bluuebunny/crossref_metadata_2025 huggingface dataset:
Some new features of the passive scalar mixing in a turbulent flow
Authors: Danaila L. , Le Gal P. , Anselmet F. , Plaza F. , Pinton J. F.
Published: July 2002

We analyze experimentally the statistical properties of a turbulent mixing created in the gap between two counter-rotating disks at a Taylor Reynolds number Rλ≈400. Local isotropy is investigated for the inertial and dissipative scales r, using two tests, one applied on C(r), the correlation coefficient between temperature increments and velocity increments, and the other one on S(r), the temperature increment skewness factor. When heating one of the disks and cooling the other one, either positive, negative or almost null values of C and S can be obtained at small scales as a direct result of the presence of several temperature sources. In particular, we emphasize the fact that null or small values for these quantities in the inertial range are an evidence of local isotropy of the temperature field. In these cases, we use the Vaienti et al. equation [Physica D 73, 99 (1994)] for the evolution of the temperature increments probability density functions (PDFs) to predict the inertial and dissipative range PDFs, using an initial PDF, and two measurable closure functions. The intermittent behavior quantified through these statistics is well reproduced by the numerical integration of this evolution equation.
Stability of oscillatory two-phase Couette flow: Theory and experiment
Authors: King Michael R. , Leighton David T. , McCready Mark J.
Published: July 2002

The interfacial instability due to viscosity stratification is studied experimentally in a closed Couette geometry. A vertical interface is formed between two concentric cylinders with density-matched fluids of unequal viscosity. The outer cylinder is rotated with a time-harmonic motion, causing spatially periodic disturbances of the interface. The wavelengths and growth rates predicted by linear theory agree well with experimental results. Application of Fjo/rtoft’s inflection point theorem shows the neutral stability curves to be consistent with an internal instability occurring in the less viscous phase. Because the standard Floquet theory yields only time-averaged growth rates, the instantaneous behavior of the system is examined numerically. This reveals the flow to be unstable to a disturbance which has a maximum that oscillates between the interface and a location within the less viscous fluid. Surprisingly, it is found that interfacial wave amplification originates with the internal disturbance, and is not directly caused by interfacial shear. This unsteady instability may explain the growth of waves in “transient” process flows, e.g., fluids encountering changing flow geometry. It is also demonstrated that in the long wave limit the problem of steady-plus-oscillatory plate motion is simply additive. This implies that it is possible to use oscillations to stabilize steady waves over a limited range of parameter values, but only when the less viscous phase is adjacent to the moving boundary.
Conditional moment closure for large eddy simulation of nonpremixed turbulent reacting flows
Authors: Bushe W. Kendal , Steiner Helfried
Published: July 2002

A method for closing the chemical source terms in the filtered governing equations of motion is proposed. Conditional filtered means of quantities appearing in the chemical reaction rate expressions are approximated by assuming that these conditional filtered means are constant for some ensemble of points in the resolved flow field; for such an ensemble, integral equations can be solved for the conditional filtered means. These conditional filtered means are then used to approximate the conditional filtered mean of the chemical source term by invoking the Conditional Moment Closure hypothesis. The filtered means of the chemical source terms are obtained by integrating their conditional filtered means over the filtered density function of the conditioning variable(s). The method is applied to direct numerical simulation results to directly compare the prediction of the reaction rates with the actual filtered reaction rates. The results of this a priori test appear to show that the method is capable of predicting the filtered reaction rates with adequate accuracy—even in the presence of heat release, and local extinction phenomena. This is especially true for predictions obtained using two conditioning variables.
A two-time-scale turbulence model for compressible flows: Turbulence dominated by mean deformation interaction
Authors: Grégoire O. , Souffland D. , Gauthier S. , Schiestel R.
Published: July 2002

The multiple-time-scale concept is applied to develop a turbulence model for compressible flows. Transport equations for the turbulent kinetic energies and the energy transfer rates are linked to each domain of the turbulent spectrum. The model coefficients are calibrated, with respect to simple flows, by using a new method which takes advantage of the spectral character of the model. One innovation of this method is to use, as a component, the CG model [V. M. Canuto and I. Goldman, Phys. Rev. Lett. 54, 430 (1985)] which gives the large scale spectrum as a function of the instability-generating turbulence. Then, the two-time-scale model, with its complete set of coefficients, has been successfully applied to the simulation of plane mixing layers and homogeneous shear flows. A significant issue of this work is the study of the behavior of the two-time-scale model when a shock wave interacts with a homogeneous turbulence. We first compare model results with experimental data for a 2.8 Mach number interaction [D. Alem, Ph.D. thesis, Université de Poitiers, 1995]. The decrease of the integral length scale, predicted by the linear analysis, is reproduced with the two-time-scale model, which, moreover, recovered the rate of reduction measured by Alem. The amplification of the turbulence level through the shock wave is also consistent with the measurements. Then, we confront our results with a direct numerical simulation of the shock–turbulence interaction at M=1.2 [S. Lee et al., J. Fluid Mech. 251, 533 (1993)]. The spectrum of the turbulence injected in the inflow region of the direct numerical simulation appeared to be far from the freely decaying state. The two-time-scale model, which accounts for the spectral nonequilibrium effects, is able to recover the spatial decrease of turbulence in the inflow region whereas a single-time-scale model fails. Moreover, the profiles for the turbulent kinetic energy and its dissipation rate over all the calculation domain are much better reproduced with the two-time-scale model than with the primary k–ε model.
Multitemperature kinetic model for heat transfer in reacting gas mixture flows
Authors: Chikhaoui A. , Dudon J. P. , Genieys S. , Kustova E. V. , Nagnibeda E. A.
Published: July 2002

