Volume 73, Issue 3, March 2002
Index of content:
- OPTICS; ATOMS and MOLECULES; SPECTROSCOPY
73(2002); http://dx.doi.org/10.1063/1.1448898View Description Hide Description
We present a new automated device for measurement of time-resolved frequency chirp of optical transmitters. It measureschirp in real time by utilizing two temperature-tunable silicon-wafer etalons as frequency discriminators. The time-resolved frequency chirp in a range of ±12 GHz can be measured with a time resolution of approximately 40 ps.
Highly reproducible laser beam scanning device for an internal source laser desorption microprobe Fourier transform mass spectrometer73(2002); http://dx.doi.org/10.1063/1.1445868View Description Hide Description
Traditionally, mass spectrometry has relied on manipulating the sample target to provide scanning capabilities for laser desorption microprobes. This has been problematic for an internal source laser desorptionFourier transformmass spectrometer (LD-FTMS) because of the high magnetic field (7 Tesla) and geometric constraints of the superconducting magnet bore. To overcome these limitations, we have implemented a unique external laser scanning mechanism for an internal source LD-FTMS. This mechanism provides adjustable resolution enhancement so that the spatial resolution at the target is not limited to that of the stepper motors at the light source (∼5 μm/step). The spatial resolution is now limited by the practical optical diffraction limit of the final focusing lens. The scanning mechanism employs a virtual source that is wavelength independent up to the final focusing lens, which can be controlled remotely to account for focal length dependence on wavelength. A binary index provides an automatic alignment feature. The virtual source is located ∼9 ft from the sample; therefore, it is completely outside of the vacuum system and beyond the 50 G line of the fringing magnetic field. To eliminate reproducibility problems associated with vacuum pump vibrations, we have taken advantage of the magnetic field inherent to the FTMS to utilize Lenz’s law for vibrational dampening. The LD-FTMS microprobe has exceptional reproducibility, which enables successive mapping sequences for depth-profiling studies.
73(2002); http://dx.doi.org/10.1063/1.1448143View Description Hide Description
Two new straightforward methods for the evaluation of optical losses in planar waveguides are proposed and discussed. The first method exploits a single nonsliding isosceles prism and it allows the attenuation to be determined through the measurement of the power exiting the waveguide and the evaluation of the coupled power. It requires a very simple operation procedure, it allows a mode-selective determination of losses, and it presents a good accuracy provided that the sample is not too short. The second method uses end-fire coupling and it is based on the measurement of the output power together with the power back-reflected by the output face of the waveguide. The main advantage of the method is that it can be very accurate also for shorter waveguides and that its accuracy is to a high degree insensitive with respect to the optical depth of the waveguide. It provides better results in the case of high refractive indexwaveguides that give an intense back-reflected signal. Experimental results obtained with both methods on two different waveguides are presented.
- PARTICLE SOURCES, OPTICS and ACCELERATION, DETECTORS
73(2002); http://dx.doi.org/10.1063/1.1448908View Description Hide Description
Experiments have been carried out to optimize the yield of ions from the plasma produced by a 100 J laser. The laser power density on the target surface was varied between and by changing the focal spot size. Data on the production of to ions are presented for the 15 and 40 ns laser pulse durations. The 10 mA/80 μs pulses of ions (about ions per pulse) were directed into an extraction aperture of 3.4 cm in diameter for optimal irradiation conditions. This is comparable with the parameters of the MEVVA ion source. Laser ion sources can be especially attractive due to the absence of the “noise” problem, typical for the MEVVA ion source, and because of the possibility of generating higher charge states. The emittance of the lead ion beam extracted from the laser-produced plasma was measured by using a 5 J laser, which provided plasma parameters (the electron temperature, ion velocities, and the charge state distribution) close to those of the plasma generated by the 100 J laser under optimized (for the yield of ions) conditions. The emittance was measured for 30 and 50 kV extraction voltages with a 10 μs temporal resolution. The value of about 400 π mm mrad was obtained for the lead ion beam with the total current of 10 mA, and a pulse length of 75 μs for about 75% of the beam ions.
73(2002); http://dx.doi.org/10.1063/1.1445866View Description Hide Description
A long undulator is a direct way to create a brilliant synchrotron radiationsource. However, the length of undulators has been bounded by technical limitations and available space in accelerators. The first long undulator was developed at SPring-8, which is a third-generation synchrotron radiation facility equipped with four 30-m-long straight sections, making it the most brilliant x-raysource in the world. The magnets are placed inside a vacuum chamber and this makes it possible to arrange 780 periods continuously for 25 m. The absolute flux and spectrum of the output radiation were measured and compared with theory. The observed spectrum is in a good agreement with theory once the electron beam orbit was corrected for geomagnetic fields.
