bluuebunny/crossref_metadata_2025 huggingface dataset:
Low-dimensional models for flows with density fluctuations
Authors: Lumley John L. , Poje Andrew
Published: July 2002

A low-dimensional model, using the proper orthogonal, or Karhunen–Loève decomposition, has been remarkably successful in representing the behavior of the wall region of a turbulent boundary layer. We briefly summarize this work. We may hope for similar success in other flows in which coherent structures play an important role, in particular flows with density fluctuations. We sketch an approach to such a decomposition for flows with density fluctuations, suggesting various alternatives which weigh the available information differently. In such a low-dimensional model, obtaining the empirical eigenfunctions poses a problem, since they can usually be determined only from extensive measurements or direct numerical simulations. However, recent work with energy method stability theory (modified by use of an anisotropic eddy viscosity and feedback to the mean profile) has been remarkably successful in predicting the form of the empirical eigenfunctions in the isothermal boundary layer. We present here preliminary results for sheared Rayleigh–Bénard convection; these results do not include anisotropic eddy viscosities and feedback, and do not predict directly the form of the POD eigenfunctions; however, a very satisfactory comparison can be made with the second order moments obtained from a DNS.
Thermohaline convection with nonlinear salt profiles
Authors: Balmforth N. J. , Casti A. R. R. , Julien K. A.
Published: July 2002

We study the linear stability of a fluid to double-diffusive convection in the case for which the background temperature field is linear, but the mean salt distribution is a nonlinear function. A variety of idealized limits amenable to perturbative treatment are examined, and a detailed numerical study of a hyperbolic tangent profile is presented. It is shown that nonlinearity in the background salinity profile leads to pronounced competition and interaction between the linear normal modes.
Unsteady flow of an axisymmetric annular film under gravity
Authors: Housiadas Kostas , Tsamopoulos John
Published: July 2002

The unsteady flow of an annular and axisymmetric film under gravity is examined. This moving boundary problem is solved by mapping the inner and the outer interface of the film in the radial direction onto fixed ones and by transforming the governing equations accordingly. The ratio of the film thickness to its inner radius at the exit of the die is small in relevant processes with polymer melts. This ratio, ε, is used as the small parameter in a perturbation expansion of the general Navier–Stokes equations. Forces applied on the film include gravity, surface tension, inertia, and viscous forces. Their ratios give rise to three dimensionless numbers, St, Ca, and Re. When these dimensionless numbers are up to order one, the base state is quite deformed and it is calculated numerically by simultaneously solving three nonlinear partial differential equations in time and the axial direction. Intuitively it is expected that when the dimensionless numbers are small the base state in the perturbation scheme is a uniformly falling film. This is confirmed by analysis and the two next orders in the perturbation scheme are calculated analytically. In both cases, it was found that increasing the St number (i) accelerates the downward motion of the film, (ii) deflects its inner and outer surfaces towards its axis of symmetry, and (iii) decreases its thickness around the middle of its length. The latter effect may lead to breakup of the film in two parts. It was also found that increasing the Ca number deflects these two interfaces towards its axis of symmetry and increases its thickness monotonically with time and the axial distance. Increasing the Re number from zero, but to not very large values, generally decelerates the film and decreases its deflection from the vertical. Given typical fluid properties and process conditions the St number is up to O(ε0), i.e., much larger than the other two dimensionless numbers, and affects the film shape more significantly.
Three-dimensional thermal convection caused by spacecraft rotation in a rectangular enclosure with rigid walls
Authors: Yuferev V. S. , Polovko Yu. A. , Sazonov V. V.
Published: July 2002

The influence of the spacecraft’s rotation on the convection in a nonuniformly heated fluid under microgravity conditions has been studied. This influence is determined by action of the buoyancy force generated by the resultant microacceleration vector and the inertia force related to variations in the angular rotational velocity of the spacecraft. It is shown that the solenoidal part of the resulting rotational force acting upon the nonuniformly heated fluid strongly depends on the angle between the temperature gradient at some point in the fluid and the radius vector of this point relative to the spacecraft’s centroid. To minimize the influence of the spacecraft’s rotation, it is necessary to make this angle close to zero. The thermal convection which could have arisen in a rectangular box with rigid walls under rotation of the Space Shuttle orbiter has been simulated numerically. It is shown that the intensity of this kind of convection may be rather appreciable.
Exact analytic solutions to turbulent particle flow equations
Authors: Hyland K. E. , McKee S. , Reeks M. W.
Published: July 2002

A technique for solving a certain class of partial differential equations is used to obtain exact solutions for particle dispersion in an unbounded Gaussian random flow field in which the mean flow is a simple shear and the turbulence is homogeneous and stationary. Results are compared with those of a simple random walk simulation and those obtained from tracking particles in a simple developing shear generated by LES.
Spontaneous formation of equatorial jets in freely decaying shallow water turbulence
Authors: Iacono R. , Struglia M. V. , Ronchi C.
Published: July 2002

Results of high-resolution, long time integrations of the shallow water equations in spherical geometry are presented. Final quasi-steady states of the freely decaying evolution are found, characterized by the emergence of robust equatorial jets and by the breaking of cyclone–anticyclone symmetry. It is shown that these two phenomena are intimately related and that global prevalence of anticyclonic vorticity necessarily leads to the formation of a westward (retrograde) equatorial jet.
Reynolds number dependence of the second-order turbulent pressure structure function
Authors: Antonia R. A. , Bisset D. K. , Orlandi P. , Pearson B. R.
Published: July 2002

