bluuebunny/crossref_metadata_2025 huggingface dataset:
Reconnection of magnetic field-lines in an elongated toroidal plasma
Authors: Ikezi H. , Schwarzenegger K. F.
Published: March 2003

A poloidal quadrupole field is applied to a tokamak discharge in order to study the magnetic field-line reconnection. Elongation of the poloidal cross-section of the plasma is observed when the quadrupole field strength is below a critical value. As the quadrupole field is increased above this critical field strength, reconnection occurs and the magnetic axis changes from being the center of elliptic magnetic surfaces (i.e., an ’’O’’ point) to the center of symmetry of hyperbolic surfaces (an ’’X’’ point). The reverse process is also observed.
Guiding-center Hamiltonian for large gyroexcursion particles in mirror configurations
Authors: Mynick H. E.
Published: March 2003

An explicit expression is derived for the guiding-center Hamiltonian K, an expression valid for particles having gyroexcursion ρ comparable to the system size, such as the high-energy ring particles in the Berta experiment. The form of K is quite simple. It depends on the magnetic field strength B and its radial derivatives, evaluated at an average radius of the particle from the symmetry axis, and it allows for a slow, but otherwise arbitrary, variation of B along the axis. The adiabatic invariant of the gyromotion is related not to the full kinetic energy of the particle normal to the field, as is μ≡(1/2)mv⊥2/B in small-ρ theories, but rather to the small additional kinetic energy due to the radial wobble of the particle about a constant-radius orbit.
Extension of weakly nonlinear theory of solitary wave propagation
Authors: Joseph R. I. , Adams R. C.
Published: March 2003

A localized wave propagating horizontally at speed c through a fluid of arbitrary depth H which consists of a lower vertical layer (thickness L) with a weak-depth-dependent density below an upper layer of constant density is considered. The flow is characterized by Long’s equation for the stream function ψ. Exact analytic results are obtained for ψ through O[1/(h−1)] and the wave speed through order O[1/(h−1)2], h=H/L. The terms in ψ of O[1/(h−1)2] are obtained numerically. Using these results the range of validity of such a perturbative approach is examined.
Harmonic emission from adiabatically collapsing Langmuir solitons
Authors: Hafizi B. , Goldman Martin V.
Published: March 2003

Numerical studies of radiation at 2ωp from a Langmuir envelope collapsing adiabatically in three dimensions show that the emissivity is higher than expected. A volume emissivity obtained from an approximate density of collapsing packets leads to favorable comparisons with measurements of type-III solar radio bursts.
Cerenkov and anomalous Doppler effects in the relaxation of an electron beam
Authors: Muschietti L. , Appert K. , Vaclavik J.
Published: March 2003

The interplay between the Cerenkov and anomalous Doppler interactions in the relaxation of a warm electron beam is investigated by numerical means. The most important feature in the interplay is found to be a nonelastic isotropization. A simple semianalytical model which allows one to estimate various quantities relevant to the relaxation process is also presented.
Wavenumber variations of spatially damped Rayleigh–Bénard convection
Authors: Atten P. , Wesfreid J. E.
Published: March 2003

Under subcritical conditions, it is shown that a spatial variation of the velocity amplitude of convective rolls is accompanied by a modification in their wavenumber which takes values lower than the critical one.
A differentiable trajectory approximation to turbulent diffusion
Authors: Costanza Vicente , Seinfeld John H.
Published: March 2003

The problem of turbulent diffusion is posed as determining the time evolution of the probability density of the concentration given those for the fluid velocity components, sources, and the initial concentration. At each time, all variables are elements of the Hilbert space Lℛ2(ℛ3), and a finite-dimensional approximation based on expansions in orthonormal basis functions is developed. An expression for the joint probability density of all the Fourier coefficients is derived, the evaluation of which is shown to be particularly straightforward. Diffusion of material from a single source in an unbounded mildly turbulent fluid is considered as an application.
Bifurcation of elliptical equilibria
Authors: Clemente R. A. , Milovich J. L.
Published: March 2003

It is shown that a bifurcation of the equilibrium solutions for straight elliptical plasma columns with a diffuse current profile exists for a suitable choice of the currents in the external conductors.
Fast ion production by suprathermal electrons in laser fusion plasmas
Authors: True M. A. , Albritton James R. , Williams E. A.
Published: March 2003

The acceleration of ions by two isothermal species of electrons, thermal and suprathermal, in a laser-fusion plasma is investigated. The disassembly of an initially stationary plasma slab, including charge separation effects, is described. Before the rarefaction wave has penetrated appreciably into the slab, an interior Debye sheath which separates the suprathermal and thermal electrons, as well as an exterior sheath at the ion-vacuum interface is found. As the rarefaction approaches the center of the slab, the dynamics is modified due to the conservation of species number and total energy. The energetics of the long-time evolution is described by a quasi-neutral similarity solution appropriate to finite two-electron species systems.
Fluid flow of ablated plasma from planar targets
Authors: Herbst M. J. , Grun J.
Published: March 2003

Using a novel diagnostic technique, the streamlines of the flow of plasma from a laser-irradiated target are observed for the first time. Implications for far-field ion measurements and similarity to a simple fluid flow are noted.
Particle end loss from linear theta pinches
Authors: Commisso R. J. , Bartsch R. R. , Ekdahl C. A. , Freese K. B. , McKenna K. F. , Miller Guthrie , Siemon R. E.
Published: March 2003

