bluuebunny/crossref_metadata_2025 huggingface dataset:
Torque and forces resulting from the rotation of two longitudinally corrugated cylinders separated by a viscous fluid
Authors: Wang C. Y. , Tsai C. F.
Published: July 2002

Two concentric corrugated cylinders are separated by a viscous fluid. When the inner cylinder is rotated, unsteady and steady torque and forces may be generated. Assuming low Reynolds numbers and small amplitude corrugations, the Navier–Stokes equations are solved by perturbations to second order. The torque and forces are dependent on the gap width, the amplitude of corrugations, and especially the number of waves of the corrugations.
Localized stability criterion for kink modes in systems with small shear
Authors: Hastie R. J. , Johnson J. L.
Published: July 2002

A localized magnetohydrodynamic stability criterion for ideal kink instabilities is determined for systems where the safety factor has a local minimum on a rational surface with no pressure gradient. These modes are stable in the cylindrical limit, but toroidal effects can make them unstable. They could provide a partial explanation for the rapid current penetration observed in tokamaks.
Induced azimuthal electric fields in a tandem mirror with throttle coils
Authors: Myra J. R.
Published: July 2002

A tandem mirror with an axisymmetric center cell separated from quadrupole regions by a large mirror ratio throttle coil is considered. Passing particles induce an azimuthal electric field in the center cell that destroys the axisymmetry of the guiding center orbits. It is shown that the resulting neoclassical resonant-plateau transport is small if dielectric resonances can be avoided.
Entrance-flow invariance in a tapering elliptical slit
Authors: Snyder B. , Olson D. E.
Published: July 2002

Patterns of laminar flow development in the entrance region of an elliptical slit have been determined experimentally by hot-wire anemometry. The inlet cross section is circular, tapering downstream into elliptical sections of increasing eccentricity. The cross-sectional area is maintained constant throughout, to facilitate an assessment of wall effects on the developing flow patterns. Flow development along the major axis is found to lag the change in geometric contour, so that axisymmetric inlet profiles do not acquire the elliptical symmetry predicted by conformal mapping. Nevertheless, flow distributions are invariant in the mean: axial partial flows through downstream elliptical rings are nearly the same as on corresponding circular rings at the inlet. This near-invariance holds approximately even for uniformly-curved pipe sections in which intense swirl is present, and so may have more general application to conduit flows than just for the examples reported here.
Renormalized turbulence theory of ion pressure gradient driven drift modes
Authors: Hong Bong-Guen , Choi Duk-In , Horton W.
Published: July 2002

From the nonlinear gyrokinetic equation we formulate the renormalized turbulence equation for the ηi -mode drift wave instability. The study shows that the dominant nonlinear damping mechanism is from the E×B convection of the pressure fluctuation and that the kinetic modifications to the fluid E×B mode coupling, studied earlier, shift the spectrum toward the shorter wavelengths. Balancing the linear growth rate with the nonlinear damping rate at the linearly most unstable region, we calculate the anomalous ion thermal conductivity, which exceeds the neoclassical plateau formula and gives a value of the same order as that previously computed by Horton, Choi, and Tang [Phys. Fluids 26, 1077 (1981)], but with a kinetic enhancement factor. Also, the thermal conductivity formula remains finite for vanishing density gradient.
On the Hamiltonian structure of ion-acoustic plasma waves and water waves in channels
Authors: Menyuk Curtis R. , Chen Hsing-Hen
Published: July 2002

It is shown that the Hamiltonian structure of ion-acoustic waves and channel waves may be used to derive the Hamiltonian structure of the Korteweg–de Vries equation and its higher-order corrections. The Hamiltonian approach used here is more systematic and less laborious than standard methods for deriving the Korteweg–de Vries equation. It is also more revealing. In particular, it is shown that the Poisson bracket of the corrected equations equals the Korteweg–de Vries Poisson bracket at every order. It is also shown that the corrected equations become nonlocal at sufficiently high order.
Single beam, single detector diagnostic for plasma density profiles across a magnetic field gradient
Authors: Bellan P. M. , Schalit M. A.
Published: July 2002

A scheme is proposed whereby the density profile of a tokamak or mirror plasma is mapped to nonlinear sidebands of a diagnostic wave, so that the sideband frequency spectrum is an image of the density profile traversed by the wave. The diagnostic relies on a monochromatic, high-frequency (ω≳ωce), pulsed wave source to determine the density profile across a magnetic field gradient. In principle, the technique offers time-dependent direct density profiles having high spatial resolution. However, the feasibility of the diagnostic is limited by the intense power and the rapid rise time required of the pulsed diagnostic wave. For parameters relevant to fusion plasmas, and for the particular scattering geometry suggested here, the technique requires, roughly, 30 mJ pulses of 2 nsec duration. The required power can be reduced by a factor of roughly 104 by applying signal averaging techniques. The rise time of the pulsed source must be short in comparison to one period of the diagnostic wave.
Drift and tearing modes in a sheared cylinder
Authors: Cary John R. , Newberger Barry S.
Published: July 2002