Second-order pressure structure functions, estimated from direct numerical simulations of homogeneous and nonhomogeneous turbulent flows, exhibit a significant Reynolds number dependence in the dissipative range. This dependence mainly reflects the contribution from low wave numbers to the instantaneous pressure gradient.
The stability of steady, helical vortex filaments in a tube
Authors: Sarasúa L. G. , Schifino A. C. Sicardi , González R.
Published: July 2002

The nonlinear conditions for the development of helical vortex filaments in a circular tube are considered. The helical flow is assumed to be irrotational, except in a vortex filament of infinitesimal core area. By introducing an appropriate image for this helical vortex filament, the boundary condition on the material frontier is satisfied. By assuming an axisymmetric flow upstream and imposing the conservation laws, a dependence between the helix pitch and the nonlinear amplitude of the helical vortex developed downstream is obtained. Our results show that only helical flows with the pitch in a certain range of values are allowed. The dependence of this range on the swirl ratio and on the tube cross section is considered. We discuss the usefulness of the nonlinear analysis of the allowed flows to explain experimental results and to complement the usual linear study of stability.
Spontaneous mean flow shear amplification in turbulent plasmas
Authors: Gruzinov A. V. , Diamond P. H. , Lebedev V. B.
Published: July 2002

The spontaneous generation of shear flows in turbulent plasmas is discussed. In the case of the Hasegawa–Mima model of turbulence, mean shear (S=dVy/dx) generation minimizes the classical Ginzburg–Landau free-energy functional F=∫ dx(−D1S2+D2S2x+BS4). The process of shear flow generation thus closely resembles the mean-field theory of a second-order phase transition.
A global dynamo in a reversed-field pinch plasma
Authors: Nagata A. , Sato K. I. , Kubota S. , Ando T. , Arimoto H. , Amano T. , Saito K. , Shiina S. , Matsuoka A. , Nagatsu M. , Tsukishima T.
Published: July 2002

It is found that a reversed-field pinch (RFP) dynamo uniformly generates most of the toroidal flux in a RFP plasma with a high magnetic Reynolds (S) number. Hereafter, this is referred to as the global dynamo. The toroidal flux generated by the global dynamo is proportional to the plasma current. The effective inductance in a RFP plasma increases with S number and remains constant against the changing plasma current. This means that the pinch parameter Θ is held constant, that is, the global dynamo has a self-organizing effect sustaining a particular RFP configuration. By comparing simulation results and analysis of magnetic fluctuations, it is confirmed that a global dynamo is generated by the nonlinear evolution of the single-helicity of the m=1 mode alone.
Modeling of Nova indirect drive Rayleigh–Taylor experiments
Authors: Weber S. V. , Remington B. A. , Haan S. W. , Wilson B. G. , Nash J. K.
Published: July 2002

The growth due to the Rayleigh–Taylor (RT) instability of single-wavelength surface perturbations on planar foils of brominated CH [CH(Br)] and fluorosilicone (FS) was measured. The foils were accelerated by x-ray ablation with temporally shaped drive pulses. A range of initial amplitudes (a0) and wavelengths (λ) have been used. This paper focuses upon foils with small a0/λ, which exhibit substantial growth in the linear regime, and are most sensitive to the calculated growth rate. The CH(Br) foils exhibit slower RT perturbation growth because opacity differences result in a larger ablation velocity and a longer density scale length than for FS. Tabulated opacities from detailed atomic models, OPAL [Astrophys. J. 397, 717 (1992)] and super transition array (STA) [Phys. Rev. A 40, 3183 (1989)] were employed. Unlike previous simulations which employed the average atom (XSN) opacity treatment, parameter adjustments to fit experimental data no longer appear necessary. Nonlocal thermodynamic equilibrium (NLTE) effects do not appear to be important. Other variables which may affect the modeling, such as changes of the equation of state and radiation drive spectrum, were also examined. The current calculational model, which incorporates physically justified choices for these calculational ingredients, agrees with the Nova single wavelength RT perturbation growth data.
Small amplitude modified electron acoustic solitary waves in semirelativistic plasmas
Authors: Sah O. P. , Goswami K. S.
Published: July 2002

The properties of small-amplitude, modified-electron acoustic solitary waves in an unmagnetized plasma consisting of relativistic drifting electrons and nondrifting thermal ions are investigated. The influences of relativistic effects and ion inertia on the propagation of such waves are examined. It is found that rarefactive types of soliton solutions exist only when the electron drift velocity is less than the ion thermal velocity.
The controlling parameter of a wiggler-pumped free-electron laser with an axial-guide magnetic field
Authors: Gou San-kui
Published: July 2002

