bluuebunny/crossref_metadata_2025 huggingface dataset:
Modified Born-Oppenheimer basis for nonadiabatic coupling: Application to the vibronic spectrum of HD+
Authors: Tuvi Inbal , Band Y. B.
Published: July 2002

Nonadiabatic matrix elements, when computed using a Born-Oppenheimer (BO) basis, do not vanish asymptotically because the motion of the electrons with the nuclei at large internuclear separations is not taken into account. We apply a method suggested by Delos [Rev. Mod. Phys. 53, 287 (1981)] to include the effect of electron translation factors in a quantum-mechanical framework, thus correcting the BO basis to incorporate proper boundary conditions. We calculate the nonadiabatic matrix elements for H2+ and its isotopic variants. We focus our calculations on HD+, for which experimental results exist, and calculate its vibronic spectrum. This is the first application of this method to calculate high precision spectroscopic information for molecular systems.
Comment on “Remarks on the information entropy maximization method and extended thermodynamics” [J. Chem. Phys. 108, 5834 (1998)]
Authors: Robertson Baldwin
Published: July 2002

By making the short-correlation-time and short-correlation-distance approximations, we show how the Liouville equation can be used to produce a theory that describes ireversible processes even though the equation is invariant under time reversal. Thus B. C. Eu’s claims about the consistency of the equations of motion formalism in nonequilibrium statistical mechanics are not well supported.
Collisional effects on angular momentum orientation in acetylene X̃ 1Σg+ (ν2″=1,j″). I. Preparation, detection and conservation in single collisions
Authors: Rudert Armin D. , Martin José , Gao Wen-Bin , Halpern Joshua B. , Zacharias Helmut
Published: July 2002

The effect of rotationally inelastic collisions on the orientation of the angular momentum is investigated for C2H2 (ν2=1, j). The orientation of the angular momentum in initial rotational states ji=7 to 13 is prepared by stimulated Raman pumping using circularly polarized light and S-branch transitions. After allowing an appropriate time for collisions to occur the orientation is probed by laser-induced fluorescence in the Ã 1Au←X̃ 1Σg transition. For acetylene the theoretically calculated and the measured initial orientation, A0(1), ranges between 0.7 and 0.9. A substantial part of the orientation is conserved in rotationally inelastic collisions, which could be measured up to |Δj|=6 with sufficient signal-to-noise ratio. These results are compared with previous measurements on the conservation of alignment in collisions. Rotationally inelastic collisions with |Δj|=2 result in an average rotation of the orientation vector by about 30°. The amount of orientation that remains after a rotationally inelastic collision is found to depend primarily on |Δj|. A Δmj=0 propensity rule often used in models of rotational energy transfer is found to be inconsistent with the measurements. Also a model which proposes a minimum change of the classical angle φ between j⃗ and the quantization axis does not reproduce well the experimental data. Finally, a microscopic model for Δmj-transitions is presented which reconciles both alignment and orientation measurements.
A time-dependent discrete variable representation method
Authors: Adhikari Satrajit , Billing Gert D.
Published: July 2002

We have developed a novel discrete variable representation (DVR) method where not only the amplitudes of the wave function at the DVR grid points can change but also the positions of these grid points can move as a function of time. Since the Gauss–Hermite basis set is used as the primitive basis functions (PBF) to construct the DVR basis set, the method appears as a semiclassical one with a small number of PBF but converges very fast to the quantum with an increasing PBF. We have investigated the dynamics of a reaction coordinate with or without coupling to a heat bath of harmonic oscillators to demonstrate the validity of the proposed method. The excellent agreement of the calculated tunneling probabilities with numbers obtained by traditional quantum grid method (FFT) and the fast computability of the present method compared to the latter are remarkable.
Solutions of the generalized Bogomol’nyi equations via monotone iterations
Authors: Wang Sheng , Yang Yisong
Published: July 2002

This paper is concerned with the numerical solutions of the classical Abelian Higgs vortex model in R2 in the critical coupling phase. A globally convergent monotone iterative method will be presented to approximate finite energy multivortex solutions of both the classical and the generalized Bogomol’nyi equations. In the latter context, it is illustrated through a series of numerical examples that the Higgs potential density function may be adjusted to realize in a wide range fairly different magnetic concentration pictures of the model.
Redundancy of the quantum level gauge condition
Authors: Kachkachi H. , Kachkachi M.
Published: July 2002

The manifold Γ̂ defined by the equations of motion (EM) of the gauge and ghost fields w.r.t. the gauge-fixed action is regarded as a supercanonical line bundle over the manifold Γ defined by the EM of the gauge fields only w.r.t. the classical action. In this language the BRST operator Q is a local section of the bundle Γ̂. Using this we give a local expression for Q as being, on the other hand, the nilpotent exterior derivative on Γ with the ghost field Ψ as its generator. This fiber bundle setup allows us to prove that any ‘‘second level’’ gauge condition, i.e., a gauge condition on Γ̂ is equivalent to a gauge on the base manifold Γ and thus does not break the BRST symmetry of the quantized theory.
A connection between quantum dynamics and approximation of Markov diffusions
Authors: Morato L. M. , Ugolini S.
Published: July 2002

