bluuebunny/crossref_metadata_2025 huggingface dataset:
Multicomponent model of the resistive hose instability
Authors: Sharp William M. , Lampe Martin , Uhm Han S.
Published: March 2003

A new model of beam dynamics is presented for treating the resistive hose instability in intense, self-pinched electron or ion beams. The beam is modeled as a superposition of rigid, independently moving components with different radii. This multicomponent model exhibits the localized resonances and other analytic properties of a Vlasov treatment. The new model is used to calculate the eigenfunctions and the dispersion relation for hose instability of an axially uniform beam propagating in a medium of specified conductivity profile with any specified return current. Wave growth is found to be significantly increased by return current or by a narrow conductivity profile. The results are compared to those of earlier models.
Comments on ‘‘Soliton propagation in slowly varying media’’
Authors: Karpman V. I. , Maslov E. M.
Published: March 2003

A comment is made on the soliton propagation in slowly varying media.
Evidence for large heat fluxes from the mass ablation rate of laser-irradiated spherical targets
Authors: Goldsack T. J. , Kilkenny J. D. , MacGowan B. J. , Cunningham P. F. , Lewis C. L. S. , Key M. H. , Rumsby P. T.
Published: March 2003

The mass ablation rate from spherical targets irradiated at wavelengths of 1.05 and 0.53 μm has been measured with particle and with x-ray diagnostics. The specific mass ablation rate is found to be proportional to I0.7a with little wavelength dependence for a given value of Ia, the absorbed irradiance. Comparisons with simulations shows that heat fluxes up to and in excess of 10% of the free streaming value are occurring. Values of the ablation pressure are derived.
Instabilities driven by the parallel variation of the electrostatic potential in tandem mirrors
Authors: Lee X. S. , Catto P. J. , Aamodt R. E.
Published: March 2003

It is demonstrated that the free energy associated with the sheared flow induced by the parallel variations of drift frequencies can provide a new destabilizing term. In particular, we show the existence of a magnetohydrodynamic-like instability which may occur in the mirror end plugs or thermal barrier region of a tandem mirror fusion device.
Numerical simulation of tokamak electron dynamics
Authors: Miner W. H. , Winsor N. K. , Bernstein I. B.
Published: March 2003

In a tokamak, the electron distribution deviates from a Maxwellian because magnetically untrapped electrons run away parallel to the applied electric field. A new method for calculating the dynamics of the electron distribution is presented. This method is novel because it can treat systems with comparable numbers of trapped and untrapped electrons, and an electric field comparable with (but smaller than) the Dreicer field. The electron distribution function and the plasma resistivity are presented for representative values.
Shear-ballooning stability analysis of low-β plasmas
Authors: Yoshikawa Shoichi
Published: March 2003

A model of magnetic configurations that has both shear and variation of the field curvature along the magnetic field is constructed. The stability analysis of this system is performed by expanding the perturbation function in φ0 which is constant along the field and φ1 which varies sinusoidally along the field line. The shear term is retained in the form of differential operations in ξ, the coordinate parallel to the pressure gradient. After several simplifications, the equation reduces to a fourth-order differential equation. The eigenvalue of this equation is obtained numerically. The calculated critical β plotted versus the well depth parameter, h, makes a smooth transition betweeen h<0 (maximum average B) and h≳0 (minimum average B). Using the same technique, the nondivergent solution to the localized shear mode (Suydam mode) is also obtained by retaining the inertia term.
Equilibrium and stability of nonaxisymmetric minimum-B mirrors
Authors: D’Ippolito D. A. , Freidberg J. P.
Published: March 2003

Analytic three-dimensional mirror equilibria are calculated from the tensor-pressure fluid equations by means of an asymptotic expansion procedure. The class of equilibria described here is of interest to the minimum-B end cells of current tandem mirror experiments. It is shown that in the shallow-well approximation, a small plasma pressure can produce a relative change of order unity in the vacuum magnetic well, thereby leading to interchange instability. In addition, if the axial dependence of the quadrupole field is not chosen in a specific way, ’’long thin’’ corrections set an upper limit on the maximum allowable plasma radius. The proper choice of quadrupole field to avoid this limitation is discussed, and an analytic expression is derived for the interchange beta limit.
Exchange of energy and momentum between gases at different temperatures
Authors: Bampi F. , Morro A.
Published: March 2003

The interaction mechanisms between gases at different temperatures are investigated within the thermodynamic theory of mixtures. Conditions a priori on the phenomenological coefficients are derived by having recourse to the Galilean invariance and the entropy principle for mixtures. The results so obtained, which, owing to their thermodynamic origin, must hold for any kinetic derivation, turn out to be satisfied by known kinetic expressions.
Comments on ‘‘Propagation of intense ion beams in straight and tapered z-discharge plasma channels’’
Authors: Winterberg F.
Published: March 2003

A comment is presented on the propagation of intense ion beams in straight and tapered z−discharge plasma channels.(AIP)
Breakdown of quasilinear theory for incoherent 1-D Langmuir waves
Authors: Laval G. , Pesme D.
Published: March 2003

