bluuebunny/crossref_metadata_2025 huggingface dataset:
Penetrative convection in an internally heated layer of water near the maximum density point
Authors: Normand Christiane , Azouni Aza
Published: July 2002

A model for penetrative convection in which a stably stratified layer of fluid is bounded by two unstable layers is considered. This configuration is obtained in a horizontal water layer around its density extremum when a quadratic temperature profile is maintained by internal heating. A linear stability analysis shows that either stationary or oscillatory modes set in at the onset of instability depending on the values of the control parameter. Moreover, two types of stationary modes have been identified that differ by the value of their critical wave number and the number of vertical cells. These results are discussed in the light of recent studies on related topics.
Taylor–Goertler instability for thin-film flows over moving curved beds
Authors: Rigby D. , Eagles P. M.
Published: July 2002

A theory for steady-state thin-film flow on moving curved beds is developed and disturbances of the Goertler type are studied. It is found that flow on both concave and convex beds, moving in either direction relative to the fluid flux, can support these disturbances for a range of values of the bed velocity. The critical Taylor–Goertler numbers are calculated.
Instability of a supersonic vortex sheet inside a circular duct
Authors: Chang Chien-Cheng , Kuo Chih-Yu
Published: July 2002

A theoretical study is carried out for investigating spatial instabilities of a supersonic vortex sheet inside a circular duct. The sheet is cylindrical in shape, separating the flow into an inner region and an outer one of uniform properties. It is found that there is one family of subsonic (Kelvin–Helmholtz) instability waves which are accompanied by two families of neutral modes. Two families of supersonic instability waves can be identified to be associated with two other families of neutral modes. A mathematical analogy indicates that instability modes at high frequencies bear resemblance to those obtained by Tam and Hu [J. Fluid. Mech. 203, 51 (1989).] for plane mixing layers inside a rectangular channel. Geometric effects of both the vortex sheet and the outer confinement are significant only at relatively low frequencies of disturbances. In addition, extensive parametric study reveals interesting features of the dependence of the instability waves on the density ratio, velocity ratio, radius ratio of the inner and outer regions, and three-dimensional disturbances.
Kinetic stability theory of space-charge-limited flow
Authors: Antonsen T. M. , Chang C. L.
Published: July 2002

The effect of a spread in energy of the emitted beam on the stability of the space-charge-limited diode is investigated. It is found that for small thermal spreads the so-called emittron or transit time instability persists and is correctly described by cold fluid equations. Thermal effects become important when ωT∼ε−1/4T, where ω is the mode frequency, T is a typical particle transit time, and εT is the ratio of the thermal energy of the beam to potential energy associated with the applied voltage. For modes with frequencies above this value, phase mixing of the particles causes a dramatic reduction in growth rate, such that other damping mechanisms will be able to stabilize the high-frequency modes. The analysis presented here applies specifically to transverse electromagnetic (TEM) modes. However, the results can be expected to qualitatively describe other modes as well.
Stabilization of lower hybrid drift modes by finite parallel wavenumber and electron temperature gradients in field-reversed configurations
Authors: Farengo R. , Guzdar P. N. , Lee Y. C.
Published: July 2002

The effect of finite parallel wavenumber and electron temperature gradients on the lower hybrid drift instability is studied in the parameter regime corresponding to the TRX-2 device [Fusion Technol. 9, 48 (1986)]. Perturbations in the electrostatic potential and all three components of the vector potential are considered and finite beta electron orbit modifications are included. The electron temperature gradient decreases the growth rate of the instability but, for kz=0, unstable modes exist for ηe(=T′en0/Ten0)>6. Since finite kz effects completely stabilize the mode at small values of kz/ky(≂5×10−3), magnetic shear could be responsible for stabilizing the lower hybrid drift instability in field-reversed configurations.
Reply to the comments of González and Gratton
Authors: Eckhoff Knut S.
Published: July 2002

It is shown how the comments of González and Gratton on the stability of the slab geometry are related to the author’s previous paper [Phys. Fluids 30, 3673 (1987)].
Alfvén-type wave motion induced by the Hall effect
Authors: Goodman Michael L.
Published: July 2002

The Hall effect is studied as a correction to ideal magnetohydrodynamics (MHD) in the context of how it affects the linear stability of the cylindrical pinch in the simple case of a static, homogeneous equilibrium state. The effects of compressibility and electron pressure are included. The presence of the electron pressure gives rise to an electric field tangent to the boundary of the plasma. This introduces an additional boundary condition in the case of a perfectly conducting plasma boundary. Imposing this boundary condition eliminates the wave solutions presented in this paper. With respect to large radial wavenumber, the accumulation point of the slow magnetoacoustic wave frequency spectrum is changed from its finite value in ideal MHD to infinity by the Hall effect. The Hall effect gives rise to linear waves that do not exist in ideal MHD. Specifically, the Hall effect induces azimuthally symmetric, compressible, Alfvén-type wave propagation. The frequency spectrum of these waves is discrete and infinite and is a singular perturbation of the incompressible Alfvén wave spectrum in ideal MHD. These Alfvén-type waves do not exist if the plasma is incompressible or if H≡K⋅B=0, where K is the wave vector of the perturbation and B is the equilibrium magnetic field.
Numerical simulation of ion cyclotron range of frequencies electron heating in the JET tokamak
Authors: Romero Hugo A. , Scharer John E.
Published: July 2002