It is observed that the existence of an attracting set in the class of solutions to the stochastic Lagrangian variational principle leads to a natural problem of convergence of diffusions in the Carlen class. It is then shown how dynamical properties enable one to prove some convergence results in the two-dimensional Gaussian case.
Unstable geodesics and topological field theory
Authors: Yasui Yukinori , Takahashi Satoshi
Published: July 2002

A topological field theory is used to study the cohomology of mapping space. The cohomology is identified with the Becchi–Rouet–Stora–Tyutin cohomology realizing the physical Hilbert space and the coboundary operator given by the calculations of tunneling between the perturbative vacua. The method is illustrated by a simple example.
Exotic Majorana spinors in (3+1)-dimensional space–times
Authors: Heß Georg
Published: July 2002

Inequivalent spin structures on Lorentzian manifolds are studied and in particular the possibility of defining inequivalent Majorana spinors associated with real irreducible representations of the Clifford algebra corresponding to a metric of signature (3,1) is analyzed. Such exotic complex (Dirac) spinors were first discussed in 1979. It is shown that exotic real (Majorana) spinors can be defined in a consistent way, too. The main idea is the use of a ‘‘chiral’’ U(1) group instead of a ‘‘complex’’ U(1) which contains all the relevant topological information. This result may be interesting in the context of the geometry of supergravity–matter coupling.
Interpolation and partial differential equations
Authors: Maligranda Lech , Persson Lars Erik , Wyller John
Published: July 2002

One of the main motivations for developing the theory of interpolation was to apply it to the theory of partial differential equations (PDEs). Nowadays interpolation theory has been developed in an almost unbelievable way {see the bibliography of Maligranda [Interpolation of Operators and Applications (1926–1990), 2nd ed. (Luleå University, Luleå, 1993), p. 154]}. In this article some model examples are presented which display how powerful this theory is when dealing with PDEs. One main aim is to point out when it suffices to use classical interpolation theory and also to give concrete examples of situations when nonlinear interpolation theory has to be applied. Some historical remarks are also included and the relations to similar results are pointed out.
Quantum Lobachevsky planes
Authors: Leitenberger Frank
Published: July 2002

We classify all SL(2,R)-covariant Poisson structures on the Lobachevsky plane with respect to all multiplicative Poisson structures on SL(2,R) and describe quantizations for all these Poisson structures.
One dimensional periodic Dirac Hamiltonians: Semiclassical and high-energy asymptotics for gaps
Authors: Nenciu Gheorghe , Purice Radu
Published: July 2002

In this article we consider a one-dimensional Dirac operator with a potential of Gevrey class α and study the semiclassical and high-energy asymptotics of the spectral gaps for a region of energies that in the Schrödinger case corresponds to unbounded motion. An exponential upper bound for the gap’s widths as well as the asymptotic expansion of their positions are derived for both cases; the first two terms in the asymptotic expansions are explicitly written down for the scalar potential case.
Integral equation methods for the inverse problem with discontinuous wave speed
Authors: Aktosun Tuncay , Klaus Martin , van der Mee Cornelis
Published: July 2002

The recovery of the coefficient H(x) in the one-dimensional generalized Schrödinger equation d2ψ/dx2+k2H(x)2ψ=Q(x)ψ, where H(x) is a positive, piecewise continuous function with positive limits H± as x→±∞, is studied. The large-k asymptotics of the wave functions and the scattering coefficients are analyzed. A factorization formula is given expressing the total scattering matrix as a product of simpler scattering matrices. Using this factorization an algorithm is presented to obtain the discontinuities in H(x) and H′(x)/H(x) in terms of the large-k asymptotics of the reflection coefficient. When there are no bound states, it is shown that H(x) is recovered from an appropriate set of scattering data by using the solution of a singular integral equation, and the unique solvability of this integral equation is established. An equivalent Marchenko integral equation is derived and is shown to be uniquely solvable; the unique recovery of H(x) from the solution of this Marchenko equation is presented. Some explicit examples are given, illustrating the recovery of H(x) from the solution of the singular integral equation and from that of the Marchenko equation.
On the Poisson integrals representation in the classical statistical mechanics of continuous systems
Authors: Gielerak R. , Rebenko A. L.
Published: July 2002

Description of the grand canonical Gibbs ensemble for classical continuous systems in terms of the nonlocally perturbed infinite-divisible generalized random fields is presented. The equivalence of the traditional description with the ones presented here on the level of DLR equations is established. The antiferromagnetism for the purely repulsive interactions has been observed. Finally, the usefulness of our description for an analysis of the high-temperature cluster expansion has been demonstrated.
Random perturbations of iterated maps
Authors: Salazar-Anaya Gelasio , Urías Jesús
Published: July 2002

Random perturbations of Markov processes on systems of contractive maps are considered. The existence of a unique invariant measure for the randomly perturbed process is proved under a very weak assumption on perturbations. In particular, the proof holds for the full class of translationally invariant perturbations.
Conditional symmetry and spectrum of the one-dimensional Schrödinger equation
Authors: Zhdanov R. Z.
Published: July 2002

