Showing papers in "Review of Scientific Instruments in 1992"

Hyper‐Rayleigh scattering in solution

Abstract: The experimental setup for the implementation of a new technique to determine the hyperpolarizability of nonlinear optical molecules in solution is presented The new technique, hyper‐Rayleigh scattering in solution [K Clays and A Persoons, Phys Rev Lett 66, 2980 (1991)], has the advantage over the electric‐field‐induced second‐harmonic generation technique that the dipole moment μ and the second hyperpolarizability γ do not have to be independently determined to obtain the first hyperpolarizability β No electric field is needed to lower the intrinsic symmetry of the isotropic solution, leading to a simpler cell design and a simpler local field factor The internal reference method, when applicable, completely eliminates the local field factor With the technique presented, values of the first hyperpolarizability β of 23×10−30 esu for para‐nitroaniline (PNA), 105×10−30 esu for 4‐methoxy‐4’‐nitrostilbene (MONS), and 95×10−30 esu for 4‐hydroxy‐4’‐nitrostilbene (HONS) dissolved in chloroform have been obtained

Flash pyrolysis nozzle for generation of radicals in a supersonic jet expansion

Abstract: A method for the high‐temperature generation of reactive species in supersonic molecular beams has been developed. Pyrolysis temperatures (25–1800 °C) can be maintained for longer than 100 h for extended experiments. Short contact time (≊10 μs) of the seeded (<1 Torr partial pressure in 2 atm inert carrier gas) precursor molecules suppresses recombination of the product radicals. Number densities for radicals are estimated to be ≊1014 cm−3 at the sonic orifice with radical fluxes of ≊1016 s−1.

Cluster ion sources (invited)

Abstract: Clusters from gases as well as from metal vapors can be obtained from an expanding nozzle flow with the appropriate set of flow field conditions, characterized by a condensation scaling parameter Γ*. Cluster ion beams are characterized by a low specific charge. Accelerated cluster ion beams allow formation of energetic particle beams in the interesting range of 0.01–10 keV/atom, and the specific characteristics of cluster ion beams have led to new applications in science and technology. In this paper a status report on clusters and cluster ions from nozzle sources is given. As example the construction and results of a source for high‐intensity silver cluster beams to be used for thin film formation are described.

Design and operation of the multipulse Thomson scattering diagnostic on DIII‐D (invited)

Abstract: This paper describes the design and operation of a 40 spatial channel Thomson scattering system that uses multiple 20‐Hz Nd:YAG lasers to measure the electron temperature and density profiles periodically throughout an entire plasma discharge. As many as eight lasers may be fired alternately for an average measurement frequency of 160 Hz, or they may be fired in rapid succession (<10 kHz), producing a burst of pulses for measuring transient events. The high spatial resolution (1.3 cm) and wide dynamic range (10 eV–20 keV) enable this system to resolve large electron density and temperature gradients formed at the plasma edge and in the scrape‐off layer during H‐mode operation. These features provide a formidable tool for studying L–H transitions, edge localized modes (ELMs), beta limits, transport, and disruptions in an efficient manner suitable for large tokamak operation where shot‐to‐shot scans are impractical. The scattered light is dispersed by interference filter polychromators and detected by silicon avalanche photodiodes. Laser control and data acquisition are performed in real time by a VME‐based microcomputer. Data analysis is performed by a MicroVAX 3400. Additional features of this system include real‐time analysis capability, full statistical treatment of error bars based on the measured background light, and laser beam quality and alignment monitoring during plasma operation. Results of component testing, calibration, plasma operation, and error analysis are presented.

