bluuebunny/crossref_metadata_2025 huggingface dataset:
Stimulated Raman backscatter in ultraintense, short pulse laser–plasma interactions
Authors: Wilks S. C. , Kruer W. L. , Williams E. A. , Amendt P. , Eder D. C.
Published: July 2002

The stimulated Raman backscatter instability that occurs when short (∼50–100 fs), intense (I≂2×1017 W/cm2) laser pulses are focused into a high density, low temperature plasma is reported on in this article. Results from linear theory, numerical differencing of the coupled mode equations, and one-dimensional particle-in-cell simulations will be discussed. It is found that the growth rate is a function of position along the pulse, and that the maximum growth rate at the peak of the pulse is reduced from the homogeneous long pulse value, γ0 by a factor of 1/√2. A criterion is obtained that predicts when the product of the pulse length and the growth rate is large enough so that the backscatter instability will heat the background electrons to a significant amount above the initial temperature. The usual hot electron component associated with this instability will also be discussed.
The sheath potential formation in the presence of a hot plasma flow
Authors: Senda Ikuo
Published: July 2002

The plasma–wall transition in the presence of a hot plasma flow toward a wall is investigated. The distribution function of the ion in the sheath potential is derived from the Boltzmann equation which includes the effect the ionization and charge–exchange interactions between the plasma and neutral. The presheath potential in the absence of the charge–exchange interaction is discussed in order to understand the qualitative behavior of the presheath potential. The profile of the sheath potential from the plasma to the wall is obtained by solving the Poisson equation numerically and the dependence of the solution on the parameters, such as the ion flow velocity, temperatures and the neutral density, is presented. Using solutions, the Bohm criterion and the power transmission factor at the wall are discussed.
Oriented s-p excitation in electron–ion collisions in dense plasmas
Authors: Jung Young-Dae
Published: July 2002

In dense plasmas the orientation and alignment phenomena for direct s-p excitation in electron–hydrogenic ion collisions are investigated using a first-order semiclassical straight-line path approximation. The interaction potential between the projectile and bound electrons is obtained by a nonspherical Debye–Hückel model. Angular momentum orientation parameter L⊥ for direct s-p excitation is determined for various Debye lengths and projectile energies. The plasma-screening effect appreciably reduces a favoring of the s→p−1 transition, especially, for high-energy projectiles.
The plasma transport equations derived by multiple time-scale expansions and turbulent transport. I. General theory
Authors: Edenstrasser J. W.
Published: July 2002

A multiple time-scale derivative expansion scheme is applied to the dimensionless Fokker–Planck equation and to Maxwell’s equations, where the parameter range of a typical fusion plasma was assumed. Within kinetic theory, the four time scales considered are those of Larmor gyration, particle transit, collisions, and classical transport. The corresponding magnetohydrodynamic (MHD) time scales are those of ion Larmor gyration, Alfvén, MHD collision, and resistive diffusion. The solution of the zeroth-order equations results in the force-free equilibria and ideal Ohm’s law. The solution of the first-order equations leads under the assumption of a weak collisional plasma to the ideal MHD equations. On the MHD-collision time scale, not only the full set of the MHD transport equations is obtained, but also turbulent terms, where the related transport quantities are one order in the expansion parameter larger than those of classical transport. Finally, at the resistive diffusion time scale the known transport equations are arrived at including, however, also turbulent contributions.
Angular dependence of stimulated Brillouin scattering in homogeneous plasma
Authors: Giacone R. E. , McKinstrie C. J. , Betti R.
Published: July 2002

The angular dependence of stimulated Brillouin scattering (SBS) in a finite homogeneous plasma is studied. For parameters typical of inertial confinement fusion experiments, the initial evolution of SBS is well approximated by a one-dimensional model. In the context of this linear model, the threshold intensity of the absolute instability and the steady-state spatial growth rate of the convective instability are both independent of the scattering angle. However, the saturation time of the convective instability exhibits a strong inverse dependence on the scattering angle: Forward SBS always occurs in the transient regime and the intensity of the scattered light is less than that predicted by a steady-state analysis. In particular, no light is emitted in the propagation direction of the incident wave.
Evolution of a Maxwellian plasma driven by ion-beam-induced ionization of a gas
Authors: Oliver B. V. , Ottinger P. F. , Rose D. V.
Published: July 2002

The ionization of gas by intense (MeV, kA/cm2) ion beams is investigated for the purpose of obtaining scaling relations for the rate of rise of the electron density, temperature, and conductivity of the resulting plasma. Various gases including He, N, and Ar at pressures of order 1 torr have been studied. The model is local and assumes a drifting Maxwellian electron distribution. In the limit that the beam to gas density ratio is small, the initial stage of ionization occurs on the beam impact ionization time and lasts on the order of a few nanoseconds. Thereafter, ionization of neutrals by the thermal electrons dominates electron production. The electron density does not grow exponentially, but proceeds linearly on a fast time scale tth=U/(vbρ dE/dx) associated with the time taken for the beam to lose energy U via collisional stopping in the gas, where U is the ionization potential of the gas, vb is the beam velocity, ρ is the gas mass density, and dE/dx is the mass stopping power in units of eV cm2/g. This results in a temperature with a slow time dependence and a conductivity with a linear rise time proportional to tth.
Evidence for precursor plasma formation resulting from heterogeneous current channels in wire array loads
Authors: Yadlowsky E. J. , Moschella J. J. , Hazelton R. C. , Settersten T. B. , Spanjers G. G. , Deeney C. , Failor B. H. , LePell P. D. , Davis J. , Apruzese J. P. , Whitney K. G. , Thornhill J. W.
Published: July 2002