An ion cyclotron range of frequencies (ICRF) full-wave code and an anisotropic quasilinear Fokker–Planck code are used to simulate the time-dependent rise in the electron and ion temperatures during a sawtooth period corresponding to initial Ohmic equilibrium conditions in the JET tokamak [in Plasma Physics and Controlled Nuclear Fusion Research, 1986 (IAEA, Vienna, 1987), Vol. 1, p. 44]. Results from the 1-D power deposition studies indicate that less than 10% of the launched fast wave energy is directly absorbed by the e1ectrons via transit time damping and mode conversion processes over a wide range of the JET plasma equilibrium parameters and wave spectrum. In contrast, the Fokker–Planck results indicate a very fast time scale for the formation of energetic minority tail ions which slow down primarily on electrons. It is concluded that the process of minority ion tail formation and electron drag best explains the experimentally observed rates of electron heating as seen in JET. To further substantiate this point, numerical Fokker–Planck results are presented that yield time-dependent electron and ion temperature rises, which are in good agreement with central core experimenta1 measurements for the appropriate range of observed ICRF power density, energy confinement time, and plasma and wave parameters.
Self-similar solutions of ion-beam-driven plasma expansion
Authors: Kärcher B. , Kull H. J.
Published: July 2002

Self-similar solutions of ion-beam-driven plasma expansions, separating in mass and time coordinates, are presented. Model equations are derived for the general class of separable flows with prescribed energy source. Their solutions show, representatively, the transition from an initial heating to an asymptotic expansion regime described by constant energy ratios. The dependence of the asymptotic energy ratios on the temporal form of the beam pulse is examined and maximum heating is found for exponentially growing pulses. The spatial flow profiles are discussed for uniform and nonuniform energy deposition with application to a realistic energy loss formula that is Bragg peaked. Comparison with numerical simulations suggests a general asymptotic validity of the present similarity results.
Flow pattern induced by a piston impulsively moving in a perfectly conducting inviscid radiating gas
Authors: Singh L. P. , Sharma V. D. , Ram Rishi
Published: July 2002

A self-similar method is used to study numerically the flow pattern resulting from the impulsive motion of a plane and a cylindrical piston through a perfectly conducting inviscid radiating gas permeated by a transverse magnetic field. The effects of thermal radiation are treated by the differential approximation to the radiative transfer equation. Numerical solutions in the region between the shock and the piston are presented for the cases of general opacity and the transparent limit. The influence of thermal radiation, the magnetic field strength, and the adiabatic heat exponent on the flow pattern for various cases are assessed. The general behavior of the velocity and density distribution remains essentially unaffected by the interaction between the radiative and magnetogasdynamic phenomena. However, the pressure profiles are greatly affected by the interaction; the effect of the magnetic interation is to diminish the effects of radiative transfer.
Radiation of partially ionized atomic hydrogen
Authors: Soon W. H. , Kunc J. A.
Published: July 2002

A nonlinear collisional-radiative model for determination of production of electrons, positive and negative ions, excited atoms, and spectral and continuum line intensities in stationary partially ionized atomic hydrogen is presented (11 000 K≤Te≤15 000 K, 1010 cm−3≤Nt ≤1018 cm−3). Transport of radiation is included by coupling the rate equations for production of the electrons, ions, and excited atoms with the radiation escape factors, which are not constant but depend on plasma conditions. It is found that the contribution of the negative ion emission to the total continuum emission can be important. Comparison of the calculated total continuum emission coefficient, including the negative ion emission, is in good agreement with experimental results.
Absorption and emission of plasma waves in mode conversion layers at general harmonics of the ion cyclotron frequency
Authors: Cho Suwon , Swanson D. G.
Published: July 2002

The mode conversion-tunneling equation is solved numerically with the resonant absorption term at general harmonics of the ion cyclotron frequency. The numerical technique is based on the Laplace contour integral and direct integration. Results for various scattering parameters as well as absorbed fractions of incident energy are presented. As in the second and the third harmonic cases, simple expressions for some scattering parameters are found for higher harmonic numbers. Finally, emission ratios are calculated from absorption results and the reciprocity relations.
The stability of a line-tied force-free magnetic field in an unbounded region of space
Authors: Aly J. J.
Published: July 2002

Sufficient criteria of linear ideal magnetohydrodynamic stability are derived for a three-dimensional nonlinear force-free magnetic field B occupying an infinite region Ω, which is either a half-space or an ‘‘exterior domain’’; all the field lines of B are assumed to be firmly rooted in the boundary ∂Ω of Ω (line-tying condition), but their topological pattern may be otherwise quite arbitrary. Roughly speaking, a field characterized by a length scale Λ is found to be stable if α0 Λ<O(1), where α0 is a number measuring the magnitude of the force-free function α=B−2(B⋅∇×B). The stability criteria established here have some important astrophysical applications (solar corona, magnetosphere of a compact object, etc.), which are briefly discussed.
Effects of velocity-space particle loss in field-reversed configurations
Authors: Hsiao Ming-Yuan , Chiang Pi-Ren
Published: July 2002

