bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of ion acoustic wave reflection from a cylindrical conductor
Authors: Schott L.
Published: March 2003

Using a novel interferometer method it is observed that sinusoidal ion acoustic waves are reflected (i.e., scattered backward) from the ion sheath surrounding a cylindrical wire immersed in a quiescent plasma. When the wavelength is decreased the ratio between reflected and incident amplitude increases slowly at first, reaches a maximum, and then decreases rapidly. Near this maximum the interferometer method is calibrated with the conventional sine burst method. When an extended plane conductor is present in the plasma in addition to the wire, reflection still occurs at the wire but no reflected signal from the plane conductor is detected.
Quenching of two-plasmon decay and stimulated-Raman-scattering instabilities by profile modification
Authors: Meyer J. , Bernard J. E. , Hilko B. , Houtman H. , McIntosh G. , Popil R.
Published: March 2003

The development of the electron density distribution in a CO2-laser irradiated underdense N2 gas jet is investigated interferometrically and is correlated to the observation of fast electrons generated by the two-plasmon decay and stimulated-Raman-scattering instabilities. During the 2 nsec laser pulse the quarter critical density scale length initially increases but soon ponderomotive forces steepen the profile resulting in a quenching of the fast electron emission.
Helical axis stellarators with noninterlocking planar coils
Authors: Reiman Allan H. , Boozer Allen H.
Published: March 2003

The properties of helical axis stellarator fields generated by unlinked, planar coils are described. It is shown that such fields can have a magnetic well and large rotational transform, implying large equilibrium and stability beta limits.
Soliton propagation on vortex cores and the Hasimoto soliton
Authors: Leibovich S. , Ma H. Y.
Published: March 2003

Weakly nonlinear, nonaxisymmetric wave propagation on vortex cores is considered under the assumption of inviscid, incompressible flow. The particular vortex ‘‘waveguide’’ upon which the waves are assumed to propagate has a vorticity distribution typical of experimentally realized concentrated vortices. The waves are modulated, and the modulation envelope is governed by the nonlinear Schrödinger equation. Solitons are found to be possible in a restricted band of wavenumbers having both a high and low wavenumber cutoff. It is found that the vortex deformations resulting from this analysis can be matched with Hasimoto’s soliton, which is based upon a local induction approximation and therefore ignores the vorticity dynamics in the core. This allows one to determine the ratio of Biot–Savart cutoffs which appears as an undetermined constant in Hasimoto’s theory: in effect, this calibrates the Hasimoto soliton. In addition, the present analysis shows that the group velocity of solitons with deflections small compared to the core radius is at most 0.292 Γ/2πa, where Γ is the circulation of the vortex, and a is the core radius; this may explain recent experimental observations of Hopfinger, Browand, and Gagne [J. Fluid Mech. 125, 505 (1982)].
Direct observation of shock splitting in a vapor–liquid system
Authors: Thompson Philip A. , Kim Yoon-Gon
Published: March 2003

The splitting of a single liquefaction shock wave into a vapor-phase forerunner shock and a following condensation shock is observed photographically in shock-tube experiments. The behavior of the two-shock system is qualitatively described by equilibrium shock conditions, but the state downstream of the forerunner shock is found to be far from equilibrium. It is proposed that this state is limited by the critical supersaturation for homogeneous nucleation. The overall pressure jump for the two-shock system is much greater than would be predicted by an equilibrium calculation.
Definition of mean free path for real gases
Authors: Bird G. A.
Published: March 2003

Attention is drawn to the inconsistency in the conventional procedure for the definition of a mean free path in a real gas through the classical hard sphere result. It is shown that the variable cross-section hard sphere, or VHS, model can be used to define a mean free path that properly accounts for the temperature exponent of the coefficient of viscosity of the gas. In addition, the VHS model is shown to have advantages over the classical inverse power law models for numerical and analytical studies.
Flow and diffusion of a rarefied gas mixture in an annular tube in the transition region
Authors: Kanki Tatsuo , Iuchi Satoru
Published: March 2003

A new explicit equation, which covers the entire range of pressures, for diffusion accompanied by the flow of a rarefied gas binary mixture in an annular tube is obtained. The analysis is based on the local equilibrium moment method. The equation is, for illustration of validity of the theory, applied to the flow of a single gas and shown to agree well with the experimental data and to be closely related to accurate numerical solution of the BGK equation.
Modification of Korteweg–de Vries solitons in plasmas by resonant particles
Authors: Lynov J. P.
Published: March 2003

A perturbed Korteweg–de Vries equation describing the influence of resonant electrons on a Trivelpiece–Gould soliton is solved numerically, and the results are compared with predictions from the perturbation analysis of the equation by Karpman and Maslov. The range of parameters for which the perturbation approach is valid is determined, and it is shown that for parameters that are within the perturbation validity limits the modifying effects on the soliton are negligible. However, it is demonstrated that the theoretical results from the perturbation analysis are good approximations even in cases of strong soliton modification. Examples are given both of soliton damping and soliton amplification, and it is shown that the numerical solutions also can be interpreted as describing the modification of an ion-acoustic soliton by resonant particles.
Quasilinear saturation effects on electron cyclotron wave damping of the ordinary mode in tokamak plasmas
Authors: Fidone I. , Meyer R. L. , Granata G.
Published: March 2003

