bluuebunny/crossref_metadata_2025 huggingface dataset:
A priori tests of a new dynamic subgrid-scale model for finite-difference large-eddy simulations
Authors: Salvetti M. V. , Banerjee S.
Published: July 2002

This work focuses on subgrid-scale (SGS) modeling for finite-difference large-eddy simulations, employing filters in physical space. When a filter in physical space is used, an overlap is allowed between the unresolved and the resolved scales. For such a filter, all the three terms in the classical decomposition of the SGS stress tensor are present: the Leonard and cross-terms, due to the overlap between scales, and the true SGS Reynolds tensor, expressing the pure effect of the small scales. A dynamic subgrid-scale stress model is proposed, for finite-difference large-eddy simulation of incompressible and compressible flows in which the Leonard and cross-parts of the SGS stress tensor are assumed to be proportional to the resolved part (the ‘‘modified Leonard term’’), which is computed explicity. The SGS Reynolds stress is modeled by the eddy-viscosity Smagorinsky model. The two unknown parameters in this model are computed dynamically, as in Germano et al. [Phys. Fluids A 3, 1790 (1991)], but using a least squares technique. The model is tested using direct numerical simulation data for fully developed turbulent incompressible flows in presence of solid boundaries and free surfaces, and for compressible homogeneous turbulence. A ‘‘box filter’’ in physical space is used. Other SGS models are also tested, viz. the dynamic model of Germano et al. (DSM), and its compressible extension by Moin et al. [Phys. Fluids A 3, 2746 (1991)], and the dynamic mixed model in Zang et al. [Phys. Fluids A 5, 3186 (1993)] (DMM) and its compressible version developed here. Results on the behavior of the different models with regard to energy exchanges and correlation with the exact SGS stresses are presented for different filter widths. In particular high correlation is found between the modified Leonard and cross-terms thus justifying the basic assumption made in the model.
Thermocapillary migration of bubbles and drops at moderate values of the Marangoni number in reduced gravity
Authors: Balasubramaniam R. , Lacy Claud E. , Woniak Günter , Subramanian R. Shankar
Published: July 2002

Experiments were performed on the motion of isolated drops and bubbles in a Dow-Corning silicone oil under the action of an applied temperature gradient in a reduced gravity environment aboard the NASA Space Shuttle in orbit. Images of the interior of the test cell during these experiments were recorded on cine film and later analyzed to obtain data on the migration velocity as a function of size and the applied temperature gradient. The data are presented in scaled form. Predictions are available in the case of gas bubbles, and it is found that the scaled velocity decreases with increasing Marangoni number qualitatively as expected even though there are quantitative discrepancies. The scaled velocity also appears to approach a theoretical asymptote predicted in the limit of large values of the Marangoni number for Stokes motion. Finally, sample results from a preliminary experiment on a pair of drops are presented. They display the remarkable feature that a small drop which leads a large drop in a temperature gradient can significantly retard the motion of the large trailing drop while itself moving as though it is virtually unaffected by the presence of the large drop.
Simulation of shock-generated instabilities
Authors: Baltrusaitis R. M. , Gittings M. L. , Weaver R. P. , Benjamin R. F. , Budzinski J. M.
Published: July 2002

Direct 2-D numerical simulation of the fluid instability of a shock-accelerated thin gas layer shows flow patterns in agreement with experimental images. The Eulerian-based hydrodynamics code features Adaptive Mesh Refinement that allows the code to follow the vorticity generation and the complex flow resulting from the measured initial perturbations. These experiments and simulations are the first to address in quantitative detail the evolution of the Richtmyer–Meshkov instability in a thin fluid layer, and to show how interfluid mixing and vorticity production depend sensitively on initial perturbations in the layer.
The effect of the numerical scheme on the subgrid scale term in large-eddy simulation
Authors: Salvetti M. V. , Beux F.
Published: July 2002

A priori tests are performed to study the effect of the filter on the subgrid scale term in large-eddy simulation. Several filters, corresponding to finite-difference schemes of different order of accuracy, are applied to direct numerical simulation data. In particular, the effect of the filter on the importance of the Leonard term and on its correlation with the subgrid scale stress is investigated to evaluate, for different numerical schemes, the capabilities of subgrid scale models accounting for this term, such as scale-similarity or mixed models.
Streamline topologies near simple degenerate critical points in two-dimensional flow away from boundaries
Authors: Bro/ns Morten , Hartnack Johan Nicolai
Published: July 2002

Streamline patterns and their bifurcations in two-dimensional incompressible flow are investigated from a topological point of view. The velocity field is expanded at a point in the fluid, and the expansion coefficients are considered as bifurcation parameters. A series of nonlinear coordinate changes results in a much simplified system of differential equations for the streamlines (a normal form) encapsulating all the features of the original system. From this, we obtain a complete description of bifurcations up to codimension three close to a simple linear degeneracy. As a special case we develop the theory for flows with reflectional symmetry. We show that all the patterns obtained can be realized in steady Navier–Stokes or Stokes flow, but an unresolved difficulty arises in the symmetric case for Navier–Stokes flow. The theory is applied to the patterns and bifurcations found numerically in two recent studies of Stokes flow in confined domains.
Shock formation within sonoluminescence bubbles
Authors: Vuong Vi Q. , Szeri Andrew J. , Young David A.
Published: July 2002

