Volume 72, Issue 5, May 2001
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
72(2001); http://dx.doi.org/10.1063/1.1368843View Description Hide Description
This study concerns the attenuation of the incident power from a free-electron laser(FEL) in steps of up to two orders of magnitude, while maintaining optical beam geometry. We have developed and evaluated the properties of a FELattenuation filter system, which can be attached and detached, with the position of the focusing lens and the target sample fixed. As a result, we have established that our FELattenuation filter system enables two orders of magnitude of power control for infrared wavelengths between 5.5 and 6.5 μm. We conclude that the attenuationsystem is extremely useful for various FEL applications.
The new ultraviolet spectral responsivity scale based on cryogenic radiometry at Synchrotron Ultraviolet Radiation Facility III72(2001); http://dx.doi.org/10.1063/1.1361081View Description Hide Description
The recently completed upgrade of the Synchrotron Ultraviolet Radiation Facility (SURF III) at the National Institute of Standards and Technology (NIST) has improved the accuracy of radiometric measurements over a broad spectral range from the infrared to the soft x ray. The beamline 4 at SURF III is a cryogenic-radiometer based radiometric facility for the ultraviolet (UV)spectral range. The upgrade of SURF III has allowed us to use beamline 4 to improve the detectorspectral power responsivity scales in the wavelength range from 125 to 320 nm. The achieved combined relative standard uncertainty is better than 0.5% over most of this spectral range. This is a significant improvement over the more than 6% relative standard uncertainty in this spectral range of the current scales maintained at the Spectral Comparator Facility (SCF) in the Optical Technology Division and the Far UVCalibration Facility in the Electron and Optical Physics Division. The new UV scale of beamline 4 was subsequently intercompared and transferred to the SCF and to the Far UVCalibration Facility to improve their UV scales and ensure consistency within NIST. The new scale established at beamline 4 improves NIST’s calibration capabilities for environmental monitoring, astrophysics, and the UV industry. The new scale also includes wavelengths such as 193 and 157 nm excimer laser wavelengths, which are of particular interest to the semiconductor photolithography industry.
72(2001); http://dx.doi.org/10.1063/1.1362439View Description Hide Description
The design and construction of a lithium ion attachment mass spectrometer that is used to study the occurrence of free radical intermediates and molecular and atomic species (which are not found under ordinary conditions) on a real-time basis in the gas phase are described. The newly developed device exhibits several advantages over conventional mass spectrometers. The performance of this equipment was extensively tested and optimized, and experimental tests show that the efficiency and sensitivity of the instrument are moderately high. The main applications and possible future perspectives of this approach are briefly reviewed.
72(2001); http://dx.doi.org/10.1063/1.1364667View Description Hide Description
We describe the realization of a magnetometer based on optical pumping with an optical frequency modulated laserdiode light. Performances (amplitude and frequency spatial isotropy and sensitivity) of our device are presented and compared with those of other scalar magnetometers.
- PARTICLE SOURCES, OPTICS and ACCELERATION
Production and characterization of highly intense and collimated cluster beams by inertial focusing in supersonic expansions72(2001); http://dx.doi.org/10.1063/1.1361082View Description Hide Description
Intense and collimated supersonic cluster beams have been produced by exploiting inertial focusing effects. To this goal we have developed and tested a novel focusing nozzle (focuser). Using this device with a pulsed microplasma cluster source we have obtained cluster beams with a divergence of 10 mrad and average densities of pulsed) corresponding to deposition rates of 2 nm/s at 300 mm distance from the source nozzle. With a focusing nozzle cluster thermal relaxation and mass distribution in a supersonic expansion can be controlled. We have measured the cluster transverse velocities, with extremely high precision, by characterizing the cluster beam deposition on a substrate by an atomic force microscope. Besides the relevance for the understanding of relaxation processes in expanding jets, the inertial focusing of clusters has several important consequences for the synthesis of nanostructured films with controlled structure and for all the experimental techniques requiring intense and collimated cluster beams. Due to its simplicity the focusing nozzle presented here can be used with a wide variety of cluster sources.
