bluuebunny/crossref_metadata_2025 huggingface dataset:
The characterization of multiphase fluid transport in a porous solid by pulsed gradient stimulated echo nuclear magnetic resonance
Authors: Tessier Jean J. , Packer Ken J.
Published: July 2002

Pulsed magnetic field gradient stimulated echo nuclear magnetic resonance (NMR) measurements are reported for the steady-state flow and diffusion of two and three phases (water, dodecane, N2 gas) within a sample of a Fontainebleau sandstone. The stimulated echo dependence on the gradient pulse area, q, is used to derive the displacement probability, PΔ(X) for fixed observation times Δ, with the displacements X being measured along the macroscopic pressure gradient. An extensive range of NMR experiments was carried out, starting with single-phase flow of either water (an aqueous solution of NaCl 3% w/v) or oil (dodecane) for various relative saturation states. Following these experiments, PΔ(X) were acquired for water and oil when both phases were forced to flow through the sandstone. Finally, NMR measurements were performed in which three phases (oil, water and N2 gas) were flowing simultaneously. Using the NMR data it was possible to evaluate the physical importance of parameters such as wettability, spreading and phase saturations on the transport phenomena. To our knowledge, these experiments represent the first comprehensive NMR study of multiphase flow in porous media, and the extent of the information obtained is providing a strong experimental basis to validate and develop accurate modelling of fluid transport in porous solids.
Thermoacoustics: Transient regimes and singular temperature profiles
Authors: Bauwens Luc
Published: July 2002

The stationary, periodic solution to the problem of oscillating flow of a conducting fluid, in a duct closed at one end and with periodical mass flow rate at the other end, is known to exhibit a singularity in the mean temperature at the closed end when transverse conduction in the fluid and the duct wall is taken into account, but longitudinal conduction is neglected. For instance, a solution of that type was originally suggested by Gifford and Longsworth as a prototype for pulse-tube refrigeration. Whether these stationary singular solutions are physical or mere theoretical curiosities depend upon the existence of a scenario leading to such a limit cycle. To address that question, a transient theory is formulated, using the narrow duct approximation. The results show that at least for constant fluid thermal conductivity, all singular profiles generated by the mechanism under study are linearly stable. For round tubes, the temperature profile is shown to evolve, from an arbitrary initial value, toward an equilibrium profile which results in balanced energy fluxes.
Experimental study of the kinematic mixing of two fluids in a 2D enclosure
Authors: Ilegbusi Olusegun J. , Mat Mahmut D.
Published: July 2002

A simple experiment is described to investigate the buoyancy driven mixing of two fluids separated by a valve in a 2D (two-dimension) enclosure. The experiment is performed for a range of Grashof numbers, Gr ranging from 1 to 3.7×105 and a valve speed of Vval=0.05 m/s. Three distinct flow regimes are investigated within the parametric range considered, depending on the Grashof number: Chaotic (Gr⩾3.7×105), convective (1<Gr<3.7×105) and diffusive (Gr<1). A novel image processing technique is used to measure the mix fraction, interface elongation, and mixing width. The technique is based on the analysis of the color intensity of images obtained from the video recording. Mixing is quantified by analysis of the interface length and mixing width, from which the molecular mixing rate and Lyapunov exponents are deduced. The chaotic regime is found to be efficiently mixed compared with the convective and diffusive regimes.
Ultrasound scattering by a swirling jet
Authors: Oljaca M. , Gu X. , Glezer A. , Baffico M. , Lund F.
Published: July 2002

Recent analytical work has shown that when an acoustic plane wave propagates through a rotational flow field there is a linear relationship between the Fourier component of the scattered acoustic pressure and the Fourier transform in space and time of the vorticity component that is normal to the plane defined by the wave vectors of the incident and scattered acoustic waves. Hence, ultrasound scattering can be used as a non-intrusive spectral probe of vorticity and potentially as a tool for direct measurements of vorticity distributions. Some aspects of this technique have been tested in a swirling air jet emanating from a 2.54 cm diameter nozzle where the swirl is generated upstream of the jet nozzle by a rotating paddle. For a given exit volume flow rate, swirl numbers up to 0.4 are realized. Radial distributions of the streamwise and tangential velocity components downstream of the jet exit plane are measured using two-component hot-wire anemometry and the corresponding distributions of streamwise vorticity are computed. A nominally plane ultrasonic wave field is generated normal to the jet axis by a transmitter having a 16 cm square aperture. The scattered ultrasound in the radial direction is measured at a number of streamwise and azimuthal stations. In accord with the theory, the normalized amplitude of the scattered acoustic wave is a linear function of the magnitude of the centerline vorticity at the exit plane of the jet, and is independent of the intensity of the incident wave field. Fourier components of the vorticity distribution are directly measured by varying the scattering angle and are in good agreement with theoretical predictions.
Time scales of stratified turbulent flows and relations between second-order closure parameters and flow numbers
Authors: Heinz Stefan
Published: July 2002