73(2002); http://dx.doi.org/10.1063/1.1435841View Description Hide Description
An electrostatic energy analyzer is described that allows parallel acquisition of the energy spectrum of charged particles over a wide range of energies and over the complete range of azimuthal directions. The analyzer is similar to the cylindrical mirroranalyzer except that a linear potential variation is applied in the axial direction to the outer cylinder. The analyzer can also be used in a second-order focusing mode to analyze a narrow range of energies with higher energy resolution. A position sensitive detector is required, the shape of which depends on the way in which the analyzer is used. Comparisons with existing electron energy analyzers are given.
73(2002); http://dx.doi.org/10.1063/1.1427036View Description Hide Description
The afterglow is known to produce pulses of highly charged ions suited for injection to accelerator facilities. We tested a new technique of magnetic pulsed extraction (PUMAEX) in our plateau-ECR ion source PECRIS III. This source has a large resonance volume due to a homogeneous magnetic field in its center. Compared to the afterglow of PECRIS III we reached higher ion currents. The length of the PUMAEX pulses depends on the electric current pulse in the PUMAEX coil, we used pulse lengths of 0.25–1 ms. The PUMAEX coil is a small coil which is placed around the extraction hole in the plasmaelectrode. The absolute value of the static magnetic field near the extraction hole can be reduced below the value in the center of the source by a current pulse in the PUMAEX coil. Thus the magnetic confinement of the ECR plasma is opened specifically in the direction of the extraction hole. The plasma therefore escapes from the source more directed to the extraction hole than in the afterglow. Since the coil is very near to the location where the magnetic field is changed, it was possible to use a small coil, which could be placed in the plasmaelectrode of PECRIS III without major modifications. Currently we are building a new multifrequency plateau ECRIS. Using eight frequencies between 17 and 18 GHz we intend to improve the electron heating in the large resonance volume. The highest frequency will be set to the resonance frequency of cold electrons at the center of the source. The lower frequencies will match the resonance frequencies of more energetic electrons as those electrons have a greater relativistic mass. In the near future we will test the PUMAEX technique with this new source.
73(2002); http://dx.doi.org/10.1063/1.1430272View Description Hide Description
Electron cyclotron resonanceion sources (ECRIS) are sophisticated plasma devices, with a complex magnetic structure. The designer of an ECRIS usually solves the following problems: (i) the conception of an efficient magnetic structure; (ii) the design of an efficient extraction system. To solve the first problem numerical codes are either free or commercially available; we briefly summarize the characteristics of these codes, with particular emphasis on cryogenic devices. The second problem is much more tricky, since it is not only an electrostatic problem, as it also involves the plasma. Different codes are, however, able to treat the problem, with some assumptions concerning the plasma; these assumptions are discussed. These codes can be very useful for the new ECRIS generation, where the current densities become very high. But today there is no program available for the calculation of the charge state distribution of the ions, the magnetic structure being given; only the experience of the designer and a certain knowledge of conception rules can lead to some (rough) estimate of the future performances. We present in this paper the first version of such a code, where the input parameters of the code are the technological parameters. These inputs being given, the code solves the self-consistent problem of the plasma creation (through the resonant interaction of the electrons with the wave), the production of the ions and their transport to the extraction zone. The first results of this code are presented and discussed. This article shows that it will be soon possible to have reliable predictions of the performances of an ECRIS.
Stabilization of ion–ion instability with the aim of improvement transportation characteristics of a negative ion beam73(2002); http://dx.doi.org/10.1063/1.1430509View Description Hide Description
In the transportation of ion beams in a rarefied gas the plasma is formed due to the ionization of gas particles by the beam ions, and this plasma partially compensates the space charge of the ions (and thus improves the conditions of beam transportation). Moreover, in the case of a negative ion beam at a certain critical value of gas pressure, the electric field in the beam changes its sign so that at high pressure the beam is transported in the focusing field. The regime of high pressures is attractive also for the reason that ion–electron and ion–ion instabilities, which develop at that, are not harmful—they may cause just a little growth of the beam emittance. However, in this regime losses of the beam ions due to recharging are essential. As to pressures lower than the critical one, at which these losses are not so essential, the beam transportation is practically impossible due to large-scale ion instability which in certain time points results in almost complete decompensation of the beam space charge. In the present proceeding the way of stabilization of this instability is proposed, which gives the possibility of negative ion beam transportation at low pressures. Mechanism of the stabilization consists in diminishing the wavelength of unstable oscillations by means of the shielding action of the electrons introduced from the heated emitter and held in the beam by a periphery magnetic field. Experiments performed with the beam of hydrogen negative ions with 30 mA current and 10 keV energy have shown that introducing the electrons into the beam, which is transported at low pressure, really provides a essential reduction of ion–ion oscillations amplitude and, correspondingly, improves transportation characteristics of the beam.