The basic equations of a wiggler-pumped free-electron laser with an axial-guide magnetic field are given. It is shown that the evolution of this configuration is governed by a controlling parameter which contains the effects of the wiggler, the electron beam, and the guide field. Using the bi-average over the all electrons and the initial phase, the critical value of the controlling parameter is obtained. Further, the evolution property of the radiation field is described.
Time-resolved x-ray imaging of high-power laser-irradiated underdense silica aerogels and agar foams
Authors: Koch J. A. , Estabrook K. G. , Bauer J. D. , Back C. A. , Klein L. , Rubenchik A. M. , Hsieh E. J. , Cook R. C. , MacGowan B. J. , Moody J. D. , Moreno J. C. , Kalantar D. , Lee R. W.
Published: July 2002

This article presents the results of experiments in which a high-power laser was used to irradiate low density (4–9 mg/cm3) silica aerogel and agar foam targets. The laser–solid interaction and energy transport through the material were monitored with time-resolved imaging diagnostics, and the data show the production and propagation of an x-ray emission front in the plasma. The emission-front trajectory data are found to be in significant disagreement with detailed simulations, which predict a much more rapid heating of the cold material, and the data suggest that this discrepancy is not explainable by target inhomogeneities. Evidence suggests that energy transport into the cold material may be dominated by thermal conduction; however, no completely satisfactory explanation for the discrepancies is identified, and further experimental and theoretical research is necessary in order to resolve this important problem in laser–plasma interaction physics.
Analysis of alpha particle-driven toroidal Alfvén eigenmodes in Tokamak Fusion Test Reactor deuterium–tritium experiments
Authors: Fu G. Y. , Cheng C. Z. , Budny R. , Chang Z. , Darrow D. S. , Fredrickson E. , Mazzucato E. , Nazikian R. , Wong K. L. , Zweben S.
Published: July 2002

The toroidal Alfvén eigenmodes (TAE) are calculated to be stable in the presently obtained deuterium–tritium plasmas in the Tokamak Fusion Test Reactor (TFTR) [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)]. However, the core localized TAE mode can exist and is less stable than the global TAE modes. The beam ion Landau damping and the radiative damping are the two main stabilizing mechanisms in the present calculation. In future deuterium–tritium experiments, the alpha-driven TAE modes are predicted to occur with a weakly reversed shear profile.
Challenge to a tabletop high-energy laser wake-field accelerator
Authors: Nakajima Kazuhisa
Published: July 2002

Recently there has been a great interest in laser–plasma accelerators as possible next generation particle accelerators because of their potential for ultrahigh accelerating gradients and a compact size compared with conventional accelerators. Particle acceleration occurs through interaction with large-amplitude plasma waves driven by an intense short laser pulse, known as a laser wake-field acceleration mechanism. Our recent experiments have demonstrated ultrahigh-gradient electron acceleration in the accelerating gradient of the order of 30 GeV/m excited by a multiterawatt laser pulse to obtain 18 MeV electrons. The current research is in progress to achieve a high-energy particle acceleration to the energies more than a GeV in a tabletop scale by the use of a 100 fs, 2 TW tabletop-terawatt laser system.
Observation of wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak
Authors: Ivers T. H. , Eisner E. , Garofalo A. , Kombargi R. , Mauel M. E. , Maurer D. , Nadle D. , Navratil G. A. , Sankar M. K. V. , Su M. , Taylor E. , Xiao Q. , Bartsch R. R. , Reass W. A. , Wurden G. A.
Published: July 2002

The High Beta Tokamak-Extended Pulse (HBT-EP) experiment [J. Fusion Energy 12, 303 (1993)] combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic (MHD) instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma (minor radius a) resulted in the suppression of β-limiting disruptions for discharges in which b/a<1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with β values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating m=2, n=1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/n=2/1 instabilities by a factor of 2.
Rayleigh–Taylor instability in a finely structured medium
Authors: Ryutov D. D.
Published: July 2002

The Rayleigh–Taylor instability in a finely layered material, with the layers oriented normally to the direction of the gravity force, is considered. It turns out that, in such a system, velocity perturbation in the most dangerous modes contains a significant and strongly nonuniform shear component that causes an increase of the viscous dissipation. Growth rates of the Rayleigh–Taylor instability for some specific examples of these fine structures are found. The conclusion is drawn that, although the viscous dissipation indeed increases, it remains insufficient to strongly reduce the growth rate of the large-scale perturbations. Possible situations where this conclusion would become invalid (and where the fine structure would produce a stronger stabilization of the global mode) are discussed.
A δf Monte Carlo method to calculate plasma parameters
Authors: Sasinowski Maciek , Boozer Allen H.
Published: July 2002

A δf Monte Carlo code has been developed which efficiently calculates plasma currents by finding the deviation, f̂, of the exact distribution function, f, from the Maxwellian, F, with ψ the toroidal magnetic flux enclosed by a pressure surface and H the Hamiltonian. The particles in the simulation are followed with a traditional Monte Carlo scheme consisting of an orbit step in which new values for the positions and momenta are obtained and a collision step in which a Monte Carlo equivalent of the Lorentz operator is applied to change the pitch of each particle. The dual-species code includes momentum conservation and was used to study the bootstrap and two Fourier components of the Pfirsch–Schlüter current in nonaxisymmetric magnetic field configuration. The effects of the toroidal ripple on the orbits of the particles as well as on the bootstrap current were examined.
Measurement of single mode imprint in laser ablative drive of a thin Al foil by extreme ultraviolet laser radiography
Authors: Wolfrum E. , Wark J. , Zhang J. , Kalantar D. , Key M. H. , Remington B. A. , Weber S. V. , Neely D. , Rose S. , Warwick J. , MacPhee A. , Lewis C. L. S. , Demir A. , Lin J. , Smith R. , Tallents G. J.
Published: July 2002

