bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of some experimental results in cavitation
Authors: Wentzell R. A. , Lastman G. J.
Published: March 2003

Some experimental phenomena associated with cavitation are examined. It is shown that these experimental results can be duplicated by the response of a single spherical bubble subjected to a suitable incident tension wave.
Conditional analysis of intermittency in the near wake of a circular cylinder
Authors: Boisson H. C. , Chassaing P. , Minh H. Ha
Published: March 2003

Some experimental results of intermittency in the near wake (X/D=2 and 4) of a circular cylinder at a Reynolds number of 52 300 are presented. Both conventional and phase-averaged properties of the intermittency are given. From the conditional analysis of turbulent/nonturbulent events, some features of the phase control of the intermittency by the periodic motion due to the vortex shedding are emphasized.
Kinetic theory of evaporation and condensation for a cylindrical condensed phase
Authors: Onishi Yoshimoto , Sone Yoshio
Published: March 2003

A problem of evaporation and condensation for a two-phase system of a vapor gas and its cylindrical condensed phase is considered on the basis of the Boltzmann–Krook–Welander equation when the Knudsen number of the system is small. The analysis was carried out based on the nonlinear equation so that the results can be applied to the case where the Reynolds number of the flow field produced by the evaporation and condensation is finite. The distributions of pressure, temperature, and velocity of the gas are obtained together with the mass and energy flows for both evaporation and condensation cases. Even for very weak condensation, the linearized theory is invalid, and the present nonlinear analysis is required.
Universal mode with diffusive electrons: Linear instability and nonlinear saturation
Authors: Beasley C. O. , Molvig K. , van Rij W. I.
Published: March 2003

The effect of spatial electron diffusion on the stability properties of the universal drift mode in a sheared magnetic field are studied using an initial value code, TEDIT. In previous studies of this problem Hirshman and Molvig relied on an approximation to the electron resonance function equivalent to making a Krook approximation for the spatial diffusion operator, D ∂2/∂x2. The present work treats the diffusion operator precisely and also allows the treatment of a realistic parallel velocity dependence of the diffusion coefficient, D=D(v∥). For the case of a velocity-independent diffusion coefficient, the qualitative features found by Hirshman and Molvig are observed. The modes with kyρi>1 destabilize at small values of the diffusion coefficient and saturate at higher values, corresponding to several orders of magnitude in D. There are quantitive discrepancies with the previous work that, near the saturation point, can be accounted for reasonably well by a simple asymptotic theory. However, when the code uses a more realistic form, D=D0 (ve/‖v∥‖) exp(−v2c/v2∥)+Dc, where Dc corresponds to the (small) collisional diffusion, and D0 parametrizes the turbulence level, then a quantitative difference is observed. Instability persists down to zero turbulence levels, D0=0. This is essentially linear instability due to collisional diffusion alone.
Electromagnetic effects on the self-modulation of nonlinear lower hybrid waves
Authors: Hsu P. , Kuehl H. H.
Published: March 2003

Electromagnetic effects on the self-modulation of nonlinear lower hybrid waves in an inhomogeneous plasma are studied for both broad and narrow spectrum excitations. For broad spectrum excitation, the complex modified Korteweg–de Vries equation is modified by two additional terms due to the electromagnetic correction and inhomogeneity. Numerical solutions of this equation for typical tokamak parameters show that these terms suppress soliton formation. For narrow spectrum excitation, the electromagnetic correction produces an additional dispersive term in the differential equation governing the wave envelope. This term opposes thermal dispersion, resulting in significant self-modulation. Numerical solutions show constriction and splitting of the envelope as well as spreading of the Fourier spectrum.
Quasilinear momentum and energy transport
Authors: Mahajan S. M. , Hazeltine R. D. , Hitchcock D. A.
Published: March 2003

The equations of quasilinear momentum and energy transport are studied to show that a suprathermal level of low-frequency fluctuations can cause highly enhanced electron energy transport without effectively changing the plasma resistivity.
Solitary surface waves on a plasma cylinder
Authors: Gradov O. M. , Stenflo L.
Published: March 2003

By considering electrostatic surface waves propagating along a plasma cylinder, it is demonstrated that solitary variations in the cylinder radius may appear. The properties of these slow perturbations are determined by the surface wave intensities.
Hot plasma decoupling condition for long wavelength modes
Authors: Berk H. L. , Van Dam J. W. , Spong D. A.
Published: March 2003

The stability of layer modes is analyzed for z-pinch and bumpy cylinder models. These modes have long wavelength across the layer and are flute-like along the field line. The stability condition can be expressed in terms of the ratio of hot to core plasma density. It is shown that to achieve conditions close to the Lee–Van Dam, Nelson core beta limit, one needs a considerably smaller hot-to-core plasma density ratio than is required to achieve stability at zero core beta.
Ion heating due to parametrically driven ion wave turbulence
Authors: Mizuno K. , DeGroot J. S.
Published: March 2003

Ion heating is studied in an unmagnetized and nonisothermal plasma (Te0≫Ti0). The ion temperature increases linearly with time while the ions sample the finite turbulence. The ions are significantly heated (Ti/Ti0 ≳10) when ion wave turbulence is large (Δn/n≳10%). The measured results agree with a simplified theory and one-dimensional particle simulation results.
Flow through rotating straight pipes of a circular cross section
Authors: Duck P. W.
Published: March 2003