Drift and tearing modes in a sheared cylindrical collisionless plasma column are studied. A set of differential equations in the radial coordinate is derived with small gyroradius and low-β expansion. The finite-β effects include curvature drifts, gradient-B drifts, and the parallel magnetic field perturbation. Algebraic elimination reduces the resulting set of equations to a fourth-order system. Analysis shows that bad curvature does not drive the collisionless modes unstable.
One-dimensional theory and simulation of acceleration in relativistic electron beam Raman scattering
Authors: Abe Takashi
Published: July 2002

Raman scattering by a parallel relativistic electron beam was examined analytically and by using the numerical simulation. Incident wave energy can be transferred not only to the scattered electromagnetic wave but also to the beam. That is, the beam can be accelerated by the Doppler-shifted plasma oscillation accompanied by the scattered wave. The energy conversion rates for them were obtained. They increase with the γ value of the electron beam. For the larger γ values of the beam, the energy of the incident wave is mainly transferred to the beam, while in smaller γ, the energy conversion rate to the scattered wave is about 0.2 times that to the beam. Even in smaller γ, the total energy conversion rate is about 0.1.
Energy confinement in the reversed field pinch
Authors: Ghosh Kamal Kumar , Ray Dipankar
Published: July 2002

The coupled equations for energy confinement in the reversed field pinch plasma obtained by Miller [Plasma Phys. Controlled Fusion 26, 1119 (1984)] are solved analytically. Although the analytic solutions are cumbersome, many noteworthy properties of the curves can be found easily and are given here.
Visualization of the vortex street behind a circular cylinder at low Reynolds numbers
Authors: Freymuth P. , Finaish F. , Bank W.
Published: July 2002

Vortical filament development behind a circular cylinder immersed in steady flow is documented in photographic sequences at Reynolds numbers 70, 120, and 300.
Parametric scattering in electron-cyclotron resonance heating
Authors: Heikkinen J. A. , Karttunen S. J.
Published: July 2002

The problem of parametric scattering of the extraordinary and ordinary waves near electron-cyclotron frequency off ion-cyclotron waves is investigated. Convective thresholds for an inhomogeneous plasma are obtained analytically in the case when the scattered ordinary or extraordinary waves and the ion-cyclotron wave are weakly damped. There are no restrictions on the angles the waves make with the ambient magnetic field. The dependence of the thresholds on the plasma density, electron and ion temperatures, and propagation angles is described. It is found that in present-day electron-cyclotron resonance heating experiments (I≲1 kW/cm2) the thresholds are evidently too high for parametric scattering, but in high-intensity (I≳10 kW/cm2) experiments the possibility for scattering exists.
Acoustic and entropy wave input–output relationships for a Faraday magnetohydrodynamic generator. II. Results
Authors: Barton J. P.
Published: July 2002

Calculated results for the linear response of an ideal, nonviscous, non-heat-conducting, subsonic, quasi-one-dimensional Faraday magnetohydrodynamic (MHD) generator duct to inputted acoustic and entropy (temperature) waves are presented for two typical MHD generator duct designs: the large, high efficiency, central station power plant design and the small, high power density compact design. The effects of applied magnetic field strength and frequency are considered. The results indicate that increased applied magnetic field strength provides a stabilizing (attenuating) effect for pure transmission. However, at low frequencies and high applied magnetic field strengths, temperature variations associated with inputted entropy waves may generate appreciable acoustic output. This entropy wave to acoustic wave interaction could cause a significant increase in overall MHD generator system unsteadiness.
Attenuation of a compressional sound wave in the presence of a fractal boundary
Authors: Koch Donald L.
Published: July 2002

The attenuation of a compressional sound wave propagating through a fluid bounded by a solid surface of fractal dimension 2≤df<3 is studied. At high sound wave frequency ω, the attenuation is caused primarily by the viscous dissipation within a boundary layer near the solid surface of thickness δ=(μ/ρω)1/2, where μ is the viscosity and ρ the density of the fluid. Because the surface area ‘‘seen’’ by the boundary layer increases with decreasing δ, one might expect the attenuation γ to scale in a self-similar manner with δ, i.e., γ∼δ2-da, where 2≤da<3. A multiple scales analysis based on a wide separation in the length scales of successively smaller levels of surface structure is used to determine the dependence of the attenuation on the boundary layer thickness. While the possibility of a self-similar scaling of the attenuation is confirmed, the attenuation exponent da is generally quite different from the fractal dimension df. In fact the presence of a fractal surface area df≠2 is neither a necessary nor sufficient criterion for a self-similar scaling of the attenuation da≠2.
Higher eigenmodes in the Blasius boundary-layer stability problem
Authors: Hultgren Lennart S.
Published: July 2002

The higher spatial-stability eigenmodes for the Blasius boundary layer are examined by using asymptotic theory and an infinite number of modes are found. The asymptotic results are shown to be in close agreement with results from a direct numerical solution of the Orr–Sommerfeld problem. The asymptotic theory would therefore provide an efficient tool in exploratory searches for the eigenvalues.
The structure of a turbulent shear layer embedded in turbulence
Authors: Tavoularis S. , Corrsin S.
Published: July 2002

