bluuebunny/crossref_metadata_2025 huggingface dataset:
Influences of iron doping on the photorefractive properties of KTa1−xNbxO3 crystals
Authors: Guan Qingcai , Wang Jiyang , Lian Yingwu , Yang Changxing , Ye Peixian
Published: July 2002

The influences of iron doping on the photorefractive properties of KTa1−xNaxO3 (KTN) crystals were studied. The two-wave-mixing gain coefficients and the grating formation rates were measured in undoped and Fe-doped KTN samples. Photorefractive parameters, such as the effective carrier number density, were determined for these samples. It was shown that the gain coefficient increases significantly with the doped Fe content, but the grating formation rate decreases with it, and that all the influences of iron doping originate mainly from the variation of the effective carrier number density.
Optical gain anisotropy in serpentine superlattice nanowire-array lasers
Authors: Hu S. Y. , Miller M. S. , Young D. B. , Yi J. C. , Leonard D. , Gossard A. C. , Petroff P. M. , Coldren L. A. , Dagli N.
Published: July 2002

We report optical gain measurement from a serpentine superlattice nanowire-array laser sample, grown by molecular beam epitaxy on a 2°-off (100)GaAs vicinal substrate. Gain spectra, obtained from in-plane ridge-waveguide lasers with stripes either parallel or perpendicular to the nanowire arrays at 1.4 K, showed that the optical gain for the TM mode became greater than that of the TE mode when the optical cavity was placed along the nanowire direction. This provides strong evidence that the lateral quantum confinement in the serpentine superlattice is stronger than the vertical quantum confinement.
One-dimensional to one-dimensional tunnelling between electron waveguides
Authors: Eugster C. C. , del Alamo J. A. , Rooks M. J. , Melloch M. R.
Published: July 2002

We report the observation of controlled electron tunnelling between two closely spaced one-dimensional (1D) electron waveguides implemented using a split-gate scheme on a high mobility AlGaAs/GaAs heterostructure. The 1D to 1D tunnelling current shows a distinct bumpy pattern when the electronic subband population in the two waveguides is modulated. These results are consistent with a picture in which tunnelling primarily occurs when the 1D subbands in both waveguides line up in energy as expected from energy and momentum conservation rules.
Laser polymer ablation threshold lowered by nanometer hot spots
Authors: Wen Xiaoning , Hare David E. , Dlott Dana D.
Published: July 2002

The effects of nanometer-sized hot spots on laser polymer ablation are studied. Two polymer thin films are synthesized, containing sensitizers which absorb intense 1.064 μm optical pulses. One film contains a molecular dye sensitizer. The other contains graphic nanoparticle sensitizers (≤220 nm diam). When longer duration pulses (150 ns) which do not produce hot spots are used, both films have the same ablation threshold. When shorter (23 ps) pulses are used, the ablation threshold for graphite films is a factor of 3 less than in dye films, attributed to enhanced polymer thermal decomposition in the vicinity of the larger graphite hot spot.
Randomly oriented Angstrom-scale microroughness at the Si(100)/SiO2 interface probed by optical second harmonic generation
Authors: Dadap J. I. , Doris B. , Deng Q. , Downer M. C. , Lowell J. K. , Diebold A. C.
Published: July 2002

Femtosecond pulses from a Kerr–Lens mode-locked Ti:sapphire laser are used to generate second harmonic from a series of native-oxidized Si(100)/SiO2 and hydrogen-terminated Si(100) samples prepared with systematically varied interfacial microroughness with root-mean-square feature heights ranging from 0.6 to 4.3 Å. Rotationally anisotropic second harmonic signals using different polarization configurations were measured in air and correlated with atomic force microscopy measurements. The results demonstrate rapid, noncontact, noninvasive measurement of Angstrom-level Si(100)/SiO2 interface roughness by optical second harmonic generation.
Room-temperature sharp line electroluminescence at λ=1.54 μm from an erbium-doped, silicon light-emitting diode
Authors: Zheng B. , Michel J. , Ren F. Y. G. , Kimerling L. C. , Jacobson D. C. , Poate J. M.
Published: July 2002

We report the first room-temperature sharp line electroluminescence of an erbium-doped silicon light-emitting diode at λ=1.54 μm. The electroluminescence originates from an internal f-shell transition of Er3+. The wavelength and linewidth are relatively independent of temperature. The light intensity saturates at a drive current density of 5 A/cm2 due to the long excited state lifetime of Er3+. As the temperature increases from 100 K to room temperature, the light intensity decreases significantly.
Maskless patterning of indium tin oxide layer for flat panel displays by diode-pumped Nd:YLF laser irradiation
Authors: Takai M. , Bollmann D. , Haberger K.
Published: July 2002

An indium tin oxide (ITO) layer on a lime glass substrate for flat panel displays has been patterned without a mask by scanning Nd:YLF (neodymium-doped yttrium-lithium-fluoride) laser irradiation in a pulsed mode. Both fundamental and frequency doubled lines of 1.047 μm and 523.5 nm were compared for processing. SEM (scanning electron microscopy) and surface stylus observation revealed that only the top ITO layer could be removed without substrate etching. A finer patterning was possible for irradiation of a 523.5 nm line because of the better focusing feature, though higher laser energy density was required for this line than for a 1.047 μm line because of the lower light absorption.
Carrier relaxation in InGaAs heterostructures
Authors: Sucha G. , Bolton S. R. , Chemla D. S. , Sivco D. L. , Cho A. Y.
Published: July 2002

