bluuebunny/crossref_metadata_2025 huggingface dataset:
Arnold diffusion in a torus with time-varying fields
Authors: Lichtenberg Allan J.
Published: July 2002

The interaction of magnetic field line stochasticity with drift islands can lead to Arnold diffusion of the drift island centers in a toroidal magnetic field. The basic formulas for this interaction have been derived and the diffusion rate determined. The formalism has been applied to calculating the parallel diffusion of an electron beam, which is subject both to magnetic stochasticity and a time-varying electric field. It is shown that the resulting Arnold diffusion can be readily measurable.
Amplification of ion-acoustic solitary waves by an ion beam
Authors: Zhang C. Y. , Kuehl H. H.
Published: July 2002

The effects of a cold ion beam on an ion-acoustic solitary wave whose potential is large enough to reflect the ion beam are studied in the regime in which the solitary-wave response to the beam is adiabatic. It is shown that the solitary wave can be amplified by a large factor, and that it is accelerated until its velocity equals the beam velocity.
Ion-temperature-gradient-driven turbulence in partially ionized plasmas with sheared parallel flow
Authors: Shukla P. K.
Published: July 2002

The ion-temperature-gradient(ITG)-driven electrostatic modes are investigated in the presence of the equilibrium sheared parallel flow in partially ionized magnetoplasmas. It is found that the source of free energy stored in the ion-temperature inhomogeneity can be coupled to electrostatic fluctuations due to the ion–neutral drag. The present collisional instability can possibly account for nonthermal fluctuations and anomalous cross-field ion thermal transport in low-temperature partially ionized magnetoplasmas.
Beam transmission in a high-power collective free-electron laser
Authors: Freund H. P.
Published: July 2002

A recent free-electron laser amplifier experiment conducted at the Massachusetts Institute of Technology [M. E. Conde and G. Bekefi, Phys. Rev. Lett. 67, 3082 (1991)] has demonstrated high-power operation without recourse to a tapered wiggler field. The experimental configuration consisted in the propagation of an intense electron beam (750 keV/300 A with a nominal axial energy spread of 1.5%) through a cylindrical waveguide in the presence of both a helical wiggler (Bw≤1.8 kG and λw=3.18 cm) and an axial guide magnetic field (B0≤12 kG). The experiment operated with the axial guide field oriented both parallel and antiparallel to the direction of the wiggler field, and the maximum efficiency was obtained for the antiparallel (i.e., reversed-field) configuration. The reversed-field case demonstrated an output power of 61 MW at 33.39 GHz for an efficiency of approximately 27%. The performance in the more usual parallel alignment of the fields was much less and peak power levels of only about 4 MW were obtained for both the weak (group I) and strong (group II) field regimes of the axial guide field. A detailed analytical characterization of this experiment has been presented in a previous work [H. P. Freund and A. K. Ganguly, IEEE Trans. Plasma Sci. PS-20, 245 (1992)] in which substantial agreement was found between the theory and the experiment for the reversed-field configuration. However, some discrepancies existed for the group I and II cases, and it was conjectured that some problem with beam transport existed for these configurations which led to an increased beam energy spread. In this paper, the question of beam transport in this experiment is analyzed. It is shown that beam transport is not a problem for the reversed-field configuration. However, substantial beam losses are found in the group I and II regimes, both in the entry taper region of the wiggler and due to high-power electromagnetic waves.
Fueling and heating of tandem mirror end cells using rf at the ion-cyclotron frequency
Authors: Golovato S. N. , Breun R. A. , Ferron J. R. , Goulding R. H. , Hershkowitz N. , Horne S. F. , Yujiri L.
Published: July 2002

Fueling and heating of tandem mirror end cells by rf at the ion-cyclotron frequency have been studied experimentally in the Phaedrus Tandem Mirror. The end cell density is found to increase linerly with rf voltage. The total plasma energy is observed to increase with rf power with no evidence of saturation at high power. The plasma axial length decreases with increasing rf power down to a length of approximately the distance between the two resonance locations in the end cell. The highest density and average ion energy are achieved with the resonance closest to the midplane. It is necessary to assume that the rf electric field at the resonance decreases with increasing density and with distance from the antenna in order to model the ion particle and power balance. The particle and power balance model predicts that dense, hot end cells may be maintained with E+∼1 V/cm and a fueling efficiency of a few percent when the resonance is close to the midplane because of better ion confinement and small plasma volume. Monte Carlo simulation of the trapping process shows that E2+ ∝T1.5ic is required to maintain a given rf trapping efficiency.
Marfes: Radiative condensation in tokamak edge plasma
Authors: Drake J. F.
Published: July 2002

Marfes are toroidally symmetric bands of high density radiating plasma that form at the edge of tokamak plasmas. The marfe results from a process of radiative condensation: A local increase in the plasma density increases the radiation rate and lowers the temperature, allowing the density to rise further to maintain pressure balance. It is demonstrated that the marfe onsets when the plasma density exceeds a critical threshold that is just below the density limit, in agreement with observations. This threshold results from a balance between condensation and cross-field thermal flux from the central hot plasma. Finally, it is noted that radiative condensation is also the driving mechanism of solar prominences and other astrophysical objects.
Linear stability analysis of nonhomentropic, inviscid compressible flows
Authors: Djordjevic V. D. , Redekopp L. G.
Published: July 2002

