Volume 72, Issue 9, September 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1396661View Description Hide Description
We specifically developed a diode-pumped, Q-switched Nd:YAG laser to act as a pump source for a terahertz-wave parametric generator. A single Q-switched laser pulse (pulse width 12 ns, pulse repetition frequency 40 Hz) produced an output energy of 12 mJ. Utilizing this pump source, we were able to construct a compact (650×350 mm) terahertz-wave parametric generator using crystals. We achieved a broad terahertz-wave spectrum, ranging from 1.2 to 2.4 THz, with a pump energy of 12 mJ.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
72(2001); http://dx.doi.org/10.1063/1.1396662View Description Hide Description
A neutral effusive source was assembled in a crossed molecular-beam apparatus in order to perform atom–molecule electron transfercollisions and its calibration by time-of-flight mass spectrometry. This source consists of a stainless-steel oven where the sublimation takes place. Typical operation temperatures are in the range of 723 up to 888 K. Through this setup the study of the ion pair formation process (where K is a potassium atom) can be performed in a wide range of collision energies (10 up to 500 eV). The results show evidence of negative ion formation (being the ratio of intensities between the parent ion and fragment ions) clearly dependent on the collision energy used.
72(2001); http://dx.doi.org/10.1063/1.1387254View Description Hide Description
Design and analysis of a new compensating element for improving the electron pulse front and compressing the pulse duration in a femtosecond photoelectron gun are described. The compensating element is a small metallic cylindrical cavity in which an external voltage is applied in such a way that a special electric field forms and interacts with the electron pulse. This electric field reduces the distances between the faster and slower electrons inside the cavity and efficiently compensates for electron pulse broadening caused by the photoelectron energy spread and space charge effects. Poisson’s equation and the equation of motion are solved to obtain the electron trajectories. Results highlight the important design parameters of the new compensating element and show its feasibility in compressing electron pulses in the femtosecond regime.
- NUCLEAR PHYSICS, FUSION and PLASMAS
72(2001); http://dx.doi.org/10.1063/1.1386896View Description Hide Description
Calibration procedures for Thomson scattering systems based on television-like cameras, so-called TVTS systems, are described. The TVTS systems of the Rijnhuizen Tokamak Project (RTP), the TJ-II stellarator, and the Torus Experiment for Technology Oriented Research (TEXTOR)tokamak combine a 10–15 J ruby laser as a source with an intensified charge coupled device camera as a detector. A tungsten strip lamp in combination with an integrating sphere is used to calibrate all pixels of the camera relatively to each other. Rayleigh scattering on hydrogen or nitrogen is used to perform an absolute calibration of the complete detection system. Great emphasis is placed on possible systematic errors on the determination of the electron temperature and density due to the calibration, such as tungsten lamp temperature, detoriation of the detection window, long term stability, laser beam alignment, and detector linearity. The long term stability of the system was tested by comparing different sets of calibration factors. Over a period of 1/2 yr the values of the calibration factors varied by less than 5%. Using the same Thomson scattering data but different sets of calibration factors the values varied even less than 1%. A two camera technique was used to search for possible unknown systematic errors in the determination of profiles. It appears that these systematic errors are about half of the observational error on Density fluctuations can be determined with an accuracy equal to the statistical error of ∼3%, while the systematic error on appeared to be ∼10%, which can be corrected for using interferometer data. As a result, these TVTS systems can measure and profiles with ∼100 (RTP) or 120 (TEXTOR) spatial elements along the full plasma diameter with observational errors on of ∼6% in the range of 25 eV–6 keV, at and 10 J laser energy.
Rotating magnetic field oscillator system for current drive in the translation, confinement, and sustainment experiment72(2001); http://dx.doi.org/10.1063/1.1389491View Description Hide Description
The experimental setup and test results for the ∼125 MW rotating magnetic fieldcurrent drive system of the Translation, Confinement and Sustainment Experiment at the University of Washington are described. The oscillator system, constructed at Los Alamos National Laboratory, drives two tank circuits potential, maximum circulating current in each tank to date) operated 90° out of phase to produce a 54 G rotating magnetic field with a frequency of 163 kHz Programmable waveform generators control “hot deck” totem pole drivers that are used to control the grid of 12 Machlett 8618 magnetically beamed triode tubes. This setup allows the current to be turned on or off in less than 100 ns (∼6°). Both tank circuits are isolated from the current source by a 1:1 air core, transmission line transformer. Each tank circuit contains two saddle coils (combined inductance of 1.6 μH), and radio frequency capacitors (580 nF). Test results are presented for three conditions: no external load, a resistive external load, and a plasma load. A SPICE model of the oscillator system was created. Comparisons between this model and experimental data are given.
