bluuebunny/crossref_metadata_2025 huggingface dataset:
Refined algebraic quantization in the oscillator representation of SL(2, ℝ)
Authors: Louko Jorma , Rovelli Carlo
Published: July 2002

We investigate refined algebraic quantization (RAQ) with group averaging in a constrained Hamiltonian system with unreduced phase space T*R4 and gauge group SL(2, ℝ). The reduced phase space ℳ is connected and contains four mutually disconnected “regular” sectors with topology R×S1, but these sectors are connected to each other through an exceptional set, where ℳ is not a manifold and where ℳ has non-Hausdorff topology. The RAQ physical Hilbert space Hphys decomposes as Hphys≃⊕Hi, where the four subspaces Hi naturally correspond to the four regular sectors of ℳ. The RAQ observable algebra Aobs, represented on Hphys, contains natural subalgebras represented on each Hi. The group averaging takes place in the oscillator representation of SL(2, ℝ) on L2(R2,2), and ensuring convergence requires a subtle choice for the test state space: the classical analog of this choice is to excise from ℳ the exceptional set while nevertheless retaining information about the connections between the regular sectors. A quantum theory with the Hilbert space Hphys and a finitely generated observable subalgebra of Aobs is recovered through both Ashtekar’s algebraic quantization and Isham’s group theoretic quantization.
Free energy of a screened ion pair
Authors: Brüggen M. , Gough D. O.
Published: July 2002

We calculate the effect of weak electrostatic screening of ions in a plasma. The original calculation by Salpeter is based on a linearization of the Poisson–Boltzmann equation for the screened electrostatic potential. This approximation is valid where the potential is small, but is formally invalid in the vicinity of the ions where the solutions for the potential and the associated charge distribution diverge. In reality, quantum exclusion must prevent the divergence of the charge density of the screening cloud. Nevertheless, in the limit in which screening is weak, Salpeter’s value for the total screening energy is essentially correct. Here we extend Salpeter’s calculation to account approximately for both quantum-mechanical exclusion in the vicinity of the ions, using what we call the Poisson–Boltzmann–Fermi–Dirac approximation, and the polarization of the screening cloud. By matching the solution onto an expression for the two-center Poisson–Boltzmann charge distribution far from the ions we are able to construct a consistent solution over all space. We obtain the first-order term in the expansion of that solution, from which we calculate the free-energy associated with the screened ion pair.
Local twistors and the conformal field equations
Authors: Frauendiener Jörg , Sparling George A. J.
Published: July 2002

In this paper we establish the connection between Friedrich’s conformal field equations and the conformally invariant formalism of local twistors.
Abelian BF-theory and spherically symmetric electromagnetism
Authors: Bojowald Martin
Published: July 2002

Three different methods to quantize the spherically symmetric sector of electromagnetism are presented: First, it is shown that this sector is equivalent to Abelian BF-theory in four spacetime dimensions with suitable boundary conditions. This theory, in turn, is quantized by both a reduced phase space quantization and a spin network quantization. Finally, the outcome is compared with the results obtained in the recently proposed general quantum symmetry reduction scheme. In the magnetically uncharged sector, where all three approaches apply, they all lead to the same quantum theory.
Algebraic linearization of dynamics of Calogero type for any Coxeter group
Authors: Caseiro R. , Françoise J.-P. , Sasaki R.
Published: July 2002

Calogero–Moser systems can be generalized for any root system (including the noncrystallographic cases). The algebraic linearization of the generalized Calogero–Moser systems and of their quadratic (respectively quartic) perturbations are discussed.
SL
(3,
C
)
generator matrix elements in a Pauli subgroup basis
Authors: de Guise H. , Patera J. , Sharp R. T.
Published: July 2002

We construct SL(3,C) basis states reduced according to its finite subgroup ℘3. Matrix elements of sl(3,C) generators are calculated between ℘3 basis states.
Algebraic theory of product integrals in quantum stochastic calculus
Authors: Hudson R. L. , Pulmannová S.
Published: July 2002

Motivated by the search for solutions of the quantum Yang–Baxter equation, an algebraic theory of quantum stochastic product integrals is developed. The product integrators are formal power series in an indeterminate h whose coefficients are elements of the Lie algebra ℒ labelling the usual integrators of a many-dimensional quantum stochastic calculus. The product integrals are also formal power series in h, whose coefficients are finite iterated additive stochastic integrals which act on the exponential domain in the Fock space of the calculus and which represent elements of the universal enveloping algebra 𝒰 of ℒ. They obey a multiplication rule suggested by the quantum Itô product formula, and are characterized among all such formal power series by a grouplike property.
Luminescence and ODMR studies of X-traps for triplet excitons in organic crystals
Authors: Avdeenko A. A. , Buravtseva L. M. , Gorobchenko V. S. , Eremenko V. V. , Izvekov S. V. , Kravchenko A. E. , Pyshkin O. S. , Sugakov V. I.
Published: July 2002