The quasilinear theory of electron cyclotron wave absorption for the ordinary mode propagating normal to the tokamak magnetic field is investigated. A two-dimensional initial value code has been developed to investigate the distortion of the electron momentum distribution from an initial Maxwellian. It is shown that for powers of interest for electron heating in future tokamaks, Coulomb electron collisions are not sufficient to avoid quasilinear flattening of the electron distribution; therefore, there is a reduction in the damping rate as compared to the linear case.
Equilibrium statistical mechanics treatment of a ‘‘modified’’ two-dimensional guiding center plasma
Authors: Calinon R. , Merlini D.
Published: March 2003

Equilibrium properties of a two-dimensional guiding center plasma are reinvestigated in detail by means of the BBGKY hierarchy. A new equation of state is presented and compared with the results of the Monte Carlo simulation and the correlation energy is computed. A heuristic generalization of the the dispersion relation is investigated by means of different truncation schemes of the hierarchy. Analytical and numerical results support the appearance of periodic states for a neutral system in an unbounded domain at a moderately high value of the coupling parameter γc∼8.
An electromagnetic integral equation: Application to microtearing modes
Authors: Farengo R. , Lee Y. C. , Guzdar P. N.
Published: March 2003

The integral equation technique previously developed for electrostatic drift waves to study low-frequency electromagnetic perturbation is extended. When σe≫σi (as is the case for tearing modes) the problem can be reduced to the simultaneous solution of an integral and a differential equation. Using a Fourier representation for φ̃(x), a differential equation is derived from Ampere’s law for a modified Green’s function that contains the magnetic effects. This equation is solved simultaneously with an integral equation (corresponding to the quasineutrality condition in k space) to obtain the eigenvalues and corresponding eigenfunctions. When applied to the study of microtearing modes this method gave, for the same values of the parameters, larger growth rates than those of the usual differential approximation.
Energy conservation and related constraints in drift wave turbulence
Authors: Thayer David R. , Molvig K.
Published: March 2003

The problem of energy conservation for the renormalization of the drift wave instability in a sheared magnetic field is considered. It has been suggested previously that there is a connection between a certain constraint on the nonlinear term in the drift kinetic equation and energy conservation. Arguments are presented to dissolve this connection; and in turn, energy conservation is formulated in the physically meaningful statistically averaged sense. Finally, energy conservation is proven for the system of nonlinear equations, renormalized by the normal stochastic approximation, describing the drift wave instability.
Equilibrium and stability properties of high-beta torsatrons
Authors: Carreras B. A. , Hicks H. R. , Holmes J. A. , Lynch V. E. , Garcia L. , Harris J. H. , Hender T. C. , Masden B. F.
Published: March 2003

Equilibrium and stability properties of high-beta torsatrons are investigated using numerical and semianalytical techniques based on the method of toroidal averaging. The averaged equilibria are compared with those obtained using full three-dimensional codes. Good agreement is obtained, thus validating the averaged method approach. The stability of plasmas for configurations with different aspect ratios and numbers of field periods is studied. The role of the vertical field is also studied in detail. The main conclusion is that for moderate-aspect-ratio torsatrons (Ap≲8), the self-stabilizing effect of the magnetic axis shift is large enough to open a direct path to the second stability region.
Beam–ion and alpha-particle effects on microinstabilities in tokamaks
Authors: Rewoldt G. , Tang W. M.
Published: March 2003

The analysis of electromagnetic kinetic toroidal eigenmodes for general magnetohydrodynamic (MHD) equilibria has been extended to include the effects of non-Maxwellian equilibrium distribution functions. This is necessary to properly represent the response of the hot-beam–ion species produced during neutral-beam injection heating and the response of the alpha-particle species in a thermonuclear plasma. The influence of these components on low-frequency microinstabilities is investigated for realistic cases typical of the PLT and PDX tokamak experiments.
Theory of axially symmetric probes in a collisionless magnetoplasma: Aligned spheroids, finite cylinders, and disks
Authors: Rubinstein J. , Laframboise J. G.
Published: March 2003

A theory is presented for current collection by electrostatic probes in a collisionless, Maxwellian plasma containing a uniform magnetic field B, where the probes are spheroids or finite cylinders whose axis of symmetry is aligned with B, or disks perpendicular to B. The theory yields upper-bound and adiabatic-limit currents for the attracted particle species. For the repelled species, it yields upper and lower bounds. This work is an extension of existing theory for spherical probes by Rubinstein and Laframboise.
Effects of the relativistic correction to the resonance condition on electron cyclotron current drive
Authors: Cairns R. A. , Owen J. , Lashmore-Davies C. N.
Published: March 2003

