bluuebunny/crossref_metadata_2025 huggingface dataset:
Bumpy torus transport in the low collision frequency limit
Authors: Hazeltine R. D. , Catto Peter J.
Published: March 2003

A variational formulation of transport for a closed field line, large aspect ratio bumpy torus in the low collision frequency limit is presented. The relatively large radial excursions made by those particles with canceling magnetic and electric poloidal drifts requires generalization of previous neoclassical techniques. General expressions for the transport coefficients of both charge species are given. Explicit numerical coefficients are evaluated under certain simplifying assumptions for both the banana and nonbanana species.
Enhanced current penetration in tokamaks
Authors: Hitchcock D. A. , Mahajan Swadesh M. , Hazeltine R. D.
Published: March 2003

The rate at which current penetrates to the interior of a plasma during the current rise phase of a discharge is found to be greatly enhanced by anomalous viscosity due to electromagnetic turbulence.
Enhancement of free electron laser radiation by applying a direct current electric field
Authors: Lin A. T.
Published: March 2003

The intensity of the electromagnetic radiation produced by a free electron laser can be substantially increased by applying a dc electric field at the time of saturation. By using a proper dc electric field strength, a substantial number of trapped electrons can become clamped in the decelerating phase and continuously transfer the dc electric field energy into the locked high frequency radiation. A modulational instability is observed when the radiation intensity exceeds some threshold, and ultimately terminates the conversion process.
Plasma rotation in reversed-field theta pinches
Authors: Steinhauer Loren C.
Published: March 2003

Field reversed plasmas have been observed to spin up and develop a destructive rotating m=2 instability. While the instability threshold has been reasonably well characterized, the cause of rotation remains obscure. Of the several mechanisms proposed, the following is treated; end shorting on open field lines spins up that portion of the plasma; viscous drag then spins up the plasma on closed field lines. Theoretical models are developed to describe these processes. The predicted behavior falls into three regimes. For low aspect ratio (≲2.8) the plasma is highly viscous, rotates as a rigid body, and spins up to instability in a few Alfvén transit times. At intermediate aspect ratio (2.8 to 3.5) the plasma is weakly viscous; it reaches instability in the viscous transfer time which lengthens very rapidly for higher aspect ratio. At high aspect ratio (≳3.5 to 4) spin up is slower that the resistive decay of the plasma, and instability should not arise. Most recent experiments fall into the strongly viscous regime and correlate with its predicted characteristics. However, one series of experiments exhibited no apparent rotational instability, and also produced the large plasma radii characteristic of high aspect ratio. The present model in the high aspect ratio, weakly viscous regime is consistent with these observations.
Stochastic model theory for coherent turbulent structures in circular jets
Authors: Plaschko Peter
Published: March 2003

A stochastic model based on a linearized theory is developed. It is assumed that the large scale structure of the turbulence is created randomly at the jet orifice and that its subsequent convection downstream may be described in terms of quantities predicted by a linearized inviscid stability theory for slowly diverging flows. The process of the generation of turbulence is modeled by the imposition of a stochastic initial condition for the corresponding stability problem. The application of this condition secures the homogeneity of mean values of the turbulent quantities with respect to the angular coordinate; the contribution of each spiral mode is then proportional to a certain spectral function, the latter being an unknown function within the framework of the present study. Since experiments have shown that the turbulence of jets is far from being adequately represented by white noise conditions, the spectral functions must be determined by fitting to experimental data. It is shown that the model is capable of predicting the experimental trends for circumferential and radial coherence functions fairly well. However, complications arise with respect to the axial coherence functions and an attempt is made to alleviate the difficulties by either a cutoff assumption or by a process of continuation of the inviscid quantities.
Two-point probability density function closure applied to a diffusive-reactive system
Authors: Kuo Ying-Yan , O’Brien Edward E.
Published: March 2003

The Ievlev multipoint probability density function closure proposal is applied at the two-point level to the mass balance equation for a single stochastically distributed reactant undergoing molecular diffusion and nonlinear reaction. The solution is shown to satisfy the major realizability properties of a probability density function and to be unique. In the case of a linear reaction, it produces the exact solution for an initially multivariate normal distribution. By short-time numerical integration of a second-order reaction with diffusion, the solution is shown to represent molecular mixing more satisfactorily than do existing one-point theories.
Rayleigh–Taylor stability with mass and heat transfer
Authors: Hsieh D. Y. , Ho S. P.
Published: March 2003

A nonlinear analysis of the Rayleigh–Taylor stability of two thermally conducting fluids is presented. Mass transfer across the interface due to the evaporation and condensation process is explicitly taken into account. The method of multiple scale expansion is employed for the analysis. The stability criterion is expressible in terms of various competing parameters representing the equilibrium heat flux, latent heat of evaporation, vapor pressure, gravity, surface tension, densities of the fluids, and thermal properties of the fluids. It is found that when the heat flux is strong enough and when the heat transfer mechanism is efficient enough, the system can be stabilized in the regimes where it would be linearly unstable. The study has also shed light on the validity of the quasi-equilibrium approximation employed in a previous nonlinear study of the stability problem. The implication for the phenomenon of boiling heat transfer is also discussed.
Mixture approach to a model for sediment transport
Authors: Bampi F. , Morro A.
Published: March 2003

