bluuebunny/crossref_metadata_2025 huggingface dataset:
Density measurement in compressible flows using off-resonant laser- induced fluorescence
Authors: McDaniel J. C. , Baganoff D. , Byer R. L.
Published: March 2003

Measurement of molecular number density in compressible flows using laser-induced fluorescence is complicated by collisional quenching of the excited state. It is shown that by exciting the fluorescence off-resonance the signal becomes proportional to number density and independent of collisional effects. Quantitative measurement of density in an underexpanded jet of nitrogen is demonstrated using off-resonant fluorescence from iodine seed molecules irradiated with the 514.5 nm line of the argon-ion laser.
Analytic model of the radiation-dominated decay of a compact toroid
Authors: Auerbach Steven P.
Published: March 2003

A simple analytic model of the radiation-dominated decay of a compact torus is presented. Several striking features of various experiments (finite lifetime, linear current decay, and insensitivity of the lifetime to density or stored magnetic energy) are explained by the model. A simple criterion is given for the maximum tolerable impurity density.
An exact solution for the drag of a sphere in low Reynolds number flow with strong uniform suction or blowing
Authors: Dukowicz John K.
Published: March 2003

An analytical expression is obtained for the drag of a sphere in low Reynolds number flow with arbitrarily strong uniform suction or blowing.
Bubble migration in a rotating, liquid-filled sphere
Authors: Annamalai P. , Subramanian R. S. , Cole R.
Published: March 2003

Experiments and analysis concerned with the trajectory of a small gas bubble in a rotating, liquid- filled glass sphere show that the bubble migrates inward toward the axis of rotation, thus providing a possible centering mechanism for hollow shell formation processes. The migration time is found to be approximately proportional to the parameter (1/ω)(ν/a2eω), where ν is the kinematic viscosity of the liquid, ae the bubble radius, and ω the rate of rotation.
Near-field analysis of a compressive supersonic ramp
Authors: Emanuel George
Published: March 2003

Steady, two-dimensional, inviscid, supersonic flow is analyzed for a compressive turn where the wall is contoured to provide a centered compression fan. The focal point of the compression is the origin of the usual (primary) oblique shock wave, a slipstream, and a secondary pressure disturbance. This disturbance can be an expansion, a weak solution shock, or a strong solution shock. In the vicinity of the focal point (the near field) there are seven possibilities, one of which is no real solution. For small wall turn angles, there is a unique near-field solution where the primary shock is the weak solution. In this case the secondary disturbance, whose strength is quite small, is either an expansion or a weak solution oblique shock wave. For larger turn angles, two near-field solutions are possible, and for still larger angles, none. At relatively large wall turn angles, where the usual oblique shock equations still provide an attached solution, the near-field equations do not have a solution when the Mach number is sufficiently large.
New observations on hysteresis effects in Taylor–Couette flow
Authors: Mullin T. , Pfister G. , Lorenzen A.
Published: March 2003

In this experimental study, laser-velocimetry has been used to investigate hysteresis phenomena presented by steady flows of a viscous fluid. Dealing with Taylor–Couette flow between concentric cylinders with differential rotation, the experiments were focused on the change-over between steady states featuring four and six cells as the length of the annular domain is varied continuously. The observations are in broad qualitative agreement with the predictions of abstract mathematical models, but they display new features which appear to be beyond the scope of any quantitative theoretical results yet available.
Nonequilibrium probability distributions for randomly forced two- dimensional flows
Authors: Thompson Philip D.
Published: March 2003

The general Liouville equation for the evolution of the probability distribution of an ensemble of randomly forced inviscid two-dimensional flows is specialized for a particular spectrum of random forcing. In that case, the probability distribution depends only on a linear combination of kinetic energy and enstrophy, and the Liouville equation reduces to a one-dimensional ’’diffusion- like’’ equation that can be solved analytically. Random stirring of a fluid initially at rest is treated as an example.
Heat transfer in a rarefied gas enclosed between parallel plates: Role of boundary conditions
Authors: Loyalka S. K. , Thomas J. R.
Published: March 2003

The influence of boundary conditions of accomodation coefficients and Maxwellian diffuse specular reflection on heat transfer through a rarefied gas enclosed between two parallel plates is examined. An exact expression for heat transfer for accomodation coefficient boundary conditions and the Bhatnagar–Gross–Krook (BGK) model is constructed by using results of Cercignani and Pagani and Thomas, Chang, and Siewert. These results are compared with some variational results of Cipolla and Cercignani and some exact results of Thomas, Chang, and Siewert and Thomas for the BGK model and Maxwellian diffuse specular reflection boundary conditions. It is concluded that the two boundary conditions provide results that agree within about 3% for a range of Knudsen numbers and boundary parameters. It is found that the variational results are remarkably accurate for the BGK model and both types of boundary conditions. Further, it is noted that the heat transfer between parallel plates with different accommodation coefficients at the two surfaces can be calculated exactly by using a harmonic mean for each surface.
Rarefaction wave in relativistic gasdynamics
Authors: Granik A.
Published: March 2003

The steady flow in relativistic gasdynamics is studied. There are two possible regimes of flow: (1) a uniform flow and (2) the rarefaction wave. The rarefaction wave is similar to one studied in the Newtonian limit and is confined to a sectorial region. The maximum angle of this sector is smaller and the pressure drop is greater than in the Newtonian case. For the ultra-relativistic fluid the results are described with simple formulae.
Theory of a spherical probe in a collisionless magnetoplasma
Authors: Rubinstein J. , Laframboise J. G.
Published: March 2003

