bluuebunny/crossref_metadata_2025 huggingface dataset:
Nonparallel linear stability analysis of Long’s vortex
Authors: Fernandez-Feria R.
Published: July 2002

A nonparallel linear stability analysis of a family of self-similar vortex cores which includes Long’s vortex as a particular member is performed using parabolized stability equations (PSE). The resulting streamwise variation of both the spatial growth rate and the axial wave number of the different unstable modes is compared with the results from a local spatial stability analysis which also takes into account the effects of viscosity and of the streamwise variation of the basic flow, so that the effect of the history of the disturbances on their stability is quantified. It is shown that this last effect is negligible for high Reynolds numbers, but becomes increasingly important as the Reynolds number decreases, especially for very small growth rates. The marching method used to solve the PSE is computationally much faster than the standard methods for solving the nonlinear eigenvalue problem resulting from the local stability equations. As a new result, the local spatial calculations reveal the existence of unstable counter-rotating spiral modes with negative group velocities for Type II Long’s vortices (that is, vortices with negative streamwise velocity at the axis), thus showing that these flows are subcritical in Benjamin’s sense. This kind of instability does not appear for Type I vortices, which can only sustain non-axisymmetric convective instabilities, and are therefore supercritical. Thus, the spatial stability analysis establishes a fundamental distinction between Type I and Type II Long’s vortices.
Stability of periodically compressed vortices at low Mach number
Authors: Leblanc Stéphane , Le Penven Lionel
Published: July 2002

Stability analysis of circular and elliptical vortices periodically compressed axially in their plane reveals, at low Mach number, two distinct mechanisms of three-dimensional instability. The first one is a manifestation of the elliptical instability, modified by compression. The second one, which exists also in the circular case, is a resonance between the frequency of compression and the intrinsic rotation rate of the uncompressed vortex.
Homogeneous pattern selection and director instabilities of nematic liquid crystal polymers induced by elongational flows
Authors: Forest M. Gregory , Wang Qi , Zhou Hong
Published: July 2002

We characterize homogeneous patterns, their stability, and phase transitions in nematic liquid crystal polymers (LCPs) with imposed elongational flows. We combine the flow-induced analysis of order parameters by See et al. [J. Chem. Phys. 92, 792 (1990)], Bhave et al. [J. Rheol. 37, 413 (1993)], Rey [Macromol. Theory Simul. 4, 857 (1995)], and Wang [J. Non-Newtonian Fluid Mech. 22, 147 (1997)], with the pure nematic, full tensor analysis of Shimada et al. [J. Chem. Phys. 88, 7181 (1988)]. To make contact with these seminal studies, we select a moment-averaged Doi kinetic model for flows of rod-like nematic LCPs with a quartic short-range intermolecular potential; the connection with alternative kinetic and continuum models for flows of LCPs is noted. New elongation-induced director instabilities are revealed for patterns previously identified as candidates for stable pattern selection. From a full tensor analysis, we determine the complete phase diagram for homogeneous patterns in the parameter space of LCP concentration and elongation rate. With respect to experimental predictions, in axial extension, biaxial patterns exist but they are all unstable and the only stable patterns are uniaxial; in planar extension, above a moderate concentration the only stable nematic patterns are biaxial.
Two-dimensional stagnation flow around a vertical plate above a plane wall
Authors: Jeong Jae-Tack
Published: July 2002

Two-dimensional slow viscous stagnation flow around a vertical plate above a plane wall is investigated based on the Stokes approximation. An exact formal expression of the stream function is obtained by use of the three-part Wiener–Hopf technique. From the formal expression obtained, the streamline patterns around the plate are shown and the force exerted on the plate is calculated. The stress distributions on the boundaries are also calculated. By examining the limiting case of very small distance between the wall and the plate, the formation of the Moffatt’s viscous eddies near the corner is explained.
Point vortex motion on a sphere with solid boundaries
Authors: Kidambi R. , Newton P. K.
Published: July 2002

We consider the motion of a point vortex on the surface of a sphere with solid boundaries. This problem is of interest in oceanography, where coherent vortex structures can persist for long times, and move over distances large enough so that the curvature of the Earth becomes important (see Gill [1982], Chaos [1994]). In this context, the boundary is a first step in modeling the presence of coastlines and shores using inviscid theory. Using the equations of motion for the vortex projected onto the stereographic plane, we construct the appropriate Green’s function using classical image method ideas, as long as the domain has certain symmetry properties. After the solution is obtained in the stereographic plane, it is projected back down to the sphere, yielding the sought after solution to the problem. We demonstrate the utility of the method by solving for the vortex trajectories and streamlines for several canonical examples, including a spherical cap, longitudinal wedge, half longitudinal wedge, channel, and rectangle. The results are compared with the corresponding ones in the physical plane in order to highlight the effect of the spherical geometry.
Axisymmetric capillary waves on thin annular liquid sheets. II. Spatial development
Authors: Mehring C. , Sirignano W. A.
Published: July 2002