Heat transfer in a high temperature reacting gas flow is investigated taking into account the influence of strong vibrational and chemical nonequilibrium. Rapid and slow vibrational energy exchanges in a mixture of molecular gases with realistic molecular spectra are taken into account and the deviation from the Boltzmann distribution over vibrational levels is studied. A kinetic theory approach is developed for the modeling of transport properties of a reacting mixture of polyatomic gases and a generalized multitemperature model is given. This theoretical model is applied for the analysis of the heat transfer and diffusion behind a strong shock wave propagating in air. The heat conductivity, diffusion coefficients, and heat flux are calculated on the basis of this model and compared to the one-temperature approach. The influence of anharmonicity of molecular vibrations is evaluated.
Relations between intermittency and structure function exponents in turbulence
Authors: Hayot F. , Jayaprakash C.
Published: July 2002

We derive from the Navier–Stokes equation an exact equation satisfied by the dissipation rate correlation function, 〈ε(x+r,t+τ)ε(x,t)〉, which we study in the equal time limit, for homogeneous, isotropic turbulence. We exploit its mathematical similarity to the corresponding equation derived from the one-dimensional stochastic Burgers equation to show that the main intermittency exponents are μ1=2−ζ6 and μ2=2z̃4−ζ4, where the ζ’s are exponents of velocity structure functions and z̃4 is a dynamical exponent characterizing the fourth order structure function. We discuss the role of sweeping and Galilean invariance in determining the intermittency exponents.
Electron beam acceleration by nonresonantly driven relativistic electron plasma waves
Authors: Sugaya R.
Published: July 2002

Acceleration and heating of a relativistic electron beam induced by nonlinear Landau damping of intense electromagnetic waves in a plasma are investigated theoretically based on kinetic wave equations and momentum-space diffusion equations derived from relativistic Vlasov–Maxwell equations. Two electromagnetic waves excite nonresonantly a beat-wave driven relativistic electron plasma wave with a phase velocity near the speed of light [vp=c(1−γ−2p)1/2, γp=ω/ωpe]. This wave interacts nonlinearly with the electron beam and accelerates effectively it to a highly relativistic energy γpmec2. When the beat-wave frequency equals the electron plasma frequency, the beam acceleration and the beat-wave energy density become maximum, and they are equal to those by stimulated Raman scattering.
Steady states of a low collisionality, small Larmor radius, Ohmically driven axisymmetric system
Authors: Weitzner Harold
Published: July 2002

A long mean free path, small Larmor radius expansion of the coupled Fokker–Planck equations for ions and electrons is performed for a steady-state, axisymmetric, Ohmically driven tokamak with small energy sources. The expansion is different from, but related to, the secondary long mean free expansion of the usual neoclassical transport theories. Two scalings of loop voltage are proposed, where the ratio of the Ohmic current to the bootstrap current is either of order one, or large. The second scaling is presently more appropriate. The expansion of the kinetic equations can be carried to all orders, and there are four equations for the four lowest order profile functions n(ψ), Ti(ψ), Te(ψ), and φ(ψ). For comparable Ohmic and bootstrap currents Ti(ψ)=Te(ψ). For larger loop voltage the temperature difference may be nonzero, depending on the necessary energy sinks. The electron energy flux is calculated and scales differently from neoclassical theory giving much larger transport. The energy confinement time varies in magnitude, depending on the loop voltage and the energy sinks. Values comparable with present experiments appear possible.
Bifurcations leading to stochasticity in a cyclotron-maser system
Authors: Pakter R. , Corso G. , Caetano T. S. , Dillenburg D. , Rizzato F. B.
Published: July 2002

This paper is concerned with the orbital dynamics of electrons in a cyclotron maser [C. Chen, Phys. Rev. A 46, 6654 (1992)] with modulated maser fields. Amplitude modulation is a natural result of wave–particle energy exchanges, and for typical system parameters, the nonlinear bifurcations of periodic orbits are investigated as the modulation level increases. Attention is focused on primary stable orbits exhibiting the same periodicity as the modulation for low modulational levels. This interest is related to the fact that the destruction of these orbits is generally associated with considerable spread of chaos over the phase space. It is found that two groups of such orbits do exist, each group located in a particular region of the phase space. As the modulation level grows, the overall behavior can be classified as a function of the modulation frequency. If this frequency is large there are two orbits in the group; one undergoes an infinite cascade of period doubling bifurcations and the other simply collapses with neighboring unstable orbits. If the frequency is small the number of orbits is larger; the collapsing orbit is still present and some of the others may fail to undergo the period doubling cascade.
Plasma opening switch conduction scaling
Authors: Weber B. V. , Commisso R. J. , Goodrich P. J. , Grossmann J. M. , Hinshelwood D. D. , Ottinger P. F. , Swanekamp S. B.
Published: July 2002

