bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of an internal angular momentum on the rotation of a symmetrical top
Authors: Chiang R. C.
Published: July 2002

The existence of an internal angular momentum induces nutations and periodic deviations from a mean precession. Motions are classified into three cases. In Case I, the nutation is regular during precessions so that the motion is a wobbling, the top behaves triaxially. This triaxiality may be involved in the triaxial deformations of nuclear shapes in nuclear physics. Case II is a limiting case of Case I at an infinite period of nutation. In Case III, the body symmetry axis is over nutated to cross over and to oscillate around the invariable plane. It is an overnutated wobbling. These three cases can be determined by whether the ratio of the internal angular momentum to the total angular momentum is less than or greater than a critical value.
Induced matter theory and embeddings in Riemann flat space–times
Authors: Abolghasem G. , Coley A. A. , Mc Manus D. J.
Published: July 2002

A class of five-dimensional space-times that contain four-dimensional hypersurfaces whose intrinsic metrics are of cosmological interest is investigated. First, the five-dimensional space–time is assumed to be Riemann flat—the problem of determining the intrinsic metrics of the four-dimensional hypersurfaces then becomes a problem of embedding in flat space–time. Second, the Riemann flat solutions are used as a starting point to find solutions to Einstein’s vacuum field equations in five dimensions that are not Riemann flat. In particular, a new general class of five-dimensional vacuum solutions is found.
Time-dependent multipoles and their application for radiation from volume source distributions
Authors: Heyman Ehud , Devaney A. J.
Published: July 2002

The radiation from a pulsed source distribution is expressed directly in the time-domain using a sum of time-dependent spherical (multipole) wave functions. Two alternative expressions for the time-dependent multipole moments (the excitation pulses) are derived. It is shown how they are related to the time-dependent plane-wave spectrum of the source (obtained via a Radon transform of the source distribution in the four space-time coordinates). Furthermore, the time-dependent multipole moments, and thereby the total time-dependent field outside the source region, are completely determined by the time-dependent radiation pattern. The series convergence is addressed by showing that the high order multipole moments tend to the quasistatic extension of the static multipole moments. This also puts an upper limit on the spatial resolution that can be achieved by a source distribution with specified size and pulse length.
Poisson brackets of Wilson loops and derivations of free algebras
Authors: Rajeev S. G. , Turgut O. T.
Published: July 2002

We describe a finite analog of the Poisson algebra of Wilson loops in Yang–Mills theory. It is shown that this algebra arises in an apparently completely different context: as a Lie algebra of vector fields on a noncommutative space. This suggests that noncommutative geometry plays a fundamental role in the manifestly gauge invariant formulation of Yang–Mills theory. We also construct the deformation of the algebra of loops induced by quantization, in the large-Nc limit.
A proof of polynomial identities of type sl(n̂)1⊗sl(n̂)1/sl(n̂)2
Authors: Foda Omar , Okado Masato , Warnaar S. Ole
Published: July 2002

We present a proof of polynomial identities related to finite analogs of the branching functions of the coset sl(n̑)1⊗sl(n̑)1/sl(n̑)2.
Orthogonal polynomials and the finite Toda lattice
Authors: Kasman Alex
Published: July 2002

The choice of a finitely supported distribution is viewed as a degenerate bilinear form on the polynomials in the spectral parameter z and the matrix representing multiplication by z in terms of an orthogonal basis is constructed. It is then shown that the same induced time dependence for finitely supported distributions which gives the ith KP flow under the dual isomorphism induces the ith flow of the Toda hierarchy on the matrix. The corresponding solution is an N particle, finite, nonperiodic Toda solution where N is the cardinality of the support of c plus the sum of the orders of the highest derivative taken at each point.
Constrained KP models as integrable matrix hierarchies
Authors: Aratyn H. , Ferreira L. A. , Gomes J. F. , Zimerman A. H.
Published: July 2002

We formulate the constrained KP hierarchy (denoted by cKPK+1,M) as an affine sl̂(M+K+1) matrix integrable hierarchy generalizing the Drinfeld–Sokolov hierarchy. Using an algebraic approach, including the graded structure of the generalized Drinfeld–Sokolov hierarchy, we are able to find several new universal results valid for the cKP hierarchy. In particular, our method yields a closed expression for the second bracket obtained through Dirac reduction of any untwisted affine Kac–Moody current algebra. An explicit example is given for the case sl̂(M+K+1), for which a closed expression for the general recursion operator is also obtained. We show how isospectral flows are characterized and grouped according to the semisimple non-regular element E of sl(M+K+1) and the content of the center of the kernel of E.
Generating functionals of physical vertex operators in superstring
Authors: Naito Seichi
Published: July 2002

We define N operators as operators that are already normally ordered with respect to the Friedan–Martinec–Shenker spinor operator. With the help of thus defined N operators, we can give the generating functional of physical vertex operators (GFPVO) of fermionic particles (i.e., in the Ramond sector of the superstring). We also propose GFPVO of bosonic particles (i.e., in the Neveu–Schwarz sector of the superstring), which is simply obtained by supergeneralizing GFPVO in the bosonic string.
Coherent states map for MIC–Kepler system
Authors: Odzijewicz A. , Świȩtochowski M.
Published: July 2002

