bluuebunny/crossref_metadata_2025 huggingface dataset:
On the inhomogeneous two-plasmon instability
Authors: Simon A. , Short R. W. , Williams E. A. , Dewandre T.
Published: March 2003

The two-plasmon instability in warm inhomogeneous plasma for a normally incident pump is considered. The complex eigenfrequencies of the absolute instability are obtained by reducing the linearized fluid equations to a Schrödinger equation in wavenumber space. These eigenvalues are obtained in several ways. One is by combining a perturbation expansion in powers of the reciprocal scale length with WKB theory. The resulting algebraic equations are solved by three analytical approximations and by direct numerical solution. A second way is by analysis of the Schrödinger equation using an interactive WKB computer code. A third way is by the use of a shooting code. These methods are all used and compared for threshold curves and growth rates above threshold. Some eigenfunction forms are also obtained. The threshold is near (v0/ve)2k0 L =3, and varies weakly with β≂v4e/v20c2, rising from near 2 to about 4 over six decades of variation of β. The corresponding critical value of (ky/k0)2 is near 0.2/β over this range. Above threshold, there is a smooth variation of the growth rate with (ky/k0)2, peaking at some intermediate value. The perturbation method is in good agreement there with more exact calculations. Experimental implications of these results are discussed.
Rayleigh–Taylor instability in an inhomogeneous ablatively accelerated fluid
Authors: Colombant D. G. , Manheimer Wallace M.
Published: March 2003

The reduction in Rayleigh–Taylor instability growth rate because of density inhomogeneity in an incompressible ablatively accelerated plasma is calculated over a very wide range of physical parameters. In the low-wavenumber regime, the effect of the density gradient is to give a significant (about a factor of 2) reduction in growth rate as seen in fluid simulations.
Stimulated Brillouin backscattering in long-scale-length plasmas
Authors: Colombant D. G. , Manheimer Wallace M. , Gardner J.
Published: March 2003

Stimulated Brillouin backscattering in long-scale-length plasmas is calculated using a wave kinetic model. In these calculations use is made of computed hydrodynamic profiles for these plasmas. Results are presented for the parameters of the joint Naval Research Laboratory–Lawrence Livermore National Laboratory experiments and for reactor size pellets of 5 mm characteristic length. Means of reducing high levels of backscattering for the latter case are investigated.
Collisionless electron shocks in electron-beam–plasma systems
Authors: Singh Nagendra , Schunk R. W.
Published: March 2003

One-dimensional Vlasov simulations show that when an electron beam is suddenly injected into a plasma, a fast-moving monotonic shock forms during the early stage of the transient plasma response. In the shock, the ions are nearly immobile. The shock appears to be an electrostatic electron-beam–plasma mode. The shock evolves from an initial positive potential perturbation, which is supported by an ion burst. The steepening of the perturbation into a shock is characterized by an electron-beam–plasma mode.
Characteristic modes and fundamental singularities of partial differential equations
Authors: Clauser Francis H.
Published: March 2003

Systems of linear partial differential equations with constant coefficients, like their ordinary differential equation counterparts, can be characterized by the properties of the matrices that form the coefficients of the differential operators. The question arises: Do the matrix operators that result from partial differential equations possess eigenvalues and eigensolutions in the same way that ordinary differential matrix operators do? The answer to this question is explored in some detail using as an example the linearized flow of a viscous fluid. It is shown that eigenfactors do exist for these equations, and that, of necessity, these involve hypercomplex algebra. This fact introduces significant new features to the problem. It is shown that eigenmodes exist and that each of these has its distinctive fundamental singularity. The fluid mechanical significance of these is examined in some detail. In addition, a representative group of other partial differential equations is examined and their eigenmodes and fundamental singularities are determined. It is shown that a number of basic differences exist between the eigenfunction theory for ordinary and for partial differential equations.
Turbulence: Space-time statistical properties and behavior in supersonic flows
Authors: Favre Alexandre
Published: March 2003

Following a scientific cooperation beginning in 1954 with L. S. G. Kovasznay, and in his memory, in this paper is presented a survey of two of the main areas of research to which he contributed: (1) turbulence properties deduced through statistical conditional sampling and space-time measurements of correlations and contingencies; these include celerities, optimum correlations, memories, and coherent structures; and (2) the study of turbulence in supersonic flows initiated by Kovasznay; this include the use of mass-weighted averages, decrease of turbulence in expansion, increase in compression, strong Reynolds analogy extension, and temperature–velocity correlation conservation.
Viscosity renormalization in the Brinkman equation
Authors: Koplik Joel , Levine Herbert , Zee A.
Published: March 2003

The Brinkman equation purports to describe low-Reynolds-number flow in porous media in situations where velocity gradients are non-negligible. The equation involves modifying the usual Darcy law by the addition of a standard viscosity term whose coefficient is usually identified with the pure-fluid viscosity. It is argued instead that the porous medium induces a renormalization of viscosity, which is calculated in the dilute limit and separately in a self-consistent approximation. The effective Brinkman viscosity is found to decrease from the pore-fluid value. The calculation fails at low porosity but agrees at least in part with experiment. In addition, the relationship between the Brinkman equation and the phenomenological boundary condition of Beavers and Joseph is discussed and it is pointed out that their experimental configuration provides a simple means of measuring viscosity renormalization.
Movement of a semipermeable vesicle through an osmotic gradient
Authors: Anderson John L.
Published: March 2003