The description of turbulent mixing and chemical reactions by Lagrangian probability density function methods offers some significant advantages over other methods, mainly due to the simulation of mixing processes and the exact treatment of chemical transformations. A key problem of such methods is the information on the time scales of processes, because they determine the dynamics and intensity of mixing. This question is considered for stratified flow. Different models are presented for the development of these time scales in time and their stationary spatial patterns in dependence on shear and stratification. The model predictions are shown to be in agreement with large-eddy simulations of stratified homogeneous shear flow. Two further applications of these models are considered: the description of transitions between flow regimes (characterized by different scaling quantities) in the stationary atmospheric surface layer and, second, the simulation of buoyant plume rise. It is shown that the predictions of the stationary frequency model agree with measured data. The consideration of limit cases of this model leads to connections between second-order closure parameters and (critical) flow numbers that characterize these transitions. These relationships are shown to be very advantageous for the application of closure models. A new flow number that characterizes the transition to free convective flow under unstable stratification is introduced here in analogy to the critical gradient Richardson number, which characterizes the onset of turbulence in stably stratified flow. The second application provides a new theory for buoyant plume rise. Two parameters that describe the turbulent mixing in the entrainment and extrainment stages of plume rise are explained as ratios of the relevant time scales. The two-thirds power law of buoyant plume rise, which is observed for nonturbulent and neutrally stratified flow, is obtained without having to make ad hoc assumptions. For turbulent flow, the plume’s leveling-off is calculated in accord with measurements.
Numerical investigation of the formation of three-dimensional structures in stably-stratified mixing layers
Authors: Cortesi A. B. , Yadigaroglu G. , Banerjee S.
Published: July 2002

A temporally-growing mixing layer has been directly simulated with a pseudospectral technique, for initial bulk Richardson numbers from 0.0 to 0.2 and for Prandtl numbers from 0.00535 to 2.2. Several different initial conditions for the velocity fluctuations were imposed. For the two-dimensional (2-D) case only purely-deterministic conditions were used, whereas purely-deterministic, combined deterministic-random, or purely-random conditions were imposed in the three-dimensional (3-D) cases. The numerical procedure allowed fields with very different characteristic lengths to be resolved, with spectral accuracy maintained. The evolution of the velocity, active (temperature), and passive scalar fields were followed independently by adaptively redistributing collocation points in the regions of high shear and rapid scalar variations. The vertical boundary conditions were imposed at infinity to eliminate any boundary-layer effects and an exponential mapping was used to translate infinite physical space into finite computational space. The birth and time evolution of the longitudinal structures have been investigated. Variations in the initial modal forcing are reflected in different outcomes from the competition between core- and braid-centered instabilities in unstratified flow. For relatively strong fundamental forcing (compared to the 3-D forcing) the most unstable mode is braid-centered, whereas if the fundamental forcing is weak the most-unstable, core-centered mode determines the overall three-dimensionalization of the flow by generating large deformations of the main vortex cores. In subcritically-stratified air and water flow, instead, the braid-centered instability supersedes the core-centered instability, whatever the initial forcing. In stratified liquid sodium the shear-aligned convective instabilities observed in air and water are not excited. The conversion of potential into kinetic energy by convective overturning in the braid region does not occur because the high thermal conduction precludes the existence of the unstably-stratified regions necessary to drive the instabilities. The initial conditions, the Richardson, and the Prandtl numbers accordingly play a significant role in the free-shear-layer evolution and need to be explicitly considered for modeling purposes.
Advection of mass fraction in forced, homogeneous, compressible turbulence
Authors: Cai X. D. , O’Brien E. E. , Ladeinde F.
Published: July 2002

In nearly isothermal, compressible turbulence of a nondense gas the mass fraction of an embedded passive scalar satisfies the same formal conservation equation as a passive scalar in incompressible turbulence. Direct numerical simulation of this system shows that the compressible turbulence modes are less efficient than the incompressible in transporting scalar spectral content from large to small scales. It is argued that the cause of this outcome is the reduced size of the integral length scale of the compressible velocity components vis á vis that of the incompressible velocity components, and this also explains the experimentally observed ineffectiveness of the dilatational velocity modes in determining scalar flux in homogeneous, compressible turbulence with a uniform mean scalar gradient.
Development of a point plume in the presence of background rotation
Authors: Fernando H. J. S. , Chen R-r. , Ayotte B. A.
Published: July 2002

This paper describes a laboratory study on the evolution of a point turbulent plume placed at the free surface of a homogeneous fluid layer in the presence of background rotation. It is shown that the plume initially evolves as if there is no rotation. However, the rotational effects become important after the plume descends a vertical distance hc1≈3.3(B/Ω3)1/4 for a normalized time Ωtc1≈2.4, whence the vertical descent rate of the plume is reduced while maintaining approximately the same lateral growth rate. Here Ω is the rate of background rotation and B is the specific buoyancy flux of the plume. The rotational effects inhibit the lateral growth of the plume at a time Ωtc2≈5.5, when the maximum plume width is bc≈1.4(B/Ω3)1/4. Thereafter, the vertical descent continues and the plume evolves into a cylindrical shape while developing a cyclonic circulation in and around it, except near the plume front. Upon reaching the bottom surface after traveling a fluid depth of H, the plume deflects, propagates horizontally, and becomes unstable breaking up into anticyclonic eddies. Studies carried out for the case of H<hc1 show that this instability is initiated at a horizontal length scale proportional to the Rossby deformation radius of the deflected flow, and hence it is of baroclinic type. These eddies appear to align vertically with the cyclonic eddies formed by the barotropic instability of the surface rim current, thus producing heton-like structures. The influence of the diameter d0 of the plume on the flow evolution is also studied, and it is shown that plumes with aspect ratio h/d0<12 (where h is the vertical extent) can be approximated as point plumes. Scaling arguments are advanced to explain the results. Some geophysical applications of the study are also discussed.
Asymptotic growth of disturbances from spatially compact source in a skewed mixing layer
Authors: Lu Ganyu , Lele Sanjiva K.
Published: July 2002