The coherent states map for MIC–Kepler system is constructed. The quantization of this system is given by the coherent states method.
On solutions of constrained Kadomtsev–Petviashvili equations: Grammians
Authors: Loris Ignace , Willox Ralph
Published: July 2002

We show the existence of Grammian-type solutions for the (vector) k-constrained Kadomtsev–Petviashvili (KP) equations. To introduce the method we give a novel proof for the presence of Grammian solutions for the bilinear l-modified KP hierarchies.
On rotational coherent states in molecular quantum dynamics
Authors: Morales Jorge A. , Deumens Erik , Öhrn Yngve
Published: July 2002

Coherent states suitable for the description of molecular rotations are developed and their connection to similar coherent states in the literature are explored. In particular their quasiclassical properties are developed. The use of such coherent states in time-dependent electron nuclear dynamics studies of molecular collision processes is discussed.
Frames from imprimitivity systems
Authors: Aniello Paolo , Cassinelli Gianni , De Vito Ernesto , Levrero Alberto
Published: July 2002

Let (P,V) be an irreducible imprimitivity system for a group H based on a dual group Â of an Abelian group A and acting on a Hilbert space ℋ. Given ψ∈H, we find necessary and sufficient conditions in order that the set of vectors {∫Â〈â,a〉¯Vh−1 dP(â)ψ : a∈A, h∈H} be a frame in ℋ. Moreover, we apply these results to some examples that are considered in the literature in the context of square-integrability modulo a coset space.
On the braided Fourier transform on the n-dimensional quantum space
Authors: Carnovale Giovanna
Published: July 2002

We work out in detail a theory of integrability on the braided covector Hopf algebra and the braided vector Hopf algebra of type An introduced by Majid. Using a braided Fourier transform very similar to the one defined by Kempf and Majid we obtain n-dimensional analogs of results by Koornwinder expressing the correspondence between products of the q2-Gaussian gq2(x_) times monomials, and products of the q2-Gaussian Gq2(∂_) times q2-Hermite polynomials under the transform. We invert the correspondence by finding a suitable inversion, different from the one of Kempf and Majid. We show that with this transforms, whenever n⩾2, the Plancherel measure will depend on the parity of the power series that we are transforming.
Coherent current states in a mesoscopic four-terminal Josephson junction
Authors: Zareyan Malek , Omelyanchuk A. N.
Published: July 2002

A theory is offered for the ballistic 4-terminal Josephson junction. The studied system consists of a mesoscopic two-dimensional normal rectangular layer which is attached on each side to the bulk superconducting banks (terminals). A relation is obtained between the currents through the different terminals, that is valid for arbitrary temperatures and junction sizes. The nonlocal coupling of the supercurrents leads to a new effect, specific for the mesoscopic weak link between two superconducting rings: an applied magnetic flux through one of the rings produces a magnetic flux in the other ring even in the absence of an external flux through the other one. The phase dependent distributions of the local density of Andreev states, of the supercurrents and of the induced order parameter are obtained. The “interference pattern” for the anomalous average inside the two-dimensional region can be regulated by the applied magnetic fluxes or the transport currents. For some values of the phase differences between the terminals, the current vortex state and the two-dimensional phase slip center appear.
Onset of periodic convection in a vertical slot
Authors: Simpkins P. G. , Godreau J. E.
Published: July 2002

New observations of the convective states in a vertical slot in the neighborhood of the transition to multicellular motion are reported. Experiments in large Prandtl number fluids reveal that the departure from unicellular flow develops gradually in the central region of the slot with an initial closure of the distorted primary flow. Subsequently, as the Rayleigh number increases, a multiplicity of transverse stationary eddies occur in the same vicinity within the primary flow. Similar observations in an air-filled slot display a different response. In this case the initial perturbation of the base flow, resulting in closure of the unicellular motion takes place near the end walls. These and other observations illustrate that differences arise because of the Prandtl number. Measurements of the thermal stratification in the core of the oil-filled cavity are in good agreement with recent numerical solutions, based on the boundary layer approximation. The results are also found to be in reasonable accord with the neutral curve predicted from a linear stability analysis.
Rayleigh–Taylor instability of a continuously stratified fluid under a general rotation field
Authors: Dávalos-Orozco L. A. , Aguilar-Rosas J. E.
Published: July 2002

An estimate of the upper bound of the growth rate of instability in a continuously unstable stratified fluid under a general rotation field is obtained. An example for exponential stratification is given. For magnitudes of the wavenumber k slightly above kc (where kc is the critical value for instability) it is shown that the angle of propagation for the perturbation associated to a maximum growth rate is θmax =0° (180°), where θmax is measured relative to a fixed horizontal component of rotation. For larger values of k bifurcation occurs with two maxima of the same magnitude appearing at angles different from θ=0° but symmetrically located. Increasing k leads to a unique maximum at θmax =90° (−90°). The bifurcation tends to disappear as the vertical component of rotation is increased.
Two-particle description of turbulence, Markov property, and intermittency
Authors: Novikov E. A.
Published: July 2002