The current density profiles in the presence of the velocity-space particle loss (VSPL) in field-reversed configurations (FRC’s) are presented; self-consistent, one-dimensional magnetic and electric fields are also obtained by solving Ampère’s law. It is found that the magnetic field structure of FRC’s can still be maintained in the presence of the VSPL. However, since ions are less confined than electrons in the regions away from the field null, the electric field can be developed in favor of the direction pointing away from the field null. This electric field structure is suggested to explain the origin of the rotation.
Overview of results from the ATF torsatron
Authors: Colchin R. J. , Murakami M. , Anabitarte E. , Anderson F. S. B. , Bell G. L. , Bell J. D. , Bigelow T. S. , Crume E. C. , Dunlap J. L. , Dyer G. R. , England A. C. , Fisher P. W. , Gabbard W. A. , Glowienka J. C. , Goulding R. H. , Halliwell J. W. , Hanson G. R. , Harris J. H. , Haste G. R. , Hidalgo-Vera C. , Hillis D. L. , Hiroe S. , Horton L. D. , Howe H. C. , Hutchinson D. P. , Isler R. C. , Jernigan T. C. , Kwon M. , Langley R. A. , Lee D. K. , Lyon J. F. , Lue J. W. , Ma C. H. , Menon M. M. , Morris R. N. , Mioduszewski P. K. , Neilson G. H. , Qualls A. L. , Rasmussen D. A. , Rogers P. S. , Shaw P. L. , Shepard T. D. , Simpkins J. E. , Sudo S. , Thomas C. E. , Uckan T. , Wade M. R. , Wilgen J. B. , Wing W. R. , Yamada H. , Zielinski J. J.
Published: July 2002

Experiments involving plasma improvement, confinement scaling, bootstrap currents, and edge fluctuations have been carried out in the Advanced Toroidal Facility (ATF) torsatron [Fusion Technol. 10, 179 (1986)]. Average densities n̄e≤9×1019 m−3 have been obtained, with global energy confinement times τ*E≤20 msec. Confinement times generally follow the stellarator/torsatron empirical scaling law, τSL =0.17×P−0.58n0.69eB0.84a2R0.75 (with τSL in seconds, power P in megawatts, density ne in 1020 m−3, and plasma radius a and major radius R in meters). Gas injection during neutral beam injection (NBI) causes increases in ne, so that τ*E does not decrease during NBI. Edge plasma fluctuations are found to exhibit a mode change near the peak of the energy confinement time. Plasma currents observed during electron cyclotron heating have been identified as bootstrap currents.
Radiation-driven inertial fusion targets for implosion experiments: Theoretical analysis and numerical simulations
Authors: Tahir N. A. , Arnold R. C.
Published: July 2002

This paper presents numerical simulations of compression, ignition, and burn of a radiation-driven inertial fusion capsule suitable for use in a heavy-ion-beam-driven laboratory microfusion facility to achieve breakeven (thermonuclear energy output=input radiation energy). These simulations have been carried out using a one-dimensional, three-temperature, Lagrangian computer code medusa-kat [J. Appl. Phys. 60, 898 (1986)]. The basic capsule design is simple and consists of a thin gold microballoon coated with a beryllium ablator. A high-pressure gaseous deuterium–tritium (DT) fuel is filled in the capsule. The capsule is driven by a shaped radiation pulse having a prepulse corresponding to a radiation temperature of 100 eV and a main pulse with a temperature of 300 eV. A parameter study of the capsule gain, G versus input radiation energy over a range 52–76 kJ, has been carried out. The fuel mass has also been varied over a range 0.3–0.8 μg. It has been found that G∼1 can be achieved with an absorbed radiation energy of 70 kJ provided that the fuel mass lies between 0.5 and 0.7 μg. These simulations have also been repeated over the same parameter range but using an unshaped pulse with a constant radiation temperature of 300 eV. An overall reduction of 10%–30% has been observed in the gain curves in this case.
Full wave field solutions and power conservation for the ion cyclotron range of frequencies (ICRF) in nonuniform plasmas
Authors: Sund Richard S. , Scharer John E.
Published: July 2002

The subject of linear wave propagation and its associated power conservation in a slab geometry for waves in the ion cyclotron range of frequencies is treated. The governing differential equations and conservation relation are obtained using a Taylor series representation of the field evaluated to third order in the parameter, gyroradius/wavelength, which is assumed to be small for the cases examined here. This approach correctly incorporates the effects of transverse nonuniformity and is valid for all values of k∥. The local power conservation relation follows from a definition of local power absorption and a new companion general expression for kinetic flux based on fundamental principles. These expressions are evaluated to second order in gyroradius/wavelength in a numerical code, and results are presented for 3He fundamental minority and majority second harmonic cases. For fundamental minority 3He absorption, substantial reflection and mode conversion is found for lower parts of the k∥ spectrum with strong absorption, especially for the ion-Bernstein wave for k∥ >5 m−1. For second harmonic heating at higher 3He concentrations, strong absorption is found for lower values of k∥ with reduced reflection and mode conversion. For plasmas with substantial ion tail formation, tail absorption is found to dominate the absorption process with negligible mode conversion or reflections.
Large-scale structures in line-focused plasma
Authors: Kieffer J. C. , Chaker M. , Pépin H. , Baldis H. A. , Enright G. D. , Lafontaine B. , Villeneuve D. M.
Published: July 2002