We present time-resolved measurements of carrier dynamics in bulk and quantum wells InGaAs, using differential absorption spectroscopy. We find that the carrier thermalization time is 200–300 fs regardless of layer width for sample thickness ranging from 100 to 6000 Å. The efficiency of screening relative to phase space filling is larger in the bulk than in the quantum wells.
Spin-valve sensors exchange-biased by ultrathin TbCo films
Authors: Freitas P. P. , Leal J. L. , Melo L. V. , Oliveira N. J. , Rodrigues L. , Sousa A. T.
Published: July 2002

Ultrathin TbCo films are used to exchange-bias spin-valve sensors as an alternative to the commonly used FeMn films. Magnetoresistance values of 7.8% and sensitivities of 2.5%/Oe are demonstrated for spin-valve stripes where the pinned NiFe layer is exchange biased by a 120-Å thick TbCo film. The exchange field can be fine tuned by changing the TbCo layer thickness. A well-biased and linear spin-valve sensor was fabricated, with a height of 2 μm and a trackwidth of 25 μm, where the free layer is biased by the sense current.
InTlP — a proposed infrared detector material
Authors: van Schilfgaarde M. , Chen An-Ban , Krishnamurthy S. , Sher Arden
Published: July 2002

In1−xTlxP is proposed as a promising material for infrared detectors. A number of key optical and structural properties are studied within local density-functional theory. In1−xTlxP at x=0.67 and In1−xTlxAs at x=0.15 are estimated to have a gap of 0.1 eV. Their binding energies are larger than that of InSb, and they are found to form stable zinc-blende alloys for all x. In1−xTlxP nearly lattice matches to InP, and offers the potential to integrate detector array and read-out circuit.
Sulphur passivation of gallium antimonide surfaces
Authors: Dutta P. S. , Sangunni K. S. , Bhat H. L. , Kumar Vikram
Published: July 2002

Improvement in optical and electrical properties were observed after sulphur passivation of gallium antimonide surface. Enhancement of photoluminescence intensity up to 60 times, reduction in surface state density by two orders of magnitude, and reverse leakage currents by a factor of 20–30 were obtained as a result of surface passivation. While the reduction of surface recombination is attained, the surface is not unpinned.
Long minority carrier lifetimes in 6H SiC grown by chemical vapor deposition
Authors: Kordina O. , Bergman J. P. , Henry A. , Janzén E.
Published: July 2002

The room-temperature minority carrier lifetimes have been measured on 6H SiC epitaxial layers with residual n-type doping ranging from below 1014 cm−3 to above 1017 cm−3. Lifetimes as high as 0.45 μs have been achieved for thick low-doped material. The samples were grown by chemical vapor deposition using silane and propane or silane and methane in a hydrogen ambient. The minority carrier lifetimes were measured by monitoring the decay of the near-band gap room-temperature luminescence of the samples after excitation of a short laser pulse.
Microcomputer-based spectrum-resolved second-harmonic generation correlator for fast measurement of ultrashort pulses
Authors: Watanabe Akira , Tanaka Shosaku , Kobayashi Hiroshi , Ishida Yuzo , Yajima Tatsuo
Published: July 2002

A new spectrum-resolved second-harmonic generation correlator using a single-line image sensor array and a microcomputer has been developed. It allows us acquisition of many spectrum-resolved second-harmonic generation correlation traces in a single scan of the optical delay. Using this correlator, the pulse properties of a colliding pulse mode-locked ring dye laser have been investigated. It was found that the measuring time can be reduced by an order of magnitude compared with the conventional repeat-scan method.
Ruby thermometer for cryobaric diamond-anvil cell
Authors: Weinstein B. A.
Published: July 2002

The ruby R-line luminescence, commonly used as a manometer in diamond-anvil cell experiments, is shown also to be a useful thermometer over the range 10–100 K. Measurements up to 120 kbar in a solid Ar pressure medium demonstrate that the intensity ratio of the two R lines is thermally activated; in agreement with elementary theory the activation energy is given by the R1–R2 splitting. Based on this dependence, the in-situ temperature of the diamond-anvil cell sample chamber can be determined with ∼10% accuracy over 10–100 K. To apply the ruby thermometer method, no additional effort is needed beyond that already incurred in the standard ruby pressure measurement. The method remains applicable under conditions of moderately nonhydrostatic pressure. The problem of laser heating is discussed.
Ultraviolet and visible single-shot autocorrelator based on multiphoton ionization
Authors: Bourne O. L. , Alcock A. J.
Published: July 2002

The construction and performance of a simple and compact single-shot autocorrelator suitable for use in both the UV and visible regions of the optical spectrum is described. This device has a temporal range of over 500 ps with a lower limit of 25 ps. Some suggestions are made to reduce this lower limit to the subpicosecond domain.
Velocimetry of fast surfaces using Fabry–Perot interferometry
Authors: McMillan C. F. , Goosman D. R. , Parker N. L. , Steinmetz L. L. , Chau H. H. , Huen T. , Whipkey R. K. , Perry S. J.
Published: July 2002