The initial current flow in single and double wire loads exploded by current generators with dI/dt in the range of 1010–1012 A/s has been investigated using laser absorption and interferometric techniques. The results clearly show heterogeneous current channel formation with a plasma corona surrounding a predominantly neutral core. In the two wire load case, the corona is observed to separate from the core and prematurely implode on axis. Parasitic current channels were observed when material, outgassed by a low level prepulse current, is ionized to form a current carrying plasma channel that temporarily shields the wire from the main current pulse. In a complementary study on a terawatt generator, the radiation spectra from Mg coated Al wire array loads contained Mg K-shell lines 10 ns before similar Al lines were observed, confirming the load straggling effects implied by the laser imaging measurements. The results are consistent with theoretical predictions of heterogeneous current channels and precursor plasma formation that lead to a softer implosion, reduced ion density, and reduced x-ray yield.
Plasma channel formation by short pulse laser
Authors: Annou R. , Tripathi V. K. , Srivastava M. P.
Published: July 2002

A short pulse laser propagating through a preformed plasma exerts a radial ponderomotive force on the electrons and creates a low density plasma channel via ambipolar diffusion. If the radial intensity profile of the laser beam is clamped, i.e., assumed Gaussian in r with fixed half-width r0, then closed form analytic solution of the evolution of plasma density in the channel is obtained in the perturbation approximation, Δn≪n0, where Δn is the modification in electron density over its equilibrium value n0, by solving the linearized equation of continuity. The relevance of the channel for low spot size beams self-focusing is examined.
Role of edge turbulence in detached divertor plasmas
Authors: Gang F. Y. , Sigmar D. J. , Krasheninnikov S. I.
Published: July 2002

The role of edge turbulence in detached divertor plasmas is investigated. It is shown that the edge turbulence, through poloidal transport of parallel momentum, can produce a significant plasma pressure drop along the magnetic field lines toward the divertor plate, a feature that characterizes the detached divertor plasma regime.
Effect of elastic scattering on stochastic ion heating by electrostatic waves
Authors: Chia Ping-Kun , Schmitz L. , Conn R. W.
Published: July 2002

The dynamics of an ion interacting with an electrostatic wave propagating perpendicularly to a static magnetic field in the presence of elastic ion-neutral scattering is investigated. Superharmonic (ω/ω0∈I) and subharmonic (ω/ω0=r/s≤1) perturbations are studied. Here, ω and ω0 are the wave and the ion cyclotron frequency, respectively, and r and s are relatively prime integers. Without elastic collisions, the ion is found to exhibit either quasi-regular or stochastic motion in phase space depending on the wave frequency, wave amplitude, and the initial position and velocity of the particle. Quasi-regular and stochastic behavior are found to be exclusive with respect to each other. In the presence of elastic collisions, the entire phase space becomes accessible to the ion irrespective of the initial conditions. The heating rate is generally found to depend on the ratio γm/ω (where γm is the momentum loss rate) and is enhanced compared to the collisionless case.
Hamiltonian description of the topology of drift orbits of relativistic particles in a tokamak
Authors: de Rover Machiel , Lopes Cardozo Niek J. , Montvai Attila
Published: July 2002

Using classical perturbation theory, a Hamiltonian description of the guiding center motion of relativistic electrons in a torus with an axially symmetric magnetic field is derived. The magnetic field itself generates concentric circular flux surfaces. This description enables us to assess the behavior of the drift orbits such that previously unknown details follow almost naturally due to the introduction of higher order terms in the description. The rotational transform of the orbit for a given particle is found to decrease when the particle energy increases and the radius of the particle orbit is almost constant during acceleration. As a consequence, the minor radius of a drift surface with a (fixed) rotational transform 1/qD is always smaller than the radius of the magnetic flux surface with q=qD from which the particle started with zero energy. At a critical energy, the drift surface has collapsed to a point in the poloidal cross section. Above this critical energy there is no particle orbit with the commensurate rotational transform 1/qD. When the symmetry of the magnetic field is broken by harmonic perturbations with amplitudes that increase towards the plasma edge, the resonant magnetic surfaces and drift surfaces break up, and form chains of islands. The Hamiltonian description was set up in such a way that it allows the computation of the width of the drift islands.
Transport in sheared stochastic magnetic fields
Authors: Vanden Eijnden E. , Balescu R.
Published: July 2002

The transport of test particles in a stochastic magnetic field with a sheared component is studied. Two stages in the particle dynamics are distinguished depending on whether the collisional effects perpendicular to the main field are negligible or not. Whenever the perpendicular collisions are unimportant, the particles show a subdiffusive behavior which is slower in the presence of shear. The particle dynamics is then inhomogeneous and non-Markovian and no diffusion coefficient may be properly defined. When the perpendicular collision frequency is small, this subdiffusive stage may be very long. In the truly asymptotic stage, however, the perpendicular collisions must be accounted for and the particle motion eventually becomes diffusive. Here again, however, the shear is shown to reduce the anomalous diffusion coefficient of the system.
Magnetohydrodynamic flow past a moving conducting sphere in the presence of a magnetic field
Authors: Ranger K. B.
Published: July 2002

