bluuebunny/crossref_metadata_2025 huggingface dataset:
Alpha-particle losses from toroidicity-induced Alfvén eigenmodes. Part I: Phase-space topology of energetic particle orbits in tokamak plasma
Authors: Hsu C. T. , Sigmar D. J.
Published: July 2002

Phase-space topology of energetic particles in tokamak plasma with arbitrary shape of cross section is studied based upon the guiding center theory. Important phase-space boundaries such as prompt loss boundary, trapped passing boundary, and other boundaries between classes of nonstandard orbits (e.g., pinch and stagnation orbits) are studied. This phase-space topology information is applied to the study of anomalous phase-space diffusion due to finite amplitude Alfvén wave fluctuations of energetic particles. The separatrix between trapped and circulating particles contributes dominantly to the losses.
The mechanism of self-sustainment in collisional drift wave turbulence
Authors: Scott Bruce D.
Published: July 2002

Although collisional drift waves in a sheared slab configuration are linearly damped, the corresponding turbulence is self-sustaining if initialized at an electrostatic potential fluctuation amplitude of eφ/T≳0.3ρs/Ln, much less than that of observed fluctuations. Within the context of two-dimensional sheared slab magnetic geometry, computational investigations into the amplitude threshhold and the nonlinear mode structure reveal the shear-induced self-organization that is responsible for efficiently tapping the free energy in the temperature and density gradients. They also show that the turbulence is nondiffusive in character, since the important assumptions behind turbulence-as-diffusion are all violated. Many important features of experimentally observed tokamak edge fluctuations are reproduced by these single resonant surface nonlinear dynamics, suggesting that model components not treated may have an effect more additive than qualitative. The ingredients giving rise to self-organized turbulence, shear-localized long-wavelength modes of some width Δ0<λ, and isotropic intermediate wavelength modes with λ∼Δ0, in direct, coherent interaction, are at least potentially shared by most of the viable candidates for anomalous tokamak transport. Consequently, such transport is unlikely to bear much resemblance to the simpler models currently in use.
Excitation of high-n toroidicity-induced shear Alfvén eigenmodes by energetic particles and fusion alpha particles in tokamaks
Authors: Fu G. Y. , Cheng C. Z.
Published: July 2002

The stability of high-n toroidicity-induced shear Alfvén eigenmodes (TAE) in the presence of fusion alpha particles or energetic ions in tokamaks is investigated. The TAE modes are discrete in nature, and thus can easily tap the free energy associated with energetic particle pressure gradient through wave particle resonant interaction. A quadratic form is derived for the high-n TAE modes using gyrokinetic equation. The kinetic effects of energetic particles are calculated perturbatively using the ideal magnetohydrodynamic (MHD) solution as the lowest-order eigenfunction. The finite Larmor radius (FLR) effects and the finite drift orbit width (FDW) effects are included for both circulating and trapped energetic particles. It is shown that, for circulating particles, FLR and FDW effects have two opposite influences on the stability of the high-n TAE modes. First, they have the usual stabilizing effects by reducing the wave particle interaction strength. Second, they also have destabilizing effects by allowing more particles to resonate with the TAE modes. It is found that the growth rate induced by the circulating alpha particles increases linearly with the toroidal mode number n for small kθρα, and decreases as 1/n for kθρα≫1. The maximum growth rate is obtained at kθρα on the order of unity, and is nearly constant for the range of 0.7≤vα/vA≤2.5. On the other hand, the trapped particle response is dominated by the precessional drift resonance. The bounce resonant contribution is negligible. The growth rate peaks sharply at the value of kθρα, such that the precessional drift resonance occurs for the most energetic trapped particles. The maximum growth rate due to the energetic trapped particles is comparable to that of circulating particles. Finally, the effect of the two-dimensional wave structure of TAE modes is considered by using the Wentzel–Kramers–Brillouin (WKB) method.
Chaotic behavior driven by an external periodic oscillation in a current-carrying unstable ion sheath
Authors: Ohno N. , Komori A. , Kono M. , Kawai Y.
Published: July 2002

A quasiperiodic route to chaos is investigated experimentally and theoretically in a current-carrying unstable ion sheath. A quasiperiodic regime with two frequencies is realized by a coherent instability and an external periodic oscillation. Chaos appears directly from the quasiperiodic regime when the amplitude of the external oscillation is increased, making nonlinear effects more important. A correlation dimension and the largest Lyapunov exponent, obtained experimentally in the chaotic regime, confirm that the observed chaos is deterministic and the attractor is strange. A period-doubling route to chaos is also observed after the instability is quenched with the external oscillation by increasing the amplitude of the external oscillation further. These experimental results are in fair agreement with a theory that describes ion dynamics in the ion sheath.
J×B heating by very intense laser light
Authors: Kruer W. L. , Estabrook Kent
Published: July 2002

Plasma heating by the oscillating component of the pondermotive force of a very intense light wave is discussed.
Experimental studies of field-reversed configuration translation
Authors: Rej D. J. , Armstrong W. T. , Chrien R. E. , Klingner P. L. , Linford R. K. , McKenna K. F. , Sherwood E. G. , Siemon R. E. , Tuszewski M. , Milroy R. D.
Published: July 2002