72(2001); http://dx.doi.org/10.1063/1.1396665View Description Hide Description
An ultrafast quenched plastic scintillation detector was used to measure the fusion fuel ion temperature at low implosion (DT) neutron yield in the initial experiment performed at the Shenguang II laser facility. The typical temperatures of exploding pusher targets for direct drive were around 4 keV and the uncertainties were ±15–23%. The detection efficiency of the detector to DTneutrons was calibrated at a K-400 accelerator. The time response function of the detection system was calibrated by implosion neutrons from a DT-filled capsule, which can be regarded as a δ function pulsed neutron source due to its much narrower pulse width than that of the measuredneutrontime-of-flight spectrum.
- MICROSCOPY and IMAGING
72(2001); http://dx.doi.org/10.1063/1.1389497View Description Hide Description
We present the design and performance of a high-pressurescanning tunneling microscope (HP–STM), which allows atom-resolved imaging of metal surfaces at pressures ranging from ultrahigh vacuum (UHV) to atmospheric pressures–1000 mbar) on a routine basis. The HP–STM is integrated in a gold-plated high-pressure cell with a volume of only ∼0.5 l, which is attached directly to an UHV preparation/analysis chamber. The latter facilitates quick sample transfer between the UHV chamber and the high-pressure cell, and allows for in situ chemical and structural analysis by a number of analytical UHV techniques incorporated in the UHV chamber. Reactant gases are admitted to the high-pressure cell via a dedicated gas handling system, which includes several stages of gas purification. The use of ultrapure gasses is essential when working at high pressures in order to achieve well-defined experimental conditions. The latter is demonstrated in the case of H/Cu(110) at atmospheric pressures where impurity-related structures were observed.
A technique to investigate inhomogeneity in materials: An arrangement of microtip and scanning electron microscope72(2001); http://dx.doi.org/10.1063/1.1389490View Description Hide Description
Amorphous and polycrystallinematerials show different electrical properties when the contact area is in the submicron range. A large number of instruments (such as combinations of scanning electron microscope and scanning tunneling microscope) have been employed for such an investigation. A distinct disadvantage of these instruments is that they are rather expensive. In view of this, we have developed an instrumentation, which can be used to measure electrical behavior of any material in the submicron scale. This was achieved by installing a simple tip-holding arm in the scanning electron microscope; electrical meters (picoammeter and an LCR bridge) were used to investigate the behavior. We have studied amorphous carbon films grown in our laboratory, using this technique.
72(2001); http://dx.doi.org/10.1063/1.1392341View Description Hide Description
The article describes the combination of a Besocke-type scanning tunneling microscope(STM) with a scanning electron microscope(SEM) in an ultrahigh vacuum (UHV) environment. The open design of the Besocke STM allows the SEM to be implemented as an add-on of a high resolution electron column and a secondary electron detector. The combined instrument is capable of atomic resolution imaging by STM and real time SEM imaging. SEM resolution down to about 80 nm was achieved. Simultaneous operation of STM and SEM is possible. The operation and performance of the combined instrument is illustrated by a variety of examples. Although the instrument is suitable for a wide range of applications where a combination of atomic resolution with lower magnification imaging is required, its operation in an UHV environment makes it particularly appropriate for the study of reactive metal surfaces.
