bluuebunny/crossref_metadata_2025 huggingface dataset:
Two- and three-dimensional instabilities of the cylinder wake in an aligned magnetic field
Authors: Mutschke G. , Gerbeth G. , Shatrov V. , Tomboulides A.
Published: July 2002

Two- and three-dimensional (2-D and 3-D) instabilities in the wake of a circular cylinder placed in an electrically conducting fluid and subjected to a constant magnetic field aligned with the freestream are investigated numerically. Increasing magnetic fields suppress 2-D instability (vortex shedding), whereas 3-D instabilities are influenced in a more complex way. In the presence of a magnetic field, 3-D instability has been detected below the 2-D stability threshold. This can lead to a reversal of the order of instabilities, i.e., 3-D instability appears at lower Re than 2-D instability.
On the relaxation of laminar jets at high Reynolds numbers
Authors: Og̃uz Hasan
Published: July 2002

In this paper we study several problems involving two dimensional and axisymmetric jets issuing in a medium with negligible dynamical effects: (1) a two-dimensional jet entirely bounded by free surfaces, (2) a two-dimensional jet bounded by a free surface and a wall, (3) an axisymmetric jet and (4) a hollow annular jet in a tube. The theoretical study is based on a new technique in which, on the basis of a boundary layer approximation, the velocity components are expanded into a suitable set of eigenfunctions. The deformation of the jet, that is assumed to be a slow function of the distance from the jet exit, is handled by a scaled coordinate system. For the case of the axisymmetric jet, which is the only one previously considered in the literature, this technique is found to be in excellent agreement with previously published, much more computationally demanding results. An experiment involving a vertical water jet has been carried out to further verify the accuracy of the current technique. In particular the behavior very near the exit was found to be in excellent agreement with the experiment at high Reynolds numbers. The fact that the jet relaxation time is much shorter than the time obtained from dimensional arguments is explained with the aid of the semi-analytical structure of the method. In the case of the annular wall jets, the length of the inner gaseous region is found to be finite depending on the Froude number.
Hydroelastic instability threshold in a turbulent boundary layer over a compliant coating
Authors: Reutov V. P. , Rybushkina G. V.
Published: July 2002

In this paper we are concerned with a linear problem of hydroelastic instability in a turbulent boundary layer over a compliant coating of a ribbon-like material. A dispersion relation for hydroelastic waves is written in terms of complex stiffness coefficients that characterize the deformation of the compliant surface and the turbulent flow response to the wavy surface flexure. A membrane-type approximation is proposed for the stiffness coefficient of solid coating at sufficiently low frequencies. In order to evaluate the turbulent flow response, the eddy-viscosity model in a curvilinear coordinate system is employed. The quasipotential approximation for the description of flow stiffness is confirmed on the basis of the computations. The critical flow speed and the parameters of the hydroelastic waves at the instability threshold are determined both within general definitions for the stiffness coefficients and for their approximations. The computational results are compared with available experimental observations.
New experimental observations of vortical motions in transverse jets
Authors: Kelso R. M. , Lim T. T. , Perry A. E.
Published: July 2002

Using dye visualization techniques, we have observed vortex structures within the jet region of round transverse jets which, to the best of our knowledge, have not been reported before. These structures confirm the existence of a pair of negative bifurcation lines in the surface flow of the pipe and the conjecture of Abramovich (1963) concerning additional vortical motions within the jet itself.
The three-dimensional vortical nature of atmospheric and oceanic turbulent flows
Authors: Dritschel David G. , de la Torre Juárez Manuel , Ambaum Maarten H. P.
Published: July 2002

Using a novel numerical method at unprecedented resolution, we demonstrate that structures of small to intermediate scale in rotating, stratified flows are intrinsically three-dimensional. Such flows are characterized by vortices (spinning volumes of fluid), regions of large vorticity gradients, and filamentary structures at all scales. It is found that such structures have predominantly three-dimensional dynamics below a horizontal scale L≈12LR, where LR is the so-called Rossby radius of deformation, equal to the characteristic vertical scale of the fluid H divided by the ratio of the rotational and buoyancy frequencies f/N. The breakdown of two-dimensional dynamics at these scales is attributed to the so-called “tall-column instability” [D. G. Dritschel and M. de la Torre Juárez, J. Fluid. Mech. 328, 129 (1996)], which is active on columnar vortices that are tall after scaling by f/N, or, equivalently, that are narrow compared with LR. Moreover, this instability eventually leads to a simple relationship between typical vertical and horizontal scales: for each vertical wave number (apart from the vertically averaged, barotropic component of the flow) the average horizontal wave number is equal to f/N times the vertical wave number. The practical implication is that three-dimensional modeling is essential to capture the behavior of rotating, stratified fluids. Two-dimensional models are not valid for scales below LR.
Inviscid instability of the Batchelor vortex: Absolute-convective transition and spatial branches
Authors: Olendraru Cornel , Sellier Antoine , Rossi Maurice , Huerre Patrick
Published: July 2002