The problem of the laminar flow through a circular pipe, rotating with constant angular velocity about an axis perpendicular to the axis of the pipe, is studied. The full equations of motion are solved numerically, using a Fourier decomposition in the angular direction. As a test of the numerical scheme, the similar problem of flow through a loosely coiled pipe is also studied, and the results obtained are seen to agree well with the existing solutions.
Hydrodynamic stability of the melt during solidification of a binary alloy
Authors: Hurle D. T. J. , Jakeman E. , Wheeler A. A.
Published: March 2003

The equations governing melt stability during solidification of a binary alloy are shown to resemble those characterizing the Bénard problem with permeable boundaries. Critical conditions for the onset of stationary convection are calculated for the case of a planar interface. Long wavelength modes are preferred when the segregation is small.
Synchrotron emission from a relativistic loss-cone distribution with application to the ELMO Bumpy-Torus Experiment
Authors: Winske D. , Boyd D. A.
Published: March 2003

Analytic expressions for the power emitted per unit volume per solid angle per unit frequency at perpendicular propagation as synchrotron radiation from a relativistic loss-cone distribution of electrons are derived and compared with exact numerical results. These expressions are useful for understanding how the radiation scales and for inferring properties of the plasma from measurements of the radiation. As an example, it is shown that the temperature and loss-cone angle of the electron distribution in the ELMO Bumpy Torus (EBT) ring can be determined in a straightforward fashion from the ratio of radiation in the two polarizations at two frequencies. Also, it is demonstrated that the loss-cone angle can be determined from the frequency at which this emission ratio is a minimum, even if the emission ratio itself cannot be measured very accurately.
Tearing modes in a general cross-section force-free cylindrical tokamak
Authors: Lee J. K. , Chu M. S. , Helton F. J.
Published: March 2003

The stability of magnetohydrodynamic tearing modes for a large aspect ratio force-free tokamak with general cross section is studied in terms of an energy principle. Determinations of the marginal stability boundary Δ′ and island size using this general two-dimensional method give good agreement with those of known methods for circular equilibria. Results for noncircular equilibria are also presented.
Measurement of growth rate of plasma drift wave excited by slow electrons
Authors: Barrett P. J. , Allen G. R.
Published: March 2003

Measurements are presented on drift waves which are driven unstable by an annular beam of slow electrons. The growth rate of the most unstable mode varies linearly with the injection velocity of the beam, in accordance with fluid theory.
Plasma convection and line-tying
Authors: Fornaca S.
Published: March 2003

The directed motion of a low-β plasma across a toroidal magnetic field is examined as the degree of electrical contact between the plasma and a conducting end wall (line-tying) is varied in a controlled manner. Experimentally, little difference is found between the motion of a completely isolated plasma and that of a plasma in contact with a wall that does not emit electrons. As the degree of line-tying is increased by making the end wall electron-emitting, the cross-field convection of the plasma is increasingly suppressed. Beyond a critical emission current density, no further suppression is observed. When the system is in the surface-line-tying configuration, the suppression of the directed cross-field motion is accompanied by a large increase in the radial transport of plasma.
Vaporization mode and state of the ablatant of a deuterium pellet in tokamak discharges
Authors: Chang C. T.
Published: March 2003

The ablation of a deuterium pellet under prevailing tokamak discharge conditions is shown to be a dynamic phase transition process. An alternative boundary condition at the pellet surface is formulated. Computational results based on the new boundary condition showed that the state of the ablatant in the subsonic region depends on the boundary condition used but is hardly affected in the supersonic region. The scaling law of the pellet ablation rate, therefore, is insensitive to the boundary condition imposed.
Influence of collisions and incoherence on ion-cyclotron transport in mirror plasmas
Authors: Cohen Bruce I. , Cohen Ronald H. , Rognlien Thomas D.
Published: March 2003

The effects of classical Coulomb collisions and wave incoherency on drift-cyclotron-loss-cone (drift-cone) mode anomalous transport are studied analytically and by simulation. Pitch-angle collisions have a cumulative effect on the gyrophase angle, and the spatially varying magnetic field in a mirror well contributes a large geometrical amplification of the gyrophase-angle diffusion. Analytical calculations of this effect for particles with arbitrary pitch angle are confirmed by Monte Carlo simulations. However, for the levels of drift-cone turbulence observed in the 2XIIB magnetic mirror experiment and for its relative collisionality, the effect of pitch-angle collisions on anomalous transport is small. In particular, heating of ions to 40 keV energies cannot be explained by this mechanism. Wave incoherency also destroys superadiabaticity, leads to velocity transport, and provides an approximate model of the finite frequency bandwidth of drift-cone turbulence, e.g., due to bursting effects in 2XIIB. Monte Carlo simulations corroborate the analytical transport theory presented. The simulations exhibit significantly enhanced velocity-space transport and heating of ions to nearly 40 keV energies due to wave incoherency for parameters at the limit of applicability to the 2XIIB experiment. These same considerations of resonant wave-particle interactions are also directly relevant to electron and ion-cyclotron resonance heating and to velocity diffusion driven by unstable Alfvén-ion-cyclotron waves in other mirror experiments.
Electron confinement studies on the EBT-S Bumpy Torus Experiment using soft x-ray techniques
Authors: Hillis D. L. , Haste G. R. , Berry L. A.
Published: March 2003

