bluuebunny/crossref_metadata_2025 huggingface dataset:
Propagation of a nonrelativistic electron beam in a plasma in a magnetic field
Authors: Okuda H. , Horton R. , Ono M. , Ashour-Abdalla M.
Published: July 2002

Propagation of a nonrelativistic electron beam in a plasma in a strong magnetic field has been studied using electrostatic one-dimensional particle simulation models. Electron beams of finite pulse length and of continuous injection are followed in time to study the effects of beam–plasma interaction on the beam propagation. For the case of pulsed beam propagation, it is found that the beam distribution rapidly spreads in velocity space generating a plateaulike distribution with a high energy tail extending beyond the initial beam velocity. This rapid diffusion takes place within a several amplification length of the beam–plasma instability given by (ωpω2b) −1/3V0, where ωp, ωb, and V0 are the target plasma, beam–plasma frequencies, and the beam drift speed. This plateaulike distribution, however, becomes unstable as the high energy tail electrons free-stream, generating a secondary beam. A similar process is observed to take place for the case of continuous beam injection when the beam density is small compared with the total density nb/nt<1. In particular, the electron velocity distribution is found monotonically decreasing in energy, having a high energy tail whose energy reaches twice the initial beam energy. Such an electron distribution is also seen in laboratory experiments and in computer simulations performed for a uniform, periodic system. When the beam density is increased so that the beam current exceeds the thermal return current, enbV0≳enevt, where ne and vt are the density and thermal speed of the ambient electrons, beam propagation becomes much slower due to the electric field generated by the excess charges associated with the beam electrons.

Beam electrons are reflected from the ambient plasma as if they are bouncing off a rigid wall. When the beam velocity is increased while holding the beam density constant, simulations show that the beam current can exceed significantly the return current generated by the thermal electrons enevt. It is shown that the electric field generated by the beam–plasma instability accelerates the ambient electrons opposite to the beam propagation, thereby enhancing the return current.
Diagnosis of mildly relativistic electron velocity distributions by electron cyclotron emission in the Alcator C tokamak
Authors: Kato K. , Hutchinson I. H.
Published: July 2002

Electron cyclotron emission from nonthermal electron distributions has been measured with a vertical view and control of multiple reflections. The observed intensities at the first few harmonics provide information that can be used to deduce the electron distribution function. Although harmonic overlap restricts the detail that can be obtained when the distribution is very energetic, a fitting procedure enables useful results to be obtained. The distribution functions during Ohmic runaway and lower-hybrid current drive and heating are presented. The current-drive distributions are not inconsistent with theoretical Fokker–Planck calculations, giving perpendicular and parallel ‘‘temperatures’’ around 60 keV and 200 keV, respectively. The runaway cases have quite similar distributions.
A comparison of velocity measurements using a laser anemometer and a hot-film probe, with application to grid-stirring entrainment experiments
Authors: Atkinson Joseph F. , Damiani Leonardo , Harleman Donald R. F.
Published: July 2002

A laser-Doppler anemometer has been used to measure the spatial decay of velocity fluctuations generated by a vertically oscillating grid in a laboratory tank. These velocity data were found to be in agreement with earlier studies using hot-film probes, though a comparison of the length scales of the motions obtained with the two different probes is difficult. Normalized entrainment rates follow similar trends using values obtained either with the laser or with the hot film, though quantitatively some differences are seen. It is concluded that true turbulence does not exist for distances less than about two mesh spacings from the grid.
Numerical calculation of Mercier beta limits in spheromaks
Authors: Mayo R. M. , Marklin G. J.
Published: July 2002

By iteratively solving the Grad–Shafranov equation and the Mercier criterion, stable pressure profiles may be determined self-consistently with spheromak equilibria. No assumption is made about the shape of the pressure profile and an individual determination is made for that value of the pressure that drives the plasma marginally stable everywhere. These limits determine the pressure profile that provides the largest 〈β〉vol allowed by Mercier modes. Particular attention is given to the CTX spheromak [Phys. Rev. Lett. 51, 39 (1983)]. The results on spheromak Mercier stability limits leading to 〈β〉vol =1.5% for sustained spheromaks are summarized. In addition, 〈β〉vol =7.0% has been calculated for spheromaks with current holes.
Two- and three-dimensional tokamak equilibrium modifications driven by the localized ponderomotive force of applied radio frequency waves
Authors: Myra J. R.
Published: July 2002

Large ponderomotive forces can occur when waves in the ion cyclotron range of frequencies (ICRF) are employed in stabilization or high power heating schemes. It is shown that the magnetohydrodynamic (MHD) equilibrium of the plasma is modified when the rf antenna is localized poloidally and/or toroidally. In the two-dimensional (2-D) problem (toroidally uniform rf) a modified Grad–Shafranov equation describes ponderomotively driven magnetic surface distortions. At electric field strengths of interest for ICRF stabilization, the equilibrium distortions are small provided that Lrf≪a and the poloidal localization of the rf is not severe. The character of the 3-D problem (toroidally and poloidally varying rf) depends on whether or not the ponderomotive force couples to rational surfaces within the plasma. For the case without rational surfaces, the 3-D component of the distortions is found to be small by 1/q2a relative to the 2-D component. In the case where rational surfaces are excited, magnetic islands result. A threshold condition for island overlap determines the rf electric field at which ergodicity may be expected. It is shown that ergodicity of the edge plasma may occur for some cases of interest for heating and stabilization, but should be controllable.
Periodic solutions for three sedimenting spheres
Authors: Caflisch Russel E. , Lim Chjan , Luke Jonathan H. C. , Sangani Ashok S.
Published: July 2002

