bluuebunny/crossref_metadata_2025 huggingface dataset:
Asymptotic behavior of a class of inhomogeneous scalar field cosmologies
Authors: Ibáñez J. , Olasagasti I.
Published: July 2002

The asymptotic behavior of a class of inhomogeneous scalar field cosmologies with a Liouville type of potential is studied. We define a set of new variables for which the phase space of the system of Einstein equations is bounded. This allows us to perform a complete analysis of the evolution of these cosmologies. We also discuss the extension of the cosmic no-hair theorem.
Magnetic structure of ferro–antiferromagnet interface. 1. Layered antiferromagnet
Authors: Grechnev A. G. , Kovalev A. S.
Published: July 2002

The ferro-antiferromagnetic interface is studied by using the classical discrete model. Collinear and noncollinear magnetic configurations of the interface are considered and critical values of parameters are obtained for which a collinear–noncollinear transition takes place.
On the theory of equilibrium magnetoelastic domain structure in easy-plane antiferromagnet
Authors: Gomonaj E. V. , Loktev V. M.
Published: July 2002

Macroscopic magnetoelastic domain structure of the defectless layered antiferromagnet of CoCl2-type with the “easy-plane” magnetic anisotropy is studied theoretically in the framework of phenomenological approach. In assumption of mobile domain walls, the finite-size effects are shown to result in the formation of a stable domain structure that changes reversibly under the action of the external magnetic field and can be treated as equilibrium. It is found that in antiferromagnets, where (in contrast to ferromagnets) long-range forces of magnetic origin are absent, the domain structure and its collective behavior are governed by elasticity. Field dependence of a domain structure, magnetostriction and low-frequency AFMR of poly- and monodomain samples are calculated, the external magnetic field being directed perpendicular to the main symmetry axis of the crystal. The results obtained are in qualitative agreement with the available experimental data.
On the genesis of droplet stream microspeed dispersions
Authors: Orme Melissa
Published: July 2002

Mechanisms that govern uniform droplet generation have been investigated. Droplet formation of viscous low vapor pressure fluids has been achieved by imposing sinusoidal pressure disturbances on a capillary stream. The capillary stream breakup and subsequent droplet propagation took place in a vacuum so that there were no significant interactions with the surrounding atmosphere. Microfluctuations of the droplet speeds have been examined after the droplets have traveled approximately 30 000 droplet diameters, and can vary from 1×10−6 to 1×10−4 times the average droplet speed depending on forcing conditions. Growth rate measurements were made of the streams radial disturbance prior to breakup, and were found to be intrinsically related to the measurements of the droplet stream’s speed microfluctuations. From this relation, it is suggested that studies of the characteristics of the droplet stream gives information about the capillary wave instabilities, which lead to droplet formation. A model that describes the microfluctuations is developed and is in excellent agreement with the experimentally obtained values. The model suggests that the source of droplet stream microspeed dispersions is the growth of a noise disturbance that modulates the otherwise controlled periodic pressure perturbation, which initiates droplet breakup.
Vorticity production at the edge of a slit by sound waves in the presence of a low-Mach-number bias flow
Authors: Fukumoto Yasuhide , Takayama Mitsuhiro
Published: July 2002

Vorticity production by low-frequency sound waves impinging on a screen perforated with a slit is studied in the presence of a steady low-Mach-number mean flow through it. A Kutta condition is invoked at the rim of the slit to determine the strength of the shed vortex sheets. It is found that, unlike the case of a circular aperture, the strength depends not only on the Strouhal number but also on the Mach number. An attempt is made to apply the results to the problem of transmission of sound waves through a uniformly perforated screen.
Inertia effect in two-dimensional MHD channel flow under a traveling sine wave magnetic field
Authors: Ueno Kazuyuki
Published: July 2002

The inertia effect peculiar to the MHD flow under a traveling sine wave transverse magnetic field between two parallel walls is investigated analytically and numerically. Solutions for steady flow of incompressible viscous fluid are obtained on the coordinate system moving with the traveling magnetic field. The flow is classified by interaction parameter N into three categories. (i) In the case of N≫H3a (Ha : average Hartmann number), inertia effect is negligibly small and the local force balance in each cross section is the same as that in the Hartmann flow characterized by Hl (Hl : local Hartmann number which is spatially periodic). (ii) In the case of 1≲N≲H3a, appreciable inertia effect appears in the region where the local magnetic field is smaller than N−1/3B0 (B0 : the amplitude of sine wave magnetic field). (iii) In the case of N≪1, a new type boundary layer appears, where the periodic part of the Lorentz force is balanced by inertia force. The inertia effect to the power output and the efficiency of an MHD induction generator is very small in spite of the remarkable change in the flow field.
Surface-tension-induced mixing following coalescence of initially stationary drops
Authors: Anilkumar A. V. , Lee C. P. , Wang T. G.
Published: July 2002