The linear stability of inviscid, compressible shear flows is studied. Some previous results for homentropic flows are extended to include adiabatic flows with variable temperature. Specific neutral solutions are obtained for both a shear layer and a wake in the subsonic regime that are stability boundaries. Unstable solutions are calculated for both streamwise and oblique disturbances in the shear layer flow. Other neutrally stable solutions are presented, which do not correspond to stability boundaries, describing stationary oscillations of supersonic shear flows.
Turbulence spectrum of a passive temperature field: Results of a numerical simulation
Authors: Chasnov J. , Canuto V. M. , Rogallo R. S.
Published: July 2002

The spectrum of a passive temperature field, G(k), has been determined by a numerical simulation using three kinds of isotropic turbulent velocity fields. For a time independent and Gaussian velocity field, the resulting G(k) has the form G(k)=G0εθε2/3 χ−3k−17/3, with G0=0.33±0.02 Ko, confirming the prediction of Batchelor, Howells, and Townsend [J. Fluid Mech. 5, 134 (1959)]. For a velocity field developed through the Navier–Stokes equations and then frozen in time, G(k) has the same form as above, but with G0 =0.39±0.03 Ko. Finally, for a velocity field developed concurrently with the temperature field, G(k) collapses onto the spectrum obtained using a frozen, developed velocity field only for high enough values of the conductivity χ. For lower values of χ, the power law behavior of G(k) is less clear.
On passive scalar derivative statistics in grid turbulence
Authors: Tong Chenning , Warhaft Z.
Published: July 2002

The probability density function, and related statistics, of scalar (temperature) derivative fluctuations in decaying grid turbulence with an imposed cross-stream, passive linear temperature profile, is studied for a turbulence Reynolds number range, Rel, varying from 50 to 1200, (corresponding to a Taylor Reynolds number range 30<Rλ<130). It is shown that the temperature derivative skewness in the direction of the mean gradient, Sθy has a value of 1.8±0.2 (twice the value observed in shear flows), and has no significant variation with Reynolds number. The ratio of the temperature derivative standard deviation along the gradient to that normal to it is approximately 1.2±0.1 also, with no variation with Re. The kurtosis of the derivatives increases approximately as Re0.2l. The results show that the rare, intense temperature deviations that produce the skewed scalar derivative, increase in frequency, but their area fraction (of the total field) becomes smaller as the Reynolds number increases. Thus, since Sθy remains constant, they become sharper and more intense, occurring deeper in the tails of the probability density function. Measurements in a thermal mixing layer, which has a nonlinear mean temperature profile, are also presented, and these show a similar value of Sθy to the linear profile case. The experiments broadly confirm the two-dimensional numerical simulations of Holzer and Siggia [Phys. Fluids (in press)], as well as other recent simulations, although there are some differences.
Numerical analysis of a supersonic rarefied gas flow past a flat plate
Authors: Aoki Kazuo , Kanba Kohsuke , Takata Shigeru
Published: July 2002

A uniform supersonic flow of a rarefied gas past a flat plate at zero angle of attack is considered, and the steady behavior of the gas around the plate is investigated numerically on the basis of the Boltzmann–Krook–Welander equation (or the so-called BGK model) and the diffuse reflection boundary condition. An accurate finite-difference analysis, which gives the correct description of the discontinuity of the velocity distribution function of the gas molecules occurring in the gas, is carried out, and the features of the flow field (the velocity distribution function and the macroscopic variables such as the density, temperature, and flow velocity of the gas), in particular, those around the leading and trailing edges, are clarified for a wide range of the Knudsen number. The drag acting on the plate and the energy transferred to it are also obtained accurately. In addition, on the basis of the results for small Knudsen numbers, the behavior of the gas around the leading edge of a semi-infinite plate is investigated in detail.
Crossover between surface tension and gravity-driven instabilities of a thin fluid layer on a horizontal cylinder
Authors: de Bruyn John R.
Published: July 2002

A thin annular layer of fluid coating a cylinder is subject to two different instabilities. One, driven by surface tension, is analogous to the Rayleigh instability of a liquid jet. The other is the Rayleigh–Taylor instability, which is driven by gravity. Measurements of the wavelength and growth rate of periodic patterns of droplets which develop as a result of the instability of such a fluid layer are reported for cylinders with radius r in the range 0.0011<r<1.27 cm. For small r the wavelength and growth rate of the pattern are in agreement with theoretical predictions for the surface-tension-driven instability. For large r, the Rayleigh–Taylor instability is observed. At intermediate r there is a region of crossover between the two limiting cases.
Viscous parallel flows in finite aspect ratio Hele-Shaw cell: Analytical and numerical results
Authors: Gondret P. , Rakotomalala N. , Rabaud M. , Salin D. , Watzky P.
Published: July 2002

The parallel flow of one or two fluids of contrasted viscosities through a rectangular channel of large aspect ratio is studied. The usual result for an infinite aspect ratio is that the velocity profile is parabolic throughout the gap and flat in the other direction. For a finite aspect ratio a deviation from this usual profile is found in boundary layers along the edges of the channel or close to the interface. The extension of these boundary layers is of the order of the small dimension of the channel. In the two-fluid case we find, however, that the velocity profile at the interface is strictly a parabola. The velocity profiles obtained by a 3-D lattice BGK simulation are successfully compared to the analytical results in the one- and two-fluid cases.
Impulsive and pressure-driven transient flows in closed ducts
Authors: Pedrizzetti Gianni , Domenichini Federico
Published: July 2002

