bluuebunny/crossref_metadata_2025 huggingface dataset:
Wave front layer stripping approach to inverse scattering for the wave equation
Authors: Weston Vaughan H. , Jonsson B. Lars G.
Published: September 2002

The inverse problem involving a point pulse source exterior to a scattering medium, where the velocity c(x) is continuous, is considered. The layer stripping approach is applied to thin curvilinear layers whose surfaces are the primary wave fronts [with c(x) continuous, the reflected wave fronts will be secondary, i.e., of lower order singularity]. It is shown that the layer stripping approach can be used in the time-domain inverse problem without employing the added complexity of having to perform wave splitting. Furthermore the procedure to determine the normal derivative of c from the asymptotic short time behavior of the field quantities on an adjacent wave front surface has been simplified compared to earlier wave splitting methods. Uniqueness results are given.
Monte Carlo collision operators for use with exact trajectory integrators
Authors: Boozer Allen H.
Published: September 2002

Classical transport theory in a spherical torus differs from that in a standard tokamak because in the spherical torus the gyroradius has a width comparable to that of a drift orbit, the banana width. In addition, the operating spherical torus experiments have gradient scales that are comparable to the gyroradius of the ions. Transport theories should use the full kinetic equation, which can be solved numerically using Monte Carlo methods. Such a code for the spherical torus could be based on Alan Glasser’s unpublished ORBIT code, which solves the full orbit equations, plus a Monte Carlo equivalent of the collision operator. This paper derives an appropriate Monte Carlo collision operator for use in such a code.
Strong absorption, intense forward-Raman scattering and relativistic electrons driven by a short, high intensity laser pulse through moderately underdense plasmas
Authors: Rousseaux C. , Rabec le Gloahec M. , Baton S. D. , Amiranoff F. , Fuchs J. , Gremillet L. , Adam J. C. , Héron A. , Mora P.
Published: September 2002

The propagation of a short and intense laser pulse (1.057 μm, 350 fs, 1017 W/cm2–2×1019 W/cm2) through preformed undercritical plasmas (≈5%–40% of nc) has been experimentally investigated on the 100-TW laser facility at the Laboratoire pour l’Utilisation des Lasers Intenses. The transmission and reflection of the 1 μm laser pulse, the forward- and backward-Raman (respectively, F-SRS and B-SRS) scattered light and the emission of fast electrons are reported. Significant absorption occurs in these plasmas, which is found to increase with the laser intensity. B-SRS is strongly driven at 1017 W/cm2 and gradually decreases at higher intensities. It is shown that the transmission is low and only weakly dependent on the laser intensity. In contrast, the forward Raman scattering continuously increases with the laser intensity, up to 7% of the incident energy at 2×1019 W/cm2 in the lowest density case. The relativistic electrons accelerated in the forward direction appear to be correlated with the F-SRS. The experimental data are discussed in the light of recent theoretical and numerical investigations, indicating that intense electron heating is likely to play a major role in the temporal growth or inhibition of the instabilities. The theoretical predictions are in agreement with the experiments.
Inhomogeneous M-theory cosmologies
Authors: Coley Alan A. , van den Hoogen R. J.
Published: September 2002

We study a class of inhomogeneous and anisotropic G2 string cosmological models. In the case of separable G2 models we show that the governing equations reduce to a system of ordinary differential equations. We focus on a class of separable G2 M-theory cosmological models, and study their qualitative behavior (a class of models with time-reversed dynamics is also possible). We find that generically these inhomogeneous M-theory cosmologies evolve from a spatially inhomogeneous and negatively curved model with a nontrivial form field toward spatially flat and spatially homogeneous dilaton-moduli-vacuum solutions with trivial form fields. The late time behavior is the same as that of spatially homogeneous models previously studied. However, the inhomogeneities are not dynamically insignificant at early times in these models.
Method of solving the homogeneous Bloch equation
Authors: Kobayashi M.
Published: September 2002

A method of solving the Bloch equation with infinite relaxation times is given. Applying this method we have found unknown analytic solutions as well as the well-known solutions.
Schwinger terms in gravitation in two dimensions as a consequence of the gravitational anomaly
Authors: Kohlprath Emmanuel
Published: September 2002

We compute the Schwinger term in the gravitational constraints in two dimensions, starting from the path integral in Hamiltonian form and the Einstein anomaly.
Pseudopotential approach to nonlinear dust acoustic waves in dusty plasma
Authors: Roychoudhury Rajkumar , Maitra Sarit
Published: September 2002

A method is outlined to derive Sagdeev’s pseudopotential which can take into account the dusty charge fluctuation. Numerical solutions of Sagdeev’s master equations are obtained to show the existence of solitary waves. Also, for small amplitude, explicit analytical expressions for solitary waves and double layers are obtained.
Low frequency dusty plasma modes in a uniform magnetic field
Authors: Wang Xiaogang , Wang Zheng-Xiong , Wang Chun-Hua , Guo Bin
Published: September 2002

Low frequency electrostatic waves of dusty plasmas in a uniform magnetic field are investigated. Since in conventional experiment parameter ranges the dust particles can hardly be magnetized, a “moderately magnetized” assumption of magnetized electrons and ions and unmagnetized dust particles is applied to such dusty plasmas. Effects of the neutral gas damping and the ion drag force on dust particles are taken into account. In the frequency range of ion waves, both dust-ion-acoustic waves propagating parallel to the magnetic field and the ion cyclotron waves are damped due to the ion drag on dust particles. In the very low dust frequency range, a new mode of slow dust waves propagating across the field line is found. The mode is damped to zero frequency in the long wavelength range due to the neutral gas damping and ion drag.
Embedding of the brane into six dimensions
Authors: Gogberashvili Merab
Published: September 2002