The excited triplet states of shallow X-traps in neat single crystals of 4,4-dichlorobenzophenone were studied at liquid helium temperature by luminescence spectroscopy and magnetic resonance with optical detection. The vibrational phosphorescence bands, the polarization of phosphorescence spectra, and the triplet zero-field splitting parameters of these traps have been analyzed. A specific model of the structure defect (a molecule is turned by 180° on its long axis) has been considered in organic crystals of benzophenone type which consist of molecules whose dipole moments compensate each other in a unit cell. The calculated depth of such a dipole X-trap for triplet excitons in 4,4-dichlorobenzophenone crystals at the account of a defect molecule relaxation upon its slewing was found to be comparable with the experimental one.
Negative thermal expansion of fullerite C60 at liquid helium temperatures
Authors: Aleksandrovskii A. N. , Esel’son V. B. , Manzhelii V. G. , Udovidchenko B. G. , Soldatov A. V. , Sundqvist B.
Published: July 2002

The thermal expansion of fullerite C60 has been measured in the temperature range 2–9 K. A compacted fullerite sample with a diameter of about 6 mm and height of 2.4 mm was used. It was found that at temperatures below ∼3.4 K the linear thermal expansion coefficient becomes negative. At temperatures above 5 K our results are in good agreement with the available literature data. A qualitative explanation of the results is proposed.
Magnetostriction of superconductors (a review)
Authors: Eremenko V. V. , Sirenko V. A. , Szymczak H. , Nabialek A.
Published: July 2002

The results of magnetostriction studies of different superconducting compounds made during the last several years are presented. Special attention is paid to giant magnetostriction and “superconducting background” against which it emerges. The results are compared with the traditional magnetostriction components in magnetic materials. Possibilities of using magnetostriction measurements for analysis of the mixed state in hard superconductors and practical applications of the results of such measurements are considered.
Spectra of coupled magnetoelastic waves of a biaxial strongly anisotropic ferromagnet with biquadratic interaction
Authors: Mitsaı̆ Yu. N. , Fridman Yu. A. , Kozhemyako O. V. , Kosmachev O. A.
Published: July 2002

The spectra of coupled magnetoelastic waves in a biaxial ferromagnet with biquadratic interaction are investigated. The phase diagrams of the system under investigation are constructed as functions of biquadratic exchange. It is shown that for a strong Heisenberg exchange (exceeding the biquadratic interaction), phase transitions in the system occur only through a decrease in the magnitude of the magnetization vector. For a strong biquadratic exchange (exceeding the Heisenberg interaction), orientational phase transitions are possible, which are reduced to a reorientation of the principal axis of the tensor of quadrupole magnetic moments.
Localization of phonons and low-frequency sound attenuation in layered crystals
Authors: Chulkin E. P. , Zhernov A. P. , Kulagina T. N.
Published: July 2002

It is shown that specific interference processes emerging in an anharmonic layered lattice in the localization regime leads to a considerable renormalization of the reciprocal phonon lifetime. This mechanism may dominate over the standard anharmonic mechanism. The coefficient of low-frequency sound attenuation in an insulator with diagonal disorder is discussed.
An analysis of particle trajectories in computer-simulated turbulent channel flow
Authors: Bernard Peter S. , Ashmawey Mohamed F. , Handler Robert A.
Published: July 2002

The origin of Reynolds stress in turbulent channel flow is analyzed using several ensembles of particle paths computed in a direct numerical simulation. The time interval over which the paths are calculated is shown by several criteria to be sufficiently long so that complete mixing of the particle momenta with the surroundings has occurred. The corresponding mixing length is determined and found to be within the range required by a gradient transport model. However, a small fraction of the particles, which tend to be associated with highly vortical sweep and ejection events, travel well beyond the mixing length and collectively make a major nongradient contribution to Reynolds stress. It is suggested that further analysis of these motions may lead to useful formulas for predicting the nongradient component of momentum transport.
Intermittency exponents and generalized dimensions of a directly simulated fully developed turbulence
Authors: Hosokawa Iwao , Yamamoto Kiyoshi
Published: July 2002

The qth-order intermittency exponents and generalized dimensions of isotropic turbulence are calculated for ‖q‖≤30 with possible error bounds based on the data of the direct numerical simulation, which was recently executed on the 1283 grid of a supercomputer to investigate a decaying isotropic turbulence. The intermittency exponents are compared with the theoretical and experimental values known thus far. The corresponding f-α spectrum is also presented and compared with the p model.
Marangoni convection in a rotating spherical geometry
Authors: Cloot A. , Lebon G.
Published: July 2002

New results concerning the problem of surface-tension-driven instability in a rotating spherical fluid shell are presented. The fluid layer is assumed to be Boussinesq-like, heated from the inner sphere and placed in a microgravity environment (Marangoni problem). Small-amplitude disturbances are considered and the normal mode problem is solved numerically by means of a finite difference technique. Marginal stability curves are computed and surstability is exhibited. Onset of convection is subordinated to several parameters like the thickness of the layer, its angular velocity, and its viscous and diffusive properties; all these effects receive a special treatment.
Stochastic geometric properties of scalar interfaces in turbulent jets
Authors: Miller Paul L. , Dimotakis Paul E.
Published: July 2002