72(2001); http://dx.doi.org/10.1063/1.1366633View Description Hide Description
A novel method which measures beam axial and rotational velocity for any large-orbit beam with a common guiding center is described. Time-integrated beam velocity information is obtained using a beam block and phosphor scintillator. Results using this method are compared to capacitive probe measurements and trajectory modeling for the electron beam for a cyclotron autoresonance maseroscillator.
Plasma diagnostics at electron cyclotron resonance ion sources by injection of laser ablated fluxes of metal atoms72(2001); http://dx.doi.org/10.1063/1.1361087View Description Hide Description
Short pulses of neutral particles generated by laser ablation of metal targets have been injected into the Frankfurt 14 GHz electron cyclotron resonance (ECR) ion source. Rise/fall times of pulses of highly charged ions of Cd and Mg were registered as a function of microwave power and gas pressure. From a comparison of the measured data to numerical simulations, values of the electron density and the temperature in the ECR plasma were estimated to be about and a few keV. The effective electron temperature increases with increasing microwave power and decreases with increasing gas pressure. The electron density is only a weak function of the microwave power, but increases significantly with the gas pressure. In the gas-mixing mode of operation, an improved confinement of lowly charged ions was observed.
- NUCLEAR PHYSICS, FUSION and PLASMAS
Ion acoustic wave velocity measurement of the concentration of two ion species in a multi-dipole plasma72(2001); http://dx.doi.org/10.1063/1.1340021View Description Hide Description
The concentration of two species in a multi-dipole plasma was determined by measuring the ion acoustic wave group velocity and the electron temperature. The wave was launched from a grid immersed in the plasma and was detected by a Langmuir probe. Electron temperature was found separately from an characteristic trace. The measurements were performed in helium/xenon and argon/xenon plasmas. Typical parameters of the plasma were density plasma potential of 3–5 V, and pressure range from 1 to 20 mTorr. The accuracy of the measurement was from 2% to 4% depending on the mass difference between the two species and how accurately the group velocity and electron temperature are measured.
72(2001); http://dx.doi.org/10.1063/1.1362437View Description Hide Description
Experimental observation has been carried out to see the effect of magnetic field and grid biasing voltage in controlling the sheath thickness in a magnetized plasma system. The experiment is carried out in a stainless steel chamber which is divided into two regions by a mesh grid, via the source region and the diffused region. The characteristic behavior of the ion rich sheath formed across the grid under various conditions of the applied magnetic field and grid biasing voltage has been investigated experimentally. It has been observed that at both conditions of increasing magnetic field and grid biasing voltage, sheath width expands in the source region, whereas in the diffused region, no such noticeable variation has been found. This study has been accompanied by the measurement of the electron temperature in both regions of the chamber via the source region and the diffused region with the help of the Langmuir probe.Plasma is produced in the source region and it penetrates into the diffused region through the grid. It has been found that the electron temperature decreases with increasing magnetic field in the source region while kept at a constant grid biasing voltage. However, in the diffused region the opposite variation has been observed. The variation of electron temperature with grid biasing voltage in both regions is not very significant.
72(2001); http://dx.doi.org/10.1063/1.1366631View Description Hide Description
A new method to estimate the negative ion density in reactive gas plasmas with a Langmuir probe is proposed. This method has the advantage that the negative ion density is evaluated only by taking the ratio of the ion saturation–electron saturation current ratio obtained from the curve of the Langmuir probemeasured in an electronegative-gas mixture plasma to that measured in a reference noble gas plasma. The negative ion density in a double plasma is estimated utilizing this method. Furthermore, the negative ion density measured with this method is confirmed to agree with that calculated from the measured phase velocity of the ion acoustic wave (fast mode) in the double plasma, where positive and negative ion masses are obtained from the spectrum analysis with a quadrupole mass spectrometer.
72(2001); http://dx.doi.org/10.1063/1.1362438View Description Hide Description
An interferometric registration scheme for dynamic plasma pressuremeasurements has been developed and tested. The sensitive element of this scheme is an acoustic quartz rod. A description of the method is presented. The main features of the proposed scheme are the absence of a calibration procedure, the small size of the sensitive area, and the possibility of separating the studied object from the measuring setup. Experimental results for a capillary discharge plasma in a polyethylene slab with three different diameters of the channel, namely, 1.5, 2, and 3 mm are shown and illustrate the feasibility of the approach. Pressures in the range 30–600 bar were recorded.