Embedding of the brane metric into Euclidean (2+4)-space is found. Brane geometry can be visualized as the surface of the hypersphere in six dimensions which “radius” is governed by the cosmological constant. Minkowski space in this picture is placed on the intersection of this surface with the plane formed by the extra space-like and time-like coordinates.
Chronon corrections to the Dirac equation
Authors: Galiautdinov Andrei A. , Finkelstein David R.
Published: September 2002

The Dirac equation is not semisimple. We therefore regard it as a contraction of a simpler decontracted theory. The decontracted theory is necessarily purely algebraic and nonlocal. In one simple model the algebra is a Clifford algebra with 6N generators. The quantum imaginary ℏi is the contraction of a dynamical variable whose backreaction provides the Dirac mass. The simplified Dirac equation is exactly Lorentz invariant but its symmetry group is SO(3, 3), a decontraction of the Poincaré group, and it has a slight but fundamental nonlocality beyond that of the usual Dirac equation. On operational grounds the nonlocality is ∼10−25 s in size and the associated mass is about the Higgs mass. There is a nonstandard small but unique spin–orbit coupling ∼1/N, whose observation would be some evidence for the simpler theory. All the fields of the standard model call for similar nonlocal simplification.
Simulation of neoclassical tearing modes (NTMs) in the DIII-D tokamak. I. NTM excitation
Authors: Popov A. M. , La Haye R. J. , Liu Y. Q. , Murakami M. , Popova N. N. , Turnbull A. D.
Published: September 2002

Nonlinear self-consistent magnetohydrodynamic (MHD) stability simulations of neoclassical tearing modes (NTM) are performed with the three-dimensional (3-D) MHD code Nonlinear Full Toroidal Code (NFTC) [Phys. Plasmas 8, 3605 (2001)] in real geometry. The numerical model is based on the nonlinear 3-D MHD equations including neoclassical effects: bootstrap current perturbations, and the transport and the polarization current thresholds. Neoclassical terms are included in the basic equations for magnetic field and pressure. An effective fully implicit numerical scheme allows the transport profile to evolve self-consistently with the nonlinear MHD instabilities and externally applied sources. A direct comparison of NTM evolution with experimental observations in different DIII-D discharges is performed. As a result, the nonlinear NTM stability diagram—dependences of the critical and the saturated island width on plasma current profile parameters are obtained. The stability criterion for the time evolution of the plasma profiles against neoclassical tearing modes is also investigated.
Simulation of neoclassical tearing modes in the DIII-D tokamak. II. Suppression by radially localized electron cyclotron current drive
Authors: Popov A. M. , La Haye R. J. , Liu Y. Q. , Murakami M. , Popova N. N. , Turnbull A. D.
Published: September 2002

The problem of neoclassical tearing mode (NTM) suppression by a radially localized toroidal current from electron cyclotron current drive (ECCD) is considered. Simulation of NTM stabilization by ECCD is performed with the Nonlinear Fully Toroidal Code (NFTC) [Phys. Plasmas 8, 3605 (2001)] for DIII-D discharges. The optimal parameters are determined for the radially localized ECCD current required to reduce the NTM instability. The time response and nonlinear evolution of the magnetic island width for ECCD and the required modulation phasing, the current drive location with respect to the rational surface, and the width of the spatial distribution are determined for both monotonic q-profile and negative central shear discharges.
Comment on “Kinetic effects on the parametric decays of circularly polarized electromagnetic waves in an electron–positron plasma” [Phys. Plasmas 9, 2534 (2002)]
Authors: Stenflo L. , Shukla P. K.
Published: September 2002

The physics of decay and modulational instabilities of radiation in an unmagnetized electron–positron plasma is clarified, thereby providing a deeper understanding of the instabilities that have been recently studied by Muñoz and Gomberoff [Phys. Plasmas 9, 2534 (2002)].
Response to “Comment on ‘Kinetic effects on the parametric decays of circularly polarized electromagnetic waves in an electron–positron plasma’ ” [Phys. Plasmas 9, 4413 (2002)]
Authors: Muñoz Vı́ctor , Gomberoff Luis
Published: September 2002

Some of the issues raised by Stenflo and Shukla in their Comment are addressed in this Response.
Probe measurements of low-frequency plasma potential and electric field fluctuations in a magnetized plasma
Authors: Ratynskaia S. V. , Demidov V. I. , Rypdal K.
Published: September 2002

A system of two cylindrical probes aligned along the magnetic field, and equipped with insulating end plugs, is proposed for measurement of low-frequency fluctuations of the electrostatic field in a magnetized plasma. It is demonstrated by modeling and experiments that the plug probe floats close to the plasma potential. The electric field component in a given direction is obtained by subtracting the plasma potentials obtained on two spatially separated plug probes. The probe system is applied to low-frequency electrostatic fluctuations in a simple magnetized torus, and reveals the presence of global oscillations, large scale propagating structures (m=1 modes), and developed turbulence with power-law spectra. Two different mode branches for the fluctuations are identified by comparing results from plug probes with results from conventional probes. Sources of errors arising from applying floating potential of conventional probes for electric field measurements are pointed out and discussed.
Anisotropic geodesic fluid spheres in general relativity
Authors: Herrera L. , Martin J. , Ospino J.
Published: September 2002