The transient incompressible flow starting from rest inside an irregular circular duct is here analyzed when a finite discharge or a finite pressure jump is instantaneously imposed. The flow field is solved numerically in the axisymmetric approximation and a new technique to impose a prescribed pressure drop in the vorticity-stream-function formulation is introduced. The differences in the unsteady regimes leading to the same asymptotic flow are outlined.
Coalescence limited by hydrodynamics
Authors: Nikolayev Vadim S. , Beysens Daniel A.
Published: July 2002

We consider an assembly of liquid drops imbedded in another immiscible liquid of similar viscosity. It is shown that a coalescence between two drops induces another coalescence when the average distance between the drops is less than a threshold value, resulting in a “chain reaction” of coalescences. The threshold value is calculated using a “shell” model that is based on the boundary integral approach. Another “many-drop” model is developed to test the shell approximation. We show that, although the shell model is adequate, its results can be improved by lowering the shell surface tension.
A thermo-chemical exploration of a two-dimensional reacting supersonic mixing layer
Authors: Chakraborty Debasis , Upadhyaya H. V. Nagaraj , Paul P. J. , Mukunda H. S.
Published: July 2002

The hypervelocity two-dimensional reacting supersonic mixing layer experiments of Erdos et al. with a H2/air stream have been simulated with model free fine grid calculations on a N–S solver with full and single step chemistry. Response of the flow to fluctuations in the in-flow stream is utilized to examine chemistry fluid flow interactions. A favourable comparison of the computation with experimentally measured wall static pressure and heat transfer data along with flow picture forms the basis for further analysis. Insight into the mean flow thermal and reaction properties is provided from the examination of large scale structures in the flow in which the hydrogen stream is at 103 K flowing at 2.4 km/s (M=3.09) and the air stream is at 2400 K flowing at 3.8 km/s (M=3.99). The chemistry-flow interaction is dominated by large stream kinetic energy and affects the mean properties including the temperature profiles across the mixing layer. Single step chemistry, in comparison to full chemistry, is inadequate to describe ignition and early combustion processes, but seems reasonable for describing mixing and combustion downstream. Fast chemistry approximation coupled with mixture fraction based on hydrogen element seems to predict H2 mean profiles well; but this is shown to be due to the insensitivity of YH2 to progress of the reaction. This approximation under-predicts YO2 though the general shape of the profile is maintained. Mixture fraction variable approach is shown to be inadequate for the prediction of the H2O mass fraction because of the effect of non-normal diffusion. Finite chemistry conditions are shown to prevail throughout the domain of the mixing layer. It appears that use of mixture fraction approach may be inadequate to compute high speed reacting turbulent flows.
4 Carlo direct simulation of rotational relaxation of diatomic molecules using classical trajectory calculations: Nitrogen shock wave
Authors: Koura Katsuhisa
Published: July 2002

For accurate and realistic simulations of the rotational relaxation of diatomic molecules, the direct simulation Monte Carlo (DSMC) method is coupled with the classical trajectory calculations (CTC) for full three-dimensional collisions of two rigid diatomic molecules. The CTC-DSMC method with the null-collision technique is examined by simulating the rotational relaxation of nitrogen in an isolated cell over a wide range of temperature using the extended Morse and Lennard-Jones potentials. Although both the potentials yield almost the same results, the extended Morse potential is more efficient than the extended Lennard-Jones potential for use in the CTC-DSMC method. The CTC-DSMC method with the extended Morse potential is applied to the simulation of the rotational relaxation of nitrogen through low and high Mach number shock waves. The CTC-DSMC results obtained using available values of the potential parameters agree well with the experimental results of Robben and Talbot.
Sand ripples in an oscillating annular sand–water cell
Authors: Scherer M. A. , Melo F. , Marder M.
Published: July 2002

Sand ripples, familiar from ocean floors, are studied by oscillating an annular sand–water cell. Empirical rules for the onset and disappearance of ripples are uncovered and compared with simple dimensional arguments, and with prior experimental and theoretical work. In addition the dynamical behavior of sand ripples are investigated, and the existence of a secondary instability where small and large wavelengths coexist in space is shown. Finally, by visualizing the fluid flow above the sand ripples the paper describes vortices which are an integral part of the ripple formation and show that there is a strong coupling between two different types of vortices and the sand ripple pattern itself.
Equilibrium characteristics in a non-neutral plasma
Authors: Bhattacharyya S. N.
Published: July 2002

The system of equations governing the axisymmetric toroidal equilibrium of a non-neutral plasma is shown to become hyperbolic when the poloidal velocity of the plasma exceeds a sound speed.
Overrelaxation phenomena during the formation of reversed-field pinch plasmas
Authors: Mazur S. , Nordlund P.
Published: July 2002

Experiments on the Extrap T1 reversed-field pinch (RFP) [Phys. Scr. 49, 224 (1994)] have shown that the formation of the RFP configuration is quite sensitive to the relative programming of the toroidal field and Ohmic heating circuits. In this paper, new measurements of the evolution of the current density profile and of the spectral structure of the fluctuations during the setup phase of RFP plasmas in the T1 experiment are presented. These measurements improve the understanding of the role of different spectral components in the dynamics of RFP formation. Under unfavorable (slow) setup conditions, comparatively high energy is accumulated in m=1 internal kinks prior to reversal of the edge toroidal field. At reversal, nonlinearly driven m=0 modes trigger a rapid broadening of the m=1 spectrum. This behavior is associated with a violent suppression of the current density in the core, leading to an overrelaxation of the discharge involving a hollowing of the parallel current density profile. The setup conditions are found to affect the volt-second consumption and plasma/wall interaction during RFP formation, as well as the flat-top discharge performance.
Modeling magnetically insulated devices using flow impedance
Authors: Mendel C. W. , Rosenthal S. E.
Published: July 2002

