bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic guiding, focusing and compression of an intense charge-neutral ion beam
Authors: Robertson Scott
Published: March 2003

The motion of an intense, charge- and current-neutralized ion beam propagating axially into a solenoidal magnetic field in vacuum is shown to be described by one of two models depending on whether or not the beam magnetic skin depth c/ωpe exceeds half the beam radius. For a less-intense beam having a large skin depth, a new theoretical model is presented which shows that a solenoidal field penetrates to the beam axis and acts as a lens. If a minimum beam density is exceeded, the electrons and ions are brought to a common focus as a result of quasineutrality. The collective focal length is the geometric mean of the focal lengths for the two species when traveling alone. For an intense beam having a small skin depth, the applied field is excluded from the center of the beam channel by a skin current and the resulting magnetic pressure gradient compresses the beam. This case is similar to the theta pinch and is described by a snowplow model. The two models are verified by experiments with a 150 kV, 0.5–5 A/cm2 proton beam from a magnetically insulated diode. Possible applications in magnetic fusion, inertial fusion, and magnetospheric physics are discussed.
Turbulence with a spectral gap
Authors: Pouquet A. , Frisch U. , Chollet J. P.
Published: March 2003

Under what conditions turbulent motions produced by two different mechanisms can coexist and have energy spectra separated by a gap, as often observed in astro- and geophysical flows, are investigated. The dynamical consistency of such a gap is illustrated using a two-point statistical closures and some experiments are suggested.
Equation of motion for a small rigid sphere in a nonuniform flow
Authors: Maxey Martin R. , Riley James J.
Published: March 2003

The forces on a small rigid sphere in a nonuniform flow are considered from first prinicples in order to resolve the errors in Tchen’s equation and the subsequent modified versions that have since appeared. Forces from the undisturbed flow and the disturbance flow created by the presence of the sphere are treated separately. Proper account is taken of the effect of spatial variations of the undisturbed flow on both forces. In particular the appropriate Faxen correction for unsteady Stokes flow is derived and included as part of the consistent approximation for the equation of motion.
Alpha particle loss and energy deposition in tandem mirrors
Authors: Santarius John F. , Callen J. D.
Published: March 2003

The bounce averaged Fokker–Planck equation is solved for alpha particles slowing down in a background plasma of elecrons and ions in the central cell of a tandem mirror. The problem is formulated for a general, monotonic magnetic field, and then solved numerically for the square-well case. This gives a distribution function in energy and lambda (magnetic moment/energy). The resulting loss of alpha particles by scattering into the magnetic loss cone and the distribution of alpha-particle energy to ions and electrons are given. For typical tandem mirror reactor parameters (Te ∼30 keV, φ∼100 keV, R∼4) about half of the alpha particles are born in or scatter into the loss cones, but less than 25% of the total alpha particles’ energy is lost through these mechanisms.
Effect of rotation on the stability of a bounded cylindrical layer of fluid heated from below
Authors: Buell Jeffrey C. , Catton Ivan
Published: March 2003

The onset of steady natural convection in a rotating cylindrical volume of fluid completely bounded by rigid surfaces is examined for moderate Taylor numbers (Ta≤2×106) and aspect ratios (A≤2). The critical Rayleigh number for three dimensional disturbances is found to be lower than that for the radially unbounded problem by up to a factor of six. The thermal boundary condition on the lateral walls is shown to have a greater effect here than in the nonrotating case.
Drift-resistive interchange and tearing modes in cylindrical geometry
Authors: Finn John M. , Manheimer Wallace M. , Antonsen Thomas M.
Published: March 2003

Resistive interchange and tearing modes are studied including the effects of electron parallel pressure gradient and ion polarization drift (Hall terms) as well as finite plasma compressibility and perpendicular resistivity. It is found that for unfavorable curvature and weak tearing forces (positive but small Δ′) all modes are stabilized provided the effects of the Hall (drift) terms are large enough. The principal stabilizing influences are plasma compression and coupling to outgoing drift waves. The relevance to reversed field pinch and spheromak experiments is discussed.
Nonlinear electron Landau damping of ion-acoustic solitons
Authors: Meiss J. D. , Morrison Philip J.
Published: March 2003

Several authors have treated kinetic effects associated with the ion-acoustic soliton; e.g., Ott and Sudan investigated linear electron Landau damping and Karpman and Lotko have looked at damping due to ion reflection. Here an O’Neil-type frozen wave calculation that includes effects associated with electron orbits in a soliton is presented. This calculation differs from previous ones in that the usual three time scale argument is made: ωpe ≫ωbe ≫γL. The orbit effects included in this ordering become important at the modest amplitude eΦ/Te≳(me/mi).2 Saturation at finite amplitude is predicted.
Solitary drift waves in the presence of magnetic shear
Authors: Meiss J. D. , Horton W.
Published: March 2003

The two-component fluid equations describing electron-drift and ion-acoustic waves in a nonuniform magnetized plasma are shown to possess nonlinear two-dimensional solitary wave solutions. In the presence of magnetic shear, radiative shear damping is exponentially small in Ls/Ln for solitary drift waves, in contrast to linear waves.
Geometry effects on ion heating at ion cyclotron frequencies
Authors: Weitzner Harold
Published: March 2003

