bluuebunny/crossref_metadata_2025 huggingface dataset:
Cyclic flow oscillations in a system of repeatedly branching channels
Authors: Brücker Ch. , Riethmuller M. L.
Published: July 2002

The unsteady nature of the flow within a cascade of bifurcating branches was investigated for Reynolds numbers of 900 and 2700 in the mother branch. The cascade consists of three generations of planar symmetric bifurcations with a fixed branching angle and ratio of branch length to width. Chronological flow field measurements were carried out using digital-particle-image-velocimetry (DPIV) to determine the time-dependent velocity distribution within the branches. Though the flow input was steady, low-frequency flow oscillations were found within the branches which were coupled with the dynamics of the separation regions and the cross-talk between the communicating daughter branches. The toggling flip-flop character of the flow was explained by means of a feedback loop in which the cyclic growth, wash-out and re-generation of the separation regions is assumed to be the inner motor for the periodic continuation. These local excitation mechanisms were suggested to be similar to those causing the generation of low-frequency oscillations in shear-layers impinging on edges like in a rectangular cavity. The results demonstrate that unsteady effects and excited oscillations play an additional role in uneven flow partitioning in flow bifurcation cascades, however, they do not favor any flow partitioning in a preferential pathway constant in time
Mixing enhancement in compressible shear layers via sub-boundary layer disturbances
Authors: Island T. C. , Urban W. D. , Mungal M. G.
Published: July 2002

Mixing enhancement results are presented for compressible (convective Mach number 0.63) planar shear layers perturbed by 2D and 3D disturbances located within the supersonic-side splitter tip boundary layer. The disturbances were parametrically varied in shape, spacing, and thickness, and for each geometry time-resolved end-, side-, and plan-view visualizations of mixed fluid were obtained. The mixing layer thickness and growth rate are measured directly from the averaged images. As an indicator of the pressure loss induced by each disturbance geometry, the streamwise static pressure distribution is also recorded. The visualizations reveal that discrete 3D disturbances induce appreciable spanwise convolution, streamwise structure, and thickening of the mixing layer with disturbances as thin as 5% of the boundary layer displacement thickness. The optimal disturbance appears to have an angle of 30° to the streamwise direction and be located at the splitter tip, rather than upstream. Panoramic side-views show that the far-field growth rate increases (45% in one case) for certain discrete 3D disturbances but not 2D disturbances, despite equivalent area blockage. For the most promising geometry, quantitative measurements of the mixing layer thickness, probability of mixed fluid, and mixing efficiency were made using cold chemistry planar laser-induced fluorescence. The perturbed layer shows a slight improvement (7%) in mixing efficiency and a large increase (48%) in layer thickness, indicating that gains in the total amount of mixed fluid occur primarily by layer thickening.
Linear instability of a particle-laden mixing layer with a dynamic dispersed phase
Authors: Dimas A. A. , Kiger K. T.
Published: July 2002

The linear, inviscid, spatial instability of a mixing layer uniformly laden with a dilute concentration of heavy particles is studied numerically. The effect of the particles is modeled using an ensemble averaged Eulerian description of the velocity field and Stokes’ drag formula to compute an averaged force, and the carrier fluid and the particle motions are assumed to be fully coupled. The behavior of the linear instability (for a given mean shear) depends on two dimensionless parameters: Cf, representing the product of the inverse Stokes number and mass loading, and Cp, representing the inverse Stokes number. For finite values of Cf and large values of Cp, the particles respond as fluid elements and the growth rate is equal to the one of the single-phase flow, while decreasing Cp results in a growth rate decrease. The growth rate also decreases with increasing Cf. Beyond certain critical values of increasing Cf and decreasing Cp, a second unstable low-frequency mode appears which is distinct from the fundamental mode. The fully coupled character of the instability reveals three important aspects of the particle effect on the flow structure: (1) the particle concentration field is organized into alternating bands of increased and decreased concentration corresponding to the braid and core regions of the vortices, respectively, with peak perturbations occurring at intermediate Cp values (0.01⩽Cp⩽0.1), (2) the streamwise particle velocity is higher than the streamwise fluid velocity for a substantial range of Cp values and every finite Cf, and (3) the modification of the fluid vorticity field structure with respect to the corresponding field in single-phase flow is driven by the divergence of the particle velocity field.
Statistical mechanical descriptions of turbulent coagulation
Authors: Wang Lian-Ping , Wexler Anthony S. , Zhou Yong
Published: July 2002

A fundamental tenet of statistical mechanics is that the rate of collision of two objects is related to the expectation value of their relative velocities. In pioneering work by Saffman and Turner [J. Fluid Mech. 1, 16 (1956)], two different formulations of this tenet are used to calculate the collision kernel Γ between two arbitrary particle size groups in a turbulent flow. The first or spherical formulation is based on the radial component wr of the relative velocity w between two particles: Γsph=2πR2〈|wr|〉, where wr=w⋅R/R, R is the separation vector, and R=|R|. The second or cylindrical formulation is based on the vector velocity itself: Γcyl=2πR2〈|w|〉, which is supported by molecular collision statistical mechanics. Saffman and Turner obtained different results from the two formulations and attributed the difference to the form of the probability function of w used in their work. A more careful examination reveals that there is a fundamental difference between the two formulations. An underlying assumption in the second formulation is that the relative velocity at any instant is locally uniform over a spatial scale on the order of the collision radius R, which is certainly not the case in turbulent flow. Therefore, the second formulation is not expected to be rigorously correct. In fact, both our analysis and numerical simulations show that the second formulation leads to a collision kernel about 25% larger than the first formulation in isotropic turbulence. For a simple uniform shear flow, the second formulation is about 20% too large. The two formulations, however, are equivalent for treating the collision rates among random molecules and the gravitational collision rates.
Monte Carlo direct simulation of rotational relaxation of nitrogen through high total temperature shock waves using classical trajectory calculations
Authors: Koura Katsuhisa
Published: July 2002