The momentumless coalescence of drops of the same liquid, separated by an immiscible host, is studied experimentally. Observations show that for low-viscosity drops of unequal sizes, there is considerable mixing following coalescence, with the smaller drop penetrating the larger drop as a vortex. The extreme case of coalescence of a small drop with the bulk of the same liquid at a flat interface with an immiscible liquid is studied in detail. The penetration depths of small drops (1–5 mm) following coalescence are measured and correlated with theoretical predictions. It is found that in the range of the investigation, the penetration depth is proportional to the 5/4 power of drop diameter and inversely proportional to the square root of the drop viscosity.
Absorption and current drive by electron cyclotron waves at upshifted and downshifted resonances in the lower hybrid current drive regime
Authors: Farina Daniela , Pozzoli Roberto
Published: July 2002

The linear analysis of the electron cyclotron (EC) wave absorption and current drive in the lower hybrid (LH) current drive regime is performed. To evaluate the absorbed power, the full relativistic dispersion relation is solved using a model function determined by the balance between the collisional and the quasilinear diffusion induced by the LH waves. The current density driven by the EC wave is computed by means of the adjoint method. The absorbed power, the current drive efficiency, and the driven current density are investigated as a function of the propagation angle and of the ratio Y between the electron cyclotron frequency and the frequency of the EC wave. The analysis is centered on the comparison between the downshifted (Y>1) and upshifted (Y<1) resonant interaction with the suprathermal tail. The general features of the results can be explained by means of the investigation of the location of the EC resonant curve in momentum space. The largest absorption and current drive efficiencies are found in the upshifted resonance condition, when the EC wave propagation is directed toward the suprathermal tail.
Energy and nonlinearity considerations for the enhanced plasma wave model of Raman scattering
Authors: Simon Albert , Short Robert W.
Published: July 2002

Raman scattering in nonuniform laser-produced plasma has been explained [Phys. Fluids 29, 1704 (1986)] as ordinary Raman scattering of the laser light from enhanced plasma waves in the corona. Here, a nonlinear model of the saturated plasma waves and of the fast electron pulses, which create the enhancement, is developed. The saturation level is normalized by use of absolute intensity measurements on the Nova laser [Phys. Rev. Lett. 60, 1018 (1988)], and the fast electron pulse parameters are chosen in agreement with the observed enhanced frequency band. The power in the fast electrons will then be consistent with both that in the laser source and in the drain to the saturated waves. There is agreement with all features of this Nova experiment, including some preliminary measurements of the angular variation of the scattered Raman intensity.
Microwave absorption in stellarators with a non-Maxwellian electron distribution function
Authors: Alejaldre C. , Castejón F.
Published: July 2002

The magnetic configurations of two stellarators, ATF [Fusion Technol. 10, 170 (1986)] and TJ-II [Fusion Technol. 13, 315 (1986)], are used as examples to study the influence that a superthermal component in the electron distribution function has upon the electron cyclotron resonance heating absorption process for microwaves propagating in the first and second harmonic of the electron cyclotron frequency. The non-Maxwellian tail is simulated by adding a drifting Maxwellian to a bulk distribution function and an analytical form of the weakly relativistic dielectric tensor is obtained. The quantitative, and more importantly, the qualitative differences between nonrelativistic and relativistic models are also presented. A numerical analysis shows that an increase of up to 50% in the absorption power with respect to the pure Maxwellian models, can be obtained if sufficient superthermal populations are present in the plasma.
Depressed collector experiments on a quasioptical gyrotron
Authors: Hargreaves T. A. , Fliflet A. W. , Fischer R. P. , Barsanti M. L.
Published: July 2002

A simple, single-stage depressed collector has been tested on the quasioptical gyrotron (QOG) experiment at the Naval Research Laboratory (NRL). This is the first application of a depressed collector to a high-power gyrotron, and was relatively easily accomplished due to the natural separation of the electron beam and the output radiation in the QOG. Collector efficiencies as high as 50% and overall efficiencies up to 16% were observed. The output power reached 431 kW with an overall efficiency of 13% and a collector efficiency of 41%. The collector efficiency was limited in this experiment due to interception of approximately 15% of the electron beam on an undepressed section of the beam guide, a problem readily correctable with a small change in the beam guide dimensions. If this part of the electron beam was collected at the collector potential, the overall and collector efficiencies would increase to 16% and 55%, respectively. The maximum collector efficiency would increase to 58% (at high output power) and the best overall efficiency would increase to 20% (at lower output power).
Experimental study of laser and wavelength dependences of laser–plasma coupling, transport, and ablation processes in planar gold targets
Authors: Dahmani Faiz
Published: July 2002

Gold-layered-target experiments at 1.06 μm have been performed in order to study the laser and wavelength dependence of the laser-light absorption, the energy transport, and the ablation at irradiances of 1012–1015 W/cm2. The results of the measured electron temperature and mass-ablation rate can be put in the form Te(eV) ≂506[IL(W/cm2)/1014]0.27 ×[λL(μm)]0.25 and ṁ(kg/cm2 sec) ≂162[IL(W/cm2)/1014]5/9 [λL(μm)]−4/3. A good agreement with the self-similar solutions presented by Sigel et al. [Phys. Fluids B 2, 199 (1990)] indicates a radiative transport for λL<0.044 μm and a thermal transport for λL>0.044 μm. In the latter case (λL>0.044 μm), a very good agreement is also obtained for 1010 W/cm2 ≤IL ≤1016 W/cm2 with the model of Dahmani et al. [Phys. Fluids B 3, 1232 (1991)]. The results presented here clarify some aspects concerning the x-ray conversion and transport in high-Z plasmas.
Observation of whistler instability in the Phaedrus-B tandem mirror plasma
Authors: Cui B. , Hershkowitz N. , Roberts D. R. , Litwin C. , Keil D.
Published: July 2002