It is shown that the hypothesis of independent increments for velocity, which is widely used by many authors [e.g., A. M. Obukhov, Adv. Geophys. 6, 113 (1959)] in the Lagrangian description of turbulence, is inconsistent with the Navier–Stokes equations in a fundamental way. A more general Lagrangian description of turbulent velocity such as the Markov process with dependent increments, which recognizes the condition of incompressibility and the important phenomenon of intermittency, is proposed. A model of intermittent relative motion of fluid particles in turbulent flow is presented. The high-order Lagrangian moments and the probability distribution are obtained. The distribution for the intermittent vorticity is also proposed.
Higher-order terms in the anisotropic representation of Reynolds stresses
Authors: Horiuti Kiyosi
Published: July 2002

Higher-order terms in the k-ε model for the anisotropic representation (AR) of the Reynolds stresses are investigated. Second-order terms in AR have been introduced to express the anisotropy of the Reynolds stresses, however they include no direct influence on the eddy viscosity. This Letter focuses on third-order AR in particular, by incorporating new additional eddy viscosity terms. The proposed model was tested in plane channel flow, and compared to a numerical database generated using a large eddy simulation technique. It was found that third-order AR may be used as an alternative method to reduce the magnitude of the eddy viscosity in the buffer layer region by acting similarly to the Van Driest damping function, commonly used in k-ε models. The relationship of AR to the algebraic stress model is also discussed.
Bifurcations of numerically simulated thermocapillary flows in axially symmetric float zones
Authors: Kazarinoff Nicholas D. , Wilkowski Joseph S.
Published: July 2002

Two-dimensional time-dependent flow in an axially symmetric liquid bridge of Si at zero g with flat ends at the melting temperature θM of Si driven by motions of the free, lateral surface is studied numerically. The system is energized by a ring heater H at temperature θH>θM that surrounds the midcircle m of the bridge. The radiation from H creates a ∇σS(δS is surface temperature), which in turn creates a ∇σ (σ is surface tension) that drives oscillations of its free surface and thermocapillary flow in the bridge. For aspect ratios A of (1)/(2) , (2)/(3) , and 1 it is found that as the relevant control parameter Δθ≡θH−θM is increased, the flow in the bridge bifurcates differently depending on A. For A= (1)/(2) and (2)/(3) the flow bifurcates from steady state to small-amplitude periodic oscillations, breaking symmetry, to large-amplitude, period-tripled ones, to their subharmonics, then becomes aperiodic, and finally chaotic. Toroidal cells in the bridge oscillate to-and-fro along its axis as a compound pendulum, and they change in shape, number, and strength of rotation over the period of an oscillation. For A=1, the bifurcation is from steady state to asymmetric oscillations steadily growing in amplitude as Δθ increases and then directly to semiperiodic ones and thence to chaos. Agreement between the present results and recent experimental and numerical ones, are also described.
Upstream-advancing waves generated by three-dimensional moving disturbances
Authors: Lee Seung-Joon , Grimshaw Roger H. J.
Published: July 2002

The wave field resulting from a surface pressure or a bottom topography in a horizontally unbounded domain is studied. Upstream-advancing waves successively generated by various forcing disturbances moving with near-resonant speeds are found by numerically solving a forced Kadomtsev–Petviashvili (fKP) equation, which shows in its simplest form the interplay of a basic linear wave operator, longitudinal and transverse dispersion, nonlinearity, and forcing. Curved solitary waves are found as a slowly varying similarity solution of the Kadomtsev–Petviashvili (KP) equation, and are favorably compared with the upstream-advancing waves numerically obtained.
Probability distribution functions in turbulent convection
Authors: Balachandar S. , Sirovich L.
Published: July 2002

Results of an extensive investigation of probability distribution functions (pdf’s) for Rayleigh–Bénard convection, in the hard turbulence regime, are presented. It is seen that the pdf’s exhibit a high degree of internal universality. In certain cases this universality is established within two Kolmogorov scales of a boundary. A discussion of the factors leading to universality is presented.
The settling of nonspherical particles in a cellular flow field
Authors: Mallier R. , Maxey M.
Published: July 2002

The motion of rigid spheroidal particles settling under gravity in a spatially periodic, cellular flow field has been studied. The particles are sufficiently small that their motion relative to the surrounding fluid satisfies the conditions for local Stokes flow, and the force and couple on the particle are linearly related to the local flow conditions. The motion of each particle depends on the orientation of its symmetry axis, which turns in response to the local vorticity and rate of strain. For spherical shapes the cellular flow field generally can hold particles in permanent suspension, as they move in simple closed paths, over a significant portion of each cell. By comparison, for nonspherical shapes this suspension is greatly reduced, though not eliminated. The individual particles undergo a tumbling motion as they settle which, at large enough aspect ratios, is found to be chaotic.
Drifting vortices in ramped Taylor vortex flow: Quantitative results from phase equation
Authors: Paap Hans-Georg , Riecke Hermann
Published: July 2002

The phase equation approach for the description of patterns in a spatially varying environment is tested for realistic setups. To this end the phase equation for axisymmetric Taylor vortex flow with spatially varying cylinder radii (spatial ramps) is derived and solved for various geometries which allow a detailed comparison with recent experiments. The wave number selected by subcritical ramps and its dependence on the geometry is determined. A suitable choice of the ramp allows the selection of wave numbers for which the pattern is unstable with respect to a wavelength changing instability (e.g., Eckhaus instability). This leads to a drift of the pattern. The drift velocity is calculated as a function of the Reynolds number for different geometries. Without any adjustable parameters the results for the selected wave numbers as well as for the drift velocities agree well with recent experiments. The calculations suggest the possibility of spatiotemporal chaos in suitably ramped systems.
Flapping model of scalar mixing in turbulence
Authors: Kerstein Alan R.
Published: July 2002