The growth of disturbances in a skewed mixing layer, i.e., the shear layer between two streams with different velocity magnitudes and directions, is examined. Due to the three-dimensionality of the mean flow, the natural amplification direction for spatially amplifying waves needs to be determined. This issue can be resolved through investigation of the asymptotic growth of disturbances from a spatially compact source. The results show that the disturbances grow in a wedge-shaped region centered at the mean convection direction for both incompressible and compressible mixing layers. For incompressible unskewed mixing layers, the normal to the instability wave fronts is parallel to the amplification direction. The wave front normal is oblique to the amplification direction for skewed mixing layers and/or mixing layers with moderate compressibility.
Thermal separation in near-axis boundary layers with intense swirl
Authors: Herrada M. A. , Pérez-Saborid M. , Barrero A.
Published: July 2002

Swirling flows have a wide range of applications and exhibit a variety of interesting features. Gas cooling near the axis in these flows, the so-called Ranque–Hilsch effect, is one of them. To gain insight into this phenomenon, we have analyzed the thermal, near-axis boundary layer of a gas jet driven by a class of conical inviscid quasi-incompressible flows whose axial and azimuthal velocity components, w and v, and stagnation temperature, Tt, behave near the axis as w=W0rm−2,v=LW0rm−2, and Tt−Tr=T0r2(m−2), where z and r are the axial and radial coordinates, L is the Squire number directly related to the swirl strength, m is any real number such as 1⩽m<2, Tr is a reference temperature, and W0 and T0 are arbitrary dimensional constants; W0 is assumed to be positive while T0 may be either positive or negative. To simplify the boundary layer analysis, low Mach number flows with small relative variations in the gas density have been considered. Radial profiles of axial and azimuthal velocity components, and static and stagnation temperatures are found to depend on the Squire parameter L, the Prandtl number, Pr, and the rest of the parameters of the problem. Even for the case of inviscid vortices with positive values of T0, for which the stagnation temperature increases towards the axis, is found that the stagnation temperature decreases substantially in the vortex core for some range of values of both L and Pr (Ranque–Hilsch effect) when the effect of both heat conduction and the work done by viscous forces are taken into account. It is also found that there exists an optimum value Lop for which the cooling effect reaches a sharp maximum and that small deviations of L from Lop reduce drastically the cooling effect. The appropriate tuning of Lop can be dramatically important for the efficient operation of Ranque–Hilsch tubes. The influence of the Prandtl number and the rest of the parameters of the problem has been also considered.
Clebsch representation near points where the vorticity vanishes
Authors: Graham C. Robin , Henyey Frank S.
Published: July 2002

We demonstrate that there is no Clebsch representation in any neighborhood of a generic vanishing point of the vorticity. This result is placed in the context of the Hamiltonian formulation of fluid mechanics. For stratified fluids, the analogous representation does exist, both locally and globally, under suitable hypotheses.
Stability of two-dimensional strip casting processes
Authors: Plaschko Peter , Schaflinger Uwe
Published: July 2002

The solidification of molten materials is of great significance in modern metallurgical engineering. We study disturbances of a process that is characterized by three disparate lengths: the solidification length L, the wavelength Λ, and the depth of the slab δ∝. The present analysis is motivated by the relation δ∞<Λ≪L, which is always true in practical strip casting processes. This justifies the use of an asymptotic expansion based on shallow water equations for long waves to describe the linear stability of disturbances. The leading-order equations govern a quasiparallel flow. In this limit we found two different types of disturbances: a weakly damped stable mode that runs downstream and a strongly damped perturbation traveling upstream. We focus on the downstream moving mode and show that this disturbance is strongly frequency dependent. Although the velocity disturbances are damped, there is a regime of parameters where the perturbations of the displacement grow in the horizontal direction. In our analyses we found a region of preferred frequencies. The displacement grows at these frequencies faster than for neighboring frequencies. The wavelengths of disturbances oscillating at these preferred frequencies are in qualitative agreement with the experimental observation of the wavelengths of harmonically varying grooves in the completely solidified material.
Using cavitation to measure statistics of low-pressure events in large-Reynolds-number turbulence
Authors: La Porta A. , Voth Greg A. , Moisy F. , Bodenschatz Eberhard
Published: July 2002

