bluuebunny/crossref_metadata_2025 huggingface dataset:
Shock wave–thermal inhomogeneity interactions: Analysis and numerical simulations of sound generation
Authors: Grasso F. , Pirozzoli S.
Published: July 2002

In the present paper the interaction of a moving planar shock wave with a cylindrical thermal inhomogeneity is analyzed. Due to the interaction, both vorticity and sound waves are generated. The main focus of the paper is on the generation and propagation of the acoustic waves and their dependence on shock strength and inhomogeneity intensity. An acoustic analogy of the problem has been formulated as a basis of comparison for the structure of the acoustic pressure field in the limit of either weak shocks or weak inhomogeneities. An extensive numerical study has been carried out in order to understand the influence of both shock strength and intensity of the inhomogeneity. The results show that the acoustic field obeys a two-stage evolution that occurs on a time scale that is independent of shock and inhomogeneity intensities. The comparison of the computed far-field pressure with the theoretical result shows that for weak inhomogeneities the sound is well predicted by the acoustic analogy regardless of the shock strength.
Approximations for turbulent energy and temperature variance dissipation rates in grid turbulence
Authors: Zhou T. , Antonia R. A.
Published: July 2002

Using a combination of a transverse vorticity (ω3) probe and a pair of parallel cold wires, simultaneous approximations, denoted as εap and εθap, to the energy dissipation rate ε and the temperature variance dissipation rate εθ, are obtained in decaying grid turbulence at a Taylor microscale Reynolds number of about 52. While there are important differences between the spectra of either εap or εθap and those of their isotropic counterparts ε′ and εθ′, the correlation between εap and εθap is as small as that between ε′ and εθ′. The large discrepancies, which exist in the literature, for the correlation coefficient between the locally averaged values of ε and εθ reflect a dependence on the flow type as well as on the Reynolds number. Whereas εap is strongly correlated with ω32, the correlation between ε′ and ω32 is weak. The correlation between εθap and ω32 is comparable to that between εθ′ and ω32. The effect different choices of ε and εθ have on the refined similarity hypothesis (RSH) (Kolmogorov, 1962) and its extension (RSHP) to a passive scalar is also examined. By reference to a nearly complete ε obtained with a three-component vorticity probe, RSH is more closely satisfied by εap than ε′. In contrast, RSHP appears to be approximately satisfied, regardless of which approximations are used for ε and εθ.
Prediction of turbulent free shear flow statistics using a simple stochastic model
Authors: Kerstein Alan R. , Dreeben Thomas D.
Published: July 2002

One-dimensional turbulence, a stochastic simulation of turbulent flow evolution based on application of a mixing-length-type hypothesis to individual turbulent eddies, is used to predict transverse profiles of single-point statistics up to third order for two time-developing planar free shear flows, a mixing layer and a wake. Comparison of computed results to statistics obtained from direct numerical simulations of these flows indicates that the model, despite its simplicity, captures important features of turbulent free shear flow structure. Implications concerning the possible universality of some aspects of turbulent shear flow are discussed.
Effect of a nonuniform resistive wall on the stability of tokamak plasmas
Authors: Fitzpatrick Richard
Published: July 2002

The effect of a nonuniform resistive wall on the stability of plasma magnetohydrodynamical (MHD) modes is examined. For the case of a tokamak plasma interacting with a wall possessing toroidally nonuniform electrical resistance, the kink mode dispersion relation is found to reduce to a surprisingly simple form, provided that the scale of variation of the resistance is sufficiently large. The influence of a wall with toroidal gaps on tokamak plasma stability is investigated in some detail. Under some circumstances, kink modes are found to ‘‘explode’’ through the gaps with ideal growth rates. A similar investigation is made for a modular wall constructed of alternate thick and thin sections.
Evaluation of the Coulomb logarithm using cutoff and screened Coulomb interaction potentials
Authors: Ordonez C. A. , Molina M. I.
Published: July 2002

The Coulomb logarithm is a fundamental plasma parameter which is commonly derived within the framework of the binary collision approximation. The conventional formula for the Coulomb logarithm, λ=ln Λ, takes into account a pure Coulomb interaction potential for binary collisions and is not accurate at small values (λ<10). However, a more exact Fokker–Planck equation was recently presented by Li and Petrasso which is accurate at small Coulomb logarithm values (λ≳2) [Phys. Rev. Lett. 70, 3063 (1993)]. This theory and computer simulations which are accurate for small Coulomb logarithm values provide the motivation for a more precise evaluation of the Coulomb logarithm. In the present work, the Coulomb logarithm is evaluated more precisely by using a cutoff Coulomb interaction potential. The result is compared to an exact numerical evaluation of the Coulomb logarithm which considers a screened Coulomb interaction potential. Fits to the numerical results are also provided. The fitted formula λ=ln(0.6 Λ) is recommended for most applications providing values within 4% of the exact numerical values for λ≳2. This formula is easily implemented by using 0.6λD in place of λD (the Debye length) in the conventional formula for the Coulomb logarithm.
Probabilistic approach to Monte Carlo operators
Authors: Tessarotto Massimo , White Roscoe B. , Zheng Lin-Jin
Published: July 2002

