Volume 75, Issue 2, February 2004
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY; PHOTON DETECTORS
Optical single-angle plane-wave transmittances/reflectances from Schwarzschild objective variable-angle measurements75(2004); http://dx.doi.org/10.1063/1.1641160View Description Hide Description
Photonic crystalstructures and other nanoscale and microscale optical structures are centrally important to future devicetechnology. The fundamental infrared single-angle plane-wave experimental characterization of these structures is needed to evaluate the analysis, design, and fabrication progress on these devices. The very small sizes of these devices necessitates focusing the infrared probe light typically with a Schwarzschild reflecting objective. The small spot size inherently requires the large range of incident angles associated with the objective. In this work, a variable-angle measurement method is presented for obtaining the optical single-angle plane-wave transmittances/reflectances. The primary steps in this method are (1) calculating the reference sample single-angle plane-wave transmittance/reflectance, (2) measuring the composite transmittances/reflectances of a reference sample over a range of objective angles of incidence, (3) calculating the intensity-angular-weighting coefficients for the objective using the Moore–Penrose (overdetermined linear equations)matrix inversion technique, (4) measuring the composite transmittances/reflectances of a sample-under-test over a range of objective angles of incidence, and (5) calculating the single-angle plane-wave transmittances/reflectances using the Moore–Penrose matrix inversion technique.
75(2004); http://dx.doi.org/10.1063/1.1642749View Description Hide Description
A velocity-map imaging system for time-resolvedphotoelectron spectroscopy is presented and discussed. Nanosecond gating of microchannel plates synchronized with the excitation laser in a velocity-map imagingspectrometer has been used to generate time-resolvedimages of electrons emitted from photoexcited xenon atoms and small carbon cluster anions. These images represent a time slice in the kinetic energy spectrum and reveal the differing time scales of competing electron emission processes.
75(2004); http://dx.doi.org/10.1063/1.1642742View Description Hide Description
Powerful lasers in the far-infrared wavelength range (47.6 and have been developed to measure the plasma density in the Large Helical Device at National Institute for Fusion Science and future plasma devices such as the International Thermonuclear Experimental Reactor. The intensification of these lasers has been done by cooling the laser tube wall, adding He as the buffer gas, and using a chemical-vapor-deposited diamond output window. The output powers for the 57.2 and lasers have been found to be 1.6 and 0.8 W, respectively.
- PARTICLE SOURCES, OPTICS and ACCELERATION; PARTICLE DETECTORS
75(2004); http://dx.doi.org/10.1063/1.1634362View Description Hide Description
A fast infrared array spectrometer (FIAS) with a thermoelectrically cooled 160-element PbSe detector was demonstrated using measurements of instantaneous infrared radiation intensities simultaneously over the 1.8–4.9 μm wavelength range at a sampling rate of 390 Hz. A three-point second-degree Lagrange interpolation polynomial was constructed to calibrate the FIAS because of the nonlinear response of the infrared array detector to the incident radiation beam. This calibration method gave excellent measurements of blackbody radiationspectra except for a narrow band at wavelength of 4.3 μm due to absorption by room carbon dioxide, which is one of the two major gas radiation peaks (2.7 and 4.3 μm) from the lean premixed hydrocarbon/air combustion products in the midinfrared spectrum. Therefore, the absorption coefficient of room carbon dioxide was conveniently measured on site with the blackbody reference source, and was used in the calibration of the FIAS and also in the calculations of the radiationspectra. Blackbody tests showed that this procedure was effective in correcting for the room carbon dioxide absorption in the radiationspectra measured by the FIAS. For an example of its application, the calibrated FIAS was used to measure spectral radiation intensities from three lean premixed laminar flames and a premixed turbulent jet flame for which reference data with a grating spectrometer were available for comparison. The agreement between the FIAS measurements and the reference data was excellent.
Cryogenic neutron detector comprising an InSb semiconductor detector and a supercritical helium-3 gas converter75(2004); http://dx.doi.org/10.1063/1.1642747View Description Hide Description
We evaluated the neutron-detection characteristics of a proposed cryogenicneutrondetector comprising an InSbsemiconductordetector and a helium-3 gas converter. The neutrondetector was operated at 4.2 K with helium-3 gas filling up to 1.5 atm, at which the density of the helium-3 nucleus corresponds to that at 160 atm at room temperature. The secondary particles generated by the reaction were successfully detected by the InSbdetector with a time response of ∼80 ns at all tested gas pressures.