The rotational relaxation of nitrogen through high total temperature shock waves is studied using the direct simulation Monte Carlo method coupled with classical trajectory calculations. The results are obtained for the extended Morse potential employed previously for low total temperature shock waves and agree well with the experimental results of Smith and Alsmeyer except Smith’s data in the rearward portion of the shock wave.
On the relationship between the mean flow and subgrid stresses in large eddy simulation of turbulent shear flows
Authors: Shao L. , Sarkar S. , Pantano C.
Published: July 2002

The present study sheds light on the subgrid modeling problem encountered in the large eddy simulation (LES) of practical flows, where the turbulence is both inhomogeneous and anisotropic due to mean flow gradients. The subgrid scale stress (SGS) tensor, the quantity that is key to the success of LES, is studied here in such flows using both analysis and direct numerical simulation (DNS). It is shown that the SGS tensor, for the case of inhomogeneous flow, where the filtering operation is necessarily performed in physical space, contains two components: a rapid part that depends explicitly on the mean velocity gradient and a slow part that does not. The characterization, rapid and slow, is adopted by analogy to that used in the modeling of the pressure–strain in the Reynolds-averaged Navier–Stokes equations. In the absence of mean flow gradients, the slow part is the only nonzero component and has been the subject of much theoretical study. However, the rapid part can be important in the inhomogeneous flows that are often encountered in practice. An analytical estimate of the relative magnitude of the rapid and slow components is derived and the distinct role of each component in the energy transfer between the resolved grid scales and the unresolved subgrid scales is identified. Results that quantify this new decomposition are obtained from DNS data of a turbulent mixing layer. The rapid part is shown to play an important role when the turbulence is in a nonequilibrium state with turbulence production much larger than dissipation or when the filter size is not very small compared to the characteristic integral scale of the turbulence, as in the case of practical LES applications. More importantly, the SGS is observed to be highly anisotropic due to the close connection of the rapid part with the mean shear. The Smagorinsky eddy viscosity and the scale-similarity models are tested by performing a priori tests with data from DNS of the mixing layer. It is found that the scale-similarity model correctly represents the anisotropic energy transfer between grid and subgrid scales that is associated with the rapid part, while the eddy viscosity model captures the dissipation associated with the slow part. This may be a physical reason for the recent successes of the mixed model (Smagorinsky plus scale similarity) reported in the literature.
The spreading of a non-isothermal liquid droplet
Authors: Benintendi Steven W. , Smith Marc K.
Published: July 2002

The effect of the slip coefficient and the mobility capillary number on the spreading of a thin axisymmetric liquid droplet with uniform heating/cooling of the solid surface is examined. The results show that increasing the slip coefficient reduces the spreading/shrinking behavior of the droplet and that the final equilibrium states are slip dependent. These results are explained by the development of a return flow inside the droplet. We show how a speed-dependent slip coefficient can be used to remove the dependence of the final state on the slip coefficient. It is also shown that increasing the mobility capillary number decreases the spreading/shrinking rate of the droplet. For thermocapillary-driven droplets, there is a capillary-number-dependent time delay for the onset of motion. The entire effect of the mobility capillary number on the spreading process is explained in terms of the deformability of the free surface.
Time-dependent simulations of point explosions with heat conduction
Authors: Shestakov A. I.
Published: July 2002

A hydrodynamic-diffusion code is used to simulate a point explosion. The gas motion is governed by both hydrodynamics and nonlinear heat conduction and is a combination of the well-known, self-similar Taylor–Sedov spherically expanding shock wave and the spherically expanding thermal wave. Two problems are discussed. In the first problem, a similarity solution exists if the diffusion coefficient is given in terms of powers of density and temperature which also define the ambient spatial density profile. If the initial explosion energy is small, the diffusive effect is limited to a region behind the shock. However, if the explosion energy is large, the thermal front precedes the hydrodynamic front, which is then an isothermal shock. In the second problem, the initial density is constant and the diffusion coefficient depends on only a power of the temperature. In this case, the solution is not self-similar; in early times, heat conduction dominates; in late times—hydrodynamics. The problems were previously analyzed by Reinicke and Meyer-ter-Vehn in terms of similarity variables.
Experiments on the Richtmyer–Meshkov instability: Wall effects and wave phenomena
Authors: Brouillette M. , Bonazza R.
Published: July 2002