A general method for constructing Monte Carlo collision operators is formulated based on the introduction of a stochastic Liouville equation. The approach is applied to the investigation of a suitable discretized gyrokinetic equation describing the dynamics of a strongly rotating multispecies plasma, in a toroidally axisymmetric configuration. Improved expressions are obtained for the Monte Carlo collision operators for general non-normal (in v space) coordinate systems. As an application, the important case of the bounce-averaged gyrokinetic equation is discussed, in various cases of interest. Finally, using an approximate collision operator, the friction coefficients are evaluated and expressed in terms of energy and momentum restoring coefficients, thus yielding for the Monte Carlo operators a representation convenient for their numerical implementation.
Finite-β modification of the ion-temperature-gradient-driven instability in a sheared slab geometry
Authors: Reynders John V. W.
Published: July 2002

A coupled system of linear integral equations are derived from the electromagnetic gyrokinetic equations and solved in straight and sheared slab geometries. Simulation results of a finite-β modified ion-temperature-gradient-driven instability in a sheared geometry are presented. It is shown for typical tokamak parameters that the fundamental l=0 slab mode can be completely stabilized at β≊1% while the l=1 slab mode persists.
A limited closure relation for anisotropic plasmas from the Earth’s magnetosheath*
Authors: Gary S. Peter , Anderson Brian J. , Denton Richard E. , Fuselier Stephen A. , McKean Michael E.
Published: July 2002

The ions of the terrestrial magnetosheath are typically observed to have bi-Maxwellian velocity distributions with T⊥≳T∥, where ⊥ and ∥ denote directions perpendicular and parallel to the background magnetic field. Observations of the highly compressed magnetosheath have demonstrated an inverse correlation between the proton temperature anisotropy and the proton parallel β. Hybrid computer simulations have shown that wave–particle scattering by the proton cyclotron anisotropy instability can yield similar correlations between these two parameters. Although these correlations are due to nonlinear plasma processes, they correspond to a threshold of this instability that can be determined from linear Vlasov theory. This proton anisotropy/β correlation represents a limited closure relation for anisotropic magnetohydrodynamic plasma models.
Plasma sheaths near a negatively step biased electrode—Dynamic behavior for planar and spherical geometry
Authors: Skorik M. A. , Hershkowitz N.
Published: July 2002

A numerical solution of fluid equations coupled with Poisson’s equation is used to study the temporal evolution of plasma sheaths near a negatively biased electrode. Special attention is paid to long-time behavior of the sheath and to mechanisms of sheath stabilization in a collisionless plasma. The crucial influence of the system geometry on the sheath behavior and the asymptotic potential profile are established. Dynamic structures in plasma and geometrical factors are found to provide presheath ion acceleration which determine the conditions at the sheath boundary and play an important role in sheath stabilization.
Three-dimensional particle-in-cell simulations of applied-B ion diodes
Authors: Pointon T. D. , Desjarlais M. P. , Seidel D. B. , Slutz S. A. , Coats R. S. , Kiefer M. L. , Quintenz J. P.
Published: July 2002

The three-dimensional particle-in-cell code quicksilver [Seidel et al., Computational Physics, edited by A. Tenner (World Scientific, Singapore, 1991), p. 475] has been used to study applied-B ion diodes. The impedance behavior of the diode in these simulations is in good agreement with both analytic theory and experiments at peak power. The simulations also demonstrate the existence of electromagnetic instabilities which induce divergence in the ion beam. Early in time, there is an instability at high frequency relative to the ion transit time τi, and the resulting beam divergence is low. However, later in time, the system makes a transition to an instability with a frequency close to 1/τi, and the ion beam divergence rises to an unacceptably high value. The transition is associated with the build-up of electron space charge in the diode, and the resulting increase in the beam current density enhancement (J/JCL). Using different schemes to inhibit the electron evolution, the transition has both been postponed and permanently eliminated, resulting in Li+1 ion beams with a sustained divergence of ∼10 mrad at an energy of ∼10 MeV.
On the Alfvén resonance and its existence
Authors: Rauf S. , Tataronis J. A.
Published: July 2002

In this Brief Communication, the continuous spectra of ideal magnetohydrodynamics are revisited in order to point out and discuss equilibrium configurations in which the Alfvén continuum is characterized by a nonlogarithmic spatial singularity in the velocity fluctuation of the plasma. This occurs in a compressible sheet pinch with a shearless equilibrium magnetic field directed parallel to a specified magnetic surface wave vector. It is shown that the component of the velocity vector parallel to the direction of inhomogeneity is bounded at the Alfvén resonance layers. The velocity vector possesses spatial singularities, but the singularities are restricted to its component perpendicular to both the direction of the spatial inhomogeneity and the equilibrium magnetic field. The singularities are found to be delta functions. Comments are also made on the Alfvén resonance in the incompressible plasma and its relationship to the continuous spectra of the compressible plasma.
Power deposition in high-density inductively coupled plasma tools for semiconductor processing
Authors: Jaeger E. F. , Berry L. A. , Tolliver J. S. , Batchelor D. B.
Published: July 2002