Plasma opening switch (POS) experiments performed on the Hawk generator [Commisso et al., Phys. Fluids B 4, 2368 (1992)] (750 kA, 1.2 μs) determine the dependence of the conduction current and conduction time on plasma density, electrode dimensions, and current rise rate. The experiments indicate that for a range of parameters, conduction is controlled by magnetohydrodynamic (MHD) distortion of the plasma, resulting in a low density region where opening can occur, possibly by erosion. The MHD distortion corresponds to an axial translation of the plasma center-of-mass by half the initial plasma length, leading to a simple scaling relation between the conduction current and time, and the injected plasma density and POS electrode dimensions that is applicable to a large number of POS experiments. For smaller currents and conduction times, the Hawk data suggest a non-MHD conduction limit that may correspond to electromagnetohydrodynamic (EMH) field penetration through the POS plasma.
Axial propagation of helicon waves
Authors: Light Max , Sudit Isaac D. , Chen Francis F. , Arnush Donald
Published: July 2002

Traveling- and standing-wave characteristics of the wave fields have been measured in a helicon discharge using a five-turn, balanced magnetic probe movable along the discharge axis z. Helical and plane-polarized antennas were used, and the magnitude and direction of the static magnetic field were varied, yielding three primary results. (1) As the density varies along z, the local wavelength agrees with the local dispersion relation. (2) Beats in the z variation of the wave intensity do not indicate standing waves, but instead are caused by the simultaneous excitation of two radial eigenmodes. Quantitative agreement with theory is obtained. (3) The damping rate of the helicon wave is consistent with theoretical predictions based on collisions alone.
Numerical and analytical estimates for the structure functions in two-dimensional magnetohydrodynamic flows
Authors: Grauer Rainer , Marliani Christiane
Published: July 2002

In two-dimensional magnetohydrodynamic turbulence, the Kraichnan–Iroshnikov dimensional analysis suggests a linear scaling law for the exponents ζp=p/4 of the structure functions for the Elsässer variables z±=u±B. Numerical simulations are presented and higher order structure functions are calculated using the extended self-similarity hypothesis of Benzi et al. [Phys. Rev. E 48, R29 (1993)]. In addition, an estimate for the first structure function ζ1≥1/4 is derived using a geometric technique introduced by Constantin and Procaccia [Phys. Rev. E 47, 3307 (1993)] in the the context of the transport of a passive scalar in three-dimensional Navier–Stokes turbulence.
Density profile steepening due to self-generated magnetic fields in plasmas produced by laser irradiation of spherical targets
Authors: Vyas P. , Srivastava M. P.
Published: July 2002

The density variation of plasmas produced by irradiation with laser beams on spherical targets has been studied in the steady state. This study used an adiabatic plasma model with self-generated magnetic fields included in a phenomenological manner. The density variation with a radial coordinate is found to have a steep rise through the critical density, which reaches a plateau in the overdense region, then a density minimum and finally, a rise. This variation has been compared with experimentally observed values in earlier works and is found to be in fairly good agreement. The variation of plateau density with the intensity of the laser is also compared with the experimentally observed variation.
Analysis of transport in gyrokinetic tokamaks
Authors: Mynick H. E. , Parker S. E.
Published: July 2002

Progress toward a detailed understanding of the transport in full-volume gyrokinetic simulations of tokamaks is described. The transition between the two asymptotic regimes (large and small) of scaling of the heat flux with system size a/ρg reported earlier is explained, along with the approximate size at which the transition occurs. The larger systems have transport close to that predicted by the simple standard estimates for transport by drift-wave turbulence (viz., Bohm or gyro-Bohm) in scaling with a/ρg, temperature, magnetic field, ion mass, safety factor, and minor radius, but lying much closer to Bohm, which seems the result better supported theoretically. The characteristic downshift in the 〈kθ〉 spectrum observed previously in going from the linear to the turbulent phase is consistent with the numerically inferred coupling coefficients Mkpq of a reduced description of the system. An explanation of the downshift is given from the resemblance of the reduced system to the Hasegawa–Mima or Terry–Horton systems. These manifest an analogous downshift in slab geometry, and have Mkpq resembling those inferred from the gyrokinetic (GK) data.
Impurity effects on linear and nonlinear ion-temperature-gradient-driven modes
Authors: Jovanović D. , Horton W.
Published: July 2002

Linear and nonlinear stages in the development of the ion-temperature-gradient-driven drift-wave instability are studied analytically in the presence of shear flows, magnetic shear, inhomogeneity, and curvature. In the linear regime, it is shown that the toroidal ηi mode is destabilized by a small amount of impurities only if there exists an impurity buildup at the plasma edge. In the nonlinear regime two types of coherent structures are found: a generalized Hasegawa–Mima dipole vortex in the weak magnetic shear case, and a periodic, vortex-chain solution in the strong shear case, which corresponds to the saturated, large-amplitude drift-tearing mode.
Transport in gyrokinetic tokamaks
Authors: Mynick H. E. , Parker S. E.
Published: July 2002

