bluuebunny/crossref_metadata_2025 huggingface dataset:
Algebraic description of SN-invariant oscillator states
Authors: Floreanini Roberto , Lapointe Luc , Vinet Luc
Published: July 2002

A group-theoretical description of N identical harmonic oscillators on a line is presented. It provides a scheme for labeling the energy eigenstates that are invariant under the permutation group SN and for obtaining the symmetric operators that transform these degenerate eigenfunctions among themselves. The symmetry algebra that these generators form is in general polynomial. The 2- and 3-particle cases are considered in detail. For the simple 2-body problem the invariance algebra is found to be the cubic SU(2) algebra: [J0,J±]=±J±, [J+,J−]=2J0−αJ03. In the 3-body case, the permutational invariant states are characterized with the help of the subgroup chain U(3)⊃U(2)⊃O(2). The labeling and step operators are obtained from determining an integrity basis for the S3 scalar in U(U(3)). Generating functions techniques are used to that end; an eight-dimensional basis is found whose elements span the symmetry algebra of the three identical oscillator problem. These constants of motion are seen to generate a nonlinear algebra whose representation on the symmetric states is provided.
The osp(1,2)-covariant Lagrangian quantization of reducible massive gauge theories
Authors: Geyer B. , Lavrov P. M. , Mülsch D.
Published: July 2002

The osp(1,2)-covariant Lagrangian quantization of irreducible gauge theories is generalized to L-stage reducible theories. The dependence of the generating functional of Green’s functions on the choice of gauge in the massive case is discussed and Ward identities related to osp(1,2) symmetry are given. Massive first-stage theories with closed gauge algebra are studied in detail. The generalization of the Chapline–Manton model and topological Yang–Mills theory to the case of massive fields are considered as examples.
Light-induced optical absorption in the garnet Ca3Mn2Ge3O12
Authors: Bedarev V. A. , Gnatchenko S. L. , Rupp R. A. , Sugg B.
Published: July 2002

Light-induced change in the optical absorption of the garnet Ca3Mn2Ge3O12 has been found. The magnitude of the light-induced optical absorption does not depend on the polarization state of the exciting illumination. The effect persists for a long time at low temperatures and can be attributed to light-induced generation or redistribution of Mn4+ ions in the crystal.
Theory of orientational relaxation in the low-temperature phase of fullerite C60
Authors: Natsik V. D. , Podolskiy A. V.
Published: July 2002

A microscopic extension of the phenomenological model of double-well orientational states, viz., pentagonal and hexagonal configurations of molecules, which is widely used for describing the low-temperature phase of fullerite C60, is proposed. A simple kinetic equation and a set of thermodynamic relations connecting the crystal lattice deformation, the concentration of orientational excitations of molecules, and temperature are derived. Basic physical properties of the low-temperature phase, including orientational glass transition, heat capacity, thermal expansion, rheological properties and damping of elastic vibrations are described on a unified basis. The conclusions of the theory are compared with the experimental data, and empirical estimates are obtained for the parameters of double-well states and the lattice–orientational interaction. It is shown that the large values of thermal expansion and acoustic damping above the orientation glass-transition temperature of fullerite are due to high-intensity lattice–orientational interaction.
Magnetic structure of the ferro–antiferromagnet interface. II. Compensated interface
Authors: Grechnev A. G. , Kovalev A. S.
Published: July 2002

A compensated ferro–antiferromagnet interface is studied in the framework of a discrete classical spin model. The phase diagram is presented for the possible collinear and canted magnetic structures of the interface. The change in magnetization upon a transition from a collinear structure to canted form is studied.
Chaotic motion of an array of vortices above a flat wall
Authors: Conlisk A. T. , Guezennec Y. G. , Elliott G. S.
Published: July 2002

The trajectories of three inviscid point vortices above a flat wall were investigated numerically. The behavior of the trajectories was characterized by detailed power spectra, phase portraits, Poincaré sections, and estimates of the Lyapunov exponent spectrum. For certain initial conditions, the trajectories are regular and confined to a two-torus in phase space. However, chaotic behavior that sets in as one of the vortices is initially placed closer to the unstable point of the system. In the chaotic regime, the system also exhibits large scale intermittency. Analytical expressions for the near- and far-field noise, generated by the motion of the vortices, were also obtained. It was found that in the presence of a flat boundary, this motion does not contribute to any dipole field noise radiation, but is characterized by a quadrupole structure to leading order. The near-field pressure was computed numerically and it was found that the chaotic behavior of the vortex trajectories results in broadband noise.
Stokes problems of multiparticle systems: A numerical method for arbitrary flows
Authors: Tran-Cong T. , Phan-Thien N.
Published: July 2002