A theory is presented for a spherical electrostatic probe in a collisionless, Maxwellian plasma containing a uniform magnetic field. The theory yields two upper bounds and an adiabatic limit for collection of the attracted particle species (either electrons or ions). For the repelled species, it yields a lower and an upper bound. The theory is similar in concept to existing theories for cylindrical probes by Laframboise and Rubinstein. It is applicable when the ratio of probe radius to Debye length is small enough, and/or the probe potential is large enough, that no potential barriers exist near the probe. Otherwise, a theory of Sanmartin applies. The attracted-particle current in the adiabatic limit, i.e., when mean gyroradius≪Debye length, shows negative-resistance behavior. One of the upper bounds is based on the use of particle canonical angular momentum conservation to define allowed and forbidden regions for particle orbits, and generalizes an existing theory by Parker and Murphy to include particle thermal motion.
Quasisteady turbulence driven by runaway electrons
Authors: Muschietti L. , Appert K. , Vaclavik J.
Published: March 2003

The evolution of the turbulence driven by runaway electrons has been followed by means of a computer code based on the quasilinear equations. The evolution is not characterized by periodic relaxations as claimed in previous works but ends in a quasisteady turbulent, yet very persistent state, accessible from different initial conditions. This discrepancy is clarified as being due to the excessive stiffness of the moment equations used to demonstrate the relaxations. Moreover, a theory is developed to interpret the quasisteady state found.
Energy conservation theorem for electrostatic systems
Authors: Decyk Viktor K.
Published: March 2003

An energy flux vector for electrostatics is derived, its relation to the Poynting vector is pointed out, and application of the corresponding energy conservation theorem is made to electrostatic simulations.
Effective opacity of dense tokamak plasmas at the second electron cyclotron harmonic
Authors: Lazzaro E. , Ramponi G. , Giruzzi G.
Published: March 2003

The perpendicular propagation of the extraordinary wave near the second electron cyclotron harmonic is analyzed by means of a numerical code embodying a full wave relativistic scheme. Discussion of the results in comparison with the standard WKB ones is performed in view of absorption and emission applications. It is found that in the density regime called the ’’overcritical case’’ the effective optical depth scales, as an effect of the increased interaction of the extraordinary wave with a quasielectrostatic wave, differently from the WKB estimate. An explicit evaluation of reflection and transmission coefficients is given.
The shear Alfvén continuum in the low beta limit
Authors: Kieras C. E. , Tataronis J. A.
Published: March 2003

The set of equations which determines the continuous spectrum of ideal magnetohydrodynamics (MHD) in the limit of low beta for axisymmetric toroidal equilibria is examined. It is shown that when the equilibrium pressure is set equal to zero, the cusp continuum is reduced to a single point ω = 0 in the spectrum, and a single second-order differential equation, which describes the shear Alfvén continuum, survives in this limit. This reduced equation may provide a good representation of the MHD continuum for low beta plasmas.
A boundary-coupled generalization of the Newcomb stability criterion
Authors: Ray Alak , Van Hoven Gerard
Published: March 2003

In magnetohydrodynamic problems in which there are boundaries in the axial or azimuthal symmetry directions, the energy principle involves multiple nonorthogonal trial perturbations. The subsequent Euler–Lagrange coupling of these allowed excitations necessitates a generalization of the Newcomb necessary-and-sufficient stability criterion. This extension, in which the absence of a conjugate point (instead of a simple zero) provides stability, is described in this paper. An important aspect of the proof involves the treatment of certain asymmetries which arise in the presence of the anisotropy caused by the magnetic field. The general method described here has applications to finite-length axial laboratory pinches and to astrophysical plasmas with rooted magnetic fields.
Parametric instabilities of electron cyclotron waves in plasmas
Authors: Tripathi V. K. , Liu C. S.
Published: March 2003

The parametric instabilities of electron cyclotron waves in (i) the EBT ring and (ii) the large tokamaks, e.g., PLT, have been studied. In EBT, the electron cyclotron pump of finite wavenumber k0 decays into two Bernstein modes at the second harmonic cyclotron layer and can account for the heating of the ring in the initial phase. The coupling coefficient for this decay vanishes for a dipole pump, whereas the convective threshold with finite k0 is approximately 200 W/cm2. For large tokamaks, the convective threshold for various decay channels turns out to be approximately 200 kW/cm2 at 3 mm wavelength.
Instabilities induced by variation of Brunt–Väisälä frequency in compressible stratified shear flows
Authors: Narayanan A. Satya , Sachdev P. L.
Published: March 2003

The stability characteristics of a Helmholtz velocity profile in a stably stratified, compressible fluid in the presence of a lower rigid boundary are studied. A jump in the Brunt–Väisälä frequency at a level different from the shear zone is introduced and the variation of the Brunt–Väisälä frequency with respect to the vertical coordinate in the middle layer of the three-layered model is considered. An analytic solution in each of the layers is obtained, and the dispersion relation is solved numerically for parameters relevant to the model. The effect of shear in the lowermost layer of the three-layered model for a Boussinesq fluid is discussed. The results are compared with the earlier studies of Lindzen and Rosenthal, and Sachdev and Satya Narayanan. In the present model, new unstable modes with larger growth rates are obtained and the most unstable gravity wave modes are found to agree closely with the observed ones at various heights.
Some aspects of strongly heated turbulent boundary layer flow
Authors: Cheng R. K. , Ng T. T.
Published: March 2003