75(2004); http://dx.doi.org/10.1063/1.1638869View Description Hide Description
There is an increasing interest in high flux sources of metastable species in many scientific communities, for example for lithography and quantum opticsexperiments. We present a simple dc discharge design, based on microstructured electrodes (MSE), for the production of truly thermal beams of metastable atoms. Even for inlet pressures above 1 atm the discharge runs stably, at relatively modest voltages. Time-of-flight data prove that the expansion is supersonic with speed ratios up to 8.5 and internal temperatures of less than 10 K. The MSE source works equally well for many different gases like He, Ne, Kr, Ar, and Its measured yield of metastable atoms compares favorably with conventional discharge sources. In addition, its simple design holds good promise for cooling the source down to cryogenic temperatures.
A simple and compact mechanical velocity selector of use to analyze/select molecular alignment in supersonic seeded beams75(2004); http://dx.doi.org/10.1063/1.1637433View Description Hide Description
A light and compact mechanical velocity selector, of novel design, for applications in supersonic molecular-beam studies has been developed. It represents a simplified version of the traditional, 50 year old, slotted disks velocity selector. Taking advantage of new materials and improved machining techniques, the new version has been realized with only two rotating slotted disks, driven by an electrical motor with adjustable frequency of rotation, and thus has a much smaller weight and size with respect to the original design, which may allow easier implementation in most of the available molecular-beam apparatuses. This new type of selector, which maintains a sufficiently high velocity resolution, has been developed for sampling molecules with different degrees of rotational alignment, like those emerging from a seeded supersonic expansion. This sampling is the crucial step to realize new molecular-beam experiments to study the effect of molecular alignment in collisional processes.
75(2004); http://dx.doi.org/10.1063/1.1642744View Description Hide Description
Proton beams are needed in neutral-beam injection for diagnostic development of an internal magnetic field measurement. High proton fraction, low axial energy spread, current density in excess of and a parallel ion beam with cw operation are the requirements for the ion source/extraction system. A multicusp-type ion source with an external rf antenna was constructed at Lawrence Berkeley National Laboratory. A proton fraction of 85% and protoncurrent density of were achieved at 1.8 kW of rf power. Plasma parameters were measured with a rf compensated Langmuir probe.
75(2004); http://dx.doi.org/10.1063/1.1642746View Description Hide Description
A toroidal analyzer has been developed to measure angular differential cross sections for electron scattering in electron–ion collisions. Energy and angular dispersed electron images are recorded by using a two-dimensional position sensitive detector. Operation and calibration of the analyzer are described with particular emphasis on a procedure to deduce the differential cross sections from the acquired two-dimensional data. Angular differential cross sections for elastic scattering of electrons from and ions are presented in the angular range from to at a collision energy of 100 eV in the center of mass frame.
75(2004); http://dx.doi.org/10.1063/1.1638876View Description Hide Description
The basic results of numeric simulations of heavy-ion motion in cyclotron CYTRACK are presented. Computer modeling confirms the possibility of ion acceleration and effective extraction. The experimental data completely support theoretical results presented.
- NUCLEAR PHYSICS, FUSION and PLASMAS
75(2004); http://dx.doi.org/10.1063/1.1637438View Description Hide Description
Initial results from a novel device to detectdust particles settling on remote surfaces are presented. Dust particle inventories are a concern in next-step fusion devices. The increase in duty cycle will lead to a scale-up in the amount of particles generated by plasma material interactions. These particles will be chemically and radiologically hazardous and it will be important to establish that the in-vessel particle inventory is within regulatory limits. The detection device consists of two interlocking combs of closely spaced conductive traces on a Teflon circuit board. When a direct current bias is applied, impinging dust creates a transient short circuit between the traces. The increase in bias current generates a signal pulse that is counted by standard nuclear counting electronics. We present data on the response of the device in air and vacuum to carbon particles.