Exact periodic solutions are found for the relative motion of three spheres sedimenting in a Stokes fluid. Nearby solutions are found to be nearly periodic for a long time. Existence of the exact periodic solutions is proved using the point-particle approximation and symmetry properties of Stokes equations. Numerical simulations for finite-sized particles are performed using a method of multipole expansions.
The possibility of drag reduction by outer layer manipulators in turbulent boundary layers
Authors: Sahlin Alexander , Johansson Arne V. , Alfredsson P. Henrik
Published: July 2002

Experiments were carried out with the aim of investigating the possibility of obtaining a net drag reduction on a finite body by manipulating the outer layer structure of the turbulent boundary layer. The experiments were carried out in a 260 m long towing tank, where large eddy breakup devices (LEBU’s) were used in single and tandem configurations on a large flat plate. The total drag was measured directly by a force gauge, and the geometrical configuration parameters, as well as the chord Reynolds number, were varied over wide ranges. The highest chord Reynolds number tested was 260 000, which is within the range of interest for practical applications. Also, the device drag was measured directly in flight, which enabled evaluation of the performance of the manipulators, as well as determination of the total skin friction reduction. Despite a substantial skin friction reduction for certain configurations (7% averaged over the part downstream from the manipulators) as well as a low device drag, no drag reduction was found. The present results, and a critical evaluation of previously reported data, make any substantial net drag reduction by use of LEBU’s at high Reynolds numbers seem implausible.
Symmetry properties of developing three-dimensional laminar disturbances in plane Poiseuille flow
Authors: Bergström Lars , Alfredsson P. Henrik
Published: July 2002

The evolution of a finite-amplitude point-like laminar disturbance in plane Poiseuille flow is investigated using hot-wire anemometry. In contrast to earlier experiments, the initial disturbance was introduced simultaneously through both the upper and lower wall of the channel, resulting in an antisymmetric disturbance in the normal velocity. Although the initial disturbance mainly generated symmetrical streamwise velocity modes, the subsequent development of the perturbation showed a marked tendency for antisymmetry.
The initial evolution of a wave packet in a laminar boundary layer
Authors: Cohen Jacob
Published: July 2002

The initial evolution of a wave packet in a laminar boundary layer is examined. By comparing the experimental streamwise growth of individual modes with the prediction of a linear model, two kinds of nonlinearities are explored. The first of which is associated with oblique waves with a frequency range centered around half the frequency of the most amplified linear mode. This type of nonlinear growth of subharmonic waves has been observed in the past in cases in which artificial excitations of a single two-dimensional (2-D) fundamental wave, with or without simultaneous excitations of three-dimensional (3-D) subharmonic waves were used, and in the case in which the disturbance was generated by a continuous harmonic point source disturbance. However, unlike all the cases mentioned above, the nonlinear growth of subharmonic waves in the present case begins well upstream of the neutral point of the fundamental wave at branch II, long before the streamwise position, where the amplitudes of the fundamental Tollmien–Schlichting (TS) waves (in the previous cases), attained a saturation level. The second type of nonlinear growth is associated with waves having shorter wavelengths than the wavelengths of the most amplified waves. In this case the nonlinear growth of the waves is first observed at their neutral points (at branch II), characterized by an enhanced streamwise growth rate of their amplitudes. The linear model, which includes the solution of the vertical vorticity equation in addition to the Orr–Sommerfeld equation and accounts for the effect of the mean flow divergence, is found to be very successful in predicting the downstream growth of the most amplified modes and the normalized cross-stream distributions of the various modes of all three velocity components.
Experiments on single oblique laminar-instability waves in a boundary layer: Introduction, growth, and transition
Authors: Schneider Steven P. , Coles Donald
Published: July 2002

The laminar–turbulent transition in an incompressible flat-plate boundary layer was studied experimentally by using a spanwise array of computer-controlled surface heating elements to generate small disturbances. Oblique Tollmien–Schlichting waves were successfully introduced, and their downstream development into the intermittent region was studied using flush-mounted hot-film wall-shear sensors and dye flow visualization. Comparative studies of the development of single oblique waves were made for various wave angles, frequencies, and amplitudes. As these single oblique waves grew and began to break down, higher harmonics and subharmonics appeared in the wall shear. The amplitude of the subharmonic component decreased rapidly with increasing oblique-wave angle, so that a 10° oblique wave had a subharmonic amplitude an order of magnitude below that for a two-dimensional (2-D) wave. Thus, the nonlinear mechanism that produces the subharmonic is affected by the symmetry of the primary wave. Intermittency measurements, carried out farther downstream, show that a 2-D wave is most effective in moving the transition point upstream, for a given power input.
A conditional similarity concept for turbulent shear flow, with application to mixing in a round jet
Authors: Kerstein Alan R. , Schefer Robert W. , Namazian Mehdi
Published: July 2002

