bluuebunny/crossref_metadata_2025 huggingface dataset:
Plasma frequency radiation in tokamaks
Authors: Swartz K. , Hutchinson I. H. , Molvig Kim
Published: March 2003

Radiation from relativistic runaway electrons is considered as a source for plasma frequency radiation in tokamaks. Two specific mechanisms, Cerenkov emission and radiation produced by nonlinear coupling of plasma and acoustic waves, are studied. In many cases the Cerenkov emission provides a reasonable spectral fit. It can also be used to measure the runaway current, and to estimate the runaway cutoff velocity. The nonlinear emission is found to be negligible unless the acoustic waves are enhanced by about two orders of magnitude above the thermal level (or the plasma waves correspondingly enhanced above their superthermal level). Some observations, though, indicate the need for significant nonlinear (or other) emission in addition to the Cerenkov emission. For some typical Alcator data, the Cerenkov model gives a runaway current of 2.5% of the Ohmic current, and a cutoff energy of 1.5 MeV.
Atmospheric type modes in laser fusion targets
Authors: Scannapieco A. J.
Published: March 2003

The fluid stability of laser-fusion targets is considered as a stability problem of a pseudo-planetary atmosphere. Three atmospheric type modes are studied; acoustic, gravity, and Lamb modes. The changing character (i.e., growing, oscillatory, or growing-oscillatory) of each is investigated as a function of the density-gradient scale length H of the fluid. A growing class of modes is found which is distinct from the gravity (i.e., Taylor) mode if a gradient in entropy exists in the fluid. These modes are shown to be overstable Lamb modes. Also, the gravity mode is only stable for a distinct band of values of H. These values, at which the density and Lamb modes change character, are derived from the dispersion relation for the modes. Finally, the consequences for laser targets are discussed.
Observations of the onset of flow instabilities in constricted tubes
Authors: Lopez J. L.
Published: March 2003

A general stability boundary is determined which establishes the minimum Reynolds number Ri and percent area ratio for the onset of flow instabilities. Both nonaxisymmetrix and axisymmetric disturbances were observed. The existance of a finite extent turbulence region is also described.
Exact symmetry of the second-order nonlinear conductivity for a relativistic turbulent plasma
Authors: Brandt Howard E.
Published: March 2003

The second-order nonlinear conductivity tensor for a relativistic turbulent plasma is shown to have an exact symmetry which, neglecting resonant wave-particle interaction, becomes the widely known symmetry related to the nondissipative nature of the nonlinear current.
Comments on ‘‘Adiabatic modifications to plasma turbulence theory’’
Authors: Krommes John A.
Published: March 2003

Comments on ’’adiabatic modification’s’’ to plasma turbulence theory is presented. (AIP)
Reconnection conditions for a coaxial plasma gun
Authors: Berk Herbert L. , Hammer James H. , Shearer James W.
Published: March 2003

A fluid model for the flow conditions necessary to form a compact torus from the plasma ejected by a coaxial plasma gun is developed. This is done by finding the conditions for which the steady-flow equations break down. Results are found for two cases; variable external flux and variable outer-wall radius.
Relaxation toward states of minimum energy in a compact torus
Authors: Riyopoulos S. , Bondeson A. , Montgomery D.
Published: March 2003

A finite-difference, resistive, magnetohydrodynamic code is used to follow the long-time evolution of decaying nonequilibrium states inside a rigid, perfectly conducting cylindrical boundary. The energy-to-magnetic helicity ratio decays toward a minimum, in accord with a conjecture of Taylor. The magnetic Reynolds number is considerably higher than the mechanical Reynolds number for the regime considered. The energy, which is mostly magnetic, tends to decay in bursts associated with current filamentation and magnetic reconnection.
Electrostatic modification of variational principles for anisotropic plasmas
Authors: Antonsen T. M. , Lee Y. C.
Published: March 2003

The kinetic energy principle is modified by the presence of an equilibrium electrostatic potential and the inclusion of the quasineutrality condition. By identifying terms that are fluid-like (local) and kinetic (nonlocal), one can express the change in potential energy, ΔW, in a form which is both variational and minimizing and also displays, clearly, the electrostatic effects on the various free energy sources. The kinetic energy principle determines the stability of the plasma to perturbations that grow on the time scale associated with the streaming of particles along field lines. Recently, a new variational principle has been proposed which determines the stability of the plasma to perturbations that grow on the time scale associated with the drifting of particles across field lines. The modifications of this principle due to electrostatic effects will also be considered, and cases in which necessary and sufficient conditions for instability can be found will be discussed.
Reynolds number dependence of velocity structure functions in turbulent shear flows
Authors: Antonia R. A. , Satyaprakash B. R. , Chambers A. J.
Published: March 2003

