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Review of Scientific Instruments

Issue 10 • Date Oct 2002

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Displaying Results 1 - 25 of 46
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Electric probes for plasmas: The link between theory and instrument

    Page(s): 3409 - 3439
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    Electric probe methods for diagnostics of plasmas are reviewed with emphasis on the link between the appropriate probe theories and the instrumental design. The starting point is an elementary discussion of the working principles and a discussion of the physical quantities that can be measured by the probe method. This is followed by a systematic classification of the various regimes of probe operation and a summary of theories and methods for measurements of charged particle distributions. Application of a single probe and probe clusters for measurements of fluid observables is discussed. Probe clusters permit both instantaneous and time-averaged measurements without sweeping the probe voltage. Two classes of applications are presented as illustrations of the methods reviewed. These are measurements of cross sections and collision frequencies (plasma electron spectroscopy), and measurements of fluctuations and anomalous transport in magnetized plasma. © 2002 American Institute of Physics. View full abstract»

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  • Ultrabroadband collection and illumination optics for Raman and photoluminescence spectroscopy in the 200–700 nm wave band

    Page(s): 3440 - 3448
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    The design and construction of an optical instrument is described to enable accurate and reliable resonance Raman scattering and photoluminescence measurements to be acquired in the 200–700 nm (1.77–6.2 eV) wave band, with minimum user intervention between changes of the excitation wavelength. An f/6 line or point focus is formed at the sample. f/2 achromatic catadioptric optics image the scattered light to the entrance slit of an f/7.5 spectrometer. An ultraviolet sensitive charge coupled device camera enables imaging and alignment of samples within the excitation laser region, providing an imaging resolution of better than 13 μm (results presented elsewhere suggest a resolution limit of 7 μm). The use of an aperture plate allows pseudo-confocal operation, leading to considerable improvement in depth discrimination when interrogating extended volume scatterers. This mode of operation furthermore eliminates stray light derived from the Rayleigh line, thereby enabling line focus spectra to be obtained to very low wave numbers. The optics present a unique method of interrogating a sample with an extended lateral and well-defined axial interaction region with a high collection efficiency (optical throughput of ≫41%), simultaneously offering broadband achromatic spectral response in collection from 200 to 700 nm. © 2002 American Institute of Physics. View full abstract»

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  • Frequency stabilization of an extended cavity semiconductor diode laser for chirp cooling

    Page(s): 3449 - 3453
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    We present a technique to stabilize in an atomic transition the chirping frequency of a narrow-band semiconductor diode laser. The technique is demonstrated to chirp-cool 85Rb atoms used for loading a magneto-optical trap. The stabilization process eliminates long-term fluctuations and drifts in the number of atoms caught in the trap. This is a simple, easy-to-implement, and robust method for wide range of laser cooling experiments employing frequency chirping. © 2002 American Institute of Physics. View full abstract»

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  • Automated flexure testing of axially rotated optical fiber gratings

    Page(s): 3454 - 3457
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    The design and performance of an automated system for flexure testing of optical fiber gratings is described. In addition to flexing (bending) a fiber grating through a specified curvature range, the system can change the axial rotational orientation of the grating relative to the plane of curvature. Flexure is accomplished by holding the grating against a smooth plastic platform. A linear stage deflects the center of the pinned platform to create a curved surface. Two small weights, hanging on the optical fiber, hold the fiber grating against the curved platform and provide constant tension on the optical fiber over the entire range of curvatures. The fiber grating is axially rotated to a different orientation by returning the platform to zero curvature, removing the weights from the optical fiber, and rotating the optical fiber about its axis using a pair of rotation stages. After replacing the weights on the optical fiber, flexure testing resumes at the new axial rotational orientation. The grating transmission spectrum during flexure is monitored with a broadband source and optical spectrum analyzer. All of these steps are done in an automated fashion (unattended) under computer program control. The testing system allows complete characterization of the fiber grating response for curvatures from 0 to 4 m-1 and for axial rotational orientations from 0° to 360°. Use of this automated test fixture eliminates the need for human intervention during the measurements and greatly decreases the testing time while still allowing complete characterization of the flexure response with axial rotational orientation as a parameter. Additional advantages of the testing system include an absolute zero curvature starting position, an absence of hysteresis effects, and accurate frictionless rotation. © 2002 American Institute of Physics. View full abstract»

