bluuebunny/crossref_metadata_2025 huggingface dataset:
Simplified scattering coefficient expressions for a spherical particle located on the propagation axis of a fifth-order Gaussian beam
Authors: Schaub Scott A. , Barton John P. , Alexander Dennis R.
Published: July 2002

Expanding on developments presented in an earlier paper, theoretical expressions for the scattering coefficients of a homogeneous, absorbing, spherical particle irradiated by a fifth-order Gaussian beam are presented for the special case of the particle center located on the propagation axis of the beam. For this case, evaluation of two-dimensional surface integrals, required in computing the scattering coefficients for the most general particle location, is reduced to a computationally more efficient one-dimensional integral. For a typical size parameter α=πd/λ=17, the CPU time required for calculation of scattering coefficients is reduced by a factor of ∼1500 by using the simplified coefficient expressions. In addition, computation of electromagnetic field components is reduced from double summation to single summation expressions, further simplifying the field calculations.
Thickness effect of amorphous Si film on formation of 7×7 superlattice surface during its solid phase epitaxial growth
Authors: Shigeta Yukichi , Maki Kunisuke
Published: July 2002

Low-energy electron diffraction intensity from crystallized Si film after solid phase epitaxial growth of its amorphous phase was measured. The intensity profile always shows a formation of 7×7 reconstructed structure, and the intensity I depends on the thickness of the amorphous Si film, d: the value of I from the crystallized film with d<30 Å is the same as that from the 7×7 reconstructed Si (111) substrate; I decreases exponentially with the increase of d (30 Å<d<200 Å). The change of I is discussed from the viewpoint of the crystallization of the amorphous film composed of strongly distorted microcrystalline-like grains which have a uniform orientation due to the proximity effect of the Si (111) substrate.
Correlations between the interfacial chemistry and current-voltage behavior of n-GaAs/liquid junctions
Authors: Tufts Bruce J. , Casagrande Louis G. , Lewis Nathan S. , Grunthaner Frank J.
Published: July 2002

Correlations between the surface chemistry of etched, (100) oriented n-GaAs electrodes and their subsequent photoelectrochemical behavior have been probed by high-resolution x-ray photoelectron spectroscopy. GaAs photoanodes were chemically treated to prepare either an oxide-free near stoichiometric surface, a surface enriched in zero-valent arsenic (As0), or a substrate-oxide terminated surface. The current-voltage (I-V) behavior of each surface type was subsequently monitored in contact with several electrolytes.
Visible light emission from semiconducting polymer diodes
Authors: Braun D. , Heeger A. J.
Published: July 2002

We report visible light emission from Shottky diodes made from semiconducting polymers, confirming the discovery by the Cambridge group [Nature 347, 539 (1990)]. Our results demonstrate that light-emitting diodes can be fabricated by casting the polymer film from solution with no subsequent processing or heat treatment required. Electrical characterization reveals diode behavior with rectification ratios greater than 104. We propose that tunneling of electrons from the recitifying metal contact into the gap states of the positive polaron majority carriers dominates current flow and provides the mechanism for light emission.
Auger electron spectroscopy and microscopy with probe-size limited resolution
Authors: Hembree G. G. , Drucker J. S. , Luo F. C. H. , Krishnamurthy M. , Venables J. A.
Published: July 2002

High spatial resolution Auger electron spectra and images have been obtained by optimizing secondary electron collection efficiency in a scanning transmission electron microscope (STEM). We describe an ultrahigh vacuum, 100 keV STEM which is capable of collecting Auger electron spectra and images with edge resolutions of ≤5 nm. Typical spectra and images from the Ge/Si(100) and Ag/Si(100) film growth systems are presented and discussed. These images demonstrate high-resolution elemental mapping which can be used in the study of surface processes on the nanometer scale.
Mole fractions of H, CH3, and other species during filament-assisted diamond growth
Authors: Hsu W. L.
Published: July 2002

Molecular beam mass spectrometry has been used to determine the concentrations of several free-radical and stable gaseous species in a hot-filament reactor under diamond-growth conditions. At a filament temperature of 2600 K, the H-atom concentration in the proximity of the growth surface was found to decrease with increasing addition of methane to the feed gas, dropping by more than an order of magnitude when the methane percentage was increased from 0.4% to 7.2%. Concurrent with this decrease, large changes in the concentration of the hydrocarbon species were observed. All numerical modeling results that have been reported to date do not include heterogeneous reactions on the filament and have failed to predict or explain these observations. Also from the measured mole fractions, the reaction H+CH4↔H2+CH3 was found to be in nonequilibrium near the surface, with the reverse rate ranging from approximately three to sixteen times larger than the forward rate. We attribute the cause of this nonequilibrium to heterogeneous reactions on the surface of the substrate.
Analysis of nonohmic current-voltage characteristics in a Cu-compensated GaAs photoconductor
Authors: Mazzola Michael S. , Roush Randy A. , Stoudt David C. , Griffiths Scott F.
Published: July 2002