A measurement method of absolute hydrogen atom density in plasmas by -photon laser-induced fluorescence spectroscopy72(2001); http://dx.doi.org/10.1063/1.1367356View Description Hide Description
A technique of -photon laser-induced fluorescence (LIF) spectroscopy has been developed for measuring hydrogen atom density in plasmas. In this method, the following two-step excitation scheme is employed to excite ground-state H atoms to the state. In the first step, H atoms at the ground state are excited to the state by two photons at a wavelength of 243 nm. Subsequently, the state is excited to the state by the third photon at 486 nm in the second step. Fluorescence emission at 486 nm line) is detected to determine the H atom density at the ground state. Since the wavelength used in the first-step excitation is the half of the wavelength used in the second-step excitation, one tunable laser with a system for second harmonic generation can be utilized in the measurement. The absolute density was evaluated by comparing the intensity of LIF emission from H with that from Xe at a known gas pressure. The present method is suitable for diagnostics of reactive plasmas since optical dissociation of molecules and radicals can be avoided because of the low photon energy of the laser radiation.
A three-dimensional Gaussian-beam ray-tracing program for designing interferometer/polarimeter plasma diagnostics72(2001); http://dx.doi.org/10.1063/1.1361083View Description Hide Description
We have developed a three-dimensional Gaussian-beam ray-tracing program to aid in the design of infrared, far-infrared, and millimeter waveinterferometer and polarimeterdiagnostic systems for magnetic confinementfusion relevant plasma physicsexperiments. An overview of the program is presented along with a description of the ray-tracing algorithm. A model is developed for the case of diffraction of a Gaussian beam off a cylindrical grating and is shown to be in good agreement with experimental measurements. The program has been used to aid the design of the scanning-grating interferometer system for the H-1NF heliac experimental plasma device. The program is written in the Research Systems Inc. Interactive Data Language and, on a typical modern personal computer, is able to trace and render the ∼50 element three-view 44-beam H-1NF interferometer optical system in about one minute.
72(2001); http://dx.doi.org/10.1063/1.1362457View Description Hide Description
We present a general analysis of possible variants of refractive index gradient (RING) diagnostics with a laser beam probe. Using a differential bicell photodiode as a detector, the sensitivity, dynamic range, and geometric restrictions of RING deflectometry have been found for lensless, one lens, and “three-telescope” optical schemes. The three-telescope method is found to be the most flexible and easily aligned. If the refracted/deflected laser beam cross section in the back focal plane of the output lens is also recorded using a fast framing camera, measurements of the beam deflection and its spatial frequency spectrum after passing through the refractive medium can be obtained simultaneously. A general relation is presented between the Fourier transform of a Gaussian beam by the output lens and the spatial frequency spectrum of the inhomogeneities. From these considerations, we present the specific design of a RING diagnostic for study of anode plasma evolution in a magnetically insulated ion diode on the Cornell Beam Research Accelerator (COBRA) (800 kV, 80 ns pulse). The maximum density gradient was found to be located about 0.4 mm from the anode surface at the peak of the diode voltage pulse. The electron density at this position was about The transverse spatial frequency distribution of the probe laser beam after passing through the anode plasma was recorded using the optical Fourier transform technique. This experiment demonstrates that the combined integrated deflection. (RING) and Fourier transform optical techniques can give a great deal of information about this thin (∼1 mm) anode plasma layer.
- MICROSCOPY and IMAGING
Element-specific imaging of magnetic domains at 25 nm spatial resolution using soft x-ray microscopy72(2001); http://dx.doi.org/10.1063/1.1351840View Description Hide Description
The combination of magnetic circular dichroism as a magnetic contrast mechanism and a transmission x-ray microscope allows imaging of magnetic structures with lateral resolutions down to 25 nm. Results on magneto-optical layers system with thermomagnetically written bits of various sizes were obtained at the x-ray microscope XM-1 at the Advanced Light Source in Berkeley, CA. The results prove the thermal stability of the bits in the recording process. Furthermore the capability of soft x-raymicroscopy with respect to the achievable lateral resolution, element specificity and sensitivity to thin magnetic layers is demonstrated. The potential of imaging in applied magnetic fields for both out-of-plane and in-plane magnetized thin magnetic films is outlined.