This article describes the use of the Fabry–Perot laser interferometer in the fringe mode to measure velocities of fast-moving reflecting surfaces, and includes a review of previously published work. We begin by describing the theory of the Doppler shift that applies to these situations, and include an experimental test of whether surface normal direction affects Doppler shift. Formulas are derived for the analysis of the effects of shocked, dispersive, moving transparent media on velocity measurements, including expressions for the velocity of light in a moving medium with moving boundaries. The Fabry–Perot method is compared with other techniques such as the VISAR interferometer. We then describe in detail a standard configuration developed at our facilities, discuss other configurations using optical fibers and more than one cylinder lens, and describe a new laser amplifier developed specifically for velocimetry. Methods of alignment, instrument calibration, surface preparation, and operation are included. Next, we present several methods of analysis, the choice of which depends on the absolute accuracy required, and examine many sources of possible error. These analytical techniques allow the motion of surfaces of reflectivity of at least 1% to be measured with an absolute precision of 0.5%. Time resolution can be a few nanoseconds, and is traded off for velocity resolution. One can make continuous velocity records of surfaces whose reflectivity under shock loading decreases to less than 1% of its initial value. We have simultaneously recorded three distinct velocity–time histories without ambiguity.
Bolometric measurements in JT-60
Authors: Nishitani T. , Nagashima K. , Sugiyama T. , Hara M. , Takeuchi H. ,   JT-60 Team
Published: July 2002

Radiation losses from the main plasma of the JAERI Tokamak-60 (JT-60) were measured by a 15-channel bolometer array and those from divertor chambers in the ion and electron drift sides were each measured by two bolometers. The bolometric detector consisting of three layers (5-μm-thick gold absorber, 7-μm-thick polyimide foil, and 0.1-μm-thick gold resistor grid) was developed for this measurement. In typical diverted discharges with NB heating, the ratios of power radiated from the main and divertor chambers to the absorbed power were approximately 10% and 20%, respectively. Intense poloidal asymmetries of radiated power localized near the X point outside the torus were observed in diverted discharges. The profiles of the radiated power were reconstructed by an Abel inversion with an asymmetric term. On the other hand, a poloidally asymmetric radiation band localized near the inner wall, ‘‘marfe,’’ was observed for dense plasmas in limiter discharges with neutral beam heating.
Theory of the gasket in diamond anvil high-pressure cells
Authors: Dunstan D. J.
Published: July 2002

The operation of the gasket in the diamond anvil cell (DAC) is analyzed and some practical conclusions are given, relevant to users of DACs in the submegabar pressure range. Two distinct regimes are found, one where the gasket is at the maximum thickness and the DAC is intrinsically unstable, and the other where the gasket is thin and the cell is stable in operation, so that there are good reasons to use preindented gaskets, of thickness rather less than usually recommended. Force-pressure plots are shown to give valuable information in routine use, and their interpretation is discussed.
A guarded parallel-plate instrument for measuring the thermal conductivity of fluids in the critical region
Authors: Mostert R. , van den Berg H. R. , van der Gulik P. S.
Published: July 2002

A description is given of the construction and operating procedure of a parallel-plate apparatus for the measurement of the thermal conductivity coefficient of fluids. This instrument can be operated at pressures up to 150 MPa and at temperatures down to 77 K. The use of platinum resistance temperature sensors allows measurements with temperature differences between upper and lower plate as small as 1 mK, which together with the small plate separation of 155 μm, makes the instrument suitable for both the normal fluid region as well as for the region very close to the critical point. The complete working equations for the instrument are presented and evaluated.
Origin of the poling-induced optical loss in a nonlinear optical polymeric waveguide
Authors: Teng C. C. , Mortazavi M. A. , Boudoughian G. K.
Published: July 2002

The excess waveguiding optical loss of nonlinear optical polymers induced by poling was studied both experimentally and theoretically. The loss was found to be caused by the inhomogeneities in the orientation of the nonlinear optical moiety after poling and is strongly dependent on the uniformity of the guiding and cladding films and the magnitude of the poling field.
A commercial system for automated nitrogen isotope-ratio analysis by the Rittenberg technique
Authors: Mulvaney R. L. , Fohringer C. L. , Bojan V. J. , Michlik M. M. , Herzog L. F.
Published: July 2002

A sequential sample treatment system is described that permits use of an isotope-ratio mass spectrometer for automated Rittenberg analyses of ammonium salts. This system is based on the design of McInteer and Montoya [Recent Developments in Mass Spectrometry in Biochemistry, Medicine and Environmental Research, edited by A. Frigerio (Elsevier, Amsterdam, 1981), Vol. 7, p. 343]. An x-y table and a set of valves under computer control are used to automatically perform hypobromite oxidations, introduce the N2 thus liberated to the mass spectrometer for isotope-ratio analysis, and effect spent sample pumpaway. Analyses can be performed on samples containing 20 μg of N. When operated with a Nuclide Model 3-60-RMS double-collector mass spectrometer, the standard deviation for measurements at the natural abundance level (12 samples, 50 or 100 μg of N per sample) ranged from 0.0002 to 0.000 06 at. % 15N. Throughput capacity is 100–250 samples per day, and unattended operation can continue for at least three days.
Thermal diffusivity measurements of molten salts using a three-layered cell by the laser flash method
Authors: Ohta Hiromichi , Ogura Gaku , Waseda Yoshio , Suzuki Mustumi
Published: July 2002