In modern pulsed power systems the electric field stresses at metal surfaces in vacuum transmission lines are so high that negative surfaces are space-charge-limited electron emitters. These electrons do not cause unacceptable losses because magnetic fields due to system currents result in net motion parallel to the electrodes. It has been known for several years that a parameter known as flow impedance is useful for describing these flows. Flow impedance is a measure of the separation between the anode and the mean position of the electron cloud, and it will be shown in this paper that in many situations flow impedance depends upon the geometry of the transmission line upstream of the point of interest. It can be remarkably independent of other considerations such as line currents and voltage. For this reason flow impedance is a valuable design parameter. Models of impedance transitions and voltage adders using flow impedance will be developed. Results of these models will be compared to two-dimensional, time-dependent, particle-in-cell simulations.
Implosions, equilibria, and stability of rotating, radiating Z-pinch plasmas
Authors: Velikovich A. L. , Davis J.
Published: July 2002

The effects of uniform rotation on the dynamics, equilibria and stability of cylindrically symmetric, radiating Z-pinch plasmas are studied. Rotation changes the Bennett and Pease–Braginskii equilibria qualitatively, eliminating radiative collapse for both quasisteady and dynamic plasmas. In particular, a steady rotating plasma column can support any current above the Pease–Braginskii value, with Ohmic heating balanced by radiative losses. Stabilizing effect of rotation on the m=0 mode of Rayleigh–Taylor instability of a hollow plasma shell was found for long perturbation wavelengths.
Multifaceted asymmetric radiation from the edge: A strong condensation phenomenon
Authors: Chen X. P. , Shi B. R. , Gao Q. D.
Published: July 2002

Profile effects of the radiation, temperature, and the effect of atomic processes on thermal condensation instability in the tokamak edge are considered. The most important influence comes from a strong condensation effect due to the atomic processes and particle transport.
Higher order nonlocal formalism for linear analysis of a magnetized multispecies plasma with inhomogeneous flows
Authors: Gavrishchaka V. V. , Ganguli G. I. , Bakshi P. M. , Koepke M. E.
Published: July 2002

The formalism necessary to study the collective properties of a plasma system with inhomogeneous flows is nonlocal and generally in the form of an integrodifferential equation. Usually the eigenvalue condition is reduced to a second-order differential equation for simplicity. While the gross physical behavior of the system can be obtained from the second-order differential equation level of description, higher-order corrections are necessary for greater accuracy. The limit in which the scale-size of the velocity inhomogeneity is large compared to the ion gyroradius is considered and a transverse flow profile sharply localized in space (“top-hat” profile) is assumed. In this limit, a simple analytical method for the solution of the general eigenvalue condition to all orders is developed. A comparison of the properties of the solutions obtained from the second-order differential equation level of description with those obtained from higher orders is presented. Both the resonant (dissipative) and the nonresonant (reactive) effects of velocity shear are considered. It is found that while the overall features are well represented by the second-order level of description, the higher-order corrections moderate the destabilizing effects due to velocity shear, which can be quite significant in some cases.
Multipactor electron discharge physics using an improved secondary emission model
Authors: Gopinath V. P. , Verboncoeur J. P. , Birdsall C. K.
Published: July 2002

This study details particle-in-cell simulations of the multipactor effect and discharges created using this effect. A detailed secondary emission model which accounts for the energy and impact angle dependence of yield is developed to simulate accurately the phenomenon. It is shown that a steady state multipactor can be built up from a very low density initial electron distribution and an oscillatory steady state can be achieved. Steady state is reached when the cavity detuning and space charge of the built-up beam balance the phase focusing. The results for a more accurate secondary electron distribution are compared with simpler analytic models and extended. It is shown that, while the energy spread of secondary electrons differs from the monoenergetic analytical model, the weighted average energy is the same. Resistive loading of the circuit due to multipacting electrons is presented. The behavior of the system with Q values of between 20 and 200 is discussed.
Radiation fronts in tokamak divertor plasmas
Authors: Krasheninnikov S. I. , Batishcheva A. A. , Simakov A. N.
Published: July 2002

The impurity radiation from a tokamak divertor can be significantly enhanced due to the formation of a V-shaped impurity radiation front and the effects of perpendicular plasma heat conduction. For some conditions, the transition to a V-shaped radiation front can have a bifurcation character, such that the impurity radiation region jumps from the divertor targets to the entrance into the divertor. The divertor geometry can play a significant role in the formation of a V-shaped impurity radiation front. The analysis of the energy transport and impurity radiation suggest that divertor geometries with “vertical” target, neutral gas recirculation where sidewalls, or their combination are the most attractive from the point of view of the formation of a V-shaped radiation front and the plasma energy loss.
Characterization of diagnostic hole-closure in Z-pinch driven hohlraums
Authors: Baker K. L. , Porter J. L. , Ruggles L. E. , Chandler G. A. , Deeney Chris , Vargas M. , Moats Ann , Struve Ken , Torres J. , McGurn J. S. , Simpson W. W. , Fehl D. L. , Jobe D. O. , Chrien R. E. , Matuska W. , Idzorek G. C.
Published: July 2002