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  • Compact focusing von Hamos spectrometer for quantitative x-ray spectroscopy

    Page(s): 3458 - 3463
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    A compact focusing crystal spectrometer based on the von Hamos scheme is described. Cylindrically curved mica and graphite crystals with a radius of curvature of R=20 mm are used in the spectrometer. A front illuminated charge-coupled device (CCD) linear array detector makes this spectrometer useful for real-time spectroscopy of laser-produced plasma x-ray sources within the wavelength range of λ=1.8–10 Å. Calibration of crystals and the CCD linear array makes it possible to measure absolute photon fluxes. X-ray spectra in an absolute intensity scale were obtained from Mg, Ti, and Fe laser-produced plasmas, with a spectral resolution λ/δλ=800–2000 for the mica and λ/δλ=200–300 for graphite crystal spectrometers. The spectrometer has high efficiency in a wide spectral range, it is compact (40 mm diam, 150 mm length), easy to align, and flexible. The spectrometer is promising for absolute spectral measurements of x-ray radiation of low-intensity sources (femtosecond laser-produced plasmas, micropinches, electron-beam–ion-trap sources, etc.). © 2002 American Institute of Physics. View full abstract»

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  • The design and performance of an x-ray micro-focusing system using differentially deposited elliptical mirrors at the National Synchrotron Light Source

    Page(s): 3464 - 3468
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    The National Synchrotron Light Source X13 straight section houses two insertion devices: a small-gap undulator (IVUN) and an elliptically polarized wiggler. The low vertical β function at the center of the straight section, where IVUN is located, produces an extremely small vertical source size [full width at half maximum (FWHM)=15.5 μm]. To take advantage of the small source size for a wide range of x-ray microbeam applications, a Kirkpatrick–Baez pair of 10 cm long elliptically figured mirrors, fabricated using a differential-deposition technique, was designed and implemented. A FWHM focused spot size of 3 μm (vertical) by 9 μm (horizontal) was achieved. The performance of this microfocusing system and selected applications are reported. © 2002 American Institute of Physics. View full abstract»

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  • A cryostat for low-temperature spectroscopy of condensable species

    Page(s): 3469 - 3473
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    A simple experimental setup for the production of cold samples for spectroscopy is described. The samples are deposited under vacuum on a cold metallic surface whose temperature is controlled between 80 and 323 K by varying the heat flow balance between a liquid nitrogen reservoir and a power transistor. Tests of temperature stability and thermal inertia, as well as a set of reflection–absorption infrared and thermal desorption spectra, are reported as a demonstration of the performance of the system. © 2002 American Institute of Physics. View full abstract»

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  • The Torsatron TJ-K, an experiment for the investigation of turbulence in a toroidal low-temperature plasma

    Page(s): 3474 - 3481
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    The Torsatron TJ-K is a toroidal device operated with a magnetically confined low-temperature plasma to study turbulence and wave propagation. The magnetic configuration is the same as for a fusion plasma device. However, the plasma parameters are such that probes can be used in the entire plasma cross section to measure the detailed spatiotemporal structure of the turbulence. At the same time, the dimensionless parameters governing turbulence are in the same range as those of the fusion plasma edge. The objective of the experiment is to obtain data, which allow a close comparison with drift-wave turbulence simulations. Furthermore, the propagation of the helicon heating wave in a toroidal geometry as well as excited gap Alfvén modes will be studied. The article presents the experimental setup and the diagnostics. It is demonstrated that the TJ-K plasma has parameters and profiles relevant to achieve the objectives of the project. First results of turbulence and wave studies are presented. © 2002 American Institute of Physics. View full abstract»

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  • Measurements of magnetic field fluctuations using an array of Hall detectors on the TEXTOR tokamak