A strong case has been made by several authors that sharp, spherically symmetric shocks converging on the center of a spherical bubble driven by a strong acoustic field give rise to rapid compression and heating that produces the brief flash of light known as sonoluminescence. The formation of such shocks is considered. It is found that, although at the main collapse the bubble wall does indeed launch an inwardly-traveling compression wave, and although the subsequent reflection of the wave at the bubble center produces a very rapid temperature peak, the wave is prevented from steepening into a sharp shock by an adverse gradient in the sound speed caused by heat transfer. It is shown that the mathematical characteristics of the flow can be prevented from accumulating into a shock front by this adverse sound speed gradient. A range of results is presented for a variety of bubble ambient radii and sound field amplitudes suggested by experiments. The time scale of the peak temperature in the bubble is set by the dynamics of the compression wave: this is typically in the range 100–300 ps (FWHM) in concert with recent measurements of the sonoluminescence pulse width.
Polynomial expansions for the isotropic Boltzmann equation and invariance of the collision integral with respect to the choice of basis functions
Authors: Ender A. Ya. , Ender I. A.
Published: July 2002

A new approach to the development of the nonlinear moment method for solving the Boltzmann equation is advanced. The approach is based on the invariance of the collision integral with respect to the choice of basis functions. It is shown that the matrix elements corresponding to the moments of the nonlinear collision integral are not independent, being related to each other by simple recursion relations. The latters are used for calculation of the matrix elements with large indices in the case of arbitrary power potentials. It is demonstrated that a numerical calculation procedure which enables constructing the distribution function up to eight–ten thermal velocities.
The equation of state of a foam
Authors: Aref Hassan , Vainchtein Dmitri L.
Published: July 2002

The equation of state of a dry foam with ideal gas in the bubbles, proposed by Ross, is proved. This equation suggests that a foam with a free boundary will expand to a maximum volume if the external pressure is lowered at constant temperature. The same foam enclosed in a container, on the other hand, can be expanded further but, as the volume is increased, will eventually become unstable. We speculate that this instability leads to a “bubble differentiation” transition of the kind observed by Herdtle in numerical simulations of two-dimensional foam with ideal gas in the bubbles. In the simulations the foam separated into two “phases,” one with a large number of small bubbles, another with a small number of very large bubbles.
Weakly nonlinear simulation of planar stratified flows
Authors: King Michael R. , McCready Mark J.
Published: July 2002

The interfacial behavior of two-fluid, planar flows is studied by numerical integration of weakly-nonlinear amplitude equations derived via eigenfunction expansion of the governing equations. This study extends the range of classic Stuart–Landau theories by the inclusion of a spectrum of modes allowing all possible quadratic and cubic interactions. Results are obtained for four cases where linear and Stuart–Landau theories do not give a complete description; gas–liquid and oil–water pressure driven flow, matched-density liquid–liquid Couette flow, and the region of gas–liquid flow near resonance that switches from supercritical to subcritical. It is found that integration of amplitude equations gives better qualitative and quantitative agreement with experiments than Stuart–Landau theory. Further, the distinctively different behaviors of these systems can be understood in terms of the spectrum of nonlinear coefficients. In gas–liquid channel flow a low wave number wave is destabilized through quadratic interaction with the mean flow mode. For liquid–liquid Poiseuille flow, a low wave number wave is destabilized through cubic interactions with higher modes. For depth and viscosity ratios where liquid–liquid Couette flow is unstable to long waves and for which the growth rates are not too large, simulation results predict that the waves grow to a statistically steady state where there is no preferred wave number. Stabilization is provided by an apparently self-similar cascade of energy to higher modes that are linearly stable, explaining why no visible waves occur in experiments done in this region. While Stuart–Landau theory provides no prediction of wave amplitude above criticality for subcritical cases, simulations show that wave saturation at small amplitude is possible and suggests that subcritical predictions may not mean that steady waves do not exist.
Spin-up in a rectangular container with an internal cylindrical obstacle
Authors: Suh Y. K. , van Heijst G. J. F.
Published: July 2002

This paper describes a study of the spin-up of a free-surface fluid in a rectangular container in which an internal cylindrical obstacle is mounted. Laboratory experiments have been carried out for a variety of obstacle positions. It was found that the flow evolution during the adjustment process leading to the final state of solid-body rotation is crucially dependent on the obstacle position. As found in previous studies, in the absence of any obstacle the relative flow becomes organized in a domain-filling regular cellular pattern, upon which it decays according to the well-known spin-up mechanism provided by the Ekman layer at the tank bottom. Since the obstacle acts both as a barrier and as a source of viscously produced wall vorticity, the formation of the cell pattern is in most cases drastically influenced (either impeded or promoted) by the solid obstacle. Theoretical predictions of the flow pattern in the starting stage agree very well with the laboratory observations.
Lower-hybrid-drift instability operative in the geomagnetic tail
Authors: Yoon Peter H. , Lui Anthony T. Y. , Chang Chia-Lie
Published: July 2002