In this article we investigate the partial closure of diagnostic holes in Z-pinch driven hohlraums. These hohlraums differ from current laser-driven hohlraums in a number of ways such as their larger size, greater x-ray drive energy, and lower temperature. Although the diameter of the diagnostic holes on these Z-pinch driven hohlraums can be much greater than their laser-driven counterparts, 4 mm in diameter or larger, radiation impinges on the wall material surrounding the hole for the duration of the Z pinch, nearly 100 ns. This incident radiation causes plasma to ablate from the hohlraum walls surrounding the diagnostic hole and partially obscure this diagnostic hole. This partial obscuration reduces the effective area over which diagnostics view the hohlraum’s radiation. This reduction in area can lead to an underestimation of the wall temperature when nonimaging diagnostics such as x-ray diodes and bolometers are used to determine power and later to infer a wall temperature. In this article we describe the techniques used to characterize the hole-closure in these hohlraums and present the experimental measurements of this process.
Effects of neutral particles on edge dynamics in Alcator C-Mod plasmas
Authors: Boivin R. L. , Goetz J. A. , Hubbard A. E. , Hughes J. W. , Hutchinson I. H. , Irby J. H. , LaBombard B. , Marmar E. S. , Mossessian D. , Pitcher C. S. , Terry J. L. , Carreras B. A. , Owen L. W.
Published: July 2002

Neutral particle densities and energy losses have been measured in the Alcator C-Mod tokamak [Hutchinson et al., Phys. Plasmas 1, 1511 (1994)]. Their effect on the formation and evolution of the edge barrier which accompanies the enhanced confinement regime are discussed. The neutrals can enter the edge dynamics through the particle, momentum, and energy balance. Neutral densities of up to 5×1016 m−3 have been measured in the edge barrier region. Neutrals enter the local dynamics around most of the periphery, not just at the X-point. High resolution measurements of the ionization profile have been obtained for the region near the separatrix. The profile shifts inside the separatrix as the plasma is making a transition from low-to high-mode confinement (H-mode) regimes, partly accounting for the dramatic rise in edge density. The measured neutral density is large enough to affect the bulk ion momentum by charge exchange, and thereby introduces a negative radial electric field at the edge. At the same time, significant edge heat flux, carried by the neutrals, contributes to the measured power loss. At very high edge densities, this loss mechanism could contribute to quenching H-modes.
Ultrasonic pole figure for the texture of an aluminum alloy
Authors: Hirao M. , Hara N.
Published: July 2002

An approximate method is proposed to calculate the pole figure diagrams based on the elastic anisotropy measured by ultrasonic velocity experiments. An example is taken from the surface texture of rolled plates of aluminum alloy 7075-T651, for which the angular dependence of the Rayleigh wave is measured in the rolling plane and analyzed to obtain the expansion coefficients of orientation distribution function. The ultrasonic pole figures are favorably compared with the conventional x-ray pole figures, both demonstrating a single texture centered over (001)[110] orientation.
Investigation of nanocrystalline iron materials by Mössbauer spectroscopy
Authors: Herr U. , Jing J. , Birringer R. , Gonser U. , Gleiter H.
Published: July 2002

Nanocrystalline materials, which have been proposed to represent a new solid state structure, are investigated by Mössbauer spectroscopy. Nanocrystalline materials are polycrystals with a crystal size of typically 1–10 nm. These materials consist of two components of comparable volume fractions: a crystalline component and an interfacial component, formed by the atoms located either in the crystals or in the interfacial regions between them. As the atomic configurations of both components are different, two kinds of Mössbauer spectra are expected. Iron nanocrystalline material is found to exhibit a two-component Mössbauer spectrum, consisting of a crystalline component and a second one with different Mössbauer parameters. The Mössbauer parameters of the second subspectrum are consistent with the model of the interfacial component of a nanocrystalline material.
Negative photoconductivity in high electron mobility transistors
Authors: Chang C. S. , Fetterman H. R. , Ni D. , Sovero E. , Mathur B. , Ho W. J.
Published: July 2002

High electron mobility transistors are sensitive to light since illumination ionizes deep donor centers and increases the drain current. In this letter the first observation of negative photoconductivity, i.e., drain current decreasing with light, will be reported. The current-voltage characteristics were enhanced by shining white light onto the devices showing negative photoconductivity.
Decoration of flux pinning positions in YBa2Cu3O7−δ superconductors
Authors: Jou C. J. , Weber E. R. , Washburn J. , Soffa W. A.
Published: July 2002

Flux pinning in the sintered high Tc oxide superconductors, YBa2Cu3O7−δ, in the vortex state has been demonstrated for the first time using the Bitter pattern technique and scanning electron microscopy. External magnetic fields of 150 or 800 G perpendicular to the specimen surfaces were applied during the evaporation of cobalt after the specimens cooled with liquid nitrogen had reached a temperature below Tc. All specimens thus decorated with cobalt show nonuniformly distributed flux pinning positions as would be expected due to the porosity and multiphase character of the specimens as well as the relatively low field applied.
Role of the cAMP signaling pathway in the regulation of gonadotropin-releasing hormone secretion in GT1 cells
Authors: Vitalis Elizabeth A. , Costantin James L. , Tsai Pei-San , Sakakibara Hideya , Paruthiyil Sreenivasan , Iiri Taroh , Martini Jean-Francois , Taga Michiyoshi , Choi Amy L. H. , Charles Andrew C. , Weiner Richard I.
Published: July 2002