We develop an algebraic approach to studying the spectral properties of the stationary Schrödinger equation in one dimension based on its high-order conditional symmetries. This approach makes it possible to obtain in explicit form representations of the Schrödinger operator by n×n matrices for any n∈N and, thus, to reduce a spectral problem to a purely algebraic one of finding eigenvalues of constant n×n matrices. The connection to so-called quasiexactly solvable models is discussed. It is established, in particular, that the case, when conditional symmetries reduce to high-order Lie symmetries, corresponds to exactly solvable Schrödinger equations. A symmetry classification of Schrödinger equation admitting nontrivial high-order Lie symmetries is carried out, which yields a hierarchy of exactly solvable Schrödinger equations. Exact solutions of these are constructed in explicit form. Possible applications of the technique developed to multidimensional linear and one-dimensional nonlinear Schrödinger equations are briefly discussed.
Three-dimensional Riemannian manifolds with constant principal Ricci curvatures ρ1=ρ2≠ρ3
Authors: Bueken Peter
Published: July 2002

The main aim of this paper is the study of (nonhomogeneous) three-dimensional Riemannian manifolds with constant principal Ricci curvatures ρ1=ρ2≠ρ3. An error in a recent paper by McManus is pointed out and corrected, and it is shown that the techniques introduced by McManus provide a very simple method to obtain the complete local classification of these manifolds, which was first given by Kowalski.
Dynamical entropy of generalized quantum Markov chains over infinite dimensional algebras
Authors: Park Yong Moon , Shin Hyun Hye
Published: July 2002

We compute the dynamical entropy in the sense of Connes, Narnhofer, and Thirring of generalized quantum Markov chain over infinite dimensional algebras. For the case in which the transition expectation is defined by a set of conditional density amplitudes, we show that the dynamical entropy is equal to the mean entropy. Thus we extend the result of Park [Lett. Math. Phys. 32, 63–74 (1994)] to non-AF type quantum Markov chains. We employ the main method developed in Park [Lett. Math. Phys. 32, 63–74 (1994)] with necessary modifications.
KP constraints from reduced multi-component hierarchies
Authors: Willox R. , Loris I.
Published: July 2002

The (m-vector) k-constrained Kadomtsev–Petviashvili (KP) hierarchy is shown to be a “pseudo”-reduction of the (m+1)-component KP hierarchy. To facilitate the implementation of this reduction on the level of the solutions, the typical multi-component KP solutions are mapped onto solutions of a Toda molecule-type equation from which (Wronskian and Grammian) solutions for the constrained KP hierarchy follow. The reduction of the associated linear systems is discussed and its importance for the choice of bilinear representation of the reduced systems is explained.
Verification of the theory of Brownian motion of a particle through a potential barrier in a viscous medium during experimental study of dislocation acoustic relaxation in normal and superconducting niobium
Authors: Natsik V. D. , Pal-Val P. P.
Published: July 2002

The Kramers theory (1940) describing a thermally activated escape of a particle from a potential well in a viscous medium is used for explaining the acoustic absorption peak discovered by Kramer and Bauer (1967) in niobium in the liquid helium temperature range. It is shown that the properties of the peak observed in experiments correspond to the model of resonant interaction of acoustic vibrations with dislocation kink chains. Kinks, viz., pseudoparticle excitations on dislocation lines, perform diffusive drift in the second-order Peierls relief under the action of acoustic vibrations, experiencing simultaneously viscous drag exerted by conduction electrons. The possibility of a sharp controllable change in electron viscosity during a superconducting transition can be used for verifying the conclusions of the Kramers theory in experimental investigations of the dislocation contribution to attenuation of sound. It is found that at low temperatures, the conditions for the observation of the anomaly predicted by Kramers are created for kinks in niobium: the diffusion mobility of particles increases with the dynamic drag coefficient. This circumstance makes it possible to explain one of the most interesting properties of the Kramer–Bauer peak which could not be interpreted correctly till now, i.e., the displacement of the peak towards low temperatures during a magnetic field induced transition of Nb from the superconducting to the normal state. The possibilities of observation of quantum diffusion of kinks in acoustic experiments are also considered briefly.
Low-temperature dislocation and magnetomechanical acoustic effects in high purity Fe single crystals
Authors: Pal-Val P. P.
Published: July 2002

The effect of preliminary plastic deformation εpl<3% and an external magnetic field H⩽3.35 kOe on linear and nonlinear acoustic properties of high purity Fe single crystals is studied. Measurements were made at frequencies ∼90 kHz in the temperature interval 5.5–300 K using the two-component composite oscillator technique. The wave vector of the longitudinal standing wave and the magnetic field direction coincide with the crystallographic axis 〈731〉. In the linear range, the main attention is paid to the influence of the deformation level and subsequent low-temperature recovery on parameters of the relaxation α-peak near 50 K. The increase in εpl leads to a considerable rise in the height of the peak, while the height and temperature of the peak are considerably lowered in the recovery process. An increase in the amplitude ε0 of the ultrasonic deformation beyond a certain threshold ε0c is accompanied by the emergence of strong amplitude dependences of acoustic parameters of the crystals under investigation. As a result of plastic deformation, the amplitude dependences of the decrement and the Young’s modulus are displaced towards lower amplitudes. Magnetoacoustic effects in a saturating external field are practically independent of the amplitude and display a weak monotonic dependence on temperature in the region of the α-peak. It is concluded that the amplitude dependences as well as the α-peak are of the dislocation nature. Possible microscopic mechanisms of the observed effects are discussed.
Upstream-advancing nonlinear waves in an axisymmetric resonant flow of rotating fluid past an obstacle
Authors: Hanazaki Hideshi
Published: July 2002