The effect on electron cyclotron current drive of the relativistic correction arising from the velocity dependence of the electron cyclotron frequency has been investigated. This correction is shown to be important, particularly for waves propagating close to perpendicular to the magnetic field, for plasmas in the keV range and above. The steady-state current due to the asymmetric heating mechanism described by Fisch and Boozer has been calculated. The most striking effect of the relativistic correction is that the current driven in the plasma in response to a given wave amplitude is, in an inhomogeneous field, no longer antisymmetric about the cyclotron resonance layer. This means that it is possible to drive a nonzero total current without strong absorption, and also that the total current depends strongly on whether the wave is incident from the low-field or the high-field side.
Quasilinear theory of current-driven ion-acoustic turbulence in a magnetized collisional plasma
Authors: Sakharov A. S. , Kuhn S.
Published: March 2003

The stationary spectra of current-driven ion-acoustic turbulence in a magnetized plasma (with the electron drift velocity vd along B0) are studied within the framework of quasilinear theory. In particular, a plasma with strongly magnetized electrons (ωce>kdvTe) and unmagnetized ions (ω>ωci) well above the threshold of instability (vd/vTe≫vs/vTe, νin/ωpi) is considered. It is found that the waves are mainly excited in a narrow angular region almost perpendicular to vd (i.e., cos θ≂vs/vd≪1), which is in agreement with experimental results by Gekelman and Stenzel [Phys. Fluids 21, 2014 (1978)] and Batanov et al.
Ponderomotive effects in nonneutral plasmas
Authors: Lamb B. M. , Morales G. J.
Published: March 2003

The ponderomotive effects which arise in a single species plasma, i.e., a nonneutral plasma, are considered. The important difference from a neutral plasma is that quasineutral density cavities given by δn/n0≂−‖E‖2/16πn0T cannot arise in a single species plasma. Instead, it is found that the ponderomotive force is balanced by self-consistent space charge fields, and results in δn≂∇2‖E∥‖2/16πmω2, where ω is the frequency and E∥ refers to the wave electric field parallel to the confining magnetic field. In addition to the density rearrangement, the zero order E×B rotation is modified. The self-consistent nonlinear state has been found for a large-amplitude l=0 axially standing Gould–Trivelpiece wave. The linear dispersion relation is modified by the presence of the large-amplitude wave and results in a nonlinear frequency shift. This shift produces an interesting hysteresis effect in the nonlinear resonant response of the plasma when the frequency of the external driver is swept slowly.
Detailed spectra of high-power broadband microwave radiation from interactions of relativistic electron beams with weakly magnetized plasmas
Authors: Kato Keith G. , Benford Gregory , Tzach David
Published: March 2003

Prodigious quantities of microwave energy distributed uniformly across a wide frequency band are observed when a relativistic electron beam (REB) penetrates a plasma. Typical measured values are 20 MW total for Δν≂40 GHz with preliminary observations of bandwidths as large as 100 GHz. An intense annular pulsed REB (I≂128 kA; r≂3 cm; Δr≂1 cm; 50 nsec FWHM; γ≂3) is sent through an unmagnetized or weakly magnetized plasma column (nplasma∼1013 cm−3). Beam-to-plasma densities of 0.01≤nbeam/nplasma≤2 are used, the higher values of this range being an unconsidered region for most previous theoretical and experimental efforts. For these higher nb/np values, the observed emission with ω≫ωp and weak harmonic structure is wholly unanticipated from Langmuir scattering or soliton collapse models. A model of Compton-like boosting of ambient plasma waves by the beam electrons, with collateral emission of high-frequency photons, qualitatively explains these spectra. Power emerges largely in an angle ∼1/γ, as required by Compton mechanisms. As nb/np falls, ωp−2ωp structure and harmonic power ratios consistent with soliton collapse theories appear. With further reduction of nb/np only the ωp line persists. Thus a transition occurs in spectral behavior from the weak to strong turbulence theories advocated for type-III solar burst radiation, and further into a regime we characterize as superstrong REB–plasma interactions, is observed. For frequencies slightly below the broadband region, an ωp line is observed with high power (approximately 1 MW); the line disappears in an external Bz∼400 G. Changing γb over a range of 2.2–3.7 has little effect on the spectra.
Density and temperature profiles within laser-produced plasmas in the classical-transport regime
Authors: Burkhalter P. G. , Herbst M. J. , Duston D. , Gardner J. , Emery M. , Whitlock R. R. , Grun J. , Apruzese J. P. , Davis J.
Published: March 2003

Using planar polystyrene targets with embedded Al tracer dots, improved x-ray spectroscopic measurements of density and temperature profiles in laser-produced plasmas were obtained. Time-integrated, spatially resolved spectra of the Al tracer emission were collected at laser intensities where the energy absorption and transport is expected to be classical. The plasma density and temperature profiles were determined by comparing the observed x-ray line intensities with collisional-radiative equilibrium calculations. Plasma profiles were obtained for the region blowoff plasma with densities between one-tenth and twice critical for the 1.05 μ laser excitation. Near agreement is found between the measured density and temperature profiles and 2-D (cylindrical) hydrodynamic calculations with energy absorption via classical inverse bremsstrahlung and energy transport via classical thermal conduction.
Confinement of a high-beta plasma column
Authors: Brunel F. , Tajima T.
Published: March 2003