Analysis of end-loss data from Los Alamos linear theta-pinch experiments gives the result that Linear Scyllac with mirrors had a longer confinement time than previously reported. New data from Scylla IV-P suggest an interpretation that gives significantly longer particle decay times.
Wave fronts of radiated electromagnetic modes in a magnetoplasma waveguide
Authors: Ohnuma Toshiro , Watanabe Tsuguhiro , Sanuki Heiji
Published: March 2003

Wave fronts and ray trajectories of electromagnetic modes radiated from a local source are obtained in an inhomogeneous magnetoplasma waveguide. They are investigated numerically using a kinetic theory at frequencies below the electron cyclotron frequency.
Experiments on the propagation of composite internal wave trains
Authors: Müller Michael R. , Karlsson S. K. F.
Published: March 2003

Experiments have been performed in a two-layer stably stratified flow where the mean velocity surpasses the phase speed of all internal waves of mode one. Results show the existence of two wave trains propagating in the direction of the mean flow (excited by a two-dimensional wave maker), which are identified by the modulation of the amplitude of the linearly superposed wave trains. Measurements of the composite wave speed and dispersion relation are presented. Wave speeds are shown to be very near the mean. The qualitative behavior of composite wave trains is shown to appear similar to that of unstable Kelvin–Helmholtz waves where spatial growth is dominant, leading to possible misinterpretation.
Shape of a drop in an electric field
Authors: Miksis Michael J.
Published: March 2003

The shape of an axisymmetric dielectric drop in a uniform electric field is computed numerically. The problem is formulated as a nonlinear integro-differential system of equations. They are discretized and the resulting algebraic system is solved by Newton’s method. The results show that when the dielectric constant ε is larger than a critical value εc, the drop develops two obtuse-angled conical points at its ends for a certain field strength. For ε<εc, the drop elongates and retains its original nearly prolate spheroidal shape without developing conical points as the field is increased. The numerical results are in good agreement with the moment and two-point approximations. The energy, volume, and area of the drop are computed, and the two-dimensional case is also treated.
Recurrence, dimensionality, and Lagrange stability of solutions of the nonlinear Schrödinger equation
Authors: Thyagaraja A.
Published: March 2003

The scope and limitations of elementary methods for establishing qualitative properties of solutions of the nonlinear Schrödinger equation are discussed and clarified. The concept of recurrence is shown to be related to Lagrange and Poisson stability of the system. Lagrange stable motions in any number of spatial dimensions are shown to be recurrent in general.
Derivation of the mode conversion-tunneling equation from the Vlasov equation
Authors: Swanson D. G.
Published: March 2003

Instead of using the uniform warm plasma dispersion relation with an inverse Fourier transform to form the mode conversion-tunneling equation, the equation is developed directly from the Vlasov equation and Maxwell’s equations. The plasma is assumed uniform parallel to B0. The expansion parameters are the Larmor orbit and the scale length, keeping terms to order ρ2L and L−1 = (1/ωc)(dωc/dx). For the special case with ω≃2ωc in a single species plasma, the asymptotic form of the mode conversion-tunneling equation is unchanged, but localized third and first derivative terms appear even as absorption vanishes.
Nonlinear saturation of the drift cyclotron loss-cone instability. II. Comparison with PR-6 data
Authors: Myer R. C. , Simon Albert
Published: March 2003

The single-mode time-asymptotic nonlinear saturation of the drift-cyclotron loss-cone instability is considered. In this model, there is a uniform magnetic field as well as mirror losses. The linear stability boundary, and the saturation levels of unstable oscillations and floating potential near this boundary, are determined. The theory is then compared with observations on the PR-6 mirror experiment. Good agreement is obtained using a simple energy model for the slow time variation of the equilibrium distribution function.
Field-reversed experiments (FRX) on compact toroids
Authors: Armstrong W. T. , Linford R. K. , Lipson J. , Platts D. A. , Sherwood E. G.
Published: March 2003

Equilibrium, stability, and confinement properties of compact toroids produced in field-reversed theta-pinch experiments (FRX) are reported. Two experimental facilities, FRX-A and FRX-B, have been used to study highly elongated compact toroid plasmas confined in a purely poloidal field geometry. Spatial scans and fill pressure scaling of the equilibrium plasma parameters are presented. Plasma conditions range from Te∼150 eV, Ti∼800 eV, nm∼1×1015 cm−3 to Te∼100 eV, Ti∼150 eV, nm∼4×1015 cm−3. Typical confined plasma dimensions are: major radius R∼4 cm, minor radius a∼2 cm, and total length 35–50 cm. The plasma configuration remains in a stable equilibrium for up to 50 μsec followed by the destructive n = 2 rotational instability. The stable period prior to the onset of the rotational mode is up to one hundred times greater than characteristic Alfvén transit times of the plasma. This stable period increases and the mode growth rate decreases with increased a/ρi (where ρi is the ion gyroradius). Agreement of experimental and theoretical mode frequencies for the instability is observed. Preferential particle loss has been proposed as a likely cause of rotation. The particle inventory at the onset of the instability is consistent with this hypothesis. The particle loss rate is also consistent with the predicted anomalous transport near the separatrix. Contributions to rotational instability from diffusion, end-shorting, equipartition, and compression are also discussed.
Eigenvalues of the Rayleigh–Bénard and Marangoni problems
Authors: Rosenblat S. , Homsy G. M. , Davis Stephen H.
Published: March 2003