An analysis of observations of nonohmic current conduction in a copper-compensated GaAs (GaAs:Cu) photoconductive switch is presented. It is demonstrated that conduction during illumination and at modest current densities can be attributed to an optically injected plasma influenced by single-injection contact effects. However, at higher current densities, a double-injection model is more suitable. We provide further evidence that the transition from a single-injection process to a double-injection process is accompanied by the formation of at least one current filament with cross-sectional area of 5×10−4 cm2, and a peak current density greater than 103 A/cm2. We finish by suggesting avalanche breakdown at the n+−i anode junction as a possible mechanism for the onset of significant hole injection at the anode, a condition necessary to justify the use of a double-injection-dominated transport model in an essentially n-i-n device.
Flux focusing effects in planar thin-film grain-boundary Josephson junctions
Authors: Rosenthal Peter A. , Beasley M. R. , Char K. , Colclough M. S. , Zaharchuk G.
Published: July 2002

We have studied the magnetic interference of the critical currents of synthetic planar thin-film grain-boundary Josephson junctions. We find that the effective area of these junctions scales as the square of the width w in contrast to the usual w(2λ+d) dependence of sandwich-type Josephson junctions. This behavior is a simple consequence of the magnetic response of thin-film superconductors to perpendicular applied fields. A model based on the London theory yields the observed behavior. In addition, we find the correction to the interference pattern due to the effect of the corners.
Photorefractive effect at 633 nm in semi-insulating cadmium sulfide
Authors: Tayebati P. , Kumar J. , Scott S.
Published: July 2002

We report the first observation of the photorefractive effect in cadmium sulfide at low optical power. The maximum gain coefficient measured with no applied field at 633 nm is 0.3 cm−1. The response time scales sublinearly with intensity and is independent of grating spacing. At a maximum intensity of 80 mW/cm2, a response time of 0.9 ms is obtained. The photorefractive sensitivity is the largest among photorefractive material operating in the visible range.
Low dark current step-bound-to-miniband transition InGaAs/GaAs/AlGaAs multiquantum-well infrared detector
Authors: Yu Larry S. , Wang Y. H. , Li Sheng S. , Ho Pin
Published: July 2002

A new step-bound-to-miniband (SBTM) transition multiquantum-well long-wavelength infrared photodetector (LWIP) using a lightly strained In0.07Ga0.93As quantum well and a short-period GaAs/Al0.4Ga0.6As superlattice barrier structure has been developed. The new structure created a potential ‘step’ in the superlattice barrier to block the undesirable electron tunneling current from the heavily doped ground state in the quantum well, which results in a significant reduction in the device dark current. The measured absorbance spectra and photocurrent are in good agreement with our theoretical predictions. The peak detectivity D* at λ=10.5 μm was found to be 2.1×1010 cm √Hz/W at Vb=5 V and T=63 K.
YBa2Cu3O7 thin-film step junctions on MgO substrates
Authors: Edwards J. A. , Satchell J. S. , Chew N. G. , Humphreys R. G. , Keene M. N. , Dosser O. D.
Published: July 2002

Josephson junctions have been made by the evaporation of epitaxial YBa2Cu3O7 over steps etched in (001) MgO. Such junctions differ from those grown on perovskite substrates in that the c-axis of the film grows approximately normal to the local substrate surface, so the junction comprises two tilt grain boundaries. Results are reported for junctions formed by this process which showed Josephson currents at temperatures up to 89 K and resistively shunted junctionlike current-voltage curves. A weak magnetic field applied normal to the substrate gave Fraunhofer patterns indicative of good current uniformity across the junction. The dc SQUIDs show deep modulation, and promising noise performance.
Picosecond large-signal switching characteristics of a pseudomorphic AlGaAs/InGaAs modulated doped field effect transistor
Authors: Jackson M. K. , Frankel M. Y. , Whitaker J. F. , Mourou G. A. , Hulin D. , Antonetti A. , Van Hove M. , De Raedt W. , Crozat P. , Hafdallah H.
Published: July 2002

We present the first comprehensive study of the large-signal switching characteristics of an AlGaAs/InGaAs modulation-doped field-effect transistor on a picosecond time scale. Electro-optic sampling is used to measure drain voltage response to a steplike gate input with a 2.8 ps rise time, at various dc biases. A large-signal switching time of 6.2 ps is obtained. Features deleterious to high-frequency device operation are observed, related to equivalent circuit parameters, and reduced by appropriate choice of operating point.
Photoinduced current transient spectroscopy of boron doped diamond
Authors: Glesener J. W.
Published: July 2002

A Schottky device fabricated on a natural type 2b diamond was characterized using photoinduced current transient spectroscopy (PICTS). A trapping level, presumed to be due to boron, with an activation energy of 0.29 eV was found from PICTS. Resistance measurements on the same device produced an activation energy of 0.38 eV. The two methods, while being described by the same exponential temperature dependence, have prefactors with different physical origins. Since the activation energy measured by PICTS is proportional to σeffe−E/kT, the difference in the measured values could be attributed to a temperature dependence in the ‘‘effective’’ hole capture cross-section σeff. This report also demonstrates the potential of PICTS in identifying an electrically active trap in a sample whose high series resistance renders it difficult to characterize using capacitance based methods.
Pulsed laser deposition of oriented VO2 thin films on R-cut sapphire substrates
Authors: Borek Mark , Qian F. , Nagabushnam V. , Singh R. K.
Published: July 2002