Measurements are reported on the growth rate and the turbulent characteristics of a two-dimensional, free shear layer generated by a nonuniform array of parallel turbulent jets and wakes. Although bounded by roughly isotropic turbulence and not having any detectable initial periodicity, this layer develops weak, plane, periodic vortices that grow in relative strength and scale downstream. Their scale and frequency match those reported for shear layers that begin with strong vortices.
A new approach to the effect of sound on vortex dynamics
Authors: Lund Fernando , Zabusky Norman J.
Published: July 2002

Two questions are asked, and answered, concerning the dynamics of vortex filaments in a three-dimensional slightly compressible fluid with constant sound speed: What is the flow (including the sound) generated by a vortex in prescribed motion? What is the response of a test vortex to an external flow? The first question is answered by a convolution of a Green’s function with a source localized at the filament; the second by a differential equation valid in the localized induction approximation. A variational principle encompassing both problems is also provided.
Formation of cylindrical shock waves in nonequilibrium magnetogasdynamic flow
Authors: Shankar Rama , Dixit Lalitha
Published: July 2002

Following Chu [in Non-Equilibrium Flows, edited by P. P. Wegener (Dekker, New York, 1970), Vol. II, p. 37], the phenomena associated with the propagation of acceleration waves in nonequilibrium magnetogasdynamic flow, which is induced by the motion of a piston advancing with finite acceleration into a constant state of rest, are studied. Also studied is the characteristic path by using the characteristics of the governing quasilinear system as the reference coordinate system. A differential equation governing the growth and decay of an acceleration wave is derived. The critical time when the breakdown of the characteristic solution occurs in the neighborhood of the leading frozen characteristic is obtained; that is, when all the characteristics will pile up at the wave front to form a shock wave. It is shown that for the slow relaxation process the effect of the magnetic field is to slow down the motion of the breakdown point along the leading frozen characteristic, and thus to increase the cylindrical shock formation time. However, the effect of the magnetic field on expansion waves is to enhance the decay rate. For the quick relaxation process, however, the effect of the magnetic field on compressive waves is to cause an early shock formation, while the effect on expansion waves is to decrease the decay rate.
The linear theory of the circular free-electron laser
Authors: Saito H. , Wurtele J. S.
Published: July 2002

A small signal theory of a free-electron laser (FEL) with a rotating electron beam in a uniform axial magnetic field and in an azimuthal wiggler field (the ‘‘circular’’ FEL) is developed. The analysis includes the low and high gain regimes and the influence of longitudinal space-charge forces. It is found that the circular FEL instability has two regimes: a strong pump regime and a negative mass dominated regime. The negative mass regime replaces the weak pump (Raman) regime found for the usual FEL geometry in which the electron beam propagates in the axial direction (the ‘‘linear’’ FEL). The dispersion relation is evaluated, and the resulting growth rates are compared with those of the linear FEL. For a cold beam, at fixed output frequency, the growth rate in the strong pump regime is larger, by a factor of γ2/3, in the circular FEL. The negative mass instability is shown to increase the growth rate and modify the bandwidth of the circular FEL in the strong pump regime. However, the circular FEL performance is found to be more sensitive to the energy spread than the linear FEL.
Thermocapillary effects in liquid bridges. II. Deformation of the interface and capillary instability
Authors: Rybicki A. , Floryan J. M.
Published: July 2002

Surface deformation and capillary instability of nonisothermal liquid bridges have been analyzed. The results show that the maximum size of the bridge is limited by the capillary instability. When the length of the bridge increases, a nonstationary surface deformation may occur before the capillary instability sets in, and it may lead to an earlier breakup of the bridge. Detailed calculations have been carried out for cylindrical bridges. It has been found that such bridges bulge at the cold end and constrict at the hot end. An asymptotic form of surface deformation has been identified in the case of very short bridges.
Diffraction, self-focusing, and the geometrical optics limit in laser produced plasmas
Authors: Marchand R. , Rankin R. , Capjack C. E. , Birnboim A.
Published: July 2002

The effect of diffraction on the self-modulation of an intense laser beam in an initially uniform hydrogen plasma is investigated. A formalism is used in which the diffraction term in the paraxial wave equation can be arbitrarily reduced by the use of a weight factor ι. In the limit where ι approaches zero, it is shown that the paraxial wave equation correctly reduces to the geometrical optics limit and that the problem then becomes formally equivalent to solving the ray-tracing equations. When ι=1, the paraxial wave equation takes its usual form and diffraction is fully accounted for. This formalism is applied to the simulation of self-modulation of an intense laser beam in a hydrogen plasma, for which diffraction is shown to be significant.
Lyapunov’s stability of nonlinear cold plasma waves in the high-frequency limit
Authors: Aragão F. R. F. , Schindler K.
Published: July 2002

The stability theory of Lyapunov is used to investigate the linear stability of Bernstein–Greene–Kruskal equilibria in the cold plasma limit. A suitable Lyapunov functional is found for the case of a high-frequency limit, i.e., for unperturbed ions, implying stability.
Cold relativistic helically symmetric steady flows
Authors: Weitzner H. , Fruchtman A. , Amendt P.
Published: July 2002