Experiments were conducted in which the behavior of scalar interfaces in turbulent jets was examined, using laser-induced fluorescence (LIF) techniques. The experiments were carried out in a high Schmidt number fluid (water), on the jet centerline, over a jet Reynolds number range of 1000≤Re≤24 000. Both two-dimensional scalar data, c(r,t) at fixed x/d, and one-dimensional scalar data, c(t) at fixed x/d and r/x, were analyzed using standard one- and two-dimensional fractal box-counting algorithms. Careful treatment was given to the handling of noise. Both long and short records as well as off-centerline measurements were also investigated. The important effect of threshold upon the results is discussed. No evidence was found of a constant (power-law) fractal dimension over the range of Reynolds numbers studied. On the other hand, the results are consistent with the computed behavior of a simple stochastic model of interface geometry.
Two-phase flow analysis of self-similar turbulent mixing by Rayleigh–Taylor instability
Authors: Freed Nathan , Ofer Dror , Shvarts Dov , Orszag Steven A.
Published: July 2002

The evolution of a randomly perturbed interface between unbounded incompressible fluids undergoing Rayleigh–Taylor instability is analyzed numerically and theoretically. Two-dimensional simulation results, obtained with an interface tracking code, are presented and compared with a theoretical model based on Young’s two-phase flow description of the mixing process. The simplifications implied by self-similarity and by high drag enable simple analytic results to be obtained for the profiles of the average volume fractions and velocities of the two materials as a function of penetration depth. Agreement of the analytic results with the simulation data is demonstrated for a wide range of density ratios.
Nonequilibrium gas flows. I: A detailed validation of Monte Carlo direct simulation for monatomic gases
Authors: Erwin Daniel A. , Pham-Van-Diep Gerald C. , Muntz E. Phillip
Published: July 2002

One-dimensional shock wave properties in helium and argon are predicted using Monte Carlo direct simulation. The collision model is based directly on the interatomic potential, taking angular scattering into account. The potential is assumed to be of the Maitland–Smith [n(r)−6] form. The detailed validity of the simulation is studied by comparing the predicted macroscopic and microscopic flow properties in shock waves to a wide range of available data.
The formation of U-shaped vortices on vortex tubes impinging on a wall with applications to free surfaces
Authors: Dommermuth Douglas G.
Published: July 2002

Numerical simulations of a pair of vortex tubes impinging on a no-slip wall show the formation of U-shaped vortices that are wrapped around the primary vortex tubes. The essential stages of U-vortex formation are as follows: generation of helical vortex sheets due to the onset of an instability on the primary vortex tubes, attachment of the helical vortex sheets to the separated wall boundary layer, wrapping of U-shaped vortices around the primary vortex tubes, and feeding of boundary-layer vorticity into the U vortices. The generation of U vortices helps to explain the striations and whirls that form on a clean or contaminated free surface above a pair of trailing-tip vortices.
Triad interactions in a forced turbulence
Authors: Ohkitani Koji , Kida Shigeo
Published: July 2002

An analysis of the data of a direct numerical simulation of a forced incompressible isotropic turbulence at a high Reynolds number (Rλ≊180) is made to investigate the interaction among three Fourier modes of wave numbers that form a triangle. The triad interaction is classified into six types according to the direction of the energy transfer to each Fourier mode: (+,+,−), (+,−,+), (+,−,−), (−,+,+), (−,+,−), (−,−,+), where the + (or −) denotes the energy gain (or loss) of the modes of the largest, the intermediate, and the smallest wave numbers in this order. The last three types of the interaction are very few. In the first type of the interaction, a comparable amount of energy is exchanged typically among three modes of comparable magnitude of wave numbers. In the second and third types, the magnitudes of the larger two wave numbers are comparable and much larger than the smallest one, and a great amount of energy is exchanged between the former two. This behavior of the triad interaction agrees very well with the prediction due to various quasinormal Markovianized closure theories of turbulence, and was observed before for lower Reynolds number turbulence by Domaradzki and Rogallo [Phys. Fluids A 2, 413 (1990)]. The fact that the dominant triad interactions involve different scales of motion suggests that the statistics of the small-scale motions of turbulence may be directly affected by the large-scale motions. Nevertheless, Kolmogorov’s local energy cascade argument may hold at least partially because the energy is exchanged predominantly between modes of two comparable scales in the triad interaction.
Conservation of expected momentum and energy in Monte Carlo particle simulation
Authors: Greengard Claude , Reyna Luis G.
Published: July 2002

Rarefied gas dynamics can be simulated numerically by Monte Carlo particle methods in which energy and momentum are conserved in expectation, but not exactly, through each collision. The conservation of the expected values of these moments does not imply the conservation of other expected second moments. For example, in Nanbu’s method [J. Phys. Soc. Jpn. 49, 2042 (1980)] (which has been proved to converge), the expected value of the temperature decreases through each collision step, and the relaxation of a gas calculated by this scheme leads to the zero temperature state. The decrease in expected temperature is of order O(1/N), where N is the number of simulated particles.
The nature of triad interactions in homogeneous turbulence
Authors: Waleffe Fabian
Published: July 2002