A discussion is presented of the eigenvalues of the linear Bénard–Marangoni stability problem. It is shown that the Marangoni number cannot be used as an eigenvalue parameter for the determination of an eigenvector set.
Self-trapping of intense laser beams
Authors: Anderson Dan
Published: March 2003

Some aspects of the self-trapped propagation of beams of strong electromagnetic radiation are discussed. A comparison is made between a variational approach and the commonly used paraxial ray approximation, demonstrating how the latter gives an erroneous result for the nonlinear wavenumber.
A potential overshoot of a magnetized plasma made by a disk probe
Authors: Kawaguchi M. , Tanaka H.
Published: March 2003

A potential overshoot is made in a current-carrying plasma with a longitudinal magnetic field by a floating disk probe which works as a disturber. The overshoot appears behind the disturber in the electron current. Its transverse and longitudinal scale lengths are measured and discussed.
Shape of shock wave produced by a concentrated impact on a surface
Authors: Nutt Gerald , Klein Lewis
Published: March 2003

An approximate similarity solution, derived by Raizer, of a concentrated impact (or intense explosion) at the boundary of a semi-infinite volume of a perfect gas is used to determine the propagation velocity of the shock front as a function of its position. This velocity function is then used to obtain the shape of the propagating shock wave. It is shown that dish-shaped shock fronts are formed when the movement of the gas at the surface is into the gas region and that cup-shaped shock fronts are formed when the movement is out of the gas region. Comparison of these results with the shapes of explosions and meteorite craters are discussed.
Tearing-mode stability properties of a diffuse anisotropic field-reversed ion layer at marginal stability
Authors: Chen James , Davidson R. C.
Published: March 2003

Stability properties are investigated for purely growing (Reω = 0) tearing modes at marginal stability (Imω = 0) for a rotating, nonrelativistic cylindrically symmetric ion layer immersed in an axial magnetic field. The analysis is carried out within the framework of a Vlasov-fluid model in which the electrons are described as a macroscopic cold fluid, and the layer ions are described by the Vlasov equation. Tearing-mode stability properties are calculated numerically for azimuthally symmetric perturbations about an ion layer equilibrium with temperature anisotropy. The marginal stability eigenvalue equation is solved numerically for the perturbation amplitude and the normalized axial wavenumbers at marginal stability in terms of temperature anisotropy, layer radius, and magnetic field depression on the axis. It is found that the range of unstable wavenumbers decreases as T‖‖/T⊥ is increased, and numerical estimates are made of the anisotropy required for complete stabilization.
Variational structure of the Vlasov equation
Authors: Berk H. L. , Dominguez R. R. , Maschke E. K.
Published: March 2003

The variational structure of the Vlasov–Maxwell integral equations is derived for a plasma equilibrium having two ignorable coordinates. It is shown that the kernel of the Maxwell equations is a self-adjoint integral operator. This operator may also be represented as a differential equation of arbitrary order. This representation is useful when the differential operator is truncated to finite order, yielding a system of intrinsically self-adjoint differential equations.
Time-dependent nonlinear Langmuir waves
Authors: Adam J. C. , Laval G. , Mendonca I.
Published: March 2003

The existence of nonlinear periodic Langmuir waves that are time dependent in any frame of reference is demonstrated, the phase and amplitude modulations being induced by the oscillating motion of trapped particles. Large amplitude modulations can be found even for weak trapped particles densities. Sideband instabilities are strongly modified by the oscillating motion when the energy dependence of the bounce frequency is taken into account.
Collective acceleration of protons in relativistic electron beam propagation in evacuated drift tubes
Authors: Adler R. , Nation J. A. , Serlin V.
Published: March 2003

The results are presented of a series of experiments investigating the mechanisms leading to the collective acceleration of protons occurring in electron beam propagation through evacuated drift tubes. Detailed measurements have been made of the electron and ion beam propagation and of the ion energy spectra. The results show that the acceleration cannot occur as a result of a beam head or deep potential well mechanism. It seems likely that the acceleration is due to the self-excitation of a wave on the beam, possibly the electron-ion two-stream instability.
Quasi-linear theory without the random phase approximation
Authors: Weibel E. S. , Vaclavik J.
Published: March 2003

The system of quasi-linear equations is derived without making use of the random phase approximation. The fluctuation quantities are described by the autocorrelation function of the electric field using the techniques of Fourier analysis. The resulting equations possess the necessary conservation properties, but comprise new terms which hitherto have been lost in the conventional derivations.
Nonlinear theory of high-m tearing modes
Authors: Dominguez R. R. , Rosenberg M. , Chang C. S.
Published: March 2003

A simplified nonlinear theory of short wavelength (high-m) electron temperature-gradient-driven tearing modes is presented. Saturation of the linear instability results from nonlinear mode coupling in which energy may cascade to both short and long wavelength modes. The resulting electron thermal conductivity exhibits the density scaling and absolute magnitude observed experimentally.
Low-frequency modes with high toroidal mode numbers: A general formulation
Authors: Pegoraro F. , Schep T. J.
Published: March 2003