Motivated by recent interests in the current-driven instability as a plausible trigger mechanism for current disruption in the magnetotail, an extensive linear analysis of the lower-hybrid-drift instability (LHDI) in a high plasma beta regime is conducted. In the theoretical formulation, the fundamental difference between this instability and the modified-two-stream/ion-Weibel instability is clarified. Numerical solutions of the dispersion equation for the LHDI are obtained for plasma parameters appropriate for the magnetotail. In spite of the tendency for the LHDI to be stabilized at high plasma beta, the near-Earth current sheet environment immediately before the onset of current disruption is found to be just above the unstable threshold of the LHDI. The stability analysis is further extended to a model two-dimensional current sheet of Lembège and Pellat, with parameters chosen to match the near-Earth current sheet at the critical time. It is found that the LHDI is operative over an entire current sheet, thus providing theoretical support for the plausibility that the LHDI may play a significant role in triggering current disruption in the magnetotail.
Stability of tearing modes in finite-beta plasmas
Authors: Iacono R. , Bhattacharjee A. , Ronchi C. , Greene J. M. , Hughes M. H.
Published: July 2002

The linear stability of classical tearing modes in a finite-beta, cylindrical plasma is reconsidered. The different regimes of tearing mode instability are delineated by means of critical Lundquist numbers. The well-known criterion Δ′≳Δc due to Glasser, Greene, and Johnson [Phys. Fluids 18, 875 (1975)] defines a critical Lundquist number above which the plasma is stable. However, the physical importance of this Lundquist number is diminished by the fact that there is another critical Lundquist number, which is typically an order of magnitude smaller, above which the mode changes qualitatively in that the growth rate scales linearly with resistivity, and eventually becomes indistinguishable from resistive diffusion. The effect of parallel viscosity, which reduces the stabilizing coupling to the sound wave, is also considered. For large parallel viscosity, it is shown that Δc tends to zero. Thus, the stability criterion Δ′≳Δc for a finite-beta plasma is simply replaced by the criterion Δ′≳0. These analytical results are shown to be in good agreement with numerical results.
Interaction of intense MeV light-ion beams with low-pressure gases*
Authors: Young F. C. , Hubbard R. F. , Lampe M. , Neri J. M. , Ottinger P. F. , Stephanakis S. J. , Slinker S. P. , Hinshelwood D. D. , Rose D. V. , Olson C. L. , Welch D. R.
Published: July 2002

The interaction of intense proton beams with low-pressure (0.25 to 4 Torr) background gases is studied to evaluate beam-current neutralization during transport. Electrons to neutralize the beam are provided by beam-induced ionization of the gas. In experiments with 1 MeV, 1 kA/cm2 protons, net currents outside the beam envelope and electron densities within the beam envelope are measured for helium, neon, argon, and air. Net-current fractions are 2% to 8% and ionization fractions are 0.6% to 5% for 5 to 7 kA beams. Simulations of the experiments for helium and argon suggest that fast electrons play an important role in generating a significant fraction of the return current in a halo outside the beam. As a result, net currents inside the beam may be larger than inferred from magnetic-field measurements outside the beam. Ions at the head of the beam are observed to lose more energy than expected from collisional energy losses in the background gas.
Nonideal theory of toroidal Alfvén eigenmodes
Authors: Candy J. , Rosenbluth M. N.
Published: July 2002

Ambiguities in the ideal magnetohydrodynamic (MHD) analysis of toroidal Alfvén eigenmodes (TAE) are resolved by incorporating nonideal effects (finite electron conductivity and ion gyroradius) into the MHD model of Rosenbluth et al. [Phys. Fluids B 4, 1806 (1992)]. The previous ideal theory yields a dielectric function containing branch points in the complex frequency plane, but provides no specification of the corresponding branch lines. The kinetic model represents a singular perturbation of the ideal theory, and specifies precisely the location of branch cuts in the ideal limit. Moreover, the analytic structure of the complex frequency plane for the kinetic model shows a countably infinite set of poles in place of a branch cut—with a new kinetic-type TAE mode near each pole. It has also been verified that the ideal frequency root is in most cases close to one of the kinetic roots. The damping and mode structure is determined numerically within the framework of the high-mode-number, small inverse aspect ratio, low beta, small gyroradius model. Finally, an analytic form for the damping is obtained including both continuum and nonideal effects, and agrees well with the numerical results.
Laser wake-field acceleration and optical guiding in a hollow plasma channel
Authors: Chiou T. C. , Katsouleas T. , Decker C. , Mori W. B. , Wurtele J. S. , Shvets G. , Su J. J.
Published: July 2002

The accelerating and focusing wake fields that can be excited by a short laser pulse in a hollow underdense plasma are examined. The evacuated channel in the plasma serves as an optical fiber to guide the laser pulse over many Rayleigh lengths. Wake fields excited by plasma current at the edge of the channel extend to the center where they may be used for ultrahigh gradient acceleration of particles over long distances. The wake field and equilibrium laser profiles are found analytically and compared to two-dimensional (2-D) particle-in-cell (PIC) simulations. Laser propagation is simulated over more than ten Rayleigh lengths. The accelerating gradients on the axis of a channel of radius c/ωp are of order of one-half of the gradients in a uniform plasma. For present high-power lasers, multi-GeV/m gradients are predicted.
Hall magnetohydrodynamics in space and laboratory plasmas
Authors: Huba J. D.
Published: July 2002