The structure of the pressure field of a turbulent water flow between counter-rotating disks is studied using cavitation. The flow is seeded with microscopic gas bubbles and the hydrostatic pressure is reduced until large negative pressure fluctuations trigger cavitation. Cavitation is detected via light scattering from cavitating bubbles. The spatial structure of the low-pressure events are visualized using a high-speed video system. A fast photo detector is used to measure the scaling of the cavitation statistics with the pressure. This data is used to determine the shape of the tail of the probability density function for the pressure. The tail is found to be exponential and scales more rapidly with Reynolds number than the standard deviation of the pressure. This may indicate the influence of internal intermittency.
Second-order temperature and velocity structure functions: Reynolds number dependence
Authors: Antonia R. A. , Zhou T. , Xu G.
Published: July 2002

An interpolation relation is used to fit second-order moments of temperature and velocity fluctuation increments which have been measured in three types of flows (decaying grid turbulence, cylinder wake, and circular jet) for values of the Taylor microscale Reynolds number Rλ in the range 30 (grid turbulence) to about 500 (jet). Several checks confirm the analytical framework underpinning the fit. The magnitude of the resulting scaling exponents increases with Rλ, the longitudinal one being first to asymptote to a constant value. The scaling exponent for the temperature increment is generally smaller than that for the longitudinal velocity increment but larger than that for the transverse velocity increment when Rλ⩽300. For Rλ>300, the magnitude of the scaling exponents of the temperature and transverse velocity increments are nearly equal. Within the framework of small-scale intermittency, the magnitude of the Obukhov–Corrsin “constant” increases at small Rλ in similar manner to that of the longitudinal and transverse velocity Kolmogorov constants.
On the ideal initial value problem for the neutral sheet
Authors: Dahlburg R. B.
Published: July 2002

The ideal initial value problem for the neutral sheet is considered, with the emphasis on fieldwise-independent, infinitesimal velocity disturbances. It is shown analytically that cross-field velocity perturbations persist, and that they also set up fieldwise magnetic disturbances that grow algebraically in time. When these disturbances attain finite amplitude they should distort the neutral sheet configuration, and hence they could provide an alternative route by which magnetic reconnection can become turbulent. Some supporting numerical simulations, performed at large Lundquist numbers, are then described. In an Appendix the analysis is extended to include fieldwise-dependent disturbances.
Starting oscillation conditions for gyrotron backward wave oscillators
Authors: Kou C. S.
Published: July 2002

A generalized linear theory that allows for mismatched boundary conditions was used in determining the starting oscillation conditions for gyrotron backward wave oscillators (gyro-BWO). The results reveal that the gyro-BWO interaction is normally caused by the interference between a constant-amplitude backward wave and a decaying backward wave. The latter becomes a forward-growing wave if the operating frequency is close to the waveguide cutoff frequency. The starting oscillation length for a reflection-type gyro-BWO is shorter than that of a matched-type gyro-BWO. The mismatch has more of an effect upon the onset of a gyro-BWO, which is operated close to the cutoff frequency.
Transport properties of the interacting magnetic-island model of tokamak plasmas
Authors: Gianakon T. A. , Callen J. D. , Hegna C. C.
Published: July 2002

This paper explores the equilibrium and transient transport properties of a mixed magnetic topology model for tokamak equilibria. The magnetic topology is composed of a discrete set of mostly nonoverlapping magnetic islands centered on the low-order rational surfaces. Transport across the island regions is fast due to parallel transport along the stochastic magnetic-field lines about the separatrix of each island. Transport between island regions is assumed to be slow due to a low residual cross-field transport. In equilibrium, such a model leads to a nonlinear dependence of the heat flux on the pressure gradient; a power balance diffusion coefficient which increases from core to edge; and profile resiliency. Transiently, such a model also exhibits a heat pulse diffusion coefficient larger than the power balance diffusion coefficient.
Equilibrium reconstruction of the safety factor profile in tokamaks from motional Stark effect data
Authors: Hirshman S. P. , Lee D. K. , Levinton F. M. , Batha S. H. , Okabayashi M. , Wieland R. M.
Published: July 2002

The motional Stark effect (MSE) diagnostic has been used to obtain accurate measurements of the internal magnetic pitch angle tan−1(BZ/BT) in finite-pressure tokamaks. The MSE data, together with external magnetic probe data, are used to reconstruct self-consistently the equilibrium safety factor (q) profile and, hence, the plasma current density, in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 1, 51 (1986)] and the Princeton Beta Experiment-Modified (PBX-M) tokamak [Phys. Fluids B 2, 1271 (1990)]. An efficient computational scheme, based on an inverse coordinate representation of the magnetic field, has been developed to solve the coupled nonlinear equations describing both the magnetohydrodynamic (MHD) equilibrium and the q profile, which best match all the experimental data.
Intensity scaling and saturation of stimulated Raman forward scattering
Authors: Batha S. H. , Bradley K. S. , Drake R. P. , Estabrook Kent , Kruer W. L. , Montgomery D. S. , Remington B. A.
Published: July 2002