Low-frequency waves with high toroidal mode numbers in an axisymmetric toroidal configuration are studied. In particular, the relationship between the periodicity constraints imposed by the geometry, magnetic shear, and the spatial structure of eigenmodes is investigated. By exploiting the radial translational invariance and the poloidal periodicity of the gyro-kinetic and Maxwell equations, the two-dimensional problem can be converted into a one-dimensional one, and the mode structure can be expressed in terms of a single extended poloidal variable. This representation is used in the description of electromagnetic modes with phase velocities larger than the ion thermal velocity and with frequencies below the ion gyro-frequency. Trapped particle, curvature, and compressional effects are retained. The dispersion equations for drift modes and Alfvén-type modes are given in general geometry and simplified solutions are presented in the configuration of a double periodic plane slab.
Structure in the ion-acoustic spectrum
Authors: Horton Wendell , Brock David
Published: March 2003

A mode simulation study using the renormalized turbulence theory for the current-driven ion-acoustic problem shows the presence of time-dependent structure in the wavenumber and frequency spectrum.
Toroidal plasma equilibrium with gravity
Authors: Yoshikawa Shoichi
Published: March 2003

A toroidal magnetic field configuration in a gravitational field is calculated both from a simple force-balance and by using magnetic surfaces. The configuration is found to be positionally stable in a star. The vibrational frequency near the equilibrium point is proportional to the hydrostatic frequency of a star multiplied by the ratio (WB/Wm)1/2, where WB is the magnetic field energy density, and Wm is the material pressure at the equilibrium point. It is proposed that this frequency may account for the observed solar spot cycles.
Autocorrelation and spectrum of dissipation fluctuations in a turbulent jet
Authors: Antonia R. A. , Phan-Thien N. , Satyaprakash B. R.
Published: March 2003

Measurements on the axis of a circular jet indicate that the autocorrelation of dissipation fluctuations exhibit a power-law dependence in the inertial subrange with an exponent m≃0.2, and that the corresponding spectrum is consistent with this value of m.
Do the dissipation fluctuations in high Reynolds number turbulence define a universal exponent?
Authors: Nelkin Mark
Published: March 2003

A simple physical model for the dissipation autocorrelation function is proposed. The resulting spectrum of dissipation fluctuations has a km−1 inertial range behavior. An experiment by Kholmyansky is consistent with the proposed model, and suggests that the exponent m is approximately equal to 0.25.
Modulational stability of short-crested free surface waves
Authors: Mollo-Christensen Erik
Published: March 2003

Chappelear’s solution for short-crested gravity waves of a permanent type is shown to be modulationally unstable for long-crest wavelengths and stable for short-crest wavelengths.
Rotational distribution of para-H2 in He shock wave
Authors: Koura Katsuhisa
Published: March 2003

The rotational distribution and collision number Zr of para-H2 in the He shock wave are studied using the master equation and the quantum mechanical rate constant obtained by Rabitz and Zarur. For high Mach number, at the initial stage, the rotational distribution indicates a merging pattern of two Boltzmann distributions at the lower and higher rotational levels corresponding to the pre-shock temperature T1 and the reduced temperature T2Tc/(T2+Tc), where T2 and Tc are the post-shock and rate-constant temperatures, respectively. The rotational distribution at the final stage indicates a non-Boltzmann distribution with underpopulation at the higher levels, which is also observed for low Mach number. The rotational collision number Zr over the period of the rotational relaxation is found to show a step pattern with the plateaus corresponding to the rotational collision number Zjj′ of the two-level systems (jj′) = (02), (24), (46),....
Modified Poisson eigenfunctions for electrostatic Bernstein–Greene–Kruskal equilibria
Authors: Ling Kuok-Mee , Abraham-Shrauner Barbara
Published: March 2003

The stability of an electrostatic Bernstein–Greene–Kruskal equilibrium by Lewis and Symon’s general linear stability analysis for spatially inhomogeneous Vlasov equilibria, which employs eigenfunctions and eigenvalues of the equilibrium Liouville operator and the modified Poisson operator, is considered. Analytic expressions for the Liouville eigenfuctions and eigenvalues have already been given; approximate analytic expressions for the dominant eigenfunction and eigenvalue of the modified Poisson operator are given. In the kinetic limit three methods are given: (i) the perturbation method, (ii) the Rayleigh–Ritz method, and (iii) a method based on a Hill’s equation. In the fluid limit the Rayleigh–Ritz method is used. The dominant eigenfunction and eigenvalue are then substituted in the dispersion relation and the growth rate calculated. The growth rate agrees very well with previous results found by numerical simulation and by modified Poisson eigenfunctions calculated numerically.
Two-dimensional modeling of the formation of spheromak configurations
Authors: Jardin S. C. , Park W.
Published: March 2003

A reduced set of two-dimensional magnetohydrodynamic equations has been derived describing the axisymmetric time evolution of a stable plasma evolving slowly due to resistive diffusion and changing boundary conditions. The equations are restricted to low b but allow changing topology. They are integrated in time to demonstrate a possible spheromak formation method. External circuit equations are integrated simultaneously with the plasma equations to determine the electromagnetic boundary conditions self-consistently. The effects of a finite conductivity vacuum chamber are included.
CO2 laser-driven high-density implosion experiments
Authors: Tan T. H. , McCall G. H. , Kopp R. , Ganley T. , van Hulsteyn D. , Hauer A. , Williams A. , Mitchell K. , Ladish J. S. , Giovanielli D. , Linnebur E. , Fries R. J.
Published: March 2003