It is shown that unlike the perfect fluid case, anisotropic fluids (principal stresses unequal) may be geodesic, without this implying the vanishing of (spatial) pressure gradients. Then the condition of vanishing four acceleration is integrated in noncomoving coordinates. The resulting models are necessarily dynamic, and the mass function is expressed in terms of the fluid velocity as measured by a locally Minkowskian observer. An explicit example is worked out.
Observation of low impurity diffusivity inside the neoclassical transport barrier in the Compact Helical System
Authors: Liang Y. , Ida K. , Rice J. E. , Minami T. , Funaba H. , Kado S. , Fujisawa A. , Yoshimura Y. , Nishimura S. , Isobe M. , Okamura S. , Matsuoka K.
Published: September 2002

The radial profiles of titanium Kα spectra are measured with photon counting x-ray charge coupled device cameras for plasmas with a neoclassical internal transport barrier (N-ITB) in the Compact Helical System Heliotron/Torsatron [K. Matsuoka et al., Plasma Phys. Controlled Fusion 42, 1145 (2000)]. The Kα lines are dominated by the resonance line of He-like titanium in the plasma core where a thermal transport barrier results in a high electron temperature of ∼3 keV. The impurity diffusion coefficients are evaluated from the ionization levels of titanium. The diffusion coefficient inside the neoclassical ITB is <0.03 m2/s, which is one order of magnitude lower than that of the plasma without neoclassical ITB.
Measurements of large poloidal variations of impurity density in the Alcator C-Mod H-mode barrier region
Authors: Pedersen T. Sunn , Granetz R. S. , Marmar E. S. , Mossessian D. , Hughes J. W. , Hutchinson I. H. , Terry J. , Rice J. E.
Published: September 2002

Simultaneous high resolution measurements of the soft x-ray emission at the top and outboard edge of the Alcator C-Mod tokamak [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] are presented. Impurity density profiles are derived from these measurements. The impurity density varies considerably on a flux surface in the high confinement mode (H-mode) pedestal region. The soft x-ray and impurity density pedestals are not on the same flux surface at the top and outboard edge. The pedestal widths are consistently larger at the outboard edge in the Enhanced D-alpha (EDA) H-mode, suggesting a ballooning-like character of the EDA quasi-coherent mode. The usual separation of time scales between radial and parallel impurity transport does not hold in the H-mode pedestal region of Alcator C-Mod. Thus, one should not necessarily expect that the impurity density be constant on a flux surface in this region.
A nonconfocal involutive system and constrained flows associated with the MKdV− equation
Authors: Li Yishen , Ma Wen-Xiu
Published: September 2002

By symmetry constraints, new finite-dimensional integrable systems are deduced from a Lax representation of the MKdV− equation, whose two terms containing spatial derivatives have the same sign. Lax representations are presented for the resulting finite-dimensional integrable systems and an r-matrix formulation is established for the corresponding Lax operator. From the Lax operator, a nonconfocal involutive system of functionally independent polynomial functions is constructed. Solutions of the MKdV− can be obtained by the method of separation of variables.
Effects of E×B velocity shear on electrostatic structures
Authors: Spolaore M. , Antoni V. , Cavazzana R. , Regnoli G. , Serianni G. , Spada E. , Vianello N. , Bergsåker H. , Drake J. R.
Published: September 2002

Bursts of electrostatic turbulence measured in the edge region of two reversed field pinch experiments (RFX and T2R) are observed to be associated with electrostatic structures. The corresponding E×B velocity drifts have been reconstructed showing features reminiscent of monopolar vortices. These vortices have been grouped in two classes depending on their rotation direction. It is observed that the prevalent rotation direction depends on the shear of the surrounding mean E×B drift velocity, showing a clear analogy with ordinary fluids.
Generation of magnetic field, vortices and relativistic particles by the nonpotential ponderomotive force
Authors: Tsintsadze Nodar L. , Mima Kunioki , Tsintsadze Levan N. , Nishikawa Kyoji
Published: September 2002

Effects of nonpotential ponderomotive force on kinetic phenomena in a plasma are presented. The particle distribution function is derived, which allows one to calculate the particle and heat fluxes. A novel relativistic hydrodynamic equation of motion, incorporating self-generated magnetic fields, is obtained, which admits no frozen-in condition. The generation of electron fluid vortices by a laser pulse in initially unmagnetized plasma is demonstrated. The merging, decay, and annihilation of vortices (filaments) are discussed. The launching of plasma jets by the vortex structures is disclosed. Self-consistent problem of magnetic, modulational, and filamentational instabilities is studied. The decay of photons into photons and the helicon waves are also discussed. The relevance of this work to recent PetaWatt experiments in connection with high-energy particle generation is pointed out. In addition, highly collimated, mass outflows (jets) are ubiquitous phenomena relevant to astrophysics.
Self-diffusion properties of neutral monatomic alkali–metal plasmas
Authors: Bouledroua M. , Zerguini T. H.
Published: September 2002