Hall magnetohydrodynamic (MHD) theory has been used to understand and describe a variety of space and laboratory plasma phenomena. Generally speaking, the theory is applicable to phenomena occurring on length scales shorter than an ion inertial length and time scales shorter than an ion cyclotron period. The theory has been successfully applied to structuring of sub-Alfvénic plasma expansions, and to rapid magnetic field transport in plasma opening switches. An overview of the underlying physics associated with the Hall term, and a brief description of recent research on the application of Hall MHD theory to space and laboratory processes is presented.
Discharge-pumped soft-x-ray laser in neon-like argon
Authors: Rocca J. J. , Tomasel F. G. , Marconi M. C. , Shlyaptsev V. N. , Chilla J. L. A. , Szapiro B. T. , Giudice G.
Published: July 2002

Starting with the discovery of x-ray lasers in 1984, laser-created plasmas remained for almost a decade, the only medium in which large amplification of soft-x-ray radiation could be obtained. In this paper the recent first demonstration of large soft-x-ray amplification in a discharge-created plasma column, realized utilizing a fast capillary discharge to collisionally excite the 46.9 nm transition of Ne-like, Ar is reviewed. Results of the parametrization of the Ar IX discharge-pumped amplifier, the study of the dynamics of its plasma column, and the measurement of the time history of the laser pulse are reported. Prospects for laser operation at shorter wavelengths are also discussed.
Advances in the simulation of toroidal gyro-Landau fluid model turbulence
Authors: Waltz R. E. , Kerbel G. D. , Milovich J. , Hammett G. W.
Published: July 2002

The gyro-Landau fluid (GLF) model equations for toroidal geometry [R. E. Waltz, R. R. Dominguez, and G. W. Hammett, Phys. Fluids B 4, 3138 (1992)] have been recently applied to study ion temperature gradient (ITG) mode turbulence using the three-dimensional (3-D) nonlinear ballooning mode representation (BMR) outlined earlier [R. E. Waltz, G. D. Kerbel, and J. Milovich, Phys. Plasmas 1, 2229 (1994)]. The present paper extends this work by treating some unresolved issues concerning ITG turbulence with adiabatic electrons. Although eddies are highly elongated in the radial direction, long time radial correlation lengths are short and comparable to poloidal lengths. Although transport at vanishing shear is not particularly large, transport at reverse global shear, is significantly less. Electrostatic transport at moderate shear is not much affected by inclusion of local shear and average favorable curvature. Transport is suppressed when critical E×B rotational shear is comparable to the maximum linear growth rate with only a weak dependence on magnetic shear. Self-consistent turbulent transport of toroidal momentum can result in a transport bifurcation at sufficiently large r/(Rq). However, the main thrust of the new formulation in the paper deals with advances in the development of finite beta GLF models with trapped electrons and BMR numerical methods for treating the fast parallel field motion of the untrapped electrons.
Nonlinearity: Basis for self-consistent modeling of surface wave produced plasmas in diffusion controlled regimes
Authors: Aliev Yu. M. , Grosse S. , Schlüter H. , Shivarova A.
Published: July 2002

A fluid model of surface wave produced discharges is presented for diffusion controlled regimes taking into account simultaneously nonlinear contributions from stepwise ionization and volume recombination. The saturation of metastable densities with growing electron density reduces the effect of step ionization and allows recombination to become effective toward high electron densities. The charged particle continuity and energy balance equations linking electron density and electric field intensity yield the plasma permittivity under conditions of strong ionization nonlinearity. This permittivity is valid for high frequency discharges in general. In the second part of the modeling this nonlinear permittivity is introduced into the electrodynamical relations for discharges maintained by the field of traveling surface waves, and subsequently the self-consistent behavior of plasma and wave characteristics along the discharge length is calculated. Both the main part of the discharge column produced by Joule heating in the plasma volume and the end section of the discharge, where the mechanism of resonance absorption of the wave power close to the tube wall can sustain the discharge, are considered.
An analytic study of the magnetohydrodynamic stability of inverse shear profiles
Authors: Gimblett C. G. , Hastie R. J. , Hender T. C.
Published: July 2002

This paper reports on the ideal magnetohydrodynamic (MHD) stability of tokamak field profiles that have a non-monotonic safety factor q(r). An analytic criterion is obtained for these ‘‘inverse shear’’ profiles by expanding in inverse aspect ratio and assuming that the minimum in q is slightly less than the m/n value of the mode under examination (m and n being the principal poloidal and toroidal mode numbers of the instability). Three terms are identified as controlling the stability of this ‘‘double kink’’; two of them are stabilizing and due, respectively, to field line bending and the interaction of average favorable curvature with the pressure gradient. The possibility of instability comes from the third term which is due to toroidal coupling and is ballooning in character. The analytic results are compared with those from a fully toroidal stability code.
The dynamics of marginality and self-organized criticality as a paradigm for turbulent transport
Authors: Newman D. E. , Carreras B. A. , Diamond P. H. , Hahm T. S.
Published: July 2002