Experiments examining the interaction of shock waves with an interface separating two gases of different densities are reported. Flow visualization by the schlieren method and x-ray densitometry reveals that important secondary effects are introduced by the experimental apparatus, especially at the walls of the shock tube from shock wave/boundary layer interaction below, above, and at the interface itself. These effects can impair the observation of the primary phenomenon under study and can lead to the overall deformation of the interface. In particular, the thickness of the viscous boundary layer at the interface is computed using a familiar shock tube turbulent boundary layer model and the occurrence of bifurcation of reflected waves below and above the interface is successfully predicted based on classical bifurcation arguments. The formation of wall vortical structures at the interface is explained in terms of baroclinic vorticity deposition resulting from the interaction of reflected waves with the interface distorted by the boundary layer. This mechanism of wall vortex formation can also explain observed test gas contamination in reflected shock tunnels when shock wave bifurcation is absent. In general, it is found that most of the side effects of the experimental investigation of the Richtmyer–Meshkov instability can be alleviated by performing experiments in large test sections near atmospheric initial pressure.
An analytical study of Mach reflection in nonequilibrium steady flows
Authors: Grasso F. , Paoli R.
Published: July 2002

In the present work, we study the influence of both thermal and chemical relaxation phenomena on steady Mach reflections. To characterize the dependency of the scales of the phenomenon upon the relaxation we develop a quasi-one-dimensional model for a nonequilibrium diatomic gas coupled with the three shock theory. The results show that Mach reflections are strongly affected by vibrational relaxation and to a lesser extent by chemical excitation. The Mach stem height and its location, and the extent of the subsonic pocket are found to depend both on the geometrical setup and on the relaxation length, and do not scale linearly with the geometrical size as found in the ideal case. In addition, the relaxation is found to cause a forward displacement of the Mach stem thus counteracting Mach reflection instabilities. The nonlinearity has also been confirmed by a perturbation analysis of the equations for small Damköhler numbers.
Lee-wave breaking over obstacles in stratified flow
Authors: Eiff Olivier S. , Bonneton Philippe
Published: July 2002

Experimental results are presented on the lee-wave breaking process which occurs at low Froude numbers when uniform and strongly stratified flow approaches two-dimensional and quasi two-dimensional Gaussian-shaped obstacles. It was found that the lee-wave breaking process is essentially independent of the two-dimensional and the quasi two-dimensional shape of the obstacles. The attainment of the critical condition where the steepening wave becomes statically unstable does not mark a threshold to breakdown. Instead, the wave remains dynamically stable for several buoyancy periods, overturning into an “S”-shape with maximum overturning reaching about 55° past the vertical. It is observed that the primary instability forms a quasi two-dimensional spanwise vortex over the central portion of the obstacles and is mainly shear driven. The quasi two-dimensional spanwise vortex persists for a few buoyancy periods before undergoing a three-dimensional convective instability, similar to a Rayleigh–Taylor instability. As a result, an array of toroidal vortex structures aligned parallel to the obstacle crest forms. These vortex structures of size ∼3H are inclined into the flow yielding three strong components of vorticity.
Production and characterization of large plasmas from gas bag targets on Nova
Authors: Kalantar D. H. , Klem D. E. , MacGowan B. J. , Moody J. D. , Montgomery D. S. , Munro D. H. , Shepard T. D. , Stone G. F. , Failor B. H. , Hsing W. W.
Published: July 2002

Large plasmas are created by illuminating gas-filled thin-walled balloon-like targets using the Nova laser [E. Campbell et al., Rev. Sci. Instrum. 57, 2101 (1986)]. The targets consist of a 5000–6000 Å skin surrounding 1 atm of neopentane, which, when ionized, becomes a plasma with an electron density of 1021 electrons/cm3. X-ray images of the gas bag target are used to evaluate the size and uniformity of the plasma by comparison with LASNEX [R. M. More, J. Quant. Spectrosc. Radiat. Transfer 27, 345 (1982)] simulations. The gas bags are heated with converging and diverging beam spots. The most uniform plasmas are created by illuminating the target with large converging beam spots that overlap to cover most of the surface of the gas bag. The gas bag plasma is heated to a peak temperature of approximately 3.5 keV, with 25 kJ of 3ω laser light in a 1 ns square pulse.
Magnetodynamics of multicomponent plasma
Authors: Rudakov L.
Published: July 2002

Magnetic field penetration and dynamics of plasma components under magnetic pressure are studied. Plasma ions with the highest Z/M turn out to move ahead of the centrum of mass and transfer the frozen-in magnetic field together with accompanying electrons. Problems of both nonlinear magnetic field diffusion in plasmas and of magnetic field redistribution in a thin shell accelerated by magnetic pressure are considered.
Thermal bifurcation of scrape-off layer plasma and divertor detachment
Authors: Krasheninnikov S. I. , Catto P. J. , Helander P. , Sigmar D. J. , Soboleva T. K.
Published: July 2002

Models to investigate the main features of plasma–neutral interactions in the recycling region of a tokamak divertor are developed for the two opposite extremes of fluid and Knudsen neutrals. Both neutral models show that a reduction of the heat flux into the hydrogen recycling region below a critical value leads to bifurcation (or rapid change) of the plasma parameters near the target. This bifurcation causes behavior in the scrape-off layer, which is in agreement with the following main features of detached divertor regimes in current tokamak experiments: (i) strong decrease of the plasma temperature near the target, (ii) plasma pressure drop in the recycling region, and (iii) strong decrease of the target heat load and plasma flux onto the target. It is also shown that in the Knudsen limit, the neutral density in the divertor region cannot exceed a maximum density, which is of the order of 1–2×1013 cm−3 for current experiments.
Finite-amplitude Alfvén waves in a dissipative inhomogeneous plasma
Authors: Rauf S. , Tataronis J. A.
Published: July 2002