We have deposited highly oriented VO2 thin films on R-cut sapphire substrates by the pulsed laser ablation technique. The stabilization of pure VO2 is complicated due to a large number of distinct phases in the vanadium oxide system. As many as 13 different phases ranging from oxygen stoichiometry of V4O to V2O5 can exist in this system. The deposition parameters for fabrication of phase pure, highly oriented VO2 thin films on R-cut sapphire substrates by the pulsed laser deposition technique are discussed. A KrF laser (wavelength λ=248 nm, pulse duration τ=15×10−9 s) was used to ablate a pure vanadium target in an atmosphere containing specific ratios of oxygen and argon. The partial pressure of oxygen in the chamber was critical in stabilizing the VO2 phase. X-ray diffraction and other microstructural characterization revealed that the films were single phase and strongly oriented with the (200) planes parallel to the sapphire substrate. Short transition widths of 2 K and resistivity changes of 3×104 have been observed, thus indicating the growth of highly oriented VO2 films.
Theory of zone-folded optical transitions in semiconductor superlattices
Authors: Lazzouni M. E. , Sham L. J.
Published: July 2002

A one-dimensional two-band tight-binding model is used to develop simple criteria governing the optical transition strength of a direct gap formed by zone folding in a superlattice of alternating layers of two indirect band-gap semiconductors. The model study explains the weak optical transitions calculated for the III-V superlattices as being due to the similarities of the constituents and predicts strong optical transitions for large band discontinuities and type I alignment, particularly in GaP/Si2. The optical strength in the Si2/Ge2 superlattice is found to depend strongly on the growth direction and on strain. The oscillator strength calculated by the simple model is checked in two cases against a more elaborate calculation using the sp3d5 empirical tight-binding band structure and atomic wave functions.
Sample current maximum at the critical angle of x-ray total reflection
Authors: Kawai Jun , Hayakawa Shinjiro , Suzuki Setsuo , Kitajima Yoshinori , Takata Yasutaka , Urai Teruo , Meaeda Kuniko , Fujinami Masanori , Hashiguchi Yoshihiro , Gohshi Yohichi
Published: July 2002

The x-ray photoemitted electron intensity and sample current of a GaAs wafer are measured at 2 and 3 keV excitation photon energies while varying the glancing angle of the incident x rays. It is found that the sample current and the x-ray photoemitted electron intensity have similar behavior with respect to the glancing angle. The sample current as well as the photoemission intensity have a maximum at the critical angle of the x-ray total reflection. This is the first report of the observation of the existence of a sample current maximum at the critical angle of x-ray total reflection.
Micro-Raman spectroscopy of electromigration-induced oxygen vacancy aggregation in YBa2Cu3O7−δ
Authors: Moeckly B. H. , Buhrman R. A. , Sulewski P. E.
Published: July 2002

We describe the results of micro-Raman spectroscopy and optical microscopy studies of basal-plane chain-oxygen vacancy motion in YBa2Cu3O7−δ thin films under the influence of a high current bias near 300 K. Above a threshold level this bias causes vacancy aggregation and then long-range displacement, leading to an enhancement of oxygen order in the region of highest current density and the complex accumulation of oxygen vacancies in the region where the electromigration force is near the threshold level.
Single-polarity charge sensing in ionization detectors using coplanar electrodes
Authors: Luke P. N.
Published: July 2002

A new method to preferentially sense the collection of single-polarity charge carriers in ionization detectors is presented. It uses coplanar electrodes to closely emulate the function of Frisch grids commonly employed in gas and liquid ionization detectors. The coplanar electrode structure allows for easy implementation on semiconductor detectors. This method can be used to obtain good energy resolution from detectors in which only one polarity type of carriers is efficiently collected. Experimental results using a CdZnTe detector demonstrate the effectiveness of this method.
Cross-section transmission electron microscope observations of diamond-turned single-crystal Si surfaces
Authors: Shibata Takayuki , Ono Atsushi , Kurihara Kenji , Makino Eiji , Ikeda Masayuki
Published: July 2002

Diamond turning was carried out along the [110] direction on the (001) plane of single-crystal silicons at extremely small depths of cut of 100 and 500 nm. Cross-section transmission electron microscope observations revealed that turning had converted the layers of the surface into an amorphous structure directly, as well as in continuous chips. The thickness of the layers was about 150 nm in spite of the depth of cut. Under the amorphous layer, another damaged layer with many dislocations was formed, the thickness of which was about 2 and 3 μm at 100- and 500-nm cutting depth, respectively. At 500-nm cutting depth, microcracks were formed through the accumulation of excessive dislocations. At both depths of cut, however, the dislocations were mostly oriented along the 〈110〉 directions within the {111} planes. It was found that ductile-regime turning could be achieved by amorphization and deformation based on the {111}〈110〉 slip systems.
Observation of dislocation-mediated layer-by-layer interface growth
Authors: Tong X. , Gibson J. M.
Published: July 2002

The growth of thin Pd2Si films on Si(111) surfaces is studied using in situ transmission electron microscope under ultrahigh vacuum conditions. No immediate reaction of deposited Pd with Si is observed at room temperature. At ∼200 °C, uniform Pd2Si films can be formed. The thin Pd2Si films are found to grow into strained islands at elevated temperatures. Interfacial misfit dislocations associated with interfacial steps propagate across the strained islands, causing the islands to grow layer-by-layer at the interface. The strain fields associated with the misfit dislocations are believed to be responsible for this behavior.
Thermal conduction in metallized silicon-dioxide layers on silicon
Authors: Käding O. W. , Skurk H. , Goodson K. E.
Published: July 2002