Rayleigh scattering and laser Doppler velocimetry were used to study density and velocity statistics in a strongly heated turbulent boundary layer with free stream velocity U∞ = 19 m/sec and wall temperature Tw = 1100 °K. Mean and rms fluctuation profiles of density and velocity were found to be self-preserving, and the velocity profiles were not significantly affected by wall heating. Streamwise velocity profiles, and mean temperature profiles deduced from density data were compared with Townsend’s self-preserving analysis. It was found that wall heating caused a reduction in Reynolds stress within the thermal layer. This phenomenon was explained with reference to the thermal layer structures observed in high speed schlieren movies.
Resistive wall effect on the stability of planar relativistic Brillouin flow
Authors: Chernin David P.
Published: March 2003

Previous treatments of relativistic Brillouin flow stability are extended to the case of nonzero anode resistivity, for the case of wave propagation across the equilibrium magnetic field. It is found that transverse magnetic (TM) waves are unstable at frequencies below the equilibrium cyclotron frequency, unlike the case for a perfectly conducting anode. Transverse electric (TE) waves, always oscillatory in the case of a perfectly conducting anode, are damped; an explicit expression for the damping rate, valid for large conductivities, is given.
Statistics of charged particles in external random longitudinal electric fields
Authors: Doveil F. , Grésillon D.
Published: March 2003

Charged particle diffusion in a given one-dimensional Gaussian random longitudinal electric field is investigated by three different methods; quasilinear theory, resonance broadening theory, and direct numerical integration of trajectories. Analytical predictions of both theories are expressed using a random walk approach. An analytic frequency and wavenumber spectrum of constant shape but variable amplitude, related to experimental observations, is used for comparison between the three methods. They all agree at low amplitude. At larger amplitude, resonance broadening theory shows the right tendency for the maximum diffusion coefficient to increase less rapidly than the mean square field. However, the exact value diverges from quasilinear and resonance broadening predictions as soon as the product of the quasilinear correlation time multiplied by mean bounce frequency increases above 0.4.
Electrostatic plasma wave propagation in a density modulated plasma
Authors: Majeski R. P. , Ellis R. F.
Published: March 2003

A model which treats the formation of wave sidebands on a propagating electron plasma wave in the case where the plasma density is modulated by a low-frequency perturbation is developed. A new mechanism for production of an asymmetric distribution of sidebands appears, and circumstances under which the original wave may reconstitute itself are described. An experimental study of electrostatic plasma waves in a weakly ionized plasma, where the density is modulated either by external means or by the presence of an unstable drift wave, is in agreement with the model.
Stimulated Raman scattering in large plasmas
Authors: Phillion D. W. , Banner D. L. , Campbell E. M. , Turner R. E. , Estabrook K. G.
Published: March 2003

In long pulse, high-energy experiments (4000 J, 2 nsec, 5×1014 W/cm2, 1.064 μm) at the Shiva laser facility, several percent of the laser light has been observed to be Raman scattered. The spectrum of the Raman-scattered light was measured from 1.48 to 2.17 μm. The Raman scattering occurred principally at electron densities much lower than the quarter-critical electron density. The high-energy electrons expected in Raman scattering were observed indirectly in these experiments via their bremsstrahlung radiation. Additional experiments show that the Raman instability has a much lower intensity threshold for longer laser pusle length and larger laser spot size. Raman light measurements for 5320 Å irradiated disk target experiments are also reported. The light near 2λ0 or 1.064 μm had both a red- and blue-shifted component. At high intensities, Raman scattering also occurred in the very underdense plasma.
Nonlinear effects of multifrequency hydrodynamic instabilities on ablatively accelerated thin shells
Authors: Verdon C. P. , McCrory R. L. , Morse R. L. , Baker G. R. , Meiron D. I. , Orszag S. A.
Published: March 2003

Two-dimensional numerical simulations of ablatively accelerated thin-shell fusion targets, susceptible to rupture and failure by Rayleigh–Taylor instability, are presented. The results show that nonlinear effects of Rayleigh–Taylor instability are manifested in the dynamics of the ’’bubble’’ (head of the nonlinear fluid perturbation) rather than in the dynamics of the spike (tail of the perturbation). The role of multiwavelength perturbations on the shell is clarified, and rules are presented to predict the dominant nonlinear mode-mode interactions which limit shell performance. It is also shown that the essential dynamics of strongly driven flows are governed by the classical Rayleigh–Taylor instability of an ideal, incompressible, thin fluid layer.
Observation of the parametric decay instability during electron cyclotron resonance heating on the Versator II tokamak
Authors: McDermott F. S. , Bekefi G. , Hackett K. E. , Levine J. S. , Porkolab M.
Published: March 2003

Observations are reported on a nonlinear, three-wave interaction process occurring during high-power electron cyclotron heating in the Versator II tokamak. The measured spectra and the threshold power are consistent with a model in which the incident power in the extraordinary mode of polarization decays at the upper-hybrid resonance layer into a lower-hybrid wave and an electron Bernstein wave.
Effect of finite aspect ratio on the neoclassical ion thermal conductivity in the banana regime
Authors: Chang C. S. , Hinton F. L.
Published: March 2003