The effects of an inhomogeneous background magnetic field and resistivity on finite-amplitude Alfvén waves are studied in terms of the derivative nonlinear Schrödinger equation. It is shown that a weak inhomogeneity in the background magnetic field introduces a perpendicular shear in the resulting waves. This causes the waves to split apart, and a longitudinal current, which increases linearly in time, is produced as a consequence. Resistivity, however, restricts this unbounded growth of current and causes the waves and the current to decay exponentially at later times. The connection of these results to previous results on the Alfvén continuum and low-frequency current drive are discussed in the paper. In the paper the effects of resistivity on the modulational instability of time harmonic solutions of the derivative nonlinear Schrödinger equation are also explored. Both numerical and analytical results indicate that resistivity will decrease the growth rate of envelope perturbations. At large enough values, it can even suppress the perturbations.
Anomalous absorption due to ion-acoustic decay instability in a large-scale plasma relevant to laser fusion
Authors: Mizuno K. , Young P. E. , Estabrook K.
Published: July 2002

The anomalous attenuation length of the laser light due to the ion-acoustic decay instability (IADI) can be much smaller than the instability width with a relatively weak laser intensity (Iλ2∼1014 W μm2/cm2) in a large-scale plasma. An instability width of only a few laser wavelengths is sufficient to absorb most of the laser energy. Two-dimensional electromagnetic computer simulation results and experimental results are consistent with strong absorption by the IADI.
Existence of core localized toroidicity-induced Alfvén eigenmode
Authors: Fu G. Y.
Published: July 2002

The core-localized toroidicity-induced Alfvén eigenmode (TAE) is shown to exist at finite plasma pressure due to finite aspect ratio effects in tokamak plasma. The new critical beta for the existence of the TAE mode is given by α≊3ε+2s2, where ε=r/R is the inverse aspect ratio, s is the magnetic shear and α=−Rq2dβ/dr is the normalized pressure gradient. In contrast, previous critical α is given by α≊s2. In the limit of s≪√r/R, the new critical α is greatly enhanced by the finite aspect ratio effects.
Expansion of a negative ion plasma into a vacuum
Authors: El-Zein Yasser , Amin Ali , Kim Hyun-Soo , Yi Seungjun , Lonngren Karl E.
Published: July 2002

The expansion of a plasma consisting of positive ions, negative ions, and electrons into a vacuum is solved numerically. In addition to the expected self-similar expansion found in a quasineutral plasma expansion model, a burst of positive ions is accelerated into the vacuum and a burst of negative ions is accelerated into the plasma. These accelerated ions arise since quasineutrality is not imposed in the simulation.
Time-dependent scenario modeling of the Tokamak Physics Experiment using a theory-based transport model
Authors: Kinsey Jon , Kritz Arnold , Bonoli Paul , Porkolab Miklos
Published: July 2002

Predictive transport and current drive simulations are presented for the time evolution of the temperature and density profiles in the Tokamak Physics Experiment (TPX). [W. M. Nevins et al., Proceedings of the International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Würzburg, 1992 (International Atomic Energy Agency, Vienna, 1992), Vol. 3, p. 279]. A distinguishing feature of this study is that we use a theoretically derived transport model that has been empirically calibrated against Ohmic, low-confinement and high-confinement mode discharges from seven different tokamaks. Heating and deposition profiles predicted by the ACCOME current drive and magnetohydrodynamic equilibrium code [R. S. Devoto et al., Nucl. Fusion 32, 773 (1992)] are incorporated into the time-dependent BALDUR one-and-one-half dimensional transport code [C. E. Singer et al., Comput. Phys. Commun. 49, 275 (1988)]. We consider various scenarios of fast wave, lower hybrid, and neutral beam heating and current drive and evaluate the effects on the evolution of density and temperature profiles.
Mass loading in velocity shears
Authors: Perez-de-Tejada H. , Durand-Manterola H.
Published: July 2002

An analysis is presented on the motion of contaminant ions picked up by the solar wind within velocity shears. An expression is obtained for the ion density in terms of the flow speed across the velocity shear. It is argued that enhanced densities occur in the region where the flow speed is small and that local values may become significantly larger than the ion density outside the velocity shear. A comparison is made with measurements obtained across the plasma wake of comet Giacobini–Zinner with the International Cometary Explorer (ICE) spacecraft. Within the velocity shear of that comet’s ionosheath the cometary ion density becomes large and reaches values that are different from those expected when the ion population depends only on the ionization of a uniform outflow of neutral particles from the cometary nucleus. The enhanced plasma density expected within the velocity shear is compatible with the shape of the density profile of contaminant ions detected in that region.
Scrape-off layer profile modifications by convective cells
Authors: Myra J. R. , D’Ippolito D. A.
Published: July 2002