Experimental results of whistler instability studies in the Phaedrus-B tandem mirror plasma [Phys. Fluids B 2, 787 (1990)] during electron cyclotron resonance heating (ECH) and ion cyclotron range of frequency (ICRF) heating are compared with theoretical models that predict whistler instability. The experimental results suggest that the whistler instability was caused by ‘‘warm’’ anisotropic electrons.
Some stationary solutions of two-fluid magnetohydrodynamic equations
Authors: Avinash K.
Published: July 2002

Some stationary solutions of two-fluid magnetohydrodynamic equations are constructed using generalized helicity invariants. Solutions corresponding to the Z pinch, Bennett pinch, θ pinch, etc., are constructed. The Z-pinch-like solution is identical to Weibel’s solution [Phys. Fluids 2, 52 (1959)], while the Bennett-pinch-like solution contains nonuniform axial drifts. By constructing the same solutions from the Vlasov–Maxwell system of equations, it is shown that the results obtained here are consistent with those of Mahajan [Phys. Fluids B 1, 43 (1989)]. Similarly, new θ-pinch-like solutions are constructed and the relation of these to earlier work is discussed.
Ion beam stabilization of rotational instability in a field-reversed configuration with rigid rotation
Authors: Ohnishi Masami , Ishida Akio , Akasaka Toshihiko
Published: July 2002

The rotational instability of a field-reversed configuration (FRC) can be suppressed by applying a multipole magnetic field. The multipole field, however, breaks the axisymmetry and may compromise configuration. An alternative method using injected ‘‘beam’’ ions would preserve the symmetry. This method is studied here within the framework of a multifluid model for which a variational principle has been developed and solved using the Rayleigh–Ritz technique. This approach leads to an analytic solution for a rigid-rotor equilibrium and allows the straightforward derivation of marginal stability conditions. This was not possible with a previous hybrid simulation which, though more complete, was cumbersome to apply. It is found that if the ratio of the rotational frequency of beam ions to that of the background ions exceeds a critical value, the radial displacement of the plasma and beam ions are opposite, and the rotational instability can be suppressed. The effect of compressibility of beam ions on the stability is also examined. The stability analysis is applied to present or near-term experimental devices and a future reactor. The beam energy and current need only be a small fraction of those of the background plasma in order to stabilize the rotational instability. These results are in qualitative agreement with previous results from a hybrid particle simulation.
Electromagnetic ion beam instabilities: II
Authors: Gary S. Peter , Madland Christian D. , Tsurutani Bruce T.
Published: July 2002

This paper extends the results of Gary et al. [Phys. Fluids 27, 1852 (1984)]. That paper examined the linear theory of electromagnetic instabilities driven by an ion beam streaming along a magnetic field in a homogeneous Vlasov plasma, emphasizing the parametric dependence of the instability growth rates. This paper considers the parametric dependence of the real frequency at maximum growth of two such instabilities with right-hand polarization. It is shown that the right-hand resonant ion beam instability can have maximum growth at frequencies near the ion-cyclotron frequency if the beam–main component relative drift speed is about twice the Alfvén speed and at least one of the following conditions holds: the ion beta or the beam–main component relative temperature are sufficiently small, or the perpendicular-to-parallel beam temperature ratio is sufficiently large. These results support the identification of the right-hand resonant instability as the source of the large amplitude magnetic fluctuations observed upstream of slow shocks in the Earth’s magnetotail.
Inductive inhibition of cold-plasma stabilization of curvature-driven modes in finite-length plasmas
Authors: Guest G. E. , Miller R. L. , Caponi M. Z.
Published: July 2002

A modified quasistatic theory that incorporates inductive effects in earlier electrostatic models connects the conventional electrostatic and magnetohydrodynamic (MHD) pictures of line tying by cold plasma. The modified theory predicts that curvature-driven flute modes in mirror-confined plasmas can be stabilized by moderate concentrations of cold plasma if the beta of the hot, mirror-confined plasma is less than a critical value. The maximum stable beta for an idealized stratified model of a hot-ion plasma, separated from conducting end walls by cold plasma, is given approximately by βcrit≂(4RpRc/LhLc) ×[ω2pe(cold)/ (k2⊥c2+ω2pe(cold))]. For ω2pe(cold) ≪k2⊥ c2, this result is identical with the predictions of electrostatic models. In the opposite limit, the result resembles the predictions of ideal MHD models, namely, that unstable curvature-driven modes can occur even in the presence of dense cold plasmas if the hot-plasma beta exceeds a limiting value estimated to be π2RpRc/L2.
Parametric instabilities in resonant absorption
Authors: Adam J. C. , Heron A.
Published: July 2002

The resonant absorption of electromagnetic waves in an inhomogeneous plasma is studied using a 2-D particle code. For the studied flux (Φ>8×1014 W/cm2), it is shown that it is impossible to separate resonant absorption from parametric instabilities. Effects of these instabilities on the absorption of incident energy and heating of particles are considered.
Stimulated Raman scattering in the presence of stimulated Brillouin scattering
Authors: Calder A. C. , Barnard A. J.
Published: July 2002