Hydromagnetic aspects of confinement of a high-beta linear plasma column, including the reverse field configurations, are investigated. The previous theoretically predicted diverging confinement time of a sharp-boundary plasma column as β→1 is removed by properly taking into account the magnetic tension effect near the column throats. The obtained end loss rate is tested through simulation: the present simulation results in slab geometry are in good agreement with theory and are also close to other simulation and experimental values. The confinement time τ of a plasma with thickness a and length L at  β=1 is found to be τ= 3/4 (3/2π)1/2(L/2cs)(L/a)1/2. When the magnetic field inside the column is reversed, the plasma end loss much improves: the end loss induces the reconnection of the field lines near the throats, which produces closed field lines (islands). Those islands are found to be unstable against the tilting instability. The tilting-induced reconnection of the island field line and the mirror field line is a very fast process even for negligibly resistive plasmas and this helps to rapidly spill out the plasma confined in the islands. The significance of these processes for the spheromaks and reversed field (theta) pinches is discussed.
Collisional diffusion of a plasma in multipolar and picket fence devices
Authors: Koch C. , Matthieussent G.
Published: March 2003

A collisional diffusion model of a low-β plasma in a two-dimensional magnetic field is worked out under the assumption of joint diffusion of electrons and ions in the plane of the magnetic field. This model is applied to multidipole and picket fence discharges; density and potential profile are computed. The main features observed experimentally are in qualitative agreement with this computation: a leak width proportional to the hybrid gyroradius and the existence of potential hills between the cusps and poor plasma confinement.
Time-resolved observations of stimulated Raman scattering from laser-produced plasmas
Authors: Turner R. E. , Phillion D. W. , Campbell E. M. , Estabrook Kent
Published: March 2003

Temporally and spectrally resolved measurements of stimulated Raman scattering from high-intensity, 0.532 μm laser–plasma experiments on disk targets are reported. The observed scattering is principally from densities below quarter-critical, and is observed to occur nearly simultaneously over a wide range of wavelengths (0.72 μm<λ<0.9 μm), in short (<50 psec) bursts. Several possible explanations of these data are briefly discussed.
Ablative acceleration of planar targets to high velocities
Authors: Grun J. , Obenschain S. P. , Ripin B. H. , Whitlock R. R. , McLean E. A. , Gardner J. , Herbst M. J. , Stamper J. A.
Published: March 2003

Laser irradiated targets are ablatively accelerated to velocities near those required for fusion pellet implosions while remaining relatively cool and uniform. The target velocities and velocity profiles are measured using a double-foil method, which is described in detail. Also, the ablation plasma flow from the target surface is spatially resolved, and the scalings with absorbed irradiance of the ablation pressure, ablation velocity, and mass ablation rate are determined. Results are compared with hydrodynamic code calculations.
Nonlinear hydrodynamic pressure on an accelerating plate
Authors: Chwang Allen T.
Published: March 2003

A nonlinear theory is developed to determine accurately the hydrodynamic pressure on an accelerating vertical plate. The plate is assumed to move horizontally towards the fluid with an acceleration which may be expressed in a power series in time. The fluid is initially at rest and the plate motion is impulsive. When the horizontal plate displacement is small in comparison with the undisturbed fluid depth, the method of small-time expansions is adopted to obtain analytical solutions up to and including the third-order velocity potential. The free surface profile of the fluid is also determined. An example for the constant acceleration case is presented to illustrate the method.
Rayleigh–Taylor instability with spatially varying acceleration: An illustration
Authors: Baker L.
Published: March 2003

An illustrative but realistic case of Taylor instability in an atmosphere in which the density increases exponentially with height and the gravitational acceleration varies with height, g(z)=g1+g2 exp(±kgz), is considered. This model problem is of interest both in astrophysical contexts (e.g., an extended accretion disk) and magnetically accelerated configurations in which the field may diffuse into the material being accelerated. Both bounded and unbounded atmospheres are considered. For the case g=g0 exp(−kgz), the growth rate is reduced compared to the case with constant g=g1=g0 exp(−kgzm), where zm is the height of the maximum of the velocity eigenfunction, by more than would be expected.
Onset of convective instabilities in binary liquid mixtures with fast chemical reactions
Authors: Gitterman M. , Steinberg V.
Published: March 2003

Analytic expressions are found for the criteria for the onset of stationary and oscillatory convection in binary mixtures with chemical reaction. The reaction is assumed to be fast compared with the diffusion rate. It is found that both stationary and oscillatory instabilities can occur as the first bifurcation, depending on the sign and the value of the heat of reaction. An endothermic polymerization, which produces the heavy component (polymer) near the warm boundary, is an example of a reaction that can cause either stationary or oscillatory convection when the system is heated from above or below, respectively. Numerical estimates are given for both stationary and oscillatory instabilities.
Consistent modeling of scalars in turbulent flows
Authors: Pope S. B.
Published: March 2003

Conserved, passive scalars with equal diffusivities evolve independently according to the same linear transport equation. This observation leads to independence and linearity principles that impose constraints on the construction of consistent turbulence closure approximations. It is shown that the functional form of the scalar dissipation equation is more restrictive than has generally been assumed, and that the consistently modeled equation leads to qualitatively incorrect results for the decay of scalar fluctuations. It is also shown that the scalar-pressure-gradient correlation can be obtained from the velocity-pressure-gradient correlation, and thus an independent model for the scalar correlation is not required. (This last result depends upon an additional assumption that is certainly valid for nearly homogeneous flows.)
Evolution of the thermal field in a turbulent wake downstream from an asymmetrically heated plate
Authors: Morel R. , Rey C. , Wallace J. M.
Published: March 2003