We studied the signaling pathways coupling gonadotropin-releasing hormone (GnRH) secretion to elevations in cAMP levels in the GT1 GnRH-secreting neuronal cell line. We hypothesized that increased cAMP could be acting directly by means of cyclic nucleotide-gated (CNG) cation channels or indirectly by means of activation of cAMP-dependent protein kinase (PKA). We showed that GT1 cells express the three CNG subunits present in olfactory neurons (CNG2, -4.3, and -5) and exhibit functional cAMP-gated cation channels. Activation of PKA does not appear to be necessary for the stimulation of GnRH release by increased levels of cAMP. In fact, pharmacological inhibition of PKA activity caused an increase in the basal secretion of GnRH. Consistent with this observation activation PKA inhibited adenylyl cyclase activity, presumably by inhibiting adenylyl cyclase V expressed in the cells. Therefore, the stimulation of GnRH release by elevations in cAMP appears to be the result of depolarization of the neurons initiated by increased cation conductance by cAMP-gated cation channels. Activation of PKA may constitute a negative-feedback mechanisms for lowering cAMP levels. We hypothesize that these mechanisms could result in oscillations in cAMP levels, providing a biochemical basis for timing the pulsatile release of GnRH.
The nucleoporin Nup98 associates with the intranuclear filamentous protein network of TPR
Authors: Fontoura Beatriz M. A. , Dales Samuel , Blobel Günter , Zhong Hualin
Published: July 2002

The Nup98 gene codes for several alternatively spliced protein
precursors. Two
in vitro
translated and
autoproteolytically cleaved precursors yielded heterodimers of
Nup98-6kDa peptide and Nup98-Nup96. TPR (translocated promoter region)
is a protein that forms filamentous structures extending from nuclear
pore complexes (NPCs) to intranuclear sites. We found that
in
vitro
translated TPR bound to
in vitro
translated Nup98 and, via Nup98, to Nup96. Double-immunofluorescence
microscopy with antibodies to TPR and Nup98 showed colocalization. In
confocal sections the nucleolus itself was only weakly stained but
there was intensive perinucleolar staining. Striking spike-like
structures emanated from this perinucleolar ring and attenuated into
thinner structures as they extended to the nuclear periphery. This
characteristic staining pattern of the TPR network was considerably
enhanced when a myc-tagged pyruvate kinase-6kDa fusion protein was
overexpressed in HeLa cells. Double-immunoelectron microscopy of these
cells using anti-myc and anti-TPR antibodies and secondary gold-coupled
antibodies yielded row-like arrangements of gold particles. Taken
together, the immunolocalization data support previous electron
microscopical data, suggesting that TPR forms filaments that extend
from the NPC to the nucleolus. We discuss the possible implications of
the association of Nup98 with this intranuclear TPR network for an
intranuclear phase of transport.
The second
STE12
homologue of
Cryptococcus neoformans
is
MATa
-specific and plays an important role in virulence
Authors: Chang Y. C. , Penoyer L. A. , Kwon-Chung K. J.
Published: July 2002

Cryptococcus neoformans STE12α
, a homologue of
Saccharomyces cerevisiae STE12
, exists only in
MAT
α strains. We identified another
STE12
homologue,
STE12a
, which is
MATa
specific. As in the case with
Δ
ste12α
, the mating efficiency for
Δ
ste12a
was reduced significantly. The
Δ
ste12a
strains surprisingly still
mated with
Δ
ste12α
strains. In
MATα
strains,
STE12a
functionally
complemented
STE12α
for mating efficacy, haploid
fruiting, and regulation of capsule size in the mouse brain.
Furthermore, when
STE12a
was replaced with two copies of
STE12α
, the resulting
MATa
strain
produced hyphae on filament agar.
STE12a
regulates mRNA
levels of several genes that are important for virulence including
CNLAC1
and
CAP
genes.
STE12a
also modulates enzyme activities of phospholipase
and superoxide dismutase. Importantly, deletion of
STE12a
markedly reduced the virulence in mice, as is the
case with
STE12
α. Brain smears of mice infected with
the
Δ
ste12a
strain showed yeast cells
with a considerable reduction in capsule size compared with those
infected with
STE12a
strains. When the disrupted locus
of
ste12a
was replaced with a wild-type
STE12a
gene, both
in vivo
and
in
vitro
mutant phenotypes were reversed. These results suggest
that
STE12a
and
STE12α
have similar
functions, and that the mating type of the cells influences the alleles
to exert their biological effects.
C. neoformans
, thus,
is the first fungal species that contains a mating-type-specific
STE12
homologue in each mating type. Our results
demonstrate that mating-type-specific genes are not only important for
saprobic reproduction but also play an important role for survival of
the organism in host tissue.
From transforming growth factor-β signaling to androgen action: Identification of Smad3 as an androgen receptor coregulator in prostate cancer cells
Authors: Kang Hong-Yo , Lin Hui-Kuan , Hu Yueh-Chiang , Yeh Shuyuan , Huang Ko-En , Chang Chawnshang
Published: July 2002