By considering the effect of finite aspect ratio, the neoclassical ion thermal conductivity has been found to be significantly larger than reported by Hazeltine, Hinton, and Rosenbluth. A simple correction to the expression of Hinton and Hazeltine is suggested.
Accuracy of the mean field approximation and the physical effect of Prandtl number in Bénard convection
Authors: McDonough J. M. , Catton I.
Published: March 2003

The mean field equations, which contain no explicit Prandtl number dependence, are shown to provide reasonably accurate predictions of average heat transport for Rayleigh numbers from 2000 to 18000 over a wide range of Prandtl numbers, provided physically observed wavenumbers are used in the calculations. It is concluded from this that the physical significance of Prandtl number in Bénard convection is principally in setting the ’’preferred’’ wavenumber, i.e., the size of the convection cell.
The nonaxisymmetric (buckling) flow regime of fast capillary jets
Authors: Stockman Michael G. , Bejan Adrian
Published: March 2003

This paper reports an experimental study of the nonaxisymmetric flow of a fast liquid jet discharging into the atmosphere. The nonaxisymmetric shape of the jet was photographed and subjected to a wavelength analysis. The results of the wavelength analysis demonstrate that the jet shape is governed by a narrow band of wavelengths associated with a characteristic value λmax which scales with the jet diameter D. It is shown that the experimental observations are in agreement with predictions based on hydrodynamic stability theory and buckling theory.
On the linear inviscid stability of rotating Poiseuille flow
Authors: Maslowe S. A. , Stewartson K.
Published: March 2003

This paper continues an earlier study by S. A. Maslowe of inviscid asymmetric disturbances to flow with rotation along a pipe. It is shown that there is a class of neutral modes with a critical layer near the wall of the pipe when the azimuthal wavenumber is large and negative. Analytic and numerical properties of these modes are studied and found to lead to consistent results, notable among which is the severe singularity in the disturbance velocity at the critial level. Some of the properties of the most unstable modes are also examined.
Turbulence structure in unstably-stratified open-channel flow
Authors: Komori Satoru , Ueda Hiromasa , Ogino Fumimaru , Mizushina Tokuro
Published: March 2003

The buoyancy effects on the turbulence structure in unstably-stratified shear flow generated by cooling the top of the layer in an open channel were investigated. Measurements were made of the characteristics of the turbulent fluctuations; turbulence intensities, correlation coefficients, joint probability density functions, and coherence-phase relationships. The results show that the buoyancy-driven motions cause substantial changes in the turbulence structure. Of the buoyancy-driven motions, the intermittent downward motion contributes mainly to the turbulent transport of heat in the vertical direction. A parameter which characterizes the stability dependence of the turbulence structure is the local gradient Richardson number Ri, and the turbulence quantities are well correlated with Ri. A theoretical spectral equation model is applied to the stratified flow and well compared with the experimental results.
A nonstationary solution to Liouville’s equation for a randomly forced two-dimensional flow
Authors: Carnevale George F.
Published: March 2003

Liouville’s equation for randomly forced two-dimensional flow with Rayleigh friction is examined. An exact nonstationary solution is presented for a special form of the forcing and zero energy initial condition. This solution is such that the fluctuation-dissipation relation is valid at all times.
Motion of a sphere in a rarefied gas
Authors: Lea K. C. , Loyalka S. K.
Published: March 2003

The problem of the motion of a single spherical particle in a rarefied gas is considered, and results for the velocity profiles and the drag forces for all rarefactions are obtained using the Bhatanagar–Gross–Krook model and diffuse reflection at the surface of the sphere. Results for the drag forces are in good agreement with previously available results of Cercignani, Pagani, and Bassanini. The velocity profile results are also in good agreement with the results of Sone and Aoki who had considered the slip limit.
Enforced gyroresonance of electrons in large-amplitude whistler waves
Authors: Wang Y. D.
Published: March 2003

Phase trapping of ’’cold’’ electrons in a finite amplitude whistler wave is possible.
Nonlinear saturation of drift waves in sheared magnetic field
Authors: Tange T. , Nishikawa K. , Sen A. K.
Published: March 2003

A nonlinear saturation mechanism of electrostatic drift waves in a sheared magnetic field is presented. The basic physical mechanism is the coupling of the unstable, lowest radial mode to higher modes which are damped because of their short radial wavelengths. Stationary spectra are obtained analytically and applied to the current-driven drift wave instability.
Decay instability of an ion-cyclotron wave in a plasma
Authors: Sharma O. P. , Tripathi V. K. , Liu C. S.
Published: March 2003

An electromagnetic ion-cyclotron wave, near the second harmonic cyclotron frequency of majority ions in the central region of a tokamak, is seen to decay efficiently into two Bernstein modes. The coupling coefficient for this decay vanishes for a dipole pump, hence a rigorous kinetic theory for decay instabilities of a finite wavenumber pump is developed. For parameters characterizing the Princeton Large Torus (PLT) experiment, the uniform medium growth rate at 200 kW turns out to be 3 μsec, whereas, the threshold due to plasma inhomogeneity is 100 kW. The nonresonant decay into ion acoustic waves is also significant, though relatively less important. For a pump wave near the cyclotron frequency of majority ions, nonlinear Landau damping and oscillating two stream instability turn out to be very important.
Plasma–wall transition in an oblique magnetic field
Authors: Chodura R.
Published: March 2003