The vertical thermal conductivities of thermally grown (TG) and chemical vapor deposited (CVD) silicon-dioxide layers 20 to 200 nm thick are measured using a simple, noncontact photothermal technique. The conductivities of TG and CVD layers are less by as much as 18% and 30%, respectively, than the conductivity of bulk fused silicon dioxide. No significant thickness dependence is observed. The thermal boundary resistance between the oxide layers and silicon is shown to be negligibly small. The boundary resistance of gold layers sputtered directly onto TG oxide is considerably larger than that of gold layers evaporated on TG oxide with a 20-nm chromium adhesion layer, and is comparable to internal resistances of the oxide layers.
Reactive pulsed laser deposition of CNx films
Authors: Zhao X.-A. , Ong C. W. , Tsang Y. C. , Wong Y. W. , Chan P. W. , Choy C. L.
Published: July 2002

Carbon nitride (CNx) films were prepared by reactive pulsed laser deposition at nitrogen partial pressure PN2varying from 0 to 300 mTorr. It is found that the atomic fraction of nitrogen f in the films first increases with increasing PN2, reaches a maximum of 0.32 at PN2=100 mTorr, and then decreases to a saturated value of 0.26 at PN2≳200 mTorr. Because of the absence of energetic particles in reactive pulsed laser deposition, the limited nitrogen content cannot be attributed to preferential sputtering of nitrogen that is generally observed in particle-assisted deposition of CNx films. Infrared absorption experiments show the existence of C≡N bonds and graphitic sp2 bonds. The sp2 bonds become IR active because of symmetry breaking of graphitic rings as a consequence of nitrogen incorporation. CNx films deposited at low PN2 (e.g., 5 mTorr) are more graphitic than the diamondlike pure carbon sample deposited at PN2=0, so have a slightly narrower electron band gap Eopt and a significantly higher room-temperature electrical conductivity σR. At PN2≳200 mTorr, nitrogenation of the films is very pronounced, leading to a wide band gap (Eopt≳1.5 eV), long electron band tail (E0≳0.7 eV), and extremely low σR(<1×10−13 Ω−1 cm−1). In addition, both the hardness and Young’s modulus are greatly reduced, for example, from 41.3 and 285 GPa for the pure carbon sample to 1.2 and 23.8 GPa, respectively, at f=0.32.
Optically switched resonant tunneling diodes
Authors: Moise T. S. , Kao Y.-C. , Garrett L. D. , Campbell J. C.
Published: July 2002

The room-temperature photoinduced switching of an InGaAs/AlAs resonant-tunneling diode is demonstrated. When illuminated at an irradiance of greater than 20 W cm−2 using 1.3 μm radiation, the resonant-tunneling diode switches from a high-conductance to a low-conductance electrical state and exhibits a voltage swing of 600 mV. The switching characteristics are reversible and, in the absence of light, the detector returns to its original high conductance operating state. Small-signal optical measurements performed with the device biased prior to resonance demonstrate a 3 dB bandwidth of ∼1.5 GHz.
Constant-tension thermocouple rake suitable for use in flame mode combustion studies
Authors: Cundy V. A. , Morse J. S. , Senser D. W.
Published: July 2002

An improved design for a thermocouple rake configuration to be used in fundamental combustion studies is presented. Experience with the constant-tension thermocouple rake has nearly eliminated thermocouple sensing wire sag which is common using conventional rake configurations.
Direct formation of ordered CoPt and FePt compound thin films by sputtering
Authors: Visokay M. R. , Sinclair R.
Published: July 2002

Equiatomic CoPt and FePt alloy films were deposited by cosputtering at substrate temperatures between 25 and 640 °C. Those deposited at high temperatures (≥400 and ≥520 °C for FePt and CoPt, respectively) contained ordered intermetallic compounds with the L10 crystal structure while those deposited at lower temperatures were chemically disordered. Deposition on single crystal [001] MgO and [0001] Al2O3 resulted in [001] and [111] oriented films, respectively, for all deposition temperatures. Ordered alloys have out-of-plane magnetic easy axes and modified magneto-optic Kerr rotation spectra relative to the disordered case. A difference in the Kerr rotation spectrum is observed between ordered, but not disordered, [001] and [111] films.
Development of standard operating procedures for differential scanning calorimeters
Authors: Callanan Jane E. , Sullivan Sandra A.
Published: July 2002

This paper describes an assessment of the behavior of a differential scanning calorimeter and the development of satisfactory calibration, operation, and data reduction procedures, which depend on performance characteristics of the individual instrument. Factors that contribute to thermal lag are identified; suggestions for evaluating and compensating for it are given.
One-step absolute frequency stabilization of a Ti:sapphire laser using frequency modulation Lamb-dip spectroscopy
Authors: An Kyungwon , Dasari Ramachandra R. , Feld Michael S.
Published: July 2002

The frequency of a commercial Ti:sapphire laser is stabilized directly relative to the Lamb-dip signal from an atomic vapor cell without the Fabry–Pérot cavity as in intermediate frequency reference. The laser spectrum is measured by means of a supercavity spectrum analyzer with a full width at half-maximum (FWHM) of 110 kHz, which is independently calibrated using a ring-down technique. The measured spectrum has a FWHM of 220 kHz, thus indicating the laser linewidth to be about 55 kHz RMS.
Simple and reliable preionization technique using SiC ceramics and its application to a TEA CO2 laser for airborne DIAL
Authors: Asai Kazuhiro
Published: July 2002