Soft x-ray bremsstrahlung measurements have been performed on the ELMO Bumpy Torus (EBT-S) plasma to determine the electron temperature Te and electron density density ne using a calibrated Si(Li) detector over a wide range of operating conditions. The purpose of this paper is to outline the necessary assumptions and essential x-ray techniques that are inherent in soft x-ray measurements in order to investigate the electron heating and confinement properties of EBT-S. In addition, by utilizing the electron density as determined by the soft x-ray measurements, the previous EBT-S confinement analyses have been extended. The steady-state plasma of EBT-S is heated by microwaves using a continuous wave (cw) gyrotron that can operate up to power levels of 200 kW. From the soft x-ray measurements, both the electron temperature and density are found to increase at higher microwave power levels. For operation at microwave power levels of 200 kW, Te approaches 1 keV while ne approaches 1.2×1012 cm−3. In general, confinement properties are found to improve with increased microwave power. The data are compared with neoclassical transport scaling and the electron transport is found to be collisionless (ν/Ω<1) as well as neoclassical.
A simple ablative implosion model—shell dynamics
Authors: Mayer F. J. , Larsen J. T. , Steele J. W.
Published: March 2003

A simple model, derived from Newton’s Second Law, for the ablative implosion of a thin spherical shell is presented. The scaling dependence of the implosion time, shell velocity, and mass loss on shell dimensions and the critical physical parameter, the ablation pressure, is derived. Finally, the model is used to examine implosion energy efficiency and to describe an interesting application, wall-recoil heating of a contained fuel gas.
Orbital stability of the high-current betatron
Authors: Barak G. , Rostoker N.
Published: March 2003

Orbital stability of the high-current betatron is analyzed. In this modified betatron a toroidal magnetic field is added to the conventional betatron magnetic field. This increases substantially the space charge limit during injection. It gives rise, however, to new problems during acceleration, such as ring stability, Fermi drift, and orbital resonances. A detailed analysis is presented showing that two serious problems may arise: (i) The beam becomes highly unstable when the net focusing force on it becomes zero. The dominant instability in this region is the Fermi drift. This may, however, be utilized in extracting the beam. (ii) Errors in the vertical (betatron) magnetic field result in driven resonances that must be crossed during acceleration. The amplitude growth of the transverse betatron oscillations due to these resonances is negligible up to very high γ (low n resonances). In practice, it may become very difficult to cross the n=1 resonance. Thus, extracting the beam again making use of the Fermi drift while n>1 is preferable.
Comments on ‘‘Gravitational instability of a rotating anisotropic plasma with Hall’s effect’’
Authors: Shivamoggi Bhimsen K.
Published: March 2003

A comment is made on ‘‘Gravitational Instability of a Rotating Anisotropic Plasma with Hall’s Effect’’ (AIP)
Reply to the Comments of B. K. Shivamoggi
Authors: Lau Y. Y. , Liu C. S.
Published: March 2003

A reply is made to comments on ‘‘Stability of Shear Flow in a Magnetized Plasma’’. (AIP)
Propagation of lower hybrid resonance cones in tokamaks
Authors: Bellan P. M.
Published: March 2003

The trajectory of the lower hybrid resonance cone surface is calculated for propagation in the helical magnetic field of a tokamak. The surface is found to be a deformed cone, having symmetry axis following the magnetic field helicity. By superimposing cone fields excited by adjacent, poloidally separated point sources, the field excited by a source of finite poloidal extend is deduced. For sources having large poloidal extent, the superimposed cones destructively interfere everywhere except in a small region of constructive reinforcement which is located towards the plasma center; sources having small poloidal extent excite an individual resonance cone, which extends from near the plasma center all the way to the wall. Numerical calculations are presented for Verasator, Alcator-A and -C, JFT-2, and PLT tokamak parameters.
Stimulated Brillouin scattering in an inhomogeneous plasma with broad-bandwidth thermal noise
Authors: Ramani A. , Max C. E.
Published: March 2003

Stimulated Brillouin scattering in a plasma containing broad-bandwidth thermal noise was considered. Plasma profiles which are inhomogeneous (for example, because of a velocity gradient) were studied. At each point in the profile, a different electromagnetic wave can be resonant with the pump wave and a local ion acoustic wave, and can thus be amplified from its noise level. Some difficult points of the linear theory of this process are first examined. Then a nonlinear equation is derived, which governs the decrease of the incident-wave amplitude as it propagates toward the critical density surface. Solutions of this equation predict a maximum value of the absorbed intensity, Imax. For incident laser intensities Iin, higher than this value, only a fraction Imax/Iin is absorbed.
Steady-state turbulence with a narrow inertial range
Authors: Weatherall J. C. , Nicholson D. R. , Goldman M. V.
Published: March 2003

