bluuebunny/crossref_metadata_2025 huggingface dataset:
Parametric instabilities in the fast-wave heating of tokamaks
Authors: Tripathi V. K.
Published: March 2003

A high-power whistler wave launched into a tokomak is susceptible to resonant decay into ion-cyclotron and lower-hybrid waves when the lower-hybrid resonance layer ωlh=ω0 exists in the outer region of the plasma x2/a2≳0.6, where ω0 is the frequency of the whistler, ωlh is the lower-hybrid frequency, x is the distance away from the center of the plasma, and a is the minor radius. For Princeton Large Torus parameters, this requires a line average density n≳6×1013 cm−3 when the whistler frequency ω0≂800 MHz and the threshold pump power for the onset of the instability is 100 kW. The threshold power is determined by the convection losses and it increases nonlinearly with the temperature and very rapidly with decreasing density. The parametric instability would tend to deposit the momentum and energy of the pump wave in the outer region of the plasma.
Limiter stabilization of high-beta external kink-tearing modes
Authors: Lee J. K. , Ohyabu N.
Published: March 2003

The stabilizing effects of finite-width poloidal limiters, toroidal limiters, and general mushroom limiters are examined for high-beta finite resistivity tokamak plamas in free boundary. When the plasma pressure and resistivity are small, a poloidal limiter is effective in reducing the growth rate even with a small limiter size, while a toroidal limiter requires a large size for a comparable effect. As the plasma pressure or resistivity increases, a toroidal limiter becomes more effective in reducing the growth rate than a poloidal limiter of the same size. A small optimized mushroom limiter might have a stabilizing effect similar to a conducting shell.
Current drive and helicity injection
Authors: Jensen T. H. , Chu M. S.
Published: March 2003

Magnetic configurations with currents parallel to the magnetic field need means for current drive. The concepts of transport of helicity are found useful for considering methods for current drive. The paper describes a formalism for considering transport of helicity, methods of injection of helicity, and specific examples of arrangements for current drive.
A study of the stability of the Z pinch under fusion conditions using the Hall fluid model
Authors: Coppins M. , Bond D. J. , Haines M. G.
Published: March 2003

The Hall fluid model (a quasineutral two-fluid model with Te=0) is used to investigate the effect of the Hall term on the m=0 instability in a pure Z pinch. The problem is treated numerically by a linearized initial value code. Two different equilibria are investigated. The growth rate of the fastest growing magnetohydrodynamic mode is increased for one equilibrium and reduced for the other by the inclusion of the Hall term, and in the second case new modes with high growth rates are found. The possibility of Hall term destabilization of MHD stable equilibria is suggested.
Electron-cyclotron heating in a pulsed mirror experiment
Authors: Mauel M. E.
Published: March 2003

Experimental measurements of electron-cyclotron resonance heating (ECRH) of a highly ionized plasma in mirror geometry is compared to a two-dimensional, time-dependent, Fokker–Planck simulation. Measurements of the absorption strength of the electrons and of the energy confinement of the ions helped to specify the parameters of the code. The electron energy distribution is measured with an end-loss analyzer and a target x-ray detector. These characterize a non-Maxwellian distribution consisting of ‘‘passing’’ (10 eV<Te,p<30 eV), ‘‘warm’’ (50 eV<Te,w<300 eV), and ‘‘hot’’ (1.2 keV<Te,h<4.0 keV) electron populations. The temperature and fractional densities of the warm and hot populations depend on the absorbed power and total density. A similar distribution is calculated with the simulation program that reproduces the end-loss and x-ray signals. Both the experimental measurments and the simulation are described.
Numerical study of the precessional instabilities in a symmetric tandem mirror with hot-electron end cells
Authors: Tsang Kang , Lee X. S. , Hafizi B. , Antonsen T. M.
Published: March 2003

The axial eigenmode equation for a symmetric tandem mirror with hot electron end cells is solved numerically in a high-beta long-thin equilibrium. Stability of the precessional ballooning modes is studied in detail. It is found that stable regions in parameter space are severely restricted even for moderately long and dense center cell. OFF
Plasma current and conductivity effects on hose instability
Authors: Lampe Martin , Sharp William M. , Hubbard Richard F. , Lee Edward P. , Briggs Richard J.
Published: March 2003

Hose instability dispersion relations, which include a self-consistent treatment of the spatial and temporal evolution of plasma conductivity and plasma current, are derived for a relativistic beam propagating in weakly ionized gas. A simplified conductivity model is used which neglects temperature dependence of the electron mobility. In some regimes the results are dramatically different from those found previously for a beam propagating in a fixed conductivity channel. For example, the hose growth rate is found to decrease with increasing current Ib for a beam propagating in initially neutral gas, even though the plasma return current fraction increases rapidly with Ib. As another example, it is found that an externally driven discharge current can completely eliminate hose instability in a fixed conductivity channel, but causes only a weak decrease in growth rate when the plasma conductivity is modeled self-consistently. OFF
Turbulent structure in the edge plasma of the TEXT tokamak
Authors: Ritz Ch. P. , Bengtson Roger D. , Levinson S. J. , Powers E. J.
Published: March 2003