A conducting solid sphere is translating and rotating both with exponential time dependent decay in an electrically conducting viscous liquid. This motion takes place in the presence of a magnetic field which originates both in the interior and exterior of the moving sphere. The fluid velocity and magnetic fields are analytically constructed in a concise form, and a compact expression is given for the drag on the sphere.
Modulational instability of an electron plasma wave in a dusty plasma
Authors: Amin M. R. , Ferdous T. , Salimullah M.
Published: July 2002

The modulational instability of an electron plasma wave in a homogeneous, unmagnetized, hot, and collisionless dusty plasma has been investigated analytically. The Vlasov equation has been solved perturbatively to find the nonlinear response of the plasma particles with random static distribution of massive and charged dust grains having certain correlation. It is noticed that the growth rate of the modulational instability of the electron plasma wave through a new ultra-low-frequency dust mode is more efficient than that through the usual ion-acoustic mode in the dusty plasma.
Continuous spectrum of a non-neutral plasma column
Authors: Bhattacharyya S. N. , Bhattacharjee A.
Published: July 2002

The linearized dynamics about the cylindrical equilibrium of a non-neutral plasma in a uniform axial magnetic field is discussed. A spectral analysis shows the existence of two continua, both of which are shown to be stable. The possibility of Suydam-like rapidly oscillating solutions is also demonstrated. The Suydam solutions may imply the existence of an infinite sequence of unstable modes that belong to the discrete spectrum.
Nonlinear dynamics of an elliptic magnetic stagnation line
Authors: Wahlberg C. , Eriksson H. G. , Jiang Z. X.
Published: July 2002

The nonlinear evolution of the kink instability of a plasma with an elliptic magnetic stagnation line is studied by means of an amplitude expansion of the ideal magnetohydrodynamic (MHD) equations. A cylindrically symmetric plasma with circular field lines is used to model the magnetic field geometry close to the stagnation line. Due to the symmetry with respect to ±z, the linear stability problem of such a system has a two-folded degeneracy, with equal eigenvalues for helical kink perturbations with positive and negative polarization. It is shown that, near marginal stability, the nonlinear evolution of the instability can be described in terms of a two-dimensional potential U(X,Y), where X and Y represent the amplitudes of the perturbations with positive and negative helical polarization. The potential U(X,Y) is found to be nonlinearly stabilizing for all values of the polarization. Furthermore, in addition to the equilibrium point (X,Y)=(0,0), the nonlinear potential has eight equilibrium points in the XY-plane, four corresponding to helical polarization (X or Y=0) and four to plane polarization (|X|=|Y|). The latter equilibria have the lowest energy, indicating that plane kinks preferably should be formed as the stagnation line instability evolves. The equilibria with helical polarization agree with the bifurcated Z-pinch equilibria obtained by means of a different method in a previous paper [Plasma Phys. Controlled Fusion 35, 551 (1993)].
Linear stability of the collisionless, large Larmor radius Z-pinch
Authors: Russell P. G. F. , Arber T. D. , Coppins M. , Scheffel J.
Published: July 2002

The Vlasov fluid model is used to study the m=0 and m=1 internal and free boundary modes in a collisionless, large Larmor radius Z pinch. Two methods (initial value and variational) are employed, and give good agreement. The growth rate can be reduced from its zero Larmor radius value by a factor of up to 10 for m=1, and up to 3 for m=0. Stability thresholds and the role of resonant ions are discussed.
Enhanced coupling of the fast wave to electrons through mode conversion to the ion hybrid wave
Authors: Lashmore-Davies C. N. , Fuchs V. , Ram A. K. , Bers A.
Published: July 2002

An analysis of ion cyclotron resonance heating in a plasma containing two ion species is given. Particular emphasis is placed on the important case of comparable concentrations of the two species. The properties of ion-Bernstein waves in such a plasma are discussed. A summary of the cutoff–resonance (Budden) and cutoff–resonance–cutoff (triplet) configurations is given and an analytic expression obtained for the Budden transmission coefficient (relevant also to the triplet case), which is valid for any concentration ratio of the two-ion species. A formula for the damping coefficient of the ion hybrid wave is obtained and a ray tracing code is used to illustrate the absorption of this mode by electrons close to the two-ion hybrid resonance. The reduced, second-order fast wave equation is solved numerically, illustrating the transition from strong electron heating through mode conversion at the two-ion hybrid resonance to direct electron damping of the fast wave in a high beta plasma.
Two-dimensional time-dependent solution of Stix’s equation for the distribution function of minority ions during ion cyclotron resonant heating
Authors: Dalla S. , Ljepojevic N. N.
Published: July 2002