Convective cells (CC’s) are important in understanding density profile modifications induced by ion cyclotron range of frequencies (ICRF) antennas. This has motivated the present work in which the effect of CC’s on transport in the scrape-off layer is studied, in the regime where the density gradient scale length Ln and the cell size L are comparable. Monte Carlo simulations show that closed cell convection acts to flatten the density profile, and that open cells enhance the particle flow to the wall, depleting the density and yielding profiles similar to those measured near ICRF antennas. A new one-dimensional, two-branch model of CC transport is shown to agree well with the simulations. The model gives rise to two characteristic scale lengths, only one of which is retained in the enhanced diffusion models that are applicable for Ln≫L. The two-branch model is expected to be useful in analyzing ICRF experiments.
Collisionless damping of a quasilinear Langmuir wave packet
Authors: Khudik V. N. , Shevchenko O. A.
Published: July 2002

Collisionless damping of a one-dimensional packet of Langmuir waves with random phases localized in the direction of its propagation is described by using quasilinear equations. A peculiar minimum principle is proposed for reducing this quasilinear problem to a simple graphic analysis of the packet initial spectrum.
Low-beta equilibrium and stability for anisotropic pressure closed field line plasma confinement systems
Authors: Pastukhov V. P. , Ilgisonis V. I. , Subbotin A. A.
Published: July 2002

A formalism has been developed to analyze the equilibrium and stability of low-beta anisotropic pressure plasmas confined in closed field line magnetic systems. This formalism, based on a joint use of paraxial (long-thin) and low-beta approximations, allows self-consistent equilibrium and stability consideration of rather general magnetic systems with nonuniform axis curvature and longitudinal profiles of toroidal and multipole poloidal field. Strong pressure anisotropy, corresponding to enhanced plasma pressure in mirror cells of the system, may also be considered. Nonconventional features of anisotropic pressure equilibria have been revealed. Application of the above formalism to the recently proposed linked mirror neutron source (LMNS) confirms the basic principles of the LMNS concept, but calculations based on this formalism have appreciably corrected some LMNS parameters. The LMNS longitudinal pressure profile and magnetic field distribution are optimized.
Dynamics of a multiple-pulse-driven x-ray laser plasma
Authors: Wan A. S. , Da Silva L. B. , Moreno J. C. , Cauble R. , Celliers P. , Dalhed H. E. , Koch J. A. , Nilsen J.
Published: July 2002

In this paper we describe experimental and computational studies of multiple-pulse-driven laser plasma, which is the gain medium for a neon-like yttrium x-ray laser. Near-field emission profiles have been measured both with and without reinjection of the x-ray laser photons to couple with the amplifying medium created by later pulses using an external multilayer mirror. From the temporal and spatial evolution of the near-field emission profiles we can examine the pulse-to-pulse variation of the x-ray laser plasma due to changes in the hydrodynamics, laser deposition, and the injecting of x-ray laser photons back into an amplifying x-ray laser plasma. Using a combination of radiation hydrodynamics, atomic kinetics, and ray propagation codes, reasonable agreement has been obtained between simulations and the experimental results.
Tokamak configurations with reversed magnetic shear and a peaked current density profile
Authors: Mazzucato E.
Published: July 2002

Recent experiments suggest that the reversal of magnetic shear in the central region of a tokamak plasma has beneficial effects on plasma transport. In this Letter, it is shown that a hollow toroidal current density distribution is not a necessary condition for the existence of tokamak equilibria with reversed magnetic shear.
The blow-up rate of the envelope instability of a cold beam in a storage ring
Authors: Yang B. , Bisoffi G. , Lamanna G. , Lenisa P. , Tecchio L. , Variale V.
Published: July 2002

Envelope instability is investigated in a storage ring using particle simulation. A typical distortion pattern of the instability is observed in transverse phase planes of cold ion beams considering the storage ring both with and without ring periodicity perturbations. The dependence of the blow-up rate of the instability on the single particle phase advance of the storage ring is calculated. Typical e-folding blow-up times of the envelope instability increase from 10−5 s to 10−2 s when the single particle phase advance is smaller than 90°.
Non-uniform rotation and the resistive wall mode
Authors: Gimblett C. G. , Hastie R. J. , Van der Linden R. A. M. , Wesson J. A.
Published: July 2002

Advanced Tokamak Concepts [C. Kessel, J. Manickam, G. Rewoldt, and W. M. Tang, Phys. Rev. Lett. 72, 1212 (1994)] have been designed assuming that the ‘‘Resistive Wall Mode’’ (RWM) is stable. It has recently been shown that the RWM can be stabilized by a combination of strong uniform plasma rotation and visco-resistive dissipation. In this paper we examine the consequences of a sheared flow on the RWM, and contrast the results to the case of uniform flow. It is shown that, as for uniform flow, the rotation initially further destabilizes the resistive wall mode, but for higher rotation velocities the growth rate is reduced, and the presence of plasma dissipation may completely stabilize the mode. However, sheared rotation allows the possibility of the RWM coupling to and converting into a Kelvin-Helmholtz mode. It is shown that the position of the wall with respect to the critical position for stabilization of the external kink mode is of crucial importance.
Plasma density buildup after pellet injection
Authors: Pégourié B. , Picchiottino J.-M.
Published: July 2002