Glass microballoons, coated with various thicknesses of plastic ablator and filled with deuterium-tritium gas mixtures, were irradiated with the eight-beam Helios CO2 laser. Neutron yield, fuel temperature, compressed fuel density, and implosion time were measured as functions of ablator thickness. Hydrodynamic efficiency and preheating of these targets were estimated. Transition from fuel conditions characteristic of earlier exploding pusher experiments to those characteristic of a more nearly isentropic mode of implosion is illustrated. Fuel densities above 2 g/cm3 were achieved.
Warm plasma axial flow through a magnetic mirror
Authors: Rognlien T. D. , Brengle T. A.
Published: March 2003

Solutions are given for the characteristics of a warm plasma flowing axially through a magnetic mirror in the presence of a confined hot-ion species. The warm plasma is assumed to have negligible effect on the magnetic field. The plasma is described by four moment equations for the density, velocity, parallel temperature, and perpendicular temperature of the ions, that are solved by a time-dependent finite-difference computer code. Two basic types of solutions occur corresponding to the warm-ion density (nw) being less than the hot-ion density (nh), and nw≳nh. In the first case, the flow is nearly stagnant between the warm-ion source and the midplane, but the flow rapidly accelerates to a supersonic velocity just beyond the midplane. In the second case, the flow velocity profile is more symmetric about the midplane, and the sonic transition occurs at the mirror throat. Specific calculations are given for gas box and stream gun sources used to stabilize microstabilities and for a collisional tandem mirror.
Numerical calculation of standing waves in water of arbitrary uniform depth
Authors: Vanden-Broeck Jean-Marc , Schwartz Leonard W.
Published: March 2003

A numerical scheme is presented for the computation of time and space periodic standing waves of finite amplitude in water of arbitrary uniform depth. The dynamic and kinematic boundary conditions are used in their exact nonlinear form. The numerical procedure involves series truncation. Accurate solutions are presented for various values of the amplitude and of the depth. It is found that for some values of the depth, the frequency is not a monotonic function of the amplitude.
One-dimensional direct current resistivity due to strong turbulence
Authors: Rowland H. L. , Palmadesso P. J. , Papadopoulos K.
Published: March 2003

Strong Langmuir turbulence acts to generate localized spiky fields and ion cavities. It is shown that these cavities generate potential barriers that can trap a significant fraction of the electrons. For example, a 20% cavity traps 50% of the electrons. This can seriously change the direct current properties of a collisionless plasma and lead to the appearance of anomalous dc resistivity.
Observation of bumps on the electron distribution function during ionization
Authors: Lins G. A. W. , Kunze H.-J.
Published: March 2003

During the implosion of weakly ionized plasmas in a theta pinch, monoenergetic electron beams in the direction of the magnetic field lines are observed, the energy of the beam corresponding to the ionization energy in each case. The distribution function has been measured by incoherent Thomson scattering.
Nonlinear frequency shift induced by the lower-hybrid drift instability
Authors: Chen Yu-Jiuan , Cohen Bruce I.
Published: March 2003

It is found that a finite perturbations of the ion orbits leads to a nonlinear frequency shift that reduces the mode frequency and has a weak stabilizing effect on the lower-hybrid drift instability. This result is obtained from a self-consistent solution of the Vlasov–Poisson equations using perturbation theory in which the nonlinear dielectric function and the nonlinear temporal evolution of a single unstable mode in the low drift velocity regime are calculated analytically. The nonlinear frequency shift does not seem to be a potent saturation mechanism in a collisionless plasma, but may be more relevant when there are ion-ion collisions.
Incomplete relaxation of pinch discharges
Authors: Turner Leaf , Christiansen J. P.
Published: March 2003

Solenoidal field linkage is explained and its application to magnetic helicity is discussed. A Green’s tensor function for the magnetic vector potential of a plasma current distribution bounded by a perfect conductor is derived. A hypothesis of incomplete relaxation of a turbulent discharge is then used to establish reversed-field pinch configurations with finite beta.
Electron beam transport in a weak plasma
Authors: Carr W. , Ryan J. , Seidl M.
Published: March 2003

The propagation of an electron beam through a dilute plasma is studied experimentally. A beam is injected into a low pressure gas (P≃10−6 Torr) resulting in sufficient ions to neutralize the beam so that space charge blow-up does not occur directly. However, a low frequency beam-drift mode is unstable, which results in a radial displacement of the beam. This can produce sufficient space charge to stop beam propagation. The experimentally observed threshold is in agreement with linear theory, and the limiting current agrees with an estimate of the space charge limit for this geometry.
Taylor relaxation in a torus of arbitrary aspect ratio and cross section
Authors: Reiman Allan
Published: March 2003