Monochromatic imaging techniques are used to study quantitatively the large-scale structures produced in line-focused plasma by the intensity inhomogeneities of the driving laser. Pronounced perturbations of the mean electron density of the region of the plasma where the heliumlike emission takes place are observed along the line focus axis. These variations indicate the presence of local changes in the plasma hydrodynamics that could strongly affect the time history of the lasing action in x-ray laser recombination schemes.
The circular orbit linear theory of the axial injection orbitron maser (AXIOM) oscillator
Authors: Burke John M.
Published: July 2002

The linear theory of axial injection orbitron maser (AXIOM) oscillators is formulated analytically for circular electron orbits using the relativistic Vlasov equation and Maxwell’s equations. The theory treats transverse electric (TE) or transverse magnetic (TM) modes with sinusoidal axial field profiles. Owing to the simplicity of modeling circular electron orbits the theory is fully relativistic. The power transferred from the electron beam to the cavity fields is calculated in terms of a general distribution function and the cavity parameters. A cold beam distribution function is used to evaluate the theory. The results of the theory are presented in terms of the threshold value of the cavity Q times the electron beam current required for self-oscillation. The results show that TE modes with small azimuthal mode numbers have the strongest interaction and that modes with large azimuthal mode numbers resonate at lower electrostatic fields but require larger electron beam currents for self-oscillation.
Hot electron heating due to resonance absorption of moderate-intensity microwaves: Microwave simulation experiment of laser–plasma interaction
Authors: Mizuno Katsu
Published: July 2002

Microwave simulation experiments of laser-produced coronal plasma are presented. Extensive studies of hot electron heatings due to resonance absorption of moderate-intensity (v0/ve < 0.3, or Iλ2/Te < 3 × 1014 W/cm2 μm2/keV), p-polarized microwaves have been made. A simple theory is developed based on warm plasma wave breaking to explain the experimental results. The experiments agree well with the theory and electromagnetic computer simulation calculations. Therefore, it is shown that the microwave experimental results of resonance absorption scale to laser parameters.
A two-dimensional nonequilibrium model of cascaded arc plasma flows
Authors: Beulens J. J. , Milojevic D. , Schram D. C. , Vallinga P. M.
Published: July 2002

A nonequilibrium model is developed for the prediction of two-dimensional flow, electron and heavy particle temperatures, and number density distributions in cascaded arcs of monatomic gases. The system of strongly coupled elliptic partial differential equations describing plasma flow is solved by a numerical method based on a control volume with a nonstaggered numerical grid. The model is applied for the computation of both stagnation and flowing argon arc plasmas. The results show that the plasma in stagnation arcs is nearly in local thermal equilibrium (LTE), except very close to the wall, whereas fast flowing arc plasmas exhibit a significant degree of nonequilibrium, both close to the wall and in the inlet region. The results of the calculations are in satisfactory agreement with experimental data, both for the cases of stagnation and flowing argon cascaded arc plasmas.
Impact of magnetic turbulence on rf current drive
Authors: Giruzzi G.
Published: July 2002

The kinetic theory of rf current drive in tokamak plasmas in the presence of magnetic turbulence is investigated. The main effect of the turbulence is to cause radial diffusion of the superthermal electrons. For typical parameters of large machines, the diffusion time can be of the same order of the slowing-down time of the current carriers. In these conditions, a significant regime is identified in which the total driven current is enhanced by the effect of the radial diffusion. This is demonstrated analytically in the weak turbulence limit, and numerically confirmed in the general case, by means of a fully three-dimensional Fokker–Planck code, for both parallel and perpendicular velocity-space diffusion.
Radial diffusion induced by an ion cyclotron resonance layer in the Phaedrus-B tandem mirror
Authors: Ichimura M. , Wen Yun-Jian , Brouchous D. A. , Breun R. A. , Hershkowitz N. , Probert P. , Majeski R. , Intrator T. , Roberts D. , Keil D.
Published: July 2002

Evidence is given that the presence of an open ion cyclotron resonance layer in a tandem mirror can result in enhanced radial ion transport.
A model for nonlinear collisionless reconnection
Authors: Lovelace R. V. E. , Romanova M. M.
Published: July 2002

A model is developed for nonlinear collisionless reconnection of thin neutral layers. The model equations obey a conservation equation for the perturbation energy. The unstable solutions exhibit an initial interval of growth and subsequent large-amplitude fluctuations of the trapped magnetic flux and ion kinetic energy. The fluctuations arise from the transient formation, merging, and dissolution of magnetic islands.
Current drive by electron cyclotron waves in stellarators
Authors: Castejón F. , Alejaldre C. , Coarasa J. A.
Published: July 2002