Quasilinear theory for the usual weak warm beam instability is reconsidered. It is found that mode-coupling terms yield non-negligible corrections to the quasilinear diffusion coefficient and mode growth rates. These mode coupling effects give rise to non-Gaussian statistics that invalidate both the usual Fokker–Planck equations and the resonance broadening theories.
Inconsistency of quasilinear theory
Authors: Laval G. , Pesme D.
Published: March 2003

Mode-coupling terms yield non-negligible corrections to the quasilinear terms in the evolution of one-dimensional Langmuir turbulence generated by the weak warm beam instability.
The shape of low Reynolds number jets
Authors: Gear R. L. , Keentok M. , Milthorpe J. F. , Tanner R. I.
Published: March 2003

New experiments and new finite element computations with surface tension of jet shapes at Reynolds numbers up to 100 were made. Good agreement between experiment and computations was obtained.
A second-order theory for solitary waves in shallow fluids
Authors: Gear J. A. , Grimshaw R.
Published: March 2003

Solitary waves in density stratified fluids of shallow depth are described, to first order in wave amplitude, by the Korteweg–de Vries equation; the solution for a single solitary wave has the familiar ‘‘sech2’’ profile and a phase speed which varies linearly with the wave amplitude. This theory is here extended to second order in wave amplitude. The second-order correction to the wave profile and the phase speed and the first-order correction to the wavelength are all determined. Four special cases are discussed in detail. In certain special circumstances the first-order theory may fail due to the vanishing of the nonlinear coefficient in the Korteweg–de Vries equation. When this occurs a different theory is required which leads to an equation with both quadratic and cubic nonlinearities.
Influence of gyroradius and dissipation on the Alfvén wave continuum
Authors: Connor J. W. , Tang W. M. , Taylor J. B.
Published: March 2003

It is well known that in ideal magnetohydrodynamics there is a continuous spectrum of real frequencies associated with a singularity of the shear Alfvén waves on the surface k∥vA=ω. It is also known that the introduction of first-order gyroradius effects eliminates the continuum. In the present work the influence of the full gyroradius response and of dissipation on the continuum is examined. In the absence of dissipation it is first confirmed that if only first-order gyroradius effects are incorporated, the continuum disappears. However, when the full gyroradius response is included, this discrete spectrum vanishes, and a new continuum (associated with singularities at k∥vA=0) appears. The introduction of collisional dissipation removes the original MHD continuum leaving discrete modes whose frequency tends to zero with the collision rate as ν1/3. Collisions also remove the new continuum of the full gyroradius model leaving discrete modes whose frequency tends to zero as (log ν)−1. Collisionless Landau damping has a similar effect.
Anomalous transport from drift modes driven by temperature gradients and trapped electrons
Authors: Mahajan S. M.
Published: March 2003

Drift modes driven by trapped electrons in the presence of temperature gradients are used to construct a theory for anomalous electron transport in tokamak plasmas. Several crucial features of the observed electron transport are predicted by the theory.
The rippling instability
Authors: Hassam A. B. , Drake J. F.
Published: March 2003

The rippling instability of current carrying magnetically confined plasmas is investigated. Complete parallel electron dynamics, including the effects of parallel thermal conduction, compressibility, and thermal forces, is incorporated into the theory. The rippling instability is found to be purely growing at very low temperatures, propagates at the drift frequency for slightly higher temperatures, and is stabilized by parallel thermal conduction as the temperature increases further. An analytic expression for the stability criterion is given. The analytic theory agrees well with a numerical calculation.
Effect of trapping on breaking of large-amplitude plasma oscillations
Authors: Kono M. , Mulser P.
Published: March 2003

The effect of trapping on the wave-breaking amplitude of Langmuir oscillations is discussed in a homogeneous system on the basis of a phenomenological description of trapped particles. The significant effect of trapping is to steepen the wave profile and to enhance the wave-breaking amplitude of the electric field.
Dispersion relation for a relativistic electron beam in a plasma
Authors: Shoucri Magdi
Published: March 2003

The dispersion relation for a relativistic electron beam in a plasma, in the absence of an external magnetic field, is derived. It is shown that the plasma oscillations excited by the beam are linearly coupled to the electromagnetic TM mode.
Heat transport with cold and hot electrons allowing for flux limitation by Shvarts’ method
Authors: Shkarofsky I. P.
Published: March 2003

The analysis of Shkarofsky on hot-electron transport is extended to include a variable degree of collisionality and gradient scale length. An extended version of the method of Shvarts et al. [Phys. Rev. Lett. 47, 247 (1981)] is used whereby the perturbed electron velocity distribution is limited in magnitude to the unperturbed part when the former becomes larger than the latter. A Maxwellian distribution is adopted for the hot electrons. For the cold electrons, either a Maxwellian, [exp(−v2/v2c)], or an exp(−v5/v5c) variation is used. The heat flow has coefficients proportional to gradients in cold- and hot-electron density, to gradients in cold- and hot-electron temperature, and the free-streaming contribution. These contributions are summed for two sample sets of scale lengths. One set of results shows that in an intermediate range of collisionality, heat inhibition occurs, in that the flux limiter is decreasing substantially below the free-streaming value, rather than increasing monotonically towards the collisionless regime.
Multishock compression of solid planar targets using tailored laser pulses
Authors: Jackel S. , Salzmann D. , Krumbein A. , Eliezer S.
Published: March 2003

