Volume 73, Issue 9, September 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
73(2002); http://dx.doi.org/10.1063/1.1499536View Description Hide Description
We demonstrate that a Geiger–Müller-type bandpassphotondetector, suitable for inverse photoemission experiments, can be constructed from a entrance window that has a high-energy transmission threshold of 10.97 eV, and ethanol, a detection gas with an ionization potential of 10.48 eV. The photondetector has a mean detection energy of and a bandpass of A photondetector can also be constructed from with a mean detection energy of and a bandpass These two new detection gas/window combinations have a higher detection energy and a narrower bandpass than the detector and Since all three detectors utilize a entrance window, the photonbandpass can be changed straightforwardly by changing the detection gas. For systems that can be easily damaged by electron beams, having the freedom to open up the detectorbandpass is an advantage because it can reduce the total electron exposure time.
73(2002); http://dx.doi.org/10.1063/1.1499541View Description Hide Description
The design of a frequency chirped external cavitydiode laser (CECDL) in the Littrow configuration with an intra-cavity electro-optic crystal is presented and its tuning and noise performance is studied. The CECDL has a center wavelength of 793 nm with a 20 nm tuning range by mechanical rotation of the feedback grating about a pivot point. Rapid tuning of the CECDL is achieved electronically by voltage control of the electro-optic crystal. The electro-optic tuning response of this laser is 2.01 MHz/V and linear frequency chirps of 800 MHz ranging in duration from 3 to 337 μs are demonstrated. The maximum electro-optic tuning is set by the external cavity mode spacing of 2.4 G Hz for this laser.
73(2002); http://dx.doi.org/10.1063/1.1499537View Description Hide Description
Although the energy dispersive x-ray diffraction (EDXD) technique has proved to have several merits in comparison with its conventional angular dispersive counterpart, it has the serious drawback that its resolution is intrinsically lower than that of the latter. This makes EDXD unsuitable each time that high-resolution measurements are needed. However, a wide class of samples does not require high resolution since the diffraction peaks they produce are so wide that the further broadening due to the use of EDXD is negligible. Amorphous solids,liquids, semicrystalline materials, and nanocrystallinepowders belong to such a category. In this case, it is not worth performing sophisticated simulations to calculate the angular transfer function of the diffractometer because a simplified model is sufficient to describe the effect of the angular divergence of the x-ray beam on peak broadening. The aim of the present work is to obtain an analytic function that can be used for this purpose, allowing the collimation system to be properly set up.
Externally controlled nanosecond Xe discharge lamp equipped with a synchronous high-voltage power supply using an automobile ignition coil73(2002); http://dx.doi.org/10.1063/1.1499535View Description Hide Description
We have constructed two kinds of nanosecond discharge lamps using an ignition coil for an automobile, in which a commercially available Xe lamp that is normally operated at a direct-current (dc) mode is employed. The first one is a free-running uncontrolled-driven Xe discharge lamp, where a positive high-voltage pulse obtained from the ignition coil is applied directly to an anodeelectrode of the Xe lamp. The second one refined is a synchronized-driven Xe discharge lamp, in which a capacitanceC attached on the anodeelectrode of the lamp is charged up in a stepwise fashion to the breakdown voltage of the lamp and then an external control signal for triggering the lamp discharge is fed on a high-voltage semiconductor switch connected between the cathodeelectrode of the lamp and the ground. Unlike the conventional discharge lamp, no high-voltage dc power supply is required and, therefore, it is free from undesirable arc discharge problems. To increase the repetition frequency of the pulsed emission, two ignition coils whose operating phases are shifted by 180° with respect to each other are incorporated. When pF, the pulse width, peak power, and repetition frequency were 8.0 ns, 2.0 W, and 125 Hz, respectively, with a time jitter of 60 ns.
- PARTICLE SOURCES, OPTICS and ACCELERATION; PARTICLE DETECTORS
73(2002); http://dx.doi.org/10.1063/1.1499215View Description Hide Description
The design and manufacturing details of a new crystal deflector for proton beams are reported. The technique allows one to manufacture a very short deflector along the beam direction (2 mm). Thanks to that, multiple encounters of circulating particles with the crystal are possible with a reduced probability of multiple scattering and nuclear interactions per encounter. Thus, drastic increase in efficiency for particle extraction out of the accelerator was attained (85%) on a 70 GeV proton beam. We show the characteristics of the crystal deflector and the technology behind it.