The forced motion of semi-infinite axisymmetric thin inviscid annular liquid sheets, exiting from a nozzle or atomizer into a surrounding void under zero gravity but with constant gas-core pressure is analyzed by means of the reduced-dimension approach described in C. Mehring and W. A. Sirignano [Phys. Fluids 12, 1417 (2000)]. Linear analytical time-dependent (“limit-cycle”) solutions to the pure boundary-value problem are presented as well as linear and nonlinear numerical (transient) solutions to the mixed boundary- and initial-value problem of initially undisturbed sheets harmonically forced at the orifice or nozzle exit. Group velocities for the six independent solutions to the linear boundary-value problem are used to determine the location of boundary conditions. Numerical simulations of the linear transient problem are employed to validate these predictions. Parameter studies on sheet breakup and collapse lengths as well as on breakup and collapse times are reported. The dependence on modulation frequency, modulated disturbance amplitude, Weber number, and annular radius is presented for various cases of the mixed problem, i.e., for linearly or nonlinearly stable and unstable, dilationally or sinusoidally forced sheets. Nonlinear effects often have significant effects on breakup times and lengths or on collapse times and lengths. Nonlinear wave forms can deviate substantially from linear predictions resulting in major impacts on the size of the rings and shells that will remain after breakup.
On the decay of homogeneous isotropic turbulence
Authors: Skrbek L. , Stalp Steven R.
Published: July 2002

Decaying homogeneous, isotropic turbulence is investigated using a phenomenological model based on the three-dimensional turbulent energy spectra. We generalize the approach first used by Comte-Bellot and Corrsin [J. Fluid Mech. 25, 657 (1966)] and revised by Saffman [J. Fluid Mech. 27, 581 (1967); Phys. Fluids 10, 1349 (1967)]. At small wave numbers we assume the spectral energy is proportional to the wave number to an arbitrary power. The specific case of power 2, which follows from the Saffman invariant, is discussed in detail and is later shown to best describe experimental data. For the spectral energy density in the inertial range we apply both the Kolmogorov −5/3 law, E(k)=Cε2/3k−5/3, and the refined Kolmogorov law by taking into account intermittency. We show that intermittency affects the energy decay mainly by shifting the position of the virtual origin rather than altering the power law of the energy decay. Additionally, the spectrum is naturally truncated due to the size of the wind tunnel test section, as eddies larger than the physical size of the system cannot exist. We discuss effects associated with the energy-containing length scale saturating at the size of the test section and predict a change in the power law decay of both energy and vorticity. To incorporate viscous corrections to the model, we truncate the spectrum at an effective Kolmogorov wave number kη=γ(ε/v3)1/4, where γ is a dimensionless parameter of order unity. We show that as the turbulence decays, viscous corrections gradually become more important and a simple power law can no longer describe the decay. We discuss the final period of decay within the framework of our model, and show that care must be taken to distinguish between the final period of decay and the change of the character of decay due to the saturation of the energy containing length scale. The model is applied to a number of experiments on decaying turbulence. These include the downstream decay of turbulence in wind tunnels and a water channel, the temporal decay of turbulence created by an oscillating grid in water and the decay of energy and vorticity created by a towed grid in a stationary sample of water. We also analyze decaying vorticity data we obtained in superfluid helium and show that decaying superfluid turbulence can be described classically. This paper offers a unified investigation of decaying isotropic, homogeneous turbulence that is based on accepted forms of the three-dimensional turbulent spectra and a variety of experimental decay data obtained in air, water, and superfluid helium.
Balance of angular momentum and energy in a rotating-field generated plasma equilibrium
Authors: Kuthi A. , Zwi H. R. , Wong A. Y.
Published: July 2002

The balance of energy and angular momentum is modeled for a rigid-rotor plasma equilibrium generated by a rotating magnetic field. The model predicts that deviation from circular polarization of the rotating field and the presence of transverse stationary error fields reduce the efficiency of the diamagnetic current drive. The error field in combination with low collision frequency can prevent electron fluid corotation. In the RACETRACK experiment [Rev. Sci. Instrum. 57, 2720 (1986)], the measured power balance is in agreement with the measured rotating field polarization and the estimated magnitude of the stationary error field.
Two-dimensional kinetic modeling of a tokamak scrape-off layer with recycling
Authors: Helander P. , Catto Peter J.
Published: July 2002

The two-dimensional kinetic model of the scrape-off layer recently suggested by Catto and Hazeltine [Phys. Plasmas 1, 1882 (1994)] is extended by allowing for ion recycling at the limiter or divertor plates by an effective ion reflection coefficient. The model describes the balance between radial diffusion and streaming along the magnetic field, and the structure of the resulting scrape-off layer is found to depend on the reflection coefficient. The particle and heat loads on the limiter or divertor plates are calculated, as well as the boundary conditions on the density and temperature gradients of the core plasma. By assuming that electrons have a Maxwell–Boltzmann distribution, the radial variation of the plasma potential is determined, and the potential of the Debye sheaths formed at the limiter or divertor plates is estimated.
Free-electron laser operation in a sheath helix configuration
Authors: Sharma Anamika , Chauhan Ansuya , Tripathi V. K.
Published: July 2002

A sheath helix loaded waveguide supports a slow electromagnetic wave which can act as a wiggler for a millimeter-wave free-electron laser (FEL). A nonlocal theory of FEL interaction in the Compton regime reveals that the radiation frequency and growth rate are sensitive functions of the phase velocity of the pump (wiggler) wave and increase with increasing pump frequency. The FEL radiation frequency can be tuned by an additional parameter, viz., the pitch angle of the helix in this configuration.
Wave breaking in a cold plasma near the top of a parabolic density profile
Authors: Chiron A. , Cros B. , Matthieussent G. , Mora P.
Published: July 2002