A suite of programs for calculating x‐ray absorption, reflection, and diffraction performance for a variety of materials at arbitrary wavelengths

Abstract: One of the most useful characteristics of synchrotron radiation is the wide spectral distribution of the source. For applications involving tuned monochromatic beams it is often helpful to predict the x‐ray optical characteristics of a sample or the beam line optics at a particular wavelength. In contrast to this desire stands the fact that tabulated values for the optical parameters of interest are generally available only at wavelengths corresponding to typical x‐ray tube sources. We have developed a suite of fortran programs which calculate photoabsorption cross sections and atomic scattering factors for materials of arbitrary, uniform composition or for arbitrary layered materials. Further, the suite includes programs for calculation of x‐ray diffraction or reflection from such materials. These programs are of use for experimental planning, data analysis, and predictions of performance of beam line optical elements.

Thickness and conductivity of metallic layers from eddy current measurements

Abstract: A robust method that uses eddy current measurements to determine the conductivity and thickness of uniform conductive layers is described. The method was tested by estimating the conductivity and thickness of aluminum and copper layers on various substrate metals, and the thickness and conductivity of free‐standing foils of aluminum. The electrical impedance was measured for air‐core and ferrite‐core coils in the presence and absence of the layer for frequencies ranging from 1 kHz to 1 MHz. The thickness and conductivity of the metal layers were inferred by comparing the data taken with air‐core coils to the exact theoretical solution of Dodd and Deeds [J. Appl. Phys. 39, 2829 (1968)] using a least‐squares norm. The inferences were absolute in the sense that no calibration was used. We report experimental tests for eight different thicknesses of aluminum (20–500 μm) in free space and on four different substrates: Ti‐6Al‐4V, 304 stainless steel, copper, and 7075 aluminum, and for five different thicknesses...

igun−A program for the simulation of positive ion extraction including magnetic fields

Abstract: igun is a program for the simulation of positive ion extraction from plasmas. It is based on the well known program egun for the calculation of electron and ion trajectories in electron guns and lenses. The mathematical treatment of the plasma sheath is based on a simple analytical model, which provides a numerically stable calculation of the sheath potentials. In contrast to other ion extraction programs, igun is able to determine the extracted ion current in succeeding cycles of iteration by itself. However, it is also possible to set values of current, plasma density, or ion current density. Either axisymmetric or rectangular coordinates can be used, including axisymmetric or transverse magnetic fields.

Stroboscopic optical boxcar technique for the determination of fluorescence lifetimes

Abstract: This work describes a stroboscopic optical boxcar technique for the determination of fluorescence lifetimes which achieves performance comparable to techniques such as time‐correlated single photon counting or phase modulation. The stroboscopic technique is based on the use of a traveling wave injected into a delay line connecting the dynodes of a photomultiplier tube. The transient potential difference created between two adjacent dynodes results simultaneously in significant amplification and the generation of a ‘‘gate’’ for the amplification process. Accurate control of the timing between the flashing of the gated lamp and the computer controlled delayable triggering of the photomultiplier tube pulser allows the gate to be placed at any time position within the range of the digital delay generator. The intensity of the fluorescence emission can thus be measured as a function of time relative to the excitation flash yielding data which is very similar to that from time‐correlated single photon counting....

Mott electron polarimetry

Abstract: Electron polarimeters based on Mott scattering are extensively used in atomic and molecular, solid state, nuclear, and high‐energy physics. This use stems from the increasing realization that much additional information concerning many physical processes can be obtained through spin‐dependent measurements. In this review we discuss the basic physics and application of Mott polarimetry. A number of different Mott polarimeter designs are described that illustrate the wide range of operating energies (10 eV–1 MeV) and geometries that can be used in such instruments. The calibration of Mott polarimeters is discussed together with the potential sources of systematic error that can arise and that can limit measurement accuracies. The aim is to present a comprehensive practical guide to Mott polarimetry and the capabilities of the technique.

A new impedance spectrometer for the investigation of electrochemical systems

Abstract: The development of a computer‐controlled electrochemical impedance spectrometer, based on a Fourier transform algorithm is described. Together with a fast potentiostat the system can be used in the frequency range 1 mHz to 100 kHz. The perturbation signal is a superposition of sine waves with properly chosen frequencies. The overall measurement time is limited only by the lowest frequency in the spectrum and by the data transfer to the computer, thus, time‐resolved impedance spectra measurements can be performed. The principle of operation and technical details are presented and discussed.