In the FRX-C/T experiment [Proceedings of the 9th Symposium for Engineering Problems of Fusion Research (IEEE, New York, 1981), p. 1751], field-reversed configuration (FRC) plasmas have been formed in, and launched from, a field-reversed theta-pinch source and subsequently trapped in an adjacent confinement region. No destructive instabilities or enhanced losses of poloidal flux, particles, or thermal energy are observed for FRC total trajectories of up to 16 m. The observed translation dynamics agree with two-dimensional magnetohydrodynamic (MHD) simulations. When translated into reduced external magnetic fields, FRC’s are observed to accelerate, expand, and cool in partial agreement with adiabatic theory. The plasmas reflect from an external mirror and after each reflection, the axial kinetic energy is reduced by approximately 50%. Because of this reduction, FRC’s are readily trapped without the need of pulsed gate magnet coils.
Shear-layer-driven transition in a rectangular cavity
Authors: Neary M. D. , Stephanoff K. D.
Published: July 2002

An experimental study of the flow over a shallow rectangular cavity indicates that, between the states of periodic and fully developed turbulent flow, three different regimes of fluid motion occur as the Reynolds number increases. In the first regime, regime I, the time trace from a pressure transducer, located at the downstream corner of the cavity, varies weakly in amplitude. The frequency spectrum of the trace shows that a single frequency, its’ first harmonic, and a second frequency are selectively amplified. In the second regime, regime II, there is intermittency in the pressure time trace and the two incommensurate frequencies are further apart from each other than in regime I. The primary frequency is the result of a shear-layer instability but the secondary frequency is believed to depend on a transverse wave on the primary cavity vortex. The exchange of fluid between this vortex and the free stream is enhanced when the two waves are constructively interfering and the exchange is attenuated when the two waves are destructively interfering. If the amplitude of the transverse wave is sufficiently large and the two waves are in phase, fluid from the primary vortex is observed to burst through the shear layer giving rise to a period of apparently random motion. In the third regime, regime III, the pressure oscillations vary strongly with time, and include frequent periods of intense irregular behavior. At times, the pressure cycles have double peaks when the vortices that form in the shear layer are partially clipped by the downstream edge of the cavity. This clipping does not, however, coincide with a decay in the shear-layer oscillations, as it does in regime II.
Gyrokinetic particle simulation of ion temperature gradient drift instabilities
Authors: Lee W. W. , Tang W. M.
Published: July 2002

Ion temperature gradient drift instabilities have been investigated using gyrokinetic particle simulation techniques for the purpose of identifying the mechanisms responsible for their nonlinear saturation as well as the associated anomalous transport. For simplicity, the simulation has been carried out in a shear-free slab geometry, where the background pressure gradient is held fixed in time to represent quasistatic profiles typical of tokamak discharges. It is found that the nonlinearly generated zero-frequency responses for the ion parallel momentum and pressure are the dominant mechanisms giving rise to saturation. This is supported by the excellent agreement between the simulation results and those obtained from mode-coupling calculations, which give the saturation amplitude as ‖eΦ/Te‖ ≂(‖ωl+iγl‖/Ωi)/(k⊥ ρs)2, and the quasilinear thermal diffusivity as χi ≂γl/k2⊥, where ωl and γl are the linear frequency and growth rate, respectively, for the most unstable mode of the system. In the simulation, the time evolution of χi after saturation is characterized by its slow relaxation to a much lower level of thermal conduction. On the other hand, a small amount of electron–ion collisions, which has a negligible effect on the linear stability, can cause significant enhancement of χi in the steady state.
Solitary waves on a conducting fluid layer
Authors: Easwaran C. V.
Published: July 2002

The effect of an electric field on the long wave motion of the surface of a conducting fluid layer is considered. A Korteweg–de Vries (KdV)-type equation with coefficients depending on the applied field is derived. The speed of the solitary wave on the fluid layer is seen to be reduced by the electric field. It is found that there are two critical values of the applied voltage that lead to (i) breaking up of the solitary waves and (ii) bifurcation of solutions of the governing equations. For a given value of the imposed potential, solitary waves of elevation and depression are possible depending on the value of surface tension.
On the structure of an electrostatic spray of monodisperse droplets
Authors: Tang Keqi , Gomez Alessandro
Published: July 2002

An experimental study has been performed on the structure of an electrostatic spray of monodisperse droplets. Such a spray is established when a liquid with sufficient electric conductivity and moderate surface tension, in the present case heptane doped with an antistatic additive, is fed through a small metal tube maintained at several kilovolts relative to a ground electrode a few centimeters away. The liquid meniscus at the outlet of the capillary takes a conical shape under the action of the electric field, with a thin jet emerging from the cone tip. This jet breaks up into charged droplets that disperse into a fine spray. Flash shadowgraph of the breakup region showed that the jet initially breaks into droplets of bimodal size distribution by varicose wave instabilities. The spray monodispersity is established farther downstream by a segregation process of electrostatic and inertial nature that confines the bulk of the mass flow rate (97%) and 85% of the total current in a core of nearly monodisperse primary droplets, with the remainder in a shroud of satellites. Droplet size, axial velocity, and concentration were measured throughout the spray by phase Doppler anemometry (PDA). The complementary use of these measurements permitted the determination of the electric field via the spray momentum equation.