The interaction of ions with electromagnetic waves at frequencies on the order of the ion cyclotron frequency is studied without the assumption of geometrical optics. Instead, it is assumed that the time for an ion to travel one wavelength is large compared with the wave period. Such a scaling admits nontrivial effects of the plasma geometry and is particularly appropriate to the ELMO Bumpy Torus experiment. The relevant frequency range is from one to a moderate multiple of the ion cyclotron frequency. It is shown that the ion response may be represented by the usual cold ion conductivity and with appropriate jump conditions across the fundamental resonant surface. Nonresonant energy transfer is calculated and shown to exist under many circumstances, depending on the antenna and electron response. Expressions are given for the resonant energy absorption at the fundamental and second harmonic. At higher harmonics, the nonresonant energy transfer is large compared with the resonant absorption.
A numerical analysis of flute modes in the ELMO Bumpy Torus
Authors: El Nadi A. M. , Whitson J. C.
Published: March 2003

Allowing for realistic magnetic field and plasma density profiles, a radial eigenvalue computer code was used to investigate flute modes in the ELMO Bumpy Torus (EBT).
Slowing down of ion rings
Authors: Ruchti C. B. , Lovelace R. V.
Published: March 2003

A study is made of the slowing down of self-consistent, large-orbit ion rings due to small-angle Coulomb scattering in a dense, low-temperature plasma. A kinetic equation is derived for the ion-ring distribution function f(P,I,t), where P is the canonical angular momentum, and I is an adiabatic invariant which is an implicit function of P and the particle energy. The ratio of the time scale for the perpendicular momentum change (diffusion) to that for the parallel momentum change (drag) is m/M, with M the ion mass and m the electron mass. Therefore, drag is the dominant collisional effect, and the slowing down corresponds to incompressible flow in the (P,I) plane. This flow is calculated numerically by computing sequences of equilibria. Initially well-trapped distribution functions are found to have essentially no particle loss during the first slowing-down time τ, which is defined as the time it would take a single ring particle to slow down to zero energy. In most of the cases studied, the rings shrink in both radius and in axial length, with the field reversal increasing by ≂10% within one τ. The rate of loss of toroidal kinetic energy exceeds the loss of poloidal energy by a factor of typically 1.5–2.0. The self-consistent toroidal electric field opposes the slowing down, increasing the ring life time by 20%–50% over the single-particle slowing-down time.
Zero-frequency cyclotron wave on an intense relativistic-electron beam
Authors: Starke T. P. , Wong H. V. , Rienstra W. W. , Moore B. N. , Cornet Edward , Davis H. A.
Published: March 2003

The zero-frequency cyclotron wave is studied on an unneutralized, magnetized relativistic-electron beam. This stationary mode occurs where the Doppler-shifted beam-cyclotron dispersion branch crosses the wavenumber axis. In this experiment zero-frequency-cyclotron waves have been generated by the injection of foil (and foilless) diode beams into a drift tube. A linear-cyclotron wave-dispersion theory is presented which includes the radial electron-energy variation γ(r) due to beam space charge. The γ(r) variation causes only one propagating eigenmode to exist for each azimuthal-mode number. Numerical simulations of the beam and the cyclotron wave are described. Experimental measurements of the mode radial dependence, the wavelength scaling with magnetic field, and the wavelength scaling with beam energy are also presented.
Field saturation for arbitrary temperature in resonance absorption
Authors: Bezzerides B. , Gitomer S. J.
Published: March 2003

A nonlinear theory of a driven, inhomogeneous, one-dimensional plasma of arbitrary temperature is presented. The model is solved numerically and analytically and detailed expressions are given for the saturated electric field excited at critical density. The results of the calculation are valid for arbitrary temperature and connect in a continuous manner the limits of cold wavebreaking and linear convective saturation. Comparison of the results of the calculation with particle simulations is made and good agreement is obtained.
Stable equilibria having arbitrary q profile
Authors: Reiman Allan H. , Boozer Allen H.
Published: March 2003

It is shown that minimum energy equilibria can be constructed with any q (inverse rotational transform) profile such that 1/q is continuous. These equilibria are stable to both ideal magnetohydrodynamic (MHD) and tearing modes. In particular, explicit equilibria are presented which have a local minimum in q but are nevertheless stable.
An experimental study of entraining, stress-driven, stratified flow in an annulus
Authors: Scranton D. R. , Lindberg W. R.
Published: March 2003

An experimental study of turbulent, two-layer stratified entrainment in an annular, surface stress driven geometry is described. Density and velocity probes were used to determine interfacial locations and flow patterns. Annular geometry effects were examined using three channel widths. The entire flow system was constructed of clear plastic to facilitate observations. The measurements and observations of these flows over a wide range of Richardson number indicate a strong secondary circulation, radial interfacial tilt, and a dominant dependence of flow parameters on the annular radius. It is concluded that the entrainment results in annuli such as the one used in this study may not be directly applicable to one-dimensional entrainment processes.
A model for the turbulent energy spectrum
Authors: Driscoll Richard J. , Kennedy Lawrence A.
Published: March 2003