Wave breaking of electron plasma waves in an unmagnetized, cold, collisionless plasma with a parabolic density profile is considered when the plasma resonance is situated near the top of the profile. For this case, wave breaking takes place at a density lower than the resonant one and limits the growth of the plasma wave. The dynamics of wave breaking in such conditions are studied as a function of position of resonance in the profile and of amplitude of the driver field.
A singular perturbation approach to ion and electron confinement in trapping fields
Authors: Soběhart J. R. , Farengo R.
Published: July 2002

The confinement of ions and electrons by linear and nonlinear fields is studied. It is assumed that both species are perturbed by collisions with a neutral background gas with the resulting energy and momentum transfer between the interacting species. Asymptotic expansions, based on a singular perturbation approach in terms of the momentum exchange during collisions, are used to determine the distribution function of the confined species.
Collisionless kinetic ballooning mode equation in the low-frequency regime
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

The derivation of the fluid-like kinetic ballooning mode equation from the gyrokinetic and Maxwell equations in the low-frequency regime is revisited. The traditional results are pointed out to be ordering-inconsistent. With the theory developed in the paper, two new effects are discovered: the small parallel-ion-velocity (SPIV) and the apparent mass (AM) effects. The SPIV effect is induced by the ions with parallel velocities much smaller than the thermal velocity, while the AM effect mainly by the ions with velocities comparable with the thermal velocity. The trapped-particle contribution to the SPIV effect can be interpreted as a harmonic trapped-particle effect, which can be even more important than the traditional bounce-average trapped-particle effect. The SPIV effect is shown to be more significant in ordering than the inertia effect with the AM and finite Larmor radius modifications. The application of the equation is discussed. The comparison with the ideal magnetohydrodynamic theory is made.
Role of core toroidal rotation on the H-mode radial electric field shear, turbulence, and confinement as studied by magnetic braking in the DIII-D tokamak
Authors: La Haye R. J. , Rettig C. L. , Groebner R. J. , Hyatt A. W. , Scoville J. T.
Published: July 2002

‘‘Magnetic braking’’ of the plasma toroidal rotation in the high confinement H mode by applied resonant, low m,n=1 static error fields is used in DIII-D [Nucl. Fusion 31, 875 (1991)] as an independent control to evaluate the Er×B stabilization of microturbulence in the plasma core. In the core (ρ≲0.9) of a tokamak, the radial electric field and its shear are dominated by toroidal rotation. The fundamental quantity for shear stabilization of microturbulence is shear in the velocity of the fluctuations v⊥≊Er×B/B⋅B which in the core is v⊥≊vφBθ/ Bφ. With magnetic braking greatly decreasing the toroidal rotation and thus reducing the core radial electric field and shear, far infrared (FIR) measurements of density microturbulence show downshifting in frequency near ρ≊0.8 as a result of the reduced Doppler shift (ω≊kθEr/Bφ) and a factor of 2 increase in the turbulence level (ñ/n)2 in the period between edge localized modes (ELMs). There is also a large reduction in turbulence at an ELM which tends to compensate for the increase in turbulence with reduced radial electric field shear between ELMs. No significant change is found in H-mode plasma energy, confinement time, internal inductance li, density profile, Te profile, or Ti profile. Good H-mode confinement is maintained by the edge (ρ≳0.95) transport barrier where the reversed edge Er and high edge Er shear remain unchanged.
Recent progress of high-power millimeter wavelength gyrodevices
Authors: Goldenberg A. L. , Litvak A. G.
Published: July 2002

This presentation reviews recent progress of Russian physicists in developing electronic devices of the gyroresonance type, which are so far the most advanced sources of millimeter microwaves. It deals with the concept of the gyrotron as a device operating at higher volume resonator modes with a built-in quasioptical converter of the output radiation into a wave beam. That concept made it possible to create comparatively simple and reliable long-pulse and quasicontinuous devices with a power level of 0.5 MW in the frequency band 30–160 GHz. Analysis of the principal problems of projects for continuous-wave (CW) gyrotrons of near 1 MW output power (choice of an operating mode, requirements to an electron beam, peculiarities of built-in converters) is included, as well as the results of model experiments. For amplifying millimeter wavelength gyroklystrons, a level of several hundreds of kW at the frequency 35 GHz, and up to 65 kW at frequency 94 GHz were also achieved. Some modifications of gyrotrons for material processing and special research are described.
Diagnostics of fast-ion minority in tokamak plasmas using collective scattering from thermally excited eigenmodes
Authors: Piliya A. D. , Saveliev A. N. , Bulanin V. V.
Published: July 2002