A mixture of heat-conducting viscous inert fluids is given the structure of material with hidden variables. As a consequence, a model for sediment transport is elaborated within the approximation of sediments as a dilute suspension. Such a model turns out to be hyperbolic and allows for exceptional transverse waves.
Elongational flow of dilute drag reducing fluids in a falling jet
Authors: Usui Hiromoto , Sano Yuji
Published: March 2003

An elongational flow of dilute polymer solution in a falling jet was studied. Measurements were made of the stress and velocity distribution along the falling jet by means of a photographic technique. The results show that the relaxation time in dilute drag reducing fluids is much longer than those predicted in a simple shear flow, and suggest the possibility of using a very high value of the relaxation time in a wall turbulence model. Also, the increase in Trouton ratios determined for a simple elongational flow is possibly as high as 27 000.
Multi-shock solutions of random phase three-wave interactions
Authors: Reiman Allan , Kaup D.
Published: March 2003

Explicit solutions of the equations describing the space–time interaction of three random phase wave packets are presented. The solutions describe the collision and decay of previously known shock-like solutions. Stability of the solutions to a perturbation in initial conditions is also analyzed. For those solutions which are unstable, a closed form solution of the nonlinear evolution of the interaction is given.
Discrete spectrum for ideal magnetohydrodynamic channel flow
Authors: Kadish Abraham
Published: March 2003

Sufficient conditions are derived for the stability of cylindrically symmetric plasma equilibria with steady flow when the magnetic field is unidirectional and the flow is tangential to the field. The stability criteria involve only the flow, sound, and Alfvén speeds.
Finite-beta modifications of drift and trapped-electron modes
Authors: Rewoldt G. , Tang W. M. , Frieman E. A.
Published: March 2003

A previous two-dimensional electrostatic calculation for the spatial structure of drift and trapped-electron modes is extended to include finite-β effects. Specifically, the parallel perturbed vector potential, and the parallel Ampere’s law are added to the calculation. Illustrative results are presented. The effect of increasing β can be either stabilizing or destabilizing, depending on the parameter values chosen. For present tokamak β values, the finite-β effects are small.
Diocotron instability of a relativistic coaxial multi-ring hollow electron beam
Authors: Chen H. C. , Palmadesso P. J.
Published: March 2003

The diocotron stability properties of a relativistic coaxial multi-ring hollow electron beam are investigated using a macroscopic cold fluid description based on moment-Maxwell equations. It is found that for a broad range of beam parameters and somewhat more general type beam profiles, the growth rate of instability has a sensitive dependence on fractional charge neutralization, relative position of the rings with conducting wall, and gap-length of the rings. In the case of a sharp boundary density profile, the beam can easily be stabilized by a small fractional charge neutralization with appropriate gap-length. The growth rate can be either enhanced or reduced depending on the position and gap-length of the rings.
Multi-channel ion beam overlap
Authors: Wright Thomas P.
Published: March 2003

Calculations of ion flux enhancement from overlap of many plasma-channel-transported beams onto inertial confinement fusion targets give factors of 10 with efficiencies around 85%. Scaling relations are presented for different beam parameters and overlap geometries.
Soret diffusion and convective stability in a closed vertical cylinder
Authors: Abernathey J. R. , Rosenberger F.
Published: March 2003

The convective stability range of nonreactive binary gas mixtures (Xe:He,Xe:Ar) in a closed vertical cylinder with conducting walls heated from below was investigated. The xenon mole fraction was varied from XXe = 1 to 0.03 and from 1 to 0 for the helium and argon containing mixtures, respectively. Critical thermal Rayleigh numbers NRa1 for the onset of convective flow were obtained from high resolution differential temperature measurements. Thermal diffusion ratios were calculated, based on Lennard–Jones potentials, over the whole composition range for both gas mixtures. Minima in NRa1 occurred at the composition for which maximum relative mass separation of the components due to thermal diffusion is predicted. Specifically, the monocomponent value NRa1≊200 for a cylinder with height-to-radius ratio of 6 was reduced to 25 at XXe≊0.03, and 165 at XXe≊0.3 for the Xe:He and Xe:Ar systems, respectively. Comparison with Rayleigh–Bénard instability theory, including Soret and Dufour effects, shows that for (gas) mixtures, the lateral walls act convectively less stabilizing than for monocomponent systems. It is shown that Dufour contributions to convective stability in these gas mixtures are negligible.
Calculation of turbulent boundary layers on curved surfaces
Authors: Gibson M. M. , Jones W. P. , Younis B. A.
Published: March 2003

Published data from boundary layers on convex surfaces are used to assess the performance of a calculation method based on the solution of modeled transport equations for the Reynolds’ stresses and the dissipation rate of turbulence energy. For flows with large curvature, the model closely reproduces the suppression of turbulence and the diminished growth rate and skin friction. The recovery of flow distorted by curvature is also predicted with results broadly in accord with the measurements.
Measurements of the growth of a turbulently mixed layer in a linearly stratified fluid
Authors: Folse R. F. , Cox T. P. , Schexnayder K. R.
Published: March 2003