Time resolved radiated power during tokamak disruptions and spectral averaging of AXUV photodiode response in DIII-D75(2004); http://dx.doi.org/10.1063/1.1642745View Description Hide Description
Silicon absolute extreme ultraviolet (AXUV) photodiodes have been employed in a disruption radiometerdiagnostic for measurement of radiant power in the DIII-D tokamak with a 170 kHz bandwidth. This is motivated by a need to improve the understanding of radiative processes in tokamak disruptions. The diagnostic described in this article has a single line of sight though the central plasma. Accounting for the photon energy dependence of the AXUV photodiode responsivity is made possible by optical filtering, with the aid of spectra from an extreme ultraviolet survey spectrometer. The appropriate effective responsivity for interpretation of the data is lower than the nominal value typically used for the detector. In the current quench phase of disruptions, it is less than half the nominal value. Comparisons with results from a foil bolometer find good agreement.
75(2004); http://dx.doi.org/10.1063/1.1637459View Description Hide Description
A new type of direct current, high-density, and low electron temperature reflex plasma source, obtained as a hybrid between a modified hollow-cathode discharge and a Penning ionization gauge discharge is presented. The plasma source was tested in argon, nitrogen, and oxygen over a range pressure of discharge currents 20–200 mA, and magnetic field 0–120 Gauss. Both external parameters, such as breakdown potential and the discharge voltage–current characteristic, and its internal parameters, like the electron energy distribution function, electron and ion densities, and electron temperature, were measured. Due to the enhanced hollow-cathode effect by the magnetic trapping of electrons, the density of the bulk plasma is as high as and the electron temperature is as low as a few tenths of electron volts. The plasma density scales with the dissipated power. Another important feature of this reflex plasma source is its high degree of uniformity, while the discharge bulk region is free of an electric field.
75(2004); http://dx.doi.org/10.1063/1.1641157View Description Hide Description
In this work the experimental results of a nonequilibrium laser-plasma induced by an ultraviolet 308 nm excimer laser are reported. All measurements were performed fixing the laser energy at 70 mJ. It was concentrated on a 0.0099 cm2 spot by a convergent focal lens of 15 cm focal length. The utilized target was a 99.99% pure Cu disk. An 8 cm in diameter movable Faraday cup was developed in order to detect the plasma flow pulse at different positions along a drift tube. Analyzing the time-of-flight pulse under different cup bias voltage, we were able to distinguish the electron pulse, the suprathermal ions, and the thermal evolution of the plasma. In addition, by applying a breakdown voltage as polarizing cup voltage, we characterized the duration of the neutral component. To determine the system particle production efficiency, the total etched material per pulse, 0.235 μg, and the fractional ionization were measured. The expelled particle flux distribution was measured by an optical transmission analysis of a Cu deposited film on a glass substrate. The plasma flow was detected along its propagation axis, between 6 and 40 cm far from the target. The ablation process expelled particles with an initial velocity of 34 km/s, while the maximum ion concentration was 1 μs after the laser pulse. The plasma created propagates with a mean velocity of about 20 km/s. During the propagation, the longitudinal plasma dimension changed from 2.8 cm, near the target, to 31 cm at the maximum cup distance analyzed. At lowest distances, the cup signal wave forms presented a plateau due to the high dense plasma undergone to the space charge regime governed by the Child–Langmuir law.
75(2004); http://dx.doi.org/10.1063/1.1634353View Description Hide Description
Electrostatic probes are widely used to measure spatial plasma parameters of the quasineutral plasma in Hall thrusters and similar electric discharge devices. Significant perturbations of the plasma induced by such probes can mask the actual physics involved in the operation of these devices. In Hall thrusters, probe-induced perturbations can produce changes in the dischargecurrent and plasma parameters on the order of their steady-state values. These perturbations are explored by varying the material, penetration distance, and residence time of various probe designs. A possible cause of these perturbations appears to be the secondary electron emission, induced by energetic plasma electrons, from insulatorceramic tubes in which the probe wire is inserted. A new probe in which a low secondary electron emission material, such as metal, shields the probe ceramic tube, is shown to function without producing such large perturbations. A segmentation of this shield further prevents probe-induced perturbations, by not shortening the plasma through the conductive shield. In a set of experiments with a segmented shield probe, the thruster was operated in the input power range of 500 W–2.5 kW and discharge voltages of 200–500 V, while the probe-induced perturbations of the dischargecurrent were below 4% of its steady-state value in the region in which 90% of the voltage drop takes place.