The intensity scaling of stimulated Raman forward scattering has been measured in preformed laser-produced plasmas. It was found that predictions based on linear convective theory were consistent with the amount of scattered light measured when the laser intensity was less than about 1×1015 W/cm2. No increase in the amount of scattered light was found for higher intensities. The measured saturation level is compared to several nonlinear models: pump depletion, coupling to Langmuir and ion-acoustic waves, ponderomotive detuning, wave breaking, and particle trapping. It is found that saturation models based on ponderomotive detuning and on coupling to Langmuir and ion-acoustic waves are consistent with the experiment.
Reflectivity of laser-produced plasmas generated by a high intensity ultrashort pulse*
Authors: Sauerbrey R. , Fure J. , Le Blanc S. P. , van Wonterghem B. , Teubner U. , Schäfer F. P.
Published: July 2002

Solid state density aluminum and carbon plasmas were generated by 350 fs KrF laser pulses at intensities of 1017 W cm−2. Reflectivity, x-ray emission, and spectra of the backscattered radiation were measured as a function of the laser polarization, angle of incidence, and intensity (I). For p-polarized light, the absorption (A) is almost intensity independent for 1015 W cm−2<I<2×1017 W cm−2, and can reach values up to A=0.8. For constant laser intensity, p-polarized light is up to a factor of 15 more efficient in generating x rays than s-polarized light. A polarization and angle of incidence-dependent absorption mechanism, such as resonance absorption or vacuum heating, is consequently important. The spectral line shape and positions of the reflected radiation from the carbon plasma showed a substantial intensity dependence. It is demonstrated that the backscattered spectrum is influenced by the plasma expansion (Doppler effect) and the temporal development of the electron density.
Magnetohydrodynamic jets*
Authors: Bell A. R.
Published: July 2002

In a series of three idealized magnetohydrodynamic (MHD) calculations in cylindrical geometry, we show that an azimuthal magnetic field can produce well-collimated jets by the pinch effect. In a laser-produced plasma, the magnetic field is generated by crossed density and temperature gradients (∇n×∇T), and may be responsible for the ‘‘imprint problem.’’ In an astrophysical plasma, differential rotation associated with an accretion disk can generate the magnetic field, which may then be responsible for jet formation on stellar and galactic scales.
Electron cyclotron resonant heating: A simpler method for deriving the linear wave equations in a nonuniform magnetic field
Authors: McDonald D. C. , Cairns R. A. , Lashmore-Davies C. N.
Published: July 2002

In a recent article Cairns et al. [Phys. Fluids B 3, 2953 (1991)] gave a method for the derivation of full wave equations describing propagation through a cyclotron resonance in an inhomogeneous plasma. The simplicity of this method compares favorably with previous derivations, and the damping resulting from the variation in the magnetic field across a Larmor orbit, described by Lashmore-Davies and Dendy [Phys. Fluids B 1, 1565 (1989)], is included. The effect of the relativistic mass shift on the cyclotron frequency, which plays an important role in the electron cyclotron range of frequencies, was not taken into account, however, and the object of the present work is to remedy this omission. It is shown how equations, valid in the weakly relativistic regime, may be obtained in a rather straightforward way. Results obtained by a number of earlier workers are recovered and can be extended.
Production of fast electrons in a prototype heliac by helicon waves
Authors: Loewenhardt P. K. , Blackwell B. D. , Hamberger S. M.
Published: July 2002

The generation of plasma by helicon waves has been investigated in the Small Heliac Experimental Apparatus [Nucl. Fusion 25, 1485 (1985)]. Evidence for wave–particle interaction in both argon and helium is presented in the form of observations of the electron distribution function. A high energy bump-on-tail feature is shown to coincide with the wave phase velocity and with an energy corresponding to a large ionization cross section. It is shown that the bump-on-tail feature may be related to the ionization capability of helicon waves and may be a result of some form of radio frequency current drive.
Drift-wave-like density fluctuations in the Advanced Toroidal Facility (ATF) torsatron
Authors: Shats M. G. , Harris J. H. , Likin K. M. , Wilgen J. B. , Baylor L. R. , Bell J. D. , Ma C. H. , Murakami M. , Sarksyan K. A. , Aceto S. C. , Bigelow T. S. , Bell G. L. , Colchin R. J. , Dory R. A. , Dunlap J. L. , Dyer G. R. , England A. C. , Goldfinger R. C. , Hanson G. R. , Hutchinson D. P. , Isler R. C. , Jernigan T. C. , Langley R. A. , Lee D. K. , Lyon J. F. , Qualls A. L. , Rasmussen D. A. , Richards R. K. , Saltmarsh M. J. , Simpkins J. E. , Vander Sluis K. L. , Zielinski J. J.
Published: July 2002

Density fluctuations in low-collisionality, low-beta (β∼0.1%), currentless plasmas produced with electron cyclotron heating (ECH) in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)] have been studied using a 2 mm microwave scattering diagnostic. Pulsed gas puffing is used to produce transient steepening of the density profile from its typically flat shape; this leads to growth in the density fluctuations when the temperature and density gradients both point in the same direction in the confinement region. The wave number spectra of the fluctuations that appear during this perturbation have a maximum at higher k⊥ρs (∼1) than is typically seen in tokamaks. The in–out asymmetry of the fluctuations along the major radius correlates with the distribution of confined trapped particles expected for the ATF magnetic field geometry. During the perturbation, the relative level of the density fluctuations in the confinement region (integrated over normalized minor radii ρ from 0.5 to 0.85) increases from ñ/n∼1% when the density profile is flat to ñ/n∼3% when the density profile is steepened. These observations are in qualitative agreement with theoretical expectations for helical dissipative trapped-electron modes (DTEMs), which are drift-wave instabilities associated with particle trapping in the helical stellarator field; they suggest that trapped-electron instabilities may play a role in constraining the shape of the density profile in ATF, but have little effect on global energy confinement.
Theory of isolated, small-scale magnetic islands in a high temperature tokamak plasma
Authors: Connor J. W. , Wilson H. R.
Published: July 2002