Electron cyclotron current drive studies in stellarators are justified for the potential the method holds in the control and compensation of unwanted, pressure-induced, plasma currents, as it has been shown experimentally in Wendelstain VII-AS stellarator [Fusion Technol. 17, 62 (1990)]. In this paper, a method is described to estimate induced currents by electron cyclotron waves that is computationally fast enough to be included in a ray-tracing code, but is able to account for the complicated stellarator geometry. In particular, the modification of the current drive efficiency due to trapped particles effects is included in the calculation. A close-to-Maxwellian electron distribution function and a nearly linear regime are assumed. An application of the method to the magnetic configuration of the TJ-II stellarator under construction in Madrid [Fusion Technol. 17, 131 (1990)] is also presented.
Comments on ‘‘Ion acoustic parametric decay instability in laser-produced plasma with varying ionic charge’’ [Phys. Fluids B 3, 1983 (1991)]
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

an algebraic error is found in Equation (1) of the paper by Mizuno et al.1 Their measured threshold is analyzed following the corrected from of Equation (1). (AIP)
Energy conversion processes in the starting-up phase of externally driven reconnection
Authors: Kusano Kanya , Suzuki Yoshio , Sato Tetsuya , Nishikawa Kyoji
Published: July 2002

The energy conversion process as well as the plasma heating mechanism in the starting-up phase of driven reconnection is investigated in detail using two-dimensional magnetohydrodynamic (MHD) simulation. It is revealed that the compressional heating effect is predominant in this phase rather than the Ohmic and the viscous heating. The predominance of the compressional heating means that the plasma flow energy is considerably converted to the thermal energy. The strong compression takes place in the process during which the fast shock is generated in the downstream region of the reconnection point. The plasma heating due to nonsteady reconnection is briefly discussed as a promising mechanism for the anomalous ion heating, which is observed in reversed-field pinch (RFP) experiments.
Experimental study on the effect of nonuniform magnetic fields on the behavior of a cylindrical scrape-off layer plasma
Authors: Ohira T. , Tanaka S. , Miya K.
Published: July 2002

A boundary plasma fluid model that takes into account nonuniform magnetic fields in the previous study [T. Ohira et al., Phys. Fluids B 3, 2010 (1991)] is presented. From the numerical simulations based on the model, it was observed that cross-field particle fluxes in a cylindrical scrape-off layer (SOL) plasma are dramatically influenced by azimuthal (poloidal) changes in applied axial magnetic fields. In this study, experiments to confirm the numerical results and the phenomenon have been made. Two-dimensional (r,θ) distributions of plasma density, electron temperature, floating potential, and plasma potential in the cylindrical SOL plasma with and without nonuniform magnetic fields have been measured by 12 single probes located every 30° azimuthally. The nonuniform magnetic fields were produced by ferromagnetic amorphous shells. It was observed that the densities in the SOL plasma decrease where the axial magnetic fields have positive gradient azimuthally and increase where negative in the real experiment. In addition, the phenomenon could be simulated by the two-dimensional SOL plasma code developed in the previous study. It is thought that this might be due to azimuthal mobility flows and cross-field E×B flows which are influenced by nonuniform magnetic fields.
A model for energetic ion generation in an anode plasma
Authors: Duvall R. E. , Fruchtman A. , Maron Y. , Perelmutter L.
Published: July 2002

Mechanisms for energetic ion generation that could explain the observed ion energies in the anode plasma of a magnetically insulated ion diode [Phys. Rev. A 39, 5842 (1989)], are discussed. It is suggested that strong electric fields that result from large density gradients on few tens of micrometers near the anode cause the ion acceleration. Steady state as well as time-dependent accelerations are examined.
Space-charge-limited ion flow through an ionizing neutral layer
Authors: Duvall R. E. , Litwin C. , Maron Y.
Published: July 2002

Space-charge-limited ion flow through an ionizing layer of neutral atoms is studied. The ion flow is between two parallel conducting plates (anode and cathode) with an externally applied voltage between them. An expanding layer of neutral atoms is adjacent to the anode surface, extending a finite distance into the anode–cathode gap. All ions originate either from the anode surface or from the ionization of neutrals; electrons originate only from ionization. Electrons are strongly magnetized by an externally applied, time-independent direct current (dc) magnetic field directed across the ion flow. The ions are unmagnetized, all motion being perpendicular to the conducting plates. Two different models of the anode layer were used to analyze this problem: a multifluid steady-state model and a single fluid time-dependent model. From both models it was found that the anode surface becomes shielded after the ion flux from the ionizing layer becomes larger than the space-charge-limited flux of the reduced gap between the neutral layer and cathode. Comparison was made between the time-dependent model and results from magnetically insulated ion beam diode (MID) experiments. Using an initial areal density of neutral hydrogen and carbon equal to the final observed electron areal density, comparison was made between calculated plasma shielding times and upper bounds on the shielding time observed in experiments. It was found that a layer of neutral hydrogen must contain a minimum of 15% carbon (by number density) to explain the rapid electric field screening observed in experiments.
Calculation of βI and l
i for three-dimensional plasma configurations
Authors: Hirshman S. P.
Published: July 2002