Numerical simulations are performed on the interaction of double-step laser pulses and solid planar targets. These double-step laser pulses are routinely generated in the laboratory. The simulations show that compressions of 10 to 16 can be obtained in aluminum when the shock waves, generated by the first and second laser pulse steps, collide within the target. Optimization of the laser pulse shape results in the compression, to this magnitude, of approximately 10 μm of Al target for durations of about 0.5 nsec. Utilization of shaped laser pulses in conjunction with impedance mismatched targets that cause shock reflection or with opposing laser beams that cause oppositely directed shock collisions can result in compressions of 30 to 40.
Space-time evolution of the beam-plasma instability
Authors: Jones Michael E. , Lemons Don S. , Mostrom Michael A.
Published: March 2003

Particle-in-cell simulations of the beam-plasma instability confirm that the behavior of the interaction can be described as a wave packet that continually grows in both space and time. A consequence is that the energy deposition length of the instability becomes shorter in time, offering increased potential for this interaction to be used as an inertial fusion driver.
Analogies between transitional and turbulent boundary layers
Authors: Blackwelder Ron F.
Published: March 2003

One of the interesting aspects of transitional and turbulent boundary layers is the development of counter-rotating streamwise vortices near the wall. The most regular pattern is found in a boundary layer on a concave wall where the generation mechanism is known to be the Görtler instability. The origin of these vortices in other translational and turbulent boundary layers is presently unknown. Since the counter-rotating vortices are located in a region of strong shear, low-speed fluid is pumped away from the wall which coalesces into regions of low momentum lying between the vortices. As this pumping action continues, localized inflectional velocity profiles become apparent in the transitional and turbulent boundary layers. The oscillations which develop upon these profiles scale with the local thickness and velocity difference in the same manner as the two-dimensional steady free shear layer stability problems. The oscillations grow to large amplitude and break down into new turbulence in both the transitional and turbulent boundary layers.
Dislocations in convection and the onset of chaos
Authors: Whitehead John A.
Published: March 2003

High Prandtl number convection possesses a square flow pattern that is steady and is apparently stable to infinitesimal disturbances. This pattern is unstable to finite-amplitude disturbances, however, because a more chaotic (in time and space) spoke pattern of convection eats its way into the squares from the lateral boundaries. Experiments are described in which the breakup of the squares is initiated by dislocating one square in the middle of the apparatus with the use of a small, heated resistor. Once a critical heating rate and time is exceeded, the dislocation initiates a spoke cell which then systematically destroys neighboring square cells, resulting in the more chaotic spoke pattern. If the critical rate is not exceeded, the cell becomes severely deformed during the heating, but will relax back to a square convection cell after heating has ceased.
Preferred modes and the spreading rates of jets
Authors: Gutmark Ephraim , Ho Chih-Ming
Published: March 2003

For about a decade it has been noticed from the measurements of many jets that the value of the preferred mode and the spreading rate vary within a range of about 100%. In the present paper it is suggested that extremely low-level spatially coherent disturbances in individual facilities change the initial conditions of a laminar shear layer. The various initial conditions are able to cause different downstream developments of the jet and result in the discrepancies reported in the literature.
Controlled excitation of elliptic jets
Authors: Husain H. S. , Hussain A. K. M. F.
Published: March 2003

Studies of unexcited and excited elliptic jets reveal their characteristics to be noticeably different from circular jets, suggesting applications of excited elliptic jets for enhanced mixing and chemical reaction, and control of aerodynamic noise. The near-field turbulence characteristics, jet spread, and locations of switching of major and minor axes of the jet cross section can be drastically altered by forcing. The preferred mode and the stable pairing mode of an elliptic jet scale with the exit equivalent diameter.
An instability associated with a sudden expansion in a pipe flow
Authors: Sreenivasan K. R. , Strykowski P. J.
Published: March 2003

An instability characteristic of a fully developed laminar flow encountering a sudden expansion in a circular pipe is briefly described.
Finite-Larmor-radius magnetohydrodynamic equations for microturbulence
Authors: Hasegawa Akira , Wakatani M.
Published: March 2003

A set of nonlinear fluid equations which includes the effect of finite ion Larmor radius is derived to describe microturbulence [k⊥ ρs ≂O(1), k∥R≂O(1), n1/n0≂ ρs/Ln, and B1/B0≂ρs/R ] in an inhomogeneous plasma with a strong magnetic field of general geometry. Here ρs is the ion Larmor radius at the electron temperature, Ln is the density gradient scale length, R is the radius of curvature of the magnetic line of force, k is the wave vector, and n1/n0 and B1/B0 are relative levels of density and magnetic field perturbations.
Potentials in thermal barriers with strong electron-cyclotron heating
Authors: Cohen Ronald H.
Published: March 2003

Expressions for the potential difference Φpb between the plug and barrier regions of a tandem-mirror thermal barrier cell are derived in the limit of strong electron-cyclotron heating. It is found that Φpb varies as the 2/3 power of the ratio of the density of electrostatically trapped plug electrons to the density of thermal electrons at the barrier. Electric-field requirements for validity of the solutions are presented.
Drift-wave turbulence with wave–wave and wave–particle nonlinear effects in a sheared slab
Authors: Waltz R. E. , Dominguez R. R.
Published: March 2003