A theory for the existence of noninteracting small-scale, ‘‘drift’’ magnetic islands in a high temperature tokamak plasma is presented. This situation contrasts with that discussed by Rebut and Hugon [Plasma Phys. Controlled Fusion 33, 1085 (1991)] which involves a background ‘‘sea’’ of magnetic turbulence caused by island overlap. The islands are driven by the effect of finite ion Larmor radius on the particle drifts and they propagate with a velocity comparable to the diamagnetic velocity. In contrast with the work of Smolyakov [Plasma Phys. Controlled Fusion 35, 657 (1993)] collisions are assumed to be rare. Although the saturated island size is independent of the collision frequency in the model discussed here, collisions play a crucial role in determining the frequency of the magnetic islands. An estimate is made of the anomalous heat transport which results from the fluctuations in the electrostatic potential associated with these magnetic islands. The predicted thermal diffusivity has several, but not all, of the characteristics of the Rebut–Lallia–Watkins transport model.
Study of a low pressure nitrogen plasma jet
Authors: Domingo P. , Bourdon A. , Vervisch P.
Published: July 2002

The numerical study of a low pressure nitrogen plasma jet is performed with a well documented experiment being used as a test-case. The shortcomings of different approaches used for the kinetic scheme, the thermal non-equilibrium and the transport properties are underscored. In particular, the strong influence of the charge exchange reaction (N2+N+■N2++N) on the recombination of electrons is shown. The importance of an accurate description of the ambipolar diffusion emerges from this study in the case of strong electronic pressure gradients.
Cyclotron resonance phenomena in a non-neutral multispecies ion plasma
Authors: Sarid E. , Anderegg F. , Driscoll C. F.
Published: July 2002

Cyclotron modes of a non-neutral Mg ion plasma were studied in a long cylindrical Penning–Malmberg trap. Several modes with angular dependence exp(ilθ), l≥1, are observed near the cyclotron frequencies of the various Mg ions. The l=1 modes for the majority species are downshifted from the cyclotron frequencies, with downshifts as large as four times the diocotron frequency. These large shifts are quantitatively explained by a multispecies cold-plasma theory, including the dependence on the plasma size and composition. These dependencies allow the plasma size and composition to be obtained from the measured mode frequencies. In contrast, the l=1 downshifts for minority species are generally close to twice the diocotron frequency, and remain unexplained. Cyclotron heating of the plasma ions was also observed with a surprising effect of improving the plasma confinement.
Excitation of Alfvén cyclotron instability by charged fusion products in tokamaks
Authors: Gorelenkov N. N. , Cheng C. Z.
Published: July 2002

The spectrum of ion cyclotron emission (ICE) observed in tokamak experiments shows narrow peaks at multiples of the edge cyclotron frequency of background ions. A possible mechanism of ICE based on the fast Alfvén Cyclotron Instability (ACI) resonantly excited by high energy charged products (α-particles or protons) is presented here. Two-dimensional eigenmode analysis of ACI mode structure and eigenfrequency are obtained in the large tokamak aspect ratio limit. The ACI is excited via wave-particle resonances in phase space by tapping the fast ion velocity space free energy. The instability growth rates are computed perturbatively from the perturbed fast particle distribution function, which is obtained by integrating the high frequency gyrokinetic equation along the particle orbit. Numerical examples of ACI growth rates are presented for Tokamak Fusion Test Reactor (TFTR) [World Survey of Activities in Controlled Fusion Research [Nuclear Fusion special supplement 1991] (International Atomic Energy Agency, Vienna, 1991)] supershot plasmas. The fast ion distribution function is assumed to be singular in pitch angle near the plasma edge. The results are employed to understand the ICE in Deuterium-Deuterium (DD) and Deuterium-Tritium (DT) tokamak experiments.
Electron transport studies in stochastic magnetic fields on the Compact Auburn Torsatron
Authors: Lin H. , Gandy R. F. , Knowlton S. F. , Hartwell G. J. , Prichard D. , Sasser G. , Thomas E.
Published: July 2002

A steady-state source of electrons was installed in the Compact Auburn Torsatron (CAT) [Fusion Technol. 18, 28 (1990)]. The transport of the electrons was studied with externally applied resonant magnetic fields. Magnetic fields with primary modes n/m=1/4 and n/m=1/3, and average radial field amplitude 〈(br/Bφ)rms〉∼1% have been employed. The perturbation fields produced mixed islands and stochastic regions at the outer region of the CAT torsatron. The particle diffusion coefficient was measured and compared to stochastic transport models.
Modeling energy flow in turbulent beam–plasma experiments
Authors: Urayama Paul , Benford Gregory
Published: July 2002

