Showing papers in "Review of Scientific Instruments in 1988"

Cavity ring‐down optical spectrometer for absorption measurements using pulsed laser sources

Abstract: We have developed a technique which allows optical absorption measurements to be made using a pulsed light source and offers a sensitivity significantly greater than that attained using stabilized continuous light sources. The technique is based upon the measurement of the rate of absorption rather than the magnitude of absorption of a light pulse confined within a closed optical cavity. The decay of the light intensity within the cavity is a simple exponential with loss components due to mirror loss, broadband scatter (Rayleigh, Mie), and molecular absorption. Narrowband absorption spectra are recorded by scanning the output of a pulsed laser (which is injected into the optical cavity) through an absorption resonance. We have demonstrated the sensitivity of this technique by measuring several bands in the very weak forbidden b1Σg−X3Σg transition in gaseous molecular oxygen. Absorption signals of less than 1 part in 106 can be detected.

Application of a position-sensitive detector to atom probe microanalysis

Abstract: A position‐sensitive detector system based on a wedge‐and‐strip anode has been used to build a short flight‐path atom probe which identifies both the chemical nature and position of single atoms field evaporated from the surface of a field‐ion specimen. The detector also allows digitized field‐ion images to be obtained from the region being analyzed. The prototype instrument has a lateral resolution during analysis of substantially below 1 nm, and a depth resolution of one atomic layer. Initial applications of the instrument to the analysis of nanometer‐scale precipitates in metallic alloys has shown the capability of reconstructing the three‐dimensional microstructure and microchemistry of materials.

Force microscope using a fiber‐optic displacement sensor

Abstract: A force microscope is described which uses a fiber‐optic interferometer as the cantilever displacement sensor. Low thermal drift and reduced susceptibility to laser frequency variation are achieved due to the small (several micrometer) size of the interferometer cavity. A sensitivity of 1.7×10−4 A/(Hz)1/2 is observed for frequencies above 2 kHz. The drift rate of the sensor is on the order of 3 A/min. As an initial demonstration, laser‐written magnetic domains in a thin film sample of TbFeCo were imaged.

Wide temperature range IR spectroscopy cell for studies of adsorption and desorption on high area solids

Abstract: A new design for an infrared cell useful for studies of the spectrum of surface species on high area solids is presented. The cell is well suited over a wide temperature range (100–1000 K). Other demonstrated features of the cell include ultrahigh‐vacuum operation, temperature control to ±1 K, linear and rapid temperature programmability and low‐temperature gradients across the powdered sample. The method of sample preparation and support minimizes both heat and mass transport effects. A detailed literature search of previous infrared cell designs is included. Results of the application of the new cell design to the high‐temperature dehydroxylation of Al2O3 are given as an example of the performance.

Variable temperature scanning tunneling microscope

Abstract: A thermally compensated tube scanner scanning tunneling microscope utilizes two concentric piezoelectric tubes, one for scanning and one for coarse translation as well as fine adjustment of sample position while in tunneling range. There are no mechanical components such as springs, levers, gears, or stepper motors which are known to result in considerable vibration sensitivity and thermal drift. Consequently, the standard mode of atomic resolution operation for the device is without vibration isolation and with a thermal drift of less than 1 angstrom per hour.

Advanced plasma fluctuation analysis techniques and their impact on fusion research (invited)

Abstract: This article reviews digital spectral analysis techniques that yield experimental insight into plasma turbulence. Methods to quantify the statistical properties of the fluctuations and to measure the particle and heat flux caused by electrostatic fluctuations are presented. Furthermore, analysis techniques to study the nonlinear coupling process of turbulence and the redistribution of energy among the different modes are discussed. The impact of the analysis techniques on fusion research is demonstrated with experimental results collected with Langmuir probes, heavy‐ion beam probes, and laser scattering in the tokamak TEXT. Special emphasis is given to the characterization of the wavenumber distribution and the correlation lengths in all toroidal directions, including a first measurement of k∥ in a tokamak.