A comprehensive study of transport in full-volume gyrokinetic (GK) simulations of ion temperature gradient driven turbulence in core tokamak plasmas is presented. Though this ‘‘gyrokinetic tokamak’’ is much simpler than experimental tokamaks, such simplicity is an asset, because a dependable nonlinear transport theory for such systems should be more attainable. Toward this end, two related lines of inquiry are pursued. (1) The scalings of GK tokamaks with respect to important system parameters are studied. In contrast to real machines, the scalings of larger GK systems (a/ρs≳64) with minor radius, with current, and with a/ρs are roughly consistent with the approximate theoretical expectations for electrostatic turbulent transport which exist as yet. Smaller systems manifest quite different scalings. (2) With the goal of developing a first-principles theory of GK transport, the GK data are used to infer the underlying transport physics. The data indicate that, of the many modes k present in the simulation, only a modest number (Nk∼10) of k dominate the transport, and for each, only a handful (Np∼5) of couplings to other modes p appear to be significant, implying that the essential transport physics may be described by a far simpler system than would have been expected on the basis of earlier nonlinear theory alone. Part of this analysis is the inference of the coupling coefficients Mkpq governing the nonlinear mode interactions, whose measurement from tokamak simulation data are presented here for the first time.
Modeling of neutral hydrogen velocities in the Tokamak Fusion Test Reactor
Authors: Stotler D. P. , Skinner C. H. , Budny R. V. , Ramsey A. T. , Ruzic D. N. , Turkot R. B.
Published: July 2002

Monte Carlo neutral transport simulations of hydrogen velocities in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] are compared with experiment using the Doppler-broadened Balmer-α spectral line profile. Good agreement is obtained under a range of conditions, validating the treatment of charge exchange, molecular dissociation, surface reflection, and sputtering in the neutral gas code DEGAS [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)]. A residual deficiency of 10–100 eV neutrals in most of the simulations indicates that further study of the energetics of H+2 dissociation for electron energies in excess of 100 eV is needed.
Relativistic harmonic generation excited in the ultrashort laser pulse regime
Authors: Zeng Guihua , Shen Baifei , Yu Wei , Xu Zhizhan
Published: July 2002

The envelope equations describing the relativistic harmonic components are derived and used to analyze the third relativistic harmonic radiation in the short laser pulse regime. The results show that the pulse duration of the incident laser is an important parameter for the relativistic harmonic radiation and the harmonic radiation depends on the pumping laser pulse shape. The relativistic harmonic radiation is more efficient in the short laser pulse regime than in the long laser pulse regime.
Space-charge solitary waves in charged particle beams in resistive-wall transport channels
Authors: Suk H. , Wang J. G. , Reiser M.
Published: July 2002

The properties of solitary waves in space-charge dominated charged particle beams are investigated theoretically. Similarly to linear space-charge waves, both slow and fast solitary waves can exist. In a dissipative environment, e.g. a resistive-wall channel, it is shown that the slow space-charge solitary waves grow in amplitude, while the fast space-charge solitary waves decay. However, the growth and decay rates differ from those of the linear waves. A possible experiment with an electron beam is proposed to demonstrate these phenomena.
Lyapunov stability and thermal stability of partially relaxed fluids and plasmas
Authors: Elsässer Klaus , Spiess Peter
Published: July 2002

The relation between the Lyapunov stability of a Hamiltonian system and the thermal stability of a fluid whose temperature is controlled from outside is explored: The free energy as a functional of the correct variables (specific volume, local entropy, and some Clebsch potentials of the velocity) may serve as a Lyapunov functional, depending on the ‘‘Casimirs’’ as exchanged quantities. For a multi-species plasma one obtains a sufficient condition for stability: γ(v2/c2s)−1 <d ln T/d ln n<γ−1 for each species, where γ is the adiabatic index and cs the sound speed. Some features of partially relaxed (T=const) cylindrical plasmas are also discussed.
Relativistic wave breaking in warm plasmas
Authors: Sheng Z. M. , Meyer-ter-Vehn J.
Published: July 2002

A unified description of wave breaking for relativistic plasma waves having arbitrary phase velocity is obtained on the basis of warm relativistic electron fluid theory [Phys. Fluids 25, 846 (1982)]. Limiting cases found in the literature are reproduced.
Calculations of alpha particle loss for reversed magnetic shear in the Tokamak Fusion Test Reactor
Authors: Redi M. H. , White R. B. , Batha S. H. , Levinton F. M. , McCune D. C.
Published: July 2002

Hamiltonian coordinate, guiding center code calculations of the toroidal field ripple loss of alpha particles from a reversed shear plasma in the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)] predict 40% total alpha losses and 20% ripple diffusion losses. This is about double the loss rate of a comparable non-reversed magnetic shear plasma. High central q is found to increase alpha ripple losses as well as first orbit losses of alphas in the reversed shear simulations. Alpha ripple transport on TFTR affects ions within r/a=0.5, not at the plasma edge. The entire plasma is above threshold for stochastic ripple loss of alpha particles at birth energy in the reversed shear case simulated, so that all trapped 3.5 MeV alphas are lost stochastically or through prompt losses.
Neoclassical transport in a rotating impure plasma
Authors: Helander P.
Published: July 2002

In a toroidally rotating tokamak plasma, heavy impurity ions accumulate on the outside of each flux surface under the action of the centrifugal force. The collision frequency therefore varies over the flux surface. This circumstance is shown to enhance the neoclassical transport processes, including the bootstrap current, by making collisions occur preferentially where the magnetic field is weak.
Dynamical aspects of various solitary waves and double layers in dusty plasmas
Authors: Das G. C. , Sarma Jnanjyoti , Talukdar M.
Published: July 2002