A boundary element method (BEM) is developed and used to compute the arbitrary Stokes flow of a system of N particles of arbitrary shape and size embedded in a Newtonian fluid. The particles can be assumed to have a specified deformation (this feature is included here in anticipation of some applications in biophysics). Hydrodynamic and nonhydrodynamic forces are assumed to act in general; Brownian forces are neglected; and the Stokes flow field is assumed to be arbitrary. The method is benchmarked against the exact solution of two equal spheres in free settling motion by Goldman, Cox, and Brenner [Chem. Eng. Sci. 21, 1151 (1966)]. The handling of a finite bounding surface (i.e., arbitrary flow field, container walls) is tested with the calculation of the effective shear viscosity of a dilute suspension of monosized rigid spheres. The benchmarks show that the method performs satisfactorily. Also reported are the results for three spheres, arranged either in equispaced linear or triangular configuration (fixed or free settling under gravity). The effect of the hydrodynamic interaction with a rigid plane boundary (half-space problem) is automatically treated by the use of the special image of the Kelvin state [N. Phan-Thien, J. Elasticity 13, 231 (1983)]. This is illustrated by considering the problem of free settling of particles near a rigid no-slip planar boundary; specifically, some results for two equal spheres are reported. It is shown that the particle motion near the solid plane is fundamentally different from its motion in the far field, and thus the relative divergence of particles and the reversal of their angular velocity is observed.
Inertial wave eigenfrequencies for a nonuniformly rotating fluid
Authors: Gunn J. Sterling , Aldridge Keith D.
Published: July 2002

A perturbation method is used to calculate the inertial wave eigenfrequencies for an inviscid, nonuniformly rotating fluid. The general results from this calculation for an arbitrary departure from uniform rotation are specialized for differential rotations. The method is checked by verifying that it gave the same result as found analytically for the case of a solid body perturbation. In our analysis the eigenfrequencies are real thus verifying that the differential rotation is a stable perturbation for inertial waves. The method is used to model the experimentally observed collapse of inertial waves near resonance where the differential rotation is a rectified flow. Another application of this work is its extension to the inverse problem of determining the differential rotation of the Earth’s fluid outer core. The recent gravimetric observations of inertial waves detected after large deep earthquakes provide the necessary data for this inversion.
A note on the diffusion in fluids with wave-induced random motion. Applications to the barotropic Rossby waves
Authors: Benilov E. S. , Wolanski E.
Published: July 2002

An asymptotic theory, describing turbulent diffusion due to wave-induced random motion in incompressible or compressible fluids, is constructed. It is shown that even weakly nonlinear waves cause irreversible stretching of material lines. The results obtained are applied to the Rossby-wave-induced motion in the atmosphere or ocean. An expression for the effective coefficient of diffusivity is calculated, which indicates that the diffusion due to Rossby waves can be strongly anisotropic even for an isotropic wave spectrum.
The use of disks to approximate finite axisymmetric bodies in Stokes flow
Authors: Davis A. M. J.
Published: July 2002

The analytically intractable problem of the Stokes flow past a finite cylinder is shown computationally to be well approximated by the use of an array of disks whose spacing must be sufficient to allow a formulation in terms of integral equations of the second kind. The translating cylinder within a pipe is similarly modeled and the method also allows the effects of simple streamlining or concavity to be quantified.
Linear stability of shear profiles and relation to the secondary instability of the Dean flow
Authors: Le Cunff Cédric , Bottaro Alessandro
Published: July 2002

Solutions of the Navier–Stokes equations for flow in a curved channel have previously been computed [Bottaro, J. Fluid Mech. 251, 627 (1993)] and the spatial development of the Dean flow is available at different supercritical Reynolds number. In this work the viscous instability of local longitudinal vortex structures (obtained from the nonlinear simulations) is investigated, with a focus toward the secondary instability of Dean vortices. Such a secondary instability takes the form of streamwise traveling waves. High-frequency waves are termed twisting waves, low frequency are defined undulating waves. Instead of performing analyses in which the basic flow in the cross section is two dimensional, significant shear profiles along y and z are considered as base flows at constant x before the establishment of a fully developed state. Thus one is able to discover that the twist instability is of shear type and is caused by inflectional spanwise profiles of the streamwise velocity component. Sinuous waves are always preferred to varicose waves, and the latter mode of instability is destabilized only at large Reynolds numbers. Undulating waves are related to normal profiles of the streamwise velocity; this type of secondary instability is of centrifugal origin. Results of the analyses for both types of waves are in good agreement with experiments.
Capillary–gravity waves in a surface tension gradient. I: Discontinuous change
Authors: Gou Shiyuan , Messiter Arthur F. , Schultz William W.
Published: July 2002

When waves enter a region covered with a surfactant, the spatial variation of the surface tension causes reflection and refraction of the waves. The simplified inviscid-flow problem of two adjacent half-spaces with different surface tension coefficients is solved analytically for plane waves at oblique incidence. The linearized wave potential is described by a complex contour integral representation for which uniqueness is obtained from the requirement of energy conservation. The reflection is found to be small for some typical cases.
Toroidal effects on m=2 feedback
Authors: Hender T. C. , Cowley S. C.
Published: July 2002

Toroidal calculations on m=2 magnetic feedback are presented. These calculations are performed using a coupled Δ′ formulation and show that the feedback can have a pronounced effect on the sideband (m=1 and 3) content.
Vlasov fluid stability of the m=0 mode in a skin current Z pinch
Authors: Arber T. D. , Coppins M.
Published: July 2002

The m=0 stability of a Z pinch confined by a skin current has been studied using the Vlasov fluid model. A significant reduction in growth rate compared to ideal magnetohydrodynamics (MHD) is found. An exact analytic solution can be obtained at large ka (k is the axial wavenumber and a is the pinch radius). In this limit the Vlasov fluid growth rate (normalized to the radial ion thermal transit time) saturates to the value 0.5 (π)1/2, in contrast to the ideal MHD growth rate, which increases without limit as (k)1/2.
Dynamics of magnetic relaxation in a high-temperature, current-carrying plasma
Authors: Lee G. S. , Diamond P. H. , An Z. G.
Published: July 2002