The possibility of measuring the fusion α-particle features using the scattering from thermally excited eigenmodes has been analyzed in detail. A number of these modes are coupled predominantly to the fast-ion minority. However, the standard fluctuation theory predicts that the intensity of thermally excited waves is proportional to the ‘‘temperature’’ of this fraction and is independent of its density. Such amounts of information are insufficient for the purpose of diagnostics. Different results are obtained if the effects of plasma inhomogeneity and nonlocal coupling of the observed (induced) electron density fluctuations with ‘‘bare’’ fluctuations are taken into account. Then the fluctuation intensity becomes density dependent and both the ‘‘temperature’’ and the relative density of the fast-ion minority can be found from spectral measurements of the scattering signal. The best opportunity is provided by thermally excited ion Bernstein waves in the lower hybrid frequency region. This case was analyzed using an effective numerical code which takes into account gyrokinetic effects and treats α particles as magnetized. The results show that the above two parameters can be measured with the employment of present-day laser technology.
Evolution to turbulence in a capacitor model of resonance absorption
Authors: Gavrilov E. M. , DuBois D. F. , Rose H. A. , Rubenchik A. M.
Published: July 2002

The interaction of a powerful, subpicosecond laser pulse with plasma is modeled within the capacitor model by means of one-dimensional electrostatic particle code. The temporal profile of the laser pulse and the growth of the electron energy by several orders of magnitude were taken into account providing adequate temporal and spatial resolution. It is shown that initially, a coherent structure is excited, and growth of plasma waves results in wave breaking and acceleration of electrons followed with decreasing intervals between them, accelerating electrons in both directions. Finally the system evolves to a state of moderate Langmuir turbulence where E2/(16πnT)≪1 due to the strong heating of plasma electrons and the decrease of excited plasma wave fields.
Experimental study of the hot and warm electron populations in an electron cyclotron resonance argon–oxygen–hydrogen plasma
Authors: Friedlein R. , Herpich S. , Hiller H. , Wirth H. , Zschornack G. , Tyrroff H.
Published: July 2002

Measurements of bremsstrahlung down to 7 keV emitted from a 7.25 GHz electron cyclotron resonance discharge and the deconvolution of the spectra using a newly developed integration method yield the exact electron distribution function. Two separate electron populations with temperatures about 3 and 25 keV and with densities about (0.2...4)108 cm−3 were recorded. First, results are obtained for the behavior of these populations belonging to the mixture (argon–oxygen–hydrogen). Gas mixing increases the densities of these populations, but hardly changes the temperatures.
Propagation of electromagnetic and space-charge waves in quasiperiodic structures
Authors: Schächter Levi , Nation John A.
Published: July 2002

An analytic method to calculate the propagation of electromagnetic and space-charge waves in a quasiperiodic disk-loaded waveguide including input and output arms, is presented. The approach relies on Cauchy’s residue theorem to formulate the transmission, reflection, or the Green’s function of a system composed of radial arms and grooves which are attached to a cylindrical waveguide. The only constraint of this method is that the inner radius has to be constant; all the other parameters of the system can be arbitrarily changed. This method is particularly useful for the analysis of input and output section of a high-power traveling wave structures.
The small-parallel-ion-velocity effect on the ballooning modes with frequency lower than the magnetic drift one
Authors: Zheng L.-J. , Tessarotto M.
Published: July 2002

Collisionless kinetic ballooning mode theory is presented in the case of the frequency of modes, ω, being lower than the ion magnetic drift frequency, ωDi, in the outer region of the ballooning representation space. A fluid-like kinetic ballooning mode equation is obtained. Previous results are pointed out to be inconsistent due to the particle velocities not being distinguished from the fluid ones in the ordering analyses. In our new theory, it is found that the small parallel ion velocity (SPIV) effect, which was first discovered in the regime ωDi≪ω≪ωsi, with ωsi the ion acoustic frequency along the field lines [L.-J. Zheng and M. Tessarotto, Phys. Plasmas 1, 3928 (1994)], is a dominant effect in ordering with respect to any other kinetic effects previously considered, such as the finite Larmor radius and the bounce-average trapped particle effects. It is concluded that the eigenfrequency of the modes should be determined by the SPIV effect rather than by the fluid inertia one, which suggests that those stability analyses based on balancing the instability drives with the fluid inertia effect to determine the eigenfrequency are inapplicable.
Comments on ‘‘Impossibility of plasma instabilities in isotropic quantum plasmas’’ [Phys. Plasmas 1, 832 (1994)]
Authors: Nieves José F.
Published: July 2002

Bonitz [Phys. Plasmas 1, 832 (1994)] has recently shown that isotropic quantum plasmas described by the Linhard formula for the polarization function cannot exhibit unstable longitudinal collective excitations. This result can be valid for more general systems for which the random phase approximation or the Linhard formula for the polarization function may not be applicable.
Response to ‘‘Comments on ‘Impossibility of plasma instabilities in isotropic quantum plasmas’ ’’ [Phys. Plasmas 1, 832 (1994)]
Authors: Bonitz M.
Published: July 2002

The general stability of a spherically symmetric quantum plasma is considered. The stability criterion based on analytical properties of the dielectric function (Ref. 1) is extended to include certain generalizations of lower symmetries and reduced dimensionality and better approximations in the interaction or density. (AIP)
Radial extension of drift waves in the presence of velocity profiles
Authors: Sen S. , Weiland J.
Published: July 2002