A general theory of nontenuous cold relativistic helically symmetric steady flows is developed. Both self-fields and radial effects are included. The fluid and the Maxwell equations are reduced to a system of equations for three scalar functions: a stream function, a flux function, and an electrostatic potential. These equations are expanded under the assumption of small perpendicular momenta and a simplified set of ordinary differential equations is obtained. Two cases are studied in detail. The first case is of a system externally driven by a magnetic wiggler. The second is the self-excited system, with no externally applied wiggler. The equilibria described here are of a practical importance for free-electron lasers which employ high-density thick beams.
Ion-density fluctuations in laser-heated plasma
Authors: Al-Shiraida Y. , Offenberger A. A.
Published: July 2002

Ion turbulence generated in a CO2 laser-heated gas target plasma has been studied using ruby laser Thomson scattering. Considerable enhancement of the ion fluctuations over the thermal level was observed for two high-density plasma regimes (short and long density scale lengths). In the short scale length regime the magnitude of density fluctuations, together with temporal and spectral features of the scattered ruby light, indicate that ion-turbulence levels are sufficient to account for anomalous absorption of CO2 laser energy. In the long scale length regime, measurements of the ion-fluctuation wavenumber spectrum induced in the plane of the CO2 laser electric field, together with high-speed streak camera measurements of the Thomson scattered ruby laser light, were performed to determine general features of strong ion fluctuations in laser heated plasma. Possible mechanisms for generating the observed ion turbulence are discussed.
Effects of cylinder length on transition to doubly periodic Taylor–Couette flow
Authors: Ross Michael P. , Hussain A. K. M. Fazle
Published: July 2002

The Reynolds number for transition from singly to doubly periodic Taylor–Couette flow is studied experimentally and found to vary inversely with the aspect ratio Γ of the fluid column. This transition Reynolds number is also dependent on the number m of azimuthal waves. For a given m, the type of modulating state, which results after this transition, is not affected by Γ, nor does the transition Reynolds number differ for the different types of modulating states which can occur.
Curvature effect on third-order velocity correlations and its model representation
Authors: Chung Myung Kyoon , Park Sang Woo , Kim Kyung Chun
Published: July 2002

A computational closure model for the diffusive transport of turbulent kinetic energy in a curved streamline field is proposed as a function of a velocity time scale τv and a curvature time scale τc that is derived from the analogy between buoyancy and streamline curvature effects on turbulence. In order to determine the empirical constant of the model, the mean velocity vector field and various turbulence quantities have been measured in a curved streamline field over a two-dimensional surface mounted fence. The experimental data on the third-order correlation ∼(uiuiuj) are found to be successfully represented by the proposed closure model.
Steady bimodal convection in a cylinder at large Prandtl numbers
Authors: Buell Jeffrey C. , Catton Ivan
Published: July 2002

Steady bimodal convection of an infinite Prandtl-number Boussinesq fluid in a cylinder is considered. An asymptotic analysis similar to the one used by Buell and Catton [Phys. Fluids 29, 23 (1986)] for axisymmetric convection yields a solvability condition that determines the radial wavenumber. The analysis is valid for convection far away from the origin, the lateral boundary, and any pattern dislocations. The azimuthal wavenumber is treated as a parameter, although in real systems it is dependent on the initial and boundary conditions. Results are presented for Rayleigh numbers between 14 000 and 60 000, and for azimuthal wavenumbers between 5 and 7. It is shown that for increasing Rayleigh numbers, the selected radial wavenumber and the heat transfer tend to become independent of the azimuthal wavenumber. No quantitative experimental data are available, but one qualitative comparison is good.
Experimental investigation of line-tying effects on the spheromak tilt mode
Authors: Wysocki F. J.
Published: July 2002

A variety of passive stabilization techniques compatible with S-1 inductive spheromak formation [Phys. Rev. Lett. 46, 188 (1981)] has been evaluated for effectiveness against the magnetohydrodynamic (MHD) tilting instability. A line-tying stabilization effect has been previously identified theoretically. In the present work, the effects have been experimentally identified and investigated. The perturbed current patterns within passive stabilizing structures have been measured, and line-tying currents have been identified. The simple ring model of tilting has been expanded to include line tying, and has provided estimates for growth rates which agree well with the measured values.
Use of x-ray measurements for testing thermal transport models
Authors: Marchand R. , Havazelet D. , Capjack C. E. , Birnboim A.
Published: July 2002

The energy radiated by laser-produced Al plasmas is calculated using a one-dimensional Lagrangian code and a collisional radiative equilibrium model. The predicted spectra are used to calculate the energy transmitted through Al foils of various thicknesses, and comparisons are made with experimental data. The sensitivity of the transmitted energies to the flux limiter f, used in the simulation is assessed, and it is suggested that this could be a useful independent technique for testing heat transport models in simulations.
Theory of nonmonotonic double layers
Authors: Kim K. Y.
Published: July 2002