A reversal has been observed in the mean phase velocity of the turbulent fluctuations in the edge plasma of the TEXT tokamak. The observations can be described by a model in which the wave velocity in the lab frame is dominated by a nonuniform Er×B velocity and a gradient driven drift. The measurements also exhibit a localized instability which occurs in the region of maximum velocity shear.
Electron plasma density deduced from lower oblique resonance measurements
Authors: Thiel J. , Storey L. R. O. , Lebreton J. P.
Published: March 2003

In a magnetoplasma, the amplitude of the lower oblique resonance in the mutual impedance of two electric antennas is very sensitive to the plasma frequency. This property can be used to measure the electron density in a simple way when the electron temperature and the magnetic field are known, provided also that the probe has been properly calibrated. As an application, densities are deduced from measurements made by a mutual-impedance probe on a rocket in the auroral ionosphere. In the future, better practical design and theoretical modeling of the probe should make calibration unnecessary.
Half- and three-halves harmonic measurements from laser-produced plasmas
Authors: Turner Robert E. , Phillion Donald W. , Lasinski Barbara F. , Campbell E. Michael
Published: March 2003

Spectral measurements of the 3/2 harmonic emissions from 1.06 μm irradiated disk targets, and the 1/2 harmonic emissions from 0.53 μm irradiations, are presented. The splitting of the double-peaked spectra show a distinct target Z dependence, for constant laser irradiation parameters. Possible generation mechanisms in terms of the two-plasmon decay instability are discussed.
Thermal transport measurements in 1.05 μm laser irradiation of spherical targets
Authors: Yaakobi B. , Delettrez J. , Goldman L. M. , McCrory R. L. , Marjoribanks R. , Richardson M. C. , Shvarts D. , Skupsky S. , Soures J. M. , Verdon C. , Villeneuve D. M. , Boehly T. , Hutchinson R. , Letzring S.
Published: March 2003

Transport and implosion experiments have been conducted on the OMEGA 24-beam, uniform-irradiation facility. Thermal transport in spherical irradiation was found to be different than in comparable, single-beam target irradiation and could not be described in terms of a flux-inhibited model. Deep energy deposition in spherical irradiation (by electrons on the tail of the thermal velocity distribution) was found to lead to a temperature profile which is not as steep as predicted by a flux-inhibited model. This apparently leads to more explosive implosion (i.e., higher core temperature) than predicted by using such a model.
Experimental observations of nonlinearly enhanced 2ωUH electromagnetic radiation excited by steady-state colliding electron beams
Authors: Intrator Tom , Hershkowitz Noah , Chan Chung
Published: March 2003

Counterstreaming large-diameter electron beams in a steady-state laboratory experiment are observed to generate transverse radiation at twice the upper-hybrid frequency (2ωUH) with a quadrupole radiation pattern. The electromagnetic wave power density is nonlinearly enhanced over the power density obtained from a single beam–plasma system. Electromagnetic power density scales exponentially with beam energy and increases with ion mass. Weak turbulence theory can predict similar (but weaker) beam energy scaling but not the high power density, or the predominance of the 2ωUH radiation peak over the ωUH peak. Significant noise near the upper-hybrid and ion plasma frequencies is also measured, with normalized electrostatic wave energy density WES/neTe≊10−2.
Turbulent heating and anomalous k spectrum in the quasilinear evolution of current-driven ion-acoustic instability in a magnetic field
Authors: Ishihara O. , Hirose A.
Published: March 2003

Time evolution of ion-acoustic instability driven by a current along a magnetic field is studied within the quasilinear framework. Coupled quasilinear equations for the ion and electron distribution functions are solved numerically in a self-consistent manner for a wide range of magnetic field. Efficient electron heating both parallel and perpendicular to the magnetic field is observed. The heating perpendicular to the field is due to the electron cyclotron resonance (anomalous Doppler effect). Ions form a high-energy tail strictly through quasilinear mechanisms, as previously found in one-dimensional analysis. The tail extends in the direction perpendicular to the magnetic field as well as parallel, and is found to be responsible for quasisteady saturation of the turbulence level and also for the anomalous k spectrum dominated by cross-field modes (k⊥ >k∥).
Lagrangian formulation of transport theory: Like particle collisional transport and variational principle
Authors: Cohen R. H. , Hizanidis K. , Molvig K. , Bernstein Ira B.
Published: March 2003

A formulation of transport theory, specifically treating like-particle collisions for the lowest collisionality regime, is developed for nonsymmetric geometries, using the Lagrangian picture. This theory provides a particularly simple method for calculating transport coefficients which is useful for devices such as stellarators and tandem mirrors. It is shown that the explicit piece of the transport coefficient, processes such as test particle diffusion 〈ΔψΔψ/Δt〉, provides a rigorous upper bound to the full diagonal transport coefficient. The more complex compound, implicit processes (involving perturbed distribution functions and not accessible from a simple Monte Carlo procedure), are always inward (for normal gradients) and can only reduce the full flux from its test particle value. The implicit fluxes have an associated variational principle, maximizing this inward flux. This variational principle differs from the usual minimum principle for entropy production, although the two principles are equivalent, as is shown. An application to systems with weak asymmetry, such as stellarators, bumpy tori, and tandem mirrors is considered.
On model study of Couette flow
Authors: Hsieh D. Y. , Chen Fengsu
Published: March 2003