A theory is presented for the movement of a vesicle in response to a gradient in concentration of an impermeable molecular solute. The vesicle is a fluid sphere bounded by a rigid semipermeable membrane which prevents solute from crossing it. The external solute gradient acts as an osmotic driving force for solvent exchange across the membrane; thus, the membrane converts the solute gradient into mechanical stresses which propel the vesicle toward regions of lower solute concentration. The speed of the vesicle is essentially independent of the fluid viscosity, only weakly dependent on the vesicle size, and directly proportional to the solute concentration gradient in the external fluid.
Solutions of incompressible jets
Authors: Wang Dong-Jian
Published: March 2003

In this work a momentum source model is provided to describe the flow pattern of an incompressible fluid with a magnetic field. A restricted class of analytical solutions for a one-sided jet, a two-sided symmetric jet, and a two-sided asymmetric jet are obtained. Four kinds of analytical solutions for a two-sided jet are classified by the jet cone angle which depends on two parameters related to the Alfvénic Mach number and the momentum flux. The cone angle and the boundary between the two sides of a two-sided asymmetric jet are determined by the ratio of the momentum flux parameters on each side. From this model the mass flux, momentum flux, and energy flux entrained by the jet are calculated and the differences between the above properties on each side of a two-sided asymmetric jet are obtained as well. The rotation of the jet and the restrictions of incompressibility are discussed.
Oblique envelope solutions of the Davey–Stewartson equations in intermediate water depth
Authors: Kirby James T. , Dalrymple Robert A.
Published: March 2003

Various types of envelope solutions of modulated Stokes waves may exist depending on the relative signs of terms representing dispersive and nonlinear effects. In this study, the variation of the zeroes of the dispersive and nonlinear terms in the equation governing evolution of the envelope is determined as a function of dimensionless water depth kh and oblique angle α, and solitary and periodic oblique envelope solutions are described for the general case of intermediate water depth.
The structure of the internal wave Mach front generated by a point source moving in a stratified fluid
Authors: Gray Ernest P. , Hart Robert W. , Farrell Richard A.
Published: March 2003

The height and velocity of an internal wave (IW) generated by a point fluid source moving horizontally in a stratified fluid is calculated at asymptotically large distances behind the source, in linear approximation. Particular emphasis is given to the region near the Mach front, defined by the off-track angle θM =sin−1(cM/ V), where cM is the speed of the fastest IW and V is the source velocity. To elucidate the asymptotic IW profile in that regime, a novel uniform asymptotic expansion is developed which is valid on both sides of the Mach front. It is found that, for the fluid model employed, the asymptotic approximation for the IW height, as well as for all its derivatives, is continuous at that front, and that all these IW profiles are remarkably smooth and slowly varying functions of off-track distance in that regime.
Echo phenomenon associated with lower-hybrid wave launching
Authors: Wong K. L. , Skiff F. , Ono M.
Published: March 2003

Lower-hybrid waves at two different frequencies f1 and f2 are launched simultaneously from two localized antennas, and a third wave is observed to arise near the plasma edge at the frequency f=f2−f1. This phenomenon can be explained by an echo effect near the plasma surface.
The effect of source-sink geometry on enrichment in a gas centrifuge
Authors: Conlisk A. T.
Published: March 2003

Solutions to the mass transfer problem in a gas centrifuge are examined as a function of source-sink geometry. The model centrifuge consists of a pair of concentric circular cylinders bounded on the top and bottom by horizontal end plates; the apparatus rotates rapidly about the axis of the cylinders. During steady operation, a binary gas mixture containing species A and B is injected into the container through small axisymmetric slots located on the sidewalls. Solutions for the concentration field within the centrifuge are obtained for mechanically or thermally driven centrifuges; maximum values of the separation factor based on product and feed mass fractions are predicted as a function of feed, product, and waste port locations. Both moderate and strongly forced flows in the container are considered.
Obliquely propagating whistler instability in an inhomogeneous plasma penetrated by a helical electron beam
Authors: Jain V. K. , Shah H. A.
Published: March 2003

The excitation of the whistler instability due to normal gyroresonant interaction is studied in a warm inhomogeneous plasma in the presence of a helical electron beam. The results show that it is possible to excite oblique whistler waves propagating at large angles with respect to the ambient magnetic field. It is found that the inhomogeneity increases the frequency and growth rate of the instability.
Dynamics of the heat flux-induced ion turbulence in a laser-produced plasma
Authors: Monchicourt Patrick
Published: March 2003

A self-consistent determination of the ion turbulence level in a laser-heated, expanding plasma, has been done with an inhomogeneous, hydrodynamic model. It is shown for one μm radiation, that the heat flux-induced ion turbulence appears only for laser pulse intensities≳1014 W cm−2, with rise times ≲100 psec, for plasmas with Z≳5. When present, this ion turbulence is space and time dependent. It has a short lifetime (for instance 50 psec for Z=10), and it is confined only in the conduction region, where transport inhibition is f*e∼0.2. There is no enhancement of absorption, because convecting ion modes are strongly Landau-damped before reaching the critical density nc. On the other hand, a strong enhancement of absorption is found when suprathermal electrons are present: In that case, the suprathermal energy flux towards the dense plasma substantially raises the turbulence level near nc, and strongly increases the absorption of laser energy by inverse bremsstrahlung.
Ion charge number and flux saturation effects in the corona of a laser-irradiated pellet
Authors: Sanz J. , Sanmartín J. R.
Published: March 2003