The effect of a magnetic field on the transition layer between a plasma and an absorbing wall is studied. A numerical model is used which simulates the motion of plasma particles in the electric and magnetic fields for a prescribed particle influx at the plasma boundary. Bohm’s condition for the existence of a monotonic profile of the layer is generalized. The transition layer proves to have a double structure comprising a quasineutral magnetic presheath preceding the electrostatic Debye sheath. The magnetic presheath scales with the ion gyroradius at the sound speed and with the angle of the magnetic field. The total electric potential drop between plasma and wall proves to be fairly insensitive to the magnitude and angle of the magnetic field.
Strained spiral vortex model for turbulent fine structure
Authors: Lundgren T. S.
Published: March 2003

A model for the intermittent fine structure of high Reynolds number turbulence is proposed. The model consists of slender axially strained spiral vortex solutions of the Navier–Stokes equation. The tightening of the spiral turns by the differential rotation of the induced swirling velocity produces a cascade of velocity fluctuations to smaller scale. The Kolmogorov energy spectrum is a result of this model.
Unsteady natural convection about a sphere at small Grashof number
Authors: Sano Takao
Published: March 2003

Unsteady low-Grashof-number natural convection about a sphere is studied when the surface temperature of the sphere is suddenly increased. It is shown that the solutions for the velocity and temperature are, respectively, expressed in terms of three expansions reflecting the existence of three distinct regions in the (r, t) plane, r and t being a nondimensional radial coordinate and nondimensional time, respectively.
Determination of the density perturbation at the wall for the Rayleigh problem
Authors: Ganapol Barry D.
Published: March 2003

The Rayleigh problem, a fundamental time-dependent problem of gas kinetics, is studied in the context of the constant collision frequency BGK model. An analytical result, expressed in terms of the stationary solution, is obtained and numerically evaluated. Asymptotic solutions in both the small and large time limit are also presented and compared to results of other theories.
Stellarator equilibria with weak helical curvature
Authors: Miller Guthrie
Published: March 2003

General low-β stellarator equilibria having weak helical curvature are calculated analytically. Important properties of these equilibria are also calculated; separatrix location, stability toward transverse shifts, and flute interchange stability.
Effect of ion collisionality on ion-acoustic waves
Authors: Randall C. J.
Published: March 2003

The frequency and damping ratio of ion-acoustic waves in plasmas with moderately collisional ions is obtained by numerical solution of the linearized Fokker–Planck equation for electron-ion temperature ratios, ZTc/Ti=4, 8, 16, and 32.
Self-consistent treatment of resonance absorption in a streaming plasma
Authors: Takabe H. , Mulser P.
Published: March 2003

Resonance absorption in a streaming layered plasma is theoretically studied by means of the capacitor model. The wave and density profiles are calculated self-consistently by using the analogy of the wave equation to the Newton equation of a point mass in a two-dimensional potential. The scaling laws of the wave and density are shown as functions of the driver field, and, in addition, the wave-breaking condition is also studied.
Multimode, time-dependent analysis of quasi-optical gyrotrons and gyroklystrons
Authors: Bondeson Anders , Manheimer Wallace M. , Ott Edward
Published: March 2003

A multimode, time-dependent analysis is carried out for the quasi-optical gyrotron to assess its promise as a high-power millimeter wave generator. It is shown that both relatively high efficiences and single mode operation are possible. An alternate device configuration, the quasi-optical gyroklystron, is also analyzed and is found to increase efficiency while decreasing the requirement on high-frequency field amplitude necessary for efficient operation.
Free electron laser instability for a relativistic solid electron beam in a helical wiggler field
Authors: Uhm Han S. , Davidson Ronald C.
Published: March 2003

The free electron laser instability for a solid relativistic electron beam propagating in combined transverse helical wiggler and uniform axial guide fields is investigated within the framework of the linearized Vlasov–Maxwell equations. It is assumed that ν/γb≪1, where ν is Budker’s parameter and γbmc2 is the electron energy. Stability properties are investigated for the choice of equilibrium distribution function in which all electrons have the same value of the linear combination of transverse and helical invariants, C⊥ −2γbmωbCh =const., and a Lorentzian distribution in the axial invariant Cz. (Here ωb is a constant.) The instability growth rate is calculated including a determination of the optimum value of the ratio of beam radius to conducting wall radius (R0/Rc) for maximum growth. It is found that the maximum growth rate for a solid electron beam is comparable to that for a hollow beam with similar parameters. Moreover, the introduction of a small axial momentum spread (Δ/γbmc≊  a few percent) significantly reduces the instability growth rate.
An example for period doubling bifurcations of internal gravity waves
Authors: Ma Yan-Chow
Published: March 2003

It is shown that when the wavelength of the internal gravity wave in the propagation direction is close to some integer multiple of the characteristic half-width of the background Brünt-Väisälä frequency, a sequence of period doubling bifurcations occurs. This process transfers energy from high horizontal, low vertical wavenumber modes to low horizontal, high vertical wavenumber modes.
High-resolution spectral measurements of fluctuating ωp
e emission from Alcator
Authors: Hutchinson I. H. , Kissel S. E.
Published: March 2003