A simple empirical model is developed for the isotropic turbulent kinetic energy spectrum. The model can define the spectral distribution as a function of Reynolds number at all wavenumbers. It is compared with spectral data from different types of flows over the 13 ≤Reλ≤7000 range.
Analysis of self-similar problems of imploding shock waves by the method of characteristics
Authors: Nakamura Y.
Published: March 2003

The asymptotic self-similar form of cylindrically or spherically imploding shock waves is extracted by numerically solving non-self-similar problems. The shock wave is generated by a contracting piston with finite initial velocity. For the initial shock motion, a perturbation method is used to determine the starting condition for the numerical calculation. Propagation of the shock wave and flow field properties are obtained and the transition of the non-self-similar motion of the shock wave into the self-similar one is presented. Good agreement between self-similar exponents determined from the variation of the shock strength and those calculated by Guderley is obtained.
Evaporation and condensation on two parallel plates at finite Reynolds numbers
Authors: Aoki K. , Cercignani C.
Published: March 2003

A kinetic theory analysis of the problem described in the title indicates that the disagreement with the continuum solution of Plesset can be less severe than it is usually assumed.
Sawteeth, magnetic disturbances, and magnetic flux regeneration in the reversed-field pinch
Authors: Watt R. G. , Nebel R. A.
Published: March 2003

A comparison of experimental data and theoretical results is presented that may help to explain the underlying causes of the flux regeneration mechanism (‘‘dynamo’’) in the reversed-field pinch (RFP). Discrete events are seen on many diagnostic measurements which are coincident with observable increases in toroidal magnetic flux in discharges where the pinch parameter θ (θ≡Bθwall/〈Bφ〉) exceeds the value 1.6. By observing the relative timing and the θ dependence of these events as the discharge is returned to lower values of θ, a case may be made for associating the m=1 tearing mode with the RFP dynamo.
Observations of magnetized plasma flow through stationary background plasma
Authors: Jellison G. , Parsons C. R.
Published: March 2003

The collisionless flow of a laser-produced barium plasma through low-pressure photoionized xenon in the presence of a uniform, transverse magnetic field is studied. Deceleration of the barium is observed, and attributed to magnetically induced instabilities. Interaction close (4 cm) to the target is observed. The front thickness is observed to vary inversely with field strength. Spontaneous emission of 4554.03 Å radiation increases by more than an order of magnitude when the B field and background gas are both present, indicating significant electron heating. Theory shows that these observations can be explained by the modified two-stream instability and the magnetized ion–ion instability.
Radial guiding center drifts and omnigenity in bumpy torus confinement systems
Authors: Hazeltine R. D. , Catto P. J.
Published: March 2003

Collisional transport of a high-temperature plasma across the confining field of a bumpy torus magnetic confinement system depends sensitively upon the functional form of the radial guiding center drift, and thus upon details of the confinement geometry. A general and relatively explicit formula for the radial drift is derived, using the large aspect ratio results of a previous equilibrium study. Allowance is made for (i) arbitrary toroidal variation of the confining field; (ii) field distortion due to plasma currents; (iii) noncircular deformation of the toroidal field coils. However, the analysis pertains only to the plasma core, and not to the high-beta annuli (electron rings) which are usually present in experiments. The question of bumpy torus omnigenity—whether any bumpy torus field configuration is consistent with a vanishing, or nearly vanishing, radial drift— is also investigated. It is found that omnigenity cannot occur, at least in the vicinity of the magnetic axis.
A kinetic cross-field streaming instability
Authors: Wu C. S. , Zhou Y. M. , Tsai Shih-Tung , Guo S. C. , Winske D. , Papadopoulos K.
Published: March 2003

In a high-beta plasma the so-called modified-two-stream instability, which results from strongly magnetized electrons drifting relative to unmagnetized ions across a homogeneous magnetic field, is misnamed because the mode is highly kinetic, particularly when the relative streaming velocity exceeds the Alfvén speed of the plasma. This kinetic cross-field streaming instability is investigated in detail, examining the effect of the electromagnetic terms and the stability boundaries in both low- and high-beta plasmas. An approximate dispersion relation showing the relation of this mode to the whistler is derived and solutions of it are compared with those obtained from the exact dispersion relation. The kinetic mode, unlike the usual modified-two-stream instability, is not stabilized by electromagnetic effects when the relative electron–ion drift speed exceeds the Alfvén speed.
A unified theory of a class of mode conversion problems
Authors: Cairns R. A. , Lashmore-Davies C. N.
Published: March 2003

Many radio-frequency heating methods involve conversion of an incoming wave to another mode which only propagates within the plasma and is ultimately damped. A method previously used to consider electron cyclotron heating by the O mode is extended to other mode conversion phenomena.From the properties of the dispersion relation in the neighborhood of the mode conversion point, differential equations for the mode amplitudes are constructed, in a well-defined way, which give energy conservation in the absence of damping. An analytic solution gives the transmission and conversion coefficients in terms of parameters defining the local behavior of the dispersion relation. The technique is applied to a number of problems, showing that they can all be solved by elementary algebraic manipulations of the local dispersion relation. The phenomenon of electron cyclotron emission also falls rather naturally into this mode conversion picture. It is also suggested that the method of dealing with cyclotron emission may have some relevance to the more general problem of energy transport across a magnetic field. The technique, therefore, brings together into a unified theory a number of problems which have been treated by diverse, and usually more complicated, mathematical techniques in the existing literature.
Nonlinear evolution of the transverse instability of plane-envelope solitons
Authors: Janssen Peter A. E. M. , Rasmussen J. Juul
Published: March 2003