A ‘‘conditional similarity’’ concept for turbulent shear flow is introduced that quantifies the effect of large-scale fluctuations on flow microstructure. This concept motivates a new ‘‘conditional scaling’’ method for analyzing measured instantaneous profiles or images of motion or scalar mixing in turbulent shear flow, involving the reparametrization of each profile or image so as to separate the distinct effects of large-scale and small-scale motions. The method, applied to species concentration profiles measured in a round jet, is found to capture profile shape features that are visually apparent in the instantaneous profiles but are not preserved by other conditioning methods. Similarity hypotheses leading to reduced descriptions of flow microstructure based on conditionally scaled data are formulated and are evaluated using the measured concentration profiles. The analysis provides a quantitative estimate of the respective contributions of large-scale and small-scale motion to the overall variability of the concentration field.
Self-similar supersonic variable-density shear layers in binary systems
Authors: Kennedy Christopher A. , Gatski Thomas B.
Published: July 2002

Some characteristic features of supersonic/supersonic, laminar, variable-density shear layers are studied by examining the self-similar behavior of five nitrogen/hydrogen streams. With the Levy–Lees transformation, the flow-field variables, which include the transverse velocity and dilatation, are obtained through the solution of the coupled set of nonlinear conservation equations. The issue of the appropriate ‘‘third boundary condition,’’ first given for the supersonic/supersonic case by Ting [J. Math. Phys. 28, 153 (1959)], is addressed and implemented in the present formulation. Expressions for the thermal conductivity, viscosity, specific heat, and binary diffusion coefficients of an arbitrary mixture are utilized so that the Prandtl and Lewis numbers and the Chapman–Rubesin parameter can vary freely across the shear layer. In the particular cases considered, these three quantities varied by factors of approximately 3, 7, and 22, respectively. The region of high vorticity moves toward the less-dense hydrogen stream for large density ratios (∼9:1), and becomes nearly decoupled from the density profile. Because the vorticity is responsible for the kinematic mixing of the two streams, this mixing of the two laminar streams is likely to be inhibited. Even though laminar flows are considered here, this effect is consistent with the experimental observation that as density ratios become very large, further increases in the density ratio have no effect on the turbulent shear layer growth rate. The density and hydrogen mass-fraction profiles are quite elongated in the transverse direction. An increase in the velocity ratio exaggerates both of these effects. Results obtained in this work are compatible with earlier work on incompressible, variable-density flows; more importantly, these results qualitatively resemble those from experiments of compressible and incompressible turbulent flows.
Vorticity balance and time scales of a two-dimensional airfoil in an unsteady free stream
Authors: Shih Chiang , Ho Chih-Ming
Published: July 2002

In unsteady aerodynamic research, there exists a long list of operational variables due to the complexity of the flow. For engineering design purposes, parametric study is the conventional approach. In this investigation, an alternative approach is used. It is demonstrated here that the physical insights of the intricate unsteady aerodynamic flow can be understood from a simple fundamental vorticity balance concept. The test configuration is a stationary two-dimensional NACA 0012 airfoil placed at the static stall angle of 12° in an unsteady free stream. A local circulation measurement technique is introduced to survey the evolving vorticity field on the airfoil. The information about the vorticity convection and the surface vorticity flux over the wing provides a basic understanding regarding the variation of the unsteady lift.
On the density ratio effect on the growth rate of a compressible mixing layer
Authors: Lu Ganyu , Lele Sanjiva K.
Published: July 2002

A ‘‘curious reduction’’ in the normalized growth rate found in recent experiments on compressible mixing layers is explained using linear stability analysis. The maximum amplification rate from the stability analysis agrees well with the growth rate measured in different experiments. No anomalous reduction is observed in the normalized maximum amplification rate, suggesting that the model of growth rate for incompressible mixing layers used to normalize the compressible mixing layer data is inaccurate for extreme density ratios.
Turbulence renormalization group calculations using statistical mechanics methods
Authors: Giles M. J.
Published: July 2002

The renormalization group theory of fluid turbulence is developed from a statistical mechanical viewpoint using an exact expression for the functional probability distribution of the velocity field. The latter is similar in form to an equilibrium Gibbs distribution, and is derived by combining Lagrangian statistical mechanics with an Eulerian fluid description. It is shown that this distribution enables an RG transformation to be defined, evaluated, and analyzed using statistical mechanical techniques. The method of determining amplitudes used here also differs from previous work, in that the indeterminacy of the relevant stirring force parameters is resolved through the essential requirement that all coarse-grained distributions must yield the actual dissipation rate ℰ. Consequently, although the fixed point solutions of the dynamic RG approach are recovered, slightly different numerical results are obtained for amplitudes. The present approach yields, for example, for Kolmogorov’s constant, CK=1.44.
The response of a rotating ellipse of uniform potential vorticity to gravity wave radiation
Authors: Ford Rupert
Published: July 2002

