Volume 73, Issue 4, April 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
73(2002); http://dx.doi.org/10.1063/1.1460929View Description Hide Description
A comparison of different calibration methods for optical tweezers with the differential interference contrast (DIC) technique was performed to establish the uses and the advantages of each method. A detailed experimental and theoretical analysis of each method was performed with emphasis on the anisotropy involved in the DIC technique and the noise components in the detection. Finally, a time of flight method that permits the reconstruction of the optical potential well was demonstrated.
73(2002); http://dx.doi.org/10.1063/1.1461872View Description Hide Description
This article reports on a modified cavity phase-shift (CAPS) method for accurate and reliable characterization of high reflectance mirrors. Our approach relies on using a directly modulated Fabry–Pérot laser to circumvent the difficulties encountered in previous attempts with the CAPS method. The Fabry–Pérot laser diode ensures a constant coupling between the probe laser and test cavity modes. This results in a stable beam intensity transmitted through the test cavity allowing for accurate measurements of the phase shift from which the absolute reflectance can be determined. The experimental arrangement presented in this article is versatile and easy to use. The method is nondestructive and especially suited for the characterization of distributed Bragg reflectors(DBRs) employed in vertical-cavity optoelectronic devices. A premium feature of this method is its capability to probe a relatively small area of less than 1 mm which can be positioned anywhere across the surface of the wafer. We demonstrate the use of the method by measuring the absolute reflectance of metalorganic vapor-phase epitaxygrown AlAs/GaAs DBRs for 1.3 μm vertical-cavity lasers.
73(2002); http://dx.doi.org/10.1063/1.1459093View Description Hide Description
A low cost interrogation scheme is demonstrated for a refractometer based on an in-line fiber long period grating (LPG) Mach–Zehnder interferometer. Using this interrogation scheme the minimum detectable change in refractive index of is obtained, which is the highest resolution achieved using a fiber LPG device, and is comparable to precision techniques used in the industry including high performance liquid chromatography and ultraviolet spectroscopy.
73(2002); http://dx.doi.org/10.1063/1.1461876View Description Hide Description
A geometry-independent approach to a unified description of intensity corrections factors for the two most common surface scattering geometries is presented. Simple analytical formulas relate the representations of the Lorentz factor, polarization factor, area factor, and rod interception factor in a surface reference frame to the actual diffractometer circles. An analytical formula for the area correction for very large exit angles, as can be achieved on modern diffractometers, is derived. A test data set shows that the derived formulas provide a good description of the scattering intensity up to the edge of the Ewald sphere.
New adjustment technique for time coincidence of femtosecond laser pulses using third harmonic generation in air and its application to holograph encoding system73(2002); http://dx.doi.org/10.1063/1.1461877View Description Hide Description
The third harmonic generation of light (266 nm) is enhanced, sensitively depending on the time delay between a pair of pulses split from a single 800 nm femtosecond laser pulse, when they are focused and collided in air. This finding offers a convenient and widely applicable technique to detect temporal and spatial overlapping of two femtosecond pulses. This technique has several advantages over the conventional sum frequency generation method using nonlinear optical crystals, since it obviates the need for expensive crystals, free from phase matching, and elimination of temporal walk off. By applying it to “a holographicencoding system using an interference femtosecond laser pulse,” a periodic fringe spacing is minimized to ∼430 nm by extending the colliding angle between two-pulse beams up to ∼160 °C.
Design and performance of a THz emission and detection setup based on a semi-insulating GaAs emitter73(2002); http://dx.doi.org/10.1063/1.1459095View Description Hide Description
We have built a relatively simple, highly efficient, THz emission and detection system centered around a 15 fs Ti:sapphire laser. In the system, 200 mW of laser power is focused to a 120 μm diam spot between two silverpaint electrodes on the surface of a semi-insulating GaAs crystal, kept at a temperature near 300 K, biased with a 50 kHz, ±400 V square wave. Using rapid delay scanning and lock-in detection at 50 kHz, we obtain probe laser quantum-noise limited signals using a standard electro-optic detection scheme with a 1-mm-thick (110) oriented ZnTe crystal or a (110) oriented 0.1-mm-thick GaP crystal. The maximum THz-induced differential signal that we observe is corresponding to a THz peak amplitude of 95 V/cm. The THz average power was measured to be about 40 μW, to our knowledge, the highest power reported so far generated with Ti:sapphire oscillators as a pump source. The system uses off-the-shelf electronics and requires no microfabrication techniques.