72(2001); http://dx.doi.org/10.1063/1.1394181View Description Hide Description
We report on the design and fabrication of a coarse approach mechanism, called a piezotube walker, based on a piezoelectric tube moving inside a triangular prism shaped cavity. This walker walks like a six legged insect moving its legs one by one and then the belly following. The walker works in any orientation from horizontal to vertical and its motion is found to be linear with the applied voltage above a threshold voltage. A compact scanning tunneling microscope(STM) was fabricated using this approach mechanism. The scanner tube of the STM is mounted on the inside of the walker tube, reducing the size of the STM considerably. Topographical images with atomic resolution were obtained for layered materials like graphite and
72(2001); http://dx.doi.org/10.1063/1.1392340View Description Hide Description
We describe the design and performance of a low-cost, bidirectional temperature control system with very high stability and low thermal gradients in a working range from −10 to System elements such as thermal insulation, thermal pumps, and feedback control as well as safety features are discussed. The performance of the entire system under realistic experimental conditions is documented. Precise temperature control has led to significantly improved instrumental stability. An extension of the instrumental window towards slower measurements has been made possible. This temperature-control system is presented in the framework of the extended surface forces apparatus but is also applicable to other scientific instruments that require subnanometer positioning stability over extended periods of time.
72(2001); http://dx.doi.org/10.1063/1.1394188View Description Hide Description
In this article we formulate a general methodology for estimating the bias error distribution of a device in a measurement domain from less accurate measurements when a minimal number of standard values (typically no more than two values) are available. A new perspective is that the bias error distribution can be found as a solution of an intrinsic functional equation in a domain. Based on this theory, the scaling- and translation-based methods for determining the bias error distribution are developed. These methods are applicable to virtually any device as long as the bias error distribution of the device can be sufficiently described by a power series (a polynomial) or a Fourier series in a domain. These methods were validated through computational simulations and laboratory calibrationexperiments for a number of different devices.
- CONDENSED MATTER; MATERIALS
72(2001); http://dx.doi.org/10.1063/1.1388215View Description Hide Description
Validity of the surface tensionmeasurement technique that was proposed by Elleman et al. was experimentally verified. The technique was based on Brown and Scriven’s work on the shape evolution of rotating drops. Molten tin and aluminumdrops were levitated in high vacuum by the electrostatic levitator and rotated by applying a rotating magnetic field. This technique offers an alternative technique for those liquids where the drop oscillation technique cannot be used. As a demonstration, the technique was applied to a glass-forming alloy and its surface tension was measured down to the lower temperature where the drop oscillation technique could not be applied due to its high viscosity.
72(2001); http://dx.doi.org/10.1063/1.1389495View Description Hide Description
We have recently developed a noncontact technique capable of detecting microscopic variations in the surface potential of charged dielectric films. The technique is based on measuring the charge induced on a small capacitive probe held at a constant distance from a charged sample surface. Distance control is achieved by aerodynamic floating, which is an inexpensive and simple passive feedback system capable of maintaining a constant probe-sample separation despite minor variations in sample morphology. We have used the technique to detect the presence of microscopic electrostatic defects in organic photoreceptors, such as charge deficient spots, which are responsible for image degradation in xerographic copiers and printers. However, aerodynamic floating could be implemented in other applications requiring accurate control over probe-sample separation.
72(2001); http://dx.doi.org/10.1063/1.1394186View Description Hide Description
An accurate algorithm is described for the computation of the theoretical values of the linear four-point probe thickness correction factors for point injection of current and insulating and conducting substrates. Three expressions are presented for the insulating substrate case that yield maximum fractional errors of 0.03%, and respectively. Additional expressions are presented for the case of a film on a conducting substrate and a critical comparison of the two cases is made. The theoretical basis of the algorithms, a Euler–Maclaurin expansion, is described.
72(2001); http://dx.doi.org/10.1063/1.1394178View Description Hide Description
A capability to produce quasi-isentropic compression of solids using pulsed magnetic loading on the Z accelerator has recently been developed and demonstrated [C. A. Hall, Phys. Plasmas 7, 2069 (2000)]. This technique allows planar, continuous compression of materials to stresses approaching 1.5 Mbar. In initial stages of development, the experimental configuration used a magnetically loaded material cup or disk as the sample of interest pressed into a conductor. This installation caused distortions that limited the ability to attach interferometer windows or other materials to the rear of the sample. In addition, magnetic pressure was not completely uniform over sample dimensions of interest. A new modular configuration is described that improves the uniformity of loading over the sample surface, allows materials to be easily attached to the magnetically loaded sample, and improves the quality of data obtained. Electromagnetic simulations of the magnetic field uniformity for this new configuration will also be presented. Comparisons between data on copper to ∼300 kbar using the old and new experimental configurations will also be made. Results indicate that to within experimental error, the configurations produce similar results in the pressure-volume plane.