The Shafranov equilibrium formulas for tokamaks relate volume-averaged thermal pressure and magnetic pressure to integrals of the measurable poloidal magnetic field energy at the plasma surface. Similar integral relations are derived that are valid for a general, three-dimensional plasma confinement geometry.
Universal instability in an electron hole filled with negative ions
Authors: Krall Nicholas A.
Published: July 2002

The stability of a layer of negative ions imbedded in a magnetized charge neutral plasma is analyzed. Universal instabilities are found to persist, even in the absence of a net current or net density gradient. Experiments which inject negative ions into the ionosphere may have observed such fluctuations.
High-resolution measurement of electron velocity distribution functions in a plasma
Authors: Maffei K. C. , Griem H. R.
Published: July 2002

A Thomson scattering diagnostic capable of achieving one-discharge, high-resolution spectra through use of a microchannel plate image intensifier coupled to an optical multichannel analyzer has been implemented on a theta-pinch device. The improved resolution permits measurements of local average electron velocities and current densities. First-order relativistic corrections and anomalous velocity-space fluctuations are directly observable. The enhanced fluctuations are attributed to electrons interacting with unstable waves.
Current and electric field measurements in a double inverse pinch device
Authors: Beeler Richard G. , Nickel John C.
Published: July 2002

Current and electric field measurements have been made near the magnetic neutral line in a double inverse pinch device. The current in the central region did not show any observable Petschek-type shock structure, but instead had the appearance of a simple current sheet. A disruption of the induced current system, called the impulsive flux transfer event, was observed. Measurements of the electric field in the central region showed that the space-charge field, −∇φ, was significant, especially during the current buildup and disruption. When measurements of the total electric field, E=−Ȧ−∇φ, were combined with current density measurements, the conductivity of the plasma in the central region remained sensibly constant throughout the current disruption.
Solitons on a cylindrical pinch with Hall effect
Authors: Shen M. C.
Published: July 2002

In this paper it is shown by an asymptotic method that the solitons which appear on a cylindrical magnetohydrodynamic (MHD) pinch with axial symmetry persist when the Hall current term is included in the governing equations. On the other hand, the results presented here indicate that, under the linear stability criterion for the cylindrical pinch, nonlinear waves that have an amplitude that does not decay with time can occur.
Two types of large-amplitude ion-acoustic fluctuations generated by the two-plasmon decay instability
Authors: Meyer J. , Bernard J. E.
Published: July 2002

An experimental survey of ion-acoustic turbulence using ruby-laser Thomson scattering in a CO2-laser-produced gas target plasma reveals two types of large-amplitude ion fluctuations which arise from the action of the two plasmon decay (2ωp) instability. The first type is a direct result of the ponderomotive force generated by a pair of 2ωp decay waves driving ion waves at the difference wave vector. The second type is observed temporally delayed against the 2ωp instability and consists of a return current-driven ion-acoustic instability. The return current is a result of the electrostatic plasma potential generated during energetic electron emission by the 2ωp waves.
The development of a solitary wave in a channel of variable elliptical cross section
Authors: Cai Y. H. , Shen M. C.
Published: July 2002

In this paper the disintegration of a solitary wave in an elliptical channel is studied when it propagates from one uniform section of the channel into another through a transition section. It is found that for an elliptical channel with constant width, fission of solitons is also possible when the solitary wave moves into a uniform section of larger depth.
The role of magnetic reconnection and differential rotation in spheromak formation
Authors: Guzdar P. N. , Finn John M. , Whang K. W. , Bondeson A.
Published: July 2002

The physical processes involved in the formation of a spheromak are studied. Using one-fluid axisymmetric resistive magnetohydrodynamic equations together with a temperature equation, investigations of the essential physics aspects of the formation specifically for the new Maryland Spheromak (MS) [J. Antoniades, C. Chin-Fatt, A. DeSilva, G. Goldenbaum, R. Hess, and R. Shaw, in Proceedings of the Sixth U. S. Symposium on Compact Torus Research, Princeton, 1984 (Princeton Plasma Physics Laboratory, Princeton, NJ, 1985), p. 65] have been performed. These studies elucidate the role of differential rotation in accelerating the penetration of toroidal field Bφ and in forcing I≡ rBφ to be a flux function. Another important aspect of the formation is the magnetic reconnection that occurs at the X point between the two reversal coils. A strong toroidal current is generated by the reconnection, and the X point is converted into an O point that finally becomes the magnetic axis of the spheromak. In this latter phase of reconnection, the plasma is heated very strongly. Studies have also been done on the effect of the difference in phase and time scales for the reversal and Iz banks for the purpose of optimization. The effect of radiation from carbon and oxygen is examined and found to have very little effect on the proposed formation scheme. The results are also relatively insensitive to changes in the anomalous electron thermal conduction models used.
Comments on ‘‘Study of gain, bandwidth, and tunability of a millimeter-wave free-electron laser operating in the collective regime’’ [Phys. Fluids 2
6, 2683 (1983)]
Authors: Bekefi G. , Fajans J.
Published: July 2002