The nonlinear evolution of the transverse instability of plane envelope soliton solutions of the nonlinear Schrödinger equation is investigated. For the case where the spatial derivatives in the two-dimensional nonlinear Schrödinger equation are elliptic a critical transverse wavenumber is found above which the soliton is stable. Application of the multiple time scale method near this critical transverse wavenumber gives a dynamical equation for the nonlinear evolution of the transverse instability. Nonlinearity is found to enhance the growth of the linearly unstable mode. The results are discussed in connection with soliton collapse.
Magnetohydrodynamic equilibrium and stability of field-reversed configurations
Authors: Schwarzmeier J. L. , Barnes D. C. , Hewett D. W. , Seyler C. E. , Shestakov A. I. , Spencer R. L.
Published: March 2003

Magnetohydrodynamic equilibrium and stability studies of field-reversed configurations are presented. Experimentally realistic equilibria are calculated numerically for a plasma inside a conducting cylinder. Stability studies indicate that equilibria ranging from elliptical to highly racetrack-shaped are all unstable to the internal tilting mode.
Electron temperature differences and double layers
Authors: Chan Chung , Hershkowitz Noah , Lonngren K. E.
Published: March 2003

Electron temperature differences across plasma double layers are studied experimentally. It is shown that the temperature differences across a double layer can be varied and are not a result of thermalization of the bump-on-tail distribution. The implications of these results for electron thermal energy transport in laser-pellet and tandem-mirror experiments are also discussed.
Experiments on potential gradients in a current-carrying plasma. II. Turbulence
Authors: Hollenstein Ch. , Guyot M.
Published: March 2003

Two different types of turbulence are observed in the presence of a large potential jump within a current-carrying plasma (vd<vte) and are investigated by means of correlation measurements. The electron beam on the high-potential side induced by the potential jump leads to the high-frequency turbulence ( f=fpe). The unstable waves follow the dispersion relation ω=kvb (vb is the electron beam velocity). The low-frequency turbulence ( f<fpi) observed on the low-potential side could be identified as the ion-acoustic turbulence and is dominated by low-frequency ( f<0.5 fpi ) waves propagating perpendicular to the electron drift.
Experimental studies of spheromak formation
Authors: Bruhns H. , Chin-Fatt C. , Chong Y. P. , DeSilva A. W. , Goldenbaum G. C. , Griem H. R. , Hart G. W. , Hess R. A. , Irby J. H. , Shaw R. S.
Published: March 2003

Studies in the PS-1 spheromak configuration can be effectively formed by a combined z- and θ-pinch technique on both a fast (τformation≂τAlfvén) and a much slower timescale. The gross tilt and shift instability of the toroid can be suppressed by a combination of conduction walls, shaping the separatrix by externally applied fields, and the use of ‘‘figure-eight’’ coils. Optimum stabilty is obtained for almost spherical toroids. Maximum field-reversal times for stable, well-confined toroids are ≥40 /μsec, consistent with resistive decay. Temperatures during the stable decay are 5–10 eV; impurity radiation is an important energy-loss mechanism.
Suppression of the n=2 rotational instability in field-reversed configurations
Authors: Hoffman Alan L. , Slough J. , Harding Dennis G.
Published: March 2003

Compact toroid plasmas formed in field-reversed theta pinches are generally destroyed after 30–50 μsec by a rotating n=2 instability. In the reported experiment, instability is controlled, and the plasma destruction is avoided in the TRX-1 theta pinch through the application of octopole magnetic fields. The decay times for loss of poloidal flux and particles are unaffected by the octopole fields. These decay times are about 100 μsec based on inferences from interferometry and excluded flux measurements. The weak, rotating elliptical disturbance (controlled n=2 mode) also made possible a novel determination of the density profile near the separatrix using single-chord interferometry. The local density gradient scale length in this region is found to be about one ion gyrodiameter.
Equilibrium of a high-current electron ring in a modified-betatron accelerator
Authors: Kapetanakos C. A. , Sprangle P. , Chernin D. P. , Marsh S. J. , Haber I.
Published: March 2003

The dynamics of an ultrahigh-current electron ring in a modified-betatron configuration is addressed. This study includes analytical and numerical results for both ‘‘cold’’ and ‘‘hot’’ rings. It has been found that the walls surrounding the ring and toroidal effects play a very important role in the dynamics of the ring, even when the wall conductivity is infinite. For finite wall conductivity, the diffusion of the ring’s self-magnetic field profoundly impacts its dynamics and in general the equilibrium could be lost if means are not provided to compensate for this effect. In addition, it has been found that the toroidal-field grad B drift is not important, except when the bounce frequency is very small. The general conclusion of these studies is that equilibrium states of ultrahigh-current rings in modified-betatron configuration exist over a wide range of parameters. These states are accessible and within the reach of existing pulsed power technology.
Stability of relativistic laminar flow equilibria for electrons drifting in crossed fields
Authors: Swegle John
Published: March 2003