It was found that droplets are ejected from the jet at a relatively high velocity in a region characterized by a very intense electric field. They maintain this velocity farther downstream because of inertia, even though the field is precipitously decreasing, and ultimately decelerate under the action of the drag force and a progressively weaker electrostatic force. Velocity and concentration fields were shown to be self-similar. Comparison between the external field, due to the potential difference applied between the electrodes, and the space charge field shows that the droplet axial motion is driven primarily by the external field, whereas the droplet radial motion and, consequently, the jet lateral spreading, is controlled primarily by the space charge field. The latter is typically at least one order of magnitude smaller than the external one, except at off-axis locations near the breakup region of the spray, where the two fields can be comparable. The droplet charge distribution was also determined via the spray momentum equation and the simultaneous measurements of droplet size and velocity in a region where droplets experience negligible acceleration. The charge distribution was found to be narrow, with a ratio of standard deviation over mean of 0.15.
Flow beneath a rotating annulus
Authors: Takahashi Timothy T.
Published: July 2002

Air flow between a spinning annular disk and a flat baseplate has been studied both experimentally and computationally. The Ekman number, Ek, has been determined to be the single important dimensionless parameter required to characterize this problem. A numerical solution which predicts spacing-dependent axisymmetric flow has been developed. It has been found to agree with experimental data over a wide range of parameter space. The calculated flow predicts unusual complexity; counter-rotating recirculation cells are likely to be present.
On the dynamics of ultrathin vortex filaments
Authors: Lough Michael F.
Published: July 2002

The equations of motion for ultrathin cored vortex filaments with exponentially small core size are examined, with emphasis placed on the connection between the asymptotic equations of Klein and Majda [Physica D 49, 323 (1991); 53, 267 (1991)] and the desingularized cutoff equation for thin cored filaments. It is shown that, for the particular case of a helical filament of large pitch, the ultrathin asymptotic equations and the cutoff equation give the same answer when the latter is carefully calculated to include the usually neglected terms of order γ2 ln(a/ρ), where 1/γ is the pitch, a is the core size, and ρ is the radius of curvature. Disagreement with the Kelvin [Philos. Mag. 10, 152 (1880)] analysis of waves on vortex columns is attributed to the ultrathin requirement not allowing an overlap regime in which both analyses are valid. Ultrathin filaments of more general shape are considered, and it is shown that the cutoff equation, valid when a/ρ≪1, leads generally to the Klein and Majda equation in the ultrathin regime.
Blade coating of a power-law fluid
Authors: Ross A. B. , Wilson S. K. , Duffy B. R.
Published: July 2002

In this paper we re-examine the problem of applying a thin layer of a power-law fluid to a solid substrate by means of a simple blade coater. Specifically we use lubrication theory to examine steady plane flow of a power-law fluid in the narrow nonuniform channel formed between a fixed blade of prescribed shape and a plane substrate moving parallel to itself. The first-order asymptotic solution for the case of a weakly non-Newtonian fluid is presented. An explicit expression is obtained for the first-order pressure gradient from which the first-order contributions to several important physical quantities including the thickness of the applied fluid layer and the forces on the blade are calculated for both plane and exponentially shaped blades. In particular, we find that, depending on the shape and height ratio of the coater, the effect of weakly non-Newtonian behavior can be either to increase or to decrease both the pressure and the load from their Newtonian values. We also re-examine the approximate solutions of Hwang [Trans. ASME J. Fluids Eng. 104, 469 (1982)] and Dien and Elrod [Trans. ASME J. Lubrication Technol. 105, 385 (1983)] and make a detailed comparison between their predictions and those of the exact solution in the weakly non-Newtonian limit. We find that in this limit the Dien and Elrod approximation is usually in significantly better agreement with the exact solution than Hwang’s approximation. In the Appendix we re-examine the Dien and Elrod approximate solution for the flow of a generalized Newtonian fluid.
Similarity solutions for van der Waals rupture of a thin film on a solid substrate
Authors: Zhang Wendy W. , Lister John R.
Published: July 2002

Rupture of a thin viscous film on a solid substrate under a balance of destabilizing van der Waals pressure and stabilizing capillary pressure is shown to possess a countably infinite number of similarity solutions in each of which the horizontal lengthscale decreases like (tR−t)2/5 and the film thickness decreases like (tR−t)1/5, where tR−t is the time remaining before rupture. Only the self-similar solution corresponding to the least oscillatory curvature profile is observed in time-dependent numerical simulations of the governing partial differential equation. The numerical strategy employed to obtain the self-similar solutions is developed from far-field asymptotic analysis of the similarity equations.
Validation of acoustic-analogy predictions for sound radiated by turbulence
Authors: Whitmire Julia , Sarkar Sutanu
Published: July 2002