Particle simulations of stimulated Raman scattering (SRS) are presented in which stimulated Brillouin scattering (SBS) also occurs. Frequency spectra of forward and backscattered light are compared for various ratios of ion mass to electron mass. SBS is seen to weaken the SRS backscattering in all cases in which the plasma density profile includes the quarter-critical density. For cases in which the plasma density remains below quarter-critical, the SRS backscattering is not strongly affected by the SBS. In long-scale length, quarter-critical density cases for which mi/me=400, large amplitude density perturbations grow and persist.
Instantaneous structure of a breaking wave
Authors: Lin J.-C. , Rockwell D.
Published: July 2002

The quantitative, instantaneous structure of a stationary wave undergoing breaking in the spilling mode is characterized using high-image-density particle image velocimetry. The breaker originates from a sharp trough of the free surface. The essentially discontinuous slope of the surface, in the presence of flow separation beneath it, serves as a source of vorticity, giving rise to vorticity concentrations in a separated mixing layer. The region between this vorticity layer and the free surface is turbulent; it has insignificant levels of vorticity compared to the mixing layer.
Convection driven by forced diffusion in magnetic fluids
Authors: Odenbach Stefan
Published: July 2002

A convective flow in a suspension of magnetic particles driven by an interaction between a magnetic field gradient and a concentration gradient of the magnetic component of the fluid has been investigated. The concentration gradient itself is produced by a diffusion of the particles in the field gradient. It causes the demixing of the fluid. In the nonequilibrium states of this process a concentration gradient of the magnetic particles antiparallel to the field gradient establishes. The interaction between these two gradients gives rise to the convective flow. Experiments have been performed to prove the existence of this convection and to investigate its onset. In addition, a model has been formulated based on a time-dependent dimensionless parameter to interpret the experimental data.
Effects of turbulence length-scale distribution on scalar mixing in homogeneous turbulent flow
Authors: Cremer M. A. , McMurtry P. A. , Kerstein A. R.
Published: July 2002

The linear eddy mixing model is used to study effects of the turbulence length-scale distribution on the transient evolution of a passive scalar in a statistically steady homogeneous turbulent flow. Model simulations are carried out using both wide-band length-scale distributions reflecting high-Reynolds-number scaling, and narrow-band (in effect, low-Reynolds-number) distributions. The two cases are found to exhibit qualitative differences in mixing behavior. These differences are interpreted mechanistically. The narrow-band case yields the best agreement with published direct numerical simulation results, suggesting that those results are, in effect, low-Reynolds-number results not readily extrapolated to high-Reynolds-number mixing.
Effects of the free-stream density ratio on free and forced spatially developing shear layers
Authors: Soteriou Marios C. , Ghoniem Ahmed F.
Published: July 2002

The effects of the free-stream density ratio on the evolution of the incompressible, high Reynolds and Froude number, confined mixing layer are investigated numerically. Two-dimensional simulations of the spatially developing flow with and without external forcing are obtained using the Lagrangian transport element method. Results indicate that a nonunity density ratio alters the flow characteristics significantly. In the unforced flow, it increases the layer growth as the slow stream becomes denser, biases the speed of both the linear instability waves and the rollup eddies toward that of the denser stream, and modifies entrainment in favor of the dense fluid. These results, which are in agreement with experimental and analytical evidence, are analyzed in terms of the evolution of the vorticity field and, in particular, of the action of the mechanism of baroclinic vorticity generation. It is found that this mechanism creates vorticity of opposite signs across each eddy, which, through simple kinematical arguments, is linked to the alteration of the eddy speed and the modification of the local entrainment patterns. High-amplitude external forcing modifies the growth behavior of the layer while leaving its entrainment characteristics and the eddy speeds unaffected. In this case the layer growth is no longer monotonically varying with the free-stream density ratio. Instead, it is a strong function of the momentum ratio, reaching a minimum at a momentum ratio of unity and increasing more significantly for higher values of this parameter. Enhancement of the layer growth via forcing occurs only when the momentum ratio is substantially different from unity. It is found that the forced layer growth characteristics are related to the layer orientation, which is also a function of the momentum ratio. Using this fact and basic principles, a simple analytical model is derived to explain the numerical results. It is suggested that the unforced flow behaves differently due to its initial instability characteristics that are bypassed when forcing is present.
An instability criterion for a finite-amplitude localized disturbance in a shear flow of electrically conducting fluids
Authors: Levinski Vladimir , Rapoport Ilia , Cohen Jacob
Published: July 2002

The stability of shear flows of electrically conducting fluids, with respect to finite-amplitude three-dimensional localized disturbances, is considered. The time evolution of the fluid impulse integral, characterizing such disturbances, for the case of low magnetic Reynolds number is obtained by integrating analytically the vorticity equation. Analysis of the resulted equation reveals a new instability criterion.
Batchelor–Wood formula for negative wall velocity
Authors: Kim Sun-Chul
Published: July 2002

Extension of a formula giving the constant vorticity of the inviscid limit of steady two-dimensional Navier–Stokes flows in a circular domain is considered for slightly negative wall data. In the marginal separation case the formula is shown to be applicable with relatively small error.
Enstrophy and dissipation must have the same scaling exponent in the high Reynolds number limit of fluid turbulence
Authors: Nelkin Mark
Published: July 2002