A general paradigm, based on the concept of self-organized criticality (SOC), for turbulent transport in magnetically confined plasmas, has been recently suggested as an explanation for some of the apparent discrepancies between most theoretical models of turbulent transport and experimental observations of the transport in magnetically confined plasmas. This model describes the dynamics of the transport without relying on the underlying local fluctuation mechanisms. Computations based on a cellular automata realization of such a model have found that noise-driven SOC systems can maintain average profiles that are linearly stable (submarginal) and yet are able to sustain active transport dynamics. It is also found that the dominant scales in the transport dynamics in the absence of sheared flow are system scales rather than the underlying local fluctuation scales. The addition of sheared flow into the dynamics leads to a large reduction of the system-scale transport events and a commensurate increase in the fluctuation-scale transport events needed to maintain the constant flux. The dynamics of these models and the potential ramifications for transport studies are discussed.
A nonlinear model for the singular surface response
Authors: Jensen T. H. , La Haye R. J. , Hyatt A. W.
Published: July 2002

The issues concerning the response of a plasma, at or near a singular surface, to a magnetic perturbation with a phase velocity different from the plasma flow velocity, are important for a number of phenomena. Among these are ideal and nonideal magnetohydrodynamic stability of plasmas with shear flow or a flow relative to a resistive wall, sensitivity of rotating plasma to field errors, and the ‘‘locked mode’’ phenomenon. Models for the singular surface response have been tested against results from ‘‘magnetic braking’’ experiments in DIII-D [R. J. La Haye et al., Nucl. Fusion 32, 2119 (1992)]. Previous models are found unable to account for all of the experimental observations. A new heuristic nonlinear model presented in the paper may account for the observations. A key element in the model is turbulence developed at the singular surface; the turbulence is assumed driven by the singular layer dissipation and is assumed to impede the singular current through an anomalous resistivity. When the perturbation amplitude is sufficiently large, a positive feedback mechanism exists, since in the regime of interest, dissipation increases with decreasing singular current. For small perturbation amplitudes this mechanism is not operative so that previous models for the response may be valid.
Parametric instability of a large-amplitude nonmonochromatic Alfvén wave
Authors: Malara F. , Velli M.
Published: July 2002

The parametric instability of a finite-amplitude Alfvén wave is studied in a one-dimensional geometry. The pump wave is an exact solution of the nonlinear magnetohydrodynamic (MHD) equations, i.e., the magnetic field perturbation has a uniform intensity and rotates in the plane perpendicular to the propagation direction, but its Fourier spectrum contains several wavelengths. The weakly nonmonochromatic regime is first studied by an analytical approach. It is shown that the growth rate of the instability decreases quadratically with a parameter that measures the departure from the monochromatic case. The fully nonmonochromatic case is studied by numerically solving the instability equations, when the phase function of the pump wave has a power-law spectrum. Though the growth rate is maximum in the monochromatic case, it remains of the same order of magnitude also for wide spectrum pump waves. For quasimonochromatic waves the correction to the growth rate depends only on the spectral index of the phase function.
Plasma response to ultrarelativistic beam propagation
Authors: Lotov K. V.
Published: July 2002

Plasma response to the ultrarelativistic beam of an arbitrary density is considered assuming a slow beam current growth. The problem is reduced to a single second order differential equation, which is numerically solved for Gaussian beams of various radii and densities. Graphs obtained clarify the structure of the ion channel, give insight into charge and current neutralizations of the dense beam, and permit the analysis of the focusing force acting on the beam in a plasma.
Stability of electrostatic modes in a levitated dipole
Authors: Kesner J.
Published: July 2002

Plasma confined in a magnetic dipole is stabilized by the expansion of the magnetic flux. The stability of low beta electrostatic modes in a magnetic dipole field is examined when the distribution function is to lowest order Maxwellian. It is shown that for sufficiently gentle density and temperature gradients the configuration would be expected to be stable to magnetohydrodynamic interchange, as well as to dissipative trapped ion and collisionless trapped particle modes. These results are applicable to any magnetic configuration for which the curvature drift frequency exceeds the diamagnetic drift frequency.
Compressible hydromagnetic shear flows with anisotropic thermal pressure: Nonmodal study of waves and instabilities
Authors: Chagelishvili G. D. , Rogava A. D. , Tsiklauri D.
Published: July 2002

The evolution of linear magnetohydrodynamic waves and plasma instabilities (firehose and mirror) in a compressible, magnetized plane Couette flow with anisotropic thermal pressure is investigated. In the present study we revealed that the pressure anisotropy brings significant novelty to the effect of coupling and linear reciprocal transformation of the wave modes originally discovered [Chagelishvili, Rogava, and Tsiklauri, Phys. Rev. E 53, 6028 (1996)]. It is found that behavior of the firehose and mirror instabilities is drastically changed due to the presence of shear in the flow. These novel effects are caused by the non-normality of linear dynamics in shear flows and they have been revealed through use of the nonmodal approach.
Emittance growth due to radial density variations of an emittance-dominated electron beam in a channel with continuous acceleration and focusing
Authors: Carlsten Bruce E.
Published: July 2002

