bluuebunny/crossref_metadata_2025 huggingface dataset:
Development and operation of the twin radio frequency single electron transistor for cross-correlated charge detection
Authors: Buehler T. M. , Reilly D. J. , Starrett R. P. , Court N. A. , Hamilton A. R. , Dzurak A. S. , Clark R. G.
Published: October 2004

Ultrasensitive detectors and readout devices based on the radio frequency single electron transistor (rf-SET) combine near quantum-limited sensitivity with fast operation. Here we describe a twin rf-SET detector that uses two superconducting rf-SETs to perform fast, real-time cross-correlated measurements in order to distinguish subelectron signals from charge noise on microsecond time scales. The twin rf-SET makes use of two tuned resonance circuits to simultaneously and independently address both rf-SETs using wavelength division multiplexing and a single cryogenic amplifier. We focus on the operation of the twin rf-SET as a charge detector and evaluate the cross talk between the two resonance circuits. Real-time suppression of charge noise is demonstrated by cross correlating the signals from the two rf-SETs. For the case of simultaneous operation, the rf-SETs had charge sensitivities of δqSET1=7.5μe∕Hz and δqSET2=4.4μe∕Hz.
Optical constants of porous silicon films and multilayers determined by genetic algorithms
Authors: Torres-Costa V. , Martín-Palma R. J. , Martínez-Duart J. M.
Published: October 2004

Porous silicon (PS) can be optically described as a homogeneous mixture of air, silicon, and, eventually, silicon dioxide, which explains the great tunability of its refractive index. This behavior makes porous silicon a very interesting material for the development of interference filters for use in optoelectronic applications. For the accurate design of such filters, the precise determination of the optical constants of PS is mandatory. However, since PS is a dispersive and absorbing material, determination of its complex refractive index is not trivial. In the present work, a genetic algorithm has been used to precisely determine the complex refractive index and thickness of thin films from their reflectance spectra in the visible wavelength range. This algorithm was applied to porous silicon. The precise spectral values of the refractive index obtained for PS layers of different characteristics have been used to design and fabricate interference filters with a predetermined optical behavior. In addition, the performance of the algorithm in two different working modes is discussed.
Transparent current spreading layers for optoelectronic devices
Authors: Porch A. , Morgan D. V. , Perks R. M. , Jones M. O. , Edwards P. P.
Published: October 2004

In this paper we develop a simple model based on a leaky transmission line to assess the electrical performance of transparent current spreading layers for application to surface light emitting diodes. Figures of merit suitable for device design are obtained and these are applied to a range of material systems reported in the literature. These calculations show that materials with electrical characteristics approaching ultimate performance limits can be achieved. Furthermore, calculations of the electromagnetic absorption of conducting layers show that further improvements should be possible by selecting materials with the highest possible majority carrier mobilities, which enable films of greater thickness to be grown while maintaining high transparency, leading to very low spreading layer resistances. Ways of achieving improved mobility are also discussed.
Structural and electrical properties of Ni∕Ti Schottky contacts on silicon carbide upon thermal annealing
Authors: Roccaforte F. , La Via F. , Baeri A. , Raineri V. , Calcagno L. , Mangano F.
Published: October 2004

The evolution of the structural and electrical properties of Ni∕Ti∕SiC Schottky contacts upon thermal treatments was investigated. The samples were prepared by sequentially evaporating titanium and nickel layers onto silicon carbide (6H-SiC) substrates and were annealed in vacuum in the temperature range 400–650°C. Above 450°C a solid state reaction sets in, giving rise to the formation of nickel silicides (i.e., Ni31Si12 and Ni2Si). During reaction, by increasing annealing temperatures, the electrical characteristics of the contacts showed an increase of the Schottky barrier, along with a decrease of the device leakage current. An inversion of this trend was observed at around 600°C, which can be attributed to the inhomogeneity of the nickel silicide/SiC barrier. The scenario of the reaction of the Ni∕Ti∕SiC system is presented. The physical information obtained from the study of this bilayer can be extremely important in the control of the electrical properties of Schottky barriers for advanced devices on SiC.
Synthesis of cobalt nanoparticles by ion implantation and effects of postimplantation annealing
Authors: Jacobsohn L. G. , Hawley M. E. , Cooke D. W. , Hundley M. F. , Thompson J. D. , Schulze R. K. , Nastasi M.
Published: October 2004

A study of the synthesis of Co nanoparticles by ion implantation and the effects of postimplantation annealing was carried out. Silica was implanted with 35keV Co+ ion beams to doses ranging from 8×1015to1×1017atoms∕cm2. Nanoparticle size, distribution, and structure were ascertained via transmission electron microscopy measurements, which reveal the presence of spherical nanoparticles in both as-implanted and annealed samples. X-ray photoelectron spectroscopy show the chemical state of the nanoparticles in both as-implanted and annealed samples to be metallic. Temperature-dependent field-cooled and zero-field-cooled, susceptibility measurements indicated superparamagnetic behavior, which is analyzed by accounting for the size dispersion of the nanoparticles. Results showed that the magnetic properties of the specimen can be tailored by implantation conditions. Annealing in vacuum at 900°C for up to 10h leads to an increase of the average nanoparticle size and a broader size distribution. Mie scattering and magnetic force microscopy measurements provide evidence of a strong interaction among the nanoparticles in the annealed samples. The feasibility of obtaining small-dispersion size-controlled synthesis of Co nanoparticles at room temperature is illustrated.
Investigation of simplified thermal expansion models for compressible Newtonian fluids applied to nonisothermal plane Couette and Poiseuille flows
Authors: Bechtel Stephen E. , Cai Minghao , Rooney Francis J. , Wang Qi
Published: October 2004