Writing the Poisson equation for the pressure in the vorticity-strain form, it is shown that the pressure has a finite inertial range spectrum in the high Reynolds number limit of isotropic turbulence only if the anomalous scaling exponents μ and μω for the dissipation and enstrophy (squared vorticity) are equal. Since a finite inertial range pressure spectrum requires only very weak assumptions about high Reynolds number turbulence, it is concluded that the inference from experiment and direct numerical simulation that these exponents are different must be a finite-range scaling result which will not survive taking the high Reynolds number limit.
On the relationship of effective Reynolds number and Strouhal number for the laminar vortex shedding of a heated circular cylinder
Authors: Wang An-Bang , Trávníček Zdenek , Chia Kai-Chien
Published: July 2002

The laminar vortex shedding of airflow behind a circular cylinder with different heating temperatures was experimentally investigated with emphasis on the relationship of wake frequency and the Reynolds number. A new method to generate the two-dimensional parallel vortex shedding for the heated cylinder was developed and tested. An “effective Reynolds number” that employs a kinematic viscosity computed from an “effective temperature” is used to account for the temperature effects on the vortex shedding frequency. The present result shows that the frequency data could be successfully collapsed with the effective temperature computed by Teff=T∞+0.28(TW−T∞) for a wide range of cylinder temperatures, T∞ and TW being the free-stream temperature and cylinder surface temperature, respectively. Moreover, the relationship between Strouhal number and effective Reynolds number was found to be “universal.” The physical interpretation of Teff and the applicable region of the St–Reeff curve are discussed.
Interaction of Langmuir wave packets with streaming electrons: Phase-correlation aspects
Authors: Muschietti L. , Roth I. , Ergun R.
Published: July 2002

An analytical model of the interaction between a localized wave packet and energetic electrons is presented. Electrostatic packets of tens to a hundred wavelengths are considered in order to emulate the Langmuir waves observed in the auroral zone and in the solar wind. The phase information is retained, so the results can be applied to wave–particle correlator measurements. The perturbed distribution function is explicitly calculated and is shown to be bounded over all phase space due to a broadening of resonance ascribable to the finite extent of the packet. Its resistive part (in phase or 180° out of phase with the electric field) maximizes for v=ω/k, so that the associated bunching of electrons enables assessment of the characteristic wavelength. The changes in the wave profile due to the interaction with the energetic electrons are calculated. Broad wave packets grow or decay ‘‘self-similarly’’ with a rate given by the standard expression for a plane wave. Narrow, growing packets, on the other hand, quickly widen to sizes determined by the local distribution function. This sets a lower bound to the sizes of observed packets. Present results are supported by test-particle simulations and are in accord with recent correlator data of intense, localized Langmuir waves in the auroral zone.
The effect of sheared axial flow on the linear stability of the Z-pinch
Authors: Arber T. D. , Howell D. F.
Published: July 2002

A linear analysis of the ideal magnetohydrodynamic (MHD) stability of the Z-pinch is presented in which plasma flows are included in the equilibrium. With sheared axial flows it is found that substantial stabilization of internal modes is possible for some equilibrium profiles. For this to occur equilibria with a change in fluid velocity across the pinch radius of about Mach 2 are required. However, this ignores the surrounding vacuum and for the more realistic free boundary modes flows of about Mach 4 are required to stabilize all global MHD modes. This stabilization of MHD modes is not observed for all equilibria however. This fact, combined with the supersonic flow speeds required for stability, make it unlikely that a Z-pinch could in practice be stabilized by the introduction of sheared flow.
Investigation of physical processes limiting plasma density in high confinement mode discharges on DIII-D
Authors: Maingi R. , Mahdavi M. A. , Jernigan T. C. , La Haye R. J. , Hyatt A. W. , Baylor L. R. , Whyte D. G. , Wade M. R. , Petrie T. W. , Cuthbertson J. W. , Leonard A. W. , Murakami M. , Snider R. T. , Stambaugh R. D. , Watkins J. G. , West W. P. , Wood R. D.
Published: July 2002

A series of experiments was conducted on the DIII-D tokamak [J. L. Luxon and L. G. Davis, Fusion Technol. 8, 441 (1985)] to investigate the physical processes which limit density in high confinement mode (H-mode) discharges. The typical H-mode to low confinement mode (L-mode) transition limit at high density near the empirical Greenwald density limit [M. Greenwald et al., Nucl. Fusion 28, 2199 (1988)] was avoided by divertor pumping, which reduced divertor neutral pressure and prevented formation of a high density, intense radiation zone (MARFE) near the X-point. It was determined that the density decay time after pellet injection was independent of density relative to the Greenwald limit and increased nonlinearly with the plasma current. Magnetohydrodynamic (MHD) activity in pellet-fueled plasmas was observed at all power levels, and often caused unacceptable confinement degradation, except when the neutral beam injected (NBI) power was ⩽3 MW. Formation of MARFEs on closed field lines was avoided with low safety factor (q) operation but was observed at high q, qualitatively consistent with theory. By using pellet fueling and optimizing discharge parameters to avoid each of these limits, an operational space was accessed in which density ∼1.5×Greenwald limit was achieved for 600 ms, and good H-mode confinement was maintained for 300 ms of the density flat-top. More significantly, the density was successfully increased to the limit where a central radiative collapse was observed, the most fundamental density limit in tokamaks.
High-power transit-time oscillator: Onset of oscillation and saturation
Authors: Luginsland J. W. , Arman M. J. , Lau Y. Y.
Published: July 2002