Measurements have been made of the growth of a turbulently mixed layer in a linearly stratified fluid for Brunt–Väisälä frequencies N in the range 0.099–0.386 Hz. Turbulent mixing was induced by a horizontal grid undergoing vertical oscillations at frequencies f in the range 5.3–20.5 Hz. The experimental results support a theory due to Long which predicts the size of the mixed layer D measured from a ’’virtual’’ source plane, as a function of time t, given by D∝f1/2 N−7/18 t1/9.
Rotational distribution of N2 in Ar free jet
Authors: Koura Katsuhisa
Published: March 2003

The rotational distribution, temperature, and collision number of N2 in the Ar free jet are studied using the rotational transition cross sections of the exponential gap and power laws. The relaxation of the rotational distribution along the axis of the free jet is described by the master equation, where the velocity distribution is taken as the local Maxwell distribution. The effects of the stagnation density level n0d and temperature T0 are investigated. The rotational distribution indicates a non-Boltzmann distribution with overpopulation at the higher rotational levels. The degree of deviation from the Boltzmann distribution at the same axial distance increases with decreasing n0d and decreases with increasing T0. The rotational temperature ratio Tr/T0 depends weakly on T0, which is consistent with the experimental result of Poulsen and Miller in the N2-Ar system. The rotational collision number Zr is neither constant nor a monotonically increasing function of the translational temperature; still, the downstream Zr depends weakly on n0d and T0 consistent with the experimental result of Poulsen and Miller and is in quantitative agreement with their N2-Ar value of 2±1.
Magnetohydrodynamic Taylor vortex flow under a transverse pressure gradient
Authors: Tabeling P. , Chabrerie J. P.
Published: March 2003

Electromagnetically driven flows between infinite conducting cylinders subjected to a constant axial magnetic field B0 are considered. The linear stability problem for axisymmetric stationary disturbances is treated. In contrast with Chandrasekhar’s conjecture, the present numerical results indicate that the critical wave-number tends to infinity with the magnetic field. The nonlinear state of the flow, slighly above transition, is investigated by means of Stuart’s energy approach. Further theoretical results are compared with experiment. At criticality, excellent agreement is found between linear stability theory and experiment. Above transition, only the shape of the experimental curves is correctly predicted by Stuart’s energy method.
Nonlinear ion-acoustic waves and solitons in a magnetized plasma
Authors: Lee L. C. , Kan J. R.
Published: March 2003

A unified formulation is presented to study the nonlinear low-frequency electrostatic waves in a magnetized low-b plasma. It is found that there exist three types of nonlinear waves; (i) nonlinear ion-cyclotron periodic waves with a wave speed Vp≳Cs (ion-acoustic velocity); (ii) nonlinear ion-acoustic periodic waves with Vp<Cs cosq; and (iii) ion-acoustic solitons with Cs cosq<Vp<Cs, where q is the angle between the wave vector and the magnetic field.
Theory of m = l modes in collisionless plasmas
Authors: Basu B. , Coppi B.
Published: March 2003

Modes with azimuthal or poloidal wavenumber m = 1 can appear in the form of internal kinks that are described by the ideal magnetohydrodynamic approximation, or in the form of ’’reconnecting’’ modes that lead to the formation of magnetic islands. An analysis of these modes is developed for collisionless regimes, where the collisional electrical resistivity is negligible and the effects of finite ion inertia and mode-particle resonance play the most important role.
Collective scattering of CO2-laser light from ion-acoustic turbulence
Authors: Pots B. F. M. , Coumans J. J. H. , Schram D. C.
Published: March 2003

The spectral density function S(k,w) associated with spontaneous turbulence in a current-driven plasma of medium density (ne = 1019−10220 m −3) is measured with the scattering of the radiation of a cw low-power CO2- laser (Pi≃2 W, li = 10.6 mm). Optical homodyne detection has been used. A remarkable result is that the level of fluctuations with wave vectors k∥B is close to thermal; B is the magnetic field for plasma confinement. The level of fluctuations with k⊥B is several orders of magnitude larger than the thermal level. The phase velocity of these fluctuations is equal to the ion-acoustic velocity. A power law spectrum of the form n̄e/ne∼w−2 has been found. It is consistent with the spectrum below 1 MHz as measured by optical probes.
The electron density structure of the plasma produced on glass microballoons by 10.6 μm radiation
Authors: Fedosejevs R. , Burgess M. D. J. , Enright G. D. , Richardson M. C.
Published: March 2003

A detailed study is presented of the 0.4–40 nc plasma region created by nanosecond, 10.6 mm, 1–38 J laser pulses incident upon unfilled glass microballoon targets. Many interesting features were observed including electron density scale lengths (Lc) and step heights (Np) through nc as short as 0.3l0 and as high as 38nc, respectively. An attempt has been made to explain these parameters in terms of recent theories, and a consistent picture only seems to emerge if important roles are assigned to the superthermal plasma component and the effect of inhibited energy transport during laser irradiation.
Phase velocity of wind waves along different directions
Authors: Rebuffat D. , Ramamonjiarisoa A.
Published: March 2003