The quasisteady structure of the corona of a laser-irradiated pellet is completely determined for arbitrary Zi (ion charge number) and rc/ra (ratio of critical and ablation radii), and for heat-flux saturation factor f above approximately 0.04 . The ion-to-electron temperature ratio at rc grows sensibly with Zi; all other quantities depend weakly and nonmonotonically on Zi. For rc/ra close to unity, and all Zi of interest (Zi<47), the flow is subsonic at rc. For a given laser power W, flux saturation may decrease (low f) or increase (high f) the ablation pressure Pa relative to the value obtained when saturation is not considered; in some cases a decrease in f with W fixed increases Pa. For intermediate f (∼0.1), Pa∝(W/r2a)2/3 ρ1/3c(ρc= critical density), independently of rc/ra; for f∼0.6, Pa is larger by a factor of about (rc/ra)2/3. For rc/ra>1.2 roughly, the mass ablation rate is C(Zi)[(mi/kZi)7 u/2 K̄r11aP5a]1/6, independent of ρc and f, and barely dependent on Zi(mi is ion mass; k, Boltzmann’s constant; K̄, conductivity coefficient; and C, a tabulated function).
Plasma heating with strong poloidal Ohmic currents
Authors: Holly D. J. , Prager S. C. , Sprott J. C.
Published: March 2003

The feasibility of using strong poloidal currents to heat plasmas has been examined experimentally in Tokapole II, operating as a toroidal octupole. The plasma resistivity ranges from that of Spitzer to about 1500 times Spitzer resistivity, as predicted by mirror-enhanced resistivity theory. This allows large powers (approximately 2 MW) to be coupled to the plasma at modest current levels. However, the confinement time is reduced by the heating, apparently due to a combination of the input power location (near the walls of the vacuum tank) and fluctuation-enhanced transport.
Nonlinear saturation of Rayleigh–Taylor instability in thin films
Authors: Babchin A. J. , Frenkel A. L. , Levich B. G. , Sivashinsky G. I.
Published: March 2003

A general mechanism of nonlinear saturation of instabiities in flowing films is described using the Rayleigh–Taylor instability as an example. The combined action of flow shear and surface tension is the essence of the saturation mechanism. As a result, the streamwise perturbations of the interface that would rupture a stagnant film do not rupture a film flowing in a certain range of shear rates.
Viscosity-driven impurity transport in a stellarator
Authors: Shaing K. C.
Published: March 2003

The viscosity-driven impurity transport in a stellarator in the 1/ν and collisionless detrapping regimes is calculated. It is found that the impurity flux tends to go inward radially if all the species are in the 1/ν regime and tends to go outward if all the species are in the collisionless detrapping regime.
Structure and dynamics of round turbulent jets
Authors: Dimotakis Paul E. , Miake-Lye Richard C. , Papantoniou Dimitris A.
Published: March 2003

Laser-induced fluorescence and particle streak velocity measurements were conducted to investigate the structure and dynamics of round turbulent jets. The results suggest that the far-field region of the jet is dominated by large-scale vortical structures, which appear to be axisymmetric or helical a large part of the time. Entrainment and mixing of the reservoir fluid with the jet fluid is found to be intimately connected with the kinematics of these structures. Unmixed reservoir fluid is found to reach and cross the jet axis.
Buckling of a turbulent jet surrounded by a highly flexible duct
Authors: Anderson Ren , Bejan Adrian
Published: March 2003

An experimental and theoretical study of the static buckling of a downward flowing stream surrounded by a highly flexible duct is reported. It is shown analytically that the stream buckles into a meandering shape that depends strongly on the flow velocity in the duct. The amplitude of the buckled shape is found to be governed by the magnitude of the transverse force created by nonzero curvature at the free end of the duct. Experimental measurements confirm the buckled shape described analytically.
Heat transfer measurements in a heated jet-pipe flow issuing into a cold cross stream
Authors: Andreopoulos J.
Published: March 2003

Measurements are presented of mean temperature in a nonbuoyant heated jet issuing perpendicularly into a cold stream at velocity ratios R=0.25, 0.5, 1, and 2, and of velocity-temperature fluctuation statistics at R=0.5. The mean temperature profiles at the jet exit are found to be considerably nonuniform at low velocity ratios, indicating an early mixing between the cold and hot fluid. The extra rate of strain due to streamline curvature and the longitudinal and normal temperature gradients affect the rate of generation of temperature fluctuations and turbulent heat fluxes. There are regions of the flow where the streamlines are unstably curved but the generation of ∼(ϑ2) is damped. The eddy diffusivity is well behaved across the flow only at downstream positions where the turbulent Prandtl number has rather high values close to the wall and reaches values around 0.9 away from the wall.
Using second-sound shock waves to probe the intrinsic critical velocity of liquid helium II
Authors: Turner Timothy N.
Published: March 2003