In this paper six different theories of a Newtonian viscous fluid are investigated and compared, namely, the theory of a compressible Newtonian fluid, and five constitutive limits of this theory: the incompressible theory, the limit where density changes only due to changes in temperature, the limit where density changes only with changes in entropy, the limit where pressure is a function only of temperature, and the limit of pressure a function only of entropy. The six theories are compared through their ability to model two test problems: (i) steady flow between moving parallel isothermal planes separated by a fixed distance with no pressure gradient in the flow direction, and (ii) steady flow between stationary isothermal parallel planes with a pressure gradient. The incompressible theory admits solutions to these problems of the plane Couette/Poiseuille flow form: a single nonzero velocity component in a direction parallel to the bounding planes, and velocity and temperature varying only in the direction perpendicular to the planes. The compressible theory admits a solution of this special form to problem (i) but not problem (ii). We find that the other four constitutive limits have Couette-form solutions to (i), but only the limits of density a function of temperature and pressure a function of entropy join the incompressible fluid in admitting the Poiseuille-form solutions to (ii); the limits of density a function only of entropy and pressure a function of temperature, as with the compressible theory, do not have solutions of that form. Based on the predictions of the fully compressible theory and its limits, we assess the usefulness of the limits as simplified models of thermal expansion.
An analytical study of current-voltage characteristics and breakdown performance of GaInP∕GaAs composite collector double heterojunction bipolar transistor
Authors: Goh Y. L. , Ong D. S. , Yow H. K.
Published: October 2004

An analytical model taking into account the nonlocal dead-space effects is developed to study the dc characteristics and avalanche multiplication of GaInP∕GaAs double heterojunction bipolar transistor (DHBT) incorporating composite collector designs. The dependence of the turn-on characteristics and the multiplication onset of the HBT on the device composite layer thickness and doping densities are investigated. In this paper, optimum combinations of composite parameters are presented to obtain zero spike effect in the base-collector heterojunction conduction band and to improve output breakdown voltages. The model is then applied to the GaInP∕GaAs DHBT with AlGaAs in the composite collector, which is found to have good I-V characteristics and high operating voltage range before the onset of avalanche multiplication.
Experimental observation of nonspherically-decaying radiation from a rotating superluminal source
Authors: Ardavan A. , Hayes W. , Singleton J. , Ardavan H. , Fopma J. , Halliday D.
Published: October 2004

We describe the experimental implementation of a superluminal (i.e., faster than light in vacuo) polarization current distribution that both oscillates and undergoes centripetal acceleration. Theoretical treatments predict that the radiation emitted by each volume element of the superluminally moving distribution pattern will comprise a Čerenkov-like envelope with two sheets that meet along a cusp. Correspondingly, the emission from the experimental machine is found to be tightly beamed in both the azimuthal and polar directions. The beaming is frequency independent and has a sharply defined and unchanging geometry determined only by the speed and path of the moving distribution pattern, i.e., by the parameters governing the structure of the Čerenkov-like envelopes. In addition, over a restricted range of angles, we detect the presence of cusps in the emitted radiation. These, which are due to the focusing of wave fronts on a propagating space curve, result in the reception, during a short time period, of radiation emitted over a considerably longer period of (retarded) source time. The intensity of the radiation at these angles was observed to decline more slowly with increasing distance from the source than would the emission from a conventional antenna. The angular distribution of the emitted radiation and the properties associated with the cusps are in good quantitative agreement with theoretical models of superluminal sources once the effect of reflections from the earth’s surface are taken into account. In particular, the prediction that the beaming and the slow decay should extend into the far zone has been tested to several hundred Fresnel distances (Rayleigh ranges). The excellent agreement between the theoretical calculations and the data suggests that the apparatus achieves precise and reproducible control of the polarization current and that similar machines could be of general interest for studying and utilizing the novel effects associated with superluminal electrodynamics.
Analysis of plasma dynamics of a negative ion source based on probe measurements
Authors: Bandyopadhyay M. , Tanga A. , Falter H. D. , Franzen P. , Heinemann B. , Holtum D. , Kraus W. , Lackner K. , McNeely P. , Riedl R. , Speth E. , Wilhelm R.
Published: October 2004

Measurements and analysis of the plasma flow in an ion source made for negative ion extraction are reported in this article. The plasma flow has been measured using a Mach probe having two orthogonal probe heads. The plasma flow along the axis is driven by the electron pressure gradient, dragging along the ions via a measured ambipolar electric field against the collisional drag on the background gas. The force on the ions created by the electric field is mainly balanced by the collisional drag force. The collision between the ions and the background gas creates a pressure gradient along the flow direction. The one-dimensional plasma dynamic analysis supports the consistency of the experimental observations. The presence of a transverse magnetic filter reduces the plasma flow velocity, which could affect the negative ion production on the cesiated grid surface. A simple analysis shows that a strong plasma flow could enhance the surface production of negative ions.
Origin of defect emission identified by polarized luminescence from aligned ZnO nanorods
Authors: Hsu N. E. , Hung W. K. , Chen Y. F.
Published: October 2004