- MICROSCOPY and IMAGING
75(2004); http://dx.doi.org/10.1063/1.1637457View Description Hide Description
Because of its ultrahigh sensitivity, the optical lever detection method similar to that used in the atomic force microscope(AFM) has been widely employed as a standard technique for measuring microcantilever deflection. Along with the increasing interest in using the microcantilever as a sensing platform, there is also a requirement for a reliable calibration technique. Many researchers have used the concept of optical lever detection to construct microcantilever deflection readout instruments for chemical, physical, and biological detection. However, without an AFM piezo zscanner, it is very difficult to precisely calibrate these instruments. Here, we present a step-by-step method to conveniently calibrate an instrument using commercially available piezoresistive cantilevers. The experimental results closely match the theoretical calculation. Following this procedure, one can easily calibrate any optical cantilever deflection detection system with high reproducibility, precision, and reliability. A detailed discussion of the optical lever readout system design has been addressed in this article.
75(2004); http://dx.doi.org/10.1063/1.1642743View Description Hide Description
Seminal field ionization detection of a supersonic helium beam is reported, including intensity measurements on a continuous beam and time-of-flightmeasurements on a chopped beam. Also unique to these experiments is the care with which the field ionization tip was prepared and characterized prior to its use. Comparative measurements on a static helium gas are presented and the effective detection area of the field ionization tip is extracted for both beam and static gas. These areas are much smaller than those reported in previous experiments, suggesting that detection efficiency depends strongly on energy and momentum transfer during the initial atom–tip collision.
Quantitative characterization of friction coefficient using lateral force microscope in the wearless regime75(2004); http://dx.doi.org/10.1063/1.1637436View Description Hide Description
Absolute quantitative data from atomic force microscopy (AFM)/lateral force microscopy experiments are always difficult to obtain mainly due to the need of the normal force and the friction force calibration. In this article, we developed an experimental method which allows us to extract absolute quantitative friction data without calibrating any force when the relation between and is linear or only calibrating the normal force when the relationship is nonlinear. The technique reported here, is suitable for an atomic force microscope that has the cantilever attached to the piezotube translator and an unguided incident laser beam on the cantilever. We take advantage of the piezotube bending during a large scan (5 μm×5 μm), generally considered as an undesirable effect, to calculate a detection factor that allows the determination of quantitative tribological data. The validity of our experimental method is checked on the extensively AFM studied materials, such as muscovite, silicon, and highly oriented pyrolytic graphite. The experiments are carried out in a load range where the shear stress τ can be expressed as where μ is the friction coefficient, P is the mean contact pressure, and is a parameter related to the tip/sample adhesion. The value of μ is found to be independent of the tip geometry and the pull-off force, and always constant for a given tip/sample couple in the load range investigated.
Sub-10 nm lateral spatial resolution in scanning capacitance microscopy achieved with solid platinum probes75(2004); http://dx.doi.org/10.1063/1.1641161View Description Hide Description
Sub-10 nm resolution can be obtained in scanning capacitancemicroscopy (SCM) if the probe tip is approximately of the same size. Such resolution is observed, although infrequently, with present commercially available probes. To acquire routine sub-10 nm resolution, a solid Pt metal probe has been developed with a sub-10 nm tip radius. The probe is demonstrated by SCM imaging on a cross-sectioned 70 nm gatelength field-effect transistor(FET), a shallow implant 24 nm junction depth), and an epitaxial staircase (p, ∼75 nm steps). Sub-10 nm resolution is demonstrated on the FET device over the abrupt meeting between a silicon-on-insulator oxide layer and a neighboring Si region. Comparable resolution is observed on the implant structure, and quantitative SCM dopant profiling is performed on it with sub-10 nm accuracy. Finally, the epitaxial staircase structure is quantitatively profiled demonstrating the accuracy obtained in quantitative profiling with the tips.
Transmission electron microscopy specimen holder for simultaneous in situ heating and electrical resistance measurements75(2004); http://dx.doi.org/10.1063/1.1611616View Description Hide Description
A transmission electron microscopy(TEM) specimen holder has been developed for the measurement of the electrical resistance of a TEM sample as a function of temperature. A custom TEM heating holder was modified for this purpose. Eight feedthrough wires were passed through the specimen holder; (i) providing current to the heater, (ii) allowing for the measurement of the hot stage temperature, and (iii) enabling the measurement of the electrical resistance of the sample. This configuration creates the opportunity to directly correlate changes in the resistance to microstructural changes as a function of temperature. The working of the holder is demonstrated by studies on a phase transformation in Al–Ge films. In this way, a direct correlation was found between changes in resistance and microstructure as a function of temperature.