Coupled two-dimensional wave equations are solved on a computer to model Langmuir wave turbulence excited by a weak electron beam. The model includes wave growth due to beam–plasma interaction, and dissipation by Landau damping. The inertial range is limited to a relatively small number of modes such as could occur when the ratio of masses between the negative and positive ions is larger than in a hydrogen plasma, or when there is damping in long wavelength Langmuir waves. A steady state is found consisting of quasistable, collapsed wave packets. The effects of different beam parameters and the assumed narrow inertial range are considered. The results may be relevant to plasma turbulence observed in connection with type III solar bursts.
Electromagnetic radiation from beam–plasma instabilities
Authors: Pritchett P. L. , Dawson J. M.
Published: March 2003

The generation of electromagnetic radiation from the unstable electrostatic waves of a beam–plasma system is investigated by means of computer simulation. A two-dimensional model is employed in which the plasma is treated as a finite system and in which the effects of a continuous beam inflow are included. For the parameters of the simulation (nb/n0 ∼1%, an essentially nonrelativistic beam, and no external magnetic field), the radiation is concentrated near the plasma frequency, is emitted preferentially perpendicular to the beam, and is polarized with E parallel to the beam direction. Radiation at the second harmonic is found to be weak. The production mechanism is found to be the scattering of Langmuir waves from ion-acoustic fluctuations, and the observed radiation level agrees well with the theoretical predictions for such a three-wave coupling. The results of the numerical experiments are compared with the recent laboratory experiment of Whelan and Stenzel [Phys. Rev. Lett. 47, 95 (1981)].
Secondary instability of plane channel flow to subharmonic three-dimensional disturbances
Authors: Herbert Thorwald
Published: March 2003

A linear secondary instability mechanism is presented that leads to the occurrence of subharmonic three-dimensional disturbances in wall-bounded shear flows. The instability originates from the periodic redistribution of vorticity in the shear flow by small but finite-amplitude Tollmien–Schlichting waves. Low threshold amplitudes and other characteristics of this instability are consistent with experiments and may elucidate various obscure observations.
Separation of time-averaged turbulence components by laser-induced fluorescence
Authors: Cheng S. , Zimmermann M. , Miles R. B.
Published: March 2003

Turbulence is measured by resonant fluorescence of sodium atoms seeded into a supersonic nitrogen jet. By varying the frequency of the pumping laser, the mean square velocity, temperature, and pressure fluctuations (as well as their correlations) may be determined.
Effects of ambipolar potential on multiple mirror confinement
Authors: Najmabadi F. , Lichtenberg A. J. , Lieberman M. A.
Published: March 2003

The diffusion model for multiple mirror confinement has previously been limited to devices with a large number of cells and high mirror ratios. Ambipolar effects were assumed to reduce the confinement time by the usual factor [1+Z(Te /Ti)]−1. For the ideal multiple mirror regime, in which the mean free path is shorter than the cell length but long compared with the mirror scale length, a new discrete staircase density profile model, without those limitations, has been developed. A theory employing model distribution functions for ion and electron transport in a multiple mirror device has been used. A self-consistent ambipolar potential is included. It has been found that the ambipolar potential reduces the confinement time somewhat less than the usual ambipolar factor. Special attention is given to the last cell which acts as a boundary condition for the system. The ambipolar potential across the last mirror throat is studied and found to be of the order of 0.3kTe. For devices with small numbers of cells, significant improvements over previously calculated multiple mirror confinement times are found.
Reduction of radial losses in tandem mirrors
Authors: Myra J. R. , D’Ippolito D. A. , Catto P. J.
Published: March 2003

The condition for having strictly omnigenous magnetic fields (zero geodesic curvature and neoclassical step size) is generalized to determine the fields which give the smallest mean square neoclassical step size consistent with given boundary conditions and constraints. Because this transport minimization produces less restrictive field configurations than omnigenity, a wider class of practical applications is possible. An explicit set of ordinary differential equations is obtained for the transport-minimizing vacuum fields in long-thin tandem mirror geometry. Configurations with reduced transport are found at little cost in line averaged interchange stability. Additionally, for these configurations the constraint that no parallel current flow into the center cell (due to the Stupakov effect) may be imposed in a natural way.
Experimental study of the dissipative trapped ion instability
Authors: Navratil G. A. , Sen A. K. , Slough J.
Published: March 2003

A new, steady-state linear machine has been constructed which produces a low-density, low-temperature hydrogen plasma in the collisionality regime of the dissipative trapped ion instability. This instability was seen and identified through the observed dependence of the wave amplitude on trapped fraction, axial and radial position, and collisionality and through the scaling of the real frequency with trapped fraction. The observed radial and axial mode structure of the instability is compared with the results of a simple radial model and a calculation of the Landau damping for both trapped and transit particles in a linear geometry.
Nonlinear interfacial progressive waves near a boundary in a Boussinesq fluid
Authors: Pullin D. I. , Grimshaw R.
Published: March 2003