Although transforming growth factor-β (TGF-β) has been
identified to mainly inhibit cell growth, the correlation of elevated
TGF-β with increasing serum prostate-specific antigen (PSA) levels in
metastatic stages of prostate cancer has also been well documented. The
molecular mechanism for these two contrasting effects of TGF-β,
however, remains unclear. Here we report that Smad3, a downstream
mediator of the TGF-β signaling pathway, functions as a coregulator
to enhance androgen receptor (AR)-mediated transactivation. Compared
with the wild-type AR, Smad3 acts as a strong coregulator in the
presence of 1 nM 5α-dihydrotestosterone, 10 nM 17β-estradiol, or 1
μM hydroxyflutamide for the LNCaP mutant AR (mtAR T877A), found in
many prostate tumor patients. We further showed that
endogenous PSA expression in LNCaP cells can be induced by
5α-dihydrotestosterone, and the addition of the Smad3 further induces
PSA expression. Together, our findings establish Smad3 as an important
coregulator for the androgen-signaling pathway and provide a possible
explanation for the positive role of TGF-β in androgen-promoted
prostate cancer growth.
Somatic action potentials are sufficient for late-phase LTP-related cell signaling
Authors: Dudek Serena M. , Fields R. Douglas
Published: July 2002

A question of critical importance confronting neuroscientists today is how biochemical signals initiated at a synapse are conveyed to the nucleus. This problem is particularly relevant to the generation of the late phases of long-term potentiation (LTP). Here we provide evidence that some signaling pathways previously associated with late-LTP can be activated in hippocampal CA1 neurons without synaptic activity; somatic action potentials, induced by backfiring the cells, were found to be sufficient for phosphorylation of extracellular signal-regulated kinase-1/2 and cAMP response element-binding protein, as well as for induction of
zif268
. Furthermore, such antidromic stimulation was adequate to rescue “tagged” synapses (early-LTP) from decay. These results show that a synapse-to-nucleus signal is not necessary for late-phase LTP-associated signaling cascades in the regulation of gene expression.
Stable isotope signature of philopatry and dispersal in a migratory songbird
Authors: Graves Gary R. , Romanek Christopher S. , Rodriguez Navarro Alejandro
Published: July 2002

Stable isotope analysis is widely promoted as a practical method for tracing the geographic origins of migratory birds. However, the extent to which geospatial patterns of isotope ratios in avian tissues are influenced by age-specific, altitudinal, and temporal factors remains largely unexplored. We measured carbon (
13
C/
12
C) and nitrogen (
15
N/
14
N) isotope ratios in feathers of black-throated blue warblers (
Dendroica caerulescens
) breeding along a relatively steep altitudinal gradient in the Appalachian Mountains to evaluate the effects of altitude and year on the isotopic signatures of yearling (first breeding season) and older males (>2 years). Breeding males (
n
= 302) collected during 7 consecutive years exhibited significant age-specific and altitudinal effects in δ
13
C values and age-specific and temporal effects in δ
15
N values. The δ
13
C values of older males increased with altitude at the rate of ≈1.3‰ per 1,000 m, suggesting a high degree of year-to-year philopatry to narrow altitudinal zones, if not to breeding territories. In contrast, absence of altitudinal patterns in yearlings most likely reflects natal dispersal. Carbon isotope variation (δ
13
C = −26.07 to −20.86‰) observed along a single altitudinal transect (755 m) nearly brackets the range of δ
13
C values recorded in feathers across the North American breeding range of the warbler from Georgia to New Brunswick (11° of latitude) and from New Brunswick to Michigan (22° of longitude). These data indicate that age-specific and altitudinal effects must be considered when using δ
13
C values to delineate the geographic origin of avian species with large altitudinal and latitudinal ranges.
The δ subunit of AP-3 is required for efficient transport of VSV-G from the trans-Golgi network to the cell surface
Authors: Nishimura Noriyuki , Plutner Helen , Hahn Klaus , Balch William E.
Published: July 2002

Vesicular stomatitis virus glycoprotein (VSV-G) is a transmembrane protein that functions as the surface coat of enveloped viral particles. We report the surprising result that VSV-G uses the tyrosine-based di-acidic motif (-YTDIE-) found in its cytoplasmic tail to recruit adaptor protein complex 3 for export from the trans-Golgi network. The same sorting code is used to recruit coat complex II to direct efficient transport from the endoplasmic reticulum to the Golgi apparatus. These results demonstrate that a single sorting sequence can interact with sequential coat machineries to direct transport through the secretory pathway. We propose that use of this compact sorting domain reflects a need for both efficient endoplasmic reticulum export and concentration of VSV-G into specialized post-trans-Golgi network secretory-lysosome type transport containers to facilitate formation of viral coats at the cell surface.
A unique phospholipid organization in bovine erythrocyte membranes
Authors: Florin-Christensen J. , Suarez C. E. , Florin-Christensen M. , Wainszelbaum M. , Brown W. C. , McElwain T. F. , Palmer G. H.
Published: July 2002