The phase velocities of laboratory wind waves along various angles to the mean wind direction, determined from cross-spectral densities, appear to increase with angle.
Shape of a sail in a flow
Authors: Vanden-Broeck Jean-Marc , Keller Joseph B.
Published: March 2003

The shape of a sail and the potential flow around it are considered in two dimensions. The sail is assumed to be flexible, inextensible, and supported by a mast and a sheet. A perturbation solution is obtained for a rather tightly stretched sail at any angle of attack.
Statistical theory of magnetohydrodynamic turbulence
Authors: Miller Guthrie
Published: March 2003

The statistical approach to cylindrical magnetohydrodynamic turbulence of Montgomery, Turner, and Vahala is carried somewhat further. This leads to the general result that, in statistical equilibrium, the most probable value of the turbulently fluctuating magnetic field is force free.
Singular solutions and the continuous spectrum
Authors: Hameiri Eliezer
Published: March 2003

It is generally accepted that the continuous spectrum can be detected by the presence of nonsquare integrable solutions. This is shown to be incorrect as a general principle. A method to determine the singularity of ’’eigenfunctions’’ corresponding to the continuous spectrum is presented.
Constants of motion in a helical magnetic field
Authors: Jones Roger D.
Published: March 2003

The relationship between the ’’screw’’ symmetry of a helical magnetic field and the constants of motion of the system is obtained. In particular, the integral of motion for the symmetry is derived and the relationship between this integral and one obtained by Laird and Knox is demonstrated.
Magnetic drift instability in a hot electron plasma
Authors: Dominguez R. R. , Guest G. E.
Published: March 2003

A kinetic theory of electromagnetic flute modes in a hot electron plasma is presented. Unlike the flute-interchange mode, the electromagnetic flute mode is destabilized by warm electrons.
Magnetohydrodynamic behavior of thermonuclear fuel in a preconditioned electron beam imploded target
Authors: Lindemuth I. R. , Widner M. M.
Published: March 2003

Two-dimensional magnetohydrodynamic numerical calculations have been performed to study the fuel behavior of a preconditioned relativistic electron beam target in which, experimentally, a portion of the beam current prepulse entered the target to provide fuel preheat and magneto-thermoinsulation. The magnetohydrodynamic plasma model used includes radiation, thermal conduction, ionization, and resistive diffusion. The magnetohydrodynamic partial differential equations are solved by a computer code employing implicit finite-difference methods. The fuel is shown to develop counter-streaming vortices during the implosion phase. The computed neutron yield is in reasonable agreement with the experimental value, suggesting the origin of the neutrons to be thermonuclear. The effect of both magneto-thermoinsulation and preheat is examined.
Energy transport by weak electrostatic drift fluctuations
Authors: Gary S. Peter , Sanderson J. J.
Published: March 2003

Energy transport from weak electrostatic microfluctuations, driven unstable by density and temperature gradients perpendicular to a uniform magnetic field in a Vlasov plasma, is calculated. Using the local approximation and assuming resonance broadening as the saturation mechanism, a consistent procedure is used to evaluate and compare the cross-field energy transport due to enhanced microfluctuations from the universal, lower hybrid, and ion-acoustic density drift, and the ion, electron, and lower hybrid temperature drift instabilities. If a temperature gradient is the primary source of free energy, the resultant enhanced fluctuations cause a cross-field thermal energy loss rate which is much greater than the energy loss rate associated with cross-field particle transport.
Global stability of transient drop extraction to Marangoni instabilities
Authors: Patzer John F. , Homsy George M.
Published: March 2003

The method of energy is used to study the stability of transport in a drop which is characterized by an impulsive change in boundary temperature or concentration. Instabilities driven by interfacial-tension gradients are considered for diffusive base states in which a spherical drop is either stagnant or has an internal circulation at high Peclet number. Two stability criteria are considered for the stagnant drop; strong stability, for which an exponential decay of the disturbance energy is guaranteed; and marginal stability, for which the disturbance energy is less than or equal to its initial value. The strong stability criterion is obtained for the circulating drop. In this latter case, properties of the solution to the dynamic equations are used to constrain the class of admissible functions in the energy theory. Stability curves generated in the Marangoni number-time plane clearly show the presence of both an onset time and a decay time for the disturbances. Comparison with limited available experimental data shows that predictions based upon the theory are observed qualitatively.
Effect of velocity distribution on the stability of developing flow in a pipe
Authors: Gupta S. C. , Garg V. K.
Published: March 2003

A spatial stability study of the developing flow in a pipe shows that the critical frequency and wavenumber for the Hornbeck profile are larger than those for the Sparrow profile, but the critical Reynolds number is smaller. Results for the Hornbeck profile are found to be closer to the experimental data.
Transport equation for the joint probability density function of velocity and scalars in turbulent flow
Authors: Pope S. B.
Published: March 2003