73(2002); http://dx.doi.org/10.1063/1.1435823View Description Hide Description
Methods to collect two-dimensional time-resolvedx-ray diffractionpatterns from surfaces/interfaces were developed. Reflection surfacex-ray diffraction utilizing high brilliance x rays and a charge coupled device can achieve a time resolution as good as one second. Also, two-dimensional maps of reflectivity rocking curves can be recorded fast enough to monitor growth processes. These methods were demonstrated for the study of Ag and Pbfilms on Si surfaces.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
73(2002); http://dx.doi.org/10.1063/1.1453503View Description Hide Description
A magnetic structure composed of electromagnets and permanent magnets is proposed as an undulator to control the polarization state of synchrotron radiation. The horizontal field is produced by the permanent magnets and tuned by moving them along the undulator axis, while the vertical field is produced by the electromagnets and tuned by changing the coil current. The proposed device has two important degrees of freedom. One is that of the periodic length of the electromagnets and the other is that of the relative phase between the vertical and horizontal fields. Thanks to them, the proposed device works as various types of undulator to realize polarizationcontrol.
Characterization of a high-intensity bipolar-mode pulsed ion source for surface modification of materials73(2002); http://dx.doi.org/10.1063/1.1455137View Description Hide Description
A high-intensity pulsed ion source of TEMP-type series, operating in bipolar mode, has been developed as a unique pulsed energy source to produce a high-intensity pulsed ion beam (HIPIB) for surface modification of materials. To generate the ion beam, a specially shaped bipolar pulse, consisting of a first negative pulse and a second delayed positive pulse both of nanosecond width, is formed by a double coaxial pulse-forming line (PFL) powered with a Marx generator and supplied to a magnetically insulated ion diode (MID) by a self-magnetic field. It is found that the efficient generation of a HIPIB is mainly dependent on the delay time of the bipolar pulse, adjusted by pressure ratio in the two gas switches of a PFL, and the anode–cathode (A–K) gap distance in the self-magnetic field MID. The delay time determines the effective area on the anode surface for plasma generation and the A–K gap distance ensures the stability of the process. A proper delay time and a proper A–K gap distance are obtained by a series of experimental investigations. Under delay time from 30 to 280 ns and several different A–K gap distances, the typical wave forms of the bipolar pulses at a dc charging voltage of 45 kV to Marx generator are illustrated to clarify the effects of delay time and A–K gap distance on the ion beamgeneration. The proper A–K gap distance is not uniform, varied from 6 to 8 mm, and the corresponding proper delay time is 250 ns. The most efficient plasma generation leads to a maximum output of HIPIB with a peak ion current density of 350 A cm−2 and a beam pulse width of 75 ns (full width at half maximum), at an accelerating pulse of 220 kV with a pulse width of 100 ns.
73(2002); http://dx.doi.org/10.1063/1.1461882View Description Hide Description
The knowledge of the detection efficiency of the microchannel plate is of upmost importance in many applications using this type of particle detector. However, few attempts have been made to measure the absolute value of the detection efficiency of microchannel plates. In this article, a new way of accurately measuring the absolute value of the detection efficiency is described. When particles hit the detector in the interchannel web of the microchannel plate, secondary electrons are generated. These secondary electrons can be returned to neighboring channels by means of an electric field applied at the front face of the detector. The resulting increase in the detection efficiency has been quantified using the absolute measurement of the detection efficiency. It is found that the value of the returning field has to be carefully set in order to prevent detection artifacts that may lead to a wrong estimation of the detection efficiency. In the best conditions, the detection efficiency increase reaches 40%.