Turbulent thermal and kinematic properties of the wake of a plate, heated on one side only, have been investigated. The distributions across the wake of the mean temperature, the standard deviation of the temperature fluctuations, and the streamwise and transverse thermal fluxes are presented; similarity coordinates for all these properties are found using the locus of points where ∼(vθ)=∼(∂θ2/∂y) =0 as the thermal axis. The turbulent–nonturbulent and heated–unheated interfaces have been studied using conditional sampling. The results concerning the thermal intermittency factor are consistent with these similarity coordinates.
Two models of truncated Navier–Stokes equations on a two-dimensional torus
Authors: Franceschini Valter
Published: March 2003

Two truncations of the Navier–Stokes equations for an incompressible fluid on a two-dimensional torus are numerically investigated. The two models, an eight-mode truncation and a nine-mode extension of it, exhibit almost completely different behaviors. Only two aspects appear to be common to both. First, in contrast to all the previously studied models, turbulence takes place at low Reynolds numbers and for critical values close to each other. Second, there is a parameter interval for which the same stable periodic orbit is present in both models. The main feature of the two truncations is a very rich phenomenology, with many different bifurcations of closed orbits. Turbulence is reached through a sequence of period-doubling bifurcations in the eight-mode model, and via a tangent bifurcation in the nine-mode one. Sequences of infinite bifurcations, the presence in a very narrow range of a stable closed orbit with a quite long period, a homoclinic bifurcation, and the coexistence of strange attractors with attracting periodic orbits, appear to be the more interesting phenomena.
Rayleigh–Taylor instabilities of an accelerating, compressible, perfectly conducting plane layer
Authors: Parks Donald E.
Published: March 2003

The stability of a plane stratified slab of perfectly conducting, compressible, inviscid plasma accelerated by a magnetic field is considered. Exact closed form solutions for the growth rates of the unstable modes are determined for a γ-law gas when the undisturbed equilibrium is an isothermal atmosphere of semi-infinite extent with no frozen-in field. It is shown that the fluid slab is always unstable at wavelengths long compared with the atmospheric scale height and that the corresponding growth rates agree with those calculated in the limit of incompressible flow.
Ion thermal conductivity for a pure tokamak plasma
Authors: Bolton C. , Ware A. A.
Published: March 2003

The neoclassical and Pfirsch–Schlüter components of the ion heat conduction for a pure hydrogen tokamak plasma are recalculated without assuming large aspect ratio and without neglecting energy scattering collisions. Using a model collision operator, the conductivity is determined numerically for various collision frequencies and aspect ratios. An approximate algebraic expression is fitted to the results. Even for comparatively large aspect ratios (∼10) energy scattering increases the conductivity in the banana regime by about 50% and for small aspect ratios (approximately 3.3) the Pfirsch–Schlüter contribution causes a further increase of approximately 60%.
The growth of swirl in curved circular pipes
Authors: Olson D. E. , Snyder B.
Published: March 2003

Steady secondary currents in the entry region of curved circular pipes with different curvature ratios (R/a=4.66 and 16) have been delineated at moderate Dean numbers by pulsed-probe anemometry. When swirl is quantified using circulation loops, flow development can be scaled by the length parameter (aR)1/2. The initial growth of axial swirl seems consistent with a model of vorticity transport based on average streamline curvature, but does not progress monotonically to an asymptotic value. Instead, an abrupt relaxation occurs approximately 2(aR)1/2 downstream from the inlet, suggesting that boundary-layer current intensifies well before the axial velocity profile can be reordered by convection.
Rigid ballooning modes in tandem mirrors
Authors: Kaiser Thomas B. , Nevins William McCay , Pearlstein L. Donald
Published: March 2003

Stability of tandem mirrors to rigid-displacement ballooning modes is studied using vacuum field geometry. Results show substantial improvement in values of beta over those predicted by ‘‘infinite-m’’ theory. Previous work has shown that kinetic effects stabilize these latter short-wavelength modes, suggesting that only the rigid mode survives.
Observation of suprathermal electrons produced by stimulated Raman scattering processes
Authors: Berger R. G. , Brooks R. D. , Pietrzyk Z. A.
Published: March 2003

Fast electrons (with E≫Te ) have been observed as pulses correlated in time with stimulated Raman backscatter pulses during CO2 laser interaction with cold underdense plasmas. Time-integrated electron spectra have a Maxwellian energy distribution, with discharge to discharge temperatures of 29–105 keV. Because the density is less than quarter critical and suprathermal electrons are seen only during discharges when stimulated Raman scattering occurs, it is concluded that Raman processes create the hot electrons.
Magnetohydrodynamic theory of plasma equilibrium and stability in stellarators: Survey of results
Authors: Shafranov V. D.
Published: March 2003