Using the Chapman–Enskog method, the calculations of the coefficients of self-diffusion D of neutral alkali–metal atoms in their respective parent gas as a function of the temperature T have been performed. These calculations are more accurate, since they have been carried out by using recent interatomic potentials constructed from Rydberg–Klein–Rees data, and by taking into consideration the symmetry effects caused by the identity of the atoms undergoing the collisions and the nuclear spins. When compared to the present results, the published data of D(T) show they are overestimated. The variation law of D with T has also been investigated. For temperatures ranging from about 100 K to 3000 K, it is found that the results can be reproduced by the relation D=aTb (m2 s−1), where a=3.911×10−8 and b=0.692 for Li, a=1.774×10−8 and b=0.681 for Na, and a=7.443×10−9 and b=0.700 for K.
Impact of moisture on charge trapping and flatband voltage in Al2O3 gate dielectric films
Authors: Zafar Sufi , Callegari A. , Narayanan Vijay , Guha Supratik
Published: September 2002

High permittivity gate dielectric films exhibit significant charge trapping during electrical stressing. In this letter, the impact of low temperature postmetallization anneals on charge trapping in Al2O3 films is reported; anneals were performed in vacuum and forming gas ambient. These anneals not only reduce charge trapping but also move the flatband voltage towards an ideal value that corresponds to the difference in work function. Based on these experiments, it is concluded that absorbed moisture in the films provides hydrogen that concomitantly reduces charge trapping and fixed charge densities in Al2O3 films.
Collective correlation and screening effects on radiative recombination in classical nonideal plasmas
Authors: Jung Young-Dae
Published: September 2002

Collective correlation and plasma screening effects on the K-shell radiative recombination of the free electron with the ion in classical nonideal plasmas are investigated. The radiative recombination cross section is obtained by the principle of detailed balance with the photoionization cross section of the hydrogenic ion in nonideal plasmas. It is found that the collective and plasma screening effects significantly reduce the radiative recombination cross section in nonideal plasmas. It is also found that the collective and screening effects on the recombination cross section is almost independent of the projectile energy.
Ultrathin ferroelectric strontium bismuth tantalate films
Authors: González A. , Jiménez R. , Mendiola J. , Alemany C. , Calzada M. L.
Published: September 2002

Continuous strontium bismuth tantalate films with an average thickness of ∼36 nm have been prepared by spin-coating deposition of solutions onto Pt/TiO2/SiO2/(100)Si substrates and by rapid thermal processing. The films microstructure was formed by rod-shaped grains and fine equiaxed grains. This densely packed microstructure makes the growth of a continuous ultrathin film possible without shorts in the capacitor. The films exhibited values of coercive field of Ec∼226±40 kV/cm and of remanent polarization of Pr∼7.4±1.5 μC/cm2. They retain their Pr up to ∼105 s and do not fatigue up to ∼1010 cycles.
Integration over matrix spaces with unique invariant measures
Authors: Prosen T. , Seligman T. H. , Weidenmüller H. A.
Published: September 2002

We present a method to calculate integrals over monomials of matrix elements with invariant measures in terms of Wick contractions. The method gives exact results for monomials of low order. For higher-order monomials, it leads to an error of order 1/Nα, where N is the dimension of the matrix and where α is independent of the degree of the monomial. We give a lower bound on the integer α and show how α can be increased systematically. The method is particularly suited for symbolic computer calculation. Explicit results are given for O(N), U(N), and for the circular orthogonal ensemble.
Self-magnetization of protoplanetary accretion disk matter
Authors: Wiechen Heinz , Birk Guido T. , Kopp Andreas , Lesch Harald
Published: September 2002

Shear-flow induced friction between neutral gas and charged particle components of different mass can yield a significant self-magnetization of matter. In this paper this process is discussed with respect to protoplanetary disk matter consisting of charged massive dust grains, neutral gas, ions and electrons. Self-consistent three-dimensional multi-fluid plasma—neutral gas—dust simulations are presented taking into account typical parameters for protoplanetary accretion disk matter like that of our early solar system. The results of the simulations show that self induced magnetic fields of 10−5 Tesla up to 10−3 Tesla can be expected in a protoplanetary accretion disk on very short time scales of a few years. Thus, shear-flow induced self-magnetization can yield a significant contribution to the magnetization of the early solar system.
Neutrino-driven wakefield plasma accelerator
Authors: Serbeto A. , Rios L. A. , Shukla P. K.
Published: September 2002

A classical fluid description is used to investigate the nonlinear interaction between neutrino bursts and a relativistic collisionless cold unmagnetized plasma. It is shown that during the interaction large amplitude electron plasma waves are excited in such a way that charged particles trapped in this high gradient potential can be accelerated to extremely high energies.
Relations between the ionization or recombination flux and the emission radiation for hydrogen and helium in plasma
Authors: Goto Motoshi , Sawada Keiji , Fujimoto Takashi
Published: September 2002