Further analysis of recent experiments improves agreement between a one-dimensional kinematic turbulence model and observations. The model includes both analytic and phenomenological elements. Time behavior, magnitude of turbulent electric fields, and radiated microwaves accord with data. Scaling a three-dimensional radiation rate improves agreement. Introducing ionization from the oscillating turbulent electric field does not improve agreement, arguing that this effect does not appear in typical beam–plasma systems. The model is simple and easy to apply to existing experiments.
Energy and momentum conservation in closed plasma systems with resonant and nonresonant wave turbulence
Authors: Vladimirov Sergey V. , Nambu Mitsuhiro
Published: July 2002

Total energy and momentum balance is demonstrated for the resonant and nonresonant waves in closed plasma systems which can evolve due to the resonant interaction of particles and the resonant waves. The complete expression for the turbulent collision integral which takes into account all nonlinear (in the considered approximation) effects as well as effects due to the system’s nonstationarity is derived. Additional contribution to the correlation function of the wave fields in the nonstationary system is calculated. Change of the energy of the nonresonant waves in the time-dependent background medium is discussed.
Adiabatic self-similar expansion of dust grains in a plasma
Authors: Yu M. Y. , Luo Huaqiang
Published: July 2002

The self-similar expansion of dust grains in an equilibrium plasma is investigated by means of a fluid theory for the dust. Charging of the latter as well as pressure effects are taken into account. It is found that the dust can vanish either asymptotically or abruptly.
A combined Chapman–Enskog and Grad method. II. Ionized gases
Authors: Rodbard M. G. , Bezerra A. G. , Kremer G. M.
Published: July 2002

The laws of Ohm, Fourier, and Navier–Stokes for an ionized gas are obtained from a simple method that combines the features of the methods of Chapman–Enskog and Grad. The transport coefficients are easily determined through inversions of second-order and fourth-order tensors. This work is based on the Boltzmann equation for ionized gases, where (i) the preponderant number of collisions are those that lead to small deflections corresponding to distant encounters; and (ii) a cutoff at the Debye length has to be considered.
Evolution of magnetic islands in a Heliac
Authors: Hayashi T. , Sato T. , Gardner H. J. , Meiss J. D.
Published: July 2002

Simulations of three-dimensional equilibria in the H-1 Heliac [Fusion Technol. 17, 123 (1990)] with the hint code [Phys. Fluids B 4, 1539 (1992)] show that the size of a dangerous magnetic island should increase with plasma pressure, but that a destruction of the equilibrium at low β is avoided because the rotational transform evolves to exclude the rational surface concerned. At higher pressures there is evidence of near-resonant flux surface deformations, which may lead to an equilibrium limit. A reconnected equilibrium at still higher pressures exhibits a double island structure, which is similar to homoclinic phase portraits that have been observed after separatrix reconnection in Hamiltonian systems.
Wave focusing near the electron cyclotron frequency in hot plasmas
Authors: Vézard D. , Laurent L. , Ségui J. L. , Giruzzi G.
Published: July 2002

Microwave transmission spectra measured on the Tore Supra tokamak [Equipe Tore Supra, in Proceedings of the 12th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Nice, 1988 (International Atomic Energy Agency, Vienna, 1989), Vol. 1, p. 9] exhibit a systematic enhancement of the transmitted power at frequencies close to the electron gyrofrequency, often well beyond the corresponding level detected in vacuum. In order to explain such observations, the problem of electromagnetic wave propagation near the electron cyclotron frequency in a hot magnetized plasma is investigated in the framework of geometrical optics. The simple case of the ordinary mode injected vertically is studied by taking the finite plasma temperature and relativistic corrections to the dispersion relation into account, in contrast with the more familiar cold plasma ray-tracing approach. It is found that the ray trajectories are strongly affected by such effects in the anomalous dispersion region close to the cyclotron resonance. As a consequence of this analysis, it is shown that the local enhancement of the transmitted power observed experimentally can be explained as a focusing phenomenon related to hot-plasma corrections to the wave refractive index.
Self-consistent mean field electrodynamics of turbulent dynamos
Authors: Gruzinov A. V. , Diamond P. H.
Published: July 2002

A turbulent dynamo in a conducting fluid is accompanied by the generation of small-scale magnetic fields, which are much stronger than the mean dynamo-generated magnetic field. These small-scale fields modify the α effect in such a way as to stabilize the dynamo process, α=(α0+β0R⋅/B∇×R)/(1+R2), where α0, β0 are the standard kinematic dynamo parameters, and R is proportional to the mean magnetic field B0, R=B0/(4πρV2/Rm)1/2, ρ is the fluid density, V is the characteristic turbulent velocity, and Rm is the magnetic Reynolds number. The derivation of this formula is illustrated using a simple model—the turbulent dynamo for an asymmetrical top.
Rapid current transition in a crossed-field diode
Authors: Verboncoeur J. P. , Birdsall C. K.
Published: July 2002