A picosecond time-resolved photoluminescence microscope with detection at wavelengths greater than 1500 nm72(2001); http://dx.doi.org/10.1063/1.1366635View Description Hide Description
We describe a picosecond resolution time-resolved photoluminescence microscope with high detection sensitivity at wavelengths extending beyond 1500 nm. The instrument performs time-correlated single photon counting using an InGaAs/InP single photon avalanche diode as a detector, and provides temporal resolution of less than 300 ps (full width at half maximum) and spatial resolution down to 4 μm at a sample temperature between 4 and 300 K. Analysis of noise characteristics indicates the ability to measure the excess carrier lifetimes of semiconductor devices with excited carrier densities of less than
72(2001); http://dx.doi.org/10.1063/1.1366630View Description Hide Description
When the atomic force microscope(AFM) is used for force measurements, the driving speed typically does not exceed a few microns per second. However, it is possible to perform the AFM force experiment at much higher speed. In this article, theoretical calculations and experimental measurements are used to show that in such a dynamic regime the AFM cantilever can be significantly deflected due to viscous drag force. This suggests that in general the force balance used in a surface force apparatus does not apply to the dynamic force measurements with an AFM. We develop a number of models that can be used to estimate the deflection caused by viscous drag on a cantilever in various experimental situations. As a result, the conditions when this effect can be minimized or even suppressed are specified. This opens up a number of new possibilities to apply the standard AFM technique for studying dynamic phenomena in a thin gap.
A nondestructive technique for determining the spring constant of atomic force microscope cantilevers72(2001); http://dx.doi.org/10.1063/1.1361080View Description Hide Description
We present a simple, accurate, and nondestructive method to determine cantilever spring constants by measuring the resonant frequency before and after the addition of a thin gold layer. The method for resonating the cantilevers uses electrostatic force modulation, which has been described for conductive cantilevers, but we demonstrate it can also be applied to silicon nitride cantilevers. The variations in spring constant for cantilevers of the same type across the same wafer are also explored.
72(2001); http://dx.doi.org/10.1063/1.1367360View Description Hide Description
In this article we present a simple, nonoptical shear force detection scheme to control probe–sample distance for a scanning near-field optical microscope (SNOM). Shear force detection is realized by attaching a tapered optical fiber probe to a piezoelectric bimorph cantilever in which one piezo layer generates a maximum piezo voltage when the cantilever is excited at resonance by the other piezo layer. The amplitude of the piezo voltage will decrease as the probe approaches a sample's surface due to probe–sample interacting shear force. Keeping the piezo voltage constant provides a very sensitive method by which to control probe–sample distance. Based on the shear force detection scheme, a shear force SNOM system has been built, operating in transmission collection mode. Shear force topographic and optical images have been taken using uncoated optical fiber probes fabricated by a chemical etching technique. The results suggest that the system is very reliable, repeatable, and easy to use.
- CONDENSED MATTER; MATERIALS
72(2001); http://dx.doi.org/10.1063/1.1367363View Description Hide Description
Measurement using a Kelvin probe depends vitally on the acquisition of a zero field between the probe and the sample. The charges in the space significantly affect the measurement of the work function change via the Kelvin probe due to the induced electric field between the probe and the sample, which adds uncertainty to the probe output. A simple model is presented in this article to depict the origin of the measurement errors. Then a solution, error deduction method, is suggested to effectively eliminate the influence due to charging by introducing a reference sample. The solution is also valid to cancel the error due to the change in the work function of the probe. The experimental results verified the feasibility of the method. Also discussed are other factors that affect the state of charging within a vacuum chamber. Careful shielding is necessary even if a stable reference sample is available. In the case where no stable reference samples are available, the use of insulating materials should be kept at a minimum. Once the insulating materials are used, they should be protected from being charged as much as possible. Any charge-generating components should be isolated by appropriate shielding, if possible, to avoid any leakage of charge into the space.