin-Helmholtz instability in an inhomogeneous plasma is studied using a two-dimensional magnetohydrodynamic (MHD) simulation code which includes the Hall term. This term is important when the thickness of the boundary layer is less than an ion inertial length. In comparison to the evolution of the instability based upon ideal MHD, the Hall term generates short wavelength turbulence which inhibits vortex formation, and introduces an asymmetry into the nonlinear evolution of the instability depending upon the sign of the vorticity.

A tokamak plasma configuration is reported that simultaneously improves on the maximum stable plasma pressure, the bootstrap current contribution, and kinetic stability to temperature and density gradient driven modes in toroidal geometry. It is characterized by negative magnetic shear in the plasma interior and a peaked pressure profile. Stability to the ideal low-n external kink modes requires a conducting shell at 1.3 times the plasma minor radius. This novel plasma configuration is promising for improved plasma performance in advanced tokamak experiments.

A long, relativistic particle beam propagating in an overdense plasma is subject to the self-modulation instability. This instability is analyzed and the growth rate is calculated, including the phase relation. The phase velocity of the wake is shown to be significantly less than the beam velocity. These results indicate that the energy gain of a plasma accelerator driven by a self-modulated beam will be severely limited by dephasing. In the long-beam, strongly coupled regime, dephasing is reached in a homogeneous plasma in less than four e foldings, independent of beam-plasma parameters.

Highly charged dust grains immersed in a plasma can exhibit charge fluctuations in response to oscillations in the plasma currents flowing into them. This introduces a new physical effect, namely, the electric charge on the dust particles becomes time dependent and a self-consistent dynamical variable. The consequent modifications in the collective properties of a dusty plasma are investigated. It is shown that these effects lead to dissipative and instability mechanisms for ion waves in the plasma and can lead to interesting applications to many laboratory and astrophysical situations.

The two plasmon decay instability in a uniform CO[sub 2]-laser produced plasma is studied by Thomson scattering of ruby laser radiation. Initially the instability grows as predicted by linear theory as decay plasma wave pair of expected wave vectors. During saturation unstable plasma wave activity spreads over wide regions in [ital k] space extending in places beyond the Landau cutoff contour. Comparing the saturated plasma wave intensity distribution in [ital k] space and measurements of the angular variation of 3/2-harmonic emission confirms the mechanism in which the radiation is generated by scattering of pump photons off two plasmon decay waves.

An analytic solution describing an instability of a vacuum--overdense-plasma interface under the action of a powerful laser pulse is developed. The explicit dispersion relations and the space and time dependence of the perturbations have been obtained in the linear approximation. The influence of the instability on the interaction process is discussed and some comparisons with previous studies are presented.

The theory of the uniform electric-field distribution instability in superlattices is developed. The instability with respect to high-field domain formation was discovered some time ago by Esaki and Chang but no microscopic theory has been proposed so far. To determine the instability condition we consider first the conductivity of a superlattice with a uniform electric-field distribution for arbitrary relations between the width of the miniband, the electric potential drop per period, and the energy uncertainty due to scattering. Such a general case can be described quantitatively with the help of the density matrix if an effective electron temperature is larger than those three characteristic energies. The instability threshold corresponds to an electric field for which the separation between Stark levels in the superlattice becomes larger than the energy uncertainty due to scattering. The physical reason for the instability is that any local increase of the field leads to a larger separation between the Stark levels and to the local decrease of the current. The solution of the instability problems shows that near the instability threshold only long-wavelength fluctuations are unstable. At the initial stage of the development of the instability nonlinear effects accelerate the fluctuation growth.