On the basis of the collisional-radiative models for neutral hydrogen, and neutral and ionized helium, the relationship between the ionization flux or the recombination flux and the photon emission rate of a representative visible line of each species is investigated. It is found that both fluxes are proportional to the photon emission rate and that the proportionality factor depends rather weakly on the plasma parameters in the ranges of practical interest. This implies that the observed emission line intensity can be a good measure of the ionization flux or the recombination flux. The relation between the total radiation power rate and the ionization or recombination flux is also considered. For a hydrogen plasma in ionization balance the Balmer-α line intensity takes the maximum value near the optimum temperature of 1.3 eV, while for plasmas out of ionization balance it takes the minimum near that temperature. This latter characteristic corresponds to the recently observed “inverse edge-localized mode” in divertor plasmas. For neutral hydrogen and ionized helium, it is found that in the recombining plasma of low electron temperature, Te, and density, ne, the radiation energy close to the ionization potential of the ground state is emitted during one recombination event. In the ionizing plasma of high Te and low ne, a similar amount of energy is emitted during one ionization event. Emission line intensities of hydrogen and helium were measured in the Large Helical Device, and the time variation of ne at the initial and final phases of a discharge was estimated. The results agreed well with the interferometer measurement, and this indicated that the variation of ne was dominated by their ionization or recombination processes rather than by diffusion. The total radiation energy of hydrogen and helium in the recombining phase was found to be less than 1% of the stored energy of the plasma.
Compact expressions for the Biot–Savart fields of a filamentary segment
Authors: Hanson James D. , Hirshman Steven P.
Published: September 2002

Compact and computationally efficient expressions for the Biot–Savart magnetic field and vector potential of a single filamentary segment are derived. The expressions are singular only on the segment.
Radial and zonal modes in hyperfine-scale stellarator turbulence
Authors: Jenko F. , Kendl A.
Published: September 2002

Electromagnetic plasma turbulence at hyperfine (i.e., electron gyroradius) scales is studied in the geometry of an advanced stellarator fusion experiment, Wendelstein 7-AS [H. Renner, Plasma Phys. Controlled Fusion 31, 1579 (1989)], by means of nonlinear gyrokinetic simulations. It is demonstrated that high-amplitude radial streamers may also exist in non-tokamak devices, raising the electron heat flux to experimentally relevant values. Moreover, some statistical characteristics of the fully developed turbulence are computed, highlighting the (co-)existence, nature, and role of self-generated zonal flows and fields.
Composition of self-assembled Ge/Si islands in single and multiple layers
Authors: Schmidt O. G. , Denker U. , Christiansen S. , Ernst F.
Published: September 2002

The degree of Si alloying in vertically aligned self-assembled Ge islands increases with the number of stacked layers. We find that the Si–Ge interdiffusion coefficient increases by more than two orders of magnitude for stacked hut clusters. Furthermore, we determine the composition profiles through the center of dome-shaped islands, capped with Si. These profiles exhibit a plateau near the base and a Ge enrichment near the apex of the islands. In this case, too, the upper dome island experiences a state of increased alloying with Si.
An experimental investigation of the internal magnetic field topography of an operating Hall thruster
Authors: Peterson Peter Y. , Gallimore Alec D. , Haas James M.
Published: September 2002

Magnetic field measurements were made in the discharge channel of the 5 kW-class P5 laboratory-model Hall thruster to investigate what effect the Hall current has on the static, applied magnetic field topography. The P5 was operated at 1.6 and 3.0 kW with a discharge voltage of 300 V. A miniature inductive loop probe (B-Dot probe) was employed to measure the radial magnetic field profile inside the discharge channel of the P5 with and without the plasma discharge. These measurements are accomplished with minimal disturbance to thruster operation with the High-speed Axial Reciprocating Probe system. The results of the B-Dot probe measurements indicate a change in the magnetic field topography from that of the vacuum field measurements. The measured magnetic field profiles are then examined to determine the possible nature and source of the difference between the vacuum and plasma magnetic field profiles.
Growth and spin-resolved photoemission spectroscopy of the epitaxial α-Al2O3/Fe(110) system
Authors: Dedkov Yu. S. , Fonin M. , Rüdiger U. , Güntherodt G.
Published: September 2002

Electronic and structural properties of epitaxial ultrathin aluminum oxide layers grown on epitaxial Fe(110) films were investigated at room temperature by means of spin-, angle- and energy-resolved photoemission spectroscopy. A spin polarization of −(15±5)% near the Fermi energy EF is found for 1 (ML) of α-Al2O3 on epitaxial Fe(110). This value results from the attenuation of −80% of Fe(110) by the oxide layer thickness and the spin-independent part of the scattering cross section.
Predictive simulations of radio frequency heated plasmas of Tore Supra using the Multi-Mode model
Authors: Voitsekhovitch Irina , Bateman Glenn , Kritz Arnold H. , Pankin Alexei
Published: September 2002

Multichannel integrated predictive simulations using the Multi-Mode transport model are carried out for radio frequency heated Tore Supra tokamak discharges in which helium is the primary ion component. Lower hybrid heated discharges in which the total current is driven noninductively [X. Litaudon et al., Plasma Phys. Controlled Fusion 43, 677 (2001)] and a discharge with ion cyclotron radio frequency heating of the hydrogen minority ions [G. T. Hoang et al., Nucl. Fusion 38, 117 (1998)] are simulated. The simulations of these discharges represent the first test of the Multi-Mode model in helium plasmas with dominant electron heating. Also for the first time, the particle transport in Tore Supra discharges is computed and the density profiles are predicted self-consistently with other transport channels. It is found in these simulations that the anomalous transport driven by trapped electron mode turbulence is dominant compared to the transport driven by the ion temperature gradient turbulence. The feature of the Multi-Mode model to calculate the impurity transport self-consistently with other transport channels is used in this study to predict the influence of carbon impurity influx on the discharge evolution.
Comment on “Dust electro-acoustic and Coulomb modes in dense dusty plasmas” [Phys. Plasmas 9, 1113 (2002)]
Authors: Lee Hee J.
Published: September 2002

