Showing papers in "Review of Scientific Instruments in 1979"

Inflection-point method of interpreting emissive probe characteristics.

Abstract: The use of emissive Langmuir probes in unmagnetized and weakly magnetized multidipole plasmas is investigated. It is shown that plasma potential, plasma electron temperature, and probe temperature can be determined from one probe characteristic curve. Data indicate that the inflection point of the current‐voltage curve gives the plasma potential to an accuracy the order of the probe temperature Tw/e for weak probe emission. Effects of space‐charge limiting and contamination of the probe are presented.

Velocity sensing interferometer (VISAR) modification.

Abstract: The VISAR (velocity interferometer system for any reflector) has become a common tool used in experiments where high surface velocities must be measured. A modification that uses previously wasted interferometer light to more than double output signals and to cancel noise is described. Laser power is used more efficiently, VISAR performance in the presence of intense target self-light is improved, and only two data signals are required instead of the usual three or four. Effects of changing light intensity and fringe visibility are eliminated using a novel detection system with a simplified solution for velocity.

X-ray filter assembly for fluorescence measurements of x-ray absorption fine structure.

Abstract: Fluorescence detection, in principle, permits the detection of the extended x-ray absorption fine structure (EXAFS) of more dilute atoms than can be obtained in absorption. To take advantage of this it is necessary, in practice, to eliminate the background that normally accompanies the fluorescence signal. We describe an x-ray filter assembly that accomplishes this purpose. The unique characteristic of the assembly is a slit system that minimizes the fluorescence background from the filter. The theory of the slit assembly is presented and is found to agree with measurements made on the Fe EXAFS of a dilute sample. The filter assembly has a better effective counting rate in this case than that of a crystal monochromator design.

Absolute pressure measurements and analysis of diamonds subjected to maximum static pressures of 1.3–1.7 Mbar

Abstract: Force per unit area measurements made in the megabar pressure cell, independently of other pressure calibration systems, are consistent with the ruby R1 scale of Mao, Bell, Shaner, and Steinberg and its extrapolation to 1.4 Mbar. Physical analysis of diamond anvils removed after experiments to maximum pressures of 1.3-1.7 Mbar suggests that the nitrogen platelet concentration may be related to the strength of the diamonds. The pressure face of one of the diamonds from the 1.7-Mbar experiment was deformed plastically by a macroscopic amount.

Low-distortion resistive anodes for two-dimensional position-sensitive MCP systems.

Abstract: Microchannel plates (MCPs) are frequently employed with resistive anodes to detect charged particles or photons and yield analog signals from which event positions can be decoded. We discuss an anode geometry devised by C.W. Gear that offers theoretically distortionless encoding of Cartesian event positions into pulse charge ratios. Electron beam images taken with such an anode are shown, and confirm the usefulness of the approach.

Measurement of H- density in plasma by photodetachment.

Abstract: Techniques have been developed for measurement of the density of H− in a plasma by photodetachment. Photodetachment is detected by the increase in electron density with no change in positive ion density after a light pulse from a ruby laser. The authenticity of photodetachment signals can be assured by their comparison with known cross sections for photodetachment of H−. Interpretations of photodetachment data are less ambiguous than probe interpretations because photodetachment is not affected by the mass of positive ions and is not limited in usefulness by the Debye distance. Photodetachment measurements with time resolution and spatial resolution are straightforward.

Multiwavelength optical pyrometer for shock compression experiments

Abstract: A system for measurement of the spectral radiance of materials shocked to high pressures (∼100 GPa) by impact using a light gas gun is described. Thermal radiation from the sample is sampled at six wavelength bands in the visible spectrum, and each signal is separately detected by solid‐state photodiodes, and recorded with a time resolution of ∼10 ns. Interpretation of the records in terms of temperature of transparent sample materials is discussed. Results of a series of exploratory experiments with metals are also given. Shock temperatures in the range 4000–8000 K have been reliably measured. Spectral radiance and temperatures have been determined with uncertainties of 2%.

Polarized light interferometer for laser fusion studies.

Abstract: A polarized light interferometer, suitable for the high-speed photography of microscopic objects in laser fusion experiments, is described. Based on a Wollaston prism as beam splitter, its main advantages are its relative simplicity, the absence of alignment and stability problems, and the extended spectral range down to 0.18 mum wavelength. The interferometer has been successfully applied in CO(2) and neodymium laser plasma interaction studies with laser illumination at 0.694 mum (ruby laser) and 0.265 mum (4th harmonic of the Nd(3+) laser) wavelength.

