bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of toroidicity on resistive tearing modes
Authors: Izzo R. , Monticello D. A. , Park W. , Manickam J. , Strauss H. R. , Grimm R. , McGuire K.
Published: March 2003

A reduced set of resistive magnetohydrodynamic (MHD) equations is solved numerically in three dimensions to study the stability of tokamak plasmas. Toroidal effects are included self-consistently to leading and next order in inverse aspect ratio ε. The equations satisfy an energy integral. In addition, the momentum equation yields the Grad–Shafranov equation correct to all orders in ε. Low-beta plasmas are studied using several different q profiles. In all cases, the linear growth rates are reduced by finite toroidicity. Excellent agreement with calculations using the resistive PEST code is obtained. In some cases, toroidal effects lead to complete stabilization of the mode. Nonlinear results show smaller saturated island widths for finite aspect ratio compared to the cylindrical limit. If the current channel is wide enough so as to produce steep gradients towards the outside of the plasma, both the finite-aspect-ratio cases and cylindrical cases disrupt.
Modulation of the current-driven ion-cyclotron instability by the potential relaxation instability
Authors: Schrittwieser R.
Published: March 2003

An experimental investigation in a single-ended current-carrying Q machine is presented in which the radius of the current channel is varied. For increasing radius R (where R is normalized to the ion Larmor radius) the electrostatic ion-cyclotron instability is excited mainly at first. For R≳7, however, the potential relaxation instability mainly appears. In the transition region both instabilities are observed but the amplitude of the cyclotron instability is strongly modulated with the frequency of the relaxation instability. This is because of the periodic current limitation by the latter which nearly inhibits the electron drift necessary for the excitation of the former.
Beta and density limits in tokamaks
Authors: Strauss H. R.
Published: March 2003

Ballooning modes with toroidal mode number n=1,2 can be stable in tokamaks for all beta. Below a density limit, modes with all n can be stable for any beta, because of finite-Larmor-radius effects. The critical density, found for a particular choice of model profiles, is somewhat below the density of actual experiments, such as the ISX-B tokamak. However, finite-Larmor-radius effects can still raise the critical beta for instability and substantially restrict the number of unstable modes.
Strong spherical two-phase blowoff of liquid metals into vacuum
Authors: Hornung K.
Published: March 2003

The process of the spherical nonequilibrium expansion of a homogeneous metal liquid-vapor cloud into a vacuum is considered for high initial entropies (up to 0.9 Scrit) and high expansion velocities (several km/sec). Starting from phase equilibrium, the system successively passes through stages of increasing nonequilibrium, determined by the characteristic times of the expansion as well as heat and mass transfer and ends up in a state of maximum nonequilibrium, where there is no more contact between the phases. Results are discussed for different metallic fluids.
Collision operator for a strongly magnetized pure electron plasma
Authors: O’Neil T. M.
Published: March 2003

Collisions are discussed for a pure electron plasma which is so strongly magnetized that the Larmor radius is small compared to the classical distance of closest approach. In this regime, the most important collisions are characterized by an impact parameter which is the order of the distance of closest approach. Assuming that this distance is small compared to the interparticle spacing, a Boltzmann-like collision operator may be derived. In turn, this operator may be reduced to a Fokker–Planck operator, which is integrable. An interesting property of the collision operator results from an adiabatic invariant which is preserved during scattering. There is no energy exchange between the velocity components which are parallel to the field and the velocity components which are perpendicular to the field. This may have implications for a current effort to cool a pure electron plasma to low temperature by cyclotron radiation.
Neoclassical transport in a multiple-helicity torsatron in the low-collisionality (1/ν) regime
Authors: Shaing K. C. , Hokin S. A.
Published: March 2003

For a sufficiently high number of toroidal field periods (m/ι>l), the magnetic field of a multiple-helicity torsatron can be reduced to a simple form such that the second adiabatic invariant J can be calculated. It is found that the particle and the heat fluxes for a multiple-helicity torsatron in the low-collisionality (1/ν) regime have the same geometric dependences. An optimization of both quantities is carried out for a given equilibrium constraint. It is shown that the transport fluxes can be smaller than those of the conventional stellarator by an order of magnitude. The effect of finite plasma beta on the neoclassical fluxes is also studied.
Direct losses in the ELMO Bumpy Torus experiment
Authors: Hastings D. E. , Callen J. D.
Published: March 2003

For resonant ions in the ELMO Bumpy Torus experiment (EBT), the width of the orbits near the resonance scales like (∂Ω/∂r)−1, where Ω is the precessional drift frequency. For (∂Ω/∂r)≊0 over most of the plasma, the ions will drift vertically out of the machine due to the toroidal magnetic field. The problem of these directly lost particles is analyzed by considering collisional diffusion into a hole in velocity space. Particle and energy loss rates including boundary layer corrections due to marginally confined particles are obtained. It is shown that these direct loss rates can be comparable to diffusion rates based on banana transport.
Purely growing thermal instability of an electromagnetic wave in a collisional plasma
Authors: Sharma A. K. , Tripathi V. K.
Published: March 2003