The effect of a radially varying poloidal velocity field on the recently found radially extended toroidal drift waves [Connor et al., Phys. Rev. Lett. 70, 1803 (1993)] is investigated analytically. The role of velocity curvature (Vθ″) is found to have robust effects on the radial mode structure of the mode. For a positive value of the curvature [usually found in the high-(H) mode edges] the radial mode envelope, similar to the sheared slab case, becomes fully outgoing. On the other hand, for a negative value of the curvature [usually observed in the low-(L) mode edges] the characteristics of conventional drift waves return back. The radial mode envelope reduces to a localized Gaussian shape and the mode can be unstable again for typical (magnetic) shear values in tokamaks. Velocity shear (Vθ′) on the other hand is found to have a rather insignificant role both in determining the radial mode structure and the stability.
Quasi-helical magnetohydrodynamic equilibria in the presence of flow
Authors: Tessarotto M. , Johnson J. L. , White R. B. , Zheng L.-J.
Published: July 2002

By investigating the conservation laws of guiding-center mechanics, an extension of quasi-helical static magnetohydrodynamic equilibria to incorporate strong flow is obtained, subject to suitable ordering assumptions. Such equilibria correspond to self-consistent Fokker-Planck kinetic equilibria in the presence of a quasi-helical magnetic field. The effect of small symmetry-breaking contributions in the magnetic field is investigated.
Perpendicular ion acceleration by localized high frequency electric fields in magnetized plasmas
Authors: Reitzel K. J. , Morales G. J.
Published: July 2002

A basic process capable of explaining observations of fast perpendicular ions in a wide range of plasma environments is described. Spatial symmetry breaking perpendicular to the confining magnetic field is shown to cause irreversible energy gain for ions gyrating through an electric field having a nonuniform amplitude. The efficiency depends on the ratio of the ion Larmor radius to the scale length of the amplitude gradient, and on the scaled frequency ν≡ω/Ωi. A Landau resonance is not required, and there is no lower threshold on the electric field, because the mechanism is active in the linear regime. Theory, numerics, and particle-in-cell simulations are used to illustrate the interaction for electrostatic fields in the lower-hybrid range of frequencies, but the process does not depend on a particular type of mode.
The radial structure of a magnetically confined surface-wave plasma column
Authors: Pérès I. , Fortin M. , Margot J.
Published: July 2002

The radial structure of a steady-state surface-wave-sustained cylindrical argon plasma submitted to a static, axial magnetic field is described in the context of a hydrodynamic model using three-moment equations for electrons and two-moment equations for ions. This plasma model is coupled self-consistently to Maxwell’s equations and yields the radial profile of the electron density and temperature, as well as the radial distribution of excited species, in the 3p56d orbital configuration of argon. In this paper, the discussion focuses on the radial structure of the plasma as a function of the operating conditions (magnetic field intensity, gas pressure, wave frequency, plasma tube radius). It is found that the electron density profile is, generally, weakly modified, as these parameters are changed. In contrast, the electron temperature profile and, consequently, the excited atom density distribution are very sensitive functions of the operating conditions.
Computer experiments on dynamical cloud and space time fluctuations in one-dimensional meta-equilibrium plasmas
Authors: Rouet J. L. , Feix M. R.
Published: July 2002

The test particle picture is a central theory of weakly correlated plasma. While experiments and computer experiments have confirmed the validity of this theory at thermal equilibrium, the extension to meta-equilibrium distributions presents interesting and intriguing points connected to the under or over-population of the tail of these distributions (high velocity) which have not yet been tested. Moreover, the general dynamical Debye cloud (which is a generalization of the static Debye cloud supposing a plasma at thermal equilibrium and a test particle of zero velocity) for any test particle velocity and three typical velocity distributions (equilibrium plus two meta-equilibriums) are presented. The simulations deal with a one-dimensional two-component plasma and, moreover, the relevance of the check for real three-dimensional plasma is outlined. Two kinds of results are presented: the dynamical cloud itself and the more usual density (or energy) fluctuation spectrums. Special attention is paid to the behavior of long wavelengths which needs long systems with very small graininess effects and, consequently, sizable computation efforts. Finally, the divergence or absence of energy in the small wave numbers connected to the excess or lack of fast particles of the two above mentioned meta-equilibrium is exhibited.
Weakly relativistic solitons in a cold plasma with electron inertia
Authors: Kalita B. C. , Barman S. N. , Goswami G.
Published: July 2002

Ion-acoustic solitons have been investigated in a cold plasma in the presence of electron inertia through the derivation of the Korteweg–de Vries (KdV) equation taking into account of weakly relativistic effects. Interestingly, relativistic solitons of both compressive and rarefactive characters are found to exist at the negligible difference of u0/c and v0/c (u0, v0 being the initial speeds of streaming electrons and ions respectively, and c, the velocity of light) of the order 1×10−7.
Asymptotics of the modified plasma dispersion function generalized to real kappa
Authors: Grabbe Crockett , Venturino Ezio
Published: July 2002

The modified plasma dispersion function (MPDF), an extension to κ particle distributions of the standard plasma dispersion function (PDF), has been recently generalized to real κ. An analysis of the general form of the MPDF is made to obtain its asymptotic solution for real κ≥2. A recursion relation is derived and applied to calculate the asymptotic expression directly in terms of the PDF. A numerical analysis is made of its error relative to the exact value for a range of integer values of κ from 2 to 5, and the error is found to typically be about 8% for κ=2, 2% for κ=3, 0.4% for κ=4, and 0.2% for κ=5. The precision increases as κ increases, and generally the first four to five terms of the expansion give accurate results. Potential applications for analyzing processes in a variety of space and astrophysical plasmas are discussed.
Plasma diagnostics using synchrotron radiation in tokamaks
Authors: Fidone I. , Giruzzi G. , Granata G.
Published: July 2002