A numerical study of rotating flow past an axisymmetric obstacle in a cylindrical tube is described. Long nonlinear inertial waves propagate upstream of the obstacle even when the theoretical speed of long waves as given by the linearized equations is equal to the incoming axial flow speed. This occurs when the upstream swirling velocity field is of the Brugers vortex type and the quadratic nonlinear term in the forced KdV equation is not zero. If the flow has a rigid-body rotation, so that the quadratic nonlinear term vanishes, the nonlinear correction of the wave speed is small and cannot be identified even after large times of evolution.
Linear stability of compressible Taylor–Couette flow
Authors: Kao Kai-Hsiung , Chow Chuen-Yen
Published: July 2002

A temporal stability analysis of compressible Taylor–Couette flow is presented. The viscous flow studied in this paper is contained between two concentric cylinders of infinite length, which are rotating with different angular velocities and are kept at different surface temperatures. The linear stability analysis based on the Chebyshev collocation spectral method can be used to determine both the mode of the disturbance to be first excited and the corresponding eigenfunctions. The effects of differential rotation and temperature difference on the stability of Taylor–Couette flow are contrasted for a range of Mach numbers ranging from incompressible to Mach 3.0. The relative motion of the cylinders dramatically affects the characteristics of the Couette flow at the onset of instability. It is found that, at subsonic speeds of the inner cylinder, the only unstable mode is the axisymmetric first mode. However, more than one unstable mode appear at supersonic cylinder speeds, with the dominant one remaining the first mode. The flow is stabilized or destabilized depending upon the temperature ratio and speeds of the two cylinders. Independent of Mach number and temperature ratio, increasing Reynolds number generally promotes a destabilizing effect, indicating its inviscid nature of the Taylor–Couette flow.
Vorticity generation and evolution in shock-accelerated density-stratified interfaces
Authors: Yang Xiaolong , Chern I-Liang , Zabusky Norman J. , Samtaney Ravi , Hawley John F.
Published: July 2002

The results of direct numerical simulations of inviscid planar shock-accelerated density-stratified interfaces in two dimensions are presented and compared with shock tube experiments of Haas [(private communication, 1988)] and Sturtevant [in Shock Tubes and Waves, edited by H. Gronig (VCH, Berlin, 1987), p. 89] . Heavy-to-light (‘‘slow/fast or s/f) and light-to-heavy (‘‘fast/slow,’’ or f/s) gas interfaces are examined and early-time impulsive vorticity deposition and the evolution of coherent vortex structures are emphasized and quantified. The present second-order Godunov scheme yields excellent agreement with shock-polar analyses at early time. A more physical vortex interpretation explains the commonly used (i.e., linear paradigm) designations of ‘‘unstable’’ and ‘‘stable’’ for the f/s and s/f interfaces, respectively. The later time events are Rayleigh–Taylor like and can be described in terms of the evolution of a vortex layer (large-scale translation and rotation): asymmetric tip vortex ‘‘roll-up’’ and ‘‘binding;’’ layer ‘‘instability;’’ convective mixing; and baroclinic vorticity generation from secondary shock–interface interactions.
A numerical study of wave propagation in a confined mixing layer by eigenfunction expansions
Authors: Hu Fang Q.
Published: July 2002

It is well known that the growth rate of instability waves of a two-dimensional free shear layer is reduced greatly at supersonic convective Mach numbers. In previous works, it has been shown that new wave modes exist when the shear layers are bounded by a channel due to the coupling effect between the acoustic wave modes and the motion of the mixing layer. The present work studies the simultaneous propagation of multiple stability waves using numerical simulation. It is shown here that the coexistence of two wave modes in the flow field can lead to an oscillatory growth of disturbance energy with each individual wave mode propagating linearly. This is particularly important when the growth rates of the unstable waves are small. It is also shown here that the propagation of two neutrally stable wave modes can lead to a stationary periodic structure of rms fluctuations. In the numerical simulations presented here the forced wave modes are propagating at same frequency, but with different phase velocities. In order to track the growth of each wave mode as it propagates downstream, a numerical method that can effectively detect and separate the contribution of the individual wave is given. It is demonstrated that by a least square fitting of the disturbance field with eigenfunctions the amplitude of each wave mode can be found. Satisfactory results as compared to linear theory are obtained.
Energy dissipation in the wavelet-transformed Navier–Stokes equations
Authors: Lewalle Jacques
Published: July 2002

The wavelet transformation has been successfully applied to the Navier–Stokes equations. In the case of Gaussian wavelets, a different light is shed on the role of convective, pressure, and viscous terms. This note focuses on the energy dissipation term in the resulting energy equation. It is shown that a κ2 term, analogous to the dissipation rate in Fourier space, results for wavelets of all orders except the first. For g1 transforms, the coefficient of the dissipation term vanishes, leaving only a spectral diffusion term toward small scales.
The development of a mixing layer under the action of weak streamwise vortices
Authors: Goldstein M. E. , Mathew Joseph
Published: July 2002