An analysis of recent free-electron laser experiments in combined wiggler and strong axial magnetic fields shows that near resonance, Ω∥=kwv∥, the proximity of the cyclotron maser instability precludes an unambiguous identification of the radiation mechanism (Ω∥ is the cyclotron frequency in the axial field, kw is the wiggler wavenumber, and v∥ the axial electron velocity).
The x-ray observations of a CO2-laser-produced plasma
Authors: Meyer J. , Popil R.
Published: July 2002

The x-ray observations of a CO2-laser-produced plasma are reported. An underdense plasma is produced by the interaction of intense ≤1014 W/cm2 CO2 laser light with a laminar gas jet target. Initial observations at moderate <1013 W/cm2 intensities and low target density reveal an electron temperature scaling with laser intensity that is consistent with inverse bremsstrahlung absorption calculations. At higher intensities and target molecular density, a suprathermal electron temperature appears that is invariant with laser intensity. However, the variation of the detected x-ray signal with laser intensity can be correlated to the two-plasmon decay and stimulated Raman scattering instabilities that have been observed for this interaction.
The ablative implosion of spherical shells
Authors: Piriz Antonio Roberto
Published: July 2002

An analytic model is developed to describe the final state reached by a gas-filled spherical shell compressed by a constant ablation pressure. The model takes into account the solid compressibility and the mass loss by ablation, and gives the magnitudes that characterize the state of the gas and solid when the implosion stagnates. The model provides an adequate physics interpretation and an extension of a recently reported model [B. Ahlborn, M. H. Key, and A. R. Bell, Phys. Fluids 25, 541 (1982)]. The restrictions of the model imposed by the approximations adopted are discussed. The results are compared with available numerical simulation data and with the previous model already mentioned, and the main differences between them are discussed.
Transport scalings in reversed field pinch plasmas
Authors: Yamagishi T.
Published: July 2002

A simple saturation condition for a magnetic turbulence is derived based on the nonlinearity of electron orbit diffusion, which is applied for derivation of transport coefficients in the reversed field pinch plasmas. The scalings as well as magnitudes of the plasma temperature and energy confinement time in the collisionless limit agree fairly well with those of reversed field pinch experiments.
Parametric excitations of instabilities in bubbly flow
Authors: Mond Michael
Published: July 2002

The excitation of instabilities caused by the parametric coupling between an external source and the normal modes of bubbly flow is investigated. It is shown that the presence of two species of bubbles gives rise to two types of parametric excitations. In the first one the frequency of the external source is smaller than the larger of the natural frequencies of the species. As a result, the excited mode is nonoscillatory. In the second type the driving frequency is bigger than both natural frequencies and the resulting instability is oscillatory. The threshold amplitude of the external source is calculated as well as the growth rates and frequencies of the different types of instabilities.
Radio frequency breakdown and electron confinement time in a stellarator
Authors: Wróblewski D. , Shohet J. L.
Published: July 2002

Radio frequency ( f=350 kHz) inductive breakdown has been studied in a toroidal stellarator. The time delay between the application of the radio frequency pulse and the buildup of detectable plasma density is measured as a function of the discharge parameters. It is shown that the confinement time of energetic electrons (∼100 eV), produced during the breakdown, may be deduced from the breakdown characteristics. The experimentally determined confinement times are compared with predictions of neoclassical transport theory and are found to have functional dependence on the discharge parameters approximately consistent with plateau scaling, but to be shorter than predicted by the neoclassical diffusion model.
The linear stability of a spherical liquid layer subjected to a unipolar charge injection
Authors: Oliveri S. , Atten P.
Published: July 2002

An insulating isothermal spherical liquid layer subjected to the Coulomb force resulting from ionic injection by one electrode is considered. The electrohydrodynamic stability of this liquid layer is studied using a linear analysis. The critical conditions of stability are determined for a wide range of system parameters: from weak to strong injection, by either inner or outer electrode, and for a ratio of the radii varying over a large range of its permitted values. Injection by the outer sphere is the most unstable situation for weak injection; the converse is true for strong injection. This is explained using qualitative arguments.
Amplification of electromagnetic waves by a ring–beam distribution of moderately relativistic electrons
Authors: Shi B. R. , Gaffey J. D. , Wu C. S.
Published: July 2002

In many cases of astrophysical interest, important radiation processes have been observed in regions where the plasma frequency is much larger than the cyclotron frequency, ω2e/Ω2e ≫1. It has been found that the induced emission of the eigenmode of the cold background plasma, i.e., ω=(ω2e +k2c2)1/2, by the presence of a ring–beam distribution of energetic electrons, becomes progressively insignificant as ωe/Ωe increases. A new and important unstable beam-cyclotron mode that is excited by the energetic electrons has been found. This instability has relatively large growth rates for oblique propagation. The group velocity of the unstable mode along the ambient magnetic field is approximately equal to the parallel beam velocity. Numerical solutions of the dispersion equation are presented and discussed.
A simple composite time scale model for third-order scalar transports
Authors: Chung Myung Kyoon , Kyong Nam Ho
Published: July 2002