The temporal development of laser driven single mode perturbations in thin Al foils has been measured using extreme ultraviolet (XUV) laser radiography. 15, 30, 70 and 90 μm single modes were imprinted on 2 μm thick Al foils with an optical driver laser at 527 nm for intensities in the range 5×1012 to 1.5×1013 W cm−2. The magnitude of the imprinted perturbation at the time of shock break out was determined by fitting to the data estimated curves of growth of the Rayleigh–Taylor instability after shock break out. The efficiency of imprinting is independent of perturbation wavelength in the parameter range of this experiment, suggesting little influence of thermal conduction smoothing. The results are of interest for directly driven inertially confined fusion.
Nonevolutionary discontinuous magnetohydrodynamic flows in a dissipative medium
Authors: Markovskii S. A.
Published: July 2002

Magnetohydrodynamic discontinuities in a dissipative medium are considered from the view point of the principle of evolutionarity. For a nonevolutionary discontinuity, the problem of time evolution of a small perturbation does not have a unique solution. This means that an infinitesimal perturbation results in finite variation of the initial flow. Such a discontinuity cannot exist as a stationary configuration and it must disintegrate or transform to some other nonsteady flow. Using the kinematic properties of weakly damping high-frequency perturbations of a dissipative medium, the sufficient conditions of nonevolutionarity of the dissipative discontinuities are obtained. These conditions are shown to strengthen the known criterion of a discontinuous profile of inviscid shocks. It is suggested that time evolution of the nonevolutionary shock that contains the nonevolutionary internal discontinuity is a superposition of two oscillation processes with different characteristic times corresponding to internal disintegration and disintegration of the shock as a whole.
New mechanisms of minority ion cyclotron current drive
Authors: Carlsson J. , Hellsten T. , Hedin J.
Published: July 2002

Minority ion cyclotron current drive is studied by solving the Fokker–Planck equation in toroidal geometry keeping the ∂/∂v∥-term in the quasilinear operator, hereby including the important effects of the finite orbit widths of the tail ions and the wave-induced spatial drift and diffusion. It has previously been found that the trapped ion current and the current carried by passing ions detrapped by wave-induced v⊥-diffusion are the two dominating contributions for high levels of coupled power. In this study yet another current-drive mechanism is presented, asymmetric detrapping by inward wave-induced radial drift, which occurs for negative k∥ and is strongest for on-axis resonance where it is the totally dominating effect for high powers.
First principles justification of a “single wave model” for electrostatic instabilities
Authors: Crawford John David , Jayaraman Anandhan
Published: July 2002

The nonlinear evolution of a unstable electrostatic wave is considered for a multispecies Vlasov plasma. From the singularity structure of the associated amplitude expansions, the asymptotic features of the electric field and distribution functions are studied in the limit of weak instability, i.e., γ→0+ where γ is the linear growth rate. The asymptotic electric field is monochromatic at the wavelength of the linear mode with a nonlinear time dependence. The structure of the distributions outside the resonant region is given by the linear eigenfunction but in the resonant region the distribution is nonlinear. The details depend on whether the ions are fixed or mobile; in either case this generally derived physical picture corresponds to the single wave model originally proposed by O’Neil, Winfrey, and Malmberg [Phys. Fluids 14, 1204 (1971)] for the special case of a cold weak beam instability in a plasma of fixed ions.
On the structural stability of magnetic configurations with two null lines
Authors: Bulanov S. V. , Echkina E. Yn. , Inovenkov I. N. , Pegoraro F. , Pichushkin V. V.
Published: July 2002

The nonlinear dynamics of magnetoacoustic and Alfvén-type magnetohydrodynamic (MHD) perturbations in structurally unstable magnetic configurations is studied analytically and numerically. The nonlinear evolution of the perturbed electric current turns structurally unstable configurations into structurally stable ones. This transformation is forbidden in the framework of the ideal MHD equations, but can occur in the process of magnetic field line reconnection. MHD simulations of the transformation of configurations with two null lines (X-lines) under perturbations imposed from the boundaries show that the change in the magnetic field topology due to the magnetoacoustic perturbations is accompanied by the redistribution of the electric current curried by the Alfvén perturbations.
Effect of pump depletion and self-focusing on stimulated Brillouin scattering process in laser-plasma interactions
Authors: Mahmoud S. T. , Sharma R. P. , Kumar A. , Yadav S.
Published: July 2002

This paper presents the study of the stimulated Brillouin scattering (SBS) process when the self-focusing effects of the pump and scattered laser beams are taken into account in three dimensions within the paraxial ray approximations. Integrated SBS reflectivity is found to exhibit a maximum value at an optimum SBS gain. The reflectivity with pump depletion is less than without pump depletion and several orders of magnitude lower than the usual three-wave interaction case. The effect of the statistical distribution of the laser beam on the SBS reflectivity has also been discussed.
Modeling of magnetic island formation in magnetic reconnection experiment
Authors: Watanabe T.-H. , Hayashi T. , Sato T. , Yamada M. , Ji H.
Published: July 2002