Low-β tokamaks have drift-wave density fluctuations (ñ/n) with the characteristics of strong turbulence. To describe their spectra, earlier work [Phys. Fluids 26, 169 (1983)] with simple wave–wave nonlinear coupling models in two dimensions is extended to more physical electrostatic models for drift waves in three dimensions. The electrons are treated with a linear nearly Boltzmann response. The cold ions have nonlinear cross-field drift motion and linear parallel motion. Numerical methods based on the direct interaction approximation are applied to both wave–wave and wave–particle nonlinear effects, and the spectral intensity Ikxkyk∥ω [ñ/n=(ρs/Ln)I1/2] for a sheared slab is calculated. The shear causes a linear coupling of adjacent kx and k∥ modes. A suggested extension to toroidal geometry is made. Many similarities with experiments are found: Ik⊥ is nearly isotropic in k⊥ and peaked at k⊥ρs <O(1/3). Ion Compton scattering damps at large k⊥ρs and parallel ion motion provides critical damping at low ky ρs. Here I1/2∼O(3) is weakly dependent on the linear driving rate γ and best summarized by the mixing length model: ñ/n∼1/kxLn with D∼γ/k2x. The relative frequency spectral widths Δωk/ωk̃ are finite and not inconsistent with those observed experimentally.
Self-consistent equilibrium and adiabatic evolution of a high-current electron ring in a modified betatron
Authors: Finn John M. , Manheimer Wallace M.
Published: March 2003

The equations for the self-consistent equilibrium of a cold electron beam in an axisymmetic modified betatron are derived, where it is shown that the equilibrium is uniquely defined by specifying the particle energy and toroidal field as functions of the canonical angular momentum. The generalization to a slow evolution is derived, using magnetic moment, canonical angular momentum, and toroidal flux as invariants. Results pertaining to equilibrium, allowable energy spread, self-flux diffusion, compensation for flux diffusion by discrete coils, and acceleration are presented.
Particle lifetime scaling in field-reversed configurations based on lower-hybrid-drift resistivity
Authors: Hoffman Alan L. , Milroy R. D.
Published: March 2003

The particle lifetime scaling for field-reversed configurations is derived in the limit where classical resistivity is negligible and anomalous resistivity is dominant. This scaling is shown to have the form τN∝S4(1+C/S)3, where S is the ratio of the major radius to ion Larmor radius. The term in parentheses represents a contribution from open field line confinement which is very significant for present small-S field-reversed configurations since C has a typical value of 50. The open field line confinement only loses importance for reactor-sized plasmas with major radii approaching 1 m.
On the nature of the amplitude modulation of jet shear layer instability waves
Authors: Monkewitz Peter A.
Published: March 2003

In natural jet shear layers as well as in plane mixing layers the Kelvin–Helmholtz instability waves are commonly found to be strongly amplitude modulated. In this study the nature of the modulation of the initial shear layer instability is investigated experimentally by correlating its envelope or amplitude function across the shear layer. It is shown that its origin is a receptivity of the shear layer to external disturbances which is modulated by low frequency and not a modulated growth rate. This finding is of importance for further progress in modeling feedback phenomena.
The stochastic, one-dimensional inertial equation, the inviscid Burger’s model, and the surface intersection problem
Authors: Meecham W. C. , Thebaud L. R.
Published: March 2003

A collisionless (rarefied) gas with an initial velocity in one dimension is discussed. It is shown that this inertial problem, the problem of the multiple intersections of a line of sight with a random surface, and the inviscid Burger’s model all have the same solutions. Kahng earlier used the full Wiener–Hermite expansion and guessed a solution to the stochastic problem. It is shown, using a constructive functional derivative procedure, that this solution is a linear combination of the occurring multiple solutions, with alternating signs. It is also shown that such an alternating sum of any moment is conserved, so that the initial, marginal distribution is invariant for this forceless problem. Kahng’s later-time energy spectrum for one example, which for large k was found to be k−3, is shown to be the correct result because of square root irregularities appearing at later times for solutions of the inertial equation. Further it is shown that any Gaussian process with arbitrary initial spectrum approaches k−3 for k large, with the initial dissipation length as its characteristics length. Finally a new technique is used to obtain the Wiener–Hermite expansion for physically reasonable fields in the collisionless gas problem.
Threshold for ion heating by a perpendicular electrostatic wave
Authors: Doveil F. , Mendonça J. T.
Published: March 2003

Large-scale stochasticity thresholds are analytically estimated for the ion motion in an electrostatic wave propagating perpendicularly to a uniform magnetic field. It is confirmed that the stochasticity condition does not crucially depend on how close the wave frequency is to a cyclotron harmonic.
Steepest-descent moment method for three-dimensional magnetohydrodynamic equilibria
Authors: Hirshman S. P. , Whitson J. C.
Published: March 2003