A simple cell and easy data processing are described for measuring the thermal diffusivity of a liquid sample at high temperatures using the laser flash method. A cell consists of a liquid sample sandwiched by two metallic plates. The front surface of one metallic plate is exposed to a single pulse of beam laser and the resulting temperature rise of the back surface of the other metallic plate is measured. The logarithmic analysis proposed by James using the initial time region of the temperature response curve of a two layered cell system has been extended to apply to the present three layered cell system in order to estimate the thermal diffusivity value of a liquid sample. Measurements of distilled water and methanol were made first and the results were found to be in good agreement with the reference data. Then, the thermal diffusivities of molten NaNO3 at 593–660 K and of molten KNO3 at 621–694 K were determined and the results also appear to agree reasonably well with those reported in the literature.
Collective laser light scattering from electron density fluctuations in fusion research plasmas (invited)
Authors: Holzhauer E. , Dodel G. ,   ASDEX Team
Published: July 2002

In magnetically confined plasmas density fluctuations of apparently turbulent nature with broad spectra in wave number and frequency space are observed which are thought to be the cause for anomalous energy and particle transport across the confining magnetic field. Collective laser light scattering has been used to study the nature of these fluctuations. Specific problems of scattering from fusion plasmas are addressed and illustrated with experimental results from the 119 μm far infrared laser scattering experiment operated on the ASDEX tokamak. Using the system in the heterodyne mode the direction of propagation with respect to the laboratory frame can be determined. Spatial resolution has bean improved by making use of the change in pitch of the total magnetic field across the minor plasma radius. Special emphasis is placed on the ohmic phase where a number of parameter variations including electron density, electron temperature, toroidal magnetic field, and filling gas were performed.
Far-infrared heterodyne scattering to study density fluctuations on the DIII-D tokamak
Authors: Philipona R. , Doyle E. J. , Luhmann N. C. , Peebles W. A. , Rettig C. , Burrell K. H. , Groebner R. J. , Matsumoto H. ,   the DIII-D Group
Published: July 2002

A far-infrared collective scattering system has been installed and is operational on the DIII-D tokamak. The system consists of a 245 GHz (λ= 1.22 mm) twin frequency laser and uses heterodyne detection techniques which enables us to determine the propagation direction of fluctuations in the laboratory frame of reference. At present, three receiver channels are available to study poloidal propagating fluctuations along the entire midplane of DIII-D in a wave number range of 2≤ k ≤16 cm−1. The beam geometry of the system dictates a wave number resolution of Δk ≊ 0.7 cm−1 and a radial spatial resolution of ± 20 cm at k=10 cm−1. Fluctuation spectra and propagation direction measured during Ohmic operation were found to depend on plasma parameters. Large plasma toroidal rotation was measured which correlated with modifications in the fluctuation spectra. During H-mode operation, a dramatic drop of low-frequency fluctuations (< 250 kHz) is observed coincident with the drop in D-alpha light. This is followed by the appearance of a high- frequency component possibly connected with profile modification and rotation effects.
Insertable biplanar gradient coils for magnetic resonance imaging
Authors: Martens M. A. , Petropoulos L. S. , Brown R. W. , Andrews J. H. , Morich M. A. , Patrick J. L.
Published: July 2002

Insertable planar gradient coils offer the potential for significant performance increases in magnetic resonance imaging through higher gradient strength and shorter rise times. Using variational methods to minimize inductance, and thereby to optimize switching speeds, we have analyzed and constructed a biplanar y-gradient coil for insertion into a solenoidal magnet system where z is the magnet axis. We have also analyzed biplanar x-gradient and z-gradient coil designs using the same methods. These biplanar coils offer an advantage over a cylindrical coil of comparable diameter in that they achieve high gradient strengths with relatively short rise times while maintaining patient access. Although the requirement that the currents for the x gradient lie in the same plane as for the y and z gradients increases the stored energy by a factor of 3 with respect to the other two gradients, this stored energy is still smaller by a factor of 2 than that of a comparably constrained x-gradient cylindrical coil. The biplanar coil design offers improved linearity over its single planar coil alternative. The particular designs we have investigated are generally limited to small-volume imaging.
Picosecond response of photon-drag detectors for the 10-μm wavelength range
Authors: Kesselring R. , Kälin A. W. , Sigg H. , Kneubühl F. K.
Published: July 2002

The response times of nonresonant p-Ge detectors and of resonant photon-drag detectors made from AlxGa1−xAs/GaAs multiquantum well systems are measured with intense 300-ps optical pulses of 10-μm wavelength generated with the aid of an improved laser system based on optical free induction decay. A response time limited by the oscilloscope-amplifier bandwidth is reached with the quantum well systems. Their response time is shorter than that of a p-Ge photon-drag detector. The light-pulse traveling time limits the performance of the large p-Ge photon-drag detectors.
A complete high performance heterodyne interferometer displacement transducer for microactuator control
Authors: Charette Paul G. , Hunter Ian W. , Brenan Colin J. H.
Published: July 2002

There is a need for a high performance displacement transducer to be used in the closed loop control of a microrobot. This article describes a laser heterodyne interferometer-based transducer which resolves displacements to 0.12 nm (standard deviation) locally, in a scanning range of 10 mm or more, in a 1 Hz to 10 kHz bandwidth, under normal laboratory conditions. The device combines the long measurement range of conventional optical fringe counting techniques with the high resolution of subfringe metrology interferometers. The sensor is a complete device including optics, custom-built phase demodulator electronics, and computer algorithm for displacement computations using a novel fast lookup table technique. The lookup table algorithm uses the estimated instantaneous uncertainties on both quadrature outputs from the phase demodulator to improve the accuracy of the displacement computation. The optics and electronics are completely characterized in terms of signal-to-noise characteristics and frequency response. Experimental results are shown of an application where the transducer was used in the closed loop control of a nonlinear actuator.
Neutron spectroscopy with a large neutron time-of-flight detector array (LaNSA) (invited)
Authors: Cable Michael D. , Hatchett S. P. , Nelson M. B.
Published: July 2002