Kirchhoff’s incompressible rotating elliptical vortex solution is extended to the case of weak compressibility in the rotating f-plane shallow water equations by means of matched asymptotic expansion, using the small Froude number F as the expansion parameter. The analysis shows that there is a correction to the shape of the rotating configuration at O(F2), and a gradual elongation of the shape on a time scale F4t. When the aspect ratio of the ellipse is 4.6:1, the O(F2) perturbation to its boundary shape becomes secular, and the vortex exhibits a tendency to pinch in the middle, breaking into two separate vortices. This behavior is consistent with the weakly nonlinear analysis of Williams (Ph.D. thesis, University of Leeds, 1992), and the numerical work of Chan et al. [J. Fluid Mech. 253, 173 (1993)], for the formally equivalent problem in a two-dimensional compressible gas. When the Coriolis parameter is sufficiently large, the elongation of the ellipse may equilibrate before it reaches an aspect ratio of 4.6:1. The nature of the approach of the ellipse to its equilibrium aspect ratio is discussed in these cases, highlighting an asymmetry between cyclonic and anticyclonic vortices.
Dynamics of vortex rings in crossflow
Authors: Chang Y. K. , Vakili A. D.
Published: July 2002

The spread and mixing of a fluid jet into an ambient stream occurs at a rate which deserves further study to improve efficient mixing. Mixing enhancement techniques, such as introduction of periodic disturbances into the jet flow, are used to increase mixing between a jet and the surrounding fluid. Pulsations were generated by the periodic closing and opening of a jet flow. The dynamics and trajectories of vortex rings, formed by the pulsation of the jet in a uniform crossflow, are studied. In particular, the effects of pulsation on the development of vortex rings and their penetration in a crossflow were investigated. Detailed measurements were made using flow visualization techniques including laser-induced fluorescence and hot-film anemometry. Vortex rings generated in a crossflow at one specific frequency (1 Hz) were measured using a hot-film probe. Measurements indicated that vortex rings were fully-formed at a distance of three times the jet exit diameter. To simulate the dynamics of vortex rings in crossflows, a numerical experiment was performed based on a Lagrangian, grid-free, three-dimensional vortex element method. At low frequencies, the fluid in the vortex rings penetrated into the crossflow to a height much greater than that for either high frequency pulsation or for a steady jet. At low frequencies, interaction between sequentially generated vortex rings was negligible; therefore, each ring behaved as a single discrete vortex ring. The vortex rings moved into the flow occasionally tilting up to about 30°, depending on the ring’s strength. Numerical simulation indicated that the tilting of the ring was due to the combined effects of viscosity and the crossflow. It is postulated that the increased penetration combined with the discretization of a jet into vortex rings results in a more efficient mixing rate.
The distortion of a passive scalar by two-dimensional objects
Authors: Rahai Hamid R. , LaRue John C.
Published: July 2002

The response of a heated, nearly homogeneous and isotropic turbulent field to a nonuniform rapid distortion upstream of two-dimensional objects placed in a homogeneous grid generated flow is investigated experimentally. In particular, the effect of the nonuniform distortion on the single-point statistical properties of the velocity and a passive scalar (temperature), the axial heat flux and higher-order cross moments are presented. The nearly homogeneous, turbulent flow is produced by a biplane grid of rods and a square mesh grid of electrically heated wire that is placed downstream of the turbulence producing grid. Spatially and temporally resolved, simultaneous measurements of the streamwise turbulent velocity and temperature are obtained upstream of several two-dimensional objects along the mean stagnation streamline. The effects of the blocking and vortex stretching mechanisms on the root-mean-square (RMS) velocity for various ratios of the integral length scale to the characteristic length of the object, L0/D, are in agreement with previous results, i.e., for L0/D≪1, the velocity intensity is increased and for, L0/D≫1, it is decreased. In contrast, to within a distance equal to about one integral scale from the object, the distorted RMS temperature remains nearly a constant. When L0/D≪1, where vortex stretching is important, the axial heat flux and the diffusion of temperature variance increase as the object is approached. In contrast, when L0/D≫1, where the blockage effect is dominant, they decrease as the object is approached.
Numerical simulation of mixing enhancement in a hot supersonic jet
Authors: Grosch C. E. , Seiner J. M. , Hussaini M. Y. , Jackson T. L.
Published: July 2002

Experimental observations show that the presence of small tabs on the edge of a hot, compressible jet exiting into a slower moving, colder ambient flow can increase the rate of spreading of the jet. This suggests that the rate of mixing of the jet and the ambient fluid is also increased. In order to elucidate the physical mechanism responsible for the increased spreading rate a set of calculations was carried out within the framework of the compressible three dimensional Navier–Stokes equations. A series of grid refinements were made to assess the accuracy of the results. We first simulated the flow without the tabs, obtaining reasonable agreement with experimental measurements of the velocity. We then simulated the flow, without tabs, over a range of values of the convective Mach number in order to determine the dependence of the mixing on this parameter. Simulations with modeled tabs were also carried out. In these calculations the effect of the tabs on the flow was modeled by pairs of counter-rotating vortices. The results of these calculations indeed show that the presence of the tabs increase the spreading rate of the jet. The basic physical mechanism responsible for the enhanced spreading rate is discussed and qualitative comparisons with flow visualizations are made.
The axisymmetric and plane cases of a gas phase steadily displacing a Newtonian liquid—A simultaneous solution of the governing equations
Authors: Giavedoni Marı́a D. , Saita Fernando A.
Published: July 2002