A critical velocity truly intrinsic to liquid helium II is experimentally sought in the bulk fluid far from the apparatus walls. Termed the ‘‘fundamental critical velocity,’’ it necessarily is caused by mutual interactions which operate between the two fluid components and which are activated at large relative velocities. It is argued that flow induced by second-sound shock waves provides the ideal means by which to activate and isolate the fundamental critical velocity from other extraneous fluid–wall interactions. Experimentally it is found that large-amplitude second-sound shock waves initiate a breakdown in the superfluidity of helium II, which is dramatically manifested as a limit to the maximum attainable shock strength. This breakdown is shown to be caused by a fundamental critical velocity. Secondary effects include boiling for ambient pressures near the saturated vapor pressure or the formation of helium I boundary layers at higher ambient pressures. When compared to the intrinsic critical velocity discovered in highly restricted geometries, the shock-induced critical velocity displays a similar temperature dependence and is the same order of magnitude.
Reduced magnetohydrodynamics and the Hasegawa–Mima equation
Authors: Hazeltine R. D.
Published: March 2003

Reduced magnetohydrodynamics consists of a set of simplified fluid equations which has become a principal tool in the interpretation of plasma fluid motions in tokamak experiments. The Hasegawa–Mima equation is applied to the study of electrostatic fluctuations in turbulent plasmas. The relations between these two nonlinear models is elucidated. It is shown that both models can be obtained from appropriate limits of a third, inclusive, nonlinear system. The inclusive system is remarkably simple.
Compressibility effect on instability growth rates
Authors: Newcomb William A.
Published: March 2003

For an arbitrary magnetostatic equilibrium, instability growth rates are shown to be an increasing function of compressibility.
Growth rate of the clump instability
Authors: Tetreault David J.
Published: March 2003

The instability of phase space density granulations or clumps has recently been reported. A theory for the growth rate of the clump instability in a one-dimensional, ion–electron plasma is presented. Growth is predicted to occur in regions of velocity space where the average particle distribution functions have opposing velocity gradients. The free energy required for instability is significantly below the threshold necessary for the linear instability of ion acoustic waves. The growth rate increases with fluctuation amplitude and is proportional to the inverse of the nonlinear trapping time. The effect of particle discreteness and clump self-energy on the growth rate is discussed. The growth rate is in reasonable agreement with recent particle simulations.
The scaling of oblique plasma double layers
Authors: Borovsky Joseph E.
Published: March 2003

A systematic study of oblique nonlinear potential structures is presented. The results of particle-in-cell numerical simulations, numerical solutions to the Poisson–Vlasov equations, and Poisson–Vlasov theory all support Debye-length scaling.
Emission and absorption of electron cyclotron waves in rf current sustained tokamak plasmas
Authors: Fidone I. , Giruzzi G. , Granata G. , Meyer R. L.
Published: March 2003

Emission and absorption of electron cyclotron radiation in a tokamak plasma in the presence of a fast-electron tail is investigated. Specifically, the case in which the perpendicular temperature of the very end of the tail has a sharp rise is discussed. This situation occurs during rf current drive, and wave emission and transmission measurements may yield accurate information on the tail formation. The theory is developed for propagation in a homogeneous magnetic field and applies to the vertical direction of the tokamak. The quasilinear theory of wave absorption is also investigated. This study is motivated by the possibility of remote control of temperature anisotropy instability by electron cyclotron heating. The case of the extraordinary mode propagating from the low-magnetic-field side of the torus in the equatorial plane is numerically discussed.
Linear and quasilinear theory of second electron cyclotron harmonic heating of tokamak plasmas
Authors: Hsu J. Y. , Chan V. S. , McClain F. W.
Published: March 2003

Heating and current drive by electron cyclotron waves with the second harmonic resonance layer lying inside the plasma is studied. The full warm-plasma dispersion relation is solved numerically in a slab plasma model with assumed density and temperature profiles to study the scaling laws of heating, the spatial profiles of power deposition, and the rf-driven current. The current drive, via the anisotropic resistivity, is obtained analytically for weak rf within the Lorenz model and numerically from a Fokker–Planck code. Mode conversion from an ordinary mode to the extraordinary takes place at the incident angle of 35 deg. The allowed angular spread for efficient energy penetration is enhanced by a factor of 1.5 over that of the fundamental, and therefore makes this scheme workable and heating and current drive in high-density plasmas possible. The current drive efficiency is markedly improved when energy is damped through the electrostatic Bernstein mode.
Excitation of thermonuclear cone instabilities due to alpha-particle ripple losses in tokamaks
Authors: Lisak M. , Anderson D. , Hamnén H.
Published: March 2003

The possibility of a new set of thermonuclear loss-cone instabilities is demonstrated. These instabilities are caused by an anisotropy in the alpha-particle distribution function which arises because of alpha-particle losses due to the toroidal field ripple in tokamaks. The conditions for the excitation of a number of instabilities are studied.
Neoclassical flows and transport in nonaxisymmetric toroidal plasmas
Authors: Shaing K. C. , Callen J. D.
Published: March 2003