Emission of synchrotron radiation for electron temperature diagnostics in dense and hot tokamak plasmas is discussed. This novel diagnostic scheme is motivated by the need to overcome several limitations of the familiar first and second harmonic method, caused by cutoff, refraction, and harmonic overlap. Emission at high harmonics is not restricted by density and temperature upper limits, and the method is then particularly appropriate for reactor relevant regimes. This method yields global information on the electron temperature profile, since the source of high harmonic emission is poorly localized in space. Synchrotron radiation is emitted by electrons over a wide spectrum of energies, ranging from thermal to superthermal values, and is therefore also useful to investigate deviations of the electron velocity distribution from the Maxwellian. In contrast, the source of the optically thick low harmonic radiation is highly localized in the ordinary space near the resonance points, which implies that the energy of the emitting electrons lies in the subthermal range. The basic theory of the method is presented and illustrated by numerical applications, for plasma parameters of relevance for International Thermonuclear Experimental Reactor (ITER) [ITER-JCT and Home Teams, Plasma Phys. Controlled Fusion 37, A19 (1995)].
High-mode-number ballooning modes in a heliotron/torsatron system. II. Stability
Authors: Nakajima N.
Published: July 2002

As described in the companion paper [N. Nakajima, Phys. Plasmas 3, 4545 (1996)], in heliotron/torsatron systems that have a large Shafranov shift, the local magnetic shear is found to have no stabilizing effect on high-mode-number ballooning modes at the outer side of the torus, even in the region where the global shear is stellarator-like in nature. The disappearance of this stabilization, in combination with the compression of the flux surfaces at the outer side of the torus, leads at relatively low values of the plasma pressure to significant modifications of the stabilizing effect due to magnetic field-line bending on high-mode-number ballooning modes—specifically, that the field-line bending stabilization can be remarkably suppressed or enhanced. In an equilibrium that is slightly Mercier-unstable or completely Mercier-stable due to peaked pressure profiles, such as those used in standard stability calculations, high-mode-number ballooning modes are destabilized due to these modified stability effects, with their eigenfunctions highly localized along the field line. Highly localized mode structures such as these cause the ballooning mode eigenvalues ω2 to have a strong field line dependence (i.e., α-variation) through the strong dependence of the local magnetic curvature, such that the level surfaces of ω2(ψ,θk,α) (≤0) become spheroids in (ψ,θk,α) space, where ψ labels flux surfaces and θk is the radial wave number. Because the spheroidal level surfaces for unstable eigenvalues are surrounded by level surfaces for stable eigenvalues of high-mode-number toroidal Alfvén eigenmodes, those high-mode-number ballooning modes never lead to low-mode-number modes. In configuration space, these high-mode-number modes are localized in a single toroidal pitch of the helical coils, and hence they may experience substantial stabilization due to finite Larmor radius effects.
Hall and two-temperature magnetohydrodynamic simulation of deuterium-fiber-initiated Z pinches
Authors: Sheehey Peter , Lindemuth Irvin
Published: July 2002

Two-dimensional “cold-start” resistive magnetohydrodynamic computations of formation and evolution of deuterium-fiber-initiated Z pinches have been extended to include separate ion and electron energy equations and some finite-Larmor-radius ordered terms. In the Ohm’s law (magnetic field evolution) equation, Hall and diamagnetic pressure terms have been added, and corresponding terms have been added to the energy equation. None of the extended model computations show stabilizing effects for fiber-initiated Z pinches; in fact, further slight destabilization is noted. This continues the good agreement shown between previous computational results and experiment.
High β produced by neutral beam injection in the START (Small Tight Aspect Ratio Tokamak) spherical tokamak
Authors: Sykes Alan
Published: July 2002

The world’s first high-power auxiliary heating experiments in a tight aspect ratio (or spherical) tokamak have been performed on the Small Tight Aspect Ratio Tokomak (START) device [Sykes et al., Nucl. Fusion 32, 694 (1992)] at Culham Laboratory, using the 40 keV, 0.5 MW Neutral Beam Injector loaned by the Oak Ridge National Laboratory. Injection has been mainly of hydrogen into hydrogen or deuterium target plasmas, with a one-day campaign to explore D→D operation. In each case injection provides a combination of higher density operation and effective heating of both ions and electrons. The highest β values achieved to date in START are volume average βT∼11.5% and central beta βO∼50%. Already high, these values are expected to increase further with the use of higher beam power.
Laboratory studies of field-aligned density striations and their relationship to auroral processes
Authors: Maggs J. E. , Morales G. J. , Gekelman W.
Published: July 2002