The results of a study of the role of multiple-helicity nonlinear interaction of tearing modes and the dynamics of magnetic relaxation and dynamo activity in a high-temperature, current-carrying plasma are described. A set of fluid equations for tearing modes in the semicollisional regime is derived, and a previous resistive magnetohydrodynamic study of tearing mode turbulence is thus extended to high-temperature regimes. Because of the direct connection between fluctuation evolution and configuration evolution, a generalized nonlinear theory of the turbulent dynamo and magnetic relaxation is proposed, and a two-point 〈(∇⊥p̃)ψ̃〉-correlation evolution is determined by calculating the relaxation time τcl. This calculated relaxation time is shown to serve as the phase shift between ṽr and ψ̃ and hence to control magnetic energy relaxation and dynamo processes. Careful study of the two separated regions of kink-tearing modes (i.e., the resonant region and the exterior region) reveals the direct relationship between the equilibrium magnetic energy relaxation and average magnetic flux evolution. Thus a theoretical interpretation of the observed correlation between magnetic fluctuations and maintenance of field reversal is provided. Finally, the correlation lifetime of the flux transport term is evaluated from the renormalized two-point theory, and the saturated magnetic fluctuation level is estimated. The result shows that the saturation level is independent of or very weakly dependent on the Lundquist number S. The implications of this result for anomalous thermal transport and dynamo activity in high-temperature reversed field pinch experiments are discussed.
Trapped particle sideband studies
Authors: Tsunoda S. I. , Malmberg J. H.
Published: July 2002

The behavior of sidebands growing in the presence of a launched large amplitude wave in a beam–plasma system is studied numerically. The large amplitude wave traps the beam and both the upper and the lower sidebands become unstable. The behavior of the instability is found to be dependent on a phase relationship that exists between the two sidebands and the large amplitude wave. The essential features of the instability are well described by a simple model in which the trapped electrons are replaced by a single particle oscillating in each well of the large amplitude wave.
The collisionless sausage instability
Authors: Coppins M.
Published: July 2002

The Chew–Goldberger–Low (CGL) double adiabatic model [Proc. R. Soc. London Ser. A 236, 112 (1956)] is used to study the linear m=0 (sausage) mode in a Z pinch operating in the collisionless, small ion Larmor radius regime. The model is valid in this case since the parallel heat flow is identically zero. A necessary and sufficient condition for stability, applicable to arbitrary (anisotropic) Z-pinch equilibria, is derived and the eigenvalue equation is solved for two classes of isotropic equilibria. Growth rates are shown to be lower than those of ideal magnetohydrodynamics (MHD). It is found that, in contrast to ideal MHD, the CGL eigenfunctions are characterized by an unperturbed inner region.
Envelope solitary waves in nonuniform or dissipative media
Authors: Kuehl H. H. , Zhang C. Y.
Published: July 2002

For a weakly nonuniform medium, a solitary wave solution of the variable-coefficient nonlinear Schrödinger equation is obtained through second order in the expansion parameter. From a generalization of the second-order result, the form of the solution to all orders is derived from which the conditions for an exact, distortionless solitary wave in a nonuniform medium are obtained. By use of a transformation, these results are shown to apply a weakly dissipative medium.
Ballooning stability of anisotropic, rotating plasmas
Authors: Wang X.-H. , Bhattacharjee A.
Published: July 2002

The linearized equation of motion is given in a Lagrangian representation for a rotating plasma with anisotropic pressure. A WKB theory is developed for large-n ballooning modes in an axisymmetric configuration with field-aligned and rigid toroidal flows. In the presence of field-aligned flows, it is shown that a resonance occurs which is strongly suggestive of a generalized mirror instability. In the presence of toroidal rotation, a possible stabilizing effect is identified for P⊥>P∥. Finally, as a special case of the theory, the necessary and sufficient conditions for stability in a static, anisotropic plasma are obtained.
Analysis and experiments in support of inertial confinement fusion reactor concepts
Authors: Moses G. A. , Peterson R. R. , MacFarlane J. J.
Published: July 2002

Cost-effective and safe containment of high-yield inertial confinement fusion (ICF) microexplosions in near-term laboratory microfusion facilities (LMF) and longer-term reactors requires an understanding of the interaction of target-generated x rays and ionic debris with surrounding buffer gases and the first solid surface that faces the target. The microfireball plasma created when a target explodes in a gas atmosphere of 1–10 Torr is not in local thermodynamic equilibrium. The plasma state must be determined by coupling the radiation field to the atomic level population calculation in order to correctly predict the surface emission of the plasma. Conditions similar to those predicted for ICF target chambers can be simulated using the SATURN x-ray simulator facility [Proceedings of the 2nd International Conference on Dense Z-Pinches, AIP Conf. Proc. 195 (AIP, New York, 1989), p. 3]. Aluminum and graphite samples that represent possible first wall materials were tested in SATURN. Coated aluminum samples and four-directional graphite weaves in a carbon matrix survived the tests.
Trapped ion nonlocal effect in ion cyclotron radio-frequency heating
Authors: Chen Yan Ping , Tsai Shih Tung
Published: July 2002