New display‐type analyzer for the energy and the angular distribution of charged particles

Abstract: A new principle for analyzing the kinetic energy as well as the angular distribution of charged particles has been discovered. A new display type analyzer was constructed according to theory. It consists of an inner hemispherical grid and an outer hemispherical electrode which has a radius two times larger than that of the inner grid. Charged particles with the same kinetic energy radiate from one point to all directions inside the inner grid, are repelled by the electric field between the two spheres, and converge at the point which is symmetric to the center of the hemispheres. When an aperture is put at the point, the angular distribution of the charged particles can be observed using a two‐dimensional detector which is positioned out of the aperture. The characteristics of the analyzer are (1) the acceptance cone is ultimately wide, for instance, 2π sr for the charged particles emitted from a flat surface, (2) the pattern obtained is not distorted, and (3) the two electrodes are very simple and easy to construct.

Ultrafast emission spectroscopy in the ultraviolet by time‐gated upconversion

Abstract: We have built a new apparatus to time resolve ultrafast fluorescence following ultraviolet excitation. A synchronously pumped dye laser produces optical pulses of 1‐ps or 70‐fs full‐width half‐maximum, depending upon dyes and optical configuration. These pulses are amplified at a 8.2‐kHz repetition rate using a copper vapor laser‐pumped multipass amplifier. The resulting amplified laser pulses are frequency doubled to obtain ultrafast pulses in the ultraviolet. This ultraviolet light is used to electronically excite a sample; the resulting fluorescence is time resolved using fluorescence upconversion as the optical gating technique. A minimum 300‐fs full‐width half‐maximum instrument response function is obtained. After a brief introduction, we discuss the principles involved in this method of time resolving fluorescence. We review special considerations for femtosecond laser experimentation, and discuss the construction of our apparatus. Finally, as an example, we show how this system can be used to stud...

Photoelectron microscopy with synchrotron radiation

Abstract: Real‐time images produced by photoelectrons from metal and semiconductor samples have been generated using synchrotron radiation. A single electrostatic objective lens coupled to a two‐stage image intensifier has been used to evaluate the feasibility of combined imaging and spectroscopy with photoelectrons in the ultraviolet and soft x‐ray regions. Images were generated using photon energies ranging from 9 to 160 eV. The use of monochromatic synchrotron radiation provides a unique capability for generating image contrast by digital subtraction of images formed with photon energies above and below a core‐level binding energy. A simple electron‐optical objective lens produces images with a few microns resolution, and adequate sensitivity to image 45‐eV kinetic energy electrons using bending magnet radiation. Calculations based on these experiments show that comparable resolutions are possible for combined microscopy and spectroscopy with electrons of order 100‐eV kinetic energy using currently available bending magnet synchrotron radiation. Calculations for expected performance from undulator sources are presented.

Microwave multipolar plasmas excited by distributed electron cyclotron resonance: concept and performance

Abstract: Distributed electron cyclotron resonance or DECR, a new plasma excitation technique well adapted to microwave multipolar plasmas, is described. The novel reactor concept is based upon the use of several microwave linear applicators working at the ECR mode along the multipolar confinement magnets. At a microwave frequency of 2.45 GHz, large volumes of highly homogeneous plasmas with an electron temperature of the order of 3 eV and ion densities better than 1011 cm−3 are currently being produced in DECR reactors.

Improved hot‐wire procedure for thermophysical measurements under pressure

Abstract: A new and simplified version of the circuitry for the transient hot‐wire method is presented. The circuitry provides a wide range of currents allowing probe wires of various diameters to be used in order to match the thermal properties of the specimen to be investigated. The analysis of the temperature increase during the heat pulse is based on the exact solution for a heated wire immersed in a medium. Data are corrected for varying power. The method was tested by computer simulations and by measurements of the thermal conductivity (λ) and the heat capacity per unit volume (ρcp ) of glycerol at room temperature and atmospheric pressure, and for CsCl and NaCl at room temperature and at pressures up to 2 GPa. The results on glycerol and CsCl are in excellent agreement with previous works. The inaccuracy in λ and ρcp is estimated as 1%–2% and 3%–5%, respectively, but the standard deviation of the measurements is as low as 0.2% for λ and 1% for ρcp. The improved procedure makes it possible to detect systematic errors caused by reflection of the heat pulse from the walls of the high‐pressure cell. This error, which reveals itself by a curvature of the residual, defined as the difference between fitted function and data, was demonstrated in the case of NaCl. A theoretical estimate of the influence of perturbations due to reflection was also carried out and it was found that the error mainly affects the value of ρcp.