The flow induced by a long bubble steadily displacing a liquid confined by two closely located parallel plates or by a cylindrical tube of small diameter is numerically analyzed. The technique employed solves the complete set of governing equations simultaneously. The present analysis encompasses, and also extends, the whole range of Capillary values previously studied with various numerical techniques. The results shown uncover a type of recirculating flow pattern that appears to have been overlooked before. The effects of the inertial forces on the liquid flow rate are also assessed.
Three-dimensional, unsteady, acoustic-shear flow dynamics in a cylinder with sidewall mass addition
Authors: Staab P. L. , Kassoy D. R.
Published: July 2002

Three-dimensional internal flow dynamics are studied in a cylinder with mass injection from the sidewall. A time-dependent, harmonic, non-axisymmetric axial velocity disturbance is imposed on the endwall of the cylinder to create a non-axisymmetric velocity field. An asymptotic analysis is used to reduce the Navier-Stokes equations to more elementary forms in two regions adjacent to the endwall with distinct physical characteristics: an incompressible, inviscid and irrotational core near the endwall, and an incompressible viscous boundary layer containing all three components of vorticity adjacent to the sidewall. Solutions to these equations for disturbance frequencies associated with the lowest order axial acoustic modes show that the non-axisymmetric nature of the flow is confined to the core region adjacent to the endwall with a characteristic axial dimension on the order of the cylinder radius. Within the region, axial, radial, and azimuthal velocities exist. These non-axisymmetric effects decay exponentially fast so that only axisymmetric acoustic modes exist further downstream. These results are valid for driving frequencies below the “cut-off” value that one would find in a duct.
Branching behavior of standing waves—The signatures of resonance
Authors: Smith D. H. , Roberts A. J.
Published: July 2002

Arclength continuation methods are used to conduct a detailed branching study of standing wave solutions for fluids in a rectangular container, using depth and crest acceleration as control parameters. At each depth the applicable acceleration range extends between zero and one, and a number of multiple solution structures are uncovered. An intimate connection is established between these structures and the phenomenon of harmonic resonance.
Self-adaptation and viscous selection in concentrated two-dimensional vortex dipoles
Authors: Sipp Denis , Jacquin Laurent , Cosssu Carlo
Published: July 2002

In this Letter we deal with 2D direct numerical simulations of concentrated vortex dipoles. We show that various initial dipolar vorticity distributions evolve towards a specific family of dipoles parametrized by the dipole aspect ratio a/b, where a is the radius of the vortices based on the vorticity polar moment in half a plane and b is the separation between the vortex centroids. This convergence is achieved through viscous effects. The considered Reynolds numbers Re=Γ/ν are Re=3000 and Re=15000. Moreover, all the dipoles of this family are quasi-steady solutions of the Euler equations. Their scatter plots and drift velocities are given for a/b<0.3.
Multifaceted asymmetric radiation from the edge (MARFE’s): A general magnetohydrodynamic study in a one-dimensional tokamak model
Authors: De Ploey A. , Goossens M. , Van der Linden R. A. M.
Published: July 2002

Multifaceted asymmetric radiation from the edge (MARFE) is studied as a radiative thermal instability in a one-dimensional (1-D) cylindrical equilibrium model. In order to get a global idea of the MARFE onset, a linear analysis is performed, using a full set of magnetohydrodynamic (MHD) equations. It is found that the onset of the MARFE is accompanied by perturbations of pressure and magnetic field. These characteristics seem especially important in case the MARFE leads to a disruption and rational surfaces are affected. The experimentally observed relation between critical density and plasma current [Nucl. Fusion 24, 977 (1984)] is studied. The particular role of perpendicular thermal conduction is considered separately. Correspondence with experimentally observed behavior, like toroidal symmetry and poloidal asymmetry, is discussed.
Reply to ‘‘Comment on ‘Physics of the magnetotail current sheet’ [Phys. Fluids B 5, 2663 (1993)]’’
Authors: Chen James , Holland Daniel L.
Published: July 2002

A reply is given to the comment by Burkhart et al.1 on the current sheet catastrophe.2,3 It is the authors’ opinion that the comment is not well founded and there is no need for any corrections to the discussion given in the review paper2 or to the original paper of Holland and Chen.3 (AIP)
Revisited neoclassical transport theory for steep, collisional plasma edge profiles
Authors: Rogister André
Published: July 2002

It is shown that existing neoclassical theories in the high collisionality regime are inadequate to describe the edge plasma, where the gradient length scales are of the order of (only) a few centimeters (the ‘‘edge’’ is defined here in relation to the neutrals ionization depth). Gyrostresses indeed modify considerably the continuity, energy, and parallel momentum equations if, as is usually the case, Λ1≡νi (qR)2/ΩiLψ r is not negligible compared to unity (Lψ=LN, LT, LV, where, e.g., LN is the density length scale). Particle transport, moreover, is no longer automatically ambipolar: the radial electric field −∇V is actually determined by the ambipolarity constraint. Another parameter also usually erroneously considered to be smaller than unity is Λ2≡(me/mi)1/2 (qRνi/ci)2. The theory presented here encompasses the more general conditions, where Λ1∼1, Λ2∼1.
Rotational effects on the m=1 magnetohydrodynamic instability in spheromaks
Authors: Shumlak U. , Fowler T. K. , Morse E. C.
Published: July 2002