A simple composite time scale model for third-order scalar-velocity correlations under negligible effect of buoyancy is proposed as τ3∝τu(1+bτθ/τu), where τu is a dynamic time scale (≡q̄2/ε) and τθ is a scalar time scale (≡θ̄2/εθ). The constant b represents relative influence of τθ over τu on the spatial transport of second-order moments. Its value depends on the power of the scalar fluctuations θ in the third-order moments. The model predictions are in excellent agreement with experimental data in a turbulent scalar diffusion field behind a line heat source in a uniform shear flow.
Illustrative examples of streaklines in unsteady vortices: Interpretational difficulties revisited
Authors: Kurosaka M. , Sundaram P.
Published: July 2002

As a preventive measure against false interpretations of the streakline pattern in unsteady flows, streaklines calculated for an isolated vortex and the infinite Kármán vortex street are presented, which appear to create a visual impression of apparent growth and flow reversal, while in reality they do not exist.
Electron-cyclotron heating of a tokamak reactor with the extraordinary mode
Authors: Fidone I. , Giruzzi G. , Krivenski V. , Ziebell L. F. , Mazzucato E.
Published: July 2002

Electron-cyclotron resonance absorption for oblique propagation at frequency significantly below the central electron gyrofrequency is considered. It is shown that the extraordinary mode for external launching is suited for additional heating in a tokamak reactor. The properties of wave damping at downshifted frequencies are discussed and are found to differ from those at f≊fc. Using the ray-tracing code, the wave damping of the e mode is investigated at high densities for which strong refraction of the wave beam occurs.
Bridging in vortex reconnection
Authors: Kida S. , Takaoka M.
Published: July 2002

The mechanism of vortex reconnection is investigated by solving the Navier–Stokes equation numerically starting with a closed knotted vortex tube, which gives a nonzero helicity. A new type of vortex reconnection mechanism—bridging—is observed. Small regions of high vorticity bursting out of the vortex tube become larger and bridge different portions of the tube. A relation between the change of the helicity and the mechanism of the vortex reconnection is discussed.
Collisional statistical model for superstrong plasma turbulence
Authors: Benford Gregory
Published: July 2002

Recent turbulent beam–plasma experiments revealed that the probability distribution function for the Langmuir electric fields P(E) is not a power law in E, contrary to many predictions. Instead, P(E) ∝ exp (−E2). It is hypothesized that for turbulence that has W=〈E2〉/4πnT∼1 (‘‘superstrong’’ regime) such turbulence contains self-compressed zones of high field (‘‘cavitons’’) that can collide faster than they decay. If so, simple Markovian collisions yield the observed P(E) if decay is sufficiently slow. This implies that cavitons are much longer lived than Hamiltonian theory predicts. A break in P(E) at high E is predicted and several experiments in the superstrong regime are suggested to test these ideas.
Role of atom–atom inelastic collisions in two-temperature nonequilibrium plasmas
Authors: Kunc Joseph A.
Published: July 2002

The contribution of inelastic atom–atom collisions to the production of electrons and excited atoms in two-temperature (with electron temperature Te, atomic temperature Ta, and atomic density Na), steady-state, nonequilibrium atomic hydrogen plasma is investigated. The results are valid for plasmas having large amounts of atomic hydrogen as one of the plasma components, so that e–H and H–H inelastic collisions and interaction of these atoms with radiation dominate the production of electrons and excited hydrogen atoms. Densities of electrons and excited atoms are calculated in low-temperature plasma, with Te and Ta≤8000 K and 1016 cm−3 ≤Na≤1018 cm−3, and with different degrees of the reabsorption of radiation. The results indicate that inelastic atom–atom collisions are important for production of electrons and excited atoms in partially ionized plasmas with medium and high atomic density and temperatures below 8000 K.
Diamagnetic wells in axisymmetric mirrors
Authors: Hedrick C. L. , Owen L. W. , Quon B. H. , Dandl R. A.
Published: July 2002

Here the question of whether hot-electron rings have reversed gradients in B and ∮ dl/B in symmetric mirrors is reexamined. The focal point for this reexamination is a particular data set from a symmetric mirror device. Using standard least-squares fitting techniques, this and three other cases (having comparable stored energy) have been fitted with a five-parameter function for the perpendicular pressure. Three of the four cases show reversal of the gradients. All cases pass standard tests for goodness of fit. The probable errors, deduced by fitting a total of ten experimintal data sets, were found to be consistent with the single-measurement accuracy. Sensitivity tests for random and systematic error were performed. The errors would have to be implausibly high in order to change the conclusions regarding gradient reversal. More complex functions for the perpendicular pressure, involving more parameters, have been examined to see if radial asymmetries or pressure pedestals extending to large radius are indicated by the data and whether such hypotheses change the conclusions about gradient reversal. They do not. It is concluded that gradient reversal occurs in symmetric mirrors.
The thin-layer effect and interfacial stability in a two-layer Couette flow with similar liquids
Authors: Renardy Yuriko
Published: July 2002

The linear stability of Couette flow composed of two layers of immiscible fluids, one lying on top of the other, is considered for the special case when the two fluids have similar mechanical properties. The interfacial eigenvalue is found in closed form by considering the two-fluid problem as a perturbation of the one-fluid problem. The importance of the role played by the viscosity difference, when one of the fluids is in a thin layer, is illustrated.