A high-amplitude electromagnetic wave propagating through a collisional plasma heats the electrons to a temperature higher than that of the heavy particles. This state is, however, shown to be unstable to a purely growing temperature perturbation with k ⊥ k0; k and k0 being the wave vectors of the perturbation and the pump. The temperature perturbation modulates the local electron resistivity and generates two sidebands. The sidebands couple with the pump to heat the electrons and hence to enhance the temperature perturbation. The growth rate of the instability in a strongly ionized plasma is of the order of ν0 ωpα1/2m1/4/2ω0(1+α) m1/4i, where α≂v20/3c2s is a nonlinearity parameter, v0 and cs are the electron oscillatory and ion thermal velocities, ν0 and ωp are the electron collision and plasma frequencies, ω0 is the frequency of the pump, and m and mi are the electron and ion mass.
Wave precession as a consequence of the nonlinearly developed birefringence in plasmas
Authors: Chakraborty B. , Paul S. N.
Published: March 2003

In plasmas, even in the absence of any static magnetic field, due to nonlinearities of the medium, the left and right circular polarization components of a strong electromagnetic wave are found to have different dispersion rates. Using this property of nonlinearly induced birefringence, the rate of precessional rotation and wavenumber shift of a strong elliptically polarized electromagnetic wave passing through a cold, homogeneous, collision-free, and unmagnetized plasma are derived analytically.
Discrete Alfvén eigenmode spectrum in magnetohydrodynamics
Authors: Mahajan S. M. , Ross David W. , Chen Gwo-Liang
Published: March 2003

The conditions for the existence of global Alfvén eigenmodes below the continuum are investigated, and an analytical dispersion relation describing the modes is obtained. In cylindrical geometry, the curvature together with gradients of the equilibrium current (magnetic shear), and finite ω/ωci effects are responsible for these modes which could play an important part in Alfvén wave heating of tokamak plasmas.
A solution to the linear wave conversion problem in weakly inhomogeneous plasmas
Authors: Gambier D. J. D. , Schmitt J. P. M.
Published: March 2003

The wave equation for electromagnetic and electrostatic waves propagating in a weakly inhomogeneous plasma is studied near a conversion layer (any layer where the WKB approximation breaks down). The wave equation is approximated by a Laplace equation, [P(i d/dx)−xQ(i d/dx)]E(x)=0, where P and Q are polynomials of arbitrary order. The general solution is given in terms of contour integrals. Simple transformations on sets of contours allows connection of the WKBJ asymptotic solutions on both sides of the conversion layer. The mathematical technique is general (matrix operations) and is well suited to numerical computation. The method is illustrated by solving the fourth-order problems met in ion-cyclotron fast-wave and lower-hybrid wave conversion.
The approach of a sphere to an interface
Authors: O’Neill M. E. , Ranger K. B.
Published: March 2003

A solution to the problem posed when a rigid sphere normally approaches an interface between immiscible viscous fluids is presented when the deformation of the interface is small compared with the sphere radius or its minimum clearance from the undistorted interface. The complete boundary conditions at the interface of continuity of velocity and tangential stress together with discontinuity of normal stress by the surface tension times the mean curvature are applied in the first-order analysis of a perturbation scheme in which the interface has small distortion from planar. This results in a finite force on the sphere being predicted as the sphere approaches the interface.
Stability of the flow during sedimentation in inclined channels
Authors: Herbolzheimer Eric
Published: March 2003

The conditions for wave formation and growth during sedimentation in inclined channels is investigated using a linear stability analysis in the limit of large sedimentation Grashof number with the Reynolds number remaining O(1). An asymptotic solution for long waves reveals that the critical Reynolds number is proportional to the tangent of the angle of inclination of the channel times the ratio of the thickness of the thin pure-fluid layer, which forms beneath the downward-facing surface of the channel, to the width of the channel. Hence, the critical Reynolds number is very small unless the channel is extremely long and narrow. Numerical calculations indicate, however, that the maximum amplification rates are often so small that the disturbances may be convected out of the channel before they reach an observable size. This conclusion is consistent with experiments performed in our laboratory, and the observed wave inception point and the calculated maximum amplification rates exhibit remarkably similar dependence on the angle of inclination and the suspension concentration.
On the structure of wall-bounded turbulent flows
Authors: Kim John
Published: March 2003

The variable-interval time-averaging (VITA) technique developed by Blackwelder and Kaplan [J. Fluid Mech. 76, 89 (1976)] is applied to data obtained from large-eddy simulation of turbulent channel flow in an investigation of the organized structures associated with the bursting phenomenon in the near-wall region. Conditionally averaged velocities, shear stress, pressure, and vorticity are discussed in conjunction with the bursting phenomenon detected by the VITA technique. The conditionally averaged pressure reveals that the ejection process is associated with a localized adverse pressure gradient. In the plane perpendicular to the flow direction, the conditionally averaged vorticity field indicates that a pair of counterrotating streamwise vortices is being lifted through the ejection process.
Spectral relationships between velocity and temperature fluctuations in turbulent shear flows
Authors: Fulachier L. , Antonia R. A.
Published: March 2003