The stability of the electron flow in magnetically insulated transmission lines and diodes, and in microwave devices is an important issue. Presented in this paper are the results of an electromagnetic, linear stability analysis of transverse magnetic waves propagating along the direction of electron drift in a planar system described by a relativistic laminar flow equilibrium. In general, this equilibrium features cold electrons executing E×B drifts in self-consistently calculated electromagnetic fields. For this paper, a class of laminar flow equilibria in which the relativistic plasma and cyclotron frequencies are multiples of one another was chosen, and the electrons were taken to be confined to a layer placed in a vacuum gap between two conductors. This configuration is driven unstable by two separate mechanisms which tap the free energy associated with the sheared velocity field of the layer. At long wavelengths, the interaction of positive and negative energy surface waves results in the diocotron instability. At shorter wavelengths, the interaction of surface waves and sheets of electrons resonant at the local Doppler-shifted cyclotron frequencies leads to a pair of unstable branches of the dispersion relation which are referred to collectively as the magnetron instability. The scaling of the growth rates with equilibrium parameters and the relative importance of the two instabilities are discussed.
Resonance absorption in inhomogeneous plasma with randomly rippled critical surface
Authors: Dragila R.
Published: March 2003

The two-dimensional resonance absorption problem is considered for a wave incident obliquely upon an inhomogeneous plasma with a linear density gradient perturbed by random irregularities which are modulated in the direction perpendicular to both the plasma background density gradient and to the magnetic component of the incident wave. The net coefficient of plasma reflectivity is obtained in the form of a sum of the regular coefficient of reflectivity and the coefficient of anomalous reflectivity. The latter is due to conversion of coherent light into incoherent light by scattering off the random plasma density irregularities.
Experiments on the circulation and propagation of large-scale vortex rings in He II
Authors: Borner Hartwig , Schmeling Till , Schmidt Dieter W.
Published: March 2003

The normal-fluid and superfluid vorticity, produced by forcing He II out of a tube 0.8 cm in inner diameter at velocities of about 10 cm/sec, is investigated in the region in front of the tube orifice. The method of circulation measurement uses the flow-induced variations of the traveling times of second-sound shock waves. The evolution of large-scale vortex rings with a nearly homogeneous vorticity distribution is observed and measured. Their translational velocity and circulation are measured at temperatures 1.28 K≤T≤2.02 K in the region 1–7 orifice diameters downstream from the orifice. Strong evidence for a marked dependence of the normal-fluid circulation in the vortex ring on temperature and time is found. It points out the importance of the mutual friction force for both macroscopic velocity fields.
Calculation of curved shear layers with two-equation turbulence models
Authors: Rodi W. , Scheuerer G.
Published: March 2003

Three extensions of the k-ε turbulence model were tested for shear-layer flows with streamline curvature. The first version is an algebraic-stress model which was derived by simplification of modeled Reynolds-stress equations and does not contain curvature-specific empiricism. The other two versions use a modified form of the dissipation-rate equation. The test cases considered are a convexly curved boundary layer, a curved mixing layer, and wall jets on logarithmic spiral surfaces. The results show that the algebraic-stress model gives the best overall agreement in the curved part of the flow but predicts too fast a recovery after the removal of curvature.
Evolution of externally excited convective cells in plasmas
Authors: Sugai H. , Pécseli H. L. , Rasmussen J. Juul , Thomsen K.
Published: March 2003

Convective cells are excited externally in a fully ionized magnetized plasma, and their space-time evolution is investigated by two-dimensional potential measurements. A positive cell is excited externally in the ‘‘scrape-off’’ layer of a plasma column produced by surface ionization. Its interaction with the main plasma produces a new cell with negative polarity. The two cells propagate close to each other and interact only through a mutual perturbation of orbits.
New flows in a circular Couette system with co-rotating cylinders
Authors: Andereck C. David , Dickman R. , Swinney Harry L.
Published: March 2003

Five previously unobserved and unpredicted flows are found to occur in a fluid contained between concentric cylinders with the inner cylinder rotating faster than, but in the same direction as, the outer cylinder. Flow visualization and spectral studies of these flows yield the following descriptions: (1) Twisted Taylor vortices—each Taylor vortex contains from 14 to 30 apparent twists. (2) Wavy inflow boundaries—traveling azimuthal waves appear on the inflow boundary between Tayor vortices. (3) Wavy outflow boundaries—traveling azimuthal waves appear on the outflow boundary between Taylor vortices. (4) Wavelets—small-amplitude traveling azimuthal waves of high wavenumber appear on both inflow and outflow boundaries of the vortices; this flow is distinct from the wavy vortex flows observed with only the inner cylinder rotating. (5) Braided vortices—the vortices appear to wrap around one another in an irregular fashion. The first four flow patterns, produced through quasistatic changes in the inner-cylinder speed with the outer-cylinder speed fixed, can be periodic in the azimuthal and axial directions; in contrast, the braided-vortex pattern, produced through a rapid increase of the inner-cylinder speed with the outer-cylinder speed fixed, has neither axial nor azimuthal periodicity. The power spectra of all five flows contain sharp components. The ranges of cylinder speeds for the occurrence of these flows, and for some cases, the wave speeds and wavenumbers are determined.
Guiding-center equations and characteristics for particles in a small-amplitude electromagnetic wave
Authors: Rognlien Thomas D.
Published: March 2003