A two-dimensional, computationally efficient numerical model is developed to study power deposition in high-density inductively coupled plasma sources. Calculations include both inductive coupling, caused by plasma response to external coil currents, and capacitive coupling, caused by plasma response to external voltages on the coils and wafer. Variation in current along the induction coil is determined self-consistently from the integral constraint of charge conservation. Sheath phenomena are incorporated through previously published analytic models. The system behavior is analogous in some respects to that of a transmission line. Comparison with measurement suggests that this model provides a good description of self-consistent coil response when the electric field exhibits less than a quarter wavelength per coil turn.
Stability of toroidal drift-type waves in the presence of a velocity field
Authors: Sen S.
Published: July 2002

The instability associated with the toroidal drift-type wave is shown to be stabilized by a radially varying equilibrium poloidal velocity field of suitable magnitude and curvature. The radial mode envelope is now much more extended than its conventional localized Gaussian form. The role of velocity curvature (V(r)θ″) is found to have robust effects on the stability and radial structure of the mode. Velocity shear (V(r)θ′) on the other hand is found to have a rather insignificant role in this matter.
Stability and radiative performance of structured Z-pinch loads imploded on high-current pulsed power generators
Authors: Cochran F. L. , Davis J. , Velikovich A. L.
Published: July 2002

The stability and radiative performance of structured Z-pinch plasma loads heated by high-current (≳20 MA) pulsed power generators are investigated. A limited mapping of parameter space is made for the regions of stability for loads configured as thin shells, uniform fills, and multiple shells. Although large diameter thin shell loads are shown to be the most efficient radiators of K-shell x rays, they are susceptible to disruption by the Rayleigh–Taylor instability. Large diameter uniform fill loads are shown to be more stable and very good radiators.
Effects of secondary electrons due to ionization on model predictions of discharge-pumped XeCl laser
Authors: Zhu Qifeng , Imada Go , Masuda Wataru , Yatsui Kiyoshi
Published: July 2002

A self-consistent code has been utilized in modeling a discharge-pumped XeCl laser. The electron energy distribution function (EEDF) has been calculated using a time-dependent Boltzmann equation. The effects of the secondary electrons produced by ionization on EEDF, rate coefficients, species densities, laser energy, and breakdown delay time have been examined by comparing different models in a wide range of discharge parameters. Efforts have been made at explaining the discrepancy in the breakdown delay time between simulation and experiment. It is found that the secondary electrons due to ionization play an important role in determining the breakdown delay time. Furthermore, the measured breakdown delay time can be well reproduced using the model presented in this work instead of increasing the total excitation cross sections of xenon.
Passive cyclotron current generation with fish-scale structures
Authors: Kernbichler W. , Kasilov S. V.
Published: July 2002

The paper presents a theoretical and a numerical treatment of the problem of passive current drive in a fusion plasma. In such a scenario the current is driven by the self-generated cyclotron radiation. The necessary asymmetry is introduced with the help of fish-scale structures on the reflecting wall which surrounds the plasma. The problem of the photon equilibrium in a bounded system is the subject of the first part of this paper. The general case of a plasma surrounded by a reflecting wall is presented in the form of a Fredholm integral equation of the second kind. This integral equation is then simplified for small fish-scale-like structures on the first wall with the help of a probabilistic approach. A self-consistent solution for the radiation intensity distribution is obtained numerically for a cylindrical plasma model under such boundary conditions. Based on this and on local current drive efficiencies derived in a companion paper [S. V. Kasilov and W. Kernbichler, Phys. Plasmas, 3, 4115 (1996)], the radial profile of a steady toroidal current as well as overall efficiencies are computed.
Effects of plasma long-wavelength hydrodynamical fluctuations on stimulated Brillouin scattering
Authors: Maximov A. V. , Rozmus W. , Tikhonchuk V. T. , DuBois D. F. , Rose H. A. , Rubenchik A. M.
Published: July 2002

Two important effects of long wavelength velocity and density perturbations on stimulated Brillouin scattering (SBS) have been identified: (i) nonlinear interaction of the SBS-produced ion acoustic wave with velocity perturbations provides detuning of the SBS interaction, formation of ion acoustic wave satellites and its additional damping; (ii) harmonic generation of long wavelength density perturbations enhances the spectral density of ion acoustic fluctuations over a wide range of wavevectors, including SBS resonant sound waves. The first effect reduces the magnitude of the SBS gain, and the second one enhances the nonthermal noise level for SBS. Long wavelength ion acoustic perturbations reduce SBS reflectivity more dramatically than short wavelength harmonics produced by the SBS driven nonlinear sound wave.
Transport and sawtooth oscillations from rotational pumping of a magnetized electron plasma
Authors: Cluggish B. P. , Driscoll C. F.
Published: July 2002