Motivated by the fluctuating plume model of turbulent mixing downstream of a point source, a flapping model is formulated for application to other configurations. For the scalar mixing layer, simple expressions for single-point scalar fluctuation statistics are obtained that agree with measurements. For a spatially homogeneous scalar mixing field, the family of probability density functions previously derived using mapping closure is reproduced. It is inferred that single-point scalar statistics may depend primarily on large-scale flapping motions in many cases of interest, and thus that multipoint statistics may be the principal indicators of finer-scale mixing effects.
The deformation of a liquid film flowing down an inclined plane wall over a small particle arrested on the wall
Authors: Pozrikidis C. , Thoroddsen S. T.
Published: July 2002

The shape of the free surface of a liquid film flowing down an inclined plane wall over a particle captured on the wall is studied in the asymptotic limit where the size of the particle is much smaller than the film thickness, and the Reynolds number is vanishingly small. Using the boundary integral method for Stokes flow, the problem is formulated in terms of a system of four linear integral and differential equations for the elevation of the free surface and the three components of the velocity at the free surface, and is solved using a semi-iterative method. The results demonstrate that the particle causes a marked deformation of the free surface upstream right before the particle, and a surface corrugation resembling a surface wake downstream behind the particle. The detailed features of the deformation and the disturbance flow are affected strongly by the geometry of the particle, surface tension, and the inclination of the wall.
Influences of swirl on the structure and extinction of strained premixed flames. Part II: Strong rates of rotation
Authors: Kim J. S. , Libby Paul A. , Williams Forman A.
Published: July 2002

The characteristics of isenthalpic planar premixed flames in counterflowing streams involving high Reynolds numbers and either corotating or counter-rotating swirl of large magnitudes are analyzed. The method of activation energy asymptotics is applied to a one-step Arrhenius reaction with small departures from unity Lewis numbers. Density changes resulting from heat release are fully taken into account. High swirl is shown to lead to three stagnation points and regions of radial inflow in agreement with experiment. It is shown that the character of the viscous regions containing the flames and their extinction behavior are significantly influenced by swirl for strong rates of rotation. The present analysis shows that the resonances found in an earlier inviscid calculation are not physically realizable and accounts for the reversal of the lean flammability limit with rotation rate found experimentally.
A numerical study of the effect of surface tension and noise on an expanding Hele–Shaw bubble
Authors: Dai Wei-Shen , Shelley Michael J.
Published: July 2002

In this paper, the dynamics of an interface under the influence of surface tension is studied numerically for flow in the Hele–Shaw cell, where the interface separates an expanding bubble of inviscid fluid from a displaced viscous fluid. Of special interest is the long–time behavior of the so-called q-pole initial data, whose motion is explicitly known and globally smooth for the zero surface tension flow. The numerical method is spectrally accurate and based upon a boundary integral formulation of the problem, together with a special choice for the frame of motion along the interface. In 64-bit arithmetic, a transition from the formation of side branches to tip splitting is observed as the surface tension is decreased. The tip splitting occurs on a time scale that decreases with the surface tension. This is consistent with some experimental observations. However, by increasing the arithmetic precision to 128 bits, it is found that this transition occurs at a yet smaller surface tension. The tip splitting is associated with the growth of noise in the calculation at unstable scales allowed by the surface tension, and a simple linear model of this growth seems to agree well with the observed behavior. The robustness of the various observed structures to varying amounts of noise is also investigated numerically. It is found that the appearance of side branches seems to be the intrinsic effect of surface tension, and the time scales for their appearance increases as the surface tension decreases. These results suggest, with some qualification, that surface tension acts as a regular perturbation to evolution from this initial data, even for long times.
Asymptotic behavior of modulated Taylor–Couette flows with a crystalline inner cylinder
Authors: Braun R. J. , McFadden G. B. , Murray B. T. , Coriell S. R. , Glicksman M. E. , Selleck M. E.
Published: July 2002

The linear stability of a modulated Taylor–Couette system when the inner cylindrical boundary consists of a crystalline solid-liquid interface is considered. Both experimentally and in numerical calculations it is found that the two-phase system is significantly less stable than the analogous rigid-walled system for materials with moderately large Prandtl numbers. A numerical treatment based on Floquet theory is described, which gives results that are in good agreement with preliminary experimental findings. In addition, this instability is further examined by carrying out a formal asymptotic expansion of the solution in the limit of large Prandtl number. In this limit the Floquet analysis is considerably simplified, and the linear stability of the modulated system can be determined to leading order through a conventional stability analysis, without recourse to Floquet theory. The resulting simplified problem is then studied for both the narrow gap geometry and for the case of a finite gap. It is surprising that the determination of the linear stability of the two-phase system is considerably simpler than that of the rigid-walled system, despite the complications introduced by the presence of the crystal-melt interface.
Two- and three-dimensional buoyant thermocapillary convection
Authors: Mundrane M. , Zebib A.
Published: July 2002