The time-dependent flux-surface-averaged Fokker–Planck equation for the distribution function of minority ions during ion cyclotron resonant heating introduced by Stix in a classic paper [T. H. Stix, Nucl. Fusion 15, 737 (1975)] is solved keeping two dimensions (2-D) in velocity space (speed and pitch-angle). An analysis of the applicability of the method of expansion of the distribution function f in Legendre polynomials of the pitch-angle, a method suggested by Stix and subsequently used by others, is given. A full numerical 2-D solution is also calculated. It is shown that the convergence of the Legendre polynomial expansion is very slow and non-uniform with respect to both particle energy and pitch-angle, making the method impractical when f is required at energies much higher than the background plasma thermal energy. However, the iterative sequences for the moments of f are found to converge very fast. In particular, a good approximation to the pitch-angle average of the distribution function is obtained already with two terms kept in the expansion, for a wide range of heating parameters. The validity of Stix’s analytical one-dimensional approximations is analysed in detail.
Extensions and applications of the Bogema–Davidson inversion theorem
Authors: Book David L.
Published: July 2002

For a single particle species confined by a uniform magnetic field B, the Bogema–Davidson inversion theorem determines a self-consistent rigid-rotor equilibrium distribution function F(H⊥−ωrPθ) as the derivative of the given radial density profile n(r) with respect to the corresponding generalized potential ψ(r). Aside from the quadratic density profile, most forms one can choose for n(r), even if integrable, yield potentials ψ(r) that cannot be inverted. The function F can be found parametrically, however, by writing the derivative of n with respect to ψ as the ratio of the radial derivatives of n and ψ. Several nontrivial examples of physical interest are studied. Numerical methods for calculating F are presented, its asymptotic form and integration are investigated, and applications to experiment are discussed.
Geometrical approach to fluid models
Authors: Kuvshinov B. N. , Schep T. J.
Published: July 2002

Differential geometry based upon the Cartan calculus of differential forms is applied to investigate invariant properties of equations that describe the motion of continuous media. The main feature of this approach is that physical quantities are treated as geometrical objects. The geometrical notion of invariance is introduced in terms of Lie derivatives and a general procedure for the construction of local and integral fluid invariants is presented. The solutions of the equations for invariant fields can be written in terms of Lagrange variables. A generalization of the Hamiltonian formalism for finite-dimensional systems to continuous media is proposed. Analogously to finite-dimensional systems, Hamiltonian fluids are introduced as systems that annihilate an exact two-form. It is shown that Euler and ideal, charged fluids satisfy this local definition of a Hamiltonian structure. A new class of scalar invariants of Hamiltonian fluids is constructed that generalizes the invariants that are related with gauge transformations and with symmetries (Noether).
Energy description of wave-plasma interaction in the ion cyclotron range of frequency: Application to fast wave absorption and emission in tokamaks
Authors: Fraboulet D. , Bécoulet A.
Published: July 2002

An alternative description of the energy transfer between an electromagnetic wave and a tokamak plasma is presented. It involves the evaluation of local coefficients of emission and absorption and leads to rapid numerical procedures. This method allows a coherent description of emission and absorption processes and is thus particularly suited for the determination of the rf electromagnetic energy radiated from the plasma, as is shown by calculating ion cyclotron emission spectra. Furthermore, this “fluid” energy description proves also useful for the evaluation of the fast wave plasma absorption during ion cyclotron range of frequency (ICRF) heating: a fast “fluid” one-dimensional slab numerical code has been developed. It cannot cope with singularities of the wave vector such as resonance or cutoff, but it is well adapted to scenario designs of fast wave direct coupling to electrons and evaluation of partitioning of energy between species in the case of ions high harmonics.
Twist mapping for the dynamics of magnetic field lines in a tokamak ergodic divertor
Authors: Abdullaev S. S. , Finken K. H. , Kaleck A. , Spatschek K. H.
Published: July 2002

Symplectic twist mapping is proposed to model magnetic field line dynamics in the ergodic divertor at the tokamak plasma edge. The relationship between a perturbation function in the mapping and magnetic field perturbation in the tokamak is found. The mapping is specified for the Dynamic Ergodic Divertor being proposed for the Torus Experiment for Technology Oriented Research (TEXTOR-94) [Fusion Eng. Design, 37, 337 (1997)]. The spectrum of the poloidal harmonics of perturbation is assumed to be localized around the harmonics m=12. It creates the stochastic layer near the resonant magnetic surface q=3. The mapping is applied to the formation of the stochastic layer and field line diffusivity at the plasma edge. For the moderate magnetic field perturbations, the ergodic layer consists of a stochastic sea with regular Kolmogorov–Arnold–Mozer (KAM) -stability islands. The radial profiles of the Kolmogorov lengths and the field line diffusivity are studied for different perturbations. It is shown that the behavior of open field lines at the lower boundary of the stochastic layer is subdiffusive. For large perturbations a regular convective behavior of open field lines dominates over their diffusion at the large region of the ergodic layer.
Chaos in relativistic electron plasma and nonlinear dynamics in oblique propagation of electrostatic waves
Authors: Mohanty J. N. , Naik Antaryami
Published: July 2002