The main advantage of a stellarator is its capability of steady-state operation. It can be exploited as a reactor if stable plasma confinement can be achieved with β∼10%. Therefore, this limiting pressure value is a key factor in stellarator development. This paper contains a survey of current ideas on the magnetohydrodynamic equilibrium and stability properties of stellarators with sufficiently high pressure. Here, any system of nested toroidal magnetic surfaces generated by external currents is considered a stellarator. Systems produced by helical or equivalent windings, including torsatrons and heliotrons, will be called ordinary stellarators, in contrast to those with spatial axes. It is shown that adequate confinement can be achieved.
Vorticity generation by asymmetric energy deposition in a gaseous medium
Authors: Picone J. M. , Boris J. P.
Published: March 2003

We develop a detailed theory of vorticity generation by asymmetries which are present when a cylindrical pulse of energy is deposited in a gaseous medium. Three classes of asymmetry which may occur separately or in combination, depending on the experiment being analyzed, are identified. A convenient representation of the flow field in terms of one or more vortex filament pairs having identical strengths is also introduced. The equations which give the vortex filament strength for the various asymmetry classes differ only in the value of a form factor, which contains information regarding geometric effects and hydrodynamic interactions. The form factor, however, depends only weakly on the profile of the density distribution of the medium prior to energy deposition. Finally the analytic results are verified and calibrated by providing data from accurate two-dimensional simulations of several cases. The analytic and numerical calculations agree closely for all examples considered.
Hamiltonian structure for drift waves and geostrophic flow
Authors: Weinstein Alan
Published: March 2003

The equation (1−∇2)φt+J[φ, (1−∇2)φ−λ]=0 in the plane is put in Hamiltonian form with respect to a noncanonical Poisson bracket. If the function λ is a linear combination of x,y, and x2+y2, the equation can be transformed to a form very close to the vorticity equation for an ideal fluid. In particular, β-plane dynamics are transformed into f-plane dynamics.
Observation of streamwise rotation in the near-wall region of a turbulent boundary layer
Authors: Smith C. R. , Schwartz S. P.
Published: March 2003

Simultaneous top-end view visualization studies of the flow behavior in the near-wall region of a turbulent boundary layer have been done using hydrogen bubble wires and a dual-camera, high-speed video system. The studies show clearly that the low-speed streaks formed in the near-wall region are discrete regions of outward motion, with the degree of activity varying with distance from the wall. The visualizations also reveal the presence of significant streamwise rotation in the wall region for y+≳10, including frequent observation of counter-rotating streamwise structures. An apparent interdependency between these counter-rotating structures and low-speed streak formation is observed, and appears to support the suggestions of several previous investigators.
Waves in a cold pure electron plasma of finite length
Authors: Prasad S. A. , O’Neil T. M.
Published: March 2003

A theory for low-frequency electrostatic modes of a finite length pure electron plasma column is presented. The plasma is modeled as a cold uniform density cylinder with flat ends. An interesting result is that the diocotron mode can have an axial wavelength that is much larger than the plasma length. Also, for particular values of the plasma density, the axial magnetic field, and the dimensions of the plasma, the diocotron mode is degenerate with a plasma mode and this results in a strong mixing of the modes.
Ion-temperature-gradient instability in toroidal plasmas
Authors: Guzdar P. N. , Chen Liu , Tang W. M. , Rutherford P. H.
Published: March 2003

The stability of the ion-temperature gradient mode in a toroidal plasma is investigated. Using the ballooning-mode formalism, an ordinary difference-differential equation is derived which includes full kinetic effects. The equation is then examined in various limits where it reduces to an ordinary differential equation. Analytic and computational studies show that, for ηi ≡d ln Ti /d ln n≫1, toroidal effects further destabilize the mode. However, it is also found that toroidal effects give rise to higher ηi threshold compared to the slab case. Extensive numerical calculations over a wide range of parameters are carried out to delineate the regions of instability. It is concluded that the ηi instability is of relevance to present-day beam-heated tokamaks.
Integral representation for geometric optics solutions
Authors: Hazak G. , Bernstein Ira B. , Smith Timothy M.
Published: March 2003

An integral representation of the geometric optics solutions for the field of dressed particles in inhomogeneous plasma is derived. The representation is a natural generalization of the Fourier integral used for homogeneous systems. The set of plane waves is replaced by a complete orthogonal set of ‘‘quasiplane waves’’ which in practice may be constructed by using the existing ray tracing codes.
Saturation of the lower-hybrid-drift instability by mode coupling
Authors: Drake J. F. , Guzdar P. N. , Huba J. D.
Published: March 2003

A nonlinear mode-coupling theory of the lower-hybrid-drift instability is presented. It is found that the instability saturates by transferring energy from the growing, long wavelength modes to the damped, short wavelength modes. The saturation energy, mean square of the potential fluctuations, and diffusion coefficient are calculated self-consistently.
The forces due to the relative motion of two corrugated plates
Authors: Disimile Peter , Wang C. Y.
Published: March 2003