The behavior of nonlinear progressive waves at the interface between two inviscid fluids in the presence of an upper free boundary is studied as a model of waves on the thermocline. A set of relationships between the integral properties of bounded waves in a general two-fluid model is first developed and the Stokes expansion to third order is derived. The exact free boundary problem for the wave profile is then formulated within the Boussinesq approximation as a nonlinear integral equation, which is solved numerically using two different numerical methods. For finite velocity difference across the two-fluid interface bifurcation of solutions into upper and lower branch wave profiles with quite different properties is obtained. Numerically calculated wave shapes and integral properties show good agreement with third-order Stokes expansion predictions in the weakly nonlinear regime for waves which are not too long. Very long waves were found to exhibit distinct solitary wave-like features.
Large-amplitude long waves in a shear flow
Authors: Stern Melvin E. , Paldor Nathan
Published: March 2003

Large amplitude disturbances in a boundary layer shear flow are considered in an inviscid and long-wave theory. Initially weak horizontal convergences are concentrated and amplified in time, thereby increasing the maximum normal velocity (v) until it becomes comparable with the horizontal velocity. The effect is first demonstrated in a two-dimensional model having piecewise uniform vorticity. The leading edge of the compact disturbance propagates downstream more rapidly than the trailing edge, but no ‘‘quiet’’ zone appears in the center. Instead v→∞ and a tendency for wavebreaking occurs. The evolving large v pattern is consistent with observations of the laminar spike just prior to its breakdown. The long-wave theory is generalized to three-dimensional motions, and the effect of an initial spanwise divergence is such as to rationalize the initial vorticity assumed in the two-dimensional model.
Gyrational motion of disks during free-fall
Authors: Stewart Ronald E. , List Roland
Published: March 2003

Experiments with disks having diameters from 2.0 to 3.1 cm falling freely in air at speeds of 1.7–3.3 m sec−1 were conducted with a vertically driven, also falling, high-speed camera in order to show that the gyroscopic-like motion, gyration, is the most general type of rotational motion. From analysis of the films, position and orientation were measured throughout the fall and translatory and angular motions were determined, together with the forces and torques which caused the motion. The existence of gyrational motions was confirmed together with straight oscillatory and rotational fall behavior. Typical nutation/precession frequencies were in the range of 6–11 Hz while typical angular frequencies about the minor axis were 1.8–3.0 Hz. The likelihood of gyration increased with particle density. The observed gyration characteristics of disks were similar to predictions for oblate spheroids. Further, it is postulated that air influences can cause rotation about a horizontal diameter to become unstable and to consequently evolve into gyration. This gyrational motion may represent a very general mode of free fall for other shapes in a similar range of Reynolds numbers from 2000–5000.
A theoretical prediction of the fluid buckling frequency
Authors: Cruickshank J. O. , Munson B. R.
Published: March 2003

A simple theoretical model of the coiling frequency for a buckled axisymmetric fluid jet is developed. The special case of a jet dominated by viscous and gravity forces is considered, and the theoretical prediction is compared with experimental data.
A two-equation turbulence model for two-phase flows
Authors: Elghobashi S. E. , Abou-Arab T. W.
Published: March 2003

A two-equation turbulence model has been developed for predicting two-phase flows. The two equations describe the conservation of turbulence kinetic energy and dissipation rate of that energy for the carrier fluid in a two-phase flow. They have been derived rigorously from the momentum equations of the carrier fluid. Closure of the time-mean equations is achieved by modeling the turbulent correlations up to third order. The new model eliminates the need to simulate in an ad hoc manner the effects of the dispersed phase on turbulence structure. Preliminary testing indicates that the model is successful in predicting the main features of a round gaseous jet laden with uniform-size solid particles.
Asymmetries in evaporation and condensation Knudsen layer problems
Authors: Ytrehus Tor
Published: March 2003

Basic asymmetries are shown to exist between half-space problems for evaporation and those for condensation, in the regime of linear transport. The asymmetry is determined by the normalized latent heat of the substance, which is found to have a critical influence upon the flow behavior in the condensation problem. The analysis is based upon the linearized Maxwell–Boltzmann transport equation, and the results indicate nonexistence of solutions to the condensation half-space problem for substances with latent heat below a certain critical value.
Compressible Rayleigh–Taylor instability
Authors: Baker L.
Published: March 2003

The role of compressibility in modifying the growth rate of Rayleigh–Taylor instability is discussed. Compressibility can either enhance or decrease the growth rate. If the sound speeds in the two media are the same, compressibility can still have an effect, in contrast to ‘‘first-order’’ theory. Compressibility becomes less important as the density ratio between the fluids increases. Experimental results have shown that compressibility can enhance growth rates compared to the incompressible value.
Electromagnetic instability of Langmuir waves in a magnetoplasma
Authors: Shukla P. K. , Yu M. Y.
Published: March 2003

The stability of a finite amplitude electron plasma wave with respect to Alfvén wave perturbations is considered. Three-wave decay as well as modulational instabilities are investigated. Analytical expressions for the growth rates are obtained.
The whistler instability at relativistic energies
Authors: Gladd N. T.
Published: March 2003

The effects of a relativistic electron population on the temporal and spatial growth rates of the whistler instability are considered. It is found that relativistic effects tend to reduce the temporal growth rate of the whistler but are not sufficient, by themselves, to completely stabilize the mode. Relativistic effects are also found to reduce the volume of parameter space over which temporal growth can occur; the residual rate of spatial growth, however, is substantial. The results obtained are applied to two plasmas with significant relativistic electron content—the ELMO Bumpy Torus and the Tandem Mirror Experiments.
Nonlinear, single-helicity magnetic reconnection in the reversed-field pinch
Authors: Caramana E. J. , Nebel R. A. , Schnack D. D.
Published: March 2003