Predicting sound radiated by turbulence is of interest in aeroacoustics, hydroacoustics, and combustion noise. Significant improvements in computer technology have renewed interest in applying numerical techniques to predict sound from turbulent flows. One such technique is a hybrid approach in which the turbulence is computed using a method such as direct numerical simulation (DNS) or large eddy simulation (LES), and the sound is calculated using an acoustic analogy. In this study, sound from a turbulent flow is computed using DNS, and the DNS results are compared with acoustic-analogy predictions for mutual validation. The source considered is a three-dimensional region of forced turbulence which has limited extent in one coordinate direction and is periodic in the other two directions. Sound propagates statistically as a plane wave from the turbulence to the far field. The cases considered here have a small turbulent Mach number so that the source is spatially compact; that is, the turbulence integral scale is much smaller than the acoustic wavelength. The scaling of the amplitude and frequency of the far-field sound for the problem considered are derived in an analysis based on Lighthill’s acoustic analogy. The analytical results predict that the far-field sound should exhibit “dipole-type” behavior; the root-mean-square pressure in the acoustic far field should increase as the cube of the turbulent Mach number. The acoustic power normalized by the turbulent dissipation rate is also predicted to scale as turbulent Mach number cubed. Agreement between the DNS results and the acoustic-analogy predictions is good. This study verifies the ability of the Lighthill acoustic analogy to predict sound generated by a three-dimensional, turbulent source containing many length and time scales.
Internal flow of an electrostatically levitated droplet undergoing resonant shape oscillation
Authors: Chung Sang K. , Trinh Eugene H.
Published: July 2002

Experimental evidence of internal dc flow circulation within an electrostatically levitated, 3.4 mm diameter charged water drop oscillating in shape at a different amplitude is presented. The axisymmetric shape oscillations were excited using a continuously pulsed electrostatic field at the fundamental shape mode frequency (l=2) of the drop. It was found that the speed of the internal dc flow varied quadratically with respect to the oscillation amplitude without an observable minimum threshold. The implication of the observed internal flow is discussed with respect to the microgravity research facility and environment.
Fingering phenomena for driven coating films
Authors: Eres M. H. , Schwartz L. W. , Roy R. V.
Published: July 2002

A theoretical and numerical model is formulated to describe the instability and the long-time evolution of both gravity-driven and surface-shear-stress-driven thin coating films. A single evolution equation, of higher-order diffusive type, models the flow for either problem. It is derived using the lubrication approximation. For partially wetting systems, the effect of finite contact angle is incorporated in the equation using a particular disjoining pressure model. The base state, in each case, is a two-dimensional steadily propagating capillary front. Slight perturbations of the base state, applied along the front, initiate the fingering instability. Early-time results accurately reproduce the wavelengths of fastest growth and the corresponding eigenmodes as reported in published linear stability analyses. As time proceeds, depending on parameter values, various fingering patterns arise. For conditions of perfect wetting with the substrate downstream of the moving front covered with a thin precursor layer, predicted nonlinear finger evolution agrees well with published experiments. The ultimate pattern, in this case, is a steadily translating pattern of wedge-shaped fingers. Alternatively, for partially wetting systems that exhibit sufficiently large static contact angles, long straight-sided fingers or rivulets are formed. Finally, for larger contact angles, or at relatively low speeds, we predict that the flowing rivulets will become unstable and break up into strings of isolated droplets.
Fusion performance with thermal confinement scalings
Authors: Stotler D. P.
Published: July 2002

Energy confinement scalings for the thermal component of the plasma published thus far have a different dependence on plasma density and input power than do most scalings for the total plasma energy. With such thermal scalings, reactor performance (measured by the ignition margin MI , the ratio of the alpha power to the sum of the conducted and radiated losses) worsens with increasing density. The temperature dependence is significantly different as well and could lead to stable, ignited operation at temperatures lower than imagined with scalings based on the total plasma energy.
Effect of the drift wave turbulence on the evolution of the low-m tearing modes
Authors: Siva Rama Prasad P. V. , Tewari D. P.
Published: July 2002

The effect of the background drift wave turbulence on the evolution of the low-m tearing modes has been studied, in the quasilinear regime, in various limiting cases. It is found, in the cases of the m=1 classical, collisionless, and drift-tearing modes, that the turbulence introduces finite real frequencies to these modes, which are otherwise purely growing ones, but reduces their instability activity. In the case of the m≥2 classical modes, in a limit ‖α‖1/2≫ρi, the turbulence enhances the frequencies as well as the growth rates of these modes; and in the case of the m≥2 collisionless modes, in the same limit, it enhances the frequencies of these modes, but reduces their instability activity, where ‖α‖1/2 is the current channel width and ρi is the ion Larmor radius.
Absorption of laser light in overdense plasmas by sheath inverse bremsstrahlung
Authors: Yang T.-Y. Brian , Kruer William L. , More Richard M. , Langdon A. Bruce
Published: July 2002

The original sheath inverse bremsstrahlung model [P. J. Catto and R. M. More, Phys. Fluids 20, 704 (1977)] is modified by including the v×B term in the equation of motion, as the evanescent magnetic field in an overdense plasma is greater than the corresponding electric field. It is shown that the present results are significantly different from those derived without the v×B term. The v×B term is also important in interpreting the absorption mechanism. If the v×B term were neglected, the absorption of the light would be incorrectly interpreted as an increase in the transverse components of the canonical momentum, in the case of a normally incident laser light. It is also shown that both the sheath inverse bremsstrahlung and the anomalous skin effect are limiting cases of the same collisionless absorption mechanism. Results from particle-in-cell (PIC) plasma simulations are compared with the absorption coefficient calculated from the linear theory. Finally, the effects of finite density gradients are investigated by PIC simulations.
Plasma polarimetry for large Cotton–Mouton and Faraday effects
Authors: Segre S. E.
Published: July 2002