Magnetic field-aligned structures in current, density, and temperature are common features of the auroral ionospheric plasma. These structures both generate and transform low frequency waves in the plasma. The results of laboratory studies of two processes involving magnetic field-aligned density depletions (striations) that play a role in auroral plasma dynamics are presented. The first process involves the spontaneous generation of density and magnetic fluctuations at the striation edge. The nature of the fluctuations depends upon the electron plasma beta. At high beta (greater than the electron to ion mass ratio, m/M) the drift Alfvén wave is excited. At lower beta the density and magnetic field fluctuations separate and the shear Alfvén wave dominates. This process creates an environment conducive to electron acceleration along the magnetic field when the striation size is on the order of the electron skin depth because the shear Alfvén wave then has a substantial field-aligned electric field. The second process is the direct conversion of electromagnetic whistler waves to electrostatic lower hybrid waves at the striation edge. This process provides a mechanism for concentrating lower hybrid wave energy in the vicinity of striations where it may play a role in electron and ion heating.
X-band magnicon amplifier for the Next Linear Collider
Authors: Gold Steven H. , Fliflet Arne W. , Kinkead Allen K. , Hafizi B. , Nezhevenko Oleg A. , Yakovlev Viacheslav P. , Hirshfield Jay L. , True Richard
Published: July 2002

The magnicon is a scanning-beam microwave amplifier that is being developed as a high power, highly efficient microwave source for use in powering the next generation of high gradient electron linear accelerators. This article first discusses the results from a cold cathode magnicon experiment at 11.12 GHz, driven by a single-shot Marx generator. Following this, a design is presented for a new thermionic magnicon experiment to produce more than 50 MW at 11.4 GHz, using a 210 A, 500 kV beam from an ultrahigh convergence thermionic electron gun driven by a repetition-rated modulator. This new design has a predicted efficiency in excess of 60%.
Observation of the Langmuir decay instability driven by stimulated Raman scattering
Authors: Baker K. L. , Drake R. P. , Bauer B. S. , Estabrook K. G. , Rubenchik A. M. , Labaune C. , Baldis H. A. , Renard N. , Baton S. D. , Schifano E. , Michard A. , Seka W. , Bahr R. E.
Published: July 2002

Thomson scattering was used to measure the Langmuir wave spectrum driven by stimulated Raman scattering. The Thomson scattering signals measured in the experiment showed Langmuir waves with components both parallel and antiparallel to the incident laser’s wave vector, k0. The parallel component was attributed to stimulated Raman scattering. However, the Langmuir waves with components antiparallel to k0, which cannot be explained by stimulated Raman scattering, were attributed to the Langmuir decay instability (LDI). The relative amplitude of the two Langmuir wave features and their angular width supported the conclusion that the Langmuir waves traveling antiparallel to the incident laser’s wave vector were driven by the Langmuir decay instability.
Effect of the flow continuum on magnetohydrodynamic stability
Authors: Bhattacharyya S. N. , Bhattacharjee A.
Published: July 2002

The flow resonance is concealed when the linearized equation of motion in magnetohydrodynamics is written in terms of the Lagrangian displacement vector. Its contribution to the dispersion equation of a rotating cylindrical plasma column, surrounded by a thin resistive wall, is clarified by a simple model calculation, relevant for axisymmetric modes, in the Eulerian representation. Under certain assumptions the flow resonance is shown to damp the resistive wall mode, with the damping proportional to the square of the gradient of vorticity at the resonant surface. It is shown that a flow resonance can stabilize a slightly elliptical plasma, surrounded by a thin resistive wall, against axisymmetric modes.
A variational approach to parametric instabilities in inhomogeneous plasmas IV: The mixed polarization high-frequency instability
Authors: Afeyan Bedros B. , Williams Edward A.
Published: July 2002

The theories of the two plasmon decay (TPD) instability and stimulated Raman scattering (SRS) are unified and presented using a variational formulation. The mixed polarization high-frequency instability is the resulting generalization which has SRS and TPD as special cases. The most unstable mode’s properties are derived and shown as a function of perpendicular wave number. For vanishing wave number we recover the Raman backscattering result and for wave numbers a few times larger than the square root of the homogeneous plasma growth rate normalized to the pump frequency, the two plasmon decay instability modes are recovered. This transition is accompanied by a change of polarization from a purely electromagnetic transversely polarized wave in the case of Raman to a strictly electrostatic or longitudinally polarized wave in the case of TPD, and admixtures in between. Experimental signatures of these modes are given and a method is proposed by which the density scale length and electron temperature would be simultaneously inferred from the width and height of the spectral signature corresponding to these hybrid modes in an ideal experiment.
Observation of the neoclassical current in the Small Heliac Experimental Apparatus (SHEILA)
Authors: Zhang B. C. , Blackwell B. D. , Borg G. G. , Petržı́lka V.
Published: July 2002

The first measurements of internal currents in a toroidal heliac are reported. It is suggested that the toroidal current observed in the Small Heliac Experimental Apparatus (SHEILA) [Nucl. Fusion 25, 1485 (1985)] in the low density regime consists mainly of the bootstrap current and the Pfirsch–Schlüter current, the net toroidal component corresponding to the uni-directional bootstrap current, and the asymmetric radial profile of the observed current indicating a mixture of bootstrap current with the bi-directional Pfirsch–Schlüter current.
Density scaling of the threshold for locked mode instability in the presence of toroidal field ripple in a tokamak
Authors: Lazzaro E. , Campbell D. , Debenedetti M. , Savrukhin P. , Smolyakov A. I.
Published: July 2002