The transport equation for the joint probability density function of velocity and scalars is shown to provide a good basis for modeling turbulent reactive flows. As in the equation for the probability density function of the scalars alone, nonlinear reaction schemes can be treated without approximation. The advantage of considering the joint probability density function equation is that convection (by both the mean and fluctuating velocities) appears in closed form. Consequently, the gradient-diffusion assumption for turbulent transport is avoided. Closure approximations are presented for the terms involving the fluctuating pressure and viscous and diffusive mixing. These models can be expected to be reliable since they are compatible with accurate and proven Reynolds-stress models. The resulting modeled transport equation for the joint probability density function can be solved by the Monte-Carlo method for inhomogeneous flows with complex reactions.
Potential double layers formed by ion beam reflection in magnetized plasmas
Authors: Stenzel R. L. , Ooyama M. , Nakamura Y.
Published: March 2003

Experimental observation of a new potential double layer in a collisionless magnetoplasma are presented. The double layer is formed when an ion beam is injected along converging field lines and reflected at a boundary drawing electron saturation current. The double layer is detached from the boundaries, V-shaped for magnetized ions, establishes its potential self-consistently to the beam potential (fd≲Vb), is stationary over a wide potential range (1∼efd/kTe≲25), and is produced free from ionization effects. Distribution functions measured on the high potential side show drifting Maxwellians subject to the Buneman instability (vd/ve≳1) and self-consistently produced trapped particles.
Polarization of an intense space-charge-neutral ion beam incident upon a magnetic field
Authors: Wessel Frank , Robertson Scott
Published: March 2003

The collective behavior of an intense, pulsed, space-charge-neutral proton beam (100 kV, 3 A/cm2, 0.8 msec) incident upon a transversely oriented magnetic field of 0–6 kG has been determined experimentally. At 6 kG, a floating potential probe shows longitudinal polarization creating a positive sheath (virtual anode) within the field boundary having a peak electrostatic potential equal to the ion accelerating potential. The sheath thickness is approximately v/wpi, where v is the beam velocity and wpi is the ion plasma frequency. Oscillations in the sheath potential at wpi were observed as predicted by previous computer simulations. Ions were observed to be accelerated out of the sheath in all directions. Transverse polarization of the beam as predicted by Schmidt’s polarization drift model was not observed although the plasma dielectric constant was approximately 102.
Hot-electron production and resonance absorption of laser light in the shock-front targets
Authors: Kolodner Paul , Yablonovitch Eli
Published: March 2003

Laser targets, created by fast optical breakdown of shock fronts produced in an electrothermal shock tube filled with D2 or H2, have been used for studies of resonance absorption of CO2 laser light. In these targets, the critical-surface density-gradient vector always points along the shock-tube axis, leading to reproducible hot-electron emission by plasma wave breaking. The angular distribution and energy spectrum of this emission have been studied. For 200 psec risetime laser pulses, the measured scaling of hot-electron temperature with laser pulse energy agrees with predictions of models of profile steepening by the ponderomotive force of the laser light. For 20 psec risetime laser pulses, the results are consistent with a model in which the detected electron emission is dominated by a ’’chirped’’ monoenergetic burst lasting only 1 to 2 psec. In this model, the electron energy spectrum is due to the integrated temporal variation of the enery of the burst, and the hot-electron temperature is practically independent of laser pulse energy.
Relationships between integral properties of gravity-capillary interfacial waves
Authors: Hogan S. J.
Published: March 2003

Previous relationships between integral properties of gravity-capillary surface waves are shown to hold for interfacial waves, provided the definitions of the quantities involved are stated correctly.
A note on Poisson’s equation for pressure in a turbulent flow
Authors: Bradshaw P. , Koh Y. M.
Published: March 2003

It is shown that the ’’forcing function’’ (the right-hand side) of Poisson’s equation for the mean or fluctuating pressure in a turbulent flow can be divided into two parts, one related to the square of the rate of strain and the other to the square of the vorticity.
Skewness of the temperature derivative in an asymmetrically heated wake
Authors: Tavoularis Stavros , Sreenivasan K. R.
Published: March 2003

Measurements in an asymmetrically heated two-dimensional wake show that the temperature (temporal) derivative skewness becomes slightly positive in the region between the mean velocity minimum and the mean temperature maximum, while remaining appreciably negative elsewhere.
Kinetic-theory description of electron heat transfer in a plasma
Authors: Shirazian Mohammad H. , Steinhauer Loren C.
Published: March 2003

Electron heat flux in a fully ionized gas between two parallel boundaries is studied. The moment method is employed to analyze the relation between the electron heat flux and the plasma collisionality parameter L/λe, both with and without electron-electron collisions (λe is the electron mean-free-path). The electron heat flux is determined as a function of L/λe over the entire range from free flow of collisional regimes. These calculations employ various combinations of trial distribution function and collision operator. Comparisons are made between the various models and with the known result in the collisional limit. Discrepancies between the present model and the known collisional behavior are discussed, and an approximate heat flux expression is presented.
Monte Carlo evaluation of transport coefficients
Authors: Boozer Allen H. , Kuo-Petravic Gioietta
Published: March 2003

A method is developed for evaluating transport coefficients in asymmetric geometries using the Monte Carlo method. The method is applied to the stellarator.
Effect of magnetic field errors on low-frequency waves in a pure electron plasma
Authors: Keinigs Rhon
Published: March 2003