Fast recovery, high sensitivity NMR probe and preamplifier for low frequencies

Abstract: The design of a probe‐preamplifier combination which reduces the NMR receiver recovery time following a pulse by a factor of at least twenty at 5 MHz is described. The preamplifier, which has a noise figure of 1.3 dB, employs negative feedback to damp the probe coil (a typical effective Q is 7), but maintains the signal‐to‐noise ratio that one would expect from an undamped coil. In addition, the use of a short phase‐inverted pulse following the main transmitter pulse serves to reduce drastically the ring‐down of the probe for those high voltages where the preamplifier is inoperative. Optimum utilization of transmitter power in achieving fast pulse rise times is also discussed, as well as elimination of ’’phase‐glitch’’ by proper tuning and the use of a broadband transmitter.

Electro‐optical multichannel spectrometer for transient resonance Raman and absorption spectroscopy

Abstract: An optical multichannel system is described, used for time-dependent absorption measurements in the gas phase and the liquid phase and for resonance Raman spectroscopy of short-lived transient species in the liquid phase in pulse radiolysis. It consists of either an image converter streak unit or an image intensifier coupled to a TV camera with computerized data handling. The system enables the recording of time-dependent absorption spectra or resonance Raman spectra of short-lived radicals and excited states with single electron and light pulses.

Characteristics of the Berkeley multicusp ion source.

Abstract: The performance of a cubical permanent magnet generated line‐cusp ion source has been investigated for use with neutral beam injectors. This source has been operated with discharge currents greater than 500 A and ion current densities higher than 400 mA/cm2 at the extraction grid. The uniformity of the density profile across the extraction area is found to be dependent on the gas pressure. By using a fast Langmuir probe sweeping circuit, the electron temperature and the plasma density and potential have been analyzed for different discharge powers and gas pressures. The heat load on the plasma grid when it is electrically floating or connected to the negative cathode has been compared calorimetrically. The use of lanthanum hexaboride and impregnated oxide cathodes have been investigated for the purpose of long pulse operation. The phenomenon of mode flipping is found to occur quite frequently during a discharge with these magnetic‐field‐free cathodes. Species composition as a function of discharge power and chamber length is measured by a mass spectrometer.

Time-of-flight photoelectron spectroscopy of gases using synchrotron radiation.

Abstract: A gas‐phase time‐of‐flight (TOF) photoelectron spectrometer has been developed for use with synchrotron radiation. The excellent time structure of the synchrotron radiation at the Stanford Positron Electron Accelerator Ring (SPEAR) has been used as the time base for the TOF measurements. The TOF analyzer employs two multichannel plates (MCPs) in tandem as a fast electron multiplier with a matched 50‐Ω anode to form an electron detector with a timing resolution of ⩽70 ps. The spectrometer is presently capable of analyzing electrons over a wide energy range (1–50 eV) at medium energy resolution (⩽5%) and high angular resolution (±3°).

Design and characteristics of a cell for photoacoustic spectroscopy of condensed matter.

Abstract: We describe a photoacoustic cell which is simple to use, easy to construct, and which gives a great sensitivity, expressed as a signal-to-noise ratio of 3000. Calibration of the cell indicates that it can be used with confidence as a general purpose photoacoustic cell.

Rogowski coils: theory and experimental results

Abstract: The theory is given of the voltage output of a Rogowski coil excited by a current pulse flowing along the axis of the coil. In this theory the Rogowski coil is considered as a delay line. The results do not differ from those obtained usually by considering the coil as a voltage source dφ/dt with an inductive output impedance. Details are also given of the design of two Rogowski coils and their working modes are fully analyzed.

Bremsstrahlung isochromat spectroscopy using a modified XPS apparatus

Abstract: The new components required for bremsstrahlung isochromat spectroscopy (BIS) in UHV (10−12 Torr range) have been developed and incorporated into an x‐ray photoelectron spectrometer. They consist of a Pierce electron gun and a soft x‐ray photon detector, both of which have to fulfill precise geometrical conditions imposed by the already existing monochromator. A description of the design and testing of this equipment is given. The resolution and the intensity appear to be correlated so that a compromise has to be found in order to achieve high resolution with acceptable intensity. It is demonstrated on the basis of suitable spectra that the density of unoccupied states above the Fermi level can be investigated with a resolution better than 0.43 eV.

Magnetic multipole line-cusp plasma generator for neutral beam injectors.

Abstract: The magnetic multipole line-cusp device developed by MacKenzie and associates has been adapted for use as a neutral beam ion source. It has produced high-density, large volume, quiescent, uniform hydrogen plasmas, which makes it a potential candidate for use as a plasma generator for neutral beam injectors. The device is a water-cooled cylindrical copper discharge chamber (25 cm in diameter by 36 cm long) with one end enclosed by a set of extraction grids with a 15-cm-diam multi-aperture pattern. The chamber wall serves as an anode and is surrounded by an external system of rare-earth cobalt magnets arranged in a line-cusp geometry of 12 cusps; plasma is produced by electron emission from a hot cathode assembly. This source has achieved extracted beam currents of 12 A at 18.5 kV, radial plasma density uniformities of +/-5% over a 15-cm diameter, noise levels of less than +/-0.5%, and arc efficiencies (beam current/arc power) of 0.6 A/kW.