The effect of rotation on the magnetohydrodynamic stability of m=1 modes in spheromaks is calculated to reveal a threshold for rotational instability at ΩτA≊0.2, where Ω is the angular rotation frequency and τA is the Alfvén transit time. This result may explain the appearance of m=1 modes in spheromak experiments in which the flux conserver aspect ratio L/R lies well below the threshold for the tilt instability (m=n=1). For flux conservers with aspect ratios above the tilt instability threshold, rotational instability dominates when ΩτA exceeds the rotational threshold.
Electromagnetic radiation by parametric decay of upper hybrid waves in ionospheric modification experiments
Authors: Leyser T. B.
Published: July 2002

A nonlinear dispersion relation for the parametric decay of an electrostatic upper hybrid wave into an ordinary mode electromagnetic wave, propagating parallel to the ambient magnetic field, and an electrostatic low frequency wave, being either a lower hybrid wave or a high harmonic ion Bernstein wave, is derived. The coherent and resonant wave interaction is considered to take place in a weakly magnetized and collisionless Vlasov plasma. The instability growth rate is computed for parameter values typical of ionospheric modification experiments, in which a powerful high frequency electromagnetic pump wave is injected into the ionospheric F-region from ground-based transmitters. The electromagnetic radiation which is excited by the decaying upper hybrid wave is found to be consistent with the prominent and commonly observed downshifted maximum (DM) emission in the spectrum of stimulated electromagnetic emission.
Cascading of the upper hybrid/electron Bernstein wave in ionospheric heating experiments
Authors: Zhou H. L. , Huang J. , Kuo S. P.
Published: July 2002

Parametric decay of an upper hybrid/electron Bernstein pump wave into an upper hybrid/electron Bernstein sideband wave and a lower hybrid decay wave in the long-wavelength regime is studied. It is found that the process associated with the electron Bernstein pump wave has a lower threshold field than that of a similar decay process of the upper hybrid pump wave when the instability is excited in the region away from the double resonance layer. Near the double resonance layer, where the upper hybrid resonance frequency equals a harmonic of the electron cyclotron frequency, the upper hybrid wave and the electron Bernstein wave become linearly coupled, and the threshold field of the parametric decay process changes back to a similar functional dependence as that of the upper hybrid decay process. Thus, their threshold fields approach each other. When incorporated with appropriate nonlinear scattering processes, this instability process along with its cascading is proposed to be the generation mechanism for the downshifted maximum (DM), 2DM, 3DM, ... etc. features as well as the upshifted maximum (UM) feature in the stimulated electromagnetic emission spectrum observed in ionospheric heating experiments.
Tearing stability of the two-dimensional magnetotail
Authors: Harrold B. G. , Bhattacharjee A. , Wang Xiaogang
Published: July 2002

The linearized incompressible magnetohydrodynamic equations that include a generalized Ohm’s law are solved for tearing eigenmodes of a plasma sheet with a normal magnetic field (Bn). In contrast to the Harris sheet with the equilibrium magnetic field [B=B0 tanh(z/a)x̂], the two-dimensional plasma sheet with the field [B=B0 tanh(z/a)x̂+Bnẑ], in which the Bn field lies in the plane of the Bx field, has no neutral line if Bn≠0. Such a geometry is intrinsically resilient to tearing because it cannot change topology by means of linear perturbations. This qualitative geometrical idea is supported by calculations of growth rates using a generalized Ohm’s law that includes collisional resistivity and finite electron inertia as the mechanisms for breaking field lines. The presence of Bn reduces the resistive tearing mode growth rate by several orders of magnitude (assuming Bn/B0∼0.1) compared with that in the Harris sheet model (Bn=0). The growth rate scaling with Lundquist number (S) has the typical S−3/5 (S−1/3) dependence for large (small) wave numbers and very small values of Bn. For larger values of Bn, all modes behave diffusively, scaling as S−1. The collisionless electron tearing mode growth rate is found to be proportional to δ2e in the presence of significant Bn(≳10−2B0) and large kx(∼0.1a−1–0.5a−1), and becomes completely stable (γ<0) for Bn/B0≥0.2. Implications for magnetospheric substorms are discussed.
Hard x-ray emission from intense short pulse laser plasmas
Authors: Schnürer M. , Kalashnikov M. P. , Nickles P. V. , Schlegel Th. , Sandner W. , Demchenko N. , Nolte R. , Ambrosi P.
Published: July 2002

Hard x-ray emission in the range of 100 keV has been measured from plasmas produced by irradiation of solid targets with ps laser pulses up to 7×1017 W/cm2. The experimental data obtained for oblique incidence of p-polarized laser light at different illumination angles are compared to computer calculations, which include the processes of resonance absorption and vacuum heating. The scaling of hard x-ray emission with varying laser flux is consistent with the theoretical model of bremsstrahlung emission of hot electrons. From this, together with an absolute radiation dose measured with calibrated detectors, a transfer of up to 50% of the incident laser energy to suprathermal (∼10...100 keV) electrons is estimated.
Shock polar analyses of regular reflections in magnetohydrodynamics
Authors: Ogawa Hiroyuki , Fujiwara Toshi , Hayashi A. Koichi
Published: July 2002