Measurements of secondary electron emission and plasma density enhancement for plasma exposed surfaces using an optically isolated Faraday cup73(2002); http://dx.doi.org/10.1063/1.1431707View Description Hide Description
We present secondary electron yield and plasma enhancement factor data for siliconsurfaces exposed to Ar, He, and plasmas, for incident ion energies from 0.5–10 keV. A fiber-optic isolated Faraday cup was used to directly measure the ion current allowing a direct measurement of the secondary electron yield. This method automatically accounted for the effect of pulse-induced plasma density enhancement due to the ionization of neutral gas by acceleratedsecondary electrons, which we observed and measured quantitatively. The values of the secondary electron yields measured by this method were higher than published values measured by the conventional (ultraclean surface and ultrahigh vacuum) methods but lower than published values measured by previous plasma immersion ion implantation methods.
Investigation of the mechanism of current density increase in volume sources of hydrogen negative ions at cesium adding73(2002); http://dx.doi.org/10.1063/1.1431405View Description Hide Description
In the present article the influence of adding cesium into the volume and on the surface of an ion source on its emission characteristics is studied both theoretically and experimentally. It is shown that cesium in the volume at conditions of a real ion source brings in a significant contribution to kinetic processes, but weakly influences the current of ions extracted from the source. It is shown both theoretically and experimentally that an observed increase of the current of ions with cesium added is due to the conversion of fast particles at the anode surface.
73(2002); http://dx.doi.org/10.1063/1.1431406View Description Hide Description
The idea of a space-charge lens for focusing intensive negative hydrogen ion beams is presented. Estimations of focal length of the lens are performed independent of parameters of the beam and gaseous medium. The effectiveness of the focusing ion beam with current up to ∼30 mA and energy of ∼10 keV by means of such a lens is demonstrated experimentally. Measured values of focal length are in good agreement with calculated ones.
- NUCLEAR PHYSICS, FUSION and PLASMAS
73(2002); http://dx.doi.org/10.1063/1.1436132View Description Hide Description
A 20-channel electron cyclotron emission (ECE) detection system for the grating polychromator in JT-60U has been developed to measure electron temperature and its perturbations. Indium antimonide (InSb) detectors aligned in a row are cooled by liquid helium held by a 50 cm-diam cryostat, in which a hold time of the liquid helium reaches 7 weeks. Twenty-channel preamplifiers are dc-coupled ones, which enable to measure low-frequency ECE. From the comparison between the new system and the existing 20-channel detector system, where a refrigerator for cooling InSb detectors and ac-coupled preamplifiers are used, it is found that the signal-to-noise ratio is improved, and that small perturbations can be detected. By using this system, the electron temperature and its perturbations near the internal transport barrier in a reversed shear discharge are measured in detail.
73(2002); http://dx.doi.org/10.1063/1.1445867View Description Hide Description
A three-channel heterodyne O-mode reflectometer system has been developed for density fluctuationmeasurements on the JT-60U tokamak. The system consists of one fixed channel with a frequency of 34 GHz and two selectable frequency channels with frequencies in the range of 34–40 and 48–50 GHz in order to measure the change of fluctuation amplitude, fluctuationcoherence, and movement of the cutoff layer during the discharge. The reflectometer system has now become a standard diagnostic for density fluctuationmeasurements on JT-60U covering both core and edge plasma regions. The capabilities of the system are illustrated for three experimental cases: edge transport barriers in H-mode plasmas, internal transport barriers in reversed shear plasmas, and fast turbulence burst at the onset of multifaceted asymmetric radiation from the edge.