A simple circuit model is used to investigate the transit-time oscillator (TTO) driven by a high-current diode. A novel condition for the onset of oscillation is derived in terms of the diode impedance. It is shown that a low impedance is required for the production of high-power microwaves in a TTO. The initial growth is calculated, and the saturation level is numerically computed using the one-dimensional model. These one-dimensional (1-D) results are in excellent agreement with a full scale two-dimensional Particle-in-Cell simulation. The success of the much simpler 1-D model allows a close examination of the roles played by the convection current and by the displacement current, as well as the modification in the transit time due to the intense space charge within the gap.
The spectrum of the curl operator on spherically symmetric domains
Authors: Cantarella Jason , DeTurck Dennis , Gluck Herman , Teytel Mikhail
Published: July 2002

This paper presents a mathematically complete derivation of the minimum-energy divergence-free vector fields of fixed helicity, defined on and tangent to the boundary of solid balls and spherical shells. These fields satisfy the equation ∇×V=λV, where λ is the eigenvalue of curl having smallest nonzero absolute value among such fields. It is shown that on the ball the energy minimizers are the axially symmetric spheromak fields found by Woltjer and Chandrasekhar–Kendall, and on spherical shells they are spheromak-like fields. The geometry and topology of these minimum-energy fields, as well as of some higher-energy eigenfields, are illustrated.
Ion implant monitoring with thermal wave technology
Authors: Smith W. Lee , Rosencwaig Allan , Willenborg David L.
Published: July 2002

A new method, based on thermal wave technology, is used to monitor the ion implantation process in silicon. It is a noncontact, nondestructive technique that requires no special sample preparation or processing, has high sensitivity even at low dose, and provides a one-micron spatial resolution capability. This method allows, for the first time, the ability to monitor the critical ion implantation process directly on the patterned product integrated circuit wafers as well as on the usual test wafers.
Development and use of a thin-film transmission positron moderator
Authors: Lynn K. G. , Nielsen B. , Quateman J. H.
Published: July 2002

A narrow energy beam of slow positrons can be generated by moderating the beta-decay spectrum of a radioactive β+ source. We report here the development and use of the first moderator in which the low-energy positrons are not extracted from the surface facing the β+ emitter but from the opposite surface, i.e., the transmission moderator. An advantage is that such a configuration eliminates the problem of moderated positrons passing by the radioactive source. This is an essential problem when the source is physically large such as the present commerically available Na22 sources. This moderator is a 5000-Å self-supporting W (100) film. The growth and treatment of the film were found to provide a high-quality positron moderator. The slow to fast positron conversion efficiency, using a Na22 source, was found to be 4×10−4.
Influence of microstructure on the photoconductivity of glow discharge deposited amorphous SiC:H and amorphous SiGe:H alloys
Authors: Mahan A. H. , Raboisson P. , Tsu R.
Published: July 2002

We introduce a parameter obtained from infrared measurements as a means of quantifying the amount of amorphous silicon microstructure and its degree of passivation by hydrogen. Using this parameter, the photoconductivities of amorphous silicon (a-Si:H), amorphous silicon carbon (a-SiC:H), and amorphous silicon germanium (a-SiGe:H) fall on the same curve. We discuss the relevance of these results with regard to material quality.
Microfabricated structures for the i
n s
i
t
u measurement of residual stress, Young’s modulus, and ultimate strain of thin films
Authors: Allen Mark G. , Mehregany Mehran , Howe Roger T. , Senturia Stephen D.
Published: July 2002

Two microfabricated structures for the in situ measurement of mechanical properties of thin films, a suspended membrane, and an asymmetric ‘‘released structure,’’ are reported. For a polyimide film on silicon dioxide, the membrane measurements yield a residual tensile stress of 30 MPa and a Young’s modulus of 3 GPa. The released structures measure the ratio of residual stress to Young’s modulus, and yield 0.011 at strains comparable to the suspended membranes, and 0.015 at larger strains. The ultimate strain as measured by both structures is approximately 4%.
Development of transmission positron moderators
Authors: Gramsch E. , Throwe J. , Lynn K. G.
Published: July 2002

The conversion from fast to slow positrons by several thin (0.3–7 μm) film moderators has been studied. Positrons from a radioactive 22Na source enter the film where some are thermalized, consequently diffuse to the surface opposite to the source and are emitted with low energies (1–4 eV). The experimental data show that the conversion efficiency to slow positrons ranges from 1×10−4 to 9.1×10−4 for good moderators. The moderator efficiency was found to be very sensitive to annealing temperatures.
Formation of high T
c superconducting films by organometallic chemical vapor deposition
Authors: Berry A. D. , Gaskill D. K. , Holm R. T. , Cukauskas E. J. , Kaplan R. , Henry R. L.
Published: July 2002