1‐mm wave ESR spectrometer

Abstract: An ESR spectrometer operating at 250‐GHz frequency (1.22‐mm wavelength) and 9‐T magnetic field is described. The utilization of far‐infrared (FIR) technology greatly simplifies its design and performance. Good frequency and field stability are achieved by unique designs which also conveniently permit the magnetic field to be swept. The potential utility of FIR–ESR is illustrated with examples of spectra from polycrystalline and liquid samples. In the latter case the increased spectral sensitivity to motional dynamics is stressed. Several ways in which the FIR–ESR spectrometer may be improved are also discussed.

Novel technique to measure the microwave response of high Tc superconductors between 4.2 and 200 K

Abstract: This paper describes a novel technique for measurements of the complex surface impedance Z(s) of high-T(c) superconductors over an extremely wide range of temperature, from 4.2 K and at least 200 K. The technique uses a superconducting Pb-plated Cu high-Q-cavity resonator operated at an ambient temperature of 4.2 K, with the sample (typically a 3-mm-diam x 1-mm-thick disk) mounted on a sapphire rod in the center of cavity, at a high B-field and thermally insulated from the cavity walls, making it possible to control the sample temperature externally. Since the cavity characteristics are dominated by the sample properties (the contribution of the Pb-covered walls maintained at 4.2 K is negligible), measurement of the cavity Q and resonant frequency makes it possible to measure Zs as a function of the sample temperature. The Z(s) values for bulk Y1Ba2Cu3O(y) and La(1.85)Sr(0.15)CuO4 were measured over a temperature range 4.2-100 K.

Bridged loop–gap resonator: A resonant structure for pulsed ESR transparent to high‐frequency radiation

Abstract: A new and inexpensive X‐band resonator structure for pulsed electron‐spin resonance and electron‐nuclear‐double‐resonance applications is introduced The resonator consists of a bridged loop structure and is distinguished by a good reproducibility and reliability It has a high filling factor and an adjustable Q value and is transparent to radio frequency fields with an upper limit >100 MHz The inner diameter of 5 mm allows convenient sample access of standard 4‐mm quartz tubes in the temperature range between 4 and 300 K Numerical computations of the resonant frequencies, the Q values, and the three‐dimensional distributions of the electromagnetic fields are presented They are based on a discretization method for the solution of Maxwell’s equations and include all dielectric elements of the resonator structure The field homogeneity in the sample area and the influence of the dimensions and the thickness of the metallic layers on field intensity, quality factor, and resonant frequency are determined e

Scanning x‐ray microscope with 75‐nm resolution

Abstract: A scanning soft x‐ray microscope has been built and operated at the National Synchrotron Light Source. It makes use of a mini‐undulator as a bright source of 3.2‐nm photons. An electron beam fabricated Fresnel zone plate focuses the beam onto the specimen, which is scanned under computer control. The scanning stage can be moved by both piezoelectric transducers and stepping motors, and the location is monitored by a high‐speed laser interferometer. X rays transmitted through the specimen are detected using a flow proportional counter. Images of biological specimens and of artificial microstructures have been made with resolution in the 75–100‐nm range. Acquisition time for 256×256‐pixel images is about 5 min.

Real-time, vibration-compensated CO2 interferometer operation on the DIII-D tokamak

Abstract: A multichannel, two‐color, quadrature heterodyne interferometer is used to measure the line density in the DIII‐D tokamak. The unique feature of this real‐time vibration‐compensated interferometer is the combination of high speed (1 MHz), high resolution (2π/256), and wide range (±8193 fringes). Quadrature phase information from a CO2 laser (10.6 μm) and a He–Ne laser (0.63 μm) are digitized with high‐speed (6 MHz) flash digitizers. Zero crossings of the signals are counted with digital circuitry yielding quarter fringe resolution with a 4‐MHz bandwidth. Further fringe resolution of 1/256 is provided at 350 kHz by a PROM which uses the digital signals as input to a look‐up table. Analog line density is presently available at 80 kHz with a system noise equivalent phase shift of ±2/256. Error monitoring is provided for low signal amplitude and exceeding the maximum fringe rate. In addition, a method to prevent coating of in‐vessel mirrors due to plasma and vessel wall cleaning discharges has been developed.