The recent completion of the Large Neutron Scintillator Array (LaNSA) at the Nova Laser Fusion Facility has allowed low yield neutron energy spectra to be measured for ICF implosions. These spectra can be used to determine some fundamental implosion characteristics such as fuel areal density and ion temperature. Details of how neutron energy spectra are measured with an array of single particle detectors will be presented, as well as experimental data showing the application of these spectra to the determination of implosion parameters. Particular emphasis will be placed on measurements of secondary DT neutrons produced from initially pure deuterium fuel. Determination of the interacting triton energy spectrum from secondary neutron energy spectroscopy and its application to studies of fuel-pusher mixing will also be discussed.
Radiation position-sensitive detector based on plastic scintillating fibers
Authors: Ikhlef A. , Skowronek M.
Published: July 2002

The applicability of plastic scintillation fibers as a radiation position detector is evaluated for low-energy x-ray irradiation. A continuous x-ray generator with a copper anode is used as a source radiating very intense lines: Kα2=8.02 keV, Kα1=8.04 keV, Kβ1=8.9 keV. A microchannel intensified tube followed by a vidicon camera is used as the photodetector for scintillation light guided to both extremities of a fiber detector. A one-dimensional position-sensitive radiation detector is constructed as an application. The irradiation position is derived independently of the system operating conditions (gain, sensitivity, efficiency) with a relative error smaller than 4%. Using this device, the optical attenuation length is measured with a relative error less than 10%. The differences in the photon distribution at fiber terminating faces are discussed for light and for x-ray excitations with respect to their incidence angles.
Simple device for drilling holes in metal gaskets for diamond anvil cell experiments
Authors: Sahu P. Ch. , Thomaskutty K. V. , Chandra Shekar N. V. , Yousuf Mohammad , Rajan K. Govinda
Published: July 2002

With a spring-loaded platform mounted on a precision minidrilling machine, it is possible to rapidly and inexpensively drill sharp sample holes down to about 100 μm in hardened metal gaskets for diamond anvil cell experiments.
An automated apparatus for ultrasound velocity measurements improving the pulse-echo-overlap method to a precision better than 0.5 ppm in liquids
Authors: Horváth-Szabó Géza , Ho/iland Harald , Ho/gseth Einar
Published: July 2002

An improvement of the pulse-echo-overlap (PEO) method is proposed to make it accessible for computer control. The modification makes it possible to avoid the systematic and subjective errors of the original PEO method. A digital oscilloscope and a frequency synthesizer is controlled by a personal computer providing a fast automatic method for determination of ultrasonic wave transit times. Description of the method and the computer program, as well as the technical parameters is given. The Panametrics 5053A ultrasonic time intervalometer has been slightly modified to be applicable in the new configuration with a better precision. The 0.5 ppm precision velocity determination in water requires a 0.0003 °C level temperature control and reproducible wettability of the surface of the transducer. For both problems, a practical solution is proposed. Error analysis shows that the reproducibility of measurements is within 0.5 ppm.
A novel computerized viscometer/rheometer
Authors: Jimenez Jorge A. , Kostic M.
Published: July 2002

An innovative, Couette-type viscometer/rheometer was developed, designed, and fabricated with the main objective being to measure viscosity and elastic properties of low-viscous, non-Newtonian, and visco-elastic fluids, like dilute polymer solutions. The goal was to simplify and improve some existing drawbacks of commercial instruments employing several novel design solutions, particularly with regard to instrument precision and sensitivity. With a single pair of cylinders and a torsion bar stiffness of 0.116 Nm/rad, viscosities from 0.5 to 50 000 cP (centi-Poise) in a shear-rate range from 1 to 200 s−1, and oscillatory tests from 0.1 to 10 Hz, could be measured. The transducers’ electronic signals are handled by a software, developed in C language, and an IBM-PC compatible computer with a data acquisition board. The innovative design solutions (use of a cruciform torsion bar, optoelectronics sensors, and a novel alignment procedure) have improved the critical instrument performance and reduced the number and cost of components, customarily used in commercial viscometers/rheometers. The overall steady shear-rate calibration was performed using the standard-viscosity fluids of 51 and 445 cP. Initial results show high precision (around 4%), particularly at higher velocities. The oscillatory testing was performed by measuring the phase shift between the stress and strain for viscoelastic fluids and for the limiting case of a standard Newtonian fluid. The novel design has proved efficient and with potential for further improvements.
Measurement of thermal diffusivity of small, high conductivity samples using a phase sensitive technique
Authors: DeVecchio D. , Russell D. , Taborek P.
Published: July 2002