It is found that both of the band-edge and defect emission from aligned ZnO nanorods are strongly polarized, and the intensities vary with the polarization angle by the relation of a square cosine function. The intensity of the UV emission has its maximum along the c axis of a ZnO crystal. However, the intensity of the green emission is minimum in this direction. Namely, the two intensity curves are 90° out of phase. This unique characteristic of the polarization provides useful information to identify the fact that the green emission mostly occurs on the surface defects of the nanorods. This result is consistent with the currently accepted model that the green emission arises from the recombination between holes trapped at the surface defects and electrons trapped at the oxygen vacancy.
Threshold of photoelectron emission from CNx films deposited at room temperature and at 500°C
Authors: Sago Genki , Li Wanyan , Goto Keisuke , Ichikawa Yo , Ishida Yoshihisa , Kohiki Shigemi
Published: October 2004

The threshold of photoelectron emission was measured for amorphous CNx films deposited at room temperature (RT) and at 500°C. The x values of the films deposited at RT and at 500°C by magnetron sputtering of a graphite target in a mixed N2∕Ar gas were 0.6 and 0.3, respectively. Ratios of the sp2- to sp3-hybridized components of both C and N for the film deposited at 500°C were larger by ≈4 times than those for the film deposited at RT. The onsets of the electron emission by photon irradiation were 5.0 and 4.7eV for the films deposited at RT and at 500°C, respectively.
Dielectric behavior of copper tantalum oxide
Authors: Renner B. , Lunkenheimer P. , Schetter M. , Loidl A. , Reller A. , Ebbinghaus S. G.
Published: October 2004

A thorough investigation of the dielectric properties of Cu2Ta4O12, a material crystallizing in a pseudocubic, perovskite-derived structure is presented. We measured the dielectric constant and conductivity of single crystals in an exceptionally broad frequency range up to gigahertz frequencies and at temperatures from 25to500K. The detected dielectric constant is unusually high (reaching values up to 105) and almost constant in a broad frequency and temperature range. Cu2Ta4O12 possesses a crystal structure similar to CaCu3Ti4O12, the compound for which such an unusually high dielectric constant was first observed. An analysis of the results using a simple equivalent circuit and measurements with different types of contact revealed that extrinsic interfacial polarization effects, derived from surface barrier capacitors are the origin of the observed giant dielectric constants. The intrinsic properties of Cu2Ta4O12 are characterized by a (still relatively high) dielectric constant in the order of 100 and by a charge transport via the hopping conduction of the Anderson-localized charge carriers.
Approaches to confined alpha diagnostics on ITER
Authors: Fisher R. K.
Published: October 2004

Three approaches to obtain information on the confined fast alphas in the International Thermonuclear Experimental Reactor (ITER) are proposed. The first technique measures the energetic charge exchange (CX) neutrals that result from the alpha collision-induced knock-on fuel ion tails undergoing electron capture on the MeV D neutral beams planned for heating and current drive. The second technique measures the energetic knock-on neutron tail due to alphas using the lengths of the proton recoil tracks produced by neutron collisions in nuclear emulsions. The range of the 14 to 20 MeV recoil protons increases by ∼140 μm per MeV. The third approach would measure the CX helium neutrals resulting from confined alphas capturing two electrons in the ablation cloud surrounding a dense gas jet that has been proposed for disruption mitigation in ITER.
Pilot experiments for the International Thermonuclear Experimental Reactor active beam spectroscopy diagnostic
Authors: Hellermann M. von , Bock M. de , Jaspers R. , Jakubowska K. , Barnsley R. , Giroud C. , Hawkes N. C. , Zastrow K. D. , Lotte P. , Giannella R. , Malaquias A. , Rachlew E. , Tugarinov S. , Krasilnikov A. , Litnovsky A. , Philipps V. , Wienhold P. , Oelhafen P. , Temmerman G. De , Shmaenok L.
Published: October 2004

Supporting pilot experiments and activities which are currently considered or already performed for the development of the International Thermonuclear Experiment Reactor active beam spectroscopy diagnostic are addressed in this article. Four key issues are presented including optimization of spectral instrumentation, feasibility of a motional Stark effect (MSE) evaluation based on line ratios, “first-mirror” test-bed experiments at the tokamak TEXTOR, and finally the role of integrated data analysis for the conceptual layout of the change exchange recombination spectroscopy and MSE diagnostic.
Efficiency and current in a correlated ratchet
Authors: Ai Bao-Quan , Liu Guo-Tao , Xie Hui-Zhang , Wen De-Hua , Wang Xian-Ju , Chen Wei , Liu Liang-Gang
Published: October 2004