An energy principle is used to obtain the solution of the magnetohydrodynamic (MHD) equilibrium equation J×B−∇p=0 for nested magnetic flux surfaces that are expressed in the inverse coordinate representation x=x(ρ, θ, ζ). Here, θ are ζ are poloidal and toroidal flux coordinate angles, respectively, and p=p(ρ) labels a magnetic surface. Ordinary differential equations in ρ are obtained for the Fourier amplitudes (moments) in the doubly periodic spectral decomposition of x. A steepest-descent iteration is developed for efficiently solving these nonlinear, coupled moment equations. The existence of a positive-definite energy functional guarantees the monotonic convergence of this iteration toward an equilibrium solution (in the absence of magnetic island formation). A renormalization parameter λ is introduced to ensure the rapid convergence of the Fourier series for x, while simultaneously satisfying the MHD requirement that magnetic field lines are straight in flux coordinates. A descent iteration is also developed for determining the self-consistent value for λ.
Numerical simulation of axisymmetric spheromak merging
Authors: Sato Tetsuya , Oda Y. , Otsuka S. , Katayama K. , Katsurai M.
Published: March 2003

Axisymmetric merging of spheromaks is studied extensively by means of a magnetohydrodynamic code. Merging simulations of identical and different spheromaks created by the Princeton slow induction method have revealed that (1) the total magnetic energy decreases rather quickly through reconnection while the magnetic helicity is reasonably conserved; (2) a hollow structure appears in the radial q profile when two different spheromaks merge, while the q profile is doubled when the identical spheromaks merge; and (3) the postmerging toroidal flux becomes the sum of the premerging fluxes, while the postmerging poloidal flux remains the same as the larger of premerging fluxes. It is also observed that kinetic energy once converted from the magnetic energy through reconnection is returned back to the agnetic energy near the end of the merging process, this indicating a relaxation toward a lower-energy force-free equilibrium.
Vortex shedding from a spinning cylinder
Authors: Díaz F. , Gavaldà J. , Kawall J. G. , Keffer J. F. , Giralt F.
Published: March 2003

An experimental investigation of a two-dimensional turbulent wake behind a spinning cylinder at Re=9000 is carried out to determine the influence of the rotation on the initial development of the flow. Spectral analysis of the velocity data measured in the near wake shows that for peripheral velocities up to the value of the free-stream velocity, a distinct Kármán vortex activity exists within the wake, whereas for greater peripheral velocities, the Kármán activity deteriorates and disappears for values in excess of twice the free-stream velocity.
Shock-wave series for real fluids
Authors: Thompson Philip A.
Published: March 2003

Power series based on the Rankine–Hugoniot equation relate the states upstream and downstream of a shock. The usual forms for such series are limited to very weak shocks, e.g., to a shock Mach number approximately 1.1. Improvement is obtained by restricting the shock series to a form which is exact, for the special case of a perfect gas, with the lowest-order term only. Series in this form show reasonable accuracy for real fluids up to shock Mach numbers of order 1.5.
Gyrokinetic approach in particle simulation
Authors: Lee W. W.
Published: March 2003

A new scheme for particle simulation based on the gyrophase-averaged Vlasov equation has been developed. It is suitable for studying linear and nonlinear low-frequency microinstabilities and the associated anomalous transport in magnetically confined plasmas. The scheme retains the gyroradius effects but not the gyromotion; it is, therefore, far more efficient than conventional ones. Furthermore, the reduced Vlasov equation is also amenable to analytical studies.
Determination of electron and ion thermal conductivities by analysis of Alcator-A profile data
Authors: Liewer P. C. , Pfeiffer W. , Waltz R. E.
Published: March 2003

Profile data from the Alcator-A tokamak are analyzed using the onetwo transport code to study thermal conductivities and power balance. It is found that the observed central ion temperatures can be explained by assuming an ion thermal conductivity equal to the neoclassical value as calculated recently by Bolton and Ware. For low density, n̄=1×1014 cm−3, the possible anomaly in the ion conductivity is small: ±30% of the Bolton–Ware value. The anomalous electron thermal conductivity as deduced from the profile data is 〈Kexpe〉=〈nχexpe〉 ≂2.5(±1.5)×1017 cm−1 sec−1. The power balance study shows that at low densities, electron energy losses from radiation and thermal conduction are equally important, and both are larger than the exchanges loss to the ions. At higher densities, n̄≥2×1014 cm−3, electron energy losses by thermal conduction and exchange are equally important, and both are larger than the radiation loss. Convective losses, as calculated from measured particle confinement times, are negligible for all densities analyzed.
Frequency-shift of self-trapped light
Authors: Langdon A. Bruce , Lasinski Barbara F.
Published: March 2003

A decrease in the frequency of self-trapped light by as much as a factor of two has been observed in computer simulations of several physical situations. The frequency shift is due to a decrease in the resonant frequency of the plasma cavity as it deepens and expands shortly after the light becomes trapped. Three examples are presented: a model problem in an initially uniform plasma, trapping of laser light in cavities at critical density, and modulational instability and self-trapping of Raman-scattered light at quarter-critical density in a nonuniform, expanding plasma. This effect is essential in the analysis of the evolution and stability of such cavities, and such large frequency changes limit the applicability of models using a nonlinear Schrödinger equation to represent the high-frequency fields.
Quasilinear theory of inhomogeneous magnetized plasmas
Authors: Yasseen F. , Vaclavik J.
Published: March 2003