Definitive measurements have been made of cross-field transport from ‘‘rotational pumping’’ of a magnetized electron column. Rotational pumping is the collisional dissipation of the axial compressions that are caused by E×B rotation of the column through asymmetric confining potentials; it is analogous to the magnetic pumping that damps poloidal rotation in tokamaks. The transport rate is measured over a wide range of plasma parameters, including four orders of magnitude in temperature. A new theory by Crooks and O’Neil shows excellent agreement with the measured rates when the three-dimensional plasma end shapes are numerically calculated using the measured charge density profiles and temperatures. When the plasma displacement is destabilized by a resistive wall and damped by rotational pumping, a complex, nonlinear evolution is observed: a quiescent period is followed by ‘‘sawtooth’’ oscillations of displacement and temperature accompanied by ‘‘bursts’’ of radial transport. This behavior is due to the nonmonotonic temperature dependence of the transport rate.
Laser beam deflection by flow and nonlinear self-focusing
Authors: Rose Harvey A.
Published: July 2002

In general, it is found that the plasma density response to a laser beam’s ponderomotive force is shifted in the direction of a transverse flow, which in turn deflects the laser beam in that direction, and retards the flow at a temporal rate which is proportional to the beam’s spatial deflection rate. This rate is a rapidly increasing function of 〈I〉, the average laser intensity. This may result in finite beam deflections after propagation over a distance of a few speckle lengths for random phase plate optics. In current gas filled hohlraum experiments, the estimated magnitude of these effects is consistent with their experimental observation and one may need to include this physics in order to predict beam pointing which results in a symmetric capsule implosion. Flow retardation may be sufficient to exclude flow from the beam, self-limiting the deflection process.
On the conical refraction of hydromagnetic waves in plasma with anisotropic thermal pressure
Authors: Tsiklauri David
Published: July 2002

A phenomenon analogous to the conical refraction widely known in crystalo-optics and crystalo-acoustics is discovered for the magnetohydrodynamical waves in a collisionless plasma with anisotropic thermal pressure. Angle of the conical refraction is calculated for the medium under study which is predicted to be 18°26′. Possible experimental corroboration of the discovered phenomenon is discussed.
Three-dimensional stability of drift vortices
Authors: Åkerstedt H. O. , Nycander J. , Pavlenko V. P.
Published: July 2002

The three-dimensional stability of drift vortices that are two-dimensionally stable in the framework of the Hasegawa–Mima equation is studied. The three-dimensionality is caused by a coupling of the vortex to ion-acoustic waves that propagate along the magnetic field. It is concluded that this coupling does not significantly destabilize the vortex. A general stability criterion is derived that guarantees stability if the ratio between the parallel wave number k and the azimuthal mode number m is large enough. It is also found that no instability exists in the limit k→0 if the potential vorticity is a strictly decreasing function of r. Such profiles are typical for vortices that are two-dimensionally stable. Some particular profiles where the potential vorticity vanishes outside some radius are also examined. In one case an instability with m=1 is found, but a numerical solution of the eigenvalue problem shows that the maximum growth rate is very small, three orders of magnitude smaller than the angular velocity of the vortex flow.
Comparison of two resistive ballooning mode models in transport simulations
Authors: Kinsey Jon E. , Bateman Glenn , Kritz Arnold H. , Redd Aaron
Published: July 2002

Predictive transport simulations of the temperature and density profiles have been carried out for Tokamak Fusion Test Reactor (TFTR) [K. Young et al., Plasma Phys. Controlled Fusion 26, 11 (1984)] current, density, and heating power scans. Two competing resistive ballooning mode theories are considered in order to examine their intrinsic magnetic-q dependence. The theoretically derived transport model employed in this study includes drift wave contributions from the Weiland theory of trapped electron and ion temperature gradient modes, the Kwon–Biglari–Diamond neoclassical magnetohydrodynamic (MHD) theory, the Tang–Rewoldt kinetic ballooning mode theory, and either the previously used Carreras–Diamond or the recently developed Guzdar–Drake resistive ballooning mode theories. It is found that the Guzdar–Drake theory provides the correct scaling with plasma current while maintaining a scaling with density and auxiliary heating power that is consistent with experimental data from TFTR low confinement (L-mode) plasmas. A statistical analysis of the profile results for the current scan is included to give quantitative measures of how well simulations that include either the Guzdar–Drake or the Carreras–Diamond theory compare with the experimental data.
Current drive in rotating plasmas
Authors: Avinash K.
Published: July 2002

Current drive due to plane polarized waves in rotating plasmas is studied. Low frequency compressional waves are considered. It is shown that, in this case, helicity is spontaneously generated within the plasma and that one can get an efficiency which is as good as that due to circularly polarized waves.
Reduced fractional absorption and second harmonic emission in laser-produced plasmas
Authors: Ruhl H. , Cairns R. A.
Published: July 2002

Both particle-in-cell and Vlasov simulations of the collective absorption of p-polarized intense laser light incident on a plasma with a steep density profile show the fractional absorption decreasing with intensity for large angles of incidence. Here an analytical model is developed that indicates that this effect for a high-temperature plasma (Te⩾2.5 keV) is connected with the conversion of a large fraction of the incident energy into harmonic emission. Good agreement between the results of Vlasov simulations and those of the nonlinear analytical model are obtained.
Resistive wall feedback stabilization
Authors: Jensen T. H. , Fitzpatrick R.
Published: July 2002