It is pointed out that a recent paper by Verheest and Shukla [Phys. Plasmas 9, 1113 (2002)] dealing with the low frequency dusty plasma waves, dust electro-acoustic and Coulomb modes, neglects the damping which is resulted in by electron and ion captures by the grains in the grain charging process. The damping appears to be not negligible for both the electro-acoustic and Coulomb modes.
On ion injection at quasiparallel shocks
Authors: Scholer M. , Kucharek H. , Kato C.
Published: September 2002

A large number of numerical experiments has been performed in order to study the interaction of interstellar pickup protons and helium ions with quasiparallel collisionless shocks. The shocks are modeled by a one-dimensional hybrid simulation method which treats the ions as macroparticles and the electrons as a massless fluid. Solar wind alpha particles and pickup protons are included self-consistently. In addition, the particle splitting method is used for the solar wind ions so that the distribution function can be followed over more than 10 orders of magnitude. A large part of the pickup ion distribution is reflected; the reflection efficiency is very high, and can reach in cases where the pickup ion density is low as much as 50%–60%. The reflection efficiency is almost independent of magnetic field–shock normal angle. This indicates that magnetic mirroring is unimportant and does not lead to larger reflection efficiencies. The reflection efficiency of pickup protons rapidly decreases when the pickup ion density exceeds a few percent of the solar wind density. An addition of 25% pickup protons decreases the reflection coefficient for these ions to ∼10%. This represents the fact that a quasiparallel shock cannot be considered as being uncoupled from the upstream region: at high additions of pickup ions the shock structure is changed in such a way as to reflect less pickup ions. The intensity of diffuse ions upstream of a quasiparallel shock does not depend on the temperature of the core distribution. Within the framework of the present model even solar wind distributions with a hard power law tail do not produce higher intensities of diffuse ions. It is argued that this can be understood by the fact that the intrinsic self-consistency between the processes in the upstream region and at the shock transition determines the injection and reflection properties of the core solar wind distribution.
Analysis of the steady-state axial flow in the Hall thruster
Authors: Cohen-Zur Avi , Fruchtman Amnon , Ashkenazy Joseph , Gany Alon
Published: September 2002

Momentum and energy balance considerations are used to analyze the plasma flow in a Hall thruster at the limit of intense full ionization with no losses. Two cases are examined; one in which the ion velocity at the anode is zero, and the second in which an ion backflow exists near the anode. Analytical expressions are found for the current utilization and for the thruster efficiency in each case. In the first case an expression is also found for the electron temperature at the anode, while in the second case expressions are derived for the magnitude of the ion backflow into the anode and for the location of the ionization region along the channel. These dependencies are expressed in terms of the applied magnetic field and voltage, the mass flow rate, the thruster geometry and the ionization energy.
Adhesive forces investigation on a silicon tip by contact-mode atomic force microscope
Authors: Agache Vincent , Legrand Bernard , Collard Dominique , Buchaillot Lionel
Published: September 2002

An atomic force microscope operating in contact mode is used in this letter for probing the adhesive forces at the apex of a silicon nanotip with typical radius smaller than 15 nm, fabricated using a combination of high-resolution electron beam lithography and plasma dry etching. The amplitude of the forces is deduced from force versus distance curve measurements. By determining the contact point and the pull-off force from the force curves, the surface topography and the adhesive forces are simultaneously obtained at various locations on the surface. This letter reports both measurements and modeling of adhesive forces versus the contact point on the nanotip. As the nanotip is sharper and has a smaller aperture angle than the employed atomic force microscope tip, the measurements are focused on the nanotip apex.
Realization of high efficiency in a plasma-assisted microwave source with two-dimensional electron motion
Authors: Shkvarunets A. G. , Carmel Y. , Nusinovich G. S. , Abu-elfadl T. M. , Rodgers J. , Antonsen T. M. , Granatstein V. , Goebel D. M.
Published: September 2002

Conventional microwave sources utilize a strong axial magnetic field to guide an electron beam through an interaction region. A plasma-assisted slow wave microwave oscillator (Pasotron) can operate without an external magnetic field because the presence of ions neutralizes the space charge in the beam, permits the self-pinch forces to provide beam propagation, and allows for the radial motion of electrons under the action of transverse fields of the wave. While the inherent efficiency of conventional microwave sources with 1D electron flow is limited to 15%–20%, it is shown in this work that both the calculated and measured inherent efficiency of devices with 2D electron flow can be higher than 50%. Both in situ diagnostics and analysis confirmed that the enhanced efficiency is due to the fact that rf forces dominate the beam dynamics.
Direct measurement of electrical potentials in GaInP2 solar cells
Authors: Jiang Chun-Sheng , Moutinho H. R. , Geisz J. F. , Friedman D. J. , Al-Jassim M. M.
Published: September 2002