The action of weak, streamwise vortices on a plane, incompressible, steady mixing layer is examined in the large Reynolds number limit. The outer, inviscid region is bounded by a vortex sheet to which the viscous region is confined. It is shown that the local linear analysis becomes invalid at streamwise distances O(ε−1), where ε≪1 is the cross-flow amplitude, and a new nonlinear analysis is constructed for this region. Numerical solutions of the nonlinear problem show that the vortex sheet undergoes an O(1) change in position and that the solution is ultimately terminated by a breakdown in the numerical procedure. The corresponding viscous layer shows downstream thickening, but appears to remain well behaved up to the terminal location.
Choking conditions for nonuniform viscous flows
Authors: Higuera F. J. , Liñán A.
Published: July 2002

The laminar viscous compressible flow of a gas in a pipe of constant cross section joining two reservoirs is considered. As it is well known, the pipe flow becomes independent of the discharge pressure if this is lower than a critical value. Then it is found that the parabolic equations describing the high Reynolds number flow in the interior of the pipe tend to develop a singularity at the outlet, where the limiting temperature and velocity profiles satisfy a criticality condition. The structure of the nonuniform flow near the outlet consists of an effectively inviscid vortical bulk region and a viscous boundary layer of low velocity, whose thickness tends to zero at the outlet. Matching between these two regions determines the strength of the singularity. This two-layered structure is thought to be generic for nonuniform viscous flows having to satisfy a no-slip boundary condition; in particular, the thin viscous layer is universal.
Receptivity and the evolution of boundary-layer instabilities over short-scale waviness
Authors: Crouch J. D.
Published: July 2002

Perturbation methods are used to analyze the receptivity and early evolution of boundary-layer instabilities over a wavy surface. The length scale for the surface variation is considered to be much smaller than the natural instability wavelength. This provides a simplified model for distributed roughness. Acoustic disturbances in the free stream couple with steady perturbations resulting from the waviness to force traveling-wave modes. At first order, the disturbance field contains a Stokes mode resulting from the acoustic perturbation, and a pair of steady modes resulting from the surface variation. Interactions between these modes produce steady, unsteady, and traveling-wave modes at second order. Further interactions, at third order, then produce a forcing that is close to resonance with the natural instabilities. The near-resonant forcing provides for a distributed energy transfer into the natural-instability eigenmode resulting in a nonlocalized receptivity. The total amplitude resulting from the receptivity shows an effective subcritical growth. Growth downstream of the neutral point is also increased. When the waviness has an abrupt beginning, the effective growth rates become very large. Results from the analysis show qualitative features that are observed in stability experiments for boundary layers over distributed roughness.
The effect of enhanced plasma waves on Thomson scattering with a high-frequency probe laser
Authors: Drake R. P.
Published: July 2002

McIntosh et al. [Phys. Rev. Lett. 57, 337 (1986); Phys. Fluids 29, 3451 (1986)] have reported experiments in which electron-plasma waves, attributed to simulated Raman scattering, were detected by Thomson-scattering measurements. Based on threshold considerations and sample calculations, Simon and Short have argued [Phys. Fluids 31, 3371 (1988)] that a model of enhanced electron-plasma waves, driven by an electron beam in the plasma, might explain the data. In the present Brief Communication Thomson scattering from plasma waves driven by an electron beam is analyzed. It is shown that each extremum of the angle or frequency of the observed scattering can be independently used to infer the energy of the electron beam. For the present Thomson-scattering data, the inferred energies differ in both magnitude and time dependence, implying that the enhanced-plasma-wave model is inconsistent with the data.
Evaluation of the electric field generated by longitudinal plasma waves
Authors: Ganapol B. D. , Spiga G. , Oggioni S.
Published: July 2002

The stabilizing effects of the addition of short-range collisions to a partially ionized plasma are investigated through a recently developed numerical Laplace transform inversion algorithm. The technique is demonstrated using a two-stream equilibrium distribution and double Maxwellian equilibrium distributions. The regions of plasma stability obtained in previous studies are verified.
Linear surface waves propagating along discontinuities within a relativistic plasma
Authors: Dragila R. , Vukovic S.
Published: July 2002

A relativistic two-fluid theory of linear surface waves propagating along the density and/or temperature discontinuity within a relativistic plasma has been developed. The effect of relativistic temperatures can be described by an increase of the effective particle mass by a factor proportional to the enthalpy per unit rest mass. As the electron temperature increases, the surface wave frequency spectrum is downshifted and shrinks and the contribution of the ion component to the dispersion properties becomes increasingly pronounced. In addition to the ‘‘standard’’ situation in the theory of surface waves when one deals with a density discontinuity, now a surface wave-type solution formally exists even in a homogeneous plasma containing a temperature discontinuity.
The self-similarity and the non-neutrality of near-wakes in two-dimensional geometry
Authors: Kozima Hideo , Yamada Kouichi , Nakashima Kenji
Published: July 2002