Details of the increase of the plasma density following a pellet injection have been measured with high temporal resolution (16 μs) and the parallel expansion of ablated matter modeled with a four-fluid hydrodynamic code. The driving force of the expansion is the parallel pressure gradient, progressively balanced by the compression of the background plasma. It is shown that the ablated material experiences a strong transient poloidal motion (≊5×103 ms−1, ≊100 μs) as it expands along the field lines. This motion, which is induced by the pellet itself, results from the conservation of kinetic momentum: the convective motion in the sheet of ablated material (due to its positive potential with respect to the plasma) is compensated by a global drift of the whole magnetic surface. This model reproduces the main observations concerning the parallel propagation of the ablatant in the discharge. In particular, it shows that the stretching of the sheet of ablated material by the magnetic shear and the poloidal rotation is responsible for the homogenization of the density in a characteristic time of ≊1 ms. The plasma rotation measured immediately after a pellet injection is therefore not, in general, simply linked to the background radial electric field.
Analytic solutions of the magnetic annihilation and reconnection problems. II. Three-dimensional flows
Authors: Watson P. G. , Craig I. J. D.
Published: July 2002

Analytic solutions for magnetic annihilation and reconnection in three-dimensional velocity fields are constructed. Exact and approximate solutions for three-dimensional fan current reconnection can be derived using prototype fields based on simple, one-component magnetic annihilation models. Certain reconnection solutions are found to be “fast,” in the sense that the Ohmic dissipation rate scales independently of any positive power of the plasma resistivity η. Three-dimensional models can also possess novel features not contained in two-dimensional planar reconnection solutions. In particular, classes of solutions involving separated field nulls are found to possess multiple scalings for the Ohmic dissipation rate—a feature that has not been observed before. Several solutions are shown to have the potential to meet the extreme energy release characteristics of the solar flare.
Numerical observation of turbulence enhanced growth rates
Authors: Doxas Isidoros , Cary John R.
Published: July 2002

An enhancement of the velocity diffusion over the quasilinear value is observed in the regime where the autocorrelation time is much smaller than the linear growth time or resonance broadening time. The diffusion enhancement occurs when the resonance broadening time is small compared with the linear growth time. These simulations are self consistent and have enough modes to be in the continuous spectrum limit. That is, even at the initial amplitudes the intermode spacing is sufficiently small that the resonance overlap parameter is large. A possible mechanism for the enhanced diffusion (spontaneous spectrum discretization) is discussed.
Orbital resonances and chaos in a combined rf trap
Authors: Finn J. M. , Nebel R. , Glasser A. , Lewis H. R.
Published: July 2002

The rf combined trap for confining a non-neutral plasma by a combination of rf fields and a uniform magnetic field is introduced. The particle motion is studied in such a trap in which the rf fields are given by the TM011 mode. The aim is to determine limits on a focus at the center for low angular momentum particles. Such a focus allows densities in excess of the local Brillouin limit. The motion is described in terms of an effective potential consisting of the ponderomotive potential Vpon(r,z) plus a radial potential mΩc2r2/8 due to the magnetic field Bz plus a centrifugal potential pθ2/2mr2. Near the origin r=0, z=0, the equations reduce to a pair of Mathieu equations for the r and z motions. For certain parameters the system is quasispherically symmetric, i.e. the effective ponderomotive frequencies Ωr, Ωz in r and z are equal. In this case the Brillouin limit for a uniform density plasma is shown to equal the usual cylindrical limit. Numerical integration of the ponderomotive equations is shown for parameters near those giving quasispherical symmetry. There is resonance for Ωr/Ωz=1, giving m=2, n=2 islands. The islands limit the size of the focus at the origin, even for pθ=0. For particles with large effective energy Eeff, there can also be chaos, depending on the elongation L/a of the rf cavity. It is found that the deleterious effect of islands and chaos on the focus is minimized by having the elongation L/a close to unity and having particles trapped deeply in the ponderomotive well.
High-power operation of a 170 GHz megawatt gyrotron
Authors: Kreischer K. E. , Kimura T. , Danly B. G. , Temkin R. J.
Published: July 2002

Recent gyrotron oscillator experiments have achieved record powers at 170 GHz. Single mode emission with a peak output power of 1.5 MW and an efficiency of 35% has been measured. The experiment is based on a resonant TE28,8,1 cylindrical cavity situated in a 6.7 T magnetic field. Microwaves are generated in the cavity by an 83 kV annular electron beam produced by a triode-type magnetron injection gun that is capable of currents up to 50 A. Megawatt power levels with efficiencies between 30%–36% have been measured over a wide range of operating parameters for the TE28,8,1 mode. Similar results were also achieved in the neighboring TE27,8,1 mode at 166.6 GHz, and the TE29,8,1 mode at 173.5 GHz. The high output power is the result of a carefully designed electron gun with low perpendicular velocity spread (6%–10%) and a novel cavity with an output iris that is less prone to mode competition. These results are in good agreement with nonlinear multimode simulations.
On the relaxation of cold electrons and hot ions
Authors: Potapenko I. F. , Bobylev A. V. , de Azevedo C. A. , Sakanaka P. H. , de Assis A. S.
Published: July 2002