Taylor’s analysis of plasma relaxation is generalized to a toroidal plasma of arbitrary aspect ratio and cross section. A contribution to the energy incorrectly neglected by Taylor is included in the analysis. The contribution is due to the induced wall current, which must be greater than the current through the external field coils in a self-reversed z pinch. Although this contribution to the energy is substantial, it is shown to never alter the minimum energy state. Some of the general properties of the minimum energy state in a torus of arbitrary aspect ratio and cross section are also determined.
Filamentation instability in heavy ion beams
Authors: Brueckner Keith A. , Metzler Nathan , Janda Ralph
Published: March 2003

The filamentation instability for a focused heavy ion beam is analyzed with a particle simulation code. Appreciable growth is prevented by high plasma conductivity and by the nonlinear orbit mixing characteristic of the simulation code.
A diverted plasma reversed field pinch
Authors: Manheimer Wallace
Published: March 2003

This note proposes a poloidal divertor as a method of reducing the strong plasma-wall interaction inherent in a reversed field pinch.
Resonance of electromagnetic wave in inhomogeneous plasma with density fluctuations
Authors: Dragila R.
Published: March 2003

The electromagnetic field structure modification due to the random irregularities on the inhomogeneous plasma density profile is investigated for the case of oblique incidence, p polarization, and linear background plasma density profile.
Particle streak velocity field measurements in a two-dimensional mixing layer
Authors: Dimotakis Paul E. , Debussy Francois D. , Koochesfahani Manoochehr M.
Published: March 2003

Using digital image processing of particle streak photography, the streamwise and perpendicular components of the velocity field were investigated, in the mid-span plane of a two-dimensional mixing layer, with a 6:1 velocity ratio. The Reynolds number of the flow, based on the local vorticity thickness and the velocity difference across the layer, ranged from 1360 to 2520, in the plane of observation. The significant result of this experiment was that the region of vorticity bearing fluid is confined to a small fraction of the flow. A second finding, consistent with the small regions of concentrated vorticity, was the observation of instantaneous streamwise velocity reversal, in the laboratory frame, in small regions of the flow.
Current-driven microinstabilities in a neutral sheet
Authors: Winske D.
Published: March 2003

Computer simulation is used to investigate current-driven microinstabilities in a simple reversed field (neutral sheet) configuration. A long wavelength, predominantly electromagnetic, instability develops at the reversal point as a consequence of the lower hybrid drift instability, which is excited away from the reversal point where the density gradients are steep. The resulting anomalous resistivity is much smaller at the null point.
Stability of multipoles to ballooning modes with large toroidal mode number
Authors: Adler E. A. , Lee Y. C.
Published: March 2003

The stability of multipoles having a purely poloidal field to ballooning modes of high toroidal mode number is studied. First, a perturbation theory based on large m is discussed, and it is found that the first-order correction to the β limit is of order 1/m. Next, the equations for a linear quadrupole and octopole are solved and the results are compared with recent multipole experiments.
Spontaneous ion-acoustic generation in a high power, multifilament ion source
Authors: Schoenberg Kurt F. , Kunkel Wulf B.
Published: March 2003

Under certain operating conditions, a high power, multifilament ion source can effectively function as a spontaneously excited ion-acoustic resonator. The fluctuations, initially excited by a current-driven instability at the thermionic source cathode, exhibit a Lorentzian spectral profile where the resonant frequency is solely determined by the spatial cathode configuration, and the resonance half-width is set by the mean plasma ion lifetime. The ion wave-cathode interaction, responsible for the resonant behavior, entails a feedback mechanism dependent on the electrostatic wave scattering properties of thermionic filaments operated under space-charge-limited conditions. Fluctuation amplitude levels are insufficient to significantly perturb the ions and consequently, the instability does not affect the performance of a multifilament source neutral beam system.
Turbulent heating of parametric instabilities in unmagnetized plasmas
Authors: Kuo S. P. , Cheo B. R.
Published: March 2003

Consideration of the effect of a uniform pump field on the particle orbits in a Vlasov plasma leads to a modified diffusion coefficient. When the particles oscillate in the pump field, the turbulent wave phase velocity seen by the particles is Doppler shifted by multiples of ω0/k (ω0 is the pump frequency). Hence, strong interactions between the particles and various components of the turbulent field will take place. It is shown that when the pump field is sufficiently strong: E0⩾(4πn0Te)1/2, bulk heating can dominate over tail heating and excitation of electrons to energy levels higher than the normal suprathermal values is possible. This field strength is within the range of laser fusion.
Spectroscopic study of stimulated Brillouin scattering
Authors: Turechek John J. , Chen F. F.
Published: March 2003

Results are summarized of a diagnostic study of underdense plasma targets, leading to the first observation of stimulated Brillouin scattering arising from thermal-level fluctuations. Included are; (1) a detailed analysis of spectroscopic measurements of the plasma before and during laser heating, (2) computer calculations of plasma evolution during the laser pulse, (3) observations and anlaysis of Brillouin scatter, and (4) spectroscopic observations of forbidden satellite line emission during Brillouin scatter.
Limiting beta of stellarators with no net current
Authors: Strauss H. R. , Monticello D. A.
Published: March 2003

Using reduced nonlinear magnetohydrodynamic equations, finite beta, resistive l = 2 stellarator equilibria with no net current are found. Stability to low mode number internal magnetohydrodynamic modes is investigated, and beta limits comparable to tokamaks are found. Low shear equilibria appear to be substantially more stable than high shear.
Diffusion of magnetic field into an expanding plasma shell
Authors: Hussey Thomas W. , Roderick Norman F.
Published: March 2003