Results of steady, three-dimensional numerical calculations of combined buoyant and thermocapillary-driven convection in a cavity are presented. The flow is assumed to be spatially periodic along the cavity, isothermal at other vertical boundaries, and adiabatic at horizontal ones. The Prandtl number, Marangoni number to Grashof number ratio, and geometry are fixed while the Marangoni number is varied over sufficient range to demonstrate a bifurcation from two-dimensional to three-dimensional flow. Two-dimensional results obtained from full three-dimensional calculations are typical of solutions generated by two-dimensional codes, with all field variables independent of the assumed periodic direction. Three-dimensional solutions represent a distinct departure from projected two-dimensional solutions with velocity component parallel to the periodic direction comparable in magnitude to the other two components. The numerical results are in qualitative agreement with available experimental results.
Reply to the comments of Simon and Short
Authors: Drake R. P. , Turner R. E. , Lasinski B. F. , Williams E. A. , Phillion D. W. , Estabrook Kent , Kruer W. L. , Campbell E. M. , Johnston T. W.
Published: July 2002

In their comment, Simon and Short argue that their enhanced plasma wave model remains a possible explanation of the data reported in a previous paper [Phys. Fluids 31, 3130 (1988)]. While their clarifications of this model do invalidate the single, numerical argument used in that paper to reject it, a number of reasons remain to be skeptical of this model. In the present reply, these reasons are summarized.
Exact and almost exact solutions to the Vlasov–Maxwell system
Authors: Mahajan Swadesh M.
Published: July 2002

Exact and almost exact solutions to the Vlasov–Maxwell systems describing a variety of plasma configurations with density, temperature, and current gradients are presented. Possible consequences of these solutions are also discussed.
Gyrokinetics of transverse-magnetic-mode gyrotron, gyropeniotron, cyclotron autoresonance maser, and nonwiggler free-electron laser amplifiers
Authors: Zhang Shi-Chang
Published: July 2002

In this paper a comprehensive kinetic, gyrokinetics theory is presented to provide a unified description of transverse-magnetic-mode gyrotron, gyropeniotron, cyclotron autoresonance maser, and nonwiggler free-electron laser amplifiers. By introducing gyrokinetic variables, a unified dispersion equation is derived in which the effect of the guiding-center shift is taken into account. Similar to the transverse-electric-mode cases, it is found that at the zero point of the Jm−l (R̄) instability is still effective, where Jm−l is the Bessel function of order (m−l), m and l are, respectively, the azimuthal index of the mode and the index of the synchronous harmonic, and R̄ is the electron beam radius normalized by the cutoff wavenumber.
Nonlinear dynamics of electrons accelerated by resonant fields in nonuniform plasmas
Authors: Maggs J. E. , Morales G. J.
Published: July 2002

A numerical and analytical study is presented of the nonlinear phase-space trajectories of particles accelerated by resonant electric fields in a nonuniform plasma. The fields are represented by a driven Airy pattern characteristic of resonant excitation, and thus the results have relevance to several topics of current interest. The acceleration processes can be classified into three regimes depending upon the initial energy of the particles. The first type is a diffusive interaction which occurs for high-energy particles. The second type is slowing down due to temporary trapping inside potential wells having a spatially decreasing phase velocity. The third type is a phase-independent acceleration of slow particles. The comparison velocity used to classify these regimes is ωL/(kDL/√3)2/3, where ω is the resonant frequency, L the gradient scale length, and kD the Debye wave number. Simple analytic expressions are found that explain the basic features of numerical test particle orbits and illuminate the existence of the three categories of interaction.
Experimental study of a high-frequency megawatt gyrotron oscillator
Authors: Kreischer K. E. , Grimm T. L. , Guss W. C. , Mobius A. W. , Temkin R. J.
Published: July 2002

A detailed experimental study of the efficiency and output power of a pulsed gyrotron operating in the TE16,2,1 mode at 148 GHz has been conducted. A peak efficiency of 30% was achieved at 80 kV and 20 A for an output power of 480 kW. The highest output power of 925 kW, corresponding to an efficiency of 19%, was measured at 120 kV and 40 A. Two cavities with different interaction lengths (6.0λ and 4.2λ) were investigated. In both cases, agreement was found between the theoretical and experimental efficiency for beam currents up to 15–20 A. At higher currents, the experimental efficiency saturated between 20% and 25%, well below the 35%– 40% predicted by theory. No increase was obtained for modest positive or negative linear tapering of the cavity magnetic field. Measurements indicate that the beam velocity ratio decreases as beam current increases, partially explaining the reduced efficiency at higher currents. Operation in different azimuthal rotations of the cavity modes was also observed. The measured rotation was found to be consistent with the theoretical coupling between the beam and rf field.
Backscattered light near the incident laser wavelength from 0.35 μm irradiated long scale length plasmas
Authors: Young P. E. , Estabrook K. G. , Kruer W. L. , Williams E. A. , Wegner P. J. , Drake R. P. , Baldis H. A. , Johnston T. W.
Published: July 2002