A computational and analytical study of the nonlinear evolution of single-helicity, resistive, current-driven modes in the reversed-field pinch utilizing the single-fluid magnetohydrodynamic approximation is presented. In the start-up phase it is found that an off-axis current peak can be anomalously dissipated by globally reconnecting current-driven modes. The reconnection is seen to be well described by the Kadomtsev reconnection model, which is directly applied to the reversed-field-pinch regime. During the sustainment phase of the discharge, stable profiles are driven unstable by transport processes and restabilized by the resulting current-driven instabilities. The dynamics of the toroidal field is of fundamental importance in the evolution of these modes. The reversed-field-pinch profile is restabilized by two successive global reconnections: the first removes the rational surface; the second restores it. It is also found that magnetic reconnection can occur in the reversed-field pinch without a linear phase. Although the interaction of instabilities with different helicities is not directly investigated, the importance of this phenomenon in the reversed-field pinch is discussed.
Rotational instabilities in the field-reversed configuration: Results of hybrid simulations
Authors: Harned Douglas S.
Published: March 2003

Rotational instabilities in rigidly rotating field-reversed configuration (FRC) equilibria are studied using a quasineutral hybrid simulation code. We observe unstable m=2 modes at levels of ion rotation below instability thresholds predicted by finite-Larmor-radius fluid theory. Nonlinear effects are found to reduce the growth rate and lower the real frequency of the m=2 mode at large amplitude. Instabilities with m>2 have been observed for strongly reversed cases. It is also found that growth rates for these instabilities can be reduced by increasing the ratio of the plasma radius to the ion Larmor radius.
Direct measurement of anisotropy of hot electron rings using synchrotron emission
Authors: Tamor S.
Published: March 2003

It is pointed out that velocity-space anisotropy of a relativistic electron ring is reflected in the angular distribution of the intensity and polarization of the synchrotron emission, and it is suggested that this effect may be useful for direct measurement of the anisotropy. Results of sample computations for a loss-cone type of distribution are presented.
Multiple solutions and hysteresis in steady parallel viscous flows
Authors: Davis Stephen H. , Kriegsmann Gregory A. , Laurence Robert L. , Rosenblat S.
Published: March 2003

A study is made of flows of viscous fluids when viscosity varies with temperature according to an Arrhenius law, and when viscous dissipation is taken into account. It is shown that for fluids with highly temperature-sensitive viscosity the graph of shear rate against shear stress is an S-shaped curve, with either one or three values of shear rate for given shear stress. In this situation hysteresis can occur if steady unidirectional flow is maintained. The velocity and temperature profiles are calculated, and the manner in which the temperature-sensitivity parameter affects the conclusions is discussed. The calculations serve as criteria for evaluating the importance of viscous-heating effects in applications.
A novel property of the displacement thickness in three-dimensional boundary-layer theory
Authors: Cebeci Tuncer , Keller Gary L , Unt Hillar , Stewartson Keith
Published: March 2003

It is shown that the equivalent blowing velocity of a three-dimensional boundary layer can be either positive or negative near separation depending on the geometry of the flow. The situation is illustrated by a computation of the boundary layer in the planes of symmetry of a thin ellipsoid moving in the direction of its major axis. This result is of importance in estimating the effect of the boundary layer on the external flow and is distinct from the two-dimensional problem where the blowing velocity always appears to be positive near separation.
Energy flux in capillary-gravity waves
Authors: Hogan S. J.
Published: March 2003

The relative energy flux Frel of capillary-gravity waves on a moving stream is calculated for various wavelengths, as a function of wave amplitude. In some cases, abrupt changes in the sign of Frel are found to occur for a small increase in wave amplitude. The way in which the phenomena might be observed experimentally is indicated.
The spatial derivative of temperature in a turbulent flow and Taylor’s hypothesis
Authors: Browne L. W. B. , Antonia R. A. , Rajagopalan S.
Published: March 2003

Statistics of the streamwise derivative of temperature fluctuations on the centerline and at one streamwise station in the self-preserving region of a turbulent plane jet have been obtained by two techniques. In one, the derivative is approximated by the difference between the signals from two cold wires separated in the streamwise direction. In the other, the streamwise derivative is inferred from the temporal derivative and different convection velocities. Instantaneous values, root-mean-square values, skewness, and flatness factors are in reasonable agreement for a sufficiently large range of separations regardless of whether the mean velocity (Taylor’s hypothesis) or the instantaneous velocity are used as the convection velocity. Corrections for turbulence intensity, that are often applied to Taylor’s hypothesis, do not seem warranted. The difference between spectra of the temperature difference and temporal derivative spectra cannot be resolved by identifying the convection velocity with a frequency-dependent phase velocity of the temperature fluctuation.
Turbulent spots, wave packets, and growth
Authors: Chambers F. W. , Thomas A. S. W.
Published: March 2003