A general formula for the nonlocal absorption power of trapped ions averaged over a magnetic surface in ion cyclotron radio-frequency (ICRF) heating is derived. The gyrokinetic equation is adopted and a full particle orbit and wave coupling are kept. The resulting expression for a Maxwellian plasma in tokamaks is compared with Stix’s theory [Nucl. Fusion 15, 737(1975)], which shows that Stix’s theory is rather good for the fundamental cyclotron frequency heating and the nonlocal effect is not negligible for the second-harmonic cyclotron frequency heating.
Fast-wave ion cyclotron resonance heating experiments on the Alcator C tokamak
Authors: Shepard T. D. , Fiore C. L. , McDermott F. S. , Parker R. R. , Porkolab M.
Published: July 2002

In this paper, minority regime fast-wave ICRF (Ion cyclotron range of frequency) heating experiments conducted on the Alcator C tokamak [Nucl. Fusion 26, 1665 (1986)] are described. Up to 450 kW of rf (radio frequency) power at frequency f=180 MHz was injected into plasmas composed of deuterium majority and hydrogen minority ion species at magnetic fields of B0≂12 T, densities 0.8≤n̄e≤5×1020 m−3, and minority concentrations 0.25≲ηH≤8%. Typical ion temperatures were TD(0)∼1 keV, while, depending on density, typical electron temperatures were in the range Te(0)∼1.5–2.5 keV. Central deuterium ion temperature increases of ΔTD(0)=400 eV were observed at n̄e=1×1020 m−3, while significantly smaller ion temperature increases were observed at higher densities. At the highest densities, ion heating became insignificant due in part to a limitation on power handling by the antenna that became more severe with increasing density. Significant electron heating was not observed at any density. Heating of the minority species at low densities indicated severe losses, but at higher densities it was consistent with efficient collisional coupling of the rf power from the hydrogen minority species to the deuterium majority species. Analysis of the deuterium power balance indicated no significant change in the deuterium thermal transport properties as a function of rf power. This may be consistent with the fact that the rf power never exceeded the Ohmic heating (OH) contribution. However, a very strong dependence of the deuterium transport properties on density was observed, and appears to be the dominant process limiting the effectiveness of ICRF heating at high density.
Runaway electron studies in the ATF torsatron
Authors: England A. C. , Bell G. L. , Fowler R. H. , Glowienka J. C. , Harris J. H. , Lee D. K. , Murakami M. , Neilson G. H. , Rasmussen D. A. , Rome J. A. , Saltmarsh M. J. , Wilgen J. B.
Published: July 2002

Runaway electron formation and confinement occur readily in pulsed torsatrons and heliotrons because of the high loop voltages during initiation and termination of the helical and vertical fields (‘‘field ramping’’) and the inherently good containment of the electrons on the flux surfaces in the vacuum fields. This has been confirmed for the Advanced Toroidal Facility (AFT) [Fusion Technol. 10, 179 (1986)] and other stellarators by orbit calculations. Since runaway electrons can cause an unacceptable level of hard x rays near the machine, a runaway electron suppression system was incorporated in ATF. The main component of the system is a movable paddle, which is normally left in the center of the plasma chamber during the field ramps. This device, in conjunction with programmed vertical field ramping, which reduces the volume of good flux surfaces, has proved to be very effective in reducing the runaway electron population. Measurements of hard x rays from ATF have shown that the runaway electrons are produced primarily during the field ramping but that there is usually also a small steady-state runaway electron component during the‘‘flat-top’’ portion of the fields. The paddle is the main source of the hard x radiation (thick-target bremsstrahlung). There is evidence that some of the runaway electrons may be confined to islands. The maximum x-ray energy found by pulse height analysis is ∼12–15 MeV. The mean energy appears to be a few million electron volts. There is a noticeable forward peaking of the bremsstrahlung from the paddle. The limiters do not appear to be major sources of x rays.
Breakdown of a wire-to-plane discharge: Transient effects
Authors: Jog M. A. , Cohen I. M. , Ayyaswamy P. S.
Published: July 2002

A wire-to-plane discharge during the early phases of breakdown has been studied. The discharge has been modeled in a prolate spheroidal coordinate system with the wire shape taken as a hyperboloid of revolution. Four simultaneous coupled, time-dependent, nonlinear partial differential equations describe the electrical discharge. These are the conservation equations for ion and electron densities, the energy equation for electron temperature, and Poisson’s equation for the self-consistent electric field. By solving this formulation subject to appropriate initial and boundary conditions, charged particle densities and temperature variations have been obtained as the ionization progresses in the discharge. The results show that both the electron temperature and the charged particle densities increase with the progress of ionization. The effect of different wire polarities is also examined. With a positive wire polarity, the increases in electron temperature and charged particle densities are confined to regions of the discharge in the vicinity of the wire tip. With a negative wire polarity, the breakdown occurs in the entire gap at a faster rate than with a positive wire polarity. The wire polarity affects the magnitude of energy transfer between the particles.
Tearing modes in tokamaks with lower hybrid current drive
Authors: Xu X. Q. , Rosenbluth M. N.
Published: July 2002