Moments, up to order eight, of the structure function of the steamwise velocity fluctuation, have been measured in both laboratory and atmospheric turbulent shear flows. The Reynolds number dependence of structure functions evaluated for a separation equal to the Taylor microscale is closely approximated by both lognormal and β models, at least for moments up to order six. Both models predict identical inertial subrange behavior for the sixth-order structure functions. This prediction is in good agreement with experiment when the value of 0.2 is used for the universal exponent μ. This value has been obtained from correlations of the squared velocity derivative fluctuations, calculated for both laboratory and atmospheric measurements over the inertial subrange. The slope of the correlation is related to that of the sixth-order structure function, in agreement with a conjecture by Frisch, Sulem, and Nelkin. When plotted against moments of order n+2, moments of even order n exhibit a power-law behavior whose exponent is in closer agreement with the lognormal than the β model.
Effect of overtaking disturbances on the motion of a shock wave due to an intense explosion
Authors: Yousaf M.
Published: March 2003

The Chester, Chisnell, and Whitham approximation has been applied to the Sedov problem of an explosion in a medium of uniform density. It is noted that the error in the propagation parameter K is 70% or more. The reason for this discrepancy has been explained.
Evaluation of multipole moments over the current density in a tokamak with magnetic probes
Authors: Shkarofsky I. P.
Published: March 2003

Zakharov and Shafranov’s method for deriving plasma displacement and averaged shape in a tokamak from magnetic probes positioned along a contour around the plasma cross section is extended in several ways. Formulas are derived which allow vertical as well as horizontal displacements with any ratio of minor contour width to major radius, asymmetric as well as symmetric multipole moments, and explicit relations for rectangular as well as circular contours. The relations in the literature for a circular contour are also re-examined and corrected equations are given. It is shown that a rectangular contour can provide the same information.
Wall losses for a single-species plasma near thermal equilibrium
Authors: Driscoll C. F.
Published: March 2003

The effect of a radially bounding wall on a magnetically confined single-species plasma near thermal equilibrium is considered. Solutions to the like-particle collisional transport equation are obtained; the boundary conditions at the wall follow from simple physical considerations. Integral constraints on the plasma evolution imply that only a fraction of the plasma can ever be lost to the wall. Analytic estimates and numerical solutions give the scaled wall flux in terms of the unperturbed equilibrium density at the radius of the wall.
Kink instabilities in long ion layers
Authors: Harned Douglas S.
Published: March 2003

Kink instabilities in long ion layers immersed in a dense background plasma are studied. A numerical extension of the analytic model of Lovelace indicates that these instabilities will occur for values of the self-magnetic field index below those predicted previously. A quasineutral hybrid simulation code has been used to verify these lower thresholds. The simulations also show that the end of exponential growth occurs due to a nonlinear shift in the betatron frequency at large amplitude, producing an increase in layer thickness and a layer which has many non-axis-encircling ions.
Operation limits of multipole ion sources
Authors: Goede A. P. H. , Green T. S.
Published: March 2003

Experiments in which the effective anode area of a multipole ion source is reduced, in order to enhance its ionization and gas efficiency, show that there is a minimum area below which efficient operation becomes impossible. This phenomena is analyzed in terms of a model based on particle balance equations and is shown to be closely related to the reversal in plasma potential with respect to anode potential. The theoretically predicted operation regime is shown to be in reasonable agreement with that found in a variety of multipole sources. Comparison is also made with the limitations set by cathode area.
A bimodal Maxwellian distribution as the equilibrium solution of the two-particle regime
Authors: Sagara Kenshi , Tsugé Shunichi
Published: March 2003

The Maxwellian distribution known as the one-particle equilibrium solution is generalized to the two-particle regime where homogeneous macroscopic fluctuations in the density, the velocity, and the temperature prevail. The equilibrium distribution has the form of a bimodal Maxwellian, namely, the average of two Maxwellians with thermodynamic parameters greater and smaller than the mean values by the root-mean-squared fluctuations. This result is shown to hold for monatomic gases with arbitrary intermolecular potentials. Turbulent reaction rate calculated with this distribution can be by the order of magnitude large compared with the Arrhenius law, in other words, far beyond the first order smallness of the temperature fluctuation if the activation energy is sufficiently high.
Nonlinear diffusion of test particles in the presence of an external conservative force
Authors: Boffi V. C. , Spiga G.
Published: March 2003

The diffusion of certain test particles that in the presence of an external constant conservative force, undergo collisions against the field particles of an infinite homogeneous host medium as well as between themselves, is studied on the basis of an integral reformulation of the relevant stationary spatially-independent nonlinear integrodifferential Boltzmann equation. The existence and uniqueness of the solution for the distribution function of the test particles considered is investigated by an application of the contracting mapping principle for both the general nonlinear case and for the linearized one as resulting through an appropriate decomposition of the sought distribution function. Also, the case when the scattering probability is represented as a factorized expansion of finite rank is discussed in some detail.
Suppression of the drift-cyclotron instability by lower-hybrid-drift turbulence
Authors: Diamond P. H. , Myra J. R. , Aamodt R. E.
Published: March 2003

The nonlinear interaction of the drift cyclotron and lower-hybrid-drift modes is examined. A renormalized equation for the drift cyclotron ion response is derived and solved. Lower-hybrid-drift modes comprise the turbulent background. For levels of lower-hybrid-drift turbulence of eΦ/Ti∼(me/mi)1/2(ρi/L)3/4, the growth rate of the drift cyclotron mode begins to be reduced by ion cyclotron resonance broadening, which is the dominant nonlinear effect.
The interaction between topography and a nonlinearly stratified rotating fluid
Authors: Davies Peter A. , Rahm Lars
Published: March 2003