Formation of a magnetic island found in the Magnetic Reconnection Experiment (MRX) [M. Yamada, H. Ji, S. Hsu, et al., Phys. Plasmas 4, 1936 (1997)] is investigated by a magnetohydrodynamic (MHD) relaxation theory and a numerical simulation. In the cohelicity injection with a mean toroidal field, the growing process of the island into a spheromak-type configuration is explained by quasistatic transition of the force-free and minimum energy state to a state with larger normalized helicity. It also turns out that no magnetic island would be generated in the counterhelicity case. The MHD simulation with inhomogeneous electric resistivity agrees with experimental results, which clearly shows formation and growth of the magnetic island in a diffusion region where the reconnection takes place.
Acceleration of dust grains by the ponderomotive force of dust ion-acoustic waves
Authors: Shukla P. K. , Rosenberg M.
Published: July 2002

It is shown that the ponderomotive force of large amplitude dust ion-acoustic waves can generate space charge electric fields which could accelerate charged dust grains in unmagnetized dusty plasmas. Estimates are given in relation to particulate energization in low-temperature laboratory and astrophysical plasmas.
On the existence of Alfvénic solitary waves
Authors: Seyler C. E. , Lysak R. L.
Published: July 2002

It is argued that dispersive Alfvén solitary waves of the nonoscillatory type do not exist in one dimension. Previous calculations that claim existence have neglected the nonlinear ion polarization drift in the low-frequency limit. If the nonlinear ion polarization term is retained or if no low-frequency assumption is made, solitary wave solutions are not found.
Kinetic theory of interaction of high frequency waves with a rotating plasma
Authors: Chiu S. C. , Chan V. S. , Chu M. S. , Lin-Liu Y. R.
Published: July 2002

The equations of motion of charged particles of a strongly magnetized flowing plasma under the influence of high frequency waves are derived in the guiding center approximation. A quasilinear theory of the interactions of waves with rotating plasmas is formulated. This is applied to investigate the effect of radio frequency waves on a rotating tokamak plasma with a heated minority species. The angular momentum drive is mainly due to the rf-induced radial minority current. The return current by the bulk plasma gives an equal and opposite rotation drive on the bulk. Using moment equations and a small banana width approximation, the J×B drive was evaluated for the bulk plasma. Quite remarkably, although collisions are included, the net rotation drive is due to a term which can be obtained by neglecting collisions.
Collector-emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors
Authors: Chand Naresh , Fischer Russ , Morkoç Hadis
Published: July 2002

Factors affecting the collector-emitter offset voltage in AlGaAs/GaAs heterojunction bipolar transistors have been examined both experimentally and theoretically. In the offset region the collector current, instead of being zero as reported in the literature, is negative and is equal to or less than the base current. The majority of our double heterojunction transistors have negligible offset voltage (50 mV). The prime reason of unusually large offset voltages as observed in some of our devices is found to be the poor quality of the base-collector (b-c) junction which is affected by the growth and fabrication processes. The effect of the conduction-band spike on the offset voltage depends on the compositional grading and in properly graded devices is negligible. For zero offset voltage, high quality of the b-c junction is important such that its turn-on voltage is either equal to or higher than that of the emitter-base junction.
Scanning thermal profiler
Authors: Williams C. C. , Wickramasinghe H. K.
Published: July 2002

A new high-resolution profilometer has been demonstrated based upon a noncontacting near-field thermal probe. The thermal probe consists of a thermocouple sensor with dimensions approaching 100 nm. Profiling is achieved by scanning the heated sensor above but close to the surface of a solid. The conduction of heat between tip and sample via the air provides a means for maintaining the sample spacing constant during the lateral scan. The large difference in thermal properties between air and solids makes the profiling technique essentially independent of the material properties of the solid. Noncontact profiling of resist and metal films has shown a lateral resolution of 100 nm and a depth solution of 3 nm. The basic theory of the new probe is described and the results presented.
Paramagnetic centers at Si-SiO2 interfaces in silicon-on-insulator films
Authors: Carlos W. E.
Published: July 2002

Electron spin resonance measurements of silicon-on-insulator materials formed oxygen implantation are reported. The principal paramagnetic defect observed is a Pb center at the interface between Si and SiO2 precipitates in the Si film over the buried oxide layer. The 29Si central hyperfine interaction and the 29Si superhyperfine interaction with neighboring Si atoms are resolved. The results are very close to those for the Pb center at the Si-thermal oxide interface and recent theoretical calculations.
Surface reaction and recombination of the SiH3 radical on hydrogenated amorphous silicon
Authors: Perrin Jérôme , Broekhuizen Ton
Published: July 2002

Mercury photosensitized decomposition of SiH4 is used to study surface reactions of SiH3 on hydrogenated amorphous silicon (a-Si:H). The method involves modeling of gas phase production, reaction and diffusion to the walls of reactive species, in a parallel plate reactor, combined with measurements of surface reflection coefficient of SiH3, spatial density profile of SiH3, and a-Si:H deposition rate. The reaction probability of SiH3 on a-Si:H varies from 0.1 up to 0.2 in the 40–350 °C temperature domain. However, a large fraction (≥60%) of adsorbed SiH3 recombine on the surface, instead of being incorporated in the film.
Dynamics of a rf sheath in the range between the ionic and the electronic plasma frequencies
Authors: Pointu A. M.
Published: July 2002