The main objective of the study is to examine the spatio-temporal instability properties of the Batchelor q-vortex, as a function of swirl ratio q and external axial flow parameter a. The inviscid dispersion relation between complex axial wave number and frequency is determined by numerical integration of the Howard–Gupta ordinary differential equation. The absolute-convective nature of the instability is then ascertained by application of the Briggs–Bers zero-group-velocity criterion. A moderate amount of swirl is found to promote the onset of absolute instability. In the case of wakes, transition from convective to absolute instability always takes place via the helical mode of azimuthal wave number m=−1. For sufficiently large swirl, co-flowing wakes become absolutely unstable. In the case of jets, transition from absolute to convective instability occurs through various helical modes, the transitional azimuthal wave number m being negative but sensitive to increasing swirl. For sufficiently large swirl, weakly co-flowing jets become absolutely unstable. These results are in good qualitative and quantitative agreement with those obtained by Delbende et al. through a direct numerical simulation of the linear response. Finally, the spatial (complex axial wave number, real frequency) instability characteristics are illustrated for the case of zero-external flow swirling jets.
Asymptotic analysis of a three-dimensional Bridgman furnace at large Rayleigh number
Authors: Foster M. R.
Published: July 2002

We now know that the operation of a vertical Bridgman furnace, used for solidifying a dilute binary alloy, may be optimized, in the sense of minimizing radial material segregation, by a proper choice of the geometrical parameters of the insulation zone that surrounds the crystal-melt interface. We have found [Phys. Fluids 9, 683 (1997)], as an extension of the work of Tanveer [Phys. Fluids 6, 2270 (1994)], that small surface tension reduces the radial segregation but does not modify that optimization condition. Most previous analyses have been done for axisymmetric ampoules. However, in a laboratory setting, it may not be possible to assure axisymmetry, due to, say, azimuthal variations in the heat transfer or a small tilt of the ampoule axis. We find that nonaxisymmetric effects lead to optimization conditions similar to their axisymmetric counterparts. We include in the analysis geometric (Gibbs–Thomson) and concentration effects in melt temperature; the former effect decreases the segregation, and the latter increases segregation—for either axisymmetric or asymmetric thermal loading. We identify the mechanism for the trend noted by other investigators in prior numerical work: even very small asymmetry causes large increases in crystalline segregation.
Four-vortex motion with zero total circulation and impulse
Authors: Aref Hassan , Stremler Mark A.
Published: July 2002

The problem of four interacting point vortices on the unbounded plane with vanishing total circulation and vanishing impulse is reduced to a three-body problem analogous to the three-vortex problem on the unbounded plane. A “phase plane analysis” using trilinear coordinates, similar to that used for the three-vortex problem, is presented and used to discuss details of the motion. The methodology and results complement earlier analyses of the same problem by Eckhardt and Rott.
Quasilinear interaction of m=1 and m=0 modal components with the mean fields in the Extrap T1 reversed-field pinch
Authors: Nordlund P.
Published: July 2002

Experiments have been performed on the Extrap T1 reversed-field pinch [Phys. Scr. 44, 358 (1991)] where the detailed radial structure of the mean profiles, together with associated edge field fluctuation spectra, have been studied at different values of the pinch parameter 1.6≤Θ≤2.5. It is found that the parallel current density is strongly suppressed inside the resonances of the most active m=1 modes, resulting in hollow μ profiles near the axis. This effect introduces a spectral shift in n space between the dominant m=1 modes and the leading linear growth rates. At high Θ a local flattening of μ around the reversal (m=0 resonance) surface becomes apparent. As a result a second set of m=1 modes resonant near the off-axis peak in μ becomes linearly unstable. In this case, the resulting m=1 spectrum is double peaked for the internal modes. The experimental results show several characteristic similarities with recent numerical magnetohydrodynamic simulations of the reversed-field pinch dynamo.
Effects of radio-frequency-induced radial diffusion on triton burnup
Authors: Yamagiwa M.
Published: July 2002

Effects of radio-frequency (RF)-induced radial diffusion on triton burnup are studied for D(3He) plasmas with an ion cyclotron range of frequencies heating in the fundamental minority regime. The triton distribution is determined from a Fokker–Planck equation including a radial diffusion term. Provided that the RF power is extremely localized in the plasma center, tritons are driven out from the center because of RF-induced diffusion in the second harmonic resonance and the emission profile of the 14 MeV neutrons due to deuterium tritium (D–T) reactions can possibly be flattened appreciably. This may lead to verification of RF-induced radial diffusion of the ions only through neutron detection.
Gyrotrons and free electron lasers as atmospheric sensors*
Authors: Manheimer Wallace M.
Published: July 2002

Gyrotrons and free electron lasers have recently increased both the power and tunability of, respectively, millimeter wave and infrared sources by several orders of magnitude. As radiation at these wavelengths interact strongly with atmospheric aerosols and trace impurities, these sources have a great potential as atmospheric sensors. They open the possibility of increasing by at least an order of magnitude both the range and sensitivity of these atmospheric sensors.
Harmonic resonance of electrons in combined helical wiggler and axial guide fields
Authors: Shu Xiaojian
Published: July 2002

The single-particle orbits of relativistic electrons in combined helical wiggler and axial guide magnetic fields have been studied. Based on the numerical simulations of the Raman free electron laser experiment in Shanghai Institute of Optics and Fine Mechanics, a new resonance of off-axis electrons has been shown and analyzed. This resonance is found to occur in velocity space of electrons with a beat between Ω1 and 2Ω2 (Ω1 and Ω2 are the harmonic of wiggler motion and the cyclotron frequency respectively in a frame rotating with the wiggler), in other words, there is a harmonic resonance at 2Ω̄∥=3Kwν∥ (Ω̄∥ is cyclotron frequency, Kwν∥ is the wiggling frequency). This resonance depends on the radial displacement of the electron from axis, but without any large radial drifts in position space, which provides an explanation of a dip in radiation power that has been observed in the experiment.
Interaction between resonant magnetic perturbation and the rotating magnetic island
Authors: Mao Wenjin , Wang Maoquan
Published: July 2002