Employing quasipotential analysis, the Sagdeev potential equation has been derived in a multicomponent plasma consisting of free and trapped electrons and contaminated by the dust charged grains forming therein by the attachment of electrons to finite-size dust particles. Because of the free and trapped electrons in the dusty plasma, the plasma-acoustic wave exhibits the different features of various solitary waves. The Sagdeev potential equation, at a small-amplitude approximation, leads to the evaluation, by a proposed new formalism of a simple wave solution technique, of the new scenario of solitary wave propagation in a dusty plasma. It has been shown that the ordering of the nonisothermality in the dusty plasma also plays a unique role. In the case of a plasma with first-order nonisothermality, the Sagdeev potential equation derives the compressive solitary wave propagation, while for plasma with higher-order nonisothermality the method might fail to solve the Sagdeev potential equation and, thus, an alternate method is used to reveal the coexistence of compressive and rarefactive solitary waves. In addition, for certain plasma parameters, the solitary waves disappear and a double layer is expected. Again, with the better approximation in the Sagdeev potential, more features of solitary waves, known as spiky and explosive, along with the double layers, are also highlighted. The observations made of the solitary waves could be of further interest in the understanding of laboratory and space plasmas.
Dust sheared-flow-driven dissipative instability and the formation of a vortex chain in nonuniform dusty magnetoplasmas
Authors: Salimullah M. , Shukla P. K.
Published: July 2002

It is found that dust sheared flow in combination with the dust–neutral drag produce a dissipative instability of short-wavelength (in comparison with the ion gyroradius) electrostatic waves in nonuniform dusty magnetoplasmas. Furthermore, the nonlinear interactions between these finite amplitude short-wavelength waves give rise to a vortex chain which moves with a well defined constant velocity. The relevance of our investigation to laboratory and space plasmas has been pointed out.
Effects of finite feedback loop gain and bandwidth on stabilization of magnetohydrodynamic instabilities by an “intelligent shell”
Authors: Jensen T. H. , Garofalo A. M.
Published: July 2002

The “intelligent shell” [C. M. Bishop, Plasma Phys. Contr. Fusion 31, 1179 (1989)] utilizes a feedback system intended to make a resistive wall appear perfectly conducting to a plasma. It can thus be used for stabilizing modes of the plasma which are unstable when the plasma is surrounded by a resistive wall, but stable if the wall were perfectly conducting. Several concepts of magnetic confinement, such as reversed field pinches, spheromaks, and tokamaks may benefit from an intelligent shell. The paper addresses the question of the dependency of the stabilizing effect on the gain and bandwidth of the feedback circuits (assumed linear). A simple model for the phenomena involved is made and solved numerically for certain parameter values. A characteristic time of the model is a resistive time τ of the wall; the calculations suggest that an upper cutoff frequency of ∼50/τ and sufficient gain provides a stabilization similar to that of ideal circuits with infinite bandwidth and gain. Under laboratory circumstances with τ∼10−3 s it is thus practical to obtain mass produced components which make the circuits as effective as ideal circuits.
Measurements of intrinsic shot noise in a 35 GHz gyroklystron
Authors: Calame J. P. , Danly B. G. , Garven M.
Published: July 2002

Experimental measurements of electron beam shot noise in a 35 GHz, 225 kW, three-cavity gyroklystron have been obtained from both the input and output cavities. This intrinsic noise was studied in the absence of an applied carrier (i.e., at zero drive power). The spectrum of the noise emitted by the input cavity is found to have a Lorentzian shape, with peak noise power densities from the input cavity typically reaching 6.3×10−15 W/Hz (−112 dBm/Hz), and typical 3 dB bandwidths of 160 MHz. The output cavity noise spectrum is found to be equal to the input cavity noise spectrum multiplied by the measured linear frequency response of the gyroklystron. The measured noise levels at the input cavity are 0–5 dB lower than theoretical predictions for shot noise unaltered by collective effects. Furthermore, the input cavity noise power exhibits complex variations as a function of beam current, beam velocity ratio, and circuit magnetic field that are not predicted by present theory. Noise-to-carrier ratios expected in the input cavity during full power amplifier operation are inferred from the noise measurements and known values of drive power required to saturate the gyroklystron. The noise-to-carrier ratio, with typical values of −90 to −80 dBc, is found to be a strong function of the operating parameters.
Coulomb corrections on the electron–ion bremsstrahlung spectrum from a generalized Lorentzian (kappa) distribution plasma
Authors: Jung Young-Dae
Published: July 2002

Electron–ion Coulomb bremsstrahlung radiation from a generalized Lorentzian (kappa) distribution plasma is investigated. The electron–ion bremsstrahlung cross section is given by the nonrelativistic Bethe–Heitler formula and the Elwert–Sommerfeld Coulomb correction factor. The bremsstrahlung radiation rate is obtained for various temperatures and radiation photon energies. The results show that, for low photon energies, the Coulomb correction effects are more significant for low spectral index κ. However, for high photon energies, the Coulomb correction effects are found to be more significant for high spectral index κ.
Alfvén–wave particle interaction in finite-dimensional self-consistent field model
Authors: Padhye Nikhil , Horton W.
Published: July 2002