- BASIC PHENOMENA
73(2002); http://dx.doi.org/10.1063/1.1446040View Description Hide Description
Gravity-driven soap-film channels offer a convenient way to study two-dimensional hydrodynamics in the laboratory. With recently developed quantitative soap-film diagnostic techniques, velocity and thickness field information can be acquired to provide a complete description of the flow. We present a study of soap-film flow in a state-of-the-art tilted soap-film tunnel which can sustain a mean freestream velocity about 0.5 m/s with velocity fluctuations on the order of 1%. Our investigation concentrates on the evolution of the velocity and thickness profiles with downstream distance. We observe a hydrodynamically fully developed flow within a substantial range of distances and compare our results with the analytical solution based on the assumptions of linear air drag and constant film thickness. We find air drag to be the dominant dissipative mechanism in the flow, although the role of internal viscous dissipation is also apparent. Direct measurements of the air drag coefficient are in good agreement with the values inferred from the analytical solution.
73(2002); http://dx.doi.org/10.1063/1.1448141View Description Hide Description
An improved approach for extremely low fluid dosing rate on-line measurements was described. The approach can be employed to conduct in situcalibration of microfluidic devices. It is essentially a standard gravimetric method, with some modifications. The experimental setup included a high precision balance interfaced to a data sampling/processing PC. The continuous dosing rate generated from an electrochemical actuator was measured. A stable dosing rate reached as low as 0.19 μl/min (deionized water), which was lower than the water evaporation rate.
- MICROSCOPY and IMAGING
73(2002); http://dx.doi.org/10.1063/1.1448137View Description Hide Description
Millions of single-crystalsilicon cantilevers were fabricated by anisotropicetching of silicon by KOH. They could be tailored to measure from 500 nm to 100 μm in length and from 30 to 100 nm in thickness. Since the tips and the cantilevers were formed by a combination of crystal-line facets, they had very high uniformity, well-defined shape, and size. The density of the cantilevers was over 1 mil cantilevers per square centimeter. Typical mechanical characteristics of cantilevers measuring a few microns in length were spring constant a few N/m, natural frequency around 10 MHz, Q factor 5 in air, and in vacuum. The natural frequency of cantilevers within the same row differed by 0.01%. Displacement measurement of the cantilever from the back surface of the silicon substrate by an infrared Fizeau interferometer had a visibility of 0.1.
73(2002); http://dx.doi.org/10.1063/1.1445870View Description Hide Description
We describe the design of a user-friendly compact water-window x-ray microscope. The microscope is based on a liquid-jet-target laser-plasma source in combination with a normal-incidence multilayercondensermirror and high-resolution diffractive optics for the imaging. With its high mechanical and thermal stability, the instrument demonstrates enhanced resolution and potential for compact x-ray imaging with the quality of synchrotron-based microscopes. Furthermore, a new sample handling system, computer control, and other improvements facilitate application-oriented x-ray microscopy outside the synchrotron laboratory.
73(2002); http://dx.doi.org/10.1063/1.1445864View Description Hide Description
An instrument that combines the analytical power of Raman spectroscopy with the spatial resolution of the Atomic Force Microscope(AFM) is presented. This instrument is capable of resolving 50 nm scale spectral features or better by using surface enhanced Raman scattering at the AFM tip. The localized spectrochemical information allows the interpretation of the concurrently acquired friction or phase contrast AFM images. This instrument has a unique combination of features including side illumination of the tip–sample interface that permits opaque samples. As a result of precise focusing of a laser at the AFM tip–sample interface this instrument is also capable of laser beam profiling and studying optical trapping at the probe tip. Applications of this versatile instrument include chemical analysis of nanometer scale phenomena, chemical separation, and the potential for targeted single molecule spectroscopy.
- CONDENSED MATTER; MATERIALS
Velocity distribution measurement and two-wire field effects for electric deflection of a neutral supersonic cluster beam73(2002); http://dx.doi.org/10.1063/1.1447304View Description Hide Description
The basic principle of deflection of a beam of polarizable particles by an inhomogeneous electric field is straightforward, but its application to realistic beams having spatial and velocity distributions of finite widths requires special care. We present a detailed treatment of these issues as developed for a measurement of electric polarizabilities of alkali clusters in a continuous supersonic beam. We begin by describing a general technique for extracting the beamvelocity distribution from the time-of-flight profile generated by two separate 50–50 gating choppers. This method yields accurate model-free information directly from the experimental profile, without errors associated with detector delays, and with high signal throughput. We then use a simple but accurate approximation for the deflecting field variation over space, and derive analytical expressions for the deflection of beams of finite width and finite velocity spread. This allows us to evaluate the magnitude of a number of corrections to the ideal formula; the results are applicable to optimization of various electric and magnetic field deflection experiments. Finally, we demonstrate, both theoretically and experimentally, the phenomenon of local velocity variations across the beam profile which arise as a result of deflection.