A system of Lorenz-type equations is derived for the axisymmetric Taylor–Couette flow problem. It is found there is typically no Hopf bifurcation as the Reynolds number increases beyond the transition Reynolds number.
Self-consistent, three-dimensional, electrostatic and magnetostatic tandem mirror equilibria
Authors: Anderson D. V. , Rensink M. E.
Published: March 2003

The study of ambipolar electrostatic confinement of central cell ions requires equilibrium models in which the electrostatic forces are treated on an equal footing with the magnetostatic ones. The three-dimensional guiding center fluid equilibrium model has been generalized to include these ambipolar effects; quasineutrality is used to obtain the potential φ. There is no restriction to low-beta or long-thin approximations. In this multiregion, multicomponent model omnigenity is required for each species seprately; this is the less stringent requirement of multigenity.
The effect of Mach number on the spatial stability of adiabatic flat plate flow to oblique disturbances
Authors: Wazzan A. R. , Taghavi H. , Keltner Gerlina
Published: March 2003

The linearized spatial stability of adiabatic flat plate flow to the first mode of oblique disturbances is computed numerically, using finite difference techniques, in the Mach number range M=1.6 to 6.0. The most unstable wave angle ψ is found in the range ψ=46° to 60°. Stability maps, in the form of curves of constant spatial amplification rate, are presented on the frequency-Reynolds number diagram. The critical x-Reynolds number is found to decrease monotonically with M, and is best fit with the expression R1/2x=579.34 M−1.18. This decrease is found to correspond with the outward displacement of the minimum critical layer and the ‘‘inflection’’ point, y at (U′/T)′=0, from y=0.248 and 0.231 at M=1.6 to 0.754 and 0.842 at M=6.0. No transition from viscous to inviscid instability is found with increasing Mach number, rather viscous instability persists to M=6.0. Some of the results agree with those obtained earlier by Mack, but others differ, particularly computations for M>3.0.
Remarks on disputed numerical results in compressible boundary-layer stability theory
Authors: Mack Leslie M.
Published: March 2003

Past work on the influence of Mach number on the viscous and inviscid instability of flat-plate boundary layers is reviewed, and new spatial calculations are presented. These calculations support the previous view that viscosity is only stabilizing for both two- and three-dimensional first-mode waves above M1=3.0, and for second-mode waves at all Mach numbers. It is concluded that the calculations of Wazzan, Taghavi, and Keltner that show viscous instability at M1=6.0 for first-mode 50° waves, and at M1=3.0 for two-dimensional second-mode waves, are not correct.
Parametric excitation of electron acoustic waves
Authors: Sharma R. P. , Ramamurthy K. , Yu M. Y.
Published: March 2003

It is shown that for a plasma with ion temperature greater than the electron temperature, the ordinary electromagnetic wave can decay into an upper hybrid wave and an electron acoustic wave. The growth rate is larger than that of the corresponding decay into ion quasimodes for low pump powers.
Nonlinear propagation of the ordinary mode radiation
Authors: Shukla P. K.
Published: March 2003

The nonlinear interaction of the ordinary mode radiation with numerous kinds of low-frequency oscillations in a magnetized plasma is investigated. Several coupled nonlinear equations are derived. Phenomena of soliton formation and radiation collapse are examined.
Stopping power of weakly unstable plasmas
Authors: Perez J. D. , Payne G. L.
Published: March 2003

An expression for the additional contribution to the stopping power of a weakly unstable plasma due to the modification of the beam–plasma collision operator by the presence of the unstable modes is derived and evaluated for a plasma with a flowing hot-electron tail, i.e., a bump-on-tail instability. It is found that the unstable plasma oscillations do not substantially alter the screening of the beam–plasma interaction.
Major disruptions in tokamaks, a fold catastrophe?
Authors: Guzdar P. N.
Published: March 2003

Major disruptions in tokamaks are shown to occur because of a fold catastrophe in the electron energy equation. Parametric dependences for the onset of disruption predicted by this model agree well with experimental data.
Analytical solutions to 2-D Grad-Shafranov equation with variable internal magnetic structure
Authors: Pipkins James F. , Mense Allan T.
Published: March 2003

A simple analytical solution to the Grad–Shafranov equation in the cylindrical approximation is found. The simplicity is achieved through the use of inverse-elliptic cylinder coordinates and a somewhat unorthodox choice of a boundary coordinate.
Radiation growth in a millimeter-wave free-electron laser operating in the collective regime
Authors: Gold S. H. , Black W. M. , Freund H. P. , Granatstein V. L. , Kinkead A. K.
Published: March 2003

Frequency-resolved measurements of radiation growth have been performed on a millimeter-wave free-electron laser using an intense relativistic electron beam. These measurements have shown large radiation growth rates (approximately 2 dB/cm) over a broad instantaneous bandwidth (66–90 GHz), in good agreement with predictions of theory for operation in the collective regime. Growth narrowing and saturation effects have also been observed. In addition, a large increase in experimental power and efficiency has been observed to result from tapering the strength of the axial magnetic field in the sense that compensates for kinetic energy extraction from the electron beam. Direct calorimetric measurements indicate the production of ≳75 MW centered at 75 GHz with 6% experimental efficiency.
The Vlasov equation and the description of inverse bremsstrahlung
Authors: Kim S. H.
Published: March 2003