Modified guiding-center equations are derived for particles experiencing a small-amplitude electromagnetic wave of arbitrary polarization. The resulting differential equations contain the first-order wave amplitude explicitly and are relativistically correct. A simple characteristic equation is obtained for the particle trajectory in energy and magnetic-moment phase space for cases where the Doppler frequency shift for drifts perpendicular to the magnetic field is negligible. Two examples are given to illustrate the behavior of particle trajectories in momentum space.
Numerical magnetohydrodynamic studies of magnetic mirrors
Authors: Strauss H. R. , Wollman S. , Wakatani M.
Published: March 2003

Nonlinear, three-dimensional, finite-beta, tensor-pressure, magnetohydrodynamic equilibria are computed for the 2XIIB and TMX-U devices, which are a minimum-B and a tandem mirror, and stability to long-wavelength interchange modes is determined. Finite beta is found to have a relatively weak destabilizing effect. The m=4 distortions of TMX-U flux surfaces are small and tend to produce slightly diamond-shaped cross sections.
Adiabatic compression of elongated field-reversed configurations
Authors: Spencer R. L. , Tuszewski M. , Linford R. K.
Published: March 2003

The adiabatic compression of an elongated field-reversed configuration (FRC) is computed by using a one-dimensional approximation. The one-dimensional results are checked against a two-dimensional equilibrium code. For ratios of FRC separatrix length to separatrix radius greater than about ten, the one-dimensional results are accurate within 10%. To this accuracy, the adiabatic compression of FRC’s can be described by simple analytic formulas.
Harmonic generation of radiation in a steep density profile
Authors: Grebogi Celso , Tripathi Vipin K. , Chen Hsing-Hen
Published: March 2003

It has been observed experimentally that the CO2-laser Gemini and Helios systems generate very high harmonics with nearly constant efficiency over several harmonics with a cutoff near the local plasma frequency at the upper density shelf. To understand the experimental results, the strongly nonlinear electron response to a time-dependent inhomogeneous force is studied. The equation of the electron motion in Lagrangian variables is derived and the conditions on the density scale length and power intensities for wavebreaking are found. By transforming the solution back to Eulerian variables, it is shown that the source of a given high harmonic frequency is localized around the density region where the local plasma wave is at that frequency. Using the numerically obtained nonlinear current density we evaluate the intensity of the radiation coming out of the plasma and verify the nearly constant harmonic production efficiency with a sharp decrease at the local plasma wave frequency at the upper shelf density.
The instability of an electron beam passing through a resistive medium
Authors: Davey Kent R.
Published: March 2003

In the context of magnetospheric and laboratory beam-plasma dynamics, there appears to be a need to examine consistently electron beam density development when coupled to a passive resistive background medium. The problem motivating the present study of an electron beam passing through the ionosphere is one case in point; the localized density evacuations predicted may be the forerunner of a low-frequency, diffusion-dominated, excitatory mechanism in auroral activity. The contribution offered by this work lies in the technique used to follow the electron beam in space time within a Lagrangian framework via the method of characteristics, consistently feeding back the effect of the field coupling with the resistive medium. This consistency along with a simulation of the influence of a local periodic disturbance are achieved by adjusting the upstream conditions in the characteristics solution. Prediction concerning the nature of the instability from the dispersive equation are shown to agree with those predicted from this generalized technique.
Energy confinement studies in the tandem mirror experiment (TMX): Power flow
Authors: Grubb D. P. , Allen S. L. , Casper T. A. , Clauser J. F. , Coensgen F. H. , Cohen R. H. , Correll D. L. , Cummins W. C. , Davis J. C. , Drake R. P. , Foote J. H. , Futch A. H. , Goodman R. K. , Gryczkowski G. E. , Hooper E. B. , Hornady R. S. , Hunt A. L. , Karmendy C. V. , Nexsen W. E. , Pickles W. L. , Porter G. D. , Poulsen P. , Rognlien T. D. , Simonen T. C. , Slaughter D. R. , Coakley P. , Hallock G. A. , Strand O. T.
Published: March 2003

Using the measured plasma densities and energies, the flow of power between the different particle species and regions of the tandem mirror experiment (TMX) is analyzed. The power flow is described by a simple classical model modified to include: (1) a halo of cool plasma that reduces end-cell ion losses due to charge exchange on background gas, (2) instability heating of the central-cell ions both in the central cell and as they escape through the plugs, (3) electron energy transport along the field lines which is less than predicted, and (4) radial transport of the central-cell ions. Our global power balance, including all particles and regions, accounts for 87±27% of the trapped neutral-beam power.
A study of ablation by laser irradiation of plane targets at wavelengths 1.05, 0.53, and 0.35 μm
Authors: Key M. H. , Toner W. T. , Goldsack T. J. , Kilkenny J. D. , Veats S. A. , Cunningham P. F. , Lewis C. L. S.
Published: March 2003