Polarimetry of motional Stark effect and determination of current profiles in DIII-D (invited)

Abstract: The motional electric field E=v×B, where v is the velocity and B is the tokamak magnetic field, produces a strong Stark effect in spectral lines emitted by hydrogenic neutral beams. The tilt angle of the magnetic field line, a quantity related directly to the distribution of the plasma toroidal current, is deduced from a measurement of the direction of polarization of the Stark components. In the DIII‐D tokamak, the Balmer‐α line of deuterium emitted by one of the high‐power heating beams is analyzed. A multichord polarimeter measures the magnetic field pitch angle at eight spatial locations covering ∼0.6 of the nominal plasma diameter at the midplane outboard side. The diagnostic offers 2–8‐cm resolution in the major radius and 1‐ms integration time. The accuracy of the measurement of the polarization direction necessary for an adequate reconstruction of the current profiles is obtained with the use of active polarizing elements which produce high‐frequency intensity modulation with an amplitude related ...

Simultaneous void fraction measurement, bubble velocity, and size estimate using a single optical probe in gas–liquid two‐phase flows

Abstract: Optical probes are now commonly used in industrial conditions as well as in laboratory experiments. Although they have been primarily devoted to void fraction measurements, additional information could be extracted from the raw signals they deliver. For a stretched optical probe, it is shown that the modulus of the ensemble velocity of a bubble ‖V0‖ could be inferred from the rise time Tu associated with the liquid/gas transition. These two parameters are correlated thanks to piercing experiments in which the interface curvature R and the angle β between the probe and the normal to the interface are controlled. While the influence of R is negligible, the rise time is very sensitive to β. A one‐to‐one relation between Tu and ‖V0‖ is ensured for quasiperpendicular impactions which must be accordingly distinguished among the bubble signatures. The software which includes the adequate signal processing criteria is described. It gives access to void fraction and to the chord‐velocity joint distribution which p...

Application of an imaging plate to high‐pressure x‐ray study with a diamond anvil cell (invited)

Abstract: In order to obtain reliable data on d values and integrated intensities, an imaging plate (IP) has been applied to high‐pressure diffraction experiments with a diamond anvil cell using synchrotron radiation. The IP was found to be more effective than a conventional x‐ray film by a factor of 30, and more effective than an energy dispersive method by several times. In order to integrate the intensity data on the IP, a histogram method was successfully developed. This combination proved to give a lattice constant with relative accuracy of 2×10−4, and observed integrated intensities which excellently agree with calculated ones.

Reflectron time‐of‐flight mass spectrometer for laser photodissociation

Abstract: We describe a new reflectron time‐of‐flight mass spectrometer configuration for laser photodissociation of mass‐selected ions and the initial performance characteristics observed for this instrument. Ions are produced by laser photoionization within the acceleration region of the instrument or by laser vaporization in an external pulsed‐nozzle cluster ion source. Mass selection is accomplished with pulsed deflection plates at the end of an initial drift section. Laser photodissociation of selected ions takes place at the turning point in the ion trajectory in the reflectron. The transit time through a second drift section defines the fragment ion masses. Optimized operating conditions and the role of mass discrimination in this instrument are discussed.

An ion trap storage/time-of-flight mass spectrometer

Abstract: An ion trap/time‐of‐flight (TOF) mass spectrometer combination has been developed in order to combine the storage capabilities of an ion trap with the speed and resolution of a time‐of‐flight device. The ion trap is an rf quadrupole trap which operates in the total storage mode, i.e., with the dc voltage=0 on the end caps and rf voltage on the ring electrode. The trap has an ion storage time of ≥2 s at an rf potential of 310 Vpp. The stored ions are ejected into the time‐of‐flight device using a −150 V dc pulse on the exit end cap which causes the ion trajectories in the trap to become unstable. The ions are mass analyzed using either a linear or reflectron TOF. In the linear mode the resolution is 240 while in the reflectron mode a resolution of 1300 at m/z 93 is achieved. The storage capabilities of this device may have important applications towards enhancing sensitivity, the study of very slow metastable decay, and photodissociation mass spectrometry and spectroscopy.