A simple and reliable preionizing system obtained by combining the mesh cathode method and SiC ceramics has been studied. It was found that a SiC preionizer with a resistivity of 10–20 Ω cm was the most suitable for a uniform glow discharge at atmospheric pressure. This preionizing technique has been used for TEA CO2 lasers of an airborne ozone differential absorption LIDAR (DIAL) system without any maintenance for more than 3 yrs.
Scanning positron microbeam
Authors: Brandes G. R. , Canter K. F. , Horsky T. N. , Lippel P. H. , Mills A. P.
Published: July 2002

The construction and operation of a scanning positron microbeam is reported. The positrons are obtained from a doubly remoderated positron beam with a normalized brightness-per-volt of 1.8×105 e+ s−1 cm−2 rad−2 V−1 Ci−1. The beam of 8000 e+ s−1 is brought to a 10×50 μm2 focus by a sectored lens and is scanned across a test grid. Possible applications to defect spectroscopy and the observation of small samples will be discussed.
Novel approach for particle velocity and size measurement under plasma conditions
Authors: Sakuta Tadahiro , Boulos Maher I.
Published: July 2002

A novel technique is proposed for the simultaneous, in-flight, measurement of the velocity and size of individual particles under plasma conditions. The method is based on the observation of each particle through its own emission and the analysis, in the time domain, of the waveform of the light burst generated as it crosses an observation window of known dimensions. A theoretical analysis of the parameters affecting the visibility of the particles in an argon plasma showed that depending on the particle diameter and its surface temperature, its thermal visibility factor will drop sharply from 1.0 to almost zero, with the increase of the background plasma temperature. Measurements are carried out using nickel particles (d̄p=78 μm, σ=18.0 μm) injected axially into an inductively coupled rf plasma ( f=3 MHz, P=15 kW) operated using argon as the plasma gas at atmospheric pressure and under soft vacuum conditions (p=760 and 150 Torr). The results are in good agreement with particle velocity data obtained using laser Doppler anemometry. The measured, in-flight, particle size distribution is consistent with optical microscopic measurement of the particle size distribution of the injected powder.
Substrate bias effect on the capture kinetics of random telegraph signals in submicron p-channel silicon metal–oxide–semiconductor transistors
Authors: Simoen E. , Claeys C.
Published: July 2002

This letter investigates the effect of the substrate bias on the capture kinetics of random telegraph signals in submicron silicon p-channel metal–oxide–semiconductor transistors. A strong dependence of the capture time constant τc on the transverse electric field is observed. As a result, τc∼exp(−Ap) is observed experimentally, which is much stronger than the 1/p dependence predicted by simple Shockley–Read–Hall theory, whereby p is the surface density of free holes. The observations are explained tentatively by considering a field-dependent hole capture cross section. The latter may result from quantization effects induced by the transverse field in the two-dimensional inversion layer.
Evaluation of a linear photodiode array detector for synchrotron small-angle x-ray scattering measurements
Authors: Chu Benjamin , Wu Dan Q. , Howard Roy L.
Published: July 2002

A linear photodiode array position-sensitive detector for synchrotron small-angle x-ray scattering was constructed by coupling a fiber-optic face plate coated with a thin layer (∼40 μm) of phosphor (Y2O2S:Tb) onto an electrostatically focused, intensified photodiode array detector (EG&G, PARC, Model 1422) originally designed for visible light applications. The x-ray detector was thoroughly evaluated. It showed excellent linearity of response to the incident x-ray intensity (less than 1%), stable pixel uniformity (an average of ±5%), good spatial linearity (±1 per 100 pixels), and a net gain of ∼17 counts/x-ray photon at 8 keV. The detector quantum efficiency was about 1 for a signal-to-noise ratio of greater than ∼5%. No damage to individual pixels was found after exposure to a main beam of ∼1010 x-ray photons/mm2/s. A finite afterglow (≳5 s) was observed for the phosphor inside the intensifier tube. The afterglow that could limit fast time-resolved experiments could be significantly improved by replacing the present intensifier with one using a fast-decay phosphor. The detector usable length could be increased from the present 25 to 50 mm by replacing the fiber-optic face plate with a 2:1 taper.
Measurements of neutral beam species, impurities, spatial divergence, energy dispersion, pressure, and reionization using the TFTR U.S. Common Long Pulse Ion Source
Authors: Kugel H. W. , Gammel G. M. , Grisham L. R. , Kaita R. , Kamperschroer J. H. , Langley R. A. , Magee C. W. , Medley S. S. , Murphy T. J. , Roquemore A. L. , Williams M. D.
Published: July 2002

Results are given from the first comprehensive and complementary measurements using the final production U.S. Common Long Pulse Ion Sources mounted on both the TFTR neutral beam test beamline and the TFTR neutral beam injection system, with actual tokamak experimental conditions, power systems, controls, and operating methods. The set of diagnostics included water calorimetry, thermocouples, vacuum ionization gauges, photodiodes, neutron, gamma-ray, and charged particle spectroscopy, optical multichannel analysis, charge exchange spectroscopy, Rutherford backscatter spectroscopy, and implantation/secondary ion mass spectroscopy. These systems were used to perform complementary measurements of neutral beam species, impurities, spatial divergence, energy dispersion, pressure, and reionization. The measurements were performed either in the neutralizer region, where the beam contained both ions and neutrals, or in the region of the output neutral beam. The average of the neutral particle ratios in the range from 80 to 114 keV is D0[E]:D0[E/2]:D0[E/3]=53(5):27(4):20(4), where the quantities in parentheses are the average experimental uncertainties.