Two wavy plates, separated by a viscous fluid, move relative to each other in a direction normal to the corrugations. Assuming small-amplitude corrugations and low Reynolds number, the unsteady Navier–Stokes equations are solved by perturbations. It is found not only that the mean drag is increased, but that an unsteady drag and an unsteady lift are also induced by the corrugations. These fluid-induced vibrations, predicted for the first time, may be important in the design of grooved sliders.
Evidence of Görtler-type vortices in curved film flows
Authors: Schweizer Peter M. , Scriven L. E.
Published: March 2003

A flow visualization study of the onset and behavior of secondary motions in laminar flow of a liquid film descending over a concave surface is presented. The observed secondary flow resembles the Görtler vortices found in flow over a fully submerged concave surface. The dependence of the wavelength of the instability on flow parameters and the possibility that the instability mechanism is active in coating flows are discussed.
Effect of a transition between regular and stochastic electron motion on the evolution of an obliquely propagating Langmuir wave
Authors: Menyuk C. R.
Published: March 2003

The evolution of an obliquely propagating Langmuir wave is considered, with emphasis on determining the effect of a transition between regular and stochastic electron motion. The wave has three distinct evolution regimes, whose locations in parameter space depend roughly on the relative sizes of the linear damping rate γ, the electron bounce frequency ωb and the electron cyclotron frequency Ω. When ωb<γ, the wave is in the linear regime and simply damps away. When γ<ωb<Ω, the wave evolution is dominated by regular, trapped electron motion in individual resonances. It is shown that when the propagation angle increases to about 14°, the trapping oscillations present in parallel propagating waves disappear due to phase mixing of the different bounce frequencies. When γ<Ω<ωb, the wave evolution is dominated by stochastic particle motion from resonance to resonance. It is shown that when a transition is made from the trapping to the stochastic regime, the final (t=∞) wave amplitude significantly decreases, due to the increased ability of the electrons to take energy from the wave. Finally, simulation results which verify key aspects of the theory are reported.
Ponderomotive force in a warm two-fluid plasma
Authors: Lee Nam C. , Parks G. K.
Published: March 2003

A general expression for the ponderomotive force of electromagnetic field with slowly varying amplitude in a warm plasma is obtained in the collisionless two-fluid model. Compared to the existing expression obtained for a cold and stationary plasma, this result contains two additional terms for the general case. One of them is involved with the density gradient and the wavenumber derivative of the dielectric tensor of a plasma. Thus, for an inhomogeneous plasma, it accounts for the effects of spatial dispersion, hence of finite temperature of the plasma. The other term is proportional to the time derivative of the density and the frequency derivative of the dielectric tensor, and is nonzero unless the plasma is stationary or nondispersive. Inclusion of these new terms in the ponderomotive force will enable self-consistent analysis of the general behavior of nonlinear waves with finite wavelengths in a warm plasma.
Alfvén wave heating in general toroidal plasmas
Authors: Bernstein Ira B.
Published: March 2003

A variational formulation is presented within the framework of linearized ideal magnetohydrodynamics of the problem of the heating of a general toroidal plasma by Alfvén waves. The extremal value of the functional yields directly the impedance matrix seen by the driving circuits. The problem is couched in generalized coordinates in a fashion amenable to computational implementation. The energy is absorbed in boundary layers which straddle those magnetic surfaces corresponding to continuum eigenfunctions resonant with the driving frequency. The contribution of these resonant surfaces to the extremal value of the variational functional is given explicitly in terms of quantities which must be found by solving the problem in the regions external to the resonant surfaces. The structure of the theory suggests optimum ways of implementing the method.
Resonant absorption in a self-consistent density profile at moderate intensities
Authors: David François , Mora Patrick , Pellat René
Published: March 2003

A model for the nonlinear resonant absorption of an electromagnetic wave in an inhomogeneous collision-dominated plasma at moderate intensities is proposed. The wave propagation equations are numerically solved in the time-independent self-consistent density profile. A density step appears at the critical point, and a surface wave develops which tends to enhance the absorption. Extensions of the model to collisionless situations are given when the damping can be conveniently described by some phenomenological self-consistent collision frequency. Finally the effect of a plasma expansion across the critical density is briefly discussed.
Numerical simulation of slow spheromak formation: Flux control by formation speed
Authors: Sato Tetsuya , Todd A. M. M. , Okuda Hideo
Published: March 2003

Two-dimensional magnetohydrodynamic simulations of the Princeton S-1 Spheromak have been carried out for several formation speeds. It is found that the spheromak size and shape, and hence the confined poloidal and toroidal fluxes, are largely dependent upon the formation speed, provided this is moderately slow. Specifically, the total toroidal and poloidal fluxes in a spheromak increase as the formation speed is reduced. The role of externally driven reconnection as a cause for this dependence is discussed.
Magnetohydrodynamic stability of magnetic slabs
Authors: Baños A. , Adler E. A. , Fried Burton D. , Jechart I.
Published: March 2003