A formalism is presented for treating plasma polarimetry when both the Cotton–Mouton and the Faraday effects are large. For this general case it is shown that, for each measuring chord, up to nine parameters related to the plasma can be determined, instead of the usual single Faraday rotation. These parameters can be measured by a convenient modulation of the polarization state of the input radiation, and they can be used in the reconstruction of the magnetohydrodynamic equilibrium. Thus, the potential of the polarimetric diagnostic can be significantly increased, and the range of plasma conditions where the latter can be used, is extended. The importance of refraction is discussed.
Nonlinear evolution of whistler wave modulational instability
Authors: Karpman V. I. , Lynov J. P. , Michelsen P. K. , Rasmussen J. Juul
Published: July 2002

The nonlinear evolution of the modulational instability of whistler waves coupled to fast magnetosonic waves (FMS) and to slow magnetosonic waves (SMS) is investigated. Results from direct numerical solutions in two spatial dimensions agree with simplified results from a set of ordinary differential equations obtained from a Hamiltonian formulation of the governing equations. The long-time evolution of the modulational instability for the FMS-coupling shows a quasi-recurrent behavior with a slow spreading of the energy to higher and higher mode numbers. For the SMS-coupling, no recurrent behavior is found and the energy is gradually leaking to higher mode numbers while the spatial evolution of the modulation tends to develop small scale ‘‘spikes.’’
Helicon wave excitation with helical antennas
Authors: Light Max , Chen Francis F.
Published: July 2002

Components of the wave magnetic field in a helicon discharge have been measured with a single-turn, coaxial magnetic probe. Left- and right-handed helical antennas, as well as plane-polarized antennas, were used; and the results were compared with the field patterns computed for a nonuniform plasma. The results show that the right-hand circularly polarized mode is preferentially excited with all antennas, even those designed to excite the left-hand mode. For right-hand excitation, the radial amplitude profiles are in excellent agreement with computations.
Industrial applications of low-temperature plasma physics
Authors: Chen Francis F.
Published: July 2002

The application of plasma physics to the manufacturing and processing of materials may be the new frontier of our discipline. Already partially ionized discharges are used in industry, and the performance of plasmas has a large commercial and technological impact. However, the science of low-temperature plasmas is not as well developed as that of high-temperature, collisionless plasmas. In this paper several major areas of application are described and examples of forefront problems in each are given. The underlying thesis is that gas discharges have evolved beyond a black art, and that intellectually challenging problems with elegant solutions can be found.
Toroidal gyrofluid equations for simulations of tokamak turbulence
Authors: Beer M. A. , Hammett G. W.
Published: July 2002

A set of nonlinear gyrofluid equations for simulations of tokamak turbulence are derived by taking moments of the nonlinear toroidal gyrokinetic equation. The moment hierarchy is closed with approximations that model the kinetic effects of parallel Landau damping, toroidal drift resonances, and finite Larmor radius effects. These equations generalize the work of Dorland and Hammett [Phys. Fluids B 5, 812 (1993)] to toroidal geometry by including essential toroidal effects. The closures for phase mixing from toroidal ∇B and curvature drifts take the basic form presented in Waltz et al. [Phys. Fluids B 4, 3138 (1992)], but here a more rigorous procedure is used, including an extension to higher moments, which provides significantly improved accuracy. In addition, trapped ion effects and collisions are incorporated. This reduced set of nonlinear equations accurately models most of the physics considered important for ion dynamics in core tokamak turbulence, and is simple enough to be used in high resolution direct numerical simulations.
Technique for the experimental estimation of nonlinear energy transfer in fully developed turbulence
Authors: Kim J. S. , Durst R. D. , Fonck R. J. , Fernandez E. , Ware A. , Terry P. W.
Published: July 2002

A new procedure for calculating the nonlinear energy transfer and linear growth/damping rate of fully developed turbulence is derived. It avoids the unphysically large damping rates typically obtained using the predecessor method of Ritz [Ch. P. Ritz, E. J. Powers, and R. D. Bengtson, Phys. Fluids B 1, 153 (1989)]. It enforces stationarity of the turbulence to reduce the effects of noise and fluctuations not described by the basic governing equation, and includes the fourth-order moment to avoid the closure approximation. The new procedure has been implemented and tested on simulated, fully developed two-dimensional (2-D) turbulence data from a 2-D trapped-particle fluid code, and has been shown to give excellent reconstructions of the input growth rate and nonlinear coupling coefficients with good noise rejection. However, in the experimentally important case where only a one-dimensional (1-D) averaged representation of the underlying 2-D turbulence is available, this technique does not, in general, give acceptable results. A new 1-D algorithm has thus been developed for analysis of 1-D measurements of intrinsically 2-D turbulence. This new 1-D algorithm includes the nonresonant wave numbers in calculating the bispectra, and generally gives useful results when the width of the radial wave number spectrum is comparable to or less than that of the poloidal spectrum.
Symmetry experiments in gas-filled hohlraums at NOVA
Authors: Delamater N. D. , Murphy T. J. , Hauer A. A. , Kauffman R. L. , Richard A. L. , Lindman E. L. , Magelssen G. R. , Wilde B. H. , Harris D. B. , Failor B. A. , Wallace J. , Powers L. V. , Pollaine S. M. , Suter L. J. , Chrien R. , Shepard T. D. , Rose H. A. , Williams E. A. , Nelson M. B. , Cable M. D. , Moore J. B. , Salazar M. A. , Gifford K.
Published: July 2002