The corresponding neutral power ratio is P0[E]:P0[E/2]:P0[E/3]=72(9):19(3):9(2). The half widths (1/e) in the horizontal plane for the full-, half-, and third-energy components were 0.26°, 0.34°, and 0.42°, respectively. The dispersions of the full-, half-, and third-energy components were 1.20 keV, 2.35 keV, and 2.26 keV, respectively. The carbon impurity concentration in a 80 keV D0 beam was not greater than 2×10−4 per D0 beam particle, and exhibited an apparent acceleration state of C+. The oxygen impurity concentration was less than 5×10−4 per D0 beam particle, and exhibited an apparent acceleration state of O+. A variety of vacuum conditions were observed depending on the operating conditions. Typically, pressures in the transition ducts were in the range from 0.3 to 0.7×10−5 Torr at the beginning of injection pulses, and reionized power losses were in the range from 0.75% to 1.5% of incident power. At the end of injection pulses, pressures in the transition ducts were in the range from 0.6 to 2×10−5 Torr and reionized power losses were in the range from 2% to 6% of incident power. This work describes generic results, new apparatus, and advances in measurement techniques for the optimization of tokamak neutral beam heating operations and the analysis of neutral beam heated plasmas.
Time-resolved x-ray diffraction station: X-ray optics, detectors, and data acquisition
Authors: Towns-Andrews E. , Berry A. , Bordas J. , Mant G. R. , Murray P. K. , Roberts K. , Sumner I. , Worgan J. S. , Lewis R. , Gabriel A.
Published: July 2002

A new x-ray beamline has recently been built, and is now operational, on dipole magnet 2 of the Synchrotron Radiation Source (SRS) at Daresbury. This beamline takes 32 mrad of horizontal aperture from a central tangent point. The time-resolved x-ray diffraction (TRXD) Station 2.1, takes 17 mrad of horizontal aperture and is the central point of this paper. Beamline 2 has been realized as part of a SERC–MRC agreement.
Apparatus for the study of macromolecular adsorption
Authors: Mayo C. S. , Hallock R. B.
Published: July 2002

A surface plasmon adsorbate monitor (SPAM) is described which allows the adsorption of macromolecules or other adsorbates to a metal surface to be monitored. Surface plasmons are employed and the apparatus has no moving parts. The kinetics of adsorption may be studied on a time scale of seconds rather than the more common time scale of minutes; a simple improvement in computer memory access should allow temporal studies in the millisecond range. As an illustration, the adsorption of carboxyl-terminated polystyrene from a solution with acetone onto a silver surface is measured.
A cryogenically cooled photofragment fluorescence instrument for measuring stratospheric water vapor
Authors: Weinstock Elliot M. , Schwab James J. , Nee Jan Bai , Schwab M. J. , Anderson James G.
Published: July 2002

An instrument has been developed to make daytime measurements of water vapor in the stratosphere using the technique of photofragment fluorescence. While the use of photofragment fluorescence is by no means unique for measuring water vapor, its application to the stratosphere above 25 km has been limited to two instruments, neither of which are currently operational. In both cases, instrumental contamination was a major concern, especially at the higher altitudes. For this instrument, data are taken during the descent portion of a balloon flight as ambient air is drawn through an aerodynamically designed flow tube. The problem of instrument outgassing is overcome by cryogenically cooling the instrument walls and surfaces that serve as potential sources of water vapor. Additionally, a set of fast response thermistors approximately 8 cm downstream from the optical axis provide the diagnostic capability necessary to identify flow perturbations, which can perturb the measurement. A detailed description is provided of all aspects of the instrument as well as the results of its first two flights. The fluorescence sensitivity of the instrument under zero pressure conditions is about 1.5×10−8 (counts/s)/(molecules/cm3). With a typical solar background of 5000 counts/s and a fluorescence signal of 2500 count/s, a signal-to-noise ratio of approximately 50:1 is achieved for 10 s of averaging throughout the stratosphere. The descent velocity of the instrument through the stratosphere is typically from 3 to 10 m/s, and an altitude resolution of better than 100 m is achieved.
Thermal dependence of the refractive index of GaAs and AlAs measured using semiconductor multilayer optical cavities
Authors: Talghader J. , Smith J. S.
Published: July 2002

The longitudinal optical mode shift with temperature was measured in two vertical cavity surface-emitting laser (VCSEL) type optical resonators with different GaAs and AlAs layer structures. The measurements show distinct differences in the behavior of the cavities. From the data the thermal dependencies of the indices of refraction of GaAs and AlAs for wavelengths near 1 μm were determined to be (2.67±0.07)×10−4/°C and (1.43±0.07)×10−4/°C, respectively.
Electrical impedance measurements of polymer light-emitting diodes
Authors: Campbell I. H. , Smith D. L. , Ferraris J. P.
Published: July 2002