A simplified procedure is developed for calculating the ideal magnetohydrodynamic stability of a ‘‘magnetic slab’’ consisting of two planar rows of staggered parallel conductors carrying oppositely directed (equal or unequal) currents, a configuration which is the planar analog of the University of California, Los Angeles, Large Mirror Ratio Experiment (LAMEX). Instead of solving the Grad–Shafranov equation to find the magnetic field corresponding to magnetohydrodynamic equilibrium, use was made of conformal mapping techniques to obtain analytical expressions for the vector potential A for the sharp boundary case, and then the plasma was allowed to ‘‘spill over’’ into the vacuum region with a permissible pressure profile p(A). Using conventional magnetohydrodynamic analysis, based on the well-known energy principle, the maximum stable pressure gradient on each magnetic surface was determined and, from that, the optimum permissible pressure profile p(A) as a function of the spacings (horizontal and vertical) between the conductors and the currents in the two rows was also determined. Significant reductions in maximum stable beta are found compared to the sharp boundary case.
Oblique collision of cylindrical outgoing ion-acoustic solitons
Authors: Tsukabayashi I. , Nakamura Y. , Kako F. , Lonngren K. E.
Published: March 2003

Two identical cylindrical outgoing ion-acoustic solitons are transmitted in a double-plasma device to observe an oblique collision. The colliding angle of the solitons is smaller than the resonance angle given by 2 tan−1[(2δn/n)1/2], where δn/n is the normalized amplitude of the solitons. Two new waves are observed to form in the region of the interaction. The experimental results are compared with a numerical integration of a modified Boussinesq equation and with a theory of three-wave interaction.
Intense free electron laser harmonic generation in a longitudinal magnetic wiggler
Authors: Davidson Ronald C. , McMullin Wayne A.
Published: March 2003

The free electron laser instability is investigated theoretically for a tenuous relativistic electron beam propagating parallel to a longitudinal wiggler field (B0+δB sin k0z)êz . The stability analysis is based on the linearized Vlasov–Maxwell equations for perturbations about the beam equilibrium f0b =(nb/2πp⊥)δ(p⊥−γbmV⊥) δ(pz−γbmVb). For (ωcb/k0Vb)(δB/B0) of order unity or larger, it is found that the transverse electron orbits experience a strong temporal modulation at harmonics of k0vz≂k0Vb. In the stability analysis, this orbit modulation is manifest by radiation amplification occurring near the simultaneous zeroes of ω=kc and ω−kVb−ωcb=nk0Vb for the general harmonic number n, which corresponds to an upshifted (for n≥1) wavenumber k̂n=(1−Vb/c)−1(ωcb/c+nk0Vb/c). For (ωcb/k0Vb)(δB/B0) >1, the instability is typically broadband with several harmonics excited. Moreover, depending on beam and field parameters, the characteristic maximum growth rate can be substantial with Im ω/ck0=(3/2)1/2 ×[(V2⊥/4c2)(ω2pb/c2k20) nJ2n(ωcbδB/k0VbB0)]1/3.
Kinetic description of free streaming mode in an electron beam propagating through a tape helix waveguide
Authors: Uhm Han S.
Published: March 2003

Stability properties of the free streaming mode in a relativistic electron beam propagating through a cylindrical waveguide loaded with a tape helix are investigated, in connection with the traveling wave tube amplifier. The stability analysis is carried out within the framework of the linearized Vlasov–Maxwell equations for an electron distribution function, in which all electrons have a Lorentzian distribution in the axial canonical momentum. Closed algebraic dispersion relations of the free steaming mode for both annular and solid beams are obtained. One of the most important features of the analysis is that the growth rate and range of instability increase drastically as the outer conducting wall approaches to the helix. For an appropriate choice of the radii of the helix and the outer conducting wall, it is shown that the instability range can be more than triple that for a traveling wave tube without conducting wall. However, the growth rate and range of instability are substantially reduced by increasing the axial momentum spread.
Stimulated Raman backscattering in the presence of ion-acoustic fluctuations
Authors: Rozmus W. , Offenberger A. A. , Fedosejevs R.
Published: March 2003

Threshold conditions are analyzed for stimulated Raman backscattering (SRS) from an underdense plasma in the presence of ion-acoustic fluctuations. For typical ion-acoustic spectra which could result from a Brillouin instability occurring simultaneously with SRS, a reduced collisional threshold due to induced bremsstrahlung is found for the Raman instability. Similarly, for inhomogeneous plasma the phase mismatch created by a density gradient can be compensated for by an inhomogeneity in the ion-acoustic spectrum; as a consequence, the inhomogeneity threshold for absolute instability can be substantially reduced. Comparisons are made with experimental observations of stimulated Raman scattering in the presence of strong stimulated Brillouin scattering for CO2 laser-plasma interaction. It is found theoretically that for fluctuation levels typical of that accompanying SBS, induced bremsstrahlung growth rates for Langmuir waves can be of the order of collisional damping rates.
Expansion of a multi-ion plasma into a vacuum
Authors: Singh Nagendra , Schunk R. W.
Published: March 2003

Numerical solutions of the Vlasov and Poisson equations for the expansion of a plasma with O+ and H+ as the constituent ions are presented. The self-similar nature of the expansion is examined by comparing the numerical results with available theories. The formation of a plateau in the expansion region, when the H+ ions are initially minor, is investigated by varying the electron to ion temperature ratio. The numerical results show that self-similarity holds in the quasineutral regions of the plasma.