Nonlinear interaction between an external static resonant magnetic perturbation (RMP) and the corresponding rotating saturated magnetic island in tokamak plasma has been numerically simulated. The influence of flow shear driven by this nonlinear interaction is considered. It is found that applied resonant magnetic perturbations can effectively reduce the width of spontaneous magnetic island so that the amplitude of Mirnov signal is suppressed and the velocity profile is changed as well.
Impurity poloidal rotation velocity in tokamaks
Authors: Meigs A. G. , Rowan William L.
Published: July 2002

The impurity poloidal flow velocity, vθz, was measured for Ohmic discharges in banana, plateau, and Pfirsch–Schluter regimes in the Texas Experimental Tokamak (TEXT) [K. W. Gentle, Nucl. Tech./Fusion 1, 479 (1981)]. The velocities were measured spectroscopically using the Doppler shift of a C+4 emission line, and then compared with neoclassical predictions that were evaluated with sets of measured plasma profiles that included the radial electric field. Specifically, neoclassical theory was used to relate vθz to measured radial profiles, but it was not used to predict any profiles other than vθz(r). On that basis, neoclassical theory adequately describes vθz. The prediction is dominated by the impurity and ion density scale lengths. Contributions proportional to the parallel viscosity ratio do not contribute significantly to the prediction in this experiment, and thus are not tested. Similarly, though charge exchange momentum loss may significantly retard the toroidal flow velocity and introduces an unusual electric field dependence into the poloidal flow velocity, the charge exchange contribution to vθz is too small to be detected.
Investigation of breakdown voltage curves for pure helium and silane–helium mixtures
Authors: Borg Dezani V. , Ginoux J. L.
Published: July 2002

The breakdown voltage versus pressure for pure helium and several silane–helium mixtures between coaxial cylinders were measured during an alternative current corona silent discharge at low frequency (3000 Hz) from 104 to 1.5×105 Pa at constant temperature. The differences observed between pure helium breakdown voltages and silane–helium breakdown voltages were analyzed revealing a breakdown streamer mechanism.
Explosion of soliton in a magnetic field
Authors: Nishinari Katsuhiro , Abe Kanji , Satsuma Junkichi
Published: July 2002

A dynamics of a solitary pulse of the electrostatic ion cyclotron wave that propagates perpendicular to an applied magnetic field is considered. It is shown that the solitary wave will be singular in some range of parameters in the system, such as the plasma density and the magnitude of an applied magnetic field. This fact shows that there is a possibility of controlling the place where explosion of the solitary wave occurs.
Parametric excitation of a lower-hybrid drift wave in the ionosphere
Authors: Sawhney B. K. , Tripathi V. K. , Singh S. V.
Published: July 2002

Parametric excitation of a lower-hybrid drift (LHD) wave has been studied in the ionosphere due to the propagation of a high-frequency electromagnetic (EM) pump wave (with wave number k0≊0) in the presence of the electron collisions. It is considered to be a four wave interaction process of LHD, electromagnetic pump wave, and two electron plasma waves as sidebands. Nonlinear growth rate of a LHD wave is found to be higher for 1<krLe<1.5 as compared to the case krLe<1, where k and rLe, respectively, are the wave number of the LHD wave and electron Larmor radius. Wavelengths of the excited lower-hybrid drift waves lie in the range of the various irregularities observed by Jicamarca [Farley et al., J. Geophys. Res. 75, 7199 (1970)], and Altair and Tradex radars [R. T. Tsunoda, Geophys. Res. Lett. 7, 848 (1980)]. It is seen that the nonlinear growth rate drops significantly in the presence of the electron collisions.
Dynamics and dissipation of compressional Alfvén waves near magnetic nulls
Authors: Hassam A. B.
Published: July 2002

It is shown that compressional magnetohydrodynamic (MHD) wave modes excited in the macroscopic vicinity of a magnetic field null in an otherwise strongly magnetized plasma damp at a rate consistent with a Q of order ln[LVA/η], where L is the characteristic length scale of the magnetic configuration, VA is the typical Alfvén speed, η is the relevant diffusion coefficient, such as the resistivity, and by Q we mean the ratio real frequency/damping rate. This result is shown to be valid for two-dimensional nulls, where the field is zero on some line, as well as for three-dimensional nulls, i.e., for cusp-like configurations. Thus field nulls are extremely efficient convertors of compressional MHD wave energy to thermal energy.
Bootstrap current and ion thermal conductivity in an ultralow-aspect-ratio tokamak
Authors: Shaing K. C. , Hsu C. T. , Yokoyama M. , Wakatani M.
Published: July 2002

The bootstrap current Jb=−IcP′B/〈B2〉 and χi= √2NνiMiTi(Ic/e)2〈1/B2〉/‖∇ψ ‖2 are valid for both the banana and the Pfirsch–Schlüter regimes for any finite value of the collision frequency at a radius where the local aspect ratio A approaches unity. Here, I=RBt with R the major radius, Bt the toroidal magnetic field strength, and the prime denoting the derivative with respect to the poloidal flux ψ. Thus, the bootstrap current does not vanish, even in the collisional regime, when A approaches unity. The physical reason for this dramatic result is that the magnitudes of the electron and ion parallel viscosity approach infinity as A approaches unity. This also indicates that the conventional theory underestimates the magnitude of bootstrap current in an ultralow-aspect-ratio tokamak.
Observations of plasma dynamics in the vacuum spark
Authors: Chuaqui H. , Favre M. , Wyndham E. S. , Aliaga Rossel R. , Mitchell Ian , Soto L.
Published: July 2002