The relaxation process of a space uniform plasma composed of cold electrons and one species of hot ions is studied numerically using one- and two-dimensional Landau–Fokker–Planck codes. Relaxation of a monoenergetic ion beam is considered in possibly extreme temperature regimes. Special attention is paid to the deviation of the relaxation process from the classical picture, which is characterized by the close initial temperatures Te≫(me/mi)1/3Ti. The present results give quite a clear idea of the relaxation picture for any initial temperatures also in extreme temperature regimes. A difference scheme, preserving the number of particles and the energy, gives the possibility of solving the problem numerically without error accumulation, except for machine errors.
The influence of finite temperature on strong relativistic plasma waves
Authors: Khachatryan A. G.
Published: July 2002

One-dimensional nonlinear relativistic plasma waves in warm plasma are investigated both analytically and by numerical simulation on the basis of relativistic Vlasov-Poisson equations. For the case of low temperatures the problem is considered by the Laplace method. The plasma temperature effect on the character and maximum value of the permissible amplitude of the waves is found. It is shown that besides periodic waves both solitary waves and nonperiodic fields can exist in a warm plasma. The problem of strong relativistic plasma wave excitation by charged bunches is considered.
Whistler wave leakage from plasma ducts and solitons
Authors: Karpman V. I.
Published: July 2002

An analytical full wave theory of whistler wave leakage from the density ducts and solitons, aligned parallel to the ambient magnetic field, is developed. Radiation from plasma ducts and solitons are considered from a unified standpoint. Explicit asymptotic expressions for the detrapped radiation field and the amplitude attenuation, caused by the leakage, are obtained. The results are extended to more general gyrotropic media.
Relaxed state for the coaxial helicity injection current drive in toroidal plasmas
Authors: Zhang Cheng , Zhu Sizheng , Shen Linfang
Published: July 2002

The plasma relaxed state for the coaxial helicity injection current drive in toroidal configuration is investigated by applying the principle of minimum rate of energy dissipation. The comparison between theory and helicity injection current drive experiment on HIT (the Helicity Injected Tokamak) [T. R. Jarboe, Fusion Technol. 15, 7 (1989)] is presented. The calculation results indicate that the key features such as total toroidal driven current, current in closed field, current density profile, magnetic configuration, and jt/Bt profile agree quite well with experiment. Analysis shows that plasma relaxes to a noncomplete relaxation state, and the toroidal current is effectively driven in this state.
Fast reconnection of weak magnetic fields
Authors: Zweibel Ellen G.
Published: July 2002

Fast magnetic reconnection refers to annihilation or topological rearrangement of magnetic fields on a timescale that is independent (or nearly independent) of the plasma resistivity. The resistivity of astrophysical plasmas is so low that reconnection is of little practical interest unless it is fast. Yet, the theory of fast magnetic reconnection is on uncertain ground, as models must avoid the tendency of magnetic fields to pile up at the reconnection layer, slowing down the flow. In this paper it is shown that these problems can be avoided to some extent if the flow is three dimensional. On the other hand, it is shown that in the limited but important case of incompressible stagnation point flows, every flow will amplify most magnetic fields. Although examples of fast magnetic reconnection abound, a weak, disordered magnetic field embedded in stagnation point flow will in general be amplified, and should eventually modify the flow. These results support recent arguments against the operation of turbulent resistivity in highly conducting fluids.
Scaling properties of magnetic dynamo wavenumber power spectra generated by Lagrangian chaotic flows
Authors: Reyl Charles , Antonsen Thomas M. , Ott Edward
Published: July 2002

Wavenumber power spectra of the magnetic field in kinematic dynamos of Lagrangian chaotic flows are investigated. Numerical integration of the kinematic dynamo equations with magnetic Reynolds number Rm up to 105 shows that the wavenumber power spectrum of the magnetic field generated by smooth, Lagrangian chaotic flows obeys a power-law. This property is theoretically predicted by a wavepacket model, whereby magnetic field wavepackets are evolved according to ordinary differential equations.
Instability of electromagnetic R-mode waves in a relativistic plasma
Authors: Xiao Fuliang , Thorne Richard M. , Summers Danny
Published: July 2002

An explicit mathematical formalism is developed to evaluate the growth rate of field-aligned electromagnetic R-mode waves in a relativistic plasma. The methodology is valid for weak wave growth or damping when the resonant relativistic electrons comprise a small portion of the total plasma population. Numerical results are obtained for realistic plasma parameters using three distinct distribution functions for the relativistic electron population. Wave growth rates obtained by numerical integration along the resonant relativistic ellipse are shown to be substantially smaller than calculations performed under the nonrelativistic approximation. The relativistic corrections are primarily due to a reduction in the resonant electron anisotropy. Changes from the standard nonrelativistic treatment are noticeable at relatively small electron thermal energies (a few keV), and they become very significant for thermal energies above 100 keV, especially in low density regions where the plasma frequency is comparable to or lower than the electron gyrofrequency. The results have applications to wave instability in the outer radiation belts of the Earth, the inner Jovian magnetosphere, and other space plasmas where relativistic electrons are present.
Systematic trends in x-ray emission characteristics of variable-wire-number, fixed-mass, aluminum-array, Z-pinch implosions
Authors: Sanford T. W. L. , Mock R. C. , Nash T. J. , Whitney K. G. , Pulsifer P. E. , Apruzese J. P. , Mosher D. , Peterson D. L. , Haines M. G.
Published: July 2002