High-resolution spectral measurements have been made of an intense fluctuating radiation feature from Alcator C tokamak plasmas using a rapid scan Fabry–Perot interferometer. The emission occurs near the electron plasma frequency and has a very narrow band width, Δω/ω≤1% typically. Occasionally correlation of fluctuating emission with m=1 MHD oscillations is observed.
Low-frequency oscillations in the central cell of the TMX tandem mirror experiment
Authors: Hooper E. B. , Hallock G. A. , Foote J. H.
Published: March 2003

Two modes of low-frequency oscillations have been observed in the central cell of the tandem mirror experiment (TMX). A mode at about 7 kHz has m=1 and probably drives radial transport at large radii. The mode identification is uncertain. A mode at about 13 kHz has m=0. The two end plugs oscillate 180° out of phase with each other and in phase with the amplitude (envelope) of the ion cyclotron frequency oscillations in each plug. This mode is identified as a sound wave; the frequency is apparently locked to the E×B rotation frequency, probably through an associated m=1 component. Neither mode severely limits confinement in the central cell, and both may be controllable. The lower-frequency mode is sensitive to the density profile and to the fueling and is not always present. The higher-frequency mode may be less important (or absent) in devices in which the plugs are stable at the plug’s ion cyclotron frequencies.
Measurements of resistive wall and plasma drag on the axial translation of field-reversing E layers
Authors: Rej D. J. , Parker M. R. , Fleischmann H. H.
Published: March 2003

Results from detailed measurements and related analyses of the axial translation of strong E layers in the RECE–Christa experiment testing the interaction of the rings with resistive image currents in the wall and in the ring plasma are presented. The experiments were performed under a variety of experimental conditions including cases with (i) Fw≫Fp, (ii) Fw≪Fp, and (iii) Fw≂Fp (Fw and Fp denoting the retarding image forces due to interaction with wall currents and plasma currents, respectively). In the respective analyses, theoretical ring velocities are calculated from simultaneously measured ring, wall, and plasma parameters. In all cases, good qualitative and quantitative agreement [10%–20% for case (i), and 30%–50% for (ii) and (iii)] between the observed and the theoretically calculated velocities is found.
A new millimeter free electron laser using a relativistic beam with spiraling electrons
Authors: Grossman A. , Marshall T. C. , Schlesinger S. P.
Published: March 2003

For a dense, relativistic beam of spiraling electrons passing along a magnetic field B0 and through an undulator, it is demonstrated theoretically and experimentally that radiation will occur at frequency ωs given by ωs≊(ωc+2πV∥/l0)(1−V∥/c)−1, where ωc=‖eB0/mcγ‖ is the electron gyrofrequency, l0 is the undulator period, γ is the relativistic factor, γ≡[1−(V2∥/c2)  −(V2⊥/c2)]−1/2, V∥ and V⊥ are the electron velocity components parallel and transverse to the guiding magnetic field B0. Two experiments are described, one of which is in a practical oscillator configuration. Substantial power is emitted having wavelength 1.5 mm.
Estimate of shock thickness based on entropy production
Authors: Thompson Philip A. , Strock Thomas W. , Lim David S.
Published: March 2003

The shock thickness is estimated for a stationary shock in an ideal gas by equating the internal entropy production to the entropy increase found from the Rankine–Hugoniot equation. For elementary assumed profiles, the thickness is expressed by a simple formula. For realistic dependence of viscosity on temperature, results are in qualitative agreement with experiment.
Nonlinear effects at the upper-hybrid layer
Authors: Sharma R. P. , Shukla P. K.
Published: March 2003

It is well-known that an extraordinary wave propagating along the density gradient mode converts into an electron Bernstein wave at the upper-hybrid layer. This paper reports various kinds of nonlinear effects associated with the electrostatic waves. Specifically, numerous possibilities of the three-wave decay interaction and the process of modulational instabilities are considered. The growth rate and threshold are obtained analytically. A comparative study of the growth rates and thresholds is carried out so as to emphasize the importance of slow plasma response. Also investigated were the effects of the pump profile, and the plasma nonuniformities on the temporal increment. Application of this work to tokamak devices, laboratory plasmas, as well as space plasmas, is discussed.
Integral equation formulation of electromagnetic mode equations
Authors: Mahajan S. M.
Published: March 2003

The coupled set of differential equations describing the electromagnetic perturbations in tokamak plasmas is reduced to a single simple integral equation with a symmetric kernel. Obvious analytical and computational advantages are discussed.
Electrostatic waves in general magnetic field configurations
Authors: Chen Liu , Tsai Shih-Tung
Published: March 2003

A scheme for investigating linear electrostatic waves in general magnetically confined plasmas is presented. The scheme is a generalization of the low-frequency (less than the cyclotron frequency) gyrokinetic formalism of Rutherford and Frieman as well as Taylor and Hastie to arbitrary frequencies. Governing integral wave equations for slab plasmas with magnetic shear as well as axisymmetric tokamaks are then derived to illustrate the applications.
A thermal oscillating two-stream instability
Authors: Dysthe K. B. , Mjo/lhus E. , Pécseli H. L. , Rypdal K.
Published: March 2003