Understanding drive symmetry in gas-filled hohlraums is currently of interest because the baseline design of the indirect drive ignition target for the planned National Ignition Facility uses a gas-filled hohlraum. This paper reports on the results of a series of experiments performed at the Nova laser [C. Bibeau et al. Appl. Opt. 31, 5799 (1992)] facility at Lawrence Livermore National Laboratory with the goal of understanding time-dependent drive symmetry in gas filled hohlraums. Time-dependent symmetry data from capsule implosions and reemission targets in gas-filled hohlraums are discussed. Results of symmetry measurements using thin wall gas-filled hohlraums are also discussed. The results show that the gas is effective in impeding the motion of the wall blowoff material, and that the resulting implosion performance of the capsule is not significantly degraded from vacuum results. The implosion symmetry in gas differs from vacuum results with similar laser pointing indicating a shift in beam position on the hohlraum wall and hotter drive at the capsule’s poles than at the equator. A theory has been proposed to explain the observed shift as a plasma physics effect: beam steering due to filamentation and transverse plasma flows.
A new mathematical approach for shock-wave solution in a dusty plasma
Authors: Das G. C. , Dwivedi C. B. , Talukdar Madhuri , Sarma Jnanjyoti
Published: July 2002

The problem of nonlinear Burger equation in a plasma contaminated with heavy dust grains has been revisited. As discussed earlier [C. B. Dwivedi and B. P. Pandey, Phys. Plasmas 2, 9 (1995)], the Burger equation originates due to dust charge fluctuation dynamics. A new alternate mathematical approach based on a simple traveling wave formalism has been applied to find out the solution of the derived Burger equation, and the method recovers the known shock-wave solution. This technique, although having its own limitation, predicts successfully the salient features of the weak shock-wave structure in a dusty plasma with dust charge fluctuation dynamics. It is emphasized that this approach of the traveling wave formalism is being applied for the first time to solve the nonlinear wave equation in plasmas.
Bäcklund transformations and Painlevé analysis: Exact soliton solutions for the unstable nonlinear Schrödinger equation modeling electron beam plasma
Authors: Khater A. H. , Callebaut D. K. , Ibrahim R. S.
Published: July 2002

In this paper the Bäcklund transformations technique and Painlevé analysis are used to generate classes of exact soliton solutions for some nonlinear evolution equations. For the (1+1)-dimensional problem, the unstable system of plasma equations where an electron beam is injected under a high-frequency electric field is reduced to the unstable nonlinear Schrödinger (UNLS) equation. Using the Darboux–Bargmann technique, we obtain the Bäcklund transformations for UNLS equation solvable by the inverse scattering method of Zakharov–Shabat/Ablowitz–Kaup–Newell–Segur (ZS/AKNS) and the ZS/AKNS wave functions corresponding to the soliton solutions of this equation.
What is a stellarator?
Authors: Boozer Allen H.
Published: July 2002

A stellarator is a toroidal plasma confinement concept that uses effects that arise in the absence of toroidal symmetry to maintain the magnetic configuration without the need for current drive. The largest magnetic fusion machines under construction are stellarators, and the plasma parameters achieved in stellarators are second only to those in tokamaks. Stellarators are poised for rapid progress toward showing the feasibility of fusion power. The physics and mathematical concepts that are required to understand stellarators are reviewed.
Paraxial theory of self-focusing of cylindrical laser beams. I. ABCD laws
Authors: Subbarao D. , Uma R. , Singh H.
Published: July 2002

A paraxial formulation of steady-state self-focusing is presented, taking due care of the correct paraxial approximation of the plasma frequency and the resultant nonlinear refractive index of the plasma in the presence of the electromagnetic field of a high-powered laser beam. For the laser beam and plasma refractive index description, the correct momentum space of the photons in circular cylindrical geometry of the laser beam is the transformation in terms of the Laguerre-Gauss modes that lead to the correct non-Taylor series paraxial approximation of the beam and its propagation equation. For self-trapping, the same conditions as the moments and variational approaches result. The natural way to express the results for self-focusing in this approximation is in terms of the so-called ABCD laws for the beam parameters, presented here for the self-similar beam propagation of a centrally humped (Gaussian) beam. These laws define a one-complex-parameter group of transformations [the SU(2) group for the absorptionless case and the restricted Lorentz group in general] that describe the evolution of the self-focusing beam; they naturally lead to the application of the concept of geometric phase to the self-focusing beam presented in the paper.
Access to second stability region for coupled peeling-ballooning modes in tokamaks
Authors: Wilson H. R. , Miller R. L.
Published: July 2002

The peeling mode restricts access to the second stability region of the ideal ballooning mode at the tokamak plasma edge. Using a two-dimensional, high toroidal mode number eigenmode code employing a model tokamak equilibrium, it is shown that a window to second stability exists for a sufficiently deep magnetic well. The different mode structures of the various eigenmode branches are studied. In particular, when access to second ballooning stability exists, a ballooning mode perturbation at the first stability boundary can extend deep into the plasma core, and then instability is likely to result in large scale loss of plasma energy.
Existence of ion temperature gradient driven shear Alfvén instabilities in tokamaks
Authors: Zonca Fulvio , Chen Liu , Dong J. Q. , Santoro Robert A.
Published: July 2002