    Page(s): 3482 - 3489
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    Hall detectors have been used to measure the magnetic field together with its’ fluctuations in the boundary of a tokamak. The results show, that the measurements which have been performed so far, mainly by use of coils together with subsequent integration, either on-line or later by computer, can be substituted by Hall probe measurements giving the desired value of B directly. Because the integration of the coil signal becomes more and more difficult with long pulses, Hall detectors may give advantages in future fusion devices. We implemented a stack of nine Hall detectors mounted on three planes on a rod in such a way, that the three components of the magnetic field can be measured. To avoid capacitive and charge pickup from the plasma, the probes are electrically shielded. The damping due to skin effect within this shield has been taken into account. The probes have been calibrated using a known magnetic field of a straight wire driven with a LC bank. This field has been precisely measured with a Rogowski coil. The dependence of the Hall coefficient on the frequency has been measured and the pickup in the feeds due to has been withdrawn from the results. We demonstrate the method with measurements on the TEXTOR tokamak, where we could clearly detect the small stray fields associated with magnetohydrodynamic (MHD) fluctuations. On TEXTOR we have been able to detect the MHD activity preceding discharge disruptions as well as the precursors of the so called sawteeth. The results are compared to those of other diagnostics on TEXTOR as, e.g., magnetic loops and electron cyclotron emission, and they do well compare. © 2002 American Institute of Physics. View full abstract»

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  • Development of a fast valve for mitigating disruptions in tokamaks

    Page(s): 3490 - 3493
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    In support of our disruption mitigation profram, a fast gas valve has been constructed and tested on TEXTOR at FZJ Juelich. Its main features have been shown to be: (1) rapid response time: 0.5 ms; (2) amount of injected gas: variable, 2–1000 mbar×l; (3) linear dependence of the number of injected particles on the gas pressure; (4) capability of working in a strong magnetic field; (5) sort of gas: any. The valve has the standard CF 35 flange, commonly used in vacuum engineering. All the components that have contact with vacuum were made of stainless steel, except for the closing aluminum piston. To prevent gas leaking directly from the bottles to the experimental vessel there are also two safety valves, closing the bottles before the shot. The required control equipment includes a high power supply and the combined controller for the safety valves and baratrons, both being able to work with TTL control signals. During tests and experiments on TEXTOR and ASDEX-Upgrade, the valve showed successful operation with three gas types: He, Ne, Ar. © 2002 American Institute of Physics. View full abstract»

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  • An on-wafer probe array for measuring two-dimensional ion flux distributions in plasma reactors

    Page(s): 3494 - 3499
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    In plasma etching processes, the spatial distribution of ion flux across the wafer surface determines the uniformity and profile evolution when etching is ion limited. We have designed and built a two-dimensional array of planar Langmuir probes on a 200 mm diameter silicon wafer to measure the radial (r) and azimuthal (θ) variation of ion flux impinging on the wafer surface in plasma etching reactors. Herein we demonstrate the use of this probe array to obtain two-dimensional ion flux distributions in Ar, Cl2, and Cl2/HBr/He discharges in an inductively coupled plasma reactor. The results obtained using the probe array are in good agreement with Langmuir probe measurements but also reveal azimuthal asymmetries, due to irregularities in chamber geometry such as the pumping port and radio frequency coil configuration, that cannot be detected using radially movable Langmuir probes. The probe array can also be used to investigate the spatiotemporal fluctuations of the ion flux in the 1–100 Hz range. © 2002 American Institute of Physics. View full abstract»

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  • Momentum flux measuring instrument for neutral and charged particle flows

    Page(s): 3500 - 3507
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    An instrument to measure the momentum flux (total pressure) of plasma and neutral particle jets onto a surface has been developed. While this instrument was developed for magnetized plasmas, the concept works for nonmagnetized plasmas as well. We have measured forces as small as 10-4 N on a surface immersed in the plasma where small forces are due to ionic and neutral particles with kinetic energies on the order of a few eV impacting the surface. This instrument, a force sensor, uses a target plate (surface) that is immersed in the plasma and connected to one end of an alumina rod while the opposite end of the alumina rod is mechanically connected to a titanium beam on which four strain gauges are mounted. The force on the target generates torque, causing strain in the beam. The resulting strain measurements can be correlated to a force on the target plate. The alumina rod electrically and thermally isolates the target plate from the strain gauge beam and allows the strain gauges to be located out of the plasma flow while also serving as a moment arm of several inches to increase the strain in the beam at the strain gauge location. These force measurements correspond directly to momentum flux and may be used with known plasma conditions to place boundaries on the kinetic energies of the plasma and neutral particles. The force measurements may also be used to infer thrust produced by a plasma propulsive device. Stainless steel, titanium, molybdenum, and aluminum flat target plates have been used. Momentum flux measurements of H2, D2, He, and Ar plasmas produced in a magnetized plasma device have been performed. View full abstract»