Development of multicolor pyrometers to monitor the transient response of burning carbonaceous particles

Abstract: A three‐color ratio pyrometer has been developed to obtain surface temperatures and high‐temperature combustion rates of burning carbonaceous particles. The features and performance of this instrument are contrasted to those of a two‐color ratio pyrometer, constructed earlier for similar studies. The three‐color pyrometer employs a visible (0.65 μm) and two near‐infrared (0.80 and 0.975 μm) wavelengths. The instrument uses a single optical fiber to capture radiation emitted from a particle burning in a high‐temperature laminar flow furnace. Monitoring of the combustion events takes place coaxially with the particle flow, from observation windows located at the top of the furnace injectors. Thus, the temperature‐time history of burning particles can be recorded. The radiation flux is split into three beams using dichroic edge filters. Narrow (or medium) bandwidth interference filters guide monochromatic radiation to solid‐state silicon photodetectors. The associated amplification is linear and/or logarithm...

Performance of the high-resolution SX700/II monochromator

Abstract: This article reports on the high‐resolution performance of the grazing‐incidence plane grating monochromator SX700/II, installed at BESSY by the Freie Universitat Berlin, in the photon energy range from about 40 to 900 eV. The high resolving power up to 10 000 achieved with this monochromator is based on improving the figure error of the ellipsoidal focusing mirror, on reducing the vertical dimension of the beam source, and on employing a 5‐μm exit slit. We report on high‐resolution gas‐phase studies in the double‐excitation region of He, as well as at core‐excitation thresholds of Ne, Ar, Kr, and Xe in the photon‐energy range from ≂45 eV to ≂900 eV. In addition, high‐resolution core‐excitation spectra at the K thresholds of C, N, and O are presented for gas‐phase CO, N2, and O2. In all cases, high‐n Rydberg states and/or vibrational sidebands of the electronic excitations were resolved. The various contributions to the present instrumental linewidths are discussed as well as the prospects for further improvements in resolution with this monochromator.

A combined Thomson–Rayleigh scattering diagnostic using an intensified photodiode array

Abstract: A combined Thomson–Rayleigh scattering device is discussed. It consists of a Nd:YAG laser as a light source in combination with a multichannel detection technique consisting of a gated light amplifier in combination with an optical multichannel analyzer. Special attention is focused on the analysis of the measured spectra. Including convolution methods and taking into account weak coherent effects increases the dynamic range and the accuracy of the measured electron density ne and temperature Te and neutral particle density n0. Accuracies of 1%–4% for ne, 2%–6% for Te, and 10%–50% for n0 depending on the plasma condition are obtained. The dynamic range for ne is 7×1017–1021 m−3, for n0 is 1020–1023 m−3 and for Te is 1000–50 000 K.

High‐speed gated x‐ray imaging for ICF target experiments (invited)

Abstract: We describe the use of gated microchannel‐plate detectors as high‐speed framing cameras in laser‐driven inertial‐confinement‐fusion experiments. Using an array of pinholes to image the target, detectors capable of generating up to 16 individual frames with ∼90 ps resolution on a single laser shot are now in routine use. The detectors have been used to study the development of intentionally applied perturbations in laser‐driven targets. In off‐line tests new detectors have demonstrated time resolutions better than 40 ps.

New experimental setup for photoelectron spectroscopy on cluster anions

Abstract: We describe a new experimental setup for photoelectron spectroscopy on mass selected clusters. The recently developed pulsed arc cluster ion source (PACIS) for metal and semiconductor clusters is used as an anion source. The design of the PACIS is optimized for maximum intensity of cluster ion production and a minimum internal temperature of the particles. A simple modification allows vaporization of liquid and low melting point metals. The produced anions are mass selected via an inline time‐of‐flight setup with the option of using a reflectron for increased mass resolution. Photoelectron spectra of the mass selected cluster anions are collected in a ‘‘magnetic bottle’’ type electron spectrometer. First results on copper clusters are presented.