Experimental observations are presented of a vacuum spark driven by a low impedance pulse forming line delivering 100 kA to the load. A pulsed laser is used to form a preionizing plasma on the cathode. The combination of axial and radial optical streak camera observations, together with the time and space resolved soft x-ray emission, permit the evolution of the plasma dynamics, density and temperature to be measured. Three kinds of behavior are observed according to axial position. A close correlation is found between the x-ray and the optical emission, with the observation of micropinch formation. A discussion is presented in which the behavior of the vacuum spark under differing operating conditions is compared.
Response to Comments on ‘‘Alfvén ‘resonance’ reconsidered: Exact equations for wave propagation across a cold inhomogeneous plasma’’ [Phys. Plasmas 1, 3523 (1994)]
Authors: Bellan P. M.
Published: July 2002

We agree with both Goedbloed and Lifschitz (whom we will refer to as GL) and Ruderman, Goossens and Zhelyazkov (RGZ) that the apparent violation of quasi-neutrality in Ref. is not a true problem and rescind this particular argument against MHD. However, the rest of the discussion in our paper is independent of this argument regarding quasi-neutrality, and as discussed below, the conclusions in Ref. about the validity of MHD, about compressibility versus incompressibility, and about Alfvén resonance remain valid.
Propagation of nonlinear pulses in a non-neutral plasma slab
Authors: Neu S. C. , Morales G. J.
Published: July 2002

The axial propagation of linear and nonlinear pulses in a strongly magnetized non-neutral plasma slab is studied using a recently developed particle simulation code [Ramachandran et al. Phys. Fluids B 5, 2733 (1993)]. These pulses are generated by suddenly applying a negative potential to wall antennas on the confining walls. Theoretical predictions are compared to electrostatic particle simulations over a wide range of excitation amplitudes, and are shown to be in excellent agreement. A simple theoretical model is developed to describe the formation of the observed electron bursts and the associated wave breaking that arises for very large antenna potentials.
Parametric up-conversion of electromagnetic waves by a gyrating relativistic electron beam
Authors: James Litty , Jassal Lalita , Tripathi V. K.
Published: July 2002

A gyrating relativistic electron beam supports negative energy modes near the harmonics of the electron cyclotron frequency. An electromagnetic wave passing through such a beam parametrically up-converts into high frequency electromagnetic radiation, producing a beam cyclotron mode. In the case when pump frequency ω0 is away from the harmonics of cyclotron frequency, ω≠lωc/γ0 and ω≫ωc/γ0, the beam response to the pump and sideband waves is obtained by the fluid theory whereas the response at the beam cyclotron mode is kinetic. In the case when ω0∼lωc/γ0 the beam response to all three interacting waves is taken to be fully kinetic. It is seen that the normalized growth rate decreases as ω0−ωc/γ0 increases for Raman as well as Compton regimes of operation.
Fast wave propagation studies in the DIII-D tokamak
Authors: Ikezi H. , Pinsker R. I. , Chiu S. C. , DeGrassie J. S.
Published: July 2002

Fast Alfvén waves radiated from the phased array antenna in the DIII-D tokamak [L. G. Davis et al. in Proceedings of the 12th IEEE Symposium on Fusion Engineering, Piscataway, 1987 (Institute of Electrical and Electronics Engineers, New York, 1987), p. 991] and used for heating and current drive are studied by employing a Ḃ-loop array mounted on the vacuum vessel wall. The wave propagation direction controlled by the antenna phasing is clearly observed. A small divergence of the rays arising from the anisotropic nature of the fast wave is found. Comparison with a ray tracing code confirms that the ray position calculated by the code is precise up to at least one toroidal turn of the rays. Conservation of ktR, which is a basic assumption in computer codes, is tested. Although the upshift of toroidal wave number kt at small major radius R is confirmed, ktR is not well conserved. The so-called ‘‘eigenmode’’ is observed and the cause is identified. A mass density interferometer is demonstrated by employing the extraordinary fast wave.
Joule heating of a current layer due to plasma density inhomogeneities
Authors: Zabaidullin O. Z. , Vikhrev V. V.
Published: July 2002

In the present paper the magnetic field dynamics in the frame of electron magnetohydrodynamics (EMHD) theory is considered in the case of a current passing through a plasma of inhomogeneous density. It is shown: (1) due to the Hall effect the Joule heating of a current layer increases up to (ωτ)ei times the Joule heating of ordinary homogeneous plasma. (2) The EMHD magnetic field dynamics can be considered as an enhanced magnetic field diffusion in the frame of MHD theory, using ‘‘efficient’’ coefficients of plasma conductivity and of magnetic field diffusion. The ‘‘efficient’’ coefficient of the magnetic field diffusion cT/10eH in the case of magnetic field-plasma equilibrium is the same order of magnitude as the Bohm’s coefficient cT/16eH.
Comment on ‘‘Free electron laser with longitudinal wiggler and finite magnetic field in a partially filled waveguide’’ [Phys. Plasmas 3, 1130 (1996)]
Authors: Shoucri Magdi
Published: July 2002