In this paper, the effect of current drive on the tearing modes in the semicollisional regime is analyzed using the drift-kinetic equation. A collisional operator is developed to model electron parallel conductivity. For the pure tearing modes the linear and quasilinear growth rates in the Rutherford regimes have been found to have roughly the same forms with a modified resistivity as without current drive. One interesting result is the prediction of a new instability. This instability, driven by the current gradient inside the tearing mode layer, is possibly related to magnetohydrodynamic (MHD) behavior observed in these experiments.
Self-similar solutions for the supersonic implosion of a screw-pinch plasma
Authors: Venkatesan R. C. , Choe W. H.
Published: July 2002

Self-similar solutions are obtained for the supersonic compression of a plasma containing an axial current and an entrained axial magnetic field (screw pinch). These solutions represent a complete generalization of previous works done on the Z-pinch and Θ-pinch schemes. The solutions presented herein represent a screw-pinch plasma with a diffuse current profile. Three separate plasma implosion modes are identified. These modes differ qualitatively from the Z-pinch and Θ-pinch schemes. They are comprised of the plasma annulus implosion, a converging transverse magnetohydrodynamic shock, and the propagation of fast magnetosonic waves toward the axis (weak discontinuity). These modes are chosen on the basis of their ability to achieve magnetic field cumulation during the course of the implosion. Analytic expressions describing the asymptotic behavior of the hydromagnetic profiles are obtained for each of the three implosion modes. Numerical examples for selected boundary conditions, representing realistic plasma parameters for the purpose of ultrahigh magnetic field generation, are presented.
Nonlinear effects in velocity space and drift wave transport in tokamaks
Authors: Weiland J.
Published: July 2002

It is pointed out that the use of kinetic models for drift wave transport in general makes it necessary to treat consistently the transport in both real space and velocity space. For a broad class of collisionless or marginally collisionless plasmas, nonlinear effects in velocity space will effectively remove the dissipative kinetic type wave–particle resonances, leaving only a type of fluid two-stream resonance. At the same time a two-dimensional treatment of the turbulence becomes realistic. This is important for explaining the magnitude and radial profiles of diffusion coefficients in tokamaks and may also have important consequences for low-k damping and pinch effects.
On the hyperbolicity of a relativistic warm-plasma model
Authors: Pennisi S.
Published: July 2002

In this paper the hyperbolicity of a recently discovered warm-plasma model is proven, and the propagation speeds of discontinuity waves are calculated. These speeds, relative to the fluid, are 0, ±√7/15 ε, ±√3/5 ε, ±√1/3, where ε is a smallness parameter indicating the dispersion of the plasma particles.
Ion-cyclotron eigenmodes of a hot multispecies cylindrical plasma
Authors: Shoucri Magdi M.
Published: July 2002

The equations determining the ion-cyclotron eigenmodes of a uniformly magnetized homogeneous cylindrical multispecies plasma column consisting of a minority hydrogen ion species and a majority deuterium ion species are derived by including small gyroradius corrections up to the first harmonic (ω∼2ωcα). It is shown that the radial wave numbers associated with the resonance at the first harmonic introduce transverse electromagnetic modes (TEM modes) for the transverse components of the wave electromagnetic field, in addition to the transverse electric modes (Bz-mode) and transverse magnetic modes (Ez-mode) of the cylindrical plasma column. For the special case of a hydrogen–deuterium mixture, these TEM modes are characterized by two additional radial wave numbers, each of these resonating selectively with the deuterium and the hydrogen species.
Collective filamentation in induced spatial incoherence and multiple laser beam configurations
Authors: Rose Harvey A. , DuBois D. F.
Published: July 2002

The linear stability of a system of multiple overlapping laser beams is analyzed in terms of a linear dispersion relation that is valid with sufficient bandwidth. Each beam may or may not have spatial incoherence. Two beams can always act in concert to produce a linear instability equivalent to that obtained from a single beam with an intensity equal to the sum of the two beam intensities. There is a threshold intensity Icp for all beams to produce a collective ponderomotive instability equivalent to that of a single beam with the same total intensity. The collective thermal filamentation threshold, based upon classical transport, is one based on total power Pct and it seems to be easily accessible.
Interaction between perpendicular magnetohydrodynamic shocks
Authors: Hu Y. Q. , Habbal S. R.
Published: July 2002

This paper discusses the interaction between perpendicular magnetohydrodynamic shocks (S) as well as tangential discontinuities (T). The main conclusions are as follows: (1) The collision between a forward shock (S↘) and a reverse shock (S↙) produces a shock system consisting of a weaker reverse shock, a tangential discontinuity, and a weaker forward shock, namely, S↘S↙→S↙TS↘. (2) The merging of two forward shocks leads to a stronger forward shock, followed by a tangential discontinuity and a reverse wave. For a given βr (the ratio of gas pressure to magnetic pressure) of the background plasma, there exists a critical parameter γc. When the adiabatic index γ<γc, the reverse wave is a rarefaction wave (R↙). When γ≳γc, the reverse wave produced by merging of strong shocks is still a rarefaction wave but the reverse wave becomes a shock when weak shocks merge. (3) The interaction between a forward shock and a tangential discontinuity may form S↙TS↘, R↙TS↘, or TS↘, depending on the strength of the initial forward shock and the property of the initial tangential discontinuity.
Theory of pseudoclassical confinement and transition to L mode
Authors: Itoh K. , Itoh S.-I. , Yagi M. , Fukuyama A. , Azumi M.
Published: July 2002