A simple graphic method of solving the Vlasov–Poisson system associated with nonlinear eigenvalue conditions for arbitrary potential structures is presented. A general analytic formulation for nonmonotonic double layers is presented and illustrated with some particular closed form solutions. This class of double layers satisfies the time stationary Vlasov–Poisson system while requiring a Sagdeev potential, which is a double-valued function of the physical potential. It follows that any distribution function having a density representation as any integer or noninteger power series of potential can never satisfy the nonmonotonic double-layer boundary conditions. A Korteweg–de Vries-like equation is found showing a relationship among the speed of the nonmonotonic double layer, its scale length, and its degree of asymmetry.
Microwave radiation from a tunable circular free-electron laser
Authors: Hartemann F. , Bekefi G.
Published: July 2002

Measurements of coherent microwave emission from a rotating relativistic electron ring (200 kV, 100 A, 200 nsec) interacting with an azimuthally periodic wiggler magnetic field are reported. Tunable radiation is observed between 12.5 and 18 GHz at power levels of ∼5 kW. The predicted interactions occur near the crossing points of the TMlm electromagnetic waveguide modes, ω2=k2∥c2+ω2c(l,m), and the beam modes, ω=(l+N)Ω∥+k∥v∥, where ωc is the waveguide cutoff frequency, Ω∥ is the electron cyclotron frequency, k∥ is the axial wavenumber, v∥ is the axial drift velocity, and N is the number of wiggler periods. These observations are consistent with single-mode operation in the TM01 coaxial mode.
Radio frequency modification of electron loss rate and potential in mirror-confined plasmas
Authors: Parks P. B.
Published: July 2002

During ion-cyclotron heating of mirror-confined plasmas, a small rf parallel electric field (typically 1 to 50 V/m) is generated inside the plasma. A quasilinear approach is used to study analytically the influence of the enhanced velocity space parallel diffusion on the electron end-loss rate and the potential in a simple mirror configuration. The results derived in the paper have immediate application to the problem of azimuthally asymmetric rf heating and potential variations in connection with radial transport.
Experiments on the wake behind a magnetized object in a flowing plasma
Authors: Beardsley Kelly , Hill Jacqueline , Seyhoonzadeh Amir , Rosario Rachel , Forsling Peter , Lonngren Karl E.
Published: July 2002

The two-dimensional flow characteristics of a plasma passing behind a long magnetized cylinder inserted in an underdense plasma are measured. The results are contrasted with similar flow characteristics behind an unmagnetized cylinder. Significant differences are detected that can be attributed to the localized magnetic field.
Three-fluid magnetohydrodynamical simulation of plasma focus discharges
Authors: Behler K. , Bruhns H.
Published: July 2002

A two-dimensional, three-fluid code based on the two-fluid Potter code [Methods in Computational Physics (Academic, New York, 1970), Vol. 9, p. 340] was developed for simulating the plasma focus discharge. With this code it is possible to treat the neutral gas in addition to the plasma components and to model the ionization and recombination phenomena. Thus the sheet dynamics in a plasma focus can be studied and effects investigated such as the occurrence of residual gas (or plasma) density behind the current sheet in the run-down phase. This is a prerequisite to the occurrence of leak currents, which are one of the causes limiting the performance of large plasma focus devices. It is shown that fast operating foci with small dimensions behave favorably compared with the ‘‘classical’’ Mather focus [Methods of Experimental Physics (Academic, New York, 1971), Vol. 9B, p. 187] with long coaxial electrodes.
Numerical experiments on modon stability to topographic perturbations
Authors: Carnevale G. F. , Briscolini M. , Purini R. , Vallis G. K.
Published: July 2002

A summary of a numerical study of the stability of modons to topographic perturbation is presented. Previous studies have suggested a monotonic relationship between the horizontal scale of the perturbation and the amplitude needed to destroy a modon—as the scale of the perturbation increases the strength needed for destruction decreases. The results presented here show that this relationship does not hold for scales larger than the modon radius. For large-scale perturbations, the strength needed for destruction again increases. The modon is most stable to perturbations of horizontal scale either much larger or much smaller than the modon radius. Stability graphs are presented for three types of perturbations; ridges, hills, and irregular terrain.
Lower hybrid wave propagation and absorption in inhomogeneous plasmas
Authors: Roy Choudhury S.
Published: July 2002

The high-frequency dielectric tensor for a cold, multispecies, current-carrying plasma is derived in straight-tokamak geometry, including both toroidal and poloidal magnetic fields in the equilibrium. Propagation and resonant absorption of lower hybrid waves in an inhomogeneous plasma column are investigated. The earlier treatment of Grossman and Weitzner [Phys. Fluids 27, 1699 (1984)] of the resonant-layer solutions, power absorption and energy partition, and accessibility of the lower hybrid wave is generalized to include the effects of mode propagation parallel to the magnetic field and of an equilibrium plasma current. In particular, the density gradient coupled to the finite poloidal mode number and parallel wavenumber allows the lower hybrid wave to reach the hybrid layer, unlike the case of an infinite homogeneous plasma. Finally, the eigenvalue problem is solved for a homogeneous plasma column carrying zero current, and surrounded by a vacuum region delimited by a perfectly conducting shell. The spectrum of both axisymmetric and nonaxisymmetric lower hybrid oscillations is obtained for parameters characterizing the tokamak at Saha Institute for Nuclear Physics [in Proceedings of the IVth National Conference on Plasma Science and Technology (Institute for Plasma Research, Bhat, India, 1987), Paper M-25].
Magnetic tearing of plasma discharges due to nonuniform resistivity
Authors: Hassam A. B.
Published: July 2002