The frequency dependence of a turbulent Prandtl number, defined using the Reynolds shear stress and normal heat flux co-spectra, in various turbulent shear flows accentuates the inadequacy of the Reynolds analogy from a spectral point of view. For the same flows, the analogy between spectral distributions of the kinetic energy and the temperature variance is validated over a significant frequency domain. This analogy suggests a spectral parameter which is very close to unity in this frequency range for all the flows considered.
Heat and mass exchange of a droplet in a polyatomic gas
Authors: Lang H.
Published: March 2003

Lees’ method, combined with a generalized Grad’s moment procedure, is used to calculate the heat and mass transport to and from a droplet in a polyatomic gas. A near-equilibrium and quasisteady situation is assumed. The results are represented by the transport coefficients of a symmetric Onsager matrix. The coefficients depend on the collision numbers of the internal degrees of freedom. Simple results are obtained by performing approximations analogous to the Mason and Monchick analysis for the case of the heat conductivity.
Use of the stellarator expansion to investigate plasma equilibrium in stellarators
Authors: Anania G. , Johnson J. L. , Weimer K. E.
Published: March 2003

A numerical code utilizing a large-aspect-ratio, small -helical-distortion expansion is developed and used to investigate the effect of plasma currents on stellarator equilibrium. Application is made to several existing and proposed stellarators. The modification of the equilibrium properties with pressure is strongly dependent on the vacuum-field configuration. The modular stellarator model which is investigated gives somewhat discouraging results because a large rotational transform, and hence large coil deformation, is needed to achieve high-beta equilibria.
Surface waves induced by an impinging jet
Authors: Miksis Michael J. , Ting Lu
Published: March 2003

The oblique impact of a thin or slender jet on the free surface of a semi-infinite stream is studied. Based on the thickness or radius of the jet the Froude number is assumed to be large and the fluid is assumed to be viscous. By the method of matched asymptotics, the leading term of the solution behind the impact area is constructed. The perturbation solution for the stream away from the impact region and its wake is shown to be equivalent by the linear theory of surface waves with a concentrated load at the impact point and a distributed line load along the wake. These equivalent loads are consistent with considerations of mass and momentum balances and other judicial arguments.
Turbulent time scales and the dissipation rate of temperature variance in the thermal mixing layer
Authors: Elghobashi S. E. , Launder B. E.
Published: March 2003

In a turbulent flow where the mechanisms and/or the importance of the generation processes of turbulent kinetic energy k and mean square scalar variance ∼(c2) are dissimilar, no simple connection exists between the turnover time scale of the fluctuating velocity and fluctuating scalar fields. To allow the computation of the turbulent scalar field in these situations a means of calculating the scalar time is required. Here, in an extension of work by Newman, Launder, and Lumley [J. Fluid Mech. 111, 217 (1981)], the time scale is obtained via a proposed transport equation for the dissipation rate of mean square scalar variance. The modeled equation has been applied successfully to the calculation of the spread of a thermal mixing layer in grid-generated turbulence and the decay of temperature variance in a homogeneous field.
Effect of the number density of heterogeneities on the critical diameter of condensed explosives
Authors: Engelke Ray
Published: March 2003

Experiments are reported which examine the dependence of the critical diameter of a condensed explosive on the number density of heterogeneities present within the material. The form and quantity of heterogeneities present were closely controlled by constructing the heterogeneous materials from a homogeneous one. Reductions in critical diameter of over 40% were produced in some cases, even though a chemically inert material was being introduced into the explosive. The experiments indicate that the distance between heterogeneities is an important parameter in the production of a critical-diameter reduction.
Nonlinear evolution of an obliquely propagating Langmuir wave: Boundary-value problem
Authors: Menyuk C. R.
Published: March 2003

The nonlinear evolution of an obliquely propagating Langmuir wave is explored given boundary-value conditions, with emphasis on determining the effect of a transition between regular and stochastic electron motion. It is shown that when the angle of propagation is increased, the usual amplitude oscillations disappear because of destructive interference between the different, isolated resonances. It is also shown that the final (z=∞) wave amplitude decreases when there is a parameter transition from the trapping to the stochastic regime because electrons become free to travel large distances in velocity space, taking large amounts of energy from the wave.
Nonresonant electron transport in the ELMO Bumpy Torus (EBT) for multitemperature Maxwellians
Authors: Hastings D. E.
Published: March 2003

For the experimental conditions in the ELMO Bumpy Torus (EBT), it appears that the electron distribution function may not be a single-temperature Maxwellian. Neoclassical transport coefficients have been developed for the nonresonant electron species in a bumpy torus with the assumption that the lowest-order distribution function is the sum of l different Maxwellians. These show that there can be significant coupling between the different electron species. For comparison, the diffusion coefficients are contrasted to single-temperature diffusion coefficients.
Scattering of lower-hybrid waves by drift-wave density fluctuations: Solutions of the radiative transfer equation
Authors: Andrews P. L. , Perkins F. W.
Published: March 2003