Nonlinear interactions in homogeneous turbulence are investigated using a decomposition of the velocity field in terms of helical modes. There are two helical modes per wave vector and thus eight fundamental triad interactions. These eight elementary interactions fit in only two classes, ‘‘R’’ (for ‘‘reverse’’) and ‘‘F’’ (for ‘‘forward’’), depending on whether the small-scale helical modes have helicities of the same or of the opposite sign. In a single-triad interaction, the large scale is unstable when the small-scale helical modes have helicities of opposite signs (class ‘‘F’’), and the medium scale is unstable otherwise (class ‘‘R’’). It is proposed that, on average, the triple correlations in a turbulent flow correspond to these unstable states. In the limit of nonlocal triads, where one leg is much smaller than the other two, the triadic interscale energy transfer is largest for interactions of class ‘‘R.’’ In that case, most of the energy flows locally in wave number, from the medium scale to the smallest, with a comparatively small feedback on the large scale. However, integrating over all scales in an inertial range, the net effect of nonlocal interactions of class ‘‘R’’ is a reverse energy cascade from small to large scales. All other interactions transfer energy upward in wave number. In local triads, this upward energy transfer occurs primarily between modes with helicities of the same sign, through catalytic interactions with a mode whose helicity has the opposite sign. The class ‘‘F’’ interactions transfer energy to the small scales and exist only in three dimensions. The physical processes associated to both classes of interactions are discussed. It is shown that the large local transfers due to nonlocal ‘‘R’’ interactions appear in pairs of opposite signs that nearly cancel each other and the net effect corresponds to an advection in wave space.
Asymptotic behavior of the amplitude mapping closure
Authors: O’Brien Edward E. , Sahay Anupam
Published: July 2002

It is shown, by DNS calculations and physically motivated special solutions, that the amplitude mapping closure has several unsatisfactory asymptotic properties when used to close the scalar binary mixing problem. It predicts a conditional expectation that shows both a persistent symmetry when the probability density function (pdf) is not symmetric and an unphysical permanence for the original scalar bounds.
Vortex-induced disturbance field in a compressible shear layer
Authors: Papamoschou Dimitri , Lele Sanjiva K.
Published: July 2002

The disturbance field induced by a small isolated vortex in a compressible shear layer is studied using direct simulation in a convected frame. The convective Mach number, Mc, is varied from 0.1 to 1.25. The vorticity perturbation is rapidly sheared by the mean velocity gradient. The resulting disturbance pressure field is observed to decrease both in magnitude and extent with increasing Mc, becoming a narrow transverse zone for Mc≳0.8. A similar trend is seen for the perturbation velocity magnitude and for the Reynolds shear stress. By varying the vortex size, it was verified that the decrease in perturbation levels is due to the mean-flow Mach number and not the Mach number across the vortex. At high Mc, the vortex still communicates with the edges of the shear layer, although communication in the mean-flow direction is strongly inhibited. The growth rate of perturbation kinetic energy declines with Mc primarily due to the reduction in shear stress. For Mc≥0.6, the pressure dilatation also contributes to the decrease of growth rates. Calculation of the perturbation field induced by a vortex doublet revealed the same trends as in the single-vortex case, illustrating the insensitivity of the Mach-number effect to the specific form of initial conditions.
The effect of slowly varying gap width on Dean vortices
Authors: Eagles P. M.
Published: July 2002

Taylor–Dean vortices are considered in channels whose thickness and curvature vary slowly on a scale 1/δ. The effect of divergence of the channel is shown to be destabilizing and of convergence to be stabilizing. Results for neutral curves are presented for a class of particular channels, and a method of computing such curves for more general channels is given.
Stability of an acoustically levitated and flattened drop: An experimental study
Authors: Anilkumar A. V. , Lee C. P. , Wang T. G.
Published: July 2002

This is an experimental study of the flattening and breakup of liquid drops in a single-axis acoustic levitation field, and is an extension of the authors’ previous work [C. P. Lee et al., Phys. Fluids A 3, 2497 (1991); Proceedings of the 30th Aerospace Sciences Meeting and Exibit, Reno, NV (1992)], on the static shape and stability of acoustically levitated drops. Two aspects, namely (i) the variation of drop equilibrium shape with sound pressure level and, (ii) the mechanism of disintegration of small drops in intense sound fields, have been studied mainly with water drops. The drop-shape study reveals that the critical acoustic Bond number Ba,cr [Ba = A2Rs/(σρc2); A: acoustic amplitude; Rs: spherical radius of the drop; σ : surface tension of the drop liquid; ρ: density of air and; c: sound speed in air], at which a downturn in acoustic intensity occurs for the larger drops, or loss of stability occurs for the smaller drops, varies from about 2.6 (for kRs∼0.74; k: acoustic wave number) to about 3.6 (for kRs∼0.25). The corresponding nondimensional critical equatorial radius R*cr (R*=R/Rs, R: equatorial radius of the drop) varies between 1.5 and 1.4. The study also reveals that, for deformation R* greater than about 1.3, the drop assumes the shape of a disk. The study of the dynamics of disintegration of small drops reveals that, following loss of stability, the drop expands horizontally with the liquid close to the edge drawn into a sheet by acoustic suction. The sheet continuously thins during expansion and two types of waves, one in the azimuthal direction, and the other in the radial direction, are parametrically excited on it. The ensuing violent vibration shatters the drop; with the whole process having a time scale of the order of 0.5 msec. These results partially confirm the mechanism of drop disintegration postulated by Danilov and Mironov [J. Acoust. Soc. Am. 92, 2747 (1992)]. The parametric instability of the thin sheet differs from that of the well established Faraday instability of a liquid half-space, where the parametric oscillations are excited at half the frequency of the external field.
Interacting scales and energy transfer in isotropic turbulence
Authors: Zhou Ye
Published: July 2002