The transmitted current in a crossed-field gap has been characterized analytically by a number of authors. Using a one dimensional PIC simulation, we explore the behavior of the crossed-field diode at B=BHull. For mono-energetic (cold) emission, a rapid reduction of transmitted current is observed when the injected current exceeds the critical current by just 1%. The addition of a small electron temperature normal to the cathode eliminates the transition, even for kT/V∼10−5 (V=10 kV, gap = 1 cm, B=337 G, J=1.69 A/cm2), while an isotropic velocity distribution accelerates the transition.
Ionization-induced refraction in recombination x-ray lasers
Authors: Decker Chris D. , Eder David C. , London Richard A.
Published: July 2002

Ionization-induced refraction is examined in regimes relevant to recent and planned recombination x-ray lasing experiments [D. C. Eder et al., Phys. Plasmas 1, 1744 (1994)]. A model that self-consistently evolves the laser radiation as it ionizes a neutral gas is presented. The model allows for multiple ionization stages and for spatial variations in the neutral gas density appropriate for studying gas jets and laser evaporated gases. The length of highly stripped plasma is calculated in H-like Li and Li-like N systems, where the conditions are appropriate for recombination. It is found that for a given pump power there is always an optimal spot size and focal position with respect to the gas jet, which minimizes refraction and maximizes the length for which gain is possible.
Antimatter plasmas and antihydrogen
Authors: Greaves R. G. , Surko C. M.
Published: July 2002

Recent successes in confining antimatter in the form of positron and antiproton plasmas have created new scientific and technological opportunities. Plasma techniques have been the cornerstone of experimental work in this area, and this is likely to be true for the foreseeable future. Work by a number of groups on trapping antimatter plasmas is summarized, and an overview of the promises and challenges in this field is presented. Topics relating to positron plasmas include the use of positrons to study the unique properties of electron–positron plasmas, the interaction between positrons and ordinary matter, and the laboratory modeling of positron-annihilation processes in interstellar media. The availability of cold, trapped antiprotons and positrons makes possible the production of neutral antimatter in the form of antihydrogen. This is expected to enable precise comparisons of the properties of matter and antimatter, including tests of fundamental symmetries and the measurement of the interaction of antimatter with gravity.
Collisional drift waves in stellarator geometry
Authors: Lewandowski J. L. V.
Published: July 2002

Collisional drift waves, in the linear approximation and for low-β plasmas, are studied in stellarator geometry. A system of three coupled, partial differential equations, describing the perturbed density, electrostatic potential and electron temperature, is derived. This system of equations is solved as an initial-value problem along the magnetic field line. Numerical results for the toroidal heliac H1-National Facility [S. M. Hamberger et al., Fusion Technol. 17, 123 (1990)] are presented.
E×B
shearing rate in quasisymmetric plasmas
Authors: Hahm T. S.
Published: July 2002

The suppression of turbulence by the E×B shear is studied in systems with quasisymmetry using the nonlinear analysis of eddy decorrelation previously utilized in finite aspect ratio tokamak plasmas [Phys. Plasmas 2, 1648 (1995)]. The analytically derived E×B shearing rate which contains the relevant geometric dependence can be used for quantitative assessment of the fluctuation suppression in stellarators with quasisymmetry.
One-dimensional nonlinear wake fields excited in a cold plasma by charged bunches
Authors: Khachatryan A. G.
Published: July 2002

One-dimensional nonlinear plasma wake fields excited by a single bunch and by a series of bunches are considered. The essential differences between the negatively-charged bunch case and the positively-charged one are explained. The bunches with nonuniform distributions of density are investigated. The obtained results show the dependence of excited potential electric fields on bunch parameters and permit the selection of optimal parameters.
A free electron laser operated with a dual electron beam
Authors: Seo Y. , Park I. H.
Published: July 2002

The two-stream instability is introduced to a free electron laser by employing a dual beam whose two constituent beams have a slight energy difference. In the Raman regime, this instability sets up an independent beam mode, producing a radiation growth much faster than the Raman instability.
On the radiation guiding in a free-electron laser at arbitrary pump strengths
Authors: Seo Y. , Choi E. K. , Park I. H. , Namkung W.
Published: July 2002

The relative significance of the active optical guiding and the gain guiding is identified at arbitrary pump strengths. The optical guiding is observed to be turned on gradually as the pump strength increases. Power laws are found for the eigenmode radii.
Dynamics of core transport barrier formation and expansion in the DIII-D tokamak
Authors: Rettig C. L. , Burrell K. H. , Stallard B. W. , McKee G. R. , Staebler G. M. , Rhodes T. L. , Greenfield C. M. , Peebles W. A.
Published: July 2002

While core transport barriers have been created in most large tokamaks, including DIII-D [Plasma Physics and Controlled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159], the underlying physics that governs their creation, expansion, and limitations has not been fully elucidated. Although negative central magnetic shear during a discharge aids in the creation of a core transport barrier, the model that has evolved to explain these results includes synergistic effects of magnetic shear and E×B velocity shear as the central elements. In DIII-D, the core barrier initially forms over an interval of several hundred milliseconds during the current ramp, with very low power applied. The barrier subsequently expands outward if the injected power is raised above a threshold, between 2.5 and 5 MW in DIII-D. Electrostatic turbulence reduces as the shearing rate increases to exceed the local turbulence growth rate while the transport barrier expands. Both the existence of the threshold and the barrier expansion with additional power are consistent with the theory.