The dynamics of an ionic sheath submitted to a periodic field with a frequency between the ionic and the electronic plasma frequencies is analyzed under one-dimensional, noncollisional, and thick sheath assumptions. Accordingly the displacement current is calculated.
Analysis of scanning deep level transient spectroscopy
Authors: Wada K. , Ikuta K. , Osaka J. , Inoue N.
Published: July 2002

Fundamental processes such as charge generation, trapping and emission, and collection in scanning deep level transient spectroscopy (SDLTS) are analyzed. It is concluded that SDLTS is more sensitive to minority-carrier traps than to majority-carrier traps and that a two-dimensional map of SDLTS peak signal magnitude, i.e., SDLTS image, of minority-carrier traps does not necessarily show the trap concentration distribution. A practical equation for trap concentration is derived by utilizing an electron beam induced current. Concentration of a hole trap with an activation energy 0.45 eV in an n-type as-grown liquid encapsulated Czochralski GaAs is calculated to be 1×1016 cm−3.
Theoretical evaluation of high-efficiency operation of discharge-pumped vacuum-ultraviolet F2 lasers
Authors: Ohwa Mieko , Obara Minoru
Published: July 2002

We investigated theoretically the efficiency enhancement of discharge-pumped vacuum-ultraviolet F2 lasers using He/F2 mixtures in terms of the laser kinetics. As a result, the high mixture pressure is found to be essential for the high-efficiency operation. Intrinsic efficiency in excess of 1% may be obtainable. Using a 6-atm mixture of He/F2=98.5/0.15%, intrinsic efficiency of 1.5% may be obtainable with a specific output energy of 2.6 J/l.
Preparation and substrate reactions of superconducting Y-Ba-Cu-O films
Authors: Gurvitch M. , Fiory A. T.
Published: July 2002

Multiple metal-target dc magnetron sputter deposition of a metallic YBa2Cu3 alloy in pure Ar followed by ex situ oxygen annealing was used to prepare superconducting films on various substrates. This work particularly examines film-substrate reactions which are degrading to superconductivity. Better superconductors were obtained using predeposited buffer layers, notably on cubic zirconia and MgO substrates covered with Ag and Nb. Best films have Tc=80 K, metallic resistivities with resistance ratio≳2, and critical current density ≲10 kA cm−2 at 4.2 K. Normal-state resistivity measurements were made for O2 furnace temperatures up to 850 °C. Procedures for lithographically patterning these films are also described.
Diffusion coefficients and activation energies for Zn diffusion into undoped and S-doped InP
Authors: Marek H. S. , Serreze H. B.
Published: July 2002

We present results of open tube Zn diffusion into undoped and S-doped n-type InP over the temperature range 550–675 °C. The process yields reproducible results which are consistent with an interstitial-substitutional diffusion model. For the undoped samples, an activation energy of 1.52 eV and a diffusion constant of 4.9×10−2 cm2/s are obtained. For heavily S-doped samples, values of 2.34 eV and 1.4×103 cm2/s, respectively, result. The difference in activation energy which is comparable to the Fermi level difference in the two substrate types is consistent with the different diffusion mechanisms which occur in these two types of InP.
Spatial localization of impurities in δ-doped GaAs
Authors: Schubert E. F. , Stark J. B. , Ullrich B. , Cunningham J. E.
Published: July 2002

Capacitance-voltage profiles on δ-doped GaAs grown by molecular beam epitaxy reveal extremely narrow widths of ≲40 Å at room temperature. Subband structure and capacitance-voltage (C-V) profiles of δ-doped GaAs are calculated self-consistently. Experimental C-V profiles agree with self-consistent results, only if we assume that Si impurities are localized on the length scale of the lattice constant in the host GaAs zinc-blende lattice.
Photorefractive imaging of semiconductor wafers
Authors: Bylsma R. B. , Olson D. H. , Glass A. M.
Published: July 2002

Making use of the photorefractive effect, we have developed a versatile method of imaging various crystal properties of semi-insulating compound semiconductors. The magnitude and time evolution of refractive index gratings are monitored via diffraction. The observed diffraction is directly related to the electric fields present, and quantitative information concerning the spatial variation of dark conductivity, photoconductivity, and deep level absorption can be extracted. Wafers of undoped GaAs and InP:Fe have been characterized in this manner, and comparisons of images are made which demonstrate the capabilities of this technique.
As-deposited superconducting Y-Ba-Cu-O thin films on Si, Al2O3, and SrTiO3 substrates
Authors: Silver R. M. , Berezin A. B. , Wendman M. , de Lozanne A. L.
Published: July 2002

We have developed an electron beam evaporator designed to deal with the special requirements of thin films of oxide superconductors. The growing surface is sprayed with plasma-excited oxygen while the sources and rate monitors operate in a low background pressure. This allows us to reproducibly grow films of Y-Ba-Cu-O on Si, Al2O3, and SrTiO3 substrates at a substrate temperature of 540 °C which are superconducting without the need for annealing. The resistive transitions of most films show an onset of 90 K and zero resistance by 68 K. X-ray diffraction indicates a preferred orientation for growth on most Si and SrTiO3 substrates. A preliminary measurement yields critical currents of at least 104 A/cm2 at 4.2 K for a film on silicon.
Acetylation of adenovirus E1A regulates binding of the transcriptional corepressor CtBP
Authors: Zhang Qinghong , Yao Hong , Vo Ngan , Goodman Richard H.
Published: July 2002