Backscattered light at wavelengths near the incident laser wavelength of 0.35 μm irradiated thin targets has been spectrally and temporally resolved. Low-Z targets show significant backscattered signals even after the foils burn through when the density gradient scale length is approximately 1000 times the laser wavelength. The results show the importance of considering seeding mechanisms in the plasma.
Critical wave spectra for stochastic heating of a magnetized plasma
Authors: Deeskow P. , Elsässer K. , Jestczemski F.
Published: July 2002

Stochasticity of particles in a magnetized plasma as a result of many waves with common frequency ω and different wave vectors k is examined. A critical potential amplitude Φc(k)=εc/k4∥ is verified with the following property: Effective heating along the magnetic field B0 by small-amplitude waves is only possible in the velocity interval (ω/kmax) ≤v∥ ≤(ω/kmin), where kmin, kmax denote the interval for k∥ (parallel to B0) in which ‖Φ(k)‖≥‖Φc(k)‖. Heating perpendicular to B0 by circularly polarized waves with a Fourier spectrum peaked around a perpendicular wave vector k⊥ is strongly inhibited by the presence of an adiabatic invariant, but these barriers can be overcome by waves with a sufficiently broad spectrum in k⊥.
Current drive via magnetohydrodynamic helicity waves in a nonuniform plasma
Authors: Mett Richard R. , Tataronis John A.
Published: July 2002

Fundamentals of low-frequency current drive produced by a packet of arbitrarily polarized nonlinear Alfvén waves in an incompressible plasma with an inhomogeneous equilibrium magnetic field and aligned fluid velocity are explored. Analysis of the equations of magnetohydrodynamics with electrical conductivity and kinematic viscosity using an eikonal formulation yields coupled nonlinear equations for the Alfvén wave amplitude and an induced magnetic field and fluid velocity. Introduction of suitable variables allows simplification of the nonlinear system into a system of coupled linear integrodifferential equations. Particular solutions relate the plasma current to the helicity of the Alfvén wave. Two separate and distinct agents contribute to the current drive process: (1) wave dissipation, and (2) spatial inhomogeneity of the plasma medium. The plasma inhomogeneities cause a direct transfer of helicity from the applied Alfvén wave to the equilibrium magnetic field and are connected to the damping of a single component of the wave fluctuation due to phase mixing.
Resistive magnetohydrodynamic stability of stellarators with increasing plasma pressure
Authors: Charlton L. A. , Leboeuf J. N. , Lynch V. E.
Published: July 2002

A sequence of toroidally averaged stellarator equilibria with increasing values of β was studied by numerically following the time evolution. It was found that the β dependence of many different plasma parameters indicated that this sequence represented plasmas whose resistive stability properties first worsened and then improved with β signaling entry into the second stability region. A study of the radial dependence of the stability showed that the plasma was limited in the plasma interior by a mode with a large resistive ballooning component and toward the edge by a resistive interchange mode.
Estimating the runaway diffusion coefficient in the TEXT tokamak from shift and externally applied resonant magnetic-field experiments
Authors: Catto Peter J. , Myra J. R. , Wang P. W. , Wootton A. J. , Bengtson Roger D.
Published: July 2002

Two techniques are employed on the TEXT tokamak [Nucl. Fusion 29, 547 (1989); 30, 167 (1990)] to measure the runaway diffusion coefficient D: (i) displacing the plasma column, and (ii) externally applying resonant magnetic fields. Model diffusion equations for the experiments are solved to obtain analytic predictions which then can be used to interpret the measurements. The initial transient response of the x-ray flux to the perturbing shift or applied magnetic field is used to measure a D of order 1 m2 sec−1 near the edge that decreases inward.
Vlasov fluid stability of a skin current Z pinch with finite electron pressure
Authors: Arber T. D.
Published: July 2002

The linear stability of a Z-pinch confined by a skin current has been studied in the collisionless regime using the Vlasov fluid model for arbitrary m number. The trajectory integrals that are normally so formidable an aspect of Vlasov fluid analyses are greatly simplified in this equilibrium and the eigenvalue equation reduces to a dispersion relation derived from the plasma edge boundary condition. For the case Te=0, an analytic solution is found in the short-wavelength limit for the growth rate, which saturates for all m at a value of π1/2vT/2a, where vT is the ion thermal velocity and a is the pinch radius. This should be compared with the ideal magnetohydrodynamic (MHD) growth rate for the same equilibrium that increases without limit as k1/2, where k is the axial wave number. The solution with finite electron temperature is achieved by a Neumann series expansion, which is shown to converge for all finite values of Γ, the ratio of specific heats. In the short-wavelength limit with Te=Ti, the growth rates are double those for cold electrons. The deviation from this factor of two increase is always less than 10% for longer wavelengths.
Comments on ‘‘Structure of the plasma sheath in collisional to weakly collisional plasmas’’ [Phys. Fluids B 2, 647 (1990)]
Authors: Riemann K.-U. , van den Berg H.-P.
Published: July 2002

Recently, Zawaideh et al. published a paper [Phys. Fluids B 2, 647 (1990)] asserting that they had obtained, from a more accurate set of fluid equations, physically reasonable plasma sheath solutions of an oscillating nature violating Bohm’s criterion. In contrast to this statement, it is shown here that the violation of Bohm’s criterion is based on an incorrect interpretation of the plasma sheath concept and that the oscillations are due to an incorrect initial condition in the calculations.
Determination of the safety factor in sawtoothing discharges in DIII-D
Authors: Wróblewski D. , Lao L. L.
Published: July 2002