The rearrangement of current in a plasma discharge in response to resistivity nonuniformities within a magnetic surface is studied. It is shown that macroscopic magnetic islands develop about those surfaces where the nonuniformity is aligned with the magnetic field. If the nonuniformity and the field are not aligned anywhere, there is no current rearrangement; instead, relatively large plasma flows are set up. Such resistivity inhomogeneities can obtain in solar coronal loops and, in some circumstances, in tokamak discharges.
Dissipation and vorticity spectra in a turbulent wake
Authors: Antonia R. A. , Shah D. A. , Browne L. W. B.
Published: July 2002

A reasonable spectral analogy is found between the temperature dissipation and either the energy dissipation or the vorticity in the turbulent far wake of a circular cylinder. The high-wavenumber behavior of the spectra of vorticity, energy dissipation, and temperature dissipation is consistent with isotropy.
Sideband excitation in an electromagnetic wiggler pumped free electron laser
Authors: Tripathi V. K. , Liu C. S.
Published: July 2002

A kinetic formalism of sideband excitation at the multiples of bounce and cyclotron frequencies of electrons trapped in the ponderomotive potential well and a guide field is developed. A positive (negative) slope of the distribution function with parallel energy is necessary for the excitation of upper (lower) sidebands of the bounce frequency. The cyclotron harmonic sideband excitation requires large perpendicular energy; the latter, however, tends to lower the growth rate of bounce harmonic sidebands. In either case the growth rate falls off rapidly with the harmonic number.
Perturbed motions of a bubble rising in a vertical tube
Authors: Fearn R. M.
Published: July 2002

Experiments are reported in which long air bubbles rising in liquid-filled vertical tubes were perturbed by coaxial wires. Distinctly different flow regimes were found to arise as the thickness of the wire was varied. The breaking of axial symmetry observed in the experiments is discussed in the light of ideas from bifurcation theory.
Alpha-particle effects on high-n instabilities in tokamaks
Authors: Rewoldt G.
Published: July 2002

Hot alpha particles and thermalized helium ash particles in tokamaks can have significant effects on high toroidal mode number instabilities such as the trapped-electron drift mode and the kinetically calculated magnetohydrodynamic ballooning mode. In particular, the effects can be stabilizing, destabilizing, or negligible, depending on the parameters involved. In high-temperature tokamaks capable of producing significant numbers of hot alpha particles, the predominant interaction of the mode with the alpha particles is through resonances of various sorts. In turn, the modes can cause significant anomalous transport of the alpha particles and the helium ash. Here, results of comprehensive linear eigenfrequency–eigenfunction calculations are presented for relevant realistic cases to show these effects.
Stably stratified turbulence subjected to a constant area vertical expansion
Authors: Thoroddsen S. T. , Van Atta C. W.
Published: July 2002

Stably stratified grid turbulence has been passed through a constant-area vertical expansion, in a thermally stratified wind tunnel. The latter part of the test-section expands linearly in the vertical direction while contracting accordingly in the horizontal direction. The resulting reduction in stratification strength interacts with the vortex stretching due to the induced cross-strain, an effect first studied by Townsend in a homogeneous fluid. The expansion is applied to the turbulence at two different states of evolution, by using two different stratification strengths. For the weaker stratification the turbulence is strongly stirring, while for the stronger stratification it is in the restratification regime, as it enters the expansion. Both of these cases show a clear reduction in the normalized buoyancy flux. This reduces the stirring in the former case, while enhancing restratification in the latter. The vertical velocity fluctuations thereby experience qualitatively different effects due to the buoyancy forces, as the restratification adds to w′, whereas in the stirring case the buoyancy forces assist the cross-strain in reducing w′ by extracting kinetic energy from w′ and storing potential energy in the fossilizing density field.
On initial-value and self-similar solutions of the compressible Euler equations
Authors: Samtaney Ravi , Pullin D. I.
Published: July 2002

We examine numerically the issue of convergence for initial-value solutions and similarity solutions of the compressible Euler equations in two dimensions in the presence of vortex sheets (slip lines). We consider the problem of a normal shock wave impacting an inclined density discontinuity in the presence of a solid boundary. Two solution techniques are examined: the first solves the Euler equations by a Godunov method as an initial-value problem and the second as a boundary value problem, after invoking self-similarity. Our results indicate nonconvergence of the initial-value calculation at fixed time, with increasing spatial-temporal resolution. The similarity solution appears to converge to the weak ‘zero-temperature’ solution of the Euler equations in the presence of the slip line. Some speculations on the geometric character of solutions of the initial-value problem are presented.
The structure of the compressible reacting mixing layer: Insights from linear stability analysis
Authors: Day M. J. , Reynolds W. C. , Mansour N. N.
Published: July 2002