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  • A low-temperature ultrahigh vacuum scanning force microscope with a split-coil magnet

    Page(s): 3508 - 3514
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    We present the design of a scanning force microscope, which is optimized for magnetic force microscopy experiments. It can be operated at temperatures down to 5.2 K, in ultrahigh vacuum, and in magnetic fields of up to 5 T. Cooling is provided by a liquid helium bath cryostat, and the magnetic field is generated by a superconducting split-coil magnet. The design allows easy access from the side through a shutter system for fast in situ tip and sample exchange, while the microscope stays at temperatures below 25 K. The microscope itself features an all-fiber interferometric detection system, a 7.5×7.5 μm2 scan area at 5.2 K, and an xy table. The topographic resolution is demonstrated by imaging monoatomic steps on a nickel oxide surface, while magnetic contrast is shown on cobalt platinum multilayers and on a manganite perovskite film. © 2002 American Institute of Physics. View full abstract»

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  • A scanning Hall probe microscope for large area magnetic imaging down to cryogenic temperatures

    Page(s): 3515 - 3519
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    We describe the design, construction, and operation of a large area scanning Hall probe microscope that is built around a closed-cycle refrigerator capable of cooling samples down to 35 K. The system operates in a custom vacuum housing that utilizes a flexible vacuum bellows to link to external stepping motor positioning stages allowing x–y scans of 5 mm×5 mm under vacuum at 35 K, increasing to 25 mm×25 mm in atmospheric pressure at 300 K and in magnetic fields up to 0.06 T. The Hall probe can be viewed with a microscope through a window and lowered to the desired scanning height and tilted with micrometer screws. The system has an effective lateral spatial resolution of 1.25 μm and a minimum detectable field of ∼0.08 μT/√Hz. We describe the performance of the system and illustrate its potential with images of the local magnetic induction at the surface of various magnetic recording media at room temperature as well as an array of superconducting YBa2Cu3O7-δ squares at 40 K. © 2002 American Institute of Physics. View full abstract»

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  • Using the scanning probe microscope to measure the effect of relative humidity on sample stiffness

    Page(s): 3520 - 3524
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    The ability to manipulate environmental conditions while obtaining nanoscale information about a sample is invaluable. The current article describes the construction and use of a chamber to control the relative humidity while using the scanning probe microscope (SPM) to monitor changes in sample stiffness. Although the experimental setup described is simple, this work shows that the complex relationship between temperature and relative humidity needs to be fully understood for successful operation of the apparatus. In TappingMode™, increases in relative humidity caused the free amplitude of the oscillating cantilever to decrease. Force–distance measurements taken on glass, however, showed no changes in detector sensitivity, suggesting that force–distance mode could be used to measure changes sample stiffness as a function of relative humidity. Using the SPM in force–distance mode, three moisture-sensitive materials (gelatine, wool fibers, and cotton fibers), were examined as the relative humidity was gradually increased from ambient conditions to just under 100%. The results showed that each of the materials softened as the humidity increased and that the experiments were reproducible. From the data, it was estimated that gelatine passed through a glass transition at about 86% relative humidity at room temperature. Wool passed through a glass transition at about 85% relative humidity, which is consistent with previous findings. © 2002 American Institute of Physics. View full abstract»

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  • Instrumentation for direct, low frequency scanning capacitance microscopy, and analysis of position dependent stray capacitance

    Page(s): 3525 - 3533
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    We describe instrumentation for scanning capacitance microscopy (SCM), using an atomic force microscope, that is designed to make calibrated, low frequency measurements of tip–sample capacitance and spreading resistance of thin dielectric films. We also characterize spatial variations in stray capacitance Cstray that must be understood before such measurements can be interpreted. Existing SCM circuits are generally optimized for measuring dC/dV, and not for calibrated low frequency measurements of absolute capacitance. Our circuit uses a commercially available current amplifier and low frequency (∼5 kHz) lock-in detection. This circuit adds an inverted, constant amplitude current to suppress the stray displacement current from the large (∼0.5 pF) stray capacitance Cstray between the sample and the mm-sized cantilever–probe assembly. The capacitance noise spectrum is