The smoke wire technique was used to initial formation and growth of turbulent spots in a laminar boundary layer. The spots were observed to result not from the breakdown of wave packets arising at the spot generator but from longitudinal structures which preceded the wave packets. The original wave packet in turn was observed to ultimately take the form of oblique waves trailing the spot. These waves had little apparent involvement in the growth of the spot and did not appear to be associated with the formation of new spots within the region visualized.
Inertial and viscous effects in the nonlinear growth of the tearing mode
Authors: Edery D. , Frey M. , Somon J. P. , Tagger M. , Soule J. L. , Pellat R. , Bussac M. N.
Published: March 2003

The nonlinear self-similar tearing-mode solution of Rutherford is revisited. The steam function for the plasma flow including inertia, convection, and viscosity is explicitly computed. In all cases, Rutherford’s solution is asymptotically valid.
Low Reynolds number flow in a rotating tank with barriers
Authors: Munson B. R. , Sturges L. D.
Published: March 2003

Experimental and theoretical results are presented for two-dimensional creeping flow in a rotating circular tank that has stationary barriers against its wall. The type of flow pattern obtained depends on the number and size of the barriers.
Effects of a constant magnetic field on electron–electron collisions
Authors: Matsuda Kyoko
Published: March 2003

The Rostoker equation has been numerically compared with the Fokker–Planck equation for a plasma with axially symmetric velocity distributions by computing electron slowing-down rates and velocity-space diffusion rates due to collisions with background electrons. The results in a case of Ωe ∼ωpe agree well with the corresponding quantities obtained from the Fokker–Planck operator. In the case of a strongly magnetized plasma (Ωe ≫ωpe), the results indicate corrections to the Fokker–Planck coefficients which are different from the one suggested by a previous work.
Evaluation of the structure of ergodic fields
Authors: Boozer Allen H.
Published: March 2003

A method of analyzing ergodic magnetic fields is given, including a generalization of magnetic coordinates to such fields. The results of this analysis can be used in existing Monte Carlo codes to assess the enhanced transport associated with imperfect surfaces. The Hamiltonian for a general magnetic field is given as part of this analysis.
Two-dimensional equilibria of field-reversed configurations in a perfectly conducting cylindrical shell
Authors: Hewett D. W. , Spencer R. L.
Published: March 2003

Two-dimensional field-reversed equilibria bounded by a conducting cylinder are computed. The computation is made possible by using a global constraint and by using a computational algorithm that is protective of the initial guess. A pressure profile is used that has sufficient generality to match experimentally produced configurations. It is found that for some choices of separatrix radius and separatrix beta, no equilibria exist. The reasons for loss of equilibrium are discussed and an example of a configuration near loss of equilibrium conditions is given.
Reflection of an ion-acoustic soliton by plasma inhomogeneities
Authors: Kuehl H. H.
Published: March 2003

The coupled equations governing oppositely traveling, weakly nonlinear and weakly dispersive ion-acoustic waves in a weakly inhomogeneous plasma are derived. From these equations, the reflected wave due to an ion-acoustic soliton propagating in a weakly inhomogeneous plasma is obtained. It is shown that this reflected wave is small compared with both the trailing shelf and the soliton amplitude decrease due to energy transfer to the shelf.
Formation of ion-acoustic double layers
Authors: Chanteur G. , Adam J. C. , Pellat R. , Volokhitin A. S.
Published: March 2003

The formation of an ion-acoustic double layer from a potential well is studied by computer simulation and explained by a modified Korteweg–de Vries (KdV) equation taking into account the reflections of the electrons.
Experiments on potential gradients in a current-carrying plasma. I. Potential structures
Authors: Guyot M. , Hollenstein Ch.
Published: March 2003

In a current-carrying plasma with vd<vte, constant potential gradients and strong localized potential jumps have been observed under different experimental conditions. One or more potential jumps with eΦ/kTe =10–12 and a typical width of 30–500 λDe may exist. Measurements of the electron distribution function show the presence of a beam on the high-potential side. This electron beam is thermalized within 500–1000 λDe leading to strong localized electron heating. Ion heating as well as peaked anomalous resistivity are found within the jump. In the case of a constant potential gradient, ion and electron heating have been observed. However, no electron beam could be detected. The resistivity is anomalous for this case also.
Shear-driven instabilities of annular relativistic electron beams in vacuum
Authors: Mostrom Michael A. , Jones Michael E.
Published: March 2003

The study of instabilities driven by azimuthal shear (diocotron) and axial shear is extended to annular relativistic beams. The analysis is done nonrelativistically in the beam frame where the instability is assumed to be electrostatic, and the dispersion relation is then transformed back to the laboratory frame where magnetic perturbations are non-negligible. This requires keeping finite axial wavenumber kz in the analysis. The axial and azimuthal shears are related through the self-consistent equilibrium, assuming emission from an equipotential cathode. Axial shear destabilizes the diocotron modes at higher azimuthal wavenumbers l. It also produces a set of modes including l=1 modes, that are unstable as the result of wave–particle interactions. The annular configuration introduces an important second pole in the differential equation.
Theory of accelerating gap induced radial oscillations of intense electron beams
Authors: Adler R. J.
Published: March 2003