Gundestrup: A Langmuir/Mach probe array for measuring flows in the scrape‐off layer of TdeV

Abstract: Gundestrup is a Langmuir/Mach probe array which measures the flow velocity in the scrape‐off layer of Tokamak de Varennes (TdeV). It is based on the concept of a Mach probe where presheaths extending upstream and downstream from the probe, parallel to the magnetic field, attract charge to a circular array of collecting pins. The polar distribution of ion saturation currents to the circular array is used to compute the components of flow velocity in the plasma. With Gundestrup, there is an assumed flow perpendicular to the magnetic field as well as parallel to it. Equations representing the collection of charge by individual pins on the probe are presented and sample flow patterns from the scrape‐off layer are shown.

A micropipette force probe suitable for near‐field scanning optical microscopy

Abstract: In this paper it is demonstrated that glass micropipettes have unique applicability as force probes for a variety of imaging conditions and a variety of scanned tip microscopies. These probes are characterized in terms of the parameters that determine their force characteristics. Measurements are presented showing that one can readily achieve force constants of 10 N/m and it is anticipated that a reduction in this force constant by two orders of magnitude can be achieved. Such probes can be produced simply with a variety of geometries that permit a wide range of force imaging requirements to be met. Specifically, the glass micropipette probes reported in this paper are readily produced with apertures at the tip and can thus be applied to near‐field scanning optical microscopy (NSOM). This opens the possibility of the long‐awaited development of a universal feedback mechanism for NSOM.

The multichannel motional Stark effect diagnostic on TFTR

Abstract: Although the q profile plays a key role in theories of instabilities and plasma equilibrium, it has been quite difficult to measure until the recent development of the motional Stark effect (MSE) diagnostic. A multichannel motional Stark effect polarimeter system has recently been installed on the Tokamak Fusion Test Reactor (TFTR). The diagnostic can measure the magnetic field pitch angle ({gamma}{sub p} = tan{sup {minus}1} (B{sub T})/(B{sup p})) at ten radial locations. The doppler shifted D{sub alpha} radiation from a TFTR heating beam is viewed near tangential to the toroidal magnetic field via a re-entrant front surface reflecting mirror. The field of view covers from inboard of the magnetic axis to near the outboard edge of the plasma with a radial spatial resolution of 3--5 cm. A high throughput f/2 optics system results in an uncertainty for {gamma}{sub p} of {approximately}0.1{degrees}--0.2{degrees} with a time resolution of {approximately}5--10 ms. Initial pinch angle profiles from TFTR have been obtained. The MSE data is consistent with the estimated magnetic axis position from external magnetic measurements and the q=1 radius is in good agreement with the inversion radius from the electron cyclotron emission temperature measurements.

Recent advances in photoacoustic calorimetry: Theoretical basis and improvements in experimental design

Abstract: Recent developments in time‐resolved photoacoustic calorimetry (PAC) are discussed. An equation is derived relating the amplitude of the maximum photoacoustic signal to parameters of a photoacoustic cell, to physical properties of the solvents, and to the energy deposited as heat. The equation is validated by a series of scaling experiments: it correctly correlates the dependence of the PAC signal on cell thickness, on energy deposition, and on solvent properties. The dominant sources of background signal have been determined and the background signal reduced substantially relative to previous work. It is now possible to measure energies and lifetimes of transients in solutions with optical densities as low as ∼5×10−4. Data are presented for the energy of 2‐cyclopentenone triplet, a transient for which interpretable PAC measurements were previously not possible. Its triplet energy is 73.1±1.1 kcal/mole. With reduced background and a faster digital storage oscilloscope, a quite short transient lifetime, 7....