A low-dimensional Hamiltonian model is derived for the acceleration of ions in finite-amplitude Alfvén waves in a finite-pressure plasma sheet. The reduced low-dimensional wave–particle Hamiltonian is useful for describing the reaction of the accelerated ions on the wave amplitudes and phases through the self-consistent fields within the envelope approximation. As an example, we show that for a single Alfvén wave in the central plasma sheet of the Earth’s geotail, modeled by the linear pinch geometry called the Harris sheet, the time variation of the wave amplitude during the acceleration of fast protons.
Simplification of the Boltzmann collision integral
Authors: Chang Yongbin , Ordonez C. A.
Published: July 2002

Close collision events play an important role in situations where the Coulomb logarithm approaches unity. The five-fold Boltzmann collision integral takes into account close collision events. However, the five-fold Boltzmann collision integral is generally considered too complex for practical use. In a previous paper [Y. Chang, Phys. Fluids B 4, 313 (1992)], the Boltzmann integral with a Maxwellian velocity distribution function for the field particles was reduced to a three-fold integral with a kernel function. However, the kernel function was expressed in the form of an infinite series. In this brief communication, a finite form for the kernel function is obtained and the Boltzmann integral is finally reduced to a simple form exactly.
Magnetic field generated by ionization front produced by intense laser radiating gas
Authors: Zeng Guihua , He Xiantu
Published: July 2002

A general evolution equation for the quasistatic magnetic field (QMF) in a relativistic regime is derived, and an ionization front mechanism for QMF generation is proposed. The properties of the QMF produced by the ionization front mechanism are investigated numerically: The direction of the QMF changes periodically along the direction of the laser propagation, and the generated magnetic field depends on the temperature of the electron plasma and on the effective radial width of the ionization front.
The stability gap of compressible fluids and plasmas
Authors: Elsässer Klaus , Spiess Peter
Published: July 2002

The stability of ideal three-dimensional fluid and plasma equilibria is considered by using canonical (Clebsch) variables for the Hamiltonian. An invariant functional generates a canonical gauge transformation, leading to a new Hamiltonian K which is stationary for arbitrary equilibria. Nonlinear stability is checked from finite variations ΔK of K; all terms of ΔK can be kept non-negative besides one particular term which is due to entropy variations along the equilibrium flow.
Resonant Compton scattering of photons from bound electrons in weakly coupled plasmas
Authors: Jung Young-Dae
Published: July 2002

Plasma screening effects on resonant Compton scattering of photons by bound atomic electrons from the ground state of hydrogenic ions in dense weakly plasmas are investigated. The particle interaction potential in weakly coupled plasmas is obtained by the Debye–Hückel model. The screened wave functions and energy eigenvalues for the ground and excited states of the target ion are obtained by the Ritz variation method. The nonperturbative expression for the transition matrix element near resonance is obtained by the basic lowest-order photon-perburbation Hamiltonian in the electronic dipole representation. The resonant Compton scattering cross section including the plasma screening effect is found to be greater than that neglecting the plasma screening effect. It should be noted that the plasma screening effect increases the cross section at resonance.
Plasma multipole radiation model of sonoluminescence
Authors: Tsui K. H.
Published: July 2002

By considering the material at the center of a sonoluminescent bubble in a plasma state, a point source multipole radiation model is proposed to account for the light flash in each acoustic cycle. The photon number N is calculated as a function of the electron plasma temperature.
Density measurements in exploding wire-initiated plasmas using tungsten wires
Authors: Pikuz S. A. , Shelkovenko T. A. , Mingaleev A. R. , Hammer D. A. , Neves H. P.
Published: July 2002

Calibrated density measurements have been obtained of the coronal plasmas around exploding 7.5–40 μm W wires carrying 15–120 kA per wire for 30–70 ns. X-ray radiographs of the exploding wire plasmas using 2.5–10 keV photons from a Mo wire X-pinch backlighter enabled measurements of areal densities of W ranging from 2×1017/cm2, equivalent to 0.03 μm of solid density W, to about 1019/cm2. The rapidly expanding (few mm/μs) coronal plasmas surrounding the slowly expanding (<1 mm/μs) residual wire cores have areal densities up to about 2×1018/cm2. Single 7.5 μm wires tested with 100 kA had as much as 90% of the initial wire material in the coronal plasma. Coronal plasma W number densities were estimated to be up to a few times 1018/cm3, while core W densities as low as a few times 1020/cm3 were observed. With linear arrays of four (eight) 7.5 μm wires carrying 30 kA (15 kA)/wire, up to 35% (25%) of the initial W wire material was in the plasma around and between the wires at 46–48 ns after the current started. Preheating the wires to drive off adsorbed gases and hydrocarbons increased the W mass in the coronal plasma and made it more uniform then when wires were not preheated.
Acoustic modes in a collisional dusty plasma: Effect of the charge variation
Authors: Ivlev A. V. , Morfill G.
Published: July 2002