We report on the application of electrostatic force microscopy to photovoltaic devices. Profiles of electrical potentials on cross sections of a GaInP2 solar cell device were measured quantitatively and spatially resolved. Two potentials are assigned, respectively, to the p–n junction of GaInP2 and the band offset between the GaInP2 base layer and the GaAs substrate. In addition to the flattening of the p–n junction by the light irradiations, two changes of the potential that positively contribute to the open-circuit voltage of the device are found at locations close to the window and the back surface field layers.
Nanoscale charge transport in an electroluminescent polymer investigated by conducting atomic force microscopy
Authors: Lin Heh-Nan , Lin Hui-Lien , Wang Shen-Shen , Yu Liang-Sheng , Perng Gung-Yeong , Chen Show-An , Chen Sy-Hann
Published: September 2002

We report conducting atomic force microscopy study of nanoscale hole transport in an electroluminescent polymer. Imaging of current variation with a spatial resolution of around 20 nm is achieved. Local current–voltage measurements are performed and the mobilities obtained from space-charge-limited current analysis are two orders of magnitude higher than macroscopic results. A Poole–Frenkel plot shows a clear transition at a field of around 1.5×106 V/cm, which is consistent with reported values of polaron dissociation field.
Dynamical x-ray microscopy investigation of electromigration in passivated inlaid Cu interconnect structures
Authors: Schneider G. , Denbeaux G. , Anderson E. H. , Bates B. , Pearson A. , Meyer M. A. , Zschech E. , Hambach D. , Stach E. A.
Published: September 2002

Quantitative time-resolved x-ray microscopy mass transport studies of the early stages of electromigration in an inlaid Cu line/via structure were performed with about 40 nm lateral resolution. The image sequences show that void formation is a highly dynamic process, with voids being observed to nucleate and grow within the Cu via and migrate towards the via sidewall. Correlation of the real time x-ray microscopy images with postmortem high voltage electron micrographs of the sample indicates that the void nucleation occurs at the site of grain boundaries in Cu, and that the voids migrate along these grain boundaries during electromigration.
Evaluation of the thermal stability of a Cu60Hf25Ti15 metallic glass
Authors: Louzguine Dmitri V. , Inoue Akihisa
Published: September 2002

The present letter shows how a continuous heating transformation diagram for a Cu60Hf25Ti15 metallic glass can be obtained by applying an extension of the Kissinger analysis. According to the calculation this glass is completely stable in the Earth’s climate for its lifetime. This extension of the Kissinger analysis method can be applied to any other metallic glass.
InGaAsP/InP avalanche photodiodes for photon counting at 1.06 μm
Authors: McIntosh K. A. , Donnelly J. P. , Oakley D. C. , Napoleone A. , Calawa S. D. , Mahoney L. J. , Molvar K. M. , Duerr E. K. , Groves S. H. , Shaver D. C.
Published: September 2002

Geiger-mode (photon-counting) operation at 1.06 μm has been demonstrated with InGaAsP/InP avalanche photodiodes operated at room temperature. A photon detection efficiency of 33% was measured on uncoated detectors, representing an internal avalanche probability of 60%. Under identical bias conditions a dark count rate as low as 1.7 MHz was measured at 290 K, consistent with a primary dark current of <0.3 pA. Dark count rates drop by approximately 50–200× by cooling the detectors to 210 K (−63 °C).
Superhigh-frequency surface-acoustic-wave transducers using AlN layers grown on SiC substrates
Authors: Takagaki Y. , Santos P. V. , Wiebicke E. , Brandt O. , Schönherr H.-P. , Ploog K. H.
Published: September 2002

We demonstrate the operation of surface-acoustic-wave (SAW) transducers fabricated on AlN/SiC structures at frequencies as high as 19 GHz. The high SAW velocity of the AlN film is enhanced by the even higher sound velocity of the SiC substrate, enabling us to achieve these frequencies with a SAW wavelength of 400 nm.
Theory of laser gain in InGaN quantum dots
Authors: Chow W. W. , Schneider H. C.
Published: September 2002

A theory for gain is developed for wide-bandgap nitride-based quantum-dot laser structures. A semiclassical laser theory is used to describe the optical susceptibility in the presence of strong many-body Coulomb interaction and quantum-confined Stark effect. Application of this theory shows distinctly different gain behavior depending on quantum dot dimensions, because of the interplay of these effects.
Scanning force microscopy jumping and tapping modes in liquids
Authors: Moreno-Herrero F. , de Pablo P. J. , Fernández-Sánchez R. , Colchero J. , Gómez-Herrero J. , Baró A. M.
Published: September 2002

In this work theoretical considerations of the performance of scanning force microscopy jumping mode and tapping mode in liquids are discussed. A priori, jumping mode should improve in a liquid environment compared to in air while the situation for tapping mode should become worse. In order to confirm this we present jumping and tapping mode images of DNA molecules absorbed on a mica substrate immersed in water. The experiments demonstrate that jumping mode is a suitable scanning force microscopy method by which to image soft samples in liquid and that it has similar or even better performance than those exhibited by tapping, but without the complex experimental requirements of this mode.
Bonding states and electrical properties of ultrathin HfOxNy gate dielectrics
Authors: Kang Chang Seok , Cho Hag-Ju , Onishi Katsunori , Nieh Renee , Choi Rino , Gopalan Sundar , Krishnan Sid , Han Jeong H. , Lee Jack C.
Published: September 2002