Application of a microscope to Brillouin scattering spectroscopy

Abstract: In order to make a microscopic observation of the surface elastic wave on inhomogeneous opaque materials, an apparatus for Brillouin scattering spectroscopy was constructed utilizing a commercial microscope. An optical system which serves both as an inlet of a laser beam and as an outlet of the scattered light was made and incorporated in the microscope. The apparatus was proved to be particularly useful to make microscopic‐scale observations of surface elastic waves on metal films.

The Thomson scattering systems of the ASDEX upgrade tokamak

Abstract: The Thomson scattering system of the ASDEX upgrade (AUG) tokamak is described One of the main objectives of AUG is to investigate plasma wall interaction in reactor relevant discharges with a magnetic divertor The very successful Nd:YAG scattering system developed for its predecessor ASDEX, has been upgraded to give higher spatial and temporal resolution, reliability, and flexibility to different discharge conditions The system consists of two independently operating devices, each using a cluster of six lasers: One measures the electron temperature and density along three possible vertical chords alternatively through the magnetic axis, or the inner or outer boundary layer; a second chord in the equatorial plane will always cover the magnetic center even in the case of considerable Shafranov shifts An additional compact spectrometer has been designed for measurements with high radial resolution in the equatorial plane across the separatrix A third system, using the laser beams for the vertical arrang

Fast curve fitting using neural networks

Abstract: Neural networks provide a new tool for the fast solution of repetitive nonlinear curve fitting problems. In this article we introduce the concept of a neural network, and we show how such networks can be used for fitting functional forms to experimental data. The neural network algorithm is typically much faster than conventional iterative approaches. In addition, further substantial improvements in speed can be obtained by using special purpose hardware implementations of the network, thus making the technique suitable for use in fast real‐time applications. The basic concepts are illustrated using a simple example from fusion research, involving the determination of spectral line parameters from measurements of B iv impurity radiation in the COMPASS‐C tokamak.

Fiber‐optic high‐temperature sensor based on the fluorescence lifetime of alexandrite

Abstract: A fiber‐optic sensor for continuous temperature measurement from room temperature to ≳700 °C is presented. The device is based upon the excitation of the synthetic crystal alexandrite with light from a diode laser operating at 670 nm and the measurement of its fluorescence lifetime. A simple low cost optical configuration is described together with the introduction of a new effective signal processing scheme for fluorescence lifetime measurement, and a high sensitivity and repeatability are observed, at best ±1 °C.

A new scheme for Langmuir probe measurement of transport and electron temperature fluctuations

Abstract: A new scheme to extend the triple Langmuir probe technique for the measurement of electron temperature fluctuations and the fluctuation‐driven transport has been developed. The extension is aimed at reducing the phase delay error introduced by finite probe tip separations in standard triple‐probe method. The modified triple‐probe scheme provides a more reliable measurement of the temperature fluctuations for a proper interpretation of the density and potential fluctuations and the transport measurement from Langmuir probe data. New results on fluctuations have been obtained from Phaedrus‐T and TEXT‐U tokamaks.

Methods to measure properties of slow‐scan CCD cameras for electron detection

Abstract: New ways to measure detection properties of slow‐scan charge‐coupled device (CCD) cameras suitable for electron microscopy, mainly based on the statistics of single‐electron events, are discussed. The experiments concentrate on the newly introduced Gatan 679 slow‐scan CCD camera. It has been established that for this instrument (if equipped with a thin YAG scintillator) 20% of the true intensity is recorded at the highest detectable spatial frequency (Nyquist frequency), limited mostly by the modulation transfer function of the YAG scintillator. The detection quantum efficiency is 0.45 for single 100‐kV electrons and 0.15 for single 400‐kV electrons, and is approaching unity for intensities higher than ten electrons. Furthermore, nonlinearity of the response is smaller than deviations in the image intensity due to shot noise. Examples are presented illustrating the detection properties of slow‐scan CCD cameras for electron imaging, which also include high dynamic range and negligible geometric distortion.