Performance of a microscopic imaging ellipsometer

Abstract: A microscopic imaging ellipsometer has been constructed based on use of a CCD camera and framegrabber board in a PC computer The performance (sensitivity and speed) are described

The LIDAR Thomson scattering diagnostic on JET (invited)

Abstract: By combining the time‐of‐flight or LIDAR principle with a Thomson backscatter diagnostic, spatial profiles of the electron temperature and density are measured in a magnetically confined fusion plasma. This technique was realized for the first time on the JET tokamak. A ruby laser (3‐J pulse energy, 300‐ps pulse duration, 0.5‐Hz repetition rate) together with a 700‐MHz bandwidth detection and registration system yields a spatial resolution of about 12 cm. A spectrometer with six channels in the wavelength range 400–800 nm gives a dynamic range of the temperature measurements of 0.3–20 keV. The stray light problem in the backscatter geometry is overcome by spectral discrimination and gating of the photomultipliers. A ruby filter in the spectral channel containing the laser wavelength allows calibration of the vignetting along the line of sight by means of Raman scattering, enabling the measurement of density profiles. The low level of background signal due to the short integration time for a single spatial...

Construction, calibration, and testing of a four‐detector photopolarimeter

Abstract: A computer‐controlled four‐detector photopolarimeter (FDP) has been constructed using four windowless planar‐diffused Si photodiodes, operational amplifiers, an analog‐to‐digital (A/D) converter, and a personal computer with peripherals. A nonplanar light path is selected with incidence angles at the first three detectors of ∼65° and with rotations of ∼45° between the successive planes of incidence. The last detector, which is coated for minimum reflectance, intercepts the beam at a small angle and the residual light it reflects is dumped. A 1‐mW He–Ne laser beam (λ=632.8 nm) passes through the polarizing optics of an ellipsometer to provide the polarization states needed for calibration and testing. With an optimum set of calibration states, the instrument matrix A is determined. The FDP is subsequently tested and found to correctly measure the normalized Stokes parameters of a large number of states with an average absolute error of ∼0.01, which is attributed to imperfections in the calibration optics. This first prototype instrument has a precision of ∼0.2%.

``Tracking'' tunneling microscopy

Abstract: Simple modification of the electronics of a STM allows operation in various tracking modes. Profiles of steepest inclination, equal height, equipotential, electrical field lines, etc., can be traced out. The tunnel tip can also be locked to a surface extremity for indefinite time. Some of these possibilities were tested experimentally.

Measurement of current-density changes during sawtooth activity in a tokamak by far-infrared polarimetry (invited)

Abstract: Simultaneous measurements of plasma‐induced phase shifts and Faraday rotation angles along nine vertical chords by means of a far‐infrared interferometer/polarimeter have been used for some time to determine the electron density profile and the poloidal magnetic field distribution in the TEXTOR tokamak. While the sensitivity of the interferometric subsystem is sufficient to clearly detect sawtooth oscillations of the density profile, the noise level of the polarimetric signals usually obscures the appearance of sawteeth. However, owing to the high reproducibility of the sawtooth phenomenon under certain quasistationary discharge conditions, coherent‐signal‐averaging techniques can be applied to extract sawtooth‐correlated modulations of the Faraday rotation angles. After correction for density‐induced changes, these data represent the variation of the poloidal field pattern during a sawtooth cycle. Their analysis yields a periodic shift of the magnetic axis by about 1 mm, in good agreement with equilibriu...

Two approaches to construction of vibrating probes for electrical current measurement in solution

Abstract: Two systems are described that implement the vibrating probe method for recording the voltage field in electrolyte solutions. The method has a spatial resolution of 5–40 μm and very low drift. The first system is a simple device that allows inexpensive but reliable use of the method to record the field in one dimension. It is based on a metal electrode vibrating along a line, with power supplied by a single piezoelectric bimorph. The second is a more complex system, with computer support, that measures the field in two dimensions, acquires the data to the computer disk, and plots the field measurement on a video image of the experimental preparation. A π‐shaped linkage of three bimorphs is used to move the metal electrode in two dimensions. Motor micrometers with optical encoders are used to move the electrode and monitor its position.