A feedback system, which essentially makes a resistive wall appear ideally conducting, is discussed. Such a system applied to a resistive wall surrounding a plasma will stabilize certain modes which would be unstable in the absence of the feedback system. The system discussed is similar to the “intelligent shell” by Bishop [Plasma Phys. Controlled Fusion 31, 1179 (1989)]; it utilizes a number of autonomous subsystems, each covering only a fraction of the resistive wall. A model example discussed suggests that only relatively few autonomous subsystems are needed and that the requirements of the electronics appear modest.
Nonlinear density expulsion and electron heating in the nonadiabatic regime, and application to Ion Bernstein Wave heating
Authors: Myra J. R. , D’Ippolito D. A.
Published: July 2002

In the limit where a strong parallel electric field has short parallel scale lengths, the parallel electron motion becomes nonadiabatic and highly nonlinear, and the usual ponderomotive treatment of the slow time scale behavior of electrons is invalid. Here, a new nonadiabatic model for describing the resulting heating and expulsion of electrons from regions of a strong electric field is developed. The model shows that a typical electron is heated to a value characterized by the “quiver” velocity in the applied field. A nonlinear density expulsion still occurs in this nonadiabatic strong rf field limit, but exhibits an algebraic dependence on the wave amplitude in contrast to the exponential dependence that occurs in conventional ponderomotive theory. Results are applied to electrons in the edge plasma, near a high-power Ion Bernstein Wave heating antenna.
Parametric instability producing broad symmetrical structure in the spectrum of ionospheric heating-induced radiation
Authors: Kuo S. P.
Published: July 2002

A four-wave interaction process in which an O-mode electromagnetic pump decays parametrically into a lower hybrid decay mode and two-electron Bernstein sidebands is analyzed. It is shown that the instability can be excited in a spatial region near the electron Bernstein/upper hybrid double resonance and in a narrow pump frequency range slightly below the third harmonic electron cyclotron resonance. The two electron Bernstein sidebands have about the same intensity and thus, produce Broad Symmetrical Structure (BSS) in the emission spectrum after being converted into electromagnetic radiation by scattering off background field-aligned density irregularities. The results also show that the size of the instability zone becomes very small as the pump frequency operates near a cyclotron harmonic higher than the third. Thus, the converted emission will be too weak to be detected. This explains why the BSS feature in the spectrum of stimulated electromagnetic emissions (SEEs) has only been observed in the third harmonic case.
Microinstability properties of negative magnetic shear discharges in the Tokamak Fusion Test Reactor and DIII-D
Authors: Rewoldt G. , Lao L. L. , Tang W. M.
Published: July 2002

The microinstability properties of discharges with negative (reversed) magnetic shear in the Tokamak Fusion Test Reactor (TFTR) [R. J. Hawryluk et al., Plasma Physics and Controlled Nuclear Fusion Research, 1994 (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 11] and DIII-D [R. D. Stambaugh for the DIII-D Team, Plasma Physics and Controlled Nuclear Fusion Research, 1994 (International Atomic Energy Agency, Vienna, 1995), Vol. 1, p. 83] experiments with and without confinement transitions are investigated. A comprehensive kinetic linear eigenmode calculation employing the ballooning representation is employed with experimentally measured profile data, and using the corresponding numerically computed magnetohydrodynamic (MHD) equilibria. The instability considered is the toroidal drift mode (trapped-electron-ηi mode). A variety of physical effects associated with differing q-profiles are explained. In addition, different negative magnetic shear discharges at different times in the discharge for TFTR and DIII-D are analyzed. The effects of sheared toroidal rotation, using data from direct spectroscopic measurements for carbon, are analyzed using comparisons with results from a two-dimensional calculation. Comparisons are also made for nonlinear stabilization associated with shear in Er/RBθ. The relative importance of changes in different profiles (density, temperature, q, rotation, etc.) on the linear growth rates is considered.
Spatiotemporal evolution of parametric instabilities driven by short laser pulses: Two-dimensional analysis
Authors: McKinstrie C. J. , Turano E. J.
Published: July 2002

The spatiotemporal evolution of parametric instabilities such as stimulated Raman scattering is studied analytically in time and two spatial dimensions. Initial and boundary conditions are chosen to represent the entrance, propagation, and exit of a laser pulse of finite extent convecting through a homogeneous collisional plasma with definite boundaries. For most scattering angles the growth is found to be dominated by two-dimensional effects with one-dimensional amplitudes being valid for only a small range of angles corresponding to near forward and near backward scattering. The sideward convection alters the spatiotemporal character of the instability and inhibits but does not suppress its growth.
Effect of a wiggler on two-stream instability
Authors: Sharma Suresh C. , Tripathi V. K.
Published: July 2002