The dependence of the energy transfer process on the disparity of the interacting scales is investigated in the inertial and far-dissipation ranges of isotropic turbulence. The strategy for generating the simulated flow fields and the choice of a disparity parameter to characterize the scaling of the interactions is discussed. The inertial range is found to be dominated by relatively local interactions, in agreement with the Kolmogorov assumption. The far-dissipation range is found to be dominated by relatively nonlocal interactions, supporting the classical notion that the far-dissipation range is slaved to the Kolmogorov scales. The measured energy transfer is compared with the classical models of Heisenberg [Z. Phys. 124, 628 (1948)], Obukhov [Isv. Geogr. Geophys. Ser. 13, 58 (1949)] and the more detailed analysis of Tennekes and Lumley [The First Course of Turbulence (MIT Press, Cambridge, MA, 1972)]. The energy transfer statistics measured in the numerically simulated flows are found to be nearly self-similar for wave numbers in the inertial range. Using the self-similar form measured within the limited scale range of the simulation, an ‘‘ideal’’ energy transfer function and the corresponding energy flux rate for an inertial range of infinite extent are constructed. From this flux rate the Kolmogorov constant is calculated to be 1.5, in excellent agreement with experiments [A. S. Monin and A. M. Yaglom, Statistical Fluid Mechanics (MIT Press, Cambridge, MA, 1975), Vol. 2].
Structure and flow patterns in turbulent wakes
Authors: Giralt Francesc , Ferré J. A.
Published: July 2002

A pattern recognition technique is used to detect and identify patterns and structures embedded in the velocity field of a turbulent wake behind a cylinder. Two component velocities u and ν or u and w are measured with eight X-wire anemometers aligned with the vertical and spanwise directions in the wake at x/D=420 and Re=1600. The velocity patterns educed as averaged patterns or instantaneous structures are consistent with the presence of double-roller eddies, whose legs are continuously stretched by the rate of strain. Experimental u and ν data suggest that the rollers are connected at the top, with vorticity parallel to the cylinder axis, forming a horseshoe vortex. The fine-scale activity, estimated by the envelope of the second derivatives, with respect to time of the velocity signals, confirms that this kind of structure can account for both the uν correlation and the continuous entrainment of potential flow into the wake.
Electron velocity-space diffusion in a microunstable electron cyclotron resonance heated mirror plasma
Authors: Hokin S. A. , Post R. S. , Smatlak D. L.
Published: July 2002

An experimental study of the velocity-space diffusion of electrons in an electron cyclotron resonance heated (ECRH) mirror plasma, in the presence of microunstable whistler radio frequency (rf) emission, is presented. The dominant loss mechanism for hot electrons, with temperatures Th ∼400 keV, is end loss produced by rf diffusion into the mirror loss cone. In a case with 4.5 kW of applied power, this loss limits the stored energy to 120 J with a hot electron energy confinement time τEh ∼40 msec. The corresponding value associated with collisional scattering is 320 msec. Whistler microinstability rf induces up to 25% of the endloss. The hot electron temperature is limited by rf-induced end loss of high-energy electrons, and decreases with increasing rf power in strong diffusion regimes. Measurements of collisional loss agree with standard scattering theory. Weaker diffusion is seen in experiments in which the vacuum chamber walls are lined with microwave absorber than in experiments with reflecting walls. The presence of absorber produces two effects: (1) a smaller resonant region in velocity space due to a narrower spatial rf profile, and (2) a decreased level of random wall-reflected rf fields, which could lead to an enhancement of super-adiabatic effects. Other superadiabatic effects, however, such as the existence of a ‘‘warm’’ electron component with Tw∼Es and a decrease of the heating rate as Th rises above Es, where Es∼30 keV is the calculated limit to fully stochastic diffusion, are not seen in the experiment.
Computer studies of a large-scale plasmoid driven by spontaneous fast reconnection
Authors: Ugai M.
Published: July 2002

The temporal dynamics of a plasmoid is studied by computer simulations. On the basis of the spontaneous fast reconnection model, it is demonstrated that a large-scale plasmoid is formed and propagates in a long current-sheet system. The fast reconnection develops and proceeds in a limited region of the system at large. At the edge of the fast reconnection region, a supersonic plasma jet erupts drastically, giving rise to a fast shock at the interface between the plasmoid and the fast reconnection regions. Accordingly, the plasma pressure is found to be notably enhanced in the plasmoid, which allows the plasmoid to swell and propagate effectively. It is argued that the supersonic plasma jet plays a dominant role in the formation and propagation of the large-scale plasmoid.
Kinetic effects in Alfvén wave heating. Part I: Surface eigenmodes in a pure plasma
Authors: Li Wann-Quan , Ross David W. , Mahajan Swadesh M.
Published: July 2002

Kinetic effects of Alfvén wave spatial resonances near the plasma edge are investigated numerically and analytically in a linearized cylindrical tokamak model. In Part I, cold plasma surface Alfvén eigenmodes (SAE’s) in a pure plasma are examined. Numerical calculations of antenna-driven waves exhibiting absorption resonances at certain discrete frequencies are reviewed first. From a simplified kinetic equation, an analytical dispersion relation is then obtained with the antenna current set equal to zero. The real and imaginary parts of its roots, which are the complex eigenfrequencies, agree with the central frequencies and widths, respectively, of the numerical antenna-driven resonances. These results serve as an introduction to the companion paper, Part II [Phys. Fluids B 1, 2364 (1989)], in which it is shown that, in the presence of a minority species, certain SAE’s, instead of heating the plasma exterior, can dissipate substantial energy in the two-ion hybrid layer near the plasma center.
Electron acceleration by an obliquely propagating electromagnetic wave in the regime of validity of the Fokker–Planck–Kolmogorov approach
Authors: Hizanidis K. , Vlahos L. , Polymilis C.
Published: July 2002