We present a detailed study of the transport and the efficiency of a ratchet system in a periodic potential in the presence of correlated noises. The current and the efficiency of the system are investigated. It is found that, when the potential is spatially symmetric, the correlation between the two noises can induce a net transport. The efficiency shows many interesting features as a function of the applied force, the noise intensity, the external load, etc. The efficiency can be maximized as a function of noise intensity (or temperature), which shows that the thermal fluctuation can facilitate the efficiency of energy transformation.
Efficient spin extraction from nonmagnetic semiconductors near forward-biased ferromagnetic-semiconductor modified junctions at low spin polarization of current
Authors: Bratkovsky A. M. , Osipov V. V.
Published: October 2004

We study tunneling of electrons from nonmagnetic semiconductors (S) into ferromagnets (FM) through a Schottky barrier modified by a very thin heavily doped interfacial layer. It is shown that in such forward-biased FM-S junctions, the electrons with a certain spin projection can be efficiently extracted from S, wheareas electrons with the opposite spin can efficiently accumulate in S near the interface. This occurs due to the spin filtering of electrons in a tunneling process. We find conditions for most efficient extraction and accumulation of spin, and show that spin polarization of electrons near the interface can be made close to 100% in nondegenerate S at room temperature and certain bias voltages. The polarization of current in this regime is very low and decreases inversely proportionally to a total current J. At the same time, the spin density increases and spin penetration depth decreases with the current J.
Carrier and defect dynamics in photoexcited semi-insulating epitaxial GaN layers
Authors: Gaubas E. , Juršėnas S. , Miasojedovas S. , Vaitkus J. , Žukauskas A.
Published: October 2004

Transients of fast free-carrier recombination and of multitrapping processes, determined by different types of defects, have been traced by photoluminescence (PL) and contact photoconductivity (CPC) in semi-insulating GaN epitaxial layers. To eliminate effects caused by the electrodes, the CPC decays were supplemented with noninvasive microwave absorption transients. The lifetimes of fast recombination and initial free-carrier capture processes were evaluated using ultraviolet (UV) time-resolved photoluminescence transients. The UV PL band peaked at 3.42 eV with contributions from both stimulated and spontaneous emission was attributed to band-to-band recombination. At the highest excitations, the initial PL decay time exhibited a value of 880 ps due to nonradiative free-carrier recombination. The radiative centers were revealed in continuous-wave PL spectra, where the UV band was accompanied with the bands of blue (B) PL, peaked in the range of 2.82–3.10 eV, and yellow (Y) PL, peaked at 2.19 eV, ascribed to dislocations and bulk donor-acceptor recombination, respectively. The time scale of the relaxation rate exhibited a crossover from picoseconds for stimulated emission to hundreds of nanoseconds for multitrapping. In the asymptotic part, a stretched-exponent decay on the millisecond scale was observed with the disorder factor of α=0.7. The asymptotic decay is explained by competition of centers of nonradiative recombination within bulk of the material and trapping attributed to the dislocations. Behavior of the dislocation-attributed capture centers was simulated using a model of capture cross section, which depends on the excess carrier concentration via screening.
Two-dimensional scanning high-energy particle diagnostic system in Large Helical Device
Authors: Ozaki T. , Goncharov P. , Sudo S. , Shoji M. , Kawahata K. , Kaneko O. ,   the LHD Experimental Group, Murakami S.
Published: October 2004

A high-energy neutral particle measurement is one of the important diagnostics for ion temperature and high-energy particle confinement analysis. The neutral particle analyzer in the large helical device is capable of wide range scanning as a feature. We have obtained various data using the horizontal scan of the analyzer. Recently, in addition to the horizontal scan, a high-speed perpendicular scan became possible which enables acquisition of new information in the poloidal direction. Two stainless blocks are set on the opposite sides of the chain in order to balance the weight (700 kg) of the analyzer and reduce the load for the motor. Therefore a very high scan speed of 1°/s can be obtained. The scanning speed is 1°/s. By adding the vertical scan, the ion temperature profile and the radial variation of the signal loss associated with the resonant loss was obtained in preliminary experimental results.
X-ray spectroscopy for high energy-density X pinch density and temperature measurements (invited)
Authors: Pikuz S. A. , Shelkovenko T. A. , Chandler K. M. , Mitchell M. D. , Hammer D. A. , Skobelev I. Y. , Shlyaptseva A. S. , Hansen S. B.
Published: October 2004

X pinch plasmas produced from fine metal wires can reach near solid densities and temperatures of 1 keV or even more. Plasma conditions change on time scales as short as 5–10 ps as determined using an x-ray streak camera viewing a focusing crystal spectrograph or directly viewing the plasma through multiple filters on a single test. As a result, it is possible to determine plasma conditions from spectra with ∼10 ps time resolution. Experiments and theory are now coming together to give a consistent picture of the dynamics and kinetics of these high energy density plasmas with very high temporal and spatial precision. A set of diagnostic techniques used in experiments for spectrally, temporally, and spatially resolved measurements of X pinch plasmas is described. Results of plasma parameter determination from these measurements are presented. X ray backlighting of one x- pinch by another with ∼30 ps x-ray pulses enables the dynamics and kinetics to be correlated in time.
Time- and space-resolved elliptical crystal spectrometers for high energy density physics research
Authors: Lake P. W. , Bailey J. E. , Rochau G. A. , Moore T. C. , Petmecky D. , Gard P.
Published: October 2004