The ‘‘total pressure-deflection angle’’ shock polars are obtained to apply to magnetohydrodynamic (MHD) shock reflection problems. They are categorized into four kinds, based on the type of shocks; the conditions for their appearance are given as well. Among several possible reflections, the regular reflection phenomena are studied here using shock polars, revealing that only four incident-reflection combinations are possible for regular reflection. The range of upstream conditions giving a specified-combination regular reflection is shown graphically. Simultaneously, the relations among three types of incident shock and four kinds of reflected shock polars are also exhibited graphically.
Formation and sustainment of a low-aspect ratio tokamak by coaxial helicity injection
Authors: Nelson B. A. , Jarboe T. R. , Martin A. K. , Orvis D. J. , Xie J. , Zhang C. , Zhou L.
Published: July 2002

Low-aspect-ratio tokamaks with toroidal currents, Ip, up to 250 kA are formed and sustained in the Helicity Injected Tokamak experiment [Nelson et al., Phys. Rev. Lett. 72, 3666 (1994)] using coaxial helicity injection. These plasmas are produced without use of a current drive transformer. Average toroidal currents are sustained at high values, 〈Ip〉=225 kA for 2 ms, where electron thermal energies are measured up to 80 eV with spectroscopy data suggesting burnthrough to the higher ionization states of oxygen. Currents can also be sustained for longer periods at lower values, 〈Ip〉=138 kA for 7 ms. These tokamaks are characterized by a rotating, n=1 distortion, with poloidal distortions approximately following the field line pitch, which only occur on the outer bad-curvature region. Equilibrium reconstructions show these plasmas have a tokamak q profile (q0=5 – 8, q95=10 – 12, qcyl≂3.6), with a hollow toroidal current profile and up to 170 kA of closed field toroidal current in a low-aspect-ratio, A=1.68 configuration.
Trapped ion temperature gradient driven instabilities with bounce resonance
Authors: Li J. Q. , Huang L. , Ding H. C.
Published: July 2002

A kinetic analysis of ion temperature gradient modes with the trapped ion bounce resonance in tokamaks is presented under the long-wavelength limit. The dispersion relations of the toroidal and slab branches are derived by employing two-scale expansion, and their eigenfunctions are obtained analytically. The growth rates and stability thresholds of the more unstable toroidal and slab ion modes are evaluated in the cases with and without trapped electron dynamics. It is found that the minority trapped ion bounce resonances possess approximately the same effects on the toroidal and slab ion modes as the majority ion transit resonances. The nonadiabatic trapped electron dynamics does also strongly affect the toroidal ion mode while it affects the slab branch, the trapped electrons have a destabilizing effect on these ion modes. In addition, the effects of trapped electron temperature gradient on the trapped ion temperature gradient modes are considered. For high collisionality, the trapped electron temperature gradient destabilizes toroidal and slab ion modes. On the contrary, it plays a role of stabilizing to the toroidal and slab ion modes in the case of very low collisionality.
Effect of diffraction on stimulated Brillouin scattering from a single laser hot spot
Authors: Eliseev V. V. , Rozmus W. , Tikhonchuk V. T. , Capjack C. E.
Published: July 2002

A single laser hot spot in an underdense plasma is represented as a focused Gaussian laser beam. Stimulated Brillouin scattering (SBS) from such a Gaussian beam with small f/numbers 2-4 has been studied in a three-dimensional slab geometry. It is shown that the SBS reflectivity from a single laser hot spot is much lower than that predicted by a simple three wave coupling model because of the diffraction of the scattered light from the spatially localized ion acoustic wave. SBS gain per one Rayleigh length of the incident laser beam is proposed as a quantitative measure of this effect. Diffraction-limited SBS from a randomized laser beam is also discussed.
A self-consistent model for low-high transitions in tokamaks
Authors: Guzdar P. N. , Hassam A. B.
Published: July 2002

A system of equations that couples the rapidly varying fluctuations of resistive ballooning modes to the slowly varying transport of the density, vorticity and parallel momentum have been derived and solved numerically. Only a single toroidal mode number is retained in the present work. The low-mode (L-mode) phase consists of strong poloidally asymmetric particle transport driven by resistive ballooning modes, with larger flux on the outboard side compared to the inboard side. With the onset of shear flow driven by a combination of toroidal drive mechanisms as well as the Reynolds stress, the fluctuations associated with the resistive ballooning modes are attenuated leading to a strong reduction in the particle transport. The drop in the particle transport results in steepening of the density profile leading to the high-mode (H-mode).
Explanation for MARFE formation and subsequent evolution into a detached symmetric plasma edge
Authors: Stacey Weston M.
Published: July 2002