A simple theory is developed to describe the density profile of an electromagnetically imploded hollow plasma liner. The model gives a good analytic means of determing density gradient scale length and sheath thickness for use in linear and nonlinear analysis of the Rayleigh–Taylor instability.
Turbulence driven tearing modes in a tokamak plasma
Authors: Sen Abhijit , Sundaram A. K.
Published: March 2003

The influence of a random spectrum of lower hybrid waves on the evolution of tearing modes in a tokamak plasma is investigated. For rather modest levels of fluctuations, it is found that new tearing instabilities driven by turbulence are excited with significantly enhanced growth rates and comparable real frequencies. The growth rates for the new m = 1 and m = 2 modes vary as fractional powers of τR and δ, where τR is the turbulence induced diffusion time and δ is the ratio of Alfvénic time to the ion gyration period.
Drift modes in axisymmetric tandem mirrors
Authors: Horton Wendell
Published: March 2003

The drift mode analysis of the tandem mirror is developed for a large aspect ratio, axisymmetric model of the equilibrium. The axial drift wave eigenmodes are shown to change character as the plasma pressure varies with respect to the inverse aspect ratio. In the high beta regime the drift modes are transformed into a finite frequency convective cell and the flute-like ion drift wave. Quasi-linear formulas are given for the anomalous radial losses.
Kinetic effects on ballooning modes in mirror machines
Authors: Tang W. M. , Catto Peter J.
Published: March 2003

A general procedure for examining the influence of kinetic effects on the stability of magnetohydrodynamic ballooning modes in mirror machines is presented. In particular, the basic kinetic ballooning mode equation for a nonaxisymmetric, arbitrary beta system with anisotropic pressure is derived. Considering a long-thin equilibrium typical of the tandem mirror, it is shown that this governing eigenmode equation reduces to a simple form independent of wave-particle resonant effects.
Particle simulations of a finite-length free electron laser
Authors: Liewer P. C. , Lin A. T. , Dawson J. M. , Caponi M. Zales
Published: March 2003

One-dimensional particle simulations of a finite-length free electron laser show that the convectively-unstable, short-wavelength mode is the dominant instability initially for systems long compared with the growth length of the mode. Also, a generalized one-dimensional linear theory of a finite length system has been derived with all kinetic effects included. Comparison between the unstable modes seen in the simulation and those predicted by this theory are in good agreement. However, the simulations show a significantly higher efficiency than that predicted by the theory when trapping by a single mode is the saturation mechanism. This may be caused by a local dc electric field (space charge fields) that arises self-consistently as the beam slows down.
Kinetic equation for a plasma in a constant magnetic field
Authors: Shivamoggi Bhimsen K.
Published: March 2003

Frieman’s derivation of a kinetic equation for a plasma is extended to the case of a plasma in a constant external magnetic field. A uniformly convergent collision term is constructed and a systematic way of dealing with the divergences in the collision term is indicated.
Propagation of discontinuities in magnetohydrodynamics
Authors: Sharma V. D.
Published: March 2003

The influence of the transverse magnetic field on the growth and decay behavior of weak discontinuities headed by plane and cylindrical wave fronts is analyzed completely, and the various possibilities concerning the evolution of wave forms are characterized.
Comment on ‘‘Resonant parametric excitations driven by lower-hybrid fields’’
Authors: Karney Charles F. F.
Published: March 2003

Comment on parametric instability in an inhomogeneous plasma is made.(AIP)
Theory and simulation of stimulated Brillouin scatter excited by nonabsorbed light in laser fusion systems
Authors: Randall C. J. , Albritton James R. , Thomson J. J.
Published: March 2003

The noise spectrum from which stimulated Brillouin scatter grows has two sources in laser fusion plasmas; a broadband source due to ion-acoustic fluctuations, and a line source, usually much larger, which is the nonabsorbed light returning from the plasma critical surface. We give a theoretical description of stimulated Brillouin backscatter when the fluctuation source may be neglected and the scatter grows exclusively from the nonabsorbed light. Gradients of background density, velocity, and temperature are allowed. Theoretical predictions are compared to numerical simulations of scatter for parameters of recent experiments. It is found that stimulated Brillouin scatter can be greatly enhanced by the presence of a critical surface and that it can become an important part of the total energy balance.
Some interesting properties of two-dimensional turbulence
Authors: Speziale Charles G.
Published: March 2003

The effect of superimposed rigid body motions on the structure of two-dimensional turbulence is examined. It is found that with regard to the fluctuation dynamics of the flow, the rotational behavior of two-dimensional turbulence is quite different from its three-dimensional counterpart. The implications that this has on turbulence modeling are discussed briefly.
Spectral transfer and velocity derivative skewness equation for a turbulent velocity field
Authors: Driscoll Richard J. , Kennedy Lawrence A.
Published: March 2003

A derivation of the high wavenumber spectral transfer equation is given which indicates the wavenumber range of validity for this equation in terms of Reλ. A similar treatment is given for the commonly used spectral approximation for the velocity derivative skewness and the analogous scalar spectral quantities.
Variational moment solutions to the Grad–Shafranov equation
Authors: Lao L. L. , Hirshman S. P. , Wieland R. M.
Published: March 2003