The quasilinear equations are derived for an inhomogeneous magnetized plasma without using the random phase approximation and the expansion in the ratio of the Larmor radius to the inhomogeneity scale. The equations take into account both resonant and nonresonant interactions, and possess the necessary conservation properties.
Vlasov–Maxwell and Vlasov–Poisson equations as models of a one-dimensional electron plasma
Authors: Klimas Alexander J. , Cooper Jeffery
Published: March 2003

The Vlasov–Maxwell and Vlasov–Poisson systems of equations for a one-dimensional electron plasma are defined and discussed. A method for transforming a solution of one system which is periodic over a bounded or unbounded spatial interval to a similar solution of the other is constructed.
Efficiency of free-electron lasers with a scattered electron beam
Authors: Kwan Thomas J. T. , Snell Charles M.
Published: March 2003

The quality of the relativistic electron beam in a free-electron laser is an important factor in determining its efficiency of radiation production. In this work, the problem of beam quality is addressed by considering an electron beam that has a scattered velocity distribution but is otherwise monoenergetic. A newly developed particle-in-cell plasma simulation code is used to study the nonlinear stage of the free-electron laser; in particular, the efficiency of energy extraction is determined as a function of the amount of beam scattering. It is found that the mean scattering angle of the electron beam of a free-electron laser in the Raman regime is a much more sensitive parameter than expected. The efficiency of radiation production decreases rapidly for electron beams with finite scattering angles.
Macroscopic plasma behavior in a high-voltage toroidal theta pinch
Authors: Murphy D. P. , Goldenbaum G. C. , Chin-Fatt C. , Chong Y. P. , DeSilva A. W. , Griem H. R. , Hess R. A. , Merlino R. L.
Published: March 2003

The behavior of the implosion and post-implosion phases of two different toroidal high-beta plasma configurations has been investigated. The plasma is produced in a nominally axisymmetric toroidal theta pinch in which a tokamak-like low-beta plasma is heated and compressed to high beta by a fast-rising toroidal field. The two plasma cases differed from each other by the direction of their initial toroidal magnetic field relative to the direction of the main implosion magnetic field. The purpose of this investigation was to determine what factors govern the development and lifetime of the magnetic configuration during the early time post-implosion phase. It was determined from the q profiles for the parallel field case that the plasma should be unstable to the m=1 kink mode, i.e., q<1, but the normal outward shift of the plasma with respect to the external conducting wall carried it into the internal vacuum chamber wall before the kink could grow to an observable level. The magnetic fields in the case with the initial toroidal field antiparallel to the main implosion magnetic field exhibited nonaxisymmetric behavior.
Ionospheric plasma cloud dynamics via regularized contour dynamics. I. Stability and nonlinear evolution of one-contour models
Authors: Overman E. A. , Zabusky N. J. , Ossakow S. L.
Published: March 2003

The linear stability and nonlinear evolution of a regularized contour dynamical model of an ionospheric plasma cloud (or deformable dielectric) is examined. That is, the cloud is modeled by piecewise-constant ion density regions; and the regularization is accomplished with a tangential diffusion operator that models aspects of the diffusion operator in two dimensions. A complete linear stability analysis of a circular cloud shows that a single-mode excitation ‘‘cascades downward’’ in wavenumber as it grows in amplitude, a process that results from the symmetry-breaking electric field. Approximate formulas are derived for the amplitude growth and cascade-down phenomena and verified with precise numerical calculations. A simple rescaling shows that clouds with large λ (=cloud-ion density/ambient-ion density) evolve more slowly and appear more dissipative. The regularized contour-dynamical algorithm for computations in the nonlinear regime is validated against the linear analysis and truncation errors are assessed by using different spatial resolutions. Calculations in the nonlinear regime show the experimentally observed ‘‘backside’’ striations. Furthermore, at long times, a secondary structure arises on the sides of the primary striations. A comparison of simulations with different λ shows that nonlinear effects arise sooner in normalized time (but longer in real time) if λ is larger.
Drift waves in a straight stellarator
Authors: Bhattacharjee A. , Sedlak J. E. , Similon P. L. , Rosenbluth M. N. , Ross D. W.
Published: March 2003

The eigenmode structure of drift waves in a straight stellarator using the ballooning mode formalism is investigated. The electrons are assumed to be adiabatic and the ions constitute a cold, magnetized fluid. The ‘‘effective potential’’ has an overall parabolic envelope but is modulated strongly by helical ripples along B. Two classes of solutions are found: those that are strongly localized in local helical wells, and those that are weakly localized and have broad spatial extent. The weakly localized modes decay spatially due to the existence of Mathieu resonances between the periods of the eigenfunction and the modulation of the ‘‘effective potential.’’
Finite-amplitude periodic extensional flows
Authors: Powell R. L.
Published: March 2003

By specializing the class of unsteady extensional flows considered by Slattery to motions which are periodic perturbations about a rest state and using the proper representation for Noll’s simple fluid undergoing such flows, explicit formulas for the induced stresses in terms of measurable material properties were achieved. It is found that oscillations of a sufficiently large amplitude will generate steady stresses in the fluid due to its nonlinear viscoelastic properties implying that orientation effects might be found in such flows.
Improved theory of collisionless particle motion in stellarators
Authors: Mynick Harry E.
Published: March 2003