Adenovirus E1A mediates its effects on cellular transformation and
transcription by interacting with critical cellular proteins involved
in cell growth and differentiation. The amino terminus of E1A binds to
CBP/p300 and associated histone acetyltransferases such as P/CAF.
The carboxyl terminus binds to the carboxyl-terminal binding protein
(CtBP), which associates with histone deacetylases. We show that 12S
E1A can be acetylated by p300 and P/CAF and map one of the
acetylation sites to Lys-239. This Lys residue is adjacent to the
consensus CtBP binding motif, PXDLS. Mutation of Lys-239 to Gln or Ala
blocks CtBP binding
in vitro
and disrupts the E1A–CtBP
interaction
in vivo
. Peptide competition assays
demonstrated that the interaction of E1A with CtBP is also blocked by
Lys-239 acetylation. Supporting a functional role for Lys-239 in CtBP
binding, mutation of this residue to Ala decreases the ability of E1A
to block cAMP-regulated enhancer (CRE)-binding protein
(CREB)-stimulated gene expression. Finally, we demonstrate that Lys-239
is acetylated in cells by using an antibody directed against an
acetyl-Lys-239 E1A peptide. CtBP interacts with a wide variety of other
transcriptional repressors through the PXDLS motif, and, in many
instances, this motif is followed by a Lys residue. We suggest that
acetylation of this residue by histone acetyltransferases, and the
consequent disruption of repressor complexes, might be a general
mechanism for gene activation.
A central neural circuit for experience-independent olfactory and courtship behavior in
Drosophila melanogaster
Authors: Heimbeck Gertrud , Bugnon Véronique , Gendre Nanaë , Keller Andreas , Stocker Reinhard F.
Published: July 2002

We have studied the function of the major central olfactory
pathway in fruit flies. Key elements of this pathway, the projection
neurons (PNs), connect the antennal lobes with the lateral
protocerebrum both directly and indirectly, the latter via the mushroom
bodies (MBs). Transgenic expression of tetanus toxin in the majority of
PNs and few MB neurons leads to defects in odor detection and male
courtship. Considering behavioral data from flies lacking MBs, our
results argue that the direct PN-to-lateral protocerebrum pathway is
necessary and sufficient to process these experience-independent
behaviors. Moreover, the involvement of an olfactory pathway in male
courtship suggests a role of volatile attractive female pheromones in
Drosophila
.
Identification of a potent antidiuretic factor acting on beetle Malpighian tubules
Authors: Eigenheer Richard A. , Nicolson Susan W. , Schegg Kathleen M. , Hull J. Joe , Schooley David A.
Published: July 2002

Beetles, like other insects, depend on diuretic and antidiuretic
hormones to control water balance. We have isolated, using head
extracts from the beetle
Tenebrio molitor
, a peptide
that strongly inhibits fluid secretion by the Malpighian tubules of
this insect. This antidiuretic factor (ADF) appears to elicit its
effect via cGMP as a second messenger but does not stimulate NO
production. It has primary structure:
Val-Val-Asn-Thr-Pro-Gly-His-Ala-Val-Ser-Tyr-His-Val-Tyr-OH. The ADF
inhibits tubule secretion with high potency: the EC
50
is
around 10 fM. It bears no significant resemblance to other biologically
active neuropeptides. To our knowledge this is the only
endogenous insect ADF acting on Malpighian tubules to be
sequenced, and the first coleopteran (beetle) antidiuretic factor fully
characterized to date.
A fragment of human TrpRS as a potent antagonist of ocular angiogenesis
Authors: Otani Atsushi , Slike Bonnie M. , Dorrell Michael I. , Hood John , Kinder Karen , Ewalt Karla L. , Cheresh David , Schimmel Paul , Friedlander Martin
Published: July 2002

Pathological angiogenesis contributes directly to profound loss of
vision associated with many diseases of the eye. Recent work
suggests that human tyrosyl- and tryptophanyl-tRNA synthetases (TrpRS)
link protein synthesis to signal transduction pathways including
angiogenesis. In this study, we show that a recombinant form of a
COOH-terminal fragment of TrpRS is a potent antagonist of
vascular endothelial growth factor-induced angiogenesis in a mouse
model and of naturally occurring retinal angiogenesis in the neonatal
mouse. The angiostatic activity is dose-dependent in both systems. The
recombinant fragment is similar in size to one generated naturally by
alternative splicing and can be produced by proteolysis of the
full-length protein. In contrast, the full-length protein is inactive
as an antagonist of angiogenesis. These results suggest that fragments
of TrpRS, as naturally occurring and potentially nonimmunogenic
anti-angiogenics, can be used for the treatment of neovascular eye
diseases.
Activation and inhibition of G protein-coupled receptors by cell-penetrating membrane-tethered peptides
Authors: Covic Lidija , Gresser Amy L. , Talavera Joyce , Swift Steven , Kuliopulos Athan
Published: July 2002