The first growth of superconducting YBaCuO films by organometallic chemical vapor deposition is described. Metal β-diketonates were decomposed thermally on MgO substrates in an oxygen-rich atmosphere to produce amorphous brown films. Subsequent annealing in oxygen yielded dull gray films whose thickness corresponded to deposition rates of approximately 8 nm min−1. These films showed semiconductor-like behavior at higher temperatures, followed by a broad resistive transition from 80 to 36 K with the resistance becoming zero at ∼20 K. Analysis of x-ray data indicated the presence of the orthorhombic superconducting phase and various other metal oxides. Profilometer measurements yielded film thicknesses up to 950 nm, and scanning electron microscopy revealed faceted grains from 0.5 to 1.0 μm in size.
Low-loss chalcogenide glass fiber with core-cladding structure
Authors: Nishii J. , Yamashita T. , Yamagishi T.
Published: July 2002

Ge-As-Se-Te glass fiber having the core-cladding structure was prepared using a rod-in-tube method. Fibers more than 30 m in length were drawn from a crucible with a nozzle and coated with an UV curable polymer. The diameters of the core and clad were 300 and 420 μm, respectively. The transmission loss spectra of the fibers were measured by the cut-back method and 0.6 dB/m was attained at the wavelength of 8.5 μm.
The first linkage disequilibrium (LD) maps: Delineation of hot and cold blocks by diplotype analysis
Authors: Maniatis N. , Collins A. , Xu C.-F. , McCarthy L. C. , Hewett D. R. , Tapper W. , Ennis S. , Ke X. , Morton N. E.
Published: July 2002

Linkage disequilibrium (LD) provides information about positional cloning, linkage, and evolution that cannot be inferred from other evidence, even when a correct sequence and a linkage map based on more than a handful of families become available. We present theory to construct an LD map for which distances are additive and population-specific maps are expected to be approximately proportional. For this purpose, there is only a modest difference in relative efficiency of haplotypes and diplotypes: resolving the latter into 2-locus haplotypes has significant cost or error and increases information by about 50%. LD maps for a cold spot in 19p13.3 and a more typical region in 3q21 are optimized by interval estimates. For a random sample and trustworthy map the value of LD at large distance can be predicted reliably from information over a small distance and does not depend on the evolutionary variance unless the sample size approaches the population size. Values of the association probability that can be distinguished from the value at large distance are determined not by population size but by time since a critical bottleneck. In these examples, omission of markers with significant Hardy–Weinberg disequilibrium does not improve the map, and widely discrepant draft sequences have similar estimates of the genetic parameters. The LD cold spot in 19p13.3 gives an unusually high estimate of time, supporting an argument that this relationship is general. As predicted for a region with ancient haplotypes or uniformly high recombination, there is no clear evidence of LD clustering. On the contrary, the 3q21 region is resolved into alternating blocks of stable and decreasing LD, as expected from crossover clustering. Construction of a genomewide LD map requires data not yet available, which may be complemented but not replaced by a catalog of haplotypes.
Importance of compartment formation for a self-encoding system
Authors: Matsuura Tomoaki , Yamaguchi Muneyoshi , Ko-Mitamura Elizabeth P. , Shima Yasufumi , Urabe Itaru , Yomo Tetsuya
Published: July 2002

A self-encoding system designed to have strict “compartition” of the molecules, i.e., to contain only a single molecule of DNA in each compartment, was established, and its evolutionary fate was analyzed. The system comprised the
Thermus thermophilus
DNA polymerase gene as the informational molecule and its protein product replicating the gene as the functional molecule. Imposing strict compartition allows the self-encoding system to last up to at least the tenth generation, whereas the system ceased to work after the third generation when loose compartition initiated with 100 molecules was imposed. These results provide experimental evidence on the importance of compartition for the maintenance of a self-encoding system. In addition, the extent of diversity in self-replication activity of the compartments was found to be another vital difference in the evolutionary dynamics between the strict and loose compartitions. Although the system with strict compartition provides widely diversified activity of the compartments at each generation, the values of the activity diverge only within a small range in the system with loose compartition. When the variety in the activity of a compartment is small, functional selection becomes weak, and to conform Darwinian evolution may become unfeasible. Therefore, strict compartition is essential for the evolvability of a self-encoding system.
Lineage infidelity in myeloid cells with TCR gene rearrangement: A latent developmental potential of proT cells revealed by ectopic cytokine receptor signaling
Authors: King Angela G. , Kondo Motonari , Scherer David C. , Weissman Irving L.
Published: July 2002

The most immature lymphoid-committed progenitors in both the bone marrow (common lymphoid progenitor) and thymus (proT1) maintain a latent granulocyte/macrophage (G/M) differentiation potential that can be initiated by signals emanating from exogenously expressed IL-2 receptors. In this study, we investigate at which developmental stage thymocytes lose this G/M differentiation potential. We demonstrate that the next maturational stage after proT1 cells (proT2), but not preT (TN3) cells, can convert cell fate from lymphoid to myeloid in response to ectopic IL-2 receptor signaling in human IL-2Rβ transgenic mice. It is significant that approximately 10% of clonogenic G/M colonies derived from proT cells of IL-2Rβ transgenic mice have DJ rearrangement specifically at the Dβ1 but not Dβ2 segment in the TCRβ locus. No TCR gene rearrangement is observed in G/M cells from nontransgenic mice, suggesting that the G/M cells we observe in this system were truly lymphoid-committed before stimulation with IL-2. In addition, Dβ1 and Dβ2 DJ rearrangement of the TCRβ gene may be differentially regulated and thus serve as markers for distinct proT cell maturational stages.
Assembly of the neutrophil respiratory burst oxidase: A direct interaction between p67