The relativistic motion of electrons in an intense electromagnetic wave propagating obliquely to a uniform magnetic field is studied in detail. This is done within the framework of a Fokker–Planck–Kolmogorov (FPK) analytical approach as well as numerically, by integrating the equations of motion and obtaining Poincaré’s surface of section. In particular, an electron-cyclotron wave is considered. At a certain threshold value of the wave amplitude, which depends upon the angle of propagation, cyclotron resonance overlapping can lead to a diffusive acceleration to high energies. The analytically predicted diffusion rates are explicitly evaluated for various cases. For values of the wave amplitude and the propagating angle inside the analytically predicted stochastic region, the numerical results suggest that the diffusion process proceeds in stages. In the first stage the electrons are accelerated to relatively high energies by sampling the first few overlapping resonances one by one. During that stage the ensemble-averaged square deviation of the variable involved scales quadratically with time. During the second stage, which is quite long (150ω−1ce), they scale linearly with time. For much longer times (>150ω−1ce), deviation from linear scaling slowly sets in. Good agreement with the analytical model suggests that, at least for the second stage, the FPK approach is a valid approximation.
Growth rate, saturation, and radial transport measurements for the trapped ion instability
Authors: Plaut A. B. , Sen A. K. , Navratil G. A. , Scarmozzino R.
Published: July 2002

The dissipative trapped ion instability (DTII) is observed as an m=1 mode localized to the mirror cell of the Columbia Linear Machine (CLM) [Plasma Phys. 24, 185 (1982)]. A gated feedback diagnostic allows the direct measurement of the DTII linear growth rate. The DTII growth rate is measured as a function of trapped fraction, keeping all other parameters fixed, and clearly displays the scaling with trapped fraction predicted by the radially local linear dispersion relation, including ion Landau damping. The saturated DTII mode amplitude is measured concurrently with the growth rate. The saturated amplitude displays a nearly linear dependence on the measured growth rate, normalized to the DTII real frequency, and is nearly proportional to the square root of the measured growth rate. A striped particle collector is used to measure the ambipolar radial plasma flux as the trapped fraction is varied. Radial diffusion coefficients are calculated from the measured radial particle fluxes and display a scaling intermediate between the Kadomtsev weak and strong turbulence predictions.
Stability analysis of dynamic Z pinches and theta pinches
Authors: Bud’ko A. B. , Felber F. S. , Kleev A. I. , Liberman M. A. , Velikovich A. L.
Published: July 2002

The stability of diffuse, dynamic Z pinches and theta pinches is analyzed. Spectra of the growth rates of hydromagnetic instabilities of dynamic pinches are calculated. The acceleration of plasma pinches during compression and expansion strongly modifies the growth of bulk instabilities. Also, even a relatively weak axial magnetic field substantially suppresses instabilities in Z pinches, in agreement with recent experiments. The initial axial field Bz 0 needed for a stable implosion of a pinch of initial radius R0 and average current Ī is Bz 0 ≳(10–30 kG) Ī (MA)/R0(cm). The approximate analytic solutions of the linearized equations are shown to agree well with the exact numerical solutions. The stability analysis demonstrated here for dynamic pinches is broadly applicable to the stability of dynamic plasmas in general.
Growth of a spike on a laser beam in a plasma
Authors: Pandey H. D. , Tripathi V. K. , Sodha M. S.
Published: July 2002

An on-axis spike in the intensity distribution of a Gaussian laser beam grows rapidly as the beam propagates in a laser-produced plasma. The nonlinearity arises through the ponderomotive force and has a saturating behavior. For vosc ∼vth, the optimum spike radius is approximately a few c/ωp0 and the amplification length is ∼2cω/ω2p0, where ωp0 is the unperturbed plasma frequency, ω is the radiation frequency, and vosc and vth are oscillatory and thermal velocities of electrons, respectively.
Ion temperature gradient driven turbulence in the weak density gradient limit
Authors: Hamaguchi Satoshi , Horton Wendell
Published: July 2002

The anomalous heat transport arising from the ion temperature gradient driven mode or ηi-mode turbulence is extended to the range of the weak density gradient limit (ηi=Ln/LT→∞), which is appropriate for H-mode dishcarges. It is shown that the anomalous ion heat conductivity χi with Ln→∞ scales as χi=g(ρs/LT) (cTi/eB) exp(−βσ) with σ=(Te/Ti) (LT/Ls), β≂4, and g≂1. This χi scaling is the natural extension for high ηi of the scaling of χi for K=(Ti/Te) (1+ηi)≲4 obtained [Phys. Fluids B 2, 1833 (1990)] from analytical and numerical studies.
Equilibrium beta limits in Heliotron-E due to magnetic island overlap
Authors: Nakamura Y. , Wakatani M. , Hegna C. C. , Bhattacharjee A.
Published: July 2002