It is shown that the Vlasov equation based on the Lorentz force equation does not describe inverse bremsstrahlung since the force by inverse bremsstrahlung is not included in the Lorentz force equation. It is also shown that quasilinear theory based on the classical Vlasov equation can adequately explain inverse bremsstrahlung.
Kinetic model for plasmas in a strong high-frequency field
Authors: Rozmus W. , Al-Shiraida Y. , Offenberger A. A.
Published: March 2003

A kinetic model for stable, underdense plasma in the presence of a strong electromagnetic pump wave is presented. Ion dynamics are properly treated to account for enhancement in electrostatic fluctuations. Such fluctuations are important even for pump strength below threshold for the oscillating two-stream and parametric decay instabilities in the underdense region. Thresholds of these instabilities are numerically determined. The results of calculations for the static form factor S(k) electron collision terms, and the absorption coefficient are presented for stable, homogeneous plasmas. For moderate pump values (below instability threshold) enhanced fluctuation levels and increased absorption are found, which have relevance to a variety of experimental situations involving high-intensity laser, microwave, or radio wave interaction with plasmas.
Self-consistent description of solitary surface waves on a plasma cylinder
Authors: Gradov O. M. , Stenflo L. , Sünder D.
Published: March 2003

Considering a high-frequency electrostatic surface wave that propagates along a plasma cylinder, it is shown that a local change of the axial wavelength of the wave potential may cause solitary variations in the cylinder radius. The magnitudes of these surface displacements are found to depend essentially only on the wavelength perturbation of the high-frequency wave whereas their propagation velocities are defined by the wave intensity. Previous results, which only included short-wavelength oscillations, are also generalized.
Numerical and analytical demonstration of the interaction of a wave with a free-streaming perturbation
Authors: Adam J. C. , Trocheris M.
Published: March 2003

The interaction of a plasma wave with a long wavelength free-streaming perturbation is considered in the case of a one-dimensional electron plasma. A demonstration of this interaction is provided by means of both numerical simulation and analytical theory. The initial distribution function is so chosen as to give rise to an ordinary plasma wave together with a long wavelength disturbance containing a free-streaming portion. Satellites of the plasma wave are shown to be excited in agreement with earlier work of Morales and Malmberg on the ‘‘linear sideband’’ effect. This result is found to hold when the initial long wavelength disturbance is close to a pure free-streaming perturbation. In order to have an observable effect in the numerical simulation, it is necessary to increase the density of resonant particles by adding a warm beam to the Maxwellian plasma, in such a way though, as to avoid beam plasma instability. The predictions of analytical theory on the amplitudes of the satellites are compared with the results of numerical simulation and a fair amount of agreement is found both in absolute magnitude and in time evolution.
A collisional drift wave description of plasma edge turbulence
Authors: Wakatani Masahiro , Hasegawa Akira
Published: March 2003

Model mode-coupling equations for the resistive drift wave instability are numerically solved for realistic parameters found in tokamak edge plasmas. The Bohm diffusion is found to result if the parallel wavenumber is chosen to maximize the growth rate for a given value of the perpendicular wavenumber. The saturated turbulence energy has a broad frequency spectrum with a large fluctuation level proportional to κ̄ (=ρs/Ln, the normalized inverse scale length of the density gradient) and a wavenumber spectrum of the two-dimensional Kolmogorov–Kraichnan type, ∼k−3.
F-Θ pumping and field modulation experiments on a reversed field pinch discharge
Authors: Schoenberg K. F. , Buchenauer C. J. , Massey R. S. , Melton J. G. , Moses R. W. , Nebel R. A. , Phillips J. A.
Published: March 2003

Computational simulations indicate that proper programming of the toroidal and poloidal magnetic fields in a reversed field pinch (RFP) can drive a steady-state current. This current drive technique, called F-Θ pumping, assumes that the RFP plasma remains in a quasirelaxed state and that the field programming is not deleterious to pinch performace. Field modulation experiments on the Los Alamos ZT-40M experiment have shown that these assumptions are reasonable, and hence F-Θ pumping is a plausible candidate for RFP current drive.
Transport coefficients in weakly turbulent plasmas
Authors: Hazak G. , Bernstein Ira B. , Smith Timothy M.
Published: March 2003

An integral representation is derived, within the approximation of geometric optics, for the field of dressed particles in weakly convectively unstable inhomogeneous plasmas characterized by a general non-Hermitian dielectric tensor. The representation is applied in particular to derive a formula for the velocity space diffusion tensor.
On positional stability of current-carrying plasma in a helical winding
Authors: Schaffer Michael J. , Bard Wayne D.
Published: March 2003

The average force acting on a current-carrying plasma column is calculated when a straight multipolar helical winding is displaced laterally with respect to the plasma, whose average position is kept fixed by a uniform external transverse magnetic field Bx. This nonsymmetric equilibrium problem is solved for force-free j/B=const plasmas by Fourier-analyzing plasma and vacuum fields into helical harmonics and numerically solving the free boundary interface for their amplitudes. In the stellarator limit the average helical force is as predicted from stellarator expansion theory. In the new OHTE limit, both l=2 and l=3 reversed pitch pinches are repelled from the helix. A simple explanation is offered in terms of plasma diamagnetism to the helical field.
Local magnetic divertor for control of the plasma–limiter interaction in a tokamak
Authors: Zweben S. J. , Liewer P. C. , Gould R. W.
Published: March 2003