The moment equation approach to neoclassical transport theory has been generalized to nonaxisymmetric toroidal systems under the assumption of the existence of magnetic surfaces. In particular, the parallel plasma flows and bootstrap current are calculated in both the Pfirsch–Schlüter and banana regimes. It is found that both parallel plasma flows and the bootstrap current can be reduced as the toroidal bumpiness increases in an otherwise axisymmetric system.
Stability of equilibria with helical magnetic axis
Authors: Mond Michael
Published: March 2003

The stability of equilibria with a helical magnetic axis is studied. First, two families of equilibria whose applied fields have the same pitch as the magnetic axis are investigated. The two families differ from each other by the ordering of the physical variables with β. Then, the energy principle is used in order to check the stability properties of the equilibria. It is shown that low-β, low-shear systems with zero net current are stable for arbitrary values of the pressure if the deviation from straight magnetic axis is large enough.
Synchrotron emission from runaway electron distributions
Authors: Winske D. , Peter Th. , Boyd D. A.
Published: March 2003

Synchrotron emission from a relativistic anti-loss-cone (runaway) distribution is investigated numerically and compared with various analytical approximations. The results are applied to recent measurements of enhanced emission during current-drive experiments on the Princeton Large Torus (PLT) as well as to impulsive solar microwave bursts.
Nonlinear gyrokinetic equations
Authors: Dubin Daniel H. E. , Krommes John A. , Oberman C. , Lee W. W.
Published: March 2003

Nonlinear gyrokinetic equations are derived from a systematic Hamiltonian theory. The derivation employs Lie transforms and a noncanonical perturbation theory first used by Littlejohn for the simpler problem of asymptotically small gyroradius. For definiteness, only electrostatic fluctuations in slab geometry are considered; however, there is a straightforward generalization to arbitrary field geometry and electromagnetic perturbations. An energy invariant for the nonlinear system is derived, and several limiting forms are considered. The weak turbulence theory of the equations is examined. In particular, the wave kinetic equation of Galeev and Sagdeev can ony be derived by an asystematic truncation of the equations, implying that this equation fails to consider all gyrokinetic effects. The equations are simplified for the case of small but finite gyroradius and put in a form suitable for efficient computer simulation. Although it is possible to derive the Terry–Horton and Hasegawa–Mima equations as limiting cases of our theory, several new nonlinear terms absent from conventional theories appear and are discussed. The resulting theory is very similar in content to the recent work of Lee. However, the systematic nature of our derivation provides considerable insight into the structure and interpretation of the equations.
Turbulent relaxation of compressible plasmas with flow
Authors: Finn John M. , Antonsen T. M.
Published: March 2003

Relaxation of compressible plasmas to an equilibrium with flows is studied. The magnetohydrodynamic (MHD) equations with large parallel thermal conductivity and ergodic field lines show that ∫v⋅B is an invariant even with compression. Also, S=∫ρ ln( p/ργ) is the only entropy-like invariant of the MHD equations with infinite parallel thermal conductivity; S increases in time with finite thermal conductivity. The other invariants are energy, helicity ∫A⋅B, mass ∫ρ, and possibly angular momentum. Equilibria are found by extremizing energy while conserving these invariants or by maximizing entropy with the energy and other invariants as constraints. These invariants are complete in the sense of generating all equilibria that form after relaxation with ergodic field lines. For parallel flows, there are three classes of solutions characterized by the sign of dρ/dB2 and the mirror mode parameter. A sufficient condition for stability is derived. This condition is never satisfied by the class with dρ/dB2>0, indicating the possibility of unstable resistive interchanges. The class with dρ/dB2<0 is stable if generalizations of the local firehose and mirror criteria are satisfied, and if generalized Taylor helicity eigenvalues exceed those of the equilibrium. A comparison of these conditions with those of Frieman and Rotenberg is discussed.
Ballooning instabilities in hot electron plasmas
Authors: Antonsen T. M. , Lee Y. C. , Berk H. L. , Rosenbluth M. N. , Van Dam J. W.
Published: March 2003

The stability of short-wavelength, low-frequency modes in a hot electron plasma is investigated. Particular attention is devoted to the effect of the dependence of perturbed quantities along the equilibrium magnetic field lines. Two types of modes in a bumpy torus are considered: modes which are nearly flutes in each mirror cell but vary in phase from cell to cell, and modes which have significant variation in each cell. Generally speaking, allowing perturbations to vary along the field lines is destabilizing. However, sufficiently large hot electron gyroradii can stabilize all short perpendicular wavelength modes.
Destabilization of the hot-electron precessional mode in tandem mirrors and bumpy tori
Authors: Baldwin D. E. , Berk H. L.
Published: March 2003

The high-frequency precessional mode of a hot-electron-stabilized magnetic configuration has previously been shown to be stable in a window of core-plasma mass. Under conditions of frequency matching, the resulting stable negative-energy precessional wave can be destabilized by coupling to positive-energy shear-Alfvén waves. Coupling is avoided when the hot-electron precession frequency exceeds the core-plasma ion gyrofrequency.
Computer simulations of upper-hybrid and electron cyclotron resonance heating
Authors: Lin A. T. , Lin Chih Chien
Published: March 2003