Nonlinear interaction between electrostatic waves propagating obliquely to an ambient uniform magnetic field, and electron plasma in relativistic formalism are analyzed in detail. Resonances among the harmonics (l) are delineated and a qualitative expression for the overlapping parameter (K) leading to stochasticisty is analytically derived. A novel feature of this analysis is revealed in the form of threshold wave amplitude (φk), especially at the onset of chaos. The interesting profiles concerning the variation of threshold amplitude with harmonics (l), magnetic field lines (B0), and wave vector (kz) pertaining to ensemble of electrostatic waves are displayed and discussed along with the phase-space topology mappings. These findings may provide important information concerning astrophysical scenarios, which include intergalactic plasmas, cosmic plasma particles, synchrotron radiation, and pulsar plasma analysis.
Longitudinal permittivity of a tokamak plasma with elliptic and circular magnetic surfaces
Authors: Grishanov N. I. , de Azevedo C. A. , de Assis A. S.
Published: July 2002

The contributions of untrapped and three groups of trapped particles to the longitudinal permittivity of a tokamak plasma with elliptic magnetic surfaces are derived for radio frequency waves in a wide range of frequencies, mode number, and plasma parameters. The analytical expressions of the longitudinal permittivity elements are obtained by using the kinetic theory of dielectric tensor elements, where the drift kinetic equation is solved as a boundary-value problem. Considered is a collisionless plasma model of an axisymmetric tokamak with small ellipticity and a large aspect ratio. The limit to the known results for toroidal plasmas with the circular cross-section of the magnetic surfaces is shown.
Superbanana orbits and redistribution of marginally trapped fast ions during sawtooth crashes
Authors: Kolesnichenko Ya. I. , Lutsenko V. V. , Yakovenko Yu. V.
Published: July 2002

The work represents the first step in studying the effect of sawtooth crashes on the “resonant” fast ions, i.e., the ions that are trapped or marginally untrapped with respect to the helical perturbation associated with the sawtooth crash. The simplest case is considered, when the particles in the absence of perturbations are marginally trapped in the tokamak magnetic field and characterized by narrow orbits. It is shown that in the presence of perturbation the guiding centers of the banana orbits oscillate in the toroidal and radial directions, forming “superbanana” orbits. The radial width of such “superbananas” constitutes a significant part of the sawtooth mixing radius, which implies that sawteeth can strongly redistribute these particles.
Electron acceleration by a laser pulse in vacuum
Authors: Lu Q. M. , Cheng Y. , Xu Z. Z. , Wang S.
Published: July 2002

The acceleration of an electron by a circularly polarized, high-intensity ultrashort laser pulse in vacuum is studied. It appears that the energy of the electron can be accelerated significantly, and the electron moves almost along the propagation direction of the laser pulse.
Collisionless fluid closure theory and the phase velocity transform
Authors: Mattor Nathan
Published: July 2002

Fluid equations are derived that describe wave-particle resonances, which usually require kinetic theory. The phase velocity transform is introduced, which decomposes a function of space and time into a sum over components with constant phase velocity. This leads to a new technique to solve partial differential equations, similar but not identical to the Radon transform, and produces recursive solutions. This technique is applied to the collisionless drift kinetic equation, to give closure of the fluid moment hierarchy. The result is a new term in the highest moment equation that retains many nonlinear wave-particle effects not before described by fluid equations.
Rotating toroidal equilibria of quasi-neutral and non-neutral plasmas
Authors: Hurricane O. A.
Published: July 2002

Starting from the fundamental ion and electron fluid equations, a “master” equilibrium equation set is constructed for axisymmetric toroidal plasmas. These equations retain the effects of different ion and electron densities, temperatures, sheared rotation, and electric forces. The master equation is then examined in various limits including the non-neutral plasma limit. An ordering is assumed which makes the effects of the Reynolds stress, pressure, magnetic stress, and electric field stress all comparable emphasizing the natural transition from the non-neutral case to the quasi-neutral one. For sub-relativistic flows, charge separation effects are only significant for nearly force-free plasmas. A set of equilibrium equations are derived for three different cases: (A) ni≈ne, (B) ni≠ne≠0, and (C) ni=0 and ne≠0. In the pure electron plasma case, the resulting equilibrium equation includes the effects of toroidal rotation, poloidal magnetic field, and electron pressure extending the equation of Daugherty and Levy [Phys. Fluids 10, 155 (1967)].
Evolution of Langmuir soliton caused by resonant emission of ion sound wave
Authors: Karpman V. I. , Schamel H.
Published: July 2002

The slow decay of a Langmuir soliton caused by the resonant emission of an ion sound wave is studied analytically.
Chaotic phenomena of a periodic ion-acoustic soliton system
Authors: Nagasawa T.
Published: July 2002

Chaotic phenomena of a periodic ion-acoustic solitons system that is composed of a series of ion-acoustic solitons have been observed in a double-plasma device. Periodic ion-acoustic solitons become unstable by the energy gain and energy loss of the solitons, and they become chaotic. Taking account of a pair of solitons in the system, the front soliton gains energy from the reflected ions from the rear soliton, which is controllable by the rear soliton amplitude or the interval between the solitons. The energy loss of solitons results from collisions with neutral particles that vary with changing gas pressure.
Angular dependence of stimulated Brillouin scattering in a homogeneous two-dimensional plasma
Authors: Giacone R. E. , McKinstrie C. J. , Kolber T.
Published: July 2002