The existence of unstable ion temperature gradient driven Alfvén eigenmodes (AITG) is demonstrated in tokamak plasmas, which are ideally stable with respect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of numerical solutions of a set of one-dimensional integral equations along the ballooning coordinate (quasi-neutrality and parallel Ampère’s law). Furthermore, theoretical analyses of the eigenmode dispersion relation, which is given in a compact analytical form in the small ion orbit width limit (compared to the radial wavelength), provide a basis for explaining the general properties of the modes. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be not too far away from ideal MHD marginal stability.
High yield inertial confinement fusion target design for a z-pinch-driven hohlraum
Authors: Hammer James H. , Tabak Max , Wilks Scott C. , Lindl John D. , Bailey David S. , Rambo Peter W. , Toor Arthur , Zimmerman George B. , Porter John L.
Published: July 2002

Calculations are presented for a high yield inertial fusion design, employing indirect drive with a double-ended z-pinch-driven hohlraum radiation source. A high current (∼60 MA) accelerator implodes z pinches within an enclosing hohlraum. Radial spoke arrays and shine shields isolate the capsule from the pinch plasma, magnetic field, and direct x-ray shine. Our approach places minimal requirements on z-pinch uniformity and stability, usually problematic due to magneto-Rayleigh–Taylor instability. Large inhomogeneities of the pinch and spoke array may be present, but the hohlraum adequately smooths the radiation field at the capsule. Simultaneity and reproducibility of the pinch x-ray output to better than 7% are required, however, for good symmetry. Recent experiments suggest a pulse shaping technique, through implosion of a multishell z pinch. X-ray bursts are calculated and observed to occur at each shell collision. A capsule absorbing 1 MJ of x rays at a peak drive temperature of 210 eV is found to have adequate stability and to produce 400 MJ of yield. A larger capsule absorbs 2 MJ with a yield of 1200 MJ.
Vortex dynamics and zonal flows
Authors: Marcus P. S. , Kundu T. , Lee Changhoon
Published: July 2002

Two-dimensional vortex dynamics have been studied in plasmas by exploiting the analogy between fluid velocity and the E×B drift velocity. The analogy extends to geophysical flows by including physics that mimic zonal flows, dissipation and the β-effect due to the variation in the Coriolis parameter. Vortices with the same sign as the ambient zonal shear are stable, while opposite-signed vortices fragment. Rules for vortex merger derived by maximizing entropy or minimizing enstrophy do not work for vortices embedded in zonal flows. New rules based on the minimization of energy hold. When zonal flows are not imposed, and the flow is forced at small scales, large, coherent jet streams or eddies form that co-exist with turbulence. Their sizes are determined by an energy balance, not the length scales of the forcing or boundaries. The motivation for this work is to understand atmospheric and ocean vortices: Gulf stream meanders and eddies, the Antarctic ozone hole, the jet streams of Earth and Jupiter, and the Jovian Great Red Spot and White Ovals.
Photoluminescence measurements of band discontinuity in InP-InGaPAs heterostructures
Authors: Brunemeier P. E. , Deppe D. G. , Holonyak N.
Published: July 2002

Photoluminescence data on liquid phase epitaxial single-well quantum well heterostructures are presented that permit the determination of the valence-band discontinuity ΔEv in InP-In1−xGaxP1−yAsy heterostructures at several quaternary compositions matched to InP [x≊0.453y(1+0.031y)]. The ΔEv determination, which depends on a hot-electron to bound-hole recombination transition, indicates that ΔEv ≊0.35ΔEg and ΔEc ≊0.65ΔEg for all InGaAsP compositions lattice matched to InP.
Warm electron system in the n-AlGaAs/GaAs two-dimensional electron gas
Authors: Tsubaki Kotaro , Sugimura Akira , Kumabe Kenji
Published: July 2002

The energy relaxation time τε and the critical electron temperature Tc in the warm electron system of the AlGaAs/GaAs two-dimensional electron gas is studied. The critical electron temperature Tc is defined as the electron temperature where the electric field dependence of the electron mobility begins to have a higher term than the quadric term. Simple formulas are presented for giving τε and Tc from the warm electron experimental results. The calculated values of τε agree with the other reported results. The experimental relation between the lattice temperature T and the critical electron temperature Tc is expressed as Tc=1.5 T.
Electronic structure of oxygen thermal donors in silicon
Authors: Robertson J. , Ourmazd A.
Published: July 2002

The electrical activity of oxygen-related thermal donors in the model of Ourmazd, Bourret, and Schröter is shown to derive from the shallow, doubly occupied pπ state of a divalent silicon at the center of a cluster of five or more oxygens.
Low spin density amorphous hydrogenated germanium prepared by homogeneous chemical vapor deposition
Authors: Reimer J. A. , Scott B. A. , Wolford D. J. , Nijs J.
Published: July 2002