In a plasma-filled free electron laser two-stream instability is a serious consideration. The growth rate of the instability could be significantly reduced by coupling it to the electromagnetic modes/quasimodes through the wiggler. The effect is important near wiggler plasma resonance, where plasma frequency equals the product of wiggler wave number and beam velocity.
On the stability of Mercier and ballooning modes in stellarator configurations
Authors: Hegna C. C. , Nakajima N.
Published: July 2002

The stability properties of pressure-driven ballooning and Mercier modes in general stellarator configurations are studied. A perturbation method originally introduced to study tokamak stability by Greene and Chance [Nucl. Fusion 21, 453 (1981)] is generalized to three-dimensional systems. The effects of profile variation can be examined by introducing self-consistent variations in equilibrium quantities to a localized region whose amplitudes are small but whose cross-field derivative is large. In the general case, the set of equilibria are characterized by two free functions, which can be chosen to be the local variation of pressure and rotational transform profiles (or pressure gradient and the field-line-averaged parallel current). In this way, stability curves for Mercier and ballooning modes in three-dimensional equilibria are generated that are analogous to the s-α curves used in studying tokamak equilibrium.
Burgers turbulence, intermittency, and nonuniversality
Authors: Boldyrev S. A.
Published: July 2002

Recent developments in the theory of the Burgers turbulence, based on the field theoretical approach, are presented. An application to the turbulence with pressure is discussed.
Optimal design of feedback coils for the control of external modes in tokamaks
Authors: Fitzpatrick R. , Yu E. P.
Published: July 2002

A formalism is developed for optimizing the design of feedback coils placed around a tokamak plasma in order to control the resistive shell mode. It is found that feedback schemes for controlling the resistive shell mode fail whenever the distortion of the mode structure by the currents circulating in the feedback coils becomes too strong, in which case the mode escapes through the gaps between the coils, or through the centers of the coils. The main aim of the optimization process is to reduce this distortion by minimizing the coupling of different Fourier harmonics due to the feedback currents. It is possible to define a quantity α0 which parametrizes the strength of the coupling. Feedback fails for α0⩾1. The optimization procedure consists of minimizing α0 subject to practical constraints. If there are very many evenly spaced feedback coils surrounding the plasma in the poloidal direction then the optimization can be performed analytically. Otherwise, the optimization must be performed numerically. The optimal configuration is to have many, large, overlapping coils in the poloidal direction.
On the radial structure of toroidal drift modes in the presence of background flows
Authors: Davydova T. A. , Weiland J.
Published: July 2002

The influence of flow shear and curvature on toroidal drift modes has been studied for the case of strong inhomogeneity where the ballooning mode formalism does not apply. It has been found that flow curvature is often as important as the flow shear and may lead to complete cancellation of magnetic shear damping. The limit of validity of the ballooning mode formalism is explored.
Fast reconnection due to localized anomalous resistivity
Authors: Aparicio J. , Haines M. G. , Hastie R. J. , Wainwright J. P.
Published: July 2002

A three-dimensional resistive magnetohydrodynamic code has been used to model the reconnection process at the m=1, n=1 surface, in periodic cylindrical geometry. Large current densities are expected at this reconnection layer and an enhancement of the transport properties is expected if the local drift speed exceeds a critical velocity, such as some multiple of the local sound speed. This effect is modeled in these simulations by the local enhancement of the resistivity coefficient where the criterion for micro-turbulence is satisfied. It is found that the reconnection times for this type of simulation are comparable to the reconnection times for a plasma where the resistivity is enhanced everywhere, implying that the reconnection is dominated by the local resistivity value and not its gradient. An analytic scaling law of the reconnection rate for the case when the local electron drift velocity is limited to a multiple of the sound speed is presented. This model predicts that when this multiple is (mi/me)1/2, reconnection times are close to experimental values in large tokamaks. Under these conditions, electron inertia and electron viscosity can be shown to be unimportant. The onset of micro-turbulence acts as a trigger for the reconnection process, and partial reconnection can occur if the conditions for micro-turbulence cease.
Collisionless damping of localized plasma waves in laser-produced plasmas and application to stimulated Raman scattering in filaments
Authors: Short R. W. , Simon A.
Published: July 2002

Observations of stimulated Raman scattering (SRS) in laser-produced plasmas often yield results at odds with theoretical predictions. For example, SRS is commonly seen at incident laser intensities below the theoretical threshold, and the spectrum of SRS light often extends to much shorter wavelengths than models predict. To account for these anomalies it is often proposed that SRS is occurring in high-intensity, self-focused light filaments. A serious problem with this model is that plasma wave damping rates estimated on the basis of the usual Landau theory for homogeneous plasmas would seem to rule out this explanation for many cases of interest. Damping rates for plasma waves confined to small-radius filaments, however, could be significantly different than damping rates for plane waves. Using a novel method for calculating transit-time damping, this paper analyzes the collisionless damping of plasma waveguide modes in a cylinder. It is found that the actual damping rates for waveguide modes in a suitable filament model are much less than for the plane waves in a homogeneous plasma producing the same wavelength of SRS emission. Consequently, the filament model remains viable as an explanation of the anomalous SRS observations.
Modes localized to open magnetic field lines
Authors: Hameiri Eliezer
Published: July 2002