The linear evolution of stimulated Brillouin scattering (SBS) in time and two spatial dimensions was studied analytically and numerically. In order to assess the effects that the finite size of the laser beam has on SBS, wide and narrow laser-beam geometries were considered. Detailed comparisons were made between the predictions of reduced one- and two-dimensional models, which can be solved analytically, and the results of two-dimensional numerical simulations. It was found that SBS is characterized by three parameters: the spatial growth rate in the direction of the Stokes wave, the spatial damping rate of the ion-acoustic wave in the direction of the Stokes wave, and the normalized width of the interaction region. The SBS can be saturated by the damping or the lateral convection of the ion-acoustic wave, both of which limit the growth of the ion-acoustic wave (directly) and the Stokes wave (indirectly). For most scattering angles the saturation mechanism predicted by the reduced models, and the corresponding saturation time and gain factor, agree with the simulation results.
Electrostatic sheath at the boundary of a magnetized dusty plasma
Authors: Baishya S. K. , Das G. C. , Chutia Joyanti , Sarma J.
Published: July 2002

Considering the Boltzmann response of the ions and electrons in plasma dynamics and inertial dynamics of the dust charged grains embedded in a constant magnetic field, the nature of the electrostatic potential near a plasma boundary is investigated. Based on the fluid approximation, the formation, as well as the characteristic behaviors, of sheath in plasma contaminated with dust charged grains is studied. It is expected that the presence of dust charged grains will lead to a very different behavior of the sheath as compared to that of electron–ion plasma. Moreover, in the presence of an applied magnetic field the sheath exhibits novel features.
Ion cyclotron resonance frequency heating of deuterium plasmas in the Joint European Torus: Modeling of the resonant minority ion distribution function
Authors: Testa D. , Core W. G. F. , Gondhalekar A.
Published: July 2002

Experiments were carried out in JET, the Joint European Torus [J. Wesson, Tokamaks (Clarendon, Oxford, 1997), pp. 581–603], to heat deuterium plasmas using ion cyclotron resonance frequency (ICRF) heating at fundamental hydrogen and second harmonic deuterium frequencies. These plasmas contained two minority resonant ion species, protons, and nonthermal deuterons due to neutral beam injection (NBI). To determine the resonant ion energy distribution functions in such plasmas, an iterative method for solving the hot-plasma dispersion relation was developed using a complex perpendicular wave number in cylindrical geometry. The distribution function of ICRF-heated ions is strongly anisotropic and significantly hotter than that of nonresonating particles. Therefore thermal and overlapping cyclotron harmonics effects were considered in computing wave dispersion and absorption. The formal solution of the lowest-order Fokker–Planck equation is derived for the slowing-down NBI deuterons. In a companion paper these results are applied to interpretation of measurements of distribution functions of ICRF-heated ions in the JET.
Quantitative comparison of reduced-description particle-in-cell and quasilinear-Zakharov models for parametrically excited Langmuir turbulence
Authors: Sanbonmatsu K. Y. , Vu H. X. , DuBois D. F. , Bezzerides B.
Published: July 2002

The effect of kinetic processes on the saturation of parametric instabilities in an electromagnetically driven plasma is investigated. A reduced-description particle-in-cell technique is used as a benchmark to test a new quasilinear-Zakharov model which accounts for electron heating due to Landau damping by coupling the quasilinear diffusion equation to the Zakharov equations. The reduced-description particle-in-cell method utilizes a two-time-scale approximation which significantly reduces the numerical dissipation and ion noise levels. This approach allows accurate modeling of Langmuir and ion acoustic waves in regimes typically studied with Zakharov simulations. The comparison of the two models is performed for the test case of a one-dimensional homogeneous plasma driven by a spatially uniform pump in both the Langmuir decay instability cascade and collapse regimes. Good agreement is found in both weakly and strongly driven regimes for the total Langmuir wave energy and evolved electron velocity distributions. Electron heating significantly decreases saturation levels in strongly driven regimes by increasing the Landau damping rate, bringing the quasilinear-Zakharov simulations in much closer agreement with the reduced-description particle-in-cell simulations than standard Zakharov simulations, which do not account for the evolution of the electron distribution.
Enhancement of a laser-driven shock wave up to 10 TPa by the impedance-match technique
Authors: Cottet F. , Hallouin M. , Romain J. P. , Fabbro R. , Faral B. , Pepin H.
Published: July 2002

We have used the impedance-match technique to increase the shock pressure induced in an aluminum-gold target by a laser of 0.26 μm wavelength and intensity of 1015 W/cm2. With incident pressures of 4.5 TPa in aluminum, transmitted pressures of 10+4−3.5 TPa in gold are inferred from shock velocity measurements. Experiments on gold-aluminum targets, with the same irradiation conditions, verify the shock pressure decrease due to the reverse impedance-match effect.
Observations of the Rayleigh–Taylor instability in laser imploded microballoons
Authors: Wark J. S. , Kilkenny J. D. , Cole A. J. , Key M. H. , Rumsby P. T.
Published: July 2002