It is demonstrated how static magnetic field errors can resonantly drive low-frequency electrostatic waves in a pure electron plasma column. The driving mechanism is identified as a charge bunching that results from the field error. It is found that the wave amplitude increases linearly with time.
Magnetohydrodynamic stability of high-beta closed-line systems
Authors: Pritchett P. L.
Published: March 2003

The ideal magnetohydrodynamic stability of high-beta model equilibria in slab geometry with Bz = 0 and containing elongated regions of closed field lines is investigated as an initial-value problem and in terms of the energy principle. For moderate or larger elongations, flute (interchange) modes are found to be stable throughout the closed region. Co-interchange modes with the character of an m = 1 kink mode, however, are found to be unstable. The growth rate of these modes initially increases rapidly with kz and then approaches a finite limit as kz→∞. As the elongation of the closed region is increased, the growth rate decreases rapidly. Possible implications of these results for field-reversed theta pinches are discussed.
Molecular diffusion in oscillating laminar flow in a pipe
Authors: Purtell L. P.
Published: March 2003

The effect of flow oscillations on the axial diffusion of a solute in a pipe is analyzed theoretically by a perturbation method for small oscillation Reynolds numbers. The specific case of an initial step disribution in concentration is solved to second order. Numerical results of diffusion enhancement are given for several values of the parameter involved and are found to be conveniently summarized in terms of an equivalent diffusion parameter.
Turbulent boundary layer at low Reynolds number
Authors: Purtell L. P. , Klebanoff P. S. , Buckley F.T.
Published: March 2003

The results of an experimental investigation of a turbulent boundary layer with zero-pressure gradient directed toward extending the data base at low Reynolds numbers are presented. The data obtained are concerned primarily with mean-velocity distributions, skin-friction coefficients, and distributions of intensity of the longitudinal-component of the turbulent-velocity fluctuations for Reynolds numbers based on momentum thickness as low as 465. The validity, at low Reynolds numbers, of the semi-empirical laws characterizing the inner and outer regions of the boundary layer is examined.
Absolute and convective instabilities in two-dimensional free-electron lasers
Authors: Kwan Thomas J. T. , Cary John R.
Published: March 2003

Computer simulations of the free-electron laser have been carried out using a two-and-one-half dimensional fully electromagnetic relativistic particle code. It has been found that waves propagating obliquely with respect to the electron beam are always unstable with appreciable growth rates and, therefore, mode competition is an important consideration in the design of free-electron lasers. Furthermore, electromagnetic waves with group velocities opposite to the direction of electron beam propagation can be absolutely unstable. It is shown that the absolutely unstable waves can completely disrupt the electron beam and any laser oscillaton; therefore, they are extremely detrimental to the operation of free electron lasers. However, it has been found that one can stabilize the absolute instability by taking particular precautions in the design of free-electron lasers.
Oscillating two-stream instability with pump of finite extent
Authors: Nicholson Dwight R.
Published: March 2003

In one spatial dimension, the effects of finite pump length L on the oscillating two-stream instability are considered. In the absence of damping, a purely growing instability for very small pump extent is found. Under certain conditions, the finite system exhibits faster growth rates than the corresponding infinite system.
Parametric excitation of electrostatic drift waves by a low frequency oscillating magnetic field
Authors: Nag Tarasankar , Dasgupta Brahmananda
Published: March 2003

Parametric excitation of drift waves and suppression of drift instabilities in a plasma by an oscillating field of low frequency has been studied. The role of such an oscillating field of frequency, much higher than the drift frequency in determining the stability properties of the waves, has been investigated.
Growth of laser ripple in a plasma and its effect on plasma wave excitation
Authors: Sodha M. S. , Singh Tarsem , Singh D. P. , Sharma R. P.
Published: March 2003

An investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a collisionless unmagnetized plasma is presented. The effect of the position and the width of the ripple on its growth has been studied in some detail. In a special case, the growth rate of the ripple is seen to be comparable to the growth rate of filamentation instability. In addition to the growth of the ripple, the effect of the presence of the ripple on the excitation of an electron plasma wave has also been studied.
Solutions of the Helmholtz equation with boundary conditions for force-free magnetic fields
Authors: Rasband S. Neil , Turner Leaf
Published: March 2003

It is shown that the solution, with one ignorable coordinate, for the Taylor minimum energy state (resulting in a force-free magnetic field) in either a straight cylindrical or a toroidal geometry with arbitrary cross section can be reduced to the solution of either an inhomogeneous Helmholtz equation or a Grad–Shafranov equation with simple boundary conditions. Standard Green’s function theory is, therefore, applicable. Detailed solutions are presented for the Taylor state in toroidal and cylindrical domains having a rectangular cross section. The focus is on solutions corresponding to the continuous eigenvalue spectra. Singular behavior at 90 ° corners is explored in detail.
Thermonuclear burn in wall-confined plasmas
Authors: Kmetyk Lubomyra Nadia
Published: March 2003