An explanation is proposed for the experimentally observed tokamak phenomenon of a symmetric radiating edge evolving with increasing density into a multifaceted asymmetric radiation from the edge (MARFE) condition and then cooling into a detached plasma with a symmetric edge. The thermal stability impurity density limits increase with the poloidal mode number, m, of the edge plasma distribution. When the edge impurity density exceeds the m=0 mode density limit, the edge plasma evolves into a mixture of m≳0 modes which becomes more highly asymmetric as the density increases further and exceeds the density limits of successively higher modes, forcing the plasma into the remaining stable higher-m modes—the stable MARFE. If the radiative cooling of the edge is sufficient to reduce the temperature to Te∼10 eV, the various thermal modes are stabilized by the strong positive temperature dependence of the ionization cooling in the presence of neutral atoms, the impurity density limits are thereby increased, and the plasma evolves back into a stable m=0 mode. On the other hand, if the plasma becomes highly enough asymmetric before sufficient cooling takes place, nonlinear effects drive a disruptive collapse of the radial temperature distribution.
Measurement of magnetic field and velocity profiles in 3.6 kJ United Nations University/International Center For Theoretical Physics plasma focus fusion device
Authors: Mathuthu M. , Zengeni T. G. , Gholap A. V.
Published: July 2002

A Mather-type small plasma focus device was operated in air filling in the pressure range of 0.5 to 1.0 Torr and capacitor bank charging voltage of 13–15 kV. A strong focusing action was observed in this pressure range. Magnetic probe signals at various axial positions were used to estimate velocity of current sheet and axial magnetic field distribution profiles. It was observed that under the present experimental conditions the magnetic field remains constant at 0.72 T from z=0.0 cm to z=8.0 cm but falls rapidly to 0.52 T at about z=14.5 cm at a fixed radial distance of 2.65 cm. The magnetic field and velocity measurements indicate a current shedding effect—only 68.5% of the total injected current flows into the focus region. The rapid drop of the magnetic field at z=8.0 cm suggests that further (initial shedding is at the insulator) current and mass shedding in the focus tube is significant after this value of z. Experimental values of velocity of the current sheet are compared with those of the snow plough theoretical model.
Electron heating in inductively coupled discharges
Authors: Shaing K. C.
Published: July 2002

An analytic expression for the surface impedance for electron heating in inductively coupled discharges is derived from the kinetic equation with the optimum ordering that ν∼ω. Here, ν is the electron collision frequency, and ω is the frequency of the driven current inside the antenna coil. The expression is valid for both collisional and collisionless discharges, and can be employed in the modeling of the source.
Turbulent transport in mixed states of convective cells and sheared flows
Authors: Horton W. , Hu G. , Laval G.
Published: July 2002

Low-order mode coupling equations are used to describe recent computer simulations of resistive-g turbulent convection that show bifurcations for the onset of steady and pulsating sheared mass flows. The three convective transport states are identified with the tokamak confinement regimes called low-mode (L-mode), high-mode (H-mode), and edge-localized modes (ELMs). The first bifurcation (L→H) and the second bifurcation (H→ELMs) conditions are derived analytically and compared with direct solutions of the 6-ODE mode coupling equations. First an exact expression is given for the energy transfer rate from the fluctuations to the sheared mass flow through the triplet velocity correlation function. Then the time scale expansion required to derive the Markovian closure formula is given. Markovian closure formulas form the basis for the thermodynamic-like L–H models used in several recently proposed models.
Analytical calculation of Boozer magnetic coordinates for axisymmetric magnetohydrodynamic equilibria
Authors: Alladio F. , Micozzi P.
Published: July 2002

A new analytical technique for extracting the Boozer magnetic coordinates in axisymmetric magnetohydrodynamic equilibria is described. The method is based upon the correspondence between the expansion of the flux function in toroidal multipolar moments and the expansion in toroidal axisymmetric harmonics of the magnetic scalar potential χ0, which appears in the covariant representation B=∇χ0+β ∇ψT of the magnetic field. An example of calculation of Boozer magnetic coordinates is given for an experimental highly shaped high β equilibrium of DIIID [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159].
Impurity transport in Alcator C-Mod plasmas
Authors: Rice J. E. , Terry J. L. , Goetz J. A. , Wang Y. , Marmar E. S. , Greenwald M. , Hutchinson I. , Takase Y. , Wolfe S. , Ohkawa H. , Hubbard A.
Published: July 2002

Trace non-recycling impurities have been injected into Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas in order to determine impurity transport coefficients. Subsequent impurity emission has been observed with spatially scanning x-ray and Vacuum Ultra-Violet (VUV) spectrometer systems. Measured time-resolved brightness profiles of helium- and lithium-like transitions have been compared with those calculated from a transport code which includes impurity diffusion and convection in conjunction with an atomic physics package for individual line emission. During Low-Confinement-Mode (L-mode) plasmas, the transport can be characterized by pure diffusion, with coefficients ∼5000 cm2/s, reflecting the ∼20 ms decay in the x-ray and VUV line brightnesses. During High-Confinement-Modes (H-modes), the impurity confinement times are much longer, and the modelling requires that there be a strong inward convection (of order 1000 cm/s) near the plasma edge, with greatly reduced diffusion (of order 100 cm2/s), also in the region of the edge transport barrier. These edge values of the transport coefficients during H-mode are qualitatively similar to the neo-classical values. Nitrogen has also been injected, and after the H- to L-mode transition, the inner shell satellite lines of lithium-like nitrogen dominate in intensity the resonance line of helium-like N5+ in a thin shell near the plasma edge.