The dispersion equation for the eigenmodes of a waveguide partially filled with a relativistic electron beam in a magnetic field has been recently reported. It is the purpose of the present Comment to point out that this dispersion equation is incorrect.
Stability of magnetic vortices with flow in anisotropic magnetohydrodynamics
Authors: Kinney R. , McWilliams J. C. , Wolansky G.
Published: July 2002

The eigenvalue problem for linear stability of concentric radial profiles of current and vorticity in reduced forms of three-dimensional magnetohydrodynamics is solved numerically. Arbitrary relative amplitudes of the velocity and magnetic fields are considered. Vorticity profiles are unstable if nonmonotonic, but are stabilized by a poloidal magnetic field when the on-axis vertical current is at least as large as the on-axis vertical vorticity. Nonmonotonic current profiles are less efficient at stabilization. When the neutral modes have vertical structure, an added poloidal magnetic field does not stabilize the mode unless the vertical field is also moderately strong. Current profiles in which the integrated current changes sign, although spectrally stable, are shown to be nonlinearly unstable via both numerical solution and Lyapunov techniques.
Vlasov description of implosion heating
Authors: Ågren Olov
Published: July 2002

An analytical treatment of plasma heating by a fast compression of a solenoidal field is given. An annular cylindrical plasma with a radius of 1 m is considered for the initial state, since this optimizes the nonadiabatic heating. The theory predicts that a 0.13 μs compression with 3 MV circuit voltage can give as much as 200 keV mean ion kinetic energy. The plasma beta increases by a factor of 100 in the compression. With the voltage decreased to 1 MV and a 0.36 μs compression time, the calculated mean ion kinetic energy is 60 keV. The calculated number of ions per unit length that gives tolerable magnetic shielding is 4×1017 m−1. Some practical concerns such as endplate construction, control of plasma beta, electric insulation, and transportation of the compressed plasma along a tube are briefly analyzed.
Effects of variable x-ray preheat shielding in indirectly driven implosions
Authors: Landen O. L. , Keane C. J. , Hammel B. A. , Levedahl W. K. , Amendt P. A. , Colvin J. D. , Cable M. D. , Cook R. , Dittrich T. R. , Haan S. W. , Hatchett S. P. , Hay R. G. , Lerche R. A. , McEachern R. , Murphy T. J. , Nelson M. B. , Suter L. , Wallace R. J.
Published: July 2002

The performance of indirectly driven fusion capsules has been improved by mid Z doping of the plastic capsule ablator. The doping increases x-ray preheat shielding leading to a more isentropic compression, higher convergence, and higher neutron yield. A 4× increase in neutron yield is both calculated and observed as the Ge doping level is increased from 0% to 3% by atomic fraction. A predicted 40% decrease in x-ray image core size with increasing Ge content is confirmed.
Generation of poloidal magnetic field in a hot collisional plasma by inverse Faraday effect
Authors: Srivastava M. K. , Lawande S. V. , Dutta Debjani , Sarkar S. , Khan M. , Chakraborty B.
Published: July 2002

Generation of poloidal magnetic field in a hot and collisional plasma by an inverse Faraday effect is discussed. This field can either be induced by a circularly polarized laser beam (CPLB) or a plane-polarized laser beam (PPLB). For the CPLB, an average field 〈ℜx〉∼I0λ∼11.6 MG could be produced in a DT plasma for a high intensity (I0=1022 W/m2) and shorter wavelength (λ=0.35 μm) laser. This field is essentially induced by the field inhomogeneity effect and dominates over that induced by the plasma inhomogeneity effect (〈ℜx〉∼I2/30λ7/3∼2.42 MG). The collisional and thermal contribution to 〈ℜx〉 is just negligible for the CPLB. However, in the case of PPLB the poloidal field is generated only for a hot and collisional plasma and can be quite large for a longer wavelength laser (e.g., CO2 laser, λ=10.6 μm). The collisional effect induces a field 〈ℜx〉∼0.08 kG, which dominates near the turning point and is independent of the laser parameters. However, in the outer cronal region the thermal pressure effect dominates (e.g., 〈ℜx〉∼I5/30λ4/3∼3.0 MG). Further, 〈ℜx〉 for the p-polarized beam is, in general, relatively smaller than that for the s-polarized beam. Practical implications of these results and their limitations are discussed.
Stability of an inhomogeneous transverse plasma flow
Authors: Ganguli G.
Published: July 2002

The stability of a magnetized plasma that includes a sheared transverse flow is analyzed by using both fluid and kinetic formalisms. In addition to the well known Kelvin–Helmholtz modes it is found that another branch of oscillation exists which can dominate the collective effects in a plasma if the magnitude of shear in the transverse flow is sufficiently strong. The source of free energy for the new branch is an inhomogeneity in the energy density caused by the velocity shear. Kelvin–Helmholtz modes, when examined with a fluid theory, are found to be robust and therefore have dominated the analysis of plasma systems with velocity shear in both laboratory and space plasmas. However, when a kinetic formalism is applied to Kelvin–Helmholtz modes it is found that these modes are strongly Landau damped especially when the ion temperature is comparable to or larger than the electron temperature. In addition, since the Kelvin–Helmholtz mode is dependent explicitly on the second derivative of the flow it is sensitive to the profile of the flow. On the other hand, the new branch is dependent on the localized nature of the flow and hence it is less sensitive to the details of the flow profile. The two branches of oscillation are compared using both fluid and kinetic theories and their regimes of dominance discussed.
Conditions for the existence of steady state stellarators
Authors: Weitzner Harold
Published: July 2002