Simulations have identified charge-density variations as driving the dominant emittance growth mechanism for high-current, low-emittance induction linacs using solenoidal focusing, once the beam enters the emittance-dominated regime. In this paper, we use the radial equation of motion, including the nonlinearities resulting from radial density variations, to understand this effect. Nonlinearities in the beam’s radial motion while in a solenoid arise from the noncancellation of the effects from the diamagnetic axial magnetic field and the potential depression of the beam, if the beam density is nonuniform. Any initial density variation drives a logarithmic increase in additional higher-order density variations (through the differential betatron motion), and an emittance growth that scales logarithmically, or greater (even potentially faster than linear), with the axial distance along the accelerator. The growth rate depends on the beam current, the focusing force, and the accelerating gradient, and for typical machine parameters, the growth rate can be faster than linear with distance. The magnitude of the emittance growth depends critically on the matching of the beam from the injector to the beamline. This formalism leads to a criterion of how uniform the beam density has to be and how well the beam needs to be matched in order not to have an unacceptable emittance growth.
Electron surface waves in a plasma slab with uniform ion density
Authors: Cooperberg D. J.
Published: July 2002

Electron surface waves in a metal bound plasma slab have been detected and analyzed. In this work it is shown that the presence of a matrix sheath between the central quasineutral region and the metal walls allows for the propagation of surface waves analogous to those found in dielectric bound plasmas. Measurements of the dispersion relations and eigenfunctions of asymmetric and symmetric, electrostatic, surface, and body waves are made via particle-in-cell simulation of a plasma slab with sheaths. The plasma slab has finite temperature electrons and fixed ions of uniform density. The sheaths consist of electron free, fixed, uniform ion regions (“matrix sheath”) of thickness ∼λDe. A linearized Vlasov theory is developed for comparison with the simulation. It is shown that the long wavelength approximation (kλDe≪1) is not valid even for long wavelengths in the propagation direction. Collisionless damping of both surface and body waves is measured which compares well with theoretical estimates.
Two-dimensional calculation of eddy currents on external conducting walls induced by low-n external modes
Authors: Lee D.-Y. , Chance M. S. , Manickam J. , Pomphrey N. , Okabayashi M.
Published: July 2002

The results of two-dimensional calculations of eddy currents induced on external conducting walls surrounding a tokamak are reported. The computed eddy currents are generated by low-n (n=1,2,3) external ideal-magnetohydrodynamic (MHD) instabilities. For a given toroidal mode number n the eddy current patterns are found to be very similar in a variety of plasma configurations, e.g., different edge safety factors and different plasma–wall separation distances, in high beta plasmas. This result is promising for the design of active feedback coils for the stabilization of the resistive wall mode. Also, the effects of having a partial wall that has a poloidal gap on the outboard side are considered. Using the expected gap size in the proposed Korea Superconducting Tokamak Advanced Research (KSTAR) [“The KSTAR tokmak,” in Proceedings of the 17th Symposium on Fusion Engineering, San Diego, 1997 (Institute of Electrical and Electronics Engineers, New York, in press), Paper No. O3.1], the calculation shows that active coils mounted behind the partial walls (the KSTAR passive plates) cover an adequate portion of the eddy current dominant region, enabling feedback stabilization.
Toroidal rotation effect on the ballooning modes of low frequency under gyrokinetic description
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

Gyrokinetic theory for ballooning modes with frequency lower than the thermal ion bounce frequency is presented. Due to a vast difference of the centrifugal and the Coriolis forces experienced by ions and electrons, the parallel electric field effect is found to be excited even in this low frequency case. Nevertheless, it is proved that, under suitable ordering assumptions, the magnetohydrodynamic (MHD)-structure-like ballooning mode equation can still be obtained. The kinetic effects are found to appear in three aspects: enlarging the magnitude of the apparent inertia, resulting in a real shift of the marginal frequency, and contributing a stabilizing term.
Phase space structure for matched intense charged-particle beams in periodic focusing transport systems
Authors: Chen Chiping , Pakter Renato , Davidson Ronald C.
Published: July 2002

Test particle motion is analyzed analytically and numerically in the field configuration consisting of the equilibrium self-electric and self-magnetic fields of a well-matched, thin, continuous, intense charged-particle beam and an applied periodic focusing solenoidal magnetic field. The self fields are determined self-consistently, assuming the beam to have a uniform-density, rigid-rotor Vlasov equilibrium distribution. Using the Hamilton–Jacobi method, the betatron oscillations of test particles in the average self fields and applied focusing field are analyzed, and the nonlinear resonances induced by periodic modulations in the self fields and applied field are determined. The Poincaré surface-of-section method is used to analyze numerically the phase-space structure for test particle motion outside the outermost envelope of the beam over a wide range of system parameters. For vacuum phase advance σv=80°, it is found that the phase-space structure is almost entirely regular at low beam intensity (phase advance σ≳70°, say), whereas at moderate beam intensity (30°≲σ≲70°), nonlinear resonances appear, the most pronounced of which is the third-order primary nonlinear resonance. As the beam intensity is further increased (σ≲30°), the widths of the higher-order nonlinear resonances increase, and the chaotic region of phase space increases in size. Furthermore, the many chaotic layers associated with the separatrices of the primary and secondary nonlinear resonances are still divided by the remaining invariant Kolmogorov–Arnold–Moser surfaces, even at very high beam intensities. The implications of the rich nonlinear resonance structure and chaotic particle motion found in the present test-particle studies are discussed in the context of halo formation.
Improved magnetohydrodynamic stability through optimization of higher order moments in cross-section shape of tokamaks
Authors: Turnbull A. D. , Lin-Liu Y. R. , Miller R. L. , Taylor T. S. , Todd T. N.
Published: July 2002