The investigation of the scattering of lower-hybrid waves by density fluctuations arising from drift waves in tokamaks is distinguished by the presence in the wave equation of a large, random, derivative-coupling term. The propagation of the lower-hybrid waves is well represented by a radiative transfer equation when the scale size of the density fluctuations is small compared to the overall plasma size. The radiative transfer equation is solved in two limits: first, the forward scattering limit, where the scale size of density fluctuations is large compared to the lower-hybrid perpendicular wavelength, and second, the large-angle scattering limit, where this inequality is reversed. The most important features of these solutions are well represented by analytical formulas derived by simple arguments. Based on conventional estimates for density fluctuations arising from drift waves and a parabolic density profile, the optical depth τ for scattering through a significant angle, is given by τ≊(2/N2∥) (ωpi0/ω)2 (mec2/2Ti)1/2 [c/α(ΩiΩe)1/2 ], where ωpi0 is the central ion plasma frequency and Ti denotes the ion temperature near the edge of the plasma. Most of the scattering occurs near the surface. The transmission through the scattering region scales as τ−1 and the emerging intensity has an angular spectrum proportional to cos θ, where sin θ=k⊥⋅Bp/(k⊥Bp), and Bp is the poloidal field.
Spectral broadening of lower-hybrid waves by time-dependent density fluctuations
Authors: Andrews P. L. , Perkins F. W.
Published: March 2003

Lower-hybrid waves injected into tokamaks are scattered by turbulent time-dependent drift-wave density fluctuations. This scattering process gives rise to frequency shifts as well as angular deviations. In tokamaks, most of the frequency spreading occurs in a regime where the angular scattering results from multiple scattering events, each event having a small mean-square scattering angle. A radiative transport equation governing diffusion in both angle and frequency is derived. The solution is obtained in slab geometry via separation of variables and leads to a Mathieu equation. In the limit where the slab thickness l exceeds the typical distance for diffusion through a large angle ls, an explicit generalization of the diffusion solution to the radiative transfer equation is formed. This solution gives the combined angular and frequency spectrum in the scattering layer. In the thick slab limit, the rms frequency width of lower-hybrid waves emerging from the layer is given by Δω=(ωd/ ξ0ρi) N∥ [(Mi/me)βi]1/2 (l/ls), where ωd and ξ−10 are the characteristic angular frequency and correlation length of the drift-wave turbulence.
A new interpretation of the alpha effect
Authors: Keinigs R. K.
Published: March 2003

In a magnetized plasma the alpha effect represents a turbulently generated emf directed along the mean magnetic field. This emf is central to the understanding of turbulent generation of large-scale magnetic fields. The alpha effect is reevaluated in terms of ensemble-averaged properties of the magnetic fluctuation spectrum. The results indicate that it is current helicity as opposed to kinetic helicity that is required to produce an alpha effect.
Growth rate reduction of the curvature-driven flute instability by plasma blanket line tying
Authors: Segal David
Published: March 2003

The effect of an annular, line-tied blanket, on the curvature-driven flute in a magnetic mirror is considered. The blanket is assumed to be line tied to a thermoionically emitting annular end plate. Reduction of the flute growth rate is computed as function of Larmor radius, blanket radius, and axial plasma conductance through either an external plasma or mirror sheath. It is found that significant reduction in growth rate can be achieved.
Effect of collisionless detrapping on nonaxisymmetric transport in a stellarator with radial electric field
Authors: Mynick Harry E.
Published: March 2003

A derivation is given for expressions describing transport rates in a stellarator operating at low collisionality ν, in the presence of a radial electric field. It is shown that collisionless detrapping (and retrapping) of particles in superbanana orbits can substantially modify this transport. The detrapping provides a collisionless lower bound on the width of the collisional boundary layer which, in previous work, gave rise to transport coefficients scaling as ν1/2. With this lower bound, these coefficients will instead scale as ν, implying more favorable confinement than indicated by the ν1/2 result commonly employed.
Ballooning mode calculations in stellarators
Authors: Berk H. L. , Rosenbluth M. N. , Shohet J. L.
Published: March 2003

A magnetohydrodynamic (MHD) ballooning mode formalism and calculation is developed to show how a field-line-following code can be used to study MHD stability. The asymptotic analysis of the ballooning equation yields the Mercier condition. It is shown that first-order equilibrium effects on the vacuum fields from finite pressure cancel the intrinsically destabilizing term of the Mercier condition. A ballooning unstable solution is found in a Heliac configuration that has a magnetic well at zero beta and a rotational transform that increases radially outward.
Asymptotic theory of the internal kink mode in current-carrying toroidal plasmas
Authors: Crew G. B. , Ramos J. J.
Published: March 2003

An asymptotic approach to the linear stability of the ideal magnetohydrodynamic internal kink mode in large-aspect-ratio tokamak plasmas is presented. This analysis spans arbitrary values of the plasma pressure gradient, and in particular demonstrates the stabilization of the mode at sufficiently high beta.
Anisotropy effects on curvature-driven flute instabilities in a hot-electron plasma
Authors: Spong D. A. , Berk H. L. , Van Dam J. W. , Rosenbluth M. N.
Published: March 2003

The effects of finite parallel temperature are investigated for a hot-electron plasma with sufficiently large beta that the magnetic field scale length (ΔB) is small compared with the vacuum field radius of curvature (R). Numerical and analytical estimates of stability boundaries are obtained for the four possible modes that can be treated in this limit: the conventional hot-electron interchange, the high-frequency hot-electron interchange (ω>ωci), the compressional Alfvén mode, and the interacting pressure-driven interchange.
Measurements of fluctuations in the flux of runaway electrons to the PLT tokamak limiter
Authors: Barnes Cris W. , Strachan J. D.
Published: March 2003