Ruminant erythrocytes are remarkable for their choline-phospholipid anomalies; namely, low or absent phosphatidylcholine (PC) along with high sphingomyelin levels. Here, we report another anomaly in bovine erythrocytes that affects aminophospholipids: phosphatidylethanolamine (PE) shows an extreme asymmetry, with only 2% of the total present in the outer leaflet. Furthermore, we found that phospholipase A
2
, an enzyme located on the external surface of the erythrocytes, shows higher activity against PC than against PE. In addition, we observed that acylation of PE is by far the most important biosynthetic event in this system. We propose that deacylation of PE and PC by phospholipase A
2
to generate lysocompounds, followed by selective reacylation of lyso-PE in the inner leaflet, can account for the compositional and architectural peculiarities of bovine erythrocyte membranes.
Retinoic acid induces sodium/iodide symporter gene expression and radioiodide uptake in the MCF-7 breast cancer cell line
Authors: Kogai Takahiko , Schultz James J. , Johnson Laura S. , Huang Min , Brent Gregory A.
Published: July 2002

The sodium/iodide symporter (NIS) stimulates iodide uptake in normal lactating breast, but is not known to be active in nonlactating breast or breast cancer. We studied NIS gene regulation and iodide uptake in MCF-7 cells, an estrogen receptor (ER)-positive human breast cancer cell line. All-
trans
retinoic acid (tRA) treatment stimulated iodide uptake in a time- and dose-dependent fashion up to ≈9.4-fold above baseline. Stimulation with selective retinoid compounds indicated that the induction of iodide uptake was mediated by retinoic acid receptor. Treatment with tRA markedly stimulated NIS mRNA and immunoreactive protein (≈68 kDa). tRA stimulated NIS gene transcription ≈4-fold, as shown by nuclear run-on assay. No induction of iodide uptake was observed with RA treatment of an ER-negative human breast cancer cell line, MDA-MB 231, or a normal human breast cell line, MCF-12A. The iodide efflux rate of tRA-treated MCF-7 cells was slow (
t
1/2
= 24 min), compared with that in FRTL-5 thyroid cells (
t
1/2
= 3.9 min), favoring iodide retention in MCF-7 cells. An
in vitro
clonogenic assay demonstrated selective cytotoxicity with
131
I after tRA stimulation of MCF-7 cells. tRA up-regulates NIS gene expression and iodide uptake in an ER-positive breast cancer cell line. Stimulation of radioiodide uptake after systemic retinoid treatment may be useful for diagnosis and treatment of some differentiated breast cancers.
Theoretical emission line ratios for [Fe III] and [Fe VII] applicable to the optical and infrared spectra of gaseous nebulae
Authors: Keenan Francis P. , Aller Lawrence H. , Ryans Robert S. I. , Hyung Siek
Published: July 2002

Recent calculations of electron impact excitation rates and
Einstein A-coefficients for transitions among the
3
d
6
levels of Fe III and among the
3
d
2
levels of Fe VII are used to derive
theoretical emission line ratios applicable to the optical and infrared
spectra of gaseous nebulae. Results for [Fe III] are generated for
electron temperatures
T
e
=
7,000–20,000 K and densities
N
e
=
10
2
-10
8
cm
−3
, whereas those for
[Fe VII] are provided for
T
e
=
10,000–30,000 K and
N
e
=
10
2
-10
8
cm
−3
. The theoretical line
ratios are significantly different in some instances from earlier
calculations and resolve discrepancies between theory and observation
found for the planetary nebulae IC 4997 and NGC 7027.
History dependence of protein adsorption kinetics
Authors: Calonder Claudio , Tie Yanrong , Van Tassel Paul R.
Published: July 2002

The behavior of proteins at biological and synthetic interfaces is often characterized by a strong history dependence caused by long relaxation times or irreversible transitions. In this work, we introduce the rate of adsorption as a means to systematically quantify the extent, and identify the underlying causes, of history dependence. We use multistep kinetic experiments in which the
i
′th step is an exposure of a Si(Ti)O
2
surface to a flowing fibronectin or cytochrome
c
solution of concentration
c
i
for a time
t
i
(
c
i
= 0 corresponds to a rinse) and measure the protein adsorption by optical waveguide light mode spectroscopy. The rate of adsorption is sensitive to the structure of the adsorbed layer, and we observe it to greatly increase, for a given adsorbed density, when going from a first to a subsequent adsorption step. This increase is most pronounced when the duration of the initial adsorption step is long. We attribute these observations to the gradual and irreversible formation of protein clusters or locally ordered structures and use them to explain previous underestimates of kinetic data by mesoscopic model descriptions. A thorough understanding of these complex postadsorption events, and their impact on history dependence, is essential for many physiological and biotechnological processes. Optical waveguide lightmode spectroscopy is a promising technique for their macroscopic quantification.
Two-dimensional IR spectroscopy can be designed to eliminate the diagonal peaks and expose only the crosspeaks needed for structure determination
Authors: Zanni Martin T. , Ge Nien-Hui , Kim Yung Sam , Hochstrasser Robin M.
Published: July 2002

The power of two-dimensional (2D) IR spectroscopy as a structural
method with unprecedented time resolution is greatly improved by the
introduction of IR polarization conditions that completely eliminate
diagonal peaks from the spectra and leave only the crosspeaks needed
for structure determination. This approach represents a key step
forward in the applications of 2D IR to proteins, peptides, and other
complex molecules where crosspeaks are often obscured by diagonal
peaks. The technique is verified on the model compound
1,3-cyclohexanedione and subsequently used to clarify the distribution
of structures that the acetylproline-NH
2
dipeptide adopts
in chloroform. In both cases, crosspeaks are revealed that were not
observed before, which, in the case of the dipeptide, has led to
additional information about the structure of the amino group end of
the peptide.