73(2002); http://dx.doi.org/10.1063/1.1461873View Description Hide Description
A new steady-state, low-energy, and high-flux ion beam test device has been developed to study erosion, re-deposition, and hydrogen retention mechanisms of plasma facing materials under high flux conditions. Sourceplasma was produced by electron cyclotron resonance discharge with 2.45 GHz microwave. Ion extraction was made by spherical multiaperture triode electrodes to obtain high ion flux at the focal point. Ion extraction aperture geometry was designed so that high current density can be obtained in the case of low energy (<500 eV) by applying high deceleration voltage. It was demonstrated that low-energy and high-flux ion beam can be produced by increasing deceleration voltage with the fixed acceleration voltage (beam energy). In the case of hydrogen beam energy of 300 eV, beam power density has rapidly increased with a deceleration voltage over accel–decel ratio 4, and maximum beam flux is On the other hand, maximum beam flux of was obtained for 3 keV hydrogen beam. These fluxes are much higher than that of conventional mass analyzed ion beam
73(2002); http://dx.doi.org/10.1063/1.1461875View Description Hide Description
The study of negative ion production in the multicusp negative ion sources (MS) was done by the directed deposition of well-defined amount of cesium into MS, by the cesium recovery from the polluted layers in the MS and by the plasma grid masking. The data obtained evidences the surface-plasma mechanism of negative ion production in the MS. It is shown that a dynamic cesium-tungsten coverage on a plasma grid surface provides the enhanced production in the MS. The coverage is produced on the hot plasma grid surface due to coadsorption of cesium and of tungsten, evaporated from filaments. The permanent flux of cesium to plasma grid coverage is produced by the thick reservoir with a high (75%) percentage of cesium on the cold anodesurface. A relatively high cesium seed with a rate of about 20 mg/1 h/30 shots operation is necessary to support the increased production in the regular MS. Electrode processing by an additional discharge recovers the cesium from the aged coverage and enhances the production without an additional cesium seed. Recovery processing permits the use of deposited cesium more efficiently and to minimize cesium addition during the MS long-term operation. An essential decrease of cesium escape to accelerator and an improvement of injector high voltage operation is produced by introducing the shutter mask at plasma grid surface. The shutter mask experiment displayed, that >60% of beam current was born on the hot shutter mask surface. The source operation with a high negative shutter mask biasing and an increased hydrogen pressure resulted in a 35% higher yield, than of regular MS.
Spatial and temporal diagnostics of a high-brightness electron beam by technique of laser Thomson scattering73(2002); http://dx.doi.org/10.1063/1.1461879View Description Hide Description
A technique of spatial and temporal diagnostics of an electron beam was investigated experimentally by Thomson scattering of the electron beam with a femtosecond laser beam at a 90° configuration. The spatial structure of the electron beam was measured by scanning the laser across the electron beam in the Thomson scattering, and compared with imaging of the beam profile on a phosphor screen using a charge coupled device. By scanning the femtosecond laser beam in time along the electron beam, we measured the temporal density distributions of short and long electron bunches which were produced in a photocathode rf gun with laser injection phases of 30° and 55°. A subpicosecond time slice of the actual electron beam was achieved in the measurement.
73(2002); http://dx.doi.org/10.1063/1.1458043View Description Hide Description
Typical magnetic materials are experimentally characterized. Particularly our efforts are concentrated on deriving a core-loss scaling in operation region with minor loops for highly repetitive voltage modulators. The core losses are evaluated as functions of magnetization rate and flux swing using semiempirical equations based on magnetization models. A prototype induction module has been successfully operated up to 1 MHz and the loss values are adapted well to the core-loss criterion.
- NUCLEAR PHYSICS, FUSION and PLASMAS
73(2002); http://dx.doi.org/10.1063/1.1453499View Description Hide Description
A matched filter analysis has been developed to identify the amplitude and phase of magnetohydrodynamic modes in DIII-D tokamak plasmas using magnetic probe signals As opposed to conventional Fourier spatial analysis of toroidally spaced probes, this analysis includes data from both toroidally and poloidally spaced magnetic probe arrays. Using additional probes both improves the statistics of the analysis and more importantly incorporates poloidal information into the mode analysis. The matched filter is a numeric filter that matches signals from the magnetic probes with numerically predicted signals for the mode. The numerical predictions are developed using EFIT equilibrium reconstruction data as input to the stability code GATO and the vacuum field code VACUUM. Changes is the plasma equilibrium that occur on the same time scale as the mode are taken into account by modeling simple matched filter vectors corresponding to changes in total plasma current, plus vertical and horizontal plasma shifts. The matched filter method works well when there is good understanding of a mode and good modeling of its structure. Matched filter analysis results for a fast growing ideal kink mode, where equilibrium change effects are minimal, show the effectiveness of this method. A slow growing resistive-wall mode (RWM) is also analyzed using the matched filter method. The method gives good results for identifying the amplitude and phase of the RWM but the simple equilibrium vectors are insufficient for complete elimination of equilibrium changes on this time scale. An analysis of the computational requirements of the scheme indicates that real-time application of the matched filter for RWM identification will be possible.