Widths of magnetic islands induced by finite plasma pressure are computed for the Heliotron-E configuration [Phys. Rev. Lett. 53, 2242 (1984); Nucl. Fusion 27, 895 (1987)]. Equilibrium beta limits are obtained from the criterion of island overlap. Earlier experimental observations on internal disruptions in Heliotron-E are revisited.
Plasma physics issues in laboratory x-ray lasers
Authors: Rosen M. D.
Published: July 2002

Laboratories around the world are producing plasmas with atomic population inversions that are exhibiting gain at wavelengths approaching the carbon K edge near 44 Å, which is optimal for holography of wet biological samples. Often these demonstrations of gain are accompanied by anomalies that may be due to atomic physics effects, but whose possible causes due to plasma physics effects are explored here. The optical pump laser’s pulse duration ranges from the relatively long (>1 nsec) to the ultrashort femtosecond regime, and its incident irradiance varies by many orders of magnitude as well. This broad range of results is analyzed here from a unified point of view, via the use of a suite of large simulation codes. The plasma gain media exhibit many laser–plasma instabilities, including stimulated Raman scattering (which is used to infer the evolution of an exploding foil x-ray laser target’s electron temperature and density), and possibly filamentation, radiative instability, and ion acoustic turbulence, all of whose resultant nonuniformities can seriously affect the x-ray laser beam’s propagation and achievable gain-length product. Non-Maxwellian distributions of electrons and nontrivial x-ray laser line shapes may also play a role in explaining some of these anomalies.
Dynamics of electron flows and radiation fields produced by electron-beam diodes on the HERMES III accelerator
Authors: Sanford T. W. L.
Published: July 2002

Several novel diodes to control the 19 MeV, 700 kA electron beam and subsequent bremsstrahlung from the HERMES III accelerator [Ramirez et al., Digest of Technical Papers of the 6th IEEE Pulse Power Conference (IEEE, New York, 1987), p. 294] have been designed and tested for the study of nuclear radiation effects. The electron beam has been propagated over 10 m in low-pressure gas cells. A comprehensive set of diagnostics has been developed to characterize the performance of these diodes, beams, and radiation fields. The observed performance has been compared with models that combine the simulation of electron flow, using the magic particle-in-cell (PIC) code [ Pointon, J. Comput. Phys. (in press)] with simulation of electron-photon transport in bremsstrahlung targets, using the cyltran Monte Carlo code [Halbleib and Mehlhorn, Nucl. Sci. Engl. 92, 338 (1986)]. Beam transport results have been compared with iprop PIC code [B. B. Godfrey and D. R. Welch, Twelfth Conference on Numerical Simulation of Plasmas (Lawrence Livermore National Laboratory, San Francisco, 1987), Paper No. CM1] simulations. The models are able to accurately predict the detailed spatial and temporal dependence of the electron flow and the resulting radiation field. This work provides fundamental insight into the behavior of high-voltage, high-current diodes. The present status of the research is reviewed.
Nonlinear mechanisms for drift wave saturation and induced particle transport
Authors: Dimits A. M. , Lee W. W.
Published: July 2002

In previous studies, it was shown that, in the nonlinearly saturated phase of the evolution of drift instabilities in gyrokinetic particle simulations, the saturation levels and especially the particle fluxes are significantly less than predicted by quasilinear theory, and have an unexpected dependence on collisionality. In this paper, a theory is developed that explains these phenomena. The key features of the theory are as follows: The saturation level is determined by a balance between the steady-state electron and ion fluxes. The ion flux is small for levels of the potential below an E×B-trapping threshold and increases sharply once this threshold is crossed. Because of the presence of resonant electrons, the electron flux has a much smoother dependence on the potential. In the 2 (1)/(2) - dimensional (‘‘pseudo-3-D’’) geometry, the electrons are accelerated away from the resonance as they diffuse spatially, resulting in an inhibition of their diffusion, and thereby reducing the values of the potential and flux at which the fluxes can balance. Collisions and three-dimensional effects can repopulate the resonance thereby increasing the value of the particle flux.
Theory of wake-field effects of a relativistic electron beam propagating in a plasma
Authors: Uhm Han Sup , Joyce Glenn
Published: July 2002

A theoretical description of the wake-field effects is presented for a relativistic electron beam propagating through a tenuous background plasma. When the electron beam enters the plasma chamber, it expels plasma electrons from its vicinity leaving ions behind. The plasma electrons move out to the charge neutralization radius and oscillate near the radius, producing wake-field effects. In terms of plasma density, beam current, beam current rise time, and a geometrical factor of the system, the theory determines several critically important wake-field quantities, including the oscillation frequency, the wavelength, the radial profile of the induced axial electric field, and its strength. A particle simulation study of the wake-field effects is also carried out by two-dimensional particle-in-cell codes, and the simulation results are compared with the theoretical predictions. The simulation data agree remarkably well with the theoretical values. It is found from the theory and the simulation that the strength of the wake-field effects increases with beam current and plasma density. However, the wake-field strength decreases significantly as the beam current rise time increases.
Electron diamagnetic effects on the resistive pressure-gradient-driven turbulence and poloidal flow generation
Authors: Carreras B. A. , Lynch V. E. , Garcia L.
Published: July 2002