We have developed a phase sensitive technique for measuring the thermal diffusivity of thin samples which is particularly useful for high conductivity materials such as diamond. The technique is based on a relationship between the experimentally determined frequency dependent phase shift of a thermal wave, and the geometry and thermal diffusivity of the sample. Our analysis correctly describes the phase shift for both thermally thick and thermally thin samples, which greatly expands the useful frequency range of phase shift measurements. The experimental apparatus consists of common laboratory electronics, is relatively easy to implement, and is useful over a wide range of temperatures. Measurements on glass, copper, and diamond samples at room temperature show that the accuracy and precision of the technique compares favorably with standard methods.
Low damage and residue-free dry etching of 6H–SiC using electron cyclotron resonance plasma
Authors: Xie K. , Flemish J. R. , Zhao J. H. , Buchwald W. R. , Casas L.
Published: July 2002

Dry etching-induced surface damage and contamination on SiC have been investigated for electron cyclotron resonance (ECR) plasma etching and conventional reactive ion etching (RIE) using a CF4/O2 gas mixture. Auger electron spectroscopy shows that there is no residue on the ECR etched surfaces and sidewalls of the etched structures. In contrast, the conventional RIE process leaves residues containing large amounts of Al, F, and O impurities on the surfaces and the etched sidewalls. Pd Schottky diodes on the ECR etched surface show a near-ideal diode characteristics with ideality factor of 1.06, indicating a good surface quality. Pd Schottky diodes on the conventional RIE etched surface, however, have a substantially reduced barrier height from 1.05 eV for the as-grown sample to 0.64 eV and a high ideality factor of 1.27, indicating a substantially damaged surface. Significant free-carrier reduction is observed in the RIE etched sample.
A system for time- and angle-resolved photoelectron spectroscopy based on an amplified femtosecond titanium:sapphire laser system
Authors: Karlsson Henrik S. , Chiaia Gualtiero , Karlsson Ulf O.
Published: July 2002

A system for time- and angle-resolved photoelectron spectroscopy based on an amplified femtosecond titanium:sapphire laser system is described. Using this type of system, angle-resolved photoemission is extended to include the possibility of following the time development of excited electrons at and near a solid surface. Time resolution is accomplished by using pump-and-probe technique and the photoemitted electrons are energy analyzed in a time-of-flight detector. In order to perform photoemission, the near-infrared light from the titanium:sapphire laser is frequency up-converted to the vacuum ultraviolet range. This is accomplished by using the high peak power pulses from the laser system to produce short-wavelength radiation by means of harmonic generation. The system described uses cascaded frequency doubling and tripling, reaching a photon energy close to 10 eV.
Monitor for the carbon and oxygen impurities in the ASDEX Upgrade tokamak
Authors: Neu R. , Asmussen K. , Fussmann G. , Geltenbort P. , Janeschitz G. , Schönmann K. , Schramm G. , Schumacher U. ,   the ASDEX Upgrade team
Published: July 2002

A Bragg spectrometer for monitoring impurity radiation of hydrogenlike carbon and oxygen at the tokamak ASDEX Upgrade is presented. The use of fast multistrip proportional counters allows soft x-ray photon counting with count rates above 1 MHz in a simple arrangement. The absolute calibration was performed using a large area x-ray tube with a LiF anode. The concentrations of both impurities are calculated using the coronal model both for ionization and electron impact excitation. The results of the measurements concerning the long term behavior before and after conditioning of the torus walls by boronization as well as the fast plasma processes are presented.
An overview of ITER diagnostics (invited)
Authors: Young Kenneth M. , Costley A. E. ,   Members of the ITER-JCT, ITER Home Teams and the ITER Diagnostics Expert Group
Published: July 2002

The requirements for plasma measurements for operating and controlling the ITER device have now been determined. Initial criteria for the measurement quality have been set, and the diagnostics that might be expected to achieve these criteria have been chosen. The design of the first set of diagnostics to achieve these goals is now well under way. The design effort is concentrating on the components that interact most strongly with the other ITER systems, particularly the vacuum vessel, blankets, divertor modules, cryostat, and shield wall. The relevant details of the ITER device and facility design and specific examples of diagnostic design to provide the necessary measurements are described. These designs have to take account of the issues associated with very high 14 MeV neutron fluxes and fluences, nuclear heating, high heat loads, and high mechanical forces that can arise during disruptions. The design work is supported by an extensive research and development program, which to date has concentrated on the effects these levels of radiation might cause on diagnostic components. A brief outline of the organization of the diagnostic development program is given.
Initial operation of the divertor Thomson scattering diagnostic on DIII-D
Authors: Carlstrom T. N. , Hsieh C. L. , Stockdale R. , Nilson D. G. , Hill D. N.
Published: July 2002

Thomson scattering measurements of ne and Te in the divertor region of a Tokamak are reported. These data are used as input to boundary physics codes such as UEDGE and DEGAS and to benchmark the predictive capabilities of these codes. These measurements have also contributed to the characterization of tokamak disruptions. A Nd:YAG laser (20 Hz, 1 J, 15 ns, 1064 nm) is directed vertically through the lower divertor region of the DIII–D Tokamak. A custom, aspherical collection lens (f/6.8) images the laser beam from 1 to 21 cm above the target plates into eight spatial channels with 1.5 cm vertical and 0.3 cm radial resolution. Two-dimensional mapping of the divertor region is achieved by sweeping the divertor X-point location radially through the fixed laser beam location. Fiber optics carry the light to polychromators whose interference filters have been optimized for low Te measurements. Silicon avalanche photodiodes measure both the scattered and plasma background light. Temperatures and densities are typically in the range of 5–200 eV and 1–10×1019 m−3, respectively. Low temperatures, Te<1 eV, and high densities, ne>8×1020 m−3 have been observed in detached plasmas. Background light levels have not been a significant problem. Reduction of the laser stray light permits Rayleigh calibration. Because of access difficulties, no in-vessel vacuum alignment target could be used. Instead, an in situ laser alignment monitor provides alignment information for each laser pulse. Results are compared with Langmuir probe measurements where good agreement is found except for regions of high ne and low Te as measured by Thomson scattering.
Tomography by the maximum entropy method for the W7-AS and W7-X multichannel bolometer systems
Authors: Giannone L. , Elsner A. , Heinrich O. , Hirsch M. ,   W7-AS team
Published: July 2002