Ablation by laser irradiation at wavelengths λ=1.05, 0.53, and 0.35 μm has been studied from analysis of time-resolved x-ray spectra of layered targets and of ion emission. Irradiance was varied in the range 2×1013 to 2×1015 W cm−2 with constant laser power and variable focal spot size. Deductions include the effect of lateral energy transport from small focal spots and ablation rates and ablation pressures obtained both in the limit of negligible transport and when lateral transport is significant. Advantages of short wavelengths for ablatively driven implosions are quantified.
Reflection dissipation of an ion-acoustic soliton
Authors: Lotko W.
Published: March 2003

The dynamic interaction between an ion-acoustic soliton and adiabatically reflected particles is investigated. A perturbed Korteweg–de Vries (KdV) equation for the wave electric potential is derived from the Vlasov–Poisson equations and solved in the adiabatic soliton approximation. The soliton attenuation/amplification rate is found to be proportional to χi‖Φm‖1/2, where Φm is the initial soliton amplitude, and χi is the imaginary part of the linear susceptibility of ions/electrons when the wave is compressive/rarefactive mode. The amplitude variation is caused by momentum exchange between the soliton and reflected particles, as viewed from the soliton reference frame.
Perpendicular distribution of electrostatic electron-cyclotron radiation under dense plasma conditions
Authors: Lembège B.
Published: March 2003

The electrostatic potential distribution of a point source antenna is studied numerically within the first dispersion branch defined by 1<Ω<2, where Ω is the monochromatic frequency of the source normalized to the electron gyrofrequency. The whole study is performed for the direction of radiation R⊥ perpendicular to the magnetostatic field B0. The least-damped-roots approximation is used which allows the inclusion of two modes, the main mode (m) and the cyclotron harmonic mode (h), the contribution of each being essential for the total potential estimate. It appears that under dense plasma conditions, only the mode h has a low-damped part in its dispersion curves and is primarily responsible for the behavior of the total potential ‖φ‖. The potential then exhibits two different scales in its amplitude modulation as a function of R⊥ and Ω. The small-scale pattern reflects the phase variation of this mode and the large-scale pattern reflects both diffraction and directional effects. A careful diagnosis is necessary since, according to the distance R⊥, a main peak can occur at a frequency which is not the expected usual resonance frequency. Similar results are obtained for various dense plasma conditions provided that the electron plasma frequency Ωp is larger than the higher gyroharmonic of the dispersion branch of concern, i.e., presently Ωp≫2.
Unified theory of parametric excitations in magnetized plasma produced by the action of nonmonochromatic driver pump. I. Modulated driver pump
Authors: Stefan V.
Published: March 2003

The theory of parametric excitations in a magnetized plasma by the action of a driver pump with time-varying amplitude and phase is developed. As a concrete example, amplitude modulation in the form of rectangular pulse train and phase modulation in a sinusoidal form are assumed. Both nonresonant interaction, when the driven-pump carrier frequency is larger than all high eigenfrequencies of magnetized plasma, and resonant interaction, when the carrier frequency is equal to some of the higher magnetized plasma eigenfrequencies, are studied. Excitations of upper-hybrid, lower-hybrid, and oblique Langmuir waves coupled to ion-acoustic waves are considered. In all of these cases threshold values and parametric growth rates are obtained and compared with the case of a monochromatic driver pump (constant amplitude and phase). The relevance of the results obtained is discussed for laser and microwave-plasma interactions.
Unified theory of parametric excitations in magnetized plasma produced by the action of nonmonochromatic driver pump. II. Multiple driver pump
Authors: Stefan V.
Published: March 2003

Using the Vlasov–Poisson formalism a theory of parametric interaction of n driver pumps with magnetized plasmas is developed. The interaction when the plasma appears to be transparent to all driver pumps is studied. It is shown that in this case a variety of parametric instabilities exist which are absent in a single driver pump-plasma interaction. The resonant interaction is also considered. Threshold values and parametric growth rates are obtained for the case of excitation of electron-Bernstein modes coupled to ion-Bernstein modes. The relevance of the results obtained is discussed for electromagnetic wave-ionospheric plasma interaction and laboratory plasma experiments.
Oblique incidence of a strong electromagnetic wave on a cold inhomogeneous electron plasma. Relativistic effects
Authors: Bourdier A.
Published: March 2003

A method which simplifies the investigation of the form of an obliquely incident wave on a plane parallel layered plasma by reducing it to the case where the wave is normally incident is presented. The resonance at N0=Nc (critical density) is discussed. Another resonance at N0=4Nc is described.
Simulations of compressional stabilization of the interchange mode
Authors: Goede H. , Humanic D. G. , Dawson J. M.
Published: March 2003