Low collisonality methods developed earlier [Phys. Plasmas 1, 3942 (1994)] are extended and applied to the characterization of steady state, toroidally confined non-symmetric plasmas. A magnetic field is assumed which is a small perurbation of one with exact magnetic surfaces and zero shear. However, exact magnetic surfaces are not required. Two types of conditions on the system are found. In analogy with tokamaks, conditions of energy flux balance and particle flux balance are obtained. The fluxes are of magnitude comparable with those in a tokamak. Another class of constraints relates to properties of the magnetic field B. In addition to the condition of very small shear [Phys. Plasmas 2, 3595 (1995)] and the condition that on a flux surface Σ|B⋅∇B|−1 be a function of B [Phys. Plasmas 4, 575 (1997)], as extended, a hierarchy of increasingly restrictive conditions on B are found. The consequence of these constraints is explored very briefly.
Dynamic magnetic reconnection in three space dimensions: Fan current solutions
Authors: Craig I. J. D. , Fabling R. B.
Published: July 2002

The problem of incompressible, nonlinear magnetic reconnection in three-dimensional “open” geometries is considered. An analytic treatment shows that dynamic “fan current” reconnection may be driven by superposing long wavelength, finite amplitude, plane wave disturbances onto three-dimensional magnetic X-points. The nonlinear reconnection of the field is preceded by an advection phase in which magnetic shear waves drive large currents as they localize in the vicinity of the magnetic null. Analytic arguments, reinforced by detailed simulations, show that the ohmic dissipation rate can be independent of the plasma resistivity if the merging is suitably driven.
Structure and dynamics of current sheets at Alfvén resonances in a differentially rotating plasma
Authors: Wang X. , Bhattacharjee A. , Ma Z. W. , Ren C. , Hegna C. C. , Callen J. D.
Published: July 2002

Alfvén resonances, where the local flow speed relative to the boundary is equal to the local Alfvén speed, introduce novel dynamical features in a differentially rotating plasma. The spatial structure and dynamics of current sheets in such plasmas is investigated analytically as well as numerically. The current sheets at Alfvén resonances tend to power-law singularities. The growth of current sheets is algebraic in time in the linear regime and saturates in the presence of dissipation without the intervention of nonlinear effects. These results have significant implications for forced reconnection and Alfvén wave dissipation in laboratory and space plasmas.
Linear theory of gyrotron traveling wave tubes with nonuniform and lossy interaction structures
Authors: Kou C. S. , Tseng Fouries
Published: July 2002

A linear theory model is developed to analyze gyrotron traveling wave tubes (gyro-TWT) employing nonuniform interaction structures, which can be tapered wall waveguide, severed tubes, or distributed lossy wall waveguides. Effects of the wall loss, reflections caused by the nonuniformity of the waveguide, and beam–wave interactions inside the sever region are incorporated in this theory. In addition, self-excited oscillations in the gyro-TWT induced by absolute instabilities or reflections can also be studied by the same theory. Good agreements with the self-consistent particle-tracing code have been achieved. This theory can be used to provide a first-cut design of a stable gyro-TWT.
On the analogy between a system of harmonic oscillators and resonant Alfvén waves in plasmas
Authors: Ruderman M. S.
Published: July 2002

The steady state of the driven oscillations of the system consisting of N oscillators coupled by friction is considered. It is shown that for large values of N this state is asymptotically described by the F-function previously introduced in the theory of resonant magnetohydrodynamic (MHD) waves in plasmas.
Kinetic model of Alfvén wave light-ion gyroresonance heating
Authors: Chust T. , Le Quéau D.
Published: July 2002

The absorption of Alfvén waves by gyroresonant interaction with the light-ions in an ionospheric oxygen-hydrogen plasma with a parallel magnetic field gradient is reconsidered, taking into account simultaneously the plasma inhomogeneity and the finite temperature of the resonant ions (Ti≠0). A kinetic full wave equation is derived, that is valid in the vicinity where the wave frequency matches the local proton gyrofrequency. It is analytically solved for the case of a Lorentzian distribution function. The energy transmission, reflection and absorption coefficients, for waves incident from the high magnetic field side onto the gyroresonant interaction region, are found to be the same as for the cold plasma case (Ti=0). Conversely, the absorption of waves incident from the low magnetic field side is found to be enhanced and strongly depends on the ionic temperature, whereas their transmission to the high magnetic field side occurs still in the same proportion as for the cold plasma case. Elaborating local dispersion curves and evaluating the full wave solutions enable us to interpret qualitatively these results and to extrapolate them for the case of a Maxwellian distribution function. For realistic ionospheric plasma conditions a sensible increase of the gyroresonant absorption of the waves is thus found.
Dielectric tensor for inhomogeneous plasmas in inhomogeneous magnetic field
Authors: Gaelzer R. , Ziebell L. F. , Silveira O. J. G.
Published: July 2002

The derivation of explicit expressions for the effective dielectric tensor to be utilized in the dispersion relation for weakly inhomogeneous plasmas is discussed. The general expressions obtained are useful for situations with simultaneous existence of weak inhomogeneities in density and magnetic field. The particular case of a Maxwellian distribution in velocity space for the electron population is discussed, and relatively compact expressions for the dielectric tensor are obtained, which depend on the inhomogeneous plasma dispersion function introduced by [Gaelzer et al., Phys. Rev. E 55, 5859 (1997)] and ultimately on the well-known Fried–Conte function and its derivatives.
Theory of sheath in a multicomponent plasma
Authors: Mahanta M. K. , Goswami K. S.
Published: July 2002