High-order maps with acceleration for optimization of electrostatic and radio-frequency ion-optical elements73(2002); http://dx.doi.org/10.1063/1.1497499View Description Hide Description
A method has been developed to calculate accurate high-order ion-optical maps for electrostatic and radio-frequency accelerating elements. The method has been incorporated into the arbitrary-order map-based beam optics code COSY Infinity. The treatment is restricted to the case of negligible magnetic fields, as is typical of heavy-ion accelerating cavities, and does not include space charge. For validation purposes, the beam dynamics calculated for these elements is compared against ray tracing for typical beam and cavity parameters. Different from the ray-tracing approach, parameter changes of individual components typically require only recalculation of the maps of the particular components and not the entire system, and thus the method is particularly suitable for optimization. The approach developed for accurate analytical representation of the on- and off-axis electric fields of cylindrically symmetric electrostatic lenses and radio-frequency cavities is described. Some of the many possible applications for using accurate high-order map representations of Einzel lenses,electrostatic accelerating gaps, and radio-frequency accelerating structures are discussed.
73(2002); http://dx.doi.org/10.1063/1.1499209View Description Hide Description
We discuss a numerical method for the optimization of the shape of noncircular coil windings for atomic physics applications. The method relies on two main features: first, a piecewise straight modeling of the curve which describes the winding; second, the definition of a step perturbation, based on pursuit curve theory, that changes the shape of the curve while conserving its length. The evolution resulting from a sequence of random steps is piloted towards the optimized shape by means of a Monte Carlo procedure. The method allows for significant improvement of the performance of magnetic configurations and is particularly suitable for dealing with cases in which geometrical constraints play a major role. Finally, as an example, the application of the method to the important case of a clover-leaf trap is discussed.
73(2002); http://dx.doi.org/10.1063/1.1498909View Description Hide Description
The combination of the time-of-flight(TOF) technique with two-dimensional position-sensitive (XY) ion detection is employed for tracking projectile-gas collisions. The main goal is to determine the detailed distribution of local electric fields, which accelerate the gas ions produced in the collisions. A low-pressure target gas spectrometer for this goal is described and all necessary XY- TOF expressions concerning the ion dynamics are presented. Experimental results of a 150 keV neutral H beam colliding with a He–Ne–Ar mixture are compared with the calculations. The new method can also be used for mapping magnetic fields or in large collision gas cells, e.g., to increase resolution in mass spectrometry and to perform initial momentum measurements of the produced ions.
- NUCLEAR PHYSICS, FUSION and PLASMAS
73(2002); http://dx.doi.org/10.1063/1.1499543View Description Hide Description
A bichromator system consisting of two interference filters with different bandwidths centered on the line and photomultiplier detectors for measuring the time evolution of the electron density in hydrogen-containing plasmas is described. This method is based on the time variation in the width of the Stark broadened line, as a result of which the ratio of the signals in the two detector channels is a function of the plasma electron density. Various aspects of this system are discussed briefly, including background light (line and continuum), self-absorption, choice of filter bandwidths, and detection limits. The apparatus is inexpensive, easy to calibrate, and comparatively insensitive to minor changes in optical setup. It has been used for measuring the electron density in laser sparkplasmas in room air, hydrogen, and dry air plus hydrogen.
73(2002); http://dx.doi.org/10.1063/1.1499762View Description Hide Description
Penumbral imaging is a powerful technique for imaging of penetrating radiations such as neutrons and hard x ray with a high spatial resolution. Recently, the technique has also been used for measurement of laser-produced plasma density profile. The limitation is that the straightforward image or signal reconstruction will introduce a significant distortion if the penumbral image is distorted by the noise. In this article, we propose a heuristic method for reconstruction of penumbral images. Both simulation and experimental results demonstrated that the proposed method is robust and tolerant to the noise.