An experiment is described in which plasma flow to a tokamak limiter is controlled through the use of a local toroidal divertor coil mounted inside the limiter itself. This coil produces a local perturbed field BC approximately equal to the local unperturbed toroidal field BT≂3 kG, such that when BC adds to BT the field lines move into the limiter and the local plasma flow to it increases by a factor as great as 1.6, and when BC subtracts from BT the field lines move away from the limiter and the local plasma flow to it decreases by as much as a factor of 4. A simple theoretical model is used to interpret these results. Since these changes occur without significantly affecting global plasma confinement, such a control scheme may be useful for optimizing the performance of pumped limiters.
Resonant interaction of ion-acoustic solitons in three-dimensions revisited
Authors: Gabl E. F. , Tsikis Eugene K. , Lonngren Karl
Published: March 2003

Using a launching structure consisting of four large diameter hemispherical grids located at the corners of a square, it is observed that the amplitude of a resonantly produced soliton is greater than ten times the value of an individual soliton. This observation extends to three dimensions Kaup’s conjecture that the two-dimensional resonance-soliton creation requires a finite interaction tune.
Nonlinear reduced fluid equations for toroidal plasmas
Authors: Drake J. F. , Antonsen Thomas M.
Published: March 2003

Nonlinear reduced fluid equations are derived for studying resistive instabilities in large-aspect- ratio, low-beta toroidal plasmas. An ordering is developed in which plasma compressibility as well as the poloidal curvature are retained. The nonlinear equations can be linearized and used to reproduce the Mercier criterion in the large-aspect-ratio, low-beta limit. A second set of reduced equations is derived from the Braginskii fluid equations. These equations, which are very similar to the reduced magnetohydrodynamic equations, contain diamagnetic effects as well as parallel transport associated with magnetic fluctuations. Both sets of equations conserve energy exactly.
The effect of diamagnetic rotation on the nonlinear coupling of tearing modes
Authors: Hicks H. R. , Carreras B. A. , Holmes J. A.
Published: March 2003

The tokamak disruption model based on the nonlinear interaction of multiple helicity tearing modes is extended to include diamagnetic effects. This introduces mode rotation and time-varying phases during the nonlinear interaction. The basic nonlinear destabilization mechanism associated with plasma disruptions remains unaffected by diamagnetic effects.
Nonlinear interaction of toroidicity-induced drift modes
Authors: Similon P. L. , Diamond P. H.
Published: March 2003

Drift modes in toroidal geometry are destabilized by trapped electron inverse dissipation and evolve to a nonlinearly saturated state. Using renormalized one-point turbulence theory for the nonlinear gyrokinetic equation in the ballooning representation, it is shown that ion Compton scattering is an effective saturation mechanism. Ion Compton scattering transfers wave energy from short to long perpendicular wavelength, where it is absorbed by ion resonance with extended, linearly stable, long-wavelength modes. The fluctuation spectrum and fluctuation levels are calculated using the condition of nonlinear saturation. Transport coefficients and energy confinement time scalings are determined for several regimes. Specifically, the predicted confinement time density scaling for an Ohmically heated discharge increases from n3/8 in the collisionless regime to n9/8 in the dissipative trapped electron regime.
Quasilinear theory of the thermonuclear magneto-acoustic cone instability
Authors: Anderson D. , Lisak M.
Published: March 2003

An analytical investigation is made of the development of the thermonuclear magneto-acoustic cone instability in a tokamak reactor. The excitation mechanism for the instability is the anisotropy in the α-particle distribution function resulting from either direct orbit losses or ripple losses. Explicit expressions are obtained describing the time evolution of the magneto-acoustic wave energy density as well as the distribution function, the anomalous particle loss, and the density of the α particles. The results show that the quasilinear evolution of the instability causes rapid and severe α-particle losses and saturates at a high level of turbulence for the magneto-acoustic waves. It is also shown that the quasilinear diffusion of the α particles caused by the cone instabilities may result in the generation of a significant toroidal current in the plasma.
Neoclassical transport associated with collisionless detrapping in a bumpy torus
Authors: Hastings D. E.
Published: March 2003

In the low-collisionality nonresonant regime in a bumpy torus the transitional particles can make a large contribution to neoclassical transport. This contribution can be moderated by the toroidally induced radial drift which causes transitional particles to detrap and retrap in the mirror sectors. This effect leads to diffusion coefficients which are linear in the collision frequency and scale with the inverse aspect ratio instead of the more usual square of the inverse aspect ratio.
Transport of a multiple-ion species plasma in a collisionless regime in a bumpy torus
Authors: Hastings D. E.
Published: March 2003

The ion neoclassical transport coefficient in a multiple ion species plasma in a bumpy torus are calculated in the low-collisionality regime. The poloidal electric field is calculated self-consistently with collisional electrons. Both pitch angle and energy scattering are included in the collision operator.
Detailed modeling of microwave energy deposition in EBT devices
Authors: Batchelor D. B. , Goldfinger R. C. , Rasmussen D. A.
Published: March 2003