A constructive mathematical proof is given for the existence of modes localized to a single magnetic field line, which give rise to ballooning modes and to the Alfvén continuous spectrum. The unity of these two classes of modes is emphasized.
Tokamak turbulence with stochastic field lines
Authors: Beyer P. , Garbet X. , Ghendrih P.
Published: July 2002

Three-dimensional numerical simulations of ballooning turbulence in a tokamak plasma with stochastic magnetic field lines are presented. Three main features are observed. First, the level of pressure fluctuations decreases in the ergodic layer. Second, this is essentially due to a suppression of large scale structures. Finally, the turbulent heat diffusivity does not decrease in the stochastic layer due to an increase of electric field fluctuations. These observations are in agreement with turbulence measurements on Tore Supra [Equipe Tore Supra, Proceedings, 13th International Conference on Plasma Physics and Controlled Nuclear Fusion, Washington, 1990 (International Atomic Energy Agency, Vienna, 1991), Vol. 1, p. 549].
Effects of magnetic shear on electron cyclotron resonance heating in heliotron/torsatron configurations
Authors: Nagasaki K. , Ejiri A. , Mizuuchi T. , Obiki T. , Okada H. , Sano F. , Zushi H. , Besshou S. , Kondo K.
Published: July 2002

Effects of magnetic shear on electron cyclotron resonance heating (ECRH) are studied in heliotron/torsatron configurations. In such configurations, the poloidal magnetic field is comparable to the toroidal magnetic field, and varies spatially along the minor radius, making a strong magnetic shear. When high power millimeter waves are launched into a plasma, it is coupled to propagating modes at the plasma peripheral region. The existence of a transition layer between the core plasma region and the vacuum region, where the magnetic field direction is largely changed, requires accurate polarization control for good single pass absorption. The mode conversion between the propagation modes due to the magnetic shear also affects the launching conditions. The polarization control experiment by using second harmonic ECRH in Heliotron E [T. Obiki, Fusion Technol. 17, 101 (1990)] are compared with the numerical calculation in which one dimensional second order coupled equations are solved. The polarization dependence experimentally measured is in good agreement with the numerical results including the magnetic shear terms.
The geometry and symmetries of magnetohydrodynamic turbulence: Anomalies of spatial periodicity
Authors: Montgomery David C. , Bates Jason W.
Published: July 2002

It has become common to formulate theories and computations of magnetohydrodynamic turbulent effects in rectangular periodic boundary conditions, proceeding by analogy with what is seen as a useful framework for Navier–Stokes fluid turbulence. It is shown here that because of certain features of Maxwell’s equations for electrodynamics, it is inconsistent to invoke three-dimensional, rectangular, periodic boundary conditions and symmetry at the same time that the displacement current is neglected. The difficulty does not arise in the two-dimensional case. In three dimensions, the difficulty can be remedied by a reformulation in cylindrical geometry, imposing symmetry in the azimuthal and axial directions, but not in the radial one; a geometry that is closer to laboratory possibilities than the wholly three-dimensional periodic assumption. The reformulation seems particularly necessary in cases with a net flux of magnetic field and/or electric currents through the system. These cases no longer seem discontinuous from those without net magnetic fluxes or currents. The price paid is a loss of some possibilities for dimensional analysis and identification of similarity variables.
Stable plasma response to a dynamic magnetic excitation
Authors: Riconda C. , De Benedetti M. , Ottaviani M.
Published: July 2002

The asymptotic linear response of the plasma to a finite frequency external magnetic perturbation is analyzed by using the reduced magnetohydrodynamics (MHD) resistive equation with antenna boundary conditions. Different regimes are identified depending on the parameter ω/η1/3k2/3, where ω is the difference between the antenna frequency and the rotation frequency at the rational surface normalized to the poloidal Alfvén time, and η is the normalized resistivity (the inverse of the Lundquist number). The solution for ψ (magnetic flux function) at the rational surface that interpolates the known constant-ψ regime, ω≪η1/3k2/3, and the ideal regime, ω≫η1/3k2/3, is found. In the ideal regime the presence of Alfvén resonances prevents reconnection and island formation at the rational surface. The results of recent experiments in the Joint European Torus JET [P. H. Rebut et al., Nucl. Fusion 25, 1011 (1985)], where the external magnetic field perturbations are produced by the saddle coil system are also presented. These show the presence of a resonant response of the MHD stable plasma when the antenna frequency approaches the rotation frequency of the corresponding rational surface.
Modeling of particle and energy transport in the edge plasma of Alcator C-Mod
Authors: Umansky M. V. , Krasheninnikov S. I. , LaBombard B. , Lipschultz B. , Terry J. L.
Published: July 2002