73(2002); http://dx.doi.org/10.1063/1.1498906View Description Hide Description
An overview of the bolometerdiagnostic in the W7-ASstellarator is presented. Bolometers are used in plasma physics to measure radiated power losses that are mainly produced by impurity lines and neutral particles. With an array of collimated bolometers, the spatial and temporal evolution of these losses can be measured. The accuracy of the inversion of the line integrated measurements into local values necessitates a careful calibration and conversion of the measured signal into an incident power of each foil. Using transmission linetheory to determine the equivalent circuit for the sensor and cable, the equations for bolometercalibration and measurement, taking into account the cable resistance and capacitance between amplifier and detector, are derived. The ability to switch the bolometeramplifiers from measurement to calibration mode between discharges allows continuous monitoring of the calibration constants of the bolometer foil. Measurements by bolometer cameras and their Abel inversion for the radial profile of radiated power in W7-AS are presented.
- MICROSCOPY and IMAGING
Direct current scanning field emission microscope integrated with existing scanning electron microscope73(2002); http://dx.doi.org/10.1063/1.1499212View Description Hide Description
Electron field emission (FE) from broad-area metal surfaces is known to occur at much lower electric field than predicted by Fowler–Nordheim law. Although micron or submicron particles are often observed at such enhanced field emission (EFE) sites, the strength and number of emitting sites and the causes of EFE strongly depend on surface preparation and handling, and the physical mechanism of EFE remains unknown. To systematically investigate the sources of this emission, a dc scanning field emissionmicroscope (SFEM) has been built as an extension to an existing commercial scanning electron microscope(SEM) equipped with an energy-dispersive spectrometer for emitter characterization. In the SFEM chamber of ultrahigh vacuum a sample is moved laterally in a raster pattern (2.5 μm step resolution) under a high voltage anode microtip for field emission detection and localization. The sample is then transferred under vacuum by a hermetic retractable linear transporter to the SEMchamber for individual emitter site characterization. Artificial marks on the sample surface serve as references to convert coordinates of emitters in the SFEM chamber to corresponding positions in the SEMchamber with a common accuracy of in and Samples designed to self-align in sample holders are used in each chamber, allowing them to retain position registration after non-in situ processing to track interesting features. No components are installed inside the SEM except the sample holder, which does not affect the routine operation of the SEM. The apparatus is a system of low cost and maintenance and significant operational flexibility. Field emission sources from planar niobium—the material used in high-field rf superconducting cavities for particle accelerator—have been studied after different surface preparations, and significantly reduced field emitter density has been achieved by refining the preparation process based on scan results. Scans on niobium samples at are presented to demonstrate the performance of the apparatus.
73(2002); http://dx.doi.org/10.1063/1.1499757View Description Hide Description
We report on the performance of a microscope setup, which has been developed for the imaging of sum-frequency generation (SFG) from reflecting, nontransparent samples. In order to maximize the SFG intensity the sample has to be observed from one side at an angle near 60° with respect to the surface normal. The setup is designed (a) to keep focus over the full image field and (b) to compensate for the distortion of the field-of-view, both by means of a blazed grating. In contrast to “specular” SFG spectroscopy, the incident beams reflected from the sample and the generated SF light cannot be separated by angular filtering. In this setup the separation thus relies on spectral filtering only.
Development of laser assisted nanometric resolution scanning tunneling microscopy time-of-flight mass analyzer system73(2002); http://dx.doi.org/10.1063/1.1499764View Description Hide Description
This study describes a ground-breaking process that provides a direct highly localized measurement of the atomic mass on surfaces at room temperature. Employing an original system that joins a scanning tunneling microscopy(STM) device and a time-of-flight (TOF) mass analyzer, we could locally ionize surface atoms by the combination of an optical laser pulse and an appropriate voltage variation between the sample and the STM tip. Desorbed ions were accelerated and detected by a TOF chamber. Detection and discrimination of single atomic species from nanolocalized area have been demonstrated for the first time.
73(2002); http://dx.doi.org/10.1063/1.1499533View Description Hide Description
This article presents the design, identification, and control of a nano-positioning device suited to image biological samples as part of an atomic force microscope. The device is actuated by a piezoelectric stack and its motion is sensed by a linear variable differential transformer. It is demonstrated that the conventional proportional-integral control architecture does not meet the bandwidth requirements for positioning. The design and implementation of an controller demonstrates substantial improvements in the positioning speed and precision, while eliminating the undesirable nonlinear effects of the actuator. The characterization of the resulting device in terms of bandwidth, resolution, and repeatability provided illustrates the effectiveness of the modern robust control paradigm.