A theory of the self-sustained turbulence is developed for resistive plasma in toroidal devices. Pseudoclassical confinement is obtained in the low-temperature limit. As temperature increases, the current diffusivity prevails upon resistivity, and the turbulence nature changes so as to recover the L-mode transport. Comparison with experimental observation on this transition is made. The Hartmann number is also given.
New results and applications for the quasioptical gyrotron*
Authors: Fliflet Arne W. , Fischer Richard P. , Manheimer Wallace M.
Published: July 2002

The quasioptical gyrotron (QOG), which features an open resonator formed by a pair of spherical mirrors instead of the conventional gyrotron waveguide cavity, has been under development at the U. S. Naval Research Laboratory as a tunable high power millimeter-wave source for tokamak plasma heating, advanced radars, and power beaming. Results have recently been obtained for a quasioptical gyroklystron (QOGK) realized by the addition of an open-mirror prebunching resonator driven by an 85 GHz, 1.5 kW extended interaction oscillator. Efficiency enhancement by mode priming has been investigated, and efficiencies up to 19% have been obtained by increasing the frequency detuning of the operating mode. An overall efficiency of 30% was obtained by the addition of a simple depressed collector. Phase-locked operation was demonstrated at a power of 57 kW and efficiency of 16%. The high circulating power in the QOG resonator is currently being considered for use as an electromagnetic wiggler for compact infrared free-electron lasers. The QOG is also promising as a source for an active sensor of upper atmosphere trace impurities.
Ion cyclotron range of frequency heating on the Tokamak Fusion Test Reactor*
Authors: Taylor G. , Bell M. G. , Biglari H. , Bitter M. , Bretz N. L. , Budny R. , Chen L. , Darrow D. , Efthimion P. C. , Ernst D. , Fredrickson E. , Fu G. Y. , Grek B. , Grisham L. , Hammett G. , Hosea J. C. , Janos A. , Jassby D. , Jobes F. C. , Johnson D. W. , Johnson L. C. , Majeski R. , Mansfield D. K. , Mazzucato E. , Medley S. S. , Mueller D. , Nazikian R. , Owens D. K. , Paul S. , Park H. , Phillips C. K. , Rogers J. H. , Schilling G. , Schivell J. , Schmidt G. L. , Stevens J. E. , Stratton B. C. , Strachan J. D. , Synakowski E. , Wilson J. R. , Wong K. L. , Zweben S. J. , Baylor L. , Bush C. E. , Goldfinger R. C. , Hoffman D. J. , Murakami M. , Qualls A. L. , Rasmussen D. , Machuzak J. , Rimini F. , Chang Z.
Published: July 2002

The complete ion cyclotron range of frequency (ICRF) heating system for the Tokamak Fusion Test Reactor (TFTR) [Fusion Tech. 21, 1324 (1992)], consisting of four antennas and six generators designed to deliver 12.5 MW to the TFTR plasma, has now been installed. Recently a series of experiments has been conducted to explore the effect of ICRF heating on the performance of low recycling, supershot plasmas in minority and nonresonant electron heating regimes. The addition of up to 7.4 MW of ICRF power to full size (R∼2.6 m, a∼0.95 m), helium-3 minority, deuterium supershots heated with up to 30 MW of deuterium neutral-beam injection has resulted in a significant increase in core electron temperature (ΔTe=3–4 keV). Simulations of equivalent deuterium–tritium (D–T) supershots predict that such ICRF heating should result in an increase in βα(0)∼30%. Direct electron heating has been observed and has been found to be in agreement with theory. The ICRF heating has also been coupled to neutral-beam heated plasmas fueled by frozen deuterium pellets. In addition ICRF heated energetic ion tails have been used to simulate fusion alpha particles in high-recycling plasmas. Up to 11.4 MW of ICRF heating has been coupled into a hydrogen minority, high-recycling helium plasma and the first observation of the toroidal Alfvén eigenmode (TAE) instability driven by the energetic proton tail has been made in this regime.
Vlasov solution describing nonadiabatic plasma heating by magnetic compression
Authors: Persson Hans , Ågren Olov
Published: July 2002

An analytical Vlasov description of a fast magnetic compression is developed with use of the Lewis invariant for the time-dependent harmonic oscillator. Simple formulas for the nonadiabatic heating of an initial rigid rotor Maxwellian distribution are derived, providing a low β kinetic description of the oscillatory state after compression. The role of diamagnetic shielding and orbit stochasticity to relax the oscillations in the plasma after compression is considered. In addition to the adiabatic heating (which is equal to the field ratio B1/B0), a much stronger heating, proportional to (B1/B0)2, can be obtained in a nonturbulent fast compression. The ultimate need to use low initial β to achieve strong nonadiabatic heating and avoid turbulent development of instabilities follows from a formula describing the increase in β during the implosion. An evolution profile for B(t) with a constant nonadiabaticity parameter is considered for optimization of compression time (of order 5–10 μsec) and nonadiabatic heating. The radial extension of the compressed plasma is calculated. Space charge effects can arise from the differential radial motions of electrons and ions in a nonadiabatic compression, which can have important consequences. In straight devices, space charge can be removed by letting magnetic field lines connect to a conducting plate. Special attention is paid to the possibility of creating a ‘‘Migma pinch’’ with self-colliding orbits in the MeV range. The model indicates that this high energy can be reached in a compression with large field ratio, i.e., B1/B0∼103. The derived diamagnetic shielding limit, which corresponds to a compressed beta value below which shielding of the induction field can be ignored, gives a particle number per unit length of about 1017 m−1 for the Migma case. This number exceeds by several orders of magnitude the particle number achieved in the Migma IV experiment by a different beam trapping technique.
Dependence of efficiency on magnetic field in gyro-backward wave oscillators
Authors: Lin A. T. , Lin Chih-Chien
Published: July 2002