Laboratory experiments have been conducted in which a spherical solid obstacle has been towed steadily through a thermally stratified rotating fluid in order to determine the effects of the density-profile shape upon the attenuation with height of the disturbance produced by the obstacle. The existence of a cutoff level in the fluid beyond which the disturbance is not detected is illustrated, and the dependence of this level upon local variations in density profile is studied. Measurements of velocity above and behind the obstacle are reported.
The boundary layer behind the blast wave generated by a very intense explosion on a flat ground
Authors: Dyment A.
Published: March 2003

A strong explosion occurring on a flat ground generates a blast wave behind which an unsteady boundary layer sets in. It is shown that when the ground is insulated the boundary layer develops a self-similar structure and its thickness does not depend directly on time to the first approximation.
Analytic bumpy torus equilibrium
Authors: Hazeltine R. D. , Catto P. J.
Published: March 2003

The equilibrium magnetic field of a bumpy torus confinement system is analyzed by expansion in the inverse aspect ratio. Decomposition of the plasma volume into low-beta and high-beta regions permits the derivation of relatively explicit formulas, which should be useful in bumpy torus transport and stability studies. In the low-beta regions, which typically occupy most of the plasma volume, the magnetic field is computed through first order in the inverse aspect ratio, including plasma pressure correction terms. The high-beta region is treated in a scalar pressure, thin-ring approximation, without toroidal corrections; connection formulas for the change in flux-surface geometry across the high-beta annulus are obtained, together with an expression for the field inside the annulus. The analysis of both regions is completely general with respect to bump amplitude and bump shape.
Inductive ion acceleration and heating in picket fence geometry: Theory and simulations
Authors: Leboeuf J. N. , Dawson J. M. , Ratliff S. T. , Rhodes M. , Luhmann N. C.
Published: March 2003

Particle simulations and analytic theory confirm the experimental observation of preferential ion acceleration and heating by an inductive electric field Edc in picket-fence geometry. The ions which are unmagnetized over most of the current channel are freely accelerated by the inductive field; the magnetized electrons are tied to the field lines and do not run away as long as the binding ev×B/c force is greater than the detrapping inductive force eEdc. Consequently, most of the current is carried by the ions which are also Ohmically heated.
Nonlinear theory of a positive column in a magnetic field
Authors: Xu F. , Tang F. L. , Chen L. S.
Published: March 2003

A nonlinear theory of an intermediate pressure discharge column in a magnetic field is presented. Motion of the neutral gas is considered. The continuity and momentum transfer equations for charged particles and neutral particles are solved by numerical methods. The main result obtained is that the rotating velocities of ionic gas and neutral gas are approximately equal. Bohm’s criterion and potential inversion in the presence of neutral gas motion are also discussed.
Destabilization of drift waves by a nonuniform radial electric field
Authors: El-Nadi Adel , Hassan Hatem
Published: March 2003

It is shown that drift waves can be destabilized in the presence of a nonuniform electrostatic field. This may explain the anomalous diffusion observed in tokamaks.
Point vortex motions with a center of symmetry
Authors: Aref Hassan
Published: March 2003

The equations of motion for point vortices are well known to preserve certain discrete symmetries of the initial state. The case of a center of symmetry is considered in detail here, since this particular instance seems to have been overlooked in the classical literature. This symmetry provides a generalization of the early studies by Gröbli and Greenhill wherein several axes of symmetry are present, a case which leads to an effective one-body problem. The center of symmetry yields an effective two-body problem which is Hamiltonian and integrable. As an example the ‘‘double alternate ring’’ configurations, circular analogs of the vortex street introduced by Havelock, are considered. A fully nonlinear mode wherein these double rings asymptotically dissolve into freely moving vortex pairs is found analytically. The paper concludes with a discussion of the relevance of such modes to our understanding of the disintegration of vortex streets in two-dimensional flow.
Theory of cylindrical and spherical Langmuir probes in the limit of vanishing Debye number
Authors: Parrot M. J. M. , Storey L. R. O. , Parker L. W. , Laframboise J. G.
Published: March 2003

A theory has been developed for cylindrical and spherical probes and other collectors in collisionless plasmas, in the limit where the ratio of Debye length to probe radius (the Debye number λD) vanishes. Results are presented for the case of equal electron and ion temperatures. On the scale of the probe radius, the distributions of potential and density in the presheath appear to have infinite slope at the probe surface. The dimensionless current–voltage characteristic is the same for the cylinder as for the sphere, within the limits of error of the numerical results, although no physical reason for this is evident. As the magnitude of probe potential (relative to space) increases, the current does not saturate abruptly but only asymptotically; its limiting value is about 45% larger than at space potential. Probe currents for small nonzero λD approach those for zero λD only very slowly, showing power-law behavior as function of λD in the limit as λD → 0, with power-law exponents less than unity, resulting in infinite limiting derivatives with respect to λD.
Magnetohydrodynamic fluctuations near thermal equilibrium
Authors: Hameiri Eliezer , Rose Harvey A.
Published: March 2003