In the present study recycling and transport in the edge plasma of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] is modeled and analyzed with the multi-fluid code UEDGE [T. D. Rognlien et al., J. Nucl. Mater. 196–198, 347 (1992)]. Matching the experimental plasma density profiles in the scrape-off layer (SOL) requires a spatially dependent effective anomalous diffusion coefficient D⊥ growing rapidly towards the wall. The midplane pressure of neutral gas, Pmid, is a key parameter that reflects the magnitude of anomalous transport of plasma from the core. Recycling of plasma on the main chamber wall appears to be quite significant, especially in the case of high Pmid∼0.3 mTorr when the main wall provides ∼70% of recycling neutrals in the main chamber. In the upper SOL (well above the x point) draining of particles by the poloidal flow is weak and thus the particle balance is predominantly radial. For the radial heat transport it is found that energy flux carried by radial plasma convection and by charge-exchange (CX) neutrals is quite significant in SOL. In the high Pmid case, heat conduction by CX neutrals along with radial heat convection by plasma carries most of the power flux (∼75%) across the last closed flux surface. Even in the low Pmid case, heat conduction by CX neutrals dominates the radial heat flux far out in the SOL.
Magnetosheath model in the Chew–Goldberger–Low approximation
Authors: Pudovkin Mikhail I. , Besser Bruno P. , Lebedeva Valentina V. , Zaitseva Svetlana A. , Meister Claudia-Veronika
Published: July 2002

Parameters of the solar wind plasma and magnetic field in the magnetosheath are calculated for an anisotropic plasma model in the Chew–Goldberger–Low approximation. It is shown that in the case when the energy transfer between the perpendicular and parallel (with respect to the magnetic field) degrees of freedom is absent, the resulting temperature anisotropy may significantly affect the plasma density and magnetic field intensity profiles across the magnetosheath. However, in this case, the value of the temperature anisotropy (the ratio of the perpendicular to the parallel component of the temperature with respect to the magnetic field, T⊥/T‖) becomes unrealistic high. To bring agreement between the model values of the temperature anisotropy and experimental data, the existence of an intensive proton pitch-angle diffusion is assumed. In the case when the temperature anisotropy relaxation time is much smaller than the time taken by the solar wind plasma to move from the bow shock to the magnetopause, one has T⊥/T‖≈1, and the profiles of the magnetic field and plasma density along the subsolar stream line insignificantly differ from the profiles obtained for the isotropic magnetohydrodynamic (MHD) model. In an intermediate case when the relaxation time is of the order of the plasma transport time, the value of T⊥/T‖ may amount to the values observed in the magnetosheath.
On the stability of self-gravitating magnetized dusty plasmas
Authors: Salimullah M. , Shukla P. K.
Published: July 2002

The effects of a homogeneous magnetic field and the plasma nonuniformity on the dispersion relations of various electrostatic waves in self-gravitating magnetized dusty plasmas have been investigated. For this purpose, the kinetic dielectric response functions for the electrons and ions distributions have been used and the dielectric response function for the magnetized dust grains has been derived from the hydrodynamic equations that include the self-gravitational potential. Thus, extremely massive charged dust grains are subjected to both the electromagnetic and gravitational forces. Analytical studies of the dispersion relations in various frequency and wave number regimes reveal that both the magnetic fields and plasma inhomogeneities contribute to the stability of a self-gravitating dusty plasma system. The results of this investigation should be useful in understanding the stability of dusty proto-stars and dusty dark molecular clouds, which are held in strong magnetic fields and equilibrium density gradients.
Drift-Alfvén vortices with finite ion gyroradius and electron inertia effects
Authors: Kuvshinov B. N. , Pegoraro F. , Rem J. , Schep T. J.
Published: July 2002

A two-fluid plasma model is used to analyze drift-Alfvén vortices in a magnetized, inhomogeneous, warm plasma. This low-β model retains the effects of finite electron mass and of finite ion gyroradii. The vortices are described by two potentials: the electrostatic potential and one component of the vector potential. The background plasma is assumed to have locally a linear density profile. Solutions in the form of dipoles, which propagate with constant velocity across a strong, uniform magnetic field, are analyzed. A general dispersion relation between the eigenvalues inside and outside the separatrix is derived. The analysis of this dispersion relation and of the spatial vortex structure leads to a general classification of two-potential vortices. Explicit solutions are presented for dipole vortices in the limit of zero electron inertia where finite gyroradius effects are retained and in the limit of cold ions where finite electron mass is taken into account.
A new view of the solar corona from the transition region and coronal explorer (TRACE)
Authors: Golub L. , Bookbinder J. , DeLuca E. , Karovska M. , Warren H. , Schrijver C. J. , Shine R. , Tarbell T. , Title A. , Wolfson J. , Handy B. , Kankelborg C.
Published: July 2002

The TRACE Observatory is the first solar-observing satellite in the National Aeronautics and Space Administration’s (NASA) Small Explorer series. Launched April 2, 1998, it is providing views of the solar transition region and low corona with unprecedented spatial and temporal resolution. The corona is now seen to be highly filamented, and filled with flows and other dynamic processes. Structure is seen down to the resolution limit of the instrument, while variability and motions are observed at all spatial locations in the solar atmosphere, and on very short time scales. Flares and shock waves are observed, and the formation of long-lived coronal structures, with consequent implications for coronal heating models, has been seen. This overview describes the instrument and presents some preliminary results from the first six months of operation.