A theory of particle motion in stellarators is developed which, in contrast to previous work, is both realistic enough to account for collisionless detrapping, yet simple enough that most features of the orbits can be expressed in analytic, reasonably simple formulas. From the study of detrapping, a systematic, complete classification of possible orbit types emerges. The theory is valid for a class of stellarator configurations which contains the standard model traditionally envisaged, as well as somewhat more complex configurations recently found to have favorable transport properties. The reasons for the differences in transport between configurations are elucidated.
Spectroscopic determination of the viscosity of a fully ionized plasma by means of the dynamics of acceleration by a traveling wave
Authors: Hellermann M. von , Piffko C. , Stampa A.
Published: March 2003

The mechanism of plasma acceleration by a traveling wave is derived from a cold plasma magnetohydrodynamic model with finite conductivity and finite Hall parameter. It is shown that collective effects may strongly influence the dynamics of the acceleration process. Eigenmodes of the plasma column considerably improve the acceleration whereas in general, plasma turbulence, induced by the primary wave, will reduce the efficiency of the process. Both mechanisms are demonstrated in an experiment where a high-amplitude traveling-wave pulse accelerates a nearly fully ionized plasma. In this experiment the flow profile is determined by the balance of electromagnetic and viscous forces. This enables one to determine the viscosity spectroscopically by measuring the flow velocity of the plasma. The experimental value of the viscosity is in accordance with the Spitzer formula.
Ion-acoustic waves in finite geometry
Authors: Jensen R. V. , Bernstein Ira B.
Published: March 2003

The effects of finite geometry on the frequency spectrum and mode structure of ion-acoustic oscillations in an equilibrium (Te=Ti) plasma are investigated. Numerical and analytical results for one-dimensional models of collisionless plasmas with both absorbing and reflecting boundaries show that these modes are strongly damped. With absorbing boundaries the fluctuations are damped in an ion-transit time. For reflecting boundaries the frequency spectrum is identical to the heavily Landau-damped spectrum for an unbounded plasma. These theoretical calculations suggest that the weakly damped, low-frequency density fluctuations recently observed in incandescent cavities are not ion-acoustic waves.
Steady-state radial heat conduction in a Z pinch
Authors: Scudder D. W.
Published: March 2003

One-dimensional steady-state radial profiles of a classical Z pinch are calculated. The effects of ion thermal conduction and bremsstrahlung radiation losses are included self-consistently, as are Ohmic heating, pressure balance, and an equation of state. It is shown that the upper current limit of approximately 1.4 MA for an equilibrium pinch found when only bremsstrahlung losses are allowed is virtually unaffected by the inclusion of ion thermal conduction. However, solutions with lower currents are now allowed. The profile-smoothing effect of ion thermal conduction is not sufficient to produce a pressure profile stable to m=0 modes. Total losses due to radial conduction are shown to be small for a thermonuclear plasma.
Suppression of magnetic islands by rf driven currents
Authors: Reiman Allan H.
Published: March 2003

The quasilinear theory for the saturation of nonlinear tearing modes is modified to include rf driven currents. It is shown that the presence of lower hybrid driven currents can strongly suppress the growth of magnetic islands.
Growth and saturation of the two-plasmon decay instability
Authors: Baldis H. A. , Walsh C. J.
Published: March 2003

An experimental study of the two-plasmon decay instability is reported. An intense CO2 laser (10.6 μm) was used to generate the instability in a long scale length, preformed plasma. The principal experimental diagnostic measurement was time-resolved Thomson scattering, with resolution in either ω or k space. The results show that the instability evolves basically in agreement with linear theories, then saturates by the nonlinear process of mode coupling to short-wavelength ion fluctuations. At saturation, density fluctuations close to 20% are observed in both the electron and ion waves.
Unsteady natural convection in a porous layer
Authors: Poulikakos Dimos , Bejan Adrian
Published: March 2003

The focus of this paper is on the fundamentals of the transient mechanism responsible for the establishment of natural convection in a two-dimensional porous layer confined between isothermal vertical walls and insulated horizontal walls. In the first part of the paper, pure scaling arguments are used to identify the characteristic time scales of the phenomenon and, based on these scales, the evolution of the porous system to steady state. The type of steady state is shown to depend on the relative order of magnitude of the characteristic time scales. Distinct vertical boundary layers are shown to be possible if (L/H)Ra1/2>1, and distinct horizontal layers if (H/L)1/2 Ra1/4>1. In the second part of the paper, these and other scaling laws of the transient evolution to steady state are successfully tested against a series of transient numerical experiments conducted in the range H/L=0.2–5 and Ra=10–50.
A study of the steady-state reaction-zone structure of a homogeneous and a heterogeneous explosive
Authors: Engelke Ray , Bdzil John B.
Published: March 2003