The aim of this brief report is to study the formation of sheath in a multicomponent plasma. The plasma consists of electrons, positively charged ions, negatively charged massive ions, and beam electrons. It is assumed that the beam electron is ejected from the surface toward the plasma. The sheath edge potential and the floating potential are calculated. Applications of the findings are highlighted.
Dissipative electrostatic waves associated with the dust grain density in a nonequilibrium plasma
Authors: Nejoh Y. N.
Published: July 2002

A new nonlinear dissipative equation associated with the gravitational effect of dust grains is derived in a nonequilibrium plasma. Temporal evolution of the solution of this equation is shown by numerical calculation. It is shown that the nonlinear term of this equation forms the steepening and the dissipative effect damps the wave amplitude. Damping of the wave grows as the collision frequency increases. In the very low collision frequency, when the gravitational force becomes comparatively larger than the electrostatic force, the wave becomes somewhat unstable. On the other hand, when the collision frequency increases, i.e., the acceleration for the grains is comparatively larger, the behavior of the nonlinear wave becomes mild.
Drift wave simulations with reduced stellarator equilibria
Authors: Lewandowski J. L. V.
Published: July 2002

A three-field model to study drift-resistive, low-frequency waves in low-β, nonaxisymmetric plasmas [J. L. V. Lewandowski, Phys. Plasmas 4, 4023 (1997)] is used to analyze the effect of the inhomogeneities in the stellarator magnetic field on the fastest (linear) growth rate, γ. Extensive numerical calculations for a toroidal heliac show that not all Fourier components in the representation of the equilibrium configuration are important as far as γ is concerned.
Suppression of a pressure driven m=1 mode in a lower hybrid current drive plasma by electron cyclotron heating in the WT-3 tokamak
Authors: Yoshimura S. , Watanabe M. , Tanabe K. , Nakayama A. , Asakawa M. , Maehara T. , Nakamura M. , Tanaka H. , Maekawa T. , Terumichi Y.
Published: July 2002

A pressure driven m=1/n=1 mode is excited by lower hybrid current drive in the WT-3 tokamak [T. Maehara et al., Nucl. Fusion 38, 39 (1998)]. The excitation of the mode is accompanied with the decrease of the magnetic shear and with the peaking of the soft x-ray emissivity profile inside the q=1 surface. The crescent-shaped mode structure appeared on the contour map of the soft x-ray emissivity is consistent with that of the quasi-interchange mode. The m=1 mode can be suppressed by electron cyclotron heating near the q=1 surface. The range of the location of the electron cyclotron resonance layer effective for the complete suppression is much wider and the time scale for the suppression is much faster than those in the case of the suppression of the tearing mode in the ohmic heating plasma.
I. First-order calculation of the pressure tensor for cylindrical, Z-pinch plasmas
Authors: Whitney K. G.
Published: July 2002

Calculations of the pressure tensor in a two-dimensional (2D), Z-pinch geometry are presented that extend calculations previously published [K. G. Whitney, Phys. Plasmas 6, 816 (1999)], which dealt with a spherical, Inertial Confinement Fusion geometry. In a cylindrically symmetric [(r,z)] plasma, the electron pressure tensor has three independent, but coupled, components (rather than one as in a sphere). There are a variety of nonlinear contributions to the electron pressure tensor that depend bilinearly on gradients in temperature and density and on the current density. A general presciption for calculating their magnetic field and ionization state dependence is described in this paper. It is found, for example, that the contribution that comes from gradients in temperature has the same form involving three η coefficients as the classical contributions to the Z-pinch pressure tensor that come from gradients in fluid velocity. Moreover, all three coefficients of the temperature gradient terms are found to be much larger than the corresponding fluid velocity coefficients. Explicit formulas for all first-order pressure tensor coefficients are given in the appendixes.
Transport due to front propagation in tokamaks
Authors: Sarazin Y. , Garbet X. , Ghendrih Ph. , Benkadda S.
Published: July 2002

A low-dimensional transport model which emphasizes the competition between two turbulent relaxation mechanisms, namely profile relaxation and nonlinear mode coupling, is proposed. When decreasing the relative weight of this mode coupling, transport exhibits a transition from diffusive to avalanche-like regimes. The latter plasma turbulent transport can prove to be very powerful in describing experimental evidence, especially concerning transient dynamics.
The magnetized plasma permittivity tensor
Authors: Bergman Jan
Published: July 2002

An alternative form of the nonrelativistic permittivity tensor for a homogeneous magnetized plasma in thermal equilibrium is derived in the linear approximation. The derivation follows the lines of Trubnikov’s derivation of the relativistic permittivity tensor and is restricted to a collisionless electron plasma. In the case of a Maxwellian equilibrium distribution, the final form is equivalent to Gordeev’s integral form of the permittivity tensor. The analysis is, with minor modification, applicable to plasma comprised of multiple particle species. Unlike the standard formulas, the alternative formula presented in this article yields the components of the permittivity tensor by means of partial derivatives of a scalar kernel integral. Besides the cyclotron harmonic resonance frequencies, where the permittivities in the nonrelativistic approximation become singular, the alternative formula is manifestly valid for all wave frequencies without any restrictions on the wave vector and the thermal energy.
Variation of the effective Richardson constant of Pt-Si Schottky diode due to annealing treatment
Authors: Tōyama Naotake , Takahashi Tōru , Murakami Hironori , Kōriyama Hiroaki
Published: July 2002