Mass-modulated plastic microballoons have been ablatively imploded by green (0.53 μm) laser light at an absorbed irradiance of 2×1013 W cm−2. Temporally growing spikes are seen on the outside surface of the target; evidence of the occurrence of mass redistribution indicates that the spikes are due to the Rayleigh–Taylor instability. A growth rate of 0.52±0.06 of classical has been inferred.
Formation of bubbles in BF+2-implanted silicon
Authors: Nieh C. W. , Chen L. J.
Published: July 2002

Fluorine bubbles were observed in 5×1015/cm2 BF+2 -implanted (001) and (111) Si annealed at 1000–1100 °C. Bubbles were found to form in samples annealed at 1100 °C for a time as short as 10 s. The bubbles were distributed mostly near the original amorphous/crystalline interface and silicon surface. The growth of bubbles was seen to be intimately related to the presence of residual defects. Possible ramifications of bubble formation in device applications are discussed.
Inverted thermal conversion—GaAs, a new alternative material for integrated circuits
Authors: Lagowski J. , Gatos H. C. , Kang C. H. , Skowronski M. , Ko K. Y. , Lin D. G.
Published: July 2002

We developed a new type of GaAs which exhibits inverted thermal conversion (ITC), i.e., it converts from conducting to semi-insulating upon annealing at about 850 °C. In device fabrication, its low resistivity, prior to high-temperature processing, differentiates ITC from the standard semi-insulating GaAs. The ITC characteristics are obtained through control of the concentration of the midgap donor EL2 based on heat treatment and crystal growth modification. Thus, EL2 does not exist in the conducting state of ITC GaAs. Conversion to the semi-insulating state during 850 °C annealing is caused by the formation of EL2 consistent with our earlier proposed EL2 model.
High-speed electrical sampling by fs photoemission
Authors: Marcus R. B. , Weiner Andrew M. , Abeles Joseph H. , Lin Paul S. D.
Published: July 2002

We propose and demonstrate a new method for contactless sampling of high-speed electrical signals, by spectral analysis of photoelectrons emitted when a signal-carrying conductor is illuminated by ultrashort light pulses. We present time-resolved measurements of sub-ns electrical signals on a gold transmission line on GaAs using three-photon photoemission induced by 80 fs visible laser pulses, and we discuss the temporal resolution of these measurements. This method is applicable to devices and circuits on any semiconductor.
Molybdenum-silicon multilayer monochromator for the extreme ultraviolet
Authors: Barbee Troy W. , Pianetta Piero , Redaelli Renato , Tatchyn Roman , Barbee Troy W.
Published: July 2002

A two-element molybdenum-silicon multilayer monochromator has been tested in the energy range 60–110 eV on a differentially pumped bending magnet beamline having a high-energy cutoff of 3.5 keV at the Stanford Synchrotron Radiation Laboratory. The multilayer structures were sputter deposited onto 5-cm-diam (111) single-crystal silicon and contained 20 molybdenum layers (4.93 nm thick) separated by amorphous silicon layers (7.8 nm thick). A 1200 l/mm gold transmission grating was used to independently measure the wavelength passed by the monochromator. The Al L2,3 absorption edge jump was also measured using the monochromator. These experimental results are compared to model calculations for both the synchrotron source-monochromator and the synchrotron source-monochromator-Al filter configurations and excellent agreement is shown.
Background doping dependence of silicon diffusion in p-type GaAs
Authors: Deppe D. G. , Holonyak N. , Kish F. A. , Baker J. E.
Published: July 2002

Junction depth measurements via scanning electron microscopy and secondary ion mass spectroscopy are used to characterize silicon diffusion in GaAs crystals that contain varying amounts of zinc background doping. The zinc concentration is found to control the silicon diffusion process. A reason for this is suggested based on the shift in Fermi level with increased p-type doping. Also, the electric field due to the p-n junction formed at the silicon diffusion front is shown to have a large effect on the zinc background doping profile.
Deconvolution of the infrared absorption peak of the vibrational stretching mode of silicon dioxide: Evidence for structural order?
Authors: Boyd Ian W.
Published: July 2002

It is found that the characteristic infrared spectra of thermally grown silicon dioxide in the 1075 cm−1 region for films in the thickness range 28–450 Å mathematically deconvolute consistently into two distinctly separate Gaussian profiles. The derived peaks are found around 1050 cm−1 with a full width at half-maximum transmission value (FWHM) of 65 cm−1, and near 1085 cm−1 with a FWHM of 35 cm−1. The relative band areas of these two features are consistently found to be approximately 0.76 and 0.24, respectively. These observations could be supportive of at least two structural models of amorphous silicon dioxide disclaiming the generally accepted continuous random network arrangement.
Interface contribution to the capacitance of thin-film Al-Al2O3-Al trilayer structures
Authors: Hebard A. F. , Ajuria S. A. , Eick R. H.
Published: July 2002

A dual-gun reactive ion beam sputtering technique has been used to reproducibly fabricate Al2O3 dielectrics with low electrical loss for controlled thickness ranging from approximately 10 to 360 Å. The linear dependence of the reciprocal capacitance on dielectric thickness of Al-Al2O3-Al trilayer structures incorporating this dielectric reveals a significant contribution from an interfacial capacitance in series with the geometric capacitance. Room-temperature measurements of both the dc resistance and the frequency-dependent complex impedance demonstrate that, with respect to bulk, there is an enhanced frequency-dependent dielectric loss associated with this interfacial capacitance.
Structure of the single-phase high-temperature superconductor YBa2Cu3O7−δ
Authors: Beno M. A. , Soderholm L. , Capone D. W. , Hinks D. G. , Jorgensen J. D. , Grace J. D. , Schuller Ivan K. , Segre C. U. , Zhang K.
Published: July 2002