Local magnetic field pitch angle is determined in DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. I, p. 159] by measuring the direction of polarization of Stark components of the Dα (deuterium Balmer-α) line emitted by the heating neutral beam. The measurement result is incorporated into the magnetic equilibrium code to provide a constraint on the calculated profiles of current density and safety factor (inverse rotational transform). The safety factor profiles obtained with the use of the polarimetry data indicate that the safety factor on axis q0 is close to 1.0 at the onset of the sawtooth instability but may be substantially smaller than 1.0 in later stages of a discharge. The magnetic field pitch angle variations associated with the sawtooth instability were observed in some discharges with a time resolution of about 10 msec. The increase of q0 during a sawtooth crash is found to be not larger than about 0.1±0.05.
Stable solutions of nonlocal electron heat transport equations
Authors: Prasad M. K. , Kershaw D. S.
Published: July 2002

Electron heat transport equations with a nonlocal heat flux are in general ill-posed and intrinsically unstable, as proved by the present authors [Phys. Fluids B 1, 2430 (1989)]. A straightforward numerical solution of these equations will therefore lead to absurd results. It is shown here that by imposing a minimal set of constraints on the problem it is possible to arrive at a globally stable, consistent, and energy conserving numerical solution.
Core turbulence and transport studies on the Texas Experimental Tokamak
Authors: Bravenec R. V. , Gentle K. W. , Richards B. , Ross D. W. , Sing D. C. , Wootton A. J. , Brower D. L. , Luhmann N. C. , Peebles W. A. , Yu C. X. , Crowley T. P. , Heard J. W. , Hickok R. L. , Schoch P. M. , Yang X. Z.
Published: July 2002

Recent experiments on the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)] have focused on identifying the drives and transport significance of the turbulence in the interior of discharges in the neo-Alcator confinement regime. Far-infrared (FIR) scattering consistently observes density fluctuations indicative of electron drift waves, i.e., ωk∼ωe*+kθvE, where vE≡−Er/B (the electron diamagnetic frequency Doppler-shifted by the E×B poloidal plasma rotation) and an amplitude that scales inversely with the density scale length Ln. Although consistent with scattering on the power-weighted frequency ω̄, heavy-ion beam probe (HIBP) data typically indicate k̄θρs<0.1 such that ω̄≫ω̄e*+k̄θvE and n̄/ne≪1/k̄θLn. Experiments with a modulated gas feed and/or electron-cyclotron resonance heating (ECRH) seem to rule out ∇Te as the turbulent drive, although little evidence for ∇ne is apparent either. In fact, the interior fluctuations seen by the HIBP seem to depend more on edge conditions than local gradients. The quasilinear fluxes due to dissipative trapped electrons are found to be sufficient to account for the equilibrium fluxes in the region of low collisionality (νe*≪1) for a drift-wave spectrum (k̄θρs∼0.3), but not for the low wave numbers seen by the HIBP. While the estimates with k̄θρs∼0.3 predict the scaling of the electron energy flux over a range of discharge conditions with νe*≪1, the model seems unable to predict the magnitude (or phase) of the perturbed particle flux during modulated gas feed experiments. In the outer regions of the plasma where νe*≳1, the calculated quasilinear fluxes are inadequate, even including untrapped particles.
Filamentation in long scale length plasmas: Experimental evidence and effects of laser spatial incoherence
Authors: Labaune C. , Baton S. , Jalinaud T. , Baldis H. A. , Pesme D.
Published: July 2002

Signatures of filamentation have been observed in preformed plasmas using complementary diagnostics: time-resolved images of the transmitted laser light, dark field imaging, and time-resolved spectra of Raman light. This last diagnostic clearly shows the presence of small channels inside the plasma with temporal evolution in agreement with the formation of filaments. The filamentary structures disappeared when random phase plates were used in the laser beam. This result is in agreement with a theoretical analysis showing that filamentation does not grow when the speckle size is smaller than the perturbation wavelength which maximizes, in the coherent case, the filamentation growth.
Nonlinear kink mode dynamics in circular and noncircular pinches
Authors: Wahlberg Christer
Published: July 2002

It is shown that the global (free-boundary) m=1 kink instability of the ideal, magnetohydrodynamic (MHD) sharp boundary (surface current) pinch is stabilized by nonlinear effects, provided Be≲1 and βp<1, where βp=1+B2e−B2i and Bi and Be denote, respectively, the internal and external axial magnetic fields of the pinch, normalized to the poloidal magnetic field. The stabilization has to do with the bending of the interior, ‘‘frozen’’ field lines and associated volume currents induced in the pinch, and does not occur in a pure surface current model, which neglects these currents and only conserves the total magnetic flux through the pinch. It is suggested that the global, helical m=1 structures observed in various pinch experiments may have to do with the stabilizing mechanism above. The nonlinear stability has been calculated by means of a new approach to the bifurcated equilibria of the helical m=1 mode, and the method should also be useful in connection with other nonlinear, ideal MHD phenomena. The regime of nonlinear stability above corresponds to intermediate or short wavelengths of the marginal mode (ka≳1). In the opposite, long-wavelength regime, the ideal MHD model and the pure surface current model give similar results, predicting nonlinear instability for, e.g., the nearly marginal Kruskal–Shafranov mode in tokamaks, in agreement with previous theories. Effects of mode rotation as well as of a noncircular cross section of the pinch, modeling the Extrap [Fusion Technol. 16, 7 (1989)] configuration, have also been considered, extending the results of a previous, linear investigation [Phys. Fluids B 2, 1601 (1990)] to the nonlinear regime.
Measurements of backscattered light from the interaction of 0.35 μm laser light with high-Z targets
Authors: Young P. E. , Berger R. L. , Estabrook K. G.
Published: July 2002