A variational method is developed to find approximate solutions to the Grad–Shafranov equation. The surfaces of the constant poloidal magnetic flux ψ(R, Z) are obtained by solving a few ordinary differential equations, which are moments of the Grad–Shafranov equation, for the Fourier amplitudes of the inverse mapping R(ψ, ϑ) and Z(ψ, ϑ). Analytic properties and solutions of the moment equations are considered. Specific calculations using the Impurity Study Experiment (ISX-B) and the Engineering Test Facility (ETF)/International Tokamak Reactor (INTOR) geometries are performed numerically, and the results agree well with those calculated using standard two-dimensional equilibrium codes. The main advantage of the variational moment method is that it significantly reduces the computational time required to determine two-dimensional equilibria without sacrificing accuracy.
Thermal heat flux in a plasma for arbitrary collisionality
Authors: Khan S. A. , Rognlien T. D.
Published: March 2003

The thermal heat flux along a uniform magnetic field due to a temperature gradient is calculated using a Monte Carlo solution to the Fokker–Planck equation. This numerical solution, which is computed for a particular electron temperature profile, is valid for arbitrary mean-free-path, λmfp. The calculated heat flux makes a smooth transition between the analytic expressions for the short and long λmfp limits.
Centrifugal separation of a multispecies pure ion plasma
Authors: O’Neil T. M.
Published: March 2003

Consider an unneutralized column of ions (a pure ion plasma) confined by an axial magnetic field. Because of space charge, there is a large radial electric field and a consequent rotation of the plasma column. For a multispecies ion plasma, the rotation tends to produce centrifugal separation of the plasma into its component species. Self-consistent thermal equilibrium states which exhibit various degrees of separation are discussed.
Theory of free electron laser instability in a relativistic annular electron beam
Authors: Uhm Han S. , Davidson R. C.
Published: March 2003

A self-consistent theory of the free electron laser instability is developed for a hollow electron beam propagating through an undulator (multiple mirror) magnetic field. The stability analysis is carried out within the framework of the linearized Vlasov–Maxwell equations. It is assumed that the beam is thin, with radial thickness much smaller than the mean beam radius, and that ν/γb≪1, where ν is Budker’s parameter and γbmc2 is the characteristic energy of the electron beam. The dispersion relation describing the free electron laser instability in a hollow relativistic electron beam is obtained for an equilibrium distribution function in which all electrons have the same value of transverse energy and the same value of canonical angular momentum, and a Lorentzian distribution in axial momentum. It is shown that the influence of finite radial geometry plays a critical role in determining the detailed stability behavior. Moreover, the growth rate and bandwidth of the instability can be expressed in terms of Budker’s parameter ν, instead of the plasma frequency as in the case of a uniform density beam. Furthermore, it is found that free electron laser stability properties exhibit a sensitive dependence on axial momentum spread.
Variational theory of electrical conductivity and kinetic tearing modes
Authors: Chang C. S. , Dominguez R. R. , Hazeltine R. D.
Published: March 2003

A variational theory for Ohm’s law, valid for arbitrary collision frequency, is presented for a slab geometry using a pitch angle scattering collision operator. Shear and temperature gradient effects are included but toroidicity is neglected. The variational conductivity is used to examine the effects of shear and electron temperature gradients on tearing mode instabilities. It is shown that shear has a strong stabilizing effect on the so-called ’’collisional’’ and ’’semicollisional’’ electron temperature gradient tearing mode instabilities. In addition, the stabilizing effect of shear on the classical resistive and inertial tearing modes is discussed.
Closure for velocity/pressure-gradient correlations in turbulent shear flow
Authors: Gallagher B. , Magaard L. , Gutteling E.
Published: March 2003

A closure representation is developed for the velocity/pressure-gradient correlation arising in the Reynolds stress equation through the interaction of the turbulent velocity field with mean shear flow. The development has the advantage of putting the closure on a rational footing. The resulting expression is used in a simple example where nearly steady, homogeneous turbulence is present in a rectilinear mean shear flow. Reynolds stresses and various aspects of the behavior of the velocity/pressure-gradient correlation seem to be predicted correctly.
Measurement of convective cell spectra and the resultant calculated vortex diffusion coefficient
Authors: Ehrhardt A. Butcher , Post R. S.
Published: March 2003

The presence of convective cells in a purely poloidal field Levitated Octupole has been associated with diffusion that scales as Dv∝(T*/n)1/2, independent of B, where T* is an ’’effective temperature,’’ T*∝T. The electric field spectrum of the convective cells can be used to estimate the magnitude of T* and Dv. The results are in reasonable agreement with previous measurements of cross-field transport, and agree qualitatively with theortical models of vortex diffusion.
Diffusion coefficient for ions in the presence of a coherent lower hybrid wave
Authors: Antonsen Thomas M. , Ott Edward
Published: March 2003

When the amplitude of a coherent lower hybrid wave exceeds a certain stochasticity threshold, the ion motion becomes ergodic. The problem of determining the resulting diffusion coefficient is considered. For large amplitude waves it is found that the diffusion coefficient oscillates with decreasing amplitude about the quasi-linear value as the wave amplitude is increased. Furthermore, the diffusion coefficient is shown to be subject to narrow resonances at particular values of the wave amplitude.