Fluctuations in the flux of runaway electrons to the limiter are measured during many PLT (Princeton Large Torus) discharges. Oscillations at 60, 120, and 720 Hz are driven by variations in the vertical magnetic field which moves the plasma major radius. Fluctuations are seen with frequencies in the range of 2–20 kHz because of magnetohydrodynamic (MHD) magnetic islands which extend to the plasma surface. A continuous spectrum of fluctuations is observed up to 200 kHz which correlates with drift-wave turbulence. The magnitude of the driven fluctuations can be used to measure transport properties of the runaway electrons. The amplitude of electron motion due to the MHD and drift-wave oscillations, and hence a measure of the radial size of the instability, can be determined as a function of frequency. The slope of the frequency power spectrum of the drift-wave-induced fluctuations steepens with increasing runaway-electron drift orbit displacement during the current drop at the end of the discharge, and as the power in the MHD oscillations increases. A magnetic probe is used to confirm the presence of oscillating magnetic fields capable of perturbing the electron orbits.
Effects of ion-acoustic instability on light ion beam transport in deuterium channels
Authors: Colombant D. G. , Manheimer Wallace M.
Published: March 2003

Moderate density deuterium channels can be ion-acoustic unstable when bunched high-intensity light ion beams are propagated into them. The magnetohydrodynamic response of these channels is investigated under these conditions. Anomalous effects like enhanced energy deposition, anomalous electron and ion heating, and anomalous resistivity are included in the model. Only at low channel densities does the instability cause an additional slowing down of the beam. However, the hydrodynamic motion of the channel remains unaffected.
Backscattering cascade of beam modes off ambient density fluctuations
Authors: Russell D. A. , Goldman M. V.
Published: March 2003

The effects of a given nonthermal low-frequency density-fluctuation spectrum on the amplification of Langmuir waves by a ‘‘bump-on-tail’’ beam of electrons are studied. The density-fluctuation spectrum is assumed to contain a uniform distribution of wavelengths ranging from much shorter than the beam mode wavelength to of the same order. This permits multiple large-angle (back) scattering to occur. One-dimensional numerical solutions of the kinetic equations are found which yield criteria for linear saturation of the beam instability by a cascade of backscattering to high wavenumber. The relevant time scales and spectral shapes are also determined in both the stable and unstable regions. Linear damping and Cerenkov emission by a possible nonthermal tail of electrons is taken into account. An application is made to the beam modes observed simultaneously with density fluctuations off the Jovian bow shock. It is shown that the observed level of density fluctuations is sufficient to saturate the unstable Langmuir waves, although nonthermal Landau damping may prevent a cascade to very high wavenumbers.
Structure of perpendicular shocks in collisionless plasma
Authors: Leroy M. M.
Published: March 2003

The macrostructure of perpendicular shocks in the supercritical regime is investigated theoretically. The coupling between the shock precursor (foot region) associated with reflected ions and the magnetic ramp associated with electron Ohmic heating is analyzed using a model in which the ions are treated as a multifluid and the electrons as a massless fluid. The results are found to be in good agreement with a number of features exhibited by recent numerical simulations of perpendicular shocks, including the morphology of the reflected-gyrating ion stream, the presence of potential and magnetic field overshoots, and the Mach number dependence of the shock structure.
Reply to the comment by Sanmartín, Montañés, and Barrero
Authors: Manheimer Wallace M. , Colombant D. G. , Gardner J.
Published: March 2003

A reply by the authors to the comment on steady-state flow of laser-produced plasmas is presented. (AIP)
Neoclassical dielectric property of a tokamak plasma
Authors: Hinton F. L. , Robertson J. A.
Published: March 2003

The response of a tokamak plasma to a time-dependent axisymmetric radial electric field is considered. The bounce-averaged motion of magnetically trapped ions is shown to consist of a radial drift, in addition to the well-known toroidal precession. This is a neoclassical polarization drift, which is larger than the standard one by a factor B2/B2θ, the square of the ratio of total to poloidal magnetic fields. The resulting low-frequency dielectric constant is larger than the standard one by approximately the same factor, when the ions are in the banana regime of neoclassical theory.
Local electron flow to the anode in a magnetically insulated diode
Authors: Maron Y.
Published: March 2003

Local electron flux to the anode of a magnetically insulated diode is monitored. Intense electron burst to the anode and slow variations in the electron flux are observed. Unlike the slow signals the bursts are accompanied by sharp increases in microwave emission and by increases in the ion current density. The electron bursts are not affected by the presence of the anode plasma. Indications suggest that the bursts are initiated by processes in the cathode plasma.
Experimental scaling laws for ablation parameters in plane target–laser interaction with 1.06 μm and 0.35 μm laser wavelengths
Authors: Meyer B. , Thiell G.
Published: March 2003