The behavior of a hot, dense, thermonuclear reacting plasma which is contained between cold metal walls and thermally insulated by a strong magnetic field parallel to the wall is studied. The effects of thermal conduction and radiation energy losses from the plasma, convection, and a detailed treatment of alpha particle heating, as well as Ohmic heating and magnetic field diffusion, are included in the full set of one-fluid two-temperature equations. Losses parallel to the field lines are ignored since the plasma is assumed infinite in extent parallel to the walls in this one-dimensional study. The resulting initial value problem is solved numerically with a Lagrangian computer code employing finite difference methods. The dynamic behavior of the plasma is described by the time evolution of the temperature, number density, and magnetic field profiles. Cold alpha particles accumulate at the walls due to particle-wall collisions, but no atomic physics effects are considered at the plasma-wall interface. A sustained thermonuclear burn is demonstrated by the formation of an ’’ignited’’ central region in which the alpha particles are sufficiently confined so that they heat and maintain a high plasma temperature.
Tollmien–Schlichting wave cancellation
Authors: Milling Robert W.
Published: March 2003

An experiment was conducted in which Tollmien–Schlichting type disturbances in the boundary layer on a flat plate with zero pressure gradient were shown to be nearly canceled by interference with a second wave, 180 ° out-of-phase with the original disturbance.
Randomly gapped wall stabilization for time-dependent magnetic confinement fields
Authors: Bellan Paul M.
Published: March 2003

An azimuthally continuous conducting wall cannot be used to provide stabilization for time-dependent magnetic confinement fields, such as the recently proposed traveling, compressing mirror [P. M. Bellan, Phys. Rev. Lett. 43, 858 (1979)], because, unlike static confinement fields, time-dependent confinement fields cannot penetrate such a wall. If the wall has a gap, then time-dependent fields can penetrate, but the gap produces an azimuthal asymmetry of the field which can cause enhanced particle losses. By splitting the wall into a large number of short insulated axial sections, each with a randomly oriented azimuthal gap, (i) the time-dependent confining field can penetrate the wall, (ii) wall stabilization is obtained, and (iii) azimuthal symmetry is maintained.
Cold ion space charge sheaths
Authors: Auer Peter L.
Published: March 2003

The influence of collisions or of a cross-magnetic field on the well known Langmuir–Childs relations is examined.
Characterization and suppression of zero-frequency cyclotron waves on relativistic electron beams
Authors: Shanahan William R.
Published: March 2003

Zero-frequency cyclotron waves, which can be excited by a variety of static perturbations, produce large-scale disruption of relativistic electron beam equilibria. A study of their basic nonlinear properties and an examination of methods with which to achieve their effective suppression are presented. A simple set of envelope equations based on the signal-particle equations of motion is derived. Both analytic and numerical solutions are obtained for the case of a uniform magnetic guide field. The envelope equations are then used to investigate methods of suppression. It is found that although a spatially steeply rising magnetic field is a key component in achieving such suppression, initial beam momentum flaring plays a crucial role. Relativistic, fully electromagnetic particle-in-cell simulations designed to examine methods of suppression are presented.
Observation of a decay instability of an ion beam-plasma system
Authors: Kawai Y. , Kono M. , Hollenstein Ch.
Published: March 2003

A decay type instability is observed in a linearly stable ion beam-plasma system. The spatial development of the instability agrees with the solution of the mode coupling equations describing a three-wave interaction.
Numerical simulations of electrostatic hydrogen cyclotron instabilities
Authors: Okuda H. , Cheng C. Z. , Lee W. W.
Published: March 2003

Both one- and two-dimensional particle simulation models have been used to study the nonlinear behavior of the electrostatic hydrogen cyclotron instabilities driven by the electron current along the magnetic field. It is found that the instability saturates as a result of electron velocity space diffusion along the magnetic field. The cyclotron waves remain highly coherent in the nonlinear stage. When the electron drift speed is comparable to the thermal speed, substantial ion heating as well as particle cross-field diffusion comparable to Bohm diffusion has been observed. Comparisons of the simulation results with the theoretical predictions and the observations in both laboratory and space plasmas are discussed.
Toroidal drift modes driven by ion pressure gradients
Authors: Horton Wendell , Choi Duk-In , Tang W. M.
Published: March 2003

Ion pressure gradient-driven drift modes are analyzed for their parametric dependence on the shear, the toroidal aspect ratio, and the pressure gradient using the ballooning toroidal mode theory. An approximate formula for the anomalous ion thermal conductivity is derived for the turbulent regime.
Electron-cyclotron resonance heating in plasmas with arbitrary stratification of the magnetic field
Authors: Friedland L. , Porkolab M.
Published: March 2003

Cyclotron resonance absorption in a plasma with arbitrary stratification of the magnetic field is considered. Using the absorption coefficient derived for perpendicular stratification, simple analytic estimates of the absorption can be made for artrary stratification.
Stagnation regions of separation
Authors: O’Brien V.
Published: March 2003

Vorticity contours are used to show that two-dimensional separation falls into three stagnation region classes for steady flows; one type on fixed walls and two others for moving walls. The existing data on laminar unsteady flows indicate that they fill the same three classes.
Experimental and theoretical studies of the effects of nonuniformities in equilibrium magnetohydrodynamic flows
Authors: Rosenbaum M. , Shamma S. E. , Louis J. F.
Published: March 2003