Velocimetry of fast surfaces using Fabry–Perot interferometry

Abstract: 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 inclu...

Simple piezoelectric translation device

Abstract: We describe a piezoelectric device which allows continuous movement and high‐resolution micropositioning, without distance limitation. Both mechanical construction and the electronics for the device are very simple. The movement is obtained via a stick‐slip mechanism, and steps as small as 10 nm are obtained. A displacement speed of 0.4 mm/s has been attained, and the device was capable of carrying several times its own weight, exerting a horizontal force, or climbing a plane inclined by 7°. Due to its compact construction, the device shows prospects for miniaturization.

2π-Radian field-of-view toroidal electrostatic analyzer

Abstract: We describe a new type of electrostatic charged particle analyzer based on toroidal geometry. The analyzer features a 360°×10° (polar×azimuthal) field of view and a focal length in the polar direction significantly longer than that of similar devices based on spherical geometry. Extended focal length is a requirement for matching the electrostatic energy spectrometer with a second, mass‐resolving, analyzer. Laboratory tests show that the toroid design has a large geometric factor per unit of instrument weight, good energy‐angle resolution in the particle deflection plane, and sharp (1.0° FWHM) resolution at focus in the polar angle direction. The analyzer is well suited for use in satellite‐borne mass spectrometry and also as a stand‐alone electrostatic analyzer.

Compact, transportable, and multipurpose high‐pressure unit for UV–VIS spectroscopic measurements at pressures up to 200 MPa

Abstract: The construction of a compact, transportable, and multipurpose high‐pressure unit consisting of a pressure generating system and a four‐window optical UV–VIS high‐pressure cell for spectroscopic measurements at pressures up to 200 MPa is described. The pressure generating system can be operated with a wide range of liquids suitable as pressurizing medium. The optical cell can be used for spectroscopic and kinetic (flash photolysis and laser induced T‐jump) measurements.

Design and application of a computer‐controlled confocal scanning differential polarization microscope

Abstract: Based on the confocal scanning and differential polarization imaging technique, a computer‐controlled confocal scanning differential polarization microscope has been built. This system possesses extensive image processing capability that provides digitized regular and/or differential polarization images. This microscope combines the advantages of the confocal design and the extended electronic sensitivity of polarization modulated instrumentation to achieve the measurement of an anisotropy ratio as small as 10−5. In this paper, the basic theory and the instrument are described. In addition, some samples are used to test the actual performance of the system, and it is found that the system provides high resolution (0.3 μm at wavelength 546 nm), reduced depth of field, and high signal‐to‐noise ratio (83 dB at 10‐ms integration time) images. Finally, some biological applications are shown.

Low-temperature atomic force microscopy

Abstract: A low‐temperature atomic force microscope (LTAFM) has been constructed which is capable of resolving atomic scale features both in air at room temperature and immersed in liquid helium at 4.2 K. The instrument is of a rigid compact design, using microfabricated force‐sensing cantilevers, and can easily be adapted for operation as a scanning tunneling microscope. Initial results have demonstrated that the LTAFM can image the atomic surface structure of 2H–MoS2 at 4.2 K and room temperature. Design criteria and applications of the LTAFM are discussed.

High‐performance GaAs polarized electron source for use in inverse photoemission spectroscopy

Abstract: The design and operating properties of a GaAs polarized electron source are presented. An electron optical system is described that passes more than 80% of the emitted electrons at 10 μA to the target under low‐energy (7–20 eV) parallel beam conditions. Laser excitation can give rise to abnormal energy distributions of the photoemitted electron beam. The existence of longitudinal modes gives a possible explanation for this behavior, which can be avoided using a mode stabilized light source. The overall performance of the polarized electron source is demonstrated by inverse photoemission spectra from Ni(110).

Ruby decay‐time fluorescence thermometer in a fiber‐optic configuration

Abstract: In this article, a fluorescence decay‐time thermometer using ruby crystal as the transducer material and carefully designed signal processing, with a light‐emitting diode source and solid‐state optical detection is reported. Results over a range to 170 °C are discussed with a standard deviation in the measurement, at best, corresponding to ±0.04 K, in laboratory tests.