For a two-dimensional, stationary, unmagnetized plasma flow around an obstacle, the basic equations of a plasma (Poisson’s equation and equations of continuity and momentum) around the near-wake of the obstacle are shown to be self-similar in a limited sense in the case of a constant ion flow velocity (vi,z=constant). For hypersonic plasma flow with Mach numbers 4.0 to 17.5, the evidence of the limited self-similarity is shown experimentally for the distributions of the plasma potential, the plasma density, and ion velocity in the near-wake of the obstacle. A metallic or glass obstacle is used. Charge imbalance is found around the near-wake along the boundary with the hypersonic plasma flow. The remarkable effects of nonzero net space charge on the limited self-similarity are investigated both experimentally and theoretically.
The absorption of electron cyclotron waves in the vicinity of an extremum of the equilibrium magnetic field
Authors: Lashmore-Davies C. N. , Dendy R. O. , Hastie R. J.
Published: July 2002

Local extrema in the equilibrium magnetic field strength can occur in tokamak plasmas when the plasma beta is increased. Furthermore, the magnetic field strength experienced by an electromagnetic wave propagating along a vertical chord in lower-beta tokamak plasmas may also pass through an extremum. For both these reasons, it is of interest to calculate the absorption of electron cyclotron waves in configurations where a harmonic of the cyclotron resonance lies near an extremum of the equilibrium magnetic field strength. It is shown that strong absorption can occur for both the O mode at second harmonic resonance and the X mode at third harmonic resonance, whereas absorption at these resonances is relatively weak when the gradient in magnetic field strength is linear. The optical depth in a magnetic field with parabolic scale length L is typically enhanced with respect to that in a field with linear scale length R by a factor cL/5vTR, where vT is the electron thermal velocity and c the velocity of light. Since electron cyclotron waves can reach these resonances in high-density plasma from launch positions on the low-field side of the tokamak, the number of effective and convenient electron cyclotron heating configurations is increased when the equilibrium magnetic field strength has local extrema.
Dynamo effect and current drive due to magnetic fluctuations in sheared magnetic field
Authors: Avinash K.
Published: July 2002

It is shown that when the mean electromotive force in a plasma is evaluated in terms of the two-point correlation of driven magnetic field fluctuations (as would be appropriate for problems related to current drive by waves), there is no β effect; i.e., the terms associated with gradients of the mean field vanish. The relevance of this result to the problem of current drive is discussed.
Effects of a nonuniform open magnetic field on the plasma presheath
Authors: Sato Kunihiro , Miyawaki Fujio
Published: July 2002

Effects of a nonuniform magnetic field on the plasma presheath are numerically investigated using the plasma equation for a collisionless plasma with a finite-temperature particle source. The present calculation confirms that previously published analytical solutions [Phys. Fluids B 1, 725 (1989)] are available over a wide range of mirror ratios. Potential drop in the presheath, which depends considerably on both the magnetic strength profile and the spatial distribution of the particle source, is remarkably increased by applying an expanding magnetic field when plasma particles are generated in the inner part of the plasma. An effect of a nonuniform magnetic field on sheath formation is also discussed by using a calculated ion distribution function. If the plasma equation has no singularity at the sheath edge, its solution satisfies the generalized Bohm criterion with the inequality sign in the expanding magnetic field.
Transport of carbon ion ‘‘test particles’’ in a collision dominated tokamak plasma
Authors: Wang Jian-Hua , Marshall Thomas C.
Published: July 2002

Carbon impurity ion transport is studied in the Columbia tokamak ‘‘HBT’’ [Plasma Physics and Controlled Nuclear Fusion Research 1986 (IAEA, Vienna, 1987), Vol. 1, p. 299], using a carbon tipped probe that is inserted into the plasma (ne ≊1–3×1014 cm−3, Te ≊4–10 eV, Bt ≊0.2–0.4 T). Carbon impurity light, mainly the strong lines of CII (4267 Å, emitted by the C+ ions) and CIII (4647 Å, emitted by the C++ ions), is formed by the ablation or sputtering of plasma ions and by the discharge of the carbon probe itself. The diffusion transport of the carbon ions is modeled by measuring the space- and time-dependent spectral light emission of the carbon ions with a collimated optical beam and photomultiplier. The point of emission can be observed in such a way as to sample regions along and transverse to the toroidal magnetic field. The carbon ion diffusion coefficients are obtained by fitting the data to a diffusion transport model. It is found that the diffusion of the carbon ions is ‘‘classical’’ and is controlled by the high collisionality of the HBT plasma; the diffusion is a two-dimensional problem and the expected dependence on the charge of the impurity ion is observed.
Density fluctuations and confinement in the Nagoya Bumpy Torus (NBT-1M)
Authors: Komori A. , Iguchi H. , Fujiwara M. , Shoji T. , Hosokawa M. , Kawai Y. , Ikegami H.
Published: July 2002