A 2 1/2 -dimensional relativistic electromagnetic particle code is used to investigate the dynamic behavior of electron heating around the electron cyclotron and upper-hybrid layers when an extraordinary wave is obliquely launched from the high-field side into a magnetized plasma. With a large angle of incidence most of the radiation wave energy converts into electrostatic electron Bernstein waves at the upper-hybrid layer. These mode-converted waves propagate back to the cyclotron layer and deposit their energy in the electrons through resonant interactions dominated first by the Doppler broadening and later by the relativistic mass correction. The line shape for both mechanisms has been observed in the simulations. At a later stage, the relativistic resonance effects shift the peak of the temperature profile to the high-field side. The heating ultimately causes the extraordinary wave to be substantially absorbed by the high-energy electrons. The steep temperature gradient created by the electron cyclotron heating eventually reflects a substantial part of the incident wave energy. The diamagnetic effects due to the gradient of the mode-converted Bernstein wave pressure enhance the spreading of the electron heating from the original electron cyclotron layer.
Circular waves on a stationary disk in rotating flow
Authors: Savaş Ö.
Published: March 2003

Circular waves of a new type are observed in the disk boundary layer occurring on the end of a cylindrical cavity during impulsive spindown to rest. These waves occur around z(Ωi/ν)1/2≊3.6 and are reminiscent of Tollmien–Schlichting waves of an ordinary boundary layer. It is concluded that these waves are manifestations of the instabilities excited in the Bödewadt-type boundary layer developed over the disk.
A Lagrangian two-time probability density function equation for inhomogeneous turbulent flows
Authors: Pope S. B.
Published: March 2003

An exact equation for the Lagrangian two-time velocity joint probability density function (pdf) is derived from the Navier–Stokes equation. The pdf equation contains as an unknown the conditional expectation of the fluid acceleration. A linear Markov model is proposed which leads to a modeled equation that is consistent both with Kolmogorov’s theory in the inertial subrange and with Reynolds-stress models. The dissipation rate is obtained from the joint pdf in a way that is consistent with the modeled dissipation equation. A Monte Carlo method can be used to solve the modeled two-time pdf equation for inhomogeneous turbulent flows.
Breaking of liquid films and threads
Authors: Keller Joseph B.
Published: March 2003

Breaking of nonuniform liquid films and threads is analyzed by means of a simple equation of motion. The results are applied to the bursting film between two merging gas bubbles in a liquid, to growing holes in liquid films on solid surfaces, and to the pinching off of a liquid jet. Two of the results are compared with exact hydrodynamic calculations.
Equilibrium statistics of two-dimensional viscous flows with arbitrary random forcing
Authors: Thompson Philip Duncan
Published: March 2003

A method to solve the Liouville equation for an ensemble of two-dimensional viscous flows that are driven by random forcing with arbitrary statistics is outlined. By appropriate transformations of both the dependent variable (probability distribution) and independent variables, and by expansion of the solution in eigenfunctions of the separable part of the Liouville operator, it is found possible to reduce the problem to that of solving a simultaneous system of nonhomogeneous linear algebraic equations. The equilibrium kinetic energy and energy-transfer spectra can be calculated directly from the equilibrium probability distribution.
A general theory of magnetically insulated electron flow
Authors: Mendel C. W. , Seidel D. B. , Slutz S. A.
Published: March 2003

A theory of magnetically insulated flow is developed where two- or three-dimensional spatial variation and temporal variation are allowed as long as the spatial variations in the flow direction are over many electron gyrolengths and temporal variations are over many electron gyroperiods. These criteria are met in most flows of interest. The theory is based upon describing the electron dynamics in terms of parameters which are constants of motion in one-dimensional, time-independent flows.
Some minimum-energy toroidal equilibria
Authors: Bhattacharjee A. , Wiley J. C.
Published: March 2003

Equilibria with minimum energy are constructed from a variational principle in which the energy of a plasma is minimized subject to a recently proposed set of global invariants. The equilibrium equation is solved in axisymmetric, toroidal geometry. In order to compute toroidal equilibria the variational principle is exploited to obtain a reduced set of ordinary differential equations which we solve numerically. Tokamaklike and pinchlike solutions of minimum energy are found in toroidal geometry. Based on the ideal and resistive stability studies of the cylindrical limit of these solutions, it is argued that some of these equilibria should have robust stability to modes of low m and n number.
Energy principle with global invariants: Applications
Authors: Bhattacharjee A. , Dewar R. L. , Glasser A. H. , Chance M. S. , Wiley J. C.
Published: March 2003

Minimum-energy equilibrium states of a pressureless cylindrical plasma are constructed subject to a recently proposed set of constraints appropriate to turbulent relaxation dominated by a tearing mode of single helicity. Stability to ideal and resistive modes is examined. It is found that the relaxed states of minimum energy are always stable to the dominant mode, as well as a class of other low m and n modes, but that there are typically high n residual instabilities in the case of the pitch branch, and a residual m=3, n=2 resistive instability is found for an otherwise stable narrow window in the tokamak branch. It is argued that the residual instabilities may be compatible with experimental observations.
The first and the final fifty nanoseconds of a fast focus discharge
Authors: Decker G. , Kies W. , Pross G.
Published: March 2003