Structural [nuclear magnetic and electron spin resonance (ESR), infrared spectroscopy], optical (absorption, photoacoustic, photoluminescence), and electrical data are presented for amorphous hydrogenated germanium (a-Ge:H) prepared by homogeneous chemical vapor deposition (HOMOCVD). Like HOMOCVD prepared amorphous hydrogenated silicon, these a-Ge:H materials exhibit very low ESR spin densities and systematically varying optical gaps and hydrogen contents. Nevertheless, the material displays no subband gap photoluminescence and minimal photoconductivity, suggesting that a spinless defect may ultimately limit the device application of all types of a-Ge:H and a-(Ge,Si):H films.
Sequential observation of electromigration degradation in passivated Al stripes by interferometric optical scanning
Authors: Fortini A. , Bosmans R. , Hamel J.
Published: July 2002

It is shown that geometrical damages resulting from electrotransport in passivated Al stripes can easily be observed, with a resolution of a few nanometers, by means of an interferometric optical set allowing for sequential recordings in processing.
Microstructure of silicon implanted with high dose oxygen ions
Authors: Jaussaud C. , Stoemenos J. , Margail J. , Dupuy M. , Blanchard B. , Bruel M.
Published: July 2002

Buried implanted oxide layers have been formed by high dose implantation of oxygen ions (3×1018 ions cm−2) into 〈100〉 silicon wafers, at a constant temperature of 500 °C. The implanted layers were studied by cross-sectional transmission electron microscopy and secondary ion mass spectroscopy. The defects at both the top Si/SiO2 and the SiO2/bulk Si interfaces are shown to be SiO2 precipitates. The precipitates are unstable and can be eliminated by heat treatment, and a homogeneous top silicon layer with a low density of dislocations can be obtained.
FexN thin films with perpendicular magnetic anisotropy
Authors: Asada Seiichi , Kitada Masahiro
Published: July 2002

FexN(x=2–3) thin films with perpendicular magnetic anisotropy are prepared by rf sputtering. Although conventional rf sputtered FexN films are magnetized in the direction of the film plane, dc biasing of the substrate during rf sputtering produces magnetic anisotropy normal to the film plane.
Intramode correlation spectroscopy: A new method to study line broadening mechanisms in semiconductor lasers
Authors: Elsässer W. , Göbel E. O.
Published: July 2002

A new experimental method is presented to investigate correlations of intensity and phase fluctuations within the spectral bandwidth of one longitudinal mode of a semiconductor laser. This intramode correlation spectroscopy can be applied to study the physical processes which contribute to the linewidth of a semiconductor laser. First results for a GaAs/GaAlAs oxide stripe laser reveal the contribution of a low-frequency jitter of the lasing wavelength to the mode linewidth.
Picosecond laser induced anomalous crystallization in amorphous silicon
Authors: Kanemitsu Yoshihiko , Nakada Ichiro , Kuroda Hiroto
Published: July 2002

Two different values of thresholds for crystallization of amorphous silicon were observed by using picosecond laser pulses. One of the crystallization thresholds is found experimentally to be clearly below the amorphization threshold. The other crystallization threshold is well above the amorphization threshold. Additionally, small changes of surface height are observed even at the unannealed region far outside the central annealed region. These results imply that shock stress generated by picosecond heating is important in the crystallization process.
Wear and hardness of diamondlike coatings prepared by ion beam deposition
Authors: Koskinen J. , Hirvonen J-P. , Anttila A.
Published: July 2002

The wear rate of 10-μm-thick 12C coatings prepared by mass separated ion beam deposition has been measured to be 60 times lower than WC+Co hard metal and 10 times lower than the conventional hard coating material TiN. The hardness of the 12C coatings was measured to be at least the same as that of the natural diamonds.
Spill-over effects in planar doped barrier devices
Authors: Shur Michael
Published: July 2002

We demonstrate that electron spill over from n+ regions into an i region in planar doped barrier devices strongly affects potential distribution and a barrier height. We obtain an analytical solution for the potential distribution which takes the spill-over effects into account and shows that the barrier height may be considerably higher than follows from a simple geometrical model. The obtained formulas may be used in the design of the planar doped barrier devices.
Solid phase epitaxy of deposited amorphous Ge on GaAs
Authors: Palmstro/m C. J. , Galvin G. J.
Published: July 2002

Solid phase epitaxial growth of electron beam deposited amorphous germanium on GaAs has been obtained. Contamination at the Ge/GaAs interface is observed to impede and even prevent epitaxy of the deposited Ge layer. Complete epitaxy of the Ge was obtained by thermal annealing (400 °C for 1 h) of layers deposited on in situ sputter cleaned GaAs substrates.
Electronic structure of the Si:O4 complex as related to the thermal donors in silicon
Authors: Gomes Vivili M. S. , Leite José R.
Published: July 2002

Rigorous self-consistent electronic structure calculations were carried out for complexes containing four interstitial oxygen atoms in silicon. The isolated tetrahedral site interstitial oxygen impurity was also investigated and the results were correlated to the complexes formation. Our calculations indicate that four oxygen impurities in Td symmetry surrounding a silicon atom at the regular lattice site are deep acceptor centers. It is also found that distortions which drive the complex to one of the observed symmetries, D2d, remove the impurity levels from the gap. Therefore, we conclude that these complexes do not show thermal donor actions in silicon as has been suggested.