The stabilization of low-frequency density fluctuations is evidenced in the Nagoya Bumpy Torus (NBT-1M) [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1985), Vol. 2, p. 551] in the presence of microwave-heated hot-electron rings. Flute-type fluctuations, which are considered to be stabilized by the charge-uncovering effect of the rings, are found to cause large plasma losses, and to affect radial density profiles in the way that the lower fluctuation level yields the steeper density gradient. The particle confinement is, therefore, improved by the hot-electron rings to some extent, but is mainly determined by the plasma convection, which is expected from the discrepancy between density and potential profiles. It is also found that fluctuations in a toroidal plasma inside the ring grow when a weak negative ambipolar potential and a steep density gradient are formed, and are reduced to a low level when a deep potential well is achieved.
A closed set of conservation laws and the evolution of the electron magnetic moment in the collisionless solar wind
Authors: Alexander P.
Published: July 2002

A hydromagnetic equation system for the interplanetary collisionless solar wind is used to derive a set of conservation laws for that medium. It is found that every equation of the original system, including the closure relation, is related to one conservation law. The set that has been derived does not only include the traditional laws, but also a new one for the magnetic moment of the electrons. The conservation set is then used to obtain the space constants for the solar coronal expansion. The new law yields a constant that has not been predicted by other models.
Electron cyclotron wave scattering by a probe launched ion acoustic wave
Authors: Zhang Y. S. , Scharer J. E. , Chapman B. E.
Published: July 2002

Electromagnetic wave scattering in a plasma half-space from a propagating ion acoustic wave density perturbation with a transverse conducting boundary condition is analyzed. The analysis shows that there are two different types of scattered waves. One propagates only in the region where the plasma density is perturbed, its frequency and wave vector satisfies the conditions for a three-wave interaction process. The other propagates as an unperturbed electron cyclotron mode with either an upshifted or downshifted frequency. Experiments are performed to examine electron cyclotron wave scattering by an ion acoustic wave excited by means of a grid along the magnetic field line in a mirror machine. The backscattered waves propagating away from the density perturbation region are measured and their dependence on the ion acoustic wavelength is examined. The scattered wave in the region of ion acoustic wave propagation is examined by measuring the scattered wave field pattern when the ion acoustic wave number is much larger than that of the incident wave. The measurements demonstrate that the scattered waves indeed propagate as two different modes.
A theory of the two-beam klystron
Authors: Uhm Han S.
Published: July 2002

A theoretical model is developed for the two-beam klystron, where two concentric annular electron beams propagate through a grounded conducting tube. The beams premodulated at the first cavity enter the second cavity, exciting it. The induced voltage at the second cavity returns a part of the inner beam toward the first cavity, establishing a mutual communication between two cavities. The in-phase condition of the return current is obtained in terms of the intercavity distance. The maximum current-modulation condition at the second cavity is also obtained. These two conditions determine an optimum intercavity distance. The oscillation behavior of the first cavity is investigated, including influence of the intercavity distance on the induced gap voltage appearing on the cavity opening. The relationship between the induced voltage and the modulated return current in the opening is found from an equivalent circuit representation of the cavity impedance. The boundary between the oscillator and amplifier regions is described in the parameter space defined by the normalized return-current strength h and intercavity distance represented by the phase angle α. In the amplifier region (h sin α<1), the amplification occurs only for a narrow range of the phase angle α. The induced voltage grows exponentially in the oscillator region satisfying h sin α≳1. Introducing a nonlinear saturation term into the return current, the induced voltage is bounded in the oscillator operation.
Fast magnetic field penetration into a cylindrical plasma of a nonuniform density
Authors: Gomberoff K. , Fruchtman A.
Published: July 2002

The penetration of a magnetic field into a cylindrical plasma of a density that varies both radially and axially is studied. The magnetic field penetrates rapidly due to the Hall field, along constant nr2 lines (n is the dimensionless plasma density and r is the dimensionless radial coordinate). For a plasma that conducts between two cylindrical electrodes, it is shown that there is magnetic field penetration for both positive and negative polarity cases as long as there is penetration along the electrodes. The magnetic field evolution is found, analytically and numerically, for different time behaviors of the magnetic field at the boundaries. Ion velocities are also calculated.
Development of a high-frequency, second-harmonic gyrotron tunable up to 636 GHz
Authors: Idehara T. , Tatsukawa T. , Ogawa I. , Shimizu Y. , Makino S. , Kanemaki T.
Published: July 2002

In this Letter, a high-frequency, second-harmonic gyrotron using a 12 T superconducting magnet is described. It has achieved a maximum frequency of 636 GHz (corresponding to a wavelength of 472 μm). Operating at both the fundamental and the second harmonic of the electron cyclotron frequency enables the gyrotron to act as a high power (several 100 W), step tunable, millimeter to submillimeter wave source in the wide frequency range from 150 to 630 GHz.
Matrix elements of the linearized Fokker–Planck operator
Authors: Wong Seung Kai
Published: July 2002

Generating functions for the matrix elements of the linearized Fokker–Planck operator in the Burnett function basis are considered. They are shown to be governed by recursion relations, which allow speedy numerical evaluation of the matrix elements.
Confinement properties of positive ambipolar potential equilibria in a bumpy torus
Authors: Chang C. S. , Miller R. L.
Published: July 2002

The confinement properties of particles and energy in the ELMO Bumpy Torus in the collisionless-ion regime, characterized by positive ambipolar potential in the core region, have been investigated using a one-dimensional transport code. It is found that both particle and energy confinement can be significantly improved over the conventional negative-potential-regime values. A centrally peaked radial profile of microwave heat deposition is found to be an essential condition for positive potential equilibria.
The effect of finite β on stellarator transport
Authors: Mynick Harry E.
Published: July 2002