Hafnium oxynitride (HfOxNy) gate dielectric was prepared using reactive sputtering followed by postdeposition annealing at 650 °C in a N2 ambient. Nitrogen incorporation in the dielectric was confirmed by x-ray photoelectron spectroscopy analysis. In comparison to HfO2 of the same physical thickness, HfOxNy gate dielectric showed lower equivalent oxide thickness (EOT) and lower leakage density (J). Even after a high-temperature postmetal anneal at 950 °C, an EOT of 9.6 Å with J of 0.8 mA/cm2 @−1.5 V was obtained. In contrast, J of ∼20 mA/cm2 @−1.5 V for HfO2 with an EOT of 10 Å was observed. The lower leakage current and superior thermal stability of HfOxNy can be attributed to the formation of silicon–nitrogen bonds at the gate dielectric/Si interface and strengthened immunity to oxygen diffusion by the incorporated nitrogen.
Stability of defects in lead–zirconate–titanate after unipolar fatigue
Authors: Verdier Cyril , Lupascu Doru C. , Rödel Jürgen
Published: September 2002

Bipolar fatigue in ferroelectric lead–zirconate–titanate was previously shown to induce stable pinning centers for ferroelectric domain walls resulting in a significant loss in switchable polarization, but only fairly weak offsets were induced. The corresponding scenario after unipolar fatigue is reversed inducing fairly stable offsets while hardly any loss in domain mobility is encountered. The charged defects inducing the offsets are suggested to be located at grain boundaries, while pinning centers reducing domain wall mobility predominantly reside within the grain.
Gas-phase nanoparticle formation during AlGaN metalorganic vapor phase epitaxy
Authors: Creighton J. R. , Breiland W. G. , Coltrin M. E. , Pawlowski R. P.
Published: September 2002

Using in situ laser light scattering, we have directly observed the formation of gas-phase nanoparticles during AlN, GaN, and AlGaN metalorganic vapor phase epitaxy. The nanoparticles are sharply distributed in height 6 mm from the surface, in good agreement with a model based on a balance between thermophoretic and convective forces. By measuring the angular dependence of the scattering intensity, we determined that the AlN particle sizes were 35–50 nm, and particle densities were 1–6×108 cm−3, which corresponds to 20%–80% of the input Al being converted into nanoparticles.
Dopant profiling on semiconducting sample by scanning capacitance force microscopy
Authors: Kobayashi Kei , Yamada Hirofumi , Matsushige Kazumi
Published: September 2002

Scanning capacitance force microscopy (SCFM) capable of mapping differential capacitance (∂C/∂V) on semiconducting sample based on atomic force microscopy (AFM) without an external capacitance sensor is introduced. While an electric field alternating at an angular frequency ω is applied between the tip and the sample, an induced electrostatic force (ESF) oscillating at its third harmonic frequency (3ω) is detected by a lock-in amplifier. Owing to the fact that the magnitude of the induced ESF is proportional to the square of the magnitude of the applied electric field and the fact that the capacitance of the semiconducting sample is also modulated at ω, the amplitude and the phase of the induced ESF oscillating at 3ω contain information on ∂C/∂V. We present ∂C/∂V images on a Si test sample obtained by SCFM using both contact-mode AFM and dynamic-mode AFM, showing clear contrasts depending on species and density of dopants.
Two-dimensional photonic crystal hexagonal waveguide ring laser
Authors: Kim Se-Heon , Ryu Han-Youl , Park Hong-Gyu , Kim Guk-Hyun , Choi Yong-Seok , Lee Yong-Hee , Kim Jeong-Soo
Published: September 2002

Hexagonal ring-type resonators defined by two-dimensional photonic crystal waveguides are proposed and demonstrated. Lasing actions are observed from the photonic crystal ring resonator patterned on a free-standing slab with InGaAsP active layers emitting near 1.55 μm. For a ring resonator with 8-μm diameter, the threshold peak pump power is ∼3 mW. The quality factor of the ring resonator mode estimated from below-threshold linewidth shows that the sum of the propagation loss and the bending loss of the photonic crystal ring resonator is less than ∼260 cm−1. The photonic crystal ring resonators can be used for the characterization of the optical losses in photonic crystal waveguides.
Laser surgical effects with concentrated solar radiation
Authors: Gordon Jeffrey M. , Feuermann Daniel , Huleihil Mahmoud
Published: September 2002

Surgical tissue transformations normally produced with lasers are demonstrated experimentally with highly concentrated sunlight. Our solar surgery prototype decouples the collection and delivery of high-density photon flux: a compact (200 mm) optical system outdoors, with several watts of concentrated solar radiation transported into the operating theater by a flexible, high-transmissivity optical fiber up to 20 m long, at power densities typical of surgical lasers. Measurements of the rates of tissue coagulation, dehydration, and ablation generated with solar surgery on chicken breast are consistent with the corresponding rates produced by common visible and near-infrared surgical lasers. Solar surgery offers an inexpensive alternative to surgical lasers, albeit with limitations on feasible locations and operating periods.
Colloidal photonic crystal microchannel array with periodically modulated thickness
Authors: Mı́guez H. , Yang S. M. , Ozin G. A.
Published: September 2002

Here we report on the optical properties of a micrometer scale array of periodically modulated thickness colloidal crystals. A combination of soft-lithography, micromolding, and microfluidics techniques is employed to form rectangular microchannels that can impose a periodic modulation of the thickness of colloidal crystals grown within them. Correspondingly, a periodic fluctuation of the photonic band structure occurs at distances of the order of a few microns. Experimental evidence of this effect is obtained by optical microspectroscopy. Results are analyzed in terms of finite crystal size theoretical calculations.