    ∼0.35 aF/ Hz
    and is flat down to 1 Hz measurement frequency, with an integrated noise ≪5 aF integrated over a 1–160 Hz bandwidth. We have also used this instrumentation to identify and characterize significant (≫1 fF) variations in Cstray that must be understood in order to accurately measure aF-level variations in the nm-scale tip–sample capacitance. We find that Cstray varies with lateral probe position. This is due to tilting of the probe assembly as the piezoelectric scanner tube bends during scanning. We also find that Cstray varies significantly with probe–assembly height. This causes topography related artifacts in capacitance images of rough surfaces. However, we show that stray cap- acitance artifacts can be mostly eliminated by properly characterizing position and height dependent variations in Cstray and subtracting them from measured capacitance data. © 2002 American Institute of Physics. View full abstract»

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  • Surface plasmon resonance sensor based on the measurement of differential phase

    Page(s): 3534 - 3539
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    A differential phase detection technique has been used for measuring the optical phase change associated with the surface plasmon resonance (SPR) effect that exists in a glass-metal-dielectric stack. A typical prism-coupled SPR setup was constructed and a Mach–Zehnder interferometer was used to perform interferometric analysis between the two orthogonal polarizations in the exit beam. By stepping the optical phase of the reference arm, one can measure the phase change caused by the SPR effect. Since the reference and signal beams traverse identical optical paths except for the section where phase detection is performed, we expect that this scheme can be more robust in terms of noise immunity. The interrogation area can be enlarged to enable imaging of the SPR sensing surface. Initial phase measurement obtained from a salt-water mixture is presented to demonstrate the operation of this technique. Our SPR sensing scheme provides a refractive index measurement sensitivity of 3.1×10-6RIU which is comparable to the value reported for reflectivity-based SPR systems. The measured phase drift, which is a good indicator for the stability of the system, is within 6.6×10-3rad/h. Within our experiments, the effect of the incident angle deviated from resonance on the sensitivity of SPR system has also been studied, thus demonstrating that the choice of incident angle is an important factor for achieving highest sensitivity. © 2002 American Institute of Physics. View full abstract»

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  • Differential phase-shifting interferometry for in situ surface characterization during solution growth of crystals

    Page(s): 3540 - 3545
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    We have developed a phase-shifting interferometer for high-resolution in situ imaging of the interfacial morphology during the growth of fast-growing crystals from solution. We demonstrate that the evolution of the surface morphology can be captured as the height distribution over surface areas as large as 2×2 mm2 with a depth resolution of 3 nm and a lateral resolution down to 0.5 μm with a frequency of 10–12 surface images per second. We describe the five image phase-shifting algorithm and subsequent processing, which quantify the surface morphology and yield the height differences between surface features. We illustrate the application of the technique to the (101) face of potassium di-hydrogen phosphate crystals. We visualize and monitor the formation of step bunches on the surface of this crystal during growth. © 2002 American Institute of Physics. View full abstract»

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  • Characterization and model of ferroelectrics based on experimental Preisach density

    Page(s): 3546 - 3552
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    In this article an experimental method of characterization and modeling of ferroelectric materials is presented. The reversible and irreversible contributions of polarization are separated. The measurements of these effects are performed simultaneously giving a perfect separation. Investigations on switching behavior under different electric field strengths permit final representation of totally irreversible effects by a two-dimensional (2D) function. This function, a Preisach-type density, allows us to extract traditional information such as remanent polarization, coercive field and so on. Then, this curve is fitted to a 2D Gaussian distribution in order to provide easy implementation in simulators. Finally, a physical model is considered to interpret this experimental function in terms of switching mechanism, leading to a powerful tool for future investigation, e.g., the origin of aging. © 2002 American Institute of Physics. View full abstract»

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  • Time domain reflectrometry: Calibration techniques for accurate measurement of the dielectric properties of various materials

    Page(s): 3553 - 3562
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    In this work, an extensive theoretical and experimental analysis has been carried out on the main factors that affect the accurate evaluation of dielectric properties using time domain reflectometry (TDR). Dielectric constant measurements have been performed under different experimental conditions for various types of materials having a wide range of permittivity values. Interpretation of the results on the basis of the transmission-line theory emphasizes the importance of a suitable calibration procedure that takes into account several critical aspects, some of which seem to have been disregarded or underestimated in the literature. Qualitative and quantitative information is given on the kind of corrections needed in order to significantly improve the reliability of the TDR technique for permittivity measurements. © 2002 American Institute of Physics. View full abstract»