Backscattered light at wavelengths near the incident laser frequency of 0.35 μm irradiated 0.4 μm thick gold targets has been spectrally and temporally resolved. At the start of the laser pulse, the wavelength of the backscattered light is longer than that of the incident laser, and the wavelength subsequently increases with time. The observed scaling of the backscatter level with the incident laser intensity, the bandwidth of the backscatter spectrum, and the absolute value of the backscatter flux, are adequately described by absolute growth of the stimulated Brillouin scatter instability from thermal noise, provided that realistic hot spots in the incident laser profile are taken into account. The differences between backscattered light from high-Z and low-Z targets are discussed.
Characterization of a microsecond-conduction-time plasma opening switch
Authors: Commisso R. J. , Goodrich P. J. , Grossmann J. M. , Hinshelwood D. D. , Ottinger P. F. , Weber B. V.
Published: July 2002

This paper presents data and analyses from which emerges a physical picture of microsecond-conduction-time plasma opening switch operation. During conduction, a broad current channel penetrates axially through the plasma, moving it toward the load. Opening occurs when the current channel reaches the load end of the plasma, far from the load. During conduction, the axial line density in the interelectrode region is reduced from its value with no current conduction as a result of radial hydrodynamic forces associated with the current channel. A factor of 20 reduction is observed at opening in a small, localized region between the electrodes. When open, the switch plasma behaves like a section of magnetically insulated transmission line with an effective gap of 2 to 3 mm. Increasing the magnetic field in this gap by 50% results in an improvement of 50% in the peak load voltage and load current rise time, to 1.2 MV and 20 nsec, respectively. An erosion opening mechanism explains the inferred gap growth rate using the reduced line density at opening. Improved switch performance results when the maximum gap size is increased by using a rising load impedance.
Gyrokinetic theory of fast wave transmission with arbitrary parallel wave number in a nonuniformly magnetized plasma
Authors: Lashmore-Davies C. N. , Dendy R. O.
Published: July 2002

The gyrokinetic theory of ion cyclotron resonance is extended to include propagation at arbitrary angles to a straight equilibrium magnetic field with a linear perpendicular gradient in strength. The case of the compressional Alfvén wave propagating in a D(3He) plasma is analyzed in detail. A self-consistent local dispersion relation is obtained using a single mode description; this approach enables three-dimensional effects to be included and permits efficient calculation of the transmission coefficient. The dependence of this quantity on the species density ratio, minority temperature, plasma density, magnetic field, and equilibrium scale length is obtained. A self-consistent treatment of the variation of the field polarization across the resonant region is included. Families of transmission curves are given as a function of the normalized parallel wave number for parameters relevant to JET [see, for example, J. Jacquinot et al., Plasma Phys. 30, 1467 (1988)]. Perpendicular absorption by the minority ions is also discussed, and shown to depend on a single parameter, the ratio of the ion thermal velocity to the Alfvén speed.
Integration of the Boltzmann collision term over scattering angles
Authors: Chang Yongbin
Published: July 2002

In case the distribution function of the field particle is Maxwellian, the Boltzmann collision term for both Coulomb and smooth rigid elastic interaction is directly integrated over scattering angles without using Taylor series expansion. For Coulomb interaction, a cutoff of Δvα min is introduced instead of the usual θmin cutoff. A Boltzmann collision term of the form (∂fα/∂t)c=∫[G(v′α) −G(vα)]C(vα,v′α)dv′α is obtained.
Comparison of the density fluctuation spectrum and amplitude in TEXT with expectations for electron drift waves
Authors: Yu C. X. , Brower D. L. , Zhao S. J. , Peebles W. A. , Luhmann N. C. , Bravenec R. V. , Chen J. Y. , Lin H. , Ritz Ch. P. , Schoch P. M. , Yang X. Z.
Published: July 2002

Modifications to the spectrum and magnitude of turbulent density fluctuations in the Texas Experimental Tokamak (TEXT) [Plasma Phys. Controlled Fusion 27, 1335 (1985)], measured by far-infrared scattering, are related to changes in plasma profiles (e.g., density and potential) induced by naturally occurring magnetic islands (i.e., m=2) of varying amplitude. The statistical dispersion relation of the microturbulence that propagates in the electron diamagnetic drift direction is observed to be consistent with expectations for electron drift-wave turbulence when a spectral shift due to E×B plasma rotation is included. Observed spectral widths are several times larger than the linear mode frequency. The fluctuation amplitude is found to scale inversely with the electron density scale length.