A statistical equilibrium ensemble for a class of nonlinear, ideal, compressible magnetohydrodynamics models is constructed. If the departure from thermal equilibrium is due to helical magnetic-field configurations, then the magnetic-field fluctuation spectrum differs profoundly from that predicted by Landau’s theory of fluid fluctuations which involves zero helicity. Some of the results of that theory however are recovered, as far as fluctuations of fluid-dynamical (nonmagnetic) variables are concerned, in the linear limit of our nonlinear theory. Comparisons are also made with related models of incompressible plasma turbulence.
The gyroelastic screw pinch
Authors: Kerbel Gary D.
Published: March 2003

A study is made of a scale model in three dimensions of a guiding center plasma within the purview of gyroelastic (also known as finite gyroradius-near theta pinch) magnetohydrodynamics. The nonlinear system sustains a particular symmetry called isorrhopy which permits the decoupling of fluid modes from drift modes. Isorrhopic equilibria are analyzed within the framework of geometrical optics, resulting in local dispersion relations and ray constants. A general scheme is developed to evolve an arbitrary linear perturbation of a screw pinch equilibrium as an invertible integral transform over the complete set of generalized eigenfunctions defined naturally by the equilibrium. Details of the structure of the function space and the associated spectra are elucidated. Features of the global dispersion relation owing to the presence of gyroelastic stabilization are revealed. An energy principle is developed to study the stability of the tubular screw pinch.
Reversed-field flux-trapping in a low-compression theta pinch
Authors: Knox S. O. , Meuth H. , Ribe F. L. , Sevillano E.
Published: March 2003

Rapid field reversal is employed to trap an initial negative-magnetic-field bias flux in a 300-cm long theta pinch having an inner radius of 10 cm. A large fraction (40%–75%, depending on filling pressure) of the initial bias flux is trapped after the implosion (∼1 μsec); this fraction then diminishes to a plateau value of 25% to 35% in approximately 2 μsec. During this decay, magnetic probe measurements indicate an average anomalous resistivity one order of magnitude greater than classical. The open-line configurations obtained have large separatrix (zero enclosed flux) radii and are present for the duration of the applied magnetic field. The presence of the reversed bias field lowers the plasma temperature by a factor of two.
Tearing mode stability of elongated field-reversed plasmas without toroidal field
Authors: Berk H. L. , Schnack D. D. , Sayer J. M.
Published: March 2003

The marginal stability of long thin field-reversed plasmas to axisymmetric tearing modes is examined. It is found that stability improves greatly as flux is excluded from the plasma. Such naturally occurring states may account for the lack of tearing mode activity in many experiments. Consideration is also given as to whether, in three dimensions, the tearing mode is an instability or a condition for the breakdown of equilibrium. An example is given that demonstrates that it is an instability. Idealized analytic results and more realistic numerical results are presented.
Electromagnetic kinetic toroidal eigenmodes for general magnetohydrodynamic equilibria
Authors: Rewoldt G. , Tang W. M. , Chance M. S.
Published: March 2003

A comprehensive analysis of low-frequency, high-toroidal-mode-number linear eigenmodes for tokamaks is presented. The most significant new features of this stability study are that the calculation is interfaced with a general numerical magnetohydrodynamic equilibrium, and that it is fully electromagnetic. The ballooning formalism is employed and all important kinetic effects, including those of trapped particles, are retained. In particular, the familiar trapped-electron drift-wave frequency regime is considered and results are presented for three sequences of artificial equilibria; one of increasing β(≡plasma pressure/magnetic pressure) values; one of varying equilibrium shape, from inverse-D to circular to normal-D; and one of increasing vertical ellipticity. The analysis is then applied to a realistic (self-consistent) high-β equilibrium generated with data obtained from the ISX-B tokamak experiment. Here it is found that for the usual trapped-electron branch, kinetic microinstabilities appear to be present over a wide range of toroidal mode numbers.
Establishment of magnetic coordinates for a given magnetic field
Authors: Boozer Allen H.
Published: March 2003

A method is given for expressing the magnetic field strength in magnetic coordinates for an arbitrary toroidal, scalar pressure, equilibrium with magnetic surfaces. The field strength in magnetic coordinates is central to the study of equilibrium, stability, and transport in asymmetric plasmas. While doing these calculations, it is assumed that the plasma equilibrium is known.
Reduction of transport in a tokamak reactor by externally driven turbulence
Authors: Hershcovitch Ady , Politzer Peter A.
Published: March 2003

The possibility of curtailing trapped particle modes and reducing ripple diffusion in a tokamak reactor by external turbulence is investigated. A ’’classical’’ analysis of the problem shows that the power level required is low for ripple detrapping, but it is very high for suppression of trapped particle modes. If, however, strong turbulence effects including clump formation are considered, periodically pulsed turbulence can be used to eliminate trapped particles and significantly reduce the cross-field diffusion at an average power level which may be acceptable in a reactor.
An analytic model for laser-driven ablative implosion of spherical shell targets
Authors: Ahlborn Boye , Key M. H. , Bell A. R.
Published: March 2003

A simple analytic model is developed to describe the plasma parameters produced at peak compression when gas-filled spherical shell targets are imploded by the ablation pressure produced by irradiation with constant laser power. The range of validity of the model is discussed and, in particular, the constraints due to preheating by hot electrons. Comparison with numerical simulation verifies the accuracy of the model. The implications of the scaling of compressed plasma parameters with target and laser parameters are discussed.
Nonlinear two-dimensional sail theory
Authors: Vanden-Broeck Jean-Marc
Published: March 2003