The two-dimensional steady-state reaction-zone structure of a homogeneous and a heterogeneous explosive is studied. To do this theoretical results obtained from the Euler equations of compressible flow are combined with experimental data on steady-state detonation shock-wave speed and shape as a function of the explosive charge size. The theoretical results, constrained by the experiments, define an inverse problem for the chemical heat-release function in the reaction zone which follows the shock wave; this problem is solved. The heterogeneous explosive is made from the homogeneous one by adding small quantities of other materials. Because of this, the two explosives were closely related in many respects. In spite of this, quite large differences in the detonation characteristics are observed between the two explosives, both in the wave speed as a function of charge size and in the shape of shock-wave loci near the explosive edge. It is found that a single forward rate exponentially dependent on the inverse of the local shock pressure can ‘‘explain’’ the homogeneous case observations, but that a rate with much less state dependence near the explosive edge is necessary to understand the observations for the heterogeneous material. Near the failure diameter of both materials, prominent structures are found on the experimental shock-wave loci. These structures would seem to be incompatible with a steady flow and call into question whether two-dimensional steady-state detonations occur at all in condensed explosives near their failure diameter.
Asymptotic near-wall stress dissipation rates in a turbulent flow
Authors: Launder B. E. , Reynolds W. C.
Published: March 2003

This letter provides an analysis for the correct behavior of the dissipation tensor in the Reynolds-stress transport equation in the limit as a wall is approached. This differs from what is used conventionally, and hence is important for turbulence models that include the near-wall region.
The cancellation of a sound-excited Tollmien–Schlichting wave with plate vibration
Authors: Gedney Charles J.
Published: March 2003

An experiment is conducted in which plate vibration is used to successfully cancel a sound-excited Tollmien–Schlichting wave. With sound excitation, transition to turbulence in the boundary layer occurs at a streamwise Reynolds number (Rx) of 1.2×106. The addition of plate vibration delays the transition to Rx=1.8×106. In the laminar region of the boundary layer (at Rx=0.97×106) the addition of plate vibration reduces the level of streamwise velocity fluctuations from more than 100 times to only two times the particle velocity of the sound excitation.
Enhanced diffusion in strongly magnetized plasmas
Authors: Frömling G.
Published: March 2003

Particle diffusion transverse to a strong uniform magnetic field is studied within a three-dimensional electrostatic guiding-center model in which particle streaming along the magnetic field lines is admitted as well as transverse E×B drifting. Only the randomizing influence of thermal fluctuations excited by particle discreteness is considered. The electric field autocorrelation coefficient is obtained in the form of a modified fluctuation-dissipation theorem. It is shown that because of the competition between the parallel and perpendicular decorrelation mechanisms, the divergence of the diffusion coefficient in the previous finite-volume theories disappears. The diffusion coefficient is Bohm like (proportional to 1/B) for magnetic fields which are not too strong, and goes over to a classical variation (proportional to 1/B2) in the limit B→∞.
Linear analysis of the coalescence instability
Authors: Bondeson Anders
Published: March 2003

The linear instability of pairwise-coalescing magnetic islands is studied analytically and numerically using incompressible ideal magnetohydrodynamics. An energy principle calculation shows that for small values of the island width w, the growth rate of the instability is proportional to w3/2. Numerical simulations are presented giving detailed information about the mode structure.
Analysis of laser light scattered by cold, dense hydrogen plasmas produced in a Z pinch
Authors: Maurmann S. , Kunze H.-J.
Published: March 2003

Scattering of ruby laser radiation on cold, dense hydrogen plasmas (1.5 eV≤Te≤4 eV, 3.5×1017 cm−3≤Ne≤2×1018 cm−3) shows, in addition to the Thomson component, a contribution of light which is scattered by excited hydrogen atoms on the wing of the Hα transition. This Rayleigh component is compared with a theoretical model based on the assumption that the n=2 level is in partial local thermodynamic equilibrium with the continuum.
Nonlinear development of the resistive filamentation instability
Authors: Hughes Thomas P. , Drobot Adam T. , Ott Edward
Published: March 2003

A two-dimensional particle simulation of the resistive filamentation instability on a charge- and current-neutralized particle beam is performed. The model equations used depend essentially on only one dimensionless parameter, making a numerical survey of the parametric dependence of the nonlinear behavior of the instability feasible. The simulation results are analyzed with reference to the focusing requirement of heavy-ion beams acting as drivers for pellet fusion, and it is speculated that, under certain conditions, the nonlinear development of the instability may still allow adequate focusing of the beam.
Nonlinear capillary waves under gravity with edge constraints in a channel
Authors: Shen M. C.
Published: March 2003

An equation of the Korteweg–deVries (KdV) type is derived for the study of nonlinear surface waves under gravity with surface tension and edge constraints in a channel of variable cross section. The approach used here is based upon a nonlinear ray method. An example of a symmetric rectangular channel with variable width is given.
Formation and damping of relativistic strong shocks
Authors: Anile A. M. , Miller J. C. , Motta S.
Published: March 2003

Results are presented from a detailed study of the formation of strong relativistic shocks from simple waves and their subsequent damping. Both analytical and numerical techniques have been used. In this paper planar flow is considered and self-gravity of the waves is neglected.
Electromagnetic drift modes driven by pressure gradients in tokamaks
Authors: Horton W. , Choi Duk-In , Hong Bong-Guen
Published: March 2003

A hybrid of hydrodynamics and kinetics is used to study the effect of finite plasma pressure on the pressure-gradient driven toroidal drift modes. The linear drift modes of the system are given by a fifth-order polynomial describing the coupling of the electron-drift, the ion-acoustic, and the shear Alfvén oscillations. The characteristic frequencies, growth rates, and polarization of the electromagnetic modes are investigated as functions of the parameters of toroidicity, plasma gradients, and plasma pressure.