73(2002); http://dx.doi.org/10.1063/1.1453500View Description Hide Description
In order to measure fine scale fluctuations of visible emission in plasmas, the four-beam correlation method is proposed and its feasibility is studied. This is an extension of the conventional cross beam techniques and uses four beams (i.e., four lines of sight), which lie in two parallel planes. While two beams in the same plane provide the fluctuation levels at the intersection, two beams in different planes provide correlation as a function of the distance between the planes. Various cases are considered, and the method feasibility is shown. This method is useful for reconstructing a two- or three-dimensional wave-number spectrum. In addition, it can be used to measure the magnetic field direction assuming that the parallel correlation length is longer than the perpendicular one.
Impurity density profile measurements with a fast filter spectrometer and a modulated diagnostic beam on Tore Supra by charge exchange recombination spectroscopy73(2002); http://dx.doi.org/10.1063/1.1459092View Description Hide Description
A simple interference filter spectrometer, together with a modulated neutral diagnostic beam is used on the Tore Supra tokamak for the measurement of carbon impurity profiles by charge exchangespectroscopy. A temporal resolution of 8 ms with a good signal to noise ratio is obtained with a neutral hydrogen beam of about 1 A equivalent neutral current at the plasma edge at 45 kV. Experimental results with ion cyclotron heating show that fast changes of the heating power are followed by transient modifications of the carbon impurity profile. The evolution of the profile from peaked to hollow and back to normal can be followed step by step and a typical diffusion time of about 30 ms is measured.
73(2002); http://dx.doi.org/10.1063/1.1458046View Description Hide Description
This article describes hardware and softwaresolutions to a need which is comprised of (i) acquisition of a large volume of high speed data with multiple time scales, (ii) control of various operational parameters of device and diagnostics, and (iii) processing and management of the acquired data for a large volume plasma device. The solution relies on the base of a VXI bus and uses a standard PC with a Windows 98/NT operating system and C as the programming language. The system is networked with the existing network with the result of allowing a large data storage space of processing facilities from any terminal in the laboratory.
- BASIC PHENOMENA
73(2002); http://dx.doi.org/10.1063/1.1461883View Description Hide Description
Detection of systematic errors is an important issue in the treatment of measurement data. Existing methods such as the t-detection method and the residual error method are based on the statistics theory that requires large sample sizes and known distributions. For small sample sizes and unknown distributions, these methods could become ineffective. To solve this problem, a new method is proposed to detect the systematic errors in a measurement process, based on fuzzy set theory. In the proposed method, the distribution of measured data may be unknown and the sampling size small. Mathematical models for systematic error detection are presented together with criteria for the detection. Results of the case studies show that the proposed method is effective and the proposed criteria are consistent in identifying monotonic and periodic systematic errors.
- MICROSCOPY and IMAGING
Tuning-fork-based fast highly sensitive surface-contact sensor for atomic force microscopy/near-field scanning optical microscopy73(2002); http://dx.doi.org/10.1063/1.1462038View Description Hide Description
We have developed a surface-contact sensor on the basis of a tuning fork which differs from the previously described ones in that it has a high operating speed (up to 100 times as fast as the so-called Q limit), requires no external piezoelectric drive, has a sufficiently high sensitivity, and features a “soft” probe attachment which makes the lifetime of the probe equal to that of the standard atomic force microscopy. When using a “soft” probe with a rigidity of 0.5 N/m, one can reliably detect probe tip-to-sample distance variations as small as 0.1 nm. The resonance frequency resolution attained amounted to The rate of transient rise is (this refers to the response time of the sensor proper with the Z-coordinate feedback loop open and not to the response time of the microscope as a whole). We have theoretically substantiated the fact that the Q limit, where is the Q factor of the tuning fork proper, is not a fundamental restriction on the operating speed of the sensor. This sensor characteristic is governed by another independent quantity, namely, the quality factor of the tuning fork preamplifier system that can be varied to suit the experimenter. In that case, the fundamental force limitation on the sensitivity of the sensor, associated with its operating speed and the Q factor of the tuning fork, is