Ray-tracing studies have been combined with a simple wave power balance model to provide a complete, albeit approximate, description of microwave power deposition in the ELMO Bumpy Torus (EBT-I), EBT-Scale (EBT-S), and EBT Proof-of-Principle (EBT-P) devices. Electron cyclotron absorption of ordinary and extraordinary waves by the combined core plasma and relativistic annuli is calculated using a fully relativistic damping package developed for the rays geometrical optics code. The rays are traced in finite-beta bumpy cylinder plasma equilibria that are obtained from the Oak Ridge National Laboratory (ORNL) two-dimensional (2D) equilibrium code. These results for direct, single-pass absorption are combined with results from a statistical model for the deposition of multiple reflected and mode-converted waves to obtain estimates of the power deposited in the core, surface, and annulus plasma components. Wave absorption by the annuli and by the core components at the fundamental and second harmonic resonances, reflection, and Budden tunneling of the extraordinary mode at the right-hand cutoff, and conversion between ordinary and extraordinary modes upon wall reflection are the processes included in the power balance model. Experimental measurements of wave power flux on the cavity wall in EBT-S made with a simple microwave calorimeter are in good agreement with predictions of the model for a variety of operating configurations.
Slab model for Rayleigh–Taylor stabilization by vortex shedding, compressibility, thermal conduction, and ablation
Authors: Manheimer Wallace M. , Colombant Denis G.
Published: March 2003

Analytical models are derived for stabilization of the Rayleigh–Taylor instability of an ablatively accelerated slab. The procedure is to conserve mass, momentum, and energy flux across a perturbed interface through which plasma flows. In various regimes, the stabilization mechanism can be described as vortex shedding, compressibility, thermal conduction, and ablation. Comparisons are made with recent fluid simulations, and simple laws relevant to laser-driven ablative acceleration are derived.
The turbulent twisted magnetic flux tube gas
Authors: Bogdan T. J.
Published: March 2003

The kinematic behavior of a dilute, two-dimensional ‘‘gas’’ of parallel twisted magnetic flux tubes in a highly conducting fluid is considered. Collisions between flux tubes with the same sense of twist are inelastic due to the reconnection of their oppositely directed azimuthal magnetic field components at impact. In some collisions, the tension in the reconnected magnetic flux is sufficient to bind the two flux tubes together. Collisions between flux tubes with the opposite sense of twist are elastic. It is shown that a tenuous gas of individual twisted flux tubes condenses into two large-scale regions of magnetic field with opposite twists. This calculation illustrates the results of Montgomery and co-workers on the migration of magnetic energy towards small wavenumbers in two-dimensional magnetohydrodynamic (MHD) turbulence. In particular, this problem illustrates the dynamical nonequilibrium of the relaxation phase in turbulent MHD systems.
Effects of line-tying on free-boundary global modes in high-beta tokamak plasmas
Authors: Lee J. K.
Published: March 2003

Axisymmetric line-tying, as may occur in an H-mode discharge or in a toroidal limiter, is modeled through a rigid or conducting boundary condition. It is shown effective in reducing the linear growth rate of global free-boundary magnetohydrodynamic modes in high- βp circular cross-section tokamaks. Various parameter dependencies are also examined.
Resonant absorption of phase-mixed Alfvén surface waves in ideal and resistive magnetohydrodynamics: Initial-value problem
Authors: Steinolfson R. S.
Published: March 2003

Numerical solutions of the linearized magnetohydrodynamic equations are used to investigate resonant energy absorption in the continuous frequency spectrum. Energy in the phase-mixed surface waves resistively dissipates with the absorption time and width scaling as resistivity to the −1/3 and 1/6 powers, respectively.
General model of streamer propagation
Authors: Fernsler R. F.
Published: March 2003

The propagation of electrical streamers in gases has long been controversial. Not all of the controversial issues are fundamental, however, to streamer modeling. An analysis is presented which focuses on those issues that are fundamental such as the electron avalanche process, space-charge field distortion, and the development of a filamentary structure. The model predicts that streamer propagation is determined by the electrostatic potential driving the streamer, by the preionization generated immediately ahead of the streamer, and by the avalanche characteristics of the gas. The scaling of propagation speed with these parameters is obtained through the use of similarity and related arguments. Support for the analysis comes from experiments utilizing lasers to preionize air and guide electrical streamers.
Dynamics of entangled vortex filaments
Authors: Takaki Ryuji , Hussain A. K. M. F.
Published: March 2003

The mechanics of two co-rotating, entangled, slender vortex filaments are analyzed. An equation governing the motions of the vortices is derived on the assumption that their curvature is small. Particular solutions are obtained for two flow situations: one with and one without an external strain field.
Stokes drag on a cylinder in axial motion
Authors: Ui T. J. , Hussey R. G. , Roger R. P.
Published: March 2003

Measurements are presented for the drag on a circular cylinder moving along its axis of rotational symmetry at low Reynolds number. The influence of an exterior cylindrical boundary is taken into account by making measurements with boundaries of different diameter and using an empirical correlation to extrapolate to infinite boundary diameter. The results for cylinders with length to diameter ratio (L/D) between 4 and 100 agree well with the calculated values of Youngren and Acrivos. The results for disks (0.019<L/D<0.26) are consistent with a linear interpolation between the value for a disk of zero thickness and the experimental points of Heiss and Coull. Beads-on-a-shell calculations are presented for both axial and transverse motion of cylinders with 0≤L/D≤10. Interpolated values of the ratio of transverse to axial drag are used to obtain approximate values of transverse drag for 10<L/D<100.
Salt-finger convection in shear flow
Authors: Thangam Sivagnanam , Zebib Abdelfattah , Chen C. F.
Published: March 2003