A maximum entropy approach has been applied to the tomographic inversion of the line integrated measurements of radiation power flux by a multichannel bolometer system. This method was applied to an H-mode discharge in the W7-AS stellarator, with increasing radiation from the plasma core after the transition being observed. Also, a neutral beam injection discharge was investigated, and the time evolution of a marfe was documented. This method has also been applied to the preliminary design of a multicamera system for full tomographic reconstruction in the W7-X stellarator. In addition, measurements with wire mesh shielded metal foil bolometers have been carried out and the shielding of these bolometers from microwaves produced by the 140 GHz gyrotrons used for electron cyclotron heating has been demonstrated.
A polycapillary bending and focusing lens for neutrons
Authors: Chen-Mayer H. H. , Mildner D. F. R. , Sharov V. A. , Xiao Q. F. , Cheng Y. T. , Lindstrom R. M. , Paul R. L.
Published: July 2002

A glass polycapillary lens that both bends and focuses a cold neutron beam has been designed and constructed. The bender focuser guides part of the incident beam away from its line of sight and focuses it to a spot of width 0.65 mm at a distance 95 mm from the lens exit and 20 mm below the bottom edge of the beam path, with a gain of 20 in neutron current density. The neutron transmission characteristics of the lens have been determined with two types of position-sensitive detectors, a charge injection device, and an imaging plate. The lens has been tested with prompt gamma measurements on a gadolinium shard and titanium foil.
Phase locked backward wave oscillator pulsed beam spectrometer in the submillimeter wave range
Authors: Lewen F. , Gendriesch R. , Pak I. , Paveliev D. G. , Hepp M. , Schieder R. , Winnewisser G.
Published: July 2002

We have developed a new submillimeter wave pulsed molecular beam spectrometer with phase stabilized backward wave oscillators (BWOs). In the frequency ranges of 260–380 and 440–630 GHz, the BWOs output power varies between 3 and 60 mW. Part of the radiation was coupled to a novel designed harmonic mixer for submillimeter wavelength operation, which consists of an advanced whiskerless Schottky diode driven by a harmonic of the reference synthesizer and the BWO radiation. The resulting intermediate frequency of 350 MHz passed a low noise high electron mobility transistor amplifier, feeding the phase lock loop (PLL) circuit. The loop parameters of the PLL have been carefully adjusted for low phase noise. The half power bandwidth of the BWO radiation at 330 GHz was determined to be as small as 80 MHz, impressively demonstrating the low phase noise operation of a phase locked BWO. A double modulation technique was employed by combining an 80 Hz pulsed jet modulation and a 10–20 kHz source modulation of the BWO and reaching a minimum detectable fractional absorption of 2×10−7. For the first time, a number of pure rotational (Ka=3←2, Ka=4←3) and rovibrational transitions in the van der Waals bending and stretching bands of the Ar–CO complex were recorded.
A comparison of two high performance inverse photoemission bandpass detectors
Authors: Hill I. G. , McLean A. B.
Published: July 2002

By performing inverse photoemission experiments on the same sample at the same time with two different detectors, their performance has been directly compared. The first detector is based on one of the most promising solid-state detector designs. It is comprised of a focused mesh electron multiplier and a CaF2 window. The second detector is a Geiger–Müller tube which uses dimethyl ether and a MgF2 window. Although it has already been demonstrated that detectors based on this design work, the dimethyl ether Geiger–Müller tubes are not widely used, and we show that it is essential to compensate for detector dead time effects for the detector to be practicably useful. Once this is done, the dimethyl ether Geiger–Müller tube has a sensitivity that is approximately 20 times greater than that of the solid-state detector. Furthermore, it is easy to operate and it does not appear to suffer from the problems that are normally associated with iodine Geiger–Müller detectors.
Tokamak de Varenne’s diagnostic neutral beam injector
Authors: Sarkissian A. H. ,   D-NBI Team
Published: July 2002

A fully automated and remote controlled neutral beam injector has been designed, built, and characterized on a test stand for use as a diagnostic aid on Tokamak de Varennes (TdeV). The injector has a duoPIGatron ion source with a LaB6 coaxial cathode. A cusped magnetic field surrounding the plasma chamber is used to enhance the plasma confinement. The beam current is modulated by switching the arc current, and it has a 70 μs rise time. Two, in-house-built, titanium sublimation pumps provide up to 16 000 l s−1 pumping capacity for differential pumping between the source and the tokamak. The injector operates with its source at ground potential, and its neutralizer canal at a negative potential |Vb|⩽40 kV. An electrostatic trap biased to −6 kV with respect to the neutralizer canal retains the electrons in the neutralizer. The residual ions are collected on an ion collector that is biased to −5 kV. The design of the extraction system and the electrostatic trap was optimized with a numerical code. The injector has been operated with hydrogen and helium beams. A multi-aperture three-grid extraction system provides up to 2.4 A of proton or 1.2 A of He+ current. The maximum average proton current density extracted from the 19 extraction holes was 650 mA cm−2 at 35 kV extraction voltage, which was obtained with 60 A of arc current.
Electrical resistance standards and the quantum Hall effect
Authors: Witt Thomas J.
Published: July 2002