We report electrical impedance measurements of polymer light-emitting diodes employing the soluble, conjugated polymer poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) as the light-emitting layer. The diode structures were metal-polymer-metal structures utilizing thin gold as the transparent, positive contact, and calcium as the negative contact. The devices were fabricated using undoped, polymer active layers ∼40 nm thick. The polymer light-emitting diodes are accurately modeled as a resistor and capacitor in parallel for frequencies from 100 Hz to 1 MHz and for bias conditions from reverse bias to forward current densities of 0.1 A/cm2. The diode capacitance as a function of bias voltage is qualitatively different from conventional Schottky or p-n junction diodes; in reverse bias, the polymer layer is fully depleted and the capacitance is independent of bias; at small forward bias, traps are charged near the metallic contacts and the capacitance increases; under large forward bias, with significant electron and hole injection, the traps are neutralized and the capacitance decreases. From the magnitude of the initial increase in capacitance with forward bias the trap density is estimated to be only a few times 1016 cm−3.
What do you measure when you measure resistivity?
Authors: Koon D. W. , Knickerbocker C. J.
Published: July 2002

Resistivity measurement is a weighted averaging of local resistivities. We develop a formalism to calculate the weighting function, applying it to square van der Pauw samples and to linear and square four-point probe arrays. In each case, some regions of the sample are negatively weighted, but these regions can be reduced or eliminated by van der Pauw averaging. We discuss negative weighting, which we feel is responsible for spurious reports of superconductivity above room temperature. We show how a square four-point array can be more effective at measuring local resistivity than a linear one. Finally, we show how to apply our formalism to anisotropic materials.
Systematic procedures for atom-probe field-ion microscopy studies of grain boundary segregation
Authors: Krakauer B. W. , Seidman D. N.
Published: July 2002

A procedure is presented for systematically and reproducibly preparing alloy specimens for the study of grain boundary (GB) segregation employing both transmission electron (TEM) and atom-probe field-ion microscopies (APFIM) to examine the same GB; the procedure is illustrated for an Fe(Si) alloy. A commercially available oxygen plasma source is incorporated in the sample preparation procedure to remove all traces of hydrocarbon build-up introduced during TEM GB analysis, thus allowing controlled backpolishing after a TEM analysis. Specifications for the optimum tip geometry, i.e., how a GB is positioned in a tip via backpolishing to maximize the probability of its observation and subsequent compositional analysis via APFIM, are empirically determined: 30–200 nm for the GB-to-tip separation, and 40–80 nm for the GB diam for shank angles less than 20°. It is demonstrated that accurate quantitative APFIM analyses of an Fe-3 at. % Si alloy are possible for pulse fractions ≥15% and specimen temperatures ≤55 K. Results are presented for a Σ≊3a GB that was first analyzed via TEM to determine its five macroscopic degrees of freedom, and then analyzed via APFIM to measure an average GB segregation enhancement factor for Si of 3.51±0.34 at 823 K.
Low-noise photodiode detector for optical fluctuation diagnostics
Authors: Fonck R. J. , Ashley R. , Durst R. , Paul S. F. , Renda G.
Published: July 2002

The beam emission spectroscopy optical fluctuation diagnostic requires the highest possible quantum efficiency detector at 656 nm to minimize the photon statistical baseline limit to the detectable fluctuation level. A photoconductive photodiode detector with an extremely low-noise preamplifier and a reactive feedback circuit provides quantum efficiencies up to 70%–80% for a useful frequency range of at least 0–150 kHz with incident powers of ∼10 nW. The diodes are chosen for negligible leakage current and hence do not require active cooling. These detectors have provided increase in the sensitivity to plasma fluctuation amplitude by a factor of ∼14 over photomultipliers and a factor of 4 over large area avalanche photodiodes.
Quick measurement of NMR-coil sensitivity with a dual-loop probe
Authors: Darrasse Luc , Kassab Ghazi
Published: July 2002

The radio-frequency coil sensitivity for NMR signal detection can be defined as B1/√P, which is the magnetic field that the coil induces at a given point per unit supplied power. We propose a new method, based on the reciprocity theorem, to quickly evaluate the sensitivity of a tuned coil. The only requirement of the method is a voltage gain measurement with a small dual-loop probe, without mobilization of the NMR unit. The probe can be used to gauge and map the sensitivity of the coil with good accuracy, while monitoring its tuning frequency ω0 and its quality factor Q. Our method is particularly convenient for development and maintenance of coils for MR imaging, and more generally could be applied to other fields involving radio-frequency characterization of inductive circuits.
A method for the determination of the noise parameters in preamplifying systems for semiconductor radiation detectors
Authors: Bertuccio G. , Pullia A.
Published: July 2002

In this paper a method for disentangling the various noise components in semiconductor radiation detector–amplifier systems is described and experimentally tested. A charge amplification scheme is adopted for the measurements. It is shown how an accurate estimate of the series and parallel white noise, 1/f series noise, and f parallel noise can be quickly obtained through a multiparameter least-squares interpolation of the equivalent noise charge data of the system.
A versatile microwave-frequency-compatible scanning tunneling microscope
Authors: Stranick S. J. , Weiss P. S.
Published: July 2002