Heat capacity measurements by means of thermal relaxation method in medium temperature range

Abstract: The thermal relaxation method is used to measure heat capacities of small samples in the temperature range around room temperature. The calorimeter is designed to measure under heat exchange gas and a steplike excess heat is applied to the sample by irradiation from a lamp.

Use of variable p(H) interface to a mass spectrometer for the measurement of dissolved volatile compounds.

Abstract: A wide variety of dissolved chemicals can be measured continuously with some specificity by directly interfacing a relatively inexpensive mass spectrometer to a solution. Specifically, we describe the construction and initial use of a membrane interace to a mass spectrometer which allows the continuous measurement of the concentration of dissolved volatile compounds in buffered aqueous solution. In addition to volatile compounds that do not dissociate appreciably at p(H) 7 (e.g., ethanol, ethylene glycol), volatile acids and bases, such as acetic acid or ammonia, can be measured with the interface is operated within the range 1

Method for the determination of the specific heat of metals at low temperatures under high pressures

Abstract: Efforts are made to determine the absolute specific heat of metals as a function of pressure with an accuracy, which, for the first time, will permit a direct evaluation of the pressure variation of the electronic and phonon parameters γ and ϑD′, respectively. This is achieved by employing the ac method of Sullivan and Seidel, and choosing a suitable sample configuration within a piston‐cylinder pressure cell. Essential is the use of diamond powder as a pressure transmitting medium, which because of its low heat capacity and high thermal resistance, couples the sample loosely to the temperature bath, represented by the pressure cylinder. Thus, the thermal requirements of the ac method are met, and corrections—if necessary—are fully tractable. High sensitivity even at the lowest temperatures is obtained by the use of thin slices, prepared from standard carbon resistors, for thermometers. Although delicate, these sensors withstand pressures of more than 20 kbars and remain sensitive. A thorough comparison of results on indium with the literature data at zero pressure is undertaken. It shows that the accuracy of the present results is comparable with that from literature data. Preliminary results at pressures up to 8 kbar are shown; they will be discussed in a separate paper.

Highly reliable temperature sensor using rf-sputtered SiC thin film.

Abstract: A SiC thin-film thermistor for high-temperature use has been developed by using rf-sputtered SiC thin films. This thermistor can be used for industrial and consumer use within an operating temperature range of -100 to 450 degrees C. By using SiC thin films, the thermistor maintains high electrical stability. The resistance change is less than 3% after exposure to heat at 400 degrees C for 2000 h. In addition, the film growth technique made possible the production of a high-accuracy thermistor, i.e., thermistor coefficient < +/-0.5%, thermistor resistance < +/-1.5%.

New digital filter for the analysis of experimental data

Abstract: The extremely simple mathematical technique called ’’straightening through smoothing,’’ which is a numerical frequency filter, is generalized in order to provide a transmission function having any shape. This frequency filter requires such a small memory that it can be performed using a minicomputer or even a programmable hand held calculator and the number of channels used is not limited to a power of 2, as in the case of the fast Fourier transform. For some filtering functions the number of operations required is smaller than with the fast Fourier transform.

Shielding of longitudinal magnetic fields with thin, closely spaced, concentric cylinders of high permeability material

Abstract: Formulae for the longitudinal shielding effectiveness of N thin, closely spaced, concentric cylinders of high permeability material have been developed and experimentally tested. For shields which cannot be oriented, or which change their orientation in the ambient field, the shielding effectiveness for longitudinal fields is generally the limiting criterion and no design formulae have previously been published for more than two shields. A simple diagrammatical method of writing the shielding formula is presented. Use of these equations is demonstrated by application to the design of magnetic shields for hydrogen maser atomic clocks. Examples of design tradeoffs such as size, weight, and material thickness are discussed. Experimental data on three sets of shields fabricated by three manufacturers are presented.

X-ray magnifier.

Abstract: A method for the magnification of x‐ray radiographic images is described and demonstrated. This magnifier employs two successive asymmetric diffractions of an x‐ray beam from highly perfect silicon crystals. The two diffractions magnify the beam in two perpendicular directions. A device with a magnification of 25× is demonstrated for Cu Kα radiation. This device preserves and sometimes improves the resolution inherent in the radiographic technique. The x‐ray magnifier is particularly useful in circumventing the relatively poor spatial resolution of electro‐optical imaging systems needed for real‐time observations. Basic limits on magnification and resolution using this method are described.

Optimization of data-acquistion rates in time-correlated single-photon fluorimetry.