A theory for the oscillating two-stream instability, in which the Ohmic heating of the electrons constitutes the nonlinearity, is developed for an inhomogeneous and magnetized plasma. Its possible role in explaining short-scale, field-aligned irregularities observed in ionospheric heating experiments is emphasized. The theory predicts that the initial growth of such irregularities is centered around the level of upper hybrid resonance. Furthermore, plane disturbances nearly parallel to the magnetic meridian plane have the largest growth rates. Expressions for threshold, growth rate, and transverse scale of maximum growth are obtained. Special attention is paid to the transport theory, since the physical picture depends heavily on the kind of electron collisions which dominate. This is due to the velocity dependence of collison frequencies, which gives rise to the thermal forces.
Drift wave turbulence in a low-order k space
Authors: Terry P. W. , Horton W.
Published: March 2003

In the low-order isotropic k space introduced by Kells and Orszag for the two-dimensional Euler equation, the evolution of the fluctuations arising from the electron drift wave instability is studied. The two-dimensional drift wave model contains the E×B and polarization drift nonlinearities in the hydrodynamic ions and linear, dissipative electrons. The strength of the electron dissipation is shown to determine the spectral width and the level of the fluctuations.
The growth of the tearing mode: Boundary and scaling effects
Authors: Steinolfson R. S. , Van Hoven G.
Published: March 2003

The linear development of the resistive tearing instability in a sheet pinch is investigated numerically. Particular emphasis is placed on effects which differentiate magnetic tearing in astrophysical situations from that in laboratory devices. These include extreme values of the parameters determining the mode growth and a variety of boundary conditions. Eigenfunction profiles for long and short wavelengths are computed and the applicability of the ‘‘constant Ψ’’ approximation is investigated. Nearby conducting walls tend to validate this condition and reduce the growth rate, especially for the long wavelength modes which, otherwise, disturb a larger region of the plasma than do short wavelength modes. Finally, the growth rate p is computed for values of the magnetic Reynolds number S up to 1012 and of the dimensionless wavelength parameter α down to 10−3. The results demonstrate, without approximation, the S2/5 scaling of p at large α (constant Ψ) and the S2/3 scaling at small α (nonconstant-Ψ). The α and S variation of the growth maximum, which would provide the dominant excitation in the absence of nearby boundaries, is specified for both single- and multiple-tearing layers. The growth maximum is shown to occur in a parametric regime where the constant Ψ approximation is not valid.
Influence of magnetic shear on the lower-hybrid drift instability in finite β plasmas
Authors: Huba J. D. , Ganguli G.
Published: March 2003

A self-consistent theory of the lower-hybrid-drift instability in finite β plasmas containing magnetic shear is presented. The important finite β effects included are (1) the coupling of electrostatic and electromagnetic oscillations and (2) the orbit modification of the electrons due to ∇B. It is found that the effect of electromagnetic coupling is a destabilizing influence on the instability in a sheared field. On the other hand, the effect of electron orbit modification (i.e., electron ∇B drift-wave resonances) is a stabilizing influence. The key parameter which dictates which effect is more important is Te /Ti. In the limit Te ≪Ti, the electromagnetic effect dominates, while for Te ≥Ti, the ∇B electron drift-wave resonance is more important. The relevance of these results to reversed field plasmas is discussed.
Drift- and lower-hybrid eigenmodes in a plasma cylinder
Authors: Marchand R. , Zhang C. F. , Lee Y. C.
Published: March 2003

An integral formulation for the normal mode structure for drift-type electrostatic perturbation in a collisionless cylindrical plasma with multi-Gaussian profiles immersed in a uniform magnetic field was developed. No ordering is assumed for the plasma width parameter a, compared with a typical ion gyroradius ρi, or for the mode frequency ω compared with the ion gyrofrequency Ωi. The integral equation is solved numerically in the low-frequency drift wave limit as well as the lower-hybrid limit. Comparisions with previous slab calculations and with approximate differential calculations are presented.
Mirror stability of a hot electron ring plasma
Authors: Tsang K. T.
Published: March 2003

The free energy associated with the anisotropy in the velocity space of a microwave-heated hot electron distribution can drive the mirror mode unstable. The real frequency of this instability is of the same order as the diamagnetic drift frequency of the hot electrons.
Theory of ion-cyclotron resonances and diffusion at high energy in the 2XIIB magnetic mirror experiment
Authors: Tagger M. , White R. C.
Published: March 2003

It is shown that a monochromatic ion-cyclotron wave can cause stochastic ion motion at high energy in a mirror plasma, as observed in the 2XIIB Magnetic Mirror Experiment. In earlier studies Rosenbluth, Timofeev, and Smith predicted regular (superadiabatic) motion at energies above 10 or 20 keV, in a slab (or small Larmor radius) model. Particle motion is described here by a Hamiltonian valid for arbitrary Larmor and guiding-center radii, in cylindrical geometry, and in either axisymmetric or quadrupole magnetic fields. The weakness of the self-dug magnetic well at moderate plasma pressure is used as an expansion parameter. Analytical and numerical calculations show large, stochastic changes in the particles’ kinetic energy, in the range of parameters observed in the experiment.
Ballooning modes in ELMO Bumpy Torus
Authors: Pao Young-ping
Published: March 2003