An experimental study of the effects of thermal and velocity nonuniformities is performed in an equilibrium plasma for a range of Hall coefficients. By introducing equally spaced cold blades in the radial flow of an electrodeless magnetohydrodynamic disk deivce, it is possible to create well-defined two-dimensional wake nonuniformities with strong variations of the plasma properties in the direction normal to the magnetic field and the flow. This type of nonuniformity and orientation theoretically provides the strongest reduction of Hall coefficient and effective conductivity for high values of the Hall coefficient. This degradation which reached more than 50% in some cases, is controlled by both the level of nonuniformities and the value of the ideal Hall coefficient. The former is dependent upon the number of blades (root mean square deviation of the conductivity), and the latter is dependent upon the values of the magnetic field intensities. The results provide basic quantitative information about the effects of conductivity and velocity nonuniformities on the performance of equilibrium magnetohydrodynamic generators over a wide range of Hall coefficients. The theoretical predictions are derived from a detailed two-dimensional electrodynamic analysis and a simplified engineering model based on a generalization of Rosa’s layer model. These experiments validate the analytical studies and support the use of the theoretical layer nonuniform models in describing the effect of boundary layers on the performance of linear magnetohydrodynamic devices.
Recurrence of initial state of nonlinear ion waves
Authors: Abe Kanji , Satofuka Nobuyuki
Published: March 2003

By solving the Korteweg–deVries equation in a wide range of the ratio between the nonlinearity and the dispersion, the recurrence of the initial state of the ion wave is examined. The recurrence is assured of taking place only when the dispersion of the initial ion wave predominates over the nonlinearity. If the initial wave has strong nonlinearity compared with the dispersion, the recurrence is indistinct, and the initial monochromatic wave evolves to a turbulent state.
Relativistic limit on resonance at oblique incidence
Authors: Dragila R.
Published: March 2003

In a collisionless cold plasma, the limit has been obtained of a resonance electric field peak at oblique incidence of a p-polarized electromagnetic wave upon an inhomogeneous plasma as a result of the nonlinear relativistic treatment of plasma waves.
Toroidal effects on propagation, damping, and linear mode conversion of lower hybrid waves
Authors: Ignat D. W.
Published: March 2003

A common simplifying assumption made in the consideration of radio-frequency heating of tokamaks near the lower hybrid frequency is that the wavelength imposed by the coupling device parallel to the magnetic field is not modified by gradients along the field. In the present calculation, the parallel wavelength is allowed to vary, and important effects are found on wave penetration and damping if the toroidal aspect ratio (Rmajor/rminor) is less than approximately five. The calculation shows that heating at the center of a small aspect ratio torus is inhibited by a decrease in k∥ if waves are launched at the outside, and that it may be possible to change the plasma current via electron Landau damping with a coupler of symmetric power spectrum by placing the coupler at the top (or bottom) of the torus.
Irreversibility and transport in the lower hybrid drift instability
Authors: Drake J. F. , Lee T. T.
Published: March 2003

The dynamics of electrons in a low-frequency wave propagating perpendicular to a uniform magnetic field are studied and the implications of these results for transport and heating by the lower hybrid drift instability are explored. Below a threshold eφ/Ti≊ 0.25–1.0, all electron energy exchange with the wave is reversible and no plasma transport is possible. Above this threshold, trapping of electrons by the wave potentials takes place and causes irreversible electron heating and diffusion. These results imply that anomolous transport in inhomogeneous plasma with weak drifts (diamagnetic velocity less than the ion thermal velocity) may be substantially less than previously predicted.
Simulation of the formation of the PS-1 spheromak
Authors: Sgro A. G. , Winske D.
Published: March 2003

Several interesting characteristics of the PS-1 spheromak plasma have been inferred from one-dimensional simulations of the implosion phase of the experiment. The calculations demonstrate that metallic impurities in the pre-imploded plasma, which come from the electrodes, strongly affect the dynamics of the implosion. The simulations also indicate that the resistivity and Joule heating are much larger than their classical values, suggesting that microinstabilities are a principal source of ion heating. Finally, the calculations suggest the existence of a hot, low-density plasma between the plasma column and the wall, which carries a non-negligible fraction of the poloidal current.
Measurement of energy loss and energy diffusion of charged test particles in a field-free near equilibrium plasma
Authors: Taggart D. , Schumann L. , Brown H. H.
Published: March 2003

The parallel energy loss and parallel energy diffusion of a nearly monoenergetic, well collimated, and tenuous test particle beam in a plasma has been measured. Parallel refers to the initial beam direction. Test particles of H1 +, H2 +, H3 +, and Ar+ at energies of 1, 2, and 3 keV were utilized. The parameter X ranged from 0.28 to 3.03, where X = (test particle speed)/(plasma electron thermal speed). The near equilibrium, field-free, and highly homogeneous plasma consisted of electrons and singly ionized Cs both at a temperature of 2100 °K and a density of 1×1011 cm−3. The energy loss data are compared both to classical theories with phenomenological cutoffs and to fully convergent theories. The best agreement is obtained with the work of Itikawa and Aono. The energy diffusion results are compared to the theory of Spitzer, and to the Spitzer theory with nondominant terms added. The agreement is satisfactory except for X≳1.5.