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  • Pulsed inductive microwave magnetometer

    Page(s): 3563 - 3569
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    We describe the apparatus, software, and measurement procedures for a pulsed inductive microwave magnetometer (PIMM). PIMM can measure the dynamical properties of materials used in recording heads for magnetic storage applications, and it can be used as a general magnetodynamics diagnostic tool. PIMM uses a coplanar waveguide as both a source of fast pulsed magnetic fields and as an inductive flux sensor. Magnetic field pulses are provided by a 10 V, 55 ps risetime pulse generator; a 20 GHz digital sampling oscilloscope is used to acquire the fast pulse data; and orthogonal Helmholtz pairs provide the bias and saturating fields required for the measurement. The system can measure dynamical behavior as a function of several variables, including applied magnetic bias field, magnetic pulsed field amplitude and width, and sample orientation. Using a fast Fourier transform, PIMM can determine the frequency dependence of the complex magnetic permeability, as well as the step and impulse responses of the magnetic system. Data from 50 nm Ni–Fe and rare-earth-doped Ni–Fe thin films are presented. View full abstract»

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  • Multichannel collimator for structural investigation of liquids and amorphous materials at high pressures and temperatures

    Page(s): 3570 - 3574
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    A new multichannel collimator optimized for Paris–Edinburgh press geometry and a two-dimensional imaging plate detector have been developed at beamline ID30 at the European Synchrotron Radiation Facility. This system allows in situ collection of high quality x-ray diffraction data of liquids and amorphous materials simultaneously at high pressure and high temperature conditions. Excellent improvements in terms of signal to background ratio have been obtained. In order to illustrate the potential of this new device, preliminary results on liquid tin and lead under high pressure are presented. © 2002 American Institute of Physics. View full abstract»

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  • Diffusing wave spectroscopy method based on high-speed charge coupled device for nonergodic systems of electrorheological fluids

    Page(s): 3575 - 3578
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    Diffusing wave spectroscopy (DWS) measurement has been developed to work with a high-speed charge coupled device (CCD). A real-time measurement of autocorrelation function for a nonergodic system was performed to study statistical interaction between particles of electrorheological fluids. As a test, DWS measurements on an ergodic system were achieved using both a high-speed CCD and single-mode fiber, and the deviation of the characteristic decay times using these two methods is less than 2% for glass bead/silicone oil suspension. In the method of DWS based on a high-speed CCD, the multispeckle measurement enables one to do the ensemble average directly. The CCD with the highest rate of 8000 frames/s makes it possible to investigate the short-time behavior of nonergodic systems. The structure response time is obtained by measuring the light transmittance through samples under different electric fields. By analyzing diffusion coefficient of a sample, the force response time is also obtained. © 2002 American Institute of Physics. View full abstract»

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  • A constant-velocity Mössbauer spectrometer with controlled temperature sweep

    Page(s): 3579 - 3583
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    A constant-velocity Mössbauer spectrometer with controlled temperature sweep of the sample is presented. The equipment was developed for Mössbauer thermal scanning experiments, allowing the determination of the evolution of the magnetic hyperfine field with temperature. It is based in a three level architecture implemented in a medium sized personal computer (PC). The instrument runs the widely used Linux operating system, supporting several custom microprocessor-based boards, extending the hardware capabilities of the PC. This scheme optimizes the required time for the experiment, exploiting in the background the full capabilities of modern multitasking operating systems in terms of networking, graphical user interfaces, and data storage. The prototype was tested applying the method to the simplest case, i.e., a symmetric sextet collapsing into a singlet. It was found that required times are an order of magnitude smaller than those demanded by the conventional methodology. The results obtained for the 57Fe temperature hyperfine field dependence are in good agreement with those obtained by conventional Mössbauer spectroscopy. More complex situations must be explored both experimentally and theoretically. It must be mentioned that the presented layout is actually a general tool for automation of experiments at a relative low cost. © 2002 American Institute of Physics. View full abstract»

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Review of Scientific Instruments, published by the American Institute of Physics, is devoted to scientific instruments, apparatus, and techniques.

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Albert T. Macrander
Argonne National Laboratory