Steady two-dimensional flow past a sail is considered. The sail is assumed to be supported by two masts. The flow and the shape of the sail are determined as functions of the direction α of the flow at infinity and the Weber number λ. The full nonlinear problem is formulated as an integrodifferential equation for the shape of the sail. This equation is discretized and solved numerically by Newton’s method. Sail profiles, the slack in the sail, and the lift coefficient are presented for various values of α and λ.
Plasma heating from upper-hybrid mode conversion in an inhomogeneous magnetic field
Authors: Lin A. T. , Lin Chih-Chien , Dawson J. M.
Published: March 2003

Large-amplitude short-wavelength electrostatic fields can be generated at the upper-hybrid resonance layer due to the mode conversion process when an incident wave of appropriate frequency falls on a plasma. As a consequence, a host of nonlinear and kinetic phenomena can take place. Among them, the generation of energetic electrons due to the breaking of adiabatic electron motion in the wave field, the enhancement of electron second harmonic cyclotron heating at the double resonance (ωuh = 2ωce), and parametric instabilities in which the upper-hybrid mode decays into another upper-hybrid mode and a low-frequency mode. Investigations of these phenomena through the use of computer simulations are presented. The parametric process is observed to produce a substantial number of energetic ions localized at the resonance layer.
Quantum-kinetic theory of electron heating in plasmas by high-frequency electromagnetic waves
Authors: Kim S. H. , Wilhelm H. E.
Published: March 2003

The heating rate of electrons in laser-irradiated plasmas is derived from the quantum-mechanically extended Vlasov equation. The heating of electrons is shown to be due to (i) the electron velocity space diffusion by plasma waves, which are induced by the laser field (classical turbulent heating), (ii) the microscopic quantum mechanical interaction between each electron and the laser field in the electric field of the plasma waves (collisionless inverse bremsstrahlung), and (iii) collisional inverse bremsstrahlung in the static plasma field. A quasilinear equation for the changes of the electron distribution function by these three concomitant mechanisms is derived, and the heating rates of the electrons are calculated. It is shown that the collisionless inverse bremsstrahlung is dominant for hot plasmas in most experimental cases. The so-called anomalous heating is identified as being due to collisionless inverse bremsstrahlung, and is explicitly expressed in terms of the wavenumber of the plasma wave and the plasma parameters, when the turbulence is due to the ion acoustic instability and is stabilized by ion trapping.
Nonlinear solutions of drift cyclotron modes in mirror devices
Authors: Laedke E. W. , Spatschek K. H.
Published: March 2003

Subluminary solitary wave solutions for drift cyclotron modes are linearly unstable in one dimension, but are stabilized in the presence of sufficient nonlinear frequency shift and consistent wave amplitude. Growth rates and the stability boundary are presented.
Relativistic electron trajectories in free electron lasers with an axial guide field
Authors: Freund H. P. , Drobot A. T.
Published: March 2003

The single-particle trajectories of relativistic electrons are studied in a magnetic field configuration which consists of a uniform axial guide field as well as a helical periodic wiggler field. The orbit equations are shown to reduce to a single nonlinear differential equation for the axial velocity, which has the form of the general elliptic equation. This equation is solved, and the classes of solutions are discussed for a wide range of experimental parameters. Solutions with constant axial velocity are found only as limiting cases of the generally anharmonic oscillations in the axial velocity.
Nonlinear evolution of Buneman instability. II. Ion dynamics
Authors: Hirose A. , Ishihara O. , Langdon A. B.
Published: March 2003

The nonlinear evolution of the one-dimensional Buneman instability after saturation due to electron trapping is studied both by analysis and by computer simulation. A nonlinear dispersion relation proposed previously is shown to describe the nonlinear evolution even after the electron trapping sets in. After electron trapping is completed, amplitude oscillation at a frequency close to ωpi (ion plasma frequency) appears due to the exchange of energy between the ions and the electric field. The maximum energy acquired by the ions reaches more than 10% of the initial electron drift energy.
Eikonal expansion for an anisotropic plasma
Authors: Imre Kaya
Published: March 2003

The eikonal expansion developed by Weitzner and Batchelor is reformulated to study the cyclotron resonance in an inhomogeneous anisotropic plasma. It is shown that, to leading order, the functional dependences of the Poynting theorem, the cyclotron damping or growth [or the energy absorption (or emission)] rates on the unperturbed states are unaltered by anisotropy.
Free-boundary high-beta tokamaks. I. Free-boundary equilibrium
Authors: Goedbloed J. P.
Published: March 2003