PHOX

and cytochrome
b
558
II
Authors: Dang Pham My-Chan , Cross Andrew R. , Quinn Mark T. , Babior Bernard M.
Published: July 2002

Activation of the phagocyte NADPH oxidase complex requires assembly of the cytosolic factors p47

PHOX

, p67

PHOX

, p40

PHOX

, and Rac with the membrane-bound cytochrome
b
558
. We recently established a direct interaction between p67

PHOX

and cytochrome
b
558.
In the present study, we show that removal of the C-terminal domain of p67

PHOX

increased its binding to cytochrome
b
558
. Whereas phosphorylated p40

PHOX

alone did not bind to cytochrome
b
558
, phosphorylated p47

PHOX

did, and, moreover, it allowed the binding of p40

PHOX

to the cytochrome. Furthermore, both increased the binding of p67

PHOX

to the cytochrome. Phosphorylated p47

PHOX

thus appears to increase the binding of p67

PHOX

to cytochrome
b
558
by serving as an adapter, bringing p67

PHOX

into proximity with cytochrome
b
558
, whereas phosphorylated p40

PHOX

may increase the binding by inducing a conformational change that allows p67

PHOX

to interact fully with cytochrome
b
558
.
A dynamin-like protein (ADL2b), rather than FtsZ, is involved in
Arabidopsis
mitochondrial division
Authors: Arimura Shin-ichi , Tsutsumi Nobuhiro
Published: July 2002

Recently, the FtsZ protein, which is known as a key component in bacterial cell division, was reported to be involved in mitochondrial division in algae. In yeast and animals, however, mitochondrial fission depends on the dynamin-like proteins Dnm1p and Drp1, respectively, whereas in green plants, no potential mitochondrial division genes have been identified. BLAST searches of the nuclear and mitochondrial genome sequences of
Arabidopsis thaliana
did not find any obvious homologue of the α-proteobacterial-type
ftsZ
genes. To determine whether mitochondrial division of higher plants depends on a dynamin-like protein, we cloned a cDNA for ADL2b, an
Arabidopsis
homologue of Dnm1p, and tested its subcellular localization and its dominant-negative effect on mitochondrial division. The fusion protein of green fluorescent protein and ADL2b was observed as punctate structures localized at the tips and at the constriction sites of mitochondria in live plant cells. Cells expressing dominant-negative mutant ADL2b proteins (K56A and T77F) showed a significant fusion, aggregation, and/or tubulation of mitochondria. We propose that mitochondrial division in higher plants is conducted by dynamin-like proteins similar to ADL2b in
Arabidopsis
. The evolutional points of loss of mitochondrial FtsZ and the functional acquisition of dynamin-like proteins in mitochondrial division are discussed.
Broadly cross-reactive HIV-1-neutralizing human monoclonal Fab selected for binding to gp120–CD4–CCR5 complexes
Authors: Moulard Maxime , Phogat Sanjay K. , Shu Yuuei , Labrijn Aran F. , Xiao Xiaodong , Binley James M. , Zhang Mei-Yun , Sidorov Igor A. , Broder Christopher C. , Robinson James , Parren Paul W. H. I. , Burton Dennis R. , Dimitrov Dimiter S.
Published: July 2002

HIV-1 entry into cells involves formation of a complex between gp120 of the viral envelope glycoprotein (Env), a receptor (CD4), and a coreceptor, typically CCR5. Here we provide evidence that purified gp120
JR-FL
–CD4–CCR5 complexes exhibit an epitope recognized by a Fab (X5) obtained by selection of a phage display library from a seropositive donor with a relatively high broadly neutralizing serum antibody titer against an immobilized form of the trimolecular complex. X5 bound with high (nM) affinity to a variety of Envs, including primary isolates from different clades and Envs with deleted variable loops (V1, -2, -3). Its binding was significantly increased by CD4 and slightly enhanced by CCR5. X5 inhibited infection of peripheral blood mononuclear cells by a selection of representative HIV-1 primary isolates from clades A, B, C, D, E, F, and G with an efficiency comparable to that of the broadly neutralizing antibody IgG1 b12. Furthermore, X5 inhibited cell fusion mediated by Envs from R5, X4, and R5X4 viruses. Of the five broadly cross-reactive HIV-1-neutralizing human monoclonal antibodies known to date, X5 is the only one that exhibits increased binding to gp120 complexed with receptors. These findings suggest that X5 could possibly be used as entry inhibitor alone or in combination with other antiretroviral drugs for the treatment of HIV-1-infected individuals, provide evidence for the existence of conserved receptor-inducible gp120 epitopes that can serve as targets for potent broadly cross-reactive neutralizing antibodies in HIV-1-infected patients, and have important conceptual and practical implications for the development of vaccines and inhibitors.