X-ray spectrometers used in high energy density plasma experiments must provide high time, space, and spectral resolution while overcoming the difficulties imposed by x-ray background, debris, and mechanical shocks. At the Z facility these problems are addressed using a suite of elliptical crystal spectrometers. The elliptical geometry isolates the detector from the line of sight with a slit placed at the elliptical focus, while the sensitivity enables locating the crystal 2–4 m from the plasma source. Space and time resolution are obtained by using an array of slits to project one dimensional plasma images onto the crystal and recording the spectrally dispersed images with a gated microchannel plate detector.
Error estimation and parameter dependence of the calculation of the fast ion distribution function, temperature, and density using data from the KF1 high energy neutral particle analyzer on Joint European Torus
Authors: Schlatter Christian , Testa Duccio , Cecconello Marco , Murari Andrea , Santala Marko ,   JET-EFDA Contributors
Published: October 2004

Joint European Torus high energy neutral particle analyzer measures the flux of fast neutrals originating from the plasma core. From this data, the fast ion distribution function fifast, temperature Ti,⊥fast, and density nifast are derived using knowledge of various plasma parameters and of the cross section for the required atomic processes. In this article, a systematic sensitivity study of the effect of uncertainties in these quantities on the evaluation of the neutral particle analyzer fifast, Ti,⊥fast, and nifast is reported. The dominant parameter affecting nifast is the impurity confinement time and therefore a reasonable estimate of this quantity is necessary to reduce the uncertainties in nifast below 50%. On the other hand, Ti,⊥fast is much less sensitive and can certainly be provided with an accuracy of better than 10%.
Predicting the position of the morphotropic phase boundary in high temperature PbTiO3-Bi(B′B″)O3 based dielectric ceramics
Authors: Suchomel Matthew R. , Davies Peter K.
Published: October 2004

Two previously unreported PbTiO3-Bi(B′B″)O3 type solid solution systems possessing a high dielectric transition Curie temperature (TC) at their morphotropic phase boundary (MPB) were investigated. Both Bi(B′B″)O3 type compounds, Bi(Mg1∕2Ti1∕2)O3 and Bi(Mg1∕2Zr1∕2)O3, possess a low tolerance factor and display extensive solid solubility with PbTiO3. Dielectric and structural properties of the MPB systems were measured. Using a relationship presented in this paper, which relates the tolerance factor and the position of the MPB in PbTiO3 based systems, the approximate compositional position of the MPB in these systems was accurately predicted prior to synthesis. It is proposed that this relationship can be used as a guide in the search for PbTiO3 based MPB systems.
Direct measurements of the electrostatically and magnetically bounced ions in the tandem mirror
Authors: Ishii K. , Hagisawa K. , Takemura Y. , Kojima A. , Miyata Y. , Yoshikawa M. , Saito T. , Katanuma I. , Cho T.
Published: October 2004

Existence of the bounced ion by the plug potential (PP) bounce ion is essential in order to improve the ion confinement in the tandem mirror. The trajectories of the PP bounce ion were calculated on the assumption that the equipotential surface was slightly deviated from the equilibrium state, and it was found that the radial electric field affected the radial transport of the bounce ion. A charge exchange neutral particle analyzer was located at the inner mirror throat region as a bounce ion measurement device. The energy distribution function of the bounce ion was measured using the device, and compared with the energy distribution function of the end-loss ion.
Measurement of the magnetic field fluctuations by use of a gold neutral beam probe in the tandem mirror GAMMA 10
Authors: Kojima Atsushi , Ishii K. , Takemura Y. , Hagisawa K. , Miyata Y. , Masuda Y. , Yamaguchi T. , Itakura A. , Ichimura M. , Cho T.
Published: October 2004

The beam deflection coefficients to the magnetic field fluctuation using a gold neutral beam probe was examined by numerical simulations. The beam deflection parallel to the magnetic field line was mainly caused by the azimuthal component of the magnetic field fluctuation in a case of the beam passing through the plasma center; on the other hand, the beam passing through the plasma edge was deflected by the radial component of the magnetic field fluctuation because the radial and azimuthal velocity components of the beam depend on the injection angle into the plasma. Then, the coefficients could be described as being the function of the injection angle which indicated the radial position of the ionization point and the measured point. As a result of numerical simulation, the coefficients were estimated to be 0.1 and 1.3 for the radial and the azimuthal components of the magnetic field fluctuation in the case of the beam passing through the plasma center. We also estimated the potential effects on the beam deflection parallel to the magnetic field line by the numerical simulation because the change of the beam energy caused the beam detected point to move. We found the potential effects to be smaller than the sensitivity of the magnetic field fluctuation.
Development of boundary-layer flow in the presence of forced wavelength Görtler vortices
Authors: Mitsudharmadi H. , Winoto S. H. , Shah D. A.
Published: October 2004