The free-boundary problem of a sharp-boundary high-β tokamak plasma inside a conducting shell is solved. This problem is reduced to solving Laplace’s equation on a domain with an unknown inner boundary. Centering this boundary with respect to the center of the shell is effected by means of a Moebius transformation which facilitates the use of the fast Fourier transformation. The method exploits Green’s theorem for the linear part of the problem which is the solution of Laplace’s equation with given boundaries. The nonlinear part consists of moving the plasma boundary until pressure balance is obtained. Fast convergence to accurate results is obtained through the use of a judiciously chosen damping factor determining the response of the plasma shape to changes in the poloidal field pressure. This allows for a complete scan of the two-dimensional parameter space characterized by the plasma shift Δ and the plasma thickness a. Expressions are derived for the maximum permissible value of the poloidal beta.
Modulational instability of ion-acoustic turbulence
Authors: Yasseen F. , Vaclavik J.
Published: March 2003

The adiabatic interaction of low-frequency ion-acoustic-like perturbations with high-frequency ion-acoustic turbulence is studied. The turbulence is found to become unstable provided certain conditions are met. These conditions are derived explicitly.
Optimal plasma-β in an MHD-stable field-reversed configuration with energetic, large-orbit ions
Authors: Sparks L. , Sudan R. N. , Finn J. M.
Published: March 2003

Numerical studies of the optimum plasma β of a field-reversed configuration show that stability against interchange and ballooning modes is enhanced by the presence of a component of energetic large-orbit ring ions. Stability improves as these profiles become more peaked, and stable equilibria are obtained where the total plasma energy is twice the total ring ion energy.
Interaction of small-amplitude fluctuations with a strong magnetohydrodynamic shock
Authors: Zhuang H.-C. , Russell C. T.
Published: March 2003

The interaction of small fluctuations with a strong magnetohydrodynamic shock is considered theoretically in the most general situation using an ideal model. An analytical treatment of the cutoff conditions for penetration of the upstream fluctuation through the bow shock is presented. The situation in which there exist unique solutions of the interaction is investigated quantitatively. Numerical examples of the interaction of waves in the solar wind with the terrestrial bow shock are given to illustrate the refracted waves to be found in the earth’s magnetosheath.
Theoretical model of absorption of laser light by a plasma
Authors: Mora Patrick
Published: March 2003

A simple model of laser light absorption is described. The absorption mechanism is mainly inverse bremsstrahlung, but a crude description of resonance absorption is also included. The intensity and the wavelength dependence are emphasized, but the model takes into account the target material composition, the laser pulse length, and the focal spot radius. Plane and spherical expansion are treated. Results range from short-wavelength, low-intensity regime, where inverse bremsstrahlung absorption is total, to long-wavelength high-intensity regime, where inverse bremsstrahlung absorption is negligible. Scaling laws concerning absorption, electron temperature, and electron kinetic pressure are given in the two limiting regimes. A characteristic flux Φ*, or, alternatively, a characteristic wavelength λ* is defined which separates the two regimes, the other parameters being held constant.
Nonlinear radiation by an electron beam in the whistler range: A tentative theoretical model
Authors: Lavergnat J. , Le Queau D. , Pellat R. , Roux A.
Published: March 2003

During injection into the ionospheric plasma of an electron beam, very low-frequency electrostatic noise has been detected far away from that beam. Owing to the characteristics of this emission a direct generation mechanism based, for instance, on a linear instability seems irrelevant. A different interpretation is proposed where the observed very low-frequency noise is related to a nonlinear antenna mechanism. Large-amplitude plasma waves which are strongly confined inside the beam, at frequencies close to the plasma frequency, are nonlinearly coupled and the corresponding beating produces a lower-frequency wave which can freely escape from the beam. A detailed comparison with observations is made to support this model.
Ion source discharge performance and stability
Authors: Goebel Dan M.
Published: March 2003

Very efficient ion sources which utilize electron discharges to produce the plasma have been observed in certain cases to be susceptible to discharge impedance shifts (mode shifts). This results in unreliable plasma generation and unstable operation of the ion source. Studies have shown that the mode shifts are related to a decrease in the plasma potential to values very negative with respect to the anode potential. A general model of the ion source plasma is presented and the equilibrium plasma potential is calculated as a function of the plasma parameters and average loss rates. Parameters which contribute to negative plasma potentials are then identified. The experimentally observed constraints on gas pressure range and minimum discharge voltage at which the source will operate are predicted theoretically and the dependence demonstrated in an ion source. The negative plasma potentials are shown to be generated by enhanced thermalization of the primary electrons in the plasmas for ion sources with good electron confinement. The analysis reveals that plasma generator electrical efficiency is ultimately determined by the requirements on the minimum anode area necessary for stability of the discharge.
Use of a quadrant analysis technique to identify coherent structures in a turbulent boundary layer
Authors: Rajagopalan S. , Antonia R. A.
Published: March 2003

A quadrant analysis technique based on instantaneous products uv, uϑ, and vϑ (u, v are the longitudinal and normal velocity fluctuations, ϑ is the temperature fluctuation) is used to obtain averages, in ejection and sweep quadrants, associated with coherent structures contributing significantly to Reynolds stress and heat flux in a turbulent boundary layer. Conditional averages of u, v, ϑ and products uv, uϑ, and vϑ in ejection and sweep quandrants indicate only partial qualitative agreement with those obtained by other conditional sampling techniques, with detection based on the internal front characterized by rapid changes in velocity and temperature. Although the frequency of occurrence of ejections is nearly equal to that of sweeps, only a small percentage of the total number of ejections is followed by sweeps, whereas the percentage of ejections following sweeps is negligible.
Kinetic theory of collisionless ballooning modes
Authors: Cheng C. Z.
Published: March 2003