The dispersion relation for the acoustic modes in a collisional dusty plasma is derived, taking into account the particle charge variation and ionization. Short and long wavelengths are considered separately. It is shown that the coupling between the short-wavelength branches is weak, and for sufficiently small dust fraction they can be treated independently, as in the case of a fixed charge. In the long-wavelength limit, the dust-ion acoustic (DIA) and dust acoustic (DA) branches interact “through” the dust charging (DCh) branch, unlike the direct coupling obtained in the absence of charge variations.
Hydrodynamic regime of two-dimensional electron magnetohydrodynamics
Authors: Dastgeer Sheikh , Das Amita , Kaw Predhiman
Published: July 2002

A detailed numerical simulation studying certain aspects of turbulence in the electron magnetohydrodynamic (EMHD) model is presented. In particular, new studies have been carried out in a parameter regime where the typical turbulent length scales are comparable to or smaller than the electron skin depth. This is the regime where the EMHD turbulence has similar traits to the hydrodynamic turbulence and the wave propagation effects due to whistlers are believed to be inconsequential.
Predictions on runaway current and energy during disruptions in tokamak plasmas
Authors: Martín-Solís J. R. , Sánchez R. , Esposito B.
Published: July 2002

A simple model for a tokamak disruption, taking into account the replacement of the plasma current by the runaway current, is used to evaluate the generation and energy of the runaway population during the current quench phase of a fast disruptive event. The potential efficiency of the ripple resonance and the magnetic fluctuations for runaway current mitigation during plasma disruptions, as well as their dependence on the runaway generation mechanism, are discussed. Predictions are made for the Joint European Torus (JET) [Nucl. Fusion 25, 1011 (1985)] and the projected International Thermonuclear Experimental Reactor (ITER) [ITER EDA Agreement and Protocol 2, International Atomic Energy Agency, Vienna, 1994]. It is shown that the ripple resonance leads to a reduction in the runaway beam energy if the runaway production is dominated by the Dreicer generation process; however, the effect will be negligible if the secondary generation mechanism is included. The effect of anomalous radial runaway losses induced by enhanced magnetic fluctuations is stronger. Large enough levels of magnetic fluctuations, leading to runaway electron loss rates in excess of 103 s−1, can efficiently limit the number and energy of the runaway electrons.
A high-frequency surface wave pumped He-Ne laser
Authors: Moutoulas C. , Moisan M. , Bertrand L. , Hubert J. , Lachambre J. L. , Ricard A.
Published: July 2002

A new electrodeless He-Ne laser using a plasma produced by an electromagnetic surface wave as the active medium is described. Gain measurements are reported as a function of the pump wave frequency from 200 to 915 MHz. The dependence of laser performance on the gas mixture and pressure is also presented.
Effect of wall material on H− production in a multicusp source
Authors: Leung K. N. , Ehlers K. W. , Pyle R. V.
Published: July 2002

The effect of wall material on volume H− production in a magnetically filtered multicusp source has been investigated. Under the same discharge conditions, Al and Cu generally produce the highest H− ion current. It is shown that secondary electrons emitted from wall surfaces can account for the difference in H− yield.
Donor neutralization in GaAs(Si) by atomic hydrogen
Authors: Chevallier J. , Dautremont-Smith W. C. , Tu C. W. , Pearton S. J.
Published: July 2002

Hydrogen plasma exposure of n-type GaAs(Si) at 250 °C results in a decrease of the free-carrier concentration by several orders of magnitude. This neutralization effect has been demonstrated in silicon-doped layers grown by molecular beam epitaxy or formed by annealed implants as well as in bulk material. The same effect is produced electrochemically (H3PO4 electrolyte), whereas helium plasma exposure has no effect, thus confirming the role of hydrogen insertion. The hydrogen penetration depth into GaAs(Si) is inversely dependent on the Si concentration. Recovery of the electrical activity follows first order dissociation kinetics with a dissociation energy of 2.1 eV. Complete restoration of free-carrier concentration occurs by heating at 420 °C for less than 3 min. Extrapolated to low temperatures, these results imply many years of stability at 150 °C or below.
Elemental imaging of biological specimens using a z pinch
Authors: Weinberg Irving N. , Fisher Amnon
Published: July 2002

A small scale (5 kJ stored electrical energy) gas puff z-pinch device has been used as an intense tunable flash x-ray source for high-resolution (15-nm spatial resolution) contact microscopy of unstained biological specimens. The selection of gas injected into the device to form the plasma determined the radiated spectrum. When the gas was varied, areas of different chemical composition within the specimens were selectively imaged.
Photopyroelectric scanning microscopy
Authors: Faria I. F. , Ghizoni C. C. , Miranda L. C. M.
Published: July 2002

A new pyroelectric technique for the thermal wave scanning microscopy is presented. The potentiality of the method for probing and characterizing defects in solid samples is tested for surface and subsurface defects.
Investigation of orientation effect on contact resistance in selectively doped AlGaAs/GaAs heterostructures
Authors: Kamada M. , Suzuki T. , Nakamura F. , Mori Y. , Arai M.
Published: July 2002

We established that the contact resistance to the two-dimensional electron gas (2DEG) in selectively doped n-AlGaAs/GaAs heterostructure is crystal orientation dependent. The contact resistance in the [011] direction is the lowest and that in the [01̄1] direction is the highest. The contact resistance monotonically changes between the [011] and [01̄1] directions. We also find the sheet resistance dependence of the contact resistance.
Photothermal and photoconductive determination of surface and bulk defect densities in amorphous silicon films
Authors: Smith Z E. , Chu V. , Shepard K. , Aljishi S. , Slobodin D. , Kolodzey J. , Wagner S. , Chu T. L.
Published: July 2002