Optimization of the higher order moments—specifically the squareness—of a tokamak cross-section can significantly enhance the stability to ideal magnetohydrodynamic ballooning and kink modes. At conventional aspect ratios, it is shown that access to the second regime of ballooning stability is facilitated by moderate squareness. In a low aspect ratio, fully bootstrap current driven spherical torus, optimization of the squareness results in an increase in β of the order of 10%, leading to a configuration stable to ballooning, axisymmetric, and ideal n⩽5 kink modes at β≃67%.
Magnetic field correlations in kinematic two-dimensional magnetohydrodynamic turbulence
Authors: Schumacher Jörg , Eckhardt Bruno
Published: July 2002

The scaling properties of the second order magnetic structure function D2(B)(r) and the corresponding magnetic correlation function C2(B)(r) are derived for two-dimensional magnetohydrodynamic turbulence in the kinematic regime where the ratio of kinetic energy to magnetic energy is much larger than one. In this regime the magnetic flux function ψ can be treated as a passive scalar advected in a two-dimensional turbulent flow. Its structure function D2(ψ)(r) and the one for the magnetic field D2(B)(r) are connected by an exact relation. We calculate D2(ψ)(r) and thus D2(B)(r) within geometric measure theory over a wide range of scales r and magnetic Prandtl numbers Prm. The magnetic field correlations follow a r−4/3-scaling law and show an anticorrelation at the beginning of the Batchelor regime indicative of the formation of strongly filamented current sheets. Differences to the full dynamic regime, where the ratio of kinetic to magnetic energies is smaller than in the kinematic case, are discussed.
Anomalous and neoclassical transport suppression by the radial electric field, induced by Alfvén waves in tokamaks
Authors: Tsypin V. S. , Nascimento I. C. , Galvão R. M. O. , Elfimov A. G. , Amarante Segundo G. S. , Tendler M.
Published: July 2002

The suppression of anomalous transport and/or reduction of neoclassical transport caused by the Alfvén-wave-induced shear of the radial electric field in tokamaks is investigated. The simple quantitative estimates reported in previous papers are checked by a consistent evaluation of the profile of the radial electric field created by the resonant absorption of Alfvén waves. The radial variations of the ion viscosity and heat conductivity across the mode conversion layer are evaluated both in the banana and potato regions. In agreement with previous analytical results, it is shown that the kinetic Alfvén waves may play the role of a convenient mechanism for reduction of anomalous and neoclassical transport and formation of transport barriers in tokamaks.
Long-term, correlated emittance decrease in intense, high-brightness induction linacs
Authors: Carlsten Bruce E.
Published: July 2002

Simulations of high-brightness induction linacs often show a slow, long-term emittance decrease as the beam is matched from the electron gun into the linac. Superimposed on this long-term decrease are rapid emittance oscillations. These effects can be described in terms of correlations in the beam’s radial phase space. The rapid emittance oscillations are due to transverse plasma oscillations, which stay nearly in phase for different radial positions within the beam. The initial emittance, just after the electron gun, is dominated by nonlinear focusing within the gun introduced by the anode exit hole. Due to the large space-charge force of an intense electron beam, the focusing of the beam through the matching section introduces an effective nonlinear force (from the change in the particles’ potential energies) which counteracts the nonlinearities from the electron gun, leading to an average, long-term emittance decrease. Not all of the initial nonlinearity is removed by the matching procedure, and there are important consequences both for emittance measurements using solenoid focal length scans and for focusing the electron beam to a target.
The effect of plasma flow, compressibility, and Landau damping on resistive wall modes
Authors: Lashmore-Davies C. N. , Wesson J. A. , Gimblett C. G.
Published: July 2002

A uniform compressible plasma with a uniform flow along the magnetic field in the presence of a resistive wall is shown to be subject to two instabilities. For the first instability, the flow velocity is required to exceed the Alfvén speed, whereas for the second it need only exceed the sound speed. For a sufficiently high ion temperature, ion Landau damping is shown to stabilize the second instability associated with the sound speed.
Ponderomotive force in a moving warm two-fluid plasma
Authors: Lee Nam C.
Published: July 2002

A microscopic method is used to obtain an expression for the ponderomotive force of high frequency electromagnetic waves in a warm two-fluid collisionless plasma with nonzero fluid velocity. The result contains new terms which explicitly depend on the fluid velocity and its gradients, in addition to the familiar expressions of earlier studies. It is shown that the new terms are of the same order as the Abraham force in the smallness parameter that measures the magnitude of the flow velocity of the plasma relative to the phase velocity of the wave. The stress tensor and the momentum density vector are also derived from the newly found ponderomotive force.
Magnetic instability by the relativistic laser pulses in overdense plasmas
Authors: Sentoku Yasuhiko , Mima Kunioki , Kojima Shin-ichi , Ruhl Hartmut
Published: July 2002