A kinetic ballooning mode equation retaining full finite ion Larmor radius and ion magnetic drift resonance effects is derived by employing the high n ballooning mode formalism. It is found that the critical β is smaller than the ideal magnetohydro-dynamic critical β, except that when ηi = O(ηi≡d ln Ti/d ln N), that are identical. The finite Larmor radius effects reduce the growth rate but do not stabilize the mode. The ion magnetic drift resonance effects are destabilizing.
Spontaneously developing magnetic reconnections in a current-sheet system under different sets of boundary conditions
Authors: Ugai Masayuki
Published: March 2003

The nonlinear development of magnetic reconnection in an isolated current-sheet system is studied numerically. The reconnection processes develop from an initial small disturbance under different sets of typical boundary conditions. The following distinct macroscopic characteristics are shown. In a situation where plasma can freely flow out of or into the system, the fast reconnection mechanism by which the stored magnetic energy can explosively be released is set up. The magnitude of the initial disturbance influences the onset time of the fast reconnection but has little effect on the established configuration of fast reconnection. Also, even in the presence of impermeable walls placed at the top and bottom boundaries, the fast reconnection is eventually available, so that the fast reconnection process is regarded as a gross instability inherent to the system itself. On the other hand, in the tearing-type field geometry with impermeable walls, plasma confinement in magnetic islands prevents the fast reconnection from taking place and finally leads to an oscillation on the nonlinear saturation level.
Filamentation of space-charge-limited flow
Authors: Felber F. S. , Spivey B. A.
Published: March 2003

A linear-perturbation approach is used to calculate the growth rate of the filamentation instability for nonrelativistic space-charge-limited (Child–Langmuir) flow of electrons in a planar diode.
Radiation losses from high Z, 10.6 μm laser-irradiated microballoons
Authors: Rockett Paul D. , Priedhorsky William C. , Giovanielli Damon V.
Published: March 2003

The radiation emitted from CO2 laser-irradiated (λ = 10.6 μm), gold-coated microballoons has been measured absolutely in the photon energy range from 30 eV to 6 keV. Incident laser intensity varied from 1.6×1014 W/cm2 to 9.5×1015 W/cm2 with a 1 nsec FWHM pulse. Multi-channel x- ray diode data were statistically tested against assumed spectral forms, finding the best parameter fit corresponding to the minimum variance, chi square. The spectra were established to be blackbody with color temperatures ranging from 66 to 134 eV and with the fraction of incident energy radiated decreasing from 15% to 2.3% as laser intensity was increased across the above- mentioned range. Associated diagnostics confirmed these results. An analytical model is presented in which radiated emission from hot electron energy deposition is calculated. The calculations agree well with the measured data.
Rippling modes in the edge of a tokamak plasma
Authors: Carreras B. A. , Gaffney P. W. , Hicks H. R. , Callan J. D.
Published: March 2003

A promising resistive magnetohydrodynamic candidate for the underlying cause of turbulence in the edge of a tokamak plasma is the rippling instability. In this paper a computational model for these modes in the cylindrical tokamak approximation was developed and the linear growth and single-helicity quasi-linear saturation phases of the rippling modes for parameters appropriate to the edge of a tokamak plasma were explored. Large parallel heat conduction does not stabilize these modes; it only reduces their growth rate by a factor sacling as K−4/3∥. Nonlinearly, individual rippling modes are found to saturate by quasi-linear flattening of the resistivity profile. The saturated amplitude of the modes scales as m−1, and the radial extent of these modes grows linearly with time due to radial Ẽ×B0 convection. This evolution is found to be terminated by parallel heat conduction.
Steady flow patterns of internal solitary bulges in a stratified fluid
Authors: Kamachi Masafumi , Honji Hiroyuki
Published: March 2003

Steady streamline patterns are demonstrated of a solitary bulge forming on an interfacial transition region in fresh and salt water. The bulge is composed of a vortex pair at αa≳1.3, with α the inverse of the characteristic thickness and a the bulge amplitude.
Consequence of induced bremsstrahlung radiation
Authors: Nambu Mitsuhiro
Published: March 2003

The physical meaning of induced bremsstrahlung radiation is clarified by simple considerations. The induced radiation is best understood by picturing a collision process between effective macroparticles in a turbulent plasma. The condition for the negative absorption of an electromagnetic wave is obtained. The induced bremsstrahlung radiation is a new kind of bremsstrahlung instability. The generation mechanism of ordinary mode radiation in the presence of a coherent ion cyclotron wave is discussed. The potential importance of the newly found radiation process to plasma turbulence is stressed.
Wave fronts of electromagnetic cyclotron harmonic waves
Authors: Ohnuma T. , Watanabe T.
Published: March 2003

In an inhomogeneous high-density magnetized plasma, the spatial properties of the wave fronts and ray trajectories of electromagnetic ordinary and extraordinary cyclotron harmonic waves are investigated. Those waves which are radiated from a local source are found to have wave fronts which are almost parallel to the magnetic field. Also, the reflective properties of the electromagnetic cyclotron harmonic waves are confirmed.
Microstability properties of the EBT boundary layer
Authors: Gladd N. T. , Krall N. A.
Published: March 2003