Hot-wire measurements in the boundary layer developing on a concave surface of 2.0 m radius of curvature in the presence of forced wavelength Görtler vortices have been conducted for a free-stream velocity of 3.0 m/s. The wavelengths of vortices were preset by vertical perturbation wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices that are similar to those found in the transitional flow field. It shows the occurrence of the second instability mode that is indicated by the formation of small horseshoe eddies generated between the two neighboring vortices traveling in the streamwise direction to form mushroom-like structures as a consequence of the nonlinear growth of the Görtler vortices. The breakdown of these structures before the boundary-layer flow becomes turbulent is also shown to qualitatively predict the start of the transition in the flow. The Görtler number where the start of the transition was predicted is found to be within the range of transitional Görtler numbers previously reported for naturally developed Görtler vortices. The average of the spanwise wavelength after being normalized by ν/uτ is comparable with the generally quoted value of 100 for turbulent boundary layers.
Extraction of poloidal velocity from charge exchange recombination spectroscopy measurements
Authors: Solomon W. M. , Burrell K. H. , Gohil P. , Groebner R. J. , Baylor L. R.
Published: October 2004

An approach has been implemented on the DIII-D tokamak to allow the correct determination of the plasma poloidal velocity from charge exchange spectroscopy measurements. Unlike usual techniques, the need for detailed atomic physics calculations to properly interpret the results is alleviated. Instead, the needed atomic physics corrections are self-consistently determined directly from the measurements, by making use of specially chosen viewing chords. Modeling results are presented that were used to determine a set of views capable of measuring the correction terms. We present the analysis of a quiescent H-mode discharge, illustrating that significant modifications to the velocity profiles are required in these high ion temperature conditions. We also present preliminary measurements providing a direct comparison of the standard cross-section correction to the atomic physics calculations.
Correlated vortex chiralities in interacting permalloy dot patterns
Authors: Natali M. , Popa A. , Ebels U. , Chen Y. , Li S. , Welland M. E.
Published: October 2004

Magnetostatic interactions in different polycrystalline permalloy dot patterns for dots with diameter 900nm and thickness 30nm are investigated through magnetic force microscopy imaging, magneto-optic Kerr effect measurements, and micromagnetic simulations. Magnetization reversal occurs through vortex nucleation∕annihilation. Vortex nucleation and annihilation fields for different interacting arrays were observed to vary with pattern type and with the applied field direction and were shown to follow the prediction of a simple point-dipole model. The vortex chirality was established by imaging vortices in a small applied field. The vortex chirality distribution was determined for different arrays and a well-defined ordering phenomenon of the vortex chirality was observed in two-dot and zigzag-dot chain patterns as well as in a honeycomb array of dots. Micromagnetic simulations well account for the type of vortex chirality ordering observed and attribute it to magnetostatic interactions that induce correlated C-state magnetization configurations among nearest neighbor dots prior to vortex nucleation.
A scaling analysis of turbulent transport in laser surface alloying process
Authors: Chakraborty Nilanjan , Chatterjee Dipankar , Chakraborty Suman
Published: October 2004

A systematic procedure for scaling analysis of three-dimensional transient turbulent momentum, heat, and mass transfer in a typical laser surface alloying process is presented. With suitable choices of nondimensional parameters, the governing equations coupled with appropriate boundary conditions are first scaled, and the relative significance of various terms appearing in them are accordingly analyzed. The analysis is then utilized to predict the orders of magnitude of some important quantities, such as the velocity scale at the top surface, thermal boundary layer thickness, temperature rise in the pool, turbulent kinetic energy, and its dissipation rate at the top surface. The scaling predictions are also assessed by comparison with numerical and experimental results, and an excellent agreement is observed in order-of-magnitude sense. Using the scaling predictions, the influence of various processing parameters on the system variables can be well recognized, which enables us to develop a deeper insight into the physical problem of interest. Moreover, some of the quantities predicted from the scaling analysis can be utilized for optimized selection of appropriate grid size for full numerical simulation of the process.
On higher order passive scalar structure functions in grid turbulence
Authors: Gylfason Armann , Warhaft Zellman
Published: October 2004

The scalar structure function scaling exponent ζn is experimentally determined for n⩽10 in decaying, grid-generated wind-tunnel turbulence with a constant mean temperature gradient. The Reynolds number is varied over the range 150⩽Rλ⩽700 by using static and active grids. The results show that up to n=10 the scaling exponent does not saturate although saturation is not precluded at higher orders. There appears to be no dependence of ζn on Reynolds number and the values of ζn are the same for the transverse (along the gradient) and the longitudinal (streamwise) structure functions. A compilation of previous work shows large variation in ζn, with a few results indicating saturation and most not. Reasons for the scatter are attributed to convergence problems at high orders, effects of flow or computational domain size causing clipping of large rare fluctuations, and differences in initial and boundary conditions.
The spherical model of logarithmic potentials as examined by Monte Carlo methods
Authors: Lim Chjan C. , Nebus Joseph
Published: October 2004