Ablation parameters such as velocity, mass, momentum, pressure, and hydrodynamic efficiency have been investigated with plane targets irradiated in the range 3×1011-1015 W cm−2 with 1 nsec pulses and laser wavelengths of 1.06 μm and 0.35 μm. We show that ablation velocity, ablated mass, and momentum are in good agreement with ablation scaling laws deduced from analytical models taking into account inverse bremsstrahlung absorption below the critical density. Nevertheless, processes such as lateral conduction, hot spot, and preheat effects make inaccurate the comparison between ablation pressures, mass ablation rates, or hydrodynamic efficiencies measured for different laser wavelengths. Laser illumination nonuniformities are transmitted to the target in terms of pressure variations. The harmful consequence of a reduced lateral energy flow in 0.35 μm experiments can eclipse the increasing of ablation pressure and hydrodynamic efficiency.
Motion of small particles in a gas flow
Authors: Ishii Ryuji
Published: March 2003

A theoretical investigation into (i) the motion of a spherical particle in a gas flow with a simple shear and (ii) the motion of a spherical particle in a gas flow over a wavy wall is described. The lift force on the particle due to the rate of shear of the gas is taken into consideration. Analytical solutions for the particle motion are obtained in both cases under the conditions that the gas properties are not affected by the presence of particles and also the gas is inviscid except for its interaction with solid particles. Sample calculations are carried out for a few typical conditions.
Subharmonic generation of deep-water capillary waves
Authors: Hogan S. J.
Published: March 2003

A recent experiment by Banerjee and Korpel [Phys. Fluids 25, 1938 (1982)] indicates that McGoldrick’s [J. Fluid Mech. 21, 305 (1965)] theory of resonant interactions among deep water capillary-gravity waves may require modification. The conclusions are reexamined, and it is found that the experimental results are in overall agreement with the theory. Some systematic differences remain at low and high frequencies. Possible reasons for these discrepancies are proposed.
A symmetric regularized-long-wave equation
Authors: Seyler C. E. , Fenstermacher D. L.
Published: March 2003

A symmetric version of the regularized-long-wave equation is shown to describe weakly nonlinear ion acoustic and space-charge waves. The equation possesses hyperbolic secant squared solitary waves and has four known invariants. Numerical solutions are compared with previous results on the regularized-long-wave equation.
Observations on flow fields generated by opposed free-jets of gas mixtures
Authors: McDermott W. , Hurlbut F. C.
Published: March 2003

Spatial distributions of velocities in the neighborhood of the interference zone of nondiametrically opposed two-dimensional free-jets of argon–helium mixtures have been investigated using Doppler-shifted fluorescence radiation excited by an electron beam. Macroscopic velocities for each species were measured using a scanning Fabry–Perot interferometer system. Complementary spatial distributions of species mole fractions were determined using a mass flow probe, molecular beam sampling system, and quadrupole mass analyzer. Data were obtained over a wide range of experimental parameters, in particular, the reservoir to test region pressure ratio, 1.9 to 42, the Reynolds number based on reservoir conditions and jet width, 25 to 117, the mixture ratio, from 1% to 10% argon in helium, and the total included angle between jet axes, 120° and 90°. Detailed field plots of velocity and of mole fraction provide the basis for the construction of parameteric correlation functions for velocities and for compositions. Stage separation factors were calculated giving values comparable with those expected from ideal barrier separation processes. Processes of separation are discussed.
Turbulent effects on the chemical reaction for a jet in a nonturbulent stream and for a plume in a grid-generated turbulence
Authors: Komori Satoru , Ueda Hiromasa
Published: March 2003

Effects of turbulent mixing on a moderately slow second-order chemical reaction between nitric oxide contained in a primary jet or plume and ozone being entrained from the ambient stream are experimentally investigated in low-speed wind tunnels. The experiments are conducted both for a nonpremixed free jet injected into a nonturbulent, coflowing stream and for a nonpremixed plume emitted from a fixed source into a grid-generated turbulent stream. In each flow, mean concentrations of NO and O3 are measured. In addition, eddy diffusivities are calculated from the measured values of turbulent momentum and heat flux in a nonreactive, slightly heated jet or plume. Numerical predictions of the mean concentrations of the reactive species, based on the eddy diffusivities, are compared with the measured ones. The results show that for moderately slow chemical reactions the turbulence effects on reaction rate become significant. The mean products of the concentration fluctuations are estimated to be 1.5 and 20 times as large as those of the products of the mean concentrations for a reactive jet and for a reactive plume, respectively.
A comparison of properties of temporal and spatial temperature increments in a turbulent plane jet
Authors: Antonia R. A. , Browne L. W. B. , Britz D. , Chambers A. J.
Published: March 2003

The spatial longitudinal temperature correlation function obtained, on the center line of a turbulent plane jet in the self-preserving region of the flow, for separations extending up to about 13 Kolmogoroff microscales is in reasonable agreement with the temporal autocorrelation when Taylor’s hypothesis is used. The rms value of the spatial temperature derivative, inferred from the curvature at zero separation of the correlation, agrees well with the rms value of the time derivative. Other comparisons between temporal and spatial temperature increments include probability and spectral density functions. Reasonable agreement is found between fourth-order moments of these increments, but the magnitude of third-order moments is larger for spatial than temporal increments. Spectral density functions of spatial increments do not exhibit the periodicity in spectra of temporal increments.
Experimental observations of self-similar plasma expansion
Authors: Chan Chung , Hershkowitz Noah , Ferreira Alberto , Intrator Tom , Nelson Brian , Lonngren Karl
Published: March 2003