72(2001); http://dx.doi.org/10.1063/1.1391441View Description Hide Description
This work reports on the experimental results of a Nbphotocathode stimulated by two different lasers, of wavelengths 308 and 222 nm. A fast current detector, Rogowski coil, recorded the high output currents, while the beam angular distributions were measured by an array of small Faraday cups placed along the propagation axis of the beam. With p-polarized light, we found quantum efficiencies (QEs) of and for and respectively. The efficiency of the photocathode was measured to be higher for s-polarized laser beams than for the p-polarized ones. They resulted in QEs of and for and respectively. From the electron-beam angular divergence values, we determined the upper limit emittance. The lowest upper limit normalized emittance value found was 7 (π mm mrad), achieved by the s-polarized radiation of 308 nm. In both cases the s-polarized light generated electron beams of lower emittance.
72(2001); http://dx.doi.org/10.1063/1.1396663View Description Hide Description
We report on a new miniature, high-resolution, susceptibilitythermometer that employs as the paramagnetic material for temperature measurement below 4 K. The thermometer utilizes small permanent disk magnets to provide the required magnetic field, and has a total mass of only 2 g. The thermometer shows a resolution of 0.2 nK in a 1 Hz bandwidth and an absolute drift rate less than 50 fK/s at 2.2 K. In this article, we describe the thermometer design, assembly procedure, and experimental test results, including its thermal time constant and susceptibility to external magnetic fields.
A high-temperature high-pressure optical cell for general-purpose spectrometers designed for supercritical water experiments72(2001); http://dx.doi.org/10.1063/1.1389494View Description Hide Description
We have developed a compact size high-temperature high-pressure optical cell that can be mounted on a commercially available general-purpose spectrometer. A small electric furnace heats the high-pressure optical cell while a water-cooled thermal shielding jacket protects the spectrometer. This cooling device works quite effectively to eliminate undesirable heating of spectrometer optics and thus makes it possible to take spectra at temperatures up to 600 °C with an ordinary spectrometer. The optical cell is made of Hastelloy-X and equipped with two optical windows of synthetic sapphire. The length and the diameter of the optical path are 10 and 6 mm, respectively. As the optical cell is combined with a flow system, which allows the quick replacement of the sample solution with the reference water, the reference signal can be taken immediately after the measurement of the sample signal. The developed cell can be operated up to 60 MPa and 600 °C without any special modification to the spectrometer. The absorption spectra were taken under the supercritical water condition to demonstrate the efficiency of the optical cell.
Analysis of a drift tube at ambient pressure: Models and precise measurements in ion mobility spectrometry72(2001); http://dx.doi.org/10.1063/1.1392339View Description Hide Description
Mobility spectra for positive ions, created from a foil in purified air at ambient pressure (660 Torr) with 0.15 ppm moisture, were obtained with a drift tube with a discrete drift ring design at 250 °C as electric fields for components were individually and independently varied. Peak area, peak width, baseline intensity, drift times, and reduced mobilities were used to measure the function and performance of each component and findings were interpreted using a model for the transport of thermalized ions in weak electric fields at ambient pressure. Transit times and intensities for ions in drift tubes at ambient pressure can be understood through a detailed knowledge of the fields local to a component and derivations from theory of ion transport. Prolonged ion residence in the drift region resulted in ion transformations even for highly purified gases of low moisture at high temperature. These findings suggest that mobility spectra may be obtained with uniformly high quality and reproducibility only under conditions when ion residence time is the primary point of reference in obtaining spectra. Other regions of the drift tube were optimized and newly observed chemistry occurred in the aperture to detector region. The sampling of ions by such an ion shutter was found to inherently bias the ion distributions and alter actual lengths of drift regions. Consequently, drift lengths measured from physical configurations of drift tubes will be inadequate for precise measurements of drift times. These studies establish baseline measurements for evaluating drift tubes that should be generally applicable for optimizing performance in other drift tubes with discrete drift ring designs. Also, these results demonstrate that precise measurements in ion mobility spectrometry (IMS) will require attention to detail not heretofore carefully regarded in modern analytical IMS.