We examine Euler’s equations for inviscid fluid flow by a discretized version representing the fluid as a piecewise constant finite approximation based on Voronoi cells. The strengths of these cells are constrained to conserve circulation and enstrophy. With this model we examine by a Monte Carlo Metropolis–Hastings algorithm the dependence of the system on such parameters as the number of points, the statistical mechanics temperature, and the number of sweeps used in the simulation. Tools to examine the system include the mean nearest neighbor parity, energy, distance between extreme-valued sites, and statistical study of an individual site or of all mesh site values. In negative statistical mechanics temperatures a solid-body rotation state is found. The positive-temperature state is not as strongly organized. Numerical evidence supports our expectation of a single phase transition, between positive and negative temperatures.
Modifications of the electronic structure of GaSb surface by chalcogen atoms: S, Se, and Te
Authors: Liu Z. Y. , Gokhale A. A. , Mavrikakis M. , Saulys D. A. , Kuech T. F.
Published: October 2004

Modifications to the electronic properties and chemical structures of the GaSb surface using the chalcogen atoms S, Se, and Te were investigated theoretically and experimentally. A self-consistent density-functional theory study indicates that an adsorption of a full monolayer coverage of chalcogen atoms on a Ga-terminated surface reduces the density of gap region states significantly. A greater photoluminescence enhancement was observed from GaSb samples treated by chalcogenide (Na2S, Na2Se, or Na2Te) in a nonaqueous than in an aqueous passivation medium. X-ray photoelectron spectroscopy reveals a Ga-rich surface after a nonaqueous passivation, with sulfidization providing a higher concentration of Ga(Sb)-chalcogen bonds than does a passivation with Na2Se or Na2Te. The uptake of chalcogen during the passivation is accompanied by the loss of surface antimony. The formation of Sb-X(X=S, Se, or Te) bonds competes with X displacing surface Sb, which dominates Se or Te incorporation in the GaSb surface lattice. The passivation kinetics was analyzed on basis of a single precursor-mediated coverage-dependent chemisorption proces.
The role of the silyl radical in plasma deposition of microcrystalline silicon
Authors: Smit C. , Swaaij R. A. C. M. M. van , Hamers E. A. G. , Sanden M. C. M. van de
Published: October 2004

Expanding thermal plasma chemical-vapor deposition has been used to deposit microcrystalline silicon films. We studied the behavior of the refractive index, crystalline fraction, and growth rate as a function of the silane (SiH4) flow close to the transition from amorphous to microcrystalline silicon. It was found that the refractive index, a measure for film density, increases when the average sticking probability of the depositing radicals decreases. Furthermore, we studied the influence of the position at which SiH4 is injected in the expanding plasma on the film density. It was found that the film density becomes higher when the SiH4 is injected closer to the substrate. Both findings strongly suggest that the film density benefits from a high contribution of the SiH3 radical to the growth of microcrystalline silicon.
Instrumented sphere method for measuring thermal pressure in fluids and isotropic stresses and reaction kinetics in thermosetting resins
Authors: Merzlyakov Mikhail , Meng Yan , Simon Sindee L. , McKenna Gregory B.
Published: October 2004

A novel technique is described for measuring thermal pressure in fluids and for measuring isotropic stress development and reaction kinetics in thermosetting resins during cure and thermal cycling. The method uses a 12.7-mm-diam sealed stainless steel spherical pressure vessel to impose three-dimensional isotropic constraints. The vessel is instrumented with strain gauges and thermocouples. Both isotropic stresses and reaction kinetics during cure at cure temperatures as high as 300 °C can be measured. In addition, measurement of the isotropic stress as a function of temperature yields the thermal pressure coefficient in both the glassy and rubbery (or liquid) states. Experimental results are presented for sucrose benzoate, a pressure-transmitting oil di-2-ethylhexylsebacate and an epoxy resin. The method provides reproducible estimates for the thermal pressure coefficient and the stresses are highly isotropic. A suggestion for improved versions of the device is: thicker walled vessels can be used to increase the upper stress limit (currently at 30 MPa). Also if a lower temperature range is to be studied, then aluminum can be used as a vessel material. Since epoxy resins have better adhesion to aluminum than to stainless steel, there may be an advantage to this.
Symmetry properties of periodic orbits extracted from scattering data
Authors: Merlo O. , Jung C. , Seligman T. H.
Published: October 2004

Discrete symmetries of a system are reflected in the properties of the shortest periodic orbits. By applying a recent method to extract these from the scaling of the fractal structure in scattering functions, we show how the symmetries can be extracted from scattering data simultaneously with the periods and the Lyapunov exponents. We pay particular attention to the change of scattering data under a small symmetry breaking.
Numerical modeling of two-fluid Taylor–Couette flow with deformable capillary liquid–liquid interface
Authors: Gelfgat Alexander Yu. , Yarin Alexander L. , Bar-Yoseph Pinhas Z. , Graham Michael D. , Bai Guiyu
Published: October 2004

A two-fluid Taylor–Couette flow with a deformable interface separating two liquid layers is studied numerically by a combination of the finite volume and level set methods. Effect of the interfacial tension is accounted for. It is shown that if the layers are infinitely long, there exist stable steady states with Taylor vortices of finite strength and finite deformations of the interface. On the other hand, if the length of the layers is finite and no-slip conditions are imposed at the edges, the liquid–liquid interface becomes unstable near the edges. Data from the literature and experimental data acquired in the present work are used for comparison with the numerical predictions. A qualitative agreement between the experimental and numerical observations of this instability is obtained. The results are of potential importance for development of bioseparators employing Taylor vortices for enhancement of mass transfer of a passive scalar (say, a protein) through the interface.
Characterization of gas targets for laser produced extreme ultraviolet plasmas with a Hartmann-Shack sensor
Authors: Peth Christian , Kranzusch Sebastian , Mann Klaus , Viöl Wolfgang
Published: October 2004