Conventional plasma focus devices of different capacitor bank energy (kilojoules to megajoules) show very similar discharge behavior with respect to breakdown, compression, and pinch data. The use of SPEED 1—a plasma focus driven by a fast high impedance bank (voltage 200 kV, initial current rise >5×1012 A/sec, bank impedance 160 mΩ),—allowed the investigation of a 400 nsec discharge in a parameter range not previously accessible. Considerable improvement of current and neutron efficiency (30 A/J; 106/J) has been achieved as compared with values (≤20 A/J; 2×105/J) of conventional slow devices of equal energy (20 kJ). Exceptional compression dynamics (current derivative spike >1013 A/sec FWHM 10 nsec) is observed from electric signals. This is due to the current stabilizing effect of the high bank impedance and especially to improved current sheath structure (reduced thickness, enhanced conductivity). Current density and conductivity of the sheath depend on ignition parameters at breakdown (voltage, current rise), the low values of which are possibly responsible for the similarity of the discharge behavior of conventional devices. Conclusions on further improvements are drawn.
Frozen magnetic flux in a tokamak discharge
Authors: Osborne T. H. , Dexter R. N. , Prager S. C.
Published: March 2003

The magnetic flux at the center and the safety factor profile are found to remain stationary (or frozen) during the rapid change in electron temperature that accompanies the sawtooth oscillation. This is consistent with the classical expectation.
Experimental determination of one-dimensional spectra in high-speed boundary layers
Authors: Demetriades Anthony , Martindale William R.
Published: March 2003

The spectra of velocity and temperature fluctuations have been measured in turbulent boundary layers at Mach numbers from 3 to 9.4, momentum Reynolds number from 1000 to 36 000, and wall-to-stagnation temperature ratios from 0.36 to 0.94. The data were found to agree with the universal spectrum of low-speed isotropic flows at large wavenumbers, and with the Reynolds-number-dependent behavior of the spectrum already known to occur for small wavenumbers in low-speed shear flows.
Rapid distortion of small-scale turbulence by an axisymmetric contraction
Authors: Hultgren Lennart S. , Chen Shu-Cheng
Published: March 2003

The rapid distortion of small-scale turbulence by an axisymmetric internal contracting flow has been investigated. The analysis is an example of the modern version of rapid distortion theory in that the effects of vortex bending and tilting are included. On the axis of symmetry, the classical results are recovered. The longitudinal and lateral velocity components are suppressed and amplified, respectively, with increasing contraction ratio. These effects are enhanced with increasing radial position. The numerical calculations also show that the correlation between the streamwise and radial velocity components increases monotonically with respect to both the radial location and the contraction ratio. The longitudinal and lateral one-dimensional spectra show a shift towards higher and lower wavenumbers, respectively, as the contraction ratio increases. Here, these effects become less pronounced for larger radii. These results also show that the downstream longitudinal integral length scale based on the streamwise velocity as well as the one based on the transverse velocity increase with the radial coordinate.
Third-order conditional transport correlations in the two-dimensional turbulent wake
Authors: Fabris G.
Published: March 2003

The far turbulent wake of a cylinder was investigated. Slight heating of the cylinder permitted the use of the conditional sampling technique to gain detailed insight into the transport mechanism. All important third-order correlations of u, v, w, and θ are included. Among the interesting results are that the lateral transport of u2 and v2 fluctuations is three times as intense as that of w2, whereas the streamwise transport of u2 is only twice as intense as that of v2 and w2. On a conditional basis the highest lateral transport is of the v2 component by the intermittent turbulent zones on the wake sides. The conditional lateral transport of the temperature fluctuations by turbulent bulges is especially intense. It is believed that the data will be helpful in refining Reynolds stress closure equations in predictive methodology.
Structures and flow reversal in turbulent plane jets
Authors: Goldschmidt V. W. , Moallemi M. K. , Oler J. W.
Published: March 2003

The classical description of the longitudinal velocity distribution of fully developed turbulent jets does not conceive of the possible existence of negative velocities. Recent photographs, measurements, and analysis suggest that as far as 60 nozzle widths downstream, there may persist regions of flow reversal which are related to a coherent underlying vortical structure.
Effect of compressibility on the Rayleigh–Taylor instability
Authors: Bernstein Ira B. , Book David L.
Published: March 2003

Eigenfrequencies are calculated for infinitesimal perturbations of the system consisting of two semi-infinite regions, each filled with a constant-temperature ideal polytrope stratified exponentially against gravity. The linear growth rate for the Rayleigh–Taylor instability which occurs when the density above the interface exceeds that below it is shown in the model to vary linearly with wavenumber k as k→0. The incompressible fluid result is obtained when the adiabatic index γ approaches ∞. For finite γ (i.e., compressible fluids), the growth rates are in general larger than in the incompressible case. Numerical results and limiting cases are described which illustrate this conclusion.
Effects of random fluctuations in external magnetic field on plasma conductivity
Authors: Fenstermacher Max E. , Akcasu A. Ziya
Published: March 2003