The sub-band-gap optical absorption spectra of high-quality hydrogenated amorphous silicon (a-Si:H) films are shown to be dominated by surface and interface state absorption when measured by photothermal deflection spectroscopy (PDS), while spectra determined using the constant photocurrent method (CPM) are not. For bulk defect states (both as-deposited and light-induced), the integrated subgap absorption is approximately twice as large for PDS as for CPM. Similarly, the conversion factor relating integrated subgap absorption with neutral dangling bond density is twice as large for CPM as PDS. This factor of 2 results from CPM seeing only transitions from below midgap into the conduction band while PDS sees transitions from the valence band into states above midgap as well.
Modeling UV response of rear-surface sensitized charge-coupled devices
Authors: Peckerar M. C. , Bosiers J. T. , McCarthy D. , Saks N. S. , Michels D. J.
Published: July 2002

Rear-surface illuminated charge-coupled devices (CCD’s) are typically insensitive to radiation whose depth of penetration is less than 1000 Å. This is due to the presence of immobile charges either in the rear-surface native oxide or at the native oxide/semiconductor interface. These fixed charges channel signal charge away from the front-surface confinement minima. The result is a loss of detective quantum efficiency (DQE), which is defined as the number of signal charges actually collected divided by the number produced. The CCD can be sensitized to shallow-penetrating radiation through the use of ion implantation techniques. To model the sensitization process, four factors must be accounted for. They are: the extent of the rear-surface depletion created by the fixed charge, Debye length spreading of the mobile charge background in the semiconductor, the position of the implant maximum, and the gradient of the active implant charge before the implant maximum is reached. By judicious manipulation of these factors DQE’s as high as 20% (uncorrected for UV reflection loss) have been obtained at 1216 Å.
Correlation of interface composition and barrier height for model AuGeNi contacts to GaAs
Authors: Waldrop J. R. , Grant R. W.
Published: July 2002

Model contacts to GaAs that include nonalloyed layered structures of Au, Ge, and Ni in various combinations are used to establish a correlation between interface composition and large changes in barrier height φB. The interface Fermi level EiF and chemistry during initial contact formation were investigated by x-ray photoemission spectroscopy; the corresponding φB for the thick contact was obtained by current-voltage (I-V) measurement. The circumstances under which a thin (∼10 Å) Ge layer at the GaAs interface can produce φB =∼0.25–0.4 eV (as measured by I-V) are described. For all model contacts examined a φB range from ∼0.25 to 0.9 eV is observed. This result questions the usual assumption of a relatively fixed φB of ∼0.8 eV for the alloyed AuGeNi contact and offers an alternative explanation for the mechanism of ohmic contact formation. The conditions that define the exceptionally low φB contacts provide a guide for the design of nonalloyed tunnel ohmic contacts.
Formation of a shallow p+ layer on InP using a P/Be co-implant
Authors: Wang Kou-Wei
Published: July 2002

A novel p-type implantation using a co-implant of phosphorus and beryllium to produce a shallow p+ surface layer is reported. The results from electrical measurement show an excellent control of the carrier profile. There is no Be in-diffusion observed and an improvement of the electrical activation also resulted.
Laser-induced homoepitaxial growth of gallium arsenide films
Authors: Chu Shirley S. , Chu T. L. , Chang C. L. , Firouzi H.
Published: July 2002

Epitaxial gallium arsenide films have been deposited on single crystalline GaAs substrates of (100) orientation at 425–500 °C by ArF excimer laser-induced metalorganic chemical vapor deposition. The important process parameters include the cleanliness of the substrate surface, substrate temperature, the composition, flow rate, pressure of the reaction mixture, and the pulse energy and pulse rate of the laser. Particular attention was directed to the in situ cleaning of the substrate surface prior to the deposition process. When the AsH3/(CH3)3Ga molar ratio in the reaction mixture exceeds 10, the deposited films have a specular mirror surface and show no structural features under a scanning electron microscope. Their single crystallinity has been confirmed by transmission electron microscopy. The carrier concentration decreases with increasing AsH3/(CH3)3Ga molar ratio and with decreasing substrate temperature, as expected. The carbon concentration in the deposited films, determined by secondary ion mass spectrometry, is in the range of 5×1017 to 2×1018 cm−3.
Laser-controlled dissociation and ionization pathways in electronically excited AsH3
Authors: Koplitz B. , Xu Z. , Wittig C.
Published: July 2002

Experiments involving 193 nm AsH3 excitation are described. With ≥25 ns delay between the firing of photolysis and probe lasers, product H atoms are detected by two-frequency, two-photon ionization (121.6 nm+364.7 nm) via Lyman-α. However, temporally overlapping the photolysis and ‘‘probe’’ beams enables ionization to complete with dissociation. The resulting AsH+x signal displays a marked dependence on the near-UV frequency, and AsH+x peaks are accompanied by dips in the H-atom signal. Delay and near-UV frequency control the competition between the different pathways, and the overall effect is both easily induced and macroscopic. Applications may be found in the photoassisted growth of semiconductor materials.