The potential profiles of an expanding plasma have been measured experimentally. The profiles are shown to be self-similar.
Construction of three-dimensional vacuum magnetic fields with dense nested flux surfaces
Authors: Cary John R.
Published: March 2003

Toroidal vacuum magnetic fields are analyzed by the surface of section technique and noncanonical perturbation theory. The surface of section analysis shows that it is easy to find magnetic fields with an outermost flux surface of relatively small aspect ratio. However, as the aspect ratio is decreased, so is the rotational transform of the outermost flux surface. A new method of averaging combined with noncanonical perturbation theory shows that island overlap can account roughly for this loss of rotational transform. Finally, it is shown that one can significantly decrease the stochasticity and island structure by making small modifications to the magnetic field.
Investigation of the shear Alfvén continuum in nonaxisymmetric toroidal geometry in the low beta limit
Authors: Bernardin M. P. , Tataronis J. A.
Published: March 2003

The set of equations which determine the continuous spectrum of ideal magnetohydrodynamics (MHD) in nonaxisymmetric toroidal geometry is investigated in the low beta limit. It is shown that if the equilibrium pressure is neglected, a Sturm–Liouville equation with quasiperiodic coefficients characterizes the Alfvén continuum. In addition, it is shown that the eigenvalues are real.
Upper hybrid loss-cone instability in tandem mirror thermal barriers
Authors: Porkolab M.
Published: March 2003

Electrostatic instabilities with frequencies in the vicinity of the upper hybrid frequency are studied. Such instabilities may be excited in electron cyclotron resonance heating (ECRH) plasmas where a warm Maxwellian and a hot loss-cone component may exist simultaneously. Temporal and convective growth rates are calculated for parameters relevant to present-day tandem mirror and ELMO Bumpy Torus (EBT) devices. It is found that such modes may grow convectively to significant levels in the thermal barrier region of tandem mirrors.
Improvement in electrostatic confinement by second-harmonic electron cyclotron heating in the Phaedrus tandem mirror
Authors: Sing D , Golovato S. N. , Hershkowitz Noah , Scharer J.
Published: March 2003

One minimum-B mirror end cell of the Phaedrus tandem mirror was heated with up to 30 kW of electron cyclotron heating at ω=2ωce. The plug electron temperature was increased from 18 to 23 eV while a smaller temperature increase was measured in the central cell. Changes in ion end loss energy and current were consistent with direct measurements of increases in the east plug plasma potential. Ion confining potential increases of up to 27 V were observed. These increases were much greater than the Te/e increases and in excess of that predicted by the Boltzmann relation.
Electromagnetic oscillating two-stream instability near the electron cyclotron frequency in a plasma
Authors: Tripathi V. K. , Tsai S. T. , Liu C. S.
Published: March 2003

The electromagnetic oscillating two-stream instability of electron cyclotron waves in a plasma has been examined. The low-frequency perturbation is a purely growing electromagnetic mode having k vector parallel to the ambient B field and the sidebands are elecrostatic plasma waves. The instability is excited when either ωc, ωp≥ω0 ≥(ω2p+ω2c/4)1/2 −ωc/2 or ωu>ω0>ωc,ωp. Here ω0 is the frequency of the pump and ωc,ωp, and ωu are the electron cyclotron, plasma, and upper-hybrid frequencies, respectively. The threshold power, due to the finite extent of the pump, is a few hundred kilowatts for the parameters of large tori. The growth rate above the threshold γ≂ω0 k0v20x/8k2v2e, where v0x and ve are the oscillatory and thermal speeds of electrons.
Parametric absorption in electron cyclotron resonance heating of tokamak plasmas
Authors: Stefan V. , Bers A.
Published: March 2003

A theoretical study of parametric absorption in electron cyclotron resonance heating (ECRH) of tokamak plasmas is presented. The problem of primary and secondary decays including their temporal and spatial growth rates is studied. In the nonlinear phase of development of the excited waves, a cascading saturation mechanism is invoked for both convective and absolute instabilities. Anomalous absorption frequencies and anomalous absorption lengths are evaluated for a variety of parametric excitations.
Low-amplitude, wave-induced particle energy diffusion in an inhomogeneous magnetic field
Authors: Riyopoulos Spilios , Antonsen Thomas M. , Ott Edward
Published: March 2003

The motion of a charged particle in a static, homogeneous magnetic field and a coherent electrostatic wave, whose frequency is much greater than the particle gyrofrequency, has been studied in detail by Karney. Karney found that there is a threshold in wave amplitude above which widespread diffusion of the particle energy (i.e., heating) occurs and below which the particle energy remains bounded. In this paper the effect of an inhomogeneous (or time-varying) magnetic field is examined. It is found that under general circumstances the motion of the particle can become diffusive and unbounded for values of the wave amplitude well below the threshold in the constant magnetic field case. The dependence of this diffusion on the various parameters affecting the motion is examined.
Exact time-dependent solutions of the Vlasov–Poisson equations
Authors: Lewis H. R. , Symon K. R.
Published: March 2003