The magnetic instability driven by the relativistic electron stream generated by ultra-intense laser is investigated with the help of a two-dimensional particle-in-cell simulation, which includes the relativistic binary collision. The linear growth rate of the instability is also studied using the two-stream fluid model, which consists of a fast electron current and a return current. The growth rate is evaluated numerically from the linearized equations of the electron fluids with the Maxwell equations. The kinetic effects of electrons on the magnetic instability are found to reduce the growth rate. The growth rate is maximum at the wavelength near the plasma skin length because of the plasma kinetic effect. When the initial plasma temperature is high, the growth rates of shorter wavelengths are significantly reduced. In the collisional plasma, the growth rates of modes whose wavelength is shorter than the plasma skin length are suppressed and the spectral peak of the growth rate shifts to long wavelength modes. It is found that the results of the linear analysis agree well with two-dimensional particle simulation results in the early stage.
Suppression of resistive wall instabilities with distributed, independently controlled, active feedback coils
Authors: Cates C. , Shilov M. , Mauel M. E. , Navratil G. A. , Maurer D. , Mukherjee S. , Nadle D. , Bialek J. , Boozer A.
Published: July 2002

External kink instabilities are suppressed in a tokamak experiment by either (1) energizing a distributed array of independently controlled active feedback coils mounted outside a segmented resistive wall or (2) inserting a second segmented wall having much higher electrical conductivity. When the active feedback coils are off and the highly conducting wall is withdrawn, kink instabilities excited by plasma current gradients grow at a rate comparable to the magnetic diffusion rate of the resistive wall.
Observed enhanced fluorescence at 2177, 2163, 1923, and 1620 Å in C III by photoexcitation with Mn VI line radiation at 310 Å
Authors: Qi Niansheng , Kilic Hayrettin , Krishnan Mahadevan
Published: July 2002

Line radiation at 310.182 Å from Mn VI ions in a laser produced plasma was used to resonantly pump C III ions in an adjacent, vacuum arc discharge from the 2s 1S ground level to the 4p1P0 upper level. Enhanced fluorescence by up to a factor of 150 was measured on the 4p 1P0–3d 1D line at 2177 Å. Collisional exchanges between the n=4 levels transfer the pumped 4p population to the 4d and 4f levels. Enhanced fluorescence was also measured on the 4d 1D–3p 1P0 line at 1620 Å and on the 4f–3d lines at 2163 Å and 1923 Å, respectively.
Improved recombination lifetime of photoexcited carriers in GaAs single quantum well heterostructures confined by GaAs/AlAs short-period superlattices
Authors: Fujiwara K. , Nakamura A. , Tokuda Y. , Nakayama T. , Hirai M.
Published: July 2002

Photoluminescence (PL) decay time measurements at 77 and 300 K are reported from 6.1 nm GaAs single quantum well heterostructures (SQWH’s) confined by GaAs/AlAs short-period superlattices (SPS’s) or ternary AlGaAs alloys with similar Al content, prepared by molecular beam epitaxy. The SQW PL intensity exhibits a single exponential decay with a time constant of 1.6 ns for SQWH’s confined by SPS’s and 0.3 ns for SQWH’s confined by AlGaAs alloys at 77 K. From comparison of the decay rates in both types of the sample, it is found that the radiative recombination efficiency is improved by a factor of about 6 in SPS confined SQWH’s. This higher efficiency is attributed to the improved heterointerfaces in addition to the enhanced radiative recombination rate due to the increased overlap of electron and hole wave functions in the narrow SQW.
Chemically enhanced focused ion beam etching of deep grooves and laser-mirror facets in GaAs under Cl2 gas irradiation using a fine nozzle
Authors: Takado N. , Asakawa K. , Yuasa T. , Sugata S. , Miyauchi E. , Hashimoto H. , Ishii M.
Published: July 2002

Chlorine-enhanced GaAs maskless etching has been performed with a novel focused ion beam etching (FIBE) system. The system is composed of an air-locked ultrahigh vacuum chamber, a 30-keV Ga+ FIB column, and a fine nozzle. The nozzle irradiates a high-density Cl2 flux on a desired, small area of the sample while retaining a sufficiently low surrounding gas pressure for stable Ga+ FIB emission. Condensed Ga residues, appearing on the etched surface with no Cl2 gas, could be suppressed under Cl2 gas irradiation. Highly chemically enhanced sputtering yields (up to 50 GaAs molecules per incident ion) were obtained by selecting the optimum relationship between scanning time and Cl2 gas pressure. At the maximum yield, a deep groove (about 6 μm) with a smooth surface was obtained by line-scanning FIBE. The etching was applied to laser-mirror formation of an AlGaAs laser. A vertical mirror facet, fabricated in advance by a reactive ion beam etching, was trimmed about one micron thick by line-scanning FIBE. Light output versus current characteristics did not change before and after FIBE and the etching has been shown to be useful for laser-mirror formation.
Semiconductor topography in aqueous environments: Tunneling microscopy of chemomechanically polished (001) GaAs
Authors: Sonnenfeld Richard , Schneir J. , Drake B. , Hansma P. K. , Aspnes D. E.
Published: July 2002

Scanning tunneling microscopy (STM) of (001) GaAs samples immersed in aqueous solutions has been used to assess the effectiveness of a standard bromine-methanol chemomechanical polish to produce flat surfaces over length scales from 5 to 1000 nm. The STM images reveal irregular 100-nm features coexisting with large areas of average roughness of the order of a few nanometers. The precision, stability, and reproducibility of these images suggest that immersion STM could be used to study surface chemical processes in real time.