A gyro-backward wave oscillator with its oscillation frequency close to the waveguide cutoff frequency has been experimentally demonstrated by other investigators to attain near 20% efficiency. Computer simulations have been carried out to elucidate the mechanisms which give rise to this high efficiency regime. The result shows that one of the plausible mechanisms is the resulting large frequency mismatch (detuning) which places the electron bunch in azimuthal phase close to where the maximum electron–wave coupling occurs.
Semilocal kinetic analysis of the toroidal ion temperature gradient mode
Authors: Hirose A.
Published: July 2002

The semilocal kinetic analysis based on appropriate norms of differential operators is able to recover the growth rates of the toroidal ion temperature gradient mode found from the rigorous integral equation formulation.
A theory of ferromagnetic waveguide accelerators driven by an electron beam
Authors: Uhm Han S.
Published: July 2002

A fully self-consistent theory of ferromagnetic waveguide accelerators driven by a relativistic electron beam is developed. The theoretical analysis is based on Faraday’s law, which provides a second-order partial-differential equation of the azimuthal magnetic field, under the assumption that με≫1. Here μ and ε are the permeability and dielectric constant of the waveguide material. The azimuthal magnetic field and axial acceleration field are obtained in forms of integral equations for an arbitrary profile of the drive-beam current I(t). In the limit when the conductivity σ of the waveguide material is zero, the acceleration mechanism is similar to the typical wake-field accelerators. In this limit, the acceleration field is proportional to the square root of the parameter μ/ε and can be easily more than 200 MV/m for moderate system parameters. On the other hand, for high conductivity limit, the acceleration mechanism is the magnetic field decay, exhibiting that the electromagnetic fields are a decaying function of the time. With appropriate physical parameters, the acceleration gradient of the magnetic field-decay accelerator is also very large.
Momentum coupling in ionospheric critical ionization velocity experiments*
Authors: Brenning N. , Bolin O.
Published: July 2002

The critical ionization velocity (CIV) effect is a process that can rapidly ionize a neutral gas which moves through a magnetized plasma. The process has been studied for several decades in laboratory experiments, but presently the emphasis has moved to ionospheric injection experiments. In these experiments, the neutral gas component is released at high velocity, with respect to the ionosphere, from a rocket or a satellite. Efficient momentum coupling between the injected cloud and the ambient ionosphere is achieved by means of Alfvén waves that are launched along the magnetic field. A computer model is presented for the momentum exchange between a cloud of injected ions and the ionosphere, and the model electric fields and particle spectra are shown to agree in detail with measurements from the Critical Ionization Test II (CRIT II); [Swenson et al., Geophys. Res. Lett. 17, 2337 (1990)] ionospheric injection experiment.
Production of high-energy electrons caused by counterstreaming ion beams in an external magnetic field
Authors: Abe Takashi , Machida Shinobu
Published: July 2002

A mechanism for producing suprathermal electrons having energies comparable to ion beam energy is studied by a numerical simulation. The system consists of background ions and electrons and a beam flowing across the magnetic field. The time scale considered is less than the ion gyro period, so the ion Larmor motion is ignored. Suprathermal electrons are produced when the ion beam density is smaller than the background plasma density, through resonant interactions with the excited waves propagating obliquely to the magnetic field. The work is motivated by Alfvén’s suggestion of the critical ionization phenomenon.
Nonlinear heat conductivity in plasmas
Authors: Eu Byung Chan
Published: July 2002

It is shown that the modified moment method applied to the Vlasov–Landau equation is adequate for accounting for the heat flux inhibition in laser-driven plasmas, if the temperature profile is properly taken into account in calculating the heat flux. The temperature profiles are obtained by solving the steady-state mass, momentum, and energy balance equations in which the heat flux is a nonlinear function of the temperature gradient derived from the kinetic equation by means of the modified moment method. There can be two different temperature profiles, depending on the Mach number of the fluid speed. When the temperature profiles thus calculated are self-consistently used in the heat flux formula, the heat flux limit factors are derived. The factors are of the same order of magnitude as the experimentally inferred value of 0.03. For example, in the case of the hydrogen plasma the factor is less than 0.05 for the supersonic branch.
Parasitic excitation of ion Bernstein waves from a Faraday-shielded fast wave loop antenna
Authors: Skiff F. , Ono M. , Colestock P. , Wong K. L.
Published: July 2002

Parasitic excitation of ion Bernstein waves is observed from a Faraday-shielded fast wave loop antenna in the ion-cyclotron frequency range. Local analysis of the Vlasov–Maxwell equations demonstrates the role of the plasma density gradient in the coupling process. The effects of plasma density and of parallel wavenumber on the excitation process are investigated.
Statistical theory for magnetohydrodynamic turbulent shear flows
Authors: Yoshizawa Akira
Published: July 2002