The Vlasov–Poisson equations are relevant to collisionless plasmas and to stellar dynamics. They can be solved in one spatial dimension for an interesting class of cases by using a recent result about exact invariants of the motion of a particle in a one-dimensional potential. Lewis and Leach have given the necessary and sufficient conditions on the potential energy V(x,t) that an invariant which is quadratic in the momentum exist. For such a V(x,t), they exhibit the invariant explicitly. This result can be used to find the solutions of the one-dimensional Vlasov–Poisson equations for which the distribution functions are functions of quadratic functions of the momenta. A special case is the class of locally Maxwellian time-dependent solutions. The solutions for a single-species plasma, or a multispecies plasma where the charge to mass ratios are all equal, can be obtained by translating stationary solutions of the Vlasov–Poisson equations rigidly with an arbitrarily time-dependent displacement. If the charge to mass ratios of a multispecies plasma are unequal, then the solutions can be obtained by translating stationary solutions of modified Vlasov–Poisson equations with a displacement that depends quadratically on time. The modified Vlasov–Poisson equations include a species-dependent pseudogravity or ponderomotive force. This technique can be extended to obtain solutions of the Vlasov–Poisson or Vlasov–Maxwell equations in three dimensions.
Exact, time-dependent solutions of the one-dimensional Vlasov–Maxwell equations
Authors: Abraham-Shrauner B.
Published: March 2003

Coordinate transformations from a reference frame with electrostatic Bernstein–Greene–Kruskal equilibria produce exact, time-dependent one-particle distribution functions and electric fields that obey the nonlinear one-dimensional Vlasov–Maxwell equations. The Liouville equation invariant on which the distribution function depends is the transformed particle energy. Procedures for deriving invariants of more general momentum dependence than quadratic are suggested. The vanishing of the longitudinal current density is imposed upon the exact time-dependent Vlasov–Poisson solutions and a particular case of a time oscillatory Bernstein–Greene–Kruskal equilibria is solved. The existence of solutions with a nonconstant time-stretching factor ρ(t) is demonstrated.
The formation and expansion of a toroidal drop moving in a viscous fluid
Authors: Kojima Masami , Hinch E. J. , Acrivos Andreas
Published: March 2003

The deformation of an initially spherical liquid drop moving under the action of gravity in another fluid with which it is completely miscible is investigated under conditions of small values of the drop Reynolds number. It is found experimentally that such a drop evolves into an open torus which subsequently expands, and this phenomenon is examined theoretically for two limiting drop geometries: (i) a slightly deformed spherical drop, and (ii) a highly expanded, slender open torus. Under the assumptions of zero interfacial tension and creeping flow, the theory provides a qualitative description for the initial stages of the drop evolution [case (i)], but is unable to account for the observed drop expansion during latter stages of deformation [case (ii)]. On the other hand, if small inertial effects are retained in the analysis, the theory predicts that a slender open fluid torus possessing an arbitrary cross-sectional geometry will expand without change of shape to first order in Reynolds number. Quantitative comparisons of theoretically predicted rates of expansion with experimental measurements suggest the possible existence of a small, time-dependent interfacial tension across the drop interface.
The coupling of oscillatory plasma boundary motion to an ion acoustic wave
Authors: Braithwaite N. St. J. , Allen J. E.
Published: March 2003

The coupling of sheath-edge motion to quasineutral disturbances in the plasma, particularly in nonplanar geometry, is examined. A ‘‘characteristics’’ analysis is used to show that oscillatory sheath-edge motion in spherical geometry can give rise to sonically propagating, quasineutral compressions and rarefactions of number density.
Vlasov theory of electrostatic modes in a finite length electron column
Authors: Prasad S. A. , O’Neil T. M.
Published: March 2003

A Vlasov theory of low-frequency electrostatic modes in a finite length electron column is presented. The column is assumed to have cylindrical symmetry and flat end surfaces at which the electrons undergo specular reflection. The eigenfrequencies and the eigenfunctions are obtained as a series expansion in the small parameter R/L (where R is a characteristic radius and L the half-length). In zeroth order, the modes are simply the modes for an infinitely long column with axial wavenumbers quantized as k=nπ/L. In first order, these modes are weakly coupled. This means that a k=0 diocotron mode can share in the Landau damping of higher k modes. For certain values of the plasma parameters, two zero-order modes are degenerate and the coupling is strong.
Stability of Langmuir waves in developing ion-sound turbulence
Authors: DuBois D. F. , Pesme D.
Published: March 2003

It is shown, using weak turbulence theory, that Langmuir waves are stabilized by a nonresonant quasilinear scattering of Langmuir waves by ion-sound waves in the presence of a background of developing ion-sound turbulence. This effect, which prevents the conversion of ion-sound turbulence into Langmuir turbulence in this transient regime, was ignored in previous theories. The destabilizing effect of the so-called ‘‘induced bremmsstrahlung’’ of Langmuir waves as proposed by Nambu is furthermore shown to vanish exactly. The need for a renormalized theory in the case of steady-state ion-sound turbulence is discussed.
Excitation of a two-dimensional ion-acoustic soliton
Authors: Gabl E. F. , Bulson J. M. , Lonngren Karl
Published: March 2003

A flat, disk-shaped ion-acoustic soliton has been excited in a large collisionless plasma. The soliton appears to be stable, its amplitude is constant, and its broader width perpendicular to the direction of propagation slowly decreases as the soliton propagates.