Stability of salt-finger convection in steady shear flow is examined. Both the cases of constant and variable vertical shear are considered. The results based on linear stability analysis indicate that for the cases of variable shear profiles without inflection points, the most unstable mode is that of two-dimensional rolls aligned downshear. For this most unstable mode, a nonlinear stability analysis is carried out, and the results accurate to second order in wave amplitude are presented.
Hamiltonian formulation of reduced magnetohydrodynamics
Authors: Morrison P. J. , Hazeltine R. D.
Published: March 2003

Reduced magnetohydrodynamics (RMHD) is a principal tool for understanding nonlinear processes, including disruptions, in tokamak plasmas. Although analytical studies of RMHD turbulence are useful, the model’s impressive ability to simulate tokamak fluid behavior has been revealed primarily by numerical solution. A new analytical approach, not restricted to turbulent regimes, based on Hamiltonian field theory is described. It is shown that the nonlinear (ideal) RMHD system, in both its high-beta and low-beta versions, can be expressed in Hamiltonian form. Thus a Poisson bracket, {  ,  }, is constructed such that each RMHD field quantity ξi evolves according to ξ̇i ={ξi,H}, where H is the total field energy. The new formulation makes RMHD accessible to the methodology of Hamiltonian mechanics; it has lead, in particular, to the recognition of new RMHD invariants and even exact, nonlinear RMHD solutions. A canonical version of the Poisson bracket, which requires the introduction of additional fields, leads to a nonlinear variational principle for time-dependent RMHD.
Nonlinear mode coupling theory of the lower-hybrid-drift instability
Authors: Drake J. F. , Guzdar P. N. , Hassam A. B. , Huba J. D.
Published: March 2003

A nonlinear mode coupling theory of the lower-hybrid-drift instability is presented. A two-dimensional nonlinear wave equation is derived which describes lower-hybrid-drift wave turbulence in the plane transverse to B (k⋅B=0), and which is valid for finite β, collisional and collisionless plasmas. The instability saturates by transferring energy from growing, long wavelength modes to damped, short wavelength modes. Detailed numerical results are presented which compare favorably to both recent computer simulations and experimental observations. Applications of this theory to space plasmas, the earth’s magnetotail, and the equatorial F region ionosphere are discussed.
Theory of field-swelling instability in anisotropic plasmas
Authors: Basu B. , Coppi B.
Published: March 2003

High-β ( β>1), collisionless plasmas with slightly anisotropic temperatures (T⊥≳T∥) are shown to be subject to an instability producing transport of transverse electron thermal energy into the regions where the magnetic field is weakened by the perturbation while the plasma motion is decoupled from that of the field lines. Modes of this type can be important in producing magnetic reconnection when the effects of an effective electron viscosity or resistivity are added in. A fluid-like description for the linear and quasilinear theory of this (‘‘field-swelling’’) instability is adopted.
Nonlinear evolution of the resistive tearing mode
Authors: Steinolfson R. S. , Hoven G. Van
Published: March 2003

The nonlinear behavior of the tearing instability is investigated with numerical solutions of the resistive, incompressible, magnetohydrodynamic equations. Simulations have been completed for values of the Lundquist number S from 102 to 106 and wavelength parameter α=2πa/λ from 0.042 to 0.5. The initial state for the nonlinear computations is provided by the linear instability. In all cases, the nonlinear mode initially evolves at the linear rate, followed by a period of considerably reduced growth. At high S and low α, secondary-flow vortices, opposite in direction to the linear vortices, generate a new magnetic island centered at the initial X point. The nonlinear growth rate of the one constant-ψ (in the linear regime) solution considered is approximately an order of magnitude less than that of a comparable nonconstant-ψ solution over the same time period. The nonconstant-ψ computations indicate a reduction of from 8% to 27% of the initial shear-layer magnetic energy, with the larger reductions occurring for the longer wavelength disturbances. The constant-ψ simulation shows a reduction two orders of magnitude smaller. The island width of the nonconstant-ψ solution becomes larger than twice the scale of the initial shear layer. For all cases, the electric field parallel to the magnetic field grows, at the end of the run, to about 10−3 times the Driecer field.
On the scaling of the turbulence energy dissipation rate
Authors: Sreenivasan K. R.
Published: March 2003

From an examination of all data to date on the dissipation of turbulent energy in grid turbulence, it is concluded that, for square-mesh configuration, the ratio of the time scale characteristic of dissipation rate to that characteristic of energy-containing eddies is a constant independent of Reynolds number, for microscale Reynolds numbers in excess of about 50. Insufficient data available for other grid configurations suggest a possibility that the ratio could assume different numerical values for different configurations. This persistent effect of initial conditions on the time scale ratio is further illustrated by reference to the jet-grid data of Gad-el-Hak and Corrsin.
Flow development in a porous channel and tube
Authors: Brady J. F.
Published: March 2003