The nonlinear evolution of resistive pressure-gradient-driven turbulence with diamagnetic effects included generates a dc electric field (poloidal velocity) through the convective nonlinearity in the momentum balance equation. This radial electric field has a strong shear and contributes to the saturation of the turbulence; its effect on the saturation level of turbulence is more important than the change of the time and length scales of the modes by the direct ω* effects.
Experimental studies of stimulated Raman scattering in reactor-size, laser-produced plasmas
Authors: Darrow C. B. , Drake R. P. , Montgomery D. S. , Young P. E. , Estabrook Kent , Kruer W. L. , Johnston T. W.
Published: July 2002

In this paper the results of experiments designed to provide information regarding the scaling of the stimulated Raman scattering (SRS) instability in long-scale-length, inertial- confinement-fusion-reactor-size plasmas are described. Reactor-scale plasma conditions were experimentally simulated by exploding thin CH foils with nine beams of the Nova laser [Rev. Sci. Instrum. 57, 2101 (1986)]. Using a one-dimensional hydro model, the target and irradiation parameters were chosen to produce the largest-possible scale length plasma consistent with the energy and pulse width capabilities of Nova. The SRS emissions, driven by both the nine plasma-production beams and a tightly focused and delayed tenth beam, were recorded using an angularly resolved photodiode array and a temporally resolved optical spectrometer. These measurements verified the production of a reactor-size plasma (L/λ ≊ 5000) and indicated that the absolute levels of SRS backscatter detected in the present experiments have not increased relative to those measured in previous experiments despite a calculated sevenfold increase in the plasma density-gradient scale length. Inverse bremsstrahlung absorption of the scattered light is believed to be the dominant mechanism for this apparent saturation and, in this parameter regime, may enhance the effective thermal coupling between the incident laser and plasma densities below 0.15ncrit where absorption of the incident laser light is typically weak.
Kelvin–Helmholtz instability in systems with large effective Larmor radius
Authors: Opp Eric N. , Hassam A. B.
Published: July 2002

The Kelvin–Helmholtz instability is examined for magnetized plasmas where the effective ion Larmor radius, u/ωci, is larger than the velocity shear scale (u is the flow speed and ωci is the ion gyrofrequency). In this parameter regime, the properties of the instability are modified as compared with the small Larmor radius case and a new short wavelength branch is introduced. The instability properties and some applications are discussed.
Thermal confinement bifurcation and the L- to H-mode transition in tokamaks
Authors: Hinton F. L.
Published: July 2002

A bifurcation in the thermal confinement of tokamaks, which resembles the L- to H-mode transition, is shown to follow from properties of edge turbulence recently derived by Biglari et al. [Phys. Fluids B 2, 1 (1990)], and the standard neoclassical theory of poloidal rotation. The temperature profiles develop a pedestal at the plasma edge, and the poloidal rotation near the edge is considerably increased, when the heating power exceeds a critical value. The energy confinement time is a discontinuous function of increasing heating power, but is continuous for decreasing power (power hysteresis). Critical values of density and magnetic field are found, which must be exceeded in order for the bifurcation to occur. The scaling of the power threshold with density, magnetic field, and ion mass is similar to what is found experimentally.
Finite pressure effects on sawtooth oscillations
Authors: Park W. , Monticello D. A. , Fredrickson E. , McGuire K.
Published: July 2002

Nonlinear 3-D simulation of sawtooth oscillations shows that the helically twisted hot spot has a prominent toroidal bulge at the large major radius side, characteristic of a high-εβp instability. This bulge drives other m/n islands just outside the mixing radius resulting in a stochastic annular region. A similar effect is also found in shaped plasmas, even at low beta. These effects agree with experimental data, giving detailed support to the Kadomtsev reconnection model. This effect can also be used to explain the experimental sawtooth heat pulse ‘‘anomaly.’’ At high εβp, the stochastic region can essentially fill the whole plasma, and can result in a ‘‘high-beta disruption.’’
Interaction of a 1 psec laser pulse with solid matter
Authors: Chaker M. , Kieffer J. C. , Matte J. P. , Pépin H. , Audebert P. , Maine P. , Strickland D. , Bado P. , Mourou G.
Published: July 2002

Absorption and x-ray emission results obtained with a 1 psec pulse incident on solid targets with an intensity between 1011 and 1016 W/cm2 are presented and discussed. For I<5×1014 W/cm2, submicron density gradient scale lengths (L/λ≤0.2) are measured and the comparison of calculated and experimental values of absorption for aluminum indicates a relatively good agreement for p polarization and noticeable differences for s polarization. X-ray conversion efficiencies have been obtained in the sub-keV and keV range. At high intensities (I>1015 W/cm2) an evaluation of the plasma parameters is obtained from high resolution keV spectra. Finally, the results are discussed in the light of 1-D hydrodynamic simulations with time-dependent atomic physics.
Electrostatic trapping of plasma electrons heated by whistler-wave electron cyclotron heating
Authors: Dandl R. A. , Guest G. E.
Published: July 2002

Electron cyclotron heating (ECH) using whistler waves can lead to localized heating of plasma electrons, particularly in overdense plasmas. Quasineutrality is maintained by a positive ambipolar potential that electrostatically traps electrons and inhibits their escape from the resonant interaction region. In steady state the electron flux must equal that of the ions, which is at a speed comparable to the ion sound speed. The resulting equilibria are generally of the form of phase-space vortices whose spatial extent can be dictated by material walls or by atomic processes in the surrounding gas. In this paper the localization of electron heating is demonstrated and the nature of the resulting nonlinear, dynamic equilibria is discussed.