This review of electrical resistance standards begins with a description of classical standard resistors and their limitations. Methods of comparing resistance are described; these include bridges based on cryogenic current comparators capable of achieving statistical uncertainties approaching one part in 1010 in the measurement of resistance ratios. Such reproducibility is nearly two orders of magnitude smaller than the overall uncertainty of the most accurate determinations of the ohm from its SI definition via the calculable capacitor. The quantum Hall effect can provide an invariable reference standard of resistance linked to the fundamental physical constants. Many factors, however, limit the accuracy of practical realizations of quantized Hall resistance standards. Ultimately, the accuracy of a specific realization must be confirmed by comparison with similar standards; methods for doing this and the resulting agreement are presented. The ac techniques used in the determinations of the SI ohm by means of the calculable capacitor are now being applied to accurately link the quantized Hall resistance to the impedance of standard capacitors and thereby to provide a new reference standard of capacitance.
Soft-x-ray transmission photoelectron spectromicroscopy with the MEPHISTO system
Authors: De Stasio Gelsomina , Gilbert B. , Perfetti Luca , Fauchoux O. , Valiquer A. , Nelson T. , Capozi M. , Baudat P. A. , Cerrina F. , Chen Z. , Perfetti P. , Tonner B. P. , Margaritondo G.
Published: July 2002

We complemented with data taken in transmission mode the recently described tests of the novel spectromicroscope MEPHISTO (Microscope à Emission de Photoélectrons par Illumination Synchrotronique de type Onduleur). Transmitted x rays were converted by a photocathode into photoelectrons, which were subsequently electron-optically processed by the spectromicroscope producing submicron-resolution images. Test images demonstrated excellent contrast.
Direct measurement of the resistivity weighting function
Authors: Koon D. W. , Chan Winston K.
Published: July 2002

We have directly measured the resistivity weighting function—the sensitivity of a four-wire resistance measurement to local variations in resistivity—for a square specimen of photoconducting material. This was achieved by optically perturbing the local resistivity of the specimen while measuring the effect of this perturbation on its four-wire resistance. The weighting function we measure for a square geometry with electrical leads at its corners agrees well with calculated results, displaying two symmetric regions of negative weighting which disappear when van der Pauw averaging is performed.
Introduction to time and frequency metrology
Authors: Levine Judah
Published: July 2002

In this article, I will review the definition of time and time interval, and I will describe some of the devices that are used to realize these definitions. I will then introduce the principles of time and frequency metrology, including a discussion of some of the types of measurement hardware in common use and the statistical machinery that is used to analyze these data. I will also introduce various techniques of distributing time and frequency information, with special emphasis on the global positioning system satellites. I will then discuss the advantages of clock ensembles and a prototype time-scale algorithm. I will conclude with a discussion of how clocks are synchronized to remote servers using noisy and poorly characterized transmission channels.
Nonmagnetic piston–cylinder pressure cell for use at 35 kbar and above
Authors: Walker I. R.
Published: July 2002

A piston–cylinder cell has been constructed which makes it possible to achieve pressures of over 35 kbar at room temperature, and more than 29 kbar in a cryogenic environment. The use of a nonmagnetic cobalt–nickel alloy with an unusually high yield strength (1.97 GPa at 293 K) allows the cell to be used in the presence of large magnetic fields. It seems likely that the cobalt–nickel material can be used as a direct replacement for the celebrated (but largely unavailable) Russian nickel alloy in many applications where nonmagnetic pressure cells are needed. Such applications may also include other kinds of high pressure apparatus, such as diamond anvil and sapphire anvil cells. The pressure cell described in the present article employs the “Teflon cap” technique, and detailed information concerning the application of this method is given which can be used with a wide variety of cells of the piston–cylinder type.
High resistivity and picosecond carrier lifetime of GaAs implanted with MeV Ga ions at high fluences
Authors: Jagadish C. , Tan H. H. , Jasinski J. , Kaminska M. , Palczewska M. , Krotkus A. , Marcinkevicius S.
Published: July 2002

Rutherford backscattering spectrometry-channeling, double-crystal x-ray diffraction, optical absorption studies, and electrical measurements have been carried out on 2 MeV Ga-implanted GaAs at fluences of 1×1015 and 5×1015 cm−2. Implanted samples exhibited a strain field associated with implantation-induced damage, low resistivity due to hopping conduction and mobilities of about 1 cm2/V s. Annealing of these samples at 600 °C caused substantial recovery of postimplant damage and an increase of resistivity of more than four orders of magnitude, with mobility up to about 2600 cm2/V s. Photocarrier lifetime of annealed samples is in the few picoseconds range. These properties are strikingly similar to those of arsenic implanted GaAs, suggesting that arsenic precipitates are unlikely to be solely responsible for short carrier lifetime in the latter case. Thus Ga-implanted GaAs may be an interesting prospect for fast optoelectronic device applications.