The amplitude and phase of zero-frequency cyclotron waves induced by an accelerating gap in an electron-beam accelerator are calculated. Using Fourier-transform techniques, the amplitude is found to be directly related to the Fourier transform of the radial electric field. A simple field model reflects the basic physics of the gap and is used to find closed form solutions to the oscillation equations. A formal solution for suppressing oscillations over a finite energy range is also found.
Radiation conversion and related ablation behavior of a gold-foil target irradiated by 0.35, 0.53, 1.06, and 10.6 μm lasers
Authors: Nishimura H. , Matsuoka F. , Yagi M. , Yamada K. , Nakai S. , McCall G. H. , Yamanaka C.
Published: March 2003

Ultra-soft x-ray emission from a gold-foil target was studied by irradiating with four different wavelength lasers. The experimental data were analyzed under the blackbody approximation to evaluate the radiation temperature and the radiation conversion efficiency as functions of laser intensity and wavelength. The results show that the shorter-wavelength laser leads to a remarkable conversion of absorbed energy into radiation. The ablation behavior due to radiation and related energy transport in a gold foil is also studied.
Comments on ‘‘Adiabatic modifications to plasma turbulence theories’’
Authors: Lewis M. B.
Published: March 2003

A comment on ‘‘Adiabatic modifications to plasma turbulence theories’’ is made.
Nonresonant electron transport in the ELMO Bumpy Torus experiment using a BGK operator
Authors: Hastings D. E. , Spong D. A.
Published: March 2003

For the experimentally measured parameters in the ELMO Bumpy Torus (EBT) experiment, the electrons in the core plasma are nonresonant; that is, the average ∇B×B and E×B precessional drifts are in the same direction. Analytic transport rates exist for the electrons in the case where the ambipolar field is large relative to the plasma thermal energy. For finite fields, the diffusion coefficients have been calculated numerically. An analytic expansion away from the large field case that provides corrections to the results of Kovrizhnykh for finite fields has been developed from a BGK operator. These expressions are compared with the numerical calculations for parameters of experimental interest and it is shown that a substantially improved approximation over the Kovrizhnykh results can be obtained.
Measurements of nonlinear interactions during natural transition of a symmetric wake
Authors: Miksad R. W. , Jones F. L. , Powers E. J.
Published: March 2003

Bicoherency spectrum measurements are used to study the nonlinear dynamics of spectral broadening during natural transition of a two-dimensional wake. Dominant nonlinear interactions are identified. The results are compared with multiple instability controlled transition experiments.
Higher-order statistics of turbulent fluctuations in the plane wake
Authors: Fabris G.
Published: March 2003

Skewness and flatness factors of the velocity components, u, v, w, the temperature fluctuations θ, and of the momentum and heat transport correlations uv and vθ in the two-dimensional turbulent far wake are presented. Standard deviations σuv and σvθ are also included. Observed profiles are critically reviewed and discussed in conjunction with other available data. Among the interesting findings are a change of sign of the skewness of u and θ near the wake centerline and the existence of double minima in the flatness factors of u, v, and θ. The minimum of Kθ is only 2. Here σuv and σvθ show high and broad maxima at the centerline. The coefficients ∼((uv)2)/ ∼(u2 v2) and ∼((vθ)2)/∼(v2 θ2) were introduced to indicate the degree of correlation between the u and v fluctuations, and between the v and θ fluctuations in eddies regardless of sign. Surprisingly the highest degree of correlation is obtained at the wake centerline. The information thus provided will be helpful to workers developing use of probability density functions in prediction methodology. Quantitative physical insights are gained as well.
Stationary flow of gas at very high temperatures and relativistic speeds
Authors: Herrera M. A. , Hacyan S.
Published: March 2003

The stationary flow of a gas which is a mixture of protons, electrons, positrons, and radiation—assumed to be in thermal equilibrium—within the framework of relativistic hydrodynamics is studied. The thermodynamic variables and the critical parameters associated with the subsonic and supersonic regimes are obtained, generalizing classical results.
Drift-wave spectra in plasmas with magnetic shear
Authors: Rogister André , Hasselberg Günter
Published: March 2003

The nonlinear radial eigenmode equation governing the evolution of drift waves in a weakly turbulent state is obtained under the assumption that ion nonlinear scattering (nonlinear ion Landau damping) is the dominant saturation mechanism. The poloidal spectrum is to be derived from the eigenvalue equation. The local character of the interaction in frequencies allows the transformation of the otherwise integral wave kinetic equation (i.e., the eigenvalue equation) into a system of two first-order differential equations coupling the spectra I(kθ) and I(k°θ) if ω(k°θ)=ω(kθ). The density fluctuation spectrum is obtained analytically in the limit of the small temperature ratio Ti/Te. Some features are: (i) the spectral index at high mode numbers agrees well with experimental values; (ii) the long-wavelength spectrum peaks at kθas≂0.62 (as ≡sound Larmor radius), (iii) the cross-field diffusion is mostly induced by the high-mode-number fluctuations, whereas the major contribution to the density fluctuations is in the long wavelengths; (iv) the frequency spectrum of the beat waves peaks at ω=0 (they then have a convective cell character); and (v) the turbulence level is a factor Ln/Ls (ratio of density and shear lengths) smaller than with former theories.