An analysis of the microstability properties of the Elmo Bumpy Torus (EBT) boundary layer is made. A number of plasma characteristics, including some unique to EBT, are examined to ascertain their microstability consequences. For conditions typical of current experiments, the EBT boundary layer is found to be unstable to high-frequency drift waves, in particular, the lower-hybrid-drift instability and the drift-cyclotron instability. Properties of these modes relevant to their experimental observation are discussed.
Flow enhancement in magnetically insulated ion injectors
Authors: Poukey J. W. , Humphries S. , Lockner T. R.
Published: March 2003

Magnetically insulated gaps have proved useful for the production of high current density ion beams. Although ion flow well beyond conventional space charge limits has been observed, previous theoretical treatments have not predicted such flow enhancement at the magnetic fields typically used. One-dimensional numerical simulations are presented which demonstrate that trapped electron distributions are accessible that cancel much of the effect of ion space charge. This allows ion flow more than an order of magnitude higher than the Child limit. Two processes that result in modified electron distributions have been investigated; the use of a distributed initial electron source and electron diffusion across insulating field lines (specific instabilities were not studied). In both cases, high flow enhancement can be obtained with relatively small electron loss current.
Ideal magnetohydrodynamic stability limits of a high-β tokamak with superimposed helical fields
Authors: Sherwell D. , Freidberg J. P. , Berge G.
Published: March 2003

The stability of a high-β tokamak with a superimposed single helicity stellarator field is investigated in the context of the sharp boundary surface current model. The analysis treats external ideal magnetohydrodynamic modes including both pressure-driven ballooning effects and current-driven kink instabilities. The results show that the addition of an l = 2 field is unfavorable, lowering the allowable Ohmic heating current and decreasing the maximum stable β. The addition of an l = 3 field, however, is favorable. Both the allowable Ohmic heating current and maximum stable β are increased as the l = 3 amplitude increases.
The direct-interaction approximation and statistically steady states of three nonlinearly coupled modes
Authors: Krommes John A.
Published: March 2003

The direct-interaction approximation is used to find statistically steady states of a system of three modes with complex frequencies coupled by a quadratic nonlinearity. These states are compared to the exact predictions of an ensemble of realizations with Gaussian distributed initial conditions. The direct-interaction approximation is shown to be reasonably successful in this context.
Evolution and merger of isolated vortex structures
Authors: Overman Edward A. , Zabusky Norman J.
Published: March 2003

Numerical simulations of the instability, merger, and breaking of two piecewise-constant finite-area vortex regions (FAVR’s) are presented. An improved contour dynamical algorithm with node insertion-and-removal to maintain the a priori accuracy is used. Corotating ’’V states’’ (symmetric steady-state FAVR’s) were found to be unstable when properly perturbed if their centroid-effective radius ratio, x̄/R, is <1.6, thereby verifying an estimate of Saffman and Szeto. This causes the FAVR’s to approach at an exponential rate, merge, and reform into a stable perturbed elliptical structure with filamentary arms (to conserve angular momentum). For larger x̄/R ratios, regular perimeter oscillations were observed and estimates of an eigenfrequency of the perturbed stable V states were obtained. When regions of different vorticity density merge, the larger-density region is eventually entrained within the smaller-density region. These simulations elucidate the self-consistent close interactions of isolated vortex regions in two-dimensional high Reynolds number flows.
Analysis of the decay of temperature fluctuations in isotropic turbulence
Authors: Durbin Paul A.
Published: March 2003

The Lagrangian dispersion theory of Durbin is used to analyze experiments by Warhaft and Lumley [J. Fluid Mech. 88, 659 (1978)] and by Sreenivasan et al. [J. Fluid Mech. 100, 279 (1980)] on temperature fluctuations in grid-generated turbulence. Both theory and experiment show that the decay exponent m depends on the ratio of the initial length scales of velocity and temperature; although when this ratio is greater than 2.5 such dependence is negligible. The theory shows that m is not truly constant, but within the range covered by the experiments it is nearly so. The agreement between theory and experiment lends credence to the idea that the decay of fluctuations is controlled largely by turbulent relative dispersion.
Effect of translational temperature anisotropy on the rotational distribution of N2 in the Ar free jet
Authors: Koura Katsuhisa
Published: March 2003

The effect of the translational temperature anisotropy T∥≳T⊥ on the rotational distribution yj, temperature Tr, and collision number Zr of N2 in the Ar free jet is studied at the lower density level P0d = 15 Torr-mm using the ellipsoidal velocity distribution and the rotational transition cross section of the power law. The effect of T∥≳T⊥ is appreciable. The overpopulation of the higher rotational levels j above the Boltzmann distribution fitted to the distribution of the lower j, i.e., the degree of nonequilibrium for the case T∥≳T⊥ = Ti (i≡isentropic) is slightly less pronounced than for the case T∥ = T⊥ = Ti but more pronounced than for the case T∥ = T⊥≳Ti. The ratio yj/y0, Tr, and the terminal Zr for T∥≳T⊥ = Ti are larger than for T∥ = T⊥ = Ti but smaller than for T∥ = T⊥≳Ti.