A theory of the modification of stellarator transport caused by the presence of finite plasma pressure is developed and applied to a range of stellarator configurations. For typical planar-axis stellarators, transport can change by a factor of about 4–5 in the progression from zero pressure to the equilibrium β limit of the device. For transport-optimized configurations, the factor can be over an order of magnitude. Thus, a stellarator with transport-optimized vacuum fields can have poor confinement at the desired operating β. Without an external compensating field, increasing β tends to degrade confinement, unless the initial field structure is very carefully chosen. The theory permits correct determination of this vacuum structure, in terms of the desired structure of the field at a prescribed operating β. With a compensating external field, the deleterious effect of finite β on transport can be partially eliminated.
Evolution of the Rayleigh–Taylor instability driven by a magnetic field gradient
Authors: Amini Behrouz
Published: July 2002

It is shown here that the Rayleigh–Taylor instability in a device like a theta pinch has two different stages. The first stage is during the implosion phase of the theta pinch, and the second stage initiates after the formation of the plasma column. Termination of the first stage and evolution of the second stage from growth to a highly nonlinear stage and eventual wave breaking is presented by holographic interferometry. Though the instability in the first stage has been investigated before, here by employing microscopic theory it is shown that the magnetic field gradient is the driving mechanism of the instability in the latter stage.
Convectively unstable two-plasmon decay modes in a weakly inhomogeneous plasma
Authors: Powers L. V. , Berger R. L.
Published: July 2002

The behavior of the convective two-plasmon decay instability is studied using a kinetic theory formulation which is applicable to high-temperature plasmas. Theoretical values for the Langmuir wave amplification are obtained in several limits. The temperature-dependent effects of Landau damping and kinetic theory corrections to the coupling coefficients are studied. Landau damping significantly reduces the spectrum of unstable modes for T>1 keV, while kinetic corrections become important for T>10 keV. The nonresonant scattering of Langmuir waves by thermal electrons is calculated and is found to be comparable to the resonant convective process for T>20 keV.
Heat transfer from a cylinder in a time-dependent cross flow at low Peclet number
Authors: Bayly B. J.
Published: July 2002

The heat transfer from a constant-temperature cylinder in a uniform, time-dependent cross flow at low Peclet number is considered. The time dependence is allowed to be strong, so that the velocity fluctuations may be comparable to, or larger than, the mean flow. The first nontrivial term in the Oseen approximation is calculated using matched-expansion theory, and its physical significance is discussed. As an illustration, the time-dependent heat transfer is calculated for a steady cross flow with large sinusoidal perturbations.
The evolution of axisymmetric vortex systems in liquid He II
Authors: Fiszdon W. , Peradzynski Z. , Poppe W.
Published: July 2002

The main features of the evolution of the normal and superfluid vorticity distributions (for high line density) in liquid helium are investigated for simple systems in the case of axial symmetry. The variation of initially Gaussian and top-hat vorticity distributions for different ratios of the two components is analyzed. The ratio of the obtained characteristic time scales of the variation of the vorticity on the centerline and the spreading of the vorticity distribution depends essentially on the initial conditions.
Characteristics and structure of an axially symmetric plasma echo
Authors: Mori Ichiro
Published: July 2002

An axially symmetrical spatial plasma echo is obtained theoretically by using the Vlasov–Poisson equation system. Several features of the echo are studied by making use of this echo solution. Common linear relations between exciter grid separation l and the position of echo-amplitude maximum le are shown, and an influence of l on the amplitude is also obtained. The radial extent of the echo and the effects of the magnetic field and the plasma temperature on the echo are discussed. Most importantly, the existence of the fine structure of the echo, such as the appearance of a side lobe (besides the main lobe) in the amplitude curve and the presence of a phase gap on the interferometric phase trace were demonstrated by their appearance in this experiment. Good agreement is obtained between theory and the experimental results concerning the side lobes and the phase gaps.
Strong ion acceleration by a collisionless magnetosonic shock wave propagating perpendicularly to a magnetic field
Authors: Ohsawa Yukiharu
Published: July 2002

A 2 1/2 -dimension, fully relativistic, fully electromagnetic particle code is used to study the time evolution of a nonlinear magnetosonic pulse propagating in a direction perpendicular to a magnetic field. The pulse is excited by an instantaneous piston acceleration, and evolves in a totally self-consistent manner. A large amplitude pulse traps some ions and accelerates them parallel to the wave front. They are detrapped when their velocities become of the order of the sum of the E×B drift velocity and the wave phase velocity, where E is the electric field in the direction of the wave propagation. The pulse develops into a quasishock wave in a collisionless plasma because of dissipation caused by the resonant ion acceleration. A simple nonlinear wave theory for a cold plasma describes the shock properties observed in the simulation except for the effects of resonant ions. In particular, the magnitude of an electric potential across the shock region is derived analytically and is found to be in good agreement with our simulations. The potential jump is proportional to B2, and hence the E×B drift velocity of the trapped ions is proportional to B.