The electrical conductivity for a weakly ionized magnetized plasma in which the external magnetic field is subject to random fluctuations is calculated using a stochastic differential equation approach. Calculation of the dc conductivity for a magnetized plasma with random fluctuations in the magnitude of the external magnetic field is presented as an example. The effect of the random fluctuations in the magnetic field appears as a modification of the effective collision frequency of the plasma. The results for other cases involving ac electrical conductivity and diffusion coefficient showing similar modification of the effective collision frequency of the plasma are also presented.
Anomalous reflection of electromagnetic ion cyclotron waves from a plasma
Authors: Shukla P. K.
Published: March 2003

It is shown that an electromagnetic ion cyclotron wave can be reflected by a standing density ripple in a plasma flowing along an external magnetic field. An analytical expression for the total plasma reflectance is obtained.
Stimulated Brillouin scattering of nonducted whistlers
Authors: Antani S. N. , Kaup D. J. , Shukla P. K.
Published: March 2003

Nonlinear decay of a large amplitude nonducted whistler mode signal is considered. The decay waves are the daughter whistler and obliquely propagating ion/electron acoustic waves or a slow Alfvén wave. Expressions for the growth rates and thresholds are obtained. Application of this investigation to space plasmas is pointed out.
Ion-cyclotron resonance heating in a strongly inhomogeneous magnetic field
Authors: Nozaki K. , Fried Burton D. , Morales G. J. , Fukuyama A.
Published: March 2003

Single particle ion-cyclotron resonance heating in a strongly inhomogeneous magnetic field with constant scale length κ−1 is studied for a uniform rf field. Because the ion-cyclotron frequency is energy dependent, ions get out of resonance, exhibiting superadiabatic response with no time-averaged energy gain. However, the anharmonicity of the motion results in resonances at multiple energy values, and for rf field amplitudes above a critical value Ec, these resonances overlap, resulting in stochastic heating. Explicit analytic expressions are given for the particle orbits and for Ec as a function of energy and rf frequency.
Transport and isomorphic equilibria
Authors: Boozer Allen H.
Published: March 2003

It is shown that large classes of plasma equilibria can have identical drift orbits and associated transport. Such equilibria are called isomorphic. In particular, the neoclassical coefficients are given for all equilibria in which the magnetic field strength depends on one helicity.
Axisymmetric finite-β minimum energy equilibria of weakly toroidal discharges
Authors: Edenstrasser J. W. , Schuurman W.
Published: March 2003

A variational principle is presented for the calculation of finite-β minimum energy states of partially relaxed toroidal plasma columns. The potential energy of the plasma is minimized, with the Woltjer–Taylor invariant and the equilibrium equation as constraints, and with suitably chosen boundary conditions. A solution is given for a cylindrical plasma column with translational and rotational symmetry, embedded in a vacuum. The familiar technique of an expansion in the inverse aspect ratio is then employed to calculate the first-order toroidal correction to the cylindrical equilibrium. To this end, a general method, applicable to any zero-order solution, is used. Finally, it is shown that the results of the theory are in agreement with the data on magnetic field profiles of the reversed field pinch experiment η-β-II, and those of pressure profiles of the screw pinch experiment SPICA and of the Frascati tokamak.
Magnetohydrodynamic and double adiabatic stability of compact toroid plasmas
Authors: Grossmann William , Hameiri Eliezer , Weitzner Harold
Published: March 2003

The linear stability of compact toroids is examined in the magnetohydrodynamic and double adiabatic models. The long-thin approximation in ideal magnetohydrodynamics is used to investigate tilting and shifting modes in field reversed configurations without toroidal fields. A necessary and sufficient condition is obtained and used to show that the combination of flux surface shaping and profile flatness results in stability to a class of transverse modes in a region about the O point. The double adiabatic model yields for general confined plasmas a sufficient condition for stability in the form of six ordinary differential equations along each field line. Further reduction of the condition to a single second-order equation depends on the sign of ∂S/∂B, where S(ψ,B)=p∥B5/p3⊥ is a combination of the two entropies. Strong stabilizing effects of pressure anisotropy are shown.
Analytic theory of ablation layer instability
Authors: Baker L.
Published: March 2003

An analytic theory of the stability of thin ablation fronts is developed. Large wavenumbers are found to be stabilized by lateral heat conduction; this results in maximum growth rate of order g/v at wavenumber of order g/v2, where g is the acceleration of the front and v the ‘‘exhaust’’ velocity of the ablated material. For typical ablation front parameters the model predicts that implosions with shell aspect ratios of at least 20 should be viable.
Convection induced by insulated boundaries in a square
Authors: Quon Charles
Published: March 2003

Convection induced by insulated boundaries in a tilted square cavity is analyzed for large external Rayleigh number A. Two opposite walls of the cavity are insulated, and the other opposite walls are maintained at temperatures linear in ξ, the vertical coordinate. Two orientations of the cavity are analyzed in detail for φ<A−1/2 and φ=π/2, φ being the angle between the heated walls and the horizontal. For φ<A−1/2, the flow consists of boundary layers of thickness O(A−1/4) near the insulated wall and O(A−1/6) near the heated walls. For φ=π/2, the flow consists of double boundary layers O(A−1/6) and O(A−1/8) near the insulated walls, but no boundary layer exists near the heated walls, except at the end points of the double boundary layers. The flows for π/4 are qualitatively described. The analogy between the flows considered here and those in a rotating container is also discussed.