The spatial development of the inlet velocity profile in a uniformly porous channel and tube is investigated. For tubes which are very long compared with their radius, it is shown that above a critical Reynolds number of 2.3 the inlet velocity profile does not necessarily decay into the fully developed, similarity profile for an infinite tube. Rather, the structure of the flow throughout the entire tube is influenced by the inlet profile. This loss of validity of the similarity solution is due to the fact that the tube is of finite length and the inlet profile is not of the similarity form. The actual length of the tube is, however, unimportant. Analogous results hold for the flow in a porous channel, with a critical Reynolds number of approximately 6.
Heliac parameter study
Authors: Monticello D. A. , Dewar R. L. , Furth H. P. , Reiman A.
Published: March 2003

Helical axis stellarators (heliacs) with zero net current are found to possess very good stability properties. Helically symmetric or ‘‘straight’’ heliacs with bean-shaped cross sections have a first region of stability that reaches to 〈 β〉 of 30% or more. Those with circular cross sections have a second region of stability to Mercier modes. In addition, the stability properties of these plasma configurations as functions of pressure profile, helical aspect ratio, and helical period length are also reported.
Electrostatic probes for dense flowing plasma
Authors: Bose T. K. , Seeniraj R. V.
Published: March 2003

Plasma flowing past a blunt-body electrostatic probe will stagnate at the probe surface in a region about the axis of symmetry. Multifluid equations descriptive of a quasineutral two-temperature argon plasma flowing past a probe are presented and solved numerically. The numerical results presented in Table I may be useful in interpreting experimental data from such a probe.
Stability of space-charge limited electron flow
Authors: Antonsen Thomas M. , Miner William H. , Ott Edward , Drobot Adam T.
Published: March 2003

The stability of space-charge limited flow in planar diodes is investigated. It is found that the flow is unstable whenever the electron transit time is approximately n+ (1)/(4) , cycles where n is an integer, of the high-frequency modes supported by the diode structure. Analytical and numerical solutions are found for the resulting dispersion relation in the nonrelativistic limit. The analytical results lead to a simple physical explanation of the instability and provide accurate solutions over a wide range of parameters. Numerical solutions are also obtained for the relativistic case.
An electron conductivity model for dense plasmas
Authors: Lee Y. T. , More R. M.
Published: March 2003

An electron conductivity model for dense plasmas is described which gives a consistent and complete set of transport coefficients including not only electrical conductivity and thermal conductivity, but also thermoelectric power, and Hall, Nernst, Ettinghausen, and Leduc–Righi coefficients. The model is useful for simulating plasma experiments with strong magnetic fields. The coefficients apply over a wide range of plasma temperature and density and are expressed in a computationally simple form. Different formulas are used for the electron relaxation time in plasma, liquid, and solid phases. Comparisons with recent calculations and available experimental measurement show the model gives results which are sufficiently accurate for many practical applications.
The dependence of the drift cyclotron loss cone instability on the radial density gradient
Authors: Ferron John R. , Wong Alfred Y.
Published: March 2003

Large-amplitude fluctuations (eφrms/kTe≲2) observed near the ion cyclotron frequency and its harmonics in a magnetic mirror are identified as the drift-cyclotron loss-cone (DCLC) instability through measurements of the frequency, wavenumber, propagation direction, and dependence on the loss cone ion velocity distribution and the radial density gradient. The mode amplitude is reduced when the plasma density and radial density gradient length are near the linear stability boundary, dropping by two orders of magnitude when the density gradient scale length is increased from 2 to 100 ion gyroradii. The mode is localized radially to a region of one ion gyrodiameter width near the location of the steepest density gradient. The bursting appearance of mode amplitude is accompanied by changes in its perpendicular wavelength as the plasma parameters change with time during its decay. Mode amplitude modulation which occurs simultaneously throughout certain flux volumes indicates that the fluctuations arise from an absolute instability. While oscillations at the fundamental of the ion-cyclotron frequency occur most frequently with the largest amplitude, a dominant mode is observed near the third harmonic of the ion cyclotron frequency localized at a steep gradient near the axis. One possible interpretation is that the wavelengths of lower harmonic DCLC modes are too long to satisfy the azimuthal quantization condition.
Vortex shedding due to laser ablation
Authors: Emery Mark H. , Gardner John H. , Boris Jay P. , Cooper Arnold L.
Published: March 2003

An investigation of laser-driven targets shows the generation and subsequent shedding of strong vortex structures from the ablation layer whenever the density and pressure profiles become noncollinear. It is shown here that vortex shedding explains the inhibited linear Rayleigh–Taylor growth, the short-wavelength cutoff, dynamic stabilization and clarifies the role of the Kelvin–Helmholtz roll-up.
First experimental evidence of vortex splitting
Authors: Freymuth P. , Bank W. , Palmer M.
Published: March 2003

Movie sequences of vortex patterns visualized by smoke in a two-dimensional starting flow around an airfoil indicate the process of vortex splitting predicted by Moore and Saffman and by Christiansen and Zabusky for some vortical flows.
Convective thermocapillary instabilities in liquid bridges
Authors: Xu J-J. , Davis S. H.
Published: March 2003

An axisymmetric liquid bridge is surrounded by a passive gas. A steady shear flow is set up by imposing a temperature gradient along the bridge and driving the motion by thermocapillarity. This dynamic state is susceptible to convective instabilities that lead to propagating hydrothermal waves that feed on the underlying temperature gradients. The convective instabilities of this axisymmetric return-flow state are presented as functions of the Prandtl number of the liquid and the surface Biot number of the interface. Comparisons are made with the results of Smith and Davis for planar layers and with available experimental data.