The effect of heat treatment on the various diode parameters in a Pt-Si Schottky diode has been studied. It has been revealed from photoelectric measurements that heat treatment causes a distinct variation in the effective Richardson constant of the diode whereas Schottky barrier height scarcely shows such a large change as reported to date. Also, the value of the effective Richardson constant of a Pt-Si contact in the as-prepared state is found to depend strongly on the sputtered Pt film thickness in contrast to other metal-Si contacts such as Au-Si or Ag-Si.
High current ion beam generation and transport system
Authors: Roth I. S. , Ivers J. D. , Nation J. A.
Published: July 2002

The efficient transport of a 6.0-kA, 1.1-MV proton beam generated in a magnetically insulated diode and propagated along an axial magnetic field has been studied. The beam is better than 98% charge neutral. Focusing of the beam is observed as a result of beam transport through the radial magnetic insulation field. The application of this type of beam generation and transport system to a high current linear induction ion accelerator is discussed.
High external efficiency (36%) 5-μm mesa isolated GaAs quantum well laser by organometallic vapor phase epitaxy
Authors: Welch D. F. , Schaus C. F. , Shealy J. R.
Published: July 2002

High external power efficiencies of 36% are obtained from 5-μm mesa isolated graded-index separate-confinement heterostructure single quantum well lasers grown by organometallic vapor phase epitaxy. Threshold currents of 21 mA are reported for a 5×96 μm cavity laser with differential quantum efficiencies of as high as 80% at 6 mW/facet. In addition, high facet power densities of 13 mW/μm are reported giving single ended output powers of 65 mW from 5-μm uncoated facets. Even higher power densities of 20 mW/μm are observed for 1-μm mesa stripe lasers before probe damage occurred.
Observation of modulation speed enhancement, frequency modulation suppression, and phase noise reduction by detuned loading in a coupled-cavity semiconductor laser
Authors: Vahala Kerry , Paslaski Joel , Yariv Amnon
Published: July 2002

Simultaneous direct modulation response enhancement, phase noise (linewidth) reduction, and frequency modulation suppression are produced in a coupled-cavity semiconductor laser by the detuned loading mechanism.
Hydrogen profile in ion implanted polyethylene
Authors: Calcagno L. , Foti G.
Published: July 2002

Change of the hydrogen concentration induced by ion irradiation in the polyethylene films has been studied by MeV proton backscattering. Ion implantation of polyethylene film results in a strong breakup of chemical bonds in the chains with a large hydrogen emission leaving a carbon enriched film.These effects are confined in a cylinder around the ion track, of cross section 10−15 cm2 and a length comparable with the ion range.
Wavelength independent, optical damage immune Z-propagation LiNbO3 waveguide polarization converter
Authors: Thaniyavarn Suwat
Published: July 2002

A waveguide electro-optic TE-TM mode converter is demonstrated. The polarization conversion is independent of the operating wavelength and is compatible with a nontunable laser source. A high extinction ratio of more than 25 dB is reported. The device is fabricated on an X-cut, Z-propagation, Ti-indiffused LiNbO3 waveguide and is immune to the optical damage problem which normally limits the optical power handling capability of LiNbO3 devices.
Recombination mechanisms in Si and Si thin films determined by picosecond reflectivity measurements near Brewster’s angle
Authors: Fauchet P. M. , Nighan W. L.
Published: July 2002

A sensitive pump and probe method for measuring the transient carrier density in semiconductors is proposed and demonstrated in silicon. It relies on the magnified reflectivity changes when the picosecond probe laser is incident close to Brewster’s angle. We have measured the Auger recombination coefficient γ=2×10−31 cm6 s−1 and, in microcrystalline silicon films on insulator, the effective lifetime due to grain boundaries τ≂100 ps. Additionally, at much higher pump energies, upon melting, the reflectivity jumps by nearly one order of magnitude.
Fabrication of GaAs tunnel junctions by a rapid thermal diffusion process
Authors: Ghandhi S. K. , Huang R. T. , Borrego J. M.
Published: July 2002

A rapid thermal diffusion process for the fabrication of GaAs tunnel junctions, utilizing a doped oxide zinc source and a protective cap layer of phosphosilicate glass, is described in this letter. It is shown that tunnel junctions fabricated by this open tube process are suited for low impedance interconnects in tandem solar cells, and also for tunnel diode field-effect transistor logic applications. The resulting voltage-current characteristics in both silicon and sulfur doped n+ epilayers, and the peak current as a function of effective doping concentration, are also presented. It is shown that diodes made by this process are comparable in electrical properties to those made by molecular beam epitaxy.
Influence of external mirror on antireflection-coated phased-array semiconductor lasers
Authors: Hardy Amos , Streifer William , Osiński Marek
Published: July 2002

We have analyzed a phased-array semiconductor laser coupled to an external cavity containing a spatial filter. The results indicate that the external cavity-spatial filter combination couples the natural supermodes of the array. Thus if a single supermode were to be emitted by the antireflection-coated facet, the reflected field would couple into many supermodes of the same parity. By self-consistently modeling the laser-cavity configuration we show that substantial mode mixing occurs in practical cases and its effect is more pronounced when the coupling coefficient is small.