We have determined the crystal structure of the single-phase stoichiometric high-temperature superconductor in the Y-Ba-Cu-O system using high-resolution neutron powder diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice constants a=3.8231 Å, b=3.8864 Å, and c=11.6807 Å. The structure consists of ‘‘dimpled’’ CuO2 layers in the a-b planes loosely bonded by one-dimensional fencelike square-planar CuO3 chains in the b-c planes.
Implanted nitrogen in germanium
Authors: Stein H. J.
Published: July 2002

The first measurements of localized vibrational modes for N in crystalline Ge are reported. Nitrogen isotopes were implanted into Ge, and infrared absorption measurements were made at 80 K after laser annealing. Four isotopic mass-shifted infrared absorption bands are observed. Two of the bands in Ge are analogous to those for N pairs in crystalline Si and are assigned to N pairs in Ge.
I
n s
i
t
u measurement of the fractal dimensionality of ultrafine aerosol particles
Authors: Schmidt-Ott A.
Published: July 2002

The fractal dimension df of gas phase agglomerated particles is measured in situ for the first time. This is done by comparing aerodynamic radii R0 and Rc1 before and after tempering. By tempering, agglomerates are collapsed into close-packed clusters of df=3. It is shown that the equation M∼Rdf0, where M is the agglomerate mass, holds almost down to R0=Rc1 =Rm (monomer radius), making df a very useful quantity for aerosol characterization. The measurement yields df≂2.18 for Ag agglomerates with Rm≂7.5 nm. df is predicted to be material dependent.
Temperature dependence of Hall effect in arsenic-doped silicon at intermediate dopant density
Authors: Lin Alice L.
Published: July 2002

Temperature dependence of Hall effect in arsenic-doped silicon at intermediate dopant concentrations from 1.8×1017 to 2.5×1018 cm−3 and low compensation is investigated. Activation energies of As in Si from experimental results are compared to calculated values available in literature for arsenic concentrations up to 1×1018 cm−3 . A critical dopant density (∼5×1017 cm−3 ) is found, below which calculated results agree well with experimental results and above which the calculated values exceed experimentally determined energies. The discrepancy increases with increasing dopant density. The phenomenon is explained by the delocalization of D− states and the broadening of the D− band, which are also evident from the experimental Hall mobility results.
Decreased pulmonary vascular permeability in aquaporin-1-null humans
Authors: King Landon S. , Nielsen Søren , Agre Peter , Brown Robert H.
Published: July 2002

The molecular determinants of water permeability in the human lung are incompletely defined. Aquaporins (AQP) are water-specific membrane channel proteins. AQP1 is present in endothelial cells in the lung, including those in the vascular plexus around the airways. Rare individuals have been identified who are deficient in AQP1. High-resolution computed tomography scans of the lung were used to evaluate the response to i.v. fluid challenge in two unrelated AQP1-null individuals and five normal controls. The airways and pulmonary vessels were measured at baseline and after i.v. administration of 3 liters of saline. Increases in airway wall thickness after fluid administration reflect peribronchiolar edema formation. Both control and AQP1 null subjects had approximately a 20% increase in pulmonary vessel area in response to saline infusion, suggesting similar degrees of volume loading. Control subjects had a 44% increase in the thickness of the airway wall, consistent with peribronchiolar edema formation. In marked contrast, airway wall thickness did not change in AQP1-null subjects in response to saline infusion. These studies indicate that AQP1 is a determinant of vascular permeability in the lung, and demonstrate a role for aquaporins in human pulmonary physiology.
Circles: The replication-recombination-chromosome segregation connection
Authors: Barre François-Xavier , Søballe Britta , Michel Bénédicte , Aroyo Mira , Robertson Malcolm , Sherratt David
Published: July 2002

Crossing over by homologous recombination between monomeric circular chromosomes generates dimeric circular chromosomes that cannot be segregated to daughter cells during cell division. In
Escherichia coli
, homologous recombination is biased so that most homologous recombination events generate noncrossover monomeric circular chromosomes. This bias is lost in
ruv
mutants. A novel protein, RarA, which is highly conserved in eubacteria and eukaryotes and is related to the RuvB and the DnaX proteins, γ and τ, may influence the formation of crossover recombinants. Those dimeric chromosomes that do form are converted to monomers by Xer site-specific recombination at the recombination site
dif
, located in the replication terminus region of the
E. coli
chromosome. The septum-located FtsK protein, which coordinates cell division with chromosome segregation, is required for a complete Xer recombination reaction at
dif
. Only correctly positioned
dif
sites present in a chromosomal dimer are able to access septum-located FtsK. FtsK acts by facilitating a conformational change in the Xer recombination Holliday junction intermediate formed by XerC recombinase. This change provides a substrate for XerD, which then completes the recombination reaction.