Classical ligands bind to the extracellular surface of their cognate receptors and activate signaling pathways without crossing the plasma membrane barrier. We selectively targeted the intracellular receptor–G protein interface by using cell-penetrating membrane-tethered peptides. Attachment of a palmitate group to peptides derived from the third intracellular loop of protease-activated receptors-1 and -2 and melanocortin-4 receptors yields agonists and/or antagonists of receptor–G protein signaling. These lipidated peptides—which we have termed pepducins—require the presence of their cognate receptor for activity and are highly selective for receptor type. Mutational analysis of both intact receptor and pepducins demonstrates that the cell-penetrating agonists do not activate G proteins by the same mechanism as the intact receptor third intracellular loop but instead require the C-tail of the receptor. Construction of such peptide–lipid conjugates constitutes a new molecular strategy for the development of therapeutics targeted to the receptor–effector interface.
Discrimination against purine–pyrimidine mispairs in the polymerase active site of DNA polymerase I: A structural explanation
Authors: Minnick Dana T. , Liu Lixing , Grindley Nigel D. F. , Kunkel Thomas A. , Joyce Catherine M.
Published: July 2002

We previously identified five derivatives of Klenow fragment DNA polymerase that have lower fidelity because of amino acid substitutions in the polymerase active site. One of these has alanine substituted for the invariant Glu-710, whose side chain interacts with the deoxyribose of the incoming dNTP. Here we show that the E710A enzyme has reduced fidelity for five of the 12 possible mismatches. All but one of these involve misinsertion of pyrimidines, including two transition mismatches A-dCTP and G-dTTP. In contrast, E710A polymerase error rates for the reciprocal C-dATP and T-dGTP transition mismatches were similar to those of the wild-type enzyme. The kinetics of formation of correct base pairs and transition mismatches by the wild-type and E710A polymerases, combined with information on the structure of the DNA polymerase active site and the asymmetry of wobble base pairs, provides a plausible explanation for the differential effects of the E710A mutation on fidelity. The data suggest that the Glu-710 side chain plays a pivotal role in excluding wobble base pairs between template pyrimidines and purine triphosphates by steric clash. Moreover, this same side chain enhances the stability of incoming correct dNTPs, such that loss of this interaction on removal of the side chain leads to lower selectivity against mismatches involving incoming pyrimidines.
Nanohedra: Using symmetry to design self assembling protein cages, layers, crystals, and filaments
Authors: Padilla Jennifer E. , Colovos Christos , Yeates Todd O.
Published: July 2002

A general strategy is described for designing proteins that self
assemble into large symmetrical nanomaterials, including molecular
cages, filaments, layers, and porous materials. In this strategy, one
molecule of protein A, which naturally forms a self-assembling
oligomer, A
n
, is fused rigidly to one molecule of protein
B, which forms another self-assembling oligomer, B
m
. The
result is a fusion protein, A-B, which self assembles with other
identical copies of itself into a designed nanohedral particle or
material, (A-B)
p
. The strategy is demonstrated through the
design, production, and characterization of two fusion proteins: a
49-kDa protein designed to assemble into a cage approximately 15 nm
across, and a 44-kDa protein designed to assemble into long filaments
approximately 4 nm wide. The strategy opens a way to create a wide
variety of potentially useful protein-based materials, some of which
share similar features with natural biological assemblies.
Crystal structure of an antiparallel DNA fragment with Hoogsteen base pairing
Authors: Abrescia Nicola G. A. , Thompson Andrew , Huynh-Dinh Tam , Subirana Juan A.
Published: July 2002

We report here an alternative double-helical structure of the DNA molecule. It has been found in the d(ATA
Br
UAT) and d(ATATAT) sequences by single-crystal x-ray crystallography. This sequence is found not only in TATA boxes, but also in other regulatory regions of DNA. Bases of the two antiparallel strands form Hoogsteen pairs, with adenines in the
syn
conformation. The structure is related neither to those found in triple helices nor to parallel DNA duplexes. Its conformational parameters are very similar to those of duplex DNA in the B form. Both forms may coexist under physiological conditions, although the Hoogsteen pairing greatly influences the recognition sites on DNA. Our results demonstrate that an alternative to the classical B-DNA double helix is possible.
Protein–protein interactions monitored in mammalian cells via complementation of β-lactamase enzyme fragments
Authors: Wehrman Tom , Kleaveland Benjamin , Her Jeng-Horng , Balint Robert F. , Blau Helen M.
Published: July 2002

We have defined inactive α and ω fragments of β-lactamase that can complement to form a functional enzyme in both bacteria and mammalian cells, serving as a readout for the interaction of proteins fused to the fragments. Critical to this advance was the identification of a tripeptide, Asn-Gly-Arg, which when juxtaposed at the carboxyl terminus of the α fragment increased complemented enzyme activity by up to 4 orders of magnitude. β-Lactamase is well suited to monitoring constitutive and inducible protein interactions because it is small (29 kDa), monomeric, and assayable with a fluorescent cell-permeable substrate. The negligible background, the magnitude of induced signal caused by enzymatic amplification, and detection of signal within minutes are unparalleled in mammalian protein interaction detection systems published to date.