A table top extreme ultraviolet (EUV)-source was developed at Laser-Laboratorium Göttingen for the characterization of optical components and sensoric devices in the wavelength region from 11 to 13 nm. EUV radiation is generated by focusing the beam of a Q-switched Nd:YAG laser into a pulsed xenon gas jet. Since a directed gas jet with a high number density is needed for an optimal performance of the source, conical nozzles with different cone angles were drilled with an excimer laser to produce a supersonic gas jet. The influence of the nozzle geometry on the gas jet was characterized with a Hartmann-Shack wave front sensor. The deformation of a planar wave front after passing the gas jet was analyzed with this sensor, allowing a reconstruction of the gas density distribution. Thus, the gas jet was optimized resulting in an increase of EUV emission by a factor of two and a decrease of the plasma size at the same time.
Simple, compact, flow-through, high temperature high pressure cell for UV-Vis spectrophotometry
Authors: Suleimenov O. M.
Published: October 2004

An optical pressure cell tested at pressures up to 1000 bar is presented. The cell has been used to collect UV-Vis spectra within the temperature range 20–400°C. The cell is compact, has low volume, and atypical high-pressure window seal. To test the performance of the cell the tetrahedral-octahedral equilibria in aquenous nickel chloride solutions were studied up to 400°C and 1000 bar.
Automatic VIS- near IR laser refractometer
Authors: Sarov Y. , Sainov S. , Kostic I. , Sarova V. , Mitkov S.
Published: October 2004

This work presents a simple automatic VIS-IR laser refractometer, based on a critical-angle method. Its experimental error is smaller than 6×10−4 within the range of 1.32–1.47. Such precision of the refractometer is sufficient for the most industrial applications, for environmental protection, structural analysis, investigation of new materials, etc. Spectral refractive index measurements of different liquids are led and the calculated dispersion coefficients are reported.
Spatially resolved spectra from a new x-ray imaging crystal spectrometer for measurements of ion and electron temperature profiles (invited)
Authors: Bitter M. , Hill K. W. , Stratton B. , Roquemore A. L. , Mastrovito D. , Lee S. G. , Bak J. G. , Moon M. K. , Nam U. W. , Smith G. , Rice J. E. , Beiersdorfer P. , Fraenkel B. S.
Published: October 2004

A new type of high-resolution x-ray imaging crystal spectrometer is being developed to measure ion and electron temperature profiles in tokamak plasmas. The instrument is particularly valuable for diagnosing plasmas with purely ohmic heating and rf heating, since it does not require the injection of a neutral beam—although it can also be used for the diagnosis of neutral-beam heated plasmas. The spectrometer consists of a spherically bent quartz crystal and a two-dimensional position-sensitive detector. It records spectra of helium-like argon (or krypton) from multiple sightlines through the plasma and projects a de-magnified image of a large plasma cross section onto the detector. The spatial resolution in the plasma is solely determined by the height of the crystal, its radius of curvature, and the Bragg angle. This new x-ray imaging crystal spectrometer may also be of interest for the diagnosis of ion temperature profiles in future large tokamaks, the Korea Superconducting Tokamak Advanced Research tokamak and the International Thermonuclear Experimental Reactor, where the application of the presently used charge-exchange spectroscopy will be difficult, if the neutral beams do not penetrate to the plasma center. The article presents the results from proof-of-principle experiments performed with a prototype instrument at Alcator C-Mod.
Spectroscopic method of strain analysis in semiconductor quantum-well devices
Authors: Biermann Mark L. , Duran Steven , Peterson Kelsey , Gerhardt Axel , Tomm Jens W. , Bercha Artem , Trzeciakowski Witold
Published: October 2004

The modification of the electronic bandstructure in a semiconductor, quantum well due to an induced strain is well known. Recently, we have developed a generalized, spectroscopic-based technique for analyzing the strain condition within devices based on quantum wells. This approach couples experimental data describing interband transition energies within strained, quantum-well devices with a rigorous theoretical description of the quantum-well bandstructure. The theoretical formalism is described, and various important theoretical predictions necessary in the application of this method are given. The accuracy of the theoretical model used in this approach is critical to its success, and it is therefore necessary to ensure the validity of the theoretical formalism as applied to quantum wells under a variety of strain conditions. We show the good agreement between theory and experiment for a number of known strain conditions within quantum wells and quantum-well devices. This agreement indicates the validity of the theoretical formalism in the method being described, and the applicability of the method to a wide class of quantum-well based semiconductor devices. A key result in applying this method is the piecewise linearity of the change in interband transition energy with strain for the ranges of strain of interest. The method is then applied to the important case of packaging-induced strain in high-power diode lasers or “cm-bars” as they are sometimes known. Experimental results indicate that the method provides an excellent means of analyzing packaging-induced strain in cm-bars and similar devices.