Increasing the number of wires an order of magnitude from 10 to almost 200 while simultaneously fixing the total wire mass in annular aluminum-wire-array Z-pinch implosions on the 20 TW Saturn generator [Proceedings of the 6th International IEEE Pulsed Power Conference (Institute of Electrical and Electronics Engineers, Piscataway, NJ, 1987), p. 310] demonstrates two separate power-law trends in the measured x-ray characteristics as a function of the initial interwire gap (g). These trends are approximately independent of the array radius. When g decreases from ∼6 to 0.4 mm, the peak total radiated power increases by a factor of 20 and the total energy by a factor of 2. There is a more rapid increase in peak power and energy radiated as g decreases for gaps greater than ∼2 mm. This increase is related to a measured decrease in precursor plasma and to a calculated decreased sensitivity of the implosion to azimuthal asymmetries that occurs when individual wire plasmas begin to merge following their vaporization. The substantial increase in power arises from an inferred increase in plasma compression and can be correlated with an almost linear reduction in the calculated effective thickness of the plasma annulus near stagnation as g decreases.
Thomson scattering from laser plasmas
Authors: Glenzer S. H. , Alley W. E. , Estabrook K. G. , De Groot J. S. , Haines M. G. , Hammer J. H. , Jadaud J.-P. , MacGowan B. J. , Moody J. D. , Rozmus W. , Suter L. J. , Weiland T. L. , Williams E. A.
Published: July 2002

Thomson scattering has recently been introduced as a fundamental diagnostic of plasma conditions and basic physical processes in dense, inertial confinement fusion plasmas. Experiments at the Nova laser facility [E. M. Campbell et al., Laser Part. Beams 9, 209 (1991)] have demonstrated accurate temporally and spatially resolved characterization of densities, electron temperatures, and average ionization levels by simultaneously observing Thomson scattered light from ion acoustic and electron plasma (Langmuir) fluctuations. In addition, observations of fast and slow ion acoustic waves in two-ion species plasmas have also allowed an independent measurement of the ion temperature. These results have motivated the application of Thomson scattering in closed-geometry inertial confinement fusion hohlraums to benchmark integrated radiation-hydrodynamic modeling of fusion plasmas. For this purpose a high energy 4ω probe laser was implemented recently allowing ultraviolet Thomson scattering at various locations in high-density gas-filled hohlraum plasmas. In particular, the observation of steep electron temperature gradients indicates that electron thermal transport is inhibited in these gas-filled hohlraums. Hydrodynamic calculations which include an exact treatment of large-scale magnetic fields are in agreement with these findings. Moreover, the Thomson scattering data clearly indicate axial stagnation in these hohlraums by showing a fast rise of the ion temperature. Its timing is in good agreement with calculations indicating that the stagnating plasma will not deteriorate the implosion of the fusion capsules in ignition experiments.
Steady state off-axis sawtoothing in the Rijnhuizen Tokamak project
Authors: Meulenbroeks R. F. G. , de Baar M. R. , Beurskens M. N. A. , de Blank H. J. , Deng B. H. , Donné A. J. H. , Hogeweij G. M. D. , Lopes Cardozo N. J. , Montvai A. , Oyevaar Th.
Published: July 2002

A family of off-axis, or annular, instabilities has been studied using Thomson scattering, soft X-ray emission, and two electron cyclotron emission diagnostic systems. In the Rijnhuizen tokamak (RTP) [N. J. Lopes Cardozo et al., Plasma Physics and Controlled Nuclear Fusion Research 1992 (International Atomic Energy Agency, Vienna, 1993), Vol. 1, p. 271] these phenomena are invoked in a controlled way in discharges with specific (off-axis) deposition of electron cyclotron heating (ECH) and persist during most of the heating period, or during many current diffusion times. Based on coherent mode analysis at the crash time, the instabilities are associated with resonant surfaces near simple rational values of q (32, 2, and 3). A parameter study shows an increase of reheat rate and a decrease of sawtooth period with increasing ECH power and — in contrast to observations in other experiments — with increASING density as well.
Demonstration of a 10 kW average power 94 GHz gyroklystron amplifier
Authors: Blank M. , Danly B. G. , Levush B. , Calame J. P. , Nguyen K. , Pershing D. , Petillo J. , Hargreaves T. A. , True R. B. , Theiss A. J. , Good G. R. , Felch K. , James B. G. , Borchard P. , Cahalan P. , Chu T. S. , Jory H. , Lawson W. G. , Antonsen T. M.
Published: July 2002

The experimental demonstration of a high average power W-band (75–110 GHz) gyroklystron amplifier is reported. The gyroklystron has produced 118 AW peak output power and 29.5% electronic efficiency in the TE011 mode using a 66.7 kV, 6 A electron beam at 0.2% rf duty factor. At this operating point, the instantaneous full width at half-maximum (FWHM) bandwidth is 600 MHz. At 11% rf duty factor, the gyroklystron has produced up to 10.1 kW average power at 33% electronic efficiency with a 66 kV, 4.15 A electron beam. This represents world record performance for an amplifier at this frequency. At the 10.1 kW average power operating point, the FWHM bandwidth is 420 MHz. At higher magnetic fields and lower beam voltages, larger bandwidths can be achieved at the expense of peak and average output power.