A statistical theory is developed for magnetohydrodynamic turbulent shear flows in the presence of both mean velocity and magnetic gradients. In the case of magnetic Prandtl number unity, the correlations between fluctuating velocity and magnetic fields, which play a crucially important role in the equations for the mean velocity and magnetic fields, are calculated with the aid of the two-scale, direct-interaction approximation. The electromotive force expressing the effect of turbulent fields in the equation for mean magnetic field is found from the results. Particularly, the electromotive force related linearly to the mean magnetic field is shown to originate from the statistical inhomogeneity of fluctuating fields, which is sustained by the mean field gradients.
Electron temperature and ionization degree dependence of electron transport coefficients in monatomic gases
Authors: Erwin Daniel A. , Kunc Joseph A.
Published: July 2002

The electrical conductivity of partially ionized monatomic gases (hydrogen, oxygen, helium, and neon) has been calculated over a wide range of electron temperature Te and ionization degree x (300 °K≤Te ≤30 000 °K and 10−8≤ x ≤1) with Coulomb collisions taken into account. The knowledge of the conductivity allows for easy determination of electron mobility and diffusion coefficient in the plasmas. Limiting degrees of ionization xw(Te) and xf(Te) were established, giving the range of plasma conditions where the plasma transport properties can be considered within the limits of weakly ionized or fully ionized models.
Z pinches with multi-ion species: Ion separation and stability
Authors: Rahman H. U. , Amendt Peter , Rostoker N.
Published: July 2002

Recent experiments on gas-puff Z pinches using various ion mixtures have demonstrated that the plasma shell separates into two distinct annuli which implode concentrically. This phenomenon is quantitatively explained with the use of a semihydrodynamic model in which the electrons are treated as a collisional fluid and the ions are considered cold and collisionless. Stabilitiy in this model is investigated and an expression for the growth rate of a Rayleigh–Taylor type instability is presented. This growth rate is found to be independent of the degree of plasma compressibility and somewhat reduced in the presence of an ion mixture. Comparison of these results with the University of California, Irvine, Z-pinch experiment is discussed.
The question of consistent boundary conditions when simulating reversed field pinch dynamics
Authors: Mirin A. A.
Published: July 2002

The issue of proper boundary conditions when performing magnetohydrodynamic simulations of the reversed field pinch is examined. Of particular concern is the choice of constant current, which when combined with other commonly used boundary conditions may, under careless implementation, lead to an inconsistency. It is shown that this may cause erroneous results. Cases both with and without Hall terms are presented.
Growth of nonlinear intermittent fluctuations in linearly stable and unstable simulation plasma
Authors: Berman Robert H. , Dupree Thomas H. , Tetreault David J.
Published: July 2002

Simulations of nonlinear instability in a one-dimensional Vlasov plasma with a relative electron-ion drift velocity have been extended to parameter ranges more extensive than previously reported [Phys. Fluids 28, 155 (1985)]. Linearly stable runs, with Te/Ti =1, mi/me =100, and a system length of L=512λD show nonlinear hole instability. As in the case of mi/me =4, and L=32λD, the hole depth, double-layer potential structure, acceleration, and growth rate agree well with a theory of hole growth that results from momentum exchange from reflected particles. For a linearly unstable plasma (Te/Ti ≫1), the fluctuations and their time evolution are related to the linearly stable case, but are somewhat more complex. A spatially intermittent distribution of ‘‘wavelike fluctuations’’ grows to large amplitude and then evolve into trapped ion phase-space hole structures. The hole potentials are of order eφ/Te =1, as in the linearly stable (Te/Ti =1) runs, but because Te/Ti ≫1, are of large amplitude (eφ/Ti ≫1) relative to the ions. The hole structure and dynamics agree well with the large-amplitude extension of the small eφ/Ti hole theory.
Propagation of ion beams through a tenuous magnetized plasma
Authors: Chrien E. F. , Valeo E. J. , Kulsrud R. M. , Oberman C. R.
Published: July 2002

When an ion beam is propagated through a plasma, the question of charge neutralization is critical to its propagation. This paper considers such a problem where the plasma is magnetized with the magnetic field perpendicular to the beam. The plasma-number density and beam-number density are assumed comparable. This problem is reduced to a two-dimensional model and solved. It is shown that it should be possible to attain charge neutralization if the beam density is properly varied along itself.
Boundary-layer separation from a smooth slip surface
Authors: Dandy D. S. , Leal L. G.
Published: July 2002

Numerical results are presented for the streaming motion of an incompressible Newtonian fluid past axisymmetric ellipsoids of fixed shape, subject to the condition of zero tangential stress on the body surface. It is demonstrated that a substantial, closed-streamline wake appears behind the ellipsoid at moderate Reynolds numbers, O(20–50), when the ellipsoid is sufficiently nonspherical, and that this wake initially increases in size with increase of Reynolds number but then eventually decreases in size and disappears when the Reynolds number is sufficiently large, O(200–250) in our cases. The existence of streaming flow past a smooth, slip surface without separation is consistent with predictions from laminar boundary-layer theory. It is concluded that the recirculating wake that appears behind moderately deformed bubbles is strictly a finite Reynolds number phenomenon that cannot be predicted (or understood) via a boundary-layer analysis.