The nonlinear excitation of magnetic islands in a tokamak by external resonant magnetic perturbation is examined. The perturbation amplitude threshold for the appearance of a locked mode is determined for a tokamak with a rippled toroidal field. It is shown that the critical value of the magnetic field is increased by the ripple induced damping of toroidal rotation. Qualitative aspects of the experimentally observed reduced scaling of the threshold density for mode locking, are predicted in the regime of low viscosity and dominant plasma inertial effects. In such a case the scaling is shown to be close to ni∼B̃r1/2 rather than ni∼B̃r3/2 as predicted by the usual theory.
Intense non-neutral beam propagation in a periodic solenoidal field using a macroscopic fluid model with zero thermal emittance
Authors: Davidson Ronald C. , Stoltz Peter , Chen Chiping
Published: July 2002

A macroscopic fluid model is developed to describe the nonlinear dynamics and collective processes in an intense high-current beam propagating in the z-direction through a periodic focusing solenoidal field Bz(z+S)=Bz(z), where S is the axial periodicity length. The analysis assumes that space-charge effects dominate the effects of thermal beam emittance, Krb2≫εth2, and is based on the macroscopic moment-Maxwell equations, truncated by neglecting the pressure tensor and higher-order moments. Here, K=2NbZi2e2/γ̂b3mβb2c2 is the self-field perveance, Nb is the number of particles per unit axial length, and rb is the characteristic beam radius. Assuming a thin beam with rb≪S, azimuthally symmetric beam equilibria with ∂/∂t=0=∂/∂θ are investigated, allowing for an axial modulation of the beam density nb(r,z) and macroscopic flow velocity Vrb(r,z)êr+Vθb(r,z)êθ+Vzb(r,z)êz by the periodic focusing field. To illustrate the considerable flexibility of the macroscopic formalism, assuming (nearly) uniform axial flow velocity Vb over the beam cross section, beam equilibrium properties are calculated for two examples: (a) uniform radial density profile over the interval 0⩽r<rb(z), and (b) an infinitesimally thin annular beam centered at r=rb(z). The analysis generally allows for the azimuthal flow velocity Vθb(r,z) to differ from the Larmor frequency, and the model is used to calculate the (leading-order) correction δVzb(r,z) to the axial flow velocity for the step-function density profile in case (a) above.
Stimulated Brillouin scattering in long-scale-length laser plasmas
Authors: Chirokikh A. , Seka W. , Simon A. , Craxton R. S. , Tikhonchuk V. T.
Published: July 2002

Brillouin scattering from a preformed, inhomogeneous, expanding plasma has been investigated. Backscattered light near the incident laser wavelength (λ=1054 nm) from CH planar targets has been spectrally and temporally resolved. By varying the time delay of the interaction beam, the scattering was studied for different plasma conditions. The backscattered light is predominantly blue-shifted and appears before the peak of the laser pulse. The experimental time-integrated reflectivity of backscattered light is in the range of 1%–10% and decreases with the plasma density. The time-resolved spectra and total reflectivity were calculated using a theory of convective stimulated Brillouin scattering (SBS) in a flowing inhomogeneous plasma combined with a statistical hot spot model for the interaction beam. The plasma parameters for these calculations were provided by simulations using a two-dimensional hydrodynamic code. The calculated SBS spectra are similar to the experimental observations. The time-integrated reflectivities agree well with the experimental results for the higher peak density interactions, but are below the observations by orders of magnitude for the lowest peak density cases.
Radiation environments produced by plasma z-pinch stagnation on central targets
Authors: Brownell J. H. , Bowers R. L. , McLenithan K. D. , Peterson D. L.
Published: July 2002

A goal of pulsed-power technology is the development of an intense, megajoule level source of soft x rays for use in high-energy density physics experiments. Experimental facilities, theoretical concepts, computational tools, and diagnostics that have been developed since 1980 place pulsed power at the threshold of performing experiments of great interest to the applied physics community. In this paper the “Flying Radiation Case” approach will be presented and its predicted performance on Sandia National Laboratory’s Z-Machine [M. K. Matzen, Phys. Plasmas 4, 1519 (1997)] will be described. The effects of instability growth in the plasma during the implosion, its reassembly on a central cushion, and the plasma interactions with shaped electrodes are considered.
Destabilization of poloidal plasma rotation during electron cyclotron resonance heating in tokamaks
Authors: Liu Caigen , Qian Shangjie , Wan Huaming
Published: July 2002

The poloidal plasma rotation destabilized by electron cyclotron resonance heating (ECRH) is proposed in both collision and banana regions of tokamak plasmas. The poloidal electric field, which results in poloidally inhomogeneous ion accumulation, can be produced by ECRH due to either the resonance localization or the asymmetric rf power deposition. When the ion accumulation rate overwhelms the damping rate of the ion–ion collision on the poloidal plasma rotation, the poloidal plasma rotation will be destabilized. Based on the linearized drift kinetic equation and the fluid equations, including a model term of the density perturbation driven by rf waves, the criterion of the destabilization of the poloidal plasma rotation is analytically derived. It is shown that the poloidal plasma rotation in tokamaks can be generated by ECRH in the practical rf power level by this manner.