73(2002); http://dx.doi.org/10.1063/1.1499532View Description Hide Description
Tapping mode atomic force microscopy in liquids is enhanced using an insulated cantilever with an integrated piezoelectricmicroactuator. When vibrating the cantilever via direct force modulation by the actuator, a single resonance peak appears in the plot of rms cantilever amplitude versus excitation frequency, eliminating the spurious resonances typical of acoustic excitation in a liquid medium. This simplifies selection of the cantilever’s natural resonance frequency for improved tuning accuracy and speed. Acoustic excitation can excite cantilever modes that do not displace the tip of the cantilever but vibrate the microscope’s detection system and create unwanted liquid-coupled acoustic waves between the liquid-cell and the sample. These modes are eliminated by directly forcing the cantilever. Insulated microactuated probes offer a simple and more direct alternative solution to recently presented magnetic tuning methods.
73(2002); http://dx.doi.org/10.1063/1.1497503View Description Hide Description
An automated glitch-detection/restoration method of atomic force microscopeimages is proposed and implemented. Contrary to other manual methods, our method is based on the probability distribution of the derivative of the scanned image data. The glitches are identified as the points that deviate from a normal probability density function. The essence of the automation is calculating the distribution of the scanned image and removing the points that deviate from the normal distribution. Quantitative analysis of the original and the restored image have been performed and the degree of deformation of the restored images has also been analyzed. This technique can directly be applied to other types of scanning probe microscope equipments.
- CONDENSED MATTER; MATERIALS
Novel approach to produce polymerized hydrocarbon coatings using dielectric barrier controlled atmospheric pressure glow discharge plasma73(2002); http://dx.doi.org/10.1063/1.1499538View Description Hide Description
Conventionally, low-pressure (<1 Torr) electrical discharges are used for material processing and thin-filmdeposition. These schemes suffer mainly due to the high cost of equipment and the complexity of operations. The atmospheric pressureglow dischargeplasma is developed using a threaded styled electrode in different configurations, and these reactors are used to produce plasma polymerized coatings, required on plane substrates as self-supporting films to obtain membranes for blocking holes in cavities, and on microballoon targets, which are used as fuel containers for inertial confinement fusion, to avoid DT gas permeation. Helium gas is used as the supporting gas for formation and stabilization of atmospheric pressureglow dischargeplasma reactors. Ethylene and acetylene gases are used as monomers to produce plasma polymerized hydrocarbon films. These films are characterized using scanning electron microscopy. Plasma polymerized coatings of thickness 100 nm–10 μm with a smooth surface finish (rms<100 nm) are deposited successfully. The surface finish is further improved using a postdischarge configuration. Preliminary results are very encouraging but further progress is to be made in this area. We are also planning to extend this technique for C:H coating of microballoons, which are used as fuel containers in inertial confinement fusion.
Fiber-coupled antennas for ultrafast coherent terahertz spectroscopy in low temperatures and high magnetic fields73(2002); http://dx.doi.org/10.1063/1.1498904View Description Hide Description
For the purposes of measuring the high-frequency complex conductivity of correlated-electron materials at low temperatures and high magnetic fields, a method is introduced for performing coherent time-domain terahertz spectroscopy directly in the cryogenic bore of existing dc and pulsed magnets. Miniature fiber-coupled THz emitters and receivers are constructed and are demonstrated to work down to 1.5 K and up to 17 T, for eventual use in higher-field magnets. Maintaining the submicron alignment between fiber and antenna during thermal cycling, obtaining ultrafast (<200 fs) optical gating pulses at the end of long optical fibers, and designing highly efficient devices that work well with low-power optical gating pulses constitute the major technical challenges of this project. Data on a YBCO superconducting thin film and a high mobility two-dimensional electron gas are shown.
73(2002); http://dx.doi.org/10.1063/1.1497498View Description Hide Description
A direct measurement of strain in magnetostrictive materials yields a meaningful result only if the sample remains saturated and the magnetization is rotated between directions parallel and perpendicular to the measurement direction. A simple way to accomplish this is to use small permanent magnets mounted on a rotating shaft. Within the constraints of the limited available space for a sample, we found it most convenient to use a strain gage to detect small changes in length. With commonly available laboratory equipment we were able to achieve a sensitivity ∼0.03 ppm. There is a loading effect that reduces the apparent magnetostriction if the sample is of comparable size to the strain gage. We showed that for a typical nanocrystalline ribbon sample this correction could be experimentally estimated and was of order 20%.