A two-dimensional particle simulation study of the interchange mode is presented. The computer model utilizes a bounded, electrostatic code which calculates full particle dynamics in an inhomogeneous magnetic field. Compressional stabilization, which is well known from flux tube interchange arguments, is tested. The code results do not appear to indicate any stabilization due to plasma compression. Growth rates derived from a dispersion relation obtained from the Vlasov equation in a local approximation do not compare well with those obtained from the simulations. The simulations indicate that the interchange mode develops into vortex structures after linear growth and further show stabilization due to finite-Larmor-radius effects.
Radial transport, end loss, and ambipolarity in tandem mirrors
Authors: Myra J. R. , Catto P. J.
Published: March 2003

A simple one-dimensional (radial) model describing end loss and radial transport in a tandem mirror is developed. The model enables a self-consistent calculation of various ambipolar potential profiles and the center-cell density profile in the presence of either conducting or insulating end walls. Results of the computation reproduce many features seen in the Tandem Mirror Experiment (TMX). Signatures of radial transport are delineated, and the effect on confinement is studied over a range of parameters. Optimal and critical values in parameter space are also investigated.
The tilting mode in rigidly rotating field-reversed configurations
Authors: Clemente R. A. , Milovich J. L.
Published: March 2003

The tilting-mode stability of field-reversed configurations is analyzed taking into account plasma rotational effects that had not been included in previous theoretical treatments. It is shown that for a rigidly rotating plasma in stationary equilibrium, stability can be attained if the plasma rotational energy is of the same order as the thermal energy. Since presently available values of the rotational velocities are quite lower than required by the stabilization mechanism considered here, the contribution of this effect to the overall stability of the mode does not appear to be significant.
Linear mode conversion in laser plasmas
Authors: Kull H. J.
Published: March 2003

Optical excitation of electrostatic modes in front of an infinite step in the plasma density can lead to total absorption of the incident light. The conditions for complete mode conversion are analyzed for inhomogeneous and thermal surface layers and possible applications for laser plasma experiments are discussed.
Intensity moments by path integral techniques for wave propagation through random media, with application to sound in the ocean
Authors: Flatté Stanley M. , Bernstein David R. , Dashen Roger
Published: March 2003

Utilizing the formalism of path integrals, the intensity moments of waves propagated through anisotropic inhomogeneous random media in the presence of a deterministic wave speed variation can be formed. In the saturated scattering regime, intensity fluctuations are expected to be Rayleigh distributed to first order. A correction to Rayleigh statistics is calculated that results in a form for the intensity moments: 〈In〉=n!〈I〉n[1+ 1/2 n(n−1)γ], where γ depends on the fluctuation spectrum of the medium. It is shown that the correction can be approximated by an integral along the ray of a spectrally weighted local contribution. This local contribution, called the micropath focusing function, depends both on the spectrum of the medium, and on a Green’s function which is shown to be related to the phase curvature. In the straight line horizontal ray approximation, the ‘‘micropath focusing parameter’’ γ can be approximated by 3/(2 ln Φ) in the partially saturated regime, and by [0.83/(ln Φ)3/2](1/ΛΦ) in the saturated regime, where Φ and Λ are the strength and diffraction parameters for medium fluctuations along a ray. The internal wave spectrum is chosen as an example for fluctuations in oceanic sound transmission. Theoretical results are obtained for curved rays by numerical integration, and are compared with observations from three ocean acoustics experiments.
Nonlinear electrohydrodynamic Rayleigh–Taylor instability. II. A perpendicular field producing surface charge
Authors: Mohamed Abou El Magd A. , El Shehawey El Sayed F.
Published: March 2003

The nonlinear electrohydrodynamic stability of two superposed dielectric fluids is studied. The system is stressed by a normal electric field such that it allows for the presence of surface charges at the interface. The method of multiple scale perturbations is used to obtain two nonlinear Schrödinger equations describing the behavior of the disturbed system. The stability of the perturbed system is discussed both analytically and numerically and stability diagrams are obtained. The electric field plays a dual role in the sense that it can stabilize the system for relatively large values of wavenumbers and small values of the density ratio and vice versa. The stabilizing range of the electric field for some given density ratios is tabulated. The analogy with hydromagnetic stability is established. Expressions for the surface elevation and the cutoff wavenumber separating stable and unstable disturbances are also obtained.
Cylindrically converging shock and detonation waves
Authors: Matsuo Hideo
Published: March 2003

The non-self-similar implosion of cylindrical shock and detonation waves generated by an instantaneous energy release at a cylindrical wall is analyzed theoretically by the method of integral relations. The analysis shows that as the wave propagates towards the axis, the solutions tend to approach but never reach the self-similar implosion limit. The rate of approach appears to be slower than expected, and the region of applicability of the self-similar solution appears to be restricted to a very small region behind the front. This tendency is more pronounced for the detonation case. It is also demonstrated that for detonations where the initiation energy is negligible in comparison with the chemical energy, the Chapman–Jouguet detonation jump conditions apply at the front except near the axis and near the outer wall. The chemical heating in the detonation process increases the pressure and the temperature but considerably reduces the density near the front.
Shock damping in an ultrarelativistic gas
Authors: Granik A.
Published: March 2003

The damping process for ultrarelativistic plane, cylindrical, and spherical waves is studied with the help of a model of the asymptotic degeneration of the shock wave in a quasi-acoustic wave. Simple formulas are derived for the dependence of parameters of a shocked gas as functions of the distance of the shock front from the source.