Challenged by the need to transmit a microwave frequency signal to and from the tunnel junction of a scanning tunneling microscope (STM), an STM has been designed and built that provides minimal microwave signal loss. The STM has been operated over a wide frequency range, dc to 20 GHz, without the use of a microwave resonance cavity. This design also provides a flexible feedback scheme allowing selection of the required piezoelectric gain and stability. A reliable, easy to fabricate sample approach is also described that adds to the stability of this design. The operation of this microscope by imaging a lead silicate glass surface with a bias voltage modulation frequency and detection frequency of 7.73 GHz is demonstrated.
Switching between deep-level transient spectroscopy and feedback charge capacitance modes in a versatile time-domain spectrometer
Authors: Thurzo I. , Gmucová K.
Published: July 2002

Software-supported switching between the deep-level transient spectroscopy (DLTS) and feedback charge method (FCM) operation modes of a versatile time-domain spectrometer, while using identical circuitry, is demonstrated. No additional correction of the output signal for integrated leakage current is needed. The timing scheme design is discussed in more detail with reference to a practical example. The interval of achievable rate windows is not affected by the switching between the DLTS and FCM modes of the instrument. A few illustrative examples of DLTS and C-V plots are included.
Near-infrared diode laser airborne hygrometer
Authors: Silver Joel A. , Hovde D. Christian
Published: July 2002

We describe a new laser-based hygrometer for ambient water vapor monitoring which uses a fiber-optic coupled, near-infrared diode laser in conjunction with high frequency wavelength modulation spectroscopy. The instrument operates unattended, uses little power, can be extremely compact, and exhibits high detection sensitivity. This instrument was flown on a KC-135 aircraft for a period of six months and measured frost points at altitudes between 10 000 and 40 000 ft. The water detection sensitivity corresponds to a volume mixing ratio of 8 ppm V. Further improvements including the use of newly designed near-infrared lasers are expected to realize frost points below −100 °C.
Accurate polarization interferometer
Authors: Greco Vincenzo , Molesini Giuseppe , Quercioli Franco
Published: July 2002

A polarization interferometer which measures optical path differences with improved accuracy control is presented. The approach makes use of four signals in quadrature, computing the phase with an algorithm insensitive to laser power drifts. Experimental results of the interferometer’s performance are given under laser warmup conditions, and with long-term monitoring of optical path differences.
Ultrafast broadband frequency modulation of a continuous wave reflectometry system to measure density profiles on ASDEX Upgrade
Authors: Silva A. , Manso M. E. , Cupido L. , Albrecht M. , Serra F. , Varela P. , Santos J. , Vergamota S. , Eusébio F. , Fernandes J. , Grossmann T. , Kallenbach A. , Kurzan B. , Loureiro C. , Meneses L. , Nunes I. , Silva F. , Suttrop W. ,   the ASDEX Upgrade Team
Published: July 2002

A reflectometry system has been developed for ASDEX Upgrade to measure the plasma profile from the scrape-off layer until the bulk plasma, simultaneously at the high and low field sides. Unique features of the system are the ultrafast broadband frequency modulation of a continuous wave using solid state stable hyper abrupt tuned oscillators (down to 10 μs), high and low field side channels and fully remote control operation, via optical fiber links. Due to the special design of the transmission line, with decoupled in going and out going lines and one-antenna configuration, the system is optimized for reception and spurious reflections are eliminated. The ultrafast operation guarantees that the effect of plasma turbulence is greatly reduced. Both features determine the high performance of the diagnostic. A dedicated data acquisition system handles the large amounts of data generated by the broadband operation. Recent developments include the operation of new channels and an automatic and accurate frequency calibration circuit. Also, advanced digital signal processing techniques were applied to obtain density profiles with high spatial and temporal (20 μs) resolutions under turbulent plasma regions, e.g., the scrape-off layer. Experimental results are presented showing the great sensitivity of the diagnostic to plasma radial movements and its tolerance to vertical movements of the plasma. Density profiles measured in ELMy regimes illustrate the capabilities of the diagnostic to detect fast profile changes.
Low temperature performance of photomultiplier tubes illuminated in pulsed mode by visible and vacuum ultraviolet light
Authors: Araújo H. M. , Chepel V. Yu. , Lopes M. I. , Marques R. Ferreira , Policarpo A. J. P. L.
Published: July 2002

Detecting the ultraviolet scintillation light of the liquefied rare gases, in particular liquid xenon, requires the use of photomultiplier tubes (PMTs) in a temperature range where their operation is not ensured by manufacturers. Simultaneously, monitoring of the PMTs should be done in the visible for technical reasons. Bialkali photocathodes, that present high quantum efficiency and low dark noise, exhibit a very strong increase of electrical resistivity upon cooling that, apart from other reasons, can lead to operation failure at low temperature. Photomultiplier tubes with bialkali photocathodes and quartz windows (two EMI 9750Q and one Philips XP 2020Q) and another with S11-type photocathode and glass window (FEU 85A) were tested down to −160 °C with a pulsed light source, both in the visible region and at the xenon scintillation wavelength in the vacuum ultraviolet. For the visible range, the results obtained with bialkali photomultipliers display a sharp drop of response at about −100 °C, depending on the count rate, regardless of the wavelength, and a shift of the spectral sensitivity curve towards shorter wavelengths. Concerning the vacuum ultraviolet region, a significant increase of the response was observed between room temperature and that of the efficiency drop. For some photomultiplier tubes, the response as a function of temperature was observed to depend on their past history, probably due to mechanical effects arising in the multiplier system upon cooling. It is concluded that photomultipliers with bialkali photocathode can, in principle, be used for the detection of liquid xenon scintillation, although they have to be carefully monitored while in operation.