Previous investigations have demonstrated that a compressible reacting mixing layer can develop two instability modes in addition to the more common central mode that exists unaccompanied in incompressible nonreacting flows. These two additional modes are termed “outer” because of their association with the fast and slow free streams. Numerical simulations have shown that mixing layers dominated by outer modes have a lower global reaction rate in comparison to a flow structure governed by the central mode. Therefore, the presence of these modes has important consequences for applications in supersonic combustion. Results are presented from a parametric study of the compressible reacting mixing layer’s regime space using linear stability analysis. The focus of our work is to develop a better understanding for the combined effects of compressibility, heat release and the ratios of density, equivalence, and velocity on the instability characteristics of each mode and on the structure predicted to result in a turbulent reacting mixing layer.
Onset of stationary and oscillatory convection in a tilted porous cavity saturated with a binary fluid: Linear stability analysis
Authors: Karimi-Fard M. , Charrier-Mojtabi M. C. , Mojtabi A.
Published: July 2002

In the present work, we study the onset of double-diffusive convective regimes in a tilted rectangular cavity, filled with a porous medium, saturated by a binary fluid. Two opposite walls are maintained at different but uniform temperatures and concentrations while the two other walls are impermeable and adiabatic. When the thermal and solutal buoyancy forces are comparable in intensity but have opposite signs, the motionless double-diffusive regime with linear temperature and concentration profiles is a solution of the problem. The first part of the study consists of a linear stability analysis of the motionless regime. We determine the critical thermal Rayleigh number for the onset of stationary and oscillatory convection. Indeed, we point out that there exist primary Hopf bifurcations for the studied problem in porous medium, while in the same configuration with a fluid medium only primary stationary bifurcations exist. When the first primary bifurcation creates a steady state branch of solutions, the bifurcation is either transcritical or pitchfork depending on the aspect ratio, A and the tilt, φ of the cavity. The onset of oscillatory convection (Hopf bifurcation) depends not only on A and φ but also on the Lewis number, Le and the normalized porosity, ε. Then, we determine the parts of the (Le, ε) parameter space for which the first primary bifurcation is stationary or oscillatory. In particular, it is found that in the case Le⩾1 and for ε Le2<1 the first primary bifurcation is always a Hopf bifurcation for any A and φ except for φ=+90°. For ε Le2>1 only stationary primary bifurcations exist. In the case Le<1, zones where stationary and oscillatory primary bifurcations exist are separated by a curve depending on A and φ. The last part of this work consists of a series of numerical simulations. The onset of stationary and oscillatory convection is obtained numerically at the critical Rayleigh number predicted by linear analysis. We also verified the frequency of oscillations for several sets of dimensionless parameters. The numerical simulations show multiple subcritical solutions.
A study of parametric instability in a harmonic gyrotron: Designs of third harmonic gyrotrons at 94 GHz and 210 GHz
Authors: Saraph G. P. , Antonsen T. M. , Nusinovich G. S. , Levush B.
Published: July 2002

Mode competition can present a major hurdle in achieving stable, efficient operation of a gyrotron at the cyclotron harmonics. A type of mode interaction in which three modes at different cyclotron harmonics are parametrically coupled together is analyzed here. This coupling can lead to parametric excitation or suppression of a mode; cyclic mode hopping; or the coexistence of three modes. Simulation results are presented for the parametric instability involving modes at the fundamental, second harmonic, and third harmonic of the cyclotron frequency. It is shown that the parametric excitation can lead to stable, efficient operation of a high-power gyrotron at the third harmonic. Based on this phenomenon, two practical designs are presented here for the third harmonic operation at 94 and 210 GHz.
Theory of helix traveling wave tubes with dielectric and vane loading
Authors: Freund H. P. , Zaidman E. G. , Antonsen T. M.
Published: July 2002

A time-dependent nonlinear analysis of a helix traveling wave tube (TWT) is presented for a configuration where an electron beam propagates through a sheath helix surrounded by a conducting wall. The effects of dielectric and vane loading are included in the formulation as is efficiency enhancement by tapering the helix pitch. Dielectric loading is described under the assumption that the gap between the helix and the wall is uniformly filled by a dielectric material. The vane-loading model describes the insertion of an arbitrary number of vanes running the length of the helix, and the polarization of the field between the vanes is assumed to be an azimuthally symmetric transverse-electric mode. The field is represented as a superposition of azimuthally symmetric waves in a vacuum sheath helix. An overall explicit sinusoidal variation of the form exp(ikz−iωt) is assumed (where ω denotes the angular frequency corresponding to the wave number k in the vacuum sheath helix), and the polarization and radial variation of each wave is determined by the boundary conditions in a vacuum sheath helix. The propagation of each wave in vacuo as well as the interaction of each wave with the electron beam is included by allowing the amplitudes of the waves to vary in z and t. A dynamical equation for the field amplitudes is derived analogously to Poynting’s equation, and solved in conjunction with the three-dimensional Lorentz force equations for an ensemble of electrons. Electron beams with a both a continuous and emission-gated pulse format are analyzed, and the model is compared with linear theory of the interaction as well as with the performance of a TWTs operated at the Naval Research Laboratory and at Northrop–Grumman Corporation.
Two-dimensional analysis of the power transfer between crossed laser beams
Authors: McKinstrie C. J. , Li J. S. , Giacone R. E. , Vu H. X.
Published: July 2002