Abstract: Four different operating configurations of a time‐to‐amplitude converter were examined for performance in the time‐correlated single‐photon fluorimetric experiment. Parameters considered were the intensity/count linearity, maximum data rate, and decay distortion. The inverted configuration with an interactive source was optimum. This arrangement combined total linearity with a 37‐kHz conversion rate and did not distort the fluorescence decay. In addition, the pulse‐height analyzer could severely limit the overall system data rate and could destroy the TAC intensity/count linearity.

Versatile digital lock‐in detection technique: Application to spectrofluorometry and other fields

Abstract: A digital lock‐in detection technique which overcomes some practical limitations of the analog lock‐in amplifiers is described. An example of implementation is given, based on multichannel pulse analyzers. Some representative results are reported for two typical application fields of lock‐in techniques, namely optical spectrometry and semiconductor profiling with C–V techniques. Advantages and limitation of the technique are discussed as well as adaptation to various modulation waveforms.

Quartz tube orifice leaks for local, fast-response gas sampling to mass spectrometers.

Abstract: Simple and versatile quartz tube orifice leaks, suitable for sampling of gas mixtures to mass spectrometers, have been made by heating the tip of a quartz tube in a hydrogen–oxygen flame. With these leaks the requirement of expensive and clumsy differential pumping stages is removed. The quartz probes have been used in gas sampling from catalytic reaction cells at 1 atm to a mass spectrometer. The sampling position can be located within 0.1 mm from the catalyst. Continuous recording of the local gas composition is then achieved with a response time of about 0.05 s, and with a minimum perturbation of the gas flow. The probes have been used in ambient air and at temperatures around 1000 K for extended periods of time without deterioration or plugging. The high stability at elevated temperatures and chemical resistance seem to make these probes useful for various applications, e.g., in sampling from combustion flames. The gas flow through the leak is determined by a very short and narrow constriction at the ...

Contactless technique for the measurement of electrical resistivity in anisotropic materials.

Abstract: Measurement of basal plane electrical resistivities in quasi‐two‐dimensional materials by conventional four‐point bridge techniques is extremely difficult. In making measurements of room temperature basal plane resistivities in highly conducting highly anisotropic synthetic metals we have developed an rf inductive technique which is both simple to use and provides accurate results. At the frequency we employ (100 kHz) the method is appropriate for resistivities in the range 10 −3–10−7Ω cm. Samples are thin square plates 5 mm on a side. The system is calibrated by fitting data from a series of samples of known resistivity to semi‐empirical formulae. The theoretical motivation leading to these expressions is discussed and extensive calibration data are presented. The advantages of rf techniques in general have already been discussed. Two advantages commend the particular technique we describe here. First, it is simple in design and easily constructed. Second, with careful calibration, highly accurate resistivity measurements can be made in a very straightforward manner

Cold-stage microscopy system for fast-frozen liquids.

Abstract: The least artifact‐laden fixation technique for examining colloidal suspensions, microemulsions, and other microstructured liquids in the electron microscope appears to be thermal fixation, i.e., ultrafast freezing of the liquid specimen. For rapid‐enough cooling and for observation in TEM/STEM a thin sample is needed. The need is met by trapping a thin layer (∼100 nm) of liquid between two polyimide films (∼40 nm thickness) mounted on copper grids and immersing the resulting sandwich in liquid nitrogen at its melting point. For liquids containing water, polyimides films are used since this polymer is far less susceptible to the electron beam damage observed for the commonly used polymer films such as Formvar and collodion in contact with ice. Transfer of the frozen sample into the microscope column without deleterious frost deposition and warming is accomplished with a new transfer module for the cooling stage of the JEOL JEM‐100CX microscope, which makes a true cold stage out of a device originally inte...

Uniform stress conditions in the diamond anvil cell at 200 kilobars.

Abstract: Pressure gradients in a diamond anvil cell have been measured with a 4:1 methanol:ethanol mixture as a pressure medium up to 350 kilobars. When pressure is applied rapidly, stress gradients are shown to be negligible up to 200 kilobars and probably above. With this procedure it is possible to significantly increase the precision of pressure measurements above 100 kilobars.

Pneumatic hydrogen pellet injection system for the ISX tokamak.

Abstract: We describe the design and operation of the solid hydrogen pellet injection system used in plasma refueling experiments on the ISX tokamak. The gun‐type injector operates on the principle of gas dynamic acceleration of cold pellets confined laterally in a tube. The device is cooled by flowing liquid helium refrigerant, and pellets are formed in situ. Room temperature helium gas at moderate pressure is used as the propellant. The prototype device injected single hydrogen pellets into the tokamak discharge at a nominal 330 m/s. The tokamak plasma fuel content was observed to increase by (0.5–1.2) ×1019 particles subsequent to pellet injection. A simple modification to the existing design has extended the performance to 1000 m/s. At higher propellant operating pressures (28 bars), the muzzle velocity is 20% less than predicted by an idealized constant area expansion process.