A high mode number normal mode analysis is carried out for a straight bumpy cylinder model for the ELMO bumpy torus. Ballooning instability and steady oscillations are examined.
Electron energy transport in ion waves and its relevance to laser-produced plasmas
Authors: Bell A. R.
Published: March 2003

Electron energy transport in plasmas is examined in the context of ion waves which are intermediate between collisionless isothermal ion acoustic waves and collisional adiabatic sound waves. The conductivity is found to be much less than the Spitzer-Härm result for wavelengths less than 1000 electron mean free paths. This is expected to be relevant to laser-produced ablating plasmas in which the temperature can vary considerably over a distance of 10 to 100 mean free paths. The reduction in conductivity is independent of the wave amplitude thus differing from the reduction due to saturation found recently by numerical solution of the Fokker–Planck equation. At short wavelengths the heat flow approaches an upper limit which depends on the phase velocity of the wave. Diffusive ion wave damping is strong over a large range of wavelengths.
Parametric excitation of ion-sound quasimodes during lower-hybrid heating experiments in tokamaks
Authors: Takase Yuichi , Porkolab Miklos
Published: March 2003

Parametric decay of a lower-hybrid pump wave into another lower-hybrid wave and a low-frequency ion-sound quasimode (ωR≊k∥vti) is studied. Such an instability may be excited during high-power lower-hybrid heating experiments in tokamak plasmas and may lead to strong modification of the incident n∥ spectrum near the plasma periphery. Such an instability could explain the broadened and downshifted frequency spectrum and phase-independent heating observed in the Alcator A tokamak experiments. Although the growth rate for this decay increases like γ+Γ2∝E20 for powers slightly above threshold (Γ2 is the linear damping rate at the lower sideband and E0 is the pump electric field), for powers well above threshold (such that γ≳ωR)γ∝E2/30, and the convective thresholds are rather high. However, for inaccessible n0∥, the pump wave power is expected to accumulate on the outer surface of the plasma column where the growth rate is large and the convective thresholds are significantly reduced. In such a case the threshold pump power can become quite low and may be exceeded in experiments such as Alcator A.
The effect of the bouncing motion of electrons on electron cyclotron resonance heating
Authors: Kuo S. P. , Cheo B. R.
Published: March 2003

Cyclotron resonance heating of bouncing electrons by an obliquely incident wave field is analyzed. Continuous interaction between electrons and wave fields throughout the entire range of electron excursion has been considered in the analysis. The results show that the bouncing motion of electrons serves to alleviate the detuning effect of frequency mismatch and efficient heating can be achieved. The kinetic temperature is found to increase algebraically in t2 for the fundamental resonance and exponentially in t for the second harmonic heating. Due to the interaction between electrons and the parallel component of the wave electric field, heated electrons tend to focus to the midplane and their excursion amplitudes also become filamented in the steady state.
Ballooning modes in quadrupole tandem mirrors
Authors: Kaiser Thomas B. , Pearlstein L. Donald
Published: March 2003

Ballooning modes in tandem mirrors with quadrupole vacuum field symmetry are investigated in the limits of zero gyroradius and large aspect ratio. The eigenmode equation derived from the guiding-center energy principle is reduced to one dimension by the use of an eikonal representation of the cross-field variation of the perturbation. The eigenvalue problem is solved numerically and the structure of the local dispersion relation examined in order to derive conditions for marginal stability. Applications to the TMX-U and MFTF-B tandem-mirror experiments are presented.
Reduced equations for internal kinks in tokamaks
Authors: Izzo R. , Monticello D. A. , Strauss H. R. , Park W. , Manickam J. , Grimm R. , DeLucia J.
Published: March 2003

A reduced set of ideal magnetohydrodynamic equations is derived for large-aspect-ratio, low-β tokamaks that adequately describes the linear and nonlinear evolution of ideal internal kink modes in tokamaks.
Anisotropic pressure and finite hot-electron Larmor radius effects on hot-electron ring stability
Authors: Tsang K. T. , Lee X. S. , Catto P. J.
Published: March 2003

The effect of the anisotropic pressure of a hot-electron plasma on ballooning-interchange and compressional-Alfvén modes is investigated. General eigenmode equations for these modes are derived in the eikonal and high phase velocity limits with finite gyroradius effects retained. A local dispersion relation is obtained in the flute limit for an isotropic Maxwellian background plasma with a bi-Maxwellian hot-electron population. Stability is investigated both analytically and numerically for this high phase velocity and/or high precessional frequency model.
The role of the hot-electron distribution function in the stability of the ELMO Bumpy Torus
Authors: Nguyen K. , Kammash T.
Published: March 2003

Several recent studies of ELMO Bumpy Torus (EBT) stability show that conflicting results can emerge when different distribution functions for the hot-electron component are used. In this paper the role of the distribution function is established by examining the stability of various modes with the aid of two models. In the ‘‘slab’’ model where the magnetic field curvature is simulated by a gravitational force it is shown that the stability of the compressional Alfvén wave is insensitive to the distribution function while the interacting interchange mode is sensitive. In the ‘‘local’’ approximation in which the curvature effects enter in a natural way it is seen that the interchange modes are insensitive so long as the anisotropy of the hot electrons is large, while other modes reflect dependence on the distribution function. Finally it is demonstrated that in the ‘‘deep well’ case the results for both modes are independent of the model and of the distribution function.