The power transfer between crossed laser beams made possible by an ion-acoustic wave is studied. A simple formula is derived for the steady-state power transfer, which depends on two dimensionless parameters: the ratio of the incident beam intensities and the normalized beamwidth. Numerical simulations show that the transient power transfer is larger than the steady-state power transfer and usually oscillates in time. The convective depletion of the higher-frequency beam saturates the power transfer more quickly than the damping of the ion-acoustic wave.
Imaging of fusion reaction zone in plasma focus
Authors: Zakaullah M. , Akhtar Ijaz , Murtaza G. , Waheed A.
Published: July 2002

In a low energy (2.3 kJ) Mather-type deuterium plasma focus, neutron and charged particle emission is investigated by using time-resolved neutron detectors and time-integrated charged particle pinhole imaging camera. The time-integrated charged particle pinhole images demonstrate the varying influence of magnetohydrodynamic (MHD) instabilities vis-a-vis filling pressure. The neutron production mechanism at play strongly depends upon the pressure. At lower pressure, the plasma column is highly unstable due to MHD instabilities and the neutron emission is found to be low with fluence anisotropy exceeding 3.5. At optimum pressure (2.5 mbar for this system), an almost stable dense plasma of about 17 mm3 volume is formed about 5 mm away from the anode, with neutron emission at its highest and the fluence anisotropy lowest. At higher pressure, the plasma column is stable, although it moves away from the anode like a jet and may then be called a moving boiler. In this case, the neutron emission is lowered compared to its optimum value and fluence anisotropy is increased. The data suggest beam-target mechanism at low pressure, trapped gyrating particles at optimum pressure and a jetlike moving boiler at higher pressure.
Laser photochemical fabrication of phase-controlled 160-nm period gratings by stimulated second-order surface plasma wave scattering
Authors: Ehrlich D. J. , Brueck S. R. J.
Published: July 2002

Well-controlled 160-nm period gratings have been fabricated by employing surface plasma wave scattering obtained during laser photodeposition. Extended range phase control and simultaneous period halving have been demonstrated by seeding of second-order scattering in a manner analogous to injection locking and second-Stokes scattering. This technique may be useful for the fabrication of <100-nm period structures.
Room-temperature operation of GaAs/AlGaAs diode lasers fabricated on a monolithic GaAs/Si substrate
Authors: Windhorn T. H. , Metze G. M.
Published: July 2002

Room-temperature operation has been achieved for GaAs/AlGaAs heterostructure diode lasers fabricated on a monolithic GaAs/Si substrate. These devices, which incorporate a large optical cavity structure grown by molecular beam epitaxy directly on a Si wafer, have exhibited threshold currents as low as 775 mA and power outputs as high as 27 mW/facet in pulsed operation.
Optically detected electron cyclotron resonance in silicon
Authors: Pakulis E. J. , Northrop G. A.
Published: July 2002

The cyclotron resonance of electrons in Si has been optically detected. At resonance, there is a decrease in the luminescence from defects produced near the surface of Si which has been exposed to a dry etching plasma. The resonances are observable optically even when the lines are too broad to be detected directly via microwave absorption measurements. This result, together with the absence of hole resonances, provides clues as to the possible recombination mechanisms at the defects.
Transient response of planar integrated optoelectronic antennas
Authors: DeFonzo Alfred P. , Jarwala Madhuri , Lutz Charles
Published: July 2002

Broadband tapered slot antennas monolithically integrated on ion damaged silicon-on-sapphire substrates are driven by picosecond photoconductivity to generate and detect millimeter waves. The time-dependent electromagnetic impulse response of these transceivers is modeled by relating the antenna structure and the shape of the exciting pulse. The far-field response is observed to consist of a traveling-wave component and a standing-wave component, which is also predicted by the model.
Novel optoelectronic single quantum well devices based on electron bleaching of exciton absorption
Authors: Kastalsky A. , Abeles J. H. , Leheny R. F.
Published: July 2002

Novel planar optoelectronic devices with two-dimensional exciton absorption controlled by free-carrier-induced bleaching are proposed. Exciton-resonant light propagates along a single mode rib waveguide containing a single quantum well (SQW), the only absorbing medium in the waveguide. Three such devices operating as optical modulators are (1) a gate-controlled field-effect transistor optical modulator (FETOM), (2) an optically readable memory element, and (3) an optically switched charge storage device. The FETOM, in which free-carrier density in the SQW is controlled by gate voltage, offers high speed (37.5 ps), small size (125 μm), and low power (86 nW/MHz).
High-spectral-purity cw and pulse output from an ensemble of discrete diode lasers
Authors: Anderson Kristin K. , Rediker Robert H.
Published: July 2002

The spectrum of the output from an external-cavity laser containing five discrete diode lasers operating as a coherent ensemble has been shown to correspond to a single mode of the cavity with very narrow linewidth. For cw operation, the linewidth is within the 7.5-MHz measurement resolution. Pulsed outputs from a low-level cw baseline are reported for which the single-mode pulse output is also within the 7